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Oxygen atom exchange in reactions of OH radicals with NO and ClO
13 March 1998
Chemical Physics Letters 285 Ž1998. 138–142
Oxygen atom exchange in reactions of OH radicals with NO and ClO Joseph S. Francisco Department of Chemistry and Department of Earth and Atmospheric Sciences, Purdue UniÕersity, West Lafayette, IN 47907-1393, USA Received 3 December 1997; in final form 30 December 1997
Abstract Oxygen atom exchange between OH and ŽNO, ClO. has been investigated by ab initio molecular orbital methods. Reactants and transition states were fully optimized with second-order Møller–Plesset perturbation and coupled-cluster methods using the 6-311GŽ3df,3pd. basis set. Theoretical results predicted at these levels of theory suggest that the reaction between OH and NO should exchange rapidly because the barrier to isomerization is significantly less than the O–N bond dissociation energy of the HONO adduct. For the OH q ClO reaction, the barrier to isomerization is estimated to be 3.1 kcal moly1 above the O–Cl bond dissociation energy of the HOClO adduct. q 1998 Published by Elsevier Science B.V.
1. Introduction The mechanism for the oxygen exchange reaction between hydroxyl radicals and oxygen-containing radicals has been suggested to proceed via bound adducts w1–6x. Studies of isotopically labeled oxygen atoms in the reactants have been instrumental in experimentally deducing the mechanism for such reactions w6,7x. The first step in the mechanism involves the formation of a bound adduct. The second step involves isomerization of the adduct followed by dissociation, viz., 18
OH q XO ° H18 OXO ° HOX18 O ™ OH q X18 O.
There are two limiting cases for oxygen exchange reactions involving bound adducts. The first occurs when the barrier for hydrogen atom migration is low. In this case, the isotope exchange occurs readily. The second case occurs when a substantial barrier in-
hibits the hydrogen atom transfer. In this case, the exchange rate is a function of temperature. The reaction of OH radicals with stable molecules such as N2 O, CO 2 , OCS and O 3 does not form stable adducts w3x. While the reaction of OH radicals with open-shell species such as NO and NO 2 is known to form stable adducts w8,9x, oxygen atom exchange has been observed for NO and NO 2 . The capability of a reaction not to undergo oxygen atom exchange is attributed to the fact that the hydrogen atom is not labile that is formed in the adduct because its movement is inhibited by an energy barrier or a geometrical constraint w6x. Two critical parameters determine whether exchange occurs. The first is the energy barrier for hydrogen atom transfer Ž E . and the second is the O–X bond dissociation energy Ž D . for the HOXO complex. In this study, ab initio molecular orbital theory is used to examine the potential energy surface for the oxygen atom exchange reaction between the reaction of OH radicals with NO and ClO. The possibility of
0009-2614r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 0 1 0 - 4
J.S. Franciscor Chemical Physics Letters 285 (1998) 138–142
oxygen exchange in the OH q ClO system has not been explored experimentally. The present study should provide insight into whether this reaction could involve an HOClO adduct.
2. Computational methods Ab initio molecular orbital calculations were performed with the GAUSSIAN 94 system w10x, using the split valence 6-311G basis set which is supplemented with polarization functions. All equilibrium geometries and transition state structures were fully optimized using analytical gradient methods with second-order Møller–Plesset perturbation theory ŽMP2., with all electrons unfrozen. The second derivatives from the MP2 calculations were used with the eigenvalue following method in optimizations with coupled-cluster methods involving single and double excitations with perturbative corrections for the triples ŽCCSDŽT... Vibrational frequencies and zero-point energies were obtained from analytical second derivatives calculated at the MP2r6311GŽ3df,3pd. level of theory using the MP2r6-311GŽ3df,3pd. optimized geometry.
3. Results and discussion The geometries and reactants, products, and transition states for the reaction of HO q NO and HO q ClO reactions computed at the MP2r6311GŽ3df,3pd. and CCSDŽT.r6-311GŽ3df,3pd. levels of theory are given in Table 1. There is no experimental geometry for HOClO reported in the literature. Cox et al. w11x did report an experimental structure for HONO with the MP2r6311GŽ3df,3pd. and CCSDŽT.r6-311GŽ3df,3pd. geometries reported in Table 1. The CCSDŽT.r6-311GŽ3df,3pd. geometries have the best agreement with the experimental structure reported by Cox et al. w11x, with an rms ˚ ŽMP2. and 0.004 A˚ ŽCCSDŽT.. deviation of 0.016 A in the bond angle. No experimental structure for the HOClO adduct has been reported in the literature, although there has more recently been a report of the first vibrational spectrum of the HOClO adduct w12x. Nevertheless, these calculations suggest that the prediction of the HOClO adduct structure at the
139
Table 1 Optimized geometriesa Species
Coordinate 6-311GŽ3df,3pd. MP2
reactants and products HO NO ClO X HONO
Exp.
CCSDŽT.
H–O N–O Cl–O X N–O N–O H–O X ONO HON X HONO X Cl–O Cl–O H–O X OClO HOCl X HOClO
0.964 1.135 1.552 1.174 1.410 0.964 110.8 101.5 180.0 1.481 1.693 0.967 112.7 104.0 80.3
0.969 0.970 b 1.151 1.151b 1.581 1.570 c 1.172 1.170 d 1.421 1.432 0.964 0.958 110.6 110.7 101.7 102.1 180.0 180.0 1.506 1.706 0.967 111.8 103.8 78.9
transition states X X HONO N–O X ™HO NO N–O H–O X ONO HON HONO X X HOClO Cl–O X ™HO ClO Cl–O H–O X OClO HOCl HOClO
1.258 1.258 1.296 105.8 76.4 0.0 1.592 1.592 1.271 88.5 74.8 0.0
1.264 1.264 1.295 105.1 76.6 0.0 1.622 1.622 1.266 86.9 74.7 0.0
X
HOClO
a
˚ and angles in degrees. Bond distances in A Ref. w17x. c Ref. w18x. d Ref. w11x. b
CCSDŽT.r6-311GŽ3df,3pd. level of theory should be reasonably well predicted. The transition state for the HO q NO reactions is four-centered and proceeds through a planar ring structure as shown in Fig. 1. In the transition state for HONO isomerization, the OH bond lengthens by 34% relative to HONO at the CCSDŽT.r6311GŽ3df,3pd. level of theory. The transition state was fully optimized without symmetry constraints. The transition state, however, is symmetrical in the breaking and forming OH bonds, as well as in the NO bonds. Note that, in HONO, the NO bonds are unequal with the terminal NO bond exhibiting dou-
J.S. Franciscor Chemical Physics Letters 285 (1998) 138–142
140
Fig. 1. Transition state and transition vectors for HONO isomerization. Numbers without asterisk are MP2 values; numbers with asterisk are CCSDŽT. values.
˚ . and the inner NO ble bond character ŽNO s 1.172 A bond showing single bond character ŽNO s 1.421 ˚ .. In the transition states, the NO bonds are interA mediate between single and double bond character ˚ .. The transition state is a true firstŽNO s 1.264 A order saddle point with one imaginary frequency given in Table 2. The magnitude of the imaginary frequency is quite large Ž1869i .. An examination of the transition state vector shows that the dominant motion is characterized by hydrogen atom transfer. For the HO q ClO reaction, the transition state is also a four-centered planar structure, as shown in Fig. 2. In this transition state, the OH bond lengthens by 31% relative to HOClO at the CCSDŽT.r6311GŽ3df,3pd. level of theory. Like the HO q NO reaction, the transition state is symmetrical in the OH and ClO bond lengths. The harmonic vibrational Table 2 Vibrational frequencies Species
Vibrational frequenciesa Žcmy1 .
HO NO ClO HONO
3883 3231 845 3846, 1699, 1305, 854, 640, 604 3799, 1186, 1164, 636, 440, 333 2142, 1509, 1320, 1273, 1026, 1869i 2142, 929, 903, 885, 646, 1688i
HOClO wHONOX ™HOX NOx‡ wHOClOX ™HOX ClOx‡ a
Zero-point energy Žkcal moly1.
Fig. 2. Transition state and transition vector for HOClO isomerization. Numbers without asterisk are MP2 values; numbers with asterisk are CCSDŽT. values.
frequencies for the transition state structure verify that it is a first-order saddle point with exactly one imaginary vibrational frequency. The imaginary frequency is 1673i and the motion is characterized by hydrogen atom transfer, similar to the HO q NO transition state. Considering the magnitude of these barriers, it is very likely that tunneling will be important in the dynamics of these reactions. A simple potential energy surface diagram for oxygen atom exchange in the reaction of OH with XO Žwhere X s N and Cl. is depicted in Fig. 3. The key parameters needed to assess whether exchange could occur are HO–XO bond dissociation energy Ž D . and the barriers for hydrogen atom transfer Ž E .. The total energies for the reactants and transition states for the HO q XO reaction Žwhere X s N at Cl. are given in Table 3. The relative energies are given in Table 4. The bond dissociation energy for the HO–NO adduct is estimated as 55.8 kcal moly1 at the MP2.6-311GŽ3df,3pd. level of theory. The result reduced to 48.3 kcal moly 1 at the CCSDŽT.r6-311GŽ3df,3pd. level of theory. Clearly,
5.5 4.6 1.2 12.8 10.8 10.4 10.4
Calculated at the UMP2r6-311GŽ3df,3pf. level of theory.
Fig. 3. Potential energy surface for oxygen atom exchange in the reaction of OH with XO. Ds HO–XO bond dissociation energy and Es isomerization barrier for hydrogen atom transfer.
J.S. Franciscor Chemical Physics Letters 285 (1998) 138–142 Table 3 Total energiesa Species
6-311GŽ3df,3pd. MP2
reactants and products HO NO ClO HONO HOClO transition states X X HONO ™ HO NO X X HOClO ™ HO ClO a
CCSDŽT.
y75.63883 y129.73400 y534.79094 y205.46604 y610.49834
y75.63538 y129.71289 y534.72208 y205.42949 y610.39990
y205.42205 y610.45210
y205.38200 y610.35434
Total energies are in units of Hartrees.
higher-order correlations are important in the estimation of the energetics for these reactions. Moreover, the theoretical estimated bond dissociation energy values for HO–NO are well within the experimental range of values of 49.3 to 47.5 kcal moly1 w13,14x which suggests that the theoretical results are reasonably well predicted at the CCSDŽT.r6311GŽ3df,3pd. level of theory. The calculated hydrogen atom exchange barrier Ž E . for the OH q NO reaction is 27.3 kcal moly1 . This barrier is 21 kcal moly1 below the bond dissociation energy for the HO–NO adduct. This suggests that exchange should be rapid for the HO q NO reaction because there is no barrier to inhibit the exchange. This is consistent with the experimental observations of Smith and Williams w13x, Dransfeld et al. w3x, and Greenblatt and Howard w6x. For the OH q ClO reaction, the bond dissociation energy Ž D . is predicted to be 22.6 kcal moly1 at the CCSDŽT.r6-311GŽ3df,3pd.. At present there are
Table 4 Relative energiesa,b System
HO q NO HO q ClO
6-311GŽ3df,3pd.
D E D E
MP2
CCSDŽT.
55.8 25.1 39.0 26.1
48.3 27.3 22.6 25.7
141
no direct experimental estimates of the bond dissociation energy for the HOClO adduct. From previous ab initio studies w15x, the heat of formation for HOClO has been estimated to be 11.9 " 2. Using this value and the corresponding heats of formations for HO and ClO, an estimate of the bond dissociation energy for the HOClO adduct is 21.4 kcal moly1 . This is consistent with the CCSD Ž T . r6311GŽ3df,3pd. estimate of 22.6 kcal moly1 . The hydrogen atom exchange barrier Ž E . for the OH q ClO reaction is estimated to be 25.7 kcal moly1 at the CCSDŽT.r6-311GŽ3df,3pd. level of theory. It is interesting to note that the barriers for hydrogen atom exchange for HONO and HOClO are comparable, i.e. 27.3 versus 25.7 kcal moly1 for HONO and HOClO, respectively. This is not unexpected because the transition state involves hydrogen across oxygen atoms. Note, however, that the barrier for HOClO is 3.1 kcal moly1 above the bond dissociation energy of the HOClO adduct, i.e. D - E. In this case, the barrier for hydrogen exchange could inhibit the hydrogen atom transfer because there may not be sufficient energy to overcome the barrier Ž D - E .. The barrier for the hydrogen atom exchange is quite narrow as judged by the magnitude of the imaginary frequency Ž1673i cmy1 .. In this case the OH q ClO exchange reaction could be inhibited by a small barrier whereas the OH q NO has no such barrier. It should be noted that the reaction of OH q ClO, could proceed by the formation of HOOCl complex, since it is 8.3 kcal moly1 more stable than HOClO w15x. However, oxygen atom migration from HOOCl to form HOClO requires quite a high barrier. Preliminary ab initio studies suggest that this barrier is about 80.0 kcal moly1 above the HOOCl well and 71.7 kcal moly1 above HOClO w16x. Oxygen exchange through the HOOCl complex is therefore unlikely.
Exp.
49.3, 47.5
a Ds HO–XO bond dissociation energy and Es barrier for hydrogen atom transfer. b Relative energies in units of kcal moly1 .
4. Conclusion The bond dissociation energy and hydrogen exchange barriers have been predicted for HOXO adducts for X s N and Cl. HONO is predicted to be more bound than the HOClO adduct by 25.7 kcal moly1 . The oxygen atom exchange for the OH q
142
J.S. Franciscor Chemical Physics Letters 285 (1998) 138–142
ClO reaction, however, is predicted to be inhibited by a small barrier whereas the OH q NO reaction does not have to overcome a hydrogen atom exchange barrier. Acknowledgements The Jet Propulsion Laboratory ŽJPL. Supercomputing Project is greatly acknowledged for support of this computing research. The JPL Supercomputing Project is sponsored by JPL and the NASA Office of Space Science and Application. References w1x U.C. Sridharan, F.S. Klein, F. Kaufman, J. Chem. Phys. 82 Ž1985. 592. w2x S.M. Anderson, F.S. Klein, F. Kaufman, J. Chem. Phys. 83 Ž1985. 1648. w3x P. Dransfeld, J. Lukas, H.Gg. Wagner, Z. Naturforsch. A. Phys. Chem. 41 Ž1986. 1283. w4x H. Niki, P.D. Maker, C.M. Savage, L.P. Breitenbach, J. Phys. Chem. 88 Ž1984. 2116. w5x J.F. Gleason, C.J. Howard, J. Phys. Chem. 92 Ž1988. 3414. w6x G.D. Greenblatt, C.J. Howard, J. Phys. Chem. 93 Ž1989. 1035.
w7x G.D. Greenblatt, H. Puckerman, Y. Haas, Chem. Phys. Lett. 134 Ž1987. 593. w8x I.W.M. Smith, Chem. Phys. Lett. 49 Ž1977. 112. w9x J.F. Gleason, C.J. Howard, J. Phys. Chem. 92 Ž1988. 3414. w10x M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, and J.A. Pople, GAUSSIAN 94, Revision B.1, GAUSSIAN, Inc., Pittsburgh, PA, 1995. w11x P.A. Cox, A.H. Brittain, D.J. Finnigan, Trans. Faraday Soc. 67 Ž1971. 2179. w12x K. Johnsson, A. Engdahl, B. Nelander, J. Phys. Chem. 100 Ž1996. 3923. w13x I.W.M. Smith, M.D. Williams, J. Chem. Soc., Faraday Trans. II 81 Ž1985. 1849. w14x S.W. Benson, Thermochemical Kinetics ŽWiley, New York, 1976.. w15x J.S. Francisco, S.P. Sander, T.J. Lee, A.P. Rendell, J. Phys. Chem. 98 Ž1994. 5644. w16x J.S. Francisco, unpublished work. w17x K.P. Huber, G. Herzberg, Constants of Diatomic Molecules ŽVan Nostrand Reinhold, New York, 1979.. w18x T. Amons, E. Hirsta, Y. Movino, J. Mol. Spec. 27 Ž1968. 257.
Chemical Physics Letters 285 Ž1998. 138–142
Oxygen atom exchange in reactions of OH radicals with NO and ClO Joseph S. Francisco Department of Chemistry and Department of Earth and Atmospheric Sciences, Purdue UniÕersity, West Lafayette, IN 47907-1393, USA Received 3 December 1997; in final form 30 December 1997
Abstract Oxygen atom exchange between OH and ŽNO, ClO. has been investigated by ab initio molecular orbital methods. Reactants and transition states were fully optimized with second-order Møller–Plesset perturbation and coupled-cluster methods using the 6-311GŽ3df,3pd. basis set. Theoretical results predicted at these levels of theory suggest that the reaction between OH and NO should exchange rapidly because the barrier to isomerization is significantly less than the O–N bond dissociation energy of the HONO adduct. For the OH q ClO reaction, the barrier to isomerization is estimated to be 3.1 kcal moly1 above the O–Cl bond dissociation energy of the HOClO adduct. q 1998 Published by Elsevier Science B.V.
1. Introduction The mechanism for the oxygen exchange reaction between hydroxyl radicals and oxygen-containing radicals has been suggested to proceed via bound adducts w1–6x. Studies of isotopically labeled oxygen atoms in the reactants have been instrumental in experimentally deducing the mechanism for such reactions w6,7x. The first step in the mechanism involves the formation of a bound adduct. The second step involves isomerization of the adduct followed by dissociation, viz., 18
OH q XO ° H18 OXO ° HOX18 O ™ OH q X18 O.
There are two limiting cases for oxygen exchange reactions involving bound adducts. The first occurs when the barrier for hydrogen atom migration is low. In this case, the isotope exchange occurs readily. The second case occurs when a substantial barrier in-
hibits the hydrogen atom transfer. In this case, the exchange rate is a function of temperature. The reaction of OH radicals with stable molecules such as N2 O, CO 2 , OCS and O 3 does not form stable adducts w3x. While the reaction of OH radicals with open-shell species such as NO and NO 2 is known to form stable adducts w8,9x, oxygen atom exchange has been observed for NO and NO 2 . The capability of a reaction not to undergo oxygen atom exchange is attributed to the fact that the hydrogen atom is not labile that is formed in the adduct because its movement is inhibited by an energy barrier or a geometrical constraint w6x. Two critical parameters determine whether exchange occurs. The first is the energy barrier for hydrogen atom transfer Ž E . and the second is the O–X bond dissociation energy Ž D . for the HOXO complex. In this study, ab initio molecular orbital theory is used to examine the potential energy surface for the oxygen atom exchange reaction between the reaction of OH radicals with NO and ClO. The possibility of
0009-2614r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 0 1 0 - 4
J.S. Franciscor Chemical Physics Letters 285 (1998) 138–142
oxygen exchange in the OH q ClO system has not been explored experimentally. The present study should provide insight into whether this reaction could involve an HOClO adduct.
2. Computational methods Ab initio molecular orbital calculations were performed with the GAUSSIAN 94 system w10x, using the split valence 6-311G basis set which is supplemented with polarization functions. All equilibrium geometries and transition state structures were fully optimized using analytical gradient methods with second-order Møller–Plesset perturbation theory ŽMP2., with all electrons unfrozen. The second derivatives from the MP2 calculations were used with the eigenvalue following method in optimizations with coupled-cluster methods involving single and double excitations with perturbative corrections for the triples ŽCCSDŽT... Vibrational frequencies and zero-point energies were obtained from analytical second derivatives calculated at the MP2r6311GŽ3df,3pd. level of theory using the MP2r6-311GŽ3df,3pd. optimized geometry.
3. Results and discussion The geometries and reactants, products, and transition states for the reaction of HO q NO and HO q ClO reactions computed at the MP2r6311GŽ3df,3pd. and CCSDŽT.r6-311GŽ3df,3pd. levels of theory are given in Table 1. There is no experimental geometry for HOClO reported in the literature. Cox et al. w11x did report an experimental structure for HONO with the MP2r6311GŽ3df,3pd. and CCSDŽT.r6-311GŽ3df,3pd. geometries reported in Table 1. The CCSDŽT.r6-311GŽ3df,3pd. geometries have the best agreement with the experimental structure reported by Cox et al. w11x, with an rms ˚ ŽMP2. and 0.004 A˚ ŽCCSDŽT.. deviation of 0.016 A in the bond angle. No experimental structure for the HOClO adduct has been reported in the literature, although there has more recently been a report of the first vibrational spectrum of the HOClO adduct w12x. Nevertheless, these calculations suggest that the prediction of the HOClO adduct structure at the
139
Table 1 Optimized geometriesa Species
Coordinate 6-311GŽ3df,3pd. MP2
reactants and products HO NO ClO X HONO
Exp.
CCSDŽT.
H–O N–O Cl–O X N–O N–O H–O X ONO HON X HONO X Cl–O Cl–O H–O X OClO HOCl X HOClO
0.964 1.135 1.552 1.174 1.410 0.964 110.8 101.5 180.0 1.481 1.693 0.967 112.7 104.0 80.3
0.969 0.970 b 1.151 1.151b 1.581 1.570 c 1.172 1.170 d 1.421 1.432 0.964 0.958 110.6 110.7 101.7 102.1 180.0 180.0 1.506 1.706 0.967 111.8 103.8 78.9
transition states X X HONO N–O X ™HO NO N–O H–O X ONO HON HONO X X HOClO Cl–O X ™HO ClO Cl–O H–O X OClO HOCl HOClO
1.258 1.258 1.296 105.8 76.4 0.0 1.592 1.592 1.271 88.5 74.8 0.0
1.264 1.264 1.295 105.1 76.6 0.0 1.622 1.622 1.266 86.9 74.7 0.0
X
HOClO
a
˚ and angles in degrees. Bond distances in A Ref. w17x. c Ref. w18x. d Ref. w11x. b
CCSDŽT.r6-311GŽ3df,3pd. level of theory should be reasonably well predicted. The transition state for the HO q NO reactions is four-centered and proceeds through a planar ring structure as shown in Fig. 1. In the transition state for HONO isomerization, the OH bond lengthens by 34% relative to HONO at the CCSDŽT.r6311GŽ3df,3pd. level of theory. The transition state was fully optimized without symmetry constraints. The transition state, however, is symmetrical in the breaking and forming OH bonds, as well as in the NO bonds. Note that, in HONO, the NO bonds are unequal with the terminal NO bond exhibiting dou-
J.S. Franciscor Chemical Physics Letters 285 (1998) 138–142
140
Fig. 1. Transition state and transition vectors for HONO isomerization. Numbers without asterisk are MP2 values; numbers with asterisk are CCSDŽT. values.
˚ . and the inner NO ble bond character ŽNO s 1.172 A bond showing single bond character ŽNO s 1.421 ˚ .. In the transition states, the NO bonds are interA mediate between single and double bond character ˚ .. The transition state is a true firstŽNO s 1.264 A order saddle point with one imaginary frequency given in Table 2. The magnitude of the imaginary frequency is quite large Ž1869i .. An examination of the transition state vector shows that the dominant motion is characterized by hydrogen atom transfer. For the HO q ClO reaction, the transition state is also a four-centered planar structure, as shown in Fig. 2. In this transition state, the OH bond lengthens by 31% relative to HOClO at the CCSDŽT.r6311GŽ3df,3pd. level of theory. Like the HO q NO reaction, the transition state is symmetrical in the OH and ClO bond lengths. The harmonic vibrational Table 2 Vibrational frequencies Species
Vibrational frequenciesa Žcmy1 .
HO NO ClO HONO
3883 3231 845 3846, 1699, 1305, 854, 640, 604 3799, 1186, 1164, 636, 440, 333 2142, 1509, 1320, 1273, 1026, 1869i 2142, 929, 903, 885, 646, 1688i
HOClO wHONOX ™HOX NOx‡ wHOClOX ™HOX ClOx‡ a
Zero-point energy Žkcal moly1.
Fig. 2. Transition state and transition vector for HOClO isomerization. Numbers without asterisk are MP2 values; numbers with asterisk are CCSDŽT. values.
frequencies for the transition state structure verify that it is a first-order saddle point with exactly one imaginary vibrational frequency. The imaginary frequency is 1673i and the motion is characterized by hydrogen atom transfer, similar to the HO q NO transition state. Considering the magnitude of these barriers, it is very likely that tunneling will be important in the dynamics of these reactions. A simple potential energy surface diagram for oxygen atom exchange in the reaction of OH with XO Žwhere X s N and Cl. is depicted in Fig. 3. The key parameters needed to assess whether exchange could occur are HO–XO bond dissociation energy Ž D . and the barriers for hydrogen atom transfer Ž E .. The total energies for the reactants and transition states for the HO q XO reaction Žwhere X s N at Cl. are given in Table 3. The relative energies are given in Table 4. The bond dissociation energy for the HO–NO adduct is estimated as 55.8 kcal moly1 at the MP2.6-311GŽ3df,3pd. level of theory. The result reduced to 48.3 kcal moly 1 at the CCSDŽT.r6-311GŽ3df,3pd. level of theory. Clearly,
5.5 4.6 1.2 12.8 10.8 10.4 10.4
Calculated at the UMP2r6-311GŽ3df,3pf. level of theory.
Fig. 3. Potential energy surface for oxygen atom exchange in the reaction of OH with XO. Ds HO–XO bond dissociation energy and Es isomerization barrier for hydrogen atom transfer.
J.S. Franciscor Chemical Physics Letters 285 (1998) 138–142 Table 3 Total energiesa Species
6-311GŽ3df,3pd. MP2
reactants and products HO NO ClO HONO HOClO transition states X X HONO ™ HO NO X X HOClO ™ HO ClO a
CCSDŽT.
y75.63883 y129.73400 y534.79094 y205.46604 y610.49834
y75.63538 y129.71289 y534.72208 y205.42949 y610.39990
y205.42205 y610.45210
y205.38200 y610.35434
Total energies are in units of Hartrees.
higher-order correlations are important in the estimation of the energetics for these reactions. Moreover, the theoretical estimated bond dissociation energy values for HO–NO are well within the experimental range of values of 49.3 to 47.5 kcal moly1 w13,14x which suggests that the theoretical results are reasonably well predicted at the CCSDŽT.r6311GŽ3df,3pd. level of theory. The calculated hydrogen atom exchange barrier Ž E . for the OH q NO reaction is 27.3 kcal moly1 . This barrier is 21 kcal moly1 below the bond dissociation energy for the HO–NO adduct. This suggests that exchange should be rapid for the HO q NO reaction because there is no barrier to inhibit the exchange. This is consistent with the experimental observations of Smith and Williams w13x, Dransfeld et al. w3x, and Greenblatt and Howard w6x. For the OH q ClO reaction, the bond dissociation energy Ž D . is predicted to be 22.6 kcal moly1 at the CCSDŽT.r6-311GŽ3df,3pd.. At present there are
Table 4 Relative energiesa,b System
HO q NO HO q ClO
6-311GŽ3df,3pd.
D E D E
MP2
CCSDŽT.
55.8 25.1 39.0 26.1
48.3 27.3 22.6 25.7
141
no direct experimental estimates of the bond dissociation energy for the HOClO adduct. From previous ab initio studies w15x, the heat of formation for HOClO has been estimated to be 11.9 " 2. Using this value and the corresponding heats of formations for HO and ClO, an estimate of the bond dissociation energy for the HOClO adduct is 21.4 kcal moly1 . This is consistent with the CCSD Ž T . r6311GŽ3df,3pd. estimate of 22.6 kcal moly1 . The hydrogen atom exchange barrier Ž E . for the OH q ClO reaction is estimated to be 25.7 kcal moly1 at the CCSDŽT.r6-311GŽ3df,3pd. level of theory. It is interesting to note that the barriers for hydrogen atom exchange for HONO and HOClO are comparable, i.e. 27.3 versus 25.7 kcal moly1 for HONO and HOClO, respectively. This is not unexpected because the transition state involves hydrogen across oxygen atoms. Note, however, that the barrier for HOClO is 3.1 kcal moly1 above the bond dissociation energy of the HOClO adduct, i.e. D - E. In this case, the barrier for hydrogen exchange could inhibit the hydrogen atom transfer because there may not be sufficient energy to overcome the barrier Ž D - E .. The barrier for the hydrogen atom exchange is quite narrow as judged by the magnitude of the imaginary frequency Ž1673i cmy1 .. In this case the OH q ClO exchange reaction could be inhibited by a small barrier whereas the OH q NO has no such barrier. It should be noted that the reaction of OH q ClO, could proceed by the formation of HOOCl complex, since it is 8.3 kcal moly1 more stable than HOClO w15x. However, oxygen atom migration from HOOCl to form HOClO requires quite a high barrier. Preliminary ab initio studies suggest that this barrier is about 80.0 kcal moly1 above the HOOCl well and 71.7 kcal moly1 above HOClO w16x. Oxygen exchange through the HOOCl complex is therefore unlikely.
Exp.
49.3, 47.5
a Ds HO–XO bond dissociation energy and Es barrier for hydrogen atom transfer. b Relative energies in units of kcal moly1 .
4. Conclusion The bond dissociation energy and hydrogen exchange barriers have been predicted for HOXO adducts for X s N and Cl. HONO is predicted to be more bound than the HOClO adduct by 25.7 kcal moly1 . The oxygen atom exchange for the OH q
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ClO reaction, however, is predicted to be inhibited by a small barrier whereas the OH q NO reaction does not have to overcome a hydrogen atom exchange barrier. Acknowledgements The Jet Propulsion Laboratory ŽJPL. Supercomputing Project is greatly acknowledged for support of this computing research. The JPL Supercomputing Project is sponsored by JPL and the NASA Office of Space Science and Application. References w1x U.C. Sridharan, F.S. Klein, F. Kaufman, J. Chem. Phys. 82 Ž1985. 592. w2x S.M. Anderson, F.S. Klein, F. Kaufman, J. Chem. Phys. 83 Ž1985. 1648. w3x P. Dransfeld, J. Lukas, H.Gg. Wagner, Z. Naturforsch. A. Phys. Chem. 41 Ž1986. 1283. w4x H. Niki, P.D. Maker, C.M. Savage, L.P. Breitenbach, J. Phys. Chem. 88 Ž1984. 2116. w5x J.F. Gleason, C.J. Howard, J. Phys. Chem. 92 Ž1988. 3414. w6x G.D. Greenblatt, C.J. Howard, J. Phys. Chem. 93 Ž1989. 1035.
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