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Unexpected differential sensitivity of nuclear spin—spin coupling constants to bond stretching in methane

✍ Scribed by William T. Raynes; Jan Geertsen; Jens Oddershede

Publisher
Elsevier Science
Year
1992
Tongue
English
Weight
823 KB
Volume
197
Category
Article
ISSN
0009-2614

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✦ Synopsis

Accurate coupled-cluster calculations reveal that when one of the C-H bonds in the methane molecule is stretched by a small amount from its equilibrium bond length (a) the carbon-proton spin-spin coupling constant involving the nuclei of the stretched bond changes by less than the carbon-proton coupling constant of the three unstretched bonds, and (b) the three proton-proton coupling constants which involve the proton of the stretched bond change by less than the three proton-proton coupling constants which do not involve the proton of the stretched bond. This unexpected differential sensitivity is evidently due to electron correlation as it is absent at the RPA level. It has been observed in an experimental study of carbon-proton coupling in the methane isotopomers. Experimental results in which primary and secondary isotope effects are directly compared suggest that it is a fairly common phenomenon.

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