A Combined Spectroscopic and ab Initio Investigation of Phenylacetylene-Methylamine Complex. Observation of sigma and pi Type Hydrogen-Bonded Configurations and Fluorescence Quenching by Weak C-H center dot center dot center dot N Hydrogen Bonding

Abstract

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 center dot center dot center dot 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 pi electron density of the benzene ring accompanied by a peripheral interaction between the methyl C-H group and the pi electron density of the C C 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 center dot center dot center dot N hydrogen bonded complex is the global minimum, the N-H center dot center dot center dot pi hydrogen bonded complex is a high energy local minimum. The formation of the N-H center dot center dot center dot pi 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 center dot center dot center dot 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.