The infrared spectrum of cyclic-N3: theoretical prediction.

We have carried out the first calculations of the infrared absorption spectrum of cyclic-N(3). Accurate vibrational energies and wave functions computed with incorporation of the geometric phase effect (via gauge theory) and using an ab initio potential energy surface were employed in this work. A sophisticated fully dimensional dipole moment function was constructed using accurate ab initio calculations and a three-dimensional-spline interpolation. Transformation of the dipole moment vector function from the reference frame associated with instantaneous principal axes of inertia to the laboratory-fixed reference frame was carried out using hyperspherical coordinates. We found that the permanent dipole moment of cyclic-N(3) in the ground vibrational state is relatively small (170 mD). The excited vibrational states show permanent dipole moments in the 10-25 mD range. The most intense part of the infrared absorption spectrum is observed in the deep infrared part of spectrum, 75-275 cm(-1), where five lines exhibit absolute absorption intensities in the range between 0.5 and 1.2 km/mol. These transitions correspond to excitation of the pseudorotational progression of states. Several unique spectroscopic features discussed in the paper should help to identify cyclic-N(3) in the laboratory.