The motional collapse of the methyl C–H stretching vibration bands

摘要: The asymmetric CH stretching bands of the methyl groups a number alkane systems have been examined as function temperature. At low temperatures, two are seen for CH3 group. These collapse into one temperature is raised. Examination crystals alkanes with both odd and even numbers carbon atoms, in urea clathrates, deuterium substituted allows separation intramolecular intermolecular contributions to band splittings widths. Model Hamiltonians that express energy levels torsional coordinates external set up. used Redfield equations derive expressions widths terms parameters which give coupling various types motion. found be proportional average cos 5ϑ, where ϑ angle, contribution width due fluctuations this quantity. Comparison observed shows relaxation times motion, greater than 0.2 but less about 20 ps, fast enough yield motionally narrowed limit spectrum required derivation equations.