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

Abstract

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 ∼580 MHz. The transitions assigned to this complex were fit to obtain rotational and distortion constants for both tunneling levels: A0 = 6005.289(8), B0+ = 930.553(8), C0+ = 803.9948(6) MHz, Δ0+J = 0.075(1), Δ0+JK = 0.71(1), and δ0+j = -0.010(1) kHz and A0- = 6005.275(8), B0- = 930.546(8), C0- = 803.9907(5) MHz, δ0-J = 0.076(1), Δ0-JK 0.70(2), and δ0-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, Fab, 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⋯HOOCCCH, H13COOH⋯HOOCCCH, HCOOD⋯HOOCCCH, HCOOH⋯DOOCCCH, HCOOD⋯DOOCCCH, DCOOH⋯HOOCCCH, and DCOOD⋯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⋯O propiolic hydrogen bond length is 1.8Å and the propiolic OH⋯O formic hydrogen bond length is 1.6Å, for the equilibrium configuration. The magnitude of the dipole moment was experimentally determined to be 1.95(3) × 10 -30 C m (0.584(8) D) for the 0+ states and 1.92(5)×10-30 C m (0.576(14) D) for the 0- states. © 2011 American Institute of Physics.