Hydrogen-Bonded Complexes of Phenylacetylene with Water, Methanol, Ammonia, and Methylamine. The Origin of Methyl Group-Induced Hydrogen Bond Switching
Introduction Phenylacetylene is perhaps the simplest multifunctional molecule to investigate competitive hydrogen bonding. It has three hydrogen bonding sites in the form of the benzene ring and the acetylenic CtC bond, which can act as π acceptors, and an activated acetylenic C-H group, which can act as a σ donor. Further, with the absence of any strongly acidic/basic functional groups, the hierarchy of hydrogen sites cannot be determined on the basis of Etter and Legon-Millen rules.1,2 One of the major challenges that needs to addressed in hydrogen bonding is to know, a priori, how the individual functional groups in multifunctional molecules will behave when they are made to interact with suitable hydrogen bonding partners. In multifunctional molecules the exact hydrogen bonding patte
will be a result of competition between various possibilities. Toward the goal of comprehending the hydrogen bonding behavior of multifunctional molecules, Patwari and co-workers investigated hydrogen-bonded complexes of phenylacetylene with various solvent molecules such as water, methanol, ammonia, methylamine, and other alcohols and amines.3-5 The hydrogen-bonded complexes of phenylacetylene form a wide variety of intermolecular structures, which stem out of a subtle balance of intermolecular forces of various possible intermolecular structures.3-5 For instance, phenylacetylene forms a quasiplanar cyclic complex with water, incorporating O-H· · ·π and C-H· · ·O hydrogen bonds.3,4 In this case one of the O-H groups of a water molecule interacts with the π electron density of the CtC bond, while the C-H group of the benzene ring in the ortho position is hydrogen-bonded to the oxygen atom of the water molecule. The structure of the phenylacetylene-water complex thus is different from both the benzene-water and acetylene-water complexes,6,7 even though phenylacetylene incorporates the features of both benzene and acetylene. On the other hand, the the phenylacetylene-methanol complex is characterized by the presence of single O-H· · ·π hydrogen bond, wherein the O-H group of methanol interacts with the π electron density of the benzene ring, similar to the benzene-methanol complex.4 Phenylacetylene forms a linear C-H· · ·N “σ” hydrogen-bonded complex with ammonia,5 which is similar to acetylene-ammonia complex,8 while the phenylacetylene-methylamine complex is characterized by the presence of a N-H· · ·π hydrogen bond. In this case the N-H group of methylamine interacts with the π electron density of the benzene ring.5 Such differences in the intermolecular structures of hydrogen-bonded complexes with water, methanol, ammonia, and methylamine involving benzene derivatives have not been reported in the literature prior to these complexes of phenylacetylene. These results illustrate that, in the case of interaction with multifunctional molecules, even minimal changes in the interacting partner, such as substitution by a ubiquitous methyl group, can result in dramatic change in the intermolecular structure. The change in the intermolecular structure with the substitution of a methyl group can be perceived as methyl groupinduced hydrogen bond switching. In this article we address the underlying factors that influence the hydrogen bond switching observed in the complexes of phenylacetylene. Methodology The geometry optimizations were carried out at MP2(FC)/ aug-cc-pVDZ level of theory, and in each case frequency calculations followed to ascertain the nature of the minima obtained. Single point calculations at CCSD(T)/aug-cc-pVDZ level were carried out on the MP2 level optimized structures. The stabilization energies were corrected for zero point energy * Authors to whom correspondence should be addressed. E-mail: pavel.hobza@uochb.cas.cz (P.H.); naresh@chem.iitb.ac.in (G.N.P.). † Academy of Sciences of the Czech Republic and Center for Biomolecules and Complex Molecular Systems. ‡ Palacky´ University. § Indian Institute of Technology Bombay. 6620 J. Phys. Chem. A 2009, 113, 6620–6625 10.1021/jp900813n CCC: $40.75 2009 American Chemical Society Published on Web 05/26/2009 Downloaded by AUSTRIA CONSORTIA on July 6, 2009 Published on May 26, 2009 on http://pubs.acs.org | doi: 10.1021/jp900813n (ZPE) and basis set superposition error (BSSE). Thermochemical analysis, based on rigid rotor-harmonic oscillator-ideal gas approximations was also carried out using the vibrational frequency data obtained at MP2(FC)/aug-cc-pVDZ level of theory. Further, DFT-SAPT (symmetry adapted perturbation theory) calculations were performed with the aug-cc-pVDZ basis set.9 This method allows for the separation of interaction energies into physically well-defined components, such as those arising from electrostatic, induction, dispersion, and exchange. The DFT-SAPT interaction energy (Eint) is given as Equation 1 describes the electrostatic, exchange-repulsion, induction, exchange-induction, dispersion, and exchangedispersion terms, while the last term is a Hartree-Fock correction for higher-order contributions to the interaction energy. In the present analysis the exchange-induction and exchange-dispersion terms will be included in the parent induction and dispersion terms. All calculations mentioned above were performed using the Gaussian03 and Molpro suite of programs.10,11 Results and Discussion To begin with, the acetylenic C-H stretching region of the IR spectra of phenylacetylene and its complexes are reviewed to understand the structural assignment. The acetylenic C-H stretching regions of the infrared spectra for phenylacetylene (PHA) and its five complexes with argon (Ar), water (H2O), methanol (MeOH), ammonia (NH3), and methylamine (MeNH2) are depicted in Figure 1. These spectra were recorded using the IR-UV double resonance spectroscopic method using either fluorescence or ion detection techniques.3-5 The IR spectrum of PHA (trace A) shows the presence of two intense transitions at 3325 and 3343 cm-1, which originate from Fermi resonance coupling between the acetylenic C-H stretching vibration and a combination band comprising one quantum of CtC stretching and two quanta of CtC-H out-of-plane bend.12 A 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-1.13 In the case of PHA 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 completely remove the Fermi mixing. The Fermi resonance transitions of the PHA moiety, therefore, can be used a spectroscopic tool to probe the interactions present in various PHA complexes. Figure 1B depicts the IR spectrum of the PHA-argon complex, which is almost identical to that of bare PHA, within the experimental uncertainty of (1 cm-1.13 This implies that the binding of Ar to PHA does not perturb either the C-H or the CtC oscillators. It can therefore be inferred that the Ar atom is bound to the π electron density of the benzene ring in PHA.13 This inference is substantiated by the structure of the PHA-Ar complex determined using the high-resolution REMPI spectrum for the S1 r S0 electronic transition and microwave spectroscopy.14 The IR spectrum of PHA-H2O complex, depicted in Figure 1C, consists of a single transition at 3331 cm-1, which has been assigned to the acetylenic C-H stretching vibration. The acetylenic C-H stretching frequency shifts marginally (about 3 cm-1) upon interaction with water, coupled with the loss of Fermi resonance coupling. This implies that the H2O molecule interacts with the π electron density of the acetylenic CtC bond.3 Surprisingly, in the case of the PHA-MeOH (Figure 1D) complex, two transitions appear at 3323 and 3334 cm-1, albeit with differences in the band positions and their intensities relative to bare PHA. These transitions have been assigned to the Fermi resonance bands.4 The shift in the electronic transition for the S1r0 relative to bare PHA, the shift in the O-H stretching frequency of the MeOH moiety, and the appearance of Fermi resonance transitions in the IR spectrum indicate that MeOH interacts with the π electron density of the benzene ring in PHA, resulting in formation of a O-H· · ·π hydrogen-bonded complex.4 The positions and the intensities of the Fermi resonance bands depend on the positions of the zero-order (unperturbed) oscillators and the coupling strength. The interaction of the methanolic O-H group with the π electron density of the benzene ring in PHA is expected to affect, marginally, both the zero-order positions and the coupling strength.
