NUCLEAR MAGNETIC RESONANCE SOLVENT EFFECTS AND MOLECULAR INTERACTIONS .2. A COMPARISON OF DIPOLAR, HYDROGEN-BONDING, AND CHARGE-TRANSFER EFFECTS

Concentration-dependent nmr shifts for hydrogen-bonding, charge-transfer, and dipolar solutes in the methylbenzenes and a few nonaromatic solvents have been analyzed using a simple collision complex model. K's, Δ´s, and, in some instances, ΔH's are reported. All the interactions studied are properly classified as weak (ΔH's from 0.0 to –4.0 kcal/mol; K´s from 0.03 to 4.0 1./mol; and rotational lifetimes of order 10−11H sec). Despite such similarities, information concerning the relative arrangement of solute and solvent for each type of interaction was obtained. Criteria are suggested to distinguish the various modes of interaction. A simple electrostatic model is suggested for the interactions of dipolar solutes with dipolar and/or polarizable solvents. The solvent effects for such solutes can be explained by electric field and magnetic anisotropy terms. Hydrogen-bonding protons show much larger solvent shifts than predicted from the electrostatic model. Various explanations are considered for anomalies in solvent shifts for charge-transfer complexes. A change of average complex geometry as the interaction strength changes is the suggested explanation. © 1969, American Chemical Society. All rights reserved.