Hydrogen bonding and anesthetic potency.

Hydrogen bond strengths in terms of the proton chemical shifts of five potent inhalation anesthetics containing acidic hydrogen were measured in cyclohexane and in methanol using the proton magnetic resonance spectroscopic method. The purpose of this study is to quantitatively compare the relative polar character of potent anesthetics. The hydrogen bond shift (delta ppm) of each anesthetic is the difference in the chemical shifts of the infinitely diluted unassociated anesthetic in cyclohexane and that of the infinitely diluted hydrogen bonded anesthetic in methanol. It was found that the hydrogen bond shifts (in delta ppm) are as follows: methoxyflurane, 0.72; chloroform, 0.75; halothane, 1.06; isoflurane, 1.38; enflurane, 1.44. There is a good correlation between the hydrogen bond shifts and the clinical potencies (minimum alveolar concentration in man). The conclusion from this study is that the acidic halogenated inhalation anesthetics are more potent if they form weaker hydrogen bonds.