The title compound, O,O'-diethyl (3,4-dimethylanilin-1-yl) thiophosphonate, was obtained through the reaction of 3,4-dimethylaniline and P(S)[OC2H5]2Cl at ice-bath temperature, in a procedure similar to what was previously reported for an analogous compound. This compound was investigated by UV, IR, 31P/1H/13C NMR, mass spectrometry and X-ray crystallography techniques. The optimized molecular structure and vibrational frequencies were also investigated theoretically using Gaussian03 software package [2]. The HOMO and LUMO analysis are used to determine the charge transfer process in the molecule. Molecular stability arising from the hyperconjugative interactions and charge delocalization have been analyzed using the NBO analysis. UV absorption spectrum was also simulated using the time-dependent density functional theory (TD-DFT) at the M062X/6-311G++(d,p) level of the theory, in order to have a reasonable comparison with the experimental spectrum. It was found that some factors, such as solvent effect and intermolecular interaction affect on the observed λmax= 224 nm (5.53 eV). The highest energy occupied orbital is mainly localized on the thiophosphoryl group with a small contribution of the related –CHbenzene and –NH groups (Figure 1), while the lowest energy unoccupied orbital is mainly located on the benzene ring with some contributions of the CH3 moiety.

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