The ionic hydrogen bond. 1. Sterically hindered bonds. Solvation and clustering of protonated amines and pyridines
作者:Michael Meot-Ner、L. Wayne Sieck
DOI:10.1021/ja00348a005
日期:1983.5
2,6-dialkylpyridines, and tertiary amines were investigated in the gas phase in the absence of solvent effects. Steric hindrance causes major entropy effects unfavorable to dimerization or hydration. As long as a single confirmation exists in which the hydrogenbond can obtain optimal geometry, the bond strength is not weakened by steric crowding. However, steric crowding may result in major entropy
transfer between sterically hindered pyridines and amines proceeds through locked-rotor, low-entropy intermediates. The reactions exhibit slow kinetics (efficiencies of 0.1-0.0001) and large negativetemperaturecoefficients (up to k=CT − 8 .7 ). The rates become slower and the temperature dependencies steeper with increasing steric hindrance. The observations are reproduced by a multiple complex-switching