Chemistry Reference
In-Depth Information
vibrational frequencies and relative Raman intensities for the singlet and triplet states
of the 4-methoxyphenyl nitrenium ion to the experimental resonance Raman
spectrum vibrational frequencies and relative intensities shows a good correlation
with those calculated for the singlet 4-methoxyphenyl nitrenium ion but not for the
triplet state (see Figure 5 in Ref. 41). This and the large calculated singlet-triplet
energy difference with the singlet state being more stable, prompts us to attribute the
transient Raman spectra shown in Figure 7.12 to the singlet states of 4-methox-
yphenyl nitrenium and 4-ethoxyphenyl nitrenium ions, respectively.
Furthermore, the vibrational bands in the 1450-1650 cm
1
region have great
utility in telling the singlet and triplet states of these aryl nitrenium ions apart from
one another (see Tables 1 and 2 and Figure 5 of Ref. 41). For instance, in the 1450-
1650 cm
1
region, the singlet state of the 4-methoxyphenyl nitrenium ion has
computed vibrational bands at 1485, 1549, 1571, and 1639 cm
1
while the triplet
state has computed bands at 1467, 1478, 1605 cm
1
. For these three modes, the
transient Raman frequencies seen at 1510, 1564, and 1633 cm
1
in the 1450-1650 cm
1
region in Figure 7.12 display significantly better agreement with those calculated for the
singlet state (differences on average of 12.7 cm
1
) compared to the triplet state
(differences of about 53 cm
1
on average) and this comparison indicates that the
vibrational modes in the 1450-1650 cm
1
region are characteristic of the amount
of imine, cyclohexadienyl, and oxo character displayed by these alkoxyphenyl
nitrenium ions.
DFT calculations at the random-phase approximation
67
(RPA) were done to
predict the electronic transition energies and oscillator strengths for the singlet
state 4-methoxyphenyl nitrenium and 4-ethoxyphenyl nitrenium ions (see Table 3 in
Ref. 41 for these results) and found that the singlet state 4-methoxyphenyl nitrenium
and 4-ethoxyphenyl nitrenium ions have only one large oscillator strength electronic
transition at
292.6 nm, respectively, in the 200-1000 nm region that is
consistent with experimental results that determined these ions have strong transient
absorption bands with maxima around 290-305 nm on the ns to ms timescale.
63,64
These results give further evidence supporting our attribution of the transient
resonance Raman spectra displayed in Figure 7.12 to the singlet states of the
4-methoxyphenyl nitrenium and 4-ethoxyphenyl nitrenium ions.
Comparison of the structural properties of the singlet 4-methoxyphenyl nitrenium
and 4-ethoxyphenyl nitrenium ions to those of previously studied aryl nitrenium ions at
the same BPW91/cc-PVDZ level of calculated structures
37,39,40
shows that the imine
character of the 4-methoxyphenyl nitrenium and 4-ethoxyphenyl nitrenium ions are
very strong (C
303 and
Nbonds of about 1.3042 and 1.3040A
, respectively) and similar to that
N bond of about 1.3077 A
of the 2-fluorenyl nitrenium ion (with a C
)
39,40
but
moderately stronger than in the 4-biphenylyl nitrenium ion (1.3083A
),
40
diphenyl
nitrenium ion (1.3512A
),
37,40
N
-(4-biphenylyl)-
N
-methyl nitrenium ion (1.321A
),
37
N
-(4-methylphenyl)-
N
-methyl nitrenium ion (1.323A
),
37
N
-(4-chlorophenyl)-
N
-methyl
nitrenium ion (1.323A
),
37
and
N
-(4-methoxyphenyl)-
N
-methyl nitrenium ion
(1.317 A
).
37
In addition, the amount of C
C bond alternation (e.g., cyclohexadienyl
character) in thephenyl ring towhich thenitreniumgroup is attached isalsoverystrong in
4-alkoxyphenyl nitrenium ions and are similar to that observed in the 2-fluorenyl
