:. Volume 29,hmber
2
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LETTE.83
15 November
1974,
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CHEMICiL‘PHYSLCS
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: M~~~~IC'RE~~NANCE~%UDIES~FHYDP,~~GEN~T~~~T~RAN~FER ‘.
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E.B. ZAVELOVICH and A.I. PROKOF’E;V -’ Imtirureof Organo-Element Compounds, Acadetiy of Sciences of the VSgR,
MO’SCOW,
USSR
Received 28 June 1974 Magnetic resonance has been used to study the kinetics of hydrogen atom transfer between hydroquinone and t!xir’w~esponding oxy Gee radical. The rate constant for this process wa’!;determined and the hyperfine interaction
wnsknt
of rbutyl protons was ah&ted.
1.Introduction and theory,
called the “weak pulse” or rapid exchange case, formi:la (1 j can be transformed into
cornmody
‘.
‘. It is now possible to &dy kinetics of chemical. reactions in equilibrium systems with ,tie help of ‘. magnetic resonance techniques. It is well known that’in solutions containing diamagnetic molecules and the-corresponding free radicals hydrogen exchange may occur between these two species 111. If a hydrogen is transferred from a diamagnetic to a,paramagnetic molecule the nuclear magnetic resonance spectrum’ of the first is affected by the paramagnetic pulses generated by the unpaired electron. The theoretical background .for extracting information about reaction raies and the spin density distribution in radicals from the he
The
shapes is highly developed-[24]
= (7g/7jJ)Q2/4 ‘-
C1lr,)e,
(3)
In the first “strong pulse” case the electrsn spin resonance spectrum of the paramagnetic molecules exhibits hyperfine splitting from the group of nuclei whose line aGdth is being measured. This is an ex’ pertiental criterion for applicability of formula (2). The rate constant of the hydrogen transfer K and the lifetimes 7p and rD.are related by K[DD] =r~’
and
K[P] = +
.
(4)
In these correlations [D] and Ip] are the concentiations
_
contributionof suchanexchangeprocessto
of Ae
d&magnetic
and pnrarnagnetic species,
respectively.
the line width iri the-moregeneralcaseis givenby [3] II
1
t g ) ex =yD:l
(Q7p/2j2 + (afp/2)2
(!I
’
where TD is the lifetime of the diamagnetic, state, ip is the lifetime of the paramagnetic state and Q is the hyperfine int&action constant for the group of nti$ei whose line width is being measured. When a$2 is much greater than 1, the socalled “stro@ p*e”.or slow exchange case, formula (i) .’ -. : redu& to (l/T&=
l/TD .
‘.
(2)’
,,
2. Experimental ESR mezwrements were made on a Varian E-12A spectromete:r with temperature controller. NMR spectra were @en on a SoMet NMR-2305 spectrometer (operating at 60 MHz). HMDS was used as the inteinal standard. Line broadenings were obtained by the conventional method [ 1J.
_
3. R&&s&d
disc&ion
.‘-
:.. $5 inv+igated
IE ah&her case, when u.rp/2 is much less than i, ., ,.: .::. .. $I*,-, ,: :.. ., ._
‘,.:
amixtuie .’
_. -.
“,
‘Y
.-
of 3,6-di-tert-butyl-
VoJumc 29, number 2
CHEMICA$
quinone (quinone) and 3,6di-tert-butylpyrocatechol ,(hydroquinone) in Ccl,. Radicals in this system result from the reaction between quit-tone and hydroquinonei
m
@I+
@‘:;_&2@li
15 November
PHYSICS LETTERS
(5).
It is also possiPle for hydrogen exchange to occur between the quinone radical and the hydroquinone
!974
a dia*gnetic
and paramagnetic; state. The same mechanism was suggested for the radical-radical dimerization ‘rea.ctiofl [S]. Reactions (6) and (7) also go through the interrnediatk paramagnktic coniplex. This’compIex breaks up into exchanged or nonexchan’ged products with ‘equal probabiljtjei. It is of interest that reaction (6) is the hydrogen transfer from radical to iiqagnetic, whereas reaction (7) is the reverse process. ESR spectra of dilute solutions of our mixture exhibit hyperfine.structure from ring and hydroxyl protons; on the contrary, the interaktion of the unpaired election with protons of t-butyl groups is
change limits take place in our system. NMR spectra at various temperatures
are presented
In fact reaction (5) goes through an intermediate short-lived radical pair, which recombines to form either the starting or the rearranged molecules; or the radicals diffuse apart.
Thus the process
-
7-p
interchanges the magtietic,knv&onment of the nuclei between two different diamagnetic states 0; between :
. .
.. :’ .‘. :
Fig. l-The
: :
I
6.5
6.0
I---
I
5.5
,.
NMR speckat
_’
a
1.5
L 1.0
ppm
various temperatures.
213
Volume 29,‘number
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CHE.~IICALPHYSICS_LETTERS
2
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tributron
.,
l~_~Nbvcmber1?74.
..’
of the pulse to this transient~complex
‘:
;
.,,
is
much 1es.sthan that of the exchange reaction itself 111, :,. 1 ‘,.. ,’ The rate constarnof complex for-&ationK1 and .‘tiie rate constant of hydr0ge.n exchange K are related
d&to the fact that the complex is symmetrical, and both the hydroxyl protons are ecjui+alent. ,. Thus, the rate constant K, can be determined ‘from the hydroxyl line broadening by means’of
(!&)$~VQ.
= (Kl/4jp]
,,’
‘:’
.’
-0
I .. is the radical concentration.
m]
: tqmperatuk rises, but the increase of the concentration of the radical .., takes place &ch.faste; than theIrate ‘donstant of hydrogen e&hang& .‘:
&various
.., ..
..
‘. : ... .; i 2.6
2.8
3.0
3.4 :
3.2 l/T
.’
xl+
I
I
3.6
3.8
‘--
~e~n33;~e~fF’~~ log’of ’ _rate constant K1 versus reciprocal .. ,, ‘_...~___.___‘__.-.“O kul/niolr; -, .’
.
,\:., .,
2.’
:. ;:,
.::.
3-
temperatures the ‘concentration [R] was.
.. : : : ‘L’l& :_ ‘_. :,.:, I ‘_ .. . ,:,,” ,_ ,:. ‘.
54’.
The line .. :-width of the hydroxyl.&ons ,mcreases with ternperature since the rate constant Kr .and concentrati& [RI i&re&. According to (3) the line width of :-butyl protons should become narrower when. the -. where
‘.
7-
01)
‘.
0
lo-709-:, .g -8‘.
: : :
,’ ‘....
.. .‘.,_ .:: ‘.
._,’
.., : ,I.
..
.,
-
Volume 29, number 2
,.
prauiyl GO.01.G.
.:.
A plot show& the-change of the log of the rate constant K1 versus reciprocal temperature isgiven in fig. 3. The dependence shown in this.figure can be. . appro,xirnated by the following expression ‘K, = 10’“~g
exp(-2.SO/RT)
mole-!
s-r .
(12)
By means of the signal broadening cf the line of the t-butyl protons one can calculate the hype&me inter: action constant for this group of nuclei by resorting to formulas (3) and (12): This constant, Q,&~~, equals
1974
0.009~ G, which agrees witI; the ESR data, when ~. .’
deterrked by measuring the areas of the ESRlines. .A plot of the, log of the equilibrium constant of reaction (5) iY,.vefsus reciprocai temperature is given in tig.2.
.15 November
CHEhflCAL I+lY’sICS LEiTERS
ReferenceS ,- ”
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.:’
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-. [I] R.W. Kreilick and S.I:Weissnsn,
_‘,.
J. Am.Chem.
Sic. 8~ (1966) 2645. [2] I-M. McConnell and S.B: Bergcr, J. Chem. Phys. 27 .(1957) 230. [3] ,C.S. Jonfison Jr., J. Chem. Ph&. 39 (1963) 211 I_ [4] E: de Boer and C. hIzcLe;?n, 5. Chem. Phys. 44 (l.965) 1334. (51 D.J. Williams and k. Kteitick, I. Am.‘Chem.Soc. 90. (1968) 2775.
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