Effects of intramolecular hydrogen bonding on the
excited state dynamics of phenol chromophores
Yi Lin Yang, Yu-Chieh Ho, Yuri A. Dyakov, Wen-Hsin Hsu, and Chi-Kung Ni*1
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617 Taiwan
Yi-Lun Sun, and Wei-Ping Hu*2
Department of Chemistry and Biochemistry, National Chung Cheng University
Chia-Yi, 621 Taiwan
1.
Also at Department of Chemistry, National Tsing Hua University, Hsinchu,
Taiwan. E-mail address: ckni@po.iams.sinica.edu.tw.
2.
E-mail address: chewph@ccu.edu.tw
1
ABSTRACT
The theoretical prediction and experimental confirmation of the 1* repulsive
excited state along OH bond of phenol have large impact on the interpretation of
phenol and tyrosine photochemistry. In this work, we demonstrated that this excited
state characteristic changes significantly if the OH functional group is involved in the
formation of intramolecular hydrogen bonding on the ground state. We investigated
the excited state dynamics of 2-, 3-, and 4-hydroxyacetophenone (HAP) separately in
a molecular beam at 193 nm using multimass ion imaging techniques. H atom
elimination from the repulsive excited state and the Norrish type I reactions are the
major dissociation channels of 3-HAP and 4-HAP which do not have intramolecular
hydrogen bonding. However, H atom elimination channel is completely quenched for
2-HAP which has intramolecular hydrogen bonding. In addition, the ground state and
the excited state potential energy surfaces (PES) of HAP, 2-hydroxybenzoyl fluoride,
2-hydroxybenzoyl chloride, and 2-hydroxybenzamide were investigated using ab
initio calculations. The results also showed that the excited state potential along OH
bond distance of hydroxyl group changes significantly for the molecules with
intramolecular hydrogen bonding on the ground state. The changes include: (a) the
repulsive potential energy surface becomes an attractive potential near the ground
state equilibrium geometry, (b) the conical intersection between the first and the
second excited states along OH bond moves to a much higher energy level, and (c)
the conical intersection between the repulsive excited state and the ground state along
OH bond distance disappears. The results suggest that the interpretation of the
photochemistry for the molecules with phenol chromophore must take these effects
into consideration.
2
I. Introduction
Recent theoretical calculations1 showed that the first excited singlet state of
phenol (2A' or 1*) is bound with respect to OH bond distance, but the second
excited singlet state (1A'' or 1*) is repulsive. (In this work, A' and A'' denote for
adiabatic potential; * and 1* denote for diabatic potential.) The absorption of
UV photon corresponds to excitation to the bright state, 2A'. However, population of
the 2A' state can be transferred to the dark state 1A'' through a conical intersection. As
a consequence, UV photoexcitation results in the adiabatic dissociation from the
repulsive 1A'' state. Similar potential energy surfaces (PES) were found in indole and
pyrrole along the NH bond distance.1 The theoretical predictions were confirmed by
molecular beam experiments.2-6 The corresponding potentials are illustrated in Figure
1(a). The study has led to significant impact to this research field. For example, it
changed the explanations of PhOH(NH)3 clusters from the excited state proton
transfer to the excited state hydrogen transfer.7-11 It triggered the studies on the
photochemistry of various substituted phenol
12-16
and on the photostability of amino
acid chromophores.17-21 The existence of the conical intersections between various
electronic excited states and the repulsive 1* state opens another possible
deactivation channel for the excited state dynamics of DNA and RNA bases.22-27
Some intramolecular phototautomeric reactions were found to be driven by the
repulsive 1* state.28 Similar repulsive states were also found in N-methylindole,
N-methylpyrrole, and anisole along the CO bond distance.29
In this work, the study included the experimental investigation of
photodissociation dynamics for 2-, 3-, and 4-hydroxyacetophenone (HAP) in a
molecular beam using multimass mass ion imaging techniques, as well as the
3
theoretical calculations of the potential energy surfaces for HAP, 2-hydroxybenzoyl
fluoride, 2-hydroxybenzoyl chloride, and 2-hydroxybenzamide. We demonstrated that
the repulsive state changes significantly for those molecules in which the OH
functional group is involved in intramolecular hydrogen bonding on the ground state.
The typical changes of the potential energy surfaces are illustrated in Figure 1(b). The
repulsive 1A'' state becomes an attractive potential near the equilibrium geometry of
the ground state. The conical intersection between the bound state 2A' and the
repulsive state 1A'' is located at a much higher energy level, and the intersection
between the ground state and the 1A'' along the OH bond distance disappears.
Consequently, the OH bond cleavage on the repulsive state is shutdown completely.
The investigation of these molecules suggested that these changes are general and
they can appear on other phenol chromophores. As a result, the application of the
original repulsive potential characteristics in the interpretation of phenol chromophore
photochemistry must take the effects of intramolecular hydrogen bonding into
consideration.
II. Experiments
The experiments focused on the study of photodissociation of various HAP
isomers under collisionless conditions in order to understand the excited state
dynamics of these molecules. The experimental techniques have been described in
details.30-32 Briefly, molecules in a molecular beam were dissociated using a pulsed
UV laser beam set at 193 nm. The resulting photofragments were ionized with a
pulsed VUV laser beam at 118 nm. Photofragment masses were identified along with
their translational energy distributions using multimass ion imaging techniques.
Photolysis laser fluence in the region of 1~50 mJ/cm2 was used to determine the
4
photolysis photon number dependence of each fragment. Only fragments resulted
from one-photon absorption will be discussed in this work.
III. Calculation methods
The dissociation channels on the ground state were investigated from the
calculations of energies and of transition states and products using B3LYP/6-31G
method.33-34 Energies of these structures then were refined by the G3(MP2,CCSD)
scheme.35-36 All ground-state electronic structure calculations were performed using
the GAUSSIAN 03 package.37
The energies of the ground state geometries for various conformers were
calculated using the B3LYP/6-311+G(d,p) method. The excited-state energy
calculation was performed using the complete active space (CAS) theory with the
6-31+G(d,p) basis set. The active space consists of 10 electrons and 10 orbitals (4,
1, 3*, and 2*) for the phenol molecule, and 16 electrons and 13 orbitals (6, 2,
3*, and 2*) for the HAP molecules. A state-averaged approach with equal
weighting was applied to calculate the four lowest states of A' and A" symmetry
simultaneously. This calculation was carried out using the MOLPRO 2012 program.38
For comparison, the excited potential energy curves were also calculated using the
time-dependent (TD) density functional theory with the B3LYP functional and
6-311+G(d,p) basis set using the GAUSSIAN 09 package. Calculations at TD
B3LYP/6-311+G** level are shown in supplementary material. Oscillator strengths of
the excited-states were also calculated using the EOM-CCSD and CASPT2 methods
with the 6-31+G(d,p) basis set.
IV. Results
A. Photodissociation of hydroxyacetophenone (HAP)
5
2HAP: Photofragment ions m/z=15, 43, 65, 93, and 121 were observed from the
dissociation of 2HAP. Ion images of these fragments are shown in Figure 2.
Fragments m/z=121 and 15 represent the following dissociation channel.
C6H4OHCOCH3 + hv (193 nm)  C6H4OHCO (m=121) + CH3 (m=15)
(1)
Fragments m/z=43 and 93 represent the following dissociation channel.
C6H4OHCOCH3 + hv (193 nm)  C6H4OH (m=93) + COCH3 (m=43)
(2)
Fragment m/z=65 has a disk-like image. The width of the disk changes with delay
time. It represents the dissociative ionization of heavy fragment upon VUV
photoionization, C6H4OH (m=93) + hv (118 nm)  C5H5+ (m/z=65) + CO. The
corresponding translational energy distributions of these channels are shown in Figure
3. These two dissociation channels simply represent Norrish type I reactions
(-carbon-carbon bond cleavage) which usually occurs for carbonyl compounds upon
UV irradiation. We did not find the fragments corresponding to H atom elimination
analogous to that of phenol.
Figure 4 shows the dissociation channels, the energies of transition states,
intermediates, and dissociation energies on the ground state from ab initio calculations.
Among these dissociation channels, CH4 elimination has the lowest dissociation
barrier. However, we did not observe any products related to this channel. On the
other hand, the barrier heights of α-carbon carbon bond cleavage for Norrish type I
reactions are relatively high. They are not expected to occur easily on the ground state.
Calculations based on RRKM theory show that the branching ratios of CH3, COCH3,
and CH4 eliminations on the ground state are 0.05, 0.004, 0.946, respectively. The
branching ratio is very different from our experimental observation. It indicates that
Norrish type I reactions of 2HAP do not occur on the ground state.
6
The possibility of these dissociation channels occurred on the triplet state was
also investigated by ab initio calculations. Figure 5 shows the energies of transition
states, intermediates, and dissociation energies on the first triplet state. CH3
elimination has the lowest dissociation barrier. The barrier height of COCH3 and H
atom eliminations are slightly higher than that of CH3 channel. RRKM calculations
suggest that the branching ratio of CH3 and COCH3 elimination are 0.96 and 0.04,
respectively. It suggests that these channels may occur on triplet state, like most of the
Norrish type I reactions of aldehydes and ketones.
4HAP: Fragments m/z=43, 65, 79, 93, 107, and 135 were observed.
Photofragment ion images are shown in Figure 6. Fragment m/z=135 represents H
atom elimination channel.
C6H4OHCOCH3 + hv (193 nm)  C6H4OCOCH3 (m=135) + H
(3)
The corresponding photofragment translational energy distributions are shown in
Figure 7. It shows a large portion of available energy released in translational energy.
The maximum energy release in translational degrees of freedom is close to the
maximum available energy. The large translational energy release is consistent with
the premise for dissociation from a repulsive excited state. This channel is analogous
to the H atom elimination from the repulsive excited state of phenol.
Ion images of m/z=107, 79, and 65 are disk-like. They represent the cracking of
heavy fragment upon VUV photoionization: C6H4OCOCH3 (m=135) + hv (118 nm)
 C7H7O+ (m/z=107) + CO, C6H4OCOCH3 (m=135) + hv (118 nm)  C6H7+
(m/z=79) + 2CO, C6H4OH (m=93) + hv (118 nm)  C5H5+ (m/z = 65) + CO. The
other fragments m/z=43 and 93 have line shape image, resulting from Norrish type I
reaction (reaction 2). Translational energy distributions of these fragments were
7
shown in Figure 6. Unlike 2HAP, only one Norrish type I reaction was observed for
4HAP.
The dissociation channels, the energies of transition states, intermediates, and
dissociation energies on the ground state are shown in Figure 8. The energy barriers of
α-carbon carbon bond cleavage for Norrish type I reactions are similar to that 2HAP.
The only difference is that there is no low barrier dissociation channel, like CH4
elimination channel in 2HAP. The only Norrish type I reaction observed for 4HAP is
reaction 2, which the barrier is higher than that of reaction 1 on the ground state by
more than 20 kcal/mol. The branching ratios calculated from RRKM theory are 0.91
and 0.09 for CH3 and COCH3 elimination, respectively. It indicates that Norrish type I
reaction does not occur on the ground state. The possibility of these dissociation
channels occurred on the triplet state was also investigated by ab initio calculations.
The potential energies of these dissociation channels on the first triplet state are
illustrated in Figure 9. CH3 and COCH3 elimination channels have the lowest
dissociation barriers. RRKM calculations suggest that the branching ratio of CH3 and
COCH3 elimination are 0.97 and 0.03, respectively. The calculations do not indicate
that the Norrish type I reaction occurs on the triplet state, either. At this moment, we
do not have good explanation why reaction 1 was not observed for 4HAP.
3HAP: Photodissociation properties of 3HAP are very similar to that of 4HAP.
Photofragment ions m/z=15, 43, 65, 79, 93, 107, and 135 were observed from the
dissociation of 3HAP. Photofragment ion images are shown in Figure 10. Fragment
m/z=135 represents H atom elimination channel (reaction 3). The corresponding
photofragment translational energy distribution is illustrated in Figure 11. The large
translational energy released in H atom elimination channel also suggests that H atom
elimination from the repulsive excited state, like phenol, remains in 3HAP.
8
The rest of the fragments represent Norrish type I reaction: However, the
interpretation is not straightforward. CH3 (m=15) of reaction 1 was observed, but the
corresponding heavy fragment partner, C6H4OHCO (m=121), was not observed. On
the other hand, heavy fragment of reaction 2, C6H4OH (m=93), was observed, but the
line-shape image of light fragment partner, COCH3 (m=43), was not found. Only a
disk-like image of m/z=43 was observed. The width of the image m/z=43 does not
change with delay time. It represents the cracking of undissociated excited parent
molecules by VUV photoionization.
C6H4OHCOCH3 + hv (193 nm)  C6H4OHCOCH3*
C6H4OHCOCH3* + hv (118 nm)  C6H4OH (m=93) + COCH3+ (m/z=43)
If fragment C6H4OH (m=93) completely results from secondary dissociation,
C6H4OHCO (m=121)  C6H4OH(m=93) + CO, the momentum distribution in the
center of mass frame of fragments C6H4OH (m=93, but use m=121 in momentum
calculation) and CH3(m=15) must be similar. On the other hand, if fragment CH3
completely comes from the secondary dissociation, CH3CO  CH3 (m=15) + CO, the
momentum distribution of fragment C6H4OH (m=93) and CH3 (m=15, but use m=43
in momentum calculation) should be very close. However, we found that the
momentum distributions are very different. It suggests that both Norrish type I
reactions (reactions 1 and 2) occur for 3HAP and fragments C6H4OHCO (m=121) and
CH3CO (m=43) happen to completely undergo secondary reactions.
Details of these dissociation channels, the energies of transition states,
intermediates, and dissociation energies on the ground state and triplet state can be
found in Figure 12 and 13 respectively. They are very similar to that of 4HAP. The
branching ratio calculations from RRKM theory are 0.75 (CH3 elimination) and 0.25
9
(COCH3 elimination) on the ground state, and 0.96 (CH3 elimination) and 0.04
(COCH3 elimination) on the triplet state. Although we observed both channels, we do
not have ionization cross sections of these fragments at 118 nm. Accurate branching
ratios from experimental measurement are not available at his moment. As a result,
calculations suggest that these dissociation channels can occur on both electronic
states, but there is no direct experimental evidence about the electronic state which the
Norrish type I reactions occur.
B. Potential energy along the hydroxyl OH bond
We calculated the PES of the ground state and the singlet excited states along the
OH bond distance of hydroxyl group for various conformers of HAP,
2-hydroxybenzoyl fluoride, 2-hydroxybenzoyl chloride, and 2-hydroxybenzamide.
Calculations also include PES of phenol for comparison. The results obtained from
the CAS calculation are discussed below. The TD-B3LYP results were found to be
consistent with the CAS results, and they are included in the supplementary material.
Phenol and HAP
The adiabatic PES of phenol is illustrated in Figure 14. It shows that the lowest singlet
excited state 2A' is bound and its second excited state 1A'' is repulsive. The 1A'' state
crosses 2A' at short O
1A'
This result is the same as the previous
investigation.1 The characteristic of repulsive excited state 1A'' was used to explain
the large photofragment translational energy in the photodissociation of phenol.2
Recent theoretical and experimental39-40 studies identified an additional O-H bond
fission channel when both phenol-d5 and phenol-h6 are excited at 193.3 nm. This
dissociation channel involves nonadiabatic transition from 3A' to 2A'', and subsequent
10
dissociation on the 2A'' PES. The final phenoxyl products are generated in the second
excited (B 2A2) state. We also performed the calculations of phenol PES for the other
electronic states at higher energy levels. The results, as shown in Figure 14, are
similar to the previous calculations.
The 2HAP, 3HAP, and 4HAP molecules have several conformers. The structures
and the relative energies of these conformers are shown in Figure 15. The most stable
conformer of 2HAP (2HAP-1) has a structure that the OH functional group points to
the carboxyl functional group and forms an intramolecular hydrogen bond. The other
structures have energies at least 10 kcal/mol higher than the most stable conformer.
On the other hand, the energy differences in various conformers of 3HAP, and 4HAP
are much smaller (within 1 kcal/mol). Figure 14 shows the adiabatic PES along the
OH bond distance of these conformers. The oscillator strength and energies of these
singlet excited states are listed in supplementary material. Absorption of 193 nm
photon corresponds to the excitation to the 3A' state. One possible H atom elimination
mechanism involves the nonadiabatic coupling from 3A' to2A'', and subsequent
dissociation on 2A''. This channel is similar to the recent discovered H atom
elimination channel in phenol,40 which phenoxyl radicals are produced in the second
excited (B 2A2) state. This dissociation channel may have contribution to part of the
fragments in low translational energy distribution.
On the other hand, the maximum photofragment translational energy of 3HAP and
4HAP is close to the maximum available energy, indicating some of the final products
(phenoxyl radical chromophores) from 3HAP and 4HAP are produced in the product
electronic ground state. The dissociation mechanism must involve internal conversion
from 3A' to 2A', followed by the nonadiabatic coupling from 2A' to 1A'' through
conical intersection, and eventually dissociate on the 1A'' repulsive state.
11
Concerning 2HAP-2, 3HAP-a1, 3HAP-a2, and 4HAP-1 which do not have
intramolecular hydrogen bonding, the calculations showed that the lowest singlet
excited state 2A' is bound and the second excited state 1A'' is repulsive. The 1A'' state
crosses 2A' at short OH bond distance and then crosses the ground state at large
OH bond distance. Energies at these crossing points are ~125 and ~115 kcal/mol
relative to the ground-state energy minimum, respectively. The PES of these
conformers are very similar to that of phenol, and the excited state dynamics is
expected to be similar, too. Hydrogen atom elimination may occur via the population
transfer from the bright state 2A' to the dark state 1A'', and then dissociate adibatically
on the 1A'' state, producing ground state products (phenoxyl radical chromophore
products in its the electronic ground state). The sum of these two population transfer
processes results in the H-atom elimination on a repulsive potential energy surface,
producing ground-state products with large translational energies, as observed in
experimental measurement of 3HAP and 4HAP.
In contrast, the calculated PES of 2HAP-1 conformer is very different. The 1A''
state becomes a bound state near the ground state equilibrium geometry. The energy
required for the transition from the bound state to the repulsive state was predicted as
high as 160 kcal/mol and it does not cross the 2A' state. In addition, at large OH
bond distances, the ground-state potential energy of this hydrogen-bonded conformer
does not increase as rapidly as the non-hydrogen-bonded conformers. The
intramolecular hydrogen bonding reduces the ground-state energy significantly at
large OH distance and avoids intersecting with the 1A" state. As a result, the H atom
elimination from the repulsive excited state does not occur on the 1A" for the
conformers with intramolecular hydrogen bonding.
Other phenol chromophores
12
Similar intramolecular hydrogen bonding effects on the excited state dynamics are
also found theoretically for 2-hydroxybenzoyl fluoride, 2-hydroxybenzoyl chloride,
and 2-hydroxybenzamide. The PES of two conformers were calculated for each
molecule. Only the conformers with no intramolecular hydrogen bonding have PES
similar to that of phenol. The PES for these molecules are shown in Figure 16.
V. Discussions
Our theoretical calculations show that the conformers of HAP with hydrogen
bonding (2HAP-1) are lower in energy by 13 kcal/mol than the conformers of HAP
without hydrogen bonding (2HAP-2). Since the vibrational temperature in the
molecular beam is much lower than the room temperature (estimated to be less than
100 K), almost all 2HAP molecules have the structure like 2HAP-1 in the molecular
beam. On the other hand, the energy differences between various conformers of
3-HAP and 4-HAP are small, they coexist in the molecular beam. Calculations
showed that the excited state potential energy surfaces of all 3HAP and 4HAP
conformers are similar to that of phenol. Therefore, the excited state dynamics of
3HAP, 4HAP and phenol are expected to be the same, i.e., the population of the 2A'
state generated from absorption of UV photons can be transferred to the dark state
1A'' through a conical intersection, followed by the fast dissociation on the 1A'' state.
On the other hand, the PES of 2HAP-1 changes so much that the excited state
dynamics of 2HAP-1 is expected to be completely different from that of 3HAP and
4HAP. This is supported by our experimental observation.
Theoretical calculations have been performed on the excited-state intramolecular
proton transfer in 2-hydroxybenzoic acid (2HBA) in previous studies.42 Hydrogen
transfer along the intramolecular hydrogen bond as well as torsion and pyramidization
of the carboxyl group have been identified as the most relevant photochemical
13
reaction coordinates.41-42 Recently, we have investigated the photodissociation of 2-,
3- and 4-hydroxybenzoic acid in a molecular beam at 193 nm.43 We demonstrated that
dissociation from the excited state is effectively quenched for the conformers of
hydroxybenzoic acids with intramolecular hydrogen bonding. We also performed PES
calculations and showed that the potential energy surfaces for HBA are similar to the
corresponding PES of HAP, i.e., the PES for most of the HBA without intramolecular
hydrogen bonding are similar to that of phenol, and the PES of the most stable 2HBA
which has intramolecular hydrogen bonding are different from those of phenol.
The changes due the intramolecular hydrogen bonding are not limited to HBA
and HAP which we have performed experimental measurement. Additional theoretical
calculations of the PES for 2-hydroxybenzoyl fluoride, 2-hydroxybenzoyl chloride,
and 2-hydroxybenzamide further confirm the effects of intramolecular hydrogen
bonding, as illustrated in Figure 13. The results indicated that intramolecular
hydrogen bonding effects on the excited state dynamics is a general phenomenon for
molecules with the phenol chromophores.
The changes of PES can be rationalized from two factors. At large OH bond
distances, the ground-state potential energies of the conformers with intramolecular
hydrogen-bonding do not increase as rapidly as the non-hydrogen-bonded conformers.
The formation of zwitterionic like species such as Ph=O()...(+)HO=COH or
Ph=O()...(+)HO=COCH3 might be more favorable for the hydrogen-bonded
conformers at large OH bond distances. This reduces the ground-state energy
significantly at large OH bond distance. In addition, the interaction between the H
atom and hydrogen bond acceptor is very strong in the 1A'' excited state that it
becomes attractive at short OH bond distance. These factors result in the high energy
14
required for the transition from the bound state to the repulsive state in 1A'' and avoid
crossing between the 1A'', 2A', and ground state.
The PES also changes due to the intermolecular hydrogen bonding. However, the
effects on the excited state dynamics are different. The PES of phenol and phenol with
intermolecular hydrogen bonding, like phenol-(NH3)n and phenol-(H2O)n clusters,
have been reported.1 The second excited state also becomes a bound state for these
clusters. It crosses the first excited state, but does not cross the ground state. The
population on the first excited state from photoexcitation can be transferred to the
second excited state through the crossing between these two states, followed by
hydrogen atom transfer on the second excited state. However, as the distance between
phenol and NH3 increases, the second excited state becomes repulsive, and these
clusters dissociate into phenoxy radical and (NH3)nH or (H2O)nH. As a result, the
intermolecular hydrogen bonding does not change the H atom elimination from the
repulsive excited state. The change of repulsive state to bound state due to the
intermolecular hydrogen bonding was also found in water dimer.44
Excited state intramolecular proton transfer (ESIPT) has been studied extensively
since the first observation in methyl salicylate.45-49. One important properties of
ESIPT is the large Stokes shifted fluorescence following UV photon excitation.
According to Catalan,46 this spectral property results from both the exothermal feature
of the excited state and endothermal behaviors of the ground state along the proton
transfer coordinate. Potential energy surface for ESIPT of the ground and excited
states of 2-hydroxybenzoyl compounds have been investigated by ab initio
calculations.46 The PES of these compounds are very similar to that of 2HAP-1. On
the other hand, comparison of para-substituted compounds (with intramolecular
hydrogen bond) to ortho and mata substituted compounds (without intramolecular
15
hydrogen bond) and the investigation of excitation to high energy levels have received
little attention. The relationship of the potential energy surfaces between phenol, 2-, 3-,
and 4-HAP demonstrated in this work suggests that the exothermal feature of the
excited state along proton transfer coordinate associates to the repulsive
characteristics of the 1A'' state. This is likely true for most aromatic compounds
containing O-H functional group, according to the molecules studied in this work and
HAB in previous work.43 It provides another viewpoint on the ESIPT. On the other
hand, the major channels for 2HAP excited at 193 nm are Norrish type I reactions.
The dissociation properties are not directly related to the ESIPT.
Acknowledgement
The work was supported by the National Science Council Taiwan, under contract
NSC-97-2628-M-001-011-MY3 and NSC-97-2113-M-194-004. We thank the
National Center for High-Performance Computing (NCHC) for providing part of
computing resources.
16
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Fig. 1: (a) The potential energy surface along the OH bond distance of the hydroxyl
functional group for molecules without intramolecular hydrogen bonding. (b) The
potential energy surface along the OH bond distance of the hydroxyl functional
group with intramolecular hydrogen bonding.
Fig.2: Photofragment ion images of 2HAP. Pump and probe laser pulse delay time are
5, 6, 11, 29, and 38 s for m/z = 15, 43, 65, 93, and 121, respectively.
Fig.3: Photofragment translational energy distribution of 2HAP for reaction (a)
C6H4OHCOCH3  C6H4OH + COCH3. (b) C6H4OHCOCH3  C6H4OHCO + CH3.
Fig.4: Potential energies of reactant 2HAP, transition states, and dissociation products
on the ground state.
Fig. 5 Potential energies of reactant 2HAP, transition states, and dissociation products
on the triplet state.
Fig.6: Photofragment ion images of 4HAP. Pump and probe laser pulse delay time are
6, 38, 50, 38, 50, and 50 s for m/z = 43, 65, 79, 93, 107, and 135, respectively.
Fig.7: Photofragment translational energy distribution of 4HAP for reaction (a)
C6H4OHCOCH3  C6H4OH + COCH3. (b) C6H4OHCOCH3  C6H4OCO CH3 +H.
Fig. 8: Potential energies of reactant 4HAP, transition states, and dissociation products
on the ground state.
Fig. 9: Potential energies of reactant 4HAP, transition states, and dissociation products
on the triplet state.
Fig.10: Photofragment ion images of 3HAP. Pump and probe laser pulse delay time
are 5, 22, 34, 34, 34, 81, and 81 s for m/z = 15, 43, 65, 79, 93, 107, and 135,
respectively.
21
Fig.11: Photofragment translational energy distribution of 3HAP for reaction
C6H4OHCOCH3  C6H4OCO CH3 +H.
Fig. 12: Potential energies of reactant 3HAP, transition states, and dissociation
products on the ground state.
Fig. 13: Potential energies of reactant 3HAP, transition states, and dissociation
products on the triplet state.
Fig.14: The adiabatic potential energies of the ground state and singlet excited states
along the OH bond distance in hydroxyl group for hydroxyacetophenone (HAP) and
phenol. The solid blue squares, solid red circles, solid green triangles, and solid black
triangles represent 1A', 2A', 3A', and 4A', respectively. Open blue squares, open red
circles, open green triangles, and open black triangles represent 1A'', 2A'', 31A'', and
4A'', respectively.
Fig.15: Structures and relative energies of various HAP conformers
Fig.16: The adiabatic potential energies of the ground state and singlet excited states
along the OH bond distance in hydroxyl group for 2-hydroxybenzoyl fluoride,
2-hydroxybenzoyl chloride, and 2-hydroxybenzamide. The solid squares, solid circles,
open triangles and open circles represent 1A', 2A', 1A'', and 2A'', respectively.
22