| Abstract: | Abstract Raman frequencies and intensities in methylcyclohexane solution and infrared frequencies in Nujol mull, have been obtained and assigned for the skeletal modes of (t-C4H9 6Li)4 and (t-C4H9 7Li)4. A normal coordinate analysis, neglecting hydrogen atoms, gave an overall frequency fit of 2% with a reasonable set of valence force constants. The calculated eigenvectors were used to transform the Raman intensities into bond polarizability derivatives. Because of coordinate mixing and the inherent sign ambiguity of molecular polarizability derivatives, there are eight sets of bond polarizability derivatives which are consistent with the measured intensities. Only one set, however, gives reasonable polarizability derivatives for CC stretching and CCC bending, and also shows the requisite invariance to isotope substitution. This set gives a low value for the Li-Li polarizability derivative, and application of the delta function potential equation suggests that the extent of Li-Li bonding is small, amounting to perhaps 5% of the total bonding electron density in the Li4C4 cage. This conclusion is consistent with the failure to observe Li[sbnd]Li spin-spin coupling in (t-C4H9Li)4. |
