Conformational equilibria in formic acid and the adduct of formic acid and hexafluoroacetone, HCO2C(CF3)2OH.
Low-temperature 1H and 13C NMR spectra of formic acid (1) showed separate signals for the E and Z conformations in solvents containing a hydrogen bond acceptor, dimethyl ether. The population of E-1 (6.2% in 3:1:1 CHClF2/CHCl2F/(CH3)2O) was larger than that for 13C-labeled methyl formate in the same solvent (0.2%), which indicated that the relative populations are not determined by steric effects. The free-energy difference between the E and Z conformations of 1 was 0.9 kcal/mol. In a 1:3 CD2Cl2/(CH3)2O solvent mixture, peaks for E and Z conformations were found at low temperatures by 1H and 13C NMR for both formic acid and an adduct with hexafluoroacetone, HCO2C(CF3)2OH (2). The population of E-1 in this solvent mixture was 4.3% by 13C NMR. The carbon spectrum showed two peaks in the carbonyl carbon region of nearly equal intensities at -151.6 degrees C, with E-2 (48%) absorbing downfield of the major Z-2 (52%). The large population of E-2 confirms that electron-withdrawing groups R' in RCO2R' enhance the populations of the E-isomers. The free-energy barriers for 2 of 6.24 (E-to-Z) and 6.26 kcal/mol (Z-to-E) were determined from rate constants obtained by line shape analysis at -143.2 degrees C.
Pawar
Diwakar M
DM
Department of Chemistry, Jackson State University, P.O. Box 17910, 1400 J. R. Lynch Street, Jackson, Mississippi 39217-0510, USA.
Cain-Davis
Dalephine
D
Noe
Eric A
EA
D000096:Acetone / Q000031:analogs & derivatives* / Q000737:chemistry; D005466:Fluorocarbons / Q000737:chemistry*; D005561:Formates / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D012997:Solvents / Q000737:chemistry; D013237:Stereoisomerism; D013816:Thermodynamics; D006845:Hydrocarbons, Fluorinated
D000096:Acetone; D005466:Fluorocarbons; D005561:Formates; D012997:Solvents; C030544:formic acid; C017366:hexafluoroacetone; D006845:Hydrocarbons, Fluorinated