Chemistry Reference
In-Depth Information
(U)BPW91/cc-PVDZ calculations were done for the singlet and triplet states of
the 4-methoxyphenyl nitrenium and 4-ethoxyphenyl nitrenium ions to estimate their
total energy, optimized geometry, and vibrational frequencies. The calculated (see
Tables 1 and 2 in Ref. 41) and the experimental transient resonance Raman
vibrational frequencies are shown in Figure 7.12 The DFT calculations (see Figure
4 in Ref. 41) indicate that the singlet states of the 4-methoxyphenyl nitrenium and
4-ethoxyphenyl nitrenium ions have noticeably more imine character (e.g., stronger
C
N bonds) accompanied by more cyclohexadienyl (e.g., greater carbon
carbon
bond length alternation) and oxo (e.g., stronger C
O bonds) character compared to
their respective triplet states and these differences in structure result in appreciable
differences in the vibrational frequencies of the singlet and triplet states that allow
them to be easily distinguished from one another by inspection of their vibrational
spectroscopy.
There are cases where the ground state of the aryl nitrenium ion may be a triplet
state when a large aryl ligand or
-acceptor substituent could appreciably destabilize
the singlet state relative to the triplet state, but this is unusual and there are only a few
examples that predict or see triplet ground-state aryl nitrenium ions.
26,65
The singlet-
triplet energy differences change as a function of substituents
para
to the nitrenium
ion moiety and as the
p
-donating character of the substituent increases the singlet
state becomes increasingly stabilized over the triplet state
37,66
that can result in
larger differences in the structures of the singlet and triplet states for stronger
p
p
-donating
para
-substituent moieties such as phenyl and alkoxy groups. Single-
point energy calculations for the 4-methoxy nitrenium and 4-ethoxy nitrenium ions
were done at the (U)BPW91/cc-PVDZ optimized geometry and determined singlet-
triplet gaps of 28.0 and 28.1 kcal/mol, respectively at the (U)BPW91/cc-PVDZ level
of theory and 35.1 and 35.5 kcal/mol, respectively at the (U)MP2/cc-PVDZ level of
theory with the singlet state being more stable in each calculation and this indicates
the singlet states of the 4-methoxy nitrenium and 4-ethoxy nitrenium ions are
significantly more stable than their corresponding triplet states. These DFT results
and the very quick generation of the nitrenium ions by protonation of the initially
formed singlet aryl nitrene precursor
64
indicates that the transient resonance Raman
spectra presented in Figure 7.12 are due to the singlet state 4-methoxy nitrenium and
4-ethoxy nitrenium ions similar to the attribution of the transient absorption and
resonance Raman spectra of
the 2-fluorenyl nitrenium ion to its singlet
state.
19,22,39-41
The transient resonance Raman vibrational frequencies display much better
agreement with the calculated frequencies for the singlet 4-methoxyphenyl nitre-
nium ion (the calculated vibrational frequencies are different from the experimental
Raman frequencies by about 13.8 cm
1
on average, see Table 1 in Ref. 41) than for
the corresponding triplet state (differences of about 24.3 cm
1
on average for the
triplet state). Similar level of agreement was observed between the experimental and
calculated Raman frequencies for the singlet state of 4-ethoxyphenyl nitrenium ion
(differences of about 11 cm
1
on average, Table 2 in Ref. 41) whereas the difference
between the observed frequencies and the calculated triplet state frequencies is larger
(differences of about 16.5 cm
1
on average). Comparison of
the calculated
