Ultrafast Vibrational Spectroscopy of Aqueous Solution of Methylamine from First Principles MD Simulations

Abstract

We performed Car-Parrinello molecular dynamics (CPMD) simulations of deuterated aqueous solution of methylamine (MA) to investigate the structure, dynamics and time dependent vibrational spectra of water molecules in the first solvation shell. Our results show that the hydrogen bond of DOD…ND2 is the dominant interaction between ND2 and D2O as compared to the D2O…D2N. The hydrogen bond involving DOD…ND2 has longer lifetime (2.6 ps) than both D2O…D2N (1.1 ps) and water-water hydrogen bonds. The residence time of water molecule inside the first solvation shell of ND2 is 5.72 ps. The vibrational spectral diffusion of water molecules in the first hydration shell of the amine nitrogen of methylamine proceeds with three time scales. A short-time relaxation originates from dynamics of amine-water hydrogen bonds without breaking (90 fs), and a slower relaxation (∼1.8 ps) is due to the breaking of amine-water hydrogen bonds. Another longer time constant (∼7 ps) is due to the escape dynamics of water molecules from the first hydration shell of the amine group