scholarly journals Proton Magnetic Resonance Spectrum of 1-Penten-3-yne. INDO and CNDO/2 Molecular Orbital Calculations on Long-Range Spin–Spin Coupling Constants in Enynes

1972 ◽  
Vol 50 (12) ◽  
pp. 1863-1867 ◽  
Author(s):  
L. Ernst ◽  
H. M. Hutton ◽  
T. Schaefer
Keyword(s):  
Molecular Orbital ◽  
Long Range ◽  
Benzene Solution ◽  
Parent Compound ◽  
Proton Magnetic Resonance Spectrum ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The high resolution p.m.r. spectra of 1-penten-3-yne in carbon disulfide and in benzene solution are analyzed. Long-range spin–spin coupling constants are discussed in terms of σ and π electron contributions. Comparisons are made with the isomeric 2-methyl-1-buten-3-yne and the parent compound, vinylacetylene. The results of INDO and CNDO/2 molecular orbital calculations are compared to the experimental coupling constants. It is concluded that the π electron contribution to 5J in enyne systems is +0.6 to 0.7 Hz and that σ electron contributions are rather small, the transoid ("pseudo-zig–zag") being larger than the cisoid one. Observed allylic coupling constants in the propene derivative are compared with the calculated values, including those for propene and 2-cyanopropene, available in the literature.

Long-range Spin–Spin Coupling Constants from Amino Protons and 15N to Ring Protons in Aniline-15N and Some Derivatives. INDO Molecular Orbital Calculations

1972 ◽  
Vol 50 (16) ◽  
pp. 2575-2585 ◽  
Author(s):  
R. Wasylishen ◽  
J. B. Rowbotham ◽  
L. Ernst ◽  
T. Schaefer
Keyword(s):  
Molecular Orbital ◽  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Complete Analysis ◽  
Molecular Orbital Calculations ◽  
Orbital Theory ◽  
Planar Conformation ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

A complete analysis (8-spins) is given of the p.m.r. spectrum of aniline-15N, of the spectra of some haloanilines-15N and of 2-aminoacetophenone-15N. Intermolecular exchange of the amino protons is slow enough for observation of their spin–spin coupling to the ring protons. The magnitudes of the coupling constants between amino protons and 15N or ring protons are a measure of the geometry of the amino group. This is not true of the couplings between 15N and the ring protons. Long-range couplings computed in the CNDO/2 and INDO approximations of molecular orbital theory show points of agreement with experiment. For example, their signs and magnitudes are consistent with a nonplanar but not with a planar conformation of aniline. Couplings from 15N to ring protons are also computed for nitrobenzene.

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.

INDO Molecular Orbital Calculations of Nuclear Spin–Spin Coupling Constants Over Three Bonds Between 13C and 1H in Some Simple Molecules

1973 ◽  
Vol 51 (6) ◽  
pp. 961-973 ◽  
Author(s):  
R. Wasylishen ◽  
T. Schaefer
Keyword(s):  
Dihedral Angle ◽  
Molecular Orbital ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Methyl Proton ◽  
Coupling Path ◽  
Spin Coupling Constants ◽  
Simple Molecules ◽  
Spin Spin Coupling

Molecular orbital calculations, at the INDO level, of the spin–spin coupling constants over three bonds between carbon-13 and protons are reported for a number of simple molecules. In propane the coupling depends on dihedral angle in the Karplus manner. Fluorine substituents cause changes in the computed coupling which are best described as alternating with the number of bonds intervening between the substituent and the coupled nuclei. Finer details of this phenomenon are discussed and calculations on propyllithium are performed. Replacement of a central carbon atom in propane by a heteroatom does not radically alter the computed couplings. The presence of a carbonyl group in the coupling path results in an overestimate of the magnitude of the coupling. In propene the coupling between 13C in position 1 and a methyl proton displays a maximum when the C—H bond of the methyl group lies parallel to the π orbitals. In toluene the coupling to a methyl proton is insensitive to the dihedral angle over half its range, a result of importance to structural studies. Among other molecules under consideration are methylacetylene, propionaldehyde, and the strained bicyclobutane. It is suggested that in certain instances the mean of the predictions from the INDO and CNDO/2 procedures may agree better with experiment than will the prediction from either procedure alone. Calculations on fluorobenzene and 1,2-difluorobenzene suggest that the main experimental trends of the couplings between carbon and protons within the benzene ring are reproduced. Such is perhaps not true for the five-membered heterocycles.

Mechanisms of 1H,1H; 1H,13C; and 13C,13C spin–spin coupling constants in benzyl cyanide and some derivatives. Experimental and theoretical estimates of internal rotational potentials

1986 ◽  
Vol 64 (10) ◽  
pp. 2013-2020 ◽  
Author(s):  
Ted Schaefer ◽  
Glenn H. Penner
Keyword(s):  
Molecular Orbital ◽  
Cyano Group ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Similar Data ◽  
Orbital Theory ◽  
Benzyl Cyanide ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The mechanisms of long-range spin–spin coupling constants involving the methylene protons and the 13C nucleus of the cyano group are discussed for benzyl cyanide. Analysis of the 1H nmr spectrum of benzyl cyanide-8-13C in benzene-d6 solution yields nJ(H,CH2) and nJ(H,13CN) for n = 4–6. Similar data are reported for the 2,6-dichloro and 2,6-difluoro derivatives, together with some sign determinations. nJ(13C,13CN), n = 1–5, are given for the three compounds. It is shown that all these parameters are consistent with a small barrier to internal rotation about the [Formula: see text] bond in benzyl cyanide in solution. Computations at various levels of molecular orbital theory agree that this barrier is small. The nJ(13C, 13CN) imply a stabilization in polar solvents of the conformation in which the cyano group of benzyl cyanide lies in a plane perpendicular to the benzene plane. The molecular orbital calculations indicate a predominantly twofold nature of the internal barrier, although a significant fourfold component is also present. The coupling constants cannot discern the presence of the fourfold component for benzyl cyanide nor for its 2,6-difluoro derivative. 1J(13C,13CN) is solvent dependent. A table of the computed sidechain geometries is appended.

Nuclear Magnetic Resonance Spectra, Conformations, Spin Coupling Mechanisms, and INDO Molecular Orbital Calculations for the Monofluorobenzaldehydes and some Derivatives

1971 ◽  
Vol 49 (19) ◽  
pp. 3216-3228 ◽  
Author(s):  
R. Wasylishen ◽  
T. Schaefer
Keyword(s):  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Conformational Preferences ◽  
Coupling Parameters ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Magnetic Resonance Spectra

Precise analyses of the proton and some fluorine magnetic resonance spectra in acetone solution are reported for the three monofluorobenzaldehydes as well as for 2-chloro-6-fluorobenzaldehyde and for 4-fluoro-2-nitrobenzaldehyde. The conformational dependence of the coupling parameters allows the measurement of energy differences between the O-cis and O-trans conformations. The energy differences are in better agreement with the INDO predictions than they are with energies derived from i.r. data. Di-pole moments are computed reliably and their measurement is suggested as a good guide to conformational preferences for molecules of this kind. The spin–spin coupling constants between the aldehyde proton and the ring protons and fluorine nuclei are computed for benzaldehyde and the three monofluorobenzaldehydes by the INDO and CNDO molecular orbital approximations. In many instances the agreement between calculated and observed couplings is quantitative.

Conformational dependence of long-range spin-spin coupling constants in 2,6-dichlorobenzylfluoride

1969 ◽  
Vol 47 (19) ◽  
pp. 3688-3690 ◽  
Author(s):  
T. Schaefer ◽  
C. M. Wong ◽  
K. C. Tam
Keyword(s):  
Magnetic Resonance ◽  
Long Range ◽  
Proton Magnetic Resonance ◽  
Resonance Spectrum ◽  
Double Resonance ◽  
Proton Magnetic Resonance Spectrum ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

Double resonance experiments on the proton magnetic resonance spectrum of 2,6-dichlorobenzylfluoride yield the signs of the long-range coupling constants between the ring protons and the fluorine nuclei and protons in the fluoromethyl group. The signs and magnitudes of the long-range couplings are discussed in terms of their dependence on the conformation of the fluoromethyl group.

1H nuclear magnetic resonance and molecular orbital studies of 2-formylstyrene. Three significant nonplanar conformers at 300 K

1995 ◽  
Vol 73 (12) ◽  
pp. 2208-2216 ◽  
Author(s):  
Ted Schaefer ◽  
Scott Kroeker ◽  
David M. McKinnon
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Formyl Group ◽  
Trans Conformer ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The 1H nuclear magnetic resonance spectra of 2-formylstyrene, from dilute solutions in CS2–C6DI2 and acetone-d6, are analyzed to yield precise chemical shifts and spin–spin coupling constants. The long-range coupling constants imply a conformational distribution in which the O-trans conformer is 55% abundant in both polar and nonpolar environments. They also imply that the vinyl group, on average, is twisted out of the aromatic plane to a much larger extent than in styrene. The 6-31G* basis set gives an ab initio potential for the torsion of the vinyl moiety with a relatively deep minimum at 38° out-of-plane, for the O-cis conformer. For the O-trans conformer, two minima are found, one at 45° and another at 129.6°. Essentially the same potential is obtained with the 6-31G** basis. The latter corresponds to a close approach of the hydrogen atom of the formyl group and π orbitals or the β-carbon atom of the olefinic side chain. This local minimum is interesting in terms of a hypothesis used to explain the photochemistry of the molecule. The long-range coupling constants are consistent with the conformational properties calculated for the free molecule; they also indicate no significant difference between the conformational behaviour of the molecule in the two solvents. A proximate coupling constant of −0.16 Hz exists between the formyl and methine (α) protons. The latter is strongly deshielded in the presence of the formyl group, so that it becomes even less shielded than some of the aromatic protons. Keywords: 1H NMR, 2-formylstyrene (o-vinylbenzaldehyde); long-range spin–spin coupling constants, 2-formylstyrene; conformations, three nonplanar of 2-formylstyrene; molecular orbital calculations, conformations of 2-formylstyrene.

Information from stereospecific spin–spin couplings about the relative absorptivities of the hydroxyl stretching bands in 2-iodophenol solutions

1981 ◽  
Vol 59 (21) ◽  
pp. 3021-3025 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Timothy A. Wildman
Keyword(s):  
Free Energy ◽  
Molecular Orbital ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Temperature Dependent ◽  
Hydroxyl Proton ◽  
Spin Coupling Constants ◽  
Cis And Trans ◽  
Spin Spin Coupling

The stereospecific spin–spin coupling constants between the hydroxyl proton and the ring protons for 2-iodophenol in various solvents yield some free energy differences between the cis and trans conformations of this molecule at 305 K. Comparison with areas of the hydroxyl stretching bands in the same or similar solvents shows that the ratio of the absorptivity coefficients for the two conformers is sensitive to solvent. It is suggested that this ratio is temperature dependent and therefore apparent enthalpy differences must be considered tentative for at least some solutions. Molecular orbital calculations are consistent with the arguments concerning the absorptivity coefficients.

Experimental and theoretical estimates of the internal rotational barrier of benzyl fluoride in the vapor phase

1990 ◽  
Vol 68 (4) ◽  
pp. 581-586 ◽  
Author(s):  
Ted Schaefer ◽  
Christian Beaulieu ◽  
Rudy Sebastian ◽  
Glenn H. Penner
Keyword(s):  
Molecular Orbital ◽  
Vapor Phase ◽  
Phenyl Group ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Basis Sets ◽  
Molecular Orbital Calculations ◽  
Stable Conformer ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The twofold barrier to rotation about the [Formula: see text] bond in benzyl fluoride is deduced from the long-range 1H,1H; 1H,19F; and 13C,19F nuclear spin–spin coupling constants in solution. The barrier changes from 3.2(2) kJ/mol in the polar solvent, acetonitrile-d3, to 0.7(2) kJ/mol in the nonpolar environment provided by cyclohexane-d12. In all solutions the conformer of greatest stability has the C—F bond in a plane perpendicular to that of the phenyl group. Extrapolation of the barrier to the vapor phase, using a simple reaction field model, indicates that the most stable conformer for the free (unclustered) molecule is now that with the C—F bond in the phenyl plane and that the barrier to internal rotation is 1.1(7) kJ/mol. Molecular orbital calculations with the basis sets STO-3G, 4-21G, 4-31G, 6-31G, and 6-31G* all predict the latter conformer as that of lowest energy. However, they disagree significantly among themselves as to the height of the internal barrier. The complete geometries are given for both conformers, as computed with the 6-31G basis, and the side-chain geometries are tabulated for the planar and perpendicular conformers, as given by all the bases. Keywords: benzyl fluoride, internal rotational potential; 13C,19F spin–spin coupling constants in benzyl fluoride; benzyl fluoride, molecular orbital computations.

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