Intermolecular interactions substituent effects

Some time later the feasibility of obtaining 170 spectra for natural abundance samples at relatively low concentration was demonstrated55,56. This was an important step because intermolecular interactions can markedly affect chemical shifts and, if only electronic substituent effects are to examined, then all extraneous influences must be minimized. 

Since the electronic absorption spectra of polvsilanes and polygermanes also depend strongly on conformation (3), these materials provide a unique opportunity to study the effect of backbone conformation on the NLO properties, particularly since varying alkyl substituents can dramatically influence the backbone structure through intermolecular interactions (e.g., side chain crystallization) while causing... 

The good correlations observed between the relative rates of Table 17 and the chemical shifts of the protons in position 2 of protonated 4-substituted p n i-dines would indicate that the anisotropic contributions and the intermolecular interactions are substantially identical and that the major factor controlling both the chemical reactivity and the relative shielding of the hydrogen nuclei in the meta position to the substituent is the electron density in position 2 of the molecule. The slopes of the plots (Table 18) give a measure of the different selectivity, exclusively due to polar effects, and therefore a measure of the relative nucleophilici-ties of the free radicals involved. This interpretation is further supported by the linear correlations between the relative rates and the pKa of the 4-substituted P3nidines. 

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