Chemistry Reference
In-Depth Information
+
H
Et
2
N
O
2
N
N
Excited nitrenium ion
Et
2
N
O
2
N
SOC
forbidden
About 87 kcal/mol
About 7 kcal/mol
Et
2
N
O
2
N
H
+
N
Et
2
N
O
2
N
Ground-state nitrenium ion
SCHEME 2.7.
Singlet surface connections between open-shell and closed-shell nitrene and
open-shell and closed-shell nitrenium ion.
74
azide (Scheme 2.7)—widely used as a photoaffinity label—found that direct
irradiation of the azide generates the singlet nitrene, which further undergoes an
extremely fast solvent protonation to yield an arylnitrenium ion. In methanol, this
nitrene protonation occurs in about 5 ps, the fastest intermolecular protonation
observed to date. But from which electronic configuration of the singlet nitrene
does this protonation occur? CASSCF computations indicate that the open-shell
singlet state is the lower-energy singlet electronic state of this nitrene. The computed
adiabatic closed-shell nitrene surface is
7 kcal/mol higher in energy than the open-
shell surface. Assuming that the protonation event does not occur simultaneously
with a change in electron orbital occupation, this presents a conundrum: if the
protonation event occurs from the lower-energy open-shell singlet state, this reaction
might be expected to yield a very high-energy open-shell singlet excited state of the
nitrenium ion (87 kcal/mol higher in energy than the closed-shell singlet nitrenium
ion). Such a pathway is expected to be energetically unfavorable.
An alternative mechanism is that protonation occurs from the higher-energy
closed-shell singlet state yielding the singlet nitrenium ion in its lowest energy
closed-shell ground state. This alternative mechanism rests on two assumptions: (i)
expulsion of molecular nitrogen from the azide precursor initially populates the
higher-energy closed-shell singlet surface of the nitrene; and (ii) protonation of this
initially formed nitrene excited state occurs faster than electronic relaxation to the
open-shell singlet surface.
