Infrared Spectra of Mass-Selected Br¯−(NH_3)_n and I¯−NH_3 Clusters

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¯−NH3 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.