Charge Transfer and Blue Shifting of Vibrational Frequencies in a

Article pubs.acs.org/JPCA

Charge Transfer and Blue Shifting of Vibrational Frequencies in a Hydrogen Bond Acceptor Ashley M. Wright, Austin A. Howard, J. Coleman Howard, Gregory S. Tschumper, and Nathan I. Hammer* Department of Chemistry and Biochemistry, University of Mississippi, University, Mississippi 38677, United States S Supporting Information *

ABSTRACT: A comprehensive Raman spectroscopic/ electronic structure study of hydrogen bonding by pyrimidine with eight different polar solvents is presented. Raman spectra of binary mixtures of pyrimidine with methanol and ethylene glycol are reported, and shifts in ν1, ν3, ν6a, ν6b, ν8a, ν8b, ν9a, ν15, ν16a, and ν16b are compared to earlier results obtained for water. Large shifts to higher vibrational energy, often referred to as blue shifts, are observed for ν1, ν6b, and ν8b (by as much as 14 cm−1). While gradual blue shifts with increasing hydrogen bond donor concentration are observed for ν6b and ν8b, ν1 exhibits three distinct spectral components whose relative intensities vary with concentration. The blue shift of ν1 is further examined in binary mixtures of pyrimidine with acetic acid, thioglycol, phenylmethanol, hexylamine, and acetonitrile. Electronic structure computations for more than 100 microsolvated structures reveal a significant dependence of the magnitude of the ν1 blue shift on the local microsolvation geometry. Results from natural bond orbital (NBO) calculations also reveal a strong correlation between charge transfer and blue shifting of pyrimidine’s normal modes. Although charge transfer has previously been linked to blue shifting of the X−H stretching frequency in hydrogen bond donors, here, a similar trend in a hydrogen bond acceptor is demonstrated.

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of these have involved weak C−H···O interactions.21−25 The present study interrogates the vibrational modes in a hydrogen bond acceptor molecule that exhibits blue shifts and quantifies the role of charge transfer in the magnitude of these shifts. The study of the blue shifting associated with pyrimidine’s normal modes dates back to the 1960s when Takahashi et al. recorded infrared absorption spectra of pyrimidine (1,3-diazine, Figure 1) in

INTRODUCTION From controlling biological activity to directing molecular selfassembly, hydrogen bonding plays central roles in all areas of Nature.1−14 Here, we use the common hydrogen bond nomenclature X−H···Y to denote the donor (X−H) and acceptor (Y). Most hydrogen bonding interactions result in a decrease in vibrational energy of the X−H stretching frequency of the hydrogen bond donor, often dubbed a red shift. Hydrogen bonds resulting in an increase in vibrational energy of the X−H donor stretching frequency, frequently called a blue shift, are sometimes referred to as improper hydrogen bonds. While red and blue shifts of the X−H donor stretching frequency have been linked to X−H bond shortening and elongation, respectively, the origin of the change in X−H bond length is still debated.11,15−18 Hobza proposed a method for hydrogenbonded complexes called the H-index that helps determine if the formation of an intermolecular hydrogen bond results in a red or blue shift in the X−H donor stretching frequency.19 The H-index is the ratio of electron density transferred from the hydrogen bond donor to the X−H antibonding orbital relative to the total electron density transferred. More recently, Szostak took a slightly simpler approach and correlated the atomic charge on H, calculated through natural bond orbital (NBO) analysis, to the magnitude of the shift in the X−H stretching frequency.20 These studies have focused on the effects that hydrogen bonding has on the donor (X−H). Fewer studies have concentrated on blue shifts in hydrogen bond acceptors, and many © XXXX American Chemical Society

Figure 1. Pyrimidine (Py) monomer shown with labels for the atoms and regions of the ring. Received: February 15, 2013 Revised: May 15, 2013

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dx.doi.org/10.1021/jp401642b | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 2. Raman spectra of pyrimidine with varying mole fractions of methanol and ethylene glycol, (a) χpy = 1.00, (b) χpy = 0.85, (c) χpy = 0.70, (d) χpy = 0.50, (e) χpy = 0.30, (f) χpy = 0.10, and (g) χpy = 0.00.

several donor solvents including water.26 It was suggested at that time that hydrogen bonding interactions resulted in a considerable change of electron distribution in pyrimidine and was the origin of the shifts to higher frequencies of some of pyrimidine’s normal modes. The in-plane skeletal vibration (breathing mode), ν1, was observed to be largely affected by hydrogen bonding, with shifts reported up to 14 cm−1 in water. For this reason, it was suggested that interactions between pyrimidine and hydrogen-bonded solvent molecules involved pyrimidine’s nitrogen atoms or its delocalized π-electrons. In 2007, Schlücker et al. conducted a polarization-resolved linear Raman spectroscopy study on pyrimidine in various concentrations of water to reinvestigate the blue shift in ν1.27 These authors compared their experimental results to data from electronic structure calculations on 10 small explicit pyrimidine/ water clusters and noticed a correlation between O−H···N hydrogen bond distances and the magnitude of the blue shift in the ring breathing mode for similar structural motifs. For example, this correlation existed for structures involving water molecules clustered on one side of pyrimidine. A line shape analysis was also performed on the experimental spectra of ν1, which suggested the presence of three distinct spectral components.

The authors noted that these spectral features are consistent with free pyrimidine and “two distinct groups of hydrogenbonded species”. Earlier, in 1992 and 1995, Yarwood, Besnard, and co-workers recorded Raman spectra of pyridine, a similar heterocyclic compound containing only one nitrogen atom, in binary mixtures with ethanol28 and water.29 Interestingly, two distinct spectral components were observed for ν1. The first spectral component remained at the same location of ν1 in neat pyridine and is attributed to free pyridine remaining in solution. The second spectral component was attributed to hydrogen-bonded complexes. The free pyridine component is easily seen at high concentrations of ethanol but not at high concentrations of water. The authors explain the broad shape of the second spectral component by fitting the peak with multiple Lorentzian bands in order to represent hydrogen-bonded complexes with varying numbers of solvent molecules. In 2001, Schlücker et al. supported the assignment of the second spectral component observed for ν1 due to hydrogen-bonded complexes using density functional theory (DFT) computations.30 In 2010, we undertook an effort to elucidate the origins of the spectral shifts that can be observed in 10 normal modes of B



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pyrimidine, not just ν1, when interacting with water.31 NBO analyses were performed, and significant charge transfer from pyrimidine to the water molecules was observed. This transfer also resulted in a decrease in the population of the nitrogen lone pair orbitals as well as contractions of the bonds associated with the front and the back sides of the pyrimidine ring (see Figure 1). Comparison of the experimental polarized Raman spectra to electronic structure calculations on explicit pyrimidine/ water clusters also led to the first definitive assignment of the overlapping ν8a and ν8b modes, which exhibit different degrees of blue shifting when interacting with water. That study of pyrimidine/water interactions was extended to include over 70 microhydrated clusters of pyrimidine using DFT and a triple-ζ quality basis set.32 Those structures included up to seven water molecules and indicated that the lowest-energy structures were characterized by water molecules hydrogen bonded only to one side of the pyrimidine ring, only interacting with one of the two nitrogen atoms. However, the best agreement between the experimentally observed and computed shifts consistently resulted from hydrogen bonding motifs involving at least one water molecule interacting with both nitrogen atoms in the ring, as would be expected in the bulk phase. These clusters reproduced the experimentally observed vibrational frequency shifts of ν1 and ν8b as the mole fraction of water increased to within 1 cm−1. In contrast, structures exhibiting only C−H···O interactions at the side or the back of the ring (see Figure 1) had the poorest agreement with experiment. In pyrimidine, which has C2v symmetry, the region near C1 is referred to as the front of the ring, C6 as the back, and the area around N2 and C4 (or N3 and C5) as the side of the ring. Pyrimidine/water structures where the water molecules are clustered near the front of the ring yield results that more closely reproduce experimental shifts than structures with water molecules interacting with the side or back of the ring. This interesting result suggested that the hydrophobic effect could play a key role in directing solvation of pyrimidine in such mixtures, with the creation of so-called “iceberg water” due to the structured hydrogen-bonded network at the front of the ring.33−35 In 2011, Singh and co-workers recorded Raman spectra in the C−H stretching region of binary mixtures of water with the three diazines pyrimidine, pyrazine, and pyridazine.36 These authors reported that the C−H stretching modes in all three diazines were blue-shifted and that changes in the populations of the C−H antibonding orbitals played a significant role. Also in 2011, Singh and co-workers presented the results from a Raman spectroscopic study on pyrimidine’s ν1 mode in pyrimidine/ methanol mixtures.37 These authors observed the same three spectral components with various concentrations of pyrimidine and methanol as previously reported by Schlücker et al.27 for mixtures of pyrimidine and water. In methanol, however, these features were much more distinct and persisted at low concentrations of pyrimidine. One of these three features exhibited no shift from pure pyrimidine and was attributed to free pyrimidine molecules, with no hydrogen bonded interactions. Comparing the ν1 experimental spectra between pyrimidine/water and pyrimidine/methanol revealed the presence of all three distinct spectral components, including that of free pyrimidine, at high concentrations of methanol, while only one hydrogen-bonded peak was present at high concentrations of water. Singh and coworkers suggested that the observation of this one broad feature in pyrimidine/water mixtures at high concentrations of water originates from a large number of hydrogen-bonded

interactions due to water’s ability to form multiple hydrogen bonds. In an attempt to explain the dissimilarities between water and methanol, the authors asserted the following: “PdxWy clusters require less transfer of charge than the PdxMy clusters. This essentially means that the formation of PdxWy clusters is much easier than that of PdxMy clusters.”37 These authors also reported the results of density functional calculations on eight pyrimidine/methanol structures with up to four methanol molecules. The lowest-energy structures were found to have all of the methanol molecules on one side of the pyrimidine ring. In 2012, Singh and co-workers extended their analyses of pyrimidine’s ν1 mode to include interactions involving deuterated methanol and water.38 These results also suggested the presence of free pyrimidine as the origin of one spectral component and a variety of hydrogenbonded complexes composing others. Here, in an effort to quantify the origin of blue shifting in the normal modes of the hydrogen bond acceptor pyrimidine, we report the results of a comprehensive Raman spectroscopic/ electronic structure study. Raman spectra of 10 of pyrimidine’s normal modes (ν1, ν3, ν6a, ν6b, ν8a, ν8b, ν9a, ν15, ν16a, and ν16b in the notation of Lord et al.39) at various mole fractions of methanol and ethylene glycol are presented. These results are compared to previous studies of pyrimidine/water.27,31 The blue shift of ν1 is further examined in binary mixtures of pyrimidine with acetic acid, thioglycol, phenylmethanol, hexylamine, and acetonitrile. Electronic structure computations were performed on over 100 unique microsolvated structures to provide insight to ν1’s blue shifting. NBO calculations quantify charge transfer in order to investigate trends between electron density redistribution and blue shifting in pyrimidine’s normal modes.

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EXPERIMENTAL DETAILS Raman spectra of 10 normal modes of pyrimidine (ν1, ν3, ν6a, ν6b, ν8a, ν8b, ν9a, ν15, ν16a, and ν16b) with varying mole fractions of methanol or ethylene glycol were recorded. The 514.5 nm laser line of a Coherent Innova 200 Ar+ laser was used as the Raman excitation source. The laser light passed through a 514.5 nm laser line filter (Thorlabs) and a half-wave plate (Thorlabs) before reaching the sample. The laser power measured approximately 1 W at the sample. Commercially obtained pyrimidine (Sigma-Aldrich) was used without further purification. Spectra were collected using a fiber optic cable, placed perpendicular to the direction of laser propagation, coupled to a Princeton Instruments SP2500 triple grating (2400 grooves/mm) spectrograph and ProEM 1024 CCD camera with 5 min exposure times. We also compared the effects of a variety of other Table 1. Original Location of Select Vibrational Modes of Pyrimidine and the Maximum Shift Observed in Solvent (in cm−1)

a

C

mode

neat pyrimidine

methanol

ethylene glycol

watera

ν16b ν16a ν6b ν6a ν1 ν9a ν15 ν3 ν8b ν8a

351 401 627 682 990 1139 1160 1228 1570 1564

+2 +3 +6 +2 +12 +3 +5 +1 +8 +2

+4 +9 +3 +14 +3 +7 +1 +9 +2

+2 +2 +13 +5 +14 +5 +9 +3 +12 +6

Reference 31. dx.doi.org/10.1021/jp401642b | J. Phys. Chem. A XXXX, XXX, XXX−XXX


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solvents on pyrimidine’s ring breathing mode, ν1. These solvents include water, acetic acid, thioglycol, phenylmethanol, hexylamine, and acetonitrile. Spectra involving these six solvents were collected using a high-resolution Labview-controlled JobinYvon Ramanor HG2-S double grating (2000 grooves/mm) scanning spectrometer with photomultiplier tube detection using methods previously reported.31

and acetonitrile. Optimized pyrimidine/water structures at the same level of theory were taken from our earlier studies.31,32 NBO calculations were performed on all pyrimidine/solvent clusters containing one pyrimidine molecule in order to quantify the charge transfer between pyrimidine and the solvent molecule(s). To examine the effects of the method and basis set on our results, a subset of 10 pyrimidine/solvent structures containing one pyrimidine molecule was randomly chosen. The optimized structures, vibrational frequencies, and NBOs were recomputed for these structures at the B3LYP/6-31+G(d,2p) and M06-2X/ 6-311++G(2df,2pd) levels of theory. All three model chemistries reveal the same relationship between charge transfer and blue shifting in ν1 for this subset of 10 structures. As such, only results from the B3LYP/6-311++G(2df,2pd) computations are discussed in the following sections. Data obtained from the B3LYP/631+G(d,2p) and M06-2X/6-311++G(2df,2pd) computations can be found in the Supporting Information. While gas-phase electronic structure calculations and condensedphase experimental data cannot always be directly compared, Albert and Quack measured a high-resolution rotationally resolved gas-phase infrared spectrum of pyrimidine,41 and the vibrational frequencies are within a few wavenumbers of those in earlier liquid-phase studies. Also, our previous study investigating the vibrational shifts of pyrimidine/water mixtures when compared to neat pyrimidine31 agrees well with Maes and co-workers’ matrixisolated FTIR study of pyrimidine and pyrimidine/water complexes.42

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COMPUTATIONAL DETAILS DFT calculations were performed with the Gaussian 09 software package.40 Geometry optimizations and harmonic vibrational frequencies were performed on small pyrimidine/solvent clusters at the B3LYP/6-311++G(2df,2pd) level of theory. A pruned numerical integration grid composed of 99 radial shells and 590 angular points per shell was employed, and a threshold of 10−10 for the RMS change in the density matrix during the self-consistent field procedure was employed. Solvent molecules included methanol, ethylene glycol, acetic acid, thioglycol (2-mercaptoethanol), phenylmethanol (benzyl alcohol), hexylamine,

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RESULTS Experimental Results. Figure 2 shows experimental Raman spectra of pure pyrimidine and mixtures of pyrimidine in methanol and ethylene glycol with increasing solvent concentration. Raman spectra of the pure solvents are included to ascertain any overlap with pyrimidine modes. The maximum observed shifts for each mode in methanol and ethylene glycol are recorded in Table 1 and compared to previously published results in water.31 The maximum shift of +12 cm−1 for methanol and +14 cm−1 in water in ν1 agree with early reports by Singh et al.37 and Schlücker et al.,27 respectively. The ν1 mode of pyrimidine in mixtures at high concentrations of methanol, ethylene glycol, and water is compared in Figure 3. At χpy = 0.10, all three

Figure 3. Comparison of the ring breathing mode, ν1, of (a) pure pyrimidine to various mixtures with χpy = 0.10, (b) pyrimidine/methanol, (c) pyrimidine/ethylene glycol, and (d) pyrimidine/water.

Figure 4. Raman spectra of the ring breathing mode, ν1, of pyrimidine in various solvents. Spectra were recorded with a range of pyrimidine/solvent mole fractions, (a) χpy = 1.00, (b) χpy = 0.85, (c) χpy = 0.70, (d) χpy = 0.50, (e) χpy = 0.30, and (f) χpy = 0.10. Asterisks denote peaks from the solvent. D



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Figure 5. Minimum-energy structures of pyrimidine with 1−5 methanol molecules optimized at the B3LYP/6-311++G(2df,2pd) level of theory.

of 15 pyrimidine/methanol, 16 pyrimidine/ethylene glycol, and 20 additional hydrogen bonded systems. Naming of the theoretical structures include f, s, and b in parentheses to denote hydrogen bonding to the front, side, and back of the pyrimidine ring, as defined in Figure 1. Some names of pyrimidine/ethylene glycol and pyrimidine/thioglycol structures also include dimer or trimer to indicate that some O−H or S−H groups are hydrogen bonding with another solvent molecule rather than the pyrimidine ring. Subscripts OH and SH are used in naming pyrimidine/ thioglycol structures to denote whether the alcohol or the thiol group is donating a hydrogen bond to the nitrogen. Pyrimidine/ methanol structures 1 + 0, 2(s) + 0, 1 + 1, 3 + 0, 2(s) + 1, 3 + 1, 2(s) + 2(s), and Py2M have also been reported in an earlier study.37 Shifts in select vibrational modes, relative energies, and the change in overall charge (Δq in millielectrons or me−) on the pyrimidine molecule can be found in Tables S1−S4 in the Supporting Information. Pyrimidine/water structures used for the NBO analyses are taken from our previous studies (Table S4, Supporting Information).31,32 Of the structures that we have

discrete shifts are present in methanol and ethylene glycol, but only one is apparent in water. All three components can be seen in water, however, at intermediate dilutions.27 This situation is reminiscent of the earlier work by Singh et al.37 and suggests that a large amount of free pyrimidine is still present in methanol, while ethylene glycol exhibits features between those of methanol and water. A comparison of the effects on ν1 by the solvents water, acetic acid, thioglycol, phenylmethanol, hexylamine, and acetonitrile is shown in Figure 4. Both acetic acid and phenylmethanol exhibit Raman peaks in the vicinity of ν1, and these are indicated in Figure 4 with asterisks. Blue shifting of ν1 can be seen in water, acetic acid, thioglycol, and phenylmethanol, all solvents capable of donating strong O−H hydrogen bonds. No observable effect on ν1 is produced by hexylamine, capable of donating N−H hydrogen bonds, or acetonitrile, which is not a hydrogen bond donor (apart from very weak C−H hydrogen bonds) but has a large dipole moment. Theoretical Results. Figures 5−7 show the optimized pyrimidine/solvent structures characterized in this work consisting E



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Figure 6. Minimum-energy structures of pyrimidine with 1−4 ethylene glycol molecules optimized at the B3LYP/6-311++G(2df,2pd) level of theory.

identified, the lowest-energy structures for PyMy for y = 1−4 exhibit all of the methanol molecules on one side of the pyrimidine ring, in agreement with Singh et al.37 However, the lowest-energy structure identified for PyM5 has the methanol molecules separated, interacting with both nitrogen molecules in the pyrimidine ring. A similar transition is observed with pyrimidine/ethylene glycol where the lowest-energy structures for PyE1−3 have all of the ethylene glycol molecules on one side of the ring, but PyE4 has ethylene glycol molecules on both sides. This behavior is qualitatively different than that observed with water, where the lowest-energy structures (with up to seven water molecules) consistently have all of the solvent molecules clustered on one side of pyrimidine.32 For both methanol and ethylene glycol, structures with solvent molecules interacting with the side of the pyrimidine ring are lower in energy than structures with solvents interacting with the front, while structures with solvent molecules interacting with the back of the ring are highest in energy (see Figure 1). In addition, hydrogen bonds can form between

ethylene glycol molecules, leading to lower-energy structures (for example, dimer(s) + dimer(s) is lower in energy than 2(s) + 2(s)). Figure 8 shows structures involving two pyrimidine molecules with one or two solvent molecules. The two pyrimidine molecules in each structure are labeled a or b (except for Py2E and Py2W, where the pyrimidines are symmetrically equivalent), and the vibrational shift of the ν1 mode for each pyrimidine is listed in Table 2. For Py2M, pyrimidine a is accepting a hydrogen bond from a methanol molecule and exhibits a blue shift comparable to that observed for methanol’s 1 + 0 structure, while the ring breathing mode in pyrimidine b experiences no significant shifting. When a second methanol molecule is introduced, Py2M2, pyrimidine a and b have similar shifts. For both structures that involve two pyrimidine molecules with either one ethylene glycol or water molecule bridging in the middle, symmetric and asymmetric ring breathing motions result with shifts similar to those observed in the 1 + 0 structures. In these structures, both pyrimidine molecules are symmetrically F



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Figure 7. Minimum-energy structures of pyrimidine with various solvent molecules (acetic acid, acetonitrile, hexylamine, phenylmethanol, and thioglycol) optimized at the B3LYP/6-311++G(2df,2pd) level of theory.

the vibrational blue shifts of ν1, ν6b, and ν8b. The ν1 mode exhibits the strongest quantitative correlation with an R2 value of 0.948, while ν6b has an R2 value of 0.841 and ν8b has an R2 value of 0.799. In each case, the largest calculated shift is approximately 25 cm−1, corresponding to a charge transfer of 80−90 me−. This relationship of increasing blue shift with charge transfer was further tested by interrogating additional solvents including acetic acid, acetonitrile, hexylamine, phenylmethanol, and thioglycol (Figure 10). R2 values actually increase slightly with the additional data for these solvents. The R2 values for ν1, ν6b, and ν8b are 0.952, 0.861, and 0.824, respectively. The two solvents with no experimentally observable effect on ν1, acetonitrile and hexylamine, exhibit a significantly lower degree of charge transfer from pyrimidine than all of the other solvents examined (Table S3, Supporting Information). Interestingly, hexylamine still donates a hydrogen bond to pyrimidine (Figure 7). This interesting result that there is no spectral shift in complexes with hexylamine despite hydrogen bonds to one or both of pyrimidine’s nitrogen atoms strongly suggests that charge transfer is the origin of the blue shifts in hydrogen bond acceptors such as pyrimidine.

equivalent (C2 point group). The addition of one more solvent molecule to the side of pyrimidine b (now forming hydrogen bonds to both nitrogen atoms in the pyrimidine ring and making it no longer symmetrically equivalent to the other pyrimidine molecule) leads to an additional shift of +7 cm−1 in pyrimidine b’s ring breathing mode.

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DISCUSSION Charge Transfer. In order to understand the molecular origin of blue shifting in pyrimidine’s normal modes, we compare the extent of the vibrational shifts of the three modes exhibiting the largest blue shifts (ν1, ν6b, and ν8b) with the overall charge transfer (Δq in millielectrons or me−; see Tables S1−S4, Supporting Information) from the pyrimidine molecule to the solvent molecule(s), including methanol, ethylene glycol, and water (Figure 9). It is important to point out that these three modes all involve significant motion of the hydrogenbonded nitrogen atoms. As can be seen in Figure 9, there is a strong correlation between the magnitude of charge transfer from pyrimidine to the hydrogen-bonded solvent molecules and G



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Figure 8. Minimum-energy structures with two pyrimidine molecules and one or two solvent molecules optimized at the B3LYP/6-311+ +G(2df,2pd) level of theory. Solvents include methanol, ethylene glycol, and water. The pyrimidine molecules are labeled a and b to correspond to Table 2, except for Py2E and Py2W, where the pyrimidines are symmetrically equivalent (C2 point group).

Table 2. Vibrational Shifts (cm−1) of ν1 Compared to the Pyrimidine Monomera

We showed previously in the case of water that most of this charge transfer results from substantial decreases in the populations of the nitrogen lone pair orbitals of pyrimidine.31 Table 3 lists the NBO population changes for one resonance form of pyrimidine that have a magnitude of at least 5 me− for 1 + 1 complexes with water, methanol, ethylene glycol, thioglycol, phenylalcohol, hexylamine, and acetonitrile. (See Table S5 (Supporting Information) for additional orbitals.) The C2 symmetry of the 1 + 1 structures simplify the correlation of orbitals with those from the C2v pyrimidine molecule. Formation of the hydrogen bonds has relatively little effect on the σ and σ* orbitals in all of the complexes. Complexes with water, methanol, ethylene glycol, thioglycol, and phenylalcohol all exhibit substantial loss of electron population from the lone pair orbitals of nitrogen. In contrast are hexylamine and acetonitrile, which exhibit essentially no loss of population from the lone pair orbitals. Interestingly, there is charge rearrangement in all of the complexes with the π and π* populations associated with the CCC and NCN regions of the pyrimidine ring. This is likely due to polarization of the π-electron cloud by the solvent molecules and is not accompanied by shifts in vibrational energies. Changes in the CC and CN bond lengths induced by hydrogen bond formation are provided in Table 4. Complexes with water, methanol, ethylene glycol, thioglycol, and phenylalcohol all exhibit contraction of the C1−N2, C1−N3, C4−C6, and C5−C6 bonds by at least −0.0011 Å as well as elongation of the N2−C4 and N3−C5 bonds by at least +0.0020 Å. In contrast, the complexes with hexylamine and acetonitrile exhibit smaller contractions of the C1−N2, C1−N3, C4−C6, and C5−C6 bonds (−0.0006 to −0.0007 Å) but more significant elongation of the N2−C4 and N3−C5 bonds (up to +0.0038 Å in the case of acetonitrile). A comparison of the magnitude of charge transfer in structures involving hydrogen bonds with one nitrogen atom (2 + 0) with those exhibiting hydrogen bonds with both nitrogen

Δν1 structure

a

b

Py2M Py2M2 Py2E Py2E2 Py2W Py2W2

+8 +8 +7 +7 +6 +7

0 +8 +7 +13, +18 +6 +13

a

a and b refer to different pyrimidine molecules in the pyrimidine/ solvent structure (Figure 8) except for Py2E and Py2W, where a and b refer to the symmetric and asymmetric ring breathing motions, respectively.

atoms (1 + 1) reveals a noticeable cooperativity in this effect (Tables S1−S4, Supporting Information). Interactions with both nitrogen atoms result in a greater degree of charge transfer and a larger blue shift. On the other hand, pyrimidine/ methanol structures 1(s) + 0 and 1(f) + 0 involve weak C− H···O interactions and exhibit a slight charge transfer from the oxygen atom on methanol to pyrimidine. This is accompanied by a negligible shift in the vibrational energy levels of pyrimidine. Charge transfer has been previously linked to bond length changes and blue shifting in the X−H stretching frequency associated with hydrogen bond donors.11,15−20 Here, we see a similar correlation with certain vibrational modes of a hydrogen bond acceptor, pyrimidine, when it accepts hydrogen bonds from a variety of solvent molecules. Solvent Concentration. A gradual separation with increasing concentration of solvent between ν8a, a symmetric C−N stretch, and ν8b, an antisymmetric C−C and C−N stretch, is evident with solvation in the region of 1540−1600 cm−1 (Figure 2). We previously took advantage of this selective blue shifting to definitively assign these two modes.31 ν6b, an H



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Figure 9. Raman shift (cm−1) of ν1, ν6b, and ν8b versus the change in overall charge on a central pyrimidine molecule (Δq in me−) when clustered with water, methanol, and ethylene glycol. Modes that exhibited mixing were excluded.

Figure 10. Raman shift (cm−1) of ν1, ν6b, and ν8b versus the change in overall charge on a central pyrimidine molecule (Δq in me−) when clustered with water, methanol, ethylene glycol, acetic acid, thioglycol, phenylmethanol, hexylamine, and acetonitrile. Modes that exhibited mixing were excluded.

in-plane C−N−C/C−N−C bending mode, also exhibits a gradual blue shift with increasing solvent concentration. In contrast, ν1, the ring breathing mode, exhibits three distinct shifts, which have been previously discussed in both water27 and methanol37 studies. Singh et al. speculated that the difference from pyrimidine/methanol and pyrimidine/water mixtures at high concentrations is due to water’s ability to donate two hydrogen bonds. For structures with methanol or ethylene glycol molecules interacting with both nitrogen atoms in the pyrimidine ring, the magnitude of blue shifting for ν1 is significantly higher than their y + 0 counterparts (y representing the total number of solvent molecules). In fact, the blue shift is approximately 50% larger (Tables S1 and S2, Supporting Information) when both nitrogen atoms accept a hydrogen bond compared to that for just one nitrogen atom. Two of the three distinct spectral components observed in the Raman spectra of ν1 could therefore be due to solvent molecules hydrogen bonding on just one side of the pyrimidine ring and solvent molecules hydrogen bonding on both sides of the ring. This interpretation is supported by previous studies of pyridine.28−30 For example, pyridine has only one nitrogen atom and exhibits two distinct

spectral components for ν1 in binary mixtures with hydrogenbonded solvents. The first component has been attributed to free pyridine, and the second component is attributed to hydrogenbonded complexes. Figure 11 compares experimental Raman spectra of χpy = 0.50 and 0.10 mol fractions with scaled harmonic vibrational frequencies calculated for a number of pyrimidine/methanol and pyrimidine/ethylene glycol structures. All frequencies were scaled by 0.979, which brings the computed ν1 mode of the pyrimidine monomer into agreement with experimentally observed Raman shifts for the neat liquid. Increasing the number of overall solvent molecules in these small molecular clusters tends to increase the magnitude of the blue shift. However, the value of the blue shift is greatly dependent upon the geometry of the structures. In the case of ethylene glycol, for example, the dimer(s) + 0 structure exhibits almost the same blue shift as the 1(s) + 0, whereas the 1(s) + 1(s) yields a much larger shift. With few methanol or ethylene glycol molecules hydrogen bonding to pyrimidine, structures with all solvent molecules on one side of the ring are favored energetically. Structures with I



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Table 3. Population Changes (in me−) of the B3LYP/6-311++G(2df,2pd) NBOs of Pyrimidine upon Complexation with Two Solvent Molecules to Form the 1 + 1 Structure with Water, Methanol, Ethylene Glycol, Thioglycol, Phenylalcohol, Hexylamine, and Acetonitrilea

a

orbital

water

methanol

ethylene glycol

thioglycol

phenylalcohol

hexylamine

acetonitrile

π(C1 N3) π(N2 C4) π(C5 C6) n(N2) n(N3) π*(C1 N3) π*(N2 C4) π*(C5 C6)

+15 +4 −15 −11 −11 +7 +6 −17

+15 +4 −15 −13 −13 +7 +5 −17

+14 +5 −14 −15 −15 +6 +6 −16

+16 +5 −15 −19 −19 +6 +6 −18

+16 +5 −15 −16 −16 +6 +6 −18

+11 0 −9 −2 −2 +7 +1 −11

+14 −1 −12 −1 −1 +10 +3 −14

See Figure 1 for atom numbers.

Table 4. Changes (in Å) of the CC and CN Bond Lengths of Pyrimidine at the B3LYP/6-311++G(2df,2pd) Level of Theory That Occur upon Complexation with Two Solvent Moleculesa solvent b

pyrimidine water methanol ethylene glycol thioglycol phenylalcohol hexylamine acetonitrile

R(C1 N2)

R(C1 N3)

R(N2 C4)

R(N3 C5)

R(C4 C6)

R(C5 C6)

1.3321 −0.0017 −0.0016 −0.0015 −0.0011 −0.0012 −0.0007 −0.0006

1.3321 −0.0017 −0.0016 −0.0015 −0.0011 −0.0012 −0.0007 −0.0006

1.3332 +0.0023 +0.0024 +0.0020 +0.0022 +0.0026 +0.0030 +0.0038

1.3332 +0.0023 +0.0024 +0.0020 +0.0022 +0.0026 +0.0030 +0.0038

1.3874 −0.0011 −0.0011 −0.0011 −0.0011 −0.0011 −0.0003 −0.0005

1.3874 −0.0011 −0.0011 −0.0011 −0.0011 −0.0011 −0.0003 −0.0005

a See Figure 1 for atom numbers. Data given to four decimal places to facilitate comparison. bBond lengths of isolated pyrimidine provided as reference values.

Figure 11. Experimental spectrum at χpy = 0.50 (blue) and 0.10 (red) and scaled (0.979) harmonic vibrational frequencies represented by bars.

to a smaller third spectral component than that of ethylene glycol. Table 2 compares ν1 shifts of Py2My to Py2Ey and Py2Wy. Pyrimidine a for all six structures exhibits a blue shift comparable to that solvent’s corresponding 1 + 0 structure. All six of these pyrimidine molecules have only one nitrogen atom participating in a hydrogen bond. Shifts for pyrimidine b increase from structures with one solvent molecule to structures containing two solvent molecules. A similar trend is present for both ethylene glycol structures and water structures. Molecular clusters containing methanol do not exhibit the same shifts as those observed for ethylene glycol and water. These results are consistent with pyrimidine’s experimental Raman spectrum in methanol solution in which the first spectral component is still intense at high dilutions and a weak third spectral component is also present.

solvent molecules on both sides of the ring, interacting with both nitrogen atoms, become the lowest-energy structures (of those that we have indentified) at higher concentrations of solvent molecules. This transition from interactions with one nitrogen atom to two nitrogen atoms in the same ring agrees very well with experimental results qualitatively but does not explain the greater intensity of the third spectral component in ethylene glycol than that in methanol. This discrepancy may be due to ethylene glycol’s ability to hydrogen bond to more than one pyrimidine molecule at a time, increasing the chance that both nitrogen atoms in a single ring participate in hydrogen bonding. For example, if two 4(s,f) + 0 pyrimidine/ethylene glycol structures are side by side, there are four free O−H groups that are available to hydrogen bond to the free nitrogen on the neighboring pyrimidine. In contrast, methanol molecules do not possess the ability to donate strong hydrogen bonds with two pyrimidine molecules concurrently, leading J



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Article

Notes

CONCLUSIONS Effects of hydrogen bonding on pyrimidine’s vibrational spectrum are studied through the use of Raman spectroscopy and DFT calculations. Over 100 explicit pyrimidine/solvent structures with 8 different solvents, 7 of which can donate hydrogen bonds, are compared to experimental vibrational spectra. Whereas a gradual shift in frequency with increasing solvent concentration is observed with ν6b and ν8b, three discrete spectral components are observed for ν1 in water, methanol, ethylene glycol, and thioglycol. The different spectral components are related to the number (0, 1, or 2) of hydrogen bonds formed between the solvent molecules and the nitrogen atoms of pyrimidine. The lowest-energy spectral feature is likely due to free pyrimidine. Solvent molecules interacting with both nitrogen atoms in a single pyrimidine ring induce a larger blue shift (the highestenergy spectral component) than when only one nitrogen atom in a pyrimidine ring is participating in hydrogen bonding (the middle spectral component). DFT computations reveal that structures with solvent molecules interacting with only one nitrogen atom in the ring are often lower in energy than structures with both nitrogen atoms participating in hydrogen bonding. However, experimental spectra display a more intense third spectral component for water and ethylene glycol, suggesting significant solvent interactions with both nitrogen atoms. Unlike methanol, water and ethylene glycol can donate multiple hydrogen bonds and form extended hydrogen-bonded networks. The low-energy structures with all water or ethylene glycol molecules clustered on one side of pyrimidine’s ring possess free O−H groups capable of forming hydrogen bonds to neighboring pyrimidine molecules, thus increasing the fraction of pyrimidine molecules that are fully solvated in solution. NBO analyses are performed to quantify the degree of charge transfer between pyrimidine and the solvent molecule(s). For the three modes exhibiting the largest blue shifts, ν1, ν6b, and ν8b, a direct correlation is observed between the magnitude of the blue shifts and the overall charge transfer (Δq in me−) from the pyrimidine molecule to the solvent molecule(s). Normal modes exhibiting the largest blue shifts involve stretching motions between the hydrogen-bonded nitrogen atom and the donated hydrogen atom (i.e., stretching of the X−H···N hydrogen bond). Although the hydrogen-bonded complexes exhibit charge redistribution in the π and π* as well as changes in CC and CN bond lengths, substantial decreases in the populations of the nitrogen lone pair orbitals of pyrimidine account for the net charge transfer and are the origin of the vibrational blue shifts. Previous studies have linked charge transfer to red and blue shifts of the X−H stretching mode of the hydrogen bond donor. Here, we show a strong correlation between charge transfer and blue shifts in a hydrogen bond acceptor.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported, in part, by the National Science Foundation (EPS-0903787, CHE-0955550, and CHE-0957317). Some of the computations were performed using resources at the Mississippi Center for Supercomputing Research.

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ASSOCIATED CONTENT

S Supporting Information *

Cartesian coordinates of the optimized clusters, shifts in select vibrational modes, relative energies, change in the overall charge (Δq in millielectrons or me−) on the pyrimidine molecule, and results obtained with the smaller 6-31+G(d, 2p) basis set and M06-2X method. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail: [email protected] (G.S.T.); nhammer@olemiss. edu (N.I.H.). K



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Charge Transfer and Blue Shifting of Vibrational Frequencies in a

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