T. Kumakura et al.
ChemicalPhysicsLetters714(2019)160–165
Table 1
Observed and calculated wavenumbers (in cm−1) of 1H-fluorene in the region
between 600 and 3100 cm−1
.
Obsd.
−1
−1
Abs.
IR Int./km
mol−1
ν /cm
ν /cm
630
652
707
747
752/754
815
818
866
931
1008
1012
1020
1133
1153/1155 21.3
1189
1202
37.6
60.2
12.7
36.2
90.5
23.1
35.3
8.1
13.1
21.3
21.7
26.7
7.7
624
660
711
748
754
812
823
870
917
11.8
36.0
8.2
37.8
30.8
7.8
18.3
5.6
7.0
A’
β (ring)
A”
A”
A”
A”
A’
A”
A”
A’
γ(C2,3,4-H), CH2 rock.
γ(C2,3,4-H)
γ(C5,6,7,8-H)
γ(C5,6,7,8-H)
ν(C4a-C9a)
γ(C9-H)
γ(C9-H),γ(C5,6,7,8-H)
ν (C1-C2)
Combination?
ν (C3-C4)
ν (C3-C4)
1010
1022
1134
1154
1191
1203
1280
1354
8.8
4.7
1.5
6.6
11.6
10.3
1.9
A’
A’
A’
A’
A’
A’
A’
A’
β(C9-H)
β(H-C6-C7-H)
ν(C4b-C8a-C9)
ν(C8a-C9), β(C5-H)
ν(C2-C1-C9a-C4a)
ν(C4a-C9a), ν(C4b-C8a)
Combination?
ν(C4a-C9a),ν(C8a-
C9),ν(C6-C7)
16.7
23.1
1276/1278 14.0
1343
1352
1364
67.0
20.4
14.0
19.1
Fig. 3. Numbering of carbon atoms and optimized structure of 1H-fluorene.
Blue, red and black-colored lines have single, double bond and aromatic
characters, respectively. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
1372
5.8
A’
1398/1403 21.7
1418 11.8
1453/1457 100.0 1453
1402
1413
2.6
A’
A’
A’
A’
A’
A’
A’
β(H-C2-C3-H)
14.2
72.4
4.0
25.4
9.9
CH2 scissor.
ν(aromatic ring)
β(H-C6-C7-H)
ν(C9-C9a),ν(C6-C7)
ν(C2-C3-C4-C4a)
ν(C4b-C8a),ν(C6-
C7),ν(C9-C9a)
ν(aromatic ring)
1466
26.2
1468
1572
1584
1599
1H-fluorene absorbs longer-wavelength light than 9H-fluorene. This
expectation is consistent with the experimental results that the back-
ward isomerization from 1H-fluorene to 9H-fluorene occurs by
λ ≥ 320 nm irradiation (Fig. 2(b)) and the forward isomerization from
9H-fluorene to 1H-fluorene occurs by λ ≥ 295 nm irradiation
1562/1564 19.5
1568
1587
76.5
19.5
16.0
1609
2843
2852
3007
35.3
2.3
9.0
1616
2877
2889
3045
3056
3062
3070
3075
3084
14.7
20.3
6.1
A’
A’
A”
A’
A’
A’
A’
A’
A’
νsym(CH2)
νasym(CH2)
9H-fluorene is more stable than 1H-fluorene by ca. 117 kJ mol−1
,
2.5
5.5
ν(C-H)
and the barrier height from 9H-fluorene to 1H-fluorene is estimated to
be ca. 271 kJ mol−1 in the electronic ground state at the B3LYP/6-31+
+G(d,p) level with zero-point energy correction. The barrier height
inhibits thermal isomerization between the two isomers in low-tem-
perature Ar matrices. Another possible isomer is 3H-fluorene, the re-
lative energy of which is estimated to be ca. 104 kJ mol−1 against 9H-
fluorene, meaning that 3H-fluorene is more stable than 1H-fluorene.
However, no IR bands assigned to 3H-fluorene were detected in the
present experiment. We assume that the hydrogen-atom migration to
produce 3H-fluorene is difficult, because hydrogen-atom migration
easily occurs between next nearest carbon atoms from C1 to C9 by
photo-induced dissociation-association (PIDA) mechanism as described
below. Relative energies of other possible isomers, 2H– and 4H-
fluorene, are estimated to be 174 and 188 kJ mol−1, respectively, which
are much higher than that of 9H-fluorene (see Table S5 in Supporting
information).
The reversible isomerization between 9H-fluorene and1H-fluorene
is interesting because intramolecular hydrogen-atom migration occurs
between two carbon atoms. This is exceptionally rare case, to our
knowledge, beside the 2,4-shift at cyclohexadiene-1-thione produced
from thiophenol [31], to our knowledge, although photoinduced re-
versible intramolecular hydrogen-atom migration between a carbon
atom and a hetero atom such as O or N or between hetero atoms is well-
known and investigated by matrix-isolation spectroscopy with quantum
chemical calculations; for example, hydroxypyridines (2(1H)-pyr-
18.9
29.8
35.9
32.0
13.4
ν(C2-H),ν(C4-H)
ν(C5,6,7,8-H)
ν(C2,3,4-H)
ν(C5,6,7,8-H)
ν(C9-H)
3048
3054
3057
38.9
24.9
19.0
a
Obtained at the B3LYP/6-31++G(d,p) level. The calculated wavenumbers
are adjusted by scaling factors of 0.98 below 2000 cm−1 and 0.96 above
2000 cm−1
. Calculated wavenumbers with IR intensity less than 5.5 are
omitted. All calculation results are provided in supporting information.
b
Dominantly vibrational modes are shown in. Symbols of γ, β and ν re-
present out-of-plane bending, in-plane bending and stretching modes, respec-
tively. Numbering of atom is defined in Fig. 3.
884 cm−1 marked with “+” in Fig. 2 are due to unknown products, not
assigned to 1H-fluorene.
The optimized structure of 1H-fluorene is shown in Fig. 3; the
symmetry of 1H-fluorene is changed to Cs symmetry from C2v symmetry
of 9H-fluorene. The hydrogen-atom migration largely affects not only
the right six-membered ring, which is changed from a benzene ring to a
1,3-cyclohexadiene ring with CH2 group, but also the central five-
membered ring; the bond lengths of C9-C9a and C8a-C9 are shortened
to 1.362 and 1.465 Å, respectively, from 1.516 Å of the corresponding
bond of 9H-fluorene. It may be assumed that the delocalized π-electron
system of 1H-fluorene expands on the right six-membered ring and the
left benzene ring through the C9a-C9-C8a fragment, leading to the
conclusion that the lowest electronic transition energy of 1H-fluorene is
lower than that of 9H-fluorene [30]. Actually, the first, the second, and
the third electronic transitions of 1H-fluorene are estimated by the
time-dependent DFT calculation to be 452 nm (its estimated oscillator
strength f = 0.049), 361 nm (f = 0.266) and 284 nm (f = 0.061), re-
spectively, while the corresponding values of 9H-fluorene are 276 nm
(f = 0.162), 266 nm (f = 0.287) and 257 nm (f = 0.007), implying that
The irradiation energies for the forward and the backward reactions
are ca. 405 kJ mol−1 (295 nm) and 375 kJ mol−1 (320 nm), respec-
tively, which are sufficiently higher than the estimated dissociation
energies for the C9-H bond of 9H-fluorene and the C1-H bond of 1H-
162