Atomistic Simulations of CO Vibrations in Ices Relevant to Astrochemistry

Abstract

The experimental absorption band of carbon monoxide (CO) in mixed ices has been extensively studied in the past. The astrophysical interest in this band is related to its characteristic shape, which appears to depend on the surrounding ice structure. Herein, molecular dynamics simulations are carried out to analyze the relationship between the structure of the ice and the infrared (IR) spectrum of embedded CO molecules at different concentrations. Instead of conventional force fields, anharmonic potentials are used for the bonded interactions. The electrostatic interactions are more accurately described by means of fluctuating atomic multipole moments (up to quadrupole). The experimentally observed splitting of the CO absorption band (gas phase: 2143 cm(-1)) into a blue- (2152 cm(-1)) and a red-shifted (2138 cm(-1)) signal is also found in the simulations. Complementary atomistic simulations allow us to relate the spectra with the structural features. The distinction between interstitial and substitutional CO molecules as the origin of this splitting is found to be qualitatively correct. However, at increasing CO concentrations, additional effects-such as mutual interactions between CO molecules-become important, and the simplistic picture needs to be revised.

Spatial averaging for small molecule diffusion in condensed phase environments

“…34 By comparing with experimental results from infrared spectroscopy 35,36 it was found that CO molecules can be largely classified into occupying interstitial or substitutional sites, respectively. This leads to two bands in the infrared spectrum of CO.…”

O2 formation in cold environments

“…Initial velocities are drawn from a Maxwell-Boltzmann distribution at 50 K. All simulations are initialized as follows: a minimization of 100 steps using the steepest descent method is followed by 10 ps heating, where the temperature is increased by 10 K every ps and then relaxed for the last 5 ps, and 50 ps equilibration dynamics with rescaling of the temperatures every 2.5 ps. Then, the following different types of simulations are considered: the (rigid) TIP3P 31 or the (flexible) KKY 32,33 water model are combined with either a reproducing kernel Hilbert space (RKHS) or a Morse representation of the reactive O-O potential (vide infra) to follow the recombination and relaxation dynamics.…”

Structural Identification of Spectroscopic Substates in Neuroglobin

“…[46,47] The electrostatic interaction of CO with its environment is based on distributed multipole analysis (DMA) [48] which has been developed and successfully applied to simulations of the spectroscopic properties of CO [28,49,50] and to structurally assign spectroscopic features in photodissociated Mb-CO. In particular, an expansion of the electrostatic potential up to an atomic quadrupole moment on the C atom and an octupole moment on the O atom resulted in IR spectra in very good agreement with experimental data of CO in Mb.…”