CH-Stretching Vibrational Trends in Natural Gas Hydrates Studied by Quantum-Chemical Computations

Vibrational Raman spectroscopy of hydrocarbon CH-stretching vibrations is often used to study natural gas hydrates. In this work, CH-stretching vibrational Raman spectra of hydrocarbon molecules (CH₄, C₂H₆, C₃H₆, C₃H₈, C₄H₈, <i>i</i>-C₄H₁₀, and <i>n</i>-C₄H₁₀) encapsulated in the water cages (D, ID, T, P, H, and I) of the sI, sII, sH, and sK crystal phases are derived from quantum-chemical computations at the ωB97X-D/6-311++G­(2d,2p) level of theory. The trends of CH-stretching vibrational frequencies of hydrocarbon molecules in natural gas hydrates are found to follow the prediction by the “loose cage–tight cage” model: as the size of water cavity increases, the CH frequencies will first decrease and then increase until equal to that in the gas phase. In the “tight cage” situation, the frequency will be greater than in the gas phase; in the “loose cage” situation, the frequency will be smaller or asymptotic to that in the gas phase. Furthermore, the OH-stretching frequencies are sensitive to the H-bond configuration, and the varying strengths of H-bonds for different configurations are reflected by the frequency distribution in the corresponding subspectra