Chapter 5
Theoretical study of the intramolecular hydrogen bonding
effects on the excited state dynamics of phenol
chromophores
Abstract
Theoretical prediction of hydrogen atom elimination on the excited state of
phenol and confirmation by experimental observations gave great impact on the
explanation of the photostability of amino acid tyrosine irradiated by ultraviolet
photons. In this work, we demonstrated that this excited state characteristic changes
significantly if OH functional group is involved in the formation of intramolecular
hydrogen bonding on the ground state by the theoretical calculations. We calculated
the excited state potential energy surface of 2-, 3- and 4-hydroxybenzoic acid (HBA),
2-, 3- and 4-hydroxyacetophenone (HAP), and 2-, 3-, and 4-methoxybenzoic acid
(MOBA). The calculated results showed that the excited state potential along OH
bond distance of hydroxyl group of 3-HBA, 4-HBA, 3-HAP, 4-HAP, all MOBA
isomers and some conformers of 2-HBA, 2-HAP without intramolecular hydrogen
bonding are similar to that of phenol, indicating the repulsive characteristic of the
excited state remains the same for these molecules. However, the calculation showed
both the excited state and the ground state potential energy surfaces change
significantly for the conformers of 2-HBA and 2-HAP with intramolecular hydrogen
bonding. The changes include that the repulsive potential energy surface become an
attractive potential near the ground state equilibrium geometry, the conical
143
intersection between the first and the second excited states along OH bond (with
relaxed geometry for the rest of molecule) locates at much higher energy level, and
the conical intersection between the second excited state and the ground state along
OH bond distance disappears. These result in the change of the excited state
dynamics for the conformers with intramolecular hydrogen bonding. The calculated
results are consistent with recent photodissociation experiments using molecular beam.
Consequently, the photochemistry of phenol chromophores must take these effects
into consideration.
144
Introduction
Although aromatic amino acids such as tyrosine have large UV absorption
cross-sections, the respective fluorescence quantum yields are small; indicating the
presence of fast nonradiative processes, which efficiently quench the fluorescence.1-4
The nonradiative process is assumed to be ultrafast internal conversion.2-5 Following
the internal conversion (electronic-to-vibrational energy transfer) the highly
vibrationally excited molecules quickly dissipate their energy to surrounding
molecules through intermolecular energy transfer before chemical reactions initiate.
This so-called photostability prevents the undesired photochemical reactions for these
molecules upon the irradiation with ultraviolet photons. However, recent theoretical
calculations6-10 suggested that the low fluorescence quantum yield for phenol (the
chromophore for tyrosine) was due to dissociation from an excited electronic state
potential energy surface, rather than from fast internal conversion to the ground
electronic state. The first excited 1* state of phenol is bound with respect to OH
bond distance, and the excited 1* state is repulsive. Absorption of UV photons
corresponds to excitation to the 1* excited state whose population can be
transferred to the 1* state via a conical intersection. As a consequence, dissociation
from the 1* state provides an alternative explanation for the rapid fluorescence
quenching. Dissociation from a repulsive excited state has been verified in recent
molecular beam experiments.11-14 Since dissociation from a repulsive excited state
potential energy surface is swift, quenching is incomplete even in the condensed
phase. As a result, reactions following the generation of radicals from dissociation
become an obstacle to the photostability of amino acids. Molecular conformers
typically interconvert via hindered rotations about single bonds. Low barriers for
interconversion, relative to the energy for photoexcitation or chemical transformation,
145
result in fast equilibrium for the various conformers. Consequently, similar
photochemical properties are expected for different conformers. Only few examples
of conformationally-controlled photodissociation have been observed15-17 and little
discussion has been directed at the conformationally-dependent photostability of
amino
acids.
Very
2-hydroxybenzoic
resently,
acid
photodissociation
(2-HBA),
of
3-hydroxybenzoic
tyrosine
acid
chromophores,
(3-HBA),
and
4-hydroxybenzoic acid (4-HBA) are investigated in a molecular beam at 193 nm
using multimass ion imaging techniques by Ni and coworkers.18 The experimental
results showed that the hydrogen atom elimination from a repulsive excited state was
the major dissociation channel for the majority of the conformers. This dissociation
mechanism is similar to the proposed hydrogen atom elimination for phenol. However,
they found, unexpectedly, that the H-atom elimination channel of 2-HBA and 2-HAP
conformers with intramolecular hydrogen bonding was nearly quenched, and these
conformers showed significantly different dissociation properties. This suggested that
intramolecular hydrogen bonding plays an important role in the photostability of
amino acid chromophores absorbing UV photons. In this chapter, we reported the
calculation of the excited state potential energy surface of these molecules, and to
compare with the experimental results.
146
Calculation Methods
The ground state geometries for various conformers and the corresponding
interconversion transition states were calculated with the B3LYP/6-311+G(d,p)
method.19-20 Energies of these structures then were refined by the MLSE-TPSS1KCIS
method.21-22 All ground-state electronic structure calculations were performed using
the GAUSSIAN 03 package.23
The excited-state energy calculation was done using the time-dependent (TD)24
B3LYP theory with 6-311+G(d,p) basis set and the complete active space (CAS)
theory25 with the 6-31+G(d,p) basis set. For molecules with Cs symmetry, the active
space of the CAS calculation consists of all  electrons with two * and all *
orbitals in valance orbitals. For 2HBA, there are 12  electrons and 12 orbitals (6, 4
*, and 2*). For molecules without Cs symmetry, we used graphic program to
simulate and identify the approximate  and  orbitals, than used the same rule to
select the active electrons and orbitals. A state averaged approach with equal
weighting was applied to calculate the two lowest states of A' and A" symmetry
simultaneously. This calculation was carried out using the MOLPRO 2009 program. 26
The TD-B3LYP calculation was performed using the GAUSSIAN 03 package.
147
Results and Discussions
The hydroxybenzoic acids, 2-HBA, 3-HBA, and 4-HBA have many conformers.27
The calculated structures and relative energies are shown in Figures 1, 2, and 3,
respevtively. The proximity of the hydroxyl and carboxyl groups in 2-HBA lends
itself to intramolecular hydrogen bonding. These hydrogen bonds can be formed as
such: O-H---O=COH or OH---O(H)C=O, as for 2-HBA-1. 2HBA-2 and 2-HBA-5.
Relative energy between these conformers are about 0~5 kcal/mol. When the
hydroxyl group points opposite to the carboxyl group no hydrogen bond is formed.
Energies are higher for these conformers (e.g., 2-HBA-3, 2-HBA-4, and 2HBA-6)
than for those conformers where intramolecular molecular hydrogen bonding is
present. The hydroxyl and carboxylic groups in 3-HBA and 4-HBA are too distant
from one another for intramolecular hydrogen bonding to be a factor. As a result, even
the most stable conformers of 3-HBA and 4-HBA are 4~6 kcal/mol higher in energy
than that the conformer 2-HBA-1. The 3-HBA conformers can be divided into two
groups, depending on whether the OH portion of the carboxylic group points towards
or away from the aromatic ring. Those conformers for which OH points away from
the aromatic ring (e.g., 3-HBA-1, 3-HBA-2, 3-HBA-3, and 3-HBA-4) have energies
~5 kcal/mol lower than conformers for which OH is directed towards the aromatic
ring (e.g., 3-HBA-5, 3-HBA-6, 3-HBA-7, and 3-HBA-8). The direction of the
hydroxyl group in 3-HBA appears to be unimportant with respect to the conformation
energy. A similar trend is also observed for 4-HBA. The energy difference between
conformers with hydrogen bonding and without hydrogen bonding is sufficiently
large such that most of 2-HBA remains as 2-HBA-1 at room temperature. On the
other hand, various conformers of 3-HBA (i.e., 3-HBA-1, 3-HBA-2, 3-HBA-3 and
3-HBA-4) and 4-HBA (i.e., 4-HBA-1 and 4-HBA-2) coexist at room temperature.
148
We calculated the potential energy curves for the ground state and singlet excited
states along the OH bond of the hydroxyl group for various conformers of 2-HBA,
3-HBA, 4-HBA , along with phenol and methylphenol for comparison. Figure 4
shows the calculated potential energy curves for the various conformers. For phenol,
the Figure 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H bond distance
(~1.1 Å) and then crosses the ground state at large OH bond distance (~1.8 Å). This
result is similar to that from a previous investigation.8 That is, the calculation implies
that the hydrogen atom elimination reaction may be initiated via the population
transfer from a bright state 2A' to a dark state 1A'' at short OH distance, and then
transfer to the ground 1A' state at longer distance, and to produce ground-state
products with large translational energy. For conformers of 3-HBA, 4-HBA, and the
conformers of 2-HBA without intramolecular hydrogen bonding, potential energy
curves along the OH bond distances are very similar to that of phenol. More
specifically, the second excited state 1A'' is repulsive, crossing the 2A' state at OH
bond distance of 1.1 Å. Energy at the crossing point is about 135 kcal/mol relative to
the ground-state energy minimum. The 1A'' state and the ground state cross at OH
bond distance of 1.8 Å. Energy at this crossing point is about 120 kcal/mol above the
ground-state energy minimum.
In contrast, the potential energy surfaces change significantly for the 2-HBA
comformers with intramolecular hydrogen bonding. For the confromers 2-HBA-1 and
2-HBA-2, the 1A'' state becomes a bound state at short OH bond distances. The
energy barriers for the transfer from the bound to the repulsive state are as large as
~160 kcal/mol, thereby exceeding the 193 nm photon energy (148 kcal/mol). In
149
addition, at large OH distances the ground state potential energies of 2-HBA-1 and
2-HBA-2 do not increase as rapidly as those of 2-HBA-3, 2-HBA-4, 3-HBA and
4-HBA. This can be rationalized that at large OH distances, the formation of an
zwitterionic species like Ph=O(-)...HOR(+) might be favorable for the conformers that
possess intramolecular hydrogen bonding. This reduces the ground state (1A') energy
significantly at large OH distances and avoids the intersecting with the 1A" state.
To explore the potential energy surfaces on geometries other than vicinity of the
minimum energy structures on the ground-state, we calculated the potential energy
curves as a function of both the OH bond distance and COH angle in the hydroxyl
group of the 2-HBA-1 conformer. Two integrated 3-D plots viewed from different
angles are shown in Figure 5, and the individual curves are included in the supporting
information. Interestingly, the properties of the 1A'' state change with variations in the
COH angle. Figure 5 illustrates that the region of the potential well in the 1A" state
becomes smaller at larger COH angles, and the 1A" begins to intersect with the
ground state at a COH angle of 120. In comparison, the angle is ~108 in the
minimum energy structure on the ground state. The 1A" becomes a pure repulsive
state as the COH angle reaches 150o. This suggests that some of the 2-HBA-1
conformer can dissociate into fragment through H-atom elimination channel if the
COH angle increases during this process on the excited state. However, because the
crossing point between the 1A'' and 2A' states is as high as 150 kcal/mol (close to the
photon energy) we anticipate that in experiments the H-atom elimination is not
important for conformers of 2-HBA with intramolecular hydrogen bonding.
Additionally, for 2HBA-1 we calculated the potential energy surfaces as a function of
both OH bond distances and the angles between the hydroxyl OH bond and the
150
plane of the aromatic ring with the COH angle fixed at 108 using the TD-B3LYP
theory. As we rotate the hydroxyl group of 2-HBA out of the plane, the calculation
showed that the original 1A" state remains as a bound state at short OH bond
distances, and the ground and the dark state (1A") surfaces are still well separated.
However, if the COH angle is fixed at larger angles, for example 150, as we rotate
the hydroxyl group out of the plane, the intersection of the original 1A' and 1A"
curves (which is now 1A and 2A curves in the C1 point group) becomes an avoided
crossing at small rotational angles due to the symmetry constraint. The two curves
become well-separeted at larger rotational angles. The details of these potential curves
are included in the supporting information.
Calculations also included potential energy surfaces for 2-methylphenol. The
conformers of 2-methylphenol that we calculated have structures in which the
hydroxyl group either points towards or away from the methyl group. These structures
are intended to imitate the geometry of 2-HBA, however replacing the carboxyl group
with that of a methyl group effectively eliminates the possibility for intramolecular
hydrogen bonding. The results reveal that the potential energy surfaces for the excited
states are very similar to that for phenol and conformers of 2-HBA without
intramolecular hydrogen bonding.
We also calculated the PES for the ground state and the singlet excited states
along the OH bond distance of hydroxyl group for various conformers of HAP, HBA,
2-hydroxybenzoyl fluoride, 2-hydroxybenzoyl chloride, and 2-hydroxybenzamide as
well as the PES along the CO bond distance of methoxy group for various
conformers of MOBA, as shown in Figure 6. Calculations also include PES for
151
anisole for comparison. The calculated structures of these molecules are shown in
Figure 7.
The PES of anisole shows that the replacement of the H atom of hydroxy group in
phenol by CH3 does not change the properties of PES. The second excited state 1A''
along the CO bond remains repulsive.
For MOBA, 3HAP, 4HAP, 3HBA, 4HBA, and the conformers of 2HAP, 2HBA,
2-hydroxybenzoyl fluoride, 2-hydroxybenzoyl chloride, and 2-hydroxybenzamide
without intramolecular hydrogen bonding, calculations show 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 (or OC) bond distance and then crosses the ground state at
large OH (or OC) bond distance. Energy at the crossing point between 1A'' and 2A'
is about 130 kcal/mol relative to the ground-state energy minimum. It is about 120
kcal/mol above the ground-state energy minimum at the crossing point between 1A''
state and the ground state. The excited state dynamics is similar to that of phenol and
anisole. Hydrogen atom elimination or CH3 elimination initiates via the population
transfer from the bright state 2A' to the dark state 1A'', and then to the 1A' ground
state. The sum of these two population transfer processes results in the H-atom and
CH3 elimination on a repulsive potential energy surface, producing ground-state
products with large translational energies, as observed in experimental measurement.
By contrast, the properties of the PES are very different for the conformers of
2HAP,
2HBA,
2-hydroxybenzoyl
fluoride,
2-hydroxybenzoyl
chloride,
and
2-hydroxybenzamide with intramolecular hydrogen bonding. For these confromers,
the 1A'' state becomes a bound state near the ground state equilibrium geometry. The
formation of intramolecular hydrogen bonding reduces the ground state (1A') energy
152
significantly at large OH distances and avoids the intersecting with the 1A" state,
like 2-HBA we mentioned above. 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.
The comparison between HAP, HBA, and MOBA further points out the effects of
intramolecular hydrogen bonding. Both 2HAP-1 and 2HBA-1 contain intramolecular
hydrogen bonding. However, 2MOBA-1 does not have intramolecular hydrogen
bonding due to the replacement of H atom by methyl group. The PES of
2MOBA-1conformer, unlike 2HAP-1 and 2HBA-1, remains similar to anisole. The
dissociation along repulsive O-C bond is the major channel.
153
Conclusion
In this chapter, our calculation showed that both the ground state and the excited
state potential energy surfaces of HBA change significantly for different conformers.
For various conformers of HBA, HAP and MOBA without intramolecular hydrogen
bonding, the second excited state is a repulsive state. It crosses 2A' at short OH (or
OC) bond distance and crosses 1A' at large OH (or OC) bond distance. Hydrogen
atom elimination can occur easily on this repulsive potential energy surface. However,
the calculation showed both the excited state and the ground state potential energy
surfaces change significantly for the conformers of 2-HBA and 2-HAP with
intramolecular hydrogen bonding. The formation of intramolecular hydrogen bonding
reduces the ground state (1A') energy significantly at large OH distances due to the
formation of zwitterionic species and avoids the intersecting with the 1A" state. This
in turn closes the H elimination channel from the repulsive excited state.
The calculated results were consistent with the recent experiments by Ni and
coworkers.
Acknowledgements
The research described in this chapter has been published on
The Journal of Chemical Physics (Yang, Y.-L.; Ho, Y.-C.; Dyakov, Y.; Lee, Y.-T.; Ni,
C.-K.; Sun, Y.-L.; Hu, W.-P. J. Chem. Phys. 2011, 134, 034314.). This work is
supported by the National Science Council of Taiwan, grant number NSC
97-2113-M-194-004. We are grateful to the National Center for High-Performance
Computing (NCHC) for providing part of computation resources.
154
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(27) For different orientations of hydroxyl and carboxylic groups, both 2-HBA and
3-HBA have eight conformer. 4-HBA has only four conformers due to the
symmetry. Our calculation shows that two conformers of 2-HBA are not stable.
One has geomerty that hydroxyl group and the OH portion of carboxylic group
point towards to each other. The other has similar structure of 2-HBA-3, but the
OH portion of the carboxylic group points towards to the aromatic ring. This
sturucure is not stable from the calculation by G3 method, but it is stable in the
B3LYP/6-31G* level calculation with a very small barrier (<1 kcal/mol) and
convert to 2-HBA-6 easily.
157
Figure 1. The calculated structures and energies by B3LYP/6-311+G** (MLSE-TS) method related to 2HBA-1 of 2-HBA isomers.
2-HBA-1, 0.00(0.00) kcal/mol
2-HBA-2, 3.43(4.17) kcal/mol
2-HBA-3, 11.10(10.99) kcal/mol 2-HBA-4, 10.71(10.49) kcal/mol
2-HBA-5, 6.54(5.15) kcal/mol
2-HBA-6, 13.54(11.58) kcal/mol 2-HBA-7, 6.27(5.07) kcal/mol
158
Figure 2. The calculated structures and energies by B3LYP/6-311+G** (MLSE-TS) method related to 2HBA-1 of 3-HBA isomers.
3-HBA-1, 2.79(2.00) kcal/mol
3-HBA-2, 3.09(2.33) kcal/mol
3-HBA-3, 3.52(2.67) kcal/mol
3-HBA-4, 3.44(2.62) kcal/mol
3-HBA-5, 9.21(7.26) kcal/mol
3-HBA-6, 10.44(8.51) kcal/mol
3-HBA-7, 10.08(8.08) kcal/mol
3-HBA-8, 9.92(8.01) kcal/mol
159
Figure 3. The calculated structures and energies by B3LYP/6-311+G** (MLSE-TS) method related to 2HBA-1 of 4-HBA isomers.
4-HBA-1, 1.60(1.19) kcal/mol
4-HBA-2, 1.66(1.25) kcal/mol
4-HBA-3, 8.67(6.99) kcal/mol
160
4-HBA-4, 8.92(7.22) kcal/mol
Figure 4. The calculated potential energy curves by CAS(12,12)/6-31+G** (for HBA)
method along the OH bond of the hydroxyl group.
(a) Phenol (CAS(9,7)/6-31+G**)
300
250
kcal/mol
200
1A'
150
2A'
100
1A"
2A"
50
0
0.9
1.4
1.9
-50
Å
(b) 2-HBA-1
250
200
kcal/mol
150
1A'
2A'
100
1A"
2A"
50
0
0.9
-50
1.4
1.9
Å
(c) 2-HBA-2
161
250
200
kcal/mol
150
1A'
2A'
100
1A"
2A"
50
0
0.9
1.4
1.9
Å
(d) 2-HBA-3
300
250
200
kcal/mol
1A'
150
2A'
1A"
100
2A"
50
0
0.9
1.4
1.9
Å
(e) 2-HBA-4
162
300
250
200
kcal/mol
1A'
150
2A'
1A"
100
2A"
50
0
0.9
1.4
1.9
Å
163
(f) 3-HBA-1
300
250
200
kcal/mol
1A'
150
2A'
1A"
100
2A"
50
0
0.9
1.4
1.9
Å
(g) 4-HBA-1
300
250
200
kcal/mol
1A'
150
2A'
1A"
100
2A"
50
0
0.9
1.4
1.9
Å
164
(h) Methylphenol (CAS(10,10)/6-31+G**)
300
250
200
kcal/mol
1A'
150
2A'
1A"
100
2A"
50
0
0.9
-50
1.4
1.9
Å
165
Figure 5. The 3-D plots viewed from different angles of the potential energy curves as
a function of both the OH bond distance and COH angle in the hydroxyl group of
the 2-HBA-1 conformer
166
167
Figure 6. The energies (kcal/mol) of the ground state and singlet excited states along
the O-H bond distance (Å) in hydroxyl group for HAP, hydroxybenzoic acids,
2-hydroxybenzoyl fluoride, 2-hydroxybenzoyl chloride, and 2-hydroxybenzamide,
and along O-C bond distance for anisole and MOBA.
(a) Anisole (CAS(10,11)/6-31+G**)
300
250
kcal/mol
200
1A'
150
2A'
100
1A"
2A"
50
0
1.2
1.7
2.2
-50
Å
168
(b) 2-HAP-1 (CAS(12,12)/6-31+G**)
200
180
160
140
kcal/mol
120
1A'
100
2A'
80
1A"
60
2A"
40
20
0
-20
0.9
1.1
1.3
1.5
1.7
1.9
Å
(c) 2-HAP-2 (CAS(12,12)/6-31+G**)
250
200
kcal/mol
150
1A'
2A'
100
1A"
2A"
50
0
0.9
1.4
1.9
Å
169
(d) 3-HAP-1 (CAS(12,12)/6-31+G**)
250
200
kcal/mol
150
1A'
2A'
100
1A"
2A"
50
0
0.9
1.1
1.3
1.5
1.7
1.9
Å
(e) 4-HAP-1 (CAS(12,12)/6-31+G**)
300
250
kcal/mol
200
1A'
150
2A'
100
1A"
50
2A"
0
0.9
1.4
1.9
-50
Å
170
(f) 2-MOBA-1 (CAS(14,14)/6-31+G**)
250
200
150
kcal/mol
1A'
2A'
100
1A"
50
2A"
0
1.2
1.7
2.2
-50
Å
(g) 2-MOBA-3 (CAS(10,13)/6-31+G**)
250
200
150
kcal/mol
1
2
100
3
4
50
0
1.2
1.7
2.2
-50
Å
171
(h) 3-MOBA-1 (CAS(14,14)/6-31+G**)
250
200
150
kcal/mol
1A'
2A'
100
1A"
2A"
50
0
1.2
1.7
2.2
-50
Å
(i) 4-MOBA-1 (CAS(14,14)/6-31+G**)
250
200
150
kcal/mol
1A'
2A'
100
1A"
2A"
50
0
1.2
1.7
2.2
-50
Å
172
(j) 2-hydroxybenzoyl fluoride-1 (CAS(12,12)/6-31+G**)
250
200
kcal/mol
150
1A'
2A'
100
1A"
2A"
50
0
0.9
1.4
1.9
Å
(k) 2-hydroxybenzoyl fluoride-2 (CAS(12,12)/6-31+G**)
300
250
200
kcal/mol
1A'
150
2A'
1A"
100
2A"
50
0
0.9
1.4
1.9
Å
173
(l) 2-hydroxybenzoyl chloride-1 (CAS(13,14)/6-31+G**)
250
200
150
kcal/mol
1A'
2A'
100
1A"
2A"
50
0
0.9
1.4
1.9
-50
Å
(m) 2-hydroxybenzoyl chloride-2 (CAS(13,14)/6-31+G**)
250
200
kcal/mol
150
1A'
2A'
100
1A"
2A"
50
0
0.9
1.4
1.9
Å
174
(n) 2-hydroxybenzamide-1 (CAS(10,12)/6-31+G**)
250
200
kcal/mol
150
1A'
100
2A'
3A'
50
0
0.9
1.4
1.9
-50
Å
(o) 2-hydroxybenzamide-2 (CAS(12,12)/6-31+G**)
250
200
kcal/mol
150
1A'
2A'
100
1A"
2A"
50
0
0.9
1.4
1.9
Å
175
Figure 7. The calculated structures of HAP, hydroxybenzoic acids, 2-hydroxybenzoyl fluoride, 2-hydroxybenzoyl chloride, 2-hydroxybenzamide,
anisole and MOBA.
Anisole
2-HAP-1
2-HAP-2
3-HAP-1
4-HAP-1
2-MOBA-1
2-MOBA-3
3-MOBA-1
4-MOBA-1
2-hydroxybenzoyl
fluoride-1
176
Figure 7. (continue)
2-hydroxybenzoyl fluoride-2
2-hydroxybenzoyl
chloride-1
2-hydroxybenzoyl
chloride-2
177
2-hydroxybenzamide-1
2-hydroxybenzamide-2
Appendix
(a) The Molpro input file for the planar 2-HBA-1 for CAS(12,12)/6-31+G**
calculation is listed below. There are 72 electrons in this molecule. From HF
calculation, 72 electrons are occupied in 6orbitals and 30  orbitals. The excited
states of the → * and → * types of transitions of were considered. All six
occupied  orbitals (orbital 1A” 6A”) were included in the active space.
However, due to the limited computational resources, only the four lowest
unoccupied  orbitals (orbital 7A”  10A”) were included in the active space.
The lowest two * orbitals (orbital 31A’, 32A’) were also included in the active
space due to the  character of OH bond. Thus, the active space consists of 12 
electrons and 12 orbitals (6, 4*, and 2*).
{geometry}
basis=6-31+G**
{casscf;
occ,32,10;
closed,30,0;
WF,SYM=1;State,2
WF,SYM=2;State,2
}
178
(b) The Molpro input file of the non-planar 2-MOBA-3 for CAS(10,13)/6-31+G**
calculation is listed below. From HF calculation, 40orbitals are occupied. The
orbitals 36  48 were included in the active space. The active space was
determined by graphic molecule orbitals and CIS calculations.
{geometry}
basis=6-31+G**
{casscf;
occ,48;
closed,35;
WF,SYM=1;State,4
}
179
(c) The Molpro input file of the phenol (Cs symmetry) for CAS(8,9)/6-31+G**
calculation is listed below. The excited states of the → * and → * types of
transitions of were considered. The rule of choosing active space was the same
with that of 2-HBA-1. The active space consists of 8  electrons and 9 orbitals (4,
3*, and 2*).
{geometry}
basis=6-31+G**
{casscf;
occ,23,7;
closed,21,0;
WF,SYM=1;State,2
WF,SYM=2;State,2
}
180
(d) The Molpro input file of the phenol (C1 symmetry) for CAS(8,9)/6-31+G**
calculation. Compared with the phenol (Cs symmetry) orbitals, the occupied
orbitals 20.1, 21.1, 22.1 are assigned as  orbitals. And virtual orbitals 28.1, 29.1,
31.1, 33.1 are assigned as * orbitals. These orbitals were not included in the
active space. The “RESTRICT,6,6,20.1,21.1,22.1;” means the minimum 6
and maximum 6 electrons in the 20.1, 21.1, 22.1 orbitals.
{geometry}
basis=6-31+G**
{casscf;
occ,34;
closed,18;
RESTRICT,6,6,20.1,21.1,22.1;
RESTRICT,0,0,28.1,29.1,31.1,33.1;
WF,SYM=1;State,4
}
181