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  (12)COOH-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 Å for R(O3⋯H1) and 1.79 Å for R(H4⋯O1) were obtained. The "best fit" value for the angle(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  Å(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 6-311++G(d,p) and the results were compared to experimentally determined parameters.
Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, USA.
Adam M
AM
Daly
Bryan A
BA
Sargus
Stephen G
SG
Kukolich
D000465:Algorithms; D019281:Dimerization; D004583:Electrons; D005559:Formamides / Q000737:chemistry*; D005561:Formates / Q000737:chemistry*; D006860:Hydrogen Bonding; D016018:Least-Squares Analysis; D008872:Microwaves; D011522:Protons; D011789:Quantum Theory; D012399:Rotation; D013057:Spectrum Analysis
D005559:Formamides; D005561:Formates; D011522:Protons; C030544:formic acid; C031066:formamide