Near infrared spectra (4000–12000 cm⁻¹) of 2-thiopyrrole-l,2-dicarboximide (TPH) and its N-deuterated analogue were measured in polycrystalline form at different temperatures between 410 K and 273 K. Spectra in solution were recorded as a function of concentration and temperature. The prominent bands so obtained could be interpreted in terms of overtone and combination bands of the N-H(D) and C-O groups modes. The results in CHCl₃ and CC1₄ solutions allowed differentiation between hydrogen bonded and nonhydrogen bonded bands. A comparison of cubic potential constant K₃, mechanical anharmonicity ω_eχ_e and electrical anharmonicity μ₂/μ₁ for the stretching bands indicates that these constants are not affected much by N-deuteration.

IR, far-IR and Raman spectra of 2-thiouracil are reported and interpreted. All the thirty normal modes could be assigned. The Raman spectrum and the vibrational assignments for all the thirty modes are reported for the first time. The ring breathing and Kekule stretching modes for 2-thiouracil are observed to have lower magnitudes when compared to those for uracil which could be due to the mass effect of the sulphur atom in place of the oxygen atom.

Bond orders and valence indices have been evaluated employing Mayer’s definitions with orthogonalized atomic orbitals (OAO) obtained from Löwdin orthogonalization over an STO-3G basis set in anab initio formalism. It has been observed that the eigenvalues of the submatrices associated with bond order orbitals. natural hybrid orbitals and natural bond orbitals also reproduce the same values of the bond orders and the valence indices which in turn are quite close to the classical values. Bond orders obtained by a similarity transformation of theab initio density matrix differ appreciably in numerical magnitude.

Giambiagi’s definition of oxidation number of an atom in a molecule has been applied successfully in theab initio SCF theory to calculate oxidation numbers using STO-3G and 4–31G basis sets for some substituted benzenes involving -F, -OH, -CH₃ and -NH₂ as substituents. The present study suggests that the oxidation numbers also seem to be indicative of their orientational behaviour like the net atomic charges.