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.

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 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.

Long-range spin–spin coupling constants as an indicator of conformational preferences in ethyl and trifluoroethyl vinyl ethers

1977 ◽  
Vol 55 (15) ◽  
pp. 2835-2838 ◽  
Author(s):  
Ted Schaefer ◽  
William J. E. Parr
Keyword(s):  
Vinyl Ether ◽  
Long Range ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Ethyl Vinyl Ether ◽  
Spectroscopic Techniques ◽  
Ethyl Vinyl ◽  
Conformational Preferences ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The observed and calculated, negative, long-range spin–spin coupling constants over five bonds between olefinic and methylene protons in ethyl vinyl ether and in 2,2,2-trifluoroethyl vinyl ether are consistent with predominant s-cis planar conformations. The five-bond couplings are sensitive to the proximity of the bonds containing the coupled nuclei and are unobservably small in 1-butene where the H,H distances are somewhat larger than in the ethers. The present results concur with the arguments based on other spectroscopic techniques.

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.

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.

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.

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.

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

1979 ◽  
Vol 57 (7) ◽  
pp. 807-812 ◽  
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.

Theoretical and experimental approaches to the effects of solvation on the small internal rotational potential of benzal fluoride

1996 ◽  
Vol 74 (9) ◽  
pp. 1626-1637 ◽  
Author(s):  
Ted Schaefer ◽  
Guy M. Bernard ◽  
Younes Bekkali ◽  
David M. McKinnon
Keyword(s):  
Molecular Orbital ◽  
Long Range ◽  
Minimum Energy ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Torsional Angle ◽  
Expectation Values ◽  
Ellipsoidal Cavity ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The internal rotational potential for benzal fluoride is computed at various levels of molecular orbital theory, including correlation-gradient, MP2 (frozen core) methods. The perturbations of the potential caused by solvents are calculated with the Onsager model (ellipsoidal cavity with l = 6 in the multipole expansion) as well as with the self-consistent isodensity – polarizable continuum model (SCI–PCM). Analysis of the 1H and 19F nuclear magnetic resonance spectra in cyclohexane-d12 and acetone-d6 solutions yields long-range spin–spin coupling constants from which the expectation values of [Formula: see text] can be derived. These expectation values can be compared with those calculated from the theoretical internal rotational potential. Reasonable agreement is found for potentials obtained from MP2/6-31G* approaches in both solvent models. Long-range coupling constants between 19F and 13C nuclei are also reported and provide very rough checks of the [Formula: see text] values. For the isolated molecule an additivity scheme based on the potential for benzyl fluoride reproduces much of the potential for benzal fluoride except for a deviation caused by the rather larger relative magnitude of the fourfold component in the latter. The minimum in the potential for benzal fluoride occurs for a torsional angle, [Formula: see text] of 90° corresponding to a conformation in which the C—H bond of the side chain lies in a plane perpendicular to the phenyl plane and is rationalized on the basis of electrostatic forces. The conformations of minimum energy for the benzyl and benzal fluorides and chlorides are compared and contrasted. The magnitudes of the internal potentials of the fluorides are only a little larger than thermal energies at 300 K and can become smaller than the latter in soludon. Key words: NMR spectroscopy, of benzal fluoride; spin–spin coupling constants, long range in benzal fluoride; solvent effects, on internal rotational potential in benzal fluoride; molecular orbital computations, structure, internal rotational potential, and its solvent perturbations in benzal fluoride; benzal fluoride, 1H, 19F, and 13C NMR on, internal rotational potential, MO computations.

Sign in / Sign up

Export Citation Format

Share Document