The optimized molecular structures of cis and trans conformers of 1,3-diphenyltriazene (DPT) molecule have been calculated using ab initio Hartree–Fock (HF), density functional theory (B3LYP) and second order Müller–Pleaset (MP2) methods with 6–311++G(d,p) basis set level. The trans conformer with a planar geometry have been found more stable than the twisted cis conformer. This observation is in good agreement with experimental results. The various cis–trans interconversion mechanisms have been studied. These mechanisms include Ninversion, NAN bond rotation, 1,3-hydrogen shift and ‘‘non-constrained software proposed route”. The geometries and energies of transition states for the proposed mechanism have been calculated using DFT method. The NAN bond rotation mechanism is the energetically less demanding mechanism. The ‘‘non-constrained” interconversion route proposed by Gaussian program estimates the same energy barrier as the least energy demanding mechanism, namely NAN bond rotation and its transition state resembles more to the stable conformer, namely the trans conformer, than the transition state of the other mechanisms. To estimate the solvent effect in the liquid states the PCM method was employed. Both isomers of DPT and all transition state structures reoptimized in water as a polar solvent. All structures stabilized more in water, but order of stability did not change. Optimization of trans dimer supermolecule lead to a bound state with negative binding energy and slightly deformed trans monomer while such calculation for cis conformer was unsuccessful. Electronic transition wavelength (k) as well as oscillator strength (f ) for each conformer calculated.
