Proton Tunneling in Heterodimers of Carboxylic Acids: A Rotational Study of the Benzoic Acid–Formic Acid Bimolecule

Abstract

Tunneling effects have been measured in the pulsed jet Fourier transform microwave spectra of two isotopologues of the benzoic acid-formic acid bimolecule. The tunneling splittings are originated by the concerted proton transfer of the two carboxylic hydrogens. From the values of these splittings for the OH-OH and OD-OD species, it has been possible to model/size the barrier to the concerted double proton transfer.

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.…”

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.…”

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.…”

“…[13] The formic acid dimer is the prototype system for this kind of investigation, but the lack of dipole moment inhibits rotational investigation. [4] In some cases, the barrier to the double proton transfer has been calculated from the splittings measured in several isotopologues. [14] Tunneling splittings in homo-dimers have been measured by rotationally resolved laser-induced fluorescence in the ground and first electronic-excited states of the benzoic acid dimer, [15] and recently by pure rotational spectroscopy in the homo-dimer of acrylic 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. [3,4] Reports on the conformational equilibria in dimers of carboxylic acids have only been available for about a year, addressing two conformers of acrylic acid-formic acid, [5] acrylic acid-trifluoroacetic acid, [6] and difluoroacetic acid-formic acid, [7] as well as three conformers of acrylic acid-fluoroacetic acid [8] and four conformers of acrylic acid-difluoroacetic 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.…”