Chemistry Reference
In-Depth Information
SCHEME 2.3.
General trends that have been found to favor/disfavor four common branching
pathways of the singlet arylnitrene.
2.3.1 Structural and Electronic Influences on the Rate
of Nitrene Intersystem Crossing
Most nitrenes have triplet ground states; consequently, intersystem crossing (ISC) is
usually considered as a possible decay channel for the singlet nitrene. Since the
decay of the singlet nitrene is a competition between the rates of various decay
channels, it is important to consider the factors that affect the rate of nitrene
intersystem crossing.
First, whether the singlet nitrene adopts a closed-shell or open-shell configuration
has a strong influence on the rate of intersystem crossing. Closed-shell nitrenes tend
to have faster rates of ISC than open-shell nitrenes, sometimes by orders of
magnitude. While intersystem crossing is a “spin forbidden” process, this decay
channel can become allowed via higher-order coupling mechanisms such as vibronic
and spin-orbit coupling.
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Vibronic coupling in nitrenes is anticipated to be small
because nitrenes are monovalent; as a consequence, there are few bending modes
around the nitrene center that considerably alter the orbital structure to permit state
mixing. The difference in the rates of ISC for closed-shell and open-shell nitrenes,
therefore, is thought to arise because closed-shell nitrenes can employ spin-orbit
coupling more effectively than open-shell nitrenes. For closed-shell nitrenes, the
change in spin angular momentum on going from the singlet state to the triplet is
compensated by a change in orbital angular momentum resulting from the frontier
electron hopping between orthogonal
p
orbitals. No such angular momentum
compensation occurs for open-shell nitrenes where orbital occupation remains
