C–H⋯F Hydrogen Bond and Integral Intensities of Vinyl C–H Vi�tations in �ush��ull �Di�ethyla�inotri�uoro�ethyl Ketone and Its Deuterated Analog

e accurate analysis of infrared spectra (both wavenumbers and intensities) of (E)-4-(dimethylamino)-1,1,1-tri�uorobut-3en-2-one (DMTBN) and (E)-4-(hexadeutero-dimethylamino)-1,1,1-tri�uorobut-3-en-2-one (d6-DMTBN) revealed that besides intramolecular hydrogen bond C–Hββ ⋯ F–C in the (EE) conformer, these enaminoketones form cyclic dimers between the (EZ) and (EE) conformers due to intermolecular hydrogen bonds, namely, C–Hαα ⋯O=C and C–Hαα ⋯ F–C. Evaluation of constant KKH-complex and enthalpy of formation of these H-bonds (−ΔHHH revealed that C–Hαα ⋯O=C bond has greater KKH-complex and more negative ΔHH than C–Hαα ⋯ F–C bond (cf. 214.4M , −21.7 kJMdm, and 16.4M, −6.7 kJMdm, resp.). Consequently, stronger H-bond C–Hαα ⋯O=C is formed in the �rst place, whereas weaker H-bond C–Hαα ⋯ F–C is formed aerward. Moreover, formation of intermolecular C–Hαα ⋯ F–C hydrogen bond has in�uence on C–F vibrations, but analysis of this in�uence must take into account the fact that these vibrations in some cases are coupled with δδCHαα . True enthalpy of the equilibrium (EZ)⇌(EE) is positive (25.3 kJMdm), thus con�rming results of DFT calculations, according to which the (EZ) conformer is more stable than the (EE) one.


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(sp 1 )-H > C(sp 2 )-H > C(sp 3 )-H and increases with the number of adjacent electron-withdrawing groups.e enhancement of the C-H donor strength by neighboring electronegative groups is oen 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 ).is 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 does not play a relevant role for weak C-H⋯F interactions.e above circumstances have far-reaching consequences.Firstly, electrostatic component varies smoothly with varying geometry and diminishes only slowly with increasing distance; this leads to a pronounced soness of the hydrogen bond geometry.e C-H⋯F interactions can be easily stretched, compressed, and bent from optimal geometry.Secondly, the bonding situation is strongly dependent on the donor and acceptor polarizations; therefore, weak hydrogen bonds involving polarizable groups can be critically in�uenced by their surroundings.irdly, with falling C-H polarization, the directional electrostatic component is reduced, whereas the isotropic van der Waals component is unaffected; the net interaction therefore loses directionality.Fourthly, upon compression, the van der Waals contribution becomes repulsive and might even result in a positive (i.e., nonbonding) net energy; this will be more important than the smaller the electrostatic component is.In C-H⋯F bonds, the net charge on carbon may be negative; this is the classical hydrogen bonding situation (C  -H  ⋯F  ).Carbon may also carry a positive partial charge (C  -H  ⋯F  ); this also results in electrostatic attraction between donor and acceptor (with different directionality characteristics).e typical partial charges involved are roughly 0 to +0.2 e units on H, and depend strongly on the particular system under study [3,4].e bond energies of C-H⋯F have been much debated.e energies are small and difficult to determine experimentally [5], so that our knowledge is mainly based on computations.Earlier theoretical studies have been very inconsistent, but modern quantum chemical calculations seem to provide realistic energy estimations, at least for simple molecular systems in vacuo (i.e., isolated dimers).eoretical calculations have shown that the strength of an F⋯H bond could be estimated between 8.0 and 13.4 kJ mol 1 [5], lower value corresponding to CH⋯F hydrogen bond.For comparison, currently, there is consensus that CH⋯O energies are typically ≤8 kJ mol 1 and gradually fade away with increasing H⋯O separation [2,3].It must be stressed that the above data are valid only for the particular situation they were calculated for: hydrogen bonds are strongly in�uenced by their surroundings, such as by the solvent and by other hydrogen bonds, so that interaction strengths in the liquid or solid state will in general case deviate from values mentioned above.
Earlier [5] it has been shown that there are some crystalline structures, where the stability seems to stem from C-F⋯H-C hydrogen bonds.In many cases the distance between �uorine atom and hydrogen atom of second molecule was found to be 2.3 Å, that is, shorter than the sum of the van der Waals radii of the two atoms considered.Moreover, authors [6,7] studying various F-substituted derivatives of (E)-2,3-diphenyl propenoic acid molecules by experimental (FT-IR spectroscopy) and computational (semiempirical and DTF) methods found that �uorine engaged in C-H⋯F hydrogen bonding easily; the distance of H⋯F is 1.96 to 2.06 Å.

Experimental Section
2.1.General.Carbon tetrachloride was purchased in Aldrich.It was puri�ed using standard techniques and was dried over appropriate drying agent before use.

Infrared Spectra.
Infrared spectra were recorded on a Bruker Vertex 70 FTIR spectrometer with KBr beamsplitter and RT-DLaTGS detector at the room temperature (20 ± 1 ∘ C).For all spectra 32 scans recorded at 2 cm 1 resolution were averaged.Solution spectra were measured in carbon tetrachloride using standard NaCl cells with pathlengths 0.0022, 0.0063 0.01028, 0.0212, 0.0521, and 0.102 cm (for dilution measurements).Temperature measurements were carried out in thermostated (±0.05 ∘ C) NaCl cell with pathlength 0.00875 cm.e solutions were scanned at the same conditions as a background.e Bruker Opus soware Version 6.0 was used for all data manipulation.

Results
IR spectrum of DMTBN is presented on Figure 1.e most intensive bands are in the region of vibrations of double bonds (1700-1500 cm 1 ) and C-F bonds (1400-1000 cm 1 ), whereas C-H vibrations are very weak (3100-2800 cm 1 ).

e 3100-2700 cm 𝛿1
Region.In this region the ole�nic C-H stretching, symmetric and asymmetric CH 3 stretching bands are expected to be observed.Infrared spectrum of DMTBN shows four weak bands at 3127, 3104, 3059, and 3023 and one very strong and broad band with intricate shape at 2930 cm 1 , respectively (Figure 2).Upon deuteration of dimethylamino group four distinct C-H bands are observed at 3134, 3099, 3052, and 3019 cm 1 in infrared spectrum of (d 6 -DMTBN) (see Figure 3), whereas broad and very intensive band at 2931 cm 1 disappears, and two new bands at 2218 and 2072 cm 1 appear.As was shown in [10] theoretical calculations predict that the ole�nic CH stretching wavenumber should be higher than the wavenumbers of CH and CH  (EZ) vibrations.In 1,1,1-tri�uoro-2,4-pentanedione [11], which is exclusively in (ZZ) con�guration due to intramolecular H-bonding, the ole�nic CH  stretching mode occurs at 3120 cm −1 .Similarly, in cis-1,2-di�uoroethylene the (C-H) wavenumber is 3133 cm −1 [4], therefore we ascribed the bands at 3134 and 3099 cm −1 to (C-H  ) mode.Increase of total concentration of d 6 -DMTBN results in increase of intensity of the band at 3134 cm −1 with synchronous decrease of intensity of the band at 3099 cm −1 (Figure 3).Temperature rise induces opposite phenomenon: intensity decrease of the former band with simultaneous increase of the latter band.e same effect we have observed earlier for two (C=O) bands of d 6 -DMTBN, corresponding to (EE) and (EZ) conformers [8].In particular, we have showed that concentration increase calls forth intensity increase of (C=O) band of (EE) conformer with simultaneous intensity decrease of (C=O) band of (EZ) conformer, whereas temperature rise has adverse effect.Taking into account this fact we ascribed bands at 3134 and 3099 cm −1 to (C-H  ) of (EE) and (EZ) conformer, respectively.
Aer deconvolution of IR spectrum of d 6 -DMTBN (see Figure 4) the integrated intensity of each (C-H  ) band corresponding to (EE) and (EZ) conformer was calculated according to (1).
where  (L mol −1 cm − ) is integrated intensity of the (C-H  ) band,   (mol/L) is the concentration of the given conformer in solution,  (cm) stands for the cell pathlength.e main problem was to evaluate the concentration of each stereoisomeric form   .Since all enaminoketones studied were presented as equilibrium (EZ)⇌(EE), which depended on total concentration of enaminoketone and temperature, we used the method, proposed earlier [8] for analysis of conformational equilibria.We plotted integrals of (C-H  ) (EZ) versus integrals of appropriate (C-H  ) (EE) at various enaminoketone concentrations ( total ) provided the product  total × pathlength was constant (Figure 5).Intercepts of this plot with axes were integrals of (C-H  ) bands of the (EZ) and (EE) conformer, respectively, but for all that the concentration of the conformer equaled the total concentration of enaminoketone,  total .Knowing the cell pathlength and  total , it was easy to calculate integrated intensities  of the (C-H  ) band of each conformer, and, hence, to evaluate from (1) the concentration of each stereoisomeric form,   , at given total concentration of enaminoketone.As the consequence it became possible to evaluate the constant  eq of the equilibrium (EZ)⇌(EE).As long as this equilibrium depended on total concentration of enaminoketone [8], we estimated  eq for various concentrations of d 6 -DMTBN and plotted ln  eq versus  total .we approximated obtained plot by the (2): In Table 1 we listed parameters of (2) calculated for investigated equilibrium from integral intensities of (C-H  ).Second pair of bands, namely, at 3052 and 3019 cm −1 , we attributed to (C-H  ) bands of (EE) and (EZ) conformer, respectively.In contrast to (C-H  ) bands of these conformers an increase of total concentration of d 6 -DMTBN resulted in increase of intensity of the both bands (see Figure 3).Using obtained concentrations we calculated integral intensities of (C-H  ) bands of (EE) and (EZ) conformers, respectively.It turned out that these integral intensities were not constant but depended on total concentration of the enaminoketone (e.g., see Figures 6(a) and 6(b)).Such behavior became clear when we assumed ad hoc that (EZ) and (EE) conformers of (d 6 -DMTBN) form complexes with intermolecular H  -bonds (Scheme 1).It is well known that complex formation with participation of C-H bond results in increase of intensity [12] and leads to red shi of this band [13,14], hence integral intensities obtained were the sum of integral intensities of free and bound (C-H  ).
From (1) it follows that integral of the (C-H  ) band is expressed by (3): and (EZ) conformer and evaluated  complex in assumption that formation of H-complex is in accord with Scheme 1.
As it has turned out a value of  complex for  C-H  (EE) was higher as compared with  complex for  C-H  (EZ) (cf.127.04 and 90.57M −1 , resp.).
Increase of total concentration of enaminoketone shied the bands (C-H  ) of (EE) and (EZ) conformer to lower wavenumbers (Figures 7(a) and 7(b), whereas wavenumbers of (C-H  ) bands remained almost invariable (the shi does not exceed 1 cm −1 ).is difference in (C-H) behavior is additional vindication of H-complex formation exclusively via C-H  moiety.−1 Region.In IR spectra of (DMTBN) and (DMTBN-d 6 ) there were two weak (C=O) bands at 1686 and 1672 cm −1 (DMTBN, in CCl 4 ) and 1691 and 1671 cm −1 (d 6 -DMTBN, in CCl 4 ), respectively.Earlier [8] we ascribed (C=O) band with higher wavenumber to the (EZ) and that with lower wavenumber to the (EE) conformer, respectively.e pro�le of a very intensive (C=C) band (Figure 1) ex facte creates an impression of single band, but thorough analysis showed [8] that there are two overlapped bands at 1588 and 1593 cm −1 (DMTBN), which we attributed to the (EZ) and (EE) conformers, respectively.It is worthy to note that in highly conjugated systems -C=C-C=O (C=O) and (C=C) vibrations are strongly coupled, possessing, in large measure, the character of out-of-phase and in-phase vibrational modes, respectively [10,11,[15][16][17].Nevertheless, we shall continue to use the (C=O) and (C=C) as a convenient form of shorthand.

e 1800-1500 cm
Aer band �tting the integrated intensity of each (C=O) band was calculated according to procedure described for (C=O) bands of DMTBN [8].We calculated  eq for various concentrations of d S 1: Formation of H-bonded "circular" complex between the (EZ) and (EE) conformers.
ln  eq versus  total was exponentially rised described with (2).Parameters of this equation are listed in Table 1.Temperature measurements of infrared spectrum of d 6 -DMTBN gave an opportunity to estimate thermodynamic parameters of equilibrium Δ eq and Δ eq , which were equal to 9.86 ± 0.62 kJ M −1 and −203.4 ± 2.1 J M −1 K −1 , respectively.Method of calculation was described in detail elsewhere [8].
It is necessary to note that wavenumbers of  C=O (EZ) and  C=O (EE) band behave in different way when total concentration of d 6 -DMTBN increases.While the  C=O (EE) decreases exponentially, the  C=O (EZ) is virtually stable.Comparing dependence of  C=O (EE) and  C=O (EZ) from total d 6 -DMTBN concentration one can easily come to conclusion that the carbonyl group of (EE) conformer, in contrast to (EZ) conformer, is incorporated into intermolecular hydrogen bond formation.erefore, behavior of both   − H  (EE) (Figure 7) and  C=O (EE) (Figure 11) indicates C-H  ⋯O=C bond formation.−1 Region.In this region of infrared spectrum of (DNTBN) six strong bands at 1421, 1289, 1191, 1142, and 1074 cm −1 were observed (Figure 8).Under deuteration the band at 1441 cm −1 disappeared, therefore we ascribed it to deformation CH 3 mode.In (tri�uoroacetyl) acetone (TAA) this mode was at 1461 and 1413 cm −1 [11].In (TAA), which has the same motif -C=C-C(CF 3 )=O the band at 1280 cm −1 was attributed to C-CF 3 + C = C +   C-CH 3 .At the same time authors [11] attributed this band to C-CH 3 + C-CF 3 + CF 3 + C = C.In 2-thenoyltri�uoroacetone (2-TTA) [18] the bands at 1283, 1279, and 1274 cm −1 were assigned to OH + C-CF 3 + C = C + C-thio + 15.Hence, the band at 1290 cm −1 is mainly due to CF 3 coupled with C-CF 3 and C=C.In d 6 -DMTBN this band shied to 1281 cm −1 as a result of mass increasing of (CD 3 )N-C=C moiety.

e 1500-800 cm
Wavenumber and intensity of this band depended on total concentration of d 6 -DMTBN and temperature: increase of concentration shied this band to lower wavenumbers with simultaneous intensity increase (Table 2, Figure 9).As it has been assumed previously (vide supra) CF 3 group participates in intermolecular hydrogen bond C-H⋯F-C (Scheme 1) between (EE) and (EZ) conformer, therefore increase of total concentration of d 6 -DMTBN shied the equilibrium (EZ) + (EE)⇌(EZ)⋯(EE) to the right.Consequently, wavenumber of C-F stretching vibration lowered and the intensity of this mode increased.Temperature increase shied the equilibrium to the le, thus increasing wavenumber and decreasing intensity of C-F stretching vibration (Figure 10).e strong band at 1191 cm −1 was observed in IR spectrum of DMTBN.Deuteration induced practically no changes on the position of this band: in deuterated d 6 -DMTBN this band is shied on 1 cm −1 (1190 cm −1 , see Figure 9).We assigned this band to   CF3 slightly coupled with CH  by analogy with (TAA), where very strong infrared band at 1200 cm −1 was correlated to the 1227 cm −1 DFT calculated wavenumber, so it was assigned mainly to the CF 3 stretching mode coupled to C-CH 3 (12%), OH (14%), and CH  (20%) [10].
In the work [11] this band was assigned to   CF 3 + CH.e band at 1199 cm −1 was assigned to   CF 3 in 2-TTA [18].Increase of the concentration of (d 6 -DMTBN) shied this band to lower wavenumbers, whereas the intensity was practically invariable (Figure 10).is apparent insensitivity of intensity is a result of cancellation of two effects: increase of CF 3 intensity due to participation in intermolecular hydrogen bond C-H⋯F-C and decrease of CH  intensity assuming that this deformation vibration refers to CH  of the (EZ) conformer, which percentage decreases with concentration increase.Temperature rise shied this band to higher wavenumbers and decreased its intensity (Table 2).As it clearly can be seen from Figure 10 intensity decrease of this band is much less pronounced than that for band at 1290 cm −1 probably due to partial compensation of  a CF 3 intensity decrease by CH  (EE) intensity increase.
In spectrum of (DMTBN) there was another strong band at 1142 cm −1 , which did not change position under deuteration (Figure 9).We attributed this band to  s CF 3 coupled to CH  .In IR spectrum of (TTA) this band is at 1180 cm −1 .Authors [10] assigned this band to symmetric CF 3 stretching mode, which has been slightly coupled to CH  (10%) and OH (9%).In the work [11] this band was absent, whereas there was a band at 1160 cm −1 , which authors assigned to CH + C-C +  s CF 3 .Moreover, in IR spectra of (2-TTA) [18] the bands at 1165 and 1162 cm −1 were assigned to   CF 3 .Just as the band at 1191 cm −1 , concentration rise of (d 6 -DMTBN) shied the band at 1142 cm −1 to lower wavenumbers, whereas its intensity changed very slightly (Figure 11).Here again intensity behavior was a consequence of adverse effects: increase of CF 3 intensity due to participation in intermolecular hydrogen bond C-H⋯F-C and decrease of intensity of deformation CH  vibrations of (EZ) conformer, percentage of which decreased with increase of total (d 6 -DMTBN) concentration.Temperature rise shied this band to higher wavenumbers, simultaneously decreasing its intensity (Table 2).Again, this decrease of intensity was very similar to that of the band at 1191 cm −1 , most probably, due to partial compensation of   CF 3 intensity decrease by CH  (EZ) intensity increase (see Figure 11).At last, very strong band at 1074 cm −1 (DMTBN, Figure 9), which shied to higher wavenumbers under deuteration (to 1091 cm −1 in IR spectrum of d 6 -DMTBN) we attributed to CH  , strongly coupled to CF 3 .In IR spectrum of (TAA) this band is observed at 1110 cm −1 .Authors [10] assigned it to CH  + CF 3 (31%) + C-C (15%) mode, whereas in the work [11] the band at 1109 cm −1 was assigned to  s CF 3 + CH + C-C.In (2-TTA) [18] the band at 1110 cm −1 was assigned to CH  +  s CF 3 + C-C + 6.In contrast with previously examined bands increase of total concentration of d 6 -DMTBN resulted in shi of this band to higher wavenumbers (Table 2) with synchronous increase of its intensity (Figure 10).Bearing in mind the ability of the enaminoketone (d 6 -DMTBN) to form complex between (EE) and (EZ) conformers due to intermolecular hydrogen bond C-H⋯F-C (Scheme 1) it is clear that rise of total concentration of (d 6 -DMTBN) will increase the percentage of this complex, thus increasing both wavenumber and intensity of CH  .On the other hand, temperature rise disrupts the complex, decreasing wavenumber and intensity of deformation C-H  mode, which prevails in the band at 1047 cm −1 .e band at 906 cm −1 shied to 855 cm −1 under deuteration, hence we ascribed it to N-CH 3 + C=C-O +CF 3 + CH  by analogy with (TAA) [10,11].

Calculation of ermodynamic Parameters of C-
⋯F-C and C-  ⋯O=C Hydrogen Bonds.Enthalpies of C-H  ⋯F-C and C-H  ⋯O=C bond formation were calculated according to simple (5), derived by Iogansen [12]: where −Δ is enthalpy of hydrogen bond formation (kJ dm 3 /mol), Δ 1/ =  1/ −  0 1/ denotes the intensity enhancement of the (C-H  ) stretching vibration.Values  and  0 are integral intensity of (C-H  ) in monomer and in complex with intermolecular hydrogen bond, respectively.From temperature dependence of pro�le of (C-H  ) bands we calculated  complex (Scheme 1) for various temperatures.en from plot of ln  complex versus 1/ (Figure 12) we evaluated enthalpies (−Δ) and entropies (Δ) of aforementioned hydrogen bonds.Results obtained are listed in Table 3.

Discussion
In Table 4 where  CH / CH and  H / CH are, respectively, the derivative of the electrical dipole moment  of C-H bond and charge  H referred to hydrogen atom with respect of the interatomic distance  CH , and  0 H is an equilibrium charge referred to hydrogen atom.Gussoni et al. [19][20][21][22] found that  0 H and  H / CH are very important markers of the charge distribution inside the molecule and are directly related to the molecule structure, the vibration potential, and the molecular conformation.In particular, from (6) it follows that when the charge �ux is sizable, both  0 H and  H / CH give a considerable contribution to  str CH .When the carbon atom has either sp  or sp 3 hybridization the �ux  H / CH is practically constant and amounts approximately to −0.2 e/Å [22].Since the equilibrium charge on hydrogen is always positive in C-H bonds and turns out to be always less than 0.2 e, when carbon atom has sp  or sp 3 hybridization,  H / CH is negative, but its absolute value decreases when the charge increases and so does the  str CH .is effect is clearly observed passing from ethane to 1.1-di�uoroethylene and 1.2-di�uoroethylene ( simultaneous decrease of  str CH indicates that introduction of �uorine to an ethylene molecule induces a stronger polarization of a CH bond, which makes the  0 H charge to increase; also force constant ( CH ) increases while  0 CH and  str CH decrease [4].Backdonation also may take place, but its role in �uorinated ethylenes is insigni�cant.Induction of stronger polarization of CH bond occurs when an electronegative atom is in molecule: in�uence of nitrogen in the studied enaminoketones invokes the same effect on adjacent C  −H bond as a �uorine atom in substituted ethylenes (Table 4).
As is obvious from Table 4 wavenumber  str CH of C  −H bond for (EE) conformer is signi�cantly higher then for (EZ) conformer of (d 6 -DMTBN).Decrease of wavenumber  str CH in the (EZ) conformer as compared to it in the (EE) conformer is a result of stronger polarization of C  -H bond in former one which makes the  0 H charge increase, as a consequence  CH also increases.It should be expected that  str CH decreases when equilibrium charge  0 H increases but integral intensity T 3: ermodynamic parameters of =C-H  ⋯F−C and =C-H  ⋯O=C hydrogen bonds. str CH of the (EE) conformer is considerably larger than  str CH of the (EZ) conformer.As was stated earlier [8] H  hydrogen atom is involved in formation of intramolecular C  -H⋯F-C hydrogen bond, hence  str CH is increased in the (EE) conformer as a result of increase of the effective charge / in (7), which characterizes the additive model of Hbond [12]: where  0 / and  H / H are effective charges of C-H and H⋯F bonds, respectively.Wavenumbers  str CH of C  -H stretching vibration of the (EE) and (EZ) conformers are lower than that of C  -H mode although electron withdrawal power of tri�uorocarbonyl group is much higher comparing with dimethylamino group.It is obvious that in highly conjugated systems similar to enaminoketones [8,9] complicated variations in the charge �uxes take place, and any analysis based only on variations of the charge may turn out to be incorrect, therefore we believe that much more ECCF investigations of various conjugated molecules should be obtained before deriving any conclusion.It is notable that difference in  str C  H values of the (EE) and (EZ) conformer is small (0.6 km mol   5. the involvement of C  -H bond in intramolecular C  -H⋯F hydrogen bond.Concentration dependence of pro�le of IR spectrum of (d 6 -DMTBN) in the region of  str CH vibrations re�ects not only conformation equilibrium (EZ)⇌(EE), but also equilibrium between monomeric conformers and their Hbonded complex (see Scheme 1).Moreover, analysis of pro�le of IR spectra of (DMTBN) and (d 6 -DMTBN) in the region of carbonyl vibrations revealed that the only  str C=O of (EE) conformer shis to lower wavenumbers with increase of enaminoketone concentration (see Figure 8), whereas  str C=O of the (EZ) conformer is independent of enaminoketone  total .erefore, we reached a conclusion that carbonyl of the (EE) conformer takes part in formation of intermolecular H-bond with C-H  of the (EZ) conformer.Simultaneously intermolecular C-H  ⋯F-C bond is formed, thus cyclic dimer appears as it is presented on Scheme 1. Considering that H-complex is 1 : 1 we evaluated equilibrium constant  H-complex for both C-H  ⋯O=C and C-H  ⋯F-C hydrogen bond from (8): Equilibrium constant  H-complex for C-H  ⋯O=C H-bond formation is much higher than that for C-H  ⋯F-C bond formation (viz., 214.4 and 16.4 M −1 ).Enthalpy of hydrogen bond formation (Δ), evaluated from temperature dependence of  H-complex , revealed that Δ for C-H  ⋯O=C Hbond is more negative than that for C-H  ⋯F-C, hence the former H-bond is stronger as compared with the latter (cf.values of −21.7 and −6.7 kJ mol −1 dm 3 , resp., Table 3).5.
us, when concentration of (d 6 -DMTBN) increases, dimer between the (EE) and (EZ) conformers is formed due to intermolecular H-bonds, more strong H-bond C-H  ⋯O=C forms in the �rst place, whereas the bond C-H  ⋯F-C forms aerward.It is notable that enthalpies of H-bond formation, evaluated from Iogansen equation ( 5) and from temperature measurements, are in good agreement (see Table 3).Nevertheless, some discrepancies arise from insufficient accuracy in evaluation of  mon .
As it was established earlier [8] there is an intramolecular H-bond C-H  ⋯F-C in the (EE) conformer of (d 6 -DMTBN), therefore thermodynamic parameters evaluated for equilibrium (EZ)⇌(EE) are complex and combine parameters of an intramolecular C-H  ⋯ F-C and intermolecular H-bonds (see Scheme 1) in accord with (9).In other words, conversion of the (EE) conformer to the (EZ) conformer, which takes place during dilution of (d 6 -DMTBN) solution or at temperature rise, is accompanied with breaking of the intra-and intermolecular hydrogen bonds: where Δ total , Δ conf , Δ intra , and Δ inter are enthalpy of equilibrium (EZ)⇌(EE) evaluated experimentally from temperature measurements (Table 3), true enthalpy of this equilibrium, enthalpy of an intramolecular H-bond C-H  ⋯F−C formation, and enthalpy of an intermolecular C-H  ⋯F-C and C-H  ⋯O=C H-bond formation, respectively.Using (9) and assuming value Δ intra , to be close to that of an intermolecular C-H  ⋯F-C H-bond formation (ca −6.7 kJ mol −1 dm 3 ) we calculated Δ conf which turned out to be positive and equaled 25.3 kJ mol −1 dm 3 .is value consists of enthalpy of equilibrium (EZ)⇌(EE) evaluated experimentally for (CD 3 ) 2 N-CH=CH-C(O)CH 3 (with respect to 16.4 kJ mol −1 dm 3 [8]) where formation of intramolecular H-bond and cyclic dimer is impossible.Moreover, DFT calculations revealed that the (EZ) conformer of (DMTBN) is more stable than the (EE) conformer [8].is result conforms to positive enthalpy of equilibrium (EZ)⇌(EE) (see above), thus con�rming validity of supposition of intra-and intermolecular H-bond formation in studied enaminoketones.
Intermolecular C-H  ⋯F-C hydrogen bond formation has considerable impact on C-F vibrations at 1280 ÷ 1090 cm −1 .So far as these vibrations were coupled with deformation CH  vibrations (see Table 5), the in�uence of C-H  ⋯F-C hydrogen bond was ambiguous.e band CF 3 at 1281 cm −1 was not coupled with CH  , therefore increase of (d 6 -DNTBN) concentration changed both wavenumber and intensity of this vibration.It is well known that participation in hydrogen bonding, even relatively weak, shis the band to lower wavenumbers with synchronous intensity increase [12].As can be seen from Figure 13(a) this increase shied the band at 1281 cm −1 to lower wavenumbers signi�cantly with appreciable intensity increase (Figure 10).On the other hand, vibrations   CF 3 and   CF 3 (at 1190 and 1142 cm −1 , resp.), strongly coupled with CH  , shied to lower wavenumbers in much lesser degree (in comparison with bond at 1281 cm −1 ) when concentration of (d 6 -DNTBN) increased.Intensities of these bands also decreased insigni�cantly (Figure 10).In this case we observed the result of cancellation of two effects: increasing of concentration of complex with C-H  ⋯F-C bond and simultaneous decrease of the (EZ) conformer concentration.Contribution of CH  (EZ) decreased, thus compensating the increase of intensity of (C-F) vibration.Temperature rise disrupted C-H  ⋯F-C bond but simultaneously increased concentration of (EZ) conformer, therefore decrease of wavenumbers and intensities of these bands is negligible (see Figures 14 and 11, resp.).At the same time the band CF 3 ) at 1091 cm −1 was strongly coupled with CH.Moreover, contribution of the latter mode exceeded the contribution of the former vibration, therefore behavior of this band differed from behavior of other bands examined above.Increase of (d 6 -DNTBN) concentration shied this band to higher wavenumbers (Figure 13(d)) with simultaneous increase of band intensity (Figure 10).e observed effect was a result of involvement of C-H into intermolecular C-H⋯F-C H-bonding [23] in cyclic dimer, which concentration rised when total concentration of enaminoketone increased.Temperature increase broke this hydrogen bond, thus shiing CH to lower wavenumbers and diminishing band intensity.

Conclusion
ere are four bands in IR spectra of studied enaminoketone (DNTBN) and its deuterated analog (d 6 -DNTBN) in the region of (C-H), corresponding to  EE CH  ,  EZ CH  ,  EE CH  , and  EZ CH  ;  CH of the (EE) conformer being at higher wavenumbers than  CH of the (EZ) conformer.Wavenumbers and intensities of these bands depend not only on temperature, but also on concentration of enaminoketone, thus re�ecting presence of two kinds of equilibrium: equilibrium of the (EE) and (EZ) conformer, and equilibrium between monomeric conformers and their dimer.While variations of wavenumbers and intensities of  EE CH  and  EZ CH  vibrations are called forth by changes in the conformer equilibrium exclusively, appropriate variations of wavenumbers and intensities of  EE CH  and  EZ CH  vibrations are caused by changes of both conformer and monomer-dimer equilibrium.Increase of enaminoketone concentration shis the equilibrium (EZ)⇌(EE) to the right, thus increasing equilibrium constant,  eq .Simultaneously the equilibrium (EZ) + (EE)⇌(EZ⋯EE) is shied towards cyclic dimer, which formation is due to intermolecular C-H  ⋯O=C and C-H  ⋯F-C hydrogen bonds.Evaluation of constant  H-complex and enthalpy of formation of these H-bonds Δ) revealed that C-H  ⋯O=C bond has greater  H-complex and more negative Δ than the C-H  ⋯F-C H-bond.As a consequence, the former bond is formed at �rst, whereas the latter bond being weaker is generated much slower.Evaluation of true enthalpy of the equilibrium (EZ)⇌(EE) revealed that the value of Δ is positive, thus conforming with results of DFT calculations, according to which the (EZ) conformer is more stable than the (EE) one.Formation of the C-H  ⋯F-C hydrogen bond effects  str CF vibrations, but analy�ing this in�uence one must take into account that these vibrations in some cases are coupled with  CH  .

F 5 :
Plot of integral of (C-H  ) of (EE) versus integral of (C-H  ) of (EZ) for d 6 -DMTBN in CCl 4 .

1 F 13 :
Band at 1090 cm −Plots of  (C-F) a versus concentration of d 6 -DNTBN.a Band attribution is given in Table
mon is integral intensity of free (C-H  ),  complex is integral intensity of bound (C-H  ), and  mon and  complex -respectively, concentration of free conformer and conformer bound in complex with intermolecular H-bond.Values  mon and  complex for respective conformers were estimated from plots of integral intensities  versus total concentrations of d T 1: Parameters of (2) calculated for equilibrium (EZ)⇌(EE) from integral intensities of (C-H  ) and (C=O).6 -DMTBN (Figures 6(a) and 6(b).Using (3) together with (4) we calculated concentrations of Hcomplexes formed by (EE) 6-DMTBN using values  CO (EZ) and  CO (EE) (3.36 km M −1 and 7.69 km M −1 , resp.).Plot of F 6: Plot of A(C-H  ) versus  total for d 6 -DMTBN in CCl 4 : (a) for (EE) conformer; (b) for (EZ) conformer.
we listed  str CH , the infrared intensity due to C-H stretching in molecules containing vinyl moiety.According to Equilibrium Charges  and Charge Fluxes (  /  )

Table 4 )
. Increase of  str CH with Spectrum of enaminoketone DMTBN (thick line) and d 6 -DMTBN (thin line) in the region of C-F vibrations.T 2: Dependence of wavenumbers and intensities of CF 3 stretching vibrations on concentration of d 6 -DTNBN and temperature.F 10: Plots of integral versus  total for bands at 1280, 1190, 1142, and 1190 cm −1 of d 6 -DMTBN (in CCl 4 ).
−1 dm 3 ) T 4: Wavenumbers  str C-H and integral intensities  str C-H of C-H bonds of vinyl moiety.To ambiguities italics indicates the CH  group to which refers.In parentheses relevant isomer/conformer is denoted.C=C + CF 3 + CH  and signi�cantly lower than difference in  str C  H of respective conformers (2.8 km mol −1 dm 3 ).We attributed this fact to