Vibrational spectroscopic signatures of hydrogen bond induced NH stretch-bend Fermi-resonance in amines: The methylamine clusters and other N-H⋯N hydrogen-bonded complexes.

Abstract

The appearance of multiple bands in the N-H stretching region of the infrared spectra of the neutral methylamine dimer and trimer is a sign of NH bend-stretch anharmonic coupling. Ab initio anharmonic calculations were carried out in a step-wise manner to reveal the origin of various bands observed in the spectrum of the methylamine dimer. A seven-dimensional potential energy surface involving symmetric and asymmetric stretching and bending vibrations of both the hydrogen bond donor and the acceptor along intermolecular-translational modes was constructed using the discrete variable representation approach. The resulting spectrum of the dimer shows five bands that can be attributed to the symmetric stretching (νsym D), asymmetric stretchin (νasym D), and bending overtone (2νbend D) of the donor moiety. These appear along with the combination band arising out of bending vibrations of the donor and acceptor (νbend D + νbend A) and with the combination of the intermolecular translational mode over the donor bending overtone (νtrans + 2νbend D). The spectrum of the trimer essentially consists of all the features seen in the dimer with marginal changes in band positions. The analysis of the experimental spectra based on the two-state deperturbation model and ab initio anharmonic calculations yield a matrix element of about 40 cm-1 for the N-H bend-stretch Fermi resonance coupling. In general, the IR spectra of the hydrogen-bonded amino group depict three sets of bands that arise due to bend-stretch Fermi resonance coupling.