UREY-BRADLEY FORCE-FIELD, VALENCE FORCE-FIELD, AND ABINITIO STUDY OF INTRA-MOLECULAR FORCES IN TRI-TERT-BUTYLMETHANE AND ISOBUTANE

The relationship between intramolecular forces, molecular geometry, and vibrational frequencies as reflected in triterr-buiylmcthane (TTBM) and isobutane was investigated using empirical forcc field and ab initio calculations. The strong intramolecular forces in TTBM and the resulting large deviations in its physical properties from normal values make it an ideal system for this study. Isobutane, which consists of are tert-butyl group, but is not as overcrowded as TTBM, serves as a good model for comparison. Studying TTBM with the more recent Urey-Bradlcy (UBCFF) and valence (VCFF) consistent force fields pointed to the need for the inclusion of an anharmonic bond stretching potential in both force fields and a stretchbend interaction term in the VCFF. Ab initio calculations verified the results of the empirical force fields and showed how close non-bonded hydrogen-hydrogen interactions compress the Cm-Hm bond and raise its effective force constant. They also showed how the increased 1-3 geminal interactions induced by closing the Ht-Ct-C, bond angle stretch the Ct-Ht bond and lower its effective force constant. The modified force fields leave the calculated structures essentially unchanged, but result in dramatic improvement in the vibrational frequencies, particularly in the VCFF. Thus, both shifts toward higher frequencies and lower frequencies due to overcrowding were accounted for to within 10 cm-1 or less. The root mean square deviation of the CH stretching frequencies was reduced from 26.7 to 7.9 cm-1 in isobutane and from 39.0 to 4.7 cm-1 in TTBM. © 1979, American Chemical Society. All rights reserved.