Some unique properties of  in 2-fluorotoluene derivatives. Conformational dependence

A complete analysis of the 1H nmr spectrum of 2-fluorotoluene yields [Formula: see text] the spin–spin coupling constant between 19F and the methyl protons, as 1.99 Hz. Analysis of nmr spectra of 21 other derivatives of 2-fluorotoluene shows that [Formula: see text] can vary between 1.69 and 2.55 Hz. This strong dependence on substitution contrasts with the near invariance of other long-range couplings such as [Formula: see text], [Formula: see text], [Formula: see text] The substituent dependence is discussed in terms of coupling mechanisms. INDO MO FPT calculations of [Formula: see text] are inadequate. By means of appropriate model compounds, an adequate empirical conformational dependence is deduced for [Formula: see text] which can be used to reproduce some observed couplings. INDO MO FPT computations, in which certain off-diagonal Fock matrix elements are suppressed, are used to show that spin polarization via interacton of the methyl hydrogen orbitals is a major source of the discrepancy between theory and experiment. Some STO 3G MO calculations are reported for 2-fluorotoluene conformations.

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The probable planarity of 1,2-dimethoxybenzene in solution

Canadian Journal of Chemistry ◽

10.1139/v83-040 ◽

1983 ◽

Vol 61 (2) ◽

pp. 224-229 ◽

Cited By ~ 30

Author(s):

Ted Schaefer ◽

Reino Laatikainen

Keyword(s):

Benzene Solution ◽

Theoretical Data ◽

Spin Coupling ◽

Coupling Constant ◽

Nmr Spectrum ◽

Methyl Group ◽

H Nmr ◽

Planar Conformation ◽

Precise Analysis ◽

Spin Spin Coupling

A precise analysis of the 1H nmr spectrum of 1,2-dimethoxybenzene in benzene solution yields an accurate value for the proximate spin–spin coupling constant, [Formula: see text], between the ortho ring proton and the methyl protons. The latter also couple to other ring protons and these couplings are assessed. Comparison with some values in other anisole derivatives and with a variety of INDO MO FPT calculations of [Formula: see text] strongly implies the predominance of a planar conformation in solution. This implication disagrees with the interpretation of some other experimental and theoretical data. The mechanism of this proximate coupling is examined by the procedure of Barfield. It seems that the magnitude of the coupling is dominated by interactions involving the orbitals on the carbon atom of the methyl group.

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

Canadian Journal of Chemistry ◽

10.1139/v95-183 ◽

1995 ◽

Vol 73 (9) ◽

pp. 1478-1487 ◽

Cited By ~ 3

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.

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The proximate spin–spin coupling, 5J(F,CH3), as a quantitative conformational indicator in alkylfluorobenzenes and related compounds

Canadian Journal of Chemistry ◽

10.1139/v86-266 ◽

1986 ◽

Vol 64 (8) ◽

pp. 1602-1606 ◽

Cited By ~ 3

Author(s):

Ted Schaefer ◽

Rudy Sebastian ◽

Glenn H. Penner ◽

S. R. Salman

Keyword(s):

Nuclear Spin ◽

Spin Coupling ◽

Torsional Angle ◽

Rotational Behaviour ◽

Coupling Constant ◽

Spin Coupling Constant ◽

Spin Spin Coupling ◽

Torsional Potentials ◽

Derivatives Of ◽

Related Compounds

The through-space or proximate nuclear spin–spin coupling constant, 5J(F,CH3) = 5J, between methyl protons and ring fluorine nuclei in alkylfluorobenzenes is postulated as [Formula: see text] θ being the torsional angle for the [Formula: see text] bond. A and B are obtained from the known internal rotational behaviour in 2,6-difluoroethylbenzene and the corresponding cumene derivative. The parameterization is tested on the observed 5J in derivatives of 2,4,6-tri-tert-butyl- and 2,4,6-tri-isopropyl-fluorobenzene, in 2-chloro-6-fluoroisopropylbenzene, 2,6-difluoro-α-methylstyrene, and N-methyl-8-fluoroquinolinium halides. A prediction is made for 5J in 2,6-difluoro-tert-butylbenzene. It appears that the present parameterization allows the derivation of approximate torsional potentials from proximate couplings, for example in α,α-dimethyl-2,6-difluorobenzyl alcohol.

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Spin–spin coupling constants between side-chain and ring fluorine nuclei in some benzotrifluoride, benzal fluoride, and benzyl fluoride derivatives: coupling mechanisms

Canadian Journal of Chemistry ◽

10.1139/v79-133 ◽

1979 ◽

Vol 57 (7) ◽

pp. 807-812 ◽

Cited By ~ 13

Author(s):

Ted Schaefer ◽

Walter Niemczura ◽

Chiu-Ming Wong ◽

Kirk Marat

Keyword(s):

Nmr Spectra ◽

Coupling Constants ◽

Spin Coupling ◽

Complete Analysis ◽

Side Chain ◽

Coupling Mechanism ◽

Conformational Preferences ◽

Spin Coupling Constants ◽

Spin Spin Coupling ◽

Coupling Mechanisms

A complete analysis of the 1H and 19F nmr spectra of 2,5- and 3,4-difluorobenzotrifluoride, together with multiple resonance experiments, yields the signs and magnitudes of the long-range 19F,19F and 1H,19F spin–spin coupling constants. The coupling mechanisms are discussed. In particular, the coupling over six bonds, [Formula: see text], whose sign is interpretable in terms of a σ–π mechanism, is too large in magnitude when compared to [Formula: see text], and [Formula: see text] in the analogous compounds. These latter three couplings are consistent in sign and magnitude with what is known about hyperfine interaction constants. The magnitudes of [Formula: see text] are reported for 4-fluorobenzotrifluoride, 3-amino-4-fluorobenzotrifluoride, 3-nitro-4-fluorobenzotrifluoride, as are 6JpF,F values for p-fluorobenzal fluoride and p-fluorobenzyl fluoride. In contrast to 6JpH,CH and 6JpF,CH it seems unlikely that, unless its coupling mechanism becomes more precisely understood, 6JpF,CF will be a reliable indicator of conformational preferences.

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Remarks on the barrier to rotation about the Csp2—O bond in anisole

Canadian Journal of Chemistry ◽

10.1139/v89-173 ◽

1989 ◽

Vol 67 (7) ◽

pp. 1148-1152 ◽

Cited By ~ 28

Author(s):

Ted Schaefer ◽

Rudy Sebastian

Keyword(s):

Molecular Orbital ◽

Heavy Atom ◽

Phenyl Group ◽

Solvent Mixture ◽

Spin Coupling ◽

Basis Set ◽

Coupling Constant ◽

H Nmr ◽

Internal Barrier ◽

Spin Spin Coupling

Molecular orbital computations with the basis set 6-31G are reported for seven values of θ, the torsion angle about the [Formula: see text] bond in anisole. All bond angles and lengths are optimized but the atoms of the phenyl group are constrained to a plane. The relative energies are fit by V(θ)/kJ mol−1 = 7.78(5) sin2θ + 2.41(5) sin2 2θ − 0.54(5) sin2 3θ, where θ is zero when the heavy-atom skeleton is planar. Computations with the basis set 6-31G*(5D) for three values of θ can be reproduced by V(θ)/kJ mol−1 = 6.07 sin2θ + 2.68 sin2 2θ. These results are compared with experimental gas phase data from the literature. The analysis of the 1H nuclear magnetic resonance spectrum of anisole-α-13C in aCS2/C6D12/TMS solvent mixture yields a value of 6J(1H, 13C), the long-range spin-spin coupling constant between the 13C nucleus in the methyl group and thepara proton. Because this coupling constant is proportional to sin2 θ, it is shown, together with previous dynamic nmr measurements, that the barrier to rotation about the [Formula: see text] bond in solution cannot be purely twofold. The internal potential must also contain a fourfold term of the same sign as that of the twofold component. If the V2/V4 ratio given by the various molecular orbital computations holds in solution, then V2 is 15.0 ± 2.0 kJ/mol and V4 is 5.6 ± 2.2 kj/mol. The apparent doubling of the internal barrier in solution is perhaps unprecedented for such a simple molecule. Keywords: anisole, internal barrier in solution, anisole-α-13C, 1H NMR, conformational behaviour, MO computations.

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

Canadian Journal of Chemistry ◽

10.1139/v69-666 ◽

1969 ◽

Vol 47 (21) ◽

pp. 4005-4010 ◽

Cited By ~ 10

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.

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Proton magnetic resonance spectrum of cellulose triacetate. Temperature effects, the rotational conformation of the 6-acetoxymethyl group, and computer modelling of methyl 4,6-di-O-acetyl-β-glucopyranoside

Canadian Journal of Chemistry ◽

10.1139/v85-415 ◽

1985 ◽

Vol 63 (9) ◽

pp. 2507-2510 ◽

Cited By ~ 9

Author(s):

Vanga S. Rao ◽

Françoise Sauriol ◽

Arthur S. Perlin ◽

M. T. Phan Viet

Keyword(s):

Resonance Spectrum ◽

Computer Modelling ◽

Chemical Shifts ◽

Cellulose Triacetate ◽

Proton Magnetic Resonance Spectrum ◽

Spin Coupling ◽

Supporting Evidence ◽

Nmr Spectrum ◽

H Nmr ◽

Spin Spin Coupling

Pronounced narrowing of the resonance signals in the 400-MHz 1H nmr spectrum of cellulose triacetate in CDCl3 between 25 and 50 °C, as well as shielding and deshielding changes in 1H chemical shifts, suggest that thermal disruption of intermolecular aggregates is accompanied by a conformational modification. Analysis of the spin–spin coupling patterns clearly evident at 50 °C indicates that although there is a reversal in the chemical shifts of H-6R and H-6S relative to those for acetylated D-glucopyranose derivatives, the rotational conformations of the exocyclic 6-acetoxymethyl groups of the polymer and the model compounds all favor RHS and RCS rotameric forms. Supporting evidence for this conclusion is obtained from the 2-dimensional 1H spectrum of a trisaccharide, O-β-D-glucopyranosyl-(1 → 3)-O-β-D-glucopyranosyl-(1→ 4)-O-β-D-glucopyranose undecaacetate. Computer-generated models of methyl 4,6-di-O-acetyl-β-glucopyranoside are examined in relation to the stereochemistry of 6-acetoxymethyl groups.

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The rotational angle dependence of 5JmH,SH in benzenethiol

Canadian Journal of Chemistry ◽

10.1139/v82-269 ◽

1982 ◽

Vol 60 (15) ◽

pp. 1924-1927 ◽

Cited By ~ 10

Author(s):

Ted Schaefer ◽

Timothy A. Wildman ◽

Rudy Sebastian

Keyword(s):

Spin Coupling ◽

Benzene Derivatives ◽

Coupling Constant ◽

Spin Coupling Constant ◽

Rotational Angle ◽

Angle Dependence ◽

Meta Position ◽

Electron Contribution ◽

Spin Spin Coupling ◽

Derivatives Of

It is suggested that the five-bond spin–spin coupling constant between the sulfhydryl proton and the ring proton in the meta position is given by [Formula: see text]. Here θ is the angle by which the S—H bond twists out of the benzene plane and angular brackets indicate expectation or average values, determined by the twofold barrier to internal rotation about the C—S bond. [Formula: see text] is the π electron contribution and has a maximum at θ = 90° and [Formula: see text] is the σ electron contribution with a maximum at θ = 180° (zig-zag orientation). For eight para substituted derivatives of benzenethiol, the 5J numbers can be reproduced by this equation, which also agrees with the observed couplings in 2-hydroxybenzenethiol. In this compound θ lies near 90°. It appears that this approach will allow an extension of the J method to the evaluation of two-term potential functions in, for example, ortho substituted benzene derivatives.

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The sign of the 'through-space' spin–spin coupling between methyl protons and the fluorine nucleus in 2,6-dimethylbenzoyl fluoride

Canadian Journal of Chemistry ◽

10.1139/v76-115 ◽

1976 ◽

Vol 54 (5) ◽

pp. 800-804 ◽

Cited By ~ 15

Author(s):

Ted Schaefer ◽

Kalvin Chum ◽

Kirk Marat ◽

Roderick E. Wasylishen

Keyword(s):

Spin Coupling ◽

Coupling Constant ◽

Spin Coupling Constant ◽

Fluorine Nucleus ◽

Methyl Group ◽

Out Of Plane ◽

Spin Spin Coupling ◽

Coupling Mechanisms ◽

Carbonyl Fluoride

The spin–spin coupling constant over five formal bonds between 19F and methyl protons, [Formula: see text], in 2,6-dimethylbenzoyl fluoride is −3.1 Hz. Observation of a nonzero [Formula: see text] indicates an out-of-plane conformation for the carbonyl fluoride group and implies substantial nonbonded repulsions between the methyl and carbonyl fluoride groups. It is argued that [Formula: see text] is as small as −7 Hz when the C—F bond lies cis to a methyl group and that its magnitude is a consequence of the so-called 'through-space' coupling mechanisms. On the basis of INDO–MO–FPT computations, it is suggested that such observed couplings are a composite of large contributions of either sign and, therefore, that observed through-space 1H,19F couplings may be of either sign if conformational averaging occurs.

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