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Available online at www.sciencedirect.com

Spectrochimica Acta Part A 70 (2008) 793–798

Molecular interactions between pyrazine and n-propanol,chloroform, or tetrahydrofuran

Rui Wang a, QingZhong Li b,a, Ruiguang Wu a,GuoShi Wu a, ZhiWu Yu a,∗

a Key Laboratory of Bioorganic Phosphorous Chemistry & Chemical Biology (Ministry of Education),Department of Chemistry, Tsinghua University, Beijing 100084, China

b Science and Engineering College of Chemistry and Biology, Yantai University, Yantai 264005, China

Received 18 April 2007; received in revised form 31 August 2007; accepted 18 September 2007

bstract

The molecular interactions of pyrazine (PZ) with n-propanol, chloroform, and tetrahydrofuran (THF) have been investigated by employing ultravi-let spectroscopy and quantum chemical calculation methods. A new quantity, excess absorption coefficient, was introduced to represent the strengthf the interaction. It was found that the interaction decreased in the order: PZ–propanol > PZ–chloroform > PZ–THF. The Benesi–Hildebrandethod indicated that the interaction stoichiometries of the PZ–chloroform and PZ–THF systems were both 1:1 and the equilibrium constantsere determined to be 2.07 and 0.64 M−1, respectively. Using a nonlinear fitting method, we demonstrated that the PZ–propanol was a two-step:2 interaction pair and the equilibrium constants were determined to be 8.8 and 0.19 M−1. Quantum chemical calculations showed the existence

f hydrogen-bonding interactions in all the three system: normal N· · ·H–O hydrogen bond in the PZ–propanol system, unconventional N· · ·H–Cydrogen bond in the PZ–chloroform, and weak N–C–H· · ·O hydrogen bond in the PZ–THF system. Methodologically, we pointed out that specialare must be taken when the Benesi–Hildebrand method is used to evaluate 1:2 interactions.

2007 Elsevier B.V. All rights reserved.

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eywords: Pyrazine; Hydrogen bond; Ultraviolet spectroscopy; Quantum chem

. Introduction

Pyrazine (PZ) contains two nitrogen atoms in its aromaticing and therefore, may mimic certain properties of the nitroge-ous bases of nuclear acids. PZ and its derivatives have alsoeen used as drug and food spices [1,2]. Due to the lone pairlectrons on the two nitrogen atoms, PZ is believed to interactith proton donors; on the other hand the � C–Hs of PZ have

he potential to interact with proton acceptors. It is therefore ofnterest to ask how PZ interacts with different proton donorsnd acceptors. In this work, three organic molecules have beenelected: n-propanol as a representative of strong proton donors,hloroform as a weak proton donor, and tetrahydrofuran (THF)

s a representative of proton acceptors. These three moleculesereafter are referred to as ligands.

∗ Corresponding author. Tel.: +86 10 6279 2492; fax: +86 10 6277 1149.E-mail address: yuzhw@tsinghua.edu.cn (Z. Yu).

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386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2007.09.014

alculations; Benesi–Hildebrand method

Among the three interaction pairs, n-propanol and PZolecules can form typical hydrogen bonds. What is unknown

s their interaction stoichiometry: can one PZ molecule formydrogen bonds with two alcohol molecules?

In the case of chloroform and THF, weak hydrogen bondsan be expected when they come into contact with PZ molecule.he C–H bond of chloroform has been proved to form weakydrogen bonds with various molecules such as alcohol [3,4],cetone, and ether [5,6]. The C–D· · ·N hydrogen bond between-chloroform and PZ was detected by IR spectroscopy [7].–H· · ·N interaction has also been proposed in crystals of PZnd methyl substituted PZs [8], allowing edge–edge associationf the aromatic rings.

THF has a lone electron pair on its oxygen atom, and oneeasonable structure of PZ has negative charges on nitrogen andositive charges on aromatic hydrogen [9]. Thus, it is possible for

HF to interact with the aromatic hydrogen of PZ as the protononor to form an N–C–H· · ·O weak hydrogen bond. This typef aromatic C–H· · ·O interaction has already been reported inany systems such as thiazole–water complex [10], the dimers

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f 3-oxosultams [11], and the 1,2,4,5-tetrafluorobenzene andater system [12]. It has also been observed in lipid bilayers

13] and globular protein structures [14].In this work, UV spectroscopy and quantum chemical calcu-

ation methods have been employed to investigate the molecularnteractions of the three PZ mixtures. Attention has been paido three aspects. First, hypochromic effect [15] of the inter-ctions was examined and a new quantity, excess absorptionoefficient, was defined to probe the strength of the inter-ction. Second, Benesi–Hildebrand (B–H) method [16] haseen employed to provide information of equilibrium con-tants and stoichiometries. Particularly, both linear and nonlineartting procedures were performed to derive the equilibriumonstants of the possible 1:2 interactions. Third, quantumhemical calculations were used to confirm the presence ofoth conventional and unconventional hydrogen bonds in theystems.

. Experimental and computational method

Pyrazine (99.9%) was purchased from Acros (Belgium).hloroform, 1-propanol, THF and n-heptane with purity higher

han 99% were obtained from Beijing Chemical Reagents Com-any (China). In the pseudo-binary solutions with heptane asnert solvent, the concentration of PZ was fixed at around× 10−4 M−1, whilst the molar ratio of chloroform, 1-propanol,r THF to PZ varied from 0 to 3 × 104.

The UV absorption spectra were recorded on a Pharma-ia Biotech Ultrospec 4000 spectrophotometer (Sweden). Itsbsorbance range was confirmed to be linear up to 3.0 with aque-us solutions of potassium dichromate at wavelength 440 nm.ll the experiments were performed at 25 ◦C and were repeated

or three times. The length of absorption cell b is 0.5 cm. Aartial UV spectrum of PZ dissolved in heptane is shown inig. 1. The band of n → �* transition can be deconvoluted intoix peaks according to literature work [17] using the Peakfit.0 software. These peaks are located at 304, 308, 311, 315,22, and 327 nm, respectively. The least overlapped peak around27 nm was chosen to do the following quantitative analysis andalculations.

Benesi–Hildebrand method [16] was used to determine the

quilibrium constants and stoichiometries of the hydrogen bond-ng interactions in the PZ systems. Briefly, if we assume ahromophore-containing molecule P interacts with ligand B to

ig. 1. Ultraviolet spectrum of PZ dissolved in heptane (solid line) and theeconvoluted results (dash line).

Aatveofwsm[3faiw

ta Part A 70 (2008) 793–798

orm a 1:1 complex PB:

+ BK�PB (1)

hen 1:1 B–H equation is as follows:

bC0P

�A= 1

C0BK(εPB − εP)

+ 1

εPB − εP(2)

here �A is the absorbance change during complexation, C0P

nd C0B the initial concentrations, εP and εPB the absorption

oefficients of P and PB, respectively, and b is the length ofbsorption cell. By plotting bC0

P�A versus 1/C0B, a linear rela-

ionship can be obtained. Equilibrium constant K can then bealculated from the intercept and slope. On the other hand,he linear plot is also taken as the sufficient condition of 1:1nteraction in the stoichiometry study.

If the 1:2 interaction in the form of P + BK1�PB and PB +

K2�PB2 takes place, the K1 and K2 can be expressed as follows

1 = CPB

(C0P − CPB − CPB2 )(C0

B − CPB − 2CPB2 )(3)

2 = CPB2

CPB(C0B − CPB − 2CPB2 )

(4)

ut if it is a one-step interaction in the form of P + 2BK�PB2,

hen, we can obtain 1:2 B–H equation:

bC0P

�A= 1

C02

B K(εPB2 − εP)+ 1

εPB2 − εP(5)

It is generally believed that the bC0P�A versus (1/C0

B)2 plotould assure a straight line for a 1:2 complex. For PZ systems, it

s possible to form two hydrogen bonds between PZ and ligands,ecause pyrazine has two nitrogen atoms as proton acceptors.hus, the 1:1 and 1:2 B–H methods can help us to obtain thequilibrium constants and stoichiometries of the interactions inZ systems.

Quantum chemical calculations were performed by using theaussian 98 program [18]. 1:1 molecular pairs were considered.ll the geometries of complexes (PZ–propanol, PZ–chloroform,

nd PZ–THF) and individual molecules were optimized athe B3LYP/6-31+G(d,p) and MP2/6-31+G(d) levels. Harmonicibrational frequencies were also computed at the same lev-ls to confirm that the optimized structures were local miniman the energy surfaces. No imaginary frequencies occurredor all the optimized structures. The interaction energies �Eere corrected with zero-point vibrational energies and basis

et superposition errors (BSSE). The BSSE correction was esti-ated using the counterpoise method of Boys and Bernardi

19]. The three complexes were also calculated at the B3LYP/6-1+G(d,p) level in heptane solvent by the integral equationormalism polarized continuum model (IEF-PCM). The inter-ction energies in heptane were calculated with a methodntroduced by Wang and Duan [20]. They were then correctedith BSSE in gas phase.

R. Wang et al. / Spectrochimica Acta Part A 70 (2008) 793–798 795

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(cto secure the approximation CB ≈ CB of B–H method, the molarratio of ligands to PZ is designed to be greater than 50. Threeparallel experimental data were calculated. Presented in Fig. 4are the 1:1 and 1:2 B–H plots of a set of experimental data of the

ig. 2. The decomposed UV band around 327 nm with different molar ratios (α04. It increases from top to bottom with a multiple of three.

. Results and discussion

.1. Analysis of absorption spectra

The interactions between PZ and the three ligands can bexamined in detail by using the selected UV absorption bandround 327 nm. Shown in Fig. 2 are the dependencies of thebsorption coefficient on the molar ratio (α) of ligands to PZ. Ithows that blue-shift occurs at high molar ratios of n-propanolnd chloroform to PZ. In the case of THF, however, red-shiftccurs. The result manifests that the interaction between THFnd PZ is different from those of the other two pairs. This isecause that, in the THF system, PZ works as an electron accep-or or a proton donor, whilst in the other two systems it workss a proton acceptor. This will be elaborated in Section 3.4 byeans of quantum chemical calculations.

.2. Excess absorption coefficient

In order to compare the interactions of PZ with the threeigands quantitatively, a new quantity, excess absorption coef-cient, is introduced, as an extension of our previous work in

nfrared spectroscopy [21,22].The definition of the excess absorption coefficient originates

rom the Beer–Lambert law. The linear relationship betweenbsorbance A and concentration c holds only under proper con-itions, normally at low concentration of a solute. When theoncentration is high, the Beer–Lambert law will deviate fromhe linear relationship. All the factors causing the deviation fromeer–Lambert law can be summed up and put into a nonlin-ar parameter ε*, giving an extended Beer–Lambert law in theollowing form for a fixed unit length of light path:

= (ε0 + ε∗)c (6)

here ε0 represents the absorption coefficient at low concen-ration of a solute in an inert solvent and ε* represents thexcess absorption coefficient caused by interactions between the

hromophoric solute molecules with other molecular species orith the solute itself in the system. The new parameter ε* can

lso be considered as a representation of the hypochromicity inltraviolet spectroscopy.

FtPa

gands (A, propanol; B, chloroform; C, THF) to PZ. The range of α is from 0 to

The excess absorption coefficients of the three PZ-systemsre compared in Fig. 3. It is thus clear that the excess absorptionoefficients are zero at low concentrations of the ligands. Thereppears to have a critical molar ratio on each curve, beyondhich the absorption coefficient will deviate from linearity. The

ritical values of n-propanol, chloroform, and THF systems are0, 184, and 577, respectively. It is consistent with the decreasingequential order of the strength of molecular interactions in thehree PZ systems.

.3. Equilibrium constants and stoichiometries of thenteractions

The 1:1 B–H method (Eq. (2)) and 1:2 B–H method (Eq.5)) were used to investigate the equilibrium constants and stoi-hiometries of the interactions in the three PZ-systems. In order

0

ig. 3. Dependence of the excess molar absorptivity of the band at 327 nm onhe logarithm of the molar ratio (α). Dotted line, PZ–THF system; solid line,Z–chloroform system; dash line, PZ–propanol system. Error bars represent theveraged results of three parallel experiments.

796 R. Wang et al. / Spectrochimica Acta Part A 70 (2008) 793–798

F PZ–prA plots.

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ig. 4. 1:1 and 1:2 B–H plots at the maximum absorption (around 327 nm) of, B, and C are results of the 1:1 B–H plots. The rest are those of the 1:2 B–H

hree PZ systems. Roughly speaking, the 1:1 B–H plots are lin-ar and the 1:2 B–H plots are nonlinear, suggesting all the threenteraction pairs are in 1:1 form. A closer examination, however,hows that the 1:1 B–H plot of PZ–propanol system in Fig. 4(A)s less linear than those of the other two systems in Fig. 4(B and). This is also reflected in the average fitting correlation coeffi-ients R2 of the three systems, 0.968, 0.993, and 0.995. The firsts markedly smaller than the other two. This indicates that thenteraction pattern between PZ and propanol could be neither ane-step 1:2 interaction, nor a perfect 1:1 interaction. It mighte a two-step 1:2 interaction due to the presence of two nitrogentoms in PZ and the strong proton donating ability of the alcohol.he equilibrium constants of the PZ–chloroform and PZ–THFystems are 2.07and 0.64 M−1, respectively. Evaluation of thequilibrium constant of PZ–propanol system is discussed below.

In our recent work, we found that the 1:2 B–H plot couldenerate wrong results in some situations [23]. In order to under-tand the interaction mechanism between PZ and propanol,he nonlinear fitting method is conducted by the following

t

ig. 5. The optimized structures of PZ–propanol (A), PZ–chloroform (B), and PZ–Tiew from left of every complex is added for clarity.

opanol (A and D), PZ–chloroform (B and E), and PZ–THF system (C and F).

quation [24]:

bC0P

�A= 1 + K1C

0B + K1K2C

02

B

K1�ε1C0B + K1K2�ε2C

02

B

(7)

here b and �A are the same parameters as the ones in Eq. (2).ε1 = εPB − εP and �ε2 = εPB2 − εP, where εP, εPB and εPB2 are

bsorption coefficients of free P, 1:1 bounded and 1:2 boundedomplexes. The nonlinear fitting was performed by Origin 7.5nd it was found that K1 = 8.8 M−1 and K2 = 0.19 M−1 withtting correlation coefficient R2 = 0.9998.

Pistolis and Malliaris pointed out in their work that the dif-erentiation between 1:1 and 1:2 stoichiometries was impossiblender a particular condition as described in the following equa-

ion [25]:

K1

K2= (εPB2 − εP)2

(εPB − εP)(εPB2 − εPB)(8)

HF (C) calculated at the MP2/6-31+G(d) level in the gas phase. An additional

R. Wang et al. / Spectrochimica Acta Part A 70 (2008) 793–798 797

Table 1Interaction energies (�E), bond lengths (r), and bond angles (Φ) of the hydrogen bonds in the three molecular interaction pairs

PZ–propanol PZ–chloroform PZ–THF

MP2 6-31+G(d)�E (kJ mol−1) −17.31 −15.72 −11.94rH· · ·Y (A) 1.9676 2.1680 (3.2018) 2.2486 (2.8294)ΦX–H· · ·Y (◦) 173.1 152.1 (124.3) 140.4 (127.4)

B3LYP 6-31+G(d,p)�E (kJ mol−1) −16.59 −13.22 −5.28rH· · ·Y (A) 1.9780 2.2020 (3.9172) 2.2979 (3.1859)ΦX–H· · ·Y (◦) 178.1 177.9 (78.9) 158.9 (135.4)

B3LYP (IEFPCM) 6-31+G(d,p)�E (kJ mol−1) −21.04 −15.65 −8.28

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ote: the data in parenthesis for PZ–chloroform and PZ–THF structures are attr

sing the parameters obtained from the nonlinear fitting resultsf the PZ–propanol system we found that the above equation isot satisfied at all. Thus, the nonlinear fitting results are reliable.urther, with the help of Matlab 6.5, the molar ratio of PB toB2 at the equilibrium state can be evaluated, it is found in theange of 61–0.43 within the initial B to P molar ratio range of72–24,560 used in the present work particularly, when B to P isn the range of 10,472–24,560, PB2 will dominate the complexith the molar ratio of PB to PB2 between 1 and 0.43.

.4. Quantum chemical calculations

Molecular details of the interactions between PZ and thehree ligands have been examined by quantum chemical calcula-ions at the MP2/6-31+G(d) and B3LYP/6-31+G(d,p) levels. Theptimized structures of the three 1:1 complexes at the MP2/6-1+G(d) level in the gas phase are shown in Fig. 5. Geometricarameters and interaction energies are summarized in Table 1.

Solvation effect has been considered at the B3LYP/6-1+G(d,p) level by the integral equation formalism polarizedontinuum model (IEF-PCM). Results are also listed in Table 1.t can be seen that the solvation effect of heptane lowers interac-ion energies rather significantly. In agreement with this result,he solvent effect also shortens the bond lengths and increaseshe bond angles, but with no more than 1.4% change except forhe weakest PZ–THF interaction, where bond angle subjects anncrease from 158.9◦ to 175.3◦.

Results obtained at the B3LYP/6-31+G(d,p) level and at theP2/6-31+G(d) level all suggest that hydrogen bond forms

n each of the three molecular interaction pairs. Let us takehe results of MP2/6-31+G(d) to discuss the molecular inter-ctions. For the PZ–propanol system, the interaction energy is17.31 kJ mol−1, the N· · ·H bond length is 1.9676 A, and the· · ·H–O bond angle is 173.1◦. This is a typical conventionalydrogen bond like the hydrogen bond between azabenzene andhioacetamide [26].

For the PZ–chloroform complex, the interaction energy is15.72 kJ mol−1. There are two potential interaction modes:· · ·H–C and C–H· · ·Cl. The bond length and the bond angle in

he former case are 2.1680 A and 152.1◦, whilst those in the

Qfrh

2.1854 (4.0020) 2.3026 (4.2775)178.8 (75.6) 175.3 (123.6)

d to C–H· · ·Cl and C–H· · ·N interactions, respectively.

atter are 3.2018 A and 124.3◦, respectively. Clearly the for-er mode dominates the PZ–chloroform interaction. For theZ–THF complex, the interaction energy is −11.94 kJ mol−1.here are also two potential interaction modes: C–H· · ·O and· · ·C–H. The bond length and the bond angle in the former

ase are 2.2486 A and 140.4◦, whilst those in the latter casere 2.8294 A and 127.4◦, respectively. Clearly the former modelays a bigger role in the PZ–THF interaction. These values areimilar to those of the weak hydrogen bonds reported in litera-ures. For example, the H· · ·O bond length in the C–H· · ·O weaknteraction was reported to be in the range from 1.970 to 2.520 A27]. Calhorda once concluded that the interaction energy of–H· · ·O hydrogen bond in vacuum was in the range from −2.1

o −15.8 kJ mol−1 [28]. In another work, Raos reported that thenteraction energy of C–H· · ·O weak hydrogen bonds was typi-ally in the range from −3 to −10 kJ mol−1 [29]. According tohese criteria, we conclude that weak hydrogen bonds can formn both PZ–chloroform and PZ–THF systems.

Based on the data in Table 1, it is found that the sequen-ial order of the interaction strength in the three systems is:Z–propanol > PZ–chloroform > PZ–THF. This sequence is ingreement with that obtained from Fig. 3. Equilibrium con-tants evaluated from Fig. 4 also support this conclusion. Inther words, the theoretical results are in good agreement withhose obtained experimentally.

. Conclusions and remarks

Molecular interactions between pyrazine and three represen-ative small molecules, n-propanol, chloroform, and THF, haveeen investigated. Their relative strengths were first examinedy the newly defined excess absorption coefficient. The criti-al molar ratios of the ligands to PZ molecule were found toe 50, 184, and 577, indicating that the sequence of the inter-ction strength is PZ–propanol > PZ–chloroform > PZ–THF.

uantum chemical calculations supported this conclusion and

urther pointed out that hydrogen bonding played an importantole in the interactions. In addition to the normal N· · ·H–Oydrogen bond in the PZ–propanol system, unconventional

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· · ·H–C hydrogen bond in the PZ–chloroform system and weak–C–H· · ·O hydrogen bond in the PZ–THF system have beenemonstrated.

Using the B–H method, it was found that the interaction sto-chiometries of the PZ–chloroform and PZ–THF systems areoth 1:1. For the PZ–propanol system, however, the results areroblematic and interesting. Although the B–H method indi-ated the stoichiometry is close to 1:1, we found this is true onlyn the low molar ratio range of propanol/PZ. When the ratios greater than 10,000, 1:2 (PZ:propanol) interaction will dom-nate. Generally speaking, both 1:1 and 1:2 interactions existn this system. The literature applications of the B–H methodonsider only 1:1 and the simple one-step 1:2 (or 1:n) interac-ions. The present work demonstrated special care must be takenhen the Benesi–Hildebrand method is used to evaluate 1:2

nteractions. Individual interaction steps should be considered.The equilibrium constants of the PZ–chloroform and

Z–THF systems were 2.07 and 0.64 M−1, respectively. Theonstants of the two-step PZ–propanol interactions were deter-ined as 8.8 and 0.19 M−1.

cknowledgements

This work was supported by grants from the Natural Scienceoundation of China (NSFC: 20633080, 20675046).

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[

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ta Part A 70 (2008) 793–798

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