Chapter 5 The cause and calculation of proton chemical shifts in non-conjugated organic compounds

Abstract

The replacement of hydrogen atoms in organic molecules by substituents may cause marked relative chemical shifts Δδ of nearby protons. In high resolution proton magnetic resonance spectroscopy the chemical shift increments Δδ of protons in methyl groups are brought about mainly by the action of the electric dipole moments of substituents. This is shown for OH, Cl, >CO and CN groups in non-conjugated and non-aromatic compounds. In the case of the carbonyl group, its anisotropic magnetic susceptibility must also be taken into account. The rough agreement between the calculated and observed relative chemical shifts of methyl chloride, acetonitrile and the aldehydes and the precise agreement in the case of the ethyl derivatives and of acetone are proof that, despite some successful correlations with substituent electronegativity, no σ-inductive effect causing proton chemical shifts is involved. This is further evidence suggesting that the often invoked inductive effect is, in reality, essentially the linear electric field effect originating in the substituent dipole moment. Relative chemical shifts of single protons in the proximity of the newly introduced substituent in rigid systems, as opposed to the “rotating” methyl protons, cannot be calculated completely with the model applied to methyl protons. This is shown for the rigid system {fx1-206}in which X is the substituent. A further effect must be allowed for, which is proposed to originate in different solute-solvent interactions of the molecules with and without substituent X. Empirical rules for its assessment as a function of the torsion angle around the CC single bond of the system {fx2-206} are given. A model in which the van der Waals interaction between the proton considered and that part of the solvent molecules which is excluded by the larger volume of the substituent, as compared with the replaced hydrogen atom, is considered to be the cause of this further effect, roughly describes the decisive part. With this model the relative chemical shifts due to methyl groups as substituents may also be roughly understood. These investigations confirm that for a quantitative calculation of the relative chemical shifts of single protons in rigid systems a deeper understanding of the solute-solvent interactions is necessary. Preliminary investigations demonstrate that, in addition to specific solute-solvent interactions, some sort of a reaction field may be important in the vicinity of a polar substituent. Some rules governing the solvent dependence of protons in the investigated compounds are given and parallels with similar infrared spectroscopic studies are shown.