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 ∼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 A^0- = 6005.275(8), B^0- = 930.546(8), C^0- = 803.9907(5) MHz, δ^0-_J = 0.076(1), Δ^0-_JK 0.70(2), and δ^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⋯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.