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ISRN Spectroscopy
Volume 2013 (2013), Article ID 640896, 13 pages
http://dx.doi.org/10.1155/2013/640896
Research Article

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

Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, ul. Murmanska 1, Kiev 02094, Ukraine

Received 22 August 2012; Accepted 26 September 2012

Academic Editors: M. Mączka and S. Pandey

Copyright © 2013 Sergei Vdovenko et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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 (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 does not play a relevant role for weak C–HF interactions. The 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 softness of the hydrogen bond geometry. The C–HF 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 influenced by their surroundings. Thirdly, 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–HF bonds, the net charge on carbon may be negative; this is the classical hydrogen bonding situation (Cδ–Hδ+Fδ). Carbon may also carry a positive partial charge (Cδ+–Hδ+Fδ); this also results in electrostatic attraction between donor and acceptor (with different directionality characteristics). The 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]. The bond energies of C–HF have been much debated. The 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). Theoretical calculations have shown that the strength of an FH bond could be estimated between 8.0 and 13.4 kJ mol−1 [5], lower value corresponding to C−HF hydrogen bond. For comparison, currently, there is consensus that C−HO energies are typically ≤8 kJ mol−1 and gradually fade away with increasing HO 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 influenced 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–FH–C hydrogen bonds. In many cases the distance between fluorine 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 fluorine engaged in C–HF hydrogen bonding easily; the distance of HF is 1.96 to 2.06 Å.

Studying push-pull -dimethylaminovinyl trifluoromethyl ketone by methods of FTIR, NMR spectroscopy, and quantum chemical calculations we have ascertained that there is additional intramolecular −HF–CF2 interaction, which stabilizes (EE) conformer. Besides, quantum chemical calculations revealed the distance between hydrogen and fluorine atoms to be quite short (2.409 Å) [8]. Moreover, quantum chemical calculations for various -dialkylaminovinyl perfluoromethyl ketones also showed a distance HF in (EE) conformers to be in the range 2.379 ÷ 2.436 Å [9]. Continuing to study peculiarities of special and electronic structure of fluor containing enaminoketones we analyzed Fourier IR spectra 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) enaminoketone in the region of ν(C–H), ν(C=O) and ν(C–F) stretching vibrations. To our knowledge no such study has been carried out until this time, so we have decided to present basic IR experimental data and their interpretation concerning intra-and intermolecular hydrogen bonds of the (DMTBN) and its deuterated analog (d6-DMTBN).

2. Experimental Section

2.1. General

Carbon tetrachloride was purchased in Aldrich. It was purified using standard techniques and was dried over appropriate drying agent before use.

2.2. Synthesis of Studied Enaminoketone (DMTBN) and Its Deuterated Analog (d6-DMTBN)

The enaminoketone (DMTBN) and its deuterated analog (d6-DMTBN) have been prepared from thecorresponding alkoxy derivatives [8].

2.3. Infrared Spectra

Infrared spectra were recorded on a Bruker Vertex 70 FTIR spectrometer with KBr beamsplitter and RT-DLaTGS detector at the room temperature (°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. The solutions were scanned at the same conditions as a background. The Bruker Opus software Version 6.0 was used for all data manipulation.

3. Results

IR spectrum of DMTBN is presented on Figure 1. The 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).

640896.fig.001
Figure 1: IR spectrum of (DMTBN) in CCl4,  M,  cm.
3.1. The 3100–2700 cm−1 Region

In this region the olefinic C–H stretching, symmetric and asymmetric CH3 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 (d6-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 olefinic CH stretching wavenumber should be higher than the wavenumbers of CH3 stretching modes. Therefore, we assigned the bands of d6-DMTBN at 3134, 3099, 3052, and 3019 cm−1 to the olefinic CH stretching modes, whereas the bands at 2218 and 2072 cm−1 are attributed to deuterated dimethyl ν(C–D) stretching modes. The main problem is the particular assignment of the olefinic C–H stretching bands. There are two olefinic hydrogen atoms, namely, CHα and CHβ, in enaminoketones studied. Moreover, as it was shown earlier [8] both DMTBN and d6-DMTBN are presented in solutions as mixture of (EE) and (EZ) conformers. Hence, it is reasonable to expect four C–H bands, corresponding to CHα(EE), CHα(EZ), CHβ(EE), and CHβ(EZ) vibrations. In 1,1,1-trifluoro-2,4-pentanedione [11], which is exclusively in (ZZ) configuration due to intramolecular H-bonding, the olefinic CHα stretching mode occurs at 3120 cm−1. Similarly, in cis-1,2-difluoroethylene the ν(C–H) wavenumber is 3133 cm−1 [4], therefore we ascribed the bands at 3134 and 3099 cm−1 to ν(C–Hβ) mode. Increase of total concentration of d6-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).

640896.fig.002
Figure 2: Deconvoluted infrared spectrum of DMTBN in CCl4 in 3150–2700 cm−1 region.
640896.fig.003
Figure 3: Concentration dependence of IR spectrum of d6-DMTBN in CCl4.

Temperature rise induces opposite phenomenon: intensity decrease of the former band with simultaneous increase of the latter band. The same effect we have observed earlier for two (C=O) bands of d6-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 ν(C–Hβ) of (EE) and (EZ) conformer, respectively.

After deconvolution of IR spectrum of d6-DMTBN (see Figure 4) the integrated intensity of each ν(C–Hβ) band corresponding to (EE) and (EZ) conformer was calculated according to (1). where (L mol−1 cm−2) is integrated intensity of the ν(C–Hβ) band, (mol/L) is the concentration of the given conformer in solution, (cm) stands for the cell pathlength. The 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 ν(C–Hβ) (EZ) versus integrals of appropriate ν(C–Hβ) (EE) at various enaminoketone concentrations provided the product was constant (Figure 5). Intercepts of this plot with axes were integrals of ν(C–Hβ) bands of the (EZ) and (EE) conformer, respectively, but for all that the concentration of the conformer equaled the total concentration of enaminoketone, . Knowing the cell pathlength and , it was easy to calculate integrated intensities of the ν(C–Hβ) 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 of the equilibrium (EZ)(EE). As long as this equilibrium depended on total concentration of enaminoketone [8], we estimated for various concentrations of d6-DMTBN and plotted versus . we approximated obtained plot by the (2): In Table 1 we listed parameters of (2) calculated for investigated equilibrium from integral intensities of ν(C–Hβ).

tab1
Table 1: Parameters of (2) calculated for equilibrium (EZ)(EE) from integral intensities of (C–Hβ) and (C=O).
640896.fig.004
Figure 4: Deconvoluted infrared spectrum of d6-DMTBN in CCl4 in 3200–3000 cm−1 region.
640896.fig.005
Figure 5: Plot of integral of ν(C–Hβ) of (EE) versus integral of ν(C–Hβ) of (EZ) for d6-DMTBN in CCl4.

Second pair of bands, namely, at 3052 and 3019 cm−1, we attributed to ν(C–Hα) bands of (EE) and (EZ) conformer, respectively. In contrast to ν(C–Hβ) bands of these conformers an increase of total concentration of d6-DMTBN resulted in increase of intensity of the both bands (see Figure 3). Using obtained concentrations we calculated integral intensities of ν(C–Hα) 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 (d6-DMTBN) form complexes with intermolecular Hα-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 shift of this band [13, 14], hence integral intensities obtained were the sum of integral intensities of free and bound ν(C–Hα).

640896.sch.001
Scheme 1: Formation of H-bonded “circular” complex between the (EZ) and (EE) conformers.
fig6
Figure 6: Plot of A(C–Hα) versus for d6-DMTBN in CCl4: (a) for (EE) conformer; (b) for (EZ) conformer.

From (1) it follows that integral of the ν(C–Hα) band is expressed by (3): where is integral intensity of free ν(C–Hα), is integral intensity of bound ν(C–Hα), and and —respectively, concentration of free conformer and conformer bound in complex with intermolecular H-bond. Values and for respective conformers were estimated from plots of integral intensities versus total concentrations of d6-DMTBN (Figures 6(a) and 6(b). Using (3) together with (4) we calculated concentrations of H-complexes formed by (EE) and (EZ) conformer and evaluated in assumption that formation of H-complex is in accord with Scheme 1.

As it has turned out a value of for (EE) was higher as compared with for (EZ) (cf. 127.04 and 90.57 M−1, resp.).

Increase of total concentration of enaminoketone shifted the bands ν(C–Hα) of (EE) and (EZ) conformer to lower wavenumbers (Figures 7(a) and 7(b), whereas wavenumbers of ν(C–Hβ) bands remained almost invariable (the shift does not exceed 1 cm−1). This difference in ν(C–H) behavior is additional vindication of H-complex formation exclusively via C–Hα moiety.

fig7
Figure 7: Plot of ν(C–Hα) versus : (a) for (EE) conformer; (b) for (EZ) conformer.
3.2. The 1800–1500 cm−1 Region

In IR spectra of (DMTBN) and (DMTBN-d6) there were two weak ν(C=O) bands at 1686 and 1672 cm−1 (DMTBN, in CCl4) and 1691 and 1671 cm−1 (d6-DMTBN, in CCl4), respectively. Earlier [8] we ascribed ν(C=O) band with higher wavenumber to the (EZ) and that with lower wavenumber to the (EE) conformer, respectively. The profile of a very intensive ν(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 ν(C=O) and ν(C=C) vibrations are strongly coupled, possessing, in large measure, the character of out-of-phase and in-phase vibrational modes, respectively [10, 11, 1517]. Nevertheless, we shall continue to use the ν(C=O) and ν(C=C) as a convenient form of shorthand.

After band fitting the integrated intensity of each ν(C=O) band was calculated according to procedure described for ν(C=O) bands of DMTBN [8]. We calculated for various concentrations of d6-DMTBN using values (EZ) and (EE) (3.36 km M−1 and 7.69 km M−1, resp.). Plot of versus was exponentially rised described with (2). Parameters of this equation are listed in Table 1.

Temperature measurements of infrared spectrum of d6-DMTBN gave an opportunity to estimate thermodynamic parameters of equilibrium and , which were equal to  kJ M−1 and − J M−1 K−1, respectively. Method of calculation was described in detail elsewhere [8].

It is necessary to note that wavenumbers of (EZ) and (EE) band behave in different way when total concentration of d6-DMTBN increases. While the (EE) decreases exponentially, the (EZ) is virtually stable. Comparing dependence of (EE) and (EZ) from total d6-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. Therefore, behavior of both (EE) (Figure 7) and (EE) (Figure 11) indicates C–O=C bond formation.

3.3. The 1500–800 cm−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 CH3 mode. In (trifluoroacetyl) acetone (TAA) this mode was at 1461 and 1413 cm−1 [11]. In (TAA), which has the same motif –C=C–C(CF3)=O the band at 1280 cm−1 was attributed to . At the same time authors [11] attributed this band to . In 2-thenoyltrifluoroacetone (2-TTA) [18] the bands at 1283, 1279, and 1274 cm−1 were assigned to -thio + 15. Hence, the band at 1290 cm−1 is mainly due to coupled with and νC=C. In d6-DMTBN this band shifted to 1281 cm−1 as a result of mass increasing of (CD3)N–C=C moiety.

640896.fig.008
Figure 8: Plot of (EE) versus for d6-DMTBN in CCl4.

Wavenumber and intensity of this band depended on total concentration of d6-DMTBN and temperature: increase of concentration shifted this band to lower wavenumbers with simultaneous intensity increase (Table 2, Figure 9). As it has been assumed previously (vide supra) CF3 group participates in intermolecular hydrogen bond C–HF–C (Scheme 1) between (EE) and (EZ) conformer, therefore increase of total concentration of d6-DMTBN shifted 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 shifted the equilibrium to the left, thus increasing wavenumber and decreasing intensity of C–F stretching vibration (Figure 10). The 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 d6-DMTBN this band is shifted on 1 cm−1 (1190 cm−1, see Figure 9). We assigned this band to slightly coupled with 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 CF3 stretching mode coupled to νC–CH3 (12%), δOH (14%), and δCHα (20%) [10].

tab2
Table 2: Dependence of wavenumbers and intensities of CF3 stretching vibrations on concentration of d6-DTNBN and temperature.
640896.fig.009
Figure 9: Spectrum of enaminoketone DMTBN (thick line) and d6-DMTBN (thin line) in the region of C–F vibrations.
640896.fig.0010
Figure 10: Plots of integral versus for bands at 1280, 1190, 1142, and 1190 cm−1 of d6-DMTBN (in CCl4).
640896.fig.0011
Figure 11: Plots of integral versus t°C for bands at 1280, 1190, 1142, and 1190 cm−1 of d6-DMTBN (in CCl4).

In the work [11] this band was assigned to . The band at 1199 cm−1 was assigned to ναCF3 in 2-TTA [18]. Increase of the concentration of (d6-DMTBN) shifted this band to lower wavenumbers, whereas the intensity was practically invariable (Figure 10). This apparent insensitivity of intensity is a result of cancellation of two effects: increase of CF3 intensity due to participation in intermolecular hydrogen bond C–HF–C and decrease of intensity assuming that this deformation vibration refers to of the (EZ) conformer, which percentage decreases with concentration increase. Temperature rise shifted 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 intensity decrease by (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 coupled to . In IR spectrum of (TTA) this band is at 1180 cm−1. Authors [10] assigned this band to symmetric stretching mode, which has been slightly coupled to (10%) and (9%). In the work [11] this band was absent, whereas there was a band at 1160 cm−1, which authors assigned to . Moreover, in IR spectra of (2-TTA) [18] the bands at 1165 and 1162 cm−1 were assigned to . Just as the band at 1191 cm−1, concentration rise of (d6-DMTBN) shifted 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 intensity due to participation in intermolecular hydrogen bond C–HF–C and decrease of intensity of deformation vibrations of (EZ) conformer, percentage of which decreased with increase of total (d6-DMTBN) concentration. Temperature rise shifted 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 intensity decrease by (EZ) intensity increase (see Figure 11).

At last, very strong band at 1074 cm−1 (DMTBN, Figure 9), which shifted to higher wavenumbers under deuteration (to 1091 cm−1 in IR spectrum of d6-DMTBN) we attributed to , strongly coupled to . In IR spectrum of (TAA) this band is observed at 1110 cm−1. Authors [10] assigned it to (31%) + (15%) mode, whereas in the work [11] the band at 1109 cm−1 was assigned to . In (2-TTA) [18] the band at 1110 cm−1 was assigned to . In contrast with previously examined bands increase of total concentration of d6-DMTBN resulted in shift of this band to higher wavenumbers (Table 2) with synchronous increase of its intensity (Figure 10). Bearing in mind the ability of the enaminoketone (d6-DMTBN) to form complex between (EE) and (EZ) conformers due to intermolecular hydrogen bond C–HF–C (Scheme 1) it is clear that rise of total concentration of (d6-DMTBN) will increase the percentage of this complex, thus increasing both wavenumber and intensity of . On the other hand, temperature rise disrupts the complex, decreasing wavenumber and intensity of deformation C–Hα mode, which prevails in the band at 1047 cm−1.

The band at 906 cm−1 shifted to 855 cm−1 under deuteration, hence we ascribed it to νN–δC=C–O by analogy with (TAA) [10, 11].

3.4. Calculation of Thermodynamic Parameters of C–HαF–C and C–HαO=C Hydrogen Bonds

Enthalpies of C–F–C and C–O=C bond formation were calculated according to simple (5), derived by Iogansen [12]: where is enthalpy of hydrogen bond formation (kJ dm3/mol), denotes the intensity enhancement of the ν(C–Hα) stretching vibration. Values and are integral intensity of ν(C–Hα) in monomer and in complex with intermolecular hydrogen bond, respectively. From temperature dependence of profile of ν(C–Hα) bands we calculated (Scheme 1) for various temperatures. Then from plot of versus 1/T (Figure 12) we evaluated enthalpies and entropies of aforementioned hydrogen bonds. Results obtained are listed in Table 3.

tab3
Table 3: Thermodynamic parameters of =C–HF−C and =C–HO=C hydrogen bonds.
640896.fig.0012
Figure 12: Plot of and versus 1/T for d6-DMTBN in CCL4. . .

4. Discussion

In Table 4 we listed , the infrared intensity due to C–H stretching in molecules containing vinyl moiety. According to Equilibrium Charges and Charge Fluxes parameterization (ECCF) infrared intensity can be expressed as in work [4] where and are, respectively, the derivative of the electrical dipole moment of C–H bond and charge referred to hydrogen atom with respect of the interatomic distance and is an equilibrium charge referred to hydrogen atom. Gussoni et al. [1922] found that and 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 flux is sizable, both and give a considerable contribution to . When the carbon atom has either sp2 or sp3 hybridization the flux 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 sp2 or sp3 hybridization, is negative, but its absolute value decreases when the charge increases and so does the . This effect is clearly observed passing from ethane to 1.1-difluoroethylene and 1.2-difluoroethylene (Table 4). Increase of with simultaneous decrease of indicates that introduction of fluorine to an ethylene molecule induces a stronger polarization of a CH bond, which makes the charge to increase; also force constant increases while and decrease [4]. Backdonation also may take place, but its role in fluorinated ethylenes is insignificant. Induction of stronger polarization of CH bond occurs when an electronegative atom is in molecule: influence of nitrogen in the studied enaminoketones invokes the same effect on adjacent Cβ−H bond as a fluorine atom in substituted ethylenes (Table 4). As is obvious from Table 4 wavenumber of Cβ−H bond for (EE) conformer is significantly higher then for (EZ) conformer of (d6-DMTBN). Decrease of wavenumber in the (EZ) conformer as compared to it in the (EE) conformer is a result of stronger polarization of –H bond in former one which makes the charge increase, as a consequence also increases. It should be expected that decreases when equilibrium charge increases but integral intensity of the (EE) conformer is considerably larger than of the (EZ) conformer. As was stated earlier [8] hydrogen atom is involved in formation of intramolecular –HF–C hydrogen bond, hence is increased in the (EE) conformer as a result of increase of the effective charge in (7), which characterizes the additive model of H-bond [12]: where and are effective charges of C–H and HF bonds, respectively.

tab4
Table 4: Wavenumbers and integral intensities of C–H bonds of vinyl moiety. To avoid ambiguities italics indicates the group to which refers. In parentheses relevant isomer/conformer is denoted.

Wavenumbers of –H stretching vibration of the (EE) and (EZ) conformers are lower than that of –H mode although electron withdrawal power of trifluorocarbonyl 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 fluxes 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 values of the (EE) and (EZ) conformer is small (0.6 km mol−1 dm3) and significantly lower than difference in of respective conformers (2.8 km mol−1 dm3). We attributed this fact to the involvement of –H bond in intramolecular –HF hydrogen bond.

Concentration dependence of profile of IR spectrum of (d6-DMTBN) in the region of vibrations reflects not only conformation equilibrium (EZ)(EE), but also equilibrium between monomeric conformers and their H-bonded complex (see Scheme 1). Moreover, analysis of profile of IR spectra of (DMTBN) and (d6-DMTBN) in the region of carbonyl vibrations revealed that the only of (EE) conformer shifts to lower wavenumbers with increase of enaminoketone concentration (see Figure 8), whereas of the (EZ) conformer is independent of enaminoketone . Therefore, we reached a conclusion that carbonyl of the (EE) conformer takes part in formation of intermolecular H-bond with C– of the (EZ) conformer. Simultaneously intermolecular C–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 for both C–O=C and C–F–C hydrogen bond from (8): Equilibrium constant for C–O=C H-bond formation is much higher than that for C–F–C bond formation (viz., 214.4 and 16.4 M−1). Enthalpy of hydrogen bond formation , evaluated from temperature dependence of , revealed that for C–O=C H-bond is more negative than that for C–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 dm3, resp., Table 3). Thus, when concentration of (d6-DMTBN) increases, dimer between the (EE) and (EZ) conformers is formed due to intermolecular H-bonds, more strong H-bond C–O=C forms in the first place, whereas the bond C–F–C forms afterward. 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 .

As it was established earlier [8] there is an intramolecular H-bond C–F–C in the (EE) conformer of (d6-DMTBN), therefore thermodynamic parameters evaluated for equilibrium (EZ)(EE) are complex and combine parameters of an intramolecular C– 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 (d6-DMTBN) solution or at temperature rise, is accompanied with breaking of the intra- and intermolecular hydrogen bonds: where , , , and 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–F−C formation, and enthalpy of an intermolecular C–F–C and C–O=C H-bond formation, respectively.

Using (9) and assuming value , to be close to that of an intermolecular C–F–C H-bond formation (ca −6.7 kJ mol−1 dm3) we calculated which turned out to be positive and equaled 25.3 kJ mol−1 dm3. This value consists of enthalpy of equilibrium (EZ)(EE) evaluated experimentally for (CD3)2N–CH=CH–C(O)CH3 (with respect to 16.4 kJ mol−1 dm3 [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]. This result conforms to positive enthalpy of equilibrium (EZ)(EE) (see above), thus confirming validity of supposition of intra- and intermolecular H-bond formation in studied enaminoketones.

Intermolecular C–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 vibrations (see Table 5), the influence of C–F–C hydrogen bond was ambiguous. The band at 1281 cm−1 was not coupled with , therefore increase of (d6-DNTBN) concentration changed both wavenumber and intensity of this vibration. It is well known that participation in hydrogen bonding, even relatively weak, shifts the band to lower wavenumbers with synchronous intensity increase [12]. As can be seen from Figure 13(a) this increase shifted the band at 1281 cm−1 to lower wavenumbers significantly with appreciable intensity increase (Figure 10). On the other hand, vibrations and (at 1190 and 1142 cm−1, resp.), strongly coupled with , shifted to lower wavenumbers in much lesser degree (in comparison with bond at 1281 cm−1) when concentration of (d6-DNTBN) increased. Intensities of these bands also decreased insignificantly (Figure 10). In this case we observed the result of cancellation of two effects: increasing of concentration of complex with C–F–C bond and simultaneous decrease of the (EZ) conformer concentration. Contribution of (EZ) decreased, thus compensating the increase of intensity of ν(C–F) vibration. Temperature rise disrupted C–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 at 1091 cm−1 was strongly coupled with δCH. 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 (d6-DNTBN) concentration shifted this band to higher wavenumbers (Figure 13(d)) with simultaneous increase of band intensity (Figure 10). The observed effect was a result of involvement of C–H into intermolecular C–HF–C H-bonding [23] in cyclic dimer, which concentration rised when total concentration of enaminoketone increased. Temperature increase broke this hydrogen bond, thus shifting δCH to lower wavenumbers and diminishing band intensity.

tab5
Table 5: Assignment of some fundamental bands in IR spectrum of d6-DMTBN (in CCl4).
fig13
Figure 13: Plots of versus concentration of d6-DNTBN. attribution is given in Table 5.
fig14
Figure 14: Plots of versus t°C for d6-DNTBN in CCl4. attribution is given in Table 5.

5. Conclusion

There are four bands in IR spectra of studied enaminoketone (DNTBN) and its deuterated analog (d6-DNTBN) in the region of ν(C–H), corresponding to , and ; of the (EE) conformer being at higher wavenumbers than of the (EZ) conformer. Wavenumbers and intensities of these bands depend not only on temperature, but also on concentration of enaminoketone, thus reflecting 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 and vibrations are called forth by changes in the conformer equilibrium exclusively, appropriate variations of wavenumbers and intensities of and vibrations are caused by changes of both conformer and monomer-dimer equilibrium. Increase of enaminoketone concentration shifts the equilibrium (EZ)(EE) to the right, thus increasing equilibrium constant, . Simultaneously the equilibrium (EZ) + (EE)(EZEE) is shifted towards cyclic dimer, which formation is due to intermolecular C–O=C and C–F–C hydrogen bonds. Evaluation of constant and enthalpy of formation of these H-bonds revealed that C–O=C bond has greater and more negative than the C–F–C H-bond. As a consequence, the former bond is formed at first, whereas the latter bond being weaker is generated much slower. Evaluation of true enthalpy of the equilibrium (EZ)(EE) revealed that the value of 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–F–C hydrogen bond effects vibrations, but analyzing this influence one must take into account that these vibrations in some cases are coupled with .

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