(H, H) coupling constants in the nuclear magnetic resonance spectra of hydrocarbon groupings

Abstract

The collected data on geminal (H, H) coupling constants in substituted methanes and ethanes show that this coupling constant in methane is negative in sign, relative to vicinal coupling constants; a reassessment of the theoretical situation is required.Vicinal (H, H) coupling constants in substituted ethanes and ethylenes are qualitatively well accounted for by Karplus' theoretical treatment ; the predicted positive sign is probably correct as it leads to a predominant -, +,pattern of sign-alternation for (H, H) coupling across 2, 3 and 4 bonds. The vicinal coupling constants show variations of magnitude that are closely dependent on the electronegativity of substituent groups. Exceptions to the above pattern of sign-alternation include the positive gemha1 couplings in =CH2 and epoxide groups, and meta couplings across four bonds in benzene and heterocyclic aromatic rings.Long-range couplings across four or five bonds in unsaturated hydrocarbon groupings are mostly well accounted for by theory based on U-W configuration interaction. Some exceptional inter-nuclear couplings in saturated systems across four bonds and in unsaturated ones across five bonds that show marked stereospecifici ty, perhaps indicate residual long-range coupling through sigma bonds.* (Added in proof.)-The Dirac vector model is more likely to be relevant to long range " throughbond ' .' couplings, and Karplus' successful theory of vicinal couplings includes important terms involving H-H bonding that would not be considered on the Dirac model. However, here the predicted positive sign is the same in each case.

Correlations of Proton Coupling Constants in the Cyclopropane Ring With Electronegativity: Consideration of the Proton Resonance Spectrum of Cyclopropyllithium

“…1 is that the presence of the two chlorine atoms increases the coupling constants numerically by about 2 c/s. This is contrary t o what one expects since E dependencies of coupling constants so far published (11,(13)(14)(15)(16)(17) demand that J is inversely proportional t o E. I n the present case, however, the chlorine atoms are situated on a neighboring carbon, i.e. on a carbon bonded t o the carbons t o which the hydrogens are bonded.…”

Double and Triple Resonance Experiments on 2-Bromo-5-Chlorotoluene: Signs of the Ring and Ring-Methyl Proton Coupling Constants

“…The signs found for the ring-methyl proton couplings are in agreement wit11 the Dirac vector model ( 2 5 ) , more likely to be relevant to long range "through-bond" couplings (26). The simple molecular orbital calculation (4, 27) of the u-a interaction predicts negative values for JHqCt13' and JEI,cnln as we find, but JH,CE3m = 0.…”

Nuclear Magnetic Resonance Studies of Compounds Related to Tetramethyldiphosphine Disulphide

“…ments have been reported linking the signs of ' J p p , 'JHp, 2 J H p , or 3JHp directly to that of l J = 1 3~. However, it has been shown that the signs of coupling constants across two bonds, 2J, are more variable than those for coupling across three bonds, 3J (30). The latter are norinally positive, certainly for the (H, H) case.…”

“…Recent work with the electric-field effect (29) has confirmed this hypothesis. Considerable progress has been made in placing the signs of (H, H ) coupling constants on this scale (30,31), but values for other situations are scarce. Consequently it is difficult to relate the signs of For personal use only.…”