The Barrier to Proton Transfer in the Dimer of Formic Acid: A Pure Rotational Study

Abstract

The rotational spectra of three C-deuterated isotopologues of the dimer of formic acid have been measured, thanks to the small dipole moment induced by asymmetric H! Ds ubstitution(s). Fort he DCOOH-HCOOH species,t he concerted double proton transfer of the two hydroxy hydrogen atoms takes place between two equivalent minima and generates at unneling splitting of 331.2(6) MHz. This splitting can be reproduced by a3Dmodel with abarrier of 2559 cm À1 (30.6 kJ mol À1 )aso btained from theoretical calculations.Carboxylic acids form considerably stable dimers,t hat are sufficiently abundant at experimental conditions in as upersonic jet expansion. An umber of molecular complexes involving carboxylic acids [1][2][3][4][5][6][7][8][9][10] have been investigated by rotational spectroscopy,t ou nderstand the nature of their non-covalent interactions and to have information on their internal dynamics and on their conformational equilibria. Much attention has been paid to the complexes of carboxylic acids,m ainly to their dimers [1] and to their adducts with water. [2] Plenty of data have been obtained on the hydrogenbonded dimers involving proton tunneling,t he Ubbelohde effect and conformational equilibria. [1] HCOOH (formic acid; FA)isthe prototype of the carboxylic acids family and for this reason it is involved in most of the investigations of carboxylic acids bonded to molecules containing other functional groups, such as its adducts with H 2 O, [2a] N(CH 3 ) 3 , [3] CO 2 , [4] thesimplest aldehyde CH 2 O, [5] thesimplest amide H(CO)NH 2 , [6] ethers, [7] ketones, [8] freons, [9] azines [10] and its reactivity with alcohols. [11] An interesting feature of the dimer of FA,( FA) 2 ,i st he concerted transfer of the two hydroxylic protons,amotion in ad ouble minimum potential. It can generate tunneling doubling observable in molecular spectra, and makes them useful to studies of the tunneling process.T he precise tunneling splittings (DE 01 )m easured by microwave (MW) spectroscopy for the acrylic acid dimer [1g] and for the formic acid-benzoic acid bi-molecule [1h] have been reproduced by model calculations with barriers at or close to values obtained ab initio.In Figure 1w es how the proton transfer motion in the formic acid dimer along as ymmetric double minimum potential energy profile.

Competition Between Intra‐ and Intermolecular Hydrogen Bonding:o‐Anisic Acid⋅⋅⋅Formic Acid Heterodimer

“…[7,8] In particular,j et-cooled Fouriert ransform microwave (FTMW) spectroscopy has been demonstrated as one of the most sensitive and powerful techniques with which to obtain precise information about the structure and dynamics of molecular complexes. [9][10][11] For example, FTMW spectroscopy was utilized in the recent analysiso ft he formic acid dimer, [11] among alarge list of acid-acid heterodimers. [12][13][14][15][16][17][18][19][20] Some of the reportedw ork focuses on proton transfer, [13][14][15][16] which is relatedt og eneticm utations of DNA.…”

High-dimensional potential energy surfaces for molecular simulations: from empiricism to machine learning

“…The parametrization used here was adapted to yield the correct infrared spectrum in the gas phase [185]. Recent high-resolution work has confirmed that the barrier of 7.3 kcal mol −1 for the gas-phase PES is compatible with the tunneling splitting observed in microwave studies [186]. Such a barrier height makes spontaneous transitions rare.…”

The Hydrogen Bond and Beyond: Perspectives for Rotational Investigations of Non‐Covalent Interactions

“…However, the H!Da symmetric substitution generates at iny dipole moment, which allowed the observation of the tunneling due to the concerted double proton transfer. [56] Trimers andt etramers have been observed by Xu et al on mono- [57] and trifluorinated [58] ethanol,s tabilized by ac ombination of hydroxyl Figure 4. A) Antigeared and B) gearedi nstanton tunneling pathways [51] in the water hexamer prism,t ogether with C) the variation of the potential energy along the instanton paths.…”

“…The dimer should not be observable because of a lack of a permanent dipole moment. However, the H→D asymmetric substitution generates a tiny dipole moment, which allowed the observation of the tunneling due to the concerted double proton transfer . Trimers and tetramers have been observed by Xu et al.…”