1
24
KISELEV
It can be suggested that in the common DAR van
compared with the solute–solvent in solution is the
main reason for the reduction in the value of an appar-
ent molar volume in solution [24] and an exothermic
heat of C60 dissolution [23].
der Waals solute–solvent interactions, the different ac-
cessibility of the solvent to the steric branched struc-
tures can be the additional reason for the irregularity
in the changes of the solvation energy and the partial
molar volume. It is necessary to note that for differ-
ent planar structures of dienes and dienophiles (except
tetracyanoethylene), with the surfaces accessible for
solvation, usually weak variety of the solvent effects
on the values of partial molar volumes were observed
For the reason of all of these considerations, it can
be proposed that unexpected negative values of the ac-
tivation volume of the retro isopolar DARs and abnor-
ꢁ
mal volume ratio ꢀV /ꢀV > 1 for forward isopo-
lar DARs can be caused by the different capability
of the solvent molecules to penetrate into the large
steric branched structures of the transition states and
adducts.
[
24,30]. On the other hand, the large differences of
the partial molar volumes for the branched molec-
ular structure of adducts were detected [6–8,31,32].
Thus, the partial molar volumes of the adduct formed
from maleic anhydride with trans-1-methoxybutadiene
are as follows: 144.6 in nitromethane; 137.0 in ace-
tonitrile; 142.6 in 1.2-dichloroethane; 134.7 in 1-
I am grateful to Dr. Helena Kashaeva and Alexander Bolotov
for carrying out some measurements. I also appreciate fruitful
comments from the reviewers, which helped in Þnalizing this
paper.
3
−1
chlorobutane, and 139.7 cm mol in dimethyl car-
bonate [31]. Large solvent effects on the partial
molar volumes of adduct 7 formed from tetracya-
noethylene and cyclopentadiene were noted as well
in benzene, 159.6; toluene, 158.3; o-xylene, 160.2;
mesitylene, 159.4; chlorobenzene, 159.6; acetonitrile,
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2
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1
1
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3
−1
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7
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3
−1
umes of cycloadducts, up to 10 cm mol , which are
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4
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ꢁ=
ꢁ
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1
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−1
1
1
1
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3
−1
fullerene C60 (429 cm mol ) is larger than in all
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mol ) had been interpreted in terms of the “steric”
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3
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International Journal of Chemical Kinetics DOI 10.1002/kin