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 (d6-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？1dm3, and 16.4？M？1, ？6.7？kJ？M？1dm3, 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？1dm3), thus confirming results of DFT calculations, according to which the (EZ) conformer is more stable than the (EE) one. 1. Introduction From spectroscopic experiments Allerhand and Schleyer [1] qualitatively concluded that the ability of a C–H group to form weak hydrogen bonds depends on carbon hybridization, as C(sp1)–H > C(sp2)–H > C(sp3)–H and increases with the number of adjacent electron-withdrawing groups. The enhancement of the C–H donor strength by neighboring electronegative groups is often called “activation” of C–H. It is well known that hydrogen bonds in general are composed of different types of interactions [2]. As for all intermolecular interactions, there is a nondirectional “van-der-Waals” contribution, which is weakly bonding at long distances (by dispersion forces) and strongly nonbonding at short distances (by exchange repulsion). At their optimal geometry, van der Waals interactions contribute about 1？kJ？mol？1 to the hydrogen bond energy. An electrostatic component (dipole-dipole, dipole-charge, etc.) is directional and bonding at all distances. It reduces with increasing distance and with reducing dipole moments or charge involved. For donors like O–H or N–H, the electrostatic component is the dominant one in hydrogen bond (several kJ mol？1). This is also true for strongly polarized C–H groups (up to 8？kJ？mol？1), whereas for weakly polarized C–H groups the electrostatic component is of similar magnitude to the van der Waals contribution [3]. Only for the strongest types of hydrogen bonds does a charge-transfer component become important [2]; it
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