Chemistry Reference
In-Depth Information
4-biphenylyl nitrenium ion, and 1.454 A
for the
N
-(4-biphenylyl)-
N
-methyl nitre-
nium ion
37
which is consistent with the degree of iminocyclohexadienyl character of
the first phenyl ring (attached to the nitrenium ion moiety) and the vibrational
frequencies of the
n
11
symmetric aromatic C
C stretch vibrational modes.
The 4-biphenylyl nitrenium ion and the
N
-(4-biphenylyl)-
N
-methyl nitrenium ion
only differ in the hydrogen atom attached to the nitrogen atom of nitrenium ion being
replaced by a methyl moiety, and this substitution appreciably influences the
structure and charge delocalization of the 4-biphenylyl nitrenium ion part of the
molecule so that the C
C7 bond connecting the two phenyl
rings are moderately shorter in the 4-biphenylyl nitrenium ion (1.3083 A
N bond and the C5
and
1.4494 A
,
respectively)
than in the
N
-(4-biphenylyl)-
N
-methyl nitrenium ion
(1.321 A
and 1.454 A
, respectively)
37
which correlates with the greater amount
of C
C bond alternation seen in both phenyl rings and the higher
n
11
and
n
12
symmetric aromatic C
C stretch experimental vibrational frequencies for
4-biphenylyl nitrenium ion (1625 cm
1
n
11
and 1607 cm
1
n
12
) compared
to the
N
-(4-biphenylyl)-
N
-methyl nitrenium ion (1612 cm
1
and 1584 cm
1
from
Ref. 37). These results show that the 4-biphenylyl nitrenium ion has greater
iminocyclohexadienyl character than the
N
-(4-biphenylyl)-
N
-methyl nitrenium
ion. Oxidation of neutral biphenyl and substituted biphenyl compounds to produce
their corresponding radical cations solution at room temperature usually leads to
formation of a more planar structure with some quinoidal-like character even though
the structure is still nonplanar and an example of this is when the S
0
state of biphenyl
proceeds from an inter-ring twist angle of about 38
, an inter-ring C
for
for
C bond length
of about 1.486 A
C bond alternation in the phenyl rings
to an inter-ring twist angle of about 19
, an inter-ring C
, and a modest degree of C
C bond length of about
1.444 A
1.433-
1.379-1.403 A
) in the S
0
radical cation of biphenyl.
52-58
The quinoidal character and
charge delocalization is similar in both phenyl rings of the biphenyl radical cation
and also halo- or methyl-substituted radical cations
52-61
while the nitrenium ion
appreciably interacts with the biphenyl moiety via the phenyl ring it is attached to
and the charge delocalization is significantly different for the two phenyl rings in the
aryl nitrenium ions where the C
, and an appreciable C
C bond alternation in both phenyl rings (
C bond alternation is greater in the phenyl ring to
which the nitrogen atom is attached which also affects the amount of imine character
of the C
N bond. The results for the 4-biphenylyl nitrenium ion described here and
comparison to previous results presented for the
N
-(4-biphenylyl)-
N
-methyl nitre-
nium ion show that the
NH
þ
moiety results in more interaction and a greater
amount of iminocyclohexadienyl character than the
NCH
3
þ
moiety. The greater
stability and improved charge delocalization of aryl nitrenium ions having
para
-
phenyl ring substitution on a phenyl nitrenium ion (like the 2-flourenyl nitrenium,
4-biphenylyl nitrenium, and
N
-(4-biphenylyl)-
N
-methyl nitrenium ions) is likely due
to the ability of the biphenyl-like part of the molecule to accept and delocalize the
positive charge into both phenyl rings.
Ortho
substitution relative to the inter-ring C
C bond in halo- and methyl-
substituted biphenyl radical cations leads to a less planarity and a lower amount of
quinoidal character
59-61
, and this suggests that a similar substitution in 4-biphenylyl
