Microwave measurements of the spectra and molecular structure for the monoenolic tautomer of 1,2- cyclohexanedione

The microwave spectrum for the monoenolic tautomer of 1,2-cyclohexanedione was measured in the 4-14 GHz regime using a pulsed-beam Fourier transform (PBFT), Flygare-Balle-type microwave spectrometer. The molecular structure and moments of inertia were initially calculated using Gaussian 09 using MP2 and 6-311++G∗∗ basis sets, and these calculations were used to predict the rotational constants and microwave spectra. Rotational transition frequencies were measured and used to determine rotational constants (A, B, and C) and centrifugal distortion constants (D_J and D_K). The rotational constants for the parent isotopologue, one singly substituted deuterium and six singly substituted ¹³C isotopologues, were used in a least-squares fit to determine gas-phase structural parameters for this molecule. All hydrogen atoms were held fixed to the calculated positions, as well as the carbon atoms at positions 1 and 10 and the oxygen atoms at positions 6 and 7. The rotational constants for the parent isotopologue are A = 3161.6006(12), B = 2101.5426(3), and C = 1320.7976(4) MHz. The distortion constants obtained from the fit are D_J = 0.0436 and D_K = 0.436 kHz. Structural parameters from the MP2 calculations are in fair agreement with the measured parameters.