Computational study on C-H-aEuro broken vertical bar pi interactions of acetylene with benzene, 1,3,5-trifluorobenzene and coronene
Meta-hybrid density functional theory calculations using M06-2X/6-31+G(d,p) and M06-2X/6-311+G(d,p) levels of theory have been performed to understand the strength of C-H-aEuro broken vertical bar pi interactions of two possible types for benzene-acetylene, 1,3,5-trifluorobenzene-acetylene and coronene-acetylene complexes. Our study reveals that the C-H-...pi interaction complex where acetylene located above to the center of benzene ring (classical T-shaped) is the lowest energy structure. This structure is twice more stable than the configuration characterized by H atom of benzene interacting with the pi-cloud of acetylene. The binding energy of 2.91 kcal/mol calculated at the M06-2X/6-311+G(d,p) level for the lowest energy configuration (1A) is in very good agreement with the experimental binding energy of 2.7 +/- 0.2 kcal/mol for benzene-acetylene complex. Interestingly, the C-H-...pi interaction of acetylene above to the center of the aromatic ring is not the lowest energy configuration for 1,3,5-trifluorobenzene-acetylene and coronene-acetylene complexes. The lowest energy configuration (2A) for the former complex possesses both C-H-...pi interaction and C-(HF)-F-... hydrogen bond, while the lowest energy structure for the coronene-acetylene complex involves both pi-pi and C-H-...pi interactions. C-H stretching vibrational frequencies and the frequency shifts are reported and analyzed for all of the configurations. We observed red-shift of the vibrational frequency for the stretching mode of the C-H bond that interacts with the pi-cloud. Acetylene in the lowest-energy structures of the complexes exhibits significant red-shift of the C-H stretching frequency and change in intensity of the corresponding vibrational frequency, compared to bare acetylene. We have examined the molecular electrostatic potential on the surfaces of benzene, 1,3,5-trifluorobenzene, coronene and acetylene to explain the binding strengths of various complexes studied here.
acetylene 1,3,5-trifluorobenzene density functional theory hydrogen bond c-h center dot center dot center dot pi interactions pi-pi interactions vibrational frequency shift