A guanine-substituted nitronyl nitroxide radical forming a one-dimensional ferromagnetic chain

Article abstract of DOI:10.1039/b702606d

A stable guanine-substituted nitronyl nitroxide radical 1 has been synthesized and characterized. The single-crystal structure analyses and magnetic susceptibility measurements exhibit a one-dimensional architecture of guanine base resulting from carbonyl

Full text of DOI:10.1039/b702606d

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
A guanine-substituted nitronyl nitroxide radical forming a one-dimensional
ferromagnetic chain
Kensuke Maekawa,a Daisuke Shiomi,*a,b Tomoaki Ise,a,b Kazunobu Satoa and Takeji Takui*a
Received 19th February 2007, Accepted 3rd April 2007
First published as an Advance Article on the web 20th April 2007
DOI: 10.1039/b702606d
A stable guanine-substituted nitronyl nitroxide radical 1 has been synthesized and characterized. The
single-crystal structure analyses and magnetic susceptibility measurements exhibit a one-dimensional
architecture of guanine base resulting from carbonyl-amino hydrogen bonds in the solid state, giving a
1D ferromagnetic chain of the radical moieties.
Introduction
In molecule-based magnetism as an interdisciplinary field, molecu-
lar packing or arrangement of open-shell molecules in a crystalline
solid state crucially links the magnetic properties of molecular
assemblages such as ferromagnetism.1 Hydrogen bonding is a
promising noncovalent bonding for controlling molecular pack-
ing. Hydrogen-donating and accepting molecules such as phenol,
carboxylic acid, and pyridine have been introduced to stable
radical families of nitroxides and imino- or nitronyl nitroxides.2
Biomolecule-based architecture such as a DNA duplex with
nucleobase pairing is fascinating as well in controlling molecular
packing. Whereas biologically important molecules with unpaired
electrons have been exploited mainly in spin labeling chemistry
and biochemistry,3,4 solid-state chemistry has been reported for
some nucleobases and nucleosides substituted with nitroxide or
nitronyl nitroxide radicals.5,6 There have been several examples
of stable organic radicals with well-characterized crystal struc-
Results and discussion
tures, to which naturally-found nucleobases such as cytosine
and uracil have been introduced.5 The magnetic properties of
these compounds have been examined in view of the hydrogen-
bonded assemblage of the open-shell building blocks. As for
guanine-substituted radicals, however, little has been known about
the X-ray crystal structures and magnetic properties, while the
solution chemistry of spin-labeled guanines and guanosines has
been explored.3 Non-oligomeric N9-substituted guanines7 and
guanosine8 have a marked tendency to form infinitely extended
molecular assemblies in crystalline solid states, which results from
intermolecular two-fold hydrogen bonding at N1H–N7 and N2H–
O6. Such iteratively propagating hydrogen bonds9 are attractive
from the viewpoint of hydrogen bonding-based crystal engineering
for open-shell molecular crystalline materials. In this paper, we
report, for the first time, the synthesis, the crystal structure and the
magnetic properties of a nitronyl nitroxide radical (1) substituted
with a guanine base. The magneto-structural relationship of 1 is
discussed.
Synthesis of radical 1
The synthetic route is shown in Scheme 1. After the TBDMS-
protected hydroxylamine 4 was introduced to the N9 position
of 2-amino-6-chloropurine, the chloro group of 5 was replaced
by the methoxy group using NaOMe, and subsequent treatment
of 6 with TMSI afforded the guanine form 7. Deprotection
of 7 gave the nitronyl nitroxide radical 1. Single crystals of 1
suitable for X-ray crystallography experiments were obtained
by recrystallization from a mixed solution of chloroform and
methanol.† The crystalline solid of 1 is stable under aerated
conditions at ambient temperature.
Crystal structure of radical 1
An ORTEP drawing of the asymmetric unit of 1 is shown in Fig. 1.
The asymmetric unit contains 1 mol of CHCl3 as a crystal solvent.
In Fig. 2 is depicted the molecular packing of 1. The carbonyl-
amino hydrogen bonds, N(7)H–O(4*) and N(5)H–N(4*), between
† Crystallographic data for C21H25N7O4Cl3: M = 545.83, 0.20 × 0.30 ×
aDepartments of Materials Science and Chemistry, Graduate School of
Science, Osaka City University, Sumiyoshi-ku, Osaka, 558-8585, Japan.
E-mail: shiomi@sci.osaka-cu.ac.jp, takui@sci.osaka-cu.ac.jp; Fax: +81-6-
6605-2522; Tel: +81-6-6605-3149
3
˚
0.50 mm , Mo Ka, 183 K, monoclinic,◦P21/n, a = 8.380(7) A, b =
3
˚
˚
˚
10.583(8) A, c = 29.23(3) A, b = 99.424(10) , V = 2558(4) A , Z = 4, Dcalc
=
1.417, R1 = 0.0819, Rw = 0.1968 optimized on F2 (GOF = 1.002) for 5494
reflections (all reflections) and 341 parameters. CCDC reference number
617741. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b702606d
bPRESTO, Japan Science and Technology Agency (JST), Honcho
Kawaguchi, Saitama, 332-0012, Japan
This journal is
The Royal Society of Chemistry 2007
Org. Biomol. Chem., 2007, 5, 1641–1645 | 1641
©

View Article Online
Scheme 1 The synthesis of the guanine-substituted radical. Conditions: (i) 2,3-dimethyl-2,3-dihydroxylaminobutane, benzene, 23 h; (ii) tert-butyl-
dimethylchlorosilane, imidazole, DMF, 50 ◦C, 29 h; (iii) 2-amino-6-chloropurine, K2CO3, KI, DMF, 60 ◦C, 25 h; (iv) 28% NaOMe in methanol solution,
CH2Cl2, MeOH, rt, 35 min; (v) TMSI, DMF, MeCN, rt, 23 h; (vi) 1.0 M TBAF in THF solution, THF, DMF, rt, 14 h.
Primitive a-axis translation of the two nearest-neighboring
molecules connected by the carbonyl-amino hydrogen bonds gives
a double chain of the radical moieties, as shown in Fig. 2(b). An
intermolecular short contact close to the van der Waals contact10
˚
is found at O(1)–C(12#) = 3.439(3) A between the phenyl nitronyl
nitroxide radical moieties. A short contact between a nitroxide
oxygen atom and a carbon atom of the meta-position such as
O(1)–C(12#) of 1 is expected to give rise to an intermolecular
ferromagnetic exchange interaction, as observed in many nitronyl
nitroxide derivatives.11 No short contacts around the nitroxide
groups leading to exchange interactions were found between the
chains.
Magnetic properties of radical 1
Fig. 1 An ORTEP drawing of 1 with the thermal ellipsoids of 50%
The temperature dependence of paramagnetic susceptibility vp
probability. The hydrogen atoms of the methyl groups are omitted for
clarity.
for a randomly-oriented polycrystalline sample of 1 is shown
in Fig. 3 in the vpT vs. T plots. The vpT value at room
temperature is 0.37 emu K mol−1, as expected for 1 mol of S =
1/2 spins. An upturn of vpT was found below 30 K, indicating the
occurrence of intermolecular ferromagnetic exchange interactions.
The temperature dependence of vp was analyzed using the Curie–
the guanine moieties lead to one-dimensional chains along the
Weiss law
b-axis, as depicted in Fig. 2(a). The hydrogen bonding motif
C
T − h
˚
vp =
(1)
and its bond lengths (N(7)H–O(4*) = 2.781(3) A and N(5)H–
˚
N(4*) = 2.825(3) A) are close to those of non-oligomeric guanine
and guanosine derivatives7,8 as previously studied by X-ray single-
crystalstructureanalyses. Little magnetic interactions areexpected
to propagate via the hydrogen bonds, since the hydrogen bonding
sites of guanine have little spin density, as anticipated. The guanine
moiety plays a primary role in governing the molecular packing,
instead of propagating intermolecular exchange interactions.
where the fitting provides the Curie constant of C = 0.37 emu K
mol−1 and the Weiss constant of h = 0.16 K.
As suggested from the crystal structure analyses, an S = 1/2
Heisenberg ferromagnetic chain model12
ꢀ
H = −2J
This journal is
Si·Si+1
(2)
i
1642 | Org. Biomol. Chem., 2007, 5, 1641–1645
The Royal Society of Chemistry 2007
©

View Article Online
Fig. 3 The temperature dependence of magnetic susceptibility vp of 1
measured on a SQUID magnetometer with the static magnetic field of B =
0.1 T in the vpT vs. T plot. The solid and dotted lines represent theoretical
calculations from eqn (1) and eqn (2), respectively.
allowed to distinguish the low-dimensional model, eqn (2), from
the three-dimensional mean-field model, eqn (1). In view of the
molecular packing, the exchange parameter of 2J/kB = 0.09 K is
a good estimate for the intermolecular interaction of 1.
Conclusions
The guanine-substituted nitronyl nitroxide radical 1 has been
synthesized and isolated as a crystalline solid which is stable
under ambient atmosphere. Introduction of the radical substituent
into the nucleobase results in little disturbance to the inter-
molecular hydrogen bonding motifs as found in non-oligomeric
guanines, demonstrating marked selectivity and directionality
of the hydrogen bonding of guanine governing the molecular
packing in a crystalline solid state: a bio-inspired approach to
genuinely organic molecule-based magnetism described here has
the potential to control the molecular packing of open-shell
molecular assemblages.
Experimental
X-Ray crystallographic analysis
Fig. 2 (a) The chain of the guanine moieties running along the b-axis.
The neighboring molecules related by the two-fold screw axis along the
b-axis are connected by carbonyl-amino hydrogen bonds, as depicted by
the dotted lines. (b) The double chain of the nitronyl nitroxide moieties
along the a-axis. Intermolecular short contacts are depicted by the dotted
lines. Symmetry transformations used to generate equivalent atoms: * {-x +
1/2, y + 1/2, −z + 1/2}; # {x − 1, y, z}.
The X-ray diffraction measurements were made on a Rigaku
Mercury CCD diffractometer at 183 K with graphite monochro-
mated Mo Ka radiation up to 2hmax = 55◦. The crystal structure
was solved by direct methods (SIR92)14 and subsequent Fourier
syntheses followed by a full-matrix least-squares refinement with
the anisotropic approximation for non-hydrogen atoms. Positions
of the hydrogen atoms were calculated and included in the final
refinement. All the calculations were made using a program
package CrystalStructure by the Rigaku/Molecular Structure
Corporation.15
should be more appropriate for describing the magnetic behavior
of the molecular assemblage of 1. The parameter J in eqn (2)
denotes the intermolecular exchange coupling, which is primarily
attributed to the O(1)–C(12#) contact. The calculation with the
parameter of 2J/kB = 0.09 0.01 K is consistent with the observed
temperature dependence of susceptibility.13 The calculated curve of
the Heisenberg chain model is identical to that of the Curie–Weiss
law, as depicted in Fig. 3. When the magnitude of the exchange
interaction is very small as compared with the thermal energy of
the susceptibility measurement (J ꢀ kBT or h ꢀ T), we are not
Magnetic susceptibility measurements
Magnetic susceptibility was measured on a Quantum Design
SQUID magnetometer MPMS-XL in a temperature range of
down to 1.9 K with a static magnetic field of 0.1 T.
This journal is
The Royal Society of Chemistry 2007
Org. Biomol. Chem., 2007, 5, 1641–1645 | 1643
©

View Article Online
Synthetic procedure
The reaction mixture was stirred for 35 min at room temperature,
and the solution was washed with brine, dried over MgSO4 and
concentrated. Product 6 was obtained as a colorless solid (2.25 g,
3.35 mmol, 92.5%) and used without further purification for the
next reaction. 1H NMR (CDCl3, 400 MHz): d(ppm): −0.882 (bs,
6H), −0.051 (s, 6H), 0.782 (s, 18H), 1.143 (s, 12H), 4.083 (s, 3H),
4.287 (t, 2H), 4.446 (t, 2H), 4.548 (bs, 1H), 4.918 (s, 2H), 6.797 (d,
2H), 7.269 (d, 2H), 8.020 (s, 1H). 13C NMR (CDCl3, 300 MHz):
d(ppm): −5.097, −3.951, 17.813, 26.119, 42.563, 53.762, 65.839,
67.610, 93.268, 113.590, 115.346, 131.912, 134.584, 140.057,
153.623, 157.661, 159.295, 161.532. HRMS (FAB): m/z calcd for
C33H58N7O4Si2 ([M + H]+) 672.40, found 672.41.
All commercially available materials for the synthesis shown
in Scheme 1 were used without further purification. NMR
spectra were recorded on an LA-400 spectrometer (JEOL). High-
resolution mass spectra (HRMS) were recorded on a JMS-700T
spectrometer.
2-[4-(2-Bromoethoxy)phenyl]-4,4,5,5-tetramethylimidazolidine-
1,3-diol (3). Starting material, 4-(2-bromoethoxy)benzaldehyde
2, was prepared according to a procedure previously reported.16
A suspension of 2 (2.01 g, 8.77 mmol) and 2,3-dimethyl-2,3-
dihydroxylaminobutane (1.30 g, 8.77 mmol) in dry benzene
(120 ml) was refluxed under Ar atmosphere for 23 h to give a
colorless powder of the adduct 3. The precipitate was filtered
and washed with benzene. Product 3 was used without further
purification for the next reaction (1.79 g, 4.98 mmol, 56.8%).
2-Amino-9-(2-{4-[1-(tert-butyldimethylsilanyloxy)-4,4,5,5-tetra-
methyl-3-(1-methyl-1-trimethylsilanylethoxy)imidazolidin-2-yl]-
phenoxy}ethyl)-1,9-dihydro-purin-6-one (7). To a solution of 6
(716.5 mg, 1.07 mmol) dissolved in dry DMF (17 ml) and dry
MeCN (6 ml) was added dropwise Me3SiI (0.75 ml, 5.55 mol).
The reaction mixture was stirred at room temperature for 23 h,
and then was treated with water, basified with 10% NaOH aq.
The organic layer was extracted with a mixed solvent of CH2Cl2
and MeOH, washed with brine, and dried over MgSO4. The
solvent was evaporated under reduced pressure, and the residue
was purified on silica gel. Elution with CH2Cl2–MeOH (12 : 1)
2-[4-(2-Bromoethoxy)phenyl]-1,3-bis-(tert-butyldimethylsilanyl-
oxy)-4,4,5,5-tetramethylimidazolidine (4). To a solution of 3
(1.77 g, 4.93 mmol) in dry DMF (15.6 ml) were added tert-
butyldimethylchlorosilane (3.82 g, 25.3 mmol) and imidazole
(3.35 g, 49.2 mmol). The mixture was stirred for 29 h at 50 ◦C. The
resulting mixture was treated with water, and the solution was
extracted with hexane, and dried over MgSO4. The solvent was
evaporated under reduced pressure, and the residue was purified on
silica gel. Elution with hexane–CH2Cl2 (5 : 1) gave 4 as a colorless
1
gave 7 as a colorless solid (318.2 mg, 0.484 mmol, 45.2%). H
NMR (DMSO, 400 MHz): d(ppm): −0.900 (bs, 6H), −0.066 (s,
6H), 0.755 (s, 18H), 1.111 (s, 12H), 4.243 (d, 2H), 4.309 (d, 2H),
4.477 (bs, 1H), 6.462 (s, 2H), 6.894 (d, 2H), 7.216 (d, 2H), 7.680
(s, 1H), 10.554 (s, 1H). 13C NMR (DMSO, 300 MHz): d(ppm):
−5.083, −3.945, 17.575, 26.056, 42.088, 65.685, 67.273, 93.125,
113.672, 116.436, 131.452, 133.101, 137.697, 151.178, 153.606,
156.774, 157.942. HRMS (FAB): m/z calcd for C32H56N7O4Si2
([M + H]+) 658.39, found 658.39.
1
solid (1.71 g, 2.91 mmol, 59.0%). H NMR (CDCl3, 400 MHz):
d(ppm): −0.854 (bs, 6H), −0.045 (s, 6H), 0.788 (s, 18H), 1.145 (s,
12H), 3.798 (t, 2H), 4.232 (t, 2H), 4.559 (bs, 1H), 6.837 (d, 2H),
7.281 (d, 2H). 13C NMR (CDCl3, 300 MHz): d(ppm): −5.051,
−3.852, 17.943, 26.234, 41.776, 67.717, 68.152, 93.406, 113.956,
131.973, 134.629, 158.058.
9-(2-{4-[1-(tert-Butyldimethylsilanyloxy)-4,4,5,5-tetramethyl-
3-(1-methyl-1-trimethylsilanylethoxy)-imidazolidin-2-yl]phenoxy}-
ethyl)-6-chloro-9H-purin-2-ylamine (5). A mixture of 4 (2.37 g,
4.08 mmol), 2-amino-6-chloropurine (0.972 g, 5.73 mmol), K2CO3
(0.956 g, 6.92 mmol) and KI (0.700 g, 4.22 mmol) in dry DMF
(25 ml) was stirred for 25 h at 60 ◦C. After the precipitate was
filtered, to the filtrate were added CH2Cl2 and a small amount of
hexane, and the solution was washed with brine. The solvent was
evaporated under reduced pressure, and the residue was purified
on silica gel. Elution with CH2Cl2–MeOH (54 : 1) gave 5 as a
2-Amino-9-{2-[4-(1-oxyl-3-oxido-4,4,5,5-tetramethyl-imida-
zolidin-2-yl)phenoxy]ethyl}-1,9-dihydropurin-6-one (1). To a so-
lution of 7 (152.0 mg, 0.231 mmol) dissolved in DMF (2 ml)
and THF (14 ml) was added dropwise a 1.0 M solution of
tetrabutylammonium fluoride in THF (2.85 ml, 2.85 mol). The
reaction mixture was stirred at room temperature for 14 h, and
the solvent was evaporated, and the crude product was purified
on silica gel. Elution with CH2Cl2–MeOH (15 : 1) and CH2Cl2–
MeOH (5 : 1) gave 1 as a blue solid (79.3 mg, 0.186 mmol, 80.5%).
HRMS (FAB): m/z calcd for C20H25N7O4 ([M + H]+) 427.19, found
427.19.
1
colorless solid (1.68 g, 2.46 mmol, 60.3%). H NMR (CDCl3,
400 MHz): d(ppm): −0.896 (bs, 6H), −0.061 (s, 6H), 0.772 (s,
18H), 1.134 (s, 12H), 4.291 (t, 2H), 4.466 (t, 2H), 4.542 (bs,
1H), 5.102 (s, 2H), 6.795 (d, 2H), 7.271 (d, 2H), 7.917 (s, 1H).
13C NMR (CDCl3, 300 MHz): d(ppm): −5.051, −3.906, 17.874,
26.157, 42.883, 65.534, 67.694, 93.352, 113.590, 125.110, 132.003,
134.988, 143.164, 151.249, 153.676, 157.486, 159.036. HRMS
(FAB): m/z calcd for C32H55ClN7O3Si2 ([M + H]+) 676.35, found
676.35.
Acknowledgements
This work has been supported by Grants-in-Aid for Scientific
Research from the Ministry of Education, Sports, Culture, Science
and Technology, Japan. Financial support from PRESTO of Japan
Science and Technology Agency is also acknowledged. One of the
authors (K. M.) acknowledges Research Fellowships of the Japan
Society for the Promotion of Science for Young Scientists.
9-(2-{4-[1-(tert-Butyldimethylsilanyloxy)-4,4,5,5-tetramethyl-
3-(1-methyl-1-trimethylsilanylethoxy)-imidazolidin-2-yl]-phenoxy}-
ethyl)-6-methoxy-9H-purin-2-ylamine (6). To a solution of 5
(2.46 g, 3.62 mmol) dissolved in dry MeOH (30 ml) and dry
CH2Cl2 (30 ml) was added dropwise a solution of 28% sodium
methoxide in MeOH (7.59 ml, 37.3 mmol) at room temperature.
Notes and references
1 For overviews of molecule-based magnetism, see: (a) Magnetic Prop-
erties of Organic Materials, ed. P. M. Lahti, Marcel Dekker, New
York, 1999; (b) Molecular Magnetism, ed. K. Ito and M. Kinoshita,
1644 | Org. Biomol. Chem., 2007, 5, 1641–1645
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
Gordon and Breach, Amsterdam (Kodansha as copublisher, Tokyo),
2000; (c) Proceedings of the 9th International Conference on Molecule-
Based Magnets, Polyhedron, ed. G. Christou, Tukuba, Japan, 2004,
vol. 24.
6 Very recently, a Watson–Crick type base pair functionalized with
nitronyl nitroxide radical moieties has been synthesized: T. Ise, D.
Shiomi, K. Sato and T. Takui, Chem. Commun., 2006, 4832.
7 For example, see: (a) G. I. Birnbaum, N. G. Johansson and D. Shugar,
Nucleosides Nucleotides, 1987, 6, 775; (b) G. I. Birnbaum, M. Cygler
and D. Shugar, Can. J. Chem., 1984, 62, 2646; (c) S. Fujita, A. Takenaka
and Y. Sasada, Bull. Chem. Soc. Jpn., 1984, 57, 1707; (d) R. Destro, T. J.
Kistenmacher and R. E. Marsh, Acta Crystallogr., 1974, B30, 79.
8 U. Thewalt, C. E. Bugg and R. E. Marsh, Acta Crystallogr., 1970, B26,
1089.
2 (a) J. Cirujeda, L. E. Ochando, J. M. Amigo, C. Rovira, J. Rius and
J. Veciana, Angew. Chem., Int. Ed. Engl., 1995, 34, 55; (b) J. Cirujeda,
M. Mas, E. Molins, F. Lanfranc de Panthou, J. Laugien, G. J. Park,
C. Paulsen, P. Rey, C. Rovira and J. Veciana, J. Chem. Soc., Chem.
Commun., 1995, 709; (c) M. M. Matsushita, A. Izuoka, T. Sugwara,
T. Kobayashi, N. Wada, N. Takeda and M. Ishikawa, J. Am. Chem.
Soc., 1997, 119, 4369; (d) H. Nagashima, N. Yoshioka and H. Inoue,
Polyhedron, 2001, 20, 1151; (e) T. Otsuka, T. Okuno, K. Awaga and T.
Inabe, J. Mater. Chem., 1998, 8, 1157; (f) P. M. Lahti, J. R. Ferrer, C.
George, P. Oliete, M. Julier and F. Palacio, Polyhedron, 2001, 20, 1465;
(g) K. Hayakawa, D. Shiomi, T. Ise, K. Sato and T. Takui, J. Phys.
Chem. B, 2005, 109, 9195.
3 (a) Spin Labeling, ed. L. J. Berliner, Academic Press, New York, 1976;
(b) Spin Labeling II, ed. L. J. Berliner, Academic Press, New York, 1979;
(c) G. I. Likhtenstein, Spin Labeling Methods in Molecular Biology,
Wiley Interscience, New York, 1976.
4 For recent examples of nucleosides labeled with stable nitroxides, see:
(a) A. Okamoto, T. Inasaki and I. Saito, Tetrahedron Lett., 2005, 46,
791; (b) T. Kaneko, M. Aso, N. Koga and H. Suemune, Org. Lett., 2005,
7, 303; (c) M. Aso, T. Ikeno, K. Norihisa, M. Tanaka, N. Koga and
H. Suemune, J. Org. Chem., 2001, 66, 3513; (d) M. Aso, K. Norihisa,
M. Tanaka, N. Koga and H. Suemune, J. Chem. Soc., Perkin Trans. 2,
2000, 1637.
9 V. A. Bloomfield, D. M. Crothers and I. Tinoco, Jr., Nucleic Acids:
Structure, Properties, and Function, University Science Books, Sausal-
ito, 1999, ch. 2.
10 A. Bondi, J. Phys. Chem., 1964, 68, 441.
11 For example, see: (a) M. Tamura, Y. Hosokoshi, D. Shiomi, M.
Kinoshita, Y. Nakazawa, M. Ishikawa, H. Sawa, T. Kitazawa, A.
Eguchi, A. Nishio and K. Kajita, J. Phys. Soc. Jpn., 2003, 72, 1735;
(b) Y. Hosokoshi, M. Tamura, H. Sawa, R. Kato and M. Kinoshita,
J. Mater. Chem., 1995, 5, 41; (c) K. Awaga, T. Okuno, A. Yamaguchi,
M. Hasegawa, T. Inabe, Y. Maruyama and N. Wada, Phys. Rev. B, 1994,
49, 3975; (d) Y. Nakazawa, M. Tamura, N. Shirakawa, D. Shiomi, M.
Takahashi, M. Kinoshita and M. Ishikawa, Phys. Rev. B, 1992, 46,
8906.
12 C. A. Baker, G. S. J. Rushbrooke and H. E. Gillbert, Phys. Rev., 1964,
135A, 1272.
13 The g-factor has been fixed as g = 2.006 in the calculation of vp, which
was obtained from the ESR spectrum of a mixed solution of chloroform
and methanol.
5 (a) K. Das, M. Pink, S. Rajca and A. Rajca, J. Am. Chem. Soc., 2006,
128, 5334; (b) P. Taylor, P. M. Lahti, J. B. Carroll and V. M. Rotello,
Chem. Commun., 2005, 895; (c) D. Shiomi, M. Nozaki, T. Ise, K. Sato
and T. Takui, J. Phys. Chem. B, 2004, 108, 16606; (d) P. Taylor, P. R.
Serwinski and P. M. Lahti, Chem. Commun., 2003, 1400; (e) R. Feher,
D. B. Amabilino, K. Wurst and J. Veciana, J. Mol. Cryst. Liq. Cryst.,
1999, 334, 333.
14 A. Altomare, M. C. Burlea, M. Camalli, M. Cascarano, C. Giacovazzo,
A. Guagliardi and G. Polidre, J. Appl. Crystallogr., 1994, 27, 435.
15 CrystalStructure (ver.3.7.0): Rigaku/Molecular Structure Corporation
(2000–2004), 9009 New Trails Dr. The Woodlands TX 77381 USA.
16 V. Biju, P. K. Sudeep, K. G. Thomas and M. V. George, Langmuir,
2002, 18, 1831.
This journal is
The Royal Society of Chemistry 2007
Org. Biomol. Chem., 2007, 5, 1641–1645 | 1645
©