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

摘要

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 C ~(12)OOH- H_2~(14)NCOH 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 ~(13)COOH, HCOOD, DCOOH, and H_2~(15)NCHO isotopologues, key structural parameters were obtained from a least-squares structure fit. Hydrogen bond distances of 1.78 ? for R (O3?H) and 1.79 ? for R (H4?O1) were obtained. The "best fit" value for the (C-O-H) of formic acid is significantly larger than the monomer value of 106.9° with an optimum value of 121.7(3)°. The complex is nearly planar with inertial defect Δ=-0.158 amu ?~2. The formamide proton is moved out of the molecular plane by 15(3)° for the best fit structure. Density functional theory using B3PW91, HCTH407, and TPSS as well as MP2 and CCSD calculations were performed using 6-~1++G (d,p) and the results were compared to experimentally determined parameters.