Mono and di-substituted ammonia borane: A computational study

This work reports vibrational spectra and electronic absorption spectra along with the first and second hyperpolarizabilities for ammonia borane, mono and disubstituted ammonia boranes using quantum chemical methods. The substituents used here are Cl, Br and F. A large difference in the vibrational and electronic absorption spectra is observed upon mono and disubstitution. All the electronic transitions for the mono as well as disubstituted ammonia borane molecules are found to be σ → σ* transitions. Direction of applied field and method dependent hyperpolarizabilities are obtained. The hyperpolarizabilities are found to be higher when the field applied is along Y direction than that applied in X or Z direction. The first hyperpolarizability is found to be enhanced upon mono as well as disubstitution in ammonia borane. Mono as well as disubstituted ammonia boranes are not good materials for second order nonlinear optics.