Heavy-atom tunneling in organic transformations
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Ó Indian Academy of Sciences Sadhana (0123456789().,-volV)FT3](0123456 789().,-volV)
REVIEW ARTICLE
Heavy-atom tunneling in organic transformations SHARMISTHA KARMAKAR and AYAN DATTA* School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032, India E-mail: [email protected] MS received 11 February 2020; accepted 14 May 2020
Abstract. The pronounced effect of quantum mechanical tunneling in chemical reactions involving light atoms like hydrogen is well established. Recent studies have found that tunneling can also play a significant role for common organic transformations, where if the participating atom is carbon/nitrogen, etc. For these cases, various reaction parameters like reaction barrier, barrier width and temperature play a crucial role in determining the efficiency of tunneling. In this review, we have focused on all those organic transformations where the influential role of tunneling has been documented either computationally or experimentally. Keywords. Organic transformations; heavy-atom tunneling; reaction kinetics; kinetic isotope effect.
1. Introduction Quantum mechanical tunneling (QMT) is a fundamental physical phenomenon that is being realized as a key factor in many chemical and biological transformations. Over the last 80 years, the tunneling effects have been realized predominantly for H-atom mediated reactions like H–transfer, C–H insertion. Anomalously, large kinetic isotope effect (KIE) serve as an experimental signature for tunneling.1–6 Since the tunneling probability depends strongly on the mass of moving particle, the effects of tunneling are subtle for reactions involving heavy atoms. In the early 1900’s, it was believed that particle heavier than helium do not tunnel, as Bell mentioned in his 1933 paper that ‘‘It must, however, be emphasized that all atoms heavier than helium behave, practically speaking, classically.’’7 The major experimental evidence for heavy-atom tunneling was first reported by Buchwalter and Closs in 1975. They observed that the decay rates of triplet cyclopentane-1,3-diyl (1) to bicycle [2.1.0] are temperature-independent from 1.3 K to 20 K in matrix environment.8 The occurrence of the ring closure reaction at cryogenic temperature is a typical signature of heavy-atom tunneling through a low barrier. In 1982, another experimental finding indicated the involvement of carbon tunneling in automerization of cyclobutadiene at cryogenic temperatures.9 In the very *For correspondence
next year, Carpenter interpreted the experimental results published in 1982 paper, taking into account the tunneling effects by theoretical modeling, and he also mentioned that under experimental reaction condition, at least 97% of the reaction occurs by tunneling below 0°C, which results in negative entropy factor.10 This is one of the classic examples of heavy-atom tunneling. Slowly, the idea of ‘‘heavy-atom tunneling can affect the reaction kinetics’’ gained momentum and more theoretical supports for h
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