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Hydrogen Bonding to Multifunctional Molecules: Spectroscopic and ab Initio Investigation of 4-Ethynylbenzonitrile-(Water)1-3 Complexes Surajit Maity and G. Naresh Patwari* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India ReceiVed: June 2, 2010; ReVised Manuscript ReceiVed: July 7, 2010
The water complexes of 4-ethynylbenzonitrile (4EBzN) were investigated with IR-UV double resonance spectroscopy. Water interacts with the π electron density of the CtN group in 4EBzN, leading to the formation of a quasiplanar cyclic complex incorporating CsH · · · O and OsH · · · π hydrogen bonds. The (H2O)2 and (H2O)3 complexes of 4EBzN are characterized by the presence of hydrogen bonding bridges between the hydrogen bond donor (aromatic CsH group) and acceptor (CtN group) sites present in 4EBzN. The present structures of the water complexes are similar to the corresponding complexes of benzonitrile but are drastically different from water complexes of phenylacetylene and fluorophenylacetylenes. The infrared spectra in the O-H stretching region indicate that the stability of all the water complexes of 4EBzN is lower than that of the corresponding complexes of benzonitrile, which can be attributed to the mild electron withdrawing ability of the ethynyl group present in the para position relative to the nitrile group. Hierarchally, the CtN group has the higher propensity to form hydrogen bonding relative to the acetylenic group and the benzene ring in 4EBzN. Introduction Understanding and predicting hydrogen bonding patterns in multifunctional molecules is a challenging proposition due to the fact that several energetically closely spaced intermolecular configurations can exist on the potential energy hyper surface. On the other hand, the majority of hydrogen bonding observed in both the gas and the condensed phases can be summarized by using Etter rules.1 Further, based on the geometries of several mixed dimers obtained using rotationally resolved spectroscopy, Legon and Millen laid down rules for formation of X sH · · · B hydrogen bonding.2 Both the Etter and the Legon-Millen rules govern the hierarchy of hydrogen bond formation. We had earlier investigated several hydrogen-bonded complexes of phenylacetylene with simple solvent molecules such as water, methanol, ethanol, ammonia, alcohols, and amines.3 The hydrogen-bonded complexes of phenylacetylene form a wide variety of intermolecular structures, which manifest due to the subtle balance in the intermolecular forces in various possible configurations. Further, it was also observed that even minimal changes in the interacting partners, such as substitution by a ubiquitous methyl group, can result in dramatic change in the intermolecular structures.3,4 The structure of the phenylacetylenewater complex is characterized by the presence of OsH · · · π and CsH · · · O hydrogen bonds, wherein one of the OsH group of water moiety interacts with the π electron density of the acetylenic CtC triple bond and the aromatic CsH group in the ortho position interacts the oxygen atom of the water, leading to the formation of a quasiplanar cyclic complex.3a On the other hand, the phenylacetylene-methanol complex is characterized by the presence of a single OsH · · · π hydrogen bond, wherein the OsH group of methanol interacts with the π electron density of the benzene ring.3c Similarly, the structures of the ammonia and methylamine complexes of phenylacetylene were found to be different.3b Substitution of fluorine in both the para and ortho * To whom correspondence should be addressed. E-mail: naresh@ chem.iitb.ac.in.
positions to the ethynyl group of phenylacetylene does not alter the intermolecular structure of the water complex.5 This observation was very surprising in light of the fact that substitution of fluorine on benzene and styrene leads to dramatic changes in the intermolecular structures of the corresponding water complexes.6,7 Further, it also implies that the structures of water complexes with 2-fluoro- and 4-fluorophenylacetylene are in conflict with the Legon-Millen rules,2 since formation of a π hydrogen bond is preferred over that of a σ hydrogen bond.5 To further evaluate the propensity of hydrogen bond formation in multifunctional molecules, investigations on water complexes of 4-ethynylbenzonitrile [HCtC-C6H4-CtN, 4EBzN] were carried out. 4EBzN has four hydrogen bonding sites, of which three are π acceptors, in the form of a phenyl ring and the CtC of acetylenic and CtN of cyano groups. 4EBzN offers a unique opportunity to investigate competitive hydrogen bonding between the three π acceptor sites. Experimental and Computational Methods The details of the experimental setup have been described elsewhere.8 Briefly, helium buffer gas at 4 atm is bubbled through water kept at room temperature and passed over the vapors of 4EBzN placed in a sample holder above the pulsed nozzle. The sample holder and the pulsed nozzle were heated to 393 K to obtain sufficient vapor pressure of 4EBzN. The resulting gas mixture was expanded through a 0.5 mm diameter pulsed nozzle (Series 9, Iota One; General Valve Corporation). The electronic excitation was achieved with use of a frequency doubled output of a tunable dye laser (Narrow Scan GR; Radiant Dyes) pumped with the second harmonic of a Nd:YAG laser (Surelite I-10; Continuum). 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-320) combination, while scanning the UV laser frequency. The IR spectra were obtained with use of the fluorescence dip infrared (FDIR) spectroscopic method.9 In
10.1021/jp105081f 2010 American Chemical Society Published on Web 07/23/2010
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this method, the population of a target species is monitored by the fluorescence intensity following its electronic excitation to the S1 r S0 origin band 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, and as a result the depletion of the fluorescence signal is observed. Further, in order to separate the transitions belonging to various species present in the LIF excitation spectrum, IR-UV holeburning spectra were also recorded. 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 intensity of the fluorescence decreases when compared to the LIF excitation spectrum. The lowering of the intensity in the hole-burnt spectrum relative to the LIF excitation spectrum allows identification of relevant transitions. In our experiments the sources of tunable IR light are an idler component of a LiNbO3/KTP OPOs (Custom IR OPO; LaserSpec) pumped with the fundamental of an injection-seeded Nd:YAG laser (Brilliant-B; Quantel). The typical bandwidths of the UV laser and the LiNbO3 OPO are about 1 cm-1, while that of KTP OPO is about 3 cm-1, and the absolute frequency calibration is within (2 cm-1. The LiNbO3 OPO has a hole in its tuning curve in the frequency region of 3450-3520 cm-1; therefore KTP OPO was used for recording the IR spectra in this region. To supplement the experimental observations we have carried out electronic structure calculations using the Gaussian-03 package.10 The equilibrium structures of the monomers and various binary complexes were calculated with the DFTMPW3LYP method, using the aug-cc-pVDZ basis set. The nature of the stationary points obtained was verified by calculating the vibrational frequencies at the same level of theory. The basis set superposition error (BSSE) correction was made after geometry optimization. The stabilization energies were corrected for the zero point vibrational energy (ZPVE). For medium sized basis sets, 100% of BSSE correction is believed to often underestimate the interaction energy and 50% correction is a good empirical approximation. Therefore, we report the stabilization energies with 0%, 50%, and 100% BSSE correction.11 The calculated symmetric and antisymmetric O-H stretching frequencies of water molecule at the MPW3LYP/ aug-cc-pVDZ level were 3796 and 3906 cm-1, respectively. However, the corresponding experimental values are 3657 and 3756 cm-1. The scaling factor of 0.9625 was divided by taking the ratio of the average of experimental frequencies (3706 cm-1) to the average of calculated frequencies (3851 cm-1). The scaling factor is intended to correct for the basis set truncation, partial neglect of the electron correlation, and harmonic approximation. The IR spectra for all the optimized structures were simulated in the O-H and N-H stretching region by convoluting a Lorentzian function of width (fwhm) 2 cm-1 to the calculated stick spectrum and compared with the observed experimental spectrum for structural assignment.12 Results and Discussion A. Spectra. The fluorescence excitation spectrum of bare 4EBzN is shown in Figure 1A. There are no previous reports on the electronic spectrum of 4EBzN and the intense transition marked with an asterisk (/) at 35212 cm-1 is being assigned to the band origin transition of S1 r S0 electronic transition. Interestingly, the most intense transition appears 22 cm-1 to the blue of the band origin transition. The FDIR spectrum
Maity and Patwari
Figure 1. Fluorescence excited spectrum of (trace A) 4-ethynylbenzonitrile (4EBzN) and (trace B) LIF spectrum of 4-ethynylbenzonitrile in the presence of water. The peak marked with an asterisk (/) is due to the band origin transition of 4EBN. The other bands marked with m, w1, w2, and w3 are assigned to the transitions corresponding to 4EBzN, 4EBzN-H2O, 4EBzN-(H2O)2, and 4EBzN-(H2O)3, respectively.
recorded by monitoring the 22 cm-1 blue-shifted band was identical with that recorded by monitoring the band origin transition. The energy spacing of 22 cm-1 is too small to be assigned to a vibronic transition for a rigid molecule such as 4EBzN. This observation is very similar to that observed in the case of 2-pyridone, for which the low-frequency transition was assigned to a small geometry distortion in the excited state originating out of a single ground state conformation.13 The other transitions in the higher energy part of the spectrum can be assigned to various vibronic bands. The fluorescence excitation spectrum of 4EBzN recorded in the presence of H2O, depicted in Figure 1B, shows several new transitions, which on the basis of IR spectra in the OH stretching region and hole-burnt spectra can be assigned to those originating from 4EBzN-(H2O)1-3 complexes, vide infra. The band origin transitions of 4EBzN(H2O)1 [W1] and 4EBzN-(H2O)2 [W2] complexes are shifted to the red by 46 and 94 cm-1, relative to that band origin of 4EBzN at 35212 cm-1. Further, one-to-one correspondence can be seen between all the moderate to intense transitions of 4EBzN and the transitions of W1 and W2 complexes, which can be assigned to vibronic bands of respective water complexes. Additionally, the 121 cm-1 blue-shifted band at 35355 cm-1 marked with “W3” is assigned as to originating for the 4EBzN-(H2O)3 complex. No transitions corresponding to W3 complex were found in the lower energy part of the spectrum. FDIR spectra were recorded in both the acetylenic CsH and the OsH stretching region, monitoring the fluorescence following excitation of various bands corresponding to 4EBzN and its water complexes. Figure 2 shows the FDIR spectra in the acetylenic CsH stretching region for 4EBzN and its water complexes W1, W2, and W3. The FDIR spectrum of 4EBzN, depicted in Figure 2A, shows three transitions at 3326, 3329, and 3338 cm-1. The appearance of multiple bands is indicative of the presence of Fermi resonance and higher order couplings, similar to phenylacetylene and fluorophenylacetylenes.3,5,8,14 In the case of phenylacetylene, three prominent transitions were observed at 3318, 3325, and 3343 cm-1 accompanied by several weak transitions. These transitions were assigned to be originating from Fermi resonance between the acetylenic CsH oscillator with one quantum of CtC stretch and two quanta of
Hydrogen Bonding to Multifunctional Molecules
Figure 2. FDIR spectra of (A) 4-ethynylbenzonitrile (4EBzN) (band m), (B) 4EBzN-H2O (band W1), (C) 4EBzN-(H2O)2 (band W2), and (D) 4EBzN-(H2O)3 (band W3), in the acetylenic C-H stretching region.
Figure 3. FDIR spectra of 4EBzN-H2O in the O-H stretching region. Traces A and B were recorded with LiNbO3 and KTP OPOs, respectively. The remaining five traces represent the simulated vibrational spectra in the O-H for various calculated structures.
CsH out-of-plane bend and the other higher order coupling terms.14 The FDIR spectrum of W1 (Figure 2B) shows two slightly broader bands at 3328 and 3337 cm-1. The interaction of H2O with 4EBzN marginally alters the appearance of the spectrum, which can be ascribed to the slight changes in the zero order frequencies, which in turn affect the position and the intensities of the Fermi resonance bands, in a minor fashion. The FDIR spectra of W2 and W3 complexes, shown in Figure 2, spectra C and D, respectively, have very similar appearance, with marginal changes in the band positions and the intensities. To understand the nature of interaction of water with 4EBzN, FDIR spectra were also recorded in the O-H stretching region. The FDIR spectrum of the W1 complex, depicted in Figure 3A, shows two intense transitions at 3624 and 3733 cm-1. Comparison of this spectrum with that of other hydrogen-bonded complexes of water suggests that the bands appearing at 3624 and 3733 cm-1 can be assigned to the hydrogen-bonded and free O-H oscillators of water moiety in the W1 complex, respectively. Shown in Figure 3B is the spectrum of the W1
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Figure 4. FDIR spectra of 4EBzN-(H2O)2 in the O-H stretching region. Traces A and B were recorded with LiNbO3 and KTP OPOs, respectively. The remaining three traces represent the simulated vibrational spectra in the O-H for various calculated structures. The transition marked with an asterisk (/) is assigned to the combination band.
complex recorded under identical conditions, but with the KTP OPO. It is evident from the comparison of the two spectra that the bandwidth of KTP OPO is around three times larger than that of the LiNbO3 OPO. Figure 4A shows the FDIR spectra of the W2 complex recorded with LiNbO3 OPO, which shows three prominent transitions at 3557, 3720, and 3725 cm-1. The two bands appearing at 3720 and 3725 cm-1 can be assigned to the free OsH oscillators, rather straightforwardly. This indicates the presence of two water molecules in the W2 complex. However, only one band at 3557 cm-1 can be seen in the hydrogen-bonded region. Figure 4B also shows the FDIR spectra of the W2 complex recorded with KTP OPO. The two bands corresponding to the two free OsH oscillators coalesce and appear as a single band, which can be attributed to the larger bandwidth of KTP OPO. On the other hand, an additional band can be seen at 3501 cm-1, which could not be observed in the spectrum recorded with the LiNbO3 OPO due to a hole in the tuning curve in this energy region. The bands at 3501 and 3557 cm-1 can be assigned as two hydrogen-bonded OsH oscillators of the two water molecules in the 4EBzN-(H2O)2 complex. The FDIR spectra of the W3 complex are shown in Figure 5. Trace A was recorded with LiNbO3 OPO while trace B was recorded with KTP OPO. A congested set of transitions appear around 3720 cm-1, which can be assigned to the free OsH stretching vibrations of water molecules. Additionally, three bands observed at 3429, 3477, and 3512 cm-1 (see Figure 5B) can be assigned to the hydrogen-bonded OsH stretching vibrations present of the three water molecules of the 4EBzN(H2O)3 complex. Further, the FDIR spectra of 4EBzN-(H2O)2 and 4EBzN-(H2O)3 complexes also show the presence of a combination of bands (marked with an asterisk). Many examples exist in the literature wherein combination bands have been observed in the IR spectra of moderate to strong hydrogenbonded complexes.15 IR-UV hole-burning spectroscopy was carried out to ascertain the origin of several transitions appearing in the fluorescence excitation spectrum of 4EBzN in the presence of water and the results are presented in Figure 6. Trace A shows the fluorescence excitation spectrum of 4EBzN in the presence of water, which is identical to the spectrum shown in Figure 1B. Trace B shows
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Figure 5. FDIR spectra of 4EBN-(H2O)3 in the O-H stretching region. Traces A and B were recorded with LiNbO3 and KTP OPOs, respectively. The remaining three traces represent the simulated vibrational spectra in the O-H for various calculated structures. The bands marked with an asterisk are combination bands. The transitions marked with “/” are assigned to the combination bands and the transition marked with “#” is the acetylenic C-H stretching vibration of the complex. The gap in A is due to the hole in the tuning curve of LiNbO3 OPO.
Figure 6. (A) Fluorescence excitation spectrum of 4EBzN in the presence of water. IR-UV hole-burnt recorded by pumping the O-H stretching vibration with the IR laser fixed at (B) 3624 and (C) 3557 cm-1, prior to the exciting electronic excitation. The arrows point to the bands with reduced intensities in the hole-burnt spectrum. The series W1 and W2 correspond to 4EBzN-H2O and 4EBzN-(H2O)2 complexes, respectively.
the IR-UV hole-burnt spectrum, which was recorded by tuning the IR laser to pump the OsH vibrational transition of the W1 complex at 3624 cm-1. The transitions arising out of the W1 complex show diminished intensities, which are indicated by arrows. On the other hand, trace C shows the IR-UV hole-burnt spectrum, which was recorded by tuning the IR laser frequency fixed at the OsH stretching vibration of the W2 complex at 3557 cm-1, and the transitions showing diminished intensities are marked with arrows. Taking into account the transitions of bare 4EBzN along with W1 and W2 complexes, all the transitions except the one marked with W3 at 35354 cm-1 have been accounted for by using the hole-burning spectroscopy.
Maity and Patwari
Figure 7. Structures of 4EBzN-H2O complexes calculated at the MPW3LYP/aug-cc-pVDZ level. Stabilization energies (kJ mol-1) are shown in parentheses.
B. Structures. The geometry optimization calculations with the MPW3LYP/aug-cc-pVDZ level of theory converged onto five minima on the potential energy hypersurface for the 4EBzNH2O complex, structures of which are depicted in Figure 7. In the first structure (EBzNW-A) both 4EBzN and water molecules act as donor and acceptor. In this case one of the OsH groups of the water moiety acts as a donor to the CtN group, while the CsH group in the ortho position interacts with the O atom of water, leading to the formation of a quasiplanar cyclic complex. The second structure (EBzNW-B) is a linear OsH · · · N hydrogen-bonded complex involving the terminal N atom of the CtN group. The third complex (EBzNW-C) is once again a quasiplanar cyclic structure, similar to EBzNW-A, involving the acetylenic CtC. The fourth structure (EBzNW-D) is characterized by the presence of the CsH · · · O hydrogen bond, wherein the terminal acetylenic hydrogen acts as a hydrogen bond donor to the O atom of the water molecule. In the last structure (EBzNW-D) the OsH group of the water molecule interacts with the π electron density of the benzene ring, leading to the formation of the OsH · · · π hydrogen-bonded complex. The ZPE and BSSE corrected stabilization energies of various structures are listed in Table 1. On the energetic front, EBzNW-A is the most stable structure, while EBzNW-E is the least stable one. The FDIR spectra of W2 and W3 complexes in the acetylenic CsH stretching region reveal that the water molecules show almost no interaction with the -CtCsH group (see Figure 2), and also clearly indicate that in these complexes water molecules interact with the nitrile (CtN) group. Starting with the EBzNW-A structure, a second water molecule was added at various orientations and several initial structures of 4EBzN(H2O)2 complexes were generated. These initial structures were then optimized with the MPW3LYP/aug-cc-pVDZ level of theory followed by vibrational frequency calculation. Similarly, several initial structures of 4EBzN-(H2O)3 complexes were generated and optimized by adding two water molecules to the EBzNW-A structure. The optimized structures of 4EBzN-
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TABLE 1: Stabilization Energies (kJ mol-1), Scaled Vibrational Frequencies and their Shifts for Various 4EBzN-H2O Complexes Calculated at MPW3LYP/aug-cc-pVDZ Level of Theorya-c -∆E0 4EBzN H 2O EBzNW-A EBzNW-B EBzNW-C EBzNW-D EBzNW-E experiment (W1)
νCH
∆νCH
ν1
ν3
∆ν1
∆ν3
∑(∆νO-H)
-46 -85 -45 +1 -14 -33
-19 -29 -22 -1 -15 -23
-65 -114 -67 0 -29 -56
3347 (108) 16.0 15.6 9.2 11.1 2.5
14.8 14.7 7.8 10.1 1.8
13.5 13.8 6.5 9.1 1.0
3346 3346 3341 3268 3347 3332
(108) (111) (122) (471) (107)
-1 -1 -6 -79 0 +1
3652 3606 3567 3607 3653 3639 3624
(4) (76) (602) (91) (14) (31)
3758 3739 3729 3736 3757 3743 3733
(62) (111) (167) (135) (76) (123)
a 0%, 50%, and 100% BSSE corrected stabilization energies. b The calculated intensities (km mol-1) are shown in parentheses. experimental acetylenic C-H stretching frequency corresponds to the weighted average model.
Figure 8. Structures of 4EBzN-(H2O)2 complexes calculated at the MPW3LYP/aug-cc-pVDZ level. Stabilization energies (kJ mol-1) are shown in parentheses.
(H2O)2 and 4EBzN-(H2O)3 complexes are shown in Figures 8 and 9, respectively, while Tables 2 and 3 list their stabilization energies. In the case of 4EBzN-(H2O)2 complexes, the first structure (EBzNW2-A) is a cyclic structure, wherein both the water molecules act as donor as well as acceptor. One of the water molecules is a hydrogen bond donor to the N end of the CtN group and simultaneously is a hydrogen bond acceptor to the second water molecule. The second water molecule is a hydrogen bond acceptor for the aromatic CsH group in the ortho position to the CtN group and a hydrogen bond donor to the first water molecule. In this structure the two water molecules bridge the acceptor and donor atoms of 4EBzN though the hydrogen-bonding network. The second structure (EBzNW2-B) is a symmetrically double sided quasiplanar
c
The
Figure 9. Structures of 4EBzN-(H2O)3 complexes calculated at the MPW3LYP/aug-cc-pVDZ level. Stabilization energies (kJ mol-1) are shown in parentheses.
structure wherein two water molecules are hydrogen bonded on either side of the CtN group. In the third structure (EBzNW2-C) one of the OsH groups of water molecule interacts with the CtN group, while the CsH group in the ortho position interacts with the O atom of water. The second water molecule acts as a hydrogen bond acceptor to the second OsH group of first water molecule. In EBzNW2-C one of the water molecules is rendered in ADD hydrogen-bonding configuration. Energetically, EBzNW2-A is the most stable complex followed by EBzNW2-C and EBzNW2-B. The higher stabilization energy for the EBzNW2-A complex can be attributed to the presence of water-water hydrogen bonding along with the cooperative nature of the hydrogen-bonding bridges. In the case
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TABLE 2: Stabilization Energies (kJ mol-1) and Scaled Vibrational Frequencies for Various 4EBzN-(H2O)2 Complexes Calculated at MPW3LYP/aug-cc-pVDZ Level of Theorya-c -∆E0 EBzNW2-A EBzNW2-B EBzNW2-C experiment (W2)
47.5 30.9 30.3
45.5 28.3 28.5
43.5 25.8 26.6
νCH
ν1
ν2
ν3
ν4
3347 (110) 3348 (108) 3347 (109) 3332
3401 (387) 3610 (128) 3535 (383) 3502
3487 (628) 3612 (6) 3648 (19) 3557
3719 (129) 3739 (194) 3680 (269) 3720
3723 (58) 3740 (34) 3752 (82) 3725
a 0%, 50%, and 100%BSSE corrected stabilization energies. b The calculated intensities (km mol-1) are shown in parentheses. experimental acetylenic C-H stretching frequency corresponds to the weighted average model.
c
The
TABLE 3: Stabilization Energies (kJ mol-1) and Scaled Vibrational Frequencies for Various 4EBzN-(H2O)3 Complexes Calculated at MPW3LYP/aug-cc-pVDZ Level of Theorya-c -∆E0 EBzNW3-A EBzNW3-B EBzNW3-C experiment (W3)
76.9 61.4 69.3
73.8 58.1 66.2
70.6 54.8 63.1
νCH
ν1
ν2
ν3
ν4
ν5
ν6
3348 (119) 3348 (111) 3348 (111) 3333
3324 (668) 3414 (387) 3210 (486) 3430
3382 (706) 3505 (581) 3466 (317) 3478
3438 (1009) 3619 (54) 3519 (175) 3512
3720 (66) 3719 (136) 3596 (506) 3717
3722 (52) 3725 (61) 3710 (62) 3720
3731 (149) 3743 (114) 3728 (76) 3723
a 0%, 50%, and 100% BSSE corrected stabilization energies. b The calculated intensities (km mol-1) are shown in parentheses. experimental acetylenic C-H stretching frequency corresponds to the weighted average model.
of 4EBzN-(H2O)3 complexes the first structure, EBzNW3-A, is characterized by formation of a hydrogen-bonding bridge between the N end of the CtN group and the aromatic CsH group in the ortho position to the CtN group. In this structure three water molecules bridge the acceptor and donor atoms of 4EBzN though the hydrogen-bonding network. This structure is an expanded version of EBzNW2-A. The second structure, EBzNW3-B, is reminiscent of EBzNW2-A with the third water molecule being added on the other side of the CtN group. Alternatively, EBzNW3-B can be viewed as inserting a bridging water molecule on one side of the EBzNW2-B structure. The third structure EBzNW3-C has an interesting intermolecular configuration, wherein the three water molecules form a cyclic water trimer and the 4EBzN is incorporated at two vertices via hydrogen bonding to the CtN group and the aromatic CsH group in the ortho position to the CtN group. For this structure both 4EBzN and the water trimer unit act as both hydrogen bond donor and acceptor. C. Structural Assignment. Infrared spectroscopy is an excellent tool to establish the intermolecular structures, as it probes the local interaction present in molecules and molecular complexes. The vibrational spectra in the hydride stretching region, viz., OsH, NsH, and CsH groups, offer the advantage of inferring the hydrogen-bonded structure. 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 hydrogen-bonded complexes. The simulated IR spectra of various possible structures were compared with the experimental spectra, and the agreement between the simulated and observed vibrational spectra served as a benchmark for the structural assignment of various complexes. In the case of phenylacetylene the FDIR spectrum in the acetylenic C-H stretching region shows three prominent transitions at 3318, 3325, and 3343 cm-1 accompanied by several weaker transitions.13 Fermi resonance transitions in phenylacetylene originate from the coupling of acetylenic CsH oscillator with one quantum of CtC stretch and two quanta of CsH out-of-plane bend. Additionally several weak bands observed in the case of phenylacetylene were assigned to be arising out of higher order coupling terms.14 We had earlier reported deperturbation analysis for PHA using a two-state Fermi resonance model for the two strong transitions observed at 3325 and 3347 cm-1.3,8 However, in view of the observed
c
The
complexity of the FDIR spectrum of 4EBzN, similar to fluorophenylacetylenes,5 the two-state Fermi resonance model may not be completely appropriate. Since the primary goal of this work is to elucidate the structures of the water complexes on the basis of spectral signatures observed in the IR spectra, therefore we use a relatively simple weighted-average model to analyze the IR spectra in the acetylenic CsH stretching region.5 The FDIR spectrum of 4EBzN in the acetylenic CsH stretching region, depicted in Figure 2A, is very similar to that observed for phenylacetylene and shows three prominent bands at 3326, 3329, and 3338 cm-1. The weighted average model places the unperturbed CsH oscillator at 3331 cm-1. The FDIR spectrum of W1 (Figure 2B) shows two slightly broader bands at 3328 and 3337 cm-1. In this case the weighted average model places the unperturbed acetylenic CsH oscillator at 3332 cm-1. Thus the acetylenic CsH stretching vibration of 4EBzN shifts by 1 cm-1 to a higher frequency upon interaction with H2O. Since the appearance of Fermi resonance bands in 4EBzN involves CtC and CsH oscillators of the acetylenic moiety, any perturbation of these two oscillators will lead to changes in the characteristics of the IR spectrum in the acetylenic CsH stretching region. Therefore from the appearance of the spectrum as well as analysis with the weighted average model, it is clear that in the W1 complex H2O does not interact with the acetylene moiety of 4EBzN. Similarly for the W2 and W3 complexes, the weighted average model places the unperturbed acetylenic CsH oscillator for both the complexes at 3333 cm-1. The appearance of the FDIR spectra of the water complexes of 4EBzN in the acetylenic CsH stretching region is drastically different from that of the corresponding spectra of water complexes with phenylacetylene and 2- and 4-fluorophenylacetylenes.3-5 These spectra clearly indicate the absence of any interaction between the water molecules and the acetylene moiety of 4EBzN. The FDIR spectra of the 4EBzN-H2O complex in the O-H stretching is presented in Figure 3 along with the simulated spectra for various isomers of the 4EBzN-H2O complex and Table 1 lists all the vibrational frequencies along with their shifts. The calculated spectrum of EBzNW-A matches well in both terms of band positions and intensities with the experimental spectrum. 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, corresponding to symmetric (ν1) and antisymmetric (ν3) stretching vibrations, respectively. In the event of hydrogen bond formation to one
Hydrogen Bonding to Multifunctional Molecules of the OsH groups of the water moiety, the two frequencies will now correspond to the hydrogen-bonded and free OsH stretching vibrations. Though only one of the OsH groups is involved in hydrogen bond formation, both stretching frequencies are lowered due to partial decoupling of the two OsH oscillators. The vibrational frequencies corresponding to the hydrogen-bonded and free OsH stretching vibrations are lower than the symmetric and antisymmetric stretching vibrations, respectively. Since both OsH stretching frequencies are lowered due to hydrogen bond formation, we have used the total shift in the OsH stretching frequencies [∑(∆ν) ) (∆ν1 + ∆ν3)] as a tool to assign the intermolecular structures. Further, it must also be pointed out that in the present case the scaling factor was based on the OsH stretching vibrations on the free water molecule. Therefore it is expected that the agreement between the calculated and experimental free OsH stretching vibrations would be better than that in the case of hydrogen-bonded OsH stretching vibrations. In the case of 4EBzN-(H2O)2 and 4EBzN-(H2O)3 complexes, the present level of calculations overestimates the H2O-H2O interaction. Similar observations were made for the 2-fluorophenylacetylene-(H2O)2 complexes at the same level of theory.5 The two OsH stretching transitions in the FDIR spectra of 4EBzN-H2O complex appear at 3624 and 3733 cm-1 representing a total shift in the OsH stretching frequencies [∑(∆ν)] of 56 cm-1. From Table 1 it can be seen that the EBzNW-A structure shows ∑(∆ν) 65 cm-1, which is in good agreement with the experimental value. Additionally the EBzNW-A structure is the global minimum for the 4FPHA-H2O complex. Thus, the IR spectra in the acetylenic CsH stretching region and in the OsH stretching region and the stabilization energy clearly favors the formation of a quasiplanar cyclic complex EBzNW-A, which is characterized by the presence of hydrogen bonding between the OsH group of the water molecule with the π electron density of the CtN group along with the hydrogen bond between the CsH group of benzene ring in the ortho position with the oxygen of the water moiety. The intermolecular structure of the 4EBzN-H2O complex is similar to that of the benzonitrile-H2O complex.16,17 The comparison of simulated and experimental spectra for the 4EBzN-(H2O)2 complex in the OsH stretching region is presented in Figure 4 and Table 2 lists all the calculated vibrational frequencies. The FDIR spectra of the 4EBzN-(H2O)2 complex shows four distinct transitions at 3502, 3557, 3720, and 3725 cm-1. The first two bands can be assigned to hydrogenbonded OsH stretching vibrations, while the later two can be assigned to free OsH stretching vibrations. The EBzNW2-B structure can be considered as a double sided EBzNW-A structure, and the spectrum would consist of near doubly degenerate free and hydrogen-bonded OsH stretching frequencies (see Figure 4 and Table 2), which will lead to only two transitions in the spectrum and is different in appearance from the observed FDIR spectrum. Further, in the case of EBzNW2-C only one transition will appear in the free OsH stretching region, which once again is not in agreement with the observed spectrum. On the other hand the structure EBzNW2-A will consist of two hydrogen-bonded OsH stretching vibrations and two free OsH stretching vibrations. Therefore the observed 4EBzN-(H2O)2 complex can be assigned to the EBzNW2-A, which also happens to the energetically most favorable structure. Finally, the comparison of simulated and experimental spectra for the 4EBzN-(H2O)3 complex in the OsH stretching is presented in Figure 5, while Table 3 lists all the calculated vibrational frequencies. The FDIR spectra of the 4EBzN-(H2O)3 complex show several transitions, prominent being a bunch
J. Phys. Chem. A, Vol. 114, No. 32, 2010 8343 around 3720 cm-1 and three broad transitions at 3430, 3478, and 3512 cm-1. Once again, for the 4EBzN-(H2O)3 complex, the observed bands can be split into two sets of free and hydrogen-bonded OsH stretching vibrations. The structure EBzNW3-B is an overlay of structures EBzNW2-A and EBzNW-A, therefore one can expect hydrogen-bonded OsH stretching vibration around 3600 cm-1, corresponding to an interaction of the single H2O molecule on one side of the CtN group, which is absent in the FDIR spectrum, ruling out the possibility of the EBzNW3-B structure. On the other hand, the EBzNW3-C structure has two free and four hydrogen-bonded OsH groups. However, only three bands in the hydrogenbonded O-H stretching region can be seen in the FDIR spectrum, ruling out EBzNW3-C as a probable structure as well. The EBzNW3-A structure shows three bands in the hydrogenbonded OsH stretching region along with the crowding of the free OsH stretching vibrations around 3720 cm-1. These features are in qualitative agreement with the FDIR spectra of the 4EBzN-(H2O)3 complex. Further, EBzNW3-A is also the global minimum for the 4EBzN-(H2O)3 complexes. Therefore the EBzNW3-A is assigned as the structure responsible for the FDIR spectra shown in Figure 5. The structures of the 4EBzN-(H2O)1-3 complexes are similar to those of the corresponding benzonitrile-(H2O)1-3 complexes.16 This implies that the substitution by the CtN group on phenylacetylene, unlike fluorine, alters the structures of the water complexes. However, comparison of the present FDIR spectra with the corresponding FDIR spectra of benzonitrile complexes suggests that the hydrogen bonding in the present complexes is marginally weaker. This can be attributed to the lowering of the hydrogen bond accepting ability of the CtN group due to the presence of weakly electron withdrawing nature of the acetylenic group present in the para position. Further, it can be inferred that the CtN group has the higher propensity to form hydrogen bonds relative to the acetylenic group and the benzene ring in 4EBzN. Conclusions In summary, we have investigated water complexes of 4EBzN using IR-UV double resonance spectroscopic technique in combination with the DFT-MPWB3LYP/aug-cc-pVDZ method. The band origin transition for the S1 r S0 electronic excitation of 4EBzN occurs at 35212 cm-1. The band origin transitions for the W1 and W2 complexes are shifted to the red by 46 and 94 cm-1, respectively. The FDIR spectrum of 4EBzN shows the presence of Fermi resonance bands similar to those of phenylacetylene. Interaction water molecules in W1, W2, and W3 complexes alter Fermi resonance bands very marginally, which indicates that in these complexes water does not interact with the acetylenic moiety in 4EBzN. The FDIR spectrum of the W1 complex shows two transitions corresponding to hydrogen-bonded and free OsH stretching vibrations. The structure of the W1 complex incorporates OsH · · · π (π electron density CtN group) and CsH · · · O (CsH group of the benzene ring in the ortho position) interactions leading to the formation of a quasiplanar cyclic complex. The FDIR spectra of the W2 complex show two bands each in the hydrogen-bonded and free O-H stretching regions, while the FDIR spectra of the W3 complex show three bands each in the hydrogen-bonded and free OsH stretching regions. The analysis of the FDIR spectra with the aid of theoretical calculations leads to assignment of the hydrogen-bonded bridged structures for the W2 and W3 complexes, wherein the water molecules bridge the acceptor (N atom of the CtN) and donor (CsH on the benzene ring in
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the ortho position) groups of 4EBzN. The structures of W1, W2, and W3 complexes are similar to the corresponding water complexes of benzonitrile. Acknowledgment. S.M. thanks 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 the Department of Science and Technology (Grant No. SR/ S1/PC/23/2008) and the 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) (a) Etter, M. C. Acc. Chem. Res. 1990, 23, 120. (b) Etter, M. C. J. Phys. Chem. 1991, 95, 4601. (2) Legon, A. C.; Millen, D. J. Chem. Soc. ReV. 1987, 16, 467. (3) (a) Singh, P. C.; Bandyopadhyay, B.; Patwari, G. N. J. Phys. Chem. A 2008, 112, 3360. (b) Singh, P. C.; Patwari, G. N. J. Phys. Chem. A 2008, 112, 4426. (c) Singh, P. C.; Patwari, G. N. J. Phys. Chem. A 2008, 112, 5121. (d) Maity, S.; Sedlak, R.; Hobza, P.; Patwari, G. N. Phys. Chem. Chem. Phys. 2009, 11, 9738. (4) Sedlak, R.; Hobza, P.; Patwari, G. N. J. Phys. Chem. A 2009, 113, 6220. (5) Maity, S.; Patwari, G. N. J. Phys. Chem. A 2009, 113, 1760. (6) (a) Barth, H. D.; Buchhold, K.; Djafari, S.; Reimann, B.; Lommatzsch, U.; Brutschy, B. Chem. Phys. 1998, 239, 49. (b) Brutschy, B. Chem. ReV. 2000, 100, 3891. (7) Singh, P. C.; Maity, S.; Patwari, G. N. J. Phys. Chem. A 2008, 112, 9702. (8) Singh, P. C.; Patwari, G. N. Curr. Sci. 2008, 95, 469.
Maity and Patwari (9) Page, R. H.; Shen, Y. R.; Lee, Y. T. J. Chem. Phys. 1988, 88, 4621. (10) 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.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision A.1; Gaussian, Inc.: Wallingford, CT, 2003. (11) Kim, S. K.; Tarakeshwar, P.; Lee, J. Y. Chem. ReV. 2000, 100, 4145. (12) Molden: a pre- and post-processing program for molecular and electronic structures: Schaftenaar, G., Noordik, J. H. J. Comput.-Aided Mol. Des. 2000, 14, 123. (13) Nimlos, M. R.; Kelley, D. F.; Bernstein, E. R. J. Phys. Chem. 1989, 93, 643. (14) Stearns, J. A.; Zwier, T. S. J. Phys. Chem. A 2003, 107, 10717. (15) (a) Patwari, G. N.; Ebata, T.; Mikami, N. J. Chem. Phys. 2002, 116, 6056. (b) Carney, J. R.; Zwier, T. S. J. Phys. Chem. A 1999, 103, 9943. (16) Ishikawa, S.; Ebata, T.; Mikami, N. J. Chem. Phys. 1999, 110, 9504. (17) Helm, R. M.; Vogel, H.-P.; Neusser, H. J.; Storm, V.; Consalvo, D.; Dreizler, H. Z. Naturforsch., A: Phys. Sci. 1997, 52, 655.
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