Stereospecific Spin–Spin Coupling between Hydroxyl Protons and 19F Nuclei in o-, m- and p-Fluorophenol Derivatives. "Through-space" Interactions via the Hydrogen Bond

1975 ◽  
Vol 53 (7) ◽  
pp. 986-992 ◽  
Author(s):  
J. Brian Rowbotham ◽  
Murray Smith ◽  
Ted Schaefer
Keyword(s):  
Hydrogen Bond ◽  
Substituent Effects ◽  
Proton Exchange ◽  
Spin Coupling ◽  
Side Chain ◽  
Molecular Orbital Calculations ◽  
Coupling Mechanism ◽  
Fluorine Nucleus ◽  
Hydroxyl Protons ◽  
Spin Spin Coupling

Under conditions of slow intermolecular hydroxyl proton exchange 6JpOH,F < 0, 5JtransOH,F > 0, 5JcisOH,F < 0, 4JoOH,F < 0 in fluorophenol derivatives. 5JmOH,F is remarkably insensitive to intrinsic substituent effects, yields accurate values of conformational populations of m-fluorophenol derivatives and can be used to demonstrate a buttressing effect on the strength of the relatively weak [Formula: see text] hydrogen bond. 4JoOH,F displays a stereospecific dependence opposite to that of other couplings from a side-chain proton to a ring fluorine nucleus, is apparently very sensitive to the distance between proton and fluorine nucleus and is cited as an example of a negative through-space coupling mechanism. Approximate molecular orbital calculations give some support to the last suggestion.

Through-space or proximate 19F,19F and 19F,1H spin–spin coupling constants in some benzenesulfonyl fluoride derivatives

1976 ◽  
Vol 54 (22) ◽  
pp. 3564-3568 ◽  
Author(s):  
William J. E. Parr ◽  
Ted Schaefer
Keyword(s):  
Magnetic Resonance ◽  
Proton Magnetic Resonance ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Side Chain ◽  
Conformational Preference ◽  
Fluorine Nucleus ◽  
Spin Coupling Constants ◽  
The Difference ◽  
Spin Spin Coupling

The analysis of the fluorine and proton magnetic resonance spectra of 2,4,6-trimethylbenzenesulfonyl fluoride and of 2,5-difluorobenzenesulfonyl fluoride yields the signs and magnitudes of the spin–spin coupling constants containing a through-space component. The coupling between the fluorine nucleus and the methyl protons over five bonds is +1.9 Hz, opposite in sign to the −3.1 Hz observed for the corresponding coupling in 2,6-dimethylbenzoyl fluoride. The difference of 5 Hz is possibly a consequence of the different conformational preference of the SO2F and COF substituents. The coupling over four bonds between the fluorine nucleus on the side chain and that on the ring is +11.6 Hz in 2,5-difluorobenzenesulfonyl fluoride. It is argued that this value indicates a preference of the S—F bond for a plane lying, on average, nearly perpendicular to the benzene ring. Similar indications are noted for pentafluorobenzenesulfonyl fluoride and for pentafluorobenzenesulfinyl fluoride.

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.

Orientations of the hydroxyl and isopropyl groups in the cis and trans conformers of 2-isopropylphenol and 2-isopropyl-6-methylphenol

1981 ◽  
Vol 59 (11) ◽  
pp. 1656-1659 ◽  
Author(s):  
Ted Schaefer ◽  
Brenda M. Addison ◽  
Rudy Sebastian ◽  
Timothy A. Wildman
Keyword(s):  
Coupling Constants ◽  
Spin Coupling ◽  
Hydroxyl Group ◽  
Hydroxyl Groups ◽  
Molecular Orbital Calculations ◽  
Isopropyl Group ◽  
Phenol Derivatives ◽  
Hydroxyl Protons ◽  
Cis And Trans ◽  
Spin Spin Coupling

Measurement of the long-range spin–spin coupling constants between methine or hydroxyl protons and the ring protons in 2-isopropylphenol shows that the trans conformer is favored by 2.5 ± 0.3 kJ/mol for a 2 mol% solution in CCl4, at 305 K. In this conformer the methine C—H bond of the isopropyl group points towards the hydroxyl group. In the cis conformer, the C—H bond points away from the hydroxyl group. In 2-isopropyl-6-methylphenol the conformation in which the O—H bond lies trans to the isopropyl group is preferred by 1.2 ± 0.3 kJ/mol in free energy at 305 K in CCl4 solution. This result agrees with an additive superposition of the conformational preferences of the hydroxyl groups in the monosubstituted phenol derivatives. Molecular orbital calculations do not agree with the existence of a cis conformer.

INDO Molecular Orbital Calculations on the Conformational Dependence of Long-range Spin–Spin Coupling of Methyl Protons in Toluene, p-Fluorotoluene, and the Xylenes

1972 ◽  
Vol 50 (12) ◽  
pp. 1852-1862 ◽  
Author(s):  
R. Wasylishen ◽  
T. Schaefer
Keyword(s):  
Molecular Orbital ◽  
Perturbation Technique ◽  
Spin Coupling ◽  
Methyl Groups ◽  
Molecular Orbital Calculations ◽  
Orbital Theory ◽  
Fluorine Nucleus ◽  
Rotational Angle ◽  
Methyl Group ◽  
Spin Spin Coupling

The conformational dependence of the nuclear spin–spin coupling from methyl protons to ring protons, to the fluorine nucleus, and to protons of other methyl groups in toluene, p-fluorotoluene, and the xylenes is computed by the finite perturbation technique in the INDO approximation of molecular orbital theory. The calculated coupling over six bonds to the proton in the para position agrees quantitatively with experiment and its predicted dependence on the rotational angle of the methyl group supports a commonly assumed π electron mechanism for the transmission of spin information between the nuclei. Similar remarks apply to the fluorine nucleus in p-fluorotoluene. The couplings over five and four bonds to the protons in the meta and ortho positions display a more complex angular dependence and the former can be interpreted in terms of a dominant σ electron mechanism. The coupling between protons in different methyl groups in the ortho and meta xylenes is calculated as rather larger than the values observed in their derivatives and in the main shows the behavior expected from a π electron mechanism. Those conformations of ortho xylene in which the coupled protons are in close proximity yield computed values plausibly attributable to "direct" and/or "through-space" mechanisms. The preferred conformation of the methyl group in toluene is predicted to have a C—H bond eclipsing the plane of the aromatic ring and the calculated barriers to rotation of 0.013 kcal/mol in toluene and of 0.014 kcal/mol in p-fluorotoluene are in quantitative accord with microwave data.

scholarly journals The para substituent dependence of the internal rotational barrier in benzenethiol

1977 ◽  
Vol 55 (3) ◽  
pp. 552-556 ◽  
Author(s):  
Ted Schaefer ◽  
William J. E. Parr
Keyword(s):  
Ab Initio ◽  
Internal Rotation ◽  
Molecular Orbital ◽  
Rotational Barrier ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Molecular Orbital Calculations ◽  
Fluorine Nucleus ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

On the basis of the observed spin–spin coupling constants between the sulfhydryl and ring protons and a hindered rotor treatment of the twofold barrier to internal rotation in a series of para substituted benzenethiol derivatives, it is argued that V2 is essentially zero in p-amino-benzenethiol and is 2.5 ± 0.2 kcal/mol in p-nitrobenzenethiol; having intermediate values for the methoxy, fluoro, methyl, and bromo derivatives in solution. The results are based on an assumed relationship between the four-bond and the fictitious six-bond couplings to the sulfhydryl proton. The conclusions are consistent with the observed magnitudes of the couplings over six and seven bonds, respectively, between the sulfhydryl proton and the fluorine nucleus and the methyl protons in the appropriate derivatives; as well as with the coupling between the sulfhydryl and methyl protons in 4-bromo-3-methylbenzenethiol. The experimental barriers are compared with ab initio molecular orbital calculations of their substituent dependence.

Spin–spin coupling between proximate protons on neighbouring aromatic rings and between hydroxyl protons in 2,2′-dihydroxy-4-methoxybenzophenone. Coupling through the hydrogen bond

1976 ◽  
Vol 54 (14) ◽  
pp. 2231-2234 ◽  
Author(s):  
Ted Schaefer ◽  
Kalvin Chum
Keyword(s):  
Hydrogen Bond ◽  
Relative Orientation ◽  
Spin Coupling ◽  
Weak Hydrogen Bond ◽  
Aromatic Rings ◽  
Covalent Character ◽  
Nonbonded Interactions ◽  
Direct Mechanism ◽  
Hydroxyl Protons ◽  
Spin Spin Coupling

The doubly hydrogen bonded conformation of 2,2′-dihydroxy-4-methoxybenzophenone ensures substantial nonbonded interactions between C—H bonds on neighbouring aromatic rings and gives rise to spin–spin coupling between the protons in those bonds. Because of the relative orientation of the C—H bonds containing the coupled protons, the coupling represents a direct mechanism which probably does not depend on the orbitals of the carbon atom. The observed coupling between the hydroxyl protons, formally over eight bonds, is small and indicative of an inappreciable covalent character of the hydrogen bond, in agreement with recent conclusions that a weak hydrogen bond is best represented by electrostatic and non-bonded forces only.

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