Chemistry Reference
In-Depth Information
*
*
1000
1500
2000
Raman shift (cm
-1
)
FIGURE 7.13.
The transient resonance Raman spectrum of the 4-acetamidophenyl nitrenium
ion acquired with 320 nm probe and 266 nm pump excitation wavelengths is presented. The
assignments of the larger Raman bands are labeled (see text and Table 1 of Ref. 42 for more
details). The asterisks mark solvent and/or parent-band subtraction artifacts.
Source
: Reprinted
with permission from Ref. 42. Copyright (2003) American Chemical Society.
relative to the triplet state.
37,66
Our calculations determined singlet-triplet energy
gaps of 27.6 kcal/mol for the 4-acetamidophenyl nitrenium ion and 27.8 kcal/mol for
the 4-(
N
-methylacetamido)phenyl nitrenium ion, with the singlet species being more
stable, and this indicates the singlet states of these acetamidophenyl nitrenium ions
have much greater stability than their triplet states. This and the fast formation of the
nitrenium ions by protonation of the initially formed singlet aryl nitrene precursor
70
indicates the transient resonance Raman spectra displayed in Figure 7.13 can be
assigned to the singlet state of the 4-acetamidophenyl nitrenium ion which is similar
to assignments made or other
para
-phenyl-substituted phenyl nitrenium ions such as
the 2-fluorenyl nitrenium and 4-biphenylyl nitrenium ions.
39,40
The transient Raman
vibrational band frequencies display significantly better agreement with the fre-
quencies computed for the singlet 4-acetamidophenyl nitrenium ion than for the
corresponding triplet state (see Table 1 in Ref. 42) with the singlet computed
vibrational frequencies being different from the experimental Raman frequencies by
about 13 cm
1
on the average, compared to differences of about 19 cm
1
on average
for the triplet.
The DFT-calculated Raman intensity for the C
O stretch mode in the 1760-
1801 cm
1
region compared to intensities of the modes in the 1400-1650 cm
1
region for the 4-acetamidophenyl nitrenium ion appears to be a good indicator to
difference between the singlet and triplet ion. The calculations predict that the
1763 cm
1
C
O stretch mode is the largest Raman band and it is significantly
greater than the Raman bands in the 1470-1620 cm
1
region of the triplet state
spectrum but the experimental transient Raman spectrum has no clear band in the
C
O stretch region and the greatest bands are in the 1450-1650 cm
1
region which
indicates the experimental transient Raman spectrum is not due to the triplet state of
