basis sets in the theoretical method. These e†ects are corrected
by applying a single empirical scaling factor determined from
the mean deviation between experimental and calculated fre-
quencies. Only modes of the anion radical for which the corre-
lation between experimental and calculated frequencies is
unambiguous are involved in the determination. As for the
neutral molecule,14 only the four modes m , m , m and m
obey this criterion (see discussion below). Thus, considering
the 5 isotopic derivatives investigated, 20 frequency values are
used to establish the scaling factor. For the trans anion
radical, a mean deviation of [3.7 ^ 1.1% is determined,
leading to a scaling factor of D0.964. For the cis 22BPY~~, a
mean deviation of [4.1 ^ 2.0% is calculated, yielding a
scaling factor of D0.961. By analogy, a mean deviation of
very well calculated, within 8 ^ 8 cm~1. The excellent agree-
ment between calculation and experiment, not only for the
frequencies, but also for the isotopic shifts and the frequency
shifts on going from 22BPY to 22BPY~~ indicates that the
modiÐcations of the force constants and the redistribution of
the potential energy among the normal coordinates upon
reduction are correctly computed. This strongly supports the
optimized geometry determined for the anion radical by the
B3-LYP/6-31G(]*) method.
2
11 12
13
The vibrational characteristics of the trans conformer of the
parent neutral molecule have been studied in detail else-
where.14 The 15 a modes of the h derivative are numbered
g
8
in the order of decreasing frequencies (Table 5). As previously
discussed,14 the low frequency modes, m Èm , and the high
11 15
frequency vibrations, m and m , are typical motions of the
[
3.9 ^ 0.7% and a scaling factor of D0.963 were obtained
1
2
for the parent neutral molecule from the same set of 20 fre-
biphenyl-like ring skeleton, as in biphenyl,21 4-
phenylpyridine22 and 4,4@-bipyridine,23 with similar fre-
quencies and PEDs. For these modes, the labeling according
to the convention used by Varsanyi24 for monosubstituted
benzenes, adapted from the Wilson notation25 for benzene, is
indicated jointly with the numbering. Most of the other vibra-
tions of 22BPY involve important contributions of the CH i.p.
bendings. In contrast to biphenyl and 4,4@-bipyridine, the CH
vibrators in 22BPY are not disposed symmetrically with
respect to the axis passing through the inter-ring bond, in
such a way that the vibrational modes implicating these vibra-
tors have quite di†erent PEDs in 22BPY than in biphenyl and
4,4@-bipyridine. Moreover, for these modes, no correlation can
be established with the typical benzene vibrations and so
WilsonÏs notation cannot be used.
From the analysis of both the PEDs and the Cartesian dis-
placements, 2 types of anion vibrations may be distinguished.
The Ðrst group contains the vibrational modes which may be
correlated to the corresponding ones of the neutral molecule,
and so may be numbered in the same way in both cases
(modes m and m Èm ). Among these modes, two groups of
quencies.14 The experimental and scaled calculated fre-
quencies obtained for the 15 a modes of the trans anion
g
radical for the h , 15N , d , d and d isotopic derivatives are
8
2
2
4
8
listed in Table 5. The corresponding experimental and calcu-
lated frequencies of the neutral molecule are given for com-
parison. The vibrational correlation between modes of the
neutral and reduced species, established on the basis of the
calculated PEDs and of the Cartesian displacements, is dis-
cussed below. The PEDs of the vibrational modes, expressed
in terms of local symmetry coordinates, are also indicated for
2
2BPY~~ -h (only contributions greater than 10% are given).
8
More detailed tables are provided as Electronic Supplemen-
tary Information.¤ (The observed and scaled frequencies (with
PEDs) of the 15 a modes of trans 22BPY~~ -h , d , d and
g
8
2
4
d
are given in Tables 1S, 2S, 3S, and 4S, respectively. The
8
calculated and observed isotopic shifts with respect to the h
derivative are indicated in Tables 1SÈ4S. The experimental
8
and calculated isotopic shifts arising on going to the 15N iso-
2
topomer are also reported in Table 1S. For modes correlated
between the neutral molecule and the anion radical (see dis-
cussion below), the theoretical and experimental frequency
shifts on going from 22BPY to 22BPY~~ for each isotopomer
are given in Tables 1SÈ4S. The experimental frequencies are
also compared to the scaled calculated frequencies determined
2
5
15
vibrational modes may be di†erentiated with regard to the
reduction e†ect on frequencies.
Modes m , m , m , m Èm present nearly the same PEDs
2
8
9
11 15
and isotopic shifts in the neutral molecule and in 22BPY~~ for
each isotopomer. Their frequencies are only slightly shifted on
going from 22BPY to the anion radical. This may be
accounted for by the weak variations upon photoreduction of
the force constants related to the coordinates involved in the
PEDs of these modes (principally inter-ring and CH i.p. bend-
ings and ring i.p. deformations). Modes m and m Èm are
for the 15 a modes of the cis anion radical in Table 5S for the
1
h and d derivatives.)
8
8
For all isotopomers of the trans anion radical, there is an
almost one to one correspondence between the observed reso-
nance Raman lines and the theoretical a vibrations. Only a
g
few modes are not detected, among which the low frequency
2
11 15
motions m1
and m . All the observed lines are ascribable to a
weakly sensitive to deuteriation. Apart from m , which is a
4
15
14
calculated mode and no band splitting is found in the Raman
spectra. For the set of all totally symmetric modes of the Ðve
isotopomers, the scaled frequencies are in very good agree-
ment with the experimental values, the mean deviation being
pure inter-ring i.p. bending, they are essentially skeleton dis-
tortions of the pyridyl rings. Modes m Èm and m are prin-
1
1
13
15
cipally ring i.p. deformations and correspond typically to the
WilsonÏs modes 12, 1, 6b and 6a, respectively. Mode m is pre-
2
0
.2 ^ 1.1%, and the mean absolute average being 9 ^ 6 cm~1.
dominantly a CC bond stretching and derives from the
On the other hand, a correlation of the observed resonance
Raman spectra with the theoretical spectra calculated for the
cis 22BPY~~ anion radical is not satisfactory (Table 5S). For
WilsonÏs mode 8a. Modes m , m , m and m are observed
2
11 12
13
experimentally and assigned unambiguously on the basis of
their well-deÐned spectral region and moderate isotopic shifts,
in perfect agreement with the calculation. The soundness of
the affiliation of the experimental Raman lines in the di†erent
isotopomers is warranted by the very speciÐc Raman inten-
sities observed for these modes in resonance with the UV and
the d derivative, one of the two observed lines at 1343 and
8
1
266 cm~1 is not ascribable to a calculated mode. Moreover,
between 1150 and 1200 cm~1, only one band is detected in the
experimental spectrum at 1175 cm~1, while two frequencies
are computed at 1165 and 1191 cm~1. Further, the scaled cal-
culated frequencies are in poor agreement with the observed
values, with a mean deviation of 0.5 ^ 1.9%, and a mean
absolute average of 17 ^ 13 cm~1. These results conÐrm that
the trans planar 22BPY~~ species is the only stable conforma-
tion in solution.
visible electronic transitions of the anion (Fig. 3). Modes m
and m give rise to the most enhanced Raman peaks on probe
2
1
2
excitation at 390 nm but to very weak lines on 567 nm excita-
tion. Mode m shows a constant medium intensity with both
excitations. Its assignment is conÐrmed furthermore by a spe-
1
1
ciÐc sensitivity of its frequency to the 15N isotopic substitut-
2
ion (*l , [10 cm~1) nicely predicted by the B3-LYP
exp
For the trans conformer of 22BPY~~, very good agreement
is also obtained between the calculated and experimental iso-
calculation (*l , [11 cm~1) (Table 1S)¤. Finally, mode m
cal
13
topic shifts relative to the h derivative, with the average mean
is observed exclusively upon probing at 390 nm.
8
deviation of 5 ^ 8 cm~1. In the same way, the frequency shifts
In contrast, the PEDS of modes m and m are largely modi-
8
9
on going from the neutral molecule to the anion radical are
Ðed upon deuteriation. Mode m mainly involves C H , C H
8
3 3 4 4
4686
Phys. Chem. Chem. Phys., 2000, 2, 4682È4689
Castella-Ventura
