1H and 19F NMR conformational studies of the monofluorostyrenes in solution. Comparison with theory and vapor phase behavior

1990 ◽  
Vol 68 (8) ◽  
pp. 1383-1392 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian
Keyword(s):  
Long Range ◽  
Coupling Constants ◽  
Side Chain ◽  
Coupling Constant ◽  
Mo Calculations ◽  
Carbon Carbon Bond ◽  
Electron Contribution ◽  
Internal Barriers ◽  
Cis And Trans ◽  
Coupling Mechanisms

The parameters for the high resolution 1H and 19F NMR spectra of 2-, 3-, and 4-fluorostyrene are reported for solutions in CS2 and acetone-d6 at 300 K. The populations of the planar cis and trans conformers of 2- and 3-fluorostyrene are deduced from the long-range coupling constants involving the meta and α protons. These populations are insensitive to solvent and appear to be in reasonable agreement with previous 6-31G MO computations for the free molecule; they are also compared with populations deduced from recent rotational and vibronic spectra. The long-range coupling constants for the protons of 4-fluorostyrene imply an internal barrier to rotation about the exocyclic carbon–carbon bond very similar to that in styrene, in agreement with the 6-31G results. The signs of the coupling constants involving 19F and the protons in the side chain are reported and discussed in terms of coupling mechanisms for the three molecules. An earlier surmise, of a positive a electron contribution to the coupling constant over six bonds in an all-trans arrangement, is confirmed for the meta and trans-β protons in 3-fluorostyrene. Keywords: monofluorostyrenes, 1H and 19F NMR, conformations, long-range coupling mechanisms, MO calculations of internal barriers.

Concerning the extension of the J method for internal barriers to six-bond couplings to 19F in the 4-fluorophenyl fragment

1978 ◽  
Vol 56 (18) ◽  
pp. 2442-2446 ◽  
Author(s):  
Ted Schaefer ◽  
Werner Danchura ◽  
Walter Niemczura ◽  
James Peeling
Keyword(s):  
Ground State ◽  
Proton Magnetic Resonance ◽  
Coupling Constants ◽  
Side Chain ◽  
Fluorine Nucleus ◽  
Proton Proton ◽  
Proton Coupling ◽  
Carbon Carbon Bond ◽  
Internal Barriers

The long-range coupling constants over six bonds from the side-chain protons to the fluorine nucleus on the ring are extracted from the proton magnetic resonance spectra of p-fluorobenzyl cyanide, chloride, and bromide; of p-fluorobenzal chloride; and of p-fluoroisopropylbenzene. On the assumption that these couplings are transmitted via a σ–π mechanism, a hindered rotor treatment yields the barriers to internal rotation about the carbon–carbon bond which attaches the substituent to the benzene ring. These barriers, when compared to those derived from the analogous proton–proton coupling constants, apparently are accurate enough for the determination of ground state conformations and for a rough assessment of the energetics of conformational interconversions.

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.

Long-range coupling constants for α-13C nuclei in phenyl-X-R (X = O,S,Se,Te) derivatives. Internal rotational information

1988 ◽  
Vol 66 (7) ◽  
pp. 1641-1646 ◽  
Author(s):  
Ted Schaefer ◽  
Glenn H. Penner
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Long Range ◽  
Chemical Shifts ◽  
Coupling Constants ◽  
Photoelectron Spectra ◽  
Side Chain ◽  
Coupling Constant ◽  
Stable Conformation ◽  
Nuclear Magnetic

The long-range spin–spin coupling constant over six bonds between the 19F nucleus and the 13C nucleus in the side chain,6J(C,F), is reported for 4—F—C6H4—X—R, where X = O, S and R = CH3, CH2CH3, CH(CH3)2 and C(CH3)3 and 6J(C,F) depends on sin2 θ, where θ is the angle by which the side chain twists out-of-plane about the [Formula: see text] bond. Expectation values of sin2 θ are obtained from 6J(C,F), yielding magnitudes of the apparent twofold barrier to rotation about the [Formula: see text] bond. In these terms, the most stable conformation is that for θ = 0° for all compounds, with the exception of R = C(CH3)3 and of X = S, R = CH(CH3)2; there is effectively free rotation about the [Formula: see text] bond in isopropyl 4-fluorophenyl sulfide in acetone-d6 solution. Good correlations exist between 6J(C,F) and a number of other molecular properties, including certain differences of ionization potentials of the molecular orbitals in the ethers. In particular, the chemical shifts of C-4 are correlated with 6J(C,F). Because 5J(C,C), the coupling constant involving C-4, also depends on sin2 θ, it is measured for the methyl and ethyl selenides and tellurides, as are other 13C,13C couplings involving a 13C nucleus in the side chain. The literature values for the 13C nuclear magnetic resonance chemical shifts in alkyl phenyl selenides and tellurides can be related to θ preferences and also allow estimates of the extrema in 5J(C,C). The resultant values of arcsin [Formula: see text] for R = CH3 are in good agreement with estimates of θ obtained from electron diffraction patterns, photoelectron spectra, and nuclear magnetic resonance in the nematic phase.

Proton Magnetic Resonance Studies of Long-range Coupling Constants: Conformational Dependence of the 4J Coupling Between Side Chain and Ring Protons in Formylheterocycles

1973 ◽  
Vol 51 (4) ◽  
pp. 573-581 ◽  
Author(s):  
Bernard Pierre Roques ◽  
Suzanne Combrisson
Keyword(s):  
Proton Magnetic Resonance ◽  
Aromatic Aldehydes ◽  
Coupling Constants ◽  
Side Chain ◽  
Coupling Constant ◽  
Precise Analysis ◽  
J Coupling ◽  
Electron Contribution ◽  
Replacement Technique ◽  
J Coupling Constants

The 4J coupling constant between side chain and ring protons is studied on particular models of planar heterocyclic aldehydes.The preferential conformation of these molecules is established by use of the "zig–zag" rule for the 5J coupling constant. This allows precise analysis of the 4J coupling across cc and ct pathways.The sign of some of these constants is determined and is found to be negative in either cc or ct pathways. The use of the "methyl replacement technique" shows a dominant σ electron contribution to the 4J coupling constants. In all examples studied here, the 4J coupling value is dependent on the pathway: 4Jct (−0.30 at −0.45 Hz) > 4Jcc (−0.10 at −0.15 Hz). The existence of a stereospecificity for this coupling is in accordance with a preponderant σ electron mechanism and is useful in conformational analysis of planar aromatic aldehydes.

Long-range spin–spin coupling constants between ring protons and aldehydic protons in some para-substituted benzaldehydes

1969 ◽  
Vol 47 (21) ◽  
pp. 4005-4010 ◽  
Author(s):  
S. S. Danyluk ◽  
C. L. Bell ◽  
T. Schaefer
Keyword(s):  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Proton Proton ◽  
Proton Coupling ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Coupling Mechanisms ◽  
Benzaldehyde Derivatives

The long-range proton–proton coupling constants between the ring protons and the aldehydic proton are reported for a series of para-substituted benzaldehyde derivatives. It was found that JoH,CHO < 0 and JmH,CHO > 0. Furthermore, JoH,CHO increases in magnitude as the electron donating power of the sub-stituent increases. A similar trend is observed forJmH,CHO but the ratio of the increase to the magnitude of JmH,CHO is much less than for JoH,CHO. A good correlation is obtained between JoH,CHO and the sub-stituent parameters of Swain and Lupton.The coupling constant data are discussed in terms of σ and π coupling mechanisms and it is concluded that σ electron mechanisms are dominant for both JoH,CHO and JmH,CHO.

The internal barriers to rotation about the carbon–carbon bond in 3,5-dichlorobenzyl alcohol and selenol by the J method

1978 ◽  
Vol 56 (13) ◽  
pp. 1721-1723 ◽  
Author(s):  
Ted Schaefer ◽  
Werner Danchura ◽  
Walter Niemczura ◽  
William J. E. Parr
Keyword(s):  
Internal Rotation ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Side Chain ◽  
Carbon Carbon Bond ◽  
Carbon Carbon Bonds ◽  
Spin Coupling Constants ◽  
Internal Barriers ◽  
Spin Spin Coupling

The J method, depending on a comparison between observed spin–spin coupling constants over six bonds between protons on a side chain and para ring protons and those calculated by a hindered rotor treatment, is applied to the determination of the twofold barrier to internal rotation about the carbon–carbon bonds in 3,5-dichlorobenzyl alcohol and selenol. In the alcohol, the C—O bond prefers the benzene plane by 0.3 ± 0.2 kcal/mol whereas, in the selenol, the C—Se bond prefers a plane perpendicular to the benzene ring by 3.8 ± 0.7 kcal/mol. Comparison with the thiol suggests that a major component of the barrier arises from repulsive interactions, increasing as the size of the XH (X = O, S, Se) group increases.

1H and 13C nuclear magnetic resonance and molecular orbital studies of the internal rotational potential and of spin–spin coupling transmission in phenylallene

1995 ◽  
Vol 73 (9) ◽  
pp. 1478-1487 ◽  
Author(s):  
Ted Schaefer ◽  
Scott Kroeker ◽  
David M. McKinnon
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Molecular Orbital ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Side Chain ◽  
Coupling Constant ◽  
H Nmr ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Coupling Mechanisms

The 1H nuclear magnetic resonance spectra of phenylallene, diluted in acetone-d6 and benzene-d6, yield long-range coupling constants over as many as eight formal bonds between the ring and side-chain protons. These are discussed in terms of σ- and π-electron spin–spin coupling mechanisms, which are sensitive to the torsion angle between the allenyl and phenyl fragments. The torsion angle is assessed by means of molecular orbital computations of the internal rotational potential, whose height is calculated as 16.0 kJ/mol at the MP2/6-31G* level of correlation-gradient theory. Comparison with experimental and theoretical internal rotational potentials for styrene suggests that steric repulsions in the planar form of styrene amount to about 4 kJ/mol. In a field of 7.0 T, phenylallene is partially aligned, entailing a positive dipolar coupling constant between the methylene protons, from which absolute signs of the spin–spin coupling constants involving these protons can be inferred. Such coupling constants over seven and eight bonds, to the meta and para protons, are taken as being mediated by the extended π-electron system, providing a measure of π-electron contributions to coupling constants between meta protons and those in side chains (spin correlation). Some coupling constants between protons and 13C nuclei in the side chain, as well as between ring protons and these 13C nuclei, are also discussed in terms of spin coupling mechanisms. Solvent perturbations of one-bond proton–carbon coupling constants in the allenyl group do not follow the usual pattern in which an increase in polarity of the solvent is associated with an increase in the magnitude of the coupling constant. Keywords: 1H NMR, phenylallene; 1H NMR, long-range spin–spin coupling constants in phenylallene; phenylallene, internal rotational potential, molecular orbital computations; molecular orbital calculations, an internal rotational potential in phenylallene.

The preferred conformations in solution and rotational barriers of the phenyl moiety in 3,5-dichlorobenzyl derivatives of ammonia, dimethylamine, and dimethylarsine

1978 ◽  
Vol 56 (17) ◽  
pp. 2229-2232 ◽  
Author(s):  
Ted Schaefer ◽  
Werner Danchura ◽  
Walter Niemczura
Keyword(s):  
Long Range ◽  
Phenyl Ring ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Carbon Bond ◽  
Carbon Carbon Bond ◽  
Spin Coupling Constants ◽  
Internal Barriers ◽  
Spin Spin Coupling ◽  
Derivatives Of

The long-range spin–spin coupling constants between methylene protons and ring protons are measured in 3,5-dichlorobenzylamine, 3,5-dichlorobenzyldimethylamine, and in 3,5-dichlorobenzyldimethylarsine. The couplings over six bonds are used to derive internal barriers to rotation about the carbon–carbon bond to the phenyl ring. In the above order, they are 0.3 ± 0.3, 0.8 ± 0.2, and 3.0 ± 0.5 kcal/mol. The conformation of lowest energy in the arsine is that in which the CH2—X bond lies in a plane perpendicular to the benzene plane.

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.

Long-range coupling and conformation in some thienylmethylene and furfurylidene derivatives

1977 ◽  
Vol 30 (2) ◽  
pp. 357 ◽  
Author(s):  
GC Brophy ◽  
PJ Newcombe ◽  
RK Norris
Keyword(s):  
Aromatic Ring ◽  
Long Range ◽  
Coupling Constants ◽  
Coupling Constant ◽  
Vicinal Coupling Constant

The p.m.r, spectra of 12 2-thienylmethylene and furfurylidene derivatives are reported. The average conformation about the Ar-CHXY bond is deduced from the magnitude of the meta (and ortho) long-range benzylic coupling constants. In thiophens, conformations in which the benzylic hydrogen is in or near the plane of the aromatic ring and anti to the ring sulphur are preferred. In furans the hydrogen-in-plane conformations are again favoured. The signs of the meta and ortho benzylic coupling constants have been determined and are positive and negative respectively with respect to the vicinal coupling constant J3,4.

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