Internal barriers to rotation in 2,6-difluoroisopropyl and 2,6-difluoroethylbenzenes. Overlap of dynamic nuclear magnetic resonance (DNMR) and the J method

1982 ◽  
Vol 60 (20) ◽  
pp. 2611-2616 ◽  
Author(s):  
Ted Schaefer ◽  
Richard P. Veregin ◽  
Reino Laatikainen ◽  
Rudy Sebastian ◽  
Kirk Marat ◽  
...  
Keyword(s):  
Internal Rotation ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Temperature Dependent ◽  
Carbon Carbon Bond ◽  
Barrier Region ◽  
Yield Formula ◽  
Internal Barriers ◽  
Dynamic Nuclear Magnetic Resonance ◽  
Spin Spin Coupling

The temperature-dependent 19F nmr spectra of 2,6-difluoroisopropylbenzene yield [Formula: see text], ΔH≠, and ΔS≠ as 6.93 (5) kcal/mol, 6.1(1) kcal/mol, and −5.0(8) cal/mol K, respectively, for the internal rotation of the isopropyl group about the sp2–sp3 carbon–carbon bond. The long-range spin–spin coupling constant over six bonds, 6JpH,CH, combined with the J method gives a twofold internal potential barrier of 5.0 ± 1.6 kcal/mol at 305 K. Although in this barrier range the J method suffers from large errors, the two methods yield comparable values for the barrier height. The lineshape method is inapplicable to 2,6-difluoroefhylbenzene. The J method finds the preferred conformation and a twofold barrier of 6.0 ± 2 kcal/mol, again in a barrier region where this method is inaccurate. Relative to hydrogen, the fluorine substituents cause substantial increases in the barriers to internal rotation. Signs of the stereospecific couplings, 4JoF,CH, are determined.

The internal barriers to rotation about the carbon–carbon bond in 3,5-dichlorobenzyl alcohol and selenol by the J method

1978 ◽  
Vol 56 (13) ◽  
pp. 1721-1723 ◽  
Author(s):  
Ted Schaefer ◽  
Werner Danchura ◽  
Walter Niemczura ◽  
William J. E. Parr
Keyword(s):  
Internal Rotation ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Side Chain ◽  
Carbon Carbon Bond ◽  
Carbon Carbon Bonds ◽  
Spin Coupling Constants ◽  
Internal Barriers ◽  
Spin Spin Coupling

The J method, depending on a comparison between observed spin–spin coupling constants over six bonds between protons on a side chain and para ring protons and those calculated by a hindered rotor treatment, is applied to the determination of the twofold barrier to internal rotation about the carbon–carbon bonds in 3,5-dichlorobenzyl alcohol and selenol. In the alcohol, the C—O bond prefers the benzene plane by 0.3 ± 0.2 kcal/mol whereas, in the selenol, the C—Se bond prefers a plane perpendicular to the benzene ring by 3.8 ± 0.7 kcal/mol. Comparison with the thiol suggests that a major component of the barrier arises from repulsive interactions, increasing as the size of the XH (X = O, S, Se) group increases.

The conformational behaviour of 2-fluoro and of 2,6-difluoroacetophenone implied by proximate spin–spin coupling constants and MO calculations

1985 ◽  
Vol 63 (8) ◽  
pp. 2256-2260 ◽  
Author(s):  
Ted Schaefer ◽  
Glenn H. Penner ◽  
Timothy A. Wildman ◽  
James Peeling
Keyword(s):  
Temperature Dependence ◽  
Geometry Optimization ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Population Distributions ◽  
Carbon Carbon Bond ◽  
Acetyl Group ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The temperature dependence of [Formula: see text], the nuclear spin–spin coupling constant over five formal bonds between the methyl protons and the 19F nucleus in 2-fluoroacetophenone and 2,6-difluoroacetophenone, is modelled on the assumption that 5J is a proximate coupling and that the STO 3G MO potential functions describe the population distributions of the rotamers defined by rotation about the exocyclic sp2–sp2 carbon–carbon bond. It is assumed that 5J has a cos4 θ dependence between 0 and 90°, where θ is the angle by which the acetyl group twists out of the plane of the benzene plane. The potential function is obtained from extensive geometry optimization procedures for a range of θ values. At 305 K, nonplanar conformations are substantially populated in 2-fluoroacetophenone, according to this model, which is also consistent with the idea that the 2,6-difluoro derivative has a markedly nonplanar ground state. The model reproduces the large 5J in the monofluoro relative to the difluoro compound, as well as the much larger temperature dependence in the former.

Derivatives of diphenylmethane. Preferred conformations and barriers to internal rotation by the J method

1979 ◽  
Vol 57 (14) ◽  
pp. 1881-1886 ◽  
Author(s):  
Ted Schaefer ◽  
Walter Niemczura ◽  
Werner Danchura ◽  
Timothy A. Wildman
Keyword(s):  
Internal Rotation ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Carbon Carbon Bond ◽  
Conformational Properties ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of ◽  
Rotor Model

The long-range spin–spin coupling constants over six bonds, 6JpH,CH, in 3,5-dibromodiphenylmethane and 4,4′-difluorodiphenylmethane, respectively, imply that the ground state conformations of these molecules have C2v symmetry (gable conformations). In terms of a hindered rotor model which assumes a twofold barrier to internal rotation about the exocyclic carbon–carbon bond, the barrier in the dibromo derivatives is 1.1 ± 0.3 kcal/mol. A satisfactory fit to the temperature dependence of 6JpF,CH is found for a gable conformation. If the conformational properties of these molecules and of diphenylmethane are determined mainly by steric interactions between ortho C—H bonds on neighbouring phenyl groups, it seems likely that the results above are a first approximation to the conformational behaviour of diphenylmethane. Some molecular orbital calculations are in semiquantitative agreement with the conclusions based on coupling constants.

The preferred conformations in solution and rotational barriers of the phenyl moiety in 3,5-dichlorobenzyl derivatives of ammonia, dimethylamine, and dimethylarsine

1978 ◽  
Vol 56 (17) ◽  
pp. 2229-2232 ◽  
Author(s):  
Ted Schaefer ◽  
Werner Danchura ◽  
Walter Niemczura
Keyword(s):  
Long Range ◽  
Phenyl Ring ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Carbon Bond ◽  
Carbon Carbon Bond ◽  
Spin Coupling Constants ◽  
Internal Barriers ◽  
Spin Spin Coupling ◽  
Derivatives Of

The long-range spin–spin coupling constants between methylene protons and ring protons are measured in 3,5-dichlorobenzylamine, 3,5-dichlorobenzyldimethylamine, and in 3,5-dichlorobenzyldimethylarsine. The couplings over six bonds are used to derive internal barriers to rotation about the carbon–carbon bond to the phenyl ring. In the above order, they are 0.3 ± 0.3, 0.8 ± 0.2, and 3.0 ± 0.5 kcal/mol. The conformation of lowest energy in the arsine is that in which the CH2—X bond lies in a plane perpendicular to the benzene plane.

The spin–spin coupling mechanism for 5J(C-4, CHα) in toluene derivatives and its conformational applications

1983 ◽  
Vol 61 (12) ◽  
pp. 2773-2776 ◽  
Author(s):  
Ted Schaefer ◽  
James Peeling ◽  
Glenn H. Penner
Keyword(s):  
Internal Rotation ◽  
Spin Coupling ◽  
Coupling Mechanism ◽  
Carbon Nucleus ◽  
Conformational Preference ◽  
Carbon Bond ◽  
Carbon Carbon Bond ◽  
Spin Spin Coupling

It is shown that the spin–spin coupling over five formal bonds between carbon 4 and the α protons in toluene and its ring or sidechain substituted derivatives is transmitted by a σ–π mechanism. As such it varies as sin2 θ, where θ is the angle by which the α C—H bond twists out of the benzene plane, and can be used to determine the conformational preference and the barrier to internal rotation about the exocyclic sp2–sp3 carbon–carbon bond. The coupling is remarkably insensitive to ring substituents, even to those at carbon 4. Two measurements suggest that the coupling over six formal bonds between a β carbon nucleus and apara proton also represents a σ–π interaction

An estimate of the spin–spin coupling constant, 1J(1H,13C), in gaseous benzene

1996 ◽  
Vol 74 (8) ◽  
pp. 1524-1525 ◽  
Author(s):  
Ted Schaefer ◽  
Guy M. Bernard ◽  
Frank E. Hruska
Keyword(s):  
Nuclear Magnetic Resonance ◽  
Dielectric Constant ◽  
Magnetic Resonance ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Spin Coupling Constant ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Gaseous Benzene

An excellent linear correlation (r = 0.9999) exists between the spin–spin coupling constants 1J(1H,13C), in benzene dissolved in four solvents (R. Laatikainen et al. J. Am. Chem. Soc. 117, 11006 (1995)) and Ando's solvation dielectric function, ε/(ε – 1). The solvents are cyclohexane, carbon disulfide, pyridine, and acetone. 1J(1H,13C)for gaseous benzene is predicted to be 156.99(2) Hz at 300 K. Key words: spin–spin coupling constants, 1J(1H,13C) for benzene in the vapor phase; spin–spin coupling constants, solvent dielectric constant dependence of 1J(1H,13C) in benzene; benzene, estimate of 1J(1H,13C) in the vapor; nuclear magnetic resonance, estimate of 1J(1H,13C) in gaseous benzene.

Internal motion in benzylidene diacetate and its 2,6-dibromo derivative by DNMR and the J method

1989 ◽  
Vol 67 (6) ◽  
pp. 1022-1026 ◽  
Author(s):  
Ted Schaefer ◽  
Craig S. Takeguchi
Keyword(s):  
Double Resonance ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Maximum Magnitude ◽  
Coupling Constant ◽  
Spectral Parameters ◽  
Proton Proton ◽  
Ether Solution ◽  
Spin Spin Coupling ◽  
Dibromo Derivative

The 1H nuclear magnetic resonance spectral parameters are reported for benzylidene diacetate in CS2 and acetone-d6 solutions. The long-range spin–spin coupling constant over six formal bonds, 6J, is used to derive apparent twofold barriers to rotation about the exocyclic C(1)—C bond in the two solutions. The conformation of lowest energy has the α. C—H bond in the benzene plane. The barrier is higher in CS2 than in acetone-d6 solution, in contrast to a molecule like benzyl chloride. In the 2,6-dibromo derivative, the free energy of activation for reorientation about the bond in question is 36 kJ/mol at 165 K in dimethyl ether solution. Such a high barrier implies a very small six-bond proton–proton coupling constant for this derivative because 6J is proportional to the expectation value of sin2θ. The angle θ is zero when the α C—H bond lies in the benzene plane. 6J is −0.051 Hz in acetone-d6 solutions; its sign is determined by double resonance experiments. The question of an angle-independent component of 6J, that is, whether 6J is finite at θ = 0°, is addressed. A maximum magnitude of 0.02 Hz may be present at θ = 0° for the 2,6-dibromo derivative, although a zero magnitude is also compatible with the experimental data. In a compound with a higher internal barrier, α,α,2,6-tetrachlorotoluene, the experimental results are best in accord with a negligibly small 6J at θ = 0°. Keywords: 1H NMR of benzylidene diacetate, spin–spin coupling constants for benzylidene diacetate, DNMR, 2,6-dibromobenzylidene diacetate.

Form of the dihedral angle dependence of the spin-spin coupling constant JHNNH

1989 ◽  
Vol 25 (3) ◽  
pp. 338-341
Author(s):  
L. M. Kapkan ◽  
A. Yu. Chervinskii ◽  
T. M. Pekhtereva ◽  
Yu. I. Smirnov ◽  
A. F. Dmitruk
Keyword(s):  
Dihedral Angle ◽  
Spin Coupling ◽  
Coupling Constant ◽  
Spin Coupling Constant ◽  
Angle Dependence ◽  
Spin Spin Coupling
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