Solvation Effect on the NH Stretching Vibrations of Solvated

Article pubs.acs.org/JPCA

Solvation Effect on the NH Stretching Vibrations of Solvated Aminopyrazine, 2‑Aminopyridine, and 3‑Aminopyridine Clusters Yuji Yamada, Hiroumi Ohba, Yusuke Noboru, Shigeyuki Daicho, and Yoshinori Nibu* Department of Chemistry, Faculty of Science, Fukuoka University, Jonan-ku, Fukuoka 814-0180, Japan S Supporting Information *

ABSTRACT: The vibrational spectra of the hydrated and methanol-solvated aminopyrazine, 2-aminopyridine and 3-aminopyridine in supersonic jets have been measured in terms of IR-UV double-resonance spectroscopy. Comparing the IR spectrum of aminopyrazine with those of 2-aminopyridine and 3-aminopyridine clusters, we determine the solvation structure of aminopyrazine to be a similar cyclic structure as hydrated 2-aminopyridine clusters [Wu, et al., Phys. Chem. Chem. Phys. 2004, 6, 515]. In the case of monohydrated aminopyrazine cluster, one of the normal modes composed of the hydrogen-bonded OH and NH stretching local modes gives the anomalously weak IR intensity, which is ascribed to the cancellation of the dipole moment change between the OH and NH stretching local modes. The solvated 3-aminopyridine clusters forms the hydrogen-bond between the pyridyl nitrogen atom and the OH group, but the amino group is indirectly affected to induce slight blue shift of the NH2 stretches. This phenomenon is explained by inductive effect where the electron withdrawing from the amino group upon the solvation results in a “quinoid-like” structure of the amino group.

1. INTRODUCTION An IR measurement in the region of the OH and NH stretching vibrations is known as a powerful tool that provides a good deal of spectroscopic information on the microscopic circumstance around the XH group (X = O, N), so that it has been extensively used for elucidating solvation structures as well as intramolecular conformation determined by intra- and intermolecular interactions, such as a hydrogen bond (Hbond).1 Especially, the combination of this method and the supersonic jet method has brought about remarkable progress in research concerning solvation effects and vibrational dynamics under the manipulation of solvent number and configuration.2−7 Recently, biologically relevant molecules, such as amino acids, polypeptides, nucleic acids, and so on, in supersonic jets have been investigated in detail regarding nonrigid multiconformation and solvation structures.8−17 However, because of their structural complexity leading to a complicated IR spectrum, it is more difficult to analyze them the larger the molecular size. In addition, strongly H-bonded XH bonds exhibit various unexpected phenomena in the IR spectra, which are originated from Fermi resonance, band broadening, and so on. Thus, it is significant to investigate the origin of the observed bands in the IR spectrum. A large number of spectroscopic studies on pyridine derivatives possessing an amino group have been investigated for a long time.18−23 Among them, 2-aminopyridine (2AP) is especially known as a model molecule of a nucleobase, namely, cytosine, and its dimer has been researched from the viewpoint of a nucleobase pair.24−29 The vibrational spectra of 2AP and its solvated clusters with various solvents, such as water and ammonia, have been investigated by Brutschy’s group.29−31 © 2012 American Chemical Society

Furthermore, Lim et al. reported the comparison of the electronic transitions, ionization energies, and vibrational frequencies with those of 3-aminopyridine (3AP), where the position of the amino group to the nitrogen atom in pyridyl ring is altered.22,23 Then, in this article, we focus on aminopyrazine (APz), which has two H-bond acceptor sites in the pyrazine ring, namely the nitrogen atoms at the 2position and the 5-one with respect to the amino group, corresponding to the H-bond sites of 2AP and 3AP, respectively. It is predicted that two H-bonded clusters can exist, and that the amino group offers the former an advantage because of its cyclic structure. A main issue in this article concerns the solvation structure of solvated APz clusters with water (W) and methanol (M). We have measured the vibrational spectra in the OH/NH stretching vibrational region by applying fluorescence-detected IR (FDIR) spectroscopy to APz, 2AP, and 3AP clusters in supersonic jets. From the results obtained with density functional theory (DFT) calculations as well as the experimental ones, the determination of cluster structures and vibrational mode assignment are performed; especially, the cluster structure of APz is determined by comparing it with those of 2AP and 3AP. On the basis of these assignments, we discuss various solvation effects on the vibrational modes of the amino group that is far from the Hbonded site, not only the directly bounded amino group. Received: July 4, 2012 Revised: August 28, 2012 Published: August 31, 2012 9271

dx.doi.org/10.1021/jp306609f | J. Phys. Chem. A 2012, 116, 9271−9278

The Journal of Physical Chemistry A

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2. EXPERIMENT A detailed description of the experimental setup was given in previous papers.32,33 Briefly, the jet-cooled APz, 2AP, and 3AP clusters with water or methanol were generated in a supersonic expansion of their vapor seeded in He carrier gas at typically 3 atm into a vacuum through a pulsed nozzle (General valve, series 9). The samples were heated up to about 320 K to obtain a sufficient vapor pressure. A tunable UV laser radiation was obtained with a frequency-doubled dye laser (Sirah, CSTR-G), which was pumped by a frequency-doubled Nd:YAG laser (Spectra Physics, INDI-40). UV laser radiation was introduced into the vacuum chamber and focused at 10 mm downstream from the nozzle exit. Laser-induced fluorescence (LIF) spectra were detected with a photomultiplier tube (R-928, Hamamatsu Photonics). A tunable IR laser pulse was generated with a differential harmonic generation (DFG) in a LiNbO3 crystal (Inrad, Autotracker III) with the fundamental light of a Nd:YAG laser (Spectra Physics GCR-130) and the output of a dye laser (LAS, LDL-205) with LDS 759 dye. The tunable IR radiation was introduced from the counter propagating direction against the UV laser. IR spectra were recorded as a decrease of LIF intensity due to the vibrational transition induced by the tunable IR laser. The samples of 2AP (Tokyo Chemical Industry, 98%), 3AP (Sigma-Aldrich, 98%), and APz (Sigma-Aldrich, 98%) were used without purification. The solvent sample of CD3OH (99.8 atm% D) was purchased from Sigma-Aldrich. The geometry optimization and vibrational frequency calculations of the clusters were preformed at the B3LYP level with the 6-311++G** basis set using the Gaussian 03 package.34 Scaling factors of 0.9599, 0.9608, and 0.9576 were adopted in the vibrational frequency calculations for the monomer and the solvated clusters of APz, 2AP, and 3AP, respectively. These values enable the scaled harmonic vibrational frequencies to reproduce the observed NH2 stretching frequencies of each monomer well. In the binding energy calculations, we also corrected the basis set superposition error (BSSE) by a counter-poise method and carried out the zeropoint vibrational energies correction (ZPC). The computation was carried out using the computer facilities at the Research Institute for Information Technology, Kyushu University.

Figure 1. LIF spectra of (a) APz-W1, (b) APz-M1, (c) 2AP-W1, (d) 2AP-M1, (e) 3AP-W1, and (f) 3AP-M1. The values in parentheses indicate the frequency shift from the origin band of each monomer. Some bands originated from hydrated clusters are also observed in the spectra of the methanol-solvated ones. Asterisks in panels d and e indicate vibronic bands of 2AP-M2 and 3AP monomer, respectively.

band of 2AP-M2, which is demonstrated by the FDIR spectrum (not shown). The origin bands of both of the solvated 2AP clusters are considerably shifted from that of the monomer (33466 cm−1).29 As seen in Figure 1e,f, on the other hand, the origin bands of 3AP-W1 and -M1, which are observed at 32884 and 32856 cm−1, respectively, exhibit a rather small red shift from that of the 3AP monomer (33049 cm−1).23 The small red shift of 3AP relative to the other two chromopheres seems doubtful. We, however, observe no band derived from 1:1 clusters, even if scanning the laser frequency further to the lower region. Moreover, by means of UV−UV and IR−UV hole-burning spectroscopy (not shown), we confirm that all the bands observed in each figure belong to the monosolvated clusters and conclude that there is only a single isomer for each solvated cluster. 3.2. FDIR Spectra and Assignment of Solvation Structures. Figure 2 shows the FDIR spectra of APz, APzW1, and APz-M1, which are obtained by scanning IR frequency with fixing the UV frequency to each origin band in the LIF spectra. As for monomer one, two bands of NH2 stretching vibrations, which are classified into symmetric (NHs) and asymmetric (NHa) modes, are observed at 3442 and 3548 cm−1, respectively. These frequencies are close to those of 2AP

3. RESULTS 3.1. LIF Spectrum. Figure 1 shows the LIF spectra obtained under the condition of the sample vapor mixed with water or methanol. Those with methanol contain some bands originated from hydrated clusters due to the water vapor left in the sample holder or gas line. As seen in Figure 1a, a strong band appears at 30463 cm−1, which is shifted by −803 cm−1 relative to the APz monomer origin band at 31266 cm−1. The LIF spectrum is similar to the resonance-enhanced multiphoton ionization (REMPI) spectrum of APz-W1 reported by Tembreull, et al.,35 although the intensity ratio of vibronic bands to the origin one is somewhat different, which may be ascribed to the difference of quantum yield observation technique. By addition of methanol into the sample vapor, some bands newly appear in Figure 1b, and the strongest band at 30325 cm−1 is assigned to the origin band of APz-M1. Figure 1c,d displays the LIF spectra of 2AP-W1 and -M1, respectively. The obtained frequency of the origin band of 2AP-W1 (32547 cm−1) is identical with the value reported by Brutschy’s group.30 The additional band marked by the asterisk in Figure 1d is assigned to a vibronic 9272



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the nitrogen atom at the 1-position in pyrazine ring (Wa/Ma) or at the 4-position (Wb/Mb) since other isomers have considerably higher energies. For comparison, the calculated frequencies and binding energies of two isomers are summarized in Table 1. In order to determine which geometry is observed in our experiment, we focus on the presence or absence of the free NH2 stretching vibrational bands. Thus, the fact that we observe no band around 3442 cm−1, corresponding to NHs of the monomer in both the FDIR spectra, supports the assignment of Wa/Ma. Additionally, if these clusters had Wb/Mb structures, NHa would be somewhat blue-shifted as simulated by the DFT calculations, which give rise to the shift from 3551 to 3556/ 3557 cm−1. In fact, as will be mentioned below, 3AP-W1 and -M1 where the H-bond is formed in the same manner as Wb/ Mb exhibit slightly blue-shifted NHa bands. However, the experimental results for APz show no blue shift of the NHa band upon cluster formation. Therefore, it is concluded that the observed structures are Wa and Ma. Furthermore, these assignments are supported by the larger binding energies of Wa and Ma, as seen in Table 1. Here, it should be noted that the calculated intensity at 3343 cm−1 in Figure 2b is much weaker than that of free OH or NH stretch, although it corresponds to the stretching vibrational band involved in the H-bond. As stated in the Introduction, the IR spectra of 2AP monomer and 2AP-W1 have been reported by Wu, et al.,29,30 so we mainly refer to 2AP-M1 in this section. First, for comparison of the above-mentioned IR spectra of APz-W1 with that of 2APW1, we display the FDIR spectrum of 2AP-W1 in Figure 3a, which is inherently identical with the previous one, although some additional weak bands are observed at 3346 cm−1, 3560 cm−1, and so on. Its solvation structure has been discussed in detail by the previous study, and is determined to be the cyclic structure depicted on the right side of the figure.30 Figure 3b shows the FDIR spectrum of 2AP-M1, where main three bands are observed: one is 3542 cm−1 corresponding to the free NH stretching mode, the others are H-bonded NH and OH stretching vibrational bands at 3371 and 3305 cm−1. In this case too, several additional bands appear around 3400 cm−1, and a very weak band is at 3582 cm−1. These additional bands in the solvated clusters will be assigned in the Discussion section. The lower part of Figure 3b indicates the calculated IR spectrum for the cyclic structure of 2AP-M1, which is similar to 2AP-W1, and is coincident with the main three bands in the experimental one. This coincidence as well as the calculated result that other cluster structures are much less stable (not shown) supports the cyclic structure of 2AP-M1.

Figure 2. FDIR spectra of (a) APz monomer, (b) APz-W1, and (c) APz-M1, obtained by fixing the probe frequencies to 31266, 30463, and 30325 cm−1, respectively. The lower inserts are the calculated spectra of two typical isomers whose geometries are depicted on the right side.

mentioned below, but not to aniline (3423 and 3509 cm−1).36 As seen in Figure 2b, the monohydration gives only three sharp bands at 3409, 3548, and 3720 cm−1; furthermore, a very broad band with small IR intensity is also observed in the region of 3300 to 3450 cm−1. This broad band cannot be removed by the suppression of an influence from higher clusters, so it is considered an intrinsic band to APz-W1, which might be due to the anharmonic coupling with a large number of dark states. The newly appearing band at 3720 cm−1 is assigned to the free OH stretching vibration of water, which donates the other hydrogen atom to a H-bond acceptor site. On the other hand, the 3548 cm−1 band does not change by monohydration. Similarly, as seen in Figure 2c, APz-M1 provides two distinguishable bands and a shoulder band. The sharp band at 3541 cm−1 exhibits a small red shift from those of the monomer and its hydrated cluster (3548 cm−1). The lower parts of Figure 2b,c represent the simulated IR spectra for the optimized geometries displayed in the right side. Here, we take account of two geometries where water/methanol is bound to

Table 1. Calculated Frequenciesb in cm−1 and Their Mode Assignment of Bare and Solvated APz Clusters at the B3LYP/6-311+ +G** Levela isomer

APz

APz-W1 Wa

APz-M1 Wb

Ma

Mb

assignment

exp.

calc.

exp.

calc.

calc.

exp.

calc.

calc.

NHsym/H‑bond NHasym/free OHH‑bond OHfree Ebindd

3442 3548

3440 3551

c

3343 3544 3393 3732 5.99 (5.09)

3444 3556 3438 3731 4.28 (4.10)

3383 3541 3393

3328 3541 3409

3444 3557 3454

6.61 (6.12)

4.62 (4.81)

3548 3409 3720

a

The binding energies (Ebind) of two isomers in kcal/mol are also listed. bScaling factors are 0.9599. cThe band cannot be detected due to the very weak intensity. dThe values in parentheses indicate the calculated results at the MP2/6-311++G** level. 9273



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3518 and 3517 cm−1 in 3AP-W1 and -M1, respectively. This phenomenon is very intriguing because the amino group is not directly involved in the H-bond formation, which will be explained in the Discussion section. Another interesting issue is that there are several weak bands except for the assigned fundamental bands, for example, 3584 and 3578 cm−1 in Figure 4b,c, respectively. As mentioned above, these bands also appear in the IR spectra of the 2AP clusters. Therefore, it is considered that this phenomenon is relatively likely to be observed in the solvated clusters of the pyridine derivatives.

4. DISCUSSION From our FDIR results, we have obtained three intriguing issues. One is that the IR intensity of a H-bonded stretching vibrational mode in APz-W1 is anomalously weak. The second is concerning the slightly blue-shifted NH2 bands upon the Hbond formation of 3AP. The third is that there are several additional bands in the IR spectra for the solvated clusters of 2AP and 3AP, which cannot be assigned to fundamental bands. In the following section, we will discuss them with the calculated results. 4.1. Anomalously Weak Intensity of H-Bonded XH Band. The weak IR band intensity at 3343 cm−1 in the calculated spectrum of APz-W1 seems anomalous because the H-bonded XH stretching vibrational band is well-known to exhibit the considerably enhanced intensity, relative to the nonH-bonded one. Figure 5a illustrates two motional features of the H-bonded OH and NH stretching vibrational modes for APz-W1, which are calculated to be the vibrational frequencies of 3343 and 3393 cm−1 with the IR intensities of 14.8 and 869.5 km·mole−1, respectively. One notes that the local stretching modes of the H-bonded OH and NH bonds are mixed, and that the lower-frequency normal mode describes the cancellation of the dipole moment change between the OH and NH stretching local modes. Thus, in-phase or out-of phase between both the local modes results in a considerably enhanced or weakened IR intensity. Sinha et al. have also reported that a similar intensity pattern is observed in the mixed H-bonded OH/NH stretching vibrations of monohydrated 2-aminopurine, where a “bridging” H2O forms the H-bond network between an amino group and nitrogen atom of the H-bond acceptor.37 On the other hand, in the case of 2AP-W1 having a similar cluster structure to APzW1, both the experimental and calculated IR spectra give sufficiently strong intensities of the H-bonded OH/NH vibrations. That is because the mixing between the H-bonded OH and NH local modes is so small (Figure 5b) that the lowerfrequency vibrational mode is roughly classified into the Hbonded OH one. Here, the difference in IR spectra between hydrated APz and 2AP may be ascribed to the somewhat weaker H-bond of the OH group with the nitrogen atom in APz. For example, the calculated H-bond length of OH-N for APz-W1 is 1.9537 Å, which is longer than that for 2AP, 1.9081 Å. The more strongly H-bonded OH band in 2AP-W1 provides the larger red shift than APz, leading to the larger zero-order energy gap between the H-bonded OH and NH local modes which gives rise to the relatively small extent of the mode mixing. Therefore, we suggest that the slight difference of Hbond strength is responsible for the accidental mixing between the H-bonded OH and NH stretching local modes for APz-W1. 4.2. Slight Blue Shift of NH2 Stretch Bands Induced by Solvation to 3AP. The NH2 stretching vibrations of 3AP are slightly blue-shifted by the H-bond formation, although the Hbond is formed far from the amino group. In order to

Figure 3. FDIR spectra of (a) 2AP-W1 and (b) 2AP-M1, obtained by fixing the probe frequencies to 32547 and 32426 cm−1, respectively. The lower inserts are the calculated spectra for the optimized geometries shown in the right side.

Figure 4 shows the FDIR spectra of 3AP and its solvated clusters. The lower part of each figure indicates the simulated

Figure 4. FDIR spectra of (a) 3AP monomer, (b) 3AP-W1, and (c) 3AP-M1, obtained by fixing the probe frequencies to 33049, 32884, and 32856 cm−1, respectively. The lower inserts are the calculated spectra for the optimized geometries shown in the right side.

IR spectrum for the most stable geometry, where the solvents are bound to the nitrogen atom in the pyridine ring, as seen on the right side of the figure. The experimental and calculated results are summarized in Table 2. Since other isomers of the solvated clusters are less stable by about 2 kcal/mol than the most stable one (Figure S1, Supporting Information), we take account of the most stable one only. According to this assignment, it is found that the solvation of water and methanol to 3AP gives rise to the strong H-bonded OH bands at 3436 and 3415 cm−1, respectively. Furthermore, the free NH2 stretching vibrational bands are slightly blue-shifted; for instance, the NHa of the monomer, 3510 cm−1, is shifted to 9274



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Table 2. Calculated Frequenciesb in cm−1 and Their Mode Assignment of Bare and Solvated 2AP and 3AP Clusters at the B3LYP/6-311++G** Levela 2APc

2AP-W1d

assignment

exp.

calc.

exp.

NHsym/H‑bond NHasym/free OHH‑bond OHfree additional band

3441 3548

3439 3549

3403 3547 3315 3717 3560 3346

3373 3545 3312 3734

exp.

calc.

3371 3542 3305

3365 3543 3324

3582 3396

Ebinde

6.42 (5.41) 3AP

7.00 (6.54)

3AP-W1

assignment

exp.

calc.

exp.

NHsym/H‑bond NHasym/free OHH‑bond OHfree additional band

3421 3510

3418 3512

3429 3518 3436 3719 3584

3AP-M1 calc. 3422 3517 3407 3527

exp. 3428 3517 3415

4.89 (4.64)

a

calc. 3422 3517 3394

3578 3437

Ebinde c

2AP-M1 calc.

5.39 (5.46)

b

The binding energies (Ebind) of the clusters in kcal/mol are also listed. Scaling factors are 0.9608 and 0.9576 for 2AP and 3AP, respectively. Yamada, et al., ref 27. dWu, et al., ref 30. eThe values in parentheses indicate the calculated results at MP2/6-311++G** level.

investigate this origin, we evaluate the structural change from monomer. The calculated structural parameters of the 3AP moiety in monomer and the H-bonded clusters are listed in Table 3. Despite a very small change in the amino group, these parameters reveal that it forms a “quinoid-like” structure upon the H-bond formation. For example, the N2−C2 bond length (RN2C2) in the DFT calculation result is shortened from 1.3932 to 1.3902 Å by monohydration. Similarly, the calculated results of the larger H1−N2−C2 and H2−N2−C2 angle (denoted as AH1N2C2 and AH2N2C2, respectively) as well as the smaller dihedral angle of DH1N2C2C1 roughly implies the CNδ+H2 formation. As known broadly, comparing the XH bonds with the hybrid orbital of spx and spx′ (x < x′) in the X atom, the former is stronger, resulting in the higher frequency of the XH stretching mode. That is because spx constructs more stabilized bonding orbital by repelling with 1s orbital in the hydrogen atom. Of course, the structural changes in this case are much smaller than those upon the ionization of aniline,38 2AP22 and 3AP,23 where the planarization of the amino group is induced. Thus, as the simplest explanation for blue shift, we propose that the solvation to 3AP induces the slight decrease of the electron

Figure 5. The motional features of the H-bonded OH and NH stretching vibrational modes for (a) APz-W1 and (b) 2AP-W1.

Table 3. Some Structural Parameters Involved in the Amino Group of 3AP and Its Solvated Clusters Calculated at the B3LYP and MP2 Levels with the 6-311++G** Basis Seta B3LYP/6-311++G**

a

MP2/6-311++G**

3AP

3AP-W1

3AP-M1

3AP

3AP-W1

3AP-M1

RH1N2 RH2N2 RN1C2 RC1C2 RN1C1 AH1N2C2 AH2N2C3

1.0094 1.0091 1.3932 1.4060 1.3300 115.93 116.19

1.0092 1.0089 1.3902 1.4050 1.3315 116.29 116.50

1.0092 1.0089 1.3901 1.4050 1.3316 116.32 116.52

1.0119 1.0116 1.4000 1.4075 1.3400 113.99 114.09

1.0118 1.0114 1.3964 1.4068 1.3412 114.27 114.42

1.0117 1.0114 1.3962 1.4068 1.3411 114.33 116.44

DH1N2C2C1 DH2N2C2C3

21.91 −25.55

21.37 −24.73

21.48 −24.51

26.29 −30.52

25.29 −30.26

25.63 −29.92

The atomic notation is shown in the inset picture of Figure 4. 9275



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3AP moiety generate little change. On the other hand, it is found that the separations slightly decrease. Here, we consider that the decrease is attributed to the change in intermolecular vibrational frequencies. As listed in Table 4, the calculated frequencies of σ and τ modes reproduce these separations and their decrease by deuteration well. However, the OH bending mode, which is considered to be a major candidate for Fermi resonance, shows a large shift of about 23 cm−1, so that the overtone or combination band including it cannot reproduce the experimental result. Furthermore, it is easily considered that the Fermi resonance interactions between the OH stretch mode in the methanol moiety and intramolecular modes in the 3AP one are negligible. Actually, Ebata’s group found that the initially excited OH stretching vibrational energy in H-bonded phenol clusters is primarily redistributed mostly into the phenolic moiety and suggested that the anharmonic coupling between the OH stretch and intramolecular modes is specially important.46,47 Eventually, although their frequencies are predicted to remain unchanged upon deuteration, they are eliminated from the candidates for Fermi resonance. Thus, we suggest that these additional bands are assigned to the combination bands composed of the OH stretching mode and intermolecular vibrations, and that they are ascribed to the strong anharmonic coupling with the intermolecular vibrational modes.

density on the amino group (the calculated atomic charge by means of NBO analysis is shown in Figure 2S, Supporting Information), leading to a “quinoid-like” structure, although there may be other complicated possibilities that explain it well. Furthermore, it is reasonable to consider that the electron withdrawing caused by the H-bond is associated with a charge transmission by an electronegative substituent, i.e. inductive effect. Though the amplitude of this effect is much smaller than the substitution of the atom jointed to pyridine and aniline ring, which has been investigated by Kydd, et al.39,40 and by Cazzoli et al.,41 respectively, the frequency shifts of the NH2 stretching vibrations may be reliable as a sensitive indicator to detect the inductive effect in other systems also. 4.3. Unexpected Additional Bands in the IR Spectrum. Another intriguing issue is that the additional bands except for the fundamental bands are observed in the FDIR spectra for 2AP and 3AP clusters. Especially, 3AP-M1 provides two recognizable bands at 3578 and 3437 cm−1. As well-known, these weak IR bands except for the fundamental bands are frequently explained by the following two interactions. One is Fermi resonance between the OH or NH stretching vibration and the overtone/combination levels of the intramolecular modes, and the other is the anharmonic coupling to the lowfrequency intermolecular vibrations, leading to a “Franck− Condon-like” vibrational progression, which means the appearance of some weak combination bands of the XH stretching vibration and the intermolecular ones.42,43 The former between the fundamental OH stretch band and overtone of OH bending mode is very famous in the case of the extremely strong H-bond formation, for example, carboxylic acid dimers44 and solvated phenol cations.45 In order to elucidate which interaction is responsible for the results, we investigate the deuteration influence on the IR spectrum of 3AP-M1. Figure 6 shows the IR spectra of 3AP-M1 and −M(d3)1 where the term “M(d3)” means a deuterated methanol, CD3OH. The band frequencies and the separation values from the methanol OH stretching band to two additional bands are inset in the figure, although these values include experimental errors of ±0.5 cm−1. Because only the methyl group of methanol is deuterated, the vibrational modes for the

5. CONCLUSIONS The vibrational spectra of the hydrated and methanol-solvated 2AP, 3AP, and APz in supersonic jets have been measured in terms of FDIR spectroscopy. By the aid of DFT calculations, the determination of their solvation structures and the assignment of the vibrational bands are performed. Although the APz clusters have been mainly predicted to form two kinds of H-bonded structures where a solvent is bound to the nitrogen atom at the 1-position or at the 4-position in the pyrazine ring, by comparing the IR spectrum with those of 2AP and 3AP clusters, we conclude that the experimentally observed one is the former, which is a cyclic structure similar to the hydrated 2AP cluster. Furthermore, in the case of the monohydrated APz cluster, the lower-frequency normal modes composed of the H-bonded OH and NH stretching local modes gives the anomalously weak IR intensity, which is ascribed to the cancellation of the dipole moment change between the OH and NH stretching local modes. The solvated 3AP clusters form the H-bond between the pyridyl nitrogen atom and the OH group, but the amino group is somewhat affected by this indirect H-bond, inducing the slight blue shift of the NH2 stretches. As one of the explanations for this phenomenon, we proposed an inductive effect where the electron withdrawing from the amino group by the H-bond results in the “quinoid-like” structure of the amino group. Furthermore, comparing the change in the IR spectrum of methanol-solvated 3AP upon deuteration of the methanol solvent, it is concluded that several additional bands except for the fundamental bands observed in the IR spectra are assigned to the combination bands of the H-bonded OH stretching mode and the intermolecular ones. We suggest that this phenomenon is relatively likely to be observed in the solvated clusters of the pyridine derivatives.

Figure 6. FDIR spectra of (a) 3AP-M1 and (b) 3AP-M(d3)1 where M(d3) means CD3OH. 9276



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Table 4. Calculated Frequencies in cm−1 and Approximate Description of Intermolecular Vibrations and the OH Bending Mode (δOH) of 3AP-CH3OH (3AP-M1) and 3AP-CD3OH (3AP-M(d3)1) Clusters at the B3LYP/6-311++G** Level 3AP-M1

a

descriptiona frequencyb

τ 22.6

ρ1 23.9

β1 36.5

descriptiona frequencyb

τ 20.1

ρ1 22.6

β1 34.7

β2 96.3 3AP-M(d3)1 β2 78.7

ρ2 105.0

σ 156.5

δOH 1385.2

ρ2 91.8

σ 151.5

δOH 1361.8

See ref 5 for the nomenclature of intermolecular vibrations. bScaling factor of 0.9576.

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ASSOCIATED CONTENT

S Supporting Information *

Simulated IR spectra of three most stabilized isomers for 3aminopyridine clusters as Figure S1. Estimated atomic charge and calculated structural parameters of 3-aminopyridine monomer and its cluster are shown in Figure S2. This information is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Grant-in-aids for Young Scientist (A) (Grant No. 22750020) by from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT).

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REFERENCES

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Solvation Effect on the NH Stretching Vibrations of Solvated

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