Intermolecular hydrogen bonds between a radical and a diamagnetic matrix: CW-ESR investigations of 2,6-di-tert-butyl-4-hydroxymethylphenol

Article abstract of DOI:10.1246/bcsj.82.58

We propose a novel formation model of an intermolecular hydrogen bond between radicals and a diamagnetic matrix in the condensed phase. W-band and multi-frequency ESR studies revealed that the polycrystalline radical sample generated from PbO₂

Full text of DOI:10.1246/bcsj.82.58

58
Bull. Chem. Soc. Jpn. Vol. 82, No. 1, 58–64 (2009)
© 2009 The Chemical Society of Japan
Intermolecular Hydrogen Bonds between a Radical
and a Diamagnetic Matrix: CW-ESR Investigations
of 2,6-Di-tert-butyl-4-hydroxymethylphenol
Toshiki Yamaji,*1,Ñ Islam SM Saiful,2 Masaaki Baba,1 Seigo Yamauchi,2 and Jun Yamauchi1
1Department of Chemistry, Graduate School of Science, Kyoto University, Oiwake-cho, Kitashirakawa, Kyoto 606-8502
2Institute of Multidisciplinary, Research for Advanced Materials, Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai 980-8577
Received March 31, 2008; E-mail: toshiki-yamaji@aist.go.jp
We propose a novel formation model of an intermolecular hydrogen bond between radicals and a diamagnetic
matrix in the condensed phase. W-band and multi-frequency ESR studies revealed that the polycrystalline radical sample
generated from PbO2 oxidation of 2,6-di-tert-butyl-4-hydroxymethylphenol in toluene solution in vacuum at room
1
temperature has an anomalously large H-hyperfine interaction. It was concluded that this interaction is attributed to
hydrogen bonding between the O● which is a paramagnetic center in the molecular structure of the radicals and the para-
OH or the phenolic OH of the primary phenol. Although the phenol shows interesting radical chemical reactions unlike
other 2,6-di-tert-butylphenol derivatives, the generated radical species were successfully defined by the solution ESR
investigations. This may be the first ESR observation of an anomalously large hyperfine doublet structure coming from a
proton on an intermolecular hydrogen bond between a radical and a diamagnetic phenol.
Historically, most commercial spectrometers have always
At this point, we have systematically interpreted the powder-
pattern ESR spectra of a series of the phenoxyl radical
derivatives with tert-butyl groups at both para positions,
including the above radical, and obtained the g-tensors data
from X-, Q-, and W-band multi-frequency ESR investiga-
tions.16 The polycrystalline samples of phenoxyl radical
derivatives diluted in the corresponding phenol matrix can be
prepared either by method 1; UV-irradiation of corresponding
phenol matrix,16 or by method 2; mixing corresponding
primary phenol matrix with phenoxyl radical derivative
generated from chemical oxidation (PbO2 oxidation) of
corresponding phenol (shown in Figure 1).15,16
However, in these studies, the X-band ESR spectrum of a
polycrystalline radical sample derived from a PbO2 oxidation
of 2,6-di-tert-butyl-4-hydroxymethylphenol in toluene solution
in vacuum at room temperature showed a singular pattern, that
is to say, anomalously large doublet structure. The generated
compound was not evidently the corresponding radical, that is
2,6-di-tert-butyl-4-hydroxymethylphenoxyl radical, from the
splitting in the X-band spectrum. In this paper, we would like
to report the results of the CW-ESR studies of the polycrystal-
operated around 10 GHz at magnetic fields around 0.3 T using
electromagnets. However, over the last ten years, there has been
an increasing interest in achieving the ability to make multi-
frequency measurements at much larger magnetic fields using
super-conducting magnets.1Í14 A higher-frequency operation
potentially offers much greater absolute sensitivity as well as
higher g-factor resolution. More generally, multi-frequency
measurements often allow the field-dependent and field-
independent terms of the spin Hamiltonian to be separated
and characterized.
In The Journal of Physical Chemistry A, we recently
reported a multi-frequency ESR study of the polycrystalline
phenoxyl radical of ¡-(3,5-di-tert-butyl-4-hydroxyphenyl)-N-
tert-butylnitrone in a diamagnetic matrix,15 which is very
useful as a spin-trapping technique. As reported, we success-
fully interpreted the powder-pattern ESR spectra and deter-
mined the ESR parameters (g-tensor and hyperfine tensor)
using high-frequency ESR (Q- and W-band ESR) techniques in
addition to the conventional X-band ESR. Thus, this study
showed the significant potential of a multi-frequency ESR
approach to powder-sample radicals, in particular samples that
are difficult to acquire as single crystals, because of their high
resolution with a g-value, confirming the electronic structure of
the phenoxyl radical in terms of the ESR parameters.
O
OH
UV or PbO2
in vacuum
Ñ Present address: Polymer Standards Section, Materials Charac-
terization Division, National Metrology Institute of Japan
(NMIJ), National Institute of Advanced Industrial Science and
Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba 305-
8565
R
R
Figure 1. Oxidation scheme of a series of 2,6-di-tert-
butylphenol derivatives.
Published on the web January 14, 2009; doi:10.1246/bcsj.82.58

T. Yamaji et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 1 (2009)
also presented.
59
OH
From the W-band spectrum, two possibilities can be
considered. One is that two radical species having different
orthorhombic g-tensors are generated during the sample
preparation in the toluene solution, resulting in the W-band
spectrum being a superposition of those orthorhombic spectra
having no hyperfine structure. Another is that a radical species
with a large H-hyperfine interaction is generated, resulting in
the W-band spectrum showing a doublet structure like that of
2,6-di-tert-butylphenoxyl radical.16
CH2OH
1
Figure 2. Structural formula of the phenol parent com-
pound, 2,6-di-tert-butyl-4-hydroxymethylphenol.
line radical sample generated from PbO2 oxidation of 2,6-di-
tert-butyl-4-hydroxymethylphenol and propose a hydrogen-
bond scheme between radicals and the phenol. As seen later,
the powder-pattern X-, Q-, and W-band ESR spectra are clearly
showing a doublet structure as typical textbook data. It goes
without saying that the hydrogen-bond model proposed in this
paper can be referred in many studies related to radicals such as
phenol science and bionics.
We previously reported “Pulsed ESR study of electron spin
relaxation times of polycrystalline phenoxyl radical derivative
in a diamagnetic crystal” in the journal, Solid State Ionics.17
In that study, the analysis of a temperature dependence of the
electron spin relaxation times from the viewpoint of the
molecular motions is correct, however, we regarded a radical
generated by PbO2 oxidation as the corresponding radical,
since in the case of the other phenols,15,16 the corresponding
phenoxyl radicals can be derived by a similar way. Thus, the
present paper also corresponds to the errata of the previous
report. The details will be written in the Conclusion section.
The simulated spectra at the all-band in the former case are
not shown. The g-tensors of two species were estimated as
appropriately reproducing the W-band spectrum. The simula-
tion results may reproduce the W-band spectrum, but as the
frequency becomes lower (from Q to X-band), the degree of
reproducibility becomes lower. While a hyperfine structure
does not change to first order on perturbation by varying the
external magnetic field (band or frequency), splitting of g-
values has a linear relationship with field strength. Conse-
quently, the result understandably becomes a single peak on the
X-band level, the peaks overlapping with each other. It may
therefore be concluded that a kind of radical species was
generated which has large unpaired electron-spin density on a
single proton in the molecular frame. This conclusion can be
reached not only through high-frequency measurement but also
by carrying out multi-frequency ESR measurements. As seen in
Figure 3, the simulated results successfully reproduce the
experimental on the all-band of the phenoxyl radicals.15,16 The
ESR parameters obtained from the assignment of the W-band
spectrum are summarized in Table 1. The orientation of the
g-tensor in the radical can be defined as illustrated in a previous
paper.15,16
Materials and Methods
The structural formula of 2,6-di-tert-butyl-4-hydroxymethyl-
phenol discussed in this paper is shown in Figure 2. The phenol
sample was oxidized with lead dioxide (PbO2) in toluene solution
under vacuum at room temperature.15Í17 The toluene solutions
including generated radicals and including the primary phenol
were mixed. We then obtained the polycrystalline radical sample
diluted in a diamagnetic matrix, the primary phenol by grinding the
remaining crystal after evaporation of the toluene solvent. Finally,
it was sealed in a 5 mmº ESR sample tube under vacuum.
A solution-CW-ESR measurement of a species generated
from PbO2 oxidation of the phenol in toluene solution under
vacuum at room temperature was also carried out. A solution
spectrum generally supplies high-resolution (isotropic) data
and shows electronic structure of a radical. The observed and
simulated solution-CW-ESR spectra are shown in Figure 4.
From seeing the spectrum, it can be concluded that the
generated species is the corresponding radical, that is, 2,6-di-
tert-butyl-4-hydroxymethylphenoxyl radical. The assignment
of the isotropic hyperfine splittings is summarized in Table 2.
From the results of this toluene solution spectrum, it is seen
that the corresponding radical has notable electron spin density
at the meta-Hs and para-¡Hs and shows isotropic hyperfine
structures from those protons in toluene solution, splitting to
1:2:1 by the two equivalent para-Hs, in addition, to 1:2:1 by
the two equivalent meta-Hs. The proton at the para-OH has
little electron spin density so the hyperfine structure is within
the linewidth, resulting in no resolution. From the powder
pattern spectra, it is thus clear that the generated radical species
is not the corresponding 2,6-di-tert-butyl-4-hydroxymethyl-
phenoxyl radical, because the corresponding radical does not
Experimental
X- and Q-band ESR experiments were carried out using a JEOL
FE1XG spectrometer and a JEOL FE-3X ESR spectrometer,
respectively. W-band ESR measurement was performed using a
Bruker E600 W-band EPR spectrometer. X-band ESR experiments
were conducted with the samples in the evacuated X-band tube,
whereas Q- and W-band measurements were carried out with the
samples in the Q- and W-band tubes not evacuated after taking the
samples out of the X-band tube. In this work, the effect of air was
not considered to be a problem.16 Sealing the X-band tube also
contributed to maintain the vacuum before high-frequency ESR
measurements. The computer simulations of the ESR spectra were
conducted with the assigned spin-Hamiltonian parameters using
Bruker SimFonia V.1.25.
1
have as large a H-hyperfine doublet structure.
Incidentally, in our studies it has been found that the phenol
discussed in this paper shows interacting chemical reactions in
solution unlike the other 2,6-di-tert-butylphenol derivatives,
resulting in changes of the ESR spectrum. The spectrum
changed by keeping in contact with PbO2, while the spectra of
Results and Discussion
The experimental and simulated all band powder pattern
ESR spectra are shown in Figure 3. In the W-band spectra, the
assignments of the g- and hyperfine tensors determined later are

60
Bull. Chem. Soc. Jpn. Vol. 82, No. 1 (2009)
Hydrogen Bonds between Radicals and a Phenol
a
c
AHx
gx
AHy
20 mT
AHz
gy
b
gz
20 mT
25 mT
Figure 3. Experimental (solid lines) and simulated (dotted lines) X-, Q-, and W-band (a, b, and c) ESR spectra of the powder
sample, respectively. Experimental conditions: room temperature; microwave frequency, 9.18 GHz (X-band), 36.08 GHz (Q-band),
93.743251 GHz (W-band); center field, 328 mT (X-band), 1256 mT (Q-band), 3341.65 mT (W-band); modulation amplitude, 0.1 mT
(X-band), 0.2 mT (Q-band), 0.1 mT (W-band); modulation frequency, 100 kHz (X-, Q-, and W-bands); microwave power, 1 mW (X-
and Q-bands), 0.1265 mW (W-band).
Table 1. The ESR Parameters Obtained from the W-Band
Table 2. The Isotropic Hyperfine Splitting Constants
Obtained from the X-Band ESR Measurement of
2,6-Di-tert-butyl-4-hydroxymethylphenoxyl Radical in
Toluene Solutiona)
Powder-Pattern ESR Spectraa)
b)
gx
gy
gz
gave
2.00449
2.00689
2.00423
2.00234
Corresponding
phenoxyl radical
0.875
0.167
Ax
1.59
Ay
1.34
Az
1.25
Aave
1.39
(two Hs in para-CH2) (two meta-Hs)
a) The unit of hfs is mT.
a) The unit of A is mT. b) gave = (gx + gy + gz)/3. Aave
(Ax + Ay + Az)/3.
=
At this point, we presume that the radical species and the
diamagnetic host matrix, the primary phenol forms the hydro-
gen bond as a kind of a stabilization effect and the anomalously
large hyperfine doublet structure results from the H at the
hydrogen-bond. We shall analyze the generated polycrystalline
radical sample more in detail.
In order to define the generated radical species, the
polycrystalline radical sample was dissolved in toluene under
vacuum at room temperature. An X-band CW-ESR measure-
ment of the toluene solution was done. The observed spectrum
is shown in Figure 6.
It can be easily recognized that multi radical species exist
since this experimental ESR spectrum is not symmetric at the
center position. From amplitudes of the hyperfine splittings, it
is found that the corresponding radical does not exist. We
attempted the spectral fittings by presuming all possible
combinations of multi radical species. Finally, a superposition
of the ESR spectra coming from two possible radical species
the other phenol derivatives do not change. Polymerization of
phenoxyl radical derivatives in solution, generation of secon-
dary radicals, and ESR spectral variations are also known.20
In our studies, it had been found that in a series of 2,6-di-
tert-butylphenoxyl radical derivatives, 2,6-di-tert-butylphen-
oxyl radical (illustrated left in Figure 5) shows a large doublet
hyperfine structure resulting from the 1H at the para position.16
However, the spectra in the present paper are clearly different
from those of 2,6-di-tert-butylphenoxyl radical. For example,
the x-component of the hyperfine structure of the para-H of
2,6-di-tert-butylphenoxyl radical is not resolved, unlike the
other components, due to the relatively small splitting within
the line width. Furthermore, 2,6-di-tert-butylphenoxyl radical
can be obtained only by UV irradiation, since in solution, the
radical can instantly dimerize, becoming diamagnetic and ESR-
silent.16 So the generated radical discussed at this moment,
indeed is not 2,6-di-tert-butylphenoxyl radical.

T. Yamaji et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 1 (2009)
61
Exp
1
:
2
:
1
1 : 2 :1 : 2 :4: 2 : 1 :2 :1
320
321
322
323
324
325
326
O
Magnetic field /mT
Figure 6. Experimental X-band ESR spectrum of the
toluene solution dissolved polycrystalline sample under
vacuum at room temperature. Experimental conditions:
microwave frequency, 9.066 GHz; center field, 323 mT;
sweep-width, 5.00 mT; sweep-time, 16 min; modulation
amplitude, 0.05 mT; modulation frequency, 100 kHz;
microwave power, 1.00 mW; amp, 320; time constant,
0.3 s.
CH2OH
325
326
327
328
329
330
331
Magnetic field /mT
Figure 4. Experimental (top) and simulated (bottom) X-
band ESR spectra of 2,6-di-tert-butyl-4-hydroxymethyl-
phenoxyl radical in toluene solution under vacuum at room
temperature. The molecular structure of the radical is also
illustrated. Experimental conditions: microwave frequency,
9.40 GHz; center field, 328.084 mT; sweep-width, 5.00 mT;
sweep-time, 8 min; modulation amplitude, 0.1 mT; modu-
lation frequency, 100 kHz; microwave power, 1.00 mW;
amp, 100; time constant, 0.3 s.
center proton. However, the interaction is small (µ0.57 mT)
compared with the value of this work. A high-field ESR
spectrum of a galvinoxyl radical had been already reported,
showing no hyperfine structures.21 The observed W-band
spectrum and the obtained g-tensors are similar to those of
other phenoxyl radicals,15,16 showing that the unpaired electron
is delocalized over the ³-conjugated framework. At this point,
it can be assumed that the oxygen which is a paramagnetic
center in the molecular structures of the generated radicals
might form a hydrogen bond with the para-OH or the phenolic
OH of the host diamagnetic matrix, primary phenol in the
condensed phase and a large part of the unpaired electron spin
might be distributed over the OH, resulting in the large doublet
1H-hyperfine splitting. The presumed hydrogen-bond mecha-
nism is shown in Figure 8. It can be considered that the
hydrogen bonds break in a toluene solution due to the fast
Brownian molecular motions. We presume that the g-values
and 1H hyperfine constants of both hydrogen-bond radical
species are almost the same within the line width by
considering the isotropic data from the toluene solution
experiment and the powder pattern W-band spectrum.
As seen in Table 3, the two protons of the para-CH2 in the
radical I have a large electron spin density, (AHiso = 1.02 mT)
enough to show the visible splitting in the powder spectra. It
was however, assumed that the phenol-radical I does not show
visible hyperfine splitting except the center hydrogen-bonding
H because the electron spin density is delocalized over the
phenol and the electron spin density at the para-CH2 in
the phenol-radical I becomes small. The hyperfine coupling
constants at the CH2 can be estimated to be less than 0.51 mT
resulting in the unresolved hyperfine structure in the powder-
pattern spectra.16 Since the C of the CH2 is a sp3-hybrid carbon,
the ³-conjugated system is sectioned at the point of the C. The
quantum chemical calculation study was thus required to
confirm the above-mentioned analysis of the electron spin
O
O
C
O
Figure 5. Molecular structures of 2,6-di-tert-butylphenoxyl
radical (left) and a galvinoxyl radical (right).
successfully reproduced the observed spectrum. The results of
the spectral simulation and the molecular structures of the
presumed radical components are shown in Figure 7.
The isotropic hyperfine splitting constants of the two
radical components obtained from the spectral simulations
are summarized in Table 3. The molecular ratio of the two
radical components is found to be approximately 1:1 from the
comparison of the spectral amplitudes. It has been reported that
the radical component II, that is 2,6-di-tert-butyl-4-carboxy-
phenoxyl radical, can be generated as a secondary radical
during the chemical oxidation reactions of the primary phenol
of the present work in solution, resulting in the triple line-ESR
spectrum.20
The doublet hyperfine splitting observed in the powder
spectra is anomalously large. A galvinoxyl radical (illustrated
right in Figure 5), which is one of the most well known organic
free radicals, may have doublet hyperfine structure from its

62
Bull. Chem. Soc. Jpn. Vol. 82, No. 1 (2009)
Hydrogen Bonds between Radicals and a Phenol
Sim
1
:
:
2
:
1
1:2:1
1
1 : 2 : 2 : 1 : 1
1:2:1:1:2:1:2:4:2:2:4:2:1:2:1:1:2:1
+
OH
O
CH2
O
COOH
radical II
radical I
Figure 7. Simulated X-band ESR spectra and the molecular structure of the two radical components.
Table 3. The Isotropic Hyperfine Splitting Constants of the
Two Radical Components Obtained from the Spectral
Simulations in the Units of mT
H2
C
H2
C
HO
OH
HO
O
Radical I
1.02 (two Hs in
para-CH2)
0.481 (H in
0.173 (two
phenoric OH)
meta-Hs)
Radical II 0.169
(two meta-Hs)
phenol
radical I
density of the CH2.
Density functional theory (DFT) calculations were per-
formed using the Gaussian 03 program22 to estimate the
hyperfine coupling constants of some radicals appearing in
this work. The B3LYP method and the 6-31G(d) basis set were
used for both the structure optimizations and the parameter
calculations. All opt-calculations of the radicals successfully
converged. The B3LYP method is known to be applicable for
the calculations of organic radicals as well as transition-metal
ions.23Í25 The calculated results of the hyperfine coupling
constants at the para-CH2 in the phenol-radical I on the default
and structure-optimized molecular models were 0.248 and
0.003 mT, respectively, decreasing from the calculated results
H2
HO
C
OH
O
COOH
radical II
phenol
Figure 8. Presumed hydrogen-bond mechanism between
the O● of the radicals and the phenolic or para-OH of the
primary phenol in the condensed phase.

T. Yamaji et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 1 (2009)
63
of the radical I model. This confirmed the above-mentioned
analysis of the hyperfine coupling of the CH2 in the phenol-
radical I. The following were found, but not shown in detail.
The calculated estimates of the hyperfine coupling constants of
the protons near/away from the center (O●) of the unpaired
electron were overestimated/underestimated compared to the
experiment values. This shows that the electron spin densities
of the radicals in the solutions are actually more delocalized
over the molecules. The experimental values are between the
calculated results on the default and the structure-optimized
molecular models, close to the calculated results on the former.
The presently used calculation modeling is a simple one
(isolated)-molecular-type. It probably needs consideration of
the molecular environmental effects (the intermolecular inter-
actions or the solvent effects), such as the van der Waals force,
polarization, and steric strain from the neighboring diamagnetic
phenol or paramagnetic radical molecules. The calculated
results of the hyperfine coupling constants on the hydrogen-
bonding protons in the phenol-radical I and II were over-
estimated for the default molecular models, and underestimated
for the structure optimized models. The reason might be that
the radical molecules are actually hindered in the condensed
phase, forming the hydrogen-bonding network.26 The calcu-
lations done by using a basis set 6-31++G(d), that added
diffuse functions to a 6-31G(d), gave just a little better results
than the calculations by a 6-31G(d). Diffuse functions were
thus found to be not so efficient for the evaluation of the spin
density distributions or the hyperfine splitting constants at the
hydrogen-bonding Hs discussed in this work. Better basis-set
functions and methods need to be used for more precise
evaluation. In any case, the validity of the analysis taken in
this work was qualitatively confirmed by quantum chemical
calculations.
original polycrystalline sample (Figure 3a). The change be-
tween the dissolution state of the radical species in toluene
solution to the hydrogen-bond formation in the condensed
phase was thus found to be reversible. This result confirms the
conclusion of the present work. We have not found reports that
an anomalously large 1H doublet hyperfine splitting is observed
in ESR spectra resulting from formation of a hydrogen bond
between a paramagnetic center (O●) of radicals and an OH of a
diamagnetic molecule (phenol) in a condensed phase. This
paper thus may supply a new physical insight for a hydrogen-
bond formation between paramagnetic and diamagnetic species
to, for example, studies on reactions or structures of bio
molecules. Historically, it has been well known that a p-
benzoquinone and a hydroquinone have charge-transfer inter-
action resulting in becoming a quinhydrone, which can be
explained on the charge-transfer-complex theory established by
Mulliken. Further research, for example, ab initio calculations,
solution- and solid-state NMR, or UVÍvisible and IR spec-
troscopy investigations may be necessary to more strongly
confirm the conclusions of the present work. Studies of frontier
orbital interactions (SOMOÍHOMO and SOMOÍLUMO)27 or
thermodynamic stability might be also required for the analysis
of the interesting radical chemical reaction and the hydrogen-
bond formation. It can be considered that the SOMOÍHOMO
interaction should be predominant for the chemical reactions
and hydrogen-bond formation from the viewpoint of the inter-
orbital interaction since the SOMO generates from the HOMO
of the precursor phenol. In solid-state NMR investigations, the
proton having large electron-spin density could show the
resonance-shift by the portion of the hyperfine interaction
(so-called, paramagnetic shift), possibly accompanying the
line broadening. Solid-state pulse-NMR methods, CPMAS and
CRAMPS might be useful for more precise determination of
the structures of the diamagnetic and paramagnetic molecules
discussed in this paper. The states of the hydrogen bonds in a
toluene solution might be investigated by solution NMR
experiments (HSQC, NOESY, etc.).28 The parameters are the
hydrogen-exchange rates, the isotope shifts, the isotropic and
anisotropic chemical shifts, the quadrupolar coupling constant
of a deuteron involved in the bond, the scalar coupling
constants transmitted through the hydrogen bonds and so on.
These scalar couplings seem to correlate with the hydrogen-
bond lengths and can be used to identify the coupling partner
directly. We will carry out the above-mentioned investigations
of the primary phenol and polycrystalline radical samples.
The crystal structure of the host matrix phenol, 2,6-di-tert-
butyl-4-hydroxymethylphenol is illustrated in Figure 9.26 Our
conclusions from the present ESR studies on the formation of
the hydrogen bonding head-to-tail between the radicals and the
phenol is also confirmed from the view point of the organic
crystallographic data.
A re-evaporation of the toluene solvent was done. The X-
band ESR spectrum of the polycrystalline sample obtained by
grinding after the re-evaporation was the same as that of the
Conclusion
The CW-ESR studies of the polycrystalline radical sample
generated from PbO2 oxidation of 2,6-di-tert-butyl-4-hydroxy-
methylphenol in toluene solution under vacuum at room
temperature have been done.29 The two generated radical
species were successfully defined by analyzing the toluene
dissolution spectrum. It was concluded that the radical species
have an orthorhombic g-tensor and those paramagnetic centers
O● form the hydrogen bonding with the para- and phenolic
OH of the host matrix phenol resulting in the anomalouly large
1H-hyperfine splittings. These conclusions were also confirmed
from the crystallographic structure data and quantum chemical
calculations. It would be very interesting to further investigate
Figure 9. Crystal structure of the host matrix, 2,6-di-tert-
butyl-4-hydroxymethylphenol. This crystal structure en-
ables us to understand the abnormally large hyperfine
doublet splittings from the hydrogen-bonding Hs in the
phenol radical complex I and II.
1

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Bull. Chem. Soc. Jpn. Vol. 82, No. 1 (2009)
Hydrogen Bonds between Radicals and a Phenol
20 J. K. Becconsall, S. Clough, G. Scott, Trans. Faraday Soc.
1960, 56, 459.
21 A. Y. Bresgunov, A. A. Dubinsky, O. G. Poluektov, Y. S.
Lebedev, A. I. Prokof’ev, Mol. Phys. 1992, 75, 1123.
this proposed hydrogen-bond scheme from multiple eyes, i.e.,
other spectroscopic methods and quantum chemical calcula-
tions.
22 Cite this work as: M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A.
Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M.
Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M.
Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M.
Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E.
Knox, H. P. Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C.
Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.
Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D.
Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S.
Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
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M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe,
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phenols.15,16 It may be however concluded that since the generated
species are radicals which have tert-butyl groups, the assignments
of the minima in the temperature variations of the relaxation times
remain reasonable. We will report this discussion in the journal,
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