The 13C nuclear magnetic resonance substituent chemical shifts of monosubstituted cyclohexanes. Interpretation by characteristic vector analysis

Abstract

The set of literature ¹³C n.m.r. substituent chemical shifts (s.c.s.) of 43 monosubstituted cyclohexanes has been used in Characteristic Vector Analysis (CVA). All variability in the spectra generated by substituents and their configuration could be explained by three orthogonal characteristic vectors. Experimental s.c.s. are thus linear combinations of three independent phenomena acting at each carbon atom simultanously with varying degrees of intensity. The largest contribution, described by V_1,n is associated with the polarizing power of the substituent through σ bonds. The contribution to s.c.s. by V_2,n is due to a field-inductive effect of the substituent. The contribution associated with V_3,n is significant only for C-γ and C-δ and results in an upfield or downfield shift due to the conformation of the substituent.