Long-range 1H,19F and 13C,19F coupling constants and molecular orbital computations as indicators of internal motion in C6H5OCF3 and its 4-fluoro derivative

1991 ◽  
Vol 69 (7) ◽  
pp. 1047-1053 ◽  
Author(s):  
Ted Schaefer ◽  
Glenn H. Penner ◽  
Rudy Sebastian ◽  
James Peeling ◽  
Christian Beaulieu
Keyword(s):  
Molecular Orbital ◽  
Long Range ◽  
Solvent Polarity ◽  
Coupling Constants ◽  
Molecular Orbital Theory ◽  
Orbital Theory ◽  
H Nmr ◽  
Fluoro Derivative ◽  
Nuclear Magnetic Resonance Spectra ◽  
Nonpolar Solution

Internal rotational potentials for rotation about the Csp2—O bond in C6H5OCF3 and its 4-fluoro derivative are computed at the STO-3G and 3-12G levels of molecular orbital theory. As for anisole, the perpendicular conformer is calculated as least stable for C6H5OCF3 but the height of the internal barrier is considerably lower than in anisole. The 4-fluoro substituent reduces the twofold component significantly, indicating reduced p …π conjugation, but does not much change the values of the higher terms in the theoretical potentials. The internal potentials are rather flat with minima between about 30° and 45°, these angles measuring the twist away from planarity. The 1H, 13C, and 19F nuclear magnetic resonance spectra in polar and nonpolar solution yield long-range coupling constants, nJ(1H,19F) and nJ(13C,19F). These parameters are qualitatively consistent with relatively low barriers to rotation about the Csp2—O bonds in these molecules and with only a small dependence on solvent polarity. Key words: C6H5OCF3 and 4-F-C6H4OCF3: 1H NMR, 19F NMR, conformations, MO computations.

An increased internal rotational barrier in thiophenol caused by meta substituents

1985 ◽  
Vol 63 (9) ◽  
pp. 2471-2475 ◽  
Author(s):  
Ted Schaefer ◽  
James D. Baleja ◽  
Glenn H. Penner
Keyword(s):  
Molecular Orbital ◽  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Theory ◽  
Mo Calculations ◽  
Orbital Theory ◽  
Spin Coupling Constants ◽  
Internal Barriers ◽  
Spin Spin Coupling

The twofold internal barriers to rotation about the C—S bond in 3,5-diX-thiophenols were determined in solution from long-range spin–spin coupling constants. They are 3.4, 4.85, 5.3, 6.45, and 7.25 ± 10% kJ/mol for X = H, CH3, OCH3, F, and Cl, respectively. In 3,5-dichloro-4-hydroxythiophenol, V2 is −0.8 kJ/mol as compared to −1.9 kJ/mol in 4-methoxythiophenol. The para substituent here dominates. The observed barriers are in rough agreement with arguments based on perturbation molecular orbital theory and with MO calculations of changes in the barrier caused by substituents. The computed values appear as nearly pure twofold barriers with very small fourfold components.

The internal rotational potential in benzyl chloride

1986 ◽  
Vol 64 (7) ◽  
pp. 1322-1325
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Glenn H. Penner
Keyword(s):  
Internal Rotation ◽  
Long Range ◽  
Nmr Spectra ◽  
Benzyl Chloride ◽  
Polar Solvent ◽  
Low Energy ◽  
Coupling Constants ◽  
Molecular Orbital Theory ◽  
Orbital Theory ◽  
H Nmr

The 1H nmr spectra of benzyl chloride in dilute CS2 and acetone-d6 solutions are analyzed. The long-range coupling constants are consistent only with a low-energy conformation in which the C—Cl bond lies in a plane perpendicular to the benzene plane. Geometry optimized computations at the STO 3G level of molecular orbital theory agree with this conclusion and yield a nearly pure twofold barrier to internal rotation of 8.6 kJ/mol. In CS2 solution the long-range couplings yield 8.8 kJ/mol, rising to 11.2 kJ/mol in acetone solution. This increase in the internal barrier in a polar solvent is similar to that found for benzyl fluoride, but in the latter the barrier itself is very much smaller than in benzyl chloride.

A proton magnetie resonance and molecular orbital study of the conformational preferences of the vinylic fragment in some 2-vinylfurans and in 2-vinylthiophene

1976 ◽  
Vol 54 (20) ◽  
pp. 3216-3223 ◽  
Author(s):  
William J. E. Parr ◽  
Roderick E. Wasylishen ◽  
Ted Schaefer
Keyword(s):  
Molecular Orbital ◽  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Side Chain ◽  
Orbital Theory ◽  
Molecular Orbital Study ◽  
Conformational Preferences ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The stereospecific spin–spin coupling constants over five bonds between the α-proton in the side chain and the protons in the heterocycle in 2-vinylfuran, in its β-nitro and β-aldehydic derivatives, and in 2-vinylthiophene are used to demonstrate the preponderance of the s-trans conformers in polar and nonpolar solutions. These conclusions are compared with predictions made by molecular orbital theory at the STO-3G, INDO, CNDO/2, and MINDO/3 levels. Long-range coupling constants between the protons in the side chain and protons in the heterocycle are calculated by CNDO/2 and INDO–MO–FPT and are compared with experiment. It is concluded that the five-bond couplings involving the α-proton are most sensitive to conformation and that they are transmitted mainly via a σ electron mechanism. The other long-range coupling constants are discussed in terms of σ and π electron mechanisms. The STO-3G calculations yield barriers to internal rotation of greater than 4.8 kcal/mol.

Molecular orbital calculations and 1H nuclear magnetic resonance spectra as indicators of internal rotational potentials in some methyl derivatives of styrene

1988 ◽  
Vol 66 (4) ◽  
pp. 584-590 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Glenn H. Penner
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Long Range ◽  
Coupling Constants ◽  
Molecular Orbital Calculations ◽  
Carbon Carbon Bond ◽  
Nuclear Magnetic Resonance Spectra ◽  
Magnetic Resonance Spectra ◽  
Nuclear Magnetic

The 1H nuclear magnetic resonance spectra of the α-methyl, cis and trans-β-methyl, 4-methyl, and β,β-dimethyl styrènes are analyzed to yield long-range proton–proton coupling constants. With the assumptin that the internal rotational potential for styrene in the gas phase is unaltered in solution, a consistent treatment of over 40 of the long-range coupling constants is given in terms of the known coupling mechanisms. Expectation values of sin2 θ, where θ is the angle of twist about the exocyclic carbon–carbon bond, are presented for these molecules. These are compared with theoretical potentials at the 6-31 G level of molecular orbital theory. The present data indicate rather larger average twist angles than those in the literature. The extrema (at θ = θ° and 90°) in the angle dependent long-range coupling constants appear to be rather smaller in magnitude than are theoretical values obtained from valence bond and molecular orbital approaches.

The conformational distribution of 2-cyanobenzaldehyde and 3-cyanobenzaldehyde in solution and in the vapor

1991 ◽  
Vol 69 (7) ◽  
pp. 1039-1046 ◽  
Author(s):  
Ted Schaefer ◽  
Kerry J. Cox ◽  
Rudy Sebastian
Keyword(s):  
Long Range ◽  
Dipole Moments ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Orbital Theory ◽  
Spectral Parameters ◽  
Proton Proton ◽  
H Nmr ◽  
Proton Coupling ◽  
Spin Spin Coupling

The 1H nuclear magnetic resonance spectra of 2-cyanobenzaldehyde (2CNB) and 3-cyanobenzaldehyde (3CNB) in CS2/C6D12 and acetone-d6 solutions at 300 K yield precise stereospecific long-range proton–proton coupling constants. These are used to establish the conformational population of the o-cis and o-trans conformers of these relatively polar molecules. For example, the fractional o-cis population of 2CNB changes from 0.12(4) in CS2/C6D12 to 0.46(6) in acetone-d6, whereas that of 3CNB is 0.48(2) in both solvents. Extrapolation to the vapor phase, using a dielectric model, implies a negligible concentration of the o-cis conformer of 2CNB and a roughly 50% abundance of each conformer of 3CNB. Computations at various levels of molecular orbital theory provide estimates of the rotational barrier of the aldehyde moiety and confirm the planar structure of each conformer. The geometries of three conformers are given as obtained from the 6-31G MO basis and may be useful to molecular spectroscopists. Theoretical and experimental dipole moments are interpolated to yield estimates of their magnitudes for the four planar conformers. Somewhat less precise 1H nmr spectral parameters (than for the above solutions) are also obtained for dilute solutions in benzene-d6 at 300 K. The conformational distributions based on these parameters are compared with their only other measurement, based on dipolar moments in benzene at 298 K. Good agreement between the results of the two methods is found for 3CNB but not for 2CNB. It is suggested that specific interactions occur between benzene solvent and solute molecules, particularly for 3CNB, for which these interactions stabilize the conformer having a low dipole moment. Remarkable changes in the intraring proton–proton coupling constants occur in going from CS2/C6D12 to acetone-d6 solution. Key words: 2- and 3-cyanobenzaldehyde (2CNB and 3CNB): 1H NMR, conformations, long-range spin–spin coupling constants, MO computations.

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