On the spectral features of dangling bonds in CH₄/H₂O amorphous ice mixtures

Dangling bond (DB) bands in IR spectra, above 3600 cm-1, are a source of information on structural properties of amorphous water ice, and especially on ice mixtures of water and other frozen gases. We deal in this paper with the spectroscopic behavior of DB bands of CH4/H2O mixtures. We use ab initio methodology to predict theoretical results which are compared with experimental results. Our model mixtures are created by inserting a variable number of molecules of either species into a cell of appropriate size to reach an initial density of 1 g cm-3, which can be modified by including an empty space at the top, to simulate pores. The cell is taken as a unit cell for a solid state calculation The structure of the mixture is optimized and the IR spectrum is calculated for the converged geometry. We find two different kinds of dangling bonds, in which the O-H stretching responsible for this mode is directed either to an empty space of a pore or towards a nearby CH4 molecule, with which some interaction takes place. The spectral characteristics of these two DB types are clearly different, and follow satisfactorily the pattern observed in experimental spectra. Estimated band strengths for these DB bands are given for the first time.