Computational replication of the abnormal secondary kinetic isotope effects in a hydride transfer reaction in solution with a motion assisted H-tunneling model.
We recently reported abnormal secondary deuterium kinetic isotope effects (2° KIEs) for hydride transfer reactions from alcohols to carbocations in acetonitrile (Chem. Comm. 2012, 48, 11337). Experimental 2° KIE values were found to be inflated on the 9-C position in the xanthylium cation but deflated on the β-C position in 2-propanol with respect to the values predicted by the semi-classical transition-state theory. No primary (1°) isotope effect on 2° KIEs was observed. Herein, the KIEs were replicated by the Marcus-like H-tunneling model that requires a longer donor-acceptor distance (DAD) in a lighter isotope transfer process. The 2° KIEs for a range of potential tunneling-ready-states (TRSs) of different DADs were calculated and fitted to the experiments to find the TRS structure. The observed no effect of 1° isotope on 2° KIEs is explained in terms of the less sterically hindered TRS structure so that the change in DAD due to the change in 1° isotope does not significantly affect the reorganization of the 2° isotope and hence the 2° KIE. The effect of 1° isotope on 2° KIEs may be expected to be more pronounced and thus observable in reactions occurring in restrictive environments such as the crowded and relatively rigid active site of enzymes.
Department of Chemistry, Southern Illinois University Edwardsville , Edwardsville, Illinois 62026, United States.
Sadra
S
Kashefolgheta
Mortezaali
M
Razzaghi
Blake
B
Hammann
James
J
Eilers
Daniel
D
Roston
Yun
Y
Lu
D000097:Acetonitriles / Q000737:chemistry; D000438:Alcohols / Q000737:chemistry*; D020134:Catalytic Domain; D039301:Computers, Molecular; D003903:Deuterium / Q000737:chemistry*; D007554:Isotopes / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D012996:Solutions
D000097:Acetonitriles; D000438:Alcohols; D007554:Isotopes; D012996:Solutions; D003903:Deuterium; C032159:acetonitrile