Vibrational spectral diffusion in supercritical deuterated ammonia from first principles simulations: Roles of hydrogen bonds, free ND modes and inertial rotation of ammonia molecules

We have presented a first principles study of the vibrational spectral diffusion and related dynamical and structural properties of supercritical ammonia at two different densities by means of ab initio molecular dynamics and time series analysis. The ab initio simulations are performed using two density functionals for the electronic structure calculations: one without any dispersion corrections and the other with dispersion corrections. Apart from calculating the dynamics of frequency fluctuations of ND modes of deuterated ammonia in supercritical states, we have also at many other related quantities such as the dynamics of hydrogen bonds and free ND groups, rotational and translational motion of ammonia molecules and also some of the equilibrium quantities such as local density distributions, pair correlations, hydrogen bonding properties and dipole distribution of ammonia molecules. It is found that the hydrogen bonding between ammonia molecules decreases significantly as we move to the supercritical state of lower density from the normal liquid phase. The inclusion of dispersion interactions is found to increase the structural correlations and also the number of hydrogen bonds to some extent. Our calculations of ND frequency distributions reveal that as we move from normal to the supercritical states, the frequency distributions become narrower and show a shift toward the higher frequencies which is in general agreement with experimental results. the frequency time correlation function of the ND modes of ammonia molecules. The frequency time correlation function of the ND modes of ammonia molecules in the supercritical states is found to decay with two time scales: One around 80-100 fs and the other one falls in the region of 350-700 fs. Our calculations of the dynamics of hydrogen bonds, free ND modes and also rotational motion of ND groups in supercritical ammonia reveal that the fast hydrogen bond breaking and inertial rotational motion of ND bonds contribute to the faster component of spectral diffusion of ND bonds while the slower component originates from the dynamics of non-hydrogen-bonded free ND bonds.