Does the Csp2—Csp3 barrier in ethylbenzene have a hyperconjugative component? An indirect experimental and theoretical approach

1994 ◽  
Vol 72 (8) ◽  
pp. 1780-1784 ◽  
Author(s):  
Ted Schaefer ◽  
Robert W. Schurko ◽  
Guy M. Bernard
Keyword(s):  
Chemical Shifts ◽  
Minor Component ◽  
Phenyl Group ◽  
Electron System ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Total Magnitude ◽  
The Difference ◽  
A Minor

The proton chemical shifts and the proton spin–spin coupling constants are reported for 1-phenyl-1-butyne and 1-phenyl-1-pentyne dissolved in CS2/C6D12 and acetone-d6. The long-range coupling constants between the methylene and ring protons are used to derive the twofold barriers to internal rotation in these molecules. They are 0.5 ± 0.1 kJ/mol; the perpendicular conformer is the most stable, the one in which the C(3)—C(4) bond of the side chain lies in a plane perpendicular to the phenyl group. This energetic preference is assigned to the difference between C—C and C—H hyperconjugative interactions with the aromatic π electron system, the C—C interaction being larger. Comparison of the twofold components of the internal rotational barriers in biphenyl and diphenylacetylene, that in the latter amounting to 40% of that in the former, implies that the hyperconjugative component of the internal barrier in ethylbenzene is 1.2 ± 0.3 kJ/mol, a minor component of the total magnitude. Molecular orbital computations of the conformational energies of 1 -phenyl-1-butyne all agree that the perpendicular conformer has the lowest energy but only to the extent of 0.1 kJ/mol at most.

Methyl–Methyl Proton Spin–Spin Coupling Constants in Some Six-membered Ring Compounds. A Test for Delocalization in Borazine

1974 ◽  
Vol 52 (3) ◽  
pp. 489-496 ◽  
Author(s):  
J. Brian Rowbotham ◽  
T. Schaefer
Keyword(s):  
Electron System ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Substitution Pattern ◽  
Methyl Proton ◽  
Ring Compounds ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

Spin–spin coupling constants over five, six, and seven bonds between protons in different methyl groups are reported for the xylenes, derivatives of benzene, pyridine, pyrimidine, pyridinium salts, p-benzoquinone, and borazine. The coupling magnitudes are characteristic of the substitution pattern in the delocalized systems. Calculations at the INDO-MO-FPT level of methyl proton couplings in N- and B-methylborazines are in agreement with the available experimental evidence in indicating a relatively weak transmission of spin density information via the presumed π electron system of borazine.

Conformational analysis of the pyrrolidine ring of the N-methylthiohydantoin of 2-methylproline

1988 ◽  
Vol 53 (11) ◽  
pp. 2511-2518 ◽  
Author(s):  
Jef J. M. Sleeckx ◽  
Marc J. O. Anteunis ◽  
Frans A. M. Borremans
Keyword(s):  
Conformational Analysis ◽  
Dynamic Equilibrium ◽  
Torsional Oscillation ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Pyrrolidine Ring ◽  
Vicinal Proton ◽  
A Minor ◽  
Spin Spin Coupling

The conformational analysis of the pyrrolidine ring in 2-methylproline-N-methylthiohydantoin was achieved from the vicinal proton spin-spin coupling constants obtained from the 360 MHz 1H NMR spectrum in hexadeuterobenzene solution. Two descriptions are consistent with the experimental data. One is a large amplitude torsional oscillation centered between a β-endo envelope (C8) and a β-endo-α-exo twist (C2). The other is a 3 : 1 dynamic equilibrium between two fixed states: a (major) β-exo envelope and a minor N-endo envelope with N-endo-δ-exo twist character. The effect of the 2-methyl group on the pyrrolidine ring conformation is evaluated and discussed.

The conformational properties of some phenyl esters. Molecular orbital and nuclear magnetic resonance studies

1987 ◽  
Vol 65 (9) ◽  
pp. 2175-2178 ◽  
Author(s):  
Ted Schaefer ◽  
Glenn H. Penner
Keyword(s):  
Chemical Shifts ◽  
Phenyl Group ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Ambient Temperatures ◽  
Spin Coupling Constants ◽  
The One ◽  
Internal Barriers ◽  
Spin Spin Coupling ◽  
Planar Conformer

Extensive, geometry-optimized, STO-3G MO computations on phenyl formate imply a strongly nonplanar Z conformer (C=O bond cis to the phenyl group) at ambient temperatures. The internal barrier to rotation about the C(1)—O bond in this conformer is computed as V/kJ mol−1 = (−5.17 ± 0.27) sin2 θ − (2.42 ± 0.27) sin2 2θ, θ being zero for the planar conformer; the twofold is nearly twice as large as the fourfold component. The expectation value of θ is 58° at 300 K. The spin–spin coupling constants over six bonds between 13C and I9F nuclei in 4-fluorophenyl formate, acetate, propionate, and isobutyrate, as well as in the 2,6-dichloro-4-fluorophenyl acetate, are adduced as evidence for nonplanar conformers of these molecules. The magnitudes of these six-bond coupling constants are consistent with internal barriers to rotation about the C(1)—O bonds, which are similar in magnitude to those given by the computations on the Z conformer of phenyl formate. The energies of the planar and nonplanar E conformers, as well as the interconversion energies for [Formula: see text] isomerization, are computed. Small amounts of the nonplanar E conformer are predicted at ambient temperatures. The 13C chemical shifts and the one-bond 13C, 19F coupling constants are consistent, respectively, with only minor variations in the conformational behavior of the ester moieties caused by the fluorine substituent and by changes in the structures of these moieties themselves.

Nuclear magnetic resonance study of carbethoxypyrroles

1970 ◽  
Vol 48 (14) ◽  
pp. 2303-2305 ◽  
Author(s):  
M. W. Roomi ◽  
H. Dugas
Keyword(s):  
Chemical Shifts ◽  
Nuclear Magnetic Resonance Study ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Ring Proton ◽  
Proton Coupling ◽  
Low Field ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The chemical shifts and the ring proton coupling constants of various substituted carbethoxypyrroles are reported.The electron-withdrawing effect of the carbethoxy groups shifts the resonances of the ring substituents to low field while the inductive and mesomeric effects of the methyl groups shift the resonances to higher field. The deshielding effect is more pronounced with 2-carbethoxypyrroles than for 3-carbethoxypyrroles. The ring proton spin–spin coupling constants depend on the nature of substituents and increase with the electronegativity of the substituents. In some cases long-range coupling between the methyl side-chain protons and ring protons could be observed.

Experimental and Theoretical Estimates of Sigma and Pi Electron Contributions to Long-range Spin–Spin Coupling Constants in Coumarin and its Methyl Derivatives

1973 ◽  
Vol 51 (6) ◽  
pp. 953-960 ◽  
Author(s):  
J. B. Rowbotham ◽  
T. Schaefer
Keyword(s):  
Valence Bond ◽  
Electron System ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Orbital Theory ◽  
Coupling Path ◽  
Spin Coupling Constants ◽  
Methyl Derivatives ◽  
Spin Spin Coupling

A full analysis of the p.m.r. spectra of coumarin and five of its methyl derivatives allows the assignment of σ and π electron components to the inter-ring proton–proton spin–spin coupling constants. Magnitudes of up to 0.3 Hz are observed over a coupling path of eight bonds. The σ and π components can be combined on the basis of the appropriate valence bond structures to predict closely the observed parameters in naphthalene, suggesting only weak participation of the ether oxygen atom in the π electron system of coumarin. Calculations of the inter-ring coupling constants at the INDO and CNDO/2 levels of molecular orbital theory are helpful in the interpretation of the observed data, the agreement with experiment being quantitative in many instances. Comparison of the observed coupling constants in coumarin and styrene with the INDO results for an assumed planar form of the latter suggests that styrene exists in a nonplanar conformation.

Proton Spin–Spin Coupling Constants in Isothiazole, Isoxazole, and some of their Alkyl Derivatives

1974 ◽  
Vol 52 (5) ◽  
pp. 833-837 ◽  
Author(s):  
Roderick E. Wasylishen ◽  
J. Brian Rowbotham ◽  
Ted Schaefer
Keyword(s):  
Chemical Shifts ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Common Mechanism ◽  
Orbital Theory ◽  
Alkyl Derivatives ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Coupling Mechanisms

The signs and magnitudes of the spin–spin coupling constants over three to six bonds between protons in isothiazole, isoxazole, and in 10 of their alkyl derivatives are measured and discussed in terms of the coupling mechanisms. The chemical shifts of ring protons and methyl protons appear to arise from a common mechanism originating in the ring but are not simply related to electron densities calculated by molecular orbital theory at the CNDO/2 level of approximation.

scholarly journals Glycosaminoglycan Monosaccharide Blocks Analysis by Quantum Mechanics, Molecular Dynamics, and Nuclear Magnetic Resonance

2014 ◽  
Vol 2014 ◽  
pp. 1-11 ◽  
Author(s):  
Sergey A. Samsonov ◽  
Stephan Theisgen ◽  
Thomas Riemer ◽  
Daniel Huster ◽  
M. Teresa Pisabarro
Keyword(s):  
Molecular Dynamics ◽  
Dihedral Angle ◽  
Chemical Shifts ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Proton Proton ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
The Impact

Glycosaminoglycans (GAGs) play an important role in many biological processes in the extracellular matrix. In a theoretical approach, structures of monosaccharide building blocks of natural GAGs and their sulfated derivatives were optimized by a B3LYP6311ppdd//B3LYP/6-31+G(d) method. The dependence of the observed conformational properties on the applied methodology is described. NMR chemical shifts and proton-proton spin-spin coupling constants were calculated using the GIAO approach and analyzed in terms of the method's accuracy and sensitivity towards the influence of sulfation, O1-methylation, conformations of sugar ring, andωdihedral angle. The net sulfation of the monosaccharides was found to be correlated with the1H chemical shifts in the methyl group of the N-acetylated saccharides both theoretically and experimentally. Theωdihedral angle conformation populations of free monosaccharides and monosaccharide blocks within polymeric GAG molecules were calculated by a molecular dynamics approach using the GLYCAM06 force field and compared with the available NMR and quantum mechanical data. Qualitative trends for the impact of sulfation and ring conformation on the chemical shifts and proton-proton spin-spin coupling constants were obtained and discussed in terms of the potential and limitations of the computational methodology used to be complementary to NMR experiments and to assist in experimental data assignment.

Structural Characterization of Natural Products By Means of Quantum Chemical Calculations of NMR Parameters: New insights

2021 ◽  
Author(s):  
Fabio Luiz Paranhos Costa ◽  
Ana Carolina Ferreira de Albuquerque ◽  
Rodolfo Goetze Fiorot ◽  
Luciano Morais Lião ◽  
Lucas Haidar Martorano ◽  
...  
Keyword(s):  
Natural Products ◽  
Structural Characterization ◽  
Quantum Chemical ◽  
Chemical Shifts ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Nmr Parameters ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The calculation of NMR parameters for natural products was pioneered by Bifulco and coworkers in 2002. Since then, modelling 1H and 13C chemical shifts and spin-spin coupling constants for this...

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