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Hans-heinrich Limbach - One of the best experts on this subject based on the ideXlab platform.
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NMR studies of solid state-solvent and H/D isotope effects on hydrogen bond geometries of 1:1 complexes of collidine with carboxylic acids
Journal of Molecular Structure, 2004Co-Authors: Peter M. Tolstoy, Sergei N. Smirnov, Ilya G. Shenderovich, Nikolai S. Golubev, Gleb S. Denisov, Hans-heinrich LimbachAbstract:H and 15 N NMR spectra of 10 complexes exhibiting strong OHN hydrogen bonds formed by 15 N-labeled collidine and different proton donors, partially deuterated in mobile proton sites, have been observed by Low-Temperature NMR spectroscopy using a Low-freezing CDF3/CDF2Cl mixture as polar aprotic solvent. The folLowing proton donors have been used: HCl, formic acid, acetic acid, various substituted benzoic acids and HBF4. The sLow hydrogen bond exchange regime could be reached beLow 140 K, which alLowed us to resolve 15 N signal splittings due to H/D isotopic substitution. The valence bond order model is used to link the observed NMR parameters to hydrogen bond geometries. The results are compared to those obtained previously (Magn. Reson. Chem. 39 (2001) S18) for the same complexes in the organic solids. The increase of the dielectric constant from the organic solids to the solution (30 at 130 K) leads to a change of the hydrogen bond geometries along the geometric correlation line towards the zwitterionic structures, where the proton is partially transferred from oxygen to nitrogen. Whereas the changes of spectroscopic and, hence, geometric parameters are small for the systems which are already zwitterionic in the solid state, large changes are observed for molecular complexes which exhibit almost a full proton transfer from oxygen to nitrogen in the polar liquid solvent. q 2004 Elsevier B.V. All rights reserved.
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Low Temperature NMR studies of the structure and dynamics of a novel series of acid base complexes of hf with collidine exhibiting scalar couplings across hydrogen bonds
Journal of the American Chemical Society, 2003Co-Authors: Ilya G. Shenderovich, Peter M. Tolstoy, Sergei N. Smirnov, Nikolai S. Golubev, G S Denisov, Hans-heinrich LimbachAbstract:The Low-Temperature 1 H, 19 F, and 15 N NMR spectra of mixtures of collidine- 15 N (2,4,6- trimethylpyridine- 15 N, Col) with HF have been measured using CDF3/CDF2Cl as a solvent in the Temperature range 94-170 K. BeLow 140 K, the sLow proton and hydrogen bond exchange regime is reached where four hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3 could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants across the 19 F 1 H 19 F and 19 F 1 H 15 N hydrogen bridges have been measured which alLowed us to determine the chemical composition of the complexes. The simplest complex, collidine hydrofluoride ColHF, is characterized at Low Temperatures by a structure intermediate between a molecular and a zwitterionic complex. Its NMR parameters depend strongly on Temperature and the polarity of the solvent. The 2:3 complex (ColHFHCol)+(FHF)- is a contact ion pair. Collidinium hydrogen difluoride (ColH)+(FHF)- is an ionic salt exhibiting a strong hydrogen bond between collidinium and the (FHF)- anion. In this complex, the anion (FHF)- is subject to a fast reorientation rendering both fluorine atoms equivalent in the NMR time scale with an activation energy of about 5 kcal mol-1 for the reorientation. Finally, collidinium dihydrogen trifluoride (ColH)+(F(HF)2)- is an ionic pair exhibiting one FHN and two FHF hydrogen bonds. Together with the (F(HF)n)- clusters studied previously (Shenderovich et al., Phys. Chem. Chem. Phys. 2002, 4, 5488), the new complexes represent an interesting model system where the evolution of scalar couplings between the heavy atoms and between the proton and the heavy atoms of hydrogen bonds can be studied. As in the related FHF case, we observe also for the FHN case a sign change of the coupling constant1JFH when the F‚‚‚H distance is increased and the proton shifted to nitrogen. When the sign change occurs, that is, 1 J FH ) 0, the heavy atom coupling constant 2J FN remains very large, of the order of 95 Hz. Using the valence bond order model and hydrogen bond correlations, we describe the dependence of the hydrogen bond coupling constants, of hydrogen bond chemical shifts, and of some H/D isotope effects on the latter as a function of the hydrogen bond geometries.
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h d isotope effects on the Low Temperature NMR parameters and hydrogen bond geometries of fh 2f and fh 3f dissolved in cdf3 cdf2cl
Physical Chemistry Chemical Physics, 2002Co-Authors: Ilya G. Shenderovich, Peter M. Tolstoy, Sergei N. Smirnov, Hans-heinrich Limbach, G S Denisov, Nikolai S. GolubevAbstract:Using liquid state 1H, 2H and 19F NMR spectroscopy in the Temperature range 110–130 K we have studied the hydrogen-bonded anions (FH)2F− and (FH)3F− and their partially and fully deuterated analogs dissolved in the Low-freezing freon mixture CDF3/CDF2Cl, in the presence of (C4H9)4N+ as the counter cation. The spin multiplets of the three isotopologs HH, HD, DD of (FH)2F−, and of the four isotopologs HHH, HHD, HDD, DDD of (FH)3F− have been resolved and assigned. Thus, we were able to determine the zero-, one- and two-bond H/D isotope effects on the hydrogen and fluorine NMR chemical shifts as well as isotope effects on the scalar spin–spin hydrogen–fluorine and fluorine–fluorine coupling constants. Using the valence bond order model these NMR data are related to H/D isotope effects on the hydrogen bond geometries. A semi-quantitative interpretation of the observed long range isotope effects is proposed in terms of an anti-cooperative coupling between the hydrogen bonds within each anion. The experimental data can be rationalized in terms of an empirical NMR isotope sum rule, which is analogous to a similar rule for the vibrational frequencies.
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a solid state NMR x ray diffraction and ab initio computational study of hydrogen bond structure and dynamics of pyrazole 4 carboxylic acid chains
Journal of the American Chemical Society, 2001Co-Authors: Concepcion Focesfoces, Aurea Echevarria, Nadine Jagerovic, Ibon Alkorta, Jose Elguero, Uwe Langer, Oliver Klein, Maria Minguetbonvehi, Hans-heinrich LimbachAbstract:Using high-resolution solid-state 15N CMAS NMR, X-ray crystallography, and ab initio calculations, we have studied the structure of solid pyrazole-4-carboxylic acid (1). The crystal structure was determined at 295 and 150 K. Molecules of 1 are located on a two-fold axis, implying proton disorder of the NH and OH groups; no phase transition was observed between these two Temperatures. The compound forms quasi-linear ribbons in which the molecules are linked by cyclic hydrogen bonds between pyrazole and carboxylic acid groups with disordered hydrogen-bonded protons. Crystallography is unable to decide whether the disorder is dynamic or static. NMR shows that this disorder is dynamic, that is, consisting of very fast degenerate double proton transfers between two rapidly interconverting O−H···N and O···H−N hydrogen bridges. However, at Low Temperature, NMR shows a proton disorder−order transition where the protons are preferentially localized on given nitrogen and oxygen atoms. An amorphous phase exhibiting ...
Ilya G. Shenderovich - One of the best experts on this subject based on the ideXlab platform.
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symmetrization of cationic hydrogen bridges of protonated sponges induced by solvent and counteranion interactions as revealed by NMR spectroscopy
Chemistry: A European Journal, 2010Co-Authors: Peter M. Tolstoy, Ilya G. Shenderovich, Mariusz Pietrzak, Jens Wehling, Shushu Kong, Concepcion Lopez, Rosa M ClaramuntAbstract:The properties of the intramolecular hydrogen bonds of doubly (15)N-labeled protonated sponges of the 1,8-bis(dimethylamino)naphthalene (DMANH(+)) type have been studied as a function of the solvent, counteranion, and Temperature using Low-Temperature NMR spectroscopy. Information about the hydrogen-bond symmetries was obtained by the analysis of the chemical shifts delta(H) and delta(N) and the scalar coupling constants J(N,N), J(N,H), J(H,N) of the (15)NH(15)N hydrogen bonds. Whereas the individual couplings J(N,H) and J(H,N) were averaged by a fast intramolecular proton tautomerism between two forms, it is shown that the sum |J(N,H)+J(H,N)| generally represents a measure of the hydrogen-bond strength in a similar way to delta(H) and J(N,N). The NMR spectroscopic parameters of DMANH(+) and of 4-nitro-DMANH(+) are independent of the anion in the case of CD(3)CN, which indicates ion-pair dissociation in this solvent. By contrast, studies using CD(2)Cl(2), [D(8)]toluene as well as the freon mixture CDF(3)/CDF(2)Cl, which is liquid down to 100 K, revealed an influence of Temperature and of the counteranions. Whereas a small counteranion such as trifluoroacetate perturbed the hydrogen bond, the large noncoordinating anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate B[{C(6)H(3)(CF(3))(2)}(4)](-) (BARF(-)), which exhibits a delocalized charge, made the hydrogen bond more symmetric. Lowering the Temperature led to a similar symmetrization, an effect that is discussed in terms of solvent ordering at Low Temperature and differential solvent order/disorder at high Temperatures. By contrast, toluene molecules that are ordered around the cation led to typical high-field shifts of the hydrogen-bonded proton as well as of those bound to carbon, an effect that is absent in the case of neutral NHN chelates.
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NMR studies of solid state-solvent and H/D isotope effects on hydrogen bond geometries of 1:1 complexes of collidine with carboxylic acids
Journal of Molecular Structure, 2004Co-Authors: Peter M. Tolstoy, Sergei N. Smirnov, Ilya G. Shenderovich, Nikolai S. Golubev, Gleb S. Denisov, Hans-heinrich LimbachAbstract:H and 15 N NMR spectra of 10 complexes exhibiting strong OHN hydrogen bonds formed by 15 N-labeled collidine and different proton donors, partially deuterated in mobile proton sites, have been observed by Low-Temperature NMR spectroscopy using a Low-freezing CDF3/CDF2Cl mixture as polar aprotic solvent. The folLowing proton donors have been used: HCl, formic acid, acetic acid, various substituted benzoic acids and HBF4. The sLow hydrogen bond exchange regime could be reached beLow 140 K, which alLowed us to resolve 15 N signal splittings due to H/D isotopic substitution. The valence bond order model is used to link the observed NMR parameters to hydrogen bond geometries. The results are compared to those obtained previously (Magn. Reson. Chem. 39 (2001) S18) for the same complexes in the organic solids. The increase of the dielectric constant from the organic solids to the solution (30 at 130 K) leads to a change of the hydrogen bond geometries along the geometric correlation line towards the zwitterionic structures, where the proton is partially transferred from oxygen to nitrogen. Whereas the changes of spectroscopic and, hence, geometric parameters are small for the systems which are already zwitterionic in the solid state, large changes are observed for molecular complexes which exhibit almost a full proton transfer from oxygen to nitrogen in the polar liquid solvent. q 2004 Elsevier B.V. All rights reserved.
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Low Temperature NMR studies of the structure and dynamics of a novel series of acid base complexes of hf with collidine exhibiting scalar couplings across hydrogen bonds
Journal of the American Chemical Society, 2003Co-Authors: Ilya G. Shenderovich, Peter M. Tolstoy, Sergei N. Smirnov, Nikolai S. Golubev, G S Denisov, Hans-heinrich LimbachAbstract:The Low-Temperature 1 H, 19 F, and 15 N NMR spectra of mixtures of collidine- 15 N (2,4,6- trimethylpyridine- 15 N, Col) with HF have been measured using CDF3/CDF2Cl as a solvent in the Temperature range 94-170 K. BeLow 140 K, the sLow proton and hydrogen bond exchange regime is reached where four hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3 could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants across the 19 F 1 H 19 F and 19 F 1 H 15 N hydrogen bridges have been measured which alLowed us to determine the chemical composition of the complexes. The simplest complex, collidine hydrofluoride ColHF, is characterized at Low Temperatures by a structure intermediate between a molecular and a zwitterionic complex. Its NMR parameters depend strongly on Temperature and the polarity of the solvent. The 2:3 complex (ColHFHCol)+(FHF)- is a contact ion pair. Collidinium hydrogen difluoride (ColH)+(FHF)- is an ionic salt exhibiting a strong hydrogen bond between collidinium and the (FHF)- anion. In this complex, the anion (FHF)- is subject to a fast reorientation rendering both fluorine atoms equivalent in the NMR time scale with an activation energy of about 5 kcal mol-1 for the reorientation. Finally, collidinium dihydrogen trifluoride (ColH)+(F(HF)2)- is an ionic pair exhibiting one FHN and two FHF hydrogen bonds. Together with the (F(HF)n)- clusters studied previously (Shenderovich et al., Phys. Chem. Chem. Phys. 2002, 4, 5488), the new complexes represent an interesting model system where the evolution of scalar couplings between the heavy atoms and between the proton and the heavy atoms of hydrogen bonds can be studied. As in the related FHF case, we observe also for the FHN case a sign change of the coupling constant1JFH when the F‚‚‚H distance is increased and the proton shifted to nitrogen. When the sign change occurs, that is, 1 J FH ) 0, the heavy atom coupling constant 2J FN remains very large, of the order of 95 Hz. Using the valence bond order model and hydrogen bond correlations, we describe the dependence of the hydrogen bond coupling constants, of hydrogen bond chemical shifts, and of some H/D isotope effects on the latter as a function of the hydrogen bond geometries.
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h d isotope effects on the Low Temperature NMR parameters and hydrogen bond geometries of fh 2f and fh 3f dissolved in cdf3 cdf2cl
Physical Chemistry Chemical Physics, 2002Co-Authors: Ilya G. Shenderovich, Peter M. Tolstoy, Sergei N. Smirnov, Hans-heinrich Limbach, G S Denisov, Nikolai S. GolubevAbstract:Using liquid state 1H, 2H and 19F NMR spectroscopy in the Temperature range 110–130 K we have studied the hydrogen-bonded anions (FH)2F− and (FH)3F− and their partially and fully deuterated analogs dissolved in the Low-freezing freon mixture CDF3/CDF2Cl, in the presence of (C4H9)4N+ as the counter cation. The spin multiplets of the three isotopologs HH, HD, DD of (FH)2F−, and of the four isotopologs HHH, HHD, HDD, DDD of (FH)3F− have been resolved and assigned. Thus, we were able to determine the zero-, one- and two-bond H/D isotope effects on the hydrogen and fluorine NMR chemical shifts as well as isotope effects on the scalar spin–spin hydrogen–fluorine and fluorine–fluorine coupling constants. Using the valence bond order model these NMR data are related to H/D isotope effects on the hydrogen bond geometries. A semi-quantitative interpretation of the observed long range isotope effects is proposed in terms of an anti-cooperative coupling between the hydrogen bonds within each anion. The experimental data can be rationalized in terms of an empirical NMR isotope sum rule, which is analogous to a similar rule for the vibrational frequencies.
Peter M. Tolstoy - One of the best experts on this subject based on the ideXlab platform.
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symmetrization of cationic hydrogen bridges of protonated sponges induced by solvent and counteranion interactions as revealed by NMR spectroscopy
Chemistry: A European Journal, 2010Co-Authors: Peter M. Tolstoy, Ilya G. Shenderovich, Mariusz Pietrzak, Jens Wehling, Shushu Kong, Concepcion Lopez, Rosa M ClaramuntAbstract:The properties of the intramolecular hydrogen bonds of doubly (15)N-labeled protonated sponges of the 1,8-bis(dimethylamino)naphthalene (DMANH(+)) type have been studied as a function of the solvent, counteranion, and Temperature using Low-Temperature NMR spectroscopy. Information about the hydrogen-bond symmetries was obtained by the analysis of the chemical shifts delta(H) and delta(N) and the scalar coupling constants J(N,N), J(N,H), J(H,N) of the (15)NH(15)N hydrogen bonds. Whereas the individual couplings J(N,H) and J(H,N) were averaged by a fast intramolecular proton tautomerism between two forms, it is shown that the sum |J(N,H)+J(H,N)| generally represents a measure of the hydrogen-bond strength in a similar way to delta(H) and J(N,N). The NMR spectroscopic parameters of DMANH(+) and of 4-nitro-DMANH(+) are independent of the anion in the case of CD(3)CN, which indicates ion-pair dissociation in this solvent. By contrast, studies using CD(2)Cl(2), [D(8)]toluene as well as the freon mixture CDF(3)/CDF(2)Cl, which is liquid down to 100 K, revealed an influence of Temperature and of the counteranions. Whereas a small counteranion such as trifluoroacetate perturbed the hydrogen bond, the large noncoordinating anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate B[{C(6)H(3)(CF(3))(2)}(4)](-) (BARF(-)), which exhibits a delocalized charge, made the hydrogen bond more symmetric. Lowering the Temperature led to a similar symmetrization, an effect that is discussed in terms of solvent ordering at Low Temperature and differential solvent order/disorder at high Temperatures. By contrast, toluene molecules that are ordered around the cation led to typical high-field shifts of the hydrogen-bonded proton as well as of those bound to carbon, an effect that is absent in the case of neutral NHN chelates.
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NMR studies of solid state-solvent and H/D isotope effects on hydrogen bond geometries of 1:1 complexes of collidine with carboxylic acids
Journal of Molecular Structure, 2004Co-Authors: Peter M. Tolstoy, Sergei N. Smirnov, Ilya G. Shenderovich, Nikolai S. Golubev, Gleb S. Denisov, Hans-heinrich LimbachAbstract:H and 15 N NMR spectra of 10 complexes exhibiting strong OHN hydrogen bonds formed by 15 N-labeled collidine and different proton donors, partially deuterated in mobile proton sites, have been observed by Low-Temperature NMR spectroscopy using a Low-freezing CDF3/CDF2Cl mixture as polar aprotic solvent. The folLowing proton donors have been used: HCl, formic acid, acetic acid, various substituted benzoic acids and HBF4. The sLow hydrogen bond exchange regime could be reached beLow 140 K, which alLowed us to resolve 15 N signal splittings due to H/D isotopic substitution. The valence bond order model is used to link the observed NMR parameters to hydrogen bond geometries. The results are compared to those obtained previously (Magn. Reson. Chem. 39 (2001) S18) for the same complexes in the organic solids. The increase of the dielectric constant from the organic solids to the solution (30 at 130 K) leads to a change of the hydrogen bond geometries along the geometric correlation line towards the zwitterionic structures, where the proton is partially transferred from oxygen to nitrogen. Whereas the changes of spectroscopic and, hence, geometric parameters are small for the systems which are already zwitterionic in the solid state, large changes are observed for molecular complexes which exhibit almost a full proton transfer from oxygen to nitrogen in the polar liquid solvent. q 2004 Elsevier B.V. All rights reserved.
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Low Temperature NMR studies of the structure and dynamics of a novel series of acid base complexes of hf with collidine exhibiting scalar couplings across hydrogen bonds
Journal of the American Chemical Society, 2003Co-Authors: Ilya G. Shenderovich, Peter M. Tolstoy, Sergei N. Smirnov, Nikolai S. Golubev, G S Denisov, Hans-heinrich LimbachAbstract:The Low-Temperature 1 H, 19 F, and 15 N NMR spectra of mixtures of collidine- 15 N (2,4,6- trimethylpyridine- 15 N, Col) with HF have been measured using CDF3/CDF2Cl as a solvent in the Temperature range 94-170 K. BeLow 140 K, the sLow proton and hydrogen bond exchange regime is reached where four hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3 could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants across the 19 F 1 H 19 F and 19 F 1 H 15 N hydrogen bridges have been measured which alLowed us to determine the chemical composition of the complexes. The simplest complex, collidine hydrofluoride ColHF, is characterized at Low Temperatures by a structure intermediate between a molecular and a zwitterionic complex. Its NMR parameters depend strongly on Temperature and the polarity of the solvent. The 2:3 complex (ColHFHCol)+(FHF)- is a contact ion pair. Collidinium hydrogen difluoride (ColH)+(FHF)- is an ionic salt exhibiting a strong hydrogen bond between collidinium and the (FHF)- anion. In this complex, the anion (FHF)- is subject to a fast reorientation rendering both fluorine atoms equivalent in the NMR time scale with an activation energy of about 5 kcal mol-1 for the reorientation. Finally, collidinium dihydrogen trifluoride (ColH)+(F(HF)2)- is an ionic pair exhibiting one FHN and two FHF hydrogen bonds. Together with the (F(HF)n)- clusters studied previously (Shenderovich et al., Phys. Chem. Chem. Phys. 2002, 4, 5488), the new complexes represent an interesting model system where the evolution of scalar couplings between the heavy atoms and between the proton and the heavy atoms of hydrogen bonds can be studied. As in the related FHF case, we observe also for the FHN case a sign change of the coupling constant1JFH when the F‚‚‚H distance is increased and the proton shifted to nitrogen. When the sign change occurs, that is, 1 J FH ) 0, the heavy atom coupling constant 2J FN remains very large, of the order of 95 Hz. Using the valence bond order model and hydrogen bond correlations, we describe the dependence of the hydrogen bond coupling constants, of hydrogen bond chemical shifts, and of some H/D isotope effects on the latter as a function of the hydrogen bond geometries.
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h d isotope effects on the Low Temperature NMR parameters and hydrogen bond geometries of fh 2f and fh 3f dissolved in cdf3 cdf2cl
Physical Chemistry Chemical Physics, 2002Co-Authors: Ilya G. Shenderovich, Peter M. Tolstoy, Sergei N. Smirnov, Hans-heinrich Limbach, G S Denisov, Nikolai S. GolubevAbstract:Using liquid state 1H, 2H and 19F NMR spectroscopy in the Temperature range 110–130 K we have studied the hydrogen-bonded anions (FH)2F− and (FH)3F− and their partially and fully deuterated analogs dissolved in the Low-freezing freon mixture CDF3/CDF2Cl, in the presence of (C4H9)4N+ as the counter cation. The spin multiplets of the three isotopologs HH, HD, DD of (FH)2F−, and of the four isotopologs HHH, HHD, HDD, DDD of (FH)3F− have been resolved and assigned. Thus, we were able to determine the zero-, one- and two-bond H/D isotope effects on the hydrogen and fluorine NMR chemical shifts as well as isotope effects on the scalar spin–spin hydrogen–fluorine and fluorine–fluorine coupling constants. Using the valence bond order model these NMR data are related to H/D isotope effects on the hydrogen bond geometries. A semi-quantitative interpretation of the observed long range isotope effects is proposed in terms of an anti-cooperative coupling between the hydrogen bonds within each anion. The experimental data can be rationalized in terms of an empirical NMR isotope sum rule, which is analogous to a similar rule for the vibrational frequencies.
Erich Kleinpeter - One of the best experts on this subject based on the ideXlab platform.
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synthesis conformational preferences in gas and solution and molecular gear rotation in 1 dimethylamino 1 phenyl 1 silacyclohexane by gas phase electron diffraction ged lt NMR and theoretical calculations
Tetrahedron, 2018Co-Authors: Bagrat A. Shainyan, E N Suslova, Tran Dinh Phien, Sergey A Shlykov, Erich KleinpeterAbstract:Abstract 1-(Dimethylamino)-1-phenyl-1-silacyclohexane 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED), Low Temperature 13C NMR spectroscopy and quantum-chemical calculations. The predominance of the 1-Phax conformer (1-Pheq:1-Phax ratio of 20:80%, ΔG°(317 K) = −0.87 kcal/mol) in the gas phase is close to the theoretically estimated conformational equilibrium. In solution, Low Temperature NMR spectroscopy showed analyzable decoalescence of Cipso and C(1,5) carbon signals in 13C NMR spectra at 103 K. Opposite to the gas state in the freon solution employed (CD2Cl2/CHFCl2/CHFCl2 = 1:1:3), which is still liquid at 100 K, the 1-Pheq conformer was found to be the preferred one [(1-Pheq: 1-Phax = 77%: 23%, K = 77/23 = 2.8; −ΔG° = −RT ln K (at 103 K) = 0.44 ± 0.1 kcal/mol]. When comparing 1 with 1-phenyl-1-(X)silacylohexanes (X = H, Me, OMe, F, Cl), studied so far, the trend of predominance of the Phax conformer in the gas phase and of the Pheq conformer in solution is confirmed.
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molecular structure and conformational analysis of 1 phenyl 1 x 1 silacyclohexanes x f cl by electron diffraction Low Temperature NMR and quantum chemical calculations
Journal of Organic Chemistry, 2017Co-Authors: Bagrat A. Shainyan, Alexander V Belyakov, Yurii Sigolaev, Alexander N Khramov, Erich KleinpeterAbstract:The molecular structure and conformational preferences of 1-phenyl-1-X-1-silacyclohexanes C5H10Si(Ph,X) (X = F (3), Cl (4)) were studied by gas-phase electron diffraction, Low-Temperature NMR spectroscopy, and high-level quantum chemical calculations. In the gas phase only three (3) and two (4) stable conformers differing in the axial or equatorial location of the phenyl group and the angle of rotation about the Si–CPh bond (axi and axo denote the Ph group lying in or out of the X–Si–CPh plane) contribute to the equilibrium. In 3 the ratio Pheq:Phaxo:Phaxi is 40(12):55(24):5 and 64:20:16 by experiment and theory, respectively. In 4 the ratio Pheq:Phaxo is 79(15):21(15) and 71:29 by experiment and theory (M06-2X calculations), respectively. The gas-phase electron diffraction parameters are in good agreement with those obtained from theory at the M06-2X/aug-ccPVTZ and MP2/aug-cc-pVTZ levels. Unlike the case for M06-2X, MP2 calculations indicate that 3-Pheq conformer lies 0.5 kcal/mol higher than the 3-Phaxo...
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1,3-Dimethyl-3-silapiperidine: Synthesis, Molecular Structure, and Conformational Analysis by Gas-Phase Electron Diffraction, Low Temperature NMR, IR and Raman Spectroscopy, and Quantum Chemical Calculations
2016Co-Authors: Bagrat A. Shainyan, Nina N. Chipanina, Erich Kleinpeter, Svetlana V. Kirpichenko, Sergey A. Shlykov, Dmitriy Yu. Osadchiy, Larisa P. OznobikhinaAbstract:The first Si–H-containing azasilaheterocycle, 1,3-dimethyl-3-silapiperidine 1, was synthesized, and its molecular structure and conformational properties were studied by gas-phase electron diffraction (GED), Low Temperature NMR, IR and Raman spectroscopy and quantum chemical calculations. The compound exists as a mixture of two conformers possessing the chair conformation with the equatorial NMe group and differing by axial or equatorial position of the SiMe group. In the gas phase, the SiMeax conformer predominates (GED: ax/eq = 65(7):35(7)%, ΔG = 0.36(18) kcal/mol; IR: ax/eq = 62(5):38(5)%, ΔG = 0.16(7) kcal/mol). In solution, at 143 K the SiMeeq conformer predominates in the frozen equilibrium (NMR: ax/eq = 31.5(1.5):68.5(1.5)%, ΔG = −0.22(2) kcal/mol). Thermodynamic parameters of the ring inversion are determined (ΔG‡ = 8.9–9.0 kcal/mol, ΔH‡ = 9.6 kcal/mol, ΔS‡ = 2.1 eu). High-level quantum chemical calculations (MP2, G2, CCSD(T)) nicely reproduce the experimental geometry and the predominance of the axial conformer in the gas phase
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Synthesis, Molecular Structure, Conformational Analysis, and Chemical Properties of Silicon-Containing Derivatives of Quinolizidine
2012Co-Authors: Nataliya F. Lazareva, Bagrat A. Shainyan, Uwe Schilde, Nina N. Chipanina, Larisa P. Oznobikhina, Alexander I. Albanov, Erich KleinpeterAbstract:A silicon analog of quinolizidine 3,3,7,7-tetramethylhexahydro-1H-[1,4,2]oxazasilino[4,5-d][1,4,2]oxazasilin-9a-yl)methanol 3 was synthesized. X-ray diffraction analysis confirmed the trans configuration and Low Temperature NMR spectroscopy both the flexibility (barrier of interconversion 5.8 kcal mol–1) and the conformational equilibrium (chair–chair and chair–twist conformers) of the compound. The relative stability of the different isomers/conformers of 3 was calculated also at the MP2/6-311G(d,p) level of theory. Intra- and intermolecular hydrogen bonding in 3 and the appropriate equilibrium between free and self-associated molecules was studied in solvents of different polarity. Both the N-methyl quaternary ammonium salt and the O-trimethylsilyl derivative of 3 could be obtained and their structure determined
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polar substituent effect of the ester group on conformational equilibria of o mono substituted cyclohexanes the para substituent effect in cyclohexyl benzoates
Tetrahedron, 2008Co-Authors: Erich Kleinpeter, Ute Bolke, Andrea FrankAbstract:Abstract Together with the nonsubstituted reference compound, para -methoxy- and para -nitro cyclohexyl benzoates have been synthesized and their conformational equilibria studied by Low Temperature NMR spectroscopy and theoretical DFT calculations. The free energy differences Δ G ° between axial and equatorial conformers were examined with respect to polar substituent influences on the conformational equilibrium of O -mono-substituted cyclohexane.
Maurice Brookhart - One of the best experts on this subject based on the ideXlab platform.
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nickel catalyzed copolymerization of ethylene and vinyltrialkoxysilanes catalytic production of cross linkable polyethylene and elucidation of the chain growth mechanism
Journal of the American Chemical Society, 2017Co-Authors: Zhou Chen, Mark D Leatherman, Olafs Daugulis, Maurice BrookhartAbstract:Copolymerizations of ethylene with vinyltrialkoxysilanes using cationic (α-diimine)Ni(Me)(CH3CN)+ complexes 4a,b/B(C6F5)3 yield high molecular weight copolymers exhibiting highly branched to nearly linear backbones depending on reaction conditions and catalyst choice. Polymerizations are first-order in ethylene pressure and inverse-order in silane concentration. Microstructural analysis of the copolymers reveals both in-chain and chain-end incorporation of −Si(OR)3 groups whose ratios depend on Temperature and ethylene pressure. Detailed Low-Temperature NMR spectroscopic investigations show that well-defined complex 3b (α-diimine)Ni(Me)(OEt2)+ reacts rapidly at −60 °C with vinyltrialkoxysilanes via both 2,1 and 1,2 insertion pathways to yield 4- and 5-membered chelates, respectively. Such chelates are the major catalyst resting states but are in rapid equilibrium with ethylene-opened chelates, (α-diimine)Ni(R)(C2H4)+ complexes, the species responsible for chain growth. Chelate rearrangement via β-silyl el...
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oxygen atom transfer to a half sandwich iridium complex clean oxidation yielding a molecular product
Journal of the American Chemical Society, 2014Co-Authors: Christopher R Turlington, Peter S White, Maurice Brookhart, Joseph L TempletonAbstract:The oxidation of [Ir(Cp*)(phpy)(NCArF)][B(ArF)4] (1; Cp* = η5-pentamethylcyclopentadienyl, phpy = 2-phenylene-κC1′-pyridine-κN, NCArF = 3,5-bis(trifluoromethyl)benzonitrile, B(ArF)4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) with the oxygen atom transfer (OAT) reagent 2-tert-butylsulfonyliodosobenzene (sPhIO) yielded a single, molecular product at −40 °C. New Ir(Cp*) complexes with bidentate ligands derived by oxidation of phpy were synthesized to model possible products resulting from oxygen atom insertion into the iridium–carbon and/or iridium–nitrogen bonds of phpy. These new ligands were either cleaved from iridium by water or formed unreactive, phenoxide-bridged iridium dimers. The reactivity of these molecules suggested possible decomposition pathways of Ir(Cp*)-based water oxidation catalysts with bidentate ligands that are susceptible to oxidation. Monitoring the [Ir(Cp*)(phpy)(NCArF)]+ oxidation reaction by Low-Temperature NMR techniques revealed that the reaction involved two separate OAT...
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Oxygen Atom Transfer to a Half-Sandwich Iridium Complex: Clean Oxidation Yielding a Molecular Product
2014Co-Authors: Christopher R Turlington, Peter S White, Maurice Brookhart, Joseph L TempletonAbstract:The oxidation of [Ir(Cp*)(phpy)(NCArF)][B(ArF)4] (1; Cp* = η5-pentamethylcyclopentadienyl, phpy = 2-phenylene-κC1′-pyridine-κN, NCArF = 3,5-bis(trifluoromethyl)benzonitrile, B(ArF)4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) with the oxygen atom transfer (OAT) reagent 2-tert-butylsulfonyliodosobenzene (sPhIO) yielded a single, molecular product at −40 °C. New Ir(Cp*) complexes with bidentate ligands derived by oxidation of phpy were synthesized to model possible products resulting from oxygen atom insertion into the iridium–carbon and/or iridium–nitrogen bonds of phpy. These new ligands were either cleaved from iridium by water or formed unreactive, phenoxide-bridged iridium dimers. The reactivity of these molecules suggested possible decomposition pathways of Ir(Cp*)-based water oxidation catalysts with bidentate ligands that are susceptible to oxidation. Monitoring the [Ir(Cp*)(phpy)(NCArF)]+ oxidation reaction by Low-Temperature NMR techniques revealed that the reaction involved two separate OAT events. An intermediate was detected, synthesized independently with trapping ligands, and characterized. The first oxidation step involves direct attack of the sPhIO oxidant on the carbon of the coordinated nitrile ligand. Oxygen atom transfer to carbon, folLowed by insertion into the iridium–carbon bond of phpy, formed a coordinated organic amide. A second oxygen atom transfer generated an unidentified iridium species (the “oxidized complex”). In the presence of triphenylphosphine, the “oxidized complex” proved capable of transferring one oxygen atom to phosphine, generating phosphine oxide and forming an Ir–PPh3 adduct in 92% yield. The final Ir–PPh3 product was fully characterized
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mechanistic studies of the palladium catalyzed copolymerization of ethylene and α olefins with methyl acrylate
Journal of the American Chemical Society, 1998Co-Authors: Stefan Mecking, Lynda K Johnson, Lin Wang, Maurice BrookhartAbstract:Mechanistic aspects of palladium-catalyzed insertion copolymerizations of ethylene and α-olefins with methyl acrylate to give high molar mass polymers are described. Complexes [(N∧N)Pd(CH2)3C(O)OMe]BAr‘4 (2) or [(N∧N)Pd(CH3)(L)]BAr‘4 (1: L = OEt2; 3: L ⋮ NCMe; 4: L ⋮ NCAr‘) (N∧N ≡ ArNC(R)−C(R)NAr, e.g., Ar ⋮ 2,6-C6H3(i-Pr)2, R ⋮ H (a), Me (b); Ar‘ ⋮ 3,5-C6H3(CF3)2) with bulky substituted α-diimine ligands were used as catalyst precursors. The copolymers are highly branched, the acrylate comonomer being incorporated predominantly at the ends of branches as −CH2CH2C(O)OMe groups. The effects of reaction conditions and catalyst structure on the copolymerization reaction are rationalized. Low-Temperature NMR studies show that migratory insertion in the η2-methyl acrylate (MA) complex [(N∧N)PdMe{H2CCHC(O)OMe}]+ (5) occurs to give initially the 2,1-insertion product [(N∧N)PdCH(CH2CH3)C(O)OMe]+ (6), which rearranges stepwise to yield 2 as the final product upon warming to −20 °C. Activation parameters (ΔH⧧ = ...
