Heavy‐Atom Tunneling Through Crossing Potential Energy Surfaces: Cyclization of a Triplet 2‐Formylarylnitrene to a Singlet 2,1‐Benzisoxazole

Abstract

Not long ago, the occurrence of quantum mechanical tunneling (QMT) chemistry involving atoms heavier than hydrogen was considered unreasonable. Contributing to the shift of this paradigm, we present here the discovery of a new and distinct heavy‐atom QMT reaction. Triplet syn‐2‐formyl‐3‐fluorophenylnitrene, generated in argon matrices by UV‐irradiation of an azide precursor, was found to spontaneously cyclize to singlet 4‐fluoro‐2,1‐benzisoxazole. Monitoring the transformation by IR spectroscopy, temperature‐independent rate constants (k≈1.4×10−3 s−1; half‐life of ≈8 min) were measured from 10 to 20 K. Computational estimated rate constants are in fair agreement with experimental values, providing evidence for a mechanism involving heavy‐atom QMT through crossing triplet to singlet potential energy surfaces. Moreover, the heavy‐atom QMT takes place with considerable displacement of the oxygen atom, which establishes a new limit for the heavier atom involved in a QMT reaction in cryogenic matrices.

Spin crossover of thiophosgene via multidimensional heavy-atom quantum tunneling

“…[26][27][28] This has recently been confirmed by a set of studies of carbenes and nitrenes in matrix isolation at cryogenic temperatures. [29][30][31][32][33] Yet as we shall demonstrate, strong tunneling effects are not limited to such extreme environments but can also play a crucial role at room temperature in these nonadiabatic reactions and even allow for heavy-atom tunneling. Importantly, we find that the "corner cutting" effects first described in the context of adiabatic reactions 34 are much more important for nonadiabatic reactions and can drastically change the reaction mechanism.…”

“…This explains the widely varying quality of results obtained with NA-TST/WC that can be found in the literature. 31,45,53,57 Instanton theory avoids these issues by locating the optimal reaction mechanism at each temperature/energy of interest in a fully ab initio manner. This has allowed us to unravel the substantial role of multidimensional quantum nuclear tunneling in this reaction, which proceeds by a corner-cutting pathway.…”

“…It has been used to study spin-crossover reactions in a number of molecules. 31,45,50,[52][53][54][55][56][57] It is common to employ a one-dimensional tunneling correction, 50 known as the weak-coupling (WC) approximation, 58,59 which is based on the assumption that the diabatic states are linear functions of the predetermined reaction coordinate. This calls the qualitative picture of the reaction mechanism into question as it ignores the true shape of the PESs as well as the intrinsic multidimensional character of nuclear tunneling.…”

Heavy-Atom Quantum Tunnelling in Spin Crossovers of Nitrenes

“…They additionally found that this process is accompanied by tunnelling of hydrogen 41 or heavier atoms. 42,43 In order to simulate these nonadiabatic reactions, methods are required which go beyond the Born-Oppenheimer approximation.…”

Lewis acid catalyzed heavy atom tunneling – the case of 1H-bicyclo[3.1.0]-hexa-3,5-dien-2-one

“…The ring opening of 6 is particularly interesting since the resulting carbene 7 has a triplet ground state, and hence intersystem crossing (ISC) has to occur at some point along the reaction coordinate. [14][15][16] The lowest lying singlet state of carbene 7 is the open-shell (os) singlet state osS-7, which is predicted to be 8.4 kcal mol -1 above the triplet ground state T-7. The (planar) closed-shell singlet state csS-7 is with 26.1 kcal mol -1 at the CASPT2 level of theory much higher in energy.…”