Berechnung kinetischer Isotopeneffekte für die Reaktion von Methyljodid mit Hydroxid-Ion

Abstract

Kinetic carbon-13, deuterium, and oxygen-18 isotope effects are calculated from force constants for the SN2 reaction of CH3I with OH-. The secular equations for the vibrational frequencies of reactants and transition states are solved on an IBM 7094 computer. Values for 7 force constants of the planar CH3 moiety in the transition state (with an sp2 atom) are obtained by comparison with suitable stable molecules. The stretching force constants ƒCO = ƒ11, ƒCI = ƒ22, and ƒ12 in the transition state are chosen as linear functions of x, the degree of bond making and bond breaking, with the aid of eqs. (1) and (2). The carbon isotope effect as a function of x exhibits a flat maximum near x=0.6. If the bending force constants ƒHCO and ƒHCI are kept constant, k12/k13 is significantly lower than its maximum value only for values of x ≦ 0.3 and x ≧ 0.8. There is only a small influence of x on the deuterium isotope effect, kH/kD (compare rates of CH3I and CD3I). However, kH/kD) is strongly dependent on ƒHCO and ƒHCI . The oxygen-18 isotope effect decreases with increasing x. Experimental data of the D isotope effect — if they were available — could be used to adjust the bending force constants ƒHCO and ƒHCI (— or to adjust ƒHCO if ƒHCI is held constant). The degree of unsymmetry of the transition state could then be estimated with the aid of the C isotope effect. Experimental data of the O isotope effect would supply information on the direction of the unsymmetry — which is needed for a distinction between 2 possible values of x —. There is approximate agreement between the calculated and the experimental temperature dependence of the carbon-13 isotope effect.