EXPERIMENTAL AND THEORETICAL VIBRATIONAL STUDIES OF COVALENT X-N-3 AZIDES (X=H, F, CL, BR, I) - APPLICATION OF THE DENSITY-FUNCTIONAL THEORY AND COMPARISON WITH AB-INITIO RESULTS

The infrared spectra of gaseous chlorine azide, bromine azide and iodine azide as well as the Raman spectrum of liquid chlorine azide were recorded. The geometries and vibrational spectra of all halogen azides XN(3) (X = F, Cl, Br, I) and HN3 were computed employing density functional theory calculations (DFT) at the self-consistent level with the nonlocal exchange functional of Becke (B) and the nonlocal correlation functional of Lee-Yang-Parr(B-LYP). The results were compared with those previously reported by Frenking et al. (Inorg. Chem. 1992, 31, 3647-3655) using quantum chemical ab initio methods at the Hartree-Fock and correlated levels. The results of DFT calculations and experimentally obtained vibrational spectra indicate that the nu(X-N-3) stretching modes (X = Cl, Br, I) had previously been assigned incorrectly. The general agreement between the computed geometries at correlated levels (ab initio, MP2; DFT, B-LYP) and the observed structures is very good. Whereas the ab initio calculations predict too high frequencies due to the neglect of electron correlation and of anharmonicity and require scaling, the DFT computations at the correlated level (B-LYP) predict for all covalent azides considered the vibrational modes remarkably well and no scaling was required.