A Combined Spectroscopic and ab Initio Investigation of

Aug 26, 2010 - This article is part of the A: Klaus Muller-Dethlefs Festschrift special issue ... Double Resonance Spectroscopic and Computational Inv...
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J. Phys. Chem. A 2010, 114, 11347–11352

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A Combined Spectroscopic and ab Initio Investigation of Phenylacetylene-Methylamine Complex. Observation of σ and π Type Hydrogen-Bonded Configurations and Fluorescence Quenching by Weak C-H · · · N Hydrogen Bonding† Surajit Maity, Arghya Dey, and G. Naresh Patwari* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, 400076 India

S. Karthikeyan and Kwang S. Kim* Center for Superfunctional Materials, Department of Chemistry, Pohang UniVersity of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Korea ReceiVed: June 14, 2010; ReVised Manuscript ReceiVed: August 2, 2010

Two distinct isomers for the binary complex between phenylacetylene and methylamine were observed. The first complex is characterized by the presence of a C-H · · · N hydrogen bond between the acetylenic C-H group and the N atom of methylamine. In the second complex the N-H group of methylamine interacts with the π electron density of the benzene ring accompanied by a peripheral interaction between the methyl C-H group and the π electron density of the CtC bond. Stabilization energies and Gibbs free energies at the complete basis set (CBS) limit of the coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)] suggest that while the C-H · · · N hydrogen bonded complex is the global minimum, the N-H · · · π hydrogen bonded complex is a high energy local minimum. The formation of the N-H · · · π complex could be related to kinetic trapping or higher accessibility. Comparison of the laser induced fluorescence (LIF) excitation and the one-color-resonant two-photon ionization (1C-R2PI) spectra suggests that formation of C-H · · · N hydrogen bonding leads to fluorescence quenching in phenylacetylene, most probably due to dipolar coupling in the excited state. The binary complex between the phenylacetylene and methylamine shows interesting isomer-dependent fluorescent properties. Introduction Hydrogen bonding is ubiquitous in nature and plays a pivotal role in simple acid-base chemistry to convoluted pathways in biology.1,2 Hydrogen bonding in simple (hydrogen bond) donor and (hydrogen bond) acceptor complexes is well understood. In many cases free energy relationships between the donor and acceptor properties have also been reported.3 However, understanding the hydrogen bonding pattern in multifunctional molecules, wherein several energetically closely spaced minima can be found on the potential energy hypersurface, can be a challenging proposition. Over the past couple of years, we have investigated several hydrogen bonded complexes of phenylacetylene, which form a wide variety of intermolecular structures.4 Phenylacetylene was chosen because it has multiple hydrogen bonding sites, which cannot be classified in a hierarchical pattern, according to the Etter or Legon-Millen rules.5 Infrared spectroscopic investigations on complexes with water and methanol have indicated that the intermolecular structure is different for the two complexes. Such differences in the intermolecular structures of water and methanol complexes have not been reported for any other molecule. This implies that the intermolecular potentials are very sensitive to the nature of the approaching molecule and respond to even subtle changes such as substitution by the ubiquitous methyl group. We had earlier investigated the hydrogen-bonded complexes of ammonia and methylamine with phenylacetylene.4b †

Part of the “Klaus Mu¨ller-Dethlefs Festschrift”. * Authors to whom correspondence should be addressed. E-mail: [email protected] (G.N.P.); [email protected] (K.S.K).

These investigations reveal that ammonia forms a C-H · · · N hydrogen-bonded complex, while methylamine forms a N-H · · · π hydrogen-bonded complex. High level theoretical calculations at CCSD(T)/aug-cc-pVDZ indicate that the C-H · · · N hydrogenbonded complex is the global minimum for both ammonia and methylamine complexes.4d The observation of the N-H · · · π hydrogen-bonded complex in the case of methylamine could be related to the kinetic trapping of the high energy minimum. These calculations also indicate that for the methylamine complex the energy of the N-H · · · π hydrogen-bonded complex was higher by about 5 kJ mol-1 than that of the C-H · · · N hydrogen-bonded complex. The experimental observations were very surprising because only a high energy local minimum was observed and the global minimum could not be found. In this article we report the investigations on the methylamine complex to observe and characterize the global minimum. Experimental and Computational Methods The details of the experimental setup have been described elsewhere.6 Briefly, helium buffer gas at 4 atm was bubbled through phenylacetylene (Aldrich) followed by addition of a 50% aqueous solution of methylamine (Aldrich) kept at room temperature, and expanded through a 0.5 mm diameter pulsed nozzle (Series 9, Iota One; General Valve Corporation). Electronic excitation was achieved using a frequency doubled output of a tunable dye laser (Narrow Scan GR; Radiant Dyes) operating with Rhodamine-19 dye, pumped with the second harmonic of a Nd:YAG laser (Surelite I-10; Continuum). The LIF excitation spectra were recorded by monitoring the total fluorescence with a photomultiplier tube (9780SB+1252-5F;

10.1021/jp105439y  2010 American Chemical Society Published on Web 08/26/2010

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Electron Tubes Limited) and a filter (WG-320) combination, while the 1C-R2PI spectra were recorded by monitoring the appropriate mass signal with time-of-flight mass spectrometer using a channeltron (KBL-25RS; Sjuts) detector. The IR spectra were obtained using the ion dip infrared (IDIR) spectroscopic method.7 Further, in order to separate out the transitions belonging to various species present in the IC-R2PI spectrum, IR-UV hole-burning spectra were also recorded. The procedure involves an IR pulse tuned to a vibrational transition of a specific species of interest, while a delayed tunable UV laser probes the S1rS0 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 signal intensity decreases when compared to the IC-R2PI spectrum. The lowering of the intensity in the hole-burnt spectrum relative to the IC-R2PI spectrum allows identification of relevant transitions. In our experiments the source of tunable IR light was an idler component of a LiNbO3 OPO (Custom IR OPO; Euroscan Instruments) pumped with an injection-seeded Nd:YAG laser (Brilliant-B; Quantel). The typical bandwidth of both UV and IR lasers is about 1 cm-1, and the absolute frequency calibration is within (2 cm-1. A detailed conformational search was followed by a complete geometry optimization at the DFT/M06-2X and MP2 levels of theory using the aug-cc-pVDZ (aVDZ) basis set. In the case of DFT-D/M06-2X/aVDZ,8 the calculations were performed with ultrafine grids. Frequency calculations were carried out for several low energy isomers at the DFT/M06-2X/aVDZ and MP2/aVDZ levels. Thermochemical analysis was performed based on rigid rotor, harmonic oscillator, and ideal gas approximations. We also carried out the CCSD(T)/aVDZ single point energy calculations on the MP2/aVDZ optimized geometries. The basis set superposition error (BSSE) correction was made after geometry optimization. By carrying out MP2 calculations using the aug-cc-pVTZ (aVTZ) basis set, we estimated the MP2/CBS and CCSD(T)/CBS binding energies for the selected stable isomers with the extrapolation scheme utilizing an electron correlation error proportional to N-3 for the aug-cc-pVNZ basis set.9,10 The CCSD(T)/CBS energies were estimated by assuming that the difference in binding energies between MP2/aVDZ and MP2/CBS calculations is similar to that between CCSD(T)/aVDZ and CCSD(T)/CBS calculations.10,11 All the calculations reported in this article were performed using the GAUSSIAN-09 suite of programs.12 Results and Discussion The LIF excitation spectrum of phenylacetylene in the presence of methylamine is depicted in Figure 1A. This spectrum is identical to the one we had reported earlier.4b The most intense transition at 35876 cm-1, marked with an asterisk, is the band-origin for the S1rS0 transition of bare phenylacetylene. The transition at 35865 cm-1, marked M1, appears only in the presence of methylamine and was assigned to the bandorigin transition of the binary complex between phenylacetylene and methylamine. The 1C-R2PI spectrum of the phenylacetylene-methylamine complex recorded by monitoring the signal at 133 amu is shown in Figure 1B. This spectrum shows three intense transitions at 35788, 35865, and 35958 cm-1 and a weak transition at 35900 cm-1. The transition at 35865 cm-1, marked with M1, can be found in both the LIF excitation and the 1CR2PI spectra. Surprisingly, however, the three transitions at 35788, 35900, and 35958 cm-1, marked with M2, can only be seen in the 1C-R2PI spectrum. Repeated attempts to observe the three transitions corresponding M2 in the LIF excitation spectrum, by varying experimental conditions, did not yield

Maity et al.

Figure 1. (A) LIF excitation spectrum of phenylacetylene in the presence of methylamine. (B) 1C-R2PI spectrum of phenylacetylenemethylamine complex recorded by monitoring the mass signal at 133 amu. In A the most intense peak marked with an asterisk corresponds to the band-origin of bare phenylacetylene and the second transition marked with an asterisk is a vibronic band bare phenylacetylene. The peaks marked with M1 and M2 correspond to two different binary complexes between methylamine and phenylacetylene.

Figure 2. IDIR spectra of (A) phenylacetylene, (B) phenylacetylenemethylamine complex M1, and (C) phenylacetylene-methylamine complex M2. In A the arrow indicates the position of the unperturbed acetylenic C-H oscillator of bare phenylacetylene.

desired results. The comparison of LIF excitation and 1C-R2PI spectra reveals that the fluorescence quantum yield for the M2 complex is much lower than the detection threshold of our apparatus. Further, due to a drastic difference in the fluorescence quantum yields of M1 and M2 complexes, it can be inferred that the mode of interaction of methylamine with phenylacetylene in these two complexes are dissimilar. IDIR spectra were recorded in the acetylenic C-H stretching region, monitoring the ion signal following the R2PI process for bare phenylacetylene and the two complexes, and the results are presented in Figure 2. The IDIR spectrum of bare phenylacetylene, depicted in Figure 2A, shows two intense bands at 3326 and 3343 cm-1 accompanied by several other transitions. The appearance of multiple bands in the acetylenic C-H stretching region for bare phenylacetylene has been attributed to the Fermi resonance and other higher order couplings.13,14 In

Phenylacetylene-Methylamine Complex this case the two intense peaks originate from the Fermi resonance coupling between the acetylenic C-H stretching vibration and the combination band of one quantum of CtC stretch and two quanta of C-H out-of-plane bend. A simple two-state deperturbation analysis places the unperturbed acetylenic C-H oscillator at 3334 cm-1 with the magnitude of coupling constant to be 9 cm-1.6 In the case of phenylacetylene complexes any interaction that will perturb either the acetylenic C-H oscillator or the CtC oscillator or both will lead to disappearance of Fermi resonance transitions. However, the perturbation should be about the order of the coupling constant of 9 cm-1 or more, in order to completely remove the Fermi mixing. The presence and absence of Fermi resonance transitions of the phenylacetylene moiety, therefore, can be used a spectroscopic tool to probe the interactions present in various complexes of phenylacetylene. The IDIR spectrum of M1 complex, Figure 2B, shows a single band at 3333 cm-1. This spectrum is identical to the fluorescence dip infrared (FDIR) spectrum that we had reported earlier for the M1 complex. Further, the IDIR spectrum of the M2 complex once again shows a single band at 3195 cm-1. Clearly, both spectra of M1 and M2 complexes show single bands, which imply that the interaction of methylamine with phenylacetylene leads to loss of Fermi resonance coupling. The most important distinction between the M1 and M2 complexes however is the position of the acetylenic C-H stretching vibration. In the case of M1 complex the acetylenic C-H stretching vibration is shifted to a lower frequency by just 1 cm-1 relative to the unperturbed C-H oscillator of the bare phenylacetylene. This clearly rules out the formation of and C-H · · · N hydrogen-bonded complex involving the terminal C-H group of acetylene. On the other hand, the disappearance of Fermi mixing and the marginal shift in the acetylenic C-H stretching frequency indicates that methylamine interacts with the π electron density of the CtC bond of the acetylenic moiety in phenylacetylene. We had earlier assigned the structure of the M1 complex to a N-H · · · π (benzene π) hydrogen-bonded complex which also incorporates a peripheral interaction between the methyl C-H group and the acetylenic CtC bond. This peripheral interaction is responsible for a loss of Fermi resonance coupling. On the other hand, appearance of the acetylenic C-H stretching vibration at 3195 cm-1, which is shifted to a lower frequency by 139 cm-1, for the M2 complex clearly suggests the formation of a C-H · · · N hydrogen-bonded complex. IR-UV hole burning spectroscopy was carried out to investigate the origin of several peaks appearing in the 1C-R2PI spectrum of phenylacetylene-methylamine complex and the results are presented in Figure 3. Trace A is the 1C-R2PI spectrum of phenylacetylene-methylamine complex, which is identical to the spectrum shown in Figure 1B. Trace B is the IR-UV hole-burnt spectrum, which was recorded tuning the IR laser to pump the C-H vibrational transition at 3195 cm-1 (see Figure 2C), 100 ns prior to the R2PI-UV pulse, while scanning the UV laser. The hole-burnt spectrum shows diminished intensities for all the three bands marked M2, while the intensity of the M1 band remains unaffected. These results confirm the presence of two isomers, and the bands at 35788, 35900, and 35958 cm-1 originate from a single isomer. The lowest energy transition at 35788 cm-1 can be assigned to the band-origin of the S1rS0 electronic transition of the M2 methylamine complex, while the other two bands are the vibronic transitions. Geometry optimizations at various levels of theory locate five low-lying energy structures, which are shown in Figure 4. The first structure, PHA-MA-S1, is a C-H · · · N hydrogen-bonded

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Figure 3. (A) 1C-R2PI spectrum of phenylacetylene-methylamine complex recorded by monitoring the mass signal at 133 amu. (B) IRUV hole-burnt spectrum recorded by pumping the C-H stretching vibration of M2 complex by fixing the IR laser at 3195 cm-1. All the peaks marked M2 have reduced intensities (shown by arrows) while the intensity of the peak marked M1 remains unaffected.

Figure 4. Structures of the phenylacetylene-methylamine complex calculated at the MP2/aVDZ level.

complex, wherein the acetylenic C-H group of phenylacetylene interacts with the nitrogen atom of methylamine. In the second structure, PHA-MA-S2, the C-H group of the benzene ring in the ortho position is hydrogen-bonded to the nitrogen atom of methylamine. Additionally, one of the hydrogens on the methyl group interacts with the π acetylenic CtC bond. The third structure PHA-MA-S3 is also characterized by the presence of C-H · · · N hydrogen bonding of the C-H group in the ortho position along with the N-H · · · π (acetylenic π) interaction. The last two structures PHA-MA-S4 and PHA-MA-S5 are characterized by the presence of N-H · · · π and C-H · · · π hydrogen bonding with the π electron density of the benzene ring, respectively. Additionally, the PHA-MA-S4 structure also incorporates a peripheral C-H · · · π interaction with the acetylenic CtC bond, which is conspicuously absent in PHA-MAS5. This can be attributed to the relative orientation of the NH2

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TABLE 1: Electronic Stabilization Energies (kJ/mol) and Acetylenic C-H Stretching Frequencies (cm-1) for Various Low Energy Structures of the Phenylacetylene-Methylamine Complexa M06-2X

MP2

CCSD(T) M06-2X

-∆Ee -∆E0 -∆Ee -∆E0 PHA-MA-S1 PHA-MA-S2 PHA-MA-S3 PHA-MA-S4 PHA-MA-S5

15.3 8.9 12.5 8.8 9.6

11.6 6.0 10.4 7.3 7.2

16.3 11.0 15.3 12.3 12.3

11.7 7.9 11.8 9.7 9.2

MP2

-∆Ee

νC-H

νC-H

15.3 10.1 12.8 8.6 8.5

3188 3336 3333 3335 3335

3165 3334 3330 3332 3332

a The vibrational frequencies are scaled by the scale factor 0.939 for DFT-D/M06-2X/aVDZ and 0.958 for MP2/aVDZ to match the experimental νC-H frequency of phenylacetylene (3335 cm-1 which is the average value of two resonance frequencies, though the more accurate unperturbed peak is at 3334 cm-1).

and CH3 groups in the two structures. The stabilization energies for all the five structures, calculated at various levels are listed in Table 1 along with the acetylenic C-H stretching frequencies. Table 2 lists the energies used in the extrapolation at MP2 level. As discussed earlier, the same extrapolation scheme was also used for the CCSD(T) level. The energies and free energies for the formation at 10 and 100 K computed at the CCSD(T)/CBS levels are also listed in Table 2. In the present case, we have considered two structures to be isoenergetic if the difference in the energies is 0.5 kJ mol-1 or less. PHA-MA-S1 is the energetically most favored in almost all the levels. PHA-MAS3 is the next most stable structure, and at some levels of calculations it is the most stable structure as well, especially at MP2/aVTZ and MP2/CBS levels (see Table 2). This can be attributed to overestimation of dispersion energy by MP2 level. Further, PHA-MA-S3 also includes the interaction with acetylenic CtC bond, which could lead to the loss of Fermi resonance coupling as seen in the IDIR spectrum of M2 complex. However, three structures PHA-MA-S2, PHA-MA-S3, and PHA-MAS4 account for the interaction of methylamine moiety with the acetylenic CtC bond, leading to the disappearance of Fermi mixing. In order to assign the observed structure the IDIR spectrum in the aromatic C-H stretching region was recorded and is presented in Figure 5. The IDIR spectrum of bare phenylacetylene (Figure 5A) shows several transitions in the 3050-3100 cm-1 region and is in good agreement with the spectrum reported earlier.13 This spectrum shows a number of transitions higher than the number of aromatic C-H oscillators present in phenylacetylene, is very similar in appearance to the corresponding spectrum of aniline and toluene, and is indicative of the presence of anharmonic coupling.15 The IDIR spectrum of phenylacetylene-methylamine (M1) complex, depicted in Figure 5 (Trace B), shows fewer transitions. However, for every

Figure 5. IDIR spectra in the aromatic C-H stretching region of pehylacetylene and its complexes with methylamine. Also shown for the sake of comparison are the simulated spectra of phenylacetylene and its various complexes calculated at the DFT-D/M06-2X/aVDZ level of theory.

transition observed for the phenylacetylene-methylamine complex, there is a one-to-one correspondence with the transitions in the spectrum of bare phenylacetylene. Also shown in Figure 5 are the calculated (and simulated) spectra for the sake of comparison. To begin with, the agreement of the calculated and observed spectra of bare phenylacetylene is not satisfactory and can be attributed to the anharmonic coupling, which cannot be reproduced by the harmonic frequency calculations. We had earlier shown in the case of the binary complex of phenylacetylene with 1-azabicyclo[2.2.2]octane, the strongest peak in the aromatic C-H stretching region is around 3040 cm-1, which is an indicator of C-H · · · N hydrogen bonding involving the aromatic C-H groups.4g In the present case, the absence of a strong peak in the lower energy region, around 3040 cm-1, rules out the possibility of PHA-MA-S2 and PHA-MA-S3 as possible

TABLE 2: MP2 and CCSD(T) Binding Energies and Thermodynamic Quantities (kJ mol-1) for Low Energy Structures of the Phenylacetylene-Methylamine Complexa MP2/aVTZ PHA-MA-S1 PHA-MA-S2 PHA-MA-S3 PHA-MA-S4 PHA-MA-S5

CCSD(T)/CBSM (CCSD(T)/CBSD)

MP2/CBS

-∆Ee

-∆E0

-∆Ee

-∆E0

∆Ee

-∆E0

-∆G(10K)

-∆G(100K)

16.7 12.0 17.1 14.1 14.2

12.1 8.9 13.6 11.4 11.1

16.8 12.4 17.8 14.8 15.0

12.3 9.3 14.3 12.2 11.9

15.9 11.5 15.3 11.1 11.2

11.3 (11.6) 8.4 (7.1) 11.8 (10.6) 8.5 (7.1) 8.1 (6.1)

10.7 (11.0) 7.8 (6.5) 11.1 (10.0) 7.8 (6.5) 7.4 (5.4)

1.7 (1.2) -2.4 (-2.1) -0.4 (1.5) -4.9 (-2.8) -4.4 (-4.3)

a The BSSE corrections were made. The CCSD(T)/CBS energies were estimated by applying the correction term (the difference between MP2/aVDZ and CCSD(T)/aVDZ energies) to the MP2/CBS interaction energies which were obtained with the extrapolation scheme utilizing the election correlation error proportional to N-3 for the aug-cc-pVNZ basis set. The CCSD(T)/CBSM values are calculated based on the MP2/ aVDZ thermal energies, while the CCSD(T)/CBSD values are based on the DFT-D/aVDZ thermal energies.

Phenylacetylene-Methylamine Complex structures of the M1 complex. Further, the calculated spectrum of PHA-MA-S3 distinctly shows a set of transitions in the lower energy part of the spectrum, different than those observed in the case of bare phenylacetylene, which are not found the in the experimental spectrum. Additionally, the shift in the electronic transition for the M1 complex (11 cm-1) is smaller than the shift in the electron transition for the 1-azabicyclo[2.2.2]octane complex (31 cm-1).4g Therefore PHA-MA-S4 is the probable structure for the M1 complex. The present assignment is in agreement with our earlier assignment. The two observed complexes of methylamine are PHA-MAS1 and PHA-MA-S4. The calculated stabilization energies and the Gibbs free energy (see Tables 1 and 2) indicated that PHAMA-S1 is the most stable structure, while PHA-MA-S4 is a higher energy local minimum. Earlier it was pointed out that the M1 complex (PHA-MA-S4) is formed due possibly to the kinetic trapping. The DFT-SAPT (symmetry adapted perturbation theory) calculations suggest that electrostatic and induction energies play a dominant role in stabilizing the PHA-MA-S1 structure, while the dispersion energy has a higher contribution in stabilizing the PHA-MA-S4.4d The formation of the PHAMA-S1 structure leads to fluorescence quenching in phenylacetylene. Phenylacetylene has a long fluorescence lifetime of 54 ns,16 which is completely quenched by the formation of C-H · · · N hydrogen bonding involving the acetylenic moiety. On the other hand, the comparison of a real-time signal trace on the oscilloscope indicates that the fluorescence lifetime of phenylacetylene and the M1 complex are similar. Therefore, prima-facie the M1 complex does not alter the properties of the electronic excited state of phenylacetylene. Further, in the case of the binary complex between phenylacetylene with 1-azabicyclo[2.2.2]octane the formation of C-H · · · N hydrogen bonding involving the aromatic C-H did not lead to fluorescence quenching.4g The lowering of the fluorescence lifetime and in some case complete fluorescence quenching is associated with either the proton/hydrogen-atom transfer, electron transfer, or energy transfer dynamics in the electronic excited state.17-19 In the present case, because of the weak C-H · · · N hydrogen bonding, the possibility of proton/ hydrogen-atom transfer can be ruled out. Alkylamines, especially trimethylamine, are known to be an excellent fluorescent quencher of aromatic molecules, through an electron transfer mechanism.20 Since methylamine is closely related to triethylamine, it is probable that in the present case the electron transfer process through dipolar coupling is responsible for the fluorescence quenching. It is also important to recognize that the geometry of the C-H · · · N hydrogen-bonded complex is favorable for such a process and alters the excited state dynamics considerably. The IDIR spectrum of the M2 complex could not be obtained in the aromatic C-H stretching region even after several attempts, which was rather surprising because the corresponding spectrum for the bare phenylacetylene and the M1 complex could readily be obtained. Earlier, we had reported the observation of the C-H · · · N hydrogen-bonded complex between phenylacetylene and ammonia using 1C-R2PI and IDIR spectroscopic techniques.4b Attempts to obtain the LIF excitation spectrum of ammonia complex were futile, which indicates that the formation of the C-H · · · N hydrogen-bonded ammonia complex also leads to fluorescence quenching in phenylacetylene, which is in agreement with the present results. Conclusions In summary, investigations on the binary complex between phenylacetylene and methylamine using 1C-R2PI and IDIR

J. Phys. Chem. A, Vol. 114, No. 42, 2010 11351 spectroscopic techniques lead to observation of two distinct isomers. The two complexes show electronic shifts of 11 and 88 cm-1 to the red. In the first isomer the acetylenic C-H stretching vibration shows a low frequency shift of 139 cm-1, which can be assigned to the formation of a C-H · · · N hydrogen-bonded complex. The IDIR spectrum of the second isomer shows the loss of Fermi resonance coupling and accompanied by an almost negligible shift of 1 cm-1. The analysis of the IDIR spectra in the acetylenic C-H stretching region in the aromatic C-H stretching and suggests the presence of N-H · · · π hydrogen bonding to the π electron density of the benzene ring accompanied by a peripheral interaction of the methyl C-H group with the acetylenic CtC bond. Ab initio calculations at the CCSD(T)/CBS level indicate that C-H · · · N hydrogen-bonded complex is the global minimum, while the N-H · · · π hydrogen-bonded complex is a less stable local minimum. The formation of the N-H · · · π hydrogen-bonded complex could be related to kinetic trapping or higher accessibility. Further, the formation of C-H · · · N hydrogen bonding leads to fluorescence quenching of phenylacetylene moiety, which can be attributed to the dipolar coupling in the electronically excited state. The phenylacetylene-methylamine complex presents an interesting isomer-dependent fluorescence behavior. Acknowledgment. S.M. thanks UGC and A.D. thanks CSIR for their research fellowships. G.N.P. is presently on leave with Genesis Research Institute Inc. Japan and thanks Prof. A. Terasaki for discussion and support. This material is based upon work supported by Department of Science and Technology (Grant No.SR/S1/PC/23/2008), Board of Research in Nuclear Sciences (Grant No. 2004/37/5/BRNS/398), Council of Scientific and Industrial Research (Grant No. 01(2268)/08/EMR-II) to G.N.P., and NRF (WCU: R32-2008-000-10180-0, EPB Center: 2009-0063312) to K.S.K. Most calculations were carried out using supercomputers at KISTI (Grant No.: KSC-2008-K080002). References and Notes (1) (a) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, 1999. (b) G. A. Jeffrey, G. A. Saenger, W. Hydrogen Bonding in Biological Structures; Springer-Verlag: Berlin, 1991. (2) (a) Hobza, P.; Selzle, H. L.; Schlag, E. W. Chem. ReV. 1994, 94, 1767. (b) Kim, K. S.; Tarakeshwar, P.; Lee, J. Y. Chem. ReV. 2000, 100, 4145. (c) Brutschy, B. Chem. ReV. 2000, 100, 3891. (3) Graton, J.; Berthelot, M.; Laurence, C. J. Chem. Soc., Perkin Trans. 2001, 2, 2130. (b) Singh, P. C.; Patwari, G. N. Chem. Phys. Lett. 2006, 419, 48. (c) Fujii, A.; Ebata, T.; Mikami, N. J. Phys. Chem. A 2002, 106, 8554. (d) Parthasarathy, R.; Subramanian, V.; Sathyamurthy, N. J. Phys. Chem. A 2006, 110, 3349. (4) (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) Sedlak, R.; Hobza, P.; Patwari, G. N. J. Phys. Chem. A 2009, 113, 6620. (e) Maity, S.; Sedlak, R.; Hobza, P.; Patwari, G. N. Phys. Chem. Chem. Phys. 2009, 11, 9738. (f) Guin, M.; Patwari, G. N.; Karthikeyan, S.; Kim, K. S. Phys. Chem. Chem. Phys. 2009, 11, 11207. (g) Maity, S.; Patwari, G. N.; Karthikeyan, S.; Kim, K. S. Phys. Chem. Chem. Phys. 2010, 12, 6150. (5) Etter, M. C. Acc. Chem. Res. 1990, 23, 120. (b) Etter, M. C. J. Phys. Chem. 1991, 95, 4601. (c) Legon, A. C.; Millen, D. J. Chem. Soc. ReV. 1987, 16, 467. (6) Singh, P. C.; Patwari, G. N. Curr. Sci. 2008, 95, 469. (7) Page, R. H.; Shen, Y. R.; Lee, Y. T. J. Chem. Phys. 1988, 88, 4621. (8) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. J. Chem. Theory Comput. 2005, 2, 364. (9) (a) Helgaker, T.; Klopper, W.; Koch, H.; Noga, J. J. Chem. Phys. 1997, 106, 9639. (10) Min, S. K.; Lee, E. C.; Lee, H. M.; Kim, D. Y.; Kim, D.; Kim, K. S. J. Comput. Chem. 2008, 29, 1208.

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