HYDROXYL PROTON MAGNETIC RESONANCE STUDY OF ALIPHATIC ALCOHOLS

The constancy of the hydroxyl-carbinol proton coupling constant (JHCOH) for rotamerically homogeneous methanol in hydrogen-bonding media indicates that JHCOH variations for different alcohols in these media may be attributed to conformational changes. JHCOH measurements for sterically unhindered primary alcohols suggest that the three C-O bond rotamers are approximately equally populated. Similar measurements for unhindered secondary alcohols suggest the rotamer with the OH staggered between the two C-C bonds to be significantly less populated than the other two. For both primary and secondary alcohols substitution of alkyl groups on a carbon β to the OH consistently increases JHCOH. This is attributed to a slight but significant decrease in the equilibrium H-C-O-H dihedral angle of some of the C-O bond rotamers in which the OH is staggered between a C-H and a C-C bond. For secondary cyclic alcohols in which the hydroxyl hydrogen is constrained to be only gauche or and to the carbinol hydrogen, JHCOH values reveal the simplified Karplus relation (JHCOH = A cos2 ϕ) to be followed to a good approximation. In dimethyl sulfoxide the hydroxyl proton chemical shift, relative to that of the neat alcohol, can be correlated with the degree of steric crowding around the OH group. © 1969, American Chemical Society. All rights reserved.