The probable planarity of 1,2-dimethoxybenzene in solution

1983 ◽  
Vol 61 (2) ◽  
pp. 224-229 ◽  
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.

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

1983 ◽  
Vol 61 (1) ◽  
pp. 29-36 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Richard P. Veregin ◽  
Reino Laatikainen
Keyword(s):  
Strong Dependence ◽  
Spin Coupling ◽  
Complete Analysis ◽  
Matrix Elements ◽  
Coupling Constant ◽  
Nmr Spectrum ◽  
H Nmr ◽  
Spin Spin Coupling ◽  
Coupling Mechanisms ◽  
Derivatives Of

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.

The proximate coupling constant, 5J(H,CH3), and the torsional mobility of the thiomethyl group in some thioanisole derivatives

1991 ◽  
Vol 69 (4) ◽  
pp. 620-624 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Salman R. Salman ◽  
James D. Baleja ◽  
Glenn H. Penner ◽  
...  
Keyword(s):  
Bond Order ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Torsional Motion ◽  
Coupling Constant ◽  
Methyl Group ◽  
H Nmr ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Ortho Substituent

The proximate coupling constants, 5J, between ortho and methyl protons in thioanisole and 18 of its derivatives are discussed as conformational indicators. On the assumption that 5J varies as cos4θ, for 0° ≤ θ ≤ 90°, θ being the angle by which the methyl group twists out of the aromatic plane, 5J for θ = 0° follows as −0.43 (2) Hz from the known internal barrier in thioanisole in solution. A measurement of 5J in meta- or para-substituted thioanisole derivatives then yields an approximate value for the twofold barrier to rotation about the Csp2—S bond. For derivatives containing an ortho substituent, 5J yields an estimate of the torsion angle for the thiomethyl moiety. In some instances these angles are compared with those derived from long-range 1H, 13C and 13C, 13C coupling constants. The size of the ortho substituent appears to have only a small effect on the magnitude of 5J. The major determinant of the latter appears to be the manner in which the substituent perturbs the mobile bond order of the Csp2—S bond. Key words: spin–spin coupling constants, thioanisole derivatives; 1H NMR, thioanisole derivatives; conformations, thioanisole derivatives; conformations, torsional motion of SCH3 group.

The planar conformation of 2-methylthiobenzaldehyde in solution

1983 ◽  
Vol 61 (1) ◽  
pp. 26-28
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian
Keyword(s):  
Chemical Shifts ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Spectral Parameters ◽  
H Nmr ◽  
Heavy Atoms ◽  
Planar Conformation ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Infrared Data

The 1H nmr spectral parameters are extracted for a 4 mol% solution of 2-methylthiobenzaldehyde in CCl4 at 305 K. The long-range spin–spin coupling constants involving the aldehydic and methyl protons are consistent only with a preferred conformation in which all heavy atoms are coplanar, as are the chemical shifts of the ring and methyl protons. This conclusion contradicts previous interpretations of the dipole moment, the nmr parameters, and of the infrared data for CCl4 solutions. The present data show that the O-syn and O-anti forms of the compound are present in roughly equal proportions.

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

1989 ◽  
Vol 67 (7) ◽  
pp. 1148-1152 ◽  
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.

The range of in anisole derivatives. An indicator of torsion and compression of the methoxy group

1985 ◽  
Vol 63 (3) ◽  
pp. 782-786 ◽  
Author(s):  
Ted Schaefer ◽  
Salman R. Salman ◽  
Timothy A. Wildman ◽  
Glenn H. Penner
Keyword(s):  
Methoxy Group ◽  
Bond Angle ◽  
Average Distance ◽  
Electron Donors ◽  
Spin Coupling ◽  
Ortho Position ◽  
Coupling Constant ◽  
Spin Coupling Constant ◽  
Methyl Group ◽  
Spin Spin Coupling

In a series of anisole derivatives, [Formula: see text], the spin–spin coupling constant between the methyl protons and the ring proton in the ortho position, ranges from −0.23 to −0.38 Hz when the methyl group lies cis to the ortho C—H bond. 5J, as a proximate coupling, is sensitive to the average distance between the coupled protons. Its variation with substituent can be rationalized in terms of torsion about the Csp2—O bond and changes in the bond angles near the methoxy moiety. The theoretical 5J numbers can be empirically reproduced by a cos4 ψ function, where ψ is the angle by which the methoxy group twists out of the benzene plane. In general, large ortho substituents cause an increase in the magnitude of 5J (bond angle changes), strong π electron donors in the para position cause a decrease in the magnitude of 5J (increased torsional freedom), and π electron acceptors do the opposite (decreased torsions).

Indirekte Kernspinkopplung zwischen Protonen und Elementen der IV. Gruppe

1964 ◽  
Vol 19 (1) ◽  
pp. 139-142 ◽  
Author(s):  
Herbert Dreeskamp
Keyword(s):  
Electron Density ◽  
Strong Correlation ◽  
Valence Electron ◽  
Spin Coupling ◽  
Atomic Data ◽  
Coupling Constant ◽  
Methyl Group ◽  
Hartree Fock ◽  
Fermi Contact ◽  
Spin Spin Coupling

The indirect spin-spin coupling between protons and Ge73, spin 9/2, in the tetraedric molecule GeH4 has been measured. JGe-H = 97,6 Hz. Introducing the normalized coupling constant J′ which is obtained by dividing the measured coupling constant by the product of the magnetogyric ratios of the coupling nuclei, a strong correlation is found between this quantity for XH4 (X = C, Si, Ge, Sn, Pb) and the electron density of the valence electron at the nucleus obtained from HARTREE-FOCK calculations or experimental atomic data. This demonstrates that at least in these cases the FERMI contact contribution is dominant. For the analogeous tetramethyl compounds the same relation holds only to the extend that the C - H coupling in the methyl group is constant.

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

1985 ◽  
Vol 63 (9) ◽  
pp. 2507-2510 ◽  
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.

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

1976 ◽  
Vol 54 (5) ◽  
pp. 800-804 ◽  
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.

Intramolecular electric field effect on a1J(C,H) NMR spin-spin coupling constant. an experimental and theoretical study

1999 ◽  
Vol 37 (3) ◽  
pp. 227-231 ◽  
Author(s):  
Dora G. de Kowalewski ◽  
Valdemar J. Kowalewski ◽  
Juan E. Peralta ◽  
Gernot Eskuche ◽  
Rubén H. Contreras ◽  
...  
Keyword(s):  
Electric Field ◽  
Field Effect ◽  
Theoretical Study ◽  
Spin Coupling ◽  
Electric Field Effect ◽  
Coupling Constant ◽  
Spin Coupling Constant ◽  
H Nmr ◽  
Spin Spin Coupling

Geminal H,D spin-spin coupling and isotope effects in partially deuterated methyl-aromatic compounds

1964 ◽  
Vol 17 (1) ◽  
pp. 38 ◽  
Author(s):  
CG Macdonald ◽  
JS Shannon ◽  
S Sternhell
Keyword(s):  
Aromatic Compounds ◽  
Hydrogen Exchange ◽  
Isotope Effects ◽  
Deuterium Atom ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Deuterium Isotope Effect ◽  
Methyl Group ◽  
Catalytic Hydrogen ◽  
Spin Spin Coupling

Seventeen methyl-aromatic compounds were selectively, partially deuterated by catalytic hydrogen exchange. The proton magnetic resonance (p.m.r.) spectra of the partially deuterated compounds showed that the geminal H,D coupling constant is approximately 2.2 c/s and that it does not vary significantly with the type of substitution in the ring. The deuterium isotope effect on the chemical shift of the remaining protons in the methyl group is approximately 0.7-1.1 c/s in the upfield direction for the introduction of one deuterium atom.

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