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atom is now made redundant. The knowledge of this value is only
necessary if we aim to determine the bond dissociation enthalpy in
solution (eqns. (7) and (8)), since it cancels out when eqn. (9) is
used to derive the gas-phase value.
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(a) R. M. Borges dos Santos and J. A. Martinho Simo˜es, J. Phys.
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´
171; ( j) M. Jonsson, J. Lind and G. Merenyi, J. Phys. Chem. A,
33 There is another photoacoustic study in acetonitrile which leads to
DHosln(PhO–H) ¼ 402.3 kJ molꢀ1 (no uncertainty provided, see
refs. 2(a) and 5). Using this result and DH(ECW) from Table 2,
together with DslnHo (Hꢃ, g) ¼ 5 ꢁ 1 kJ molꢀ1, we obtain from
eqn. (9) DHo(PhO–H) ¼ 378.6 kJ molꢀ1, which is 14 kJ molꢀ1
higher than the value derived from the result in Table 2, but in
better agreement with the gas-phase (371.3 ꢁ 2.3 kJ molꢀ1) and
the high-level computational (376.1 kJ molꢀ1) results. However,
this agreement may be fortuitous. In ref. 5 Wayner et al. have used
non-time resolved PAC to probe the energetics of the net reaction
(6), despite the rate constant of reaction (5) in acetonitrile being
estimated as at least ca. 30 times slower than in benzene. The ratio
between the rate constants of hydrogen abstraction from phenol
by cumyloxy radicals in benzene and in acetonitrile is even larger,
48.3 ( D. W. Snelgrove, J. Lusztyk, J. T. Banks, P. Mulder and
K. U. Ingold, J. Am. Chem. Soc., 2001, 123, 469). A ratio equal
to 30 is estimated using an empirical equation given by the same
authors parameters (quoted from the paper by Snelgrove et al.
and from M. H. Abraham, P. L. Grellier, D. V. Prior, J. J. Morris
and P. J. Taylor, J. Chem Soc., Perkin Trans. 2, 1990, 521).
Accepting the lower ratio, and using the experimental value for
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5
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6
7
8
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R. M. Borges dos Santos, V. S. F. Muralha, C. F. Correia, R. C.
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reaction (5) in benzene, 3.3 ꢆ 108 Mꢀ1
s
ꢀ1(Landolt-Bo¨rnstein,
ed. J. A. Howard, J. C. Scaiano and H. Ficher, Springer-Verlag,
New York, 1984, New Series II/B): the rate constant of the same
reaction in acetonitrile is 1.1 ꢆ 107 Mꢀ1
s
ꢀ1. To further check
these data, we have performed TR-PAC experiments with several
phenol concentrations. Preliminary results obtained by plotting 1/
t2 vs. phenol concentration led to 4.3 ꢆ 108 and 1.0 ꢆ 107 Mꢀ1 sꢀ1
for the rate constants of reaction (5) in benzene and in acetonitrile,
confirming that reaction (5) is significantly slower in acetonitrile.
Although Wayner et al. (ref. 5) do not provide details on the phe-
nol concentrations, they have stated that these were selected to
ensure that the rate of reaction (5) would fit the limitations of
the non-time-resolved technique. Nevertheless these may have
not been met in their experiments in acetonitrile, in which case
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DrH an upper limit, implying that DHosln(PhO–H) ¼ 402.3 kJ
molꢀ1 is also an upper limit (eqn. (7)).
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