UV-induced single and double hydrogen-atom migrations in 3,6-diimino-1,4-cyclohexadiene-1,4-diamine in a low-temperature argon matrix

Article abstract of DOI:10.1016/j.cplett.2005.04.096

Isomerization induced by ultraviolet radiation due to intramolecular hydrogen-atom migration in 3,6-diimino-1,4-cyclohexadiene-1,4-diamine (DCD) was studied by matrix-isolation Fourier transform infrared (IR) spectroscopy and density functional theory (DFT) calculation. Two isomers produced by single and double inversion of the hydrogen atom(s) around the CN bond(s) were identified using the wavelength dependence of the IR band-intensity changes. In addition, the IR bands of an intermediate produced by single hydrogen-atom transfer from the amino to the imino group (C-NH2?NHC) in the hydrogen bond were measured and identified by a comparison with the calculated spectral patterns. These observations confirm stepwise photoisomerization of DCD.

Full text of DOI:10.1016/j.cplett.2005.04.096

Chemical Physics Letters 409 (2005) 52–56
www.elsevier.com/locate/cplett
UV-induced single and double hydrogen-atom migrations
in 3,6-diimino-1,4-cyclohexadiene-1,4-diamine
in a low-temperature argon matrix
*
Kenji Ujike, Satoshi Kudoh, Munetaka Nakata
Graduate School of BASE (Bio-Applications and Systems Engineering), Tokyo University of Agriculture and Technology, Naka-cho,
Koganei, Tokyo 184-8588, Japan
Received 22 March 2005; in final form 26 April 2005
Available online 23 May 2005
Abstract
Isomerization induced by ultraviolet radiation due to intramolecular hydrogen-atom migration in 3,6-diimino-1,4-cyclohexadi-
ene-1,4-diamine (DCD) was studied by matrix-isolation Fourier transform infrared (IR) spectroscopy and density functional theory
(DFT) calculation. Two isomers produced by single and double inversion of the hydrogen atom(s) around the C@N bond(s) were
identified using the wavelength dependence of the IR band-intensity changes. In addition, the IR bands of an intermediate produced
by single hydrogen-atom transfer from the amino to the imino group ðC–NH2 ꢀ ꢀ ꢀ NH@CÞ in the hydrogen bond were measured and
identified by a comparison with the calculated spectral patterns. These observations confirm stepwise photoisomerization of DCD.
Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction
diates caused by proton and hydrogen-atom transfer in
DCD has yet been reported, to our knowledge.
3,6-Diimino-1,4-cyclohexadiene-1,4-diamine (DCD,
Fig. 1) has two intramolecular hydrogen bonds between
the amino and imino groups (C–HNHꢀ ꢀ ꢀNH@C). A
problem of theoretical and experimental interest lies in
whether or not single or double, proton or hydrogen-
atom transfer occurs in this molecule. Single proton
transfer was suggested by quantum-chemical calcula-
tions [1,2], where the structure of a zwitterion intermedi-
ate caused by single proton transfer was optimized using
the AM1 and HF/3-21G and MP2/3-21G levels. Similar
proton transfer in a phenyl-substituted derivative of
DCD, azophenine, was studied by Rumpel et al. [3–5]
who analyzed its isotopic NMR and IR spectra and
suggested stepwise single hydrogen-atom transfer.
However, no spectroscopic information on the interme-
We have recently investigated the photoisomerization
of 3,5-cyclohexadiene-1,2-diimine and 2,5-cyclohexadi-
ene-1,4-diimine produced from 1,2- and 1,4-diamino-
benzenes, respectively, by UV irradiation by a joint
analysis of low-temperature matrix-isolation IR spec-
troscopy and DFT calculation [6–8]. For 3,5-cyclohex-
adiene-1,2-diimine, which has an intramolecular
hydrogen bond between the two imino groups
ðC@NH ꢀ ꢀ ꢀ NH@CÞ, UV-induced hydrogen-atom inver-
sion around the C@N bonds was observed in the IR
spectral changes upon irradiation with various wave-
lengths (Scheme 1). This finding implies that two kinds
of hydrogen-atom migration are feasible in DCD, which
has two intramolecular hydrogen bonds between the
amino and imino groups: hydrogen-atom inversion
around the C@N bonds and hydrogen-atom transfer
within the hydrogen bond, as shown in Fig. 1. In the
present Letter, we have isolated DCD in a low-temper-
ature argon matrix and measured the IR spectra of the
*
Corresponding author. Fax: +81 42 388 7349.
E-mail address: necom@cc.tuat.ac.jp (M. Nakata).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.04.096

K. Ujike et al. / Chemical Physics Letters 409 (2005) 52–56
(a) Hydrogen-atom inversion
53
H
H
H
H
H
N
N
H
N
N
H
N
N
H
N
N
H
H
H
H
H
H
H
H
N
H
N
H
N
H
N
H
N
N
N
N
H
H
H
H
Isomer C
(51.3)
Isomer D
(52.5)
Isomer B
(29.8)
Isomer A
(0.0)
(b) Hydrogen-atom transfer
H
H
H
H
H
N
N
N
H
N
N
N
H
H
H
H
H
H
N
N
H
N
H
N
N
N
H
H
H
H
Isomer A
(0.0)
Isomer E
(54.0)
Isomer A
(0.0)
Fig. 1. Hydrogen-atom migration in 3,6-diimino-1,4-cyclohexadiene-1,4-diamine. Numbers in parentheses represent relative energies in kJ molꢂ1
obtained at the DFT/B3LYP/6-31++G** level. Isomers A and E are planar, while Isomers B, C, and D are non-planar with respect to one H atom of
non-hydrogen-bonded amino groups. Isomer A is obtained by double H-atom transfer from the original Isomer A.
used to avoid thermal reactions, and UV30 and UV32
short-wavelength cutoff filters were used to select irradi-
ation wavelength. Other experimental details were re-
ported elsewhere [10–12].
H
H
H
N
N
N
H
N
N
N
H
H
DFT calculations were performed using the GAUSSIAN
03 program [13] with the 6-31++G** basis set. BeckeÕs
three-parameter hybrid density functional [14], in com-
bination with the Lee–Yang–Parr correlation functional
(B3LYP) [15], was used to optimize the geometrical
structures and to obtain IR spectral patterns.
Scheme 1.
intermediates produced by single and double hydrogen
migrations upon UV irradiation. The intermediates have
been identified by a comparison of the observed spectra
with the calculated IR spectral patterns to gain informa-
tion on the mechanism of hydrogen migrations in DCD.
3. Results and discussion
3.1. Optimized structures
2. Experimental and calculation methods
The geometrical structures of four stable isomers of
DCD shown in Fig. 1 were optimized. Isomer A, which
has two intramolecular hydrogen bonds between the
amino and imino groups, is planar. On the other hand,
Isomer B with one hydrogen bond is non-planar, where
one hydrogen atom of the non-hydrogen-bonded amino
group is ꢁ34° out of the benzene ring. Isomers C (asym-
metric) and D (symmetric) are also non-planar, and one
hydrogen atom of each amino group is ꢁ40° out of the
ring. Their relative energies are 0, 29.8, 51.8, and 52.5
kJ molꢂ1 for Isomers A, B, C, and D, respectively,
which reflect the number of intramolecular hydrogen
bonds. The barrier height from Isomer C to D was esti-
mated to be only 3.8 kJ molꢂ1; this implies that Isomer
D can isomerize to C under our experimental condi-
tions. Hence, Isomer D was disregarded in the present
analysis of the IR spectra.
A method to synthesize DCD was established by
Nietzki and Hagenbach [9]. We prepared DCD as fol-
lows: A solid sample of 1,2,4,5-tetraaminobenznene
tetrahydrochloride of 30 mg (purity > 93%, Aldrich)
was solved in 15 ml distilled water. A solid sodium
hydroxide (purity > 96%, Wako) of 17 mg was added
to the solution. After evaporation of the solvent to dry-
ness under reduced pressure, the residue was placed in a
stainless steel pipe with a heating system and warmed to
ꢁ463 K under 10ꢂ5 Pa to obtain sufficient vapor pres-
sure. The vapor DCD was mixed with argon gas (purity
99.9999%, Nippon Sanso) and deposited in a vacuum
chamber on a CsI plate, cooled by closed-cycle helium
refrigeration (CTI Cryogenics, Model M-22) at ꢁ15 K.
The matrix sample was exposed to UV light from a
superhigh-pressure mercury lamp. A water filter was

54
K. Ujike et al. / Chemical Physics Letters 409 (2005) 52–56
We also performed the DFT/B3LYP/6-31++G** cal-
(N3AC1AC2AC3AN4) and diamino part (N2A
C6AC5AC4AN1). The charge distribution on each
atom is shown in Fig. 2b. The sum of the charge in
the diimino part is about ꢂ0.55e, while that in the dia-
mino part is +0.55e. This result implies that Isomer E
is ꢁ55% ionic, where the imino and amino parts are
charged negatively and positively, respectively. This
means that a charge redistribution in the intermediate
seems to occur with transfer of either a proton or a
hydrogen atom.
culation for an intermediate, Isomer E, which is ex-
pected to be produced by single hydrogen-atom
transfer from the amino to the imino group in Isomer
A. The relative energy of Isomer E in the singlet state
is 54.0 kJ molꢂ1 higher than Isomer A, being similar to
that of Isomers C and D. The relative energy of Isomer
E in the triplet state, also calculated using the same basis
set, was found to be higher than that in the singlet state
by ꢁ49 kJ molꢂ1. Thus, only the singlet states were con-
sidered in the present analysis.
All the geometrical parameters for Isomer E were
optimized, resulting in that this isomer is planar within
1°. Then we re-optimized the structure of Isomer E with
assumption of planarity. The calculated bond lengths
are listed in Table 1, where the numbering of atoms is
given in Fig. 2a. The lengths of the C1AC6 and
3.2. Hydrogen-atom migration by UV irradiation
Seven intense bands were observed in the range of
700 and 1700 cmꢂ1 in the IR spectrum of DCD mea-
sured before UV irradiation. This spectrum is compared
in Fig. 3 with the spectral patterns for Isomers A, B, C
and E calculated by the DFT method. The pattern for
Isomer A, which is the most stable isomer with two
intramolecular hydrogen bondings, reproduces the ob-
served spectrum; the observed wavenumbers are consis-
tent with the calculated values scaled by a factor of 0.98
within 17 cmꢂ1, as listed in Table 2.
˚
C3AC4 bonds, 1.512 A, suggest single-bond character,
˚
while those of C1AC2 and C2AC3, 1.411 A, and
˚
C4AC5 and C5AC6, 1.394 A, are shorter than the single
˚
˚
CAC bonds by ꢁ0.1 A. Similarly, the C1AN3 and
C3AN4 bonds, 1.314 A, and C4AN1 and C6AN2,
˚
1.329 A, are shorter than the CAN single bonds. These
comparisons suggest that the p-conjugation system in
Isomer E is separated to the two parts; diimino part
When the matrix sample was exposed to UV light for
2 min through a UV 32 short-cut filter (k > 310 nm), a
spectral change was observed, as shown in the difference
spectrum displayed in Fig. 4a. By a comparison between
the observed and calculated patterns for the five isomers
of DCD, the increasing bands were mainly assigned to
Isomer B (Fig. 4b), while the decreasing bands were as-
signed to Isomer A. After this measurement, the sample
was further exposed to UV light through a UV30 short-
cut filter (k > 290 nm) for 2 min. As shown in Fig. 4c,
the intensities of the bands appearing at 1409, 1047
Table 1
Calculated bond lengths (in A) in Isomer E of DCD obtained at the
˚
DFT/B3LYP/6-31++G** levela,b
C1AC2
C2AC3
C3AC4
C4AC5
C5AC6
C1AC6
C1AN3
C3AN4
C4AN1
C6AN2
a
1.411
1.411
1.512
1.394
1.394
1.512
1.314
1.314
1.329
1.329
N1AH3
N1AH4
N2AH5
N2AH6
N3AH7
N4AH8
C2AH1
C5AH2
1.022
1.008
1.008
1.022
1.020
1.020
1.087
1.087
Numbering of atoms is given in Fig. 2a.
b
Other geometrical parameters are listed in Supplementary Mate-
rial. They are also available upon request.
(a)
(b)
H7
N3
H1
C2
H8
N4
-0.34
+0.93
+0.59
-0.34
-0.40
-0.22
-0.40
-0.22
C1
C6
C3
C4
H6
H3
N2
H5
C5
H2
N1
H4
+0.20
+0.20
Fig. 2. Numbering of atoms in Isomer E (a) and charge distribution on
each atom calculated at the DFT/B3LYP/6-31++G** level (b). The
p-conjugation system is separated into two parts: diimine
(N3AC1AC2AC3AN4) and diamine (N2AC6AC5AC4AN1).
Fig. 3. Observed and calculated IR spectra of DCD: (a) observed after
deposition; (b)–(e) calculated spectral patterns of Isomers, A, B, C, and
E, respectively, obtained at the DFT/B3LYP/6-31++G** using a
scaling factor of 0.98.

K. Ujike et al. / Chemical Physics Letters 409 (2005) 52–56
55
Table 2
Observed and calculated wavenumbers (in cmꢂ1) and their relative intensities of four isomers of DCD
Isomer A
Isomer B
Isomer C
Isomer E
Obs.
Calc.
Obs.
Calc.
Obs.
Calc.
Obs.
Calc.
v
Int.a
vb
Int.a
v
Int.a
vb
Int.a
v
Int.a
100
vb
Int.a
v
Int.a
vb
Int.a
1658
1641
1602
1579
1562
1561
1444
1430
1339
1301
1279
1276
1134
1123
1046
1046
863
0
31
100
0
1671
1647
1611
1597
1579
1568
1440
1412
1343
1291
1264
1251
1131
1128
1070
1044
871
1
56
100
65
27
37
1
1682
1652
1615
1611
1596
1577
1429
1399
1346
1289
1248
1232
1134
1121
1088
1043
874
3
52
5
1636
1622
1562
1543
1515
1510
1435
1412
1330
1326
1294
1286
1138
1101
1087
1019
812
3
14
100
2
1630
1585
100
90
1633
1599
1590
100
48
1637
1624c
1578
1555
1532
49
100
16
27
1607
1595
33
23
100
62
1
0
6
6
1553
55
54
0
1566
99
9
0
0
2
4
1409
1346
58
37
23
17
1
0
1342
1267
47
87
9
1342
1291
1258
9
46
44
14
26
30
0
0
0
2
37
0
1246
1145
11
33
17
0
1
1278
46
10
5
0
17
15
8
32
1
1127
26
13
0
1125
1080
1053
877
34
11
7
4
0
0
0
7
1047
844
20
74
21
1
1
872
827
52
6
24
0
55
26
0
9
859
817
847
821
871
835
0
806
780
0
2
2
43
9
0
778
777
0
800
783
15
2
823
806
778
748
7
0
3
0
730
725
1
723
708
1
722
705
0
734
729
0
0
1
15
1
2
708
0
679
1
677
724
0
a
Intensity of the strongest band is defined as 100.
A scaling factor of 0.98 is used.
May be overlapped with a band of H2O in matrix.
b
c
and 844 cmꢂ1 were found to increase more strongly by
UV30 irradiation than by UV 32 irradiation. These
bands, shown in Fig. 4d, were assigned to Isomer C.
To clarify the origin of this wavelength dependence of
the spectral changes and to identify the other bands
for Isomer C, which should exhibit similar behavior to
the above three bands, a theoretical difference spectrum
(UV30 minus UV32) was calculated, as shown in Fig. 5,
where the bands for Isomer A were subtracted computa-
tionally. A comparison of the calculated spectral pat-
terns, shown in Fig. 5b for Isomers B (down) and C
(up), with the observed patterns evidences that Isomer
C increased and B decreased when the UV wavelength
was changed from k > 310 to >290 nm. From these spec-
tral changes, we were able to make separate assignments
of the bands of Isomers B and C. The observed and cal-
culated wavelengths and their relative intensities of these
isomers are summarized in Table 2. It is concluded that
single hydrogen-atom inversion in DCD can occur upon
UV irradiation as in 1,2-diiminobenzene [7,8], because
Isomer B is now clearly identified. Though Isomer C is
also identified, it is still unclear whether it is produced
from Isomer A by a direct pathway of double hydro-
gen-atom inversion or from Isomer B by single hydro-
gen-atom inversion. If electronically excited states for
Isomers A, B, and C are common, the single and double
hydrogen-atom inversion may both be feasible in DCD.
There remain a few strong unassigned bands in the
observed difference spectrum in Fig. 5a, besides those as-
Fig. 4. IR spectral changes due to H-atom migration in DCD: (a)
difference between the spectra measured before and after UV irradi-
ation through a UV32 optical filter (k > 310 nm) for 2 min; (b)
calculated spectral pattern of Isomer B; (c) difference between the
spectra measured before and after UV irradiation through a UV 30
optical filter (k > 290 nm) for 2 min subsequently to the UV32
irradiation; (d) calculated spectral pattern of Isomer C.
signed to Isomers B and C, such as 1578 and 1278 cmꢂ1
.

56
K. Ujike et al. / Chemical Physics Letters 409 (2005) 52–56
bands are assigned to Isomers B and C formed by single
and double hydrogen-atom inversions around the C@N
bonds, respectively, and to Isomer E produced by a sin-
gle hydrogen-atom transfer from the amino to the imino
group within the intramolecular hydrogen bond.
Though the spectrum of Isomer B resembles that of Iso-
mer C, they are distinguishable using a shorter wave-
length (k > 290 nm). These observations lead to the
conclusion that single hydrogen-atom inversion and sin-
gle hydrogen-atom transfer occur in DCD upon UV
irradiation.
Acknowledgements
The authors thank Professor Kozo Kuchitsu (BASE,
Tokyo University of A&T) for his helpful discussions.
Fig. 5. Wavelength dependence of spectral changes: (a) difference
between Fig. 4a and c, where the bands of Isomer A are cancelled
computationally; (b) calculated spectral pattern of Isomer B (down)
and Isomer C (up); (c) Isomer E, which corresponds to the observed
bands marked with a closed circle in (a).
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version at doi:10.1016/
j.cplett.2005.04.096.
It is possible to assign them to Isomer E by a compari-
son with the calculated bands, marked with closed cir-
cles in Fig. 5c. Isomer E is an intermediate produced
by single hydrogen-atom transfer from the amino to
the imino group within an intramolecular hydrogen
bond, but the stability under our matrix-isolation condi-
tion seems to be sufficient to render its IR spectrum ob-
servable. The observed and calculated wavenumbers and
their relative intensities are summarized in Table 2.
We note, however, that double hydrogen-atom trans-
fer from the amino to the imino group in DCD in the
argon matrix cannot be ruled out, because the produced
isomer is identical to the original Isomer A, resulting in
no IR spectral change with isomerization. Such double
hydrogen-atom transfer may be confirmed by deutera-
tion of one the hydrogen atoms in the amino group or
by substitution with a methyl group.
References
[1] M.J.S. Dewar, K.M. Merz Jr., J. Mol. Struct. 124 (1985) 183.
[2] M.K. Holloway, C.H. Reynolds, K.M. Merz Jr., J. Am. Chem.
Soc. 111 (1989) 3466.
[3] H.-H. Limbach, J. Hennig, D. Gerritzen, H. Rumpel, J. Chem.
Soc., Faraday Dis. 74 (1982) 229.
[4] H. Rumpel, H.-H. Limbach, G. Zachmann, J. Phys. Chem. 93
(1989) 1812.
[5] H. Rumpel, H.-H. Limbach, J. Am. Chem. Soc. 111 (1989) 5429.
[6] N. Akai, S. Kudoh, M. Nakata, J. Phys. Chem. A 107 (2003)
6725.
[7] K. Ujike, S. Kudoh, M. Nakata, Chem. Phys. Lett. 396 (2004)
288.
[8] K. Ujike, N. Akai, S. Kudoh, M. Nakata, J. Mol. Struct. 735
(2005) 225.
[9] R. Nietzki, E. Hagenbach, Ber. Dtsch. Chem. Ges. 20 (1887) 328.
[10] S. Kudoh, M. Takayanagi, M. Nakata, J. Photochem. Photobiol.
A 123 (1999) 25.
4. Conclusion
[11] N. Akai, S. Kudoh, M. Takayanagi, M. Nakata, Chem. Phys.
Lett. 356 (2002) 133.
The IR spectra of DCD measured after deposition is
assigned to Isomer A, which has two intramolecular
hydrogen bonds between the amino and imino groups.
The spectral changes observed after exposure of the
matrix sample to UV irradiation (k > 310 nm) can be as-
cribed to hydrogen-atom migration. The photoproducts
[12] M. Nakata, S. Kudoh, M. Takayanagi, T. Ishibashi, C. Kato, J.
Phys. Chem. 104 (2000) 11304.
[13] M.J. Frisch et al., GAUSSIAN 03, Revision B.04, Gaussian, Inc.,
Pittsburgh, PA, 2003.
[14] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[15] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.