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: max@thch.uni-bonn.de (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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