MICROWAVE-SPECTRUM AND STRUCTURE OF THE ARGON FORMIC-ACID VAN-DER-WAALS COMPLEX

The argon-formic acid van der Waals dimer was studied by Fourier trans form microwave spectroscopy. The structure was found to be planar (or very nearly so), with the argon attracted to the acidic hydrogen and c arbonyl oxygen. This was determined from assigning the rotational spec trum of three isotopic species: normal, Ar.DCOOH, and Ar.(HCOOH)-C-13. The distance from the argon to the carbonyl oxygen was 3.71(3) Angstr om, to the acidic hydrogen 2.81(3) Angstrom, and to the center of mass of formic acid 3.732(2) Angstrom. The deuterium quadrupole coupling c onstants were determined for the formic acid-d(1) (DCOOH) complex as c hi(aa) = 135(2) kHz, chi(bb) = -75(2) kHz. The dipole moment was also determined as mu = 1.305(1) D. An induction model was employed to expl ain the decrease in the dipole moment compared to free formic acid. A dispersive model was used to rationalize the structural data. The bind ing energy of the dimer was estimated to be 144.4(2) cm(-1) from centr ifugal distortion data and a Lennard-Jones potential.