INDO Molecular Orbital Calculations of Nitrogen–Proton Spin–Spin Coupling Constants Over Two and Three Bonds. Effects of Lone-pair Orientation, of Dihedral Angles, and of Protonation

1972 ◽  
Vol 50 (18) ◽  
pp. 2989-3008 ◽  
Author(s):  
R. Wasylishen ◽  
T. Schaffer
Keyword(s):  
Molecular Orbital ◽  
Lone Pair ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Phosphorus Compounds ◽  
Molecular Orbital Calculations ◽  
Orbital Theory ◽  
Proton Coupling ◽  
Nitrogen Lone Pair

Molecular orbital theory at the INDO level of approximation is used to calculate the Fermi contact contribution to two-bond and three-bond nitrogen–proton coupling constants for a wide variety of compounds. The dependence of calculated N,H coupling constants upon the spatial orientation of the nitrogen lone-pair is examined for selected molecules. For both saturated and unsaturated compounds the calculated 2J(15N,H) is large and negative when the lone-pair is oriented cis to the proton, and is small and of either sign when the lone-pair lies trans to the proton. Calculated 3J(N,H) depends both on the orientation of the nitrogen lone-pair and on the HCCN dihedral angle. Computed 2J(N,H) and 3J(N,H) generally follow the experimentally known substituent and structural effects. It is suggested that observed phosphorus–proton coupling constants in tervalent phosphorus compounds are dependent on lone-pair orientation in a manner analogous to the corresponding nitrogen–proton coupling constants. The influence of the nitrogen lone-pair orientation on geminal proton–proton coupling constants in methylamine is computed and compared with experiment. Calculated barriers to pyramidal inversion and methyl group rotation are in reasonable agreement with available experimental data.

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.

1H nuclear magnetic resonance and molecular orbital studies of the structure and internal rotations in ethylbenzene

1987 ◽  
Vol 65 (4) ◽  
pp. 873-877 ◽  
Author(s):  
Ted Schaefer ◽  
Glenn H. Penner ◽  
Rudy Sebastian
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Magnetic Resonance ◽  
Molecular Orbital ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Orbital Theory ◽  
Spectral Parameters ◽  
Internal Barrier ◽  
Nuclear Magnetic

Rather extensive geometry-optimized computations at the STO-3G, 4-21G, 4-31G, and 6-31G levels of abinitio molecular orbital theory suggest that the fourfold component of the barrier to internal rotation about the Csp2—Csp3 bond in ethylbenzene amounts to about 20% of the twofold component. The 1H nuclear magnetic resonance spectral parameters, extracted by complete analyses of the spectra arising from the ten protons, are reported for ethylbenzene in acetone-d6, CCl4, CS2, and perfluoromethylcyclohexane solutions. The long-range proton–proton spin–spin coupling constants demonstrate that the internal barrier is insensitive to the polarity of the solvent, in contrast to polar solute molecules such as benzyl fluoride. The coupling constants do not support a dependence of the internal barrier on the internal pressure of the solvent.

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.

Importance of Orbital Complementarity in Spin Coupling through Two Different Bridging Groups in Dicopper(II) Complexes of Endogenous Alkoxo Bridging Ligand with Exogenous Carboxylate: Ab-initio and Semi-Empirical Calculations

2007 ◽  
Vol 62 (7-8) ◽  
pp. 409-416 ◽  
Author(s):  
C. Tugrul Zeyrek
Keyword(s):  
Ab Initio ◽  
Molecular Orbital ◽  
Magnetic Behaviour ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Energy Separation ◽  
Molecular Orbital Calculations ◽  
Bridging Ligand ◽  
Spin Singlet ◽  
D Orbitals

The influence of overlap interactions between the bridging ligands and the metal d orbitals on the super-exchange coupling constant are studied by means of ab-initio restricted Hartree-Fock molecular orbital calculations. The interaction between the magnetic d orbitals and the HOMOs of the carboxylate oxygen atoms are investigated in homologous asymmetrically dibridged dicopper(II) complexes which have significantly different - 2J values (the energy separation between the spin-triplet and spin-singlet states). In order to determine the nature of the fronter orbitals, extended Hückel molecular orbital (EHMO) calculations are also reported. The differences in the magnitude of the coupling constants and magnetic behaviour are rationalized in terms of the bridging ligand orbital complementary / countercomplementary concept.

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.

The Molecular Structure of Allenes and Ketenes, XIII Correlations of Carbon-Proton Spin-Spin Coupling Constants in Allenes with ab initio STO-3G Overlap Populations

1979 ◽  
Vol 34 (1) ◽  
pp. 118-120 ◽  
Author(s):  
Wolfgang Runge
Keyword(s):  
Molecular Structure ◽  
Ab Initio ◽  
Substituent Effects ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Proton Coupling ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Cl Atoms

Abstract It is shown that substituent effects on one-bond and long-range carbon-proton coupling constants in monosubstituted allenes parallel quantitatively ab initio STO-3G carbon 2s-hydrogen 1 s overlap populations, irrespectively of whether the substituents are bonded to the allenic skeleton via first-row (C, O) or second-row (Si, S, Cl) atoms.

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