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

Abstract

Four conformers of the heterodimer o-anisic acidformic acid, formedi nas upersonic expansion,h ave been probed by Fourier transform microwave spectroscopy.T wo of theseformshave the typical double intermolecularhydrogen-bond cyclic structure. The other two show the o-anisic acid moiety bearing a trans-COOH arrangement supported by an intramolecular OÀH···Ob ond to the neighbor methoxy group. In these conformers, formic acid interacts with o-anisic acid mainly through an intermolecular OÀH···O hy-drogen bond either to the OÀHortothe C=Omoieties, reinforced by other weaki nteractions. Surprisingly,t he most abundantc onformer in the supersonic expansion is the complex in which the o-anisica cid is in trans arrangement with the formic acid interacting with the OÀHg roup. Such a trans-COOH arrangement in which the intramolecular hydrogen bond dominates over the usually observed double intermolecular hydrogen bond interactionh as never been observed previously in an acid-acid dimer.Supporting information and the ORCID identification number(s) for the author(s) of this articlecan be found under: https://doi.

Conformational Panorama and Chirality Controlled Structure–Energy Relationship in a Chiral Carboxylic Acid Dimer

“…The difference may be related to: 1) much higher partial vapor pressures and dominant cis formic acid and acetic acid, favoring formation of Type 1 dimers under pre‐expansion conditions; 2) the very different slit versus circular (current) nozzle assembly used; 3) a different THFA conformational distribution where trans dominates. One may appreciate the last point by looking at an “in‐between” case of a heterodimer consisting of formic acid and o ‐anisic acid which exists in three conformations T1 ( trans ), C1, and C2 ( cis ) in decreasing abundance . While the mixed dimer containing T1 remains the strongest, noticeable deviation from a statistical distribution was reported.…”

“…Finally, we discuss Type 3 RR and RS I ‐ I in Figure which were the focus of our initial experimental efforts, based on the study of o ‐anisic acid⋅⋅⋅formic acid . However, no transitions of I – I could be identified experimentally.…”

“…Rotational spectroscopic studies of carboxylic acid dimers have focused mainly on proton tunneling and conformational diversity, where dimers adopt a cyclic double hydrogen‐bonded structure with two cis ‐COOH subunits. Only very recently, an acid dimer containing a trans ‐COOH subunit was detected experimentally . Although the list of the acid dimers studied by rotational spectroscopy is long, no study of a chiral acid dimer has thus far been reported despite several attempts, hinting at significant experimental challenges.…”

Microwave Detection of Wet Triacetone Triperoxide (TATP): Non‐Covalent Forces and Water Dynamics

“…They make possible the unambiguous discrimination between different isomers, also including tautomers, conformers, or isotopomers. The spatial mass distribution of such species results in unique moments of inertia reflected by different rotational constants and distinct individual rotational spectra, a key feature that makes these techniques powerful tools [24, 25] in cases where other techniques may struggle.…”

Conformational Landscape of m-Anisic Acid and Its Complexes with Formic Acid

“…7,8,[12][13][14][15][16][17][18][19][20][21] The most recent ones focused on the proton transfer process, 7,8,[14][15][16][17] or on the study of conformational equilibria. [18][19][20][21] The systems for which the highest number of different acid-acid complex conformations have been observed are acrylic acid-difluoroacetic acid cluster 7 and o-AA-FA complex 21 with four conformers in each case. The most interesting aspect of the last system is the observation of two forms of the complex with o-AA bearing a trans-COOH arrangement, the first time that such uncommon complex structures were observed.…”