Structural and energetic properties of the Br−–C2H2 anion complex from rotationally resolved mid-infrared spectra and ab initio calculations

Abstract

An infrared vibrational predissociation spectrum of the Br−79–C2H2 anion complex has been recorded over the 2800–3400 cm−1 range. Bands are observed that correspond to excitation of bound and free C–H stretches of an acetylene molecule engaged in a linear hydrogen bond with Br−. The band associated with the bound C–H stretch displays rotationally resolved substructure. Lower J transitions are absent from the predissociation spectrum, indicating that the upper levels lie below the dissociation threshold. Analysis leads to constants for lower and upper states: v0=2981.28, B″=0.048 84, ΔB=9.3×10−4 cm−1, and a minimum J′=28 for dissociation. The rotational constants correspond to vibrationally averaged separation between Br− and the C2H2 center of mass of 4.11 Å in the ground state and 4.07 Å in the v3 state. A dissociation energy for Br−–C2H2 of 3020±3 cm−1 is estimated from the energy of the lowest dissociating level. The spectroscopically derived data are corroborated by ab initio calculations conducted at the MP2/aug-cc-pVTZ level.