Minimum energy structures, dissociation energies, and harmonic vibrational frequencies are determined for Ar(n)-HCO+ (n = 0-6) at the MP2/aug-cc-pVDZ level of theory. In all clusters, one Ar atom occupies an energetically preferred bonding site interacting with the hydrogen of HCO+ with a linear or near-linear Ar-H-C bond angle. The subsequent Ar atoms add by forming a ring around this Ar-HCO+ moiety with the detailed geometry determined largely by Ar/Ar interactions. The positions of the first five Ar atoms can be understood in terms of the crystal structure of solid argon whereas the sixth Ar atom produces a five membered Ar ring which has no analog in the solid phase of argon. Nonmonotonic variations in sequential binding energies of the Ar atoms are rationalized in terms of variations in the Ar(n)/HCO+ interaction energy and the differing number of Ar/Ar interactions involved. The HC stretching frequency (v1) is found to undergo a large redshift with the first Ar addition and then to slowly blueshift with subsequent Ar additions in agreement with the experimental results. The identities of two previously unassigned combination bands with v1 are established and the experimental assignment of a third combination band with v1 is confirmed. (C) 2000 Elsevier Science B.V.
- Ab initio
- Ionic clusters