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

An infrared vibrational predissociation spectrum of the Br-79(-)-C2H2 anion complex has been recorded over the 2800-3400 cm(-1) range. Bands are obser ved that correspond to excitation of bound and free C-H stretches of an ace tylene molecule engaged in a linear hydrogen bond with Br-. The band associ ated with the bound C-H stretch displays rotationally resolved substructure . Lower J transitions are absent from the predissociation spectrum, indicat ing that the upper levels lie below the dissociation threshold. Analysis le ads to constants for lower and upper states: v(0)=2981.28, B'=0.048 84, Del ta B=9.3x10(-4) cm(-1), and a minimum J'=28 for dissociation. The rotationa l constants correspond to vibrationally averaged separation between Br- and the C2H2 center of mass of 4.11 Angstrom in the ground state and 4.07 Angs trom in the v(3) state. A dissociation energy for Br--C2H2 of 3020 +/- 3 cm (-1) is estimated from the energy of the lowest dissociating level. The spe ctroscopically derived data are corroborated by ab initio calculations cond ucted at the MP2/aug-cc-pVTZ level. (C) 2000 American Institute of Physics. [S0021-9606(00)00926-0].