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…ND 2 is the dominant interaction between ND 2 and D 2 O as compared to the D 2 O…D 2 N. The hydrogen bond involving DOD…ND 2 has longer lifetime (2.6 ps) than both D 2 O…D 2 N (1.1 ps) and waterwater hydrogen bonds. The residence time of water molecule inside the first solvation shell of ND 2 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 aminewater 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.[a] S. Biswas

Ab Initio Metadynamics Calculations of Dimethylamine for Probing pKb Variations in Bulk vs. Surface Environments

“…From the RDF calculations, we find that the N atom of dimethylamine can form more hydrogen bonds with water molecules in both the bulk and slab configurations than that of the aminic hydrogen. This observation is consistent with other RDF calculations of N-H moieties in water, such as methylamine 66 and NH 2 radical in aqueous environments. 67 To provide a deeper understanding of the strength and dynamic stability of the various hydrogen bonds, we can construct a hydrogen bond auto-correlation function using a population correlation function approach.…”

“…Other studies have used less stringent criteria with cut-off angles of 45 • 75 or only employed distance-based criteria between the N-H group and surrounding water molecules. 66,[76][77][78] In the present study, our RDF calculations show that the dimethylamine molecule makes a single hydrogen bond with water molecules via the N terminal group but makes less than one hydrogen bond via the H atom in the N-H group with neighboring water molecules. Due to the variation in the number of hydrogen-bonds formed by dimethylamine with the surrounding water, we have only adopted a distancebased criteria to allow more flexibility in the calculation of the hydrogen-bond autocorrelation function.…”

Investigation of the liquid-vapour interface of aqueous methylamine solutions by computer simulation methods

“…Recently, Biswas and Mallik studied the bulk phase orientational preferences of the methylamine molecules in neat methylamine [20] as well as in its aqueous solution [21] at ambient conditions by first principles molecular dynamics simulations using dispersion corrected density functional (BLYP-D). Computer simulation studies of the behaviour of methylamine at aqueous interfaces are, however, rather scarce.…”

Distinctive behavior and two-dimensional vibrational dynamics of water molecules inside glycine solvation shell

“…We performed structural calculations like radial distribution function (RDF) to understand the solvation shell demarcation. 94,104 For aminic nitrogen and carboxylic oxygen, the solvation shell criteria are N-O W distance less than 3.48Å and O C -O W distance less than 3.30Å, respectively. For methylene, the D C -O W RDF peak minimum extends up to 4.2Å.…”

“…In aqueous methylamine, a blue shi of 30 cm À1 was reported ongoing from bulk towards amine hydration shell. 104 The OD modes within the solvation shell cut-off in amine hydration (2425 cm À1 ) show a similar 15 cm À1 frequency shi corresponding to the bulk OD stretch (2410 cm À1 ) in glycine. In glycine-water system, bulk water molecules exhibit a red shi in the average vibrational stretch frequency as compared to the carboxylate hydration shell and this shi is comparable to the 34 cm À1 shi observed ongoing from the carbonyl oxygen solvation towards bulk in aqueous acetone.…”