Microwave measurements of proton tunneling and structural parameters for the propiolic acid - formic acid dimer.
Adam M. Daly, Kevin O. Douglass, Laszlo C. Sarkozy, Justin L. Neill, Matt T. Muckle, Daniel P. Zaleski, Brooks H. Pate, Stephen G. Kukolich
Microwave spectra of the propiolic acid-formic acid, (Pro-FA) doubly hydrogen bonded complex were measured in the 1-21 GHz range. Rotational spectra for seven isotopomers were obtained and fit. For the parent isotopomer, 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 approximately 580 MHz. The transitions assigned to this complex were fit to obtain rotational and distortion constants for both tunneling vibration levels. 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 vibrational 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, Fc, 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. 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 reveal a planar structure that is slightly asymmetric in the OH distances. The formic OHO propiolic hydrogen bond length is 1.8 Å and the propiolic OHO formic hydrogen bond length is 1.6 Å, for the equilibrium configurations, The magnitude of the dipole moment was experimentally determined to be 0.579(4) D for 0+ and 0.571(7) D for 0- states.
, Douglass, K.
, Sarkozy, L.
, Neill, J.
, Muckle, M.
, Zaleski, D.
, Pate, B.
and Kukolich, S.
Microwave measurements of proton tunneling and structural parameters for the propiolic acid – formic acid dimer., Journal of Chemical Physics, [online], https://doi.org/10.1063/1.3643720, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=908927
(Accessed December 3, 2023)