C-O Bond in Diphenyl Ether and Phenyl Vinyl Ether
J. Phys. Chem. A, Vol. 101, No. 30, 1997 5411
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C6H5-C3H5 f C6H5 + •C3H5
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•
C6H5CH2-C2H3 f C6H5CH2 + •C2H3
In general, the DFT calculated entropy changes (and hence heat
capacities) for the dissociation reactions (see Table 4) are more
consistent with other experimental sources. Hence, the vibra-
tional molecular models of these compounds are well-character-
ized.
Conclusions
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The experimental data show that the two carbon-oxygen
bond strengths in PVE are only slightly different at 298 K. This
originates from a compensation effect since, in terms of X-H,
the C-H bond in ethene is 2.5 kcal mol-1 stronger relative to
benzene, while the O-H bond in vinyl alcohol is 2.9 kcal mol-1
weaker than in phenol. The phenoxyl-vinyl bond (in PVE) is
2.8 kcal mol-1 weaker than the phenoxyl-phenyl bond (in DPE)
at 298 K.
The experimental and computed C-O and O-H bond
strengths appear to be not identical. Density functional theory
at B3LYP/6-31G(d) or higher level underestimates these dis-
sociation energies by ca. 5 kcal mol-1. A further optimization
of the DFT method is required to ensure proper treatment of
bonds involving the element oxygen. Computation of the BDE-
(C-H) reveals a quite smaller deviation from experimental data.
This study shows the synergism between experimental and
computational methods. Thermal techniques, such as VLPP,
require accurate vibrational frequencies to correct for the falloff
behavior and reliable heat capacity data to extrapolate to 298
K; both can be obtained by DFT calculations.
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Acknowledgment. We thank the Hochschulrechenzentrum
der Universita¨t Essen for generous allotment of computation
time and Willi Sicking, Guido Hulstman, and Louw van As for
technical assistance.
(21) For the RRKM calculation B3LYP frequencies were adjusted by a
factor of 0.97 (see refs 12 and 22). The Gaussian output revealed that low
frequencies might be due to hindered internal rotation. PVE: 19.02, 161.5,
425.05 cm-1; DPE: 22.6, 90.2, 422.6, 426.6, 489.2 cm-1. This may produce
slight errors in heat capacity calculations (see ref 10).
Supporting Information Available: Tables of applied
RRKM parameters and optimized geometries and vibrational
frequencies for DPE and PVE (2 pages). Ordering information
is given on any current masthead page.
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