Microwave spectrum and molecular structure parameters for the 1,2-cyclohexanedione (monoenolic)–formic acid dimer

The microwave spectrum for the 1,2-cyclohexanedione (monoenolic)–formic acid dimer was measured for four isotopologues in the 4.5–9 GHz range using a Flygare–Balle type spectrometer. Rotational and distortion constants (A, B, C, DJ, DJK) were obtained. Measured rotational constants were used in a least squares fit to determine some of the gas phase structural parameters of the dimer. Rotational constants and distortion constants of the parent isotopologue are A = 2415.044(18) MHz, B = 543.6907(2) MHz, C = 451.6663(2) MHz, DJ = 0.0220(13) kHz, and DJK = 0.119(31) kHz. The experimental hydrogen bond lengths are 1.97 Å, somewhat longer than the values calculated using gaussian 09 with MP2/6-311++G**.