Proton Transfer in Homodimers of Carboxylic Acids: The Rotational Spectrum of the Dimer of Acrylic Acid

Abstract

The dimer of acrylic acid can exist in two forms, depending on the entgegen or zusammen orientations of the two allyl groups. The latter one (zusammen) has a permanent value of the μ(b) dipole moment component, which allowed measuring its pulsed jet Fourier transform microwave (MW) spectrum. From the tunneling splitting, originating in the concerted proton transfer of the two carboxylic hydrogen atoms and measured for four isotopologues of such a bimolecule, we could determine the barrier and dynamics of the proton transfer.

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

“…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. The results provided by the analysis of gas phase dimers by microwave spectroscopy are twofold: (1) accurate gas phase structures of the dimer can be determined and (2) provide a description of the dynamics regarding concerted proton tunneling.…”

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.…”

“…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.…”

“…The corresponding barriers are determined to be V 3 (trans) = 99.8 (3) and V 3 (gauche) = 90.5(9) cm À1 (where V 3 is the methyl rotation barrier height). The two conformers differ from each other, depending on the trans or gauche orientation of the terminal ÀCHF 2 group.…”