An Investigation into σ and π Contributions to Long Range 1H–1H Coupling Constants in Conjugated Dienes and Diynes Using CNDO and INDO Molecular Orbital Calculations

1973 ◽  
Vol 51 (17) ◽  
pp. 2968-2974 ◽  
Author(s):  
Ian R. Peat ◽  
William F. Reynolds
Keyword(s):  
Perturbation Theory ◽  
Molecular Orbital ◽  
Long Range ◽  
Conjugated Dienes ◽  
Experimental Results ◽  
Coupling Constants ◽  
Molecular Orbital Calculations ◽  
Relative Importance ◽  
Indo Calculations ◽  
Good Agreement

Molecular orbital calculations based on finite perturbation theory in the CNDO/2 and INDO approximations are carried out for fourteen conjugated dienes and diynes for which experimental results are available. There is generally very good agreement between experimental and calculated (INDO) coupling constants. Comparison of results of CNDO/2 and INDO calculations elucidates the relative importance of σ and π pathways for the various long-range coupling constants.

An Investigation of σ and π Contributions to Long-range 1H–1H Coupling Constants in Planar Aromatic Derivatives Using CNDO and INDO Molecular Orbital Calculations

1974 ◽  
Vol 52 (13) ◽  
pp. 2403-2408 ◽  
Author(s):  
Ian R. Peat ◽  
William F. Reynolds
Keyword(s):  
Perturbation Theory ◽  
Molecular Orbital ◽  
Long Range ◽  
Coupling Constants ◽  
Molecular Orbital Calculations ◽  
Finite Perturbation ◽  
Indo Calculations

Molecular orbital calculations of coupling constants based on finite perturbation theory in the CNDO/2 and INDO approximations have been carried out for styrene, benzaldehyde, and naphthalene. INDO calculations have also been carried out in which one-center exchange integrals involved in σ–π interaction are omitted (INDO(σ) calculations). Both CNDO and INDO(σ) constants give reasonable estimates of σ contributions to long-range 1H–1H couplings, with the former giving better results when strongly stereospecific σ contributions are present. INDO calculations also give reasonable, but generally overestimated, π contributions to these couplings.

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.

Molecular orbital calculations and 1H nuclear magnetic resonance spectra as indicators of internal rotational potentials in some methyl derivatives of styrene

1988 ◽  
Vol 66 (4) ◽  
pp. 584-590 ◽  
Author(s):  
Ted Schaefer ◽  
Rudy Sebastian ◽  
Glenn H. Penner
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Long Range ◽  
Coupling Constants ◽  
Molecular Orbital Calculations ◽  
Carbon Carbon Bond ◽  
Nuclear Magnetic Resonance Spectra ◽  
Magnetic Resonance Spectra ◽  
Nuclear Magnetic

The 1H nuclear magnetic resonance spectra of the α-methyl, cis and trans-β-methyl, 4-methyl, and β,β-dimethyl styrènes are analyzed to yield long-range proton–proton coupling constants. With the assumptin that the internal rotational potential for styrene in the gas phase is unaltered in solution, a consistent treatment of over 40 of the long-range coupling constants is given in terms of the known coupling mechanisms. Expectation values of sin2 θ, where θ is the angle of twist about the exocyclic carbon–carbon bond, are presented for these molecules. These are compared with theoretical potentials at the 6-31 G level of molecular orbital theory. The present data indicate rather larger average twist angles than those in the literature. The extrema (at θ = θ° and 90°) in the angle dependent long-range coupling constants appear to be rather smaller in magnitude than are theoretical values obtained from valence bond and molecular orbital approaches.

Long-range Proton–Proton Coupling Constants in the Vinylpyridines. Conformational Preferences of the Vinyl Group and Molecular Orbital Calculations

1974 ◽  
Vol 52 (1) ◽  
pp. 136-142 ◽  
Author(s):  
J. Brian Rowbotham ◽  
Ted Schaefer
Keyword(s):  
Molecular Orbital ◽  
Long Range ◽  
Coupling Constants ◽  
Graphical Form ◽  
Molecular Orbital Calculations ◽  
Proton Proton ◽  
Meta Position ◽  
Proton Coupling ◽  
Conformational Preferences ◽  
Vinyl Group

The p.m.r. spectra of 2-vinylpyridine, 2-methyl-5-vinylpyridine, and 4-vinylpyridine are examined for long-range coupling constants between the vinylic and ring protons. Those over six, seven, and eight bonds from the vinyl group to ring or methyl protons in the para position are dominated by a π electron mechanism. Those over five bonds between the α proton and a ring proton in the meta position indicate that the vinyl group prefers to lie cis to the nitrogen in 2-vinylpyridine but trans in the 5-vinylpyridine. INDO-MO-FPT calculations of coupling constants between vinyl protons and ring protons, carbon-13, and nitrogen-14 nuclei are presented in graphical form.

Long-range proton coupling constants and molecular orbital calculations as indicators of conformational populations of benzene-1,2- and -1,3-dicarbaldehydes

1986 ◽  
Vol 64 (1) ◽  
pp. 158-163 ◽  
Author(s):  
Ted Schaefer ◽  
Glenn H. Penner ◽  
Rudy Sebastian ◽  
Craig S. Takeuchi
Keyword(s):  
Dipole Moment ◽  
Carbon Tetrachloride ◽  
Molecular Orbital ◽  
Long Range ◽  
Benzene Solution ◽  
Geometry Optimization ◽  
Coupling Constants ◽  
Molecular Orbital Calculations ◽  
Trans Form ◽  
Nmr Studies

The 1H nmr spectra of the benzene-1,2- and -1,3-dicarbaldehydes in carbon tetrachloride, benzene-d6, and acetone-d6 solutions at 300 K are analyzed. The stereospecific long-range couplings over five formal bonds between the sidechain and ring protons show that the 1,2 isomer exists as an 87:13 mixture of the cis–trans and trans–trans conformers in carbon tetrachloride. These populations are insensitive to solvent. Molecular orbital calculations utilizing extensive geometry optimization procedures imply that the cis–cis form, with proximate C=O bonds, is indeed of negligible significance as assumed in obtaining the populations of the other forms. Further calculations define a pathway of relatively low energy for interconversion of the two abundant forms, in agreement with dynamic nmr studies. For the 1,3 isomer the long-range couplings provide a check of the conformer populations deduced from dipole moment and 13C nmr studies. For example, if the cis–trans form is 70% abundant, as deduced from the dipole moment in benzene solution, then the long-range couplings imply that the population of the cis–cis conformer is insignificant.

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.

Formation of H -Addition Radicals in Adenine Derivatives

1976 ◽  
Vol 31 (5-6) ◽  
pp. 225-231 ◽  
Author(s):  
H. Zehner ◽  
W. Flossmann ◽  
E. Westhof
Keyword(s):  
Esr Spectroscopy ◽  
Coupling Constants ◽  
Molecular Orbital Calculations ◽  
X Rays ◽  
Hydrogen Atoms ◽  
Indo Calculations ◽  
Differential Stability ◽  
Purine Ring ◽  
Bonding Scheme ◽  
Adenine Derivatives

The formation of H-aduition radicals in monocrystals of 9-methyl adenine, deoxyadenosine monohydrate, and adenosine hydrochloride by irradiation with X-rays has been studied using ESR spectroscopy. In 9-methyl adenine and adenosine-HCl, hydrogen atoms add exclusively to position C8 of the imidazole part of the purine ring. On the other hand, in deoxyadenosine · H2O crystals, H-addition radicals at position C2 of the pyrimidine part of the purine ring occur together with H-addition radicals at position C8. Both radicals could be isolated by using their differential stability under warming or illumination with light. The C8-addition radical is characterized by two equivalent β-protons of 38.0 ± 0.5 G and the C2-addition radical by two non-equivalent β-protons of 34.0 ±1.0 and 50.0 ± 1.0 G. The nitrogen splittings perpendicular to the purine ring are 27.0 and 6.6 G for the C2-addition radical and 20.5 and 9.2 G for the C2-addition radical. The coupling constants of both radicals are in agreement with INDO calculations. It is further shown that the added hydrogen atom comes partly from the hydrogen bonding scheme and partly from the non­exchangeable hydrogens for the C8-addition radical. Together with additional experiments on polycrystalline samples, these results lead to the conclusion that hydrogen atoms add non-selectively to C2 and C8 of the neutral molecules, whereas protons add predominantly to C8 of anion radicals. This is supported by Hückel molecular orbital calculations.

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