Abstract
The structures of a donor–acceptor thiophene, furan, pyrrole, thiadiazole, oxadiazole and the three ring tautomers of the corresponding triazole, have been calculated using the ab initio 3-21 G basis set to give a reasonable correlation with crystallographic data where available. The non-linear optical properties of these structures have been assessed by calculating their molecular hyperpolarizabilities using a sum-over-states semiempirical approach. The results show that the 1,3,4-thiadiazole, 1,3,4-oxadiazole and 1,3,4-triazole, exhibit much larger dipole moment changes than the corresponding thiophene, furan and pyrrole on excitation, but their calculated hyperpolarizabilities are smaller mainly because of their smaller transition moments.

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