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The molecular geometries and the torsional potentials about the inter‐ring C‐C bond in α‐oligothiophenes (α‐nTh, n=2–4) have been calculated by means of conventional ab initio and density functional theory (DFT) calculations employing the hybrid B3LYP and BH&HLYP functionals. The position and the energetics of the critical points in the potential energy curve generated by rotation about the inter‐ring CC bond are shown to be dependent on the computational method. DFT calculations, in comparison with MP2 calculations, favour conjugative interactions, while steric and coulombic interactions are equally treated by both methods. On oligomerization the electron delocalisation increases slightly, the p‐charge being preferentially confined within the rings, although it is sufficient to move the molecular structure towards co‐planarization and to increase the barrier through the perpendicular conformation. The IR and Raman spectra on the relevant rotamers of α‐2Th have been computed at HF/6–31G* and B3LYP/6–31G* levels. The comparison with the experiment is excellent. It has been found that small twisting from the planar conformation has no apparent effects, while 90° twisting and isomerization to the syn‐gauche form produce significant frequency and intensity variations which could be useful probes in conformational studies. The simulated IR and Raman spectra of the α‐2Th rotamers are consistent with a smallπ‐electron delocalisation between the rings.

The geometries, relative energies, gas-phase static and dynamic dipole polarizabilities of the two most stable neutral forms and of the zwitterionic form of the twenty naturally occurring amino acids have been obtained by Density Functional and conventional ab initio HatreeFock theories using correlation consistent basis sets. Mean electronic polarizabilities (<α e >s) are encompassed in the 40-160 a.u. range and are little dependent on the amino acid framework conformation, structure and conformation. The relation between <α e > and the number of electrons in the molecule makes to classify the amino acids as one of the most polarizable family of compounds. Calculated <α e > values of the neutral forms linearly relate to the molecular volume and molecular hardness as well as, rather unexpectedly, with the experimental values in water solution, where amino acids are known to be in a zwitterionic form. Vibrational polarizabilities amount to 15-45 a.u.. They come essentially from the lowfrequency angular deformation modes of the -OH and -NH 2 groups.

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