Ab initio molecular orbital theory with the minimal STO-3G basis set was used to study the conformational preferences of f u ra n -2-, f u ra n -3 -, pyrrole-2 -, pyrrole-3-, pyrid i ne-2-, pyridine-3-, and pyrid i ne-4-carbalde hydes. The calculations yielded conformational energies, potential functions for internal rotation, rotational barriers, stabilisation energies, charge distributions, electric dipole moments, and x-overlap populations. Results were compared with available experimental data, and analysed to identify the main intramolecular factors determining the favoured conformations of these molecules.MUCH effort has been directed recently towards determining the conformational preferences of simple heteroaromatic aldehydes, and a wide variety of physical methods has been applied to problems of this nature. We have previously reported and analysed infinitedilution electric dipole moments and molar Kerr constants for several such molecules (pyridine-,l pyrrole-,2 furan-, and thiophen-carbaldehydes 3, and many other aromatic carbonyl compounds as solutes in nondipolar solvents. It is now recognised, however, that the position of equilibrium between alternative rotational isomers of a particular molecule can be markedly medium-dependent . For example, in the much-studied l C.
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