Microwave spectra and structure of the cyclopropanecarboxylic acid-formic acid dimer.

The rotational spectrum of the cyclopropanecarboxylic acid-formic acid doubly hydrogen bonded dimer has been measured in the 4-11 GHz region using a Flygare-Balle type pulsed-beam Fourier transform microwave spectrometer. Rotational transitions were measured for the parent, four unique singly substituted (13)C isotopologues, and a singly deuterated isotopologue. Splittings due to a possible concerted double proton tunneling motion were not observed. Rotational constants (A, B, and C) and centrifugal distortion constants (DJ and DJK) were determined from the measured transitions for the dimer. The values of the rotational (in MHz) and centrifugal distortion constants (in kHz) for the parent isotopologue are A = 4045.4193(16), B = 740.583 80(14), C = 658.567 60(23), DJ = 0.0499(16), and DJK = 0.108(14). A partial gas phase structure of the dimer was derived from the rotational constants of the measured isotopologues, previous structural work on each monomer units and results of the calculations.