Infrared spectra of mass-selected Br(-)-(NH3)n and I(-)-NH3 clusters.

ABSTRACT

Infrared vibrational predissociation spectra are recorded for Br(-)-(NH(3))(n) (n = 1-4) and I(-)-NH(3) clusters in the N-H stretch region (3040-3460 cm(-1)). To aid spectral assignments and clarify structures of the Br(-)-(NH(3))(n) clusters, ab initio calculations are performed at the MP2/aug-cc-pVDZ and MP2/aug-cc-pVTZ levels of theory. The Br(-)-NH(3) and I(-)-NH(3) dimers are predicted to have structures in which the NH(3) molecule is attached to the halide anion by a single hydrogen-bond. The dominant infrared band for Br(-)-NH(3) at 3171 cm(-1) corresponds to a hydrogen-bonded N-H stretch vibrational mode, whereas two weaker bands are assigned to a symmetric stretch vibration of the nonbonded N-H groups (3347 cm(-1)) and to an ammonia-based bending overtone (3293 cm(-1)) deriving infrared intensity through Fermi interaction with the H-bonded N-H stretch mode. The corresponding I(-)-NH(3) spectrum is dominated by the H-bonded N-H stretch band at 3217 cm(-1), with three weaker bands at 3240, 3305, and 3360 cm(-1) assigned to two bending overtone vibrations and the nonbonded N-H symmetric stretch vibration, respectively. Spectra of the Br(-)-(NH(3))(n), n = 2-4, clusters are similar to the I(-)-NH(3) spectrum, exhibiting evidence for strong Fermi interactions between the H-bonded N-H stretch vibrational mode and ammonia-based bending overtones. On the basis of the infrared spectra and ab initio calculations, the larger Br(-)-(NH(3))(n) clusters are deduced to have structures in which the NH(3) molecules are attached to the Br(-) by single H-bonds, but not necessarily to one other.