mination of rate constants. A test to decide if this correlation
is a result of a statistical artefact or not, is to compare the
PIA 6633), UBA (EX010, EX025 and TW10) and Volkswagen
Foundation (Germany).
slope of the graph E vs. *SE (which has dimensions of
a
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95% conÐdence level, this value covers the range 321È375 K.
The average over the kinetic data S1/T T~1 \ 306 K is outside
this range, thus the correlation shown in Fig. 6 can be con-
sidered as relevant.
1
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2
3
4
In ethÈhex mixtures, E and E have values that are char-
a1
a2
acteristic of nonpolar and polar environments, respectively,
and these values do not change, within our experimental pre-
cision, over the whole composition range. The x \ 0.02
eth
mixture already shows a value of E that corresponds to a
a
polar environment. The values of *SE follow the com-
pensation with E observed for acnÈtol mixture (Fig. 6).
5
6
a
In solution, the kinetic parameters E and *SE are the
a
result of three contributions. The Ðrst one is its intrinsic value,
corresponding to the reaction in the gas phase [the values of
*SE in this case can be estimated from the values in nonpolar
solvents: [44 J K~1 mol~1 in toluene (this work); [39 J
K~1 mol~1 (ref. 12)]. The other two contributions are related
to the polar solvent response when the less polar transition
state is generated. The second contribution arises from the
polarity of the medium and from solventÈsolute dipolar orien-
tational correlations; it involves mainly fast molecular reori-
7
entations and produces an increase of E with solvent polarity.
a
This contribution is also present in *SE, causing an increase
in its value as the more polar solvent will be more organized
around the polar species MC. The third contribution, only
possible in solvent mixtures, is due to the relaxation of the
preferential solvation shell around the highly polar MC mol-
ecule and involves a di†usional exchange of solvent species;
this is a slower process.17 This e†ect will cause an increase in
8
9
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both E and *SE in addition to the polarity e†ect, reÑecting
a
the change of solvent energy and local concentration in the
vicinity of the substrate molecule. It should be borne in mind
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From the comparison of E and *SE for neat solvents and
a
solvent mixtures it can be concluded that the third e†ect has
14 R. A. Moore, J. Lee and G. W. Robinson, J. Phys. Chem., 1985,
no signiÐcant contribution to the Ðnal values of these activa-
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values of the lifetime of the transition state compared to the
characteristic time for di†usion and mixing of the preferential
solvation shell and the bulk mixture.17
89, 3648.
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18 D. A. Fernandez and G. P. Peltzer, Electronic Instrumentation
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From the present results, the contribution of the polar
environment on the values of *SE can be estimated to be as
high as 100 J K~1 mol~1. This increase is compensated by the
increase in E to yield a weak dependence of *GE on polarity.
a
Nevertheless, if these two big e†ects in E and *SE do not
a
exactly cancel, some deviations may arise in the behaviour of
*GE, as can be seen for the acnÈtol solutions in the x
0.29 range.
\
acn
Finally, it is inescapable to assume hydrogen bonding by
ethanol plays an important role in the contrasting kinetic
behaviour of mixtures containing acetonitrile and ethanol.
Hydrogen bonding could be responsible for the stabilization
of, at least, one isomeric species of MC with its concomitant
e†ect in the kinetics of thermal decay. Surprisingly, the activa-
tion parameters associated with the fast process closely resem-
ble those obtained for dilute acnÈtol mixtures.
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23 C. J. Drummond and D. N. Furlong, J. Chem. Soc., Faraday
Acknowledgements
T rans., 1990, 86, 3613.
RFP and PFA are members of Carrera del Investigador
Cient•Ðco from Consejo Nacional de Investigaciones
Cient•Ðcas y Tecnicas (CONICET, Argentina). The work was
performed under Ðnancial support from CONICET (PIP4208,
24 R. R. Krug, W. G. Hunter and R. A. Grieger, J. Phys. Chem.,
1976, 80, 2335.
Paper 9/05273I
Phys. Chem. Chem. Phys., 1999, 1, 4955È4959
4959