Dynamic NMR spectroscopy can determine energy barriers due to internal motion over the range of about 4.5-23 kcal mol -1 . Conformational analysis of the frozen conformations can be simulated and interpreted by reliable theoretical calculations based mainly on density functional theory (DFT). The same calculations can identify transition states

The 100-and 220-MHz nmr spectra of 2,2'-bithienyl partially oriented in the nematic phase of a liquid crystalline solvent have been obtained and interpreted. The values of the direct dipolar couplings (£>,,) were found in disagreement with the presence of solely the s-trans or the s-cis conformation; a number of models taking into account the existence of a rapid equilibrium between the two forms have been discussed. It has been shown that the s-trans is the more stable conformer (70 ±5%) and the energy required for the s-trans, s-cis interconversion is about 5 ± 2 kcal/mol. The results agree with the conclusions of theoretical calculations and are consistent with X-ray and esr experimental evidences.Information on the structures and conformations of molecules with internal motions has been gained by means of the nmr spectroscopy of partially oriented molecules in liquid crystalline solvents.3•4 The possibility of using this technique for conformational analysis appears very interesting but must be tested with many examples in order to know the advantages and the limits of its applications.In the case of some bithienyl derivatives413 it has been possible to show that neither the free rotation nor the existence of a single conformer (twisted or planar) accounts for the parameters obtained from the nematic phase nmr spectrum. By allowing for some approximations it was possible to infer that both the s-trans and s-cis isomers are present with different populations, s-trans s-cis the former being the more stable. However, when symmetrically substituted derivatives of 2,2'-bithienyl are examined,413 the number of observable parameters is rather small so that the results cannot be very accurate; furthermore, no data are available for studying the barrier to the internal rotation.

The addition of several photochemically generated aryl and fluoroalkyl radicals to fullerene C 70 was studied by electron spin resonance and the results were compared with those obtained with C 60 . While simple alkyl radicals afford only three of the five expected RC 70 • regioisomers, the more reactive aryl and fluoroalkyl radicals give rise to four, except for the trifluoromethyl radical which yielded for the first time the ESR spectra of all five expected isomers. Semiempirical MO calculations and density functional theory calculations were carried out for HC 60 • and for the five HC 70 • regioisomers as models for the corresponding alkyl, aryl, and fluoroalkyl analogs in order to describe the unpaired electron distribution in these radical species and as an aid in the assignment of the observed spectra to specific RC 70 • regioisomers.