Methyl–Methyl Proton Spin–Spin Coupling Constants in Some Six-membered Ring Compounds. A Test for Delocalization in Borazine

1974 ◽  
Vol 52 (3) ◽  
pp. 489-496 ◽  
Author(s):  
J. Brian Rowbotham ◽  
T. Schaefer
Keyword(s):  
Electron System ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Substitution Pattern ◽  
Methyl Proton ◽  
Ring Compounds ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

Spin–spin coupling constants over five, six, and seven bonds between protons in different methyl groups are reported for the xylenes, derivatives of benzene, pyridine, pyrimidine, pyridinium salts, p-benzoquinone, and borazine. The coupling magnitudes are characteristic of the substitution pattern in the delocalized systems. Calculations at the INDO-MO-FPT level of methyl proton couplings in N- and B-methylborazines are in agreement with the available experimental evidence in indicating a relatively weak transmission of spin density information via the presumed π electron system of borazine.

Proton spin–spin coupling constants and intramolecular hydrogen bonding in bromine derivatives of 1,3-dihydroxybenzene

1976 ◽  
Vol 54 (14) ◽  
pp. 2228-2230 ◽  
Author(s):  
Ted Schaefer ◽  
J. Brian Rowbotham
Keyword(s):  
Hydrogen Bond ◽  
Hydrogen Bonding ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Hydroxyl Groups ◽  
Oh Groups ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling ◽  
Derivatives Of

The conformational preferences in CCl4 solution at 32 °C of the hydroxyl groups in bromine derivatives of 1,3-dihydroxybenzene are deduced from the long-range spin–spin coupling constants between hydroxyl protons and ring protons over five bonds. Two hydroxyl groups hydrogen bond to the same bromine substituent in 2-bromo-1,3-dihydroxybenzene but prefer to hydrogen bond to different bromine substituents when available, as in 2,4-dibromo-1,3-dihydroxybenzene. When the OH groups can each choose between two ortho bromine atoms, as in 2,4,6-tribromoresorcinol, they apparently do so in a very nearly statistical manner except that they avoid hydrogen bonding to the common bromine atom.

Experimental and Theoretical Estimates of Sigma and Pi Electron Contributions to Long-range Spin–Spin Coupling Constants in Coumarin and its Methyl Derivatives

1973 ◽  
Vol 51 (6) ◽  
pp. 953-960 ◽  
Author(s):  
J. B. Rowbotham ◽  
T. Schaefer
Keyword(s):  
Valence Bond ◽  
Electron System ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Orbital Theory ◽  
Coupling Path ◽  
Spin Coupling Constants ◽  
Methyl Derivatives ◽  
Spin Spin Coupling

A full analysis of the p.m.r. spectra of coumarin and five of its methyl derivatives allows the assignment of σ and π electron components to the inter-ring proton–proton spin–spin coupling constants. Magnitudes of up to 0.3 Hz are observed over a coupling path of eight bonds. The σ and π components can be combined on the basis of the appropriate valence bond structures to predict closely the observed parameters in naphthalene, suggesting only weak participation of the ether oxygen atom in the π electron system of coumarin. Calculations of the inter-ring coupling constants at the INDO and CNDO/2 levels of molecular orbital theory are helpful in the interpretation of the observed data, the agreement with experiment being quantitative in many instances. Comparison of the observed coupling constants in coumarin and styrene with the INDO results for an assumed planar form of the latter suggests that styrene exists in a nonplanar conformation.

Signs of methyl–methyl proton spin–spin coupling constants in xylene derivatives (interbenzylic coupling)

1970 ◽  
Vol 48 (6) ◽  
pp. 1002-1007 ◽  
Author(s):  
C. J. Macdonald ◽  
W. F. Reynolds
Keyword(s):  
Double Resonance ◽  
Proton Spin ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Methyl Proton ◽  
Π Interactions ◽  
Methyl Methyl ◽  
Para Xylene ◽  
Spin Coupling Constants ◽  
Spin Spin Coupling

The signs of the intermethyl proton spin–spin coupling constants in ortho, meta, and para xylene derivatives (interbenzylic coupling) have been determined by double resonance techniques. [Formula: see text] [Formula: see text] and [Formula: see text] are positive, negative, and positive, respectively, in agreement with the predictions of theories based on σ–π interactions. The magnitudes of the coupling constants are discussed in terms of the values predicted by these theories.

Proton Magnetic Resonance of the Methyl Derivatives of 2-Fluoropyridine. Sigma and Pi Electron Contributions to Spin–Spin Coupling Constants

1971 ◽  
Vol 49 (11) ◽  
pp. 1799-1803 ◽  
Author(s):  
J. B. Rowbotham ◽  
R. Wasylishen ◽  
T. Schaefer
Keyword(s):  
Nitrogen Atom ◽  
Proton Magnetic Resonance ◽  
Electron System ◽  
Coupling Constants ◽  
Spin Coupling ◽  
Electron Contribution ◽  
Spin Coupling Constants ◽  
Methyl Derivatives ◽  
Spin Spin Coupling ◽  
Derivatives Of

The p.m.r. spectra of the methyl derivatives of 2-fluoropyridine are analyzed. The signs and magnitudes of the long-range spin–spin coupling constants between the methyl protons and the ring protons and between the methyl protons and fluorine are consistent with a model in which the nitrogen atom polarizes the sigma electron system but leaves the pi electron contribution to the coupling constants relatively unchanged. There are dramatic changes in the ring proton – fluorine couplings while the couplings involving the methyl protons vary little from those in the corresponding toluene derivatives. Thus the coupling over six bonds between fluorine and methyl protons is 1.25 ± 0.03 Hz in 2-fluoro-5-methyl pyridine compared to 1.15 ± 0.02 Hz in p-fluorotoluene.

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