Conformational analysis of 2,2-difluoroethylamine hydrochloride: double gauche effect.
The gauche effect in fluorinated alkylammonium salts is well known and attributed either to an intramolecular hydrogen bond or to an electrostatic attraction between the positively charged nitrogen and the vicinal electronegative fluorine atom. This work reports the effect of adding a fluorine atom in 2-fluoroethylamine hydrochloride on the conformational isomerism of the resulting 2,2-difluoroethylamine chloride (2). The analysis was carried out using NMR coupling constants in D2O solution, in order to mimic the equilibrium conditions in a physiological medium, in the gas phase and in implicit water through theoretical calculations. Despite the presence of σCH→σ*CF and σCH→σ*CN interactions, which usually rule the hyperconjugative gauche effect in 1,2-disubstituted ethanes, the most important forces leading to the double gauche effect ((+)NH3 in the gauche relationship with both fluorine atoms) in 2 are the Lewis-type ones. Particularly, electrostatic interactions are operative even in water solution, where they should be significantly attenuated, whereas hyperconjugation and hydrogen bond have secondary importance.
Department of Chemistry, Federal University of Lavras, P. O. Box 3037, 37200-000, Lavras, MG, Brazil.
Josué M
JM
Silla
Chemistry Institute, State University of Campinas, P. O. Box 6154, 13084-971, Campinas, SP, Brazil.
Claudimar J
CJ
Duarte
Chemistry Institute, State University of Campinas, P. O. Box 6154, 13084-971, Campinas, SP, Brazil.
Rodrigo A
RA
Cormanich
Chemistry Institute, State University of Campinas, P. O. Box 6154, 13084-971, Campinas, SP, Brazil.
Roberto
R
Rittner
Department of Chemistry, Federal University of Lavras, P. O. Box 3037, 37200-000, Lavras, MG, Brazil.
Matheus P
MP
Freitas
2,2-difluoroethylamine hydrochloride; conformational analysis; gauche effect; hydrogen bonding