Infrared bands of formaldehyde dissolved in liquid krypton at cryogenic temperatures and the vibrational modes ν1, ν2, and ν5 of H2CO in comets and interstellar clouds

Abstract

The infrared spectra of formaldehyde (H2CO) dissolved in liquid krypton between 125 K and 137 K have been obtained using a Fourier transform infrared spectrophotometer and a low temperature cryostat. Backing pressures higher than 1.5 atm were necessary to liquefy Kr above 120 K. Monomeric formaldehyde in gas phase was prepared by thermal decomposition of para-formaldehyde. Peak positions (ν), wavenumber shifts between the gas and liquid solution spectra (Δν), and full widths at half maximum (fwhm) are reported. The H2CO vibrational frequencies in liquid Kr are slightly lower than the gas phase frequencies at room temperature because of the inertness of the solvent. The Gaussian 16 software package is used to determine the geometry and lowest energy of H2CO at room temperature and at the temperature of the experiment. Fundamental harmonic and anharmonic vibrational frequencies and relative intensities are calculated. The influence of the solvent on fundamental vibrational frequencies is studied using the polarizable continuum model (PCM). When used in conjunction with calculated anharmonic frequencies, the PCM model shows qualitative agreement with experimental frequency shifts and relative intensities in liquid Kr. The presence of H2CO in space is discussed in reference to the ν1 and ν5 absorption bands observed in comets, the ν1 and ν2 absorptions in interstellar clouds, and the lack of detection of the ν4 and ν6 bands of H2CO in the atmosphere of Titan. The carbonyl stretching mode ν2 is suggested as a possible candidate for confirmation and identification of H2CO in space due to its larger strength in comparison to the other modes that have been studied at low temperatures and its characteristic frequency that is completely removed from CO2, CO, H2O and CH3OH vibrational modes.