C–H ⋯ F Hydrogen Bond and Integral Intensities of Vinyl C–H Vibtations in Push-Pull β -Dimethylaminotrifluoromethyl Ketone and Its Deuterated Analog

Abstract

The accurate analysis of infrared spectra (both wavenumbers and intensities) of (E)-4-(dimethylamino)-1,1,1-trifluorobut-3-en-2-one (DMTBN) and (E)-4-(hexadeutero-dimethylamino)-1,1,1-trifluorobut-3-en-2-one (d 6 -DMTBN) revealed that besides intramolecular hydrogen bond in the (EE) conformer, these enaminoketones form cyclic dimers between the (EZ) and (EE) conformers due to intermolecular hydrogen bonds, namely, O=C and . Evaluation of constant and enthalpy of formation of these H-bonds revealed that O=C bond has greater and more negative than bond (cf. 214.4 M −1 , −21.7 kJ M −1 dm 3 , and 16.4 M −1 , −6.7 kJ M −1 dm 3 , resp.). Consequently, stronger H-bond ⋯ O=C is formed in the first place, whereas weaker H-bond is formed afterward. Moreover, formation of intermolecular hydrogen bond has influence on C–F vibrations, but analysis of this influence must take into account the fact that these vibrations in some cases are coupled with . True enthalpy of the equilibrium (EZ) ⇌ (EE) is positive (25.3 kJ M −1 dm 3 ), thus confirming results of DFT calculations, according to which the (EZ) conformer is more stable than the (EE) one.