The anharmonic bending vibration of the NaCl dimer

A vibrational analysis of the NaCl dimer at high temperature is performed, with special attention given to a quantum mechanical study of the anharmonic, large amplitude B1u out-of-plane bending mode. This mode is the primary contributor to the perpendicular amplitude correction K, and the anharmonic, large amplitude K value for the Na—Cl bond is found to be only 0.0002 Å less than the standard harmonic value of 0.0159 Å at 1130 K. This corresponds to an average out-of-plane bending angle α¯ of 10°. The results of a standard ASYM20 harmonic analysis are then applied to extract an approximate r e(Na—Cl) value for the dimer of 2.514(40) Å, in excellent agreement with the best microwave/ ab initio value of 2.515 Å. Derived non-bonded r a to r e corrections are also reasonable, although the nona e bonded distances are best combined to determine the Cl—Na—Cl angle.