ESR study of structural characteristics and intramolecular motions of tris(3,5-di-tert-butyl-4-hydroxybenzyl)amine free radical in toluene

Article abstract of DOI:10.1007/s11172-011-0070-6

The magnetic and dynamic properties of tris(2,6-di-tert-butyl-4- hydroxybenzyl)amine free radical was studied by ESR spectroscopy in the temperature range from 170 to 350 K in toluene. Symmetrical conformers in the rigid toluene matrix with characteristic angles θ₁ = 26° and θ₂ = 60° between the symmetry axis of the 2p _z -orbital on the C_α atom of the phenoxyl ring and the direction of the C_α-C_β bond of the CH ₂ fragment were determined. The activation energy of transition between the conformers was determined to be 8.2 kcal mol^-1. The correlation time of transition between the conformers was established as 8.2?10^-1 s.

Full text of DOI:10.1007/s11172-011-0070-6

Russian Chemical Bulletin, International Edition, Vol. 60, No. 3, pp. 446—448, March, 2011
446
ESR study of structural characteristics and intramolecular motions
of tris(3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxybenzyl)amine free radical in toluene
I. R. Nizameev, V. I. Morozov, and M. K. Kadirov
A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843 2) 75 2253. Eꢀmail: kadirov2004@mail.ru
The magnetic and dynamic properties of tris(2,6ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxybenzyl)amine free
radical was studied by ESR spectroscopy in the temperature range from 170 to 350 K in toluene.
Symmetrical conformers in the rigid toluene matrix with characteristic angles θ = 26° and
1
θ = 60° between the symmetry axis of the 2pzꢀorbital on the Cα atom of the phenoxyl ring and
2
the direction of the Cα—Cβ bond of the CH2 fragment were determined. The activation energy
of transition between the conformers was determined to be 8.2 kcal mol–1. The correlation time
of transition between the conformers was established as 8.2•10–1 s.
Key words: phenoxyl, free radical, ESR spectroscopy, conformers, conformational angles,
correlation time of conformational transitions.
Phenols represent a class of chemical compounds in
which chemists and industrial workers are especially inꢀ
terested. The most frequent use of substituted phenols in
industry is their application as antioxidants.1—3 The antiꢀ
oxidation properties of these phenols are due, to a great
extent, to the easy detachment of the phenolic hydrogen
atom and the efficient interaction of the phenoxyls formed
with peroxyl and alkoxyl radicals, being the main reacꢀ
tants in selfꢀoxidation.4 In some cases, intermediate
phenoxyl radicals can undergo further oxidation leading
to the formation of quinoid products.5 As shown earlier,6
the oxidation of the methoxyꢀsubstituted phenoxyl radical
is the main mechanism of photoinduced yellowing of paꢀ
per. It is known7,8 that the efficiency of antioxidation efꢀ
fect of a series of phenol stabilizers is determined by their
spatial structure. At the same time, literature data on the
spatial structure of similar compounds in solutions and on
conformations of products of their oxidation, which play
an important role in the inhibited oxidation process, are
insufficient.
fragment exert almost no effect on the reactivity and other
properties of the rest unreacted phenol fragments.9,10
Phenoxyl radicals formed from polynuclear phenols
were studied to a considerably less extent than free radiꢀ
cals of mononuclear phenols. A comparison of radicals of
alkylated bis, trisꢀ, and tetraphenols with their mononuꢀ
clear analogs9 shows that they exhibit, as a rule, no subꢀ
stantial difference in HFC constants and, hence, in spin
density distribution and predominant conformation. In
such radicals an unpaired electron is localized on only one
aromatic ring.
In this work, the free radical of tris(3,5ꢀdiꢀtertꢀbutylꢀ
4ꢀhydroxybenzyl)amine (ТТА•) was studied by the ESR
method, its isotropic HFC constants were obtained, the
preferential conformations of the phenoxyl fragment relaꢀ
tive to rotation about the Сα—Сβ bond were determined,
and the activation energy of transition between these conꢀ
formations was found.
Experimental
Tris(3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydroxybenzyl)amine (TTA)
is a polynuclear phenol, i.e., such phenols contain more
than one phenol group per molecule. When phenol fragꢀ
ments are separated from each other by an aliphatic chain
of at least three atoms, chemical changes in one phenol
Commercially available initial amine (Chem Service, USA)
was used. Free radical was obtained by the oxidation with lead
dioxide of the initial compound dissolved in toluene in a conꢀ
centration of 1•10–3 mol L–1. The procedure on oxygen removꢀ
al from the samples by freezing—evacuation—thawing out was
carried out three times.
ESR spectra were recorded on an Elexsys E500 3ꢀcm range
radiospectrometer (Bruker, Germany). A ТЕ102 rectangular resꢀ
onator was used in measurements. The choice of detection modes
was determined by requirements of undisturbed recording of the
first derivative of the ESR signal. The measurement error of
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 437—439, March, 2011.
1066ꢀ5285/11/6003ꢀ446 © 2011 Springer Science+Business Media, Inc.

ESR of trishydroxybenzylamine free radical
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 3, March, 2011
447
magnetic parameters mainly depends on errors of the frequency
meter and magnetometer, stability of resonance conditions, and
ESR line width, being 3•10–2 G for HFC constants and 1•10–4
for gꢀfactors.
The temperature variator from the package of the spectrometer
was used to detect the temperature dependence of the spectra.
Magnetic resonance parameters and relative intensities of
the spectral components were determined by computer simulaꢀ
tion of the experimental ESR spectra using the WinSim simulaꢀ
tion program, which makes it possible to find the main paraꢀ
meters of the isotropic spectrum by automatic fitting.
tions, then А1 ≠ А2 and the HFC constants (a) are not
equal. Four lines of equal intensity are seen. If А1 = А2,
i.e., the both βꢀprotons are equivalent, three lines with the
intensity ratio 1 : 2 : 1 are observed. The a values for
predominant conformers can be estimated from the ESR
spectra using the equation
аН( ) = bсos2θ,
(1)
β
where parameter b = 23.3 G for the βꢀprotons in the paraꢀ
position with respect to the phenoxyl group.9
Let us particularly consider the spectrum at 210 K
(Fig. 2). It is a doublet of doublets of quintets caused by
the interaction of an unpaired electron with two nonequivꢀ
Results and Discussion
The experimental ESR spectra of the free radical ТТА•
in the temperature range from 170 to 350 K in toluene are
shown in Fig. 1, a. In the rigid matrix below the freezing
point of neat toluene (178 K) there are (Fig. 1, b) four
broad lines, and two medium lines of them gradually broadꢀ
en with temperature (170—250 K) and, hence, decrease in
intensity and are brought together (see Fig. 1, a). One
averaged broad line appears instead of them at 250 K.
Such a character of the spectra is explained by the
existence of energetically equivalent conformers 1 and 2
between which conformational transitions occur. In this
case, angle θ is the angle between the symmetry axis of the
2pzꢀorbital on the Сα atoms and the direction of the
Сα—Нβ bond. When βꢀprotons are in nonequivalent posiꢀ
alent βꢀprotons with the HFC constants аН(β ) = 18.9 G
1
and аН(β ) = 5.8 G, two protons of the phenoxyl ring in the
2
metaꢀposition to the phenoxyl group, and the 14N nitroꢀ
gen nucleus near the Cβ atom with the constants аН(m)
= 1.5 G and аN = 1.4 G, respectively.
=
Further we will be interested in the interaction with
the βꢀprotons. This is the difference in HFC constants
with βꢀprotons which is due to the existence of various
conformers and transitions between them.
Transitions between two conformers result in the situꢀ
ation when the βꢀprotons transfer from one position to
another and thus cause modulation in the counterphase of
hyperfine splitting, due to which ESR line widths alterꢀ
nate. These transitions become intense with temperature
and their frequency becomes comparable with the differꢀ
ence in the HFC constants of the βꢀprotons. Two central
quintets (marked by dotted brackets in Fig. 2) broaden,
decrease in intensity, and are brought together until join
into one averaged line at 250 K, which increases in intenꢀ
sity with temperature, i.e., with an increase in the freꢀ
quency of the transition between the conformers, and is
resolved to a quintet at 360 K.
a
g = 2.0083
350 K
330 K
310 K
290 K
270 K
250 K
230 K
210 K
1
2
190 K
170 K
3340
3350
3360
3370
H/G
b
170 K
3340
3350
3360
3370
H/G
3340
3350
3360
3370
H/G
Fig. 2. ESR spectrum of the ТТА• radical at 210 K (1) and
simulation of this spectrum (2). Internal quintets of the spectrum
are marked by dotted brackets.
Fig. 1. Temperature dependence of the ESR spectra of the ТTА•
radical (a) and its ESR spectrum at 170 K (b).

448
Russ.Chem.Bull., Int.Ed., Vol. 60, No. 3, March, 2011
Nizameev et al.
ln(√2I 1/I0 – 1)
Let us consider the ESR spectrum of the free radical
ТTА• at 350 K (see Fig. 1). The acceleration of rotation
about the Сα—Сβ axis results in averaging of the splitting
constants: аН(β ) = 12.2 G and аН(β ) = 10.5 G. The aforeꢀ
2
mentioned cen1tral quintets are brought together still more
and join with the further temperature increase to form
a triplet of quintets.
–0.25
–0.50
–0.75
The shift can be determined from the ESR lines that
are not overlapped in the initial spectrum. The shift is
related to the lifetime τ (or the correlation time of the
process) between the conformational transitions by the
equation11
(ΔH02 – ΔHe
)
2 1/2 = 21/2/(γ τ),
(2)
e
3.00
2.95
2.90
T–1•103/K–1
where ΔΗ0 is the distance between the lines in the absence
of mutual transformations а and b, and ΔHе is the distance
between the lines in the case of mutual transformations.
To fulfill Eq. (2), it is necessary that the individual line
width of ESR lines δН would be much shorter than the
distance between the lines. Presenting the temperature deꢀ
pendence of τ in the form
Fig. 3. Arrhenius curve for the ТТА• radical.
oxybenzyl)amine at low temperatures. The temperature
dependence of the ESR spectra made it possible to calculate
the correlation time of transition between these conformꢀ
ers and to estimate the activation energy of this process.
τ = Аexp[Ea/(RT)],
(3)
References
one can construct the plot of lnτ vs 1/T. This plot is
a straight line with the slope Еа/R, i.e., the Arrhenius line
(Fig. 3), whose slope makes it possible to determine the
activation energy of transitions between conformers 1 and
2 for the free radical ТТА•. The activation energy is equal
1. E. P. Talzi, Soros. Obraz. Zh. [Soros Educational Journal],
2000, 6, 35 (in Russian).
2. G. A. Kovtun, G. F. Pustarnakova, Kataliz i Neftekhimiya
[Catalysis in Petrochemistry], 2001, 24 (in Russian).
3. V. E. Tarban´ko, S. A. Varganov, D. V. Petukhov, P. V.
Avramov, N. M. Ivanchenko, Khimiya Rastitel´nogo Syr´ya
[Chemistry of Plant Raw Materials], 1998, No. 3, 99 (in Russian).
4. V. D. Pokhodenko, V. A. Khizhnyi, V. A. Bidzilya, Usp.
Khim., 1968, 37, 998 [Russ. Chem. Rev. (Engl. Transl.),
1968, 37].
to 8.2 kcal mol–1
.
The correlation time of rotation for the case where the
atoms Нβ transfer from one position to another and thus
cause the alternation effect of the ESR line width can be
presented as follows11
:
5. I. V. Khudyakov, V. A. Kuz´min, Usp. Khim., 1975, 44, 1748
[Russ. Chem. Rev. (Engl. Transl.), 1968, 37].
,
(4)
6. I. A. Shkrob, M. C. Depew, J. K. S. Wan, Res. Chem. Interꢀ
mediates, 1992, 17, 271.
7. Ya. A. Gurvich, I. G. Arzamanova, G. E. Zaikov, Khim. Fiz.
[Chemical Physics], 1996, 15, 23 (in Russian).
8. E. A. Antonova, D. V. Paramonov, Vestn. Mosk. Gos. Univ.,
Ser. 2. Khim., 1999, 40, 283 [Vestn Mosk. Univ., Ser. Khim.
(Engl. Transl.), 1999, 40, No. 4].
9. V. A. Roginskii, Fenol´nye antioksidanty: reaktsionnaya
sposobnost´ i effektivnost´ [Phenol Antioxidants: Reactivity and
Efficiency], Nauka, Moscow, 1988, 247 (in Russian).
10. A. L. Buchachenko, Izv. Akad. Nauk SSSR, Ser. Khim., 1963,
1120 [Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.),
1963, 32].
where I 1 and I0 are the intensities of ultimate nonꢀoverꢀ
lapped lines of multiplets corresponding to the projections
–1
of the nuclear magnetic moment mI(CH2) = 1.0; T2,0
is the contribution to the line widths from other relaxation
mechanisms independent of mI(CH2); and γe is the gyroꢀ
magnetic ratio of an electron.
The correlation time τ can be calculated for radical
ТТА•, since the constants а1(Нβ) and а2(Нβ) are discernꢀ
ible at low temperatures. For temperature 210 K we obtain
by formula (4) that τ ≈ 8.2•10–10 s (2I 1 = 111.4, I0 = 97.1,
T2,0–1 = 1.91•106 Hz).
11. H. S. Gutowsky, C. H. Holm, J. Chem. Phys., 1956, 1228.
The values of conformational angles for the radical
under study can be calculated by formula (1): θ1 = 26° and
θ2 = 60°, when b = 23.3 G,9 аН(
= 18.9 G, and
β
)
1
аН(β ) = 5.8 G.
1
Thus, particular conformers were detected by the ESR
Received February 19, 2010;
spectra of the free radical tris(3,5ꢀdiꢀtertꢀbutylꢀ4ꢀhydrꢀ
in revised form October 21, 2010