Drastic Change in Electronic Transition upon Hydrogen Bond Network

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Drastic Change in Electronic Transition upon Hydrogen Bond Network Switching in 3-Aminopyridine-(HO) Clusters 2

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Yuji Yamada, Yuji Goto, Seiichi Higuchi, and Yoshinori Nibu J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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The Journal of Physical Chemistry

Drastic Change in Electronic Transition upon Hydrogen Bond Network Switching in 3-Aminopyridine−(H2O)n Clusters Yuji Yamada, Yuji Goto, Seiichi Higuchi, and Yoshinori Nibu* Department of Chemistry, Faculty of Science, Fukuoka University, Jonan-ku, Fukuoka 814-0180, Japan ABSTRACT The hydration structures of 3-aminopyridine(3AP)−(H2O)n (n = 2−4) in supersonic jets have been investigated by measuring the electronic and vibrational spectra with the aid of quantum chemical calculations. The S1−S0 electronic transition of 3AP−(H2O)2 is observed at slightly redshifted position from 3AP−(H2O)1, while further hydration induces drastic red shifts and complicated vibrational structures. We assign the cluster structures of 3AP−(H2O)2 as a cyclic structure composed of the homodromic hydrogen bond (H-bond) chain connecting the pyridyl CH bond acting as proton donor toward a pyridyl nitrogen acceptor. For 3AP−(H2O)n (n = 3, 4), on the other hand, the initial donor site in the H-bond network changes from a pyridyl CH to an amino group. The observed red shift derived from H-bond network switching can be reproduced very well with the S1−S0 origin band estimation obtained by applying geometry optimization and subsequent harmonic vibrational analysis of (TD-)DFT calculations to each electronic state of the isomer structure. It is suggested that the drastic red shift of the electronic transition upon H-bond network switching is due to much more stabilized “quinoid-like” structure in the * state by the H-bond formation of an amino group.

*Corresponding author. Fax: +81-92-865-6030 E-mail: [email protected]

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1. INTRODUCTION Hydrogen bond (H-bond) is one of the most important non-covalent interaction since it widely exists in biological system and controls three-dimensional structures of proteins, nucleobase pairs and so on, which affect their stability and biological functions.1

Gas-phase spectroscopic

studies on a pyridine and its substituted derivatives H-bonded by various protic solvents are beneficial to investigate microscopic properties of H-bond between biological relevant molecules and solvents as the simplest model system.

As for the solvation or substituent effects on excited state dynamics

as well as structural stability, the pyridine derivatives attract our interest because of notable issues about the coupling or mixing between closely-lying n* and * excited states2−4 and the geometrical change upon the electronic excitation.5 1*

Recently, Jouvet’s group demonstrated that the pyridine

excitation of Py–(H2O)n clusters produces the pyridinyl radical derived from the relatively fast

hydrogen transfer from water.6

Thus, it is possible that solvated pyridine derivatives also occur

similar reactions or relaxation controlled by particular substituent groups.

Besides, Schultz, et al.

suggested that 2-aminopyridine (2AP), thought to be a simple model molecule of Watson-Crick pair, namely adenine,7-10 showed the remarkably shortened lifetime of S1 state (*) upon dimerization and proposed that a similar relaxation mechanism might accelerates internal conversion in the excited state of DNA base pairs.11 In contrast to 2AP for which a lot of studies on H-bonded clusters with various molecules, such as water, ammonia, etc., and its dimer, have been reported by many researchers,7-15 there were fewer spectroscopic investigations about vicinally substituted 3-aminopyridine (3AP) clusters. Many molecular properties of 3AP may be different from 2AP because the relative position between the lone-pair electrons on the nitrogen atom in the pyridyl ring and the amino group influences the electronic resonance structure.16

Lim, et al. reported the comparison of the electronic transitions,

ionization energies, and vibrational frequencies of bare 3AP with those of 2AP.17

Recently,

applying fluorescence-detected IR (FDIR) spectroscopy with the aim of quantum chemical calculations, we reported the H-bonded structures of mono-hydrated and methanol-solvated 2AP, 3AP, and aminopyrazine (APz) which has two H-bond acceptor sites in pyrazine ring, namely the nitrogen atoms at the 2-position and the 5-one with respect to the amino group, corresponding to the H-bond site of 2AP and 3AP, respectively.18

As mentioned by Brutschy’s group, the mono-hydrated

2AP cluster forms a cyclic H-bond network from the amino group to nitrogen atom in a pyridyl ring,13 while the amino group in 3AP one is barely involved in H-bond between water and pyridyl nitrogen atom, resulting in the relatively weak H-bond due to the lack of the cooperative effect.19,20

In

addition, a hydration to 3AP exhibits only 165 cm−1 red shift of S1−S0 electronic transition, which is much smaller than those of 2AP and APz, 919 and 803 cm−1, respectively. Thus, it is predicted that, in larger size clusters also, the difference of substitution site provides the different microscopic Page 2

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solvation mechanism and has the accompanying influence on electronically excited states. A main issue in this article is the H-bond network structure of 3AP−(H2O)n (n = 2−4) clusters, henceforth denoted by W2−W4. We have measured the cluster size-specified electronic and vibrational spectra in the OH/NH stretching vibrational region by applying UV−UV hole-burning (HB) and FDIR spectroscopies to hydrated 3AP clusters in supersonic jets.

From the density

functional theory (DFT) calculation results as well as the experimental ones, the cluster structure determination and vibrational mode assignment were performed. We found that the drastic change in the H-bond network from W2 to W3 and W4 gave rise to the considerably red-shifted S1−S0 transitions. The time-dependent (TD) DFT calculation reproduced the experimentally observed appreciable change in the electronic transitions very well and led us to a deep insight into the electronic structure change upon the H-bond network switching. 2. EXPERIMENT A detailed description of the experimental setup was given in the previous papers.21,22 Briefly, the jet-cooled 3AP−(H2O)n clusters were generated in a supersonic expansion of the sample vapor mixed with water seeded in He carrier gas at typically 3 atm into a vacuum chamber through a pulsed nozzle with a orifice diameter of 0.8 mm (General valve, series 9). The sample of 3AP (SigmaAldrich, 98%) was heated up to about 320 K to obtain a sufficient vapor pressure. In order to suppress the water vapor pressure in carrier gas line, a water bottle was cooled to 288 K corresponding to the vapor pressure of 17 hPa. 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 the position of 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 ( = ~3 m) 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. For FDIR spectroscopy, the tunable IR pulse focused by a CaF2 lens with a focal length of 250 mm was introduced 50 ns prior to the UV laser by the counterpropagating direction method. For UV–UV HB measurement, a hole-burning UV laser obtained with the frequency-doubled output of the same dye laser used for IR one was introduced 500 ns prior to the probe UV laser. The measured UV–UV HB spectrum was strongly saturated because it was recorded at high fluence of the pump laser (~0.8 mJ/pulse), while the LIF spectrum was measured at the mild condition of the low UV laser intensity (~0.1 mJ/pulse) focused by an 800 mm lens in order to avoid saturation effect over all observed region.

FDIR or UV–UV HB spectrum was recorded as

intensity depletion of the probed band due to the vibrational or electronic transition induced by Page 3



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tunable IR or UV lasers. The geometry optimization without symmetry restriction and the subsequent vibrational frequency analysis were carried out using the molecular orbital calculation with second order Møller– Plesset perturbation (MP2) theory23 as well as the DFT method with the B3LYP,24 CAM-B3LYP,25 M06-2X,26 and B97x-D27 functionals and a Pople-type basis set with added polarization and diffuse functions, 6-311++G(d,p). Scaling factors of 0.9576 and 0.9728 were applied to free OH/NH stretching vibrational frequencies and to H-bonded ones, respectively, obtained by the harmonic frequency analysis at B3LYP/6-311++G(d,p) level. The former value enabled the scaled frequencies to reproduce the observed NH2 stretching ones of monomer, while the latter was selected to reproduce very well the H-bonded stretching bands in mono-hydrated clusters of the pyridine derivatives such as 3AP,18 2AP,13 and 2-fluoropyridine.28

In the estimation of the relative stabilization energy, we

also corrected the basis set superposition error (BSSE) by a counterpoise method and carried out the zero-point vibrational energies correction (ZPC). All the calculations were performed using the GAUSSIAN 09 package29 in the computer facilities at the Research Institute for Information Technology, Kyushu University. 3. RESULTS 3.1. Electronic Spectrum Figure 1 shows the LIF spectra obtained in the condition of the sample vapor mixed with water. The intense peak at 33049 cm–1 is the origin band of 3AP monomer, as reported in the previous paper.17

In the lower frequency region, two peaks having almost the same intensity are observed,

and a large number of peaks appear in 31200−32100 cm–1. From our previous paper and experimental results of FDIR spectra which will be mentioned in the next section, the intense peaks at 32884 and 32651 cm–1 are assigned as mono-hydrated and di-hydrated 3AP clusters, respectively, denoted by W1 and W2.

In order to analyze the complicated structure of vibronic bands observed in the lower

frequency region, UV–UV HB spectra were measured by scanning the pump UV frequency with the probe UV frequencies fixed to the relatively intense peaks at 31542 and 31240 cm–1, which are shown in Figs. 1b and c, respectively.

The result that all the peaks in LIF are in agreement with the

depletion positions in either of two UV–UV HB spectra suggests that there are two species possessing the origin bands of 31542 and 31240 cm–1, labelled as W3 and W4, respectively. Both spectra shows the following two common features; one is that the relatively intense vibronic band is observed at about 520 cm–1 higher frequency than each origin band, and this band can be assigned as 6a10 band which was observed at 493 cm–1 with sufficiently large Franck–Condon activity in the case of monomer.30

The other is the complicated vibronic structure in the lower frequency region below

300 cm–1, which is considerably different from W1 and W2 showing intense origin bands and no Page 4


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vibronic bands in this region. This different vibronic structural feature suggests that the geometry change of W3 and W4 upon the S1–S0 excitation is much larger than that of W1 and W2, and predicts that a particular site in 3AP chromophore having influence on * transition is involved in the Hbond network of W3 and W4. In order to ascertain that origin bands labeled as W3 and W4 are derived not from two isomers solvated with the same number of water molecules but from different size clusters, we investigated spectral change with increasing water vapor pressure. Raising the temperature of a water bottle increases the peak height ratio of W4 to W3 from 0.77 at 288 K to 1.38 at 293 K (Figure S1, supporting information). From the point of view that the higher vapor pressure tends to generate the larger size clusters, the experimental result provides the positive proof that the cluster size of W4 is larger than W3. 3.2. Observed and Calculated IR Spectra Figure 2 shows the FDIR spectra of hydrated 3AP clusters obtained by scanning IR frequency with fixing the UV frequency to each origin band in the LIF spectra. In addition, the peak positions observed in those spectra are listed in Table 1. For comparison, those of monomer and W1 are also listed, which was reported in the previous paper.18

As for monomer, two NH2 stretching

vibrational bands at 3421 and 3510 cm–1 are classified into symmetric (NHsym) and asymmetric (NHasym) modes, respectively. As seen in Fig. 2a, di-hydration hardly perturbs these two sharp bands, meaning that amino group is free from H-bond network, similarly to W1. However, the fact that the lower-frequency band vanishes with further hydration gives us a clear idea that one NH bond in the amino group forms the NH…O H-bond in W3 and W4. Moreover, it should be noted that the intense and broad bands at the lowest frequency position, derived from the most strongly H-bonded OH stretching vibrations of water moiety, are red-shifted dependently on the cluster size. In order to assign the cluster structures, the experimental results are compared with the calculated ones lying in the lower part of the FDIR spectra. Figure 3 shows the optimized geometries of the three most stable isomers calculated using MP2/6-311++G(d,p) level. They shows a similar motif for hydration despite the different calculation levels such as B97x-D and B3LYP functionals which contain dispersion term or not. The stabilization energies of isomers relative to the global minima (WnA; n = 2−4) obtained at various calculation levels are also listed in Table 2. All the calculation level provides the same structures as the global minima and shows similar tendency of stabilization energy difference except for M06-2X one. The vibrational spectra of these isomers simulated at B3LYP level, which has been broadly utilized to reproduce the vibrational spectrum in H-bonded cluster system including water molecules for a long time, are compared to the experimentally observed ones, as seen as stick diagrams displayed in the lower of Figs. 2a−c (those Page 5



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with other calculation levels were given in Fig. S2, Supporting information). We will discuss the cluster assignment with the aim of the calculated results in the following section. Table 1. Scaled Harmonic Vibrational Frequenciesa in cm-1 and Their Mode Assignment of Bare and Solvated 3AP Clusters Calculated at the B3LYP/6-311++G(d,p) Level. Obs. WnA WnB WnC Assignment b Monomer 3510 NHasym 3421 NHsym b n=1 3719 3719 OHfree 3518 3517 NHasym 3429 3422 NHsym 3436 3435 OH…N n=2

n=3

n=4

3720 3714 3520 3430 3471 3186

3724 3717 3521 3424 3456 3235

3723 3717 3514 3420 3471 3240

3732 3718 3442

3714 3496

3722 3720 3719 3496

3731 3722 3715 3429

3432 3368 3352 3106

3428 3364 3342 3159

3423 3316 3375 3270

3723 3722 3714 3512 3419 3455

3721 3708

3727 3726 3722 3714 3500 3405 3362 3331 3288 3058

3729 3719 3712

3523 3435 3352

3349 3137 3731 3722 3704

OHfree OHfree NHasym NHsym OH…O OH…N NH…O OHfree OHfree OHfree NHasym / NHfree NHsym OH…O NH…O OH…O OH…N

OHfree OHfree OHfree OHfree 3503 3496 3495 NHfree 3421 3507 3459 OH…O 3360 3337 3379 NH…O 3343 3290 3287 OH…O 3304 OH…O 2984 3079 3063 OH…N 3681 3666 OH(ADD)asym 3601 3588 OH(ADD)sym a Scaling factor of 0.9576 is applied to OH free, NHsym, and NHasym/NHfree, while the other H-bonded OH and NH stretching vibrations are scaled by a factor of 0.9728. bRef.

18.

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Table 2. Relative Stabilization Energies (in kcal/mol) for the Three Most Stable Isomers of 3AP−(H2O)n (n = 2−4) Calculated Using MP2, B3LYP, B97x-D, and M06-2X Calculation Level with the Basis Sets of 6-311++G(d,p). n=2 W2A W2B W2C W3A MP2 0.00. +0.93 +1.67 0.00. B3LYP 0.00 +0.95 +2.39 0.00 +1.00 +1.69 0.00 B97x-D 0.00 M06-2X 0.00 +1.33 +0.93 0.00

n=3 W3B +2.89 +3.43 +3.14 +3.57

W3C +4.00 +3.93 +4.56 +5.76

W4A 0.00. 0.00 0.00 0.00

n=4 W4B +1.04 +2.44 +1.09 +0.06

W4C +2.32 +3.93 +2.65 +1.50

4. Discussion 4.1. Assignment of Solvation Structures As seen in Fig. 3a, W2A and W2B have a similar H-bond motif where the H-bond network chain is bridged from CH bond to pyridyl nitrogen atom. However, the difference between the protondonor CH bond at the 2-position and at the 6-one gives rise to the distinguishable difference in stabilization energies (about 1 kcal/mol), which may be due to intramolecular interaction from neighboring amino group. The other candidate, W2C, which has H-bond chain among amino group and two water molecules is easily removed because of much less stable energy, and the result that the amino group is a weaker H-bond acceptor than pyridyl nitrogen atom predicts that the larger size clusters with this motif are also much unstable.

In addition to the relative energy of more stable

W2A, the free NH stretching band may give us a supporting key to determine the structural assignment. As seen in Table 1, the observed NHasym and NHsym of W2, i.e. 3520 and 3430 cm–1 respectively, are slightly blue-shifted from those of W1, 3518 and 3429 cm−1. The calculation result that the NH2 bands in W2A obtained to be 3521 and 3424 cm−1 are blue-shifted from W1 ones (3517 and 3422 cm−1) reproduces the experimental blue shift very well, while those in W2B (3514 and 3420 cm−1) are opposite. Thus, the reproduced tendency of blue/red shift supports that the cluster structure of W2 is determined as W2A. Here, the curious issue is that the difference in CH bond position forming CH...O interaction gives rise to frequency shifts of free NH stretching modes to the opposite side. In the previous paper, we found a similar blue shift in the case of mono-hydration to 3AP, and suggested that the H-bond formation between water and pyridyl nitrogen in 3AP induces the electron-withdrawing from amino group by inductive effect, resulting in “quinoid-like” structure.18

The calculation result at MP2/6-

311++G(d,p) level shows that the C−N bond length of 1.3942 Å between amino group and pyridine ring in W2A is shorter than those of W1 and W2B, 1.3963 and 1.3982 Å, respectively, and suggests that the “quinoid-like” structure is more induced in W2A. Since the main difference between W2A and W2B is the CH bond position forming CH...O interaction, it is considered that the stronger Page 7



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intramolecular interaction between amino group and adjacent CH bond is responsible for the slight blue shift of NH2 stretches. As seen in Fig. 3b, the calculated geometries of tri-hydrated 3AP clusters show that the most stable isomer has a linear H-bond network motif where three water molecules form the single chain bridged from amino group to pyridyl nitrogen atom, and has substantially large stabilization energy relative to others, in which W3B and W3C are analogous to W2C and W2B, respectively. It should be noted that the initial structure which has a single H-bond chain linking between the CH bond at the 2-position and pyridyl nitrogen like W2A is relaxed to W3A within the geometry optimization steps. Comparing the simulated W3A spectrum with the experimental one, we can obtain three common points; one is that the free NH stretching band at 3496 cm−1 is in good agreement with the observed one at 3497 cm−1. The second is that the superposed free OH bands around 3700 cm−1 are reproduced as a single band which cannot be resolved in our IR laser resolution. The third that there is no sharp NHsym band at 3430 cm−1 which was observed in W1 and W2 suggests the H-bond formation of one NH bond as a hydrogen donor, so that the isomer candidates of W3B and W3C should be removed. Therefore, based on stabilization energy and comparison of the vibrational spectra, it is concluded that the cluster structure of 3AP−(H2O)3 is assigned to be W3A. Figure 3c shows that, in three stable isomers of W4 commonly, an amino group acts as a proton donor and a pyridyl nitrogen dose as an acceptor. They has a subtle difference in the H-bond network comprised of four waters; W4A forms a homodromic linear chain similarly to W3A, while the others have a branched H-bond chain including (H2O)3 triangle moiety, which was reported to be the most stable structure in pure (H2O)3 cluster by a large number of groups.31−34

In both W4B and

W4C, a water molecule acts as single-acceptor and double-donor (ADD) and another water as doubleacceptor and single-donor (AAD). From the point of view that the larger H-bond number tends to provide more stability, the latter seems more stable at first glance. However the former constructs stronger H-bond network by about 1 kcal/mol. We consider that this contradiction is ascribed to the linear H-bond chain with smaller distortion. Thus, it can be easily predicted that further hydration such as 3AP−(H2O)n≥5 may form a branched H-bond network to be global minimum, as reported in phenol−(H2O)n≥4,35,36

2-naphthol−(H2O)5,37

benzene−(H2O)n≥6,38 and

2-pyridone−(H2O)n≥3.39

Interestingly enough, 2-pyridone−(H2O)3 exhibits two isomers; one isomer forms a three-membered water wire similar to W3A and W4A, and the other shows the branched H-bond network like W4B and W4C.39

The removal of W4B and W4C candidates can be supported by comparing the

simulated IR spectra shown in Fig. 2c. The triangle (H2O)3 moieties provide several intense bands derived from ADD in the region of about 3600 cm−1, which has been called as “window region”.40,41 However, such bands are not experimentally observed in this region.

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4.2. Fermi Resonance of the Most Red-Shifted OH Stretching Mode Based on the calculated stabilization energies as well as the vibrational spectra, we could assign the cluster structures as W2A, W3A, and W4A, namely, cyclic structures formed by homodromic linear H-bond chain where a pyridyl ring CH bond or amino group acts as a proton donor and pyridyl nitrogen dose as an acceptor. Such singly H-bonded chain tends to provide the remarkably red-shifted OH stretching mode and consequently to give rise to the Fermi resonance with OH bending overtone, leading to very complicated vibrational spectral feature in the lower frequency region of 2800−3200 cm−1. This phenomenon frequently interrupts unambiguous spectral assignment given by the harmonic vibrational analysis of quantum chemical calculation. As seen in the lower frequency region of Figs. 2a−c, in fact, the most red-shifted bands of W2A−W4A calculated to be 3235, 3159, and 3058 cm−1 disagree with the experimentally observed lowest frequency ones of 3186, 3106, and 2984 cm−1 respectively, although the tendency of lower frequency shift upon solvation seems similar. In addition, there exists more than two relatively intense bands around 3200 cm−1, which cannot be assigned by the simulated results. Vibrational analysis shows that the most red-shifted mode is the H-bonded OH stretching vibration of the water solvated to pyridyl nitrogen directly. Thus, we assign these intense bands as OH...N H-bonded mode and consider other additional bands to be derived from Fermi resonance with OH bending overtone from the spectral perspective of the band shift tendency and peak intensity. One notes that the calculated frequencies of OH2 and NH2 bending overtones for W2A−W4A are in coincidence with these additional band region of 3200−3300 cm-1 (Table S1, supporting information). Another important point is that the calculated position of the OH…N H-bonded mode is located between the observed lowest-frequency band and additional bands. Thus, it is reasonable to conclude that these OH stretching levels are red-shifted by the repulsion from higher frequency-lying overtone due to Fermi resonance, resulting in lowerfrequency shift of OH...N mode than that of the scaled harmonic frequency. 4.3. Electronic transition shift induced by H-bond network switching As stated in the Sec. 3.1, the electronic transition frequency strongly depends on the number of water in hydrated 3AP clusters. Based on the assigned cluster structures, TD-DFT calculations of 3AP−Wn (n = 1−4), which are known to be a good tool for isomer assignments as reported by Thut, et al.,42 were carried out using B3LYP and CAM-B3LYP functionals with the basis set of 6311++G(d,p). The latter is famous to reproduce electronic transitions of various aromatic molecules very well.43,44

The S1−S0 origin bands were estimated from the energy difference in both electronic

states including ZPE obtained by geometry optimization and subsequent harmonic vibrational analysis in each electronic state. As seen in Table 3, the calculated transition frequency and the red shift from W1 are compared with the experimentally observed values. Page 9



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The red shifts calculated at both levels show a similar trend that mono-hydration to W1 or W3 gives rise to about 300 cm−1 red shift, while the shift from W2 to W3 is considerably large, more than 1000 cm−1, although the absolute values are different from the observed ones. Here, it should be noted that the excited state calculation for 3AP monomer is difficult because of mixing between closely-spaced n* and * states, resulting in an excessively stable S1 state with the pyridine ring deformation (Fig. S3, supporting information).

In the case of hydrated clusters, however, the higher

energy shift of n* state ascribed to H-bond formation of n-orbital in pyridyl nitrogen atom causes pure * state to be the S1 state, leading to simpler and more accurate calculation. The * state of aromatic molecules including an amino group such as aniline is well known to form “quinoid-like” structure, and the acidity of an amino group increases relatively to that in S0.45,46

The fact implies

that the larger stabilization upon the NH...O H-bond formation in S1 than S0 provides the considerably red-shifted S1−S0 transition. In the case of WnA (n = 2−4) also, the calculated S1−S0 transitions include charge transfer component between the pyridyl ring and the amino group (Figure S4, supporting information).

Furthermore, the NH...O H-bond strengthened by the electronic excitation

in W3 and W4 provides the significant change in intermolecular configuration, resulting in long and extensively rich intermolecular vibronic structure in the LIF spectrum as seen in the lower frequency region of Fig. 1(a). Thus, the calculated result of the large shift accompanied by the H-bond network change from W2 to W3 is consistent with the experimental one and supports our assignment of cluster structure based on IR spectral analysis. Table 3. Transition Frequencies of S1−S0 Origin Bands Including Zero-Point Vibrational Energy in cm−1 and the Red Shifts Relative to 3AP−(H2O)1 Calculated with B3LYP and CAM-B3LYP/6311++G(d,p). B3LYP Origin 33178 32975 31400 31026

W1 W2 W3 W4

Shift 0 −203 −1778 −2152

CAM-B3LYP Origin Shift 35128 0 34846 −282 33438 −1690 33075 −2053

Obs. Origin 32884 32651 31542 31240

Shift 0 −233 −1342 −1644

5. CONCLUSIONS The vibrational spectra of the hydrated 3AP in supersonic jets have been measured in terms of FDIR spectroscopy. With the aid of quantum chemical calculations, the determination of their solvation structures is performed. We assign the cluster structures of 3AP−(H2O)2 as a cyclic structure formed by linear H-bond chain where a pyridyl ring CH bond act as a proton donor and a pyridyl nitrogen atom as an acceptor. For 3AP−(H2O)n (n = 3, 4), on the other hand, the initial donor site in Page 10


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the H-bond network switches from a pyridyl CH to an amino group.

The H-bond network switching

provides the considerably red-shifted S1−S0 electronic transition, which can be also reproduced well by the S1−S0 origin band estimation obtained by applying geometry optimization and subsequent harmonic vibrational analysis to each electronic state of the isomer structure predicted based on IR spectral analysis. It is suggested that the drastic shift of S1−S0 electronic transition upon H-bond network switching is due to “quinoid-like” structure in the * state more stabilized by H-bond formation of the amino group. Furthermore, the vibrational analysis clearly indicates that the most red-shifted mode is ascribed to the OH...N H-bonded OH stretching vibration of the water solvated directly to pyridyl nitrogen and that additional bands in the region of 3200−3300 cm-1 which harmonic vibrational analysis cannot assign to fundamental modes are due to Fermi resonance with OH bending overtones of water from the point of view of the band shift tendency and peak intensity. ACKNOWLEDGMENT 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). Supporting Information Available Temperature dependence of LIF spectra as Figure S1. Vibrational spectral dependence on calculation levels for the most stable isomers is shown in Figure S2. Optimized geometry in S1 state and Kohn−Sham orbitals related to S1−S0 transition for 3-aminopyridine monomer are shown in Figure S3. Kohn−Sham orbitals of HOMO and LUMO related to S1−S0 transitions for WnA (n = 2−4) isomers and their orbital energies are exhibited in Figure S4.

Calculated frequencies for NH2/OH2

bending overtones of 3-aminopyridine-(H2O)n (n = 2−4) as Table S1. This information is available free of charge via the Internet at http://pubs.acs.org. Reference (1) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; Freeman: San Francisco, 1960. (2) Stephenson, H. P. Solution Spectra and Oscillator Strengths of Electronic Transitions of Pyridine and Some Monosubstituted Derivatives. J. Chem. Phys. 1954, 22, 1077-1082. (3) Hollas, J. M.; Kirby, G. H.; Wright, R. A. Electronic Assignment as 1A’ (*)-1A’ of the 2980 Å System of 2Aminopyridine by Rotational Band Contour Analysis. Mol. Phys.1970, 18, 327-335. (4) Hager, J.; Wallace, S. C. Solvation Effects in Jet-Cooled 2-Aminopyridine Clusters: Excited-State Dynamics and Two-Color Threshold Photoionization Spectroscopy. J. Phys. Chem. 1985, 89, 3833–3841. (5) Kydd, R. A. The Amino Inversion Vibration in Aminopyridines. Spectrochim. Acta A 1979, 35, 409-413.

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Figure 1. (a) LIF spectrum of 3AP−(H2O)n (n = 0−4) and UV−UV hole burning spectra measured by probing (b) W3 and (c) W4 bands in the LIF spectrum.

Figure 2. FDIR spectra observed by probing the origin bands of (a) W2, (b) W3, and (c) W4 in LIF spectrum. The lower inserts are the simulated vibrational spectra of the most stable three isomers obtained with B3LYP/6-311++G(d,p) level. Bands with double slash and surrounded by broken lines in the figure mean the band intensity scaled by 1/10 times in order to make it easy to compare the calculated spectra with the observed one. Page 15



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Figure 3. Optimized geometries of the most stable three isomers for (a) 3AP−(H2O)2, (b) 3AP−(H2O)3, (c) 3AP−(H2O)3 clusters obtained using MP2 method with basis sets of 6-311++G(d,p).

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TOC graphic, Y. Yamada, et al.

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Drastic Change in Electronic Transition upon Hydrogen Bond Network

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