Proton N M R spectra of ]-substituted 2,4-dimethylbcnzenes (2). I-substituted 2.6dimethylbenzenes (3) and 1-substituted 2,4,6-trimethylbenzenes (4) were determined and the SCS values compared with those of monosubstituted benzenes (1). SCS of 1 are assumed to be primarily due to the effects of x-electron charge density, substituent electric field and substituent diamagnetic anisotropy, and the van der Waals interaction: thus scs scs, f SCSZ 1 scs.4 + SCSV When, however, the substituent is sterically hindered, then:
where c , , ~ may be constants predictable from the theories associated R i t h each component. By estimating SCS,,v or c,,~, a quantitative separation of SCS into their components was attempted.
It is shown, however, that the data available, as well as the nature of this approach, cannot necessarily be sufficient for this purpose.
Various effects which might also contribute to the proton N M R chemical shifts of sterically hindered molecules are also discussed.
I N T R O D U C T I O N
ALTHOUGH considerable data has been reported on the proton NMR spectra of substituted benzenes, the nature of the substituent effect which determines the substituent chemical shift (SCS)t of monosubstituted benzenes (1) has not yet been fully understood.2 In principle, the calculation of chemical shifts is based on the Ramsey e q ~a t i o n , ~" which in this case is practically impossible because of the lack of knowledge of the exact wave functions for ground and all excited states. In discussing substituent effects, the method of Saika and S1ichtersb is usually followed. This subdivides the chemical shifts into the diamagnetic and paramagnetic contributions from the atom in question and neighbouring atoms. Although there are several factors contributing to the local diamagnetic term, the most dominant factor4 for substituted benzenes seems to be the changes in .rr-electron charge density in the ring resulting from the substituent perturbation. Aromatic proton chemical shifts due to the diamagnetic term are thus most conveniently assumed to be proportional to * For Part 111, see Ref. 1. t SCS (Substituent Chemical Shift) as defined by M. J. S. Dewar and A. P. Marchand, J. A m .
Chem. SOC. 88, 3318 (1966): the difference in chemical shift for a proton in a substituted benzene and the appropriate proton shift when the substituent is hydrogen.