avoided. In the liquid phase, intersystem crossing cannot
compete with vibrational relaxation, and CHÈH bond Ðssion
follows vibrational relaxation. This predicts that the disso-
ciation rate is limited by the decay rate of thermally equili-
plane is 3.41 A, and the binding energy is 1 680 cm~1 in the S
0
state. For HET, the H atom of the OH group may be p
hydrogen-bonded to the p cloud of the benzene system. The
distance between the H atom of the OH group and the C
atom of the benzene ring adjacent to the CH(OH) group is
estimated to be 2.35 A from the following values: C H ÈC
brated levels of the S state. Dissociation actually occurs from
1
vibrationally-excited levels of the T state, and the low disso-
1
6 4
ciation yield (D10~3) is expected by competition with vibra-
(1.52 A),35 CÈO (1.43 A),36,37 OÈH (0.94 A),36 CÈCÈO
tional relaxation (D1011 s~1 rate) in the T state.
(111¡),37 CÈOÈH (110¡).36
1
The dissociation mechanism is similar to that of the CÈX
By intersystem crossing from the S state (l \ 36 400
1
0
bonds (X \ Cl, Br) for 1- and 2-(halomethyl)naphthalene
cm~1) to the T state (l \ 28 900 cm~1), ET gains an excess
1
0
excited to the S states at 266 and 299 nm in the liquid
vibrational energy (7 500 cm~1) corresponding to the energy
gap between the S and T states. If the excess energy is dis-
2
phase.29,30 For these molecules, dissociation takes place by
1
1
intersystem crossing to upper triplet states which are them-
selves, or cross to, the rr* dissociative triplet states, and is
not a†ected by the amounts of excess vibrational energies
(4 900 and 700 cm~1).
tributed over the 60 vibrational modes including the CHÈH
bond stretch mode, vibrational excitation is characterized by a
high vibrational temperature (D580 K). If a larger part of the
excess energy is distributed to the CHÈH bond stretch mode,
or if the excess energy is relaxed among the vibrational
quanta, vibrational excitation of the 59 modes other than the
CHÈH bond stretch mode is equivalent to a somewhat lower
vibrational temperature. Since internal rotation is nearly free
about the C H ÈCH CH bond, the conformation where the
For dissociation of HET, avoided crossing takes place
between the pp*(benzene) (S , AA) and np(O)r*(CÈO) (AA)
1
conÐgurations at the geometry of the stretched CHÈOH bond.
It provides the AA adiabatic potential energy surface, which
has pp*(benzene) character in the FranckÈCondon region but
evolves to np(O)r*(CÈO) character beyond the barrier along
the CHÈOH bond Ðssion coordinate. Rapid CHÈOH bond
Ðssion occurs directly on the AA adiabatic potential energy
surface from the S state of the precursor to the ground (2B
6
4
2
3
CHÈH bond lies in the symmetry (C ) plane perpendicular to
the benzene ring is as stable as the other conformations. There
s
will exist a probability that the CHÈH bond lies in the sym-
metry (C ) plane for the T state.
1
2
s
1
] 2%) state of the asymptotic products. This accounts for the
In order to cross the barrier to CHÈH bond Ðssion, the
dissociation pathway that occurs from vibrationally excited
molecule must undergo intersystem crossing to the T state.12
1
levels of the S state. The low dissociation yield (D10~3) is
The electronic wave function must change subsequently from
1
explained by competition with vibrational relaxation (D1011
the pp*(benzene) (T ) to the rr*(CÈH) conÐguration. Also, the
1
s~1 rate) in the S state. There will be a higher probability for
adiabatic potential energy surfaces must be split at the barrier
1
CHÈOH bond Ðssion, if the precursor possesses a large vibra-
to CHÈH bond Ðssion. When the CHÈH bond lies in the sym-
tional energy, i.e., if it is pumped into highly vibrationally
metry (C ) plane perpendicular to the benzene ring, the two
conÐgurations are symmetric relative to this plane. Splitting is
represented, by conÐguration mixing, with the exchange inter-
action matrix element of the form:
s
excited levels of the S state.
1
4.3. Molecular geometry
K \ Sp*
(2)r (1)(e2/r )p
(1)r* (2)T.
(3)
The photon energy is absorbed initially by the benzene
system, and electronic excitation is localized on it. The ener-
getically available ground state of the product radical has no
pp*(benzene) character. Hence, considerable electronic
rearrangement must be involved in dissociation. CHÈH and
CHÈOH bond Ðssion is possible, if an electronic change
occurs in the respective bonds during dissociation. This may
be enabled in the geometry which leads to potential coupling
to the rr*(CÈH) and np(O)r*(CÈO) conÐgurations.
benzene
ChH 12 benzene
ChH
The overlap integral is ignored. The p
orbitals are made up of six p atomic orbitals of the benzene
and p*
benzene
benzene
Ci
system. The p atomic orbital of the benzene system adjacent
Ca
to the CH group possesses a considerable electron density
2
p2 . The p orbital is at a short distance from the r
and
Ca
Ca
ChH
and p r*
Ca ChH
with them. The exchange interaction matrix element may arise
r* orbitals, and has overlap densities p r
ChH Ca ChH
from these overlap densities. The r
and r* orbitals are in
ChH
ChH
The geometry is described by internal rotations about the
C H ÈCH CH , C H ÈCH(OH)CH and CHÈOH bonds
the direction of the CHÈH bond, and the overlap densities
and exchange interaction matrix element depend on the orien-
tation of this bond. When the CHÈH bond lies in the sym-
6
4
2
3
6
4
3
and by p hydrogen-bonding between the benzene system and
OH group. These interactions are assumed from those of
related molecules.
metry (C ) plane, i.e., when the r
and r* orbitals are in
s
ChH ChH
the same plane as the p orbital, the exchange interaction
Ca
(1) For p-Ñuorotoluene, the CH group undergoes nearly
matrix element will have a maximum. This will allow potential
3
free internal rotation about the C H ÈCH bond.31 The
coupling between the pp*(benzene) and rr*(CÈH) conÐgu-
rations, and will permit the electronic wave function to change
to the rr*(CÈH) conÐguration, thereby providing the prob-
ability of CHÈH bond Ðssion.
6
4
3
barriers to internal rotation are 4.8 cm~1 in the S state
0
and 33.7 cm~1 in the S state. For ET and HET, internal
1
rotation may be nearly free about the C H ÈCH CH and
6
4
2
3
C H ÈCH(OH)CH bonds, respectively.
(2) For ethanol, the OH group experiences highly hindered
After excitation to the S state (l \ 36 600 cm~1) with a
6
4
3
1
0
266 nm photon (37 600 cm~1), HET possesses an excess vibra-
tional energy (1 000 cm~1). Part of the excess energy may be
distributed to local excitation of the CHÈOH bond stretch
mode. If the excess energy is distributed over the 60 vibra-
tional modes including the CHÈOH bond stretch mode, vibra-
tional excitation corresponds to a low vibrational temperature
(D350 K). Since internal rotation is nearly free about the
C H ÈCH(OH)CH bond, the conformation where the
internal rotation about the CH ÈOH bond.32 The conforma-
2
tion is least stable when the H atom of the OH group is
eclipsed by the CH group. The energy di†erences are 408.7
3
cm~1 between the eclipsed and gauche conformations and
452.0 cm~1 between the eclipsed and trans conformations in
the S state. For HET, highly hindered OH internal rotation
0
may occur about the CHÈOH bond.
6
4
3
(3) For the benzeneÈmethanol cluster, the p hydrogen-bond
is formed when the H atom of the OH group is donated to the
delocalized p cloud of the benzene system.33,34 The methanol
molecule is pulled o† the six-fold axis of the benzene ring, and
the H atom of the OH group is nearly on the C atom of the
benzene plane. The distance from the O atom to the benzene
CHÈOH bond lies in the symmetry (C ) plane perpendicular
to the benzene ring is as stable as the other conformations.
Because internal rotation is highly hindered about the
CHÈOH bond, the trans conformation is more stable and the
eclipsed conformation is less stable between the benzene ring
and OH group. However, the eclipsed conformation may be
s
Phys. Chem. Chem. Phys., 2000, 2, 3791È3796
3795