S. Yamaguchi et al.rChemical Physics Letters 286 (1998) 284–290
289
partial molar volume of CO was measured. The
decomposition reaction of a diazo compound is a
useful way to produce N2 in solution. Diffusion
y9
2
constant of N ŽD s5.3=10 m rs. is close to
2
N
what is expected from literature values in CCl
4
y9
2
Ž3.4=10
m rs w27x. with a correction of the
y9
2
solution viscosity Ž5.5=10 m rs..
The D H of ether formation is similar for DPDM
and diazo-2 but slightly larger for diazo-3. The
difference may be explained in terms of the geome-
tries of these compounds. As shown later in this
section, the optimum orientations of the phenyl
groups are different between the diazo and ether
forms if they can freely move. The configuration of
the phenyl groups can be relaxed with the diazo-ether
transformation for DPDM, but rigidly fixed for di-
azo-2. Diazo-3 is an interesting case. A preliminary
semi-empirical MO calculation shows that the con-
figuration of the phenyl groups determined by the
ethylene groups of diazo-3 is unstable for the diazo
form but is stable for the ether form. Hence, the D H
for ether formation is larger for diazo-3 compared
with the other diazo compounds. A more elevated
calculation will be needed to understand fully vari-
ous D H for these compounds.
It is important to note that the volume changes
depend on the diazo compounds. The volume change
decreases in an order of DPDM)diazo-3)diazo-2,
and this order is the same as the decrease of flexibil-
ity of the phenyl groups. This fact may suggest that
the volume change is induced by rearrangement of
the phenyl groups between the reactant and product.
In fact, if we calculate the Van der Waals volume
change by the method of atom increment w28x, it
predicts a negligibly small change. Therefore, the
volume change cannot be due to change in the
intrinsic molecular volume. Fig. 4 depicts the ener-
getically optimized geometries of DPDM, diphenyl-
methy methyl ether, diazo-2 and ether by the PM3
method of the MOPAC program. Apparently, the
geometries of the phenyl groups are different for
these species. The phenyl groups are closer or com-
pact for DPDM, while they are almost parallel to
each other and widely open for ether. An increase of
the void volume around the phenyl groups could be
the main cause of the relatively large volume in-
crease by the reaction. Naturally, the relative ar-
rangement of the phenyl groups of diazo-2 is fixed
Fig. 4. Optimized geometries of DPDM, diphenylmethyl methyl
ether, diazo-2, and ether from of diazo-2.
and it cannot produce an additional void volume by
the reaction.
In summary, we analyzed in detail the TG signal
observed after photoexcitation of diazo compounds
in methanol and determined the reaction enthalpy,
reaction volume, partial molar volume of nitrogen
molecule, diffusion constant of N , and diffusion
2
constant of diazo compounds. The reaction volume
contributes significantly to the signal and any analy-
sis without this contribution should lead an unrea-
sonable enthalpy change as previously reported. The
large volume change was attributed to rearrangement
of the phenyl moieties induced by the reaction from
the diazo compounds to the ethers.
Acknowledgements
Authors are indebted to Prof. Y. Tanimoto and
Dr. Y. Fujiwara in Hiroshima University for their
kindly providing us with the samples.
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