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

Abstract

We report the rotational spectra of two conformers of the acetic acid-difluoroacetic acid adduct (CH3COOH-CHF2COOH) and supply information on its internal dynamics. The two conformers differ from each other, depending on the trans or gauche orientation of the terminal -CHF2 group. Both conformers display splittings of the rotational transitions, due to the internal rotation of the methyl group of acetic acid. The corresponding barriers are determined to be V3(trans)=99.8(3) and V3(gauche)=90.5(9) cm(-1) (where V3 is the methyl rotation barrier height). The gauche form displays a further doubling of the rotational transitions, due to the tunneling motion of the -CHF2 group between its two equivalent conformations. The corresponding B2 barrier is estimated to be 108(2) cm(-1). The increase in the distance between the two monomers upon OH→OD deuteration (the Ubbelohde effect) is determined.

Interactions between Carboxylic Acids and Heteroaromatics: A Rotational Study of Formic Acid–Pyridine

“…Actually, to reproduce the rotational constants also for the HCOOD-PYR isotopologue, an increase of about 7 mÅ of the r N1O12 distance is required. This behavior is surprising, because generally an increase of the distance between the two heavy atoms involved in the hydrogen bond is observed upon deuteration (Ubbelohde effect) for double minima potential describing the proton transfer, as in dimers of carboxylic acids. − Vice versa, in the cases of complexes with the two subunits held together by single O–H···O or O–H···N single hydrogen bonds a shortening of the distance between the two heavy atoms involved in the hydrogen bond–called the reverse Ubbelohde effectis generally observed upon deuteration . However, in the case of HCOOH-PYR, plausibly a proton transfer takes place, and we are no longer dealing with a “normal” single hydrogen bond.…”

“…This aspect is pointed out in an NMR investigation of adducts of carboxylic acids with ( 15 N)PYR. 36 The authors of this paper outline that the heteronuclear scalar 1 H− 15 N coupling constants between the hydrogen bond proton and the 15 N nucleus of pyridine show that the proton is remarkably shifted from the acid to pyridine. d. Information on the Dissociation Energy.…”

“…Several molecular complexes involving carboxylic acids have been investigated by rotational spectroscopy, − to understand the nature of their noncovalent interactions and to have information on their internal dynamics and on their conformational equilibria. Most attention has been paid to their dimers − and adducts with water. − Plenty of data have been obtained on the dimers, concerning proton tunneling, the Ubbelohde effect, and conformational equilibria.…”

A butterfly motion of formic acid and cyclobutanone in the 1 : 1 hydrogen bonded molecular cluster

“…However, unexpectedly, DE 01 (HCOODÁ Á ÁCBU) = 1180.282 MHz is larger than the value of the parent species. This is probably related to the Ubbelohde effect, 40 often observed in the complexes of carboxylic acids, [8][9][10][11][12][13][14] and related to the change in the bond length of the O-HÁ Á ÁO hydrogen bond upon H -D substitution.…”

“…a CH 3 group); 1,5,18 (iii) inversion of an asymmetric group (e.g. a hydroxyl or carboxylic acid) connecting two equivalent minima 11,12,14,19 or (iv) internal motion of water in hydrated forms of carboxylic acids. [15][16][17][18][19][20][21] Here we report the direct observation in the jet-cooled spectra of huge rotational splittings arising from a relative motion of a partner as heavy as cyclobutanone (CBU) with respect to formic acid (FA) in their 1 : 1 complex (FA-CBU).…”