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An ab initio study of the reaction between FO radicals and H2O
Volume 215, llumber 1,2,3
CHEMICAL PHYSICS LETTERS
26 November 1993
An ab initio study of the reaction between FO radicals and Hz0 Joseph S. Francisco
and Yi Su
Department of Chemistry, Wayne State University, Detroit, MI 48202, USA
Received 13 July 1993; in final form 10 September 1993
Ab initio calculations are used to examine the energetics for the reaction of FO radicals and H,O. Optimized geometries have been calculated for all reactants, transition states, and products at the unrestricted second-order Msller-Plesset perturbation level of theory. Both Msller-Plesset perturbation (up to fourth-order) and quadratic configuration interaction (QCISD(T) ) methods are used to compute the energetics. The best estimate for the heat of reaction is 18.1 kcal mol- ’ at the QCISD( T ) /6-3 11 + + G (Zdf, 2d) level of theory, while the activation energy at this level is estimated as 25.7 kcal mol-‘. The atmospheric implications of this result are discussed.
1. Introduction The gas-phase chemistry of halogen monoxide (X0 where X =F, Cl and Br) has long been of atmospheric interest because of its role in catalytic cycles for ozone destruction. In recent years, there has been a plethora of work focusing on the roles of chlorine oxides and bromine oxides in stratospheric chemistry. The chemistry of inorganic fluorines, on the other hand, has not received as considerable attention. The reason is that fluorines, which result from photo-oxidation of the chlorofluorocarbons have largely been assumed to be scavenged readily by water and methane to form stable hydrogen fluoride [ 1,2 1. The hydrogen fluoride is usually rained out of the atmosphere. As a result this removes fluorine from potential catalytic cycles for the destruction of ozone, viz. F+03+F0+0z, FO+O-+F+02,
(1) (2)
FO+0,+F0,+02, FO,+O-+FO+O,,
(4) (5)
0+03-+20&
(3)
and viz. FO+Oa+FOz+02, F02+03+FO+202,
(4) (6)
203+ 302.
(7)
The above cycles are denoted FOO, cycles. Their viability depends on the likelihood of FO and FOI being removed by other species in the atmosphere. Previously we explored how probable are FO radicals to be removed by major atmospheric trace species such as HO and HO2 radicals [ 4,5 1. However, a critical knowledge of how FO radicals react with water, FO+H,O+HO+HOF,
0+0,+20,.
(3)
The analogous reactions for chlorine form an important process for ozone destruction [ 1,3]. Fluorines themselves are less likely to have potential perturbation effects on stratospheric ozone. However, there is potential atmospheric chemistry through the fluorine oxides viz. 58
(8)
is important since there is more water vapor than HO and HOz radicals in the atmosphere. This reaction fundamentally involves a hydrogen atom transfer process, which has been important in combustion chemistry. The reaction proceeds by a concerted bond breaking and making process to the transferring atom. No experimental kinetic data is
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 215, number 1,2,3
26 November 1993
CHEMICAL PHYSICS LETTERS
available for the FO+ Hz0 reaction. The study of reactions involving FO radicals is quite difficult experimentally; yet to evaluate the importance of FOO, species (FO, FOZ) in atmospheric processes, knowledge of the energetics is desirable. Consequently, in the present work, ab initio molecular orbital calculations are used to determine the activation energy barrier for the FO+HzO reaction in order to estimate the energetic feasibility and assess how important this reaction is in diminishing the involvement of fluorine oxides in atmospheric chemical processes.
and quadruple excitation using the optimized geometries obtained at the MP2/6-3 11G( d, p) level of theory. In order to remove the spin contamination from higher spin states, the spin projection method was performed by annihilating the highest spin contaminant of the unrestricted wavefunction. These energies are denoted by PMP4 [ 10 1. Quadratic configuration interaction theory using single, double, and triple excitations are also used to improve the energetics, The QCISD(T) method includes terms to correct for excitations to all orders in perturbation theory [ll].
2. Computational methods 3. Results and discussion Ab initio molecular orbital calculations were performed using the GAUSSIAN 92 system of program [ 6 1. Geometries were fully optimized at the unrestricted second-order Moller-Plesset perturbation (MP2) with all orbitals active, using 6-31G(d) [7] and 6-3 11G (d, p) [ 8 ] basis sets. Harmonic vibrational frequencies and zero-point energies for the reactants, products and transition states were computed with MP2 wavefunctions by numerical second derivatives [ 9 1. Extended electron correlation yas calculated with fourth-order Msller-Plesset perturbation theory in the space of single, double, triple
Table 1 lists the geometries for reactants, transition states and products. The transition state structure, shown in fig. 1, involves mainly three atoms in the abstraction process. They are the oxygen atom involving the FO and the HO atoms from water. The preferred approach of the incoming FO is out of the plane of the water. The newly formed O’H’ bond length is 1.108 A and the breaking H’ 0 bond in water is 1.186 A. The H’O bond which is attacked by the FO is elongated by 0.229 A; this is R 24% of its original length. The changes in the transition structure
Table 1 Optimized geometries of species involved in the FO + Hz0 reaction ‘) Species
Coordinate b,
UMP2/6-31G(d)
UMP2/6-31 lG(d, p)
Exp.
HO FO Hz0
HO FO HO HOH HO FO FOH FO H’O’ H’O HO FO’H’ O’H’O HOH’ FO’H’O HOH’O’
0.919 1.344 0.969 104.0 0.979 1.444 97.2 1.400 1.132 1.202 0.978 99.6 145.2 105.0 92.9 -25.5
0.966 1.328 0.957 102.5 0.965 1.424 97.9 1.387 1.108 1.187 0.965 101.0 154.2 104.5 90.9 -7.9
0.970 1.358 0.958 104.5 0.966 1.442 96.8
FOH
[FO+H,O]+
‘) Bond lengths in A, angles in degrees. b, See fig. 1 for atom labeling for the transition state. 59
Volume 215, number 1,2,3
CHEMICAL PHYSICS LETTERS
Fig. 1. (a) Transition state structure for FO+H,O abstraction reaction (MP2/6_31G(d) optimizedgeometry, no asterisk; MP2/ 6-31G(d, p) optimized, asterisk, see table 1 for complete list of geometrical parameters). (b) View of the transition state along the O’H’O axis (MP2/6-31G(d) optimized dihedral angle, no asterisk; MP2/6-31 lG(d, p) optimized, asterisk). Table 2 UMP2/6-3 lG* vibrational frequencies for species involved in the FO + HsO reaction Species
Frequencies (cm-’ ) a)
reactants and products 3741(3738) HO 1542(1033) FO 3918(3756), 3776 (3651), H20 1735(1594) 3713 (3578), 1404 (1355), FOH 985 (889) transition state [FO+HsO]+
3770,2174,1621,1361 921,410,351,145 2538i
Zero-point energy (kcal mol-i)
5.4 2.2 13.5 8.7
15.4
‘) Sources of experimental values: HO ref. [ 121, FO ref. [ 131 and all others ref. [ 141.
suggest that the transitional structure assumes more product-like characteristics. We have examined two other orientations of the approach of the FO radical to water. One orientation involves the fluorine relative to the terminal unattached hydrogen of water either in a cis or in a trans configuration. At the MP2/ 6-3 1G* the energy difference between cis and trans structures are 1.1 kcal mol- ‘, and the energy difference between the structure in fig. 1 and the cis structures is 1.7 kcal mol- ‘. However, an examination of the vibrational frequencies for the cis and trans structures show that these structures are second- and third-order saddle points. The out-of-plane transition state structure (fig. 1) as shown in table 2 is a true first-order saddle point for the hydrogen atom transfer process for the FO+HzO system. The im60
26 November 1993
aginary frequency of 2538i suggests that the barrier for this reaction is quite narrow and that tunneling through the barrier may be important. Relative energetics for the stationary points of the FO + Hz0 reaction are listed in table 3. These results are determined from the total energies given in table 4. The experimental heats of formation for FO, H20, HOandHOFare27.8fl [15], -57.12+0.01 [14], 9.175kO.27 [15] and 19.921 [16] kcalmol-‘, respectively. With these heats of formation, the experimental heat of reaction for the FO+HzO abstraction process is 18.6 + 0.8 kcal mol-‘. The calculated heat of reaction for the abstraction process at the UMP2/6-311G(d, p) level is 17.2 kcal mol-‘. This is 1.4 kcal mol-’ underestimated from the experimental estimate. From the UMP2/63 11G (d, p) level of theory, increase in basis set size and the form of higher-order correlation are important in improving the energetics. Increasing the basis set size by supplementing the 6-3 1lG( d, p) basis with an extra set of diffuse, d-polarization and f-polarization functions, increases the heat of reaction energy by 1.3 kcal mol-I. With correlation using full fourth-order and spin projection, or with correlation treatment using the QCISD(T) method, the basis set effect on the heat of reaction is 1.5 kcal mol-‘. The addition of f-polarization functions has the most impact on improving the energetics, which suggests that the energetics should be computed using large basis sets supplemented with f-polarization functions. Heats of reaction calculated with the use of the QCISD(T) method appear to be consistently lower than the Moller-Plesset (PMP4) results by about 0.6-0.8 kcal mol-*. With the largest basis set used, 6-3 11+ + G ( 2df, 2p) the calculated QCISD (T ) heat of reaction is 18.1 kcal mol-‘, and is in reasonable agreement with the experiment. The activation energy for the hydrogen abstraction by FO radicals from HZ0 to form HO radicals and HOF is estimated as 27.6 and 27.5 kcal mol-’ at the UMP2/6-31G(d) and UMP2/6-31 lG(d, p) levels of theory, respectively. Incorporation of higherorder electron correlation has the effect of lowering the activation energy, with either Moller-Plesset fourth-order with spin projection effects or quadratic configuration interaction methods. Improvement of the basis set size has more effect on the activation energy. The best estimated activated energy
26 November 1993
CHEMICAL PHYSICS LETTERS
Volume 215, number 1,2,3 Table 3 Heat of reaction and barrier height (kcal mol-i)
for FO+H*O reaction Heat of reaction
Activation energy
16.1 17.2
27.6 27.5
PMP4/6-31 lG(d, p) PMP4/6-311G(2d, 2p) PMP4/6-3ll++G(2d,2p) PMP4/6_311++G(2df, 2p)
17.6 17.7 18.2 18.9
25.1 25.4 26.9 26.6
QCISD(T)/6-31 lG(d, p) QCISD(T)/6-311G(2d, 2p) QCISD(T)/6_311++G(2d,2p) QCISD(T)/6-311+ +G(Zdf, 2p)
18.2 18.3 18.8 19.7
24.9 25.1 26.2 26.0
UMP2/6-31G(d) UMP2/6-311G(d,
p)
AZPE(MP2/6-31G(d))
- 1.6
-0.3 26.3 25.7
PMP4/6_311++G(2df, Zp)+AZPE QCISD(T)/6-311++G(2df,2p)+AZPE
17.3 18.1
exp.
18.650.8
Table 4 Total energy (in hartree) for species involved in the FO+HsO reaction Level of theory
HO
FO
Hz0
FOH
[FO+HxO]+
UMP2/6-31G(d) UMP2/6-31 lG(d, p) PMP4/6-31 lG(d, p) PMP4/6-31 lG(2d, 2p) PMP4/6-311+ +G(2d, 2p) PMP4/6-311+ +G(Zdf, 2p)
-75.52321 -75.59140 -75.58914 -75.60824 -75.61518 -75.63387
-
174.44249 174.57343 174.56255 174.60579 174.61489 174.66053
-76.19924 - 76.28290 - 76.27634 - 76.29945 -76.30914 - 76.32926
- 175.09286 - 175.23748 -175.22168 - 175.26878 - 175.27990 - 175.32575
-250.59780 -250.81252 -250.79889 -250.86480 -250.88110 -250.94742
QCISD(T)/6-31lG(d, p) QCISD(T)/6-311G(2d, 2p) QCISD(T)/6-311+ +G(2d, 2p) QCISD(T)/6-311+ +G(Zdf, 2p)
-75.58925 -75.60825 -75.61506 -75.63361
-
174.56281 174.60548 174.61428 174.65975
- 76.27633 -76.29910 -76.30838 - 76.32839
-
-250.79953 -250.86457 -250.88094 -250.94671
forthereactionofFO+HzO+HO+HOFis25.7kcal mol-’ at the QCISD(T)/6-311+ +G(2df, 2p)// UMP2/6-3 11G( d, p) level of theory. As mentioned previously, there exists, as yet, no experimental measurement of the activation energy for the FO+H20 reaction. Our prediction of the barrier for the FO+H*O reaction has important implications for the atmospheric chemistry of inorganic fluorine oxides. With the low temperatures prevalent in the ‘troposphere and stratosphere, the relative unreactivity of FO radicals to abstraction of hydrogen from
175.22088 175.26711 175.27758 175.32318
Hz0 prevents FO radicals from being removed by water prevalent in the atmosphere.
References [ 1 ] F.S. Rowland and M.J. Molina, Rev.+Geophys. Space Phys. 12 (1975) 1. [2] F.S. Rowland and M.J. Molina, Nature 249 (1974) 810. [3] R.J. Cicerone, Science 237 (1987) 35. [4] J.S. Francisco, J. Chem. Phys. 96 (1992) 3348. [5] J.S. Francisco, J. Chem. Phys. 98 (1993) 2198.
61
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CHEMICAL PHYSICS LETTERS
[6] M.J. Frisch, G.W. Trucks, M. Head-Gordon, P.M.W. Gill, M.W. Wong, J.B. Foresman, B.G. Johnson, H.B. Schlegel, M.A. Robb, E.S. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.J. DeFrees, J. Baker, J.J.P. Stewart and J.A. Pople, GAUSSIAN 92, Revision A (Gaussian, Pittsburgh, 1992). [7] MS. Gordon, J.S. Binkley, W.J. Pietro and W.J. Hehre, J. Am. Chem. Sot. 104 (1982) 2797. [ 81 R. Krishnan, J.S. Binkley, R. Seeger and J.A. Pople, J. Chem. Phys. 72 (1980) 650. [ 91 J.A. Pople, R. Krishnan, H.B. Schlegel and J.S. Binkley, Intern. J. Quantum Chem. Symp. 12 (1979) 225. [lo] H.B. Schlegel, J. Chem. Phys. 84 (1986) 4530.
62
26 November 1993
[ 111 J.A. Pople, M. Head-Gordon and K. Raghavachari, J. Chem. Phys. 90 (1989) 4635. [ 121 K.P. Huber and G. Henberg, Molecular spectra and molecular structure, Vol. 4. Constants of diatomic molecules (Van Nostrand, New York, 1979). [13]A.R.W.McKellar,Can. J.Phys.57 (1979) 2106. [ 141 M.W. Chase Jr., C.A. Davies, J.R. Downey Jr., D.J. Frurip, R.A. McDonald and A.N. Syverud, JANAF thermochemical tables, 3rd Ed., J. Phys. Chem. Ref. Data 14 (1985) suppl. 1. [ 15 ] Y. Zhao and J.S. Francisco, Chem. Phys. Letters 167 ( 1990) 285. [16] J.A.PopleandL.A.Curtiss,J.Chem.Phys.90 (1989) 2833.
CHEMICAL PHYSICS LETTERS
26 November 1993
An ab initio study of the reaction between FO radicals and Hz0 Joseph S. Francisco
and Yi Su
Department of Chemistry, Wayne State University, Detroit, MI 48202, USA
Received 13 July 1993; in final form 10 September 1993
Ab initio calculations are used to examine the energetics for the reaction of FO radicals and H,O. Optimized geometries have been calculated for all reactants, transition states, and products at the unrestricted second-order Msller-Plesset perturbation level of theory. Both Msller-Plesset perturbation (up to fourth-order) and quadratic configuration interaction (QCISD(T) ) methods are used to compute the energetics. The best estimate for the heat of reaction is 18.1 kcal mol- ’ at the QCISD( T ) /6-3 11 + + G (Zdf, 2d) level of theory, while the activation energy at this level is estimated as 25.7 kcal mol-‘. The atmospheric implications of this result are discussed.
1. Introduction The gas-phase chemistry of halogen monoxide (X0 where X =F, Cl and Br) has long been of atmospheric interest because of its role in catalytic cycles for ozone destruction. In recent years, there has been a plethora of work focusing on the roles of chlorine oxides and bromine oxides in stratospheric chemistry. The chemistry of inorganic fluorines, on the other hand, has not received as considerable attention. The reason is that fluorines, which result from photo-oxidation of the chlorofluorocarbons have largely been assumed to be scavenged readily by water and methane to form stable hydrogen fluoride [ 1,2 1. The hydrogen fluoride is usually rained out of the atmosphere. As a result this removes fluorine from potential catalytic cycles for the destruction of ozone, viz. F+03+F0+0z, FO+O-+F+02,
(1) (2)
FO+0,+F0,+02, FO,+O-+FO+O,,
(4) (5)
0+03-+20&
(3)
and viz. FO+Oa+FOz+02, F02+03+FO+202,
(4) (6)
203+ 302.
(7)
The above cycles are denoted FOO, cycles. Their viability depends on the likelihood of FO and FOI being removed by other species in the atmosphere. Previously we explored how probable are FO radicals to be removed by major atmospheric trace species such as HO and HO2 radicals [ 4,5 1. However, a critical knowledge of how FO radicals react with water, FO+H,O+HO+HOF,
0+0,+20,.
(3)
The analogous reactions for chlorine form an important process for ozone destruction [ 1,3]. Fluorines themselves are less likely to have potential perturbation effects on stratospheric ozone. However, there is potential atmospheric chemistry through the fluorine oxides viz. 58
(8)
is important since there is more water vapor than HO and HOz radicals in the atmosphere. This reaction fundamentally involves a hydrogen atom transfer process, which has been important in combustion chemistry. The reaction proceeds by a concerted bond breaking and making process to the transferring atom. No experimental kinetic data is
0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 215, number 1,2,3
26 November 1993
CHEMICAL PHYSICS LETTERS
available for the FO+ Hz0 reaction. The study of reactions involving FO radicals is quite difficult experimentally; yet to evaluate the importance of FOO, species (FO, FOZ) in atmospheric processes, knowledge of the energetics is desirable. Consequently, in the present work, ab initio molecular orbital calculations are used to determine the activation energy barrier for the FO+HzO reaction in order to estimate the energetic feasibility and assess how important this reaction is in diminishing the involvement of fluorine oxides in atmospheric chemical processes.
and quadruple excitation using the optimized geometries obtained at the MP2/6-3 11G( d, p) level of theory. In order to remove the spin contamination from higher spin states, the spin projection method was performed by annihilating the highest spin contaminant of the unrestricted wavefunction. These energies are denoted by PMP4 [ 10 1. Quadratic configuration interaction theory using single, double, and triple excitations are also used to improve the energetics, The QCISD(T) method includes terms to correct for excitations to all orders in perturbation theory [ll].
2. Computational methods 3. Results and discussion Ab initio molecular orbital calculations were performed using the GAUSSIAN 92 system of program [ 6 1. Geometries were fully optimized at the unrestricted second-order Moller-Plesset perturbation (MP2) with all orbitals active, using 6-31G(d) [7] and 6-3 11G (d, p) [ 8 ] basis sets. Harmonic vibrational frequencies and zero-point energies for the reactants, products and transition states were computed with MP2 wavefunctions by numerical second derivatives [ 9 1. Extended electron correlation yas calculated with fourth-order Msller-Plesset perturbation theory in the space of single, double, triple
Table 1 lists the geometries for reactants, transition states and products. The transition state structure, shown in fig. 1, involves mainly three atoms in the abstraction process. They are the oxygen atom involving the FO and the HO atoms from water. The preferred approach of the incoming FO is out of the plane of the water. The newly formed O’H’ bond length is 1.108 A and the breaking H’ 0 bond in water is 1.186 A. The H’O bond which is attacked by the FO is elongated by 0.229 A; this is R 24% of its original length. The changes in the transition structure
Table 1 Optimized geometries of species involved in the FO + Hz0 reaction ‘) Species
Coordinate b,
UMP2/6-31G(d)
UMP2/6-31 lG(d, p)
Exp.
HO FO Hz0
HO FO HO HOH HO FO FOH FO H’O’ H’O HO FO’H’ O’H’O HOH’ FO’H’O HOH’O’
0.919 1.344 0.969 104.0 0.979 1.444 97.2 1.400 1.132 1.202 0.978 99.6 145.2 105.0 92.9 -25.5
0.966 1.328 0.957 102.5 0.965 1.424 97.9 1.387 1.108 1.187 0.965 101.0 154.2 104.5 90.9 -7.9
0.970 1.358 0.958 104.5 0.966 1.442 96.8
FOH
[FO+H,O]+
‘) Bond lengths in A, angles in degrees. b, See fig. 1 for atom labeling for the transition state. 59
Volume 215, number 1,2,3
CHEMICAL PHYSICS LETTERS
Fig. 1. (a) Transition state structure for FO+H,O abstraction reaction (MP2/6_31G(d) optimizedgeometry, no asterisk; MP2/ 6-31G(d, p) optimized, asterisk, see table 1 for complete list of geometrical parameters). (b) View of the transition state along the O’H’O axis (MP2/6-31G(d) optimized dihedral angle, no asterisk; MP2/6-31 lG(d, p) optimized, asterisk). Table 2 UMP2/6-3 lG* vibrational frequencies for species involved in the FO + HsO reaction Species
Frequencies (cm-’ ) a)
reactants and products 3741(3738) HO 1542(1033) FO 3918(3756), 3776 (3651), H20 1735(1594) 3713 (3578), 1404 (1355), FOH 985 (889) transition state [FO+HsO]+
3770,2174,1621,1361 921,410,351,145 2538i
Zero-point energy (kcal mol-i)
5.4 2.2 13.5 8.7
15.4
‘) Sources of experimental values: HO ref. [ 121, FO ref. [ 131 and all others ref. [ 141.
suggest that the transitional structure assumes more product-like characteristics. We have examined two other orientations of the approach of the FO radical to water. One orientation involves the fluorine relative to the terminal unattached hydrogen of water either in a cis or in a trans configuration. At the MP2/ 6-3 1G* the energy difference between cis and trans structures are 1.1 kcal mol- ‘, and the energy difference between the structure in fig. 1 and the cis structures is 1.7 kcal mol- ‘. However, an examination of the vibrational frequencies for the cis and trans structures show that these structures are second- and third-order saddle points. The out-of-plane transition state structure (fig. 1) as shown in table 2 is a true first-order saddle point for the hydrogen atom transfer process for the FO+HzO system. The im60
26 November 1993
aginary frequency of 2538i suggests that the barrier for this reaction is quite narrow and that tunneling through the barrier may be important. Relative energetics for the stationary points of the FO + Hz0 reaction are listed in table 3. These results are determined from the total energies given in table 4. The experimental heats of formation for FO, H20, HOandHOFare27.8fl [15], -57.12+0.01 [14], 9.175kO.27 [15] and 19.921 [16] kcalmol-‘, respectively. With these heats of formation, the experimental heat of reaction for the FO+HzO abstraction process is 18.6 + 0.8 kcal mol-‘. The calculated heat of reaction for the abstraction process at the UMP2/6-311G(d, p) level is 17.2 kcal mol-‘. This is 1.4 kcal mol-’ underestimated from the experimental estimate. From the UMP2/63 11G (d, p) level of theory, increase in basis set size and the form of higher-order correlation are important in improving the energetics. Increasing the basis set size by supplementing the 6-3 1lG( d, p) basis with an extra set of diffuse, d-polarization and f-polarization functions, increases the heat of reaction energy by 1.3 kcal mol-I. With correlation using full fourth-order and spin projection, or with correlation treatment using the QCISD(T) method, the basis set effect on the heat of reaction is 1.5 kcal mol-‘. The addition of f-polarization functions has the most impact on improving the energetics, which suggests that the energetics should be computed using large basis sets supplemented with f-polarization functions. Heats of reaction calculated with the use of the QCISD(T) method appear to be consistently lower than the Moller-Plesset (PMP4) results by about 0.6-0.8 kcal mol-*. With the largest basis set used, 6-3 11+ + G ( 2df, 2p) the calculated QCISD (T ) heat of reaction is 18.1 kcal mol-‘, and is in reasonable agreement with the experiment. The activation energy for the hydrogen abstraction by FO radicals from HZ0 to form HO radicals and HOF is estimated as 27.6 and 27.5 kcal mol-’ at the UMP2/6-31G(d) and UMP2/6-31 lG(d, p) levels of theory, respectively. Incorporation of higherorder electron correlation has the effect of lowering the activation energy, with either Moller-Plesset fourth-order with spin projection effects or quadratic configuration interaction methods. Improvement of the basis set size has more effect on the activation energy. The best estimated activated energy
26 November 1993
CHEMICAL PHYSICS LETTERS
Volume 215, number 1,2,3 Table 3 Heat of reaction and barrier height (kcal mol-i)
for FO+H*O reaction Heat of reaction
Activation energy
16.1 17.2
27.6 27.5
PMP4/6-31 lG(d, p) PMP4/6-311G(2d, 2p) PMP4/6-3ll++G(2d,2p) PMP4/6_311++G(2df, 2p)
17.6 17.7 18.2 18.9
25.1 25.4 26.9 26.6
QCISD(T)/6-31 lG(d, p) QCISD(T)/6-311G(2d, 2p) QCISD(T)/6_311++G(2d,2p) QCISD(T)/6-311+ +G(Zdf, 2p)
18.2 18.3 18.8 19.7
24.9 25.1 26.2 26.0
UMP2/6-31G(d) UMP2/6-311G(d,
p)
AZPE(MP2/6-31G(d))
- 1.6
-0.3 26.3 25.7
PMP4/6_311++G(2df, Zp)+AZPE QCISD(T)/6-311++G(2df,2p)+AZPE
17.3 18.1
exp.
18.650.8
Table 4 Total energy (in hartree) for species involved in the FO+HsO reaction Level of theory
HO
FO
Hz0
FOH
[FO+HxO]+
UMP2/6-31G(d) UMP2/6-31 lG(d, p) PMP4/6-31 lG(d, p) PMP4/6-31 lG(2d, 2p) PMP4/6-311+ +G(2d, 2p) PMP4/6-311+ +G(Zdf, 2p)
-75.52321 -75.59140 -75.58914 -75.60824 -75.61518 -75.63387
-
174.44249 174.57343 174.56255 174.60579 174.61489 174.66053
-76.19924 - 76.28290 - 76.27634 - 76.29945 -76.30914 - 76.32926
- 175.09286 - 175.23748 -175.22168 - 175.26878 - 175.27990 - 175.32575
-250.59780 -250.81252 -250.79889 -250.86480 -250.88110 -250.94742
QCISD(T)/6-31lG(d, p) QCISD(T)/6-311G(2d, 2p) QCISD(T)/6-311+ +G(2d, 2p) QCISD(T)/6-311+ +G(Zdf, 2p)
-75.58925 -75.60825 -75.61506 -75.63361
-
174.56281 174.60548 174.61428 174.65975
- 76.27633 -76.29910 -76.30838 - 76.32839
-
-250.79953 -250.86457 -250.88094 -250.94671
forthereactionofFO+HzO+HO+HOFis25.7kcal mol-’ at the QCISD(T)/6-311+ +G(2df, 2p)// UMP2/6-3 11G( d, p) level of theory. As mentioned previously, there exists, as yet, no experimental measurement of the activation energy for the FO+H20 reaction. Our prediction of the barrier for the FO+H*O reaction has important implications for the atmospheric chemistry of inorganic fluorine oxides. With the low temperatures prevalent in the ‘troposphere and stratosphere, the relative unreactivity of FO radicals to abstraction of hydrogen from
175.22088 175.26711 175.27758 175.32318
Hz0 prevents FO radicals from being removed by water prevalent in the atmosphere.
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