Highlevel ab initio prediction of the structure and infrared spectra offormaldehyde–water radicalcation complexesElena L. Coitiño, Alberto Pereira, and Oscar N. Ventura Citation: J. Chem. Phys. 102, 2833 (1995); doi: 10.1063/1.468661 View online: http://dx.doi.org/10.1063/1.468661 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v102/i7 Published by the AIP Publishing LLC. Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors

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High-level ab initio prediction of the structure and infrared spectraof formaldehyde–water radical-cation complexes

Elena L. Coitinoa)Instituto de Quı´mica, Facultad de Ciencias, P.O. Box 1157, Montevideo, Uruguay

Alberto PereiraCatedra de Quı´mica Cuantica, Facultad de Quı´mica, Gral. Flores 2124, C.C. 1157, 11800 Montevideo,Uruguay

Oscar N. VenturaInstituto de Quı´mica, Facultad de Ciencias, P.O. Box 1157, Montevideo, Uruguay and Ca´tedra de Quı´micaCuantica, Facultad de Quı´mica, Gral. Flores 2124, C.C. 1157, 11800 Montevideo, Uruguay

~Received 9 May 1994; accepted 2 November 1994!

In a previous work we have identified two possible structures for the radical cation obtained byionization of hydrogen-bonded formaldehyde–water complexes@Coitino et al., J. Am. Chem. Soc.115, 9121~1993!#, a hydrogen-bonded and an addition-like complexes. We observed that the resultswere highly dependent on the method of calculation employed. Inclusion of correlation was crucialfor obtaining the correct structures of some of the complexes. In this work we used high-levelabinitio calculations in order to predict the equilibrium structure of these two complexes, thepossibility of its existence in gas phase, and the infrared spectrum to be expected in each case. Aseries of progressively more sophisticated basis sets was used to assess the effect of the quality ofthe calculations on the expected results. Also, full geometry optimization with each basis set wasperformed at the second-order Mo” ller–Plesset level, and correlation energy was calculated at thefourth-order Mo” ller–Plesset level to assess the contribution of this factor to the global result.Confirming our previous results, we found that correlation affects the hydrogen-bondedradical-cation complex more than the addition one, due to the different bonding patterns in each ofthem. Both complexes are stable—toward decomposition to the fragments or to CO1H1H3O

1—byseveral kcal/mol at all levels of theory. The hydrogen-bonded complex is more stable than theadditional one by a respectable amount~13 kcal/mol at the highest level used here!, lending supportto our previous analysis of the reactions of the former as the main channels for evolution of theformaldehyde–water radical cation. The H-bonded complex@H3O

1•••HCO•# presents twocharacteristics, very intense absorptions which should allow identification of this radical cation ifpresent in the experimental setup. These transitions are also present in the HCO• radical but theirintensity is enhanced by an order of magnitude due to the coupling with the proton in H3O

1. Weconclude that the combination of stability and characteristic infrared transitions should make thisradical-cation complex a relatively easy target for experimental determination. ©1995 AmericanInstitute of Physics.

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INTRODUCTION

One of the simplest but fundamental reactions a mecule can suffer is the loss or gaining of an electron to fothe corresponding ion radical. Recently several papers happeared in the literature on this subject.1–30With the use oflasers31 and molecular beams32,33 it is now possible to studythese ion radicals in less aggressive conditions thanelectron-impact mass-spectroscopic experiments. New teniques have been developed—i.e., matrix isolation electparamagnetic resonance34 or multiphoton ionization,35–39 forinstance—on the basis of those tools.

Several studies have been published on radical cati~RC! of oxygen-containing species~the ones in which we aremost interested!.1–5,9,21–24 However, less information wasavailable on radical cations derived from hydrogen-bond

a!Author to whom all correspondence should be addressed. Preaddress: Dipartimento di Chimica e Chimica Industriale, Univers`di Pisa, Via Risorgimento, 35. I-56100 Pisa, Italy. E-mailaurac@ibm580.icqem.pi.cnr.it

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complexes. The study of these species, due to the new tavailable, became more interesting for experimentalists atheoreticians in recent times.40–48 For instance, some workwas done on radical cations derived from the water dime46

The main process is a proton transfer from one of the momers to the other giving rise to the ion–dipole compleHO••••H3O

1. Burgers et al.23 investigated the@H2CO•••H3N#1• radical cation—within the context of a general study on the@CH5NO#1• potential energy surface~PES!—and found that it has a hydrogen-bridged structuwhich can be written as@H2CO•••H•••NH2#

1•. They alsofound a proton-transfer complex, which can be written@HCO••••NH4

1# as the most stable structure. Postmaet al.studied the radical cations derived from the vinyl alcohowater and vinyl alcohol–methanol complexes40,42 as well asthose obtained from the ketene–water dimer.41 More re-cently, Burci and Hobza have studied the radical cationsrived from methanol–water.43 Other information available onradical cations derived from H-bonded dimers can be fou

entta:

2833)/2833/8/$6.00 © 1995 American Institute of Physicst. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

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2834 Coitino, Pereira, and Ventura: Formaldehyde–water complexes

FIG. 1. Structure of the species studied in this work, showing the definitiof parameters used in Tables I–IV.2a and2a8 correspond to the structuresobtained from the H-bonded formaldehyde–water complex at the SCF aMP2 levels, respectively;2b is the structure obtained from the most stabldipole-coupled complex.

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in the works of Burgers and Terlouw44 and Heinrich andSchwarz.45

In a previous work49 we reported on a study of the radical cations derived from the formaldehyde–water dimer atheir possible decomposition pathways. We found two diffeent stable structures, one of them~2a in Fig. 1! arising fromthe conventional, H-bonded complex and the other~2b inFig. 1! from the most stable50 dipole-coupled complex. Theoptimum structure obtained for2a was dependent on theinclusion or not of correlation energy, the proton transferwater not observed in the latter case~see2a8 in Fig. 1!.However, it was not clear whether this problem was explaable on pure correlation grounds or also if the incompleness of the basis set was a cause for the drawback. Moreothe level of calculation was not high enough to accurateassess the stability and the energy difference of the strtures, and no attempt was made to investigate the possexperimental identification of these species. These aspecthe problem are the ones addressed in this work.

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METHODS

Standard unrestricted Hartree–Fock~UHF! and Mo” ller–Plesset~UMP! ab initio calculations were performed usingGAUSSIAN92.51 Complete geometry optimization of the structures presented in this work was done at each level of theusing Schelegel’s algorithm52 with several basis sets~withthe purpose of addressing the basis-set dependency ofresults!. Pople’s basis sets53 were used, starting with thesimple 3-21G up to 6-31111G(2d f ,2pd). In addition, Sa-dlej’s basis sets—developed with the goal of obtaining betelectrical properties of the monomers and, consequently, bter interaction energies and structures in dimers—were eployed. Both the medium-polarized54—(10s6p4d) con-tracted to [5s3p2d] for C, O and (6s4p) contracted to[3s2p] for H—and extended-polarized55—(24s9p6d) con-tracted to [6s4p2d] for C, O and (11s6p) contracted to[4s2p] for H—basis sets were used in this work. In alcases, spin contamination was carefully monitored a

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2835Coitino, Pereira, and Ventura: Formaldehyde–water complexes

TABLE I. Optimum geometries and total energies~in a.u.! at the SCF, MP2, and MP4~single point! levels for2a.

r1 r2 r3 r4 r5 u1 u2 u3 u4 w1 E~SCF!

SCF3-21G 1.2307 1.1701 1.0859 2.6514 0.9681 119.80 116.99 120.67 124.59 91.06 2188.520 8476-31G(d) 1.2094 1.1123 1.0876 2.8681 0.9514 118.57 116.68 118.85 126.87 90.74 2189.565 5066-311G(d,p) 1.2090 1.1132 1.0898 2.8880 0.9474 118.55 116.74 118.83 126.55 90.66 2189.588 7926-31111G(d,p) 1.2011 1.1160 1.0921 2.8848 0.9455 118.73 117.00 118.71 126.71 89.73 2189.638 8416-31111G(2d,2p) 1.2010 1.1121 1.0894 2.8926 0.9436 118.65 116.95 118.84 126.68 89.74 2189.645 2126-31111G(2d f,2p) 1.1997 1.1134 1.0899 2.8908 0.9438 118.79 117.06 119.02 126.65 89.67 2189.649 3486-31111G(2d f,2pd) 1.1994 1.1142 1.0900 2.8819 0.9439 118.78 117.08 119.01 126.68 89.67 2189.650 462Sadlej’s medium 1.2053 1.1246 1.0986 2.8731 0.9482 118.71 116.81 118.91 126.82 89.70 2189.360 901Sadlej’s extended 1.2007 1.1195 1.0939 2.8766 0.9450 118.67 116.84 118.73 126.79 89.84 2189.661 291

r1 r2 r3 r4 r5 u1 u2 u3 u4 w1 E~MP2! E~MP4//MP2!

MP23-21G 1.1867 1.9164 1.1067 2.9447 0.9908 114.98 134.80 113.43 119.23 100.922188.853 363 2188.876 5246-31G(d) 1.1763 1.8754 1.1123 2.9065 0.9883 109.97 131.28 107.68 109.12 119.922190.033 481 2190.067 5906-311G(d,p) 1.1790 1.8302 1.1058 2.8500 0.9781 105.50 131.74 102.64 110.13 118.642190.081 082 2190.117 8316-31111G(d,p) 1.1672 1.7743 1.1119 2.8079 0.9751 109.43 131.83 107.52 110.75 118.642190.174 003 2190.213 3306-31111G(2d,2p) 1.1689 1.7952 1.1056 2.8211 0.9737 106.03 131.23 103.47 108.84 120.262190.217 308 2190.257 7126-31111G(2d f,2p) 1.1672 1.7808 1.1062 2.8109 0.9742 106.29 131.64 103.95 109.66 119.532190.266 648 2190.310 2906-31111G(2d f,2pd) 1.1669 1.7719 1.1062 2.8028 0.9741 106.40 131.66 104.00 109.75 119.432190.274 433Sadlej’s medium 1.1778 1.7590 1.1227 2.7961 0.9832 103.34 131.81 100.85 108.00 121.312189.876 432 2189.917 432Sadlej’s extended 1.1704 1.7771 1.1171 2.8106 0.9793 104.64 131.42 101.71 108.41 120.712190.210 768 2190.251 634

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checked to be below normal thresholds. One may noticnonetheless, that Olivella56 has shown that spin contamination is not an important factor in the UHF or UMP determination of the geometrical structures of radical cations. Frozcore unrestricted Mo” ller–Plesset57 calculation of the correla-tion energy was done at second~UMP2! and fourth~UMP4!orders. Complete geometry optimizations were done onlythe UHF and UMP2 levels using gradient technique@UMP4~SDTQ! calculations were done at UMP2 optimum

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geometries#. One may notice that Maet al.58 observed apathological behavior of UMP2 calculations regarding thformaldehyde radical cation. However, we were ableshow59 that this conclusion does not apply to theformaldehyde–water radical cations. Force constant matricwere checked to have no negative eigenvalues. Analytisecond derivatives60 for all the species at the UMP2/6-31111G(2d,2p) level were used for calculating the theoreticainfrared~IR! spectra. Both the frequencies and intensities

TABLE II. Optimum geometries and total energies~in a.u.! at the SCF, MP2, and MP4~single point! levels for2b.

r1 r2 r3 r4 u1 u2 u3 w1 w2 E~SCF!

SCF3-21G 1.3453 1.5864 1.0758 0.9762 110.25 113.40 120.94 113.57 78.94 2188.546 0656-31G(d) 1.2986 1.5491 1.0790 0.9630 110.84 113.66 116.66 112.91 65.45 2189.574 1906-311G(d,p) 1.2993 1.5909 1.0806 0.9574 110.74 113.57 117.01 113.00 67.08 2189.593 3236-31111G(d,p) 1.2928 1.5955 1.0814 0.9553 110.86 113.73 117.34 112.97 67.45 2189.641 7556-31111G(2d,2p) 1.2926 1.5941 1.0788 0.9534 110.83 113.77 115.49 112.96 64.85 2189.648 2616-31111G(2d f,2p) 1.2920 1.5900 1.0794 0.9533 110.89 113.75 116.41 113.10 66.19 2189.653 4046-31111G(2d f,2pd) 1.2928 1.5856 1.0795 0.9538 110.89 113.68 116.45 113.24 66.27 2189.654 608Sadlej’s medium 1.2990 1.5791 1.0877 0.9585 110.97 113.53 115.50 113.53 64.57 2189.366 357Sadlej’s extended 1.2933 1.5963 1.0822 0.9585 110.83 113.70 115.75 112.90 65.00 2189.665 701

r1 r2 r3 r4 u1 u2 u3 w1 w2 E~MP2! E~MP4//MP2!

MP23-21G 1.3411 1.6446 1.0924 0.9962 112.40 113.99 117.84 113.06 70.302188.840 493 2188.869 3036-31G(d) 1.2764 1.6521 1.0968 0.9876 112.66 114.10 112.75 112.69 60.892190.016 084 2190.056 5686-311G(d,p) 1.2870 1.6664 1.0930 0.9784 112.76 114.20 114.18 112.08 62.602190.058 574 2190.101 8686-31111G(d,p) 1.2719 1.6774 1.0977 0.9743 112.93 114.55 114.71 111.89 62.642190.147 020 2190.191 9826-31111G(2d,2p) 1.2722 1.6839 1.0915 0.9727 112.86 114.55 112.49 111.81 60.662190.189 607 2190.235 6126-31111G(2d f,2p) 1.2729 1.6510 1.0933 0.9738 112.83 114.38 113.12 112.61 61.462190.240 117 2190.289 6346-31111G(2d f,2pd) 1.2729 1.6510 1.0933 0.9738 112.83 114.38 113.12 112.61 61.462190.247 643Sadlej’s medium 1.2806 1.6694 1.1078 0.9829 112.58 114.50 111.61 112.14 59.362189.850 392 2189.898 140Sadlej’s extended 1.2702 1.6982 1.1004 0.9779 112.68 114.79 112.43 111.24 60.242190.184 193 2190.231 690

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2836 Coitino, Pereira, and Ventura: Formaldehyde–water complexes

TABLE III. Relative energies~in kcal/mol! for decomposition of2a.

Basis set

With respect to H2CO1•1H2O With respect to HCO.1H3O

1 With respect to CO1H3O11H

SCF MP2 MP4/MP2 SCF MP2 MP4//MP2 SCF MP2 MP4//MP2

3-21G 234.4 257.0 251.8 216.2 217.0 216.6 225.2 221.9 222.56-31G(d) 220.9 242.3 236.7 217.9 215.8 215.5 225.1 224.4 224.56-311G(d,p) 219.4 239.3 234.0 214.5 214.4 214.3 223.5 228.5 229.16-31111G(d,p) 219.1 240.5 235.8 214.2 215.4 215.4 222.2 228.8 229.76-31111G(2d,2p) 217.8 240.4 235.7 213.7 216.0 219.1 221.8 230.6 234.66-31111G(2d f,2p) 217.8 241.2 236.4 213.4 216.3 216.3 221.7 231.7 233.06-31111G(2d f,2pd) 217.8 241.1 ••• 213.2 216.4 ••• 221.6 232.9 •••Sadlej’s medium 218.5 242.4 236.8 214.6 218.0 218.0 222.7 231.0 232.0Sadlej’s extended 218.0 241.6 236.4 213.9 217.2 217.0 221.5 230.0 231.1

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the IR absorptions were obtained in the harmonic appromation. No scaling of the force constants was done.

RESULTS AND DISCUSSION

Effect of basis set and correlation on 2a and 2b

In Tables I and II we show the optimum geometries atotal energies of the structures2a and2b calculated at eachlevel of theory. Two main effects are observed. On one sithe effect of inclusion of correlation energy on the finstructure of2a is clearly noticeable~particularly in r2 andthe angles!. For all the basis sets employed, either Pople’sSadlej’s, inclusion of correlation at the UMP2 level leadsproton transfer from the formaldehyde to water fragmen~compare2a and 2a8 in Fig. 1!. This is the most dramaticchange in geometry observed, all the parameters otherwconverging smoothly toward their limit values with increaing sophistication of the basis set. A minor difference btween UHF and UMP2 calculations can be observed in2bcomparing both C–O distances. The addition of the wamolecule to the formaldehyde radical cation is less advanaccording to UMP2 than UHF. It can be observed that 6-311G(2d f ,2pd) and extended-polarized values are in quconsistent agreement for both complexes. The only excepto this remark is the newly formed C–O bond in the radiccation2b which is larger when using the extended-polarizbasis set than with 6-31111G(2d f ,2pd).

One can notice that the SCF energy obtained withextended-polarized basis set is lower than that obtained wany of Pople’s basis sets. This is opposite at the UMP2 a

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UMP4 levels, with the values for 6-31111G(2d,2p) andbetter being lower than those for the extended-polarized basis set. This fact seems to imply that the extended-polarizebasis set is recovering less correlation energy than Poplebasis sets, a puzzling result.

In Tables III and IV are shown the relative energies of2aand 2b with respect to the following decomposition prod-ucts:

HCO•1H3O1, ~i!

CH2O1.1H2O, ~ii !

CO1H•1H3O1. ~iii !

To build these tables we have calculated the optimum geometries and energies of all the fragments at each theoreticlevel ~this information is available as additional material61!.To better interpret the meaning of these numbers we havplotted them with respect to the basis set in Fig. 2.

One can observe that after a certain level—which we calocate roughly at 6-31111G(d,p)—there are not very sig-nificant variations in the quality of the results. Sadlej’s basissets behave in a similar way than the best Pople’s basis sethe extended-polarized one being better, as expected. Tconvergence of the results obtained with the different basishows that BSSE effects are not qualitatively important, aleast for the larger basis sets~especially taking into accountthe agreement of Pople’s and Sadlej’s sets!.

TABLE IV. Relative energies~in kcal/mol! for decomposition of2b.

Basis set

With respect to H2CO1•1H2O With respect to HCO.1H3O

1 With respect to CO1H3O11H

SCF MP2 MP4/MP2 SCF MP2 MP4/MP2 SCF MP2 MP4//MP2

3-21G 250.2 249.0 247.2 232.0 28.94 212.1 241.0 213.8 218.06-31G(d) 226.4 231.4 229.7 223.3 24.84 28.55 230.6 213.5 217.66-311G(d,p) 222.3 225.2 224.0 217.3 20.31 24.30 226.3 214.4 219.16-31111G(d,p) 221.0 223.6 222.4 216.1 1.50 22.03 224.1 211.9 216.36-31111G(2d,2p) 219.7 223.0 221.8 215.6 1.39 25.21 223.7 213.2 220.76-31111G(2d f,2p) 220.4 224.5 223.4 216.0 0.38 23.38 224.3 215.0 220.16-31111G(2d f,2pd) 220.4 224.3 ••• 215.8 0.40 ••• 224.2 216.1 •••Sadlej’s medium 221.9 226.1 224.7 218.0 21.66 25.86 227.0 214.6 219.9Sadlej’s extended 220.7 224.9 223.9 216.7 20.52 24.69 224.3 213.3 218.6

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2837Coitino, Pereira, and Ventura: Formaldehyde–water complexes

FIG.2.

Effectofthebasisseton

therelativeenergies

obtained

ateach

leveloftheory

@SCF,UMP2,andUMP4~SDTQ

!#for2a

~top

ofthefigure,plotsa–c!and2b

~bottom,plotsd–f!with

respecttothedecomposition

products

CH 2O

1. 1H2O,HCO. 1H3O

1,andCO1

H. 1H3O

1.1–7are,

respectively,

Pople’sbasissets

3-21G,6-31G(

d),6-31

1G(d,p),6-3111

1G(d,p),6-3111

1G(2d,2p),6-3111

1G(2df,2p),and6-3111

1G(2df,2pd).8and9correspond

toSadlej’s

medium-polarized

andextended-polarized

basissets.

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2838 Coitino, Pereira, and Ventura: Formaldehyde–water complexes

The effect of correlation energy is especially noticeabin Fig. 2~a!, because of the proton transfer given by UMPcalculations. It is interesting to notice that the decompositioreactions

2a→HCO•1H3O1 @Fig. 2~b!#,

2b→CH2O1.1H2O @Fig. 2~d!#

~which may be looked at as the natural decomposition infragments of each of the complexes! show the smallest cor-relation energy effects, reflecting the fact that the interframent bond is the weakest in each of the complexes.

From an energetical point of view, one can observe ththe complex2a is more stable than2b @by about 13 kcal/molat the UMP4/6-31111G(2d f ,2p) level#. This fact adds sig-nificance to our previous study on the decomposition patways of2a. In fact, it can be concluded that ionization of themost stable H-bonded formaldehyde–water dimer in miconditions will give rise to the most stable radical cation2a,which will follow the reaction paths already described.49 Onthe other side, it is also easier to break the weak bond in2athan it is in2b ~216.3 vs223.4 kcal/mol at the same levelas before!. The significance of these data comes from a prvious speculation as to what may be the use of these distoradical cations. Since they have a charge, two moleculesthese species will tend to repel but, at the same time, a becan be electrically driven toward a desired place, a reactichamber, for instance. Then if one can easily get rid of thcharged fragment~and here is where the energy of decomposition into fragments comes into place! the radical frag-ments will be able to react fast to obtain the reaction produ

IR spectra and identification of 2a and 2b

We show in Table V the harmonic frequencies and intesities of the radical cations2a and2b together with those ofthe fragments HCO• and CH2O

1.. To allow an easier com-parison of the spectra, we have plotted the main lines in F

TABLE V. Harmonic frequencies~in cm21! and IR intensities~Km/mol!computed at the UMP2/6-31111G(2d,2p)//UMP2/6-31111G(2d,2p)level for the four radicals2a, 2b, H2CO

1•, and HCO.. The transitions for thelatter two radicals have been listed in front of the corresponding transitionthe complex cation radicals to facilitate the comparison.

2a HCO. 2b H2CO1.

Symm. v I v I Symm. v I v I

A9 163 74A8 104 48 A8 271 67A8 280 24 A8 523 286A9 374 5 A8 717 141A8 541 121 A9 770 2 882 1A9 622 23 A9 948 5A8 1071 236 A9 1146 6 1124 29A8 1189 47 1128 45 A8 1269 56A9 1696 37 A8 1307 50 1324 81A8 1698 38 A8 1398 17A8 2124 1178 1908 69 A8 1675 106 1628 2A8 2765 1378 2760 96 A8 3050 39A8 2946 54 A9 3150 33A8 3675 220 A8 3670 195 2879 163A9 3757 374 A9 3771 275 3025 153

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3 for both complexes. It is immediately evident that the pairof strong lines at 2124 and 2765 cm21 in 2a would allowdifferentiation of the two radical cations, because they arecompletely absent in2b. These two lines correspond mainlyto C5O and C–H stretchings, respectively, and they are alsopresent in HCO• at 1908 and 2760 cm21. However, in2a thecoupling of those vibrations with the motion of the H-bondedproton in H3O

1 enhances the intensities several times. Con-sequently, there should be no interference of the spectrum oHCO• with the identification of2a ~unless the former were inmuch too high concentration with respect to the latter!. It isinteresting to point out that a similar enhancement in inten-sity has been noted for the OH stretching in the water dimeras compared to the water monomer, essentially for the samreasons than here~coupling to the second fragment!.

Identification of2b would not be so easy as that of2abecause it does not present any intense line. The lines a3670 and 3771 cm21 can be easily masked by the one of thehydronium cation, and the other line at 523 cm21 can bemasked by that of2a if present.

CONCLUSIONS

In this work we used high-levelab initio methods tostudy the structure and IR spectra of two radical cationswhich may possibly be obtained from the H-bonded

in

FIG. 3. Main lines for the vibrational spectra of2a and2b calculated at theUMP2/6-31111G(2d,2p) level within the harmonic approximation. Har-monic frequencies are not scaled.

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.

2839Coitino, Pereira, and Ventura: Formaldehyde–water complexes

formaldehyde–water dimer. We found that all theoretical caculations agree in that both complexes~2aand2b! are stable.Convergence of the results with the improvement of the baset was demonstrated. However, independent of the quaof the basis set, the inclusion of correlation energy is essetial to get the correct structure of2a. All methods agree inthat 2a is more stable than2b by a considerable amount~ifthe correct structure of2a is considered!, 13 kcal/mol at thebest level of theory employed up to now. This lends suppoto the study we already made on the decomposition reactioof 2a.

We were also able to show that decomposition of2a intothe fragments~HCO•1H3O

1! needs only a relatively modestamount of energy~about 16 kcal/mol!. Thus, it may be dif-ficult to identify this species except in very specific condtions for the ionization. However, the IR spectrum is vercharacteristic, due to a pair of very intense transitions whiceven if present in HCO•, are much more intense because othe coupling with the hydronium cation fragment.

Since2a shows adequate stability and characteristic Ispectrum we predict that this species can be observedadequate experimental setups.

ACKNOWLEDGMENTS

We gratefully acknowledge Professor Jacopo Tomasiwhose laboratory part of the calculations were performeContinuous support to our research by the Commissionthe European Communities is gratefully acknowledged.

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61See AIP document No. PAPS JCPSA-102-2833-3 for 3 pages of tabcontaining structural parameters and energetics obtained at each levetheory for the possible fragments of decomposition~H., CO, H3O

1,CH2O

1., H2O, and HCO.!. Order by PAPS number and journal referencefrom the American Institute of Physics, Physics Auxiliary Publication Service, Carolyn Gehlbach, 500 Sunnyside Boulevard, Woodbury, New Yo11797-2999. Fax: 516-576-2223, e-mail: janis@aip.org. The price is $1.for each microfiche~98 pages! or $5.00 for photocopies of up to 30 pages,and $0.15 for each additional page over 30 pages. Airmail additionaMake checks payable to the American Institute of Physics.

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