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: A(0+) = 6005.289(8), B(0+) = 930.553(8), C(0+) = 803.9948(6) MHz, Δ(0+)(J) = 0.075(1), Δ(0+)(JK) = 0.71(1), and δ(0+)(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, H(13)COOH···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 1.8 Å and the propiolic OH···O formic hydrogen bond length is 1.6 Å, 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.

Conformational Equilibria and Large‐Amplitude Motions in Dimers of Carboxylic Acids: Rotational Spectrum of Acetic Acid–Difluoroacetic Acid

“…Recently, rotational spectroscopy has been used to investigate several of these dimers in the gas phase, which allows the description of several interesting features: 1) tunneling splittings, due to the concerted double proton transfer connecting two equivalent forms; [1][2][3][4] 2) conformational equilibria when the proton transfer, or other feasible large-amplitude motion, connects different configurations of the complex; [5][6][7][8][9][10] and 3) the Ubbelohde effect, [11] which is the structural change of the bimolecular system upon OH!OD isotopic substitution. Recently, rotational spectroscopy has been used to investigate several of these dimers in the gas phase, which allows the description of several interesting features: 1) tunneling splittings, due to the concerted double proton transfer connecting two equivalent forms; [1][2][3][4] 2) conformational equilibria when the proton transfer, or other feasible large-amplitude motion, connects different configurations of the complex; [5][6][7][8][9][10] and 3) the Ubbelohde effect, [11] which is the structural change of the bimolecular system upon OH!OD isotopic substitution.…”

“…This complication takes place, for example, in complexes involving trifluoroacetic acid. [3] Tunneling doubling has also been measured in some hetero-dimers, such as formic acid-propriolic acid, [1] formic acid-acetic acid, [2] and benzoic acid-formic acid. However, tunneling splittings in this complex have been observed when using techniques other than microwave (MW) spectroscopy, such as femtosecond degenerate four-wave mixing and Raman spectroscopy.…”

Gas phase measurements of mono-fluoro-benzoic acids and the dimer of 3-fluoro-benzoic acid

“…These frequencies were not acid-formic acid dimer. 2,3 These transitions would provide a more direct measure of ∆E, the tunneling frequency, but are likely too weak to obtain in this large complex. Both acrylic acid-homodimer and 3F-benzoic homodimer have the symmetry axis as the b-axis which is the component of the non-zero permanent dipole.…”

“…Since the publication of results for the formic acid homodimer measurements in the infrared by Havenith et al, 1 there have been studies made on heterodimers in the microwave region and one study on a homodimer. A permanent dipole moment allows the study by rotational spectroscopy and the following systems have been reported: propiolic acid-formic acid, [2][3][4] acetic acid-formic acid, 5 formic acid-fluoroacetic acid, 6 formic acid-trifluoroacetic acid, and acetic acid-trifluoroacetic acid, 7 formic acid-benzoic acid, 8 and more recently the dimers of acrylic acid with fluoroacetic acid. 9 The analysis of the homodimer of acrylic acid 10 demonstrated proton tunneling amongst identical molecules.…”

Gas Phase Measurements of Mono-Fluoro-Benzoic Acids and the Dimer of 3-Fluoro-Benzoic Acid

“…Since the publication of results for the formic acid homodimer measurements in the infrared by Havenith et al, 1 there have been studies made on heterodimers in the microwave region and one study on a homodimer. A permanent dipole moment allows the study by rotational spectroscopy and the following systems have been reported: propiolic acid-formic acid, [2][3][4] acetic acid-formic acid, 5 formic acid-fluoroacetic acid, 6 formic acid-trifluoroacetic acid, and acetic acid-trifluoroacetic acid, 7 formic acid-benzoic acid, 8 and more recently the dimers of acrylic acid with fluoroacetic acid. 9 The analysis of the homodimer of acrylic acid 10 demonstrated proton tunneling amongst identical molecules.…”