Journal of Molecular Structure xxx (2016) 1e7 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc Tautomeric and conformational properties of dipivaloylmethane Natalya V Belova a, *, Nguen Hoang Trang a, c, Heinz Oberhammer b, Georgiy V Girichev a a Ivanovo State University of Chemistry and Technology, Ivanovo, 153460, Russia €t Tübingen, 72076, Tübingen, Germany Institut für Physikalische und Theoretische Chemie, Universita c University of Education, Vietnam National University, Hanoi, Viet Nam b a r t i c l e i n f o a b s t r a c t Article history: Received 20 June 2016 Received in revised form 27 August 2016 Accepted September 2016 Available online xxx Dedicated to Prof Georgiy V Girichev on the Occasion of his 70th birthday The tautomeric and structural properties of 5-hydroxy-2,2,6,6-tetramethyl-3-heptanone, (dipivaloylmethane, C(CH3)3C(O)CH2C(O)C(CH3)3) have been studied by means of gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximation with different basis sets up to aug-cc-pVTZ) Both, quantum chemistry and GED analyses resulted in the presence of 100(5)% enol tautomer at 296(3)K Quantum chemical calculations predict the existence of two enol conformers in about equal amounts In both conformers the enol ring possesses Cs symmetry and they possess different torsional orientations of the two tert-butyl groups The experimental data refinement results in an enol tautomer, in which the tert-butyl group adjacent to the carbonyl group possesses an intermediate orientation between those in “enol1” and “enol 2” forms (torsional angle is about 30 ), and the tert-butyl group adjacent to the hydroxyl group slightly deviates from orientation in the theoretical conformers (by about 10 ) The enol ring possesses CS symmetry with a strongly asymmetric hydrogen bond The experimental geometric parameters are reproduced very closely by the B3LYP/aug-cc-pVTZ method © 2016 Published by Elsevier B.V Keywords: Dipivaloylmethane Gas phase structure Tautomeric and conformational properties Gas electron diffraction Quantum chemical calculations Introduction Tautomeric and conformational properties of b-diketones (bdicarbonyl compounds of the type R1C(O)eCH2eC(O)R2 continue to attract great interest for many years, because a large group of applications of these compounds concerns their role as an important organic reagent [1e4] On the other hand, these compounds were found to be useful chelating ligands The possibility of diketoenol tautomerization, the conformational and structural properties of b-diketones, and the nature of the strong OeH/O hydrogen bond in the enol form are also of considerable interest Keto-enol tautomerization of b-diketones has been extensively studied both experimentally and theoretically It is well known that the equilibrium between diketo and enol forms is influenced by the temperature, phase and solvent [3] Furthermore, the preference of the diketo or enol tautomeric form depends strongly on the bsubstituents R1 and R2 [5] and on the a-substituent In the present study we are interested only in the effect of different b-substituents Our previous study of the acetylacetonate (acac, CH3C(O) CH3C(O)CH3) tautomeric and conformational properties by means * Corresponding author E-mail address: belova@isuct.ru (N.V Belova) of gas-phase electron diffraction (GED) and quantum chemical calculations result in the presence of 100(3)% of the enol tautomer at 300(5)K and 64(5)% of the enol at 671(7)K [6] In the literature it has been reported that the substitution of methyl groups in acac by more bulky groups (CF3, phenyl or t-butyl) should increase the enol content According to [7,8] the larger the alkyl substituent, the greater are steric interactions, resulting in carbonyl groups to be closer to each other, and hence the tendency to form the enol tautomer increases Furthermore, D.C Nonhebel [9] has suggested that the strong electron donating effect of two t-butyl groups favors the intramolecular hydrogen bond in the enol form of dipivaloylmethane (dpm) as compared to that in acac [9] In the present study we are interested in the effect of the substitution of methyl by more bulky t-butyl groups on the tautomeric and conformational properties by investigating 2,2,6,6-tetramethyl-3,5-heptanedione, dipivaloylmethane (C(CH3)3C(O)CH2C(O)C(CH3)3) Several NMR studies of dpm result in an increase of the enol content in comparison with acac The amount of the enol form of dpm is from 92% in CDCl3[10] to 100% in neat liquid [7,8] Based on the analysis of IR spectra equilibrium constants K ¼ [enol]/[diketo] of 1.4 for acac and 6.1 for dpm in acetonitrile solution have been obtained [7] The structure of dipivaloylmethane has already been studied experimentally (GED [11]) and theoretically IR and Raman spectra of dpm in the liquid phase (neat liquid and the solution in http://dx.doi.org/10.1016/j.molstruc.2016.09.003 0022-2860/© 2016 Published by Elsevier B.V Please cite this article in press as: N.V Belova, et al., Tautomeric and conformational properties of dipivaloylmethane, Journal of Molecular Structure (2016), http://dx.doi.org/10.1016/j.molstruc.2016.09.003 N.V Belova et al / Journal of Molecular Structure xxx (2016) 1e7 CCl4) were interpreted using the results of quantum chemical calculations (DFT approach with different basis sets) [12] The authors of [12] considered 32 different enol forms and 10 diketo conformers Several of these structures were found to be unstable However, a detailed conformational analysis was not carried out Furthermore, only diketo conformers with CeCeC]O dihedral angles of 0 and 180 (planar diketo skeleton) were considered [12] According to these calculations, the enol structure with CS symmetry possesses the lowest energy, whereas the structure with C2v symmetry (with symmetric O/H/O bond) corresponds to a transition state with relative energy of 5.4 kJ/mol (B3LYP/631G(d,p)) [12] CS symmetry of dpm was also confirmed by analysis of the IR spectra [12] The broad band at 2634 cmÀ1 was assigned to n(OeH) By comparing vibrational data and quantum chemical results of acac, dpm, benzoylacetone (BA) and dibenzoylmethane (DBM), the following trend in hydrogen bond strength has been obtained: acac < BA < dpm < DBM Vakili M et al [12] suppose that the increase of the hydrogen bond strength in this series is due to enol ring stabilization when the CH3 groups in acac are substituted by bulky electron-donating tert-butyl (dpm), by one phenyl group (BA) or by two phenyl groups (DBM) Electron diffraction data, obtained in 2000 [11], were interpreted in terms of the presence of the enol form only Two different structures of the enol tautomer were considered, the first one with overall C2v symmetry (symmetric O/H/O bond) and the second one with Cs symmetry (asymmetric O_H/O bond) The authors of [11] have concluded that the C2v structure is in better agreement with the experimental data However, this result contradicts the conclusions of the authors of [12] Furthermore, according to the results of preliminarily quantum chemical calculations [13], both configurations considered in Ref [11] not correspond to minima on the potential energy surface (PES) Now, more sophisticated methods for the analysis of GED data are available, including the possibility of quantum chemical results to be used in the interpretation of experimental data This allows us to determine the structural parameters more precisely compared to the studies performed in 2000 Therefore, a re-interpretation of the electron diffraction data for dipivaloylmethane was performed Quantum chemical calculation All quantum chemical calculations were performed with the GAUSSIAN03 program set [14] In the first step the potential energy surface (PES) was scanned at the B3LYP/6-31G(d,p) level to obtain all possible diketo and enol structures of dpm To detect all diketo conformers, the torsional angles t(O1C2C1C3) and t(O2C3C1C2) were varied in steps of 20 with full optimization of all other parameters (see Fig for atom numbering) This surface possesses four minima corresponding to (ac, ac), (sc, ac), (sc, sc), and (ac, sp) conformers (see Fig 1) [“ac” stands for anticlinal and corresponds to dihedral angles t(O]CeCeC) ¼ 120 ± 30 , “sp” for synperiplanar, t(O]CeCeC) ¼ ± 30 ), “sc” for synclinal, t(O] CeCeC) ¼ 60 ± 30 ] Similarly, the PES was scanned for possible enol conformers by varying the torsional angles of the two t-butyl groups, t(C1C2C4C6) and t(C1C3C5C9), in steps of 20 and full optimization of all other parameters Two stable enol conformers were obtained with different orientation of the two t-butyl groups (see Fig 1) The t-butyl group adjacent to the hydroxyl group staggers the CeOH in both conformers and the t-butyl group adjacent to the carbonyl group eclipses the C]O bond in “enol1” and staggers this bond in “enol2” The geometries of the four diketo and the two enol conformers were fully optimized with the B3LYP and MP2 methods and 6-31G(d,p) and cc-pVTZ basis sets The relative energies (DE ¼ Eketo À Eenol) and relative free energies (DG0298 ¼ G0298keto À G0298enol) obtained with the different computational methods are summarized in Table along with the skeletal OCCC dihedral angles The values in Table demonstrate that predicted relative energies of diketo and enol tautomers depend strongly on the computational method In all cases the free energy of the diketo forms is higher than that of the enol forms (by 5.4e12.7 kJ/mol in MP2/6-31G(d,p) approximation, and by 21.4e32.5 kJ/mol in the other cases) The relative free Gibbs energy DG0298 ¼ G0298 (enol2) À G0298(enol1) between the two enol conformers varies from À0.3 kJ/mol (B3LYP/6-31G(d,p) and MP2/ccpVTZ to 0.8 kJ/mol (MP2/6-31G(d,p) Thus, all methods predict a strong predominance of the enol tautomer, and a rather equal content of the two possible enol conformers The geometric parameters for two enol conformers which were derived with the B3LYP/cc-pVTZ method are listed in Table together with experimental results Vibrational amplitudes and corrections, Dr ¼ rh1 À ra, were derived from theoretical force fields (B3LYP/ccpVTZ) by Sipachev's method (approximation with nonlinear transformation of Cartesian coordinates into internal coordinates), using the program SHRINK [15e17] Relevant values for the enol forms are listed in Table and a full list of interatomic distances, vibrational amplitudes, and vibrational corrections for the enol forms (excluding nonbonded distances involving hydrogen) are available as Supporting Information (Table S1) The NBO 5G program [18], implemented for natural orbital analysis in PC GAMESS [19], was used to obtain the net atomic charges, and to study the effect of hyperconjugation on the structure of the two enol conformers and (ac, ac) diketo conformer B3LYP/aug-cc-pVTZ wave functions were used in the NBO analyses Relevant values of second order interaction energies (E(2)) between donorÀacceptor orbitals are collected in Table Experimental part The electron diffraction patterns and the mass spectra were recorded simultaneously using the techniques described previously [20,21] Two series of GED/MS experiments at two different nozzleto-plate distances were performed The conditions of GED/MS experiments and the relative abundance of the characteristic ions of dipivaloylmethane are shown in Tables and 3, respectively A special inlet system constructed for studying liquids was used for inserting vapor to the effusion cell The temperature of the stainless steel effusion cell was measured with a W/Re-5/20 thermocouple calibrated by the melting points of Sn and Al The wavelength of electrons was determined from the diffraction patterns of polycrystalline ZnO Optical densities were measured by a computer controlled MD-100 (Carl Zeiss, Jena) microdensitometer [22] With his method the molecular intensities sM(s) were obtained in a wider s-range than in the experiment in 2000 [11]: 2.6e27.0 Å and 1.3e14.0 Å for the short and long nozzle-to-plate distance, respectively The experimental and theoretical intensities sM(s) are compared in Fig Fig presents the mass spectra which were recorded simultaneously with the GED data Analysis of the mass spectra demonstrates that only monomer molecules of dpm are present in the vapor Structure analysis The experimental radial distribution function (Fig 4) was derived by Fourier transformation of the experimental intensities Fig demonstrates that theoretical radial distribution functions for enol and diketo tautomers of dpm differ very strongly The experimental curve can be reproduced reasonably well by both enol conformers Therefore, in the least-squares refinements of the molecular intensities (LS) only the enol forms were considered Separate LS analyses were carried out for each enol conformer The Please cite this article in press as: N.V Belova, et al., Tautomeric and conformational properties of dipivaloylmethane, Journal of Molecular Structure (2016), http://dx.doi.org/10.1016/j.molstruc.2016.09.003 N.V Belova et al / Journal of Molecular Structure xxx (2016) 1e7 Fig Two enol (a,b) and diketo forms of dipivaloylmethane c) sc, sc; d) ac, ac; e) ac, sp; f) sc, ac Table Torsion angles, optimized relative energies, Gibbs free energies at 298 K of the enol and diketo tautomers of dipivaloylmethane B3LYP/6-31G(d,p) t 1(С3С1С2О1)а t 2(С2С1С3О2)а DE, (kJ/mol) DG , (kJ/mol) B3LYP/cc-pVTZ t 1(С3С1С2О1) t 2(С2С1С3О2) DE, (kJ/mol) DG , (kJ/mol) MP2/6-31G(d,p) t 1(С3С1С2О1) t 2(С2С1С3О2) DE, (kJ/mol) DG , (kJ/mol) MP2/cc-pVTZ t 1(С3С1С2О1) t 2(С2С1С3О2) DE, (kJ/mol) DG , (kJ/mol) a Enol1 Enol2 Diketo (ac,ac) Diketo (sc,ac) Diketo (sc,sc) Diketo (ac,sp) e e 0.0 0.0 e e 0.01 À0.3 91.9 91.9 32.2 30.8 72.6 96.5 29.1 24.5 64.9 64.9 30.1 23.1 90.1 1.9 34.7 25.7 e e 0 e e 0.2 0.2 89.9 89.9 37.9 32.5 65.1 92.7 33.87 28.1 61.5 61.5 32.5 23.0 88.3 0.7 36.4 26.9 e e 0 e e 2.3 0.8 93.9 93.9 10.8 12.7 86.7 108.0 9.7 7.4 60.8 60.8 11.6 5.8 94.6 18.6 13.1 7.0 e e 0 e e 1.3 À0.3 95.8 95.8 28.1 30.0 93.2 114.9 26.0 23.7 61.0 61.0 27.4 21.4 94.9 19.1 29.4 23.3 Angles in degrees For atom numbering see Fig theoretical sM(s) functions were calculated with the following assumptions Independent rh1 parameters were used to describe the molecular structure Starting parameters from B3LYP/cc-pVTZ calculations were refined The differences between all CeH and OeH bond lengths, between all CeC, as well as between all CeO bond lengths were constrained to calculated values (B3LYP/cc-pVTZ) A planar skeleton with Cs symmetry of the enol ring was assumed Vibrational amplitudes were refined in groups with fixed differences With the abovementioned assumptions three bond distances and nine bond angles (Table 4) were refined simultaneously with nine groups of vibrational amplitudes (Table 5) LS analyses for the “enol1” and “enol2” forms with geometric parameters predicted by quantum chemical calculations (B3LYP/ccpVTZ) lead to agreement factors Rf equal to 6.8% and 9.2%, respectively Refinements of the parameters of “enol1” and “enol2”, but with torsional orientation of the two t-butyl groups fixed to calculated dihedral angles, decrease Rf values to 3.5% and 6.0%, respectively The best agreement between theoretical and experimental sM(s) was obtained by refining also the dihedral angles t1(C9C5C3O2) and t2(C6C4C2O1), which describe the difference Please cite this article in press as: N.V Belova, et al., Tautomeric and conformational properties of dipivaloylmethane, Journal of Molecular Structure (2016), http://dx.doi.org/10.1016/j.molstruc.2016.09.003 N.V Belova et al / Journal of Molecular Structure xxx (2016) 1e7 Table The conditions of GED/MS experiment Nozzle-to-plate distance, mm Fast electron beam, mA Temperature of effusion cell, K Accelerating voltage, kV (Electron wavelength), Ǻ Ionization voltage, V Exposure time, s Residual gas pressure, Torr S-values range, ǺÀ1a a 338 2.31 296(3) 72.0 0.044200(5) 50 180 2.0$10À6 2.3e26.8 598 2.26 296(3) 72.4 0.044062(5) 50 150 3.3$10À6 1.3e14.0 S ¼ (4p/l)sinq/2, l is electron wavelength and q is scattering angle Table Mass spectral data recorded simultaneously with GED data for acetylacetone m/e 184 139 127 112 109 99 85 69 57 43 42 41 15 a Ion Abundance, I, % ỵ [M] [M-3CH3]ỵ [M-tb]ỵ [M-tb-CH3]ỵ [M-5CH3]ỵ [M-tb-C(O)]ỵ [tb-C(O)]ỵ [C(O)CHC(O)]ỵ [tb]ỵ [C(O)CH3]ỵ [M-2tb-C(O)]ỵ [CHC(O)]ỵ [CH3]ỵ 12.8 1.3 87.7 5.5 6.5 4.0 20.1 11.2 100 94.5 30.7 97.1 48.7 Fig Mass spectra of dipivaloylmethane recorded simultaneously with GED patterns M ¼ C11H20O2 Fig Experimental (dots) and calculated radial distribution functions for different conformers of enol and diketo tautomers of dipivaloylmethane and the residual curve for the enol form Fig Experimental (dots) and calculated (solid lines) modified molecular intensity curves and residuals (experimental-theoretical) at two nozzle-to-plate distances (L1 ¼ 598 mm, L2 ¼ 338 mm) between two conformers The best agreement factors were Rf ¼ 3.4% with the t1 ¼ 170(5) and t2 ¼ 30(4) and Rf ¼ 3.8% with t1 ¼ 170(3) and t2 ¼ 34(4) starting from the models “enol1” and “enol2”, respectively All other parameters for both models are equal within their experimental errors Thus, independent of the starting model, structural analyses results in the enol configuration of dpm, where the tert-butyl group adjacent to the carbonyl C]O bond is in an intermediate position (torsional angle t2 is about 30 ) between its orientation in “enol1”(eclipsed) and “enol 2” (staggered) The tert-butyl group adjacent to the hydroxyl CeOH bond (torsional angle t1 is about 170 ) slightly deviates from its staggered orientation in both theoretical conformers Final results of this least-squares analysis are given in Table (geometric parameters) and Table (vibrational amplitudes) A model with a symmetric O/H/O hydrogen bond and C2v symmetry of the enol skeleton of dpm was tested in a GED analysis of the experimental data The parameters from Ref [11] were taken into account This refinement led to Rf ¼ 9.6% The statistical Hamilton's criterion [23] at 0.01 significance level definitely shows that the model with C2v symmetry must be rejected, because the critical value of Rf is 3.8% Although GED intensities are rather insensitive to the position of the enolic hydrogen atom, C2v and CS structures differ strongly in their skeletal CC and CO bond distances Whereas both CC and CO distances are equal in C2v structures, all these distances are different in CS structures GED intensities are rather sensitive to these bond distances and results in a large difference between the Rf factors Several least squares refinements were performed for mixtures of enol and diketo conformers The contribution of small amounts (2%) of any keto conformer does not Please cite this article in press as: N.V Belova, et al., Tautomeric and conformational properties of dipivaloylmethane, Journal of Molecular Structure (2016), http://dx.doi.org/10.1016/j.molstruc.2016.09.003 N.V Belova et al / Journal of Molecular Structure xxx (2016) 1e7 Table Experimental and calculated geometric parameters of dipivaloylmethane.a (rh1, :h1)b r(C1eC2) r(C1eC3) r(C2eC4) r(C3eC5) r(C4eC6) r(C4eC7) r(C4eC8) r(C5eC9) r(C5eC10) r(C5eC11) r(C3eO2) r(C2eO1) r(C1eH1) r(CmeH)d r(O1eH2) r(O2/H2) r(O1/O2) : C2C1C3 : C1C2O1 : C1C3O2 : C4C2C1 : C5C3C1 : C2C4C6 :C2C4C7 :C3C5C9 :C3C5C10 C4C2C1O1 C5C3C1O2 C6C4C2O1 C9C5C3O2 1.369(3)p1c 1.442(3)(p1) 1.520(3)(p1) 1.538(3)(p1) 1.533(3)(p1) 1.544(3)(p1) 1.544(3)(p1) 1.533(3)(p1) 1.542(3)(p1) 1.542(3)(p1) 1.249(3) p2 1.328(3)(p2) 1.076(3) p3 1.091(p3) 1.009(3)(p3) 1.571(3) 2.519(3) 121.6(7)(p4) 118.0(7)(p5) 123.4(9)(p6) 128.0(14)(p7) 118.3(25)(p8) 113.4(13)(p9) 107.3(19)(p10) 110.4(45)(p11) 111.9(18)(p12) 180 180 170(5) 30(4) Table Interatomic distances, vibrational amplitudes, and vibrational corrections for the enol tautomer.a,b B3LYP/cc-pVTZ Enol 1.369 1.442 1.520 1.538 1.533 1.544 1.544 1.533 1.542 1.542 1.245 1.325 1.075 1.090 1.008 1.581 2.508 120.9 120.9 121.0 125.7 119.5 112.4 108.2 109.8 109.2 180 180 180 Enol 1.373 1.437 1.512 1.539 1.533 1.544 1.544 1.533 1.542 1.542 1.250 1.320 1.073 1.090 1.015 1.550 2.488 120.5 120.8 120.9 125.6 121.9 112.4 108.1 113.7 107.3 180 180 180 60 a Distances in Å and angles in degrees For atom numbering see Fig Uncertainties in rh1 sẳ(s2scỵ(2.5sLS)2)1/2 (ssc ¼ 0,002r, sLS estandard deviation in least-squares refinement), for angles s ¼ 3sLS c pi e parameter refined independently (pi) e parameters calculated from the independent parameter pi by a difference D ¼ pi-(pi) from the quantum chemical calculations (B3LYP/aug-cc-pVTZ) d Average value b change the Rf value significantly A least squares refinements with 5e7% contribution of a diketo tautomer lead to a increase of Rf Using Hamilton's criteria [23] with 0.01 significance level we obtain a tautomeric composition of 100(5)% enol Thus, the reinterpretation of GED experimental data confirms the quantum chemical calculations which predict an asymmetric enol ring structure of dipivaloylmethane with a localized OeH bond, independent of the computational method A calculated potential function for a hydrogen position between two oxygen atoms is discussed below Discussion The GED experiment for dipivaloylmethane, C(CH3)3eC(O) eCH2eC(O)eC(CH3)3 is consistent with the presence of 100(5)% enol form in the gas phase at 296(3) K This result is in agreement with the predictions of quantum chemical calculations Theoretical data result in the existence of two enol forms, which differ only by the orientation of the tert-butyl group adjacent to the C]O bond The GED refinement leads to a structure of dpm, in which both tertbutyl groups are rotated relative to their positions in optimized “enol1” and “enol2” structures The internal rotation of the substituents in b-diketonato complexes was discussed in Ref [24] The authors of [24] note that the standard procedures of GED structural analyses not take into account the hindered rotation of the substituents which results in experimental texp angles different from calculated equilibrium O1eH2 C1eH1 C7e H8 C8eH11 C9eH13 C9eH14 C10eH17 C11eH20 C6eH3 C6eH4 C6eH5 C11eH19 C10eH16 C7eH6 C8eH9 C8eH10 C7eH7 C9eH12 C10eH15 C11eH18 C3eО2 C2eО1 C1eC2 С1eС3 С2eС4 С5eС9 С4eС6 С3eС5 С5eС10 С5eС11 С4eС7 С4eС8 С1/О1 С1/О2 С4/О1 С5/О2 С2/С3 С9/С10 С9/С11 С10/С11 С6/С7 С6/С8 С3/С9 О1/О2 С7/С8 С3/С10 С3/С11 С2/С6 С1/С5 С1/С4 C3/O1 C2/O2 C2/C5 C3/C4 C5/O1 C4/O2 C6/C11 C4/C5 rh1 l(GED) l(B3LYP/сс-pVTZ) rh1-ra 1.009(3) 1.076(3) 1.089(3) 1.089(3) 1.090(3) 1.090(3) 1.091(3) 1.091(3) 1.091(3) 1.091(3) 1.091(3) 1.092(3) 1.092(3) 1.092(3) 1.092(3) 1.092(3) 1.092(3) 1.092(3) 1.092(3) 1.092(3) 1.249(3) 1.328(3) 1.369(3) 1.442(3) 1.520(3) 1.533(3) 1.533(3) 1.538(3) 1.542(3) 1.542(3) 1.544(3) 1.544(3) 2.312(8) 2.372(10) 2.390(18) 2.397(16) 2.455(9) 2.479(44) 2.479(44) 2.488(109) 2.505(32) 2.505(32) 2.522(20) 2.519(16) 2.543(44) 2.552(18) 2.552(18) 2.552(16) 2.559(15) 2.598(14) 2.749(13) 2.851(10) 3.834(13) 3.906(12) 4.284(14) 4.368(12)) 5.023(60) 5.157(14) 0.079(2)l1 0.074(2)l1 0.075(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.075(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.076(2)l1 0.039(2)l1 0.045(2)l1 0.045(2)l2 0.050(2)l2 0.052(2)l2 0.053(2)l2 0.053(2)l2 0.054(2)l2 0.054(2)l2 0.054(2)l2 0.054(2)l2 0.054(2)l2 0.053(2)l3 0.053(2)l3 0.060(2)l3 0.059(2)l3 0.055(2)l3 0.072(2)l3 0.072(2)l3 0.074(2)l3 0.072(2)l3 0.072(2)l3 0.067(2)l3 0.090(2)l3 0.073(2)l3 0.083(2)l3 0.083(2)l3 0.066(2)l3 0.065(2)l3 0.061(2)l3 0.072(2)l3 0.071(2)l3 0.103(10)l5 0.102(10)l5 0.084(10)l6 0.082(10)l6 0.308(12)l7 0.096(12)l7 0.080 0.075 0.076 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.077 0.076 0.077 0.077 0.077 0.077 0.040 0.044 0.044 0.049 0.051 0.052 0.052 0.053 0.053 0.053 0.053 0.053 0.053 0.054 0.061 0.059 0.055 0.072 0.072 0.074 0.072 0.072 0.067 0.091 0.074 0.083 0.083 0.067 0.065 0.062 0.073 0.071 0.066 0.065 0.080 0.078 0.293 0.081 À0.0005 0.0018 0.0016 0.0016 0.0016 0.0016 0.0016 0.0016 0.0015 0.0015 0.0015 0.0016 0.0016 0.0016 0.0016 0.0016 0.0016 0.0016 0.0016 0.0016 À0.0003 0.0007 0.0003 0.0016 0.0001 0.0004 0.0004 0.0004 0.0005 0.0005 0.0005 0.0005 0.0023 0.0027 0.0037 0.0039 0.0034 0.0028 0.0028 0.0032 0.0025 0.0025 0.0014 À0.0041 0.0029 0.0040 0.0040 0.0017 0.0022 0.0020 0.0020 0.0014 0.0100 0.0086 0.0109 0.0071 À0.0149 0.0171 a Values in Å Error limits for the amplitudes are 3s values For atom numbering see Fig b Full table of interatomic distances, vibrational amplitudes and vibrational corrections for the enol form (excluding nonbonded distances involving hydrogen) is available as Supporting Information (Table S1) values Moreover, the value of torsional texp angle can depend on a structural analysis scheme In this study we have investigated the internal rotation of tert-butyl groups in dpm The potential energy surface has been scanned with B3LYP method, the torsion angles were varied in steps of 10 Fig presents the calculated potential curves V(t) for the rotation of both ter-butyl groups The functions Please cite this article in press as: N.V Belova, et al., Tautomeric and conformational properties of dipivaloylmethane, Journal of Molecular Structure (2016), http://dx.doi.org/10.1016/j.molstruc.2016.09.003 N.V Belova et al / Journal of Molecular Structure xxx (2016) 1e7 Fig Potential curves of tert-butyl groups rotation: a) for tert-butyl group adjacent to hydroxyl group, b) for tert-butyl group adjacent to carbonyl group are rather different Whereas the curve for the tert-butyl group adjacent to hydroxyl group possesses only one minimum for eclipsed orientation (at t ¼ 0 or 120 ), the curve for the tert-butyl group adjacent to the carbonyl group possesses two minima for eclipsed (t ¼ 0 ) and staggered (t ¼ 60 ) orientation This potential curve is similar to that for tert-butyl groups in the complexes [24] For the tert-butyl group adjacent to hydroxyl group the calculated rotational barriers are 6.3 kJ/mol (B3LYP/6-31G(d,p)) and 7.2 kJ/mol (B3LYP/cc-pVTZ) For the tert-butyl group adjacent to carbonyl group these barriers are only 1.5 kJ/mol (B3LYP/6-31G(d,p)) and 1.0 kJ/mol (B3LYP/cc-pVTZ) Considering independent rotations of the t-butyl groups, and applying the calculated V(t) functions, the method developed in Ref [23] was used to derive thermal average torsional angles tav for the experimental temperature The average values for both t-butyl groups are tav(C9C5C3O2) ¼ 29.6 and tav(C6C4C2O1) ¼ 164.8 (value consistent with experimental value) These values are in very good agreement with the experimental values, considering their error limits Table compares the structural parameters of dpm, obtained in the present work, to those recommended by authors of [11] We should note that both structures considered in Ref [11] not correspond to stationary points on the PES Fig presents a calculated potential curve for the position of the hydrogen atom between two oxygen atoms in the “enol1” form derived by B3LYP/6-31G(d,p) calculations Two equivalent minima occur for localized hydrogen bonds with r(OeH) ¼ 1.016 Å, the maximum for r(O1/H) ¼ r(O2/H) ¼ 1.206 Å The height of this barrier is 8.5 kJ/mol The calculations for the “enol2” form lead to similar results, but the barrier is slightly higher (by 0.5 kJ/mol) This barrier for proton migration is rather low On the other hand, according to IR spectra of dpm in CCl4[12] the n(OeH) ~ 2634 cmÀ1 Fig Calculated (B3LYP/6-31G(d,p)) potential curve for the hydrogen atom between the two oxygen atoms in the “enol1” form The position of the hydrogen atom is described by the difference between O1/H and O2/H distances shows the preference of localized position of hydrogen near the oxygen atom, as well as the presence of the strong intramolecular hydrogen bond NBO-analyses for the different enol and diketo forms of dpm have been performed Table collects the largest values of calculated second order interaction energies (E2) between donoracceptor orbitals in several investigated forms For the enol forms the large values correspond to the interactions related to the resonance in the enol ring, i.e the interactions between p(C1eC2) and p*(C3eO2) and between Lp2(O1) and p*(C1eC2) Furthermore, the interaction between the lone pair of the carbonyl oxygen atom Lp2(O2) and the O1eH anti-bonding orbital s*(O1eH) yields a strong hydrogen bond The presence of this interaction results in the strong preference of the enol form of dpm We can note, that NBO-analyses not reveal the hyperconjugation between tertbutyl groups and C]C or C]O double bonds both in the enol and diketo forms Table presents the structural parameters for the enol form of four molecules: acetylacetone and three molecules with huge substituents The structural parameters of the enol skeleton eC3(O2)eC1eC2(O1eH) are rather similar for all molecules This demonstrates that the replacement of the methyl groups in acac by more bulky phenyl or tert-butyl groups does not cause significant changes in the geometry of the enol skeleton Table Structural parameters of the enol form of dipivaloylmethane Symmetry of the enol ring C2v [11] [Present work] [11] rg(С1eС3) rg(С1eС2) rg(С3eС5) rg(С5eС9) rg(С3eО2) rg(С2-О1) rg(О1eН2) rg(О1/О2) :C1C3O2 :C1C2O1 :C2O1H2 :C3C1C2 :C1C3C5 :C1C2C4 1.449(6) e 1.526(4) 1.558(4) 1.277(6) e 1.285(25) 2.405(18) 117.6(4) e 75(4) 120.6(4) 113.6(6) e 1.443(3) 1.369(3) 1.538(3) 1.533(3) 1.249(3) 1.328(3) 1.009(3) 2.526(11) 123.4(9) 118.0(7) 105.7 121.6(7) 118.3(25) 128.0(14) 1.502(6) 1.341(18) 1.522(6) 1.552(4) 1.244(8) 1.329(18) 1.01(5) 2.612(11) 123(2) 128(2) 86(6) 117(16) 122(1) 122(1) CS Table Selected second order perturbation energies E(2) (kJ/mol) for the enol and diketo forms of dipivaloylmethanate Е (2) (kJ/mol) p(C1eC2) ÷ p*(C3eO2) lp2(O1) ÷ p*(C1eC2) lp2(O2) ÷ s* (C1eC3) lp1(O1) ÷ s* (C1eC2) lp2(O2) ÷ s* (C3eC5) lp2(O2) ÷ s* (O1eH) Enol Enol 146.7 190.8 41.2 34.9 78.0 134.6 159.5 197.8 38.4 35.1 72.8 154.6 Diketo (sc,sc) lp2(O2) lp2(O1) lp2(O1) lp2(O2) ÷ ÷ ÷ ÷ s* (C1eC3) s* (C1eC2) s* (C2eC4) s* (C3eC5) 96.3 96.3 84.9 84.9 Please cite this article in press as: N.V Belova, et al., Tautomeric and conformational properties of dipivaloylmethane, Journal of Molecular Structure (2016), http://dx.doi.org/10.1016/j.molstruc.2016.09.003 N.V Belova et al / Journal of Molecular Structure xxx (2016) 1e7 Table Structural parameters of the enol skeleton of several b-diketones dpm acac [6] ВА [25] [2] DBM [26] [3] GED (rh1, :h1) [4] r(C1eC3) r(C1eC2) r(C3eO2) r(C2eO1) r(O1eH) :(С3C1C2) :(O2C3C1) :(O1C2C1) 1.442(3) 1.369(3) 1.249(3) 1.328(3) 1.009(3) 121.6(7) 123.4(10) 118.0(8) 1.441(3) 1.368(3) 1.248(3) 1.326(3) 1.007(3) 121.1(8) 121.0(20) 121.3(12) 1.373(3) 1.443(3) 1.308(3) 1.256(3) 1.014(4) 120.1(8) 122.1(8) 120.7(8) 1.381(3) 1.441(3) 1.317(5) 1.252(5) 1.014(5) 120.9(8) 121.0(8) 121.0(8) [5] [6] [7] [8] [9] Table Values characterizing the hydrogen bond in the enol form of b-diketones (B3LYP/ aug-cc-pVTZ).a r(O1/O2) r(O1eH) r(O2/H) Q(O1eH) Q(O2/H) q(O1) q(O2) q(R1) q(R2) q(H) q(enol)b u(O1eH) a acac dpm BA DBM 2.532 1.006 1.614 0.615 0.111 À0.655 À0.635 0.035 0.007 0.506 À0.257 2990 2.507 1.008 1.581 0.608 0.119 À0.682 À0.641 À0.013 À0.008 0.501 À0.196 2938 2.511 1.010 1.588 0.608 0.119 À0.652 À0.643 0.023 0.034 0.505 À0.269 2934 2.493 1.014 1.556 0.597 0.131 À0.652 À0.645 0.034 0.019 0.505 À0.264 2846 [10] [11] [12] [13] [14] ; r e interatomic distances, Ǻ; Q e Wiberg bond orders; q e net atomic charges, e u - harmonic frequencies, cmÀ1 b The charge of HO1eC2eC1eC3¼O2 fragment As we have mentioned above some authors claim to observe substituent effects on the properties of the hydrogen bond M Valiki et al [12] have concluded that the strength of the hydrogen bond increases in the series: acac < BA < dpm < DBM Table presents some calculated (B3LYP/aug-cc-pVTZ) parameters, which characterize the hydrogen bond and the electron density distribution in the enol ring Some lengthening of r(O1eH) and shortening of r(O/O) distances occur in the molecules with large substituents A stronger effect occurs in the O1eH harmonic vibrational frequencies This indicates some strengthening of the hydrogen bonds in dpm, BA and DBM compared to acac This effect can not be explained by the electronegativity of the substituents, since the values of the total charge of the substituents (close to the zero) and the total charge of the enol skeleton (almost equal in all four compounds) not reproduce this trend for strength of the hydrogen bonds [21] Acknowledgements [22] We acknowledge the Ministry of Education and Science of the Russian Federation for the financial support of this work (the project N 4.1385.2014/K) [23] Appendix A 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Girichev, Tautomeric and conformational properties of beta-diketones, J Mol Struct 978 (2010) 282e293 N.V Belova, H Oberhammer, N.H Trang, G.V Girichev, Tautomeric properties and gas-phase structure of. .. et al., Tautomeric and conformational properties of dipivaloylmethane, Journal of Molecular Structure (2016), http://dx.doi.org/10.1016/j.molstruc.2016.09.003 N.V Belova et al / Journal of Molecular