Following up on his previous studies of isotope effects on the ring opening of cyclopropylcarbinyl radical 1 to give 2 (see my previous post), Borden now reports on its kinetic isotope effect (KIE).1

Using the small-curvature tunneling approximation along with structures and frequencies computed at B3LYP/6-31G(d), he finds a negligible KIE at C1, consistent with little motion of C1 in the transition vector. The KIE for substitution at C4 is large (k(12C/14C)=5.46), also consistent with its large motion in the transition vector. What is surprising is the KIE for deuterium substitution at C1: 0.37. This is a large inverse isotope effect!

Analysis of the vibrational frequencies that involve the C1 hydrogens provides an explanation. In going to the TS for the ring opening, both the torsional motion about the C1-C2 bond (making the double bond) and the pyramidal motion increase in frequency. This leads to a higher activation barrier for H than D, and the inverse isotope effect.


(1) Zhang, X.; Datta, A.; Hrovat, D. A.; Borden, W. T., “Calculations Predict a Large Inverse H/D Kinetic Isotope Effect on the Rate of Tunneling in the Ring Opening of Cyclopropylcarbinyl Radical,” J. Am. Chem. Soc., 2009, 131, 16002-16003, DOI: 10.1021/ja907406q.