- Email: [email protected]
The isomerization of HOOBr to HOBrO
24 March 2000
Chemical Physics Letters 319 Ž2000. 650–654 www.elsevier.nlrlocatercplett
The isomerization of HOOBr to HOBrO Sujata Guha, Joseph S. Francisco
)
Department of Chemistry and Department of Earth and Atmospheric Sciences, Purdue UniÕersity, West Lafayette, IN 47907-1393, USA Received 8 December 1999
Abstract A second transition state for the process of isomerization of HOOBr™ HOBrO has been located theoretically, using ab initio molecular orbital methods. The energy barrier for this isomerization pathway is found to be 39.4 kcal moly1. This result suggests that the barrier is sufficiently high so as not to facilitate the isomerization of HOOBr to HOBrO. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction In 1975, Wofsy et al. w1x suggested that bromine atoms could destroy ozone more effectively than chlorine atoms, since more bromine is present in its active forms, Br and BrO, than chlorine is in its active Cl and ClO forms. It has been estimated that about ; 25% of the ozone loss in Antarctica w2x and up to 40% of ozone loss over the Arctic region during winter w3x is due to the chemical reactions involving bromine species. The coupling of bromine oxides ŽBrO x . with HO x species to destroy ozone has been of particular importance. A critical reaction is the coupling between BrO and OH radicals, which leads to increased recycling of bromine, and the eventual depletion of
) Corresponding author. Fax: q1-765-494-0239; e-mail: [email protected]
ozone. Of the following possible representations of the BrO q HO reaction pathways, BrO q OH ™ Br q HO 2
Ž 1a .
BrO q OH ™ HBr q O 2
Ž 1b .
channel 1a is postulated to be the predominant product channel, by Bogan et al. w4x, proceeding via formation of the short-lived wHOOBrx ) vibrationally excited addition complex. The HOOBr complex is the most stable of the three plausible isomeric forms of HBrO 2 Žthe other two being HOBrO and HBrO 2 ., according to computational studies performed by Lee w5x, and Guha and Francisco w6x. The transition states for the HOOBr™ HOBrO and HOBrO™ HBrO 2 isomerization pathways were, previously, theoretically investigated by Guha and Francisco w7x. It was found that even though the HOOBr and HOBrO intermediates can form during the HO q BrO reaction, the energy barrier for the isomerization of HOOBr to HOBrO is high enough
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 1 0 0 - 7
S. Guha, J.S. Franciscor Chemical Physics Letters 319 (2000) 650–654
Ž67.2 kcal moly1 . to not allow the interconversion of HOOBr to HOBrO Žand vice-versa.. There can be yet another transition state for the isomerization of HOOBr to HOBrO, and, in this Letter, we present ab initio molecular orbital results of the structure, vibrational spectrum, and energetics of this new transition state.
2. Computational methods Ab initio molecular orbital calculations were performed using the GAUSSIAN 94 program w8x. The equilibrium geometry for the HOOBr™ HOBrO isomerization process was fully optimized to better then ˚ for bond distances and 0.18 for bond angles, 0.001 A with a self-consistent field convergence of at least 10y9 on the density matrix. The QCISD Žquadratic configuration interaction with single and double excitations. method w9x was used with the 6-31GŽd. w10x, 6-311GŽd,p. w11x, and 6-311GŽ2d,2p. basis sets in the optimization of the geometry. The harmonic vibrational frequencies and intensities for the isomerization process were calculated at the QCISD level
651
of theory in conjunction with the 6-31GŽd. basis set, using the geometry calculated at the QCISDr631GŽd. level of theory. To improve the energies, single-point calculations were performed with the QCISDŽT. Žquadratic configuration interaction with single and double excitations incorporating the perturbative corrections for triple excitation. wavefunctions w9x, using the optimized geometry obtained at the QCISDr6-311GŽ2d,2p. level of theory.
3. Results and discussion 3.1. Geometry and Õibrational frequencies Computations on the HOOBr™ HOBrO isomerization transition state were performed using the QCISD method in conjunction with the 6-31GŽd., 6-311GŽd,p. and 6-311GŽ2d,2p. basis sets. The structural parameters of this transition state are listed in Table 1. This transition state is formed due to the migration of the bromine atom in HOOBr, eventually
Table 1 ˚ and degrees. for HOOBr,a HOBrO,a and the HOOBr™ HOBrO isomerization transition state Optimized geometries ŽA Species
Coordinates
HOOBr
OO BrO HO OOBr HOO HOOBr OBr HO X BrO HOBr X OBrO X HOBrO X BrO OBr HO X OBrO HOBr
X
HOBrO
wHOBrO™ HOBrOX x /
a
Ref. w7x.
Levels of theory QCISDr 6-31GŽd.
6-311GŽd,p.
6-311GŽ2d,2p.
1.428 1.910 0.980 109.3 101.2 89.5 1.883 0.980 1.705 103.5 110.2 75.2 1.735 2.371 0.989 63.8 156.8
1.405 1.909 0.965 110.1 101.7 91.3 1.875 0.965 1.693 103.4 110.7 74.2 1.725 2.378 0.974 64.7 155.5
1.422 1.878 0.963 109.2 101.1 91.3 1.849 0.962 1.677 103.9 109.7 78.0 1.705 2.360 0.971 63.3 159.2
S. Guha, J.S. Franciscor Chemical Physics Letters 319 (2000) 650–654
652
Table 2 Harmonic frequencies Žcmy1 . and infrared intensities Žkm moly1 . for HOOBr a , HOBrO a , and the HOOBr™ HOBrO isomerization transition state Species
Symmetry
Mode no.
Description
QCISDr6-31GŽd. Frequencies
Intensities
HOOBr
a
HOBrO
X
a
wHOOBr™ HOBrOX x /
a
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
HO stretch HOO bend OO stretch BrO stretch BrOO bend torsion HO stretch HOBr bend X BrO stretch BrO stretch X OBrO bend torsion HO stretch HOBr bend X BrO stretch OBr stretch Reaction coord. X OBrO bend
3663 1451 962 561 413 294 3727 1148 811 531 308 223 3566 789 697 283 486i 228
36 50 62 17 106 7 71 51 21 40 130 9 20 109 45 5 138 149
X
Y
a a
Ref. w7x.
leading to isomerization into HOBrO. Unlike the previous HOOBr™ HOBrO transition state located by Guha and Francisco w7x, this new HOOBr™ HOBrO isomerization transition state possesses several features that can be better compared to the structure of the HOBrO species, than that of stable HOOBr. The O–Br and Br–OX lengths in HOBrOX ˚ respectively, while they are are 1.849 and 1.677 A, ˚ in the HOOBr™ HOBrO transi2.360 and 1.705 A tion state structure. The H–O bond in HOBrO Ž0.962 ˚ . is a bit shorter than the H–O bond Ž0.971 A˚ . in A
the transition state, due to the structural differences between the two species. The HOBr angle is about 568 wider, while the OBrOX angle is about 458 narrower in the transition state structure than in HOBrOX . The HOOBr™ HOBrO isomerization transition state structure resembles that of HOBrO rather than HOOBr. The calculated harmonic frequencies of the HOOBr™ HOBrOX isomerization transition state is provided in Table 2, along with those of the stable HOOBr and HOBrO species. The vibrational fre-
Table 3 Total and single-point energiesa Žhartree. for HOOBr b and the HOOBr™ HOBrO isomerization transition state Levels of theory
HOOBr
wHOOBr™ HOBrOx /
QCISDr6-31GŽd. QCISDr6-311GŽd,p. QCISDr6-311GŽ2d,2p. QCISDŽT.r6-311GŽ2d,2p. a QCISDŽT.r6-311q q GŽ2df,2p. a QCISDŽT.r6-311q q GŽ3df,3pd. a
y2720.52057 y2723.09483 y2723.13726 y2723.16041 y2723.24946 y2723.27240
y2720.42722 y2722.99912 y2723.04400 y2723.08895 y2723.18139 y2723.20553
a b
Calculated using the QCISDr6-311GŽ2d,2p. geometries. Ref. w7x.
S. Guha, J.S. Franciscor Chemical Physics Letters 319 (2000) 650–654
653
Table 4 Relative energetics Žkcal moly1 . for the HOOBr™ HOBrO isomerization transition state wHOOBr™ HOBrOx /
Levels of theory
QCISDr6-31GŽd. QCISDr6-311GŽd,p. QCISDr6-311GŽ2d,2p. QCISDŽT.r6-311GŽ2d,2p. a QCISDŽT.r6-311q q GŽ2df,2p. a QCISDŽT.r6-311q q GŽ3df,3pd. a a
D Hr,08
Barrier heights
16.9 19.8 12.6 11.1 4.8 3.8
56.0 57.5 56.0 42.3 40.2 39.4
Calculated using the QCISDr6-311GŽ2d,2p. geometries.
quencies are calculated at the QCISDr6-31GŽd. level of theory. A comparison of the frequencies of the HOOBr ™ HOBrOX transition state with those of HOBrO reveals interesting features. The wHOOBr ™ HOBrOX x / structure has one imaginary frequency, suggesting that the process of isomerization is a
first-order saddle point. The H–O stretch in wHOOBr ™ HOBrOX x / occurs with a lower frequency Ž3566 cmy1 . and intensity Ž20 km moly1 . than the H–O stretch in HOBrO Ž3727 cmy1 ., since the H–O bond in the transition state is longer than the H–O bond in HOBrO. The Br–OX Ž697 cmy1 . and O–Br Ž283 cmy1 . stretches in wHOOBr™ HOBrOX x / occur at
Fig. 1. Relative energetics of the dissociation and isomerization pathways for HBrO 2 isomers.
654
S. Guha, J.S. Franciscor Chemical Physics Letters 319 (2000) 650–654
much lower frequencies than the corresponding Br– OX and O–Br stretches in HOBrOX , consistent with the longer Br–OX and O–Br bonds in the transition state. The OBrOX bend in wHOOBr™ HOBrOX x / occurs with a lower frequency Ž228 cmy1 . than the OBrOX bend in HOBrOX Ž308 cmy1 ..
BrO reaction may proceed through the exclusive formation of HOOBr Žor HOBrO., to form Br and HO 2 as the products. It is not likely for the isomerization of HOOBr to HOBrO to occur, due to the high energy barriers associated with the isomerization process.
3.2. RelatiÕe energetics 4. Conclusion The total energies for HOOBr and the HOOBr™ HOBrO transition state, calculated at the QCISD level of theory using the 6-31GŽd., 6-311GŽd,p., and 6-311GŽ2d,2p. basis sets, are provided in Table 3. Also listed in Table 3 are single-point energy values for the species at the QCISDŽT. theory level with the 6-311GŽ2d,2p., 6-311q q GŽ2df,2p., and 6-311 q q GŽ3df,3pd. basis sets, incorporating the geometrical parameters obtained at the QCISDr6-311GŽ2d,2p. level of theory. In Table 4 are listed the heats of reaction and heights of energy barriers for the HOOBr™ HOBrO transition state at the QCISD and QCISDŽT. levels of theory using various basis sets. From Table 4 it is observed that, for the HOOBr ™ HOBrO transition state, the heat of reaction at 0 K is 3.8 kcal moly1 , while the energy barrier is 39.4 kcal moly1 at the QCISDŽT.r6-311q q GŽ3df,3pd. level of theory. In our previous computations, for the first HOOBr™ HOBrO transition state, the height of the energy barrier was 67.2 kcal moly1 w7x. Thus, the second HOOBr™ HOBrO isomerization transition state appears to possess a lower energy barrier Žby ; 28 kcal moly1 . than the first, but the barrier is still sufficiently high not to allow the interconversion of HOOBr and HOBrO. For the HOBrO™ HBrO 2 transition state, the barrier height was found to be 72.2 kcal moly1 w7x, and, thus, there appears to be a higher energy barrier Žby ; 33 kcal moly1 . for the HOBrO™ HBrO 2 isomerization process over the second HOOBr™ HOBrO transition state. Fig. 1 shows a plot of the relative energetics of the dissociation and isomerization pathways of the HBrO 2 isomers and the transition states. The HO q
The equilibrium structure, vibrational spectrum, and relative energetics of another HOOBr™ HOBrO isomerization transition state has been investigated using the QCISD ab initio electronic structure method. The energy barrier for the isomerization process is found to be 39.4 kcal moly1 , which is high enough to bar the interconversion of HOOBr and HOBrO. Thus, the HO q BrO reaction proceeds via the formation of the HOOBr Žor HOBrO. intermediate to produce Br q HO 2 .
References w1x S.C. Wofsy, M.B. McElroy, Y.L. Yung, Geophys. Res. Lett. 2 Ž1975. 215. w2x J.G. Anderson, D.W. Toohey, W.H. Brune, Science 251 Ž1991. 39. w3x R.J. Salawitch, M.B. McElroy, J.H. Yatteau, S.C. Wofsy, M.R. Schoeberl, L.R. Lait, P.A. Newman, K.R. Chan, M. Loewenstein, J.R. Podolske, S.E. Srahan, M.H. Profitt, Geophys. Res. Lett. 17 Ž1990. 561. w4x D.J. Bogan, R.P. Thorn, F.L. Nesbitt, L.J. Stief, J. Phys. Chem. 100 Ž1996. 14383. w5x T.J. Lee, Chem. Phys. Lett. 262 Ž1996. 559. w6x S. Guha, J.S. Francisco, J. Phys. Chem. A 101 Ž1997. 5347. w7x S. Guha, J.S. Francisco, Chem. Phys. 247 Ž1999. 387. w8x M.J. Frisch et al., GAUSSIAN 94, Revision D.2, Gaussian, Pittsburgh, PA, 1995. w9x J.A. Pople, M. Head-Gordon, K. Raghavachari, J. Chem. Phys. 87 Ž1987. 5968. w10x W.J. Hehre, R. Ditchfield, J.A. Pople, J. Chem. Phys. 56 Ž1972. 2257. w11x R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 Ž1980. 650.
Chemical Physics Letters 319 Ž2000. 650–654 www.elsevier.nlrlocatercplett
The isomerization of HOOBr to HOBrO Sujata Guha, Joseph S. Francisco
)
Department of Chemistry and Department of Earth and Atmospheric Sciences, Purdue UniÕersity, West Lafayette, IN 47907-1393, USA Received 8 December 1999
Abstract A second transition state for the process of isomerization of HOOBr™ HOBrO has been located theoretically, using ab initio molecular orbital methods. The energy barrier for this isomerization pathway is found to be 39.4 kcal moly1. This result suggests that the barrier is sufficiently high so as not to facilitate the isomerization of HOOBr to HOBrO. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction In 1975, Wofsy et al. w1x suggested that bromine atoms could destroy ozone more effectively than chlorine atoms, since more bromine is present in its active forms, Br and BrO, than chlorine is in its active Cl and ClO forms. It has been estimated that about ; 25% of the ozone loss in Antarctica w2x and up to 40% of ozone loss over the Arctic region during winter w3x is due to the chemical reactions involving bromine species. The coupling of bromine oxides ŽBrO x . with HO x species to destroy ozone has been of particular importance. A critical reaction is the coupling between BrO and OH radicals, which leads to increased recycling of bromine, and the eventual depletion of
) Corresponding author. Fax: q1-765-494-0239; e-mail: [email protected]
ozone. Of the following possible representations of the BrO q HO reaction pathways, BrO q OH ™ Br q HO 2
Ž 1a .
BrO q OH ™ HBr q O 2
Ž 1b .
channel 1a is postulated to be the predominant product channel, by Bogan et al. w4x, proceeding via formation of the short-lived wHOOBrx ) vibrationally excited addition complex. The HOOBr complex is the most stable of the three plausible isomeric forms of HBrO 2 Žthe other two being HOBrO and HBrO 2 ., according to computational studies performed by Lee w5x, and Guha and Francisco w6x. The transition states for the HOOBr™ HOBrO and HOBrO™ HBrO 2 isomerization pathways were, previously, theoretically investigated by Guha and Francisco w7x. It was found that even though the HOOBr and HOBrO intermediates can form during the HO q BrO reaction, the energy barrier for the isomerization of HOOBr to HOBrO is high enough
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 0 0 . 0 0 1 0 0 - 7
S. Guha, J.S. Franciscor Chemical Physics Letters 319 (2000) 650–654
Ž67.2 kcal moly1 . to not allow the interconversion of HOOBr to HOBrO Žand vice-versa.. There can be yet another transition state for the isomerization of HOOBr to HOBrO, and, in this Letter, we present ab initio molecular orbital results of the structure, vibrational spectrum, and energetics of this new transition state.
2. Computational methods Ab initio molecular orbital calculations were performed using the GAUSSIAN 94 program w8x. The equilibrium geometry for the HOOBr™ HOBrO isomerization process was fully optimized to better then ˚ for bond distances and 0.18 for bond angles, 0.001 A with a self-consistent field convergence of at least 10y9 on the density matrix. The QCISD Žquadratic configuration interaction with single and double excitations. method w9x was used with the 6-31GŽd. w10x, 6-311GŽd,p. w11x, and 6-311GŽ2d,2p. basis sets in the optimization of the geometry. The harmonic vibrational frequencies and intensities for the isomerization process were calculated at the QCISD level
651
of theory in conjunction with the 6-31GŽd. basis set, using the geometry calculated at the QCISDr631GŽd. level of theory. To improve the energies, single-point calculations were performed with the QCISDŽT. Žquadratic configuration interaction with single and double excitations incorporating the perturbative corrections for triple excitation. wavefunctions w9x, using the optimized geometry obtained at the QCISDr6-311GŽ2d,2p. level of theory.
3. Results and discussion 3.1. Geometry and Õibrational frequencies Computations on the HOOBr™ HOBrO isomerization transition state were performed using the QCISD method in conjunction with the 6-31GŽd., 6-311GŽd,p. and 6-311GŽ2d,2p. basis sets. The structural parameters of this transition state are listed in Table 1. This transition state is formed due to the migration of the bromine atom in HOOBr, eventually
Table 1 ˚ and degrees. for HOOBr,a HOBrO,a and the HOOBr™ HOBrO isomerization transition state Optimized geometries ŽA Species
Coordinates
HOOBr
OO BrO HO OOBr HOO HOOBr OBr HO X BrO HOBr X OBrO X HOBrO X BrO OBr HO X OBrO HOBr
X
HOBrO
wHOBrO™ HOBrOX x /
a
Ref. w7x.
Levels of theory QCISDr 6-31GŽd.
6-311GŽd,p.
6-311GŽ2d,2p.
1.428 1.910 0.980 109.3 101.2 89.5 1.883 0.980 1.705 103.5 110.2 75.2 1.735 2.371 0.989 63.8 156.8
1.405 1.909 0.965 110.1 101.7 91.3 1.875 0.965 1.693 103.4 110.7 74.2 1.725 2.378 0.974 64.7 155.5
1.422 1.878 0.963 109.2 101.1 91.3 1.849 0.962 1.677 103.9 109.7 78.0 1.705 2.360 0.971 63.3 159.2
S. Guha, J.S. Franciscor Chemical Physics Letters 319 (2000) 650–654
652
Table 2 Harmonic frequencies Žcmy1 . and infrared intensities Žkm moly1 . for HOOBr a , HOBrO a , and the HOOBr™ HOBrO isomerization transition state Species
Symmetry
Mode no.
Description
QCISDr6-31GŽd. Frequencies
Intensities
HOOBr
a
HOBrO
X
a
wHOOBr™ HOBrOX x /
a
1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6
HO stretch HOO bend OO stretch BrO stretch BrOO bend torsion HO stretch HOBr bend X BrO stretch BrO stretch X OBrO bend torsion HO stretch HOBr bend X BrO stretch OBr stretch Reaction coord. X OBrO bend
3663 1451 962 561 413 294 3727 1148 811 531 308 223 3566 789 697 283 486i 228
36 50 62 17 106 7 71 51 21 40 130 9 20 109 45 5 138 149
X
Y
a a
Ref. w7x.
leading to isomerization into HOBrO. Unlike the previous HOOBr™ HOBrO transition state located by Guha and Francisco w7x, this new HOOBr™ HOBrO isomerization transition state possesses several features that can be better compared to the structure of the HOBrO species, than that of stable HOOBr. The O–Br and Br–OX lengths in HOBrOX ˚ respectively, while they are are 1.849 and 1.677 A, ˚ in the HOOBr™ HOBrO transi2.360 and 1.705 A tion state structure. The H–O bond in HOBrO Ž0.962 ˚ . is a bit shorter than the H–O bond Ž0.971 A˚ . in A
the transition state, due to the structural differences between the two species. The HOBr angle is about 568 wider, while the OBrOX angle is about 458 narrower in the transition state structure than in HOBrOX . The HOOBr™ HOBrO isomerization transition state structure resembles that of HOBrO rather than HOOBr. The calculated harmonic frequencies of the HOOBr™ HOBrOX isomerization transition state is provided in Table 2, along with those of the stable HOOBr and HOBrO species. The vibrational fre-
Table 3 Total and single-point energiesa Žhartree. for HOOBr b and the HOOBr™ HOBrO isomerization transition state Levels of theory
HOOBr
wHOOBr™ HOBrOx /
QCISDr6-31GŽd. QCISDr6-311GŽd,p. QCISDr6-311GŽ2d,2p. QCISDŽT.r6-311GŽ2d,2p. a QCISDŽT.r6-311q q GŽ2df,2p. a QCISDŽT.r6-311q q GŽ3df,3pd. a
y2720.52057 y2723.09483 y2723.13726 y2723.16041 y2723.24946 y2723.27240
y2720.42722 y2722.99912 y2723.04400 y2723.08895 y2723.18139 y2723.20553
a b
Calculated using the QCISDr6-311GŽ2d,2p. geometries. Ref. w7x.
S. Guha, J.S. Franciscor Chemical Physics Letters 319 (2000) 650–654
653
Table 4 Relative energetics Žkcal moly1 . for the HOOBr™ HOBrO isomerization transition state wHOOBr™ HOBrOx /
Levels of theory
QCISDr6-31GŽd. QCISDr6-311GŽd,p. QCISDr6-311GŽ2d,2p. QCISDŽT.r6-311GŽ2d,2p. a QCISDŽT.r6-311q q GŽ2df,2p. a QCISDŽT.r6-311q q GŽ3df,3pd. a a
D Hr,08
Barrier heights
16.9 19.8 12.6 11.1 4.8 3.8
56.0 57.5 56.0 42.3 40.2 39.4
Calculated using the QCISDr6-311GŽ2d,2p. geometries.
quencies are calculated at the QCISDr6-31GŽd. level of theory. A comparison of the frequencies of the HOOBr ™ HOBrOX transition state with those of HOBrO reveals interesting features. The wHOOBr ™ HOBrOX x / structure has one imaginary frequency, suggesting that the process of isomerization is a
first-order saddle point. The H–O stretch in wHOOBr ™ HOBrOX x / occurs with a lower frequency Ž3566 cmy1 . and intensity Ž20 km moly1 . than the H–O stretch in HOBrO Ž3727 cmy1 ., since the H–O bond in the transition state is longer than the H–O bond in HOBrO. The Br–OX Ž697 cmy1 . and O–Br Ž283 cmy1 . stretches in wHOOBr™ HOBrOX x / occur at
Fig. 1. Relative energetics of the dissociation and isomerization pathways for HBrO 2 isomers.
654
S. Guha, J.S. Franciscor Chemical Physics Letters 319 (2000) 650–654
much lower frequencies than the corresponding Br– OX and O–Br stretches in HOBrOX , consistent with the longer Br–OX and O–Br bonds in the transition state. The OBrOX bend in wHOOBr™ HOBrOX x / occurs with a lower frequency Ž228 cmy1 . than the OBrOX bend in HOBrOX Ž308 cmy1 ..
BrO reaction may proceed through the exclusive formation of HOOBr Žor HOBrO., to form Br and HO 2 as the products. It is not likely for the isomerization of HOOBr to HOBrO to occur, due to the high energy barriers associated with the isomerization process.
3.2. RelatiÕe energetics 4. Conclusion The total energies for HOOBr and the HOOBr™ HOBrO transition state, calculated at the QCISD level of theory using the 6-31GŽd., 6-311GŽd,p., and 6-311GŽ2d,2p. basis sets, are provided in Table 3. Also listed in Table 3 are single-point energy values for the species at the QCISDŽT. theory level with the 6-311GŽ2d,2p., 6-311q q GŽ2df,2p., and 6-311 q q GŽ3df,3pd. basis sets, incorporating the geometrical parameters obtained at the QCISDr6-311GŽ2d,2p. level of theory. In Table 4 are listed the heats of reaction and heights of energy barriers for the HOOBr™ HOBrO transition state at the QCISD and QCISDŽT. levels of theory using various basis sets. From Table 4 it is observed that, for the HOOBr ™ HOBrO transition state, the heat of reaction at 0 K is 3.8 kcal moly1 , while the energy barrier is 39.4 kcal moly1 at the QCISDŽT.r6-311q q GŽ3df,3pd. level of theory. In our previous computations, for the first HOOBr™ HOBrO transition state, the height of the energy barrier was 67.2 kcal moly1 w7x. Thus, the second HOOBr™ HOBrO isomerization transition state appears to possess a lower energy barrier Žby ; 28 kcal moly1 . than the first, but the barrier is still sufficiently high not to allow the interconversion of HOOBr and HOBrO. For the HOBrO™ HBrO 2 transition state, the barrier height was found to be 72.2 kcal moly1 w7x, and, thus, there appears to be a higher energy barrier Žby ; 33 kcal moly1 . for the HOBrO™ HBrO 2 isomerization process over the second HOOBr™ HOBrO transition state. Fig. 1 shows a plot of the relative energetics of the dissociation and isomerization pathways of the HBrO 2 isomers and the transition states. The HO q
The equilibrium structure, vibrational spectrum, and relative energetics of another HOOBr™ HOBrO isomerization transition state has been investigated using the QCISD ab initio electronic structure method. The energy barrier for the isomerization process is found to be 39.4 kcal moly1 , which is high enough to bar the interconversion of HOOBr and HOBrO. Thus, the HO q BrO reaction proceeds via the formation of the HOOBr Žor HOBrO. intermediate to produce Br q HO 2 .
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