Microwave measurements of proton tunneling and structural parameters for the propiolic acid-formic acid dimer

Microwave spectra of the propiolic acid-formic acid doubly hydrogen bonded complex were measured in the 1 GHz to 21 GHz range using four different Fourier transform spectrometers. Rotational spectra for seven isotopologues were obtained. For the parent isotopologue, a total of 138 a-dipole transitions and 28 b-dipole transitions were measured for which the a-dipole transitions exhibited splittings of a few MHz into pairs of lines and the b-type dipole transitions were split by similar to 580 MHz. The transitions assigned to this complex were fit to obtain rotational and distortion constants for both tunneling levels: A(0+) = 6005.289(8), B0+ = 930.553(8), C0+ = 803.9948(6) MHz, Delta(0+)(J) = 0.075(1), Delta(0+)(JK) = 0.71(1), and delta(0+)(j) = -0.010(1) kHz and A(0-) = 6005.275(8), B0- = 930.546(8), C0- 803.9907(5) MHz, Delta(0-)(J) = 0.076(1), Delta(0-)(JK) = 0.70(2), and delta(0-)(j) = -0.008(1) kHz. Double resonance experiments were used on some transitions to verify assignments and to obtain splittings for cases when the b-dipole transitions were difficult to measure. The experimental difference in energy between the two tunneling states is 291.428(5) MHz for proton-proton exchange and 3.35(2) MHz for the deuterium-deuterium exchange. The vibration-rotation coupling constant between the two levels, F-ab, is 120.7(2) MHz for the proton-proton exchange. With one deuterium atom substituted in either of the hydrogen-bonding protons, the tunneling splittings were not observed for a-dipole transitions, supporting the assignment of the splitting to the concerted proton tunneling motion. The spectra were obtained using three Flygare-Balle type spectrometers and one chirped-pulse machine at the University of Virginia. Rotational constants and centrifugal distortion constants were obtained for HCOOH center dot center dot center dot HOOCCCH, (HCOOH)-C-13 center dot center dot center dot HOOCCCH, HCOOD center dot center dot center dot HOOCCCH, HCOOH center dot center dot center dot DOOCCCH, HCOOD center dot center dot center dot DOOCCCH, DCOOH center dot center dot center dot HOOCCCH, and DCOOD center dot center dot center dot center dot HOOCCCH. High-level ab initio calculations provided initial rotational constants for the complex, structural parameters, and some details of the proton tunneling potential energy surface. A least squares fit to the isotopic data reveals a planar structure that is slightly asymmetric in the OH distances. The formic OH center dot center dot center dot O propiolic hydrogen bond length is 1.8 angstrom and the propiolic OH center dot center dot center dot O formic hydrogen bond length is 1.6 angstrom, for the equilibrium configuration. The magnitude of the dipole moment was experimentally determined to be 1.95(3) x 10(-30) C m (0.584(8) D) for the 0(+) states and 1.92(5) x 10(-30) C m (0.576(14) D) for the 0(-) states. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3643720]