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Ab initio investigation of the photofragmentation ofbromomethanol
Antonija Lesar a, Melanie Schnell b, Max M€uuhlh€aauser b,*, Sigrid D. Peyerimhoff b
a Department of Physical and Organic Chemistry, Jozef Stefan Institute, Jamova 39, SI-1000 Lubljana, Sloveniab Institut f€uur Physikalische und Theoretische Chemie der Universit€aat Bonn, Wegelerstrasse 12, 53115 Bonn, FRG, Germany
Received 11 July 2002
Abstract
Ab initio multi-reference configuration interaction calculations are carried out for ground and excited states of
bromomethanol BrCH2OH to investigate photofragmentation processes relevant to atmospheric chemistry. Five low-
lying excited states with vertical excitation energies between 5.8 and 7.4 eV (11A00; 13A00; 21A0; 13A0; 23A0) are found to
be highly repulsive for C–Br elongation leading to CH2OHðX2A0Þ and Br (X2P). Photodissociation along the C–O bond
leading to BrCH2 (X2B2) and OH (X2P) has to overcome a barrier of about 0.6–0.7 eV because the low-lying excited
states 11A00; 13A0 and 13A00 become repulsive only after the C–O bond is elongated by about 0.2 �AA.� 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
It is well known that halogenated compounds
play an important role in ozone depletion pro-
cesses [1]. Important halogens are chlorine and
bromine which not only destroy ozone but also
inhibit its formation by sequestering oxygen atomsin the halogen oxide forms. Bromine is even more
efficient than chlorine in removing ozone because
the majority of it remains as BrO, a form which is
active in destroying ozone [2,3].
Bromomethanol BrCH2OH, an atmospherically
important molecule, may be formed by reaction of
hydroxymethyl radicals CH2OH with atomic and
molecular bromine. The significance of BrCH2OH
is due to its involvement in the catalytic ozone
destruction cycles. It can possibly act as bromine
reservoir in stratospheric chemistry and therefore
formation of BrCH2OH would lead to a decrease
of ozone destruction.
We have recently studied the photochemistry ofchlorinated methanol derivatives Cl1þxCH2�xOH
ðx ¼ 1; 2Þ like monochloromtehnaol ClCH2OH
[4,5], dichloromethanol Cl2CHOH [6–8] and tri-
chloromethanol Cl3COH [9,10] and found that
photodissociation under chlorine re-liberation is
very probable.
Now we want to extend this work to bromo-
methanol BrCH2OH, because until now only littleis known about BrCH2OH. We want to determine
its equilibrium structure and photofragmentation
processes namely
Chemical Physics Letters 366 (2002) 350–356
www.elsevier.com/locate/cplett
* Corresponding author. Fax: +49-0228-739064.
E-mail address: [email protected] (M. M€uuhlh€aauser).
0009-2614/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.
PII: S0009 -2614 (02 )01461 -6
BrCH2OH ! Brþ CH2OH ð1Þand
BrCH2OH ! CH2BrþOH ð2Þin order to investigate its role as possible bromine
reservoir. Therefore we performed multi-referenceconfiguration interaction calculations to compute
potential energy curves for ground and excited
states for Br–C and C–O cleavage following (1)
and (2).
2. Computational techniques
The equilibrium geometries of bromomethanol
BrCH2OH and its photofragments CH2OH,
BrCH2 and OH were fully optimised at the
B3LYP/6-311G** level using the GAUSSIANAUSSIAN 98program package [11]. All optimised structures
were checked for local minima by vibrational
analyses (no imaginary frequencies).
For the calculations of excited states we used an
enlarged cc-p-VTZ basis set [12] with an additional
s-Rydberg function located at the carbon centre
and a negative ion p-function for the bromine
(cc-p-VTZ+SP). The exponents taken arearðCÞ ¼ 0:023 and arðBrÞ ¼ 0:032. On the other
hand preliminary calculations employing a smaller
polarised and enlarged cc-p-VDZ+SP basis set of
double zeta quality [12] show almost no changes in
excitation energies and transition probabilities of
the excited states examined in this work. Therefore
this more economic cc-p-VDZ+SP basis has been
used for the computations of the numerous pointson the potential energy curves examined.
The computations of the electronically excited
states were performed with the multi-reference sin-
gle and double-excitation configuration interaction
method MRD-CI implemented in the DIESELIESEL
program [13]. The selection of reference configura-
tions can be carried out automatically according to
a summation threshold. We used a summationthreshold of 0.85 which means that the sum of the
squared coefficients of all reference configurations
selected for each state (root) is above 0.85. The
number of reference configurations per irreducible
representation (IRREP) was in the range between
5 and 15. An analysis of the molecular orbitals
(MOs) involved in these selected reference config-
urations justified the prior choice of treating 20
electrons active while keeping the remaining elec-
trons in doubly occupied orbitals (frozen).
From the set of reference configurations (mains)
all single and double excitations in form of con-figuration state functions (CSFs) are generated.
From this MRD-CI space all configurations with
an energy contributions DEðT Þ above a given
treshold T were selected, i.e., the contributions of a
configuration larger than this value relative to the
energy of the reference set is included in the final
wavefunction. A selection threshold of T ¼ 5�10�8 hartree was used. The effect of those config-urations which contribute less than T ¼ 5� 10�8
hartree is accounted for in the energy computation
E(MRD-CI) by a perturbative technique [14,15].
The contribution of higher excitations is estimated
by applying a generalised Langhoff–Davidson
correction formula EðMRD-CIþQÞ ¼ EðMRD-
CIÞ � ð1� c20Þ½Eðref :Þ � EðMRD-CIÞ=c20, where c20is the sum of squared coefficients of the referencespecies in the total CI wavefunction and E(ref.) is
the energy of the reference configurations.
Two different models were considered for the
investigation of the potential energy surfaces: one
was changing the Br–C bond length stepwise from
the equilibrium distance of 1.984–10 �AA while all
other geometrical parameters were fully optimised
for the ground state using the more economic DFTmethod with the B3LYP functional instead of
CCSD(T). This is justified since we have shown in
an earlier work [4] and since it is known from
various investigations that DFT/B3LYP calcula-
tions can be used to predict overall molecular
structures quite successfully. In the second model
the potential energy curves are obtained by elon-
gating the C–O bond length stepwise from 1.382 to10 �AA. Again all other geometrical variables were
optimised for the ground state conformation.
We examined the lowest eight states of
BrCH2OH upon Br–C cleavage (1) and the lowest
six states for C–O elongation (2). The number of
CSFs directly included in the energy calculations
are as large as 340.000 (singlet) and 400.000
(triplet) selected from a total space of 1.81 million(singlet) and 3.4 million (triplet) generated con-
figurations so that transition energies of the
A. Lesar et al. / Chemical Physics Letters 366 (2002) 350–356 351
examined energy region should generally be
obtained with an error margin of less than 0.2 eV.
3. Results and discussion
In Table 1 we summarise the fragmentation
energies we obtained for the reaction (1) and (2).
The values obtained at the different theoretical
levels are in reasonable agreement with each other.
At the MRD-CI+Q level the energy needed for
C–Br fragmentation (reaction (1)) is calculated
with 77.0 kcal/mol. Including zero point energies
(ZPE) reduces this value to 73.1 kcal/mol. ForC–O cleavage (reaction (2)) the energy needed is
DEðMRD-CIþQþ DZPEÞ ¼ 78:9 kcal=mol.
Table 1
Fragmentation energies DEB (kcal/mol) on various levels of theoretical treatment (B3LYP, MRD-CI, MRD-CI+Q) as explained in the
text
Fragmentation channel CH2OHðX2A0Þ þ BrðX2PÞ CH2BrðX2B2Þ þOHðX2PÞ
DE(B3LYP) 69.9 92.7
DE(ZPE) )3.9 )7.1DE(T) 82.9 94.2
DE(MRD-CI) 79.4 92.2
DE(MRD-CI+Q) 77.0 86.0
The values have been obtained with the cc-p-VDZ+ sp basis set at the B3LYP-optimised geometries (6-311G**). Relative stabilities
as obtained in a multi-reference single and double-excitation configuration interaction (MRD-CI) including the Davidson correction
(MRD-CI+Q) are also given. E(T) is the energy at a theshold (T ¼ 10�7 hartree) as explained in the computaional techniques. Zero
point corrections are computed at the B3LYP/6-311G** level relative to BrCH2OHðDZPEÞ.
Fig. 1. Equilibrium structure of bromomethanol BrCH2OH (I) and its possible photofragments obtained from B3LYP/6-311G**
optimisation as explained in the text. The two dissociation processes studied correspond to an elongation of the Br–C distance (1) and a
breaking of the C–O bond, respectively.
352 A. Lesar et al. / Chemical Physics Letters 366 (2002) 350–356
The optimised equilibrium geometries are given
in Fig. 1 and the potential energy curves for the
pathways (1) and (2) are presented in Figs. 2 and 3.
In conjunction with the fragmentation energies
given in Table 1 it can be seen that the formation
of bromomethanol in its ground state from thefragments CH2Br and OH as well as from the
fragments CH2OH and Br is an energetically fa-
voured process. On both cuts through the ground
state potential energy surface we have not found a
barrier according to the DFT-gradient techniques.
The fragmentation process according to (1) and (2)
from the ground state surface requires more than
70 kcal/mol and is therefore very unlikely.It can be seen from Fig. 1 that along pathway
(2) the Br–C bond in CH2Br is somewhat short-
ened compared to BrCH2OH. This observation is
in line with the expected increased double bond
character for a sp2 carbon centre in BrCH2 in
comparison to a sp3 carbon centre in BrCH2OH.
Generally the geometries of the photofragments
CH2OH, BrCH2 and OH are obtained in reason-able agreement with what is known in the litera-
ture [16,17]. For example the experimental
OHð2PÞ distance is 0.96966 �AA [16] compared to
our calculated value of 0.971 �AA. The correspond-ing value of the CO distance in CH2OH is given
with 1.363 �AA at the MP2 level [17] compared to
our calculated 1.368 �AA.The geometry of BrCH2OH possesses no sym-
metry (C1) due to the occurrence of the anomeric
effect [18] but rotation of the OH-group around
the C–O axis into a CS-symmetric cis-isomer is a
very low-energy process. This cis-isomer has anonly 1.8 kcal/mol higher energy than the equilib-
rium structure at the MRD-CI+Q level.
In addition we examined the geometries of the
excited states under OH rotation. The potential
energy curves of the excited states are also found to
be very flat. As can be seen from Table 2 the dif-
ferences in the excitation energies of CS-symmetric
cis-BrCH2OH and the gauche BrCH2OHðC1Þconformation are well below 0.2 eV, i.e., they are in
the order of the error margin of the present calcu-
lations. Therefore, in what follows as well as in the
Figs. 2 and 3 we will restrict the discussion to the
cis-pathway because computations in CS-symmetry
are more economic. In addition qualitative MO
considerations often become more clear if symme-
try is involved in the calculation.The ground state of BrCH2OH is a singlet state
and thus because of spin conservation transitions
Fig. 2. Calculated MRD-CI potential energy curves of the
lowest eight states of bromomethanol BrCH2OH along a CS-
symmetric fragmentation pathway breaking the Br–C bond.
The points resemble the computed distances. The fragmenta-
tion energy DEB and the energy difference De between the two
dissociation channels are illustrated.
Fig. 3. Calculated MRD-CI potential energy curves of the
lowest six states of bromomethanol BrCH2OH along a
CS-symmetric fragmentation pathway breaking the C–O bond.
The points resemble the computed distances. The fragmenta-
tion energy DEB and the energy difference De between the two
dissociation channels are illustrated.
A. Lesar et al. / Chemical Physics Letters 366 (2002) 350–356 353
to singlet excited states are the most likely pro-cesses for photoinduced dissociation. On the other
hand recent experimental photodissociation stud-
ies [19–21] have shown that spin–orbit coupling is
nonnegligible for some bromine and chlorine spe-
cies like HOCl for which spin orbit coupling allows
a weak transition to the triplet state. Therefore, the
lowest excited triplet states of CH2BrOH are also
included in the present study.As can be seen from Fig. 2 five excited states
with calculated vertical excitation energies of
13A00 ¼ 5:84 eV, 13A0 ¼ 6:11 eV, 11A00 ¼ 6:29 eV,
21A0 ¼ 6:67 eV and 23A0 ¼ 7:37 eV are found to be
highly repulsive for Br–C elongation leading to the
dissociation channel CH2OHðX2A0Þ and BrðX2PÞ.The repulsive character of these states can be un-
derstood on the basis of qualitative MO consid-
erations already. Schematic contour diagrams of
some important occupied and virtual MOs are il-
lustrated in Fig. 4. The lowest unoccupied molec-
ular orbital LUMO 8a0 is populated in these states
and shows Br–C antibonding character. Due to a
nodal plane perpendicular to the Br–C axis thisLUMO 8a0 is r*(Br–C) type. It is populated in the13A00 and 11A00 states by 3a00 ! 8a0 excitation and
in the states 13A0 and 21A0 by 7a0 ! 8a0 transition.
The repulsive 23A0 results from 6a0 ! 8a0 excita-
tion as can be seen from Table 2 in conjunction
Fig. 4. Charge density contours of characteristic occupied valence orbitals (6a0, 7a0 and 3a00Þ and the lowest unoccupied molecular
orbital LUMO (8a0).
Table 2
Calculated transition energies DE (eV) and oscillator strengths f from the ground state X1A0 of CS-symmetric cis-BrCH2OH to its low-
lying electronic states and comparison with the corresponding transitions in C1-gauche conformation ðDEðC1ÞÞ
State Excitation DEðCSÞ f ðC1Þ DEðC1Þ
X1A0 ð7a‘Þ2ð3a00Þ2 0.0 – 0.0
11A00 3a00 ! 8a0 6.29 0.0004 6.34
21A0 7a0 ! 8a0 6.67 0.01 6.52
21A00 3a00 ! 9a0 7.59 0.02 7.55
31A00 3a00 ! 10a0 7.62 0.03 7.71
354 A. Lesar et al. / Chemical Physics Letters 366 (2002) 350–356
with Fig. 4. The dipole allowed singlet transitions
7a0 ! 8a0 are obtained with considerable oscillator
strength so that photofragmentation along the
elongation of the Br–C bond involving this exci-
tation is very likely.
The energy surfaces of the ground state and ofthe five low-lying repulsive states end up in the
same dissociation channel at 3.34 eV (77.0 kcal/
mol) which corresponds to the X2A0 ground state
of CH2OH and a separated bromine atom in its
X2P ground state. In this process the 7a0 and the
3a00 are transformed into bromine lone-pair orbi-
tals while the third lone pair results from a mixing
between the 6a0 and 8a0.The second dissociation channel that can be
seen from Fig. 2 corresponds to the first excited
state of CH2OH. This second channel is calculated
at 7.63 eV so that an energy difference of De ¼ 4:29eV is obtained. This value is in accordance with the
4.34 eV measured [22] for the first excited state of
CH2OH. The 9a0 MO, which is populated in this
case, is localised primarily in the OH region anddoes not induce Br–C separation for this reason so
that the excited states 23A00 and 21A00 are not Br–C
repulsive.
The results for the second model adopted for
the potential energy curves, i.e., the elongation of
the C–O bond as reaction coordinate, are dis-
played in Fig. 3. It is seen that the first three ex-
cited states show small barriers. They are of theorder of 0.65 eV for the 13A00 and 13A0 states and
of 0.73 eV for the 11A00 state. Photofragmentation
breaking the C–O bond has to overcome these
barriers. If the equilibrium C–O bond distance is
elongated by only 0.2 �AA or more all three states
11A00; 13A00 and 13A0 are repulsive. Together with
the ground state they correlate with the dissocia-
tion channel at 3.73 eV (86.0 kcal/mol) whichcorresponds to the ground states of BrCH2ðX2B2Þand OH (X2P). All three excited states are popu-
lated by excitation into the LUMO 8a0 which
possesses not only Br–C antibonding but also C–O
nonbonding character. Their origins are the va-
lence MOs 7a0ð3A0) and 3a00ð1A00, 1A0Þ.The second dissociation channel corresponds to
OH in its excited 2Rþ state. The calculated energydifference De of 4.17 eV is in good agreement with
the 4.05 eV reported in the literature [16].
The upper states 23A0 and 21A0 in Fig. 3 are not
repulsive and thus the consideration of such a
fragmentation into OH in its 2Rþ state is less im-
portant.
4. Summary and conclusions
Ab initio multi-reference configuration interac-
tion calculations are employed to investigate
photofragmentation of bromomethanol BrCH2
OH. Two possible fragmentations along Br–C and
C–O coordinates, namely
BrCH2OH ! Brþ CH2OH ð1Þ
and
BrCH2OH ! CH2BrþOH ð2Þhave been examined. Both processes are energeti-
cally not preferred in the ground state. On the
other hand the formation of bromomethanol ac-
cording to the reverse reactions (1) and (2) is likely,
since no barrier is found with the DFT-gradientprocedure for these formation processes. The
fragmentation energies for (1) and (2) obtained at
the different theoretical levels are in reasonable
agreement with each other. BrCH2OH is stabilised
relative to the products of (1) by 73.9 kcal/mol and
of (2) by 78.9 kcal/mol.
The equilibrium structure of BrCH2OH in its
ground state is an asymmetric gauche conforma-tion. But the CS-symmetric cis-isomer has an only
1.8 kcal/mol higher energy. The differences in the
electronic spectrum of cis-CH2BrOH and
C1-CH2BrOH are of the order of the error margin
of the present calculation (0.2 eV).
Five low-lying excited states with vertical exci-
tation energies between 5.8 and 7.4 eV (11A00;13A00; 21A0; 13A0; 23A0) are found to be highlyrepulsive for Br–C cleavage leading to the dissoci-
ation channel CH2OHðX2A0Þ and BrðX2PÞ. Therepulsive character of these states can be under-
stood on the basis of qualitative MO consider-
ations. The lowest unoccupied molecular orbital
LUMO 8a0 is antibonding r*(Br–C) type and
consequently photofragmentation breaking the
Br–C bond involving excited states in which thisrepulsive 8a0 is populated are very likely.
A. Lesar et al. / Chemical Physics Letters 366 (2002) 350–356 355
Photodissociation breaking the C–O bond has to
overcome a barrier of about 0.65–0.73 eV because
the excited states 11A00; 13A0 and 13A00 are repulsive
only if the CO distance is elongated by more than
0.2 �AA relative to the equilibrium. The processes are
similar to those found for photodissociation of thecorresponding chlorine compounds [5].
Acknowledgements
The present study is part of a NATO science
project �Study of elementary steps of radical reac-
tions in atmospheric chemistry�. The financialsupport from the NATO collaborative linkage
Grant EST.CLG.977083 is gratefully acknowl-
edged.
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