J. Phys. Chem. 1993,97, 6616-6627
6616
Theoretical Studies of Sulfurous Species of Importance in Atmospheric Chemistry. 1. Characterization of the HSO and SOH Isomers Sotiris S. Xantheas’ and Thom H. Dunning, Jr. Theory, Modeling and Simulation, Molecular Science Research Center, Pacific Northwest Laboratory,t Richland, Washington, 99352 Received: January 19, 1993
The molecular structures, relative energies, and vibrational frequencies of the two stable isomers formed by the addition of H(,S) to SO(X3Z-), namely, HSO and SOH, are computed using multiconfiguration wave functions and correlation-consistent basis sets of up to quintuple (quality. Contrary to results from previous calculations, the HSO isomer is predicted to be more stable than the S O H isomer by 5.4 kcal/mol. The barrier for the migration of the H atom between the two isomers is computed to be 46.3 kcal/mol. The transition state for the isomerization of the two species lies below the dissociation limit to H(%) and SO(X3Z-), which is calculated to be 56.2 kcal/mol above the HSO minimum. The minimum energy paths corresponding to the addition of H(,S) to SO(X3Z-)to form HSO and S O H are also computed; a barrier of 2.4 kcal/mol is predicted for the addition of hydrogen to the oxygen side of SO. Finally, upper limits for M l f o 2 9 8 for HSO and SOH are estimated at -6.1 1.3 and -0.7 f 1.3 kcal/mol, respectively. The latter implies that HSO can be involved in a catalytic cycle that depletes ozone in the atmosphere.
*
I. Introduction
particular interest is the reaction with 0~0ne3b,2~
The chemistry of atmosphericsulfur is of great interest because of its importance in environmental issues such as acid rain, chemical air pollution, and global climate change.’ In order to derive reliable models for the atmospheric sulfur cycle one must understand the detailed steps associated with the atmospheric conversion of various sulfur-containing species such as H2S and SO2 to sulfuric acid and visibility-reducing aerosol particles. Among the numerous processes involving the various anthropogenically and biogenically produced sulfur-containing species, the most important is the oxidationof reduced sulfur species such as H2S (as well as higher analogs)which is believed2 to be initiated in the atmosphere by reaction with the hydroxyl radical:
H,S
+ OH
HS + H,O
(1) The fate of the HS radical produced in (1) is less clear. A main channel for the loss of HS exists3avb via its reaction with 0 2 : ---L
HS + O,+ O H + SO
(2) However, a significant fraction (>lo%) of HS react~3b,~*.b with Os and NO2 according5 to the following:
+ 0, HSO + 0, H S + NO, -, HSO + NO HS
(3)
+
(4)
Reactions 2 and 3 are the dominant reactions in clean air, but reaction 4 can reach comparable levels in polluted air where the concentration of NO2 is significant (>40 ppb). Other schemes that can produce HSO include reactions6a.bof the following type:
+
+
0 RSH + H S O R (5) where R = CH3 and C2H5, as well as reactions’ of OCP) and O(lD) with HzS. As can be seen in the above reactions, HSO is formed in the atmosphere by a number of elementary reactions. The HSO produced in reactions 3-5 is, in turn, destroyed in reactions with a number of atmospheric constituents, 0 2 , 03,NO2, etc. Of ~~
~
~~
Pacific Northwest Laboratory is operated by Battelle Memorial Institute for the U. S.Department of Energy under Contract DE-AC06-76RLO 1830.
0022-3654/93/2097-66 16$04.00/0
HSO
+ 0,
-.
HS
+ 20,
(6) producing SH which can then continue to react with ozone according to reaction 3. Since the standard enthalpies of formation of S H and Os are within 2 kcal/mol of each other, the standard enthalpy of formation of HSO must be less than -2 kcal/mol in order for reaction 6 to be exothermic. Others have suggested that the products of reaction 6 are HSO2 and 02,3b921 The product of reactions 3-5 has been experimentally identified6asbas HSO although all previous theoretical studiess-ll consistently predicted the SOH isomer to be more stable. It has been suggested that some SOH might be formed during reaction 5,20although its detection has not been well documented. Since the theoretically predicted energy difference between the two isomers was used to interpret recent experimentaldata7 involving this species, it is desirable to obtain a reliable estimate of the energy difference between the two isomers as well as the magnitude of the barrier to their interconversion. Furthermore, since experimental studies3b of reaction 3 provided evidence of some H atom formation (
