Switching on H-Tunneling through Conformational Control

Abstract

H-tunneling is a ubiquitous phenomenon, relevant to fields from biochemistry to materials science, but harnessing it for mastering the manipulation of chemical structures still remains nearly illusory. Here, we demonstrate how to switch on H-tunneling by conformational control using external radiation. This is outlined with a triplet 2-hydroxyphenylnitrene generated in an N 2 matrix at 10 K by UV-irradiation of an azide precursor. The anti-orientation of the nitrene's OH moiety was converted to syn by selective vibrational excitation at the 2ν(OH) frequency, thereby moving the H atom closer to the vicinal nitrene center. This triggers spontaneous H-tunneling to a singlet 6-imino-2,4-cyclohexadienone. Computations reveal that such fast H-tunneling occurs through crossing the triplet-to-singlet potential energy surfaces. Our experimental realization provides an exciting novel strategy to attain control over tunneling, opening new avenues for directing chemical transformations.

Heavy-Atom Quantum Tunnelling in Spin Crossovers of Nitrenes

“…Our results highlight the shortcomings of the commonly used 41,42,[50][51][52][53]121 WC approximation to describe quantum tunnelling at low temperatures. Although the WC method can indeed give an indication of when tunnelling becomes important, it is not at all reliable for quantitative predictions away from the classical limit.…”

“…Three recent studies have presented convincing evidence for nitrene reactions in which the spin crossover is the rate-determining step. 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.…”

“…48 This constitutes the current standard approach for the calculation of spin-crossover rates. 41,42,[49][50][51][52][53] As the name of the WC method suggests, it is based on the assumption of weak spinorbit coupling between the two states, as for Fermi's golden rule. We will demonstrate that the golden-rule assumption itself is valid for the nitrene reactions under investigation.…”

IR-induced and tunneling reactions in cryogenic matrices: the (incomplete) story of a successful endeavor

“…19). The reaction was followed by IR spectroscopy and, in order to exclude the absorption of photons above the OH-rotamerization barrier of the nitrene (B1700 cm À1 ; B3LYP/ 6-311+G(2d,p) calculated data 87 ), a long-pass filter was used to block IR light above 1580 cm À1 (Fig. 20).…”

Heavy‐Atom Quantum Tunnelling in Spin Crossovers of Nitrenes**

“…[39] This constitutes the current standard approach for the calculation of spin-crossover rates. [33,34,[40][41][42][43][44] As the name of the WC method suggests, it is based on the assumption of weak spin-orbit coupling between the two states, as for Fermi's golden rule. We will demonstrate that the golden-rule assumption itself is valid for the nitrene reactions under investigation.…”