Microwave spectrum and structural parameters for the formamide-formic acid dimer

The rotational spectra for six isotopologues of the complex formed between formamide and formic acid have been measured using a pulsed-beam Fourier transform microwave spectrometer and analyzed to obtain rotational constants and quadrupole coupling parameters. The rotational constants and quadrupole coupling strengths obtained for H (COOH)-C-12-H-2 (NCOH)-N-14 are A = 5889.465 (2), B = 2148.7409(7), 1575.1234(6), eQq(aa) = 1.014(5), eQq(bb) = 1.99(1), and eQq(cc) = -3.00(1) MHz. Using the 15 rotational constants obtained for the H (COOH)-C-13, HCOOD, DCOOH, and H-2 (NCHO)-N-15 isotopologues, key structural parameters were obtained from a least-squares structure fit. Hydrogen bond distances of 1.78 angstrom for R(O-3 center dot center dot center dot H1) and 1.79 angstrom for R(H-4 center dot center dot center dot O1) were obtained. The "best fit" value for the Delta(C-O-H) of formic acid is significantly larger than the monomer value of 106.9 degrees with an optimum value of 121.7(3)degrees. The complex is nearly planar with inertial defect Delta =-0.158 amu angstrom(2). The formamide proton is moved out of the molecular plane by 15(3)degrees for the best fit structure. Density functional theory using B3PW91, HCTH407, and TPSS as well as MP2 and CCSD calculations were performed using 6-311++G(d,p) and the results were compared to experimentally determined parameters. (C) 2010 American Institute of Physics. [doi:10.1063/1.3501356]