KINETIC DEUTERIUM ISOTOPE EFFECT IN REACTION OF METHYL IODIDE WITH CYANIDE ION IN AQUEOUS SOLUTION FROM 0 TO 40 DEGREES

The kinetic α-deuterium isotope effect, kH/kD, has been measured for the reaction of CH3I (CD3I) with CN- in aqueous solution from 0 to 40°. Kinetic measurements have been carried out in reaction vessels with no gas phase. Results indicate an inverse isotope effect: kH/Kd = 0.926 (25°), with the Arrhenius parameters AH/Ad = 1.124 (±0.005), EaH --- Ead = 116 (±20) cal. The experimental data are compared with results of model calculations of isotope effects from force constants with an electronic computer. Reasonable choices of some of the transition-state force constants can be made by comparison with suitable stable molecules. The bending force constant, fHcc, is adjusted to give the best fit with the experimental isotope effect. It is planned to compare this fHcc value with bending force constants for other transition states belonging to related reactions. It is found that excellent agreement between experimental and calculated Arrhenius parameters of the isotope effect may be obtained if 5.05 rather than 5.3 mdyn/Å is used for the CH stretching force constant, fCH, of the transition state. © 1968, American Chemical Society. All rights reserved.