J. Phys. Chem. A 2010, 114, 8323–8330
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Infrared-Optical Double Resonance Spectroscopic Measurements on 2-(2′-Pyridyl)benzimidazole and its Hydrogen Bonded Complexes with Water and Methanol Mridula Guin, Surajit Maity, and G. Naresh Patwari* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076 INDIA ReceiVed: May 30, 2010; ReVised Manuscript ReceiVed: July 5, 2010
The H2O and MeOH complexes of 2-(2′-pyridyl)benzimidazole (2PBI) were investigated using laser induced fluorescence and IR-UV double resonance spectroscopic techniques. The 000 band for the S1 r S0 electronic transition of 2PBI was observed at 31 616 cm-1, and corresponding transitions for the H2O and MeOH complexes show substantial red shifts of 1059 and 1203 cm-1, respectively. A long progression up to V′ ) 5 in the 92 cm-1 vibrational mode was observed for bare 2PBI, which is considerably shortened to V′ ) 2 and V′ ) 1 for the H2O and MeOH complexes, respectively. The combined experimental and theoretical results suggest that both H2O and MeOH form cyclic complex with 2PBI incorporating N-H · · · O and O-H · · · N hydrogen bonds. Comparison with other known hydrogen-bonded H2O complexes of bifunctional aza-heteroaromatic molecules suggest that 2PBI-H2O is probably one of the strongest of all known complexes. Further, the experimental data in combination with calculations suggests that the hydrogen atom/proton transfer leading to tautomeric form might be thermodynamically spontaneous in the electronic excited state, due to explicit solvent (H2O and MeOH) participation. Introduction Proton and hydrogen atom transfer reactions are well-known phenomenon involved in simple acid-base chemistry to very convoluted pathways of proton pumps across biological membranes.1,2 Bifunctional molecules with hydrogen bond donor and acceptor groups can undergo intramolecular proton/ hydrogen atom transfer reactions. In some cases the explicit solvent participation is essential for the proton/hydrogen atom transfer process to take place in a facile manner. The binding of solvent molecules decreases the potential energy barrier for proton/hydrogen atom transfer in some molecular systems. Such a phenomenon is called “solvent-assisted proton/hydrogen atom transfer” and is considered to be very important in the enzymecatalyzed reactions.3 Bifunctional molecules with chromophoric units can undergo photoinduced proton/hydrogen atom transfer processes in the excited state as a result of change in acidity and/or basicity experienced by the constituent groups upon electronic excitation, which can be attributed to the changes in the charge redistribution and the consequent structural changes. The excited state intramolecular proton/hydrogen atom transfer processes takes place through an intramolecular hydrogen bond, which usually is a part of five- or six-membered ring, yielding a phototautomer. There are numerous examples of excited state proton/hydrogen atom transfer reactions in the condensed phase.4 Excited state proton/hydrogen atom transfer has been observed for bare 7-(2-pyridyl) indole in the gas-phase.5 Further, excited state proton/hydrogen atom transfer assisted by explicit participation of solvent molecules with hydrogen bond accepting and donating abilities, such as water and alcohols, have been observed in the gas phase. The examples include hydrogenbonded complexes of 7-hydroxyquinoline,6,7 7-azaindole,8 and 1H-pyrrolo[3,2-h]quinolone.9 2-(2′-Pyridyl)benzimidazole (2PBI) is an interesting molecule that undergoes excited state proton transfer process in aqueous * To whom correspondence should be addressed. E-mail: naresh@ chem.iitb.ac.in.
Figure 1. Possible strucutres of 2PBI.
solution under neutral pH conditions.10 On the other hand, it has been shown that the excited state proton transfer process in 2PBI is markedly enhanced under acidic conditions.11 2PBI can adopt three conformations that are shown in Figure 1. Among these, conformers 2PBI-R1 and 2PBI-R2 are rotamers. Rotamer 2PBI-R1 is more stable than 2PBI-R2, whereas the conformer 2PBI-T is the tautomeric form and is the most unstable among these three conformers, vide infra. The absorption spectra in a wide variety of solvents reveals the complete absence of tautomeric form 2PBI-T in the ground state.11 In early experiments photophysical properties of pyridylbenzimidazoles in various organic solvents have been investigated.12 In the case of 2PBI it was observed that alcoholic solvents considerably quenched the fluorescence in comparison with other solvents and also in comparison with other pyridyl isomers in alcohols. This anomalous behavior of 2PBI molecule in alcohol has been ascribed to hydrogen atom transfer from the imidazole ring to the pyridyl nitrogen atom, facilitated by alcohols by forming a bridge-type hydrogen-bonded complex.12 The acid-base character of various isomers of PBI molecules in aqueous solutions have been investigated by Prieto et al., which has led to the identification of four different protonated/ deprotonated species of 2PBI, depending on the acidity of the medium.11 Further, Datta and co-workers observed that the excited state proton transfer process can be tweaked using various microenvironments.13 It is interesting to note that the two fragments of 2PBI, namely, pyridine and benzimidazole, have nitrogen atoms of very similar basicity. The pKa values of the pyridinium ion and the N3 protonated benzimidazole cation
10.1021/jp104952v 2010 American Chemical Society Published on Web 07/26/2010
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are 5.25 and 5.532, respectively.14 This implies that both the sites can compete for the formation of hydrogen bonding in 2PBI. However, the intramolecular excited state proton/hydrogen atom transfer, either direct or solvent mediated can only occur when the proton is transferred from the N1-H of imidazole ring to the pyridine nitrogen. All the investigations on 2PBI reported in the literature have been carried out in the solution phase using steady state and time-resolved fluorescence measurements. In order to understand the intrinsic ability of 2PBI to undergo intramolecular proton/hydrogen atom transfer we have investigated 2PBI and its water and methanol complexes in the gas phase under jet-cooled conditions. Experimental and Computational Methods The details of the experimental setup have been described elsewhere.15 Briefly, helium buffer gas at 4 atm was passed over the 2PBI (Aldrich), which was heated to 393 K in a sample holder in order to obtain sufficient vapor pressure. Solvents, H2O and MeOH were kept in a solvent holder and the buffer gas was bubbled over the solvents prior to passing it over heated 2PBI. The gas mixtures were expanded into a vacuum chamber through a 0.5 mm diameter pulsed nozzle (Series 9, Iota One; General Valve Corporation). The electronic excitation of 2PBI and its complexes was achieved using a frequency-doubled output of a tunable dye laser (Narrow Scan GR; Radiant Dyes) pumped with second harmonic of a Nd:YAG laser (Surelite I-10; Continuum) operating with the DCM dye. The fluorescence excitation spectra were recorded by monitoring the total fluorescence with a photomultiplier tube (9780SB+1252-5F; Electron Tubes Limited) and a filter (WG-345) combination, while scanning UV laser frequency. The IR spectrum of the complex was obtained using fluorescence dip infrared (FDIR) spectroscopic method.16 In this method, the population of a target species is monitored by the fluorescence intensity following its electronic excitation to 000 band of S1 r S0 transition, with an UV laser pulse. A tunable IR laser pulse is introduced 100 ns prior to the UV laser pulse. When the IR frequency is resonant with the vibrational transition of the target species, the ground state population decreases, resulting in the depletion of the fluorescence signal. Further, in order to separate out the transitions belonging to various species present in the fluorescence excitation spectrum, IR-UV hole-burning spectroscopy was also carried out. In this technique, a fluorescence excitation spectrum is recorded for the region of interest. Following, an IR pulse is tuned to a vibrational transition of a specific species of interest, while a delayed tunable UV laser probes the S1 r S0 transition region. In the event of the UV laser being resonant with the transition of the same species to which the IR pulse is tuned to, the fluorescence intensity decreases in comparison with the first spectrum. The lowering of the intensity in the second spectrum relative to the first one allows identification of relevant transitions. In our experiments the source of tunable IR light is an idler component of a LiNbO3 OPO (Custom IR OPO; Laserspec), pumped with an injection-seeded Nd:YAG laser (Brilliant-B; Quantel). The calibration of the LiNbO3 OPO was carried out using a wavemeter (Laserspec) and also by recording the photoacoustic spectrum of ambient water vapor. The typical bandwidth of both UV laser and LiNbO3 OPO is about 1 cm-1, and the absolute frequency calibration of all the lasers is within (2 cm-1. To supplement the experimental observations we have carried out density functional theory based calculations using Gaussian 03 package.17 The equilibrium structures of the monomers and various binary complexes were calculated at DFT-B3LYP
Guin et al. methods using split valence 6-311+G(d) basis set. The nature of the stationary points obtained was verified by calculating the vibrational frequencies at the same level of theory. The stabilization energies were corrected for the zero-point vibrational energy (ZPE). The calculated N-H stretching frequency of most stable rotamer of 2PBI was 3647 cm-1, whereas the corresponding experimental value is 3489 cm-1. The scaling factor of 0.9566 was devised by taking the ratio of experimental frequency (3489 cm-1) to the calculated frequency (3648 cm-1). The same scaling factor was used for both N-H and O-H stretching frequencies for all the complexes reported here. The geometry optimization and vibrational frequency calculations in the excited state were carried out at RCIS/6-31G level and a factor of 0.9 was used to scale the vibrational frequencies. The scaling factor is intended to correct for the basis set truncation, partial neglect of the electron correlation and harmonic approximation. The IR spectra in the O-H and N-H stretching region were simulated by convoluting a Lorentzian function of width (fwhm) 10 cm-1 to the calculated stick spectrum and compared with the observed experimental spectrum for structural assignment.18 Further, to evaluate the direction and magnitude of the donor-acceptor interactions, the natural bond orbital (NBO) analysis for all of the complexes has been performed using NBO 5.0 program.19,20 The theory of atoms in molecules (AIM) was used to investigate the electronic densities and intermolecular hydrogen bonding interactions of all the binary complexes of 2PBI with H2O and MeOH.21 The topological properties of the electron densities for the monomer and the complexes at the bond critical points were calculated using the AIM2000 program.22 The wave functions computed at B3LYP/ 6-311+G* level of theory were used to calculate the electron density F(r) and Laplacian of F(r) at the bond critical points. Results and Discussion A. Spectra. The fluorescence excitation spectrum of 2PBI is depicted in Figure 2A. To the best of our knowledge, this is the first report of LIF excitation spectrum of 2PBI in the gasphase. The intense transition at 31 616 cm-1 was assigned to the 000 band of the S1 r S0 transition. The 000 transition is accompanied by a strong progression up to V′ ) 5 in the 92 cm-1 vibrational mode. The fluorescence excitation spectrum has almost no Franck-Condon activity in any other vibrational mode even up to 600 cm-1 energy above the 000 band. In the presence of Ar buffer gas the fluorescence excitation spectrum (Figure 2B) shows new set of transitions that are shifted to the red by 43 cm-1, of which the lowest energy transition at 31 573 cm-1 was assigned to the 000 band of the 2PBI-Ar complex. Even in the case of Ar complex a strong progression up to V′ ) 4 can be seen in the 92 cm-1 vibrational mode. The binding of Ar atom to 2PBI has almost no influence on the 92 cm-1 vibrational progression, both in terms of frequency and intensity distribution. It can be seen from Figure 2 that the V′ ) 1 band has marginally higher intensity than V′ ) 0, which is accompanied by a long progression up to V′ ) 5 indicates that the potential energy surface of S1 state is considerably shifted by along the 92 cm-1 vibrational mode relative to S0 state. The fluorescence excitation spectrum of 2PBI in the presence of H2O, depicted in Figure 2C, shows an intense transition at 30 557 cm-1 accompanied by a progression up to V′ ) 2 with vibrational energy spacing of 95 cm-1. Further 155 cm-1 vibrational mode also shows a progression up to V′ ) 2, once again with the vibrational energy spacing of 95 cm-1. On the other hand the fluorescence excitation spectrum in the presence of MeOH (Figure 2D), shows few new transitions appear in
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Figure 3. The FDIR spectra of (A) 2PBI and (B) 2PBI-Ar. The two spectra were recorded by monitoring the total fluorescence following the respective 000 bands for the S1 r S0 electronic transition at 31 616 and 31 573 cm-1, respectively.
Figure 2. Fluorescence excitation spectrum of (A) 2PBI in He buffer gas, (B) 2PBI in Ar buffer gas, (C) 2PBI in the presence of H2O, and (D) 2PBI in the presence of MeOH. For the spectra C and D, He buffer gas was used. In D the transitions on the higher energy side are due to the 2PBI-H2O complex, which arise due to contamination of water in the gas lines. The vertical lines denote the progression in the in-plane rocking mode.
the lower energy region out of which the transition at 30 413 cm-1 can be assigned to the 000 band of 2PBI-MeOH complex. A vibronic band at 100 cm-1 can be seen with no further progression. The fluorescence excitation spectrum in the presence of MeOH also shows transitions due to 2PBI-H2O complex, due to the water contamination of gas lines. The 000 transitions of H2O and MeOH complexes are shifted substantially to the red by 1059 and 1203 cm-1, respectively, relative to the bare 2PBI. The substantial shifts in the electronic transitions are indicative of large differences between the stabilization energies of the ground and first excited state complexes. One of the interesting features observed in the fluorescence excitation spectrum of 2PBI is the long progression up to V′ ) 5 (up to V′ ) 4 of the argon complex) in the 92 cm-1 vibrational frequency mode. Further, it can also be noted from Figure 2 that the V′ ) 0 is not the most intense transition. These two features indicate that the potential along this mode is substantially shifted in the electronic excited state, relative to the ground state. The vibrational frequency calculations on the excited state 2PBI, suggest that 92 cm-1 vibrational frequency mode for the bare 2PBI corresponds to the in-plane bending mode along the C-C bond connecting the benzimidazole and pyridyl rings. The corresponding vibrational frequency in the ground state is 106 cm-1. Binding of H2O and MeOH to 2PBI leads to substantial lowering in the progression up to V′ ) 2 and V′ ) 1, respectively. Additionally the frequency of this vibration also changes from 92 cm-1 in the case of bare 2PBI (and argon complex) to 95 cm-1 for the H2O complex and 100 cm-1 for the MeOH complex. The changes in the Franck-Condon pattern and vibrational frequency upon complexation with H2O and MeOH indicate that the binding of these two molecules lead to
clamping of the benzimidazole and pyridyl rings, indicating the formation of the cyclic complexes. The calculated vibrational frequencies in the ground state for the H2O and MeOH complexes are 112 and 116 cm-1, respectively. These calculations also indicate the increase in the vibrational frequency in the ground state upon complexation with water, similar to experimental observation in the excited state. The infrared spectra in the hydride stretching region were recorded using fluorescence dip infrared spectroscopic technique. The FDIR spectrum of 2PBI, Figure 3A, shows a single transition at 3489 cm-1, which corresponds to the N-H stretching vibration of 2PBI. The appearance of N-H stretching vibration at 3489 cm-1 indicates the sp2 hybridization of the nitrogen atom,23 which clearly indicates that the hydrogen atom is attached to the benzimidazole group and not the pyridyl moiety. Therefore the structure responsible for the spectrum shown in Figure 3A is either 2PBI-R1 or 2PBI-R2 (see Figure 1). The FDIR spectrum of the 2PBI-Ar complex, depicted in Figure 3B, once again shows a single transition at 3487 cm-1. This implies that the binding of Ar atom to 2PBI feebly perturbs the N-H stretching vibration of 2PBI moiety. As discussed earlier, 2PBI can exist in three forms (see Figure 1). The calculations at B3LYP/6-311+G(d) level reveal that the rotamer-1 (2PBI-R1) is the most stable form in the ground state relative to the other two forms rotamer-2 (2PBI-R2) and the tautomeric form (2PBI-T) by 38.6 and 83.3 kJ mol-1, respectively. Therefore the observed form of 2PBI can be assigned, rather straightforwardly, to 2PBI-R1. The appearance of N-H stretching vibration at 3489 cm-1 is indicative of the fact that an intramolecular hydrogen-bonded interaction between imidazole N1-H and the nitrogen atom of the pyridine ring does not exist. The FDIR spectrum of 2PBI-H2O complex, depicted in figure 4A, shows a sharp transition at 3709 cm-1, which can be assigned as the free O-H stretching of the 2PBI-H2O. Two broad transitions appearing at 3332 and 3263 cm-1 can be assigned to the hydrogen-bonded O-H and N-H stretching vibrations of the complex, respectively. Further, a weak transition at 3419 cm-1 (marked with *) can be assigned to combination band over the O-H stretching vibration. The FDIR spectrum of 2PBI-MeOH complex is depicted in Figure 4B. A broad transition centered around 3295 cm-1 can be assigned to the hydrogen-bonded O-H stretching vibration while the
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Figure 4. The FDIR spectra of (A) 2PBI-H2O and (B) 2PBI-MeOH. Also shown are the simulated spectra of various structures. Transitions marked with asterisk are assigned to the combination bands. The arrows represent the O-H and the N-H oscillators of the monomers.
transition centered around 3210 cm-1 can be assigned to the hydrogen-bonded N-H stretching vibration. The transition around 3450 cm-1 (marked with *) once again can be assigned to a combination band over the O-H stretching vibration. Many examples exist in the literature, wherein combination bands have been observed in the IR spectra of moderate to strong hydrogenbonded complexes.24 Finally, to determine the origin of several transitions appearing in the fluorescence excitation spectrum, IR-UV hole burning spectroscopy were carried out for 2PBI, 2PBI-H2O and 2PBI-MeOH. Trace A of Figure 5 shows the fluorescence excitation spectrum of 2PBI monomer and Trace B is the IR-UV hole-burnt spectrum, which was recorded by tuning the IR laser to pump the N-H vibrational transition of the 2PBI monomer at 3489 cm-1 (see Figure 3A), 100 ns prior to the exciting UV pulse, while scanning the UV laser. The IR-UV hole-burnt spectrum shows dips for all the observed transitions in the fluorescence excitation spectrum indicating the presence of single species. Figure 5C shows the fluorescence excitation spectrum of 2PBI-H2O complex, and Figure 5D shows the corresponding IR-UV hole-burnt spectrum, which was recorded by tuning the IR laser to pump the hydrogen-bonded O-H vibrational transition of the 2PBI-H2O complex at 3332 cm-1, 100 ns prior to the exciting UV pulse, while scanning the UV laser. The resulting spectrum shows dips for all the transitions and establishes the formation of single species in 2PBI-H2O complex. Similarly, Figure 5E shows the fluorescence excitation spectrum of 2PBI-MeOH complex and Figure 5F shows the IR-UV hole-burnt spectrum, which was recorded by tuning the IR laser to pump the O-H vibrational transition of the 2PBI-methanol complex at 3295 cm-1. The results of IR-UV hole burning spectroscopy once again confirm the presence of single species of 2PBI-MeOH complex. B. Structures. As mentioned earlier, 2PBI can possibly exist in three different structures (see Figure 1). Figure 6 shows the
Guin et al.
Figure 5. LIF excitation spectra of (A) 2PBI, (C) 2PBI-H2O, and (E) 2PBI-MeOH. IR-UV hole-burnt spectra of (B) 2PBI, (D) 2PBI-H2O, and (F) 2PBI-MeOH. The spectra B, D, and F were recorded by pumping the vibrational transitions of the respective species at 3489, 3332, and 3295 cm-1, respectively. In all the cases the IR-UV hole burnt spectrum shows reduced intensities for all the transition belonging to the probed species, relative to the LIF excitation spectrum. This implies the presence of single isomer for 2PBI and its complexes with water and methanol.
Figure 6. Calculated structures of 2PBI, 2PBI-H2O and 2PBI-MeOH complexes at B3LYP/6-311+G(d) level of theory.
calculated structures at B3LYP/6-311+G(d) level and Table 1 lists their relative energies in S0 and S1 states and the corresponding scaled N-H stretching frequencies. 2PBI-R1 is characterized by the presence of imidazole N-H and pyridine nitrogen which are syn to each other, while in the case of 2PBI-R2 the two groups are anti to each other. 2PBI-R1 is planar and is stable by 38.6 kJ mol-1 relative to 2PBI-R2, which is nonplanar. The nonplanarity of 2PBI-R2 can be attributed to the repulsion between the two hydrogens at 1 and 6′ positions, similar to biphenyls. In the tautomeric form
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TABLE 1: ZPE Corrected Relative Energies (kJ mol-1) of Various Structures of 2PBI and the Corresponding N-H Stretching Frequencies (cm-1) in the S0 and S1 States -∆E0 (S0)
νN-H
-∆E0 (S1)
νN-H
00.0 38.6 83.3
3489 3498 3304
00.0 51.0 10.7
3752 3563 3432
2PBI-R1 2PBI-R2 2PBI-T
optimized to evaluate their energies relative to the 2PBI-R1 complexes, which can indicate the propensity of proton/ hydrogen atom transfer through hydrogen-bonded bridges. H2O and MeOH can form two distinct complexes with 2PBI-R1, structures of which are once again shown in Figure 6 and the energetics are listed in Table 2. All the binary complexes of H2O and MeOH are cyclic in nature. The structure of 2PBI-R1-W-A is characterized by the presence of N-H · · · O hydrogen bond involving the imidazole NH group with the lone pair of electrons on water molecule and an O-H · · · N hydrogen bond, wherein the pyridine nitrogen acts an acceptor to one of the OH groups of water moiety. The intermolecular structure of the MeOH complex, 2PBI-R1-MeOH-A is similar, with marginal changes in the intermolecular geometrical parameters. The other two complexes of H2O and MeOH, 2PBIR1-H2O-B and 2PBI-R1-MeOH-B are characterized by the presence of C-H · · · O and O-H · · · N hydrogen bonds. The second set of complexes are weaker than the first set by 6.1 and 6.0 kJ mol-1 for the H2O and MeOH complexes, respectively. The structures of the H2O and MeOH complexes of 2PBI-T (2PBI-T-W and 2PBI-T-M) are also shown in Figure 6. These complexes are once again cyclic with H2O and MeOH acting both as hydrogen bond donors and acceptors. These complexes are characterized by the formation of hydrogen bonds between imidazole nitrogen and OH group of H2O/MeOH and between the NH group of pyridyl moiety with the oxygen atom of H2O/MeOH. These structures correspond to the proton/
(2PBI-T) the hydrogen atom is attached the pyridine nitrogen, which leads to loss of aromaticity in the pyridine ring, thereby substantially increasing its energy by 83.3 kJ mol-1 relative to 2PBI-R1. RCIS/6-31(G) calculations indicate that tautomer energy is considerably lowered upon electronic excitation, although it is still higher by 10.7 kJ mol-1, relative to 2PBI-R1. This observation is in contrast to7-azaindole, wherein it was found that the tautomer form, 2PBI-T, is more stable than the normal form in the excited state. This was attributed to the orbital occupancy, change in aromaticity and delocalization of the electron density upon excitation.25 On the other hand the energy difference between 2PBI-R1and 2PBI-R2 increases to 51.0 kJ mol-1 in the electronic excited state. Because only a single species of 2PBI is responsible for the fluorescence excitation spectrum and 2PBI-R1 is the most stable structure of 2PBI, the assignment is rather straightforward. Therefore, we have performed geometry optimization of complexes of 2PBI-R1 with H2O and MeOH. Further, the H2O and MeOH complexes of the tautomer, 2PBI-T, were also
TABLE 2: ZPE Corrected Stabilization Energies (kJ mol-1) and N-H and O-H Stretching Frequencies (cm-1) for Various Structures of 2PBI, 2PBI-H2O, and 2PBI-MeOHa
a
-∆E0
νN-H
∆νN-H
νO-H
∆νO-H
Σ(∆νO-H)
2PBI-R1-H2O-A
33.7
3219 (3263)
270 (226)
27.6
3332
157
2PBI-T-H2O
58.5
2871
433
2PBI-R1-MeOH-A 2PBI-R1-MeOH-B 2PBI-T-MeOH
34.1 28.2 58.9
3215 (3210) 3347 2864
274 (279) 142 440
309 (325) 26 (47) 118 33 592 29 311 (386) 141 755
335 (372)
2PBI-R1-H2O-B
3292 (3332) 3689 (3709) 3483 3682 3009 3686 3314 (3295) 3484 2870
151 621
Experimentally observed frequencies are listed in parentheses.
TABLE 3: Calculated Changes in the Electron Occupancy of Donors and Acceptors, Interaction Energy Parameters, Electron Density at the Bond Critical Points (BCP), and the Laplacian of Electron Density at BCP for Various Hydrogen Bonded Complexesa,b
a
F
32F
58.9 59.0
0.0361 0.0326
0.0262 0.0297
0.0304 0.0047
51.9 13.3
0.0327 0.0146
0.0252 0.013
0.0665 0.0386
109.0 99.4
0.0456 0.0487
0.0362 0.0294
0.0352 0.0287
59.5 53.6
0.0349 0.0326
0.0255 0.0290
0.0315 0.0067
59.0 9.2
0.0325 0.0135
0.0249 0.0117
0.0664 0.0424
116.2 95.2
0.0481 0.0463
0.0294 0.0357
interaction
∆-DO
∆-AO
2PBI-R1-H2O-A
N · · · H-O O · · · H-N
0.0366 0.0262
2PBI-R1-H2O-B
N · · · H-O O · · · H-C
2PBI-T-H2O
N · · · H-O O · · · H-N
2PBI-R1-MeOH-A
N · · · H-O O · · · H-N
2PBI-R1-MeOH-B
N · · · H-O O · · · H-C
2PBI-T-MeOH
N · · · H-O O · · · H-N
-0.018 -0.0021 -0.0317 -0.0109 -0.0022 -0.0089 -0.0395 -0.0029 -0.0559 -0.0232 -0.0088 -0.0266 -0.0183 -0.0132 -0.0059 -0.0429 -0.0117 -0.0484
(2) EifJ
∆-DO and ∆-AO correspond to change in electron occupancy of donor and acceptor orbitals. (2) given in atomic units, while EifJ is given in kJ mol-1.
b
Values of ∆-DO, ∆-AO, F, and 32F are
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hydrogen atom transferred form of the first set of complexes, 2PBI-R1-W-A and 2PBIR1-M-A. The NBO analysis is an effective tool to estimate the overlap between the lone pair orbitals of Y and the antibonding orbital of the X-H bond of a X-H · · · Y hydrogen-bonded complex. The second-order perturbative energy, E(2) ifJ, which is proportional to the extent of overlap between the donor and acceptor orbitals, is a measure of the strength of the hydrogen bond.19 Table 3 lists the changes in the electron occupancy in the lone pairs of donor and antibonding (σ*) orbitals of the acceptor along with the E(2) ifJ for all the calculated complexes shown in Figure 6. It can be seen from Table 3 that the donor orbital looses the occupancy and the acceptor gains, implying charge migration from donor to acceptor orbitals. On the basis of the net changes in the electron occupancy, it can be inferred that, in the present case, the O-H · · · N hydrogen bonds are stronger in comparison to N-H · · · O hydrogen bonds, in both the complexes of 2PBI-R1 and 2PBI-T with H2O an MeOH. This also indicates the switching of the primary interaction along with the proton/hydrogen atom transfer pathway. Further, the lower stability of 2PBI-R1-W-B and 2PBIR1-M-B complexes can be rationalized on the basis of lower charge migration from donor (lone pair on oxygen atom) to acceptor orbitals (antibonding orbital of C-H group). The topological parameters derived from AIM theory were further used to analyze the formation of various hydrogen-bonded complexes. In the present case the critical points of electron density distribution were obtained, characterized by the rank and trace of the Hessian matrix. A (3, -1) bond critical point (BCP) with positive Laplacian (32) for the electron density distribution at the bond critical point indicates the noncovalent interaction. However, in the case of very strong hydrogen bonds, which are often partly covalent in nature, the Laplacian of electron density at the BCPs can be negative. Table 3 also lists all the electron density at the BCP and its Laplacian for various interactions, which are in the desired range for the normal hydrogen bonds except the charge density value of 2PBI-T-W and 2PBI-T-MeOH complex are slightly higher in range and the 32F value of O · · · H-C interaction in 2PBI-H2O-B and 2PBI-MeOH-B complex which are slightly lower in range.26 As C-H group is a weak donor hence the small value of 32F can be rationalized. Large value of charge density in both O-H · · · N and N-H · · · O interaction in 2PBI-T-W and 2PBI-T-M complex indicate strength of hydrogen bond is higher in comparison with the other rotamer complexes. It can be noted from both the NBO and AIM calculations that the hydrogen-bonded complexes of the 2PBI-T are more stable than those of 2PBI-R1. C. Structural Assignment. Infrared spectroscopy is an excellent technique to establish the intermolecular interaction between a pair of molecules, as it probes the local interaction present in molecules and molecular complexes. The vibrational spectra in the hydride stretching region, namely, O-H, N-H, and C-H groups, offer the advantage of inferring the hydrogenbonded structure as these groups as a consequence of being directly involved in the intermolecular interaction, show characteristic shifts in their vibrational frequencies along with the broadening of the IR spectra, especially for strongly hydrogenbonded complexes.27 The simulated IR spectra of various possible structures were compared with the experimental spectra, and the agreement between the simulated and observed vibrational frequencies served as a benchmark for the structural assignment of various complexes. In the case of the water monomer, the experimentally observed two O-H stretching frequencies of the water molecule are at 3657 and 3756 cm-1,
Guin et al. corresponding to symmetric (ν1) and antisymmetric (ν3) stretching vibrations, respectively. In the event of hydrogen bond formation to one of the O-H groups of the water moiety, the two frequencies will now correspond to the hydrogen-bonded and free O-H stretching vibrations. Although only one of the O-H groups is involved in hydrogen bond formation, both stretching frequencies are lowered due to partial decoupling of the two O-H oscillators. The vibrational frequencies corresponding to the hydrogen-bonded and free O-H stretching vibrations are lower than the symmetric and antisymmetric stretching vibrations, respectively. Since both O-H stretching frequencies are lowered due to hydrogen bond formation, we have used the total shift in the O-H stretching frequencies [Σ(∆ν) ) (∆ν1 + ∆ν3)] as a tool to assign the intermolecular structures.28 Figure 4 shows the comparison on the experimental and the simulated spectra of 2PBI-R1 complexes with H2O and MeOH, and Table 2 lists the experimental and calculated vibrational frequencies along with their shifts. For 2PBI-H2O complex the experimental and calculated vibrational frequencies of both the N-H and the O-H oscillators differ considerably for the structures 2PBI-R1-W-B and 2PBI-T-W. Similar observations can be made from comparison of experimental vibrational frequencies for the 2PBI-MeOH complex with the structures 2PBI-R1R1-M-AW-B and 2PBI-TM. For both H2O and MeOH complexes the cases the simulated spectra of complex A, namely, 2PBI-R1-W-A and 2PBI-R1-M-A, is in good agreement, both in terms of band positions and the intensities, with the experimental spectra. Further, 2PBI-R1-W-A and 2PBIR1-M-A are also the most stable complexes. Therefore, the observed complexes of 2PBI with H2O and MeOH are be assigned to structures 2PBI-R1-W-A and 2PBIR1-M-A shown in Figure 6. D. Structural Consequences. One of the most interesting aspects of the LIF excitation spectra, depicted in Figure 2, is the changes in the Franck-Condon activity along the 92 cm-1, vibrational mode. In the case of bare 2PBI and its Ar complex, an extended progression up to V′ ) 5 and V′ ) 4, respectively, can be seen. On the other hand the progression can be seen only up to V′ ) 2 for the H2O complex and V′ ) 1 for the MeOH complex. The changes in the Franck-Condon pattern due to the binding of H2O and MeOH indicates that constrained motion along this particular vibrational mode, which arises due to the in-plane bending mode along the C-C bond connecting the benzimidazole and pyridyl rings. The vibrational frequency calculations of H2O and MeOH complexes indicate that the hydrogen-bonded N-H and O-H stretching frequencies are strongly coupled with very little or almost no local mode characteristics. In fact, the two observed frequencies at 3332 and 3263 cm-1 for H2O complex (see Figure 4A) can aptly be described as out-of-phase and in-phase N-H · · · O and O-H · · · N vibrations, respectively.9 Further, it can be seen from Table 2 that the agreement between the calculated and observed frequency shifts in the O-H and N-H stretching frequencies of both the H2O and MeOH complexes is not ideal. The calculated frequencies for two modes involved in the hydrogen bonding have positive and negative deviations. This implies that the present vibrational frequency calculations have not completely captured the hydrogen bonding, which perhaps can be attributed to the increased anharmonicity of strongly coupled hydrogen bonds. Wiosna-Sałyga et al. analyzed the hydrogenbonding pattern of complexes of H2O with several bifunctional aza-heteroaromatic molecules.29 The red shift of the hydrogenbonded O-H stretching frequency of H2O moiety relative to 3707 cm-1 (mean of symmetric and asymmetric stretching
2PBI Complexes with H2O and MEOH vibrations of H2O monomer) was used as measure of hydrogen bond strength. In the present 2PBI-H2O complex, the corresponding shift is 375 cm-1. This value of shift is the largest, with an exception of H2O complex of 1H-pyrrolo[3,2-h]quinolone, wherein the proton/hydrogen atom transfer occurs even in the ground state.9,29 The propensity of 2PBI to undergo proton/hydrogen atom transfer by explicit solvent participation can be understood by analyzing the relative energies of the 2PBI-R1 and 2PBI-T and their hydrogen-bonded complexes with H2O and MeOH. The stabilization energies of the H2O complexes with 2PBI-R1 and 2PBI-T are 33.7 and 58.5 kJ mol-1, respectively, while the corresponding energies for the MeOH complexes are 34.1 and 58.8 kJ mol-1, respectively. As discussed earlier the energy difference between the 2PBI-R1 and 2PBI-T structures is 83.3 kJ mol-1 (see Table 1). On the other hand the energy difference between the H2O complexes of 2PBI-R1 and 2PBI-T (viz., 2PBI-R1-W-A and 2PBI-T-W) is only 24.8 kJ mol-1, while the corresponding energy difference for the MeOH complexes is also 24.7 kJ mol-1. This implies that the binding of H2O and MeOH lowers the energy difference between 2PBI-R1 and 2PBI-T structures by 25 kJ mol-1 (see Table 2). Further, as noted earlier, the energy difference between the 2PBI-R1 and 2PBI-T structures in the excited state is only 10.7 kJ mol-1 (see Table 1). With the assumption that the hydrogen bond strength does not reverse for the 2PBI-R1 and 2PBI-T complexes with H2O and MeOH in the excited state, clearly the hydrogen-bonded complexes of the tautomer is more stable in the electronic excited state. This implies that the proton/ hydrogen atom transfer might be thermodynamically feasible in the electronic state even for the isolated hydrogen-bonded complexes in the gas-phase. We tried to obtain the IR spectra of 2PBI and its complexes with H2O and MeOH in the electronic excited state using UV-IR double resonance spectroscopic technique. We had earlier demonstrated this technique to record the IR spectrum of phenylacetylene and its argon complex in the electronic excited state.15 However, this attempt was unsuccessful due to the fact that the excited state lifetime of 2PBI and its hydrogen-bonded complexes is few picoseconds to sub-picoseconds, which cannot be possibly probed by nanosecond lasers used in our laboratory. Conclusions In summary, we have investigated H2O and MeOH complexes of 2PBI using IR-UV double resonance spectroscopic technique. The 000 bands for the S1 r S0 electronic excitation transitions of 2PBI, 2PBI-Ar, 2PBI-H2O, and 2PBI-MeOH were observed at 31 616, 31 573, 30 557, and 30 413 cm-1, respectively. The LIF excitation spectrum showed a marked reduction in the vibrational progression in the 92 cm-1 vibrational mode from V′ ) 5 for bare 2PBI to V′ ) 2 and V′ ) 1 in the case of H2O and MeOH complexes. The comparison of experimental and theoretical IR spectra in the O-H and N-H stretching region reveals that the interaction of H2O and MeOH with 2PBI, is similar to corresponding complexes of several bifunctional aza-heteroaromatic molecules reported in the literature, which incorporates O-H · · · N and N-H · · · O hydrogen bonds to the formation of a quasi-planar cyclic complex. Large shifts in the O-H and N-H stretching frequencies along with the broadening of the transitions are indicative of formation of strong hydrogen-bonds. The vibrational frequency analysis clearly indicates the coupling of hydrogen-bonded N-H · · · O and O-H · · · N oscillators. These vibrations can be classified as out-of-phase and in-phase N-H · · · O and O-H · · · N vibra-
J. Phys. Chem. A, Vol. 114, No. 32, 2010 8329 tions. The observed shift in the hydrogen-bonded O-H stretching frequency of 2PBI-H2O complex is the largest observed so far, with the exception of H2O complex of 1H-pyrrolo[3,2-h]quinolone. The experimental results combined with theoretical calculations indicate that 2PBI is a potential candidate for excited state proton/hydrogen atom transfer in the gas phase through explicit solvent participation, such as H2O and MeOH. Acknowledgment. Authors wish to thank Prof. Anindya Datta for discussions and suggestions. M.G. and S.M. thank UGC for the research fellowship. G.N.P. is presently on Lien with Genesis Research Institute Inc. Japan and thanks Prof. A. Terasaki for support and encouragement. This material is based upon work supported by Department of Science and Technology (Grant No. SR/S1/PC/23/2008) and Board of Research in Nuclear Sciences (Grant No. 2004/37/5/BRNS/398) and Council of Scientific and Industrial Research (Grant No. 01(2268)/08/ EMR-II). References and Notes (1) For example see, Atkins, P.; Overton, P.; Rourke, J.; Weller, M.; Armstrong, F. ShriVer and Atkins’ Inorganic Chemistry; Oxford University Press: Oxford, 2009. (2) Lodish, H.; Berk, A.; Matsudaira, P.; Kaiser, C. A.; Krieger, M.; Scott, M. P.; Zipurski, L.; Darnell, J. W. H. Mol. Cell. Biol.; Freeman: San Francisco, 2004. (3) Bountis, T. Proton Transfer in Hydrogen-Bonded Systems; Plenum: New York, 1992. (4) For example see, (a) Waluk, J. Acc. Chem. Res. 2003, 36, 832. (b) Agmon, N. J. Phys. Chem. A 2005, 109, 13. (5) Nosenko, Y.; Wiosna-Sayga, G.; Kunitski, M.; Petkova, I.; Singh, A.; Buma, W. J.; Thummel, R. P.; Brutschy, B.; Waluk, J. Angew. Chem., Int. Ed. Engl. 2008, 47, 6037. (6) Matsumoto, Y.; Ebata, T.; Mikami, N. J. Phys. Chem. A 2002, 106, 5591. (7) (a) Tanner, C.; Manca, C.; Leutwyler, S. Science 2003, 302, 1736. (b) Tanner, C.; Manca, C.; Leutwyler, S. Int. ReV. Phys. Chem. 2005, 24, 457. (c) Manca, C.; Tanner, C.; Coussan, S.; Bach, A.; Leutwyler, S. J. Chem. Phys. 2004, 121, 2578. (8) (a) Sakota, K.; Komoto, Y.; Nakagaki, M.; Ishikawa, W.; Sekiya, H. Chem. Phys. Lett. 2007, 435, 1. (b) Sakota, K.; Inoue, N.; Komoto, Y. H.; Sekiya, H. J. Phys. Chem. A 2007, 111, 4596. (9) Nosenko, Y.; Kunitski, M.; Riehn, C.; Thummel, R. P.; Kyrychenko, A.; Herbich, J.; Waluk, J.; Brutschy, B. J. Phys. Chem. A 2008, 112, 1150. (10) Prieto, F. R.; Mosquera, M.; Novo, M. J. Phys. Chem. 1990, 94, 8536. (11) (a) Novo, M.; Mosquera, M.; Prieto, F. R. J. Chem. Soc., Faraday Trans. 1993, 89, 885. (b) Novo, M.; Mosquera, M.; Prieto, F. R. Can. J. Chem. 1992, 70, 823. (c) Novo, M.; Mosquera, M.; Prieto, F. R. J. Phys. Chem. 1995, 99, 14726. (12) Kondo, M. Bull. Chem. Soc. Jpn. 1978, 51, 3027. (b) Brown, R. G.; Entwistle, N.; Hepworth, J. D.; Hodgson, K. W.; May, B. J. J. Phys. Chem. 1982, 86, 2418. (13) (a) Mukherjee, T. K.; Ahuja, P.; Koner, A. L.; Datta, A. J. Phys. Chem. B 2005, 109, 12567. (b) Mukherjee, T. K.; Panda, D.; Datta, A. J. Phys. Chem. B 2005, 109, 18895. (c) Mukherjee, T. K.; Datta, A. J. Phys. Chem. B 2006, 110, 2611. (d) Burai, T. N.; Mukherjee, T. K.; Lahiri, P.; Panda, D.; Datta, A. J. Chem. Phys. 2009, 131, 34504. (14) Lide, D. R. Ed., CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, 1998. (15) Singh, P. C.; Patwari, G. N. Curr. Sci. 2008, 95, 469. (16) Page, R. H.; Shen, Y. R.; Lee, Y. T. J. Chem. Phys. 1988, 88, 4621. (17) Frisch, M. J. Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
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