Abstract
AN explanation of the complex rotational fine structure of the 1306 cm.-1 fundamental vibrational band of methane1 has been found which is based on a tetrahedrally symmetrical model for the molecule. The cause of the splitting of the rotational lines is a Coriolis coupling between the rotational levels of this threefold degenerate mode of vibration v4 and the rotational levels of the infra-red inactive twofold degenerate vibration v2 at 1536 cm.-1, which lies 230 cm.-1 above it. The perturbation is appreciable in spite of the distance between the pairs of levels concerned, because the Coriolis coupling term in question is essentially a first-order term in the Hamiltonian. A general expression for this term in methane was given by one of us in a previous paper2. It involves the components of vibrational angular momentum obtained by combining the displacements of v4 with the momenta of v2 and vice versa3. It is to be noted that since the two vibrations belong to different symmetry types, the perturbation affects only the rotational levels and leaves the origin of the v4 band unaltered.
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References
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See also Teller, E., "Hand- und Jahrbuch d. Chem. Phys.", 9 II, 152.
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JAHN, H., CHILDS, W. Structure of the Methane Molecule. Nature 141, 916 (1938). https://doi.org/10.1038/141916a0
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DOI: https://doi.org/10.1038/141916a0
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