Conformational preference and internal rotation about the C1—Cα bond in phenylacetaldehyde and some benzyl alkyl ketones from 1H nuclear magnetic resonance and abinitio molecular orbital calculations

1987 ◽  
Vol 65 (3) ◽  
pp. 538-540 ◽  
Author(s):  
Glenn H. Penner
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Long Range ◽  
Phenyl Group ◽  
Aldehyde Group ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Solvent Dependence ◽  
Nuclear Magnetic

Analysis of the 1H nuclear magnetic resonance spectra of the benzyl moieties in phenylacetaldehyde, benzyl methyl ketone, benzyl ethyl ketone, benzyl isopropyl ketone, and 3,5-dichlorobenzyl tert-butyl ketone yields the long-range couplings between ring and α protons. These stereospecific couplings change very little upon replacement of the aldehydic hydrogen by various alkyl groups. The couplings for all the molecules studied fall within the ranges 4J(CH2, Ho) = −0.566 ± 0.008 Hz, 5J(CH2, Hm) = 0.278 ± 0.002 Hz, and 6J(CH2, Hp) = −0.409 ± 0.010 Hz, suggesting that in the ketones the alkyl group prefers to be trans to the phenyl ring and does not interfere with rotation about the C1—Cα bond. The long-range couplings are consistent with a potential function V(θ) = 8.4 ± 1.2 sin2 θ for two-fold rotation about the C1—Cα bond; θ is the angle between the carbonyl and benzene ring plane. Abinitio molecular orbital calculations on phenylacetaldehyde at the STO-3G level with the C=O bond cis to the phenyl group yield a potential of V(θ) = (8.65 ± 0.73) sin2 θ + (1.27 ± 0.80) sin2 2θ, rather close to the experimental potential but with a small fourfold component. The spin–spin coupling constant between the aldehydic and α protons displays a solvent dependence consistent with previously reported values. The insensitivity of 4J(CH2, Ho), 5J(CH2, Hm), and 6J(CH2, Hp) to solvent suggests that [Formula: see text] is very weakly dependent on the rotation of the aldehyde group.

1H nuclear magnetic resonance and molecular orbital studies of the conformations of 3-fluoroanisole

1989 ◽  
Vol 67 (6) ◽  
pp. 1027-1031 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Substituent Effects ◽  
Spin Coupling ◽  
Basis Set ◽  
Molecular Orbital Calculations ◽  
Minimal Basis ◽  
Parent Molecule ◽  
Nuclear Magnetic

The proximate spin–spin coupling constant between the methyl protons and the ring protons, 5J(H,OCH3), is extracted from a full analysis of the 1H and 19F nuclear magnetic resonance spectra of 3-fluoroanisole in CS2 and acetone-d6 solutions. The values of 5J(H,OCH3) imply that the less polar cis conformer is slightly more stable at 300 K than the more polar trans conformer in both solvents, in agreement with geometry-optimized STO-3G MO computations for the free molecule. The latter also find a higher barrier to internal rotation of the methoxy group for 3-fluoroanisole than for the parent molecule. The present results are compared with other measurements of the conformer ratio for the vapor and for solutions. The STO-3G and 6-31G structures of the cis and trans conformers are compared. The C—F bond length is computed more reliably with the minimal basis set, as is the COC bond angle. The internal angles of the benzene moiety are, of course, found more accurately with the 6-31G basis. The computations indicate additivity of the substituent effects on the internal angle, as found experimentally for a variety of benzene derivatives. Keywords: 1H NMR of fluoroanisole, conformations of fluoroanisole, molecular orbital calculations for fluoroanisole.

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.

1H, 19F, and 13C nuclear magnetic resonance approaches to the barrier to rotation about the Csp2—Csp3 bond in 1,1,1 -trifluoro-2-phenylethane. Proximate coupling constants and molecular orbital computations

1995 ◽  
Vol 73 (9) ◽  
pp. 1387-1394 ◽  
Author(s):  
Ted Schaefer ◽  
Paul Hazendonk ◽  
David M. McKinnon
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
13C Nuclear Magnetic Resonance ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Nuclear Magnetic

The 1H, 19Fand 13C{1H} nuclear magnetic resonance spectra of 1,1,1-trifluoro-2-phenylethane, 1, in CS2–C6D12, acetone-d6, and benzene-d6 solutions, on analysis, yield long-range coupling constants from which are derived the apparent twofold barriers to rotation about the Csp2—Csp3 bonds. The twofold barrier is 9.0(2) kJ/mol, independent of solvent, 4.0 kJ/mol larger than that of ethylbenzene, also independent of solvent. The theoretical barrier heights for the free molecules at the post-Hartree–Fock level of molecular orbital theory (frozen-core MP2/6-31G*) also differ by 4 kJ/mol, but are about 1 kJ/mol higher than the experimental estimates. The perpendicular conformer is the most stable for both molecules. Comparisons are made with the benzyl halides, in which the internal barriers are remarkably sensitive to solvent. A spin–spin coupling constant over five formal bonds, 5J(H, F), involving the ortho protons in 1, is +0.74(2) Hz and is discussed in some detail in terms of its dependence on intenuclear distances (possible through-space interactions). The solvent perturbations of 3J(H, F) and of 2J(C, F) are of opposite sign. Other long-range coupling constants or their absence are also pointed out. For example, those between 19F and 13C nuclei or protons at the meta position are effectively zero; at the para position they are significant. Keywords: 1,1,1-trifluoro-2-phenylethane; 1H, 19F, and 13C NMR; long-range spin-spin coupling constants; through-space 1H, 19F spin–spin coupling constants; internal rotational potential; molecular orbital computations of internal potential.

1H nuclear magnetic resonance and molecular orbital estimates of the internal rotational barrier in phenylpropynal in solution and in the vapor

1992 ◽  
Vol 70 (9) ◽  
pp. 2365-2369 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Robert W. Schurko
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Basis Sets ◽  
Am1 Calculations ◽  
Internal Barrier ◽  
The One ◽  
Nuclear Magnetic

The 1H nuclear magnetic resonance spectra of phenylpropynal and 1-phenylpropyne are analyzed for CS2/C6D12 and acetone-d6 solutions. The ensuing spin–spin coupling constants over eight formal bonds are used in assessing the conformational dependence of the one in the propynal derivative, as compared to the one over six bonds in benzaldehyde. The eight-bond coupling constant in phenylpropynal implies, via a hindered rotor model, that the twofold barrier to internal rotation is 5.9 ± 1.6 kJ/mol in both solutions. This number is much smaller than that for the internal barrier in benzaldehyde, reflecting the reduced π electron conjugation in phenylpropynal. Molecular orbital computations, with geometry optimization, confirm the essentially purely twofold internal barrier in the free propynal. The theoretical magnitudes are given for AM1 calculations, as well as for abinitio computations with STO-3G, 3-21G, 6-31G, and 6-31G* bases. To within experimental error, the barrier magnitudes from the split-valence basis sets agree with those obtained in solution.

13C nuclear magnetic resonance spectra of 3,6-disubstituted pyridazines

1991 ◽  
Vol 69 (6) ◽  
pp. 972-977 ◽  
Author(s):  
Gottfried Heinisch ◽  
Wolfgang Holzer
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Chemical Shifts ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Nuclear Magnetic Resonance Spectra ◽  
13C Nuclear Magnetic Resonance ◽  
Magnetic Resonance Spectra ◽  
Nuclear Magnetic

The 13C nuclear magnetic resonance spectra of 17 3,6-disubstituted pyridazine derivatives have been systematically analyzed. Chemical shifts and various 13C, 1H coupling constants are reported. Attempts were made to correlate these data with results obtained from semiempirical molecular orbital calculations as well as with substituent electronegativities and Taft's substituent constants σI and σR0. Key words: 3,6-disubstituted pyridazines, 13C NMR spectroscopy, 13C, 1H spin coupling constants.

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