Infrared spectroscopy of water clusters isolated in methane matrices: Effects of isotope substitution and annealing

Using infrared-active solvents of CH4 and CD4 for matrix isolation, we measured infrared spectra of H2O and D2O clusters at 7 K. The solute-concentration dependence of the spectrum of H2O clusters in a CH4 matrix was investigated and was used for the peak assignment. Annealing procedures were found to promote the size growth of water clusters in methane matrices for all the combinations of (H2O, CH4), (H2O, CD4), (D2O, CH4), and (D2O, CD4). We also monitored the ν3 absorption due to methane to find the annealing-induced structural change only of solid CH4. The matrix effects on the vibrations of the clusters are discussed on the basis of “Tc plots”, where their frequencies are plotted as a function of the square root of the matrix critical temperature, Tc. The obtained plots assure the validity of the assignment of the cluster peaks.