Pairwise and nonpairwise additive forces in weakly bound complexes: High resolution infrared spectroscopy of ArnDF (n=1,2,3)

Abstract

High resolution infrared spectra of the vDF=1←0 stretch in ArnDF (n=1–3) have been recorded using a slit-jet infrared spectrometer. Analysis of the rotationally resolved spectra provides vibrationally averaged geometries and vibrational origins for a DF chromophore sequentially ‘‘solvated’’ by Ar atoms. Calculations using pairwise additive Ar–Ar and Ar–DF potentials predict lowest energy equilibrium structures consistent with the vibrationally averaged geometries inferred spectroscopically. Variational calculations by Ernesti and Hutson [A. Ernesti and J. M. Hutson, Faraday Discuss. Chem. Soc. (1994)] using pairwise additive potentials predict rotational constants which are in qualitative agreement with, but consistently larger than, the experimental values. The inclusion of nonpairwise additive (three-body) terms improves the agreement, though still not to within the uncertainty of the pair potentials. The vibrational redshifts of 8.696, 11.677, and 14.461 cm−1 for n=1–3, respectively, reflect a nonlinear dependence of the redshift on the number of Ar atoms. Both the variational calculations of Ernesti and Hutson and diffusion quantum Monte Carlo calculations [M. Lewerenz, J. Chem. Phys. (in press)] using pairwise additive potentials systematically overpredict the magnitude of these redshifts, further signifying the need for corrective three-body terms. Analysis of the ArnDF (n=2,3) rovibrational line shapes reveals an upper limit to homogeneous broadening on the order of 2–3 MHz, consistent with vibrational predissociation lifetimes in excess of 50 ns.