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

Vibrational Raman spectrosopy of hydrocarbon CH-stretching vibrations is often-used to study natural gas, hydrates., In this work, CH-stretching vibrational, Raman spectra of hydrocarbon molectles (CH4, C2H6, C(3)H6, C3H8, C4H8, i-C4H10, and n-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 omega B97X-D/6-311++G(24,2p) 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.