Spectrochimica Acta Part A 64 (2006) 532–548

Water trapped in dibenzo-18-crown-6: Theoretical andspectroscopic (IR, Raman) studies

Marcin Dułaka, Rémi Bergougnantb, Katharina M. Frommb, Hans R. Hagemanna,Adeline Y. Robinb, Tomasz A. Wesołowskia,∗

a Department of Physical Chemistry, University of Geneva, 30, quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerlandb Department of Chemistry, University of Basel, 51, Spitalstrasse, CH-4056 Basel, Switzerland

Received 6 June 2005; received in revised form 11 July 2005; accepted 29 July 2005

Abstract

Experimental (IR and Raman) and theoretical (Kohn-Sham calculations) methods are used in a combined analysis aimed at refining theavailable structural data concerning the molecular guests in channels formed by stacked dibenzo-18-crown-6 (DB18C6) crown ether. Thecalculations are performed for a simplified model comprising isolated DB18C6 unit and its complexes with either H2O or H3O+ guests, whichata©

K

1

pstisboCaocwcf

(

1d

re the simplest model ingredients of a one-dimensional diluted acid chain, to get structural and energetic data concerning the formation ofhe complex and to assign the characteristic spectroscopic bands. The oxygen centers in the previously reported crystallographic structure aressigned to either H2O or protonated species.

2005 Elsevier B.V. All rights reserved.

eywords: Crown ethers; One-dimensional polymer structures; Proton conductors ; Spectrum assignment

. Introduction

Low-dimensional materials with interesting physicalroperties are a possible solution for down-scaling dimen-ions in information data storage, and are therefore morehan ever in the focus of current research. We are thusnterested in the controlled synthesis of one-dimensionaltructures via a supramolecular approach, either by com-ining hydrogen bonding with metal ion complexationr with ligand templating and �–� interactions [1–3].rown ethers and their complexes with metal cations havettracted considerable attention since the pioneering workf Pedersen [4]. They are remarkably selective on metalations, especially alkali and alkaline earth metals cations,hich is a topic of fundamental interest in coordination

hemistry and biochemistry. These macrocyclic ligandsascinate both because they can impose unusual coordination

∗ Corresponding author. Tel.: +41 223796957; fax: +41 223796518.E-mail address: tomasz.wesolowski@chiphy.unige.ch

T.A. Wesołowski).

numbers and geometries on metal ions, and because theyserve as models for metal ion transport across membranes[5]. They have also found an application as very selectiveseparation agents on mixtures of metal ions. Especially thecrown ethers with six O atoms such as 18-crown-6, dibenzo-18-crown-6 (DB18C6) and dicyclohexano-18-crown-6 areknown to form stable complexes with alkali metal salts[6–8]. They are rare examples where neutral organic ligandsare able to complex charged metal ions in a very efficientway.

As we are interested in the formation of new materials withfunctional properties, based on one-dimensional polymers[1–3,9–12] the question arose whether it was possible to stackDB18C6 ligands in such as way as to obtain one-dimensionalchannels. The potential capacity of DB18C6 to form � inter-actions upon stacking appeared to us as an interesting prop-erty in this context. As a model for biological membranes, weattempt to study these channels as proton conductors, filledwith water molecules and protonated species.

We have shown before that such a one-dimensionalchannel compound, 1∞[(H2O)⊂(DB18C6) (�2-H2O)2/2] 1∞

386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

oi:10.1016/j.saa.2005.07.060

M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548 533

Fig. 1. The unit cell of 1∞ [(H2O)⊂(DB18C6) (�2-H2O)2/2] 1∞ [(H3O)⊂(DB18C6) (�2-H2O)2/2] I3, viewed along c-axis.

[(H3O)⊂(DB18C6) (�2-H2O)2/2] I3, 1, can be obtainedupon serendipity, and that such stacked DB18C6 ethers areextremely rare [13,14]. Since the single crystal data has beenreported before [13,14], we will restrain ourselves to a shortdescription of the structure in the context of comparisonwith the theoretical results. 1 forms dark brown rod-likesingle crystals of the orthorhombic space group Pccn (No.56) with two independent half molecules in the asymmetricunit (Fig. 1). The structure consists of two different channelsof DB18C6. In one channel, the ligands are well stacked,containing three types of oxygen atoms (see Fig. 2). Two ofthem (we denote them as the oxygen atoms type A and C)are localized on the same side of the crown ether ring as thephenyl rings, and the third one (type B) in which the oxygenatoms are on the opposite side to the phenyl rings. The atomof the type C is coordinated by the oxygen atoms of the crownether, whereas the oxygen atoms of the type A and B act asbridging atoms to the next DB18C6 unit. The attribution of

F(

Fig. 3. Side view on the less well-stacked channel 2 (ch2) in 1∞[(H2O)⊂(DB18C6) (�2-H2O)2/2] 1∞ [(H3O)⊂(DB18C6) (�2-H2O)2/2] I3.

the hydrogen atoms to the oxygen sites of the type A, B, and Ccan not be made based on the X-ray experiment. In a secondchannel, parallel to the first in its propagation direction, thestacking is by far less perfect. Three oxygen types of atoms,A, C, and B are also found in the center of the channel (Fig.3), and again, the hydrogen atoms could not be attributed.It is important to notice that even though the mean O· · ·Odistances in the two channels are very similar, the less wellstacked crown ether molecules seem to be more distortedthan the other ones when looked at in detail. The distancebetween the oxygen sites B and C equals to 2.217 and2.241 Å in channels 1 and 2, respectively. The correspondingdistances between the C and A sites are longer and amount to2.409 and 2.385 Å. It is noteworthy that the distance betweenthe C and A sites matches very closely that in the H5O2+cation [15]. The counter ions, I3−, are arranged parallel to thechannels and fill the space left between the channels (Fig. 1).

X-ray diffraction studies allowed us to determine the de-tailed structure of the channel formed by stacked DB18C6units as well as the location of the oxygen atoms of the guestmolecules in the center of the channel. To get a more de-tailed picture, we turned to computer modeling and vibra-tional spectroscopy (IR and Raman). As the first step in thecomputational studies, we analyze the structural and vibra-tional properties of the simplest model of the channel, com-prising just one DB18C6 unit and one guest molecule (eitherHmrtcsaoomHttr

fi

ig. 2. Side view on the well-stacked channel 1 (ch1) in 1∞ [(H2O)⊂DB18C6) (�2-H2O)2/2] 1∞ [(H3O)⊂(DB18C6) (�2-H2O)2/2] I3.

2O or H3O+) by means of computer modelling. Such aodel allows us to reveal some of the important properties

elevant to the real system. In particular, it allows us to studyhe effect of the presence of the host in the center of therown ether on its structure and vibrational properties. Thisimplified model is especially adequate to study properties ofneutral or protonated guest molecule well separated from

ther guests in the channel. It is important to underline thatur model of the channel is not appropriate for studies ofore complicated and probably more realistic guests such as5O2+ or H7O3+ chains. In such structures, the distance be-

ween the oxygens is determined by the interactions betweenhe corresponding hydrogens and the oxygens of differentings.

The structure of the present work is the following. In therst part, we analyze the effect of binding different guests

534 M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548

Fig. 4. Optimized structures of DB18C6 and H2O·DB18C6 1q conformer.

Fig. 5. Optimized structures of H2O·DB18C6: 1b and 1m conformers.

on the geometry of the DB18C6 unit and identify the frag-ments cut out from the periodic solid corresponding to eitherH2O·DB18C6 or H3O+· DB18C6 complex. In the secondpart, we compare the calculated IR and Raman spectra ofthe studied model systems with experimental data in order toassign the experimental bands.

Table 1Calculated geometrical parameters of the H2O molecule trapped in DB18C6ether (conformers 1q, 1b, 1m) and of the isolated H2O (isol.)

Parameter 1q 1b 1m Isolated

r(O Ha) 0.981 0.979 0.987 0.974r(O Hb) 0.981 0.979 0.974 0.974∠(Ha-O Hb) 103.4 103.2 104.7 104.4

Distances (r) in Ångstroms, angles (∠) in degrees.

Table 2Calculated geometrical parameters of hydrogen bonds (conformers 1q, 1b,1m, 2t, 2t′)

Parameter 1q 1b 1m 2t 2t′

r(O· · ·O17) 3.355 3.264 2.868 2.669 2.607r(O· · ·O14) 3.354 3.259 – 2.920 2.851r(O· · ·O16) 3.217 3.367 – 2.856 2.837r(O· · ·O13) 3.217 3.364 – 2.705 2.708r(O· · ·O15) 3.216 3.363 – 2.705 2.708r(O· · ·O18) 3.217 3.364 – 2.856 2.838∠(O H· · ·O17) – 171.4 173.1 172.9 172.0∠(O H· · ·O14) – 171.4 – – –∠(O H· · ·O13) – – – 172.8 169.6∠(O H· · ·O15) – – – 172.9 169.6O denotes oxygen atom of H2O or H3O+ molecule. Distances (r) inÅngstroms, angles (∠) in degrees.

1.1. Theoretical methods

Optimized geometries, harmonic frequencies, IR intensi-ties and Raman scattering activities were determined usingKohn-Sham density functional theory [16]. The calculationswere carried out using the semi-local exchange-correlationB88-P86 functional [17,18] and polarized double-zeta splitvalence (DZVP) basis set [19]. The Gaussian 03 package wasused [20]. Minimum energy geometries were found impos-ing the tight convergence criterion on the RMS of the forces(0.00005 Hartree/Bohr or Hartree/Radian), specified by theoption #IOp(1/7 = 50) in the input route. Pruned (99,590)grid (UltraFine) was used.

To facilitate comparisons between calculated and experi-mental spectra, the theoretical ones were convolved with theGaussian-type functions with the full-width half-maximum(FWHM) equal to 12 cm−1. Absolute differential Ramanscattering cross sections which are proportional to the Ra-man intensities were calculated as described in ref. [21].Throughout this work, the reported harmonic frequencies arenot scaled.

Reported interaction energies were corrected for thebasis set superposition error (BSSE) by means of thecounterpoise technique of Boys and Bernardi [22]. The

3O+·D

Fig. 6. Optimized structures of H

B18C6: 2t and 2t′ conformers.

M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548 535

Table 3Calculated geometrical parameters of the H3O+ molecule trapped inDB18C6 (conformers 2t and 2t′) and of the isolated H3O+ (isol.)

Parameter 2t 2t′ Isolated

r(O Ha) 1.030 1.041 0.991r(O Hb) 1.020 1.017 0.991r(O Hc) 1.020 1.017 0.991∠(Ha-O Hb) 113.0 113.2 112.1∠(Hb-O Hc) 112.8 113.6 112.0∠(Hc-O Ha) 113.0 113.1 112.1

Distances (r) in Ångstroms, angles (∠) in degrees.

binding energies Eint discussed in this work are defined as:Eint = E[guest·DB18C8] − E[guest] − E[DB18C8].

1.2. Experimental measurements

In a 1:1 mixture of THF and water (10 ml), the ligandDB18C6 was reacted with HI and I2. After evaporation ofthe solvents at room temperature, 1 was obtained in quasiquantitative yield in form of dark needles of dimensions upto 2 cm×0.2 cm×0.2 cm. The crystals were used as such forspectroscopic analysis. The compound 1 appears to be stablein air, micro-Raman spectra obtained after 4 month and IRspectra obtained after 1 year on crystalline samples from thesame batch (kept in screw-capped vial) were identical to theinitial measurements.

Table 5Distances between oxygens of crown ether and the oxygen atoms ofH2O/H3O+ chain (ch1, ch2) in the crystal structure [13]

Parameter ch1 ch2

r(OC · · ·O17) 2.781 2.719r(OC · · ·O14) 2.781 2.719r(OC · · ·O16) 2.656 2.580r(OC · · ·O13) 2.656 2.580r(OC · · ·O15) 2.650 2.783r(OC · · ·O18) 2.650 2.783r(OA · · ·O17) 3.819 3.715r(OA · · ·O14) 3.819 3.715r(OA · · ·O16) 3.605 3.340r(OA · · ·O13) 3.605 3.340r(OA · · ·O15) 3.502 3.792r(OA · · ·O18) 3.502 3.792r(OB · · ·O17) 3.419 3.427r(OB · · ·O14) 3.419 3.427r(OB · · ·O16) 3.441 3.578r(OB · · ·O13) 3.441 3.578r(OB · · ·O15) 3.529 3.447r(OB · · ·O18) 3.529 3.447See Figs. 2 and 3 for definitions of the oxygen sites A, B and C. Distances(r) in Ångstroms.

Fourier transformation infrared (FT-IR) measurementon the pure solid samples were preferred to standardnujol mull or KBr-pellet transmission measurements inorder to avoid the necessity of nujol subtraction (around

Table 4Calculated geometrical parameters of the conformers 1q, 1b, 1m, 2t, 2t′, isolated DB18C6 (isol.), and experimental values (ch1, ch2) taken from [13]

Parameter 1q 1b 1m 2t 2t′ Isolated ch1 ch2

r(O17 · · ·O14) 5.938 5.508 5.842 5.371 5.440 5.813 5.545 5.430r(O16 · · ·O13) 5.491 5.727 5.581 5.471 5.452 5.589 5.311 5.136r(O15 · · ·O18) 5.490 5.725 5.581 5.472 5.453 5.589 5.295 5.553r(O17–C9) 1.430 1.433 1.437 1.454 1.457 1.428 1.429 1.409r(O14–C3) 1.430 1.433 1.428 1.434 1.434 1.428 1.429 1.409r(O17–C8) 1.430 1.433 1.437 1.454 1.457 1.428 1.418 1.444r(O14–C2) 1.430 1.433 1.428 1.434 1.434 1.428 1.418 1.444r(O15–C5) 1.376 1.376 1.374 1.403 1.402 1.374 1.382 1.356

1.384 1.385 1.374 1.382 1.3561.384 1.386 1.374 1.395 1.3801.403 1.402 1.374 1.395 1.3801.455 1.456 1.435 1.418 1.4371.438 1.438 1.435 1.418 1.4371.438 1.438 1.435 1.422 1.4321.455 1.456 1.435 1.422 1.4321.515 1.516 1.520 1.516 1.4981.515 1.516 1.520 1.516 1.4981.515 1.516 1.520 1.486 1.4541.515 1.516 1.520 1.486 1.454

11.029 10.857 10.982 10.169 10.61711.029 10.856 10.982 10.259 10.28810.811.0

111.5113.7

D

r(O18–C11) 1.376 1.376 1.376r(O16–C6) 1.376 1.376 1.376r(O13–C12) 1.376 1.376 1.374r(O15–C4) 1.440 1.434 1.436r(O18–C10) 1.440 1.434 1.434r(O16–C7) 1.440 1.434 1.434r(O13–C1) 1.440 1.434 1.436r(C4–C3) 1.519 1.519 1.520r(C10–C9) 1.519 1.519 1.520r(C2–C1) 1.519 1.519 1.520r(C8–C7) 1.519 1.519 1.520r(C29 · · ·C20) 10.971 11.102 10.984r(C28 · · ·C21) 10.971 11.102 10.984r(C29 · · ·C21) 10.881 11.015 10.898r(C28 · · ·C20) 10.881 11.015 10.891∠(C O17–C) 111.9 111.5 112.7∠(C O14–C) 111.9 111.5 112.2

∠(C O15–C) 117.5 117.1 117.3 116.5∠(C O18–C) 117.5 117.1 117.3 117.8∠(C O16–C) 117.6 117.1 117.3 117.8∠(C O13–C) 117.6 117.1 117.3 116.5

istances (r) in Ångstroms, angles (∠) in degrees.

72 10.706 10.893 10.122 10.37208 10.826 10.893 10.122 10.372

112.5 112.2 112.2 110.6113.8 112.2 112.2 110.6116.4 117.3 119.1 116.8

117.6 117.3 119.1 116.8117.6 117.3 117.2 119.8116.4 117.3 117.2 119.8

536 M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548

2900 cm−1) or complications by potential water contamina-tions.

FT-IR measurements of the solid samples were performedon a Spectrum One (Perkin Elmer) instrument equipped withthe Golden Gate Single Reflection Diamond (P/N 10 500Graseby-Specac Series) ATR set-up. The spectral resolutionwas 2–4 cm−1 and the spectral range 600–4000 cm−1. Addi-tional measurements in solution of pure DB18C6 in CH3NO3and CS2 were done using a Bio-Rad Excalibur instrument.All spectra were recorded at room temperature. Raman spec-tra were obtained using a Dilor Labram Raman microscopewith 532 nm excitation, laser power typically 0.01 mW. Someadditional measurements were done with a laboratory assem-

bled set-up consisting of an argon ion laser (488 nm excitationwavelength) and a Kaiser Optical Holospec monochromatorequipped with a liquid nitrogen cooled CCD camera. Thespectral resolution was 3–4 cm−1.

2. Results and discussions

2.1. Geometry

In this section, we analyze the influence of complexationwith DB18C6 on the geometry of the guest molecules (H2Oand H3O+) and the effect of the interactions on the DB18C6

Table 6IR region 100–1000 cm−1: conformers 1q, 2t, isolated DB18C6 (isol.), experimental (measured in crystal) frequencies of DB18C6 (exp.), and experimental(measured in crystal) frequencies of 1∞ [(H2O)⊂(DB18C6) (�2-H2O)2/2] 1∞ [(H3O)⊂(DB18C6) (�2-H2O)2/2] I3 (1)1q 2t Isolated Experimental 1 Description

– – – – – HO Hb137 (19) – – – – H2O, ring– – – – – H2O, ring– – – – – H2O, ring– 211 (19) – – – H3O+ transl., ring– 223 (21) – – – H3O+ transl., ring– 233 (66) – – – H3O+ transl., ring– 234 (9) – – –249 (26) – – – – H2O, ring

–––––––––––––––

T

– 283 (18) –– 330 (34) –331 (14) – –– – –– – –– – –367 (70) – –– – –– – –– – 482 (12)489 (155) – –– – –519 (21) – –590 (9) 592 (12) 588 (8)– – –

– – – –710 (97) 720 (92) 709 (94) 738710 (50) 720 (61) 709 (53) 738770 (28) 764 (106) 767 (20) 777776 (13) 768 (33) 773 (14) 777– 792 (82) – –– 795 (20) – –– 798 (12) – –– 820 (14) – –831 (14) – 832 (13) –– – – –862 (11) – 859 (13) –– 899 (25) – –– 915 (50) – –923 (87) – 924 (85) 915– 930 (56) – –– 938 (14) – –939 (96) – 938 (95) 931– 942 (55) – –– 948 (46) – –

he frequencies of intense lines are underlined. Absorption coefficients are given in

– H3O+ transl.–– H2O, ring– H2O–– H2O– H2O– H2O– H2O, Cph.–H out-of-plane–– H2O– H2O, ring– H2O, ring– 1,2-Disubstitued benzene– H3O+ rock

– Ha–OH739 Cph.–H out-of-plane, in-phase739 Cph.–H out-of-plane, out-of-phase783 1,2-Disubstitued benzene783 1,2-Disubstitued benzene– H3O+ rock– Ha–OH

+2 , Cph.–H out-of-plane

– Ha–OH+2 , Cph.–H out-of-plane

– CH2 on the O17 side– CH2– H3O+, CH2 on the O14 side– Cph.–H out-of-plane––911 CH2, phenyl deformation– CH2 on the O17 side– Cph.–H out-of-plane943 CH2–– CH2 on the O14 side

parentheses (in km/mol).

M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548 537

Table 7IR region 1000–1700 cm−1: conformers 1q, 2t, isolated DB18C6 (isol.), experimental (measured in crystal) frequencies of DB18C6 (exp.), and experimental(measured in crystal) frequencies of 1∞ [(H2O)⊂(DB18C6) (�2-H2O)2/2] 1∞ [(H3O)⊂(DB18C6) (�2-H2O)2/2] I3 (1)1q 2t Isolated Experimental 1 Description

– – – – – ωasC O17–C, CH2, C O str.– 1032 (100) – – – ωasC O17–C, CH2, C O str.– 1035 (77) – – – H3O+ umbrella, ring (2t on O14 side)– – – – – H3O+ umbrella, ring1043 (23) 1044.03 (40) 1045 (22) – – Cph.–H in-plane1044 (14) 1044 (29) 1046 (14) – – Cph.–H in-plane1046 (31) – 1050 (36) – –– 1048 (60) – – – H3O+ umbrella, ring on O17 side– – – – – C C str., CH21048 (100) – 1049.87 (96) – –– 1049 (154) – – – H3O+ umbrella, ring1056 (28) – 1056 (34) – – C C sym. str.– – – – –– – – – – CH2 on O17 side– 1073 (53) – – – ωasC O17–C, CH2– – – – – H3O+ umbrella, ring on O17 side1075 (22) – 1076 (12) – – CH2– 1080 (13) – – – CH2 on O14 side– – – – – ωasC O17–C, Cph.–H in-plane1113 (196) 1115 (262) 1114 (191) 1117 – Cph.–H in-plane– – – – – CH2 on O17 side, Cph.–H in-plane– – – – – CH2 on O17 side, Cph.–H in-plane– 1122 (96) – – – ωasC O14–C, Cph.–H in-plane1124 (383) – 1125 (406) 1128 1128 ωasC O17–C, ωasC O14–C– 1108 (159) – – – ωasC O14–C, Cph.–H in-plane– 1179 (226) – – – Cph.–O str., Cph.–H in-plane1206 (13) – – – – CH21212 (292) – 1212 (117) 1227 1221 CH2, phenyl deformation– 1214 (28) – – – CH2 on O14 side– 1217 (22) – – – CH2 on O17 side– – 1220 (284) – – CH2, phenyl deformation1220 (26) – 1216 (25) – – CH2– – – – – CH2 on O17 side1222 (96) – – – – CH2– 1243 (159) – – – CH2, Cph.–H in-plane– 1249 (122) – – – Cph.–O str., CH2 on O14 side, Cph.–H in-plane– 1250 (50) – – – CH2 on O17 side– 1252 (203) – – – CH2 on O14 side1253 (298) – 1252 (214) 1256 1257 Cph.–O str., CH21257 (218) – 1259 (308) 1256 1257 Cph.–O str., CH2, Cph.–H in-plane1258 (60) – 1258 (80) 1256 1257 CH2, Cph.–H in-plane1259 (170) – 1260 (111) 1256 1257 Cph.–O str., CH2, Cph.–H in-plane– 1261 (64) – – – Cph.–O str., CH2, Cph.–H in-plane– – 1263 (72) – – Cph.–O str., CH21264 (9) – – – – CH2– 1323 (14) – – – CH2 on O14 side– 1329 (36) – – – CH2 on O17 side1329 (69) – 1329 (88) 1330 1330 CH21349 (12) – 1350 (16) – – CH21433 (13) – – – – CH2 bend.– – – – – CH2 bend. on O17 side– – 1436 (37) – – CH2 bend.– – – – – CH2 bend. on O14 side1436 (54) 1442 (37) 1434 (61) – – Cph.–O str., CH2 bend., Cph.–H in-plane– 1439 (21) – – – CH2 bend. on O17 side1440 (92) – 1440 (90) 1451 1451 CH2 bend.1441 (31) 1447 (29) 1441 (38) – – CH2 bend. (outside)– – – – – CH 2 bend. on O14 side– 1444 (51) – – – CH2 bend. on O14 side– 1448 (20) – – – CH2 bend. on O17 side– 1449 (14) – – – CH2 bend. on O17 side1455 (21) – 1459 (10) – – CH2 bend.

538 M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548

Table 7 ( Continued )

1q 2t Isolated Experimental 1 Description

1494 (292) 1488 (282) 1495 (267) 1508 1506 Cph.–O str., Cph.–H in-plane1496 (107) 1490 (85) 1497 (109) 1508 1506 Cph.–O str., Cph.–H in-plane1590 (108) – 1589 (120) 1595 1597 Cph.–Cph. str.– 1591 (35) – – – H3O+ bend., Cph.–Cph. str.– 1605 (19) – – – Cph.–Cph. str. (2t - H3O+)– 1621 (48) – – – H3O+ bend.1642 (30) – – – 1621 H2O bend.– 1637 (54) – – – H 3O+ bend.

The frequencies of intense lines are underlined. Absorption coefficients are given in parentheses (in km/mol).

geometry. In all geometry optimizations, the starting geome-try of DB18C6 was taken from crystallographic data (channel1 (ch1), in ref. [13]). In view of our interest in the properties ofguests in the crystal, conformational preferences of DB18C6were not investigated further.

2.1.1. H2O·DB18C6 complexThree non-equivalent minimum energy structures have

been found (see Figs. 4 and 5). They are denoted as con-formers 1q, 1b, and 1m, throughout this work. The bindingenergies amount to −9.1, −4.7 and −3.1 kcal/mol (−11.7,

−7.1 and −4.7 kcal/mol without the correction for BSSE)for the 1q, 1b, and 1m conformers, respectively. In the 1qconformer, the water molecule is most strongly bound to thecrown ether via two bifurcated hydrogen bonds. It is alsobound to the host molecule via two hydrogen bonds in the 1bconformer, but the resulting interaction is weaker. In the 1mconformer, the water molecule interacts with the ring throughone hydrogen bond only and the interaction is the weakest.Moreover, this arrangement of the guest molecule is less rel-evant in the general context of this study because it is notobserved in the solid [13]. Table 1 collects the calculated

Fn

ig. 7. Calculated IR spectra of isolated DB18C6 (A), conformer 1q (B), conformormalization of the spectra.

er 1b (C) and conformer 1m (D). The 1589 cm−1 line of (A) was used for

M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548 539

intramolecular geometrical parameters for the bound and theisolated water molecule.

The structure of the H2O guest molecule changes uponcomplexation. The O H bonds elongate due to their partic-ipation in hydrogen bonding. It is notable that the geometryof the isolated H2O molecule derived from our calcula-tion reproduces quite reasonably the experimental values[23] (r(O H)exp = 0.957 Å versus r(O H)calc = 0.974 Å;∠(H O H)exp = 104.5◦ versus ∠(H O H)calc = 104.4◦).The numbering of atoms shown in Fig. 6 is used throughoutthis work.

2.1.2. H3O+· DB18C6 complexTwo non-equivalent minimum energy structures have been

found (see Fig. 6). They are denoted as conformers 2t and2t′, throughout this work. The interaction energies amount to−87.6 and −87.2 kcal/mol (−89.2 and −89.0 kcal/mol with-out the correction for BSSE). In the 2t conformer, the guestmolecule and the two phenyl rings of DB18C6 are on thesame side of the crown ether. In the 2t′ conformer, which isslightly less stable, the H3O+ molecule and the phenyl ringsare on opposite sides of the crown ether ring. In both con-formers, the three hydrogen bonds are not equivalent—two

are shorter and one is longer (see Table 2). All three hydrogenbonds are quasi-linear.

No minimum energy structure corresponding to theH2O·DB18C6H+ complex, i.e. where the proton is attachednot to the guest molecule but to the crown ether ring, has beenfound (see the discussion in the ref. [24], where the energy ofthe H2O·18C6H+ complex was found to be higher than theenergy of H3O+·18C6).

Table 3 collects the calculated intramolecular geometricalparameters for the bound and the isolated H3O+ cation.

The geometry of the isolated H3O+ cation derived fromour calculations reproduces reasonably well the experimen-tal values [25] (r(O H)exp = 0.974 Å versus r(O H)calc =0.991 Å; ∠(H O H)exp = 113.6◦ versus ∠(H O H)calc =112.1◦).

In the gas phase, the hydronium ion is more strongly boundto the crown than the neutral water molecule (89.2 versus9.1 kcal/mol). Obviously, gas-phase energy difference cannotbe used to determine the relative populations of the chargedand neutral guests in the bulk where other factors play a role:guest–guest interactions, interactions with the anions, pH,stoichiometry, etc. Without taking into account such effectsthe energetics derived from our gas-phase single unit model

Fs

ig. 8. Calculated IR spectra of isolated DB18C6 (A), conformer 1q (B), conformpectra.

er 2t (C). The 1329 cm−1 line of (A) was used for normalization of the

540 M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548

has little relevance to the energetics of the guest–channel in-teractions in the crystal. We recall here that the stoichiometryof the crystal material is such that only 25% of guest speciesare charged [13,14]. For this reasons, we turn to the analysisof vibrational properties of the considered guests.

2.1.3. The effect of interactions with guest on theDB18C6 geometry

Table 4 collects the key geometrical parameters of theisolated DB18C6 and the complexes with H2O or H3O+

guests. At each of the five structures corresponding to the lo-cal energy minima, the geometry of DB18C6 is different. Forinstance, the O17· · ·O14 distance which measures the overallsize of the crown ether varies between 5.371 Å (conformer2t) and 5.938 Å (conformer 1q). Binding the guest moleculeaffects also the angle formed by the phenyl planes andthat of the crown ether. This does not change significantlythe overall geometry of the whole DB18C6. For instance,the C29· · ·C21 distance between two most distant carbonsbelonging to the opposite phenyl rings varies in a narrow

Table 8IR region 1700–3800 cm−1: conformers 1q, 2t, isolated DB18C6 (isol.), experimental (measured in crystal) frequencies of DB18C6 (exp.), and experimental(measured in crystal) frequencies of 1∞ [(H2O)⊂(DB18C6) (�2-H2O)2/2] 1∞ [(H3O)⊂(DB18C6) (�2-H2O)2/2] I3 (1)1q 2t Isolated Experimental 1 Description

– 2785 (1689) – – – H3O+ str.– – – – – CH2 sym. str. on O14 side– – – – – CH2 sym. str. on O14 side2901.83 (54) – 2897.98 (46) – – CH2 sym. str.2907.57 (173) – 2904.41 (142) – – CH2 sym. str.2907.99 (71) – 2904.75 (77) – – CH2 sym. str.– – – – – CH2 sym. str. on O17 side– – – – – CH2 sym. str. on O17 side– 2933 (2349) – – – H3O+ str.– 2938 (626) – – – H3O+ str., CH2 sym. str.– 2942 (59) – – – CH2 sym. str. on O14 side2950.20 (182) – 2941.21 (151) – – CH2 sym. str.

–––––––––––

T

2950.24 (36) – 2941.34 (32)– – –2951 (10) – 2942 (36)– 2964.3 (62) –– 2964.2 (14) –2970.6 (65) – 2951.5 (160)– – 2952 (28)– 2973 (20) –– 2977 (13) –– – –– 2983 (34) –

– – – –– 2986 (232) – –– – – –– 2995 (38) – –– 2996 (25) – –– – – –– – – –– – – –– – – –– – – –– – – –3003.45 (95) – 2995.3 (103) 29503003.46 (40) – 2995.1 (43) –– 3016 (9) – –– 3018 (14) – –– – – –– 3026 (48) – –– 3033 (41) – –– 3048.93 (20) – –3139.62 (36) 3151.63 (11) 3137.91 (37) –3139.66 (15) – 3137.95 (16) –3153 (39) 3159.04 (14) 3151.32 (45) 30653626 (56) – – –3713 (326) – – –

he frequencies of intense lines are underlined. Absorption coefficients are given in

– CH2 sym. str.– CH2 str.– CH2 sym. str.– CH2 sym. str. on O17 side– CH2 sym. str. on O17 side– CH2 asym. str.– CH2 asym. str.– CH2 sym. str. on O17 side– H3O+ str.,CH2 str. on O14 side– CH2 asym. str. on O17 side– H3O+ str.,CH2 str. on O14 side

+

– H3O str.,CH2 str. on O14 side– H3O+ sym. str.,CH2 str. on O14 side– CH2 str. on O14 side– CH2 str. on O14 side– H3O+ sym. str.,CH2 str. on O14 side– CH2 asym. str. on O17 side– CH2 asym. str. on O14 side– CH2 asym. str. on O14 side– CH2 asym. str. on O17 side– H3O+ str.,CH2 str. on O14 side– CH2 asym. str.3932 CH2 asym. str.– CH2 asym. str.– CH2 asym. str. on O17 side– CH2 asym. str. on O17 side– H3O+ sym. str.– CH2 asym. str. on O17 side– CH2 asym. str. on O17 side

CH2 asym. str. on O14 side– Cph.–H str.3044 Cph.–H str.3066 Cph.–H str.3547 H2O sym. str.3624 H2O asym. str.

parentheses (in km/mol).

M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548 541

Table 9Calculated IR frequencies (cm−1) of the H2O molecule trapped in DB18C6 ether (conformers 1q, 1b, 1m), isolated H2O (isol.), experimental harmonicfrequencies of H2O (ω), and experimental fundamentals of H2O (ν)

1q 1b 1m Isolated ω ν Description

– – 130 (89) – – – HO Hb137 – – – – – H2O, ring– – 153 (15) – – – H2O, ring– 153 (13) – – – – H2O, ring249 (26) – – – – – H2O, ring331 (14) – – – – – H2O, ring– 338 (52) – – – – H2O– – 349 (93) – – – H2O367 (70) – – – – – H2O– 423 (250) – – – – H2O– 451 (48) – – – – H2O, Cph.–H out-of-plane489 (155) – – – – – H2O– 490 (80) – – – – H2O, ring519 (21) – – – – – H2O, ring– – 672 (100) – – – Ha–OH1642 (30) 1641 (26) 1622 (67) 1601 (76) 1648 1595 H2O bend.3626 (56) 3645 (43) 3513 (377) 3716 (4) 3832 3657 H2O sym. str.3713 (326) 3738 (144) 3785 (65) 3836 (39) 3943 3756 H2O asym. str.

The frequencies of intense lines are underlined. Absorption coefficients are given in parentheses (in km/mol).

Fig. 9. Experimental IR solid phase spectrum of DB18C6 (A), calculated IR spectrum of isolated DB18C6 (B) and experimental IR spectrum of DB18C6 inCH3NO3 (spectrum of the solvent was subtracted) (C).

542 M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548

range (from 10.71 Å in the 2t′ conformer to 11.02 Å in the 1bconformer).

2.1.4. The calculated structures versus solid state dataSince no coordinates of hydrogen atoms are available in

the structure derived from X-ray diffraction data, each of theoxygen atoms localized in the center of the channel can beattributed to either H2O, H3O+, or a chain-like arrangementof water molecules—the Zundel cation H5O

+2 , for instance.

In a model for the channels formed by DB18C6 in whichonly one ligand molecule is considered to be an excerpt ofthe infinite structure, three oxygen sites can be identified withC being the one in the middle of the crown ether, A being theone on the side of the phenyl rings, and B the one oppositeto A. Note that the oxygen sites A and B are not equivalentin our model (see Fig. 1). The ring-oxygen distance differsfor the three types of oxygen. Moreover, two non-equivalentchannels occur in the crystal. They will be refereed to as ch1and ch2 (see Figs. 2 and 3). The diameter O· · ·O distancesin the two channels differ by 0.258 Å (see Table 4). The rel-ative position of the oxygens of the type A are very similarin both channels. The position of the oxygen closest to thecrown ether ring (type C) was previously suggested in theliterature to be attributed to H3O+ whereas the more distantones (type A or B) to H2O [26]. Comparisons of the distances

between the guest oxygen and ring oxygens in the crystal (seeTable 5) with the corresponding distances in the most stableconformers of H2O·DB18C6 and H3O+·DB18C6 complexes(see Table 2) confirm the above attribution of the oxygen cen-ters. Therefore, the two most stable intermolecular complexesof H2O·DB18C6 (1q) and H3O+·DB18C6 (2t) analyzed bymeans of theoretical modeling in this study can be seen asfragments cut off the solid (type B and type C, respectively).The 1b conformer does not match the type A site structure(compare Tables 2 and 5). We recall now that the crystal-lographic arrangement of the C and A sites matches that ofthe isolated H5O

+2 cation (not considered explicitly in our

calculations).According to our calculations, binding of H3O+ results

in a significantly larger deformation of DB18C6 unit thanbinding of H2O. The geometry of the ring calculated in thepresence of the H3O+ guest matches much closer that of thecrystallographic structure than the one derived for the H2Oguest (see Table 4).

2.2. Vibrational spectra

IR spectra calculated at the minimum energy structures(conformers 1q, 2t, and the isolated DB18C6 molecule) arecollected in Tables 6–8. In the following sections, the effect of

Fig. 10. Experimental IR solid phase spectrum of DB18C6 (A

) and calculated IR spectrum of isolated DB18C6 (B).

M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548 543

complexation on the vibrational properties of the moleculesforming the complex will be discussed.

2.2.1. H2O·DB18C6 complexBinding H2O affects negligibly the frequencies of

IR bands of DB18C6 (see Fig. 7). For the most sta-ble conformer of the complex (i.e. 1q), in the spectralregion 600–1700 cm−1, the complexation induced fre-quency shifts of the crown ether ring frequencies donot exceed 4 cm−1 (H2O·DB18C6—1046 cm−1 versusDB18C6—1050 cm−1, H2O·DB18C6—1220 cm−1 versusDB18C6—1216 cm−1). Some modes change, however, theirintensities (e.g. 1212 cm−1), or modes at similar frequenciesdiffer in H2O·DB18C6 and DB18C6 (e.g. H2O·DB18C6—1222 cm−1 versus DB18C6—1220 cm−1). The largest shiftsoccur in the high frequency region and amount to 19 cm−1for the CH2 asymmetric stretching mode (2971 cm−1 in thecomplex versus 2952 cm−1 for the isolated DB18C6).

In the spectrum of the 1b conformer, the largest shiftsoccur for 1125 and 2904 cm−1 bands of DB18C6. They areshifted to 1114 and 2915 cm−1, respectively, in the 1b con-former. In the case of the 1m conformer, the overall pictureis similar. Some modes delocalized over the whole crown

ether ring in the case of isolated DB18C6 molecule become,however, localized on a fragment of the crown ether.

The spectral region between 1000 and 1300 cm−1 (seeFig. 8) corresponds mainly to C O stretching modes. Dueto the fact that ring oxygens can be involved in hydrogenbonding with the guest molecule, this spectral region canbe expected to differ in the isolated DB18C6 and in thecomplex [27,28]. Seven normal modes with significant IRintensities were found in DB18C6: one delocalized mode at1050 cm−1, C H vibration in the plane of the phenyl groupsat 1114 cm−1, asymmetric stretching involving C O14 Cand C O17 C groups of the crown at 1125 cm−1, two modesinvolving CH2 vibrations and deformations of phenyl groups(1212, 1220 cm−1), and two modes characterized mainly bystretching of all C O bonds where C is the phenyl carbonand O is the oxygen of the crown ether (1252 and 1259cm−1).

For the most stable conformer of H2O·DB18C6 (1q), thefrequencies 1048, 1113, 1224 cm−1 correspond to 1050, 1114and 1225 cm−1 bands in the isolated DB18C6. The inten-sity of the 1212 cm−1 band is larger than in the isolatedDB18C6, and instead of 1220 cm−1 in the isolated DB18C6,a CH2 band appears at 1222 cm−1. The 1253 and 1257 cm−1

F(

ig. 11. Calculated IR spectrum of the conformer 1q (A), experimental IR solid pha�2-H2O)2/2] I3 (B) and calculated IR spectrum of the conformer 2t (C). The 1590

se spectrum of 1∞ [(H2O)⊂(DB18C6) (�2-H2O)2/2] 1∞ [(H3O)⊂(DB18C6)cm−1 line of (A) was used for normalization of the theoretical spectra.

544 M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548

bands correspond to 1252 and 1259 cm−1 bands in the iso-lated DB18C6, and the band at 1259 cm−1 has larger intensitycompared to the intensity of the weak band at 1260 cm−1 forisolated DB18C6 (see Fig. 11).

Table 9 collects the calculated IR frequencies correspond-ing to the intramolecular degrees of freedom of the isolatedand complexed H2O. Data for intermolecular modes are alsoincluded. As expected, hydrogen bonding leads to frequencyredshifts of both symmetric and asymmetric stretching modesand a slight blue shift of the bending mode for all consideredconformers.

2.2.2. H3O+·DB18C6 complexBinding of H3O+ affects noticeably the vibrations of

DB18C6 for both 2t and 2t′ conformers. Below, we analyzethe most affected modes.

The modes involving C O C of the ring are strongly af-fected by binding of H3O+. Among six oxygen atoms, threeare involved in a strong hydrogen bond in both 2t and 2t′conformers (see Fig. 6). As a result, instead of one C O Casymmetric stretching mode occurring at 1125 cm−1 in iso-lated DB18C, four new modes appear in the complex: (i) two

modes involving C O14 C: one strong (1032 cm−1) and oneweak at 1073 cm−1, (ii) two modes involving C O17 C andin-plane vibrations of the phenyl C H groups at 1108 and1122 cm−1. The most clearly distinct new feature appears inthe spectrum of the complex, i.e. the intense 1179 cm−1 bandin which C O (C of the phenyl and O of the crown) stretch-ing and in-plane stretching of the phenyl C H are coupled.Bands in the region of 1200–1220 cm−1 almost disappear inthe complex. Additionally two bands at 1243 and 1249 cm−1appear, and the bands at 1252 and 1261 cm−1 change theirdescription compared to isolated DB18C6 (see Fig. 11).

Table 10 collects the calculated IR frequencies corre-sponding to the intramolecular degrees of freedom of H3O+for the complexed and isolated guest molecule. Data forintermolecular vibrational modes are also included. As ex-pected, hydrogen bonding leads to larger frequency redshiftsof both symmetric and asymmetric H3O+ stretching modesthan those for H2O in H2O·DB18C6.

2.2.3. Assignment of the experimental spectraThe intermolecular complexes analyzed by means of the-

oretical modeling can be seen as simple models of the real

F(

ig. 12. Calculated IR spectrum of the conformer 1q (A), experimental IR solid pha�2-H2O)2/2] I3 (B) and calculated IR spectrum of the conformer 2t (C). The 1590

se spectrum of 1∞ [(H2O)⊂(DB18C6) (�2-H2O)2/2] 1∞ [(H3O)⊂(DB18C6)cm−1 line of (A) was used for normalization of the theoretical spectra.

M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548 545

Fig. 13. Partially polarized FT-IR solid phase spectra of 1 obtained in twosuccessive experiments with crystalline samples oriented in perpendiculardirections. The difference trace (orient. 1 − orient. 2) has been shifted verti-cally for clarity.

material. Their calculated IR and Raman spectra can be ex-pected to differ from the ones measured experimentally due totwo principal reasons (i) the applied theoretical methods arenot exact—we use approximated exchange-correlation func-tional and apply the harmonic approximation for frequenciesand double-harmonic approximation for the intensities, (ii)the studied complexes are models of the real system in whichthe crown ethers are stacked forming a channel, the guestmolecules interact with each other, and the neutralizing an-ions such as I3− are used. The last three effects, present in thereal system, are neglected in our theoretical model. Moreover,experimental conditions (solvent effects, crystallization) af-fect also the observed spectra. Nevertheless, some theoreti-cally assigned lines can be identified with the experimentalbands. The solid phase spectrum of 1, together with the solidphase/solution spectrum of DB18C6 were used in the assign-ment.

2.2.3.1. IR spectra. All IR bands below were assigned onthe basis of the calculated and solid phase/solution spectraof DB18C6 (see Figs. 9 and 10). The assigned experimental

Fig. 14. Experimental Raman solid phase spectrum of DB18C6 (A), calculated Ramspectrum of 1∞ [(H2O)⊂(DB18C6) (�2-H2O)2/2] 1∞ [(H3O)⊂(DB18C6) (�2-H2O773 cm−1 line of (B) was used for normalization of the theoretical spectra.

an spectrum of the isolated DB18C6 (B), experimental Raman solid phase)2/2] I3 (C) and calculated Raman spectrum of the conformer 1q (D). The

546 M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548

bands are (see Tables 6–8): 738, 777, 915, 931, 1117, 1128,1227, 1256, 1330, 1451, 1508, 1595, 2950 and 3065 cm−1.The band structure of the region 1000–1300 cm−1 in the IRsolid phase spectrum of DB18C6 is not reproduced by cal-culations for the isolated DB18C6 molecule. The calculatedspectrum of isolated DB18C6 resembles much better the ex-perimental spectrum of DB18C6 measured in CH3NO3 (seeFig. 9). This can be explained by the presence of different con-formations of DB18C6 in the solution and the solid phase, assuggested in refs. [29,30].

As far as the spectrum of 1 is concerned, additional bandswhich can be assigned are the vibrations of H2O: the stretch-ing bands at 3547 and 3624 cm−1 and the bending mode at1621 cm−1 in the experimental spectrum. Fig. 13 indicatesalso the same polarization for the totally symmetrical bend-ing and stretching modes of the water vibrations (negativecontributions in the difference trace).

Theoretical analysis of the solid phase spectrum of 1 doesnot provide a compelling evidence of the presence of H3O+molecule. Comparison between the theoretical spectrumof 2t with the solid spectrum of 1 shows three significantdifferences. The foremost one is the mismatch of the relativeintensities of the bands 2750–3000 cm−1 between the

experimental and the calculated spectra (see Fig. 12). Inthe experimental spectrum the intensities of these bands aresmaller than the intensities of the bands in the region below1800 cm−1. In the calculated spectrum of the conformer 2tthe intensities of the bands in the 2750–3000 cm−1 regionare significantly larger than the intensities of the bands below1800 cm−1. One of the possible reasons for this mismatchmight originate from the fact that, in the real system, thevibrations of H3O+ might be coupled to the motions ofcounterions—this effect is not accounted for in our model.

Previous experimental spectra of H3O+ in crown ethersshow a very broad band (full width more than 300 cm−1)around 2850 cm−1, in addition to bands at about 2200 cm−1(assigned to the first overtone of ν2) and the bending mode(ν4) around 1700 cm−1 [31]. The integrated intensity of thebroad band around 2900 cm−1 compared to the integratedintensity of a pure C H stretching mode (with a band widthof about 10 cm−1) is obviously much larger (see Fig. 1 in ref.[32] for a relevant system). The difference trace shown in Fig.13 suggests the presence of a weak and broad band at about2900 cm−1 which might indeed be associated with H3O+.Its relatively weak intensity may be the result of the fact thatthe refined crystal structure [13,14]. indicates the presence of

Fs

ig. 15. Calculated Raman spectra of isolated DB18C6 (A), conformer 1q (B), conpectra.

former 2t (C). The 832 cm−1 line of (A) was used for normalization of the

M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548 547

Table 10Calculated IR frequencies (cm−1) of the H3O+ molecule trapped in DB18C6ether (conformers 2t, 2t′)

2t 2t′ Description

211 (19) 211 (12) H3O+ transl., ring223 (21) – H3O+ transl., ring233 (66) 232.05 (26) H3O+ transl., ring283 (18) 313 (42) H3O+ transl.– 658 (32) H3O+ rock792 (82) 809 (26) H3O+ rock795 (20) 815 (24) Ha–OH

+2 , Cph.–H out-of-plane

798 (12) 830 (28) Ha-OH+2 , Cph.–H out-of-plane

– 851 (24) H3O+, CH2 on the O14 side1035 (77) 1035 (132) H3O+ umbrella, ring (2t on O14 side)– 1042 (102) H3O+ umbrella, ring1048 (60) 1050 (20) H3O+ umbrella, ring on O17 side1049 (154) – H3O+ umbrella, ring– 1074 (39) H3O+ umbrella, ring on O17 side1591 (35) 1594 (62) H3O+ bend., Cph.–Cph. str.1605 (19) 1604 (13) Cph.–Cph. str. (2t - H3O+)1621 (48) – H3O+ bend.1637 (54) 1657 (70) H3O+ bend.2785 (1689) 2611 (1764) H3O+ str.2933 (2349) – H3O+ str.2938 (626) – H3O+ str., CH2 sym. str.2977 (13) – H3O+ str.,CH2 str. on O14 side2983 (34) 2978 (599) H3O+ str.,CH2 str. on O14 side– 2987 (921) H3O+ str.,CH2 str. on O14 side2986 (232) – H3O+ sym. str.,CH2 str. on O14 side2996 (25) – H3O+ sym. str.,CH2 str. on O14 side– 3001 (1321) H3O+ str.,CH2 str. on O14 side– 3025 (252) H3O+ sym. str.

The calculated frequencies of the isolated H3O+ are: 856 (486) (umbrellamode), 1647 (110) (bending), 3613 (483) (asym. str.), 3498 (49) (sym. str.).The frequencies of intense lines are underlined. Absorption coefficients aregiven in parentheses (in km/mol).

one cationic species for four DB18C6 molecules. However,the presence of other bands in the difference trace of Fig. 13in conjunction with this weak and broad band may also beassociated with other protonated species.

The other difference between the theoretical spectrum of2t and the solid spectrum of 1 is the presence of the 2068 cm−1band in the spectrum of 1, which is missing in all of thetheoretical spectra. This band was assigned previously to bethe first overtone of ν2 of H3O+ (umbrella mode [31]). Theexperimental spectrum of 1 shows strong bands at 943, 1067and 1128 cm−1 which all could contribute to this feature at2068 cm−1 (943 + 1128 and 2 × 1067). The origin of theband at 2068 cm−1 remains thus unclear.

Turning back to the effect of stacking (point ii), one can tryto identify the vibrations which could be expected to be themost affected by stacking. Such vibrational modes involvethe relative motion of the phenyl groups of the DB18C6 unitwith respect to their neighbors in the channel. Our calcu-lations predict such vibrations in the 20 cm−1 region, whichfalls outside the spectral region where the measurements weremade.

2.2.3.2. Raman spectra. According to refs. [29,30], the bandin the experimental crystal Raman spectrum of NaBr complex

of DB18C6 at 856 cm−1 (data in the ref. [29]) is charac-teristic to tGt conformation of the crown (the notation usedto describe conformations of the crown ethers is explainedin the Appendix A). The corresponding band appears in thespectrum of 1 at 845 cm−1 (see Fig. 14). This is in agreementwith the crystallographic structure of DB18C6 units in thechannel which is close to (tCttGttG′t)2 conformation. Weattribute calculated band for isolated DB18C6 at 832 cm−1 tothis vibration on the basis of intensity pattern, and the knownstructure of calculated molecules which is (tCttGttG′t)2.In the conformer 1q the corresponding band appears at831 cm−1. In the case of H3O+·DB18C6 (conformer 2t),the band is split into 820 and 838 cm−1 because tGt are notequivalent in the two halves of the molecule (see Fig. 15).Low frequency Raman spectra confirm the presence of linearI3−.

Similar to the IR spectra, some features in the Ramanspectra measured in the solid phase are not well reproducedby our model. They include:

• The band in the solid phase spectrum of DB18C6 at1329 cm−1 is missing in calculations. The bands in thesolid phase spectrum of DB18C6 at about 1250 cm−1 areweak, however, calculated bands in this region are strong.

• The O H stretching bands of H2O are missing in the Ra-man spectrum of 1.

3

aulmooweDHbpbdsmasc1pcWfr

. Conclusions

In principle, the three non-equivalent oxygen sites (A, B,nd C) in the center of the channel formed by stacked DB18Cnits can be attributed to various guests: H2O, H3O+ or chain-ike structures (H5O2+, H7O3+, . . .). Our geometry mini-

ization studies on a small model system comprising justne crown ether unit and one guest molecule (either H2Or H3O+) allowed us to attribute the site B to H2O. Site C,as attributed to H3O+ or other protonated species. Geom-

try minimization studies indicate that the structure of theB18C6 unit change more significantly upon binding the3O+ than in the H2O case. The crown ether ring shrinksy about 0.6 Å upon H3O+ binding. The angle between thehenyl ring planes does not change noticeably upon bindingoth considered guests. The applied computational method toerive the vibrational spectra of all considered complexes washown to be adequate as evidenced by a rather good agree-ent between the theoretical and experimental spectra (IR

nd Raman) of DB18C6. In particular, the attribution of theite B to the H2O guest based on the geometry minimization isonsistent with the fact that the theoretical IR spectrum of theq conformer of the H2O..DB18C6 complex matches the ex-erimental spectrum of 1. Site C remains, therefore, the bestandidate to be attributed to oxygens of protonated species.

hether it corresponds to an isolated H3O+ molecule or aragment of the chain-like structure H5O2+, H7O3+, etc.)emains to be clarified.

548 M. Dułak et al. / Spectrochimica Acta Part A 64 (2006) 532–548

Acknowledgement

This work was supported by Projects 200020-100352,PP002-68775/1 and 200021-105305/1 of the Swiss NationalScience Foundation.

Appendix A. The short notation to describe theconformation of crown ethers

• Small letters correspond to C O bonds, capital letters toC C bonds.

• t denotes dihedral angle A-C-O-B close to 180 or −180degrees.

• T denotes dihedral angle A-C-C-B close to 180 or −180degrees.

• G (gauche) denotes dihedral angle A-C-C-B close to 60degrees.

• G′(gauche’) denotes dihedral angle A-C-C-B close to −60degrees.

• C (cis) denotes dihedral angle A-C-C-B close to 0 degrees.For instance, the structure of 18C6 (D3d) in this conventionis described as tG′ttGttG′ttGttG′ttGt.

A

f0

R

[8] H.J. Buschmann, G. Wenz, E. Schollmeyer, Inorg. Chem. Commun. 4(2001) 53–56.

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ppendix B. Supplementary Data

Supplementary data associated with this article can beound, in the online version, at doi:10.1016/j.saa.2005.7.060.

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doi:10.1016/j.saa.2005.07.060doi:10.1016/j.saa.2005.07.060

Water trapped in dibenzo-18-crown-6: Theoretical and spectroscopic (IR, Raman) studiesIntroductionTheoretical methodsExperimental measurements

Results and discussionsGeometryH2Ocdot DB18C6 complexH3O+ cdot DB18C6 complexThe effect of interactions with guest on the DB18C6 geometryThe calculated structures versus solid state data

Vibrational spectraH2Ocdot DB18C6 complexH3O+cdot DB18C6 complexAssignment of the experimental spectraIR spectraRaman spectra

ConclusionsAcknowledgementAppendix A The short notation to describe the conformation of crown ethersAppendix B Supplementary DataReferences