3701
J. Phys. Chem. 1994,98, 3701-3706
The Dimethylsulfide-Hydroxyl Radical Reaction. An ab Initio Study Frantigek TureEek Department of Chemistry, BG-10, University of Washington, Seattle, Washington 98195 Received: December 6, 1993; In Final Form: January 31, 1994'
+
In keeping with a 1992 gas-phase study ( J . Am. Chem. SOC.1992,114,7146), MP2(FULL)/6-31 G(d) ab initio calculations find no potential energy minimum for the elusive (CH3)2S'-OH intermediate of the tropospheric reaction of dimethylsulfide (DMS) with hydroxyl radical. A dipoldipole complex, (CH3)2S-.HO', is found to be stabilized against dissociation to DMS and OH' by f i H ~ 9 = 8 17 kJ mol-', but destabilized by &298 = 68 J mol-' K-', The reaction of DMS with OH' is predicted to prefer formation of CH3SCH2' and HzO, which is 105 kJ mol-' exothermic and requires only 4 kJ mol-' activation energy. The heat of formation and adiabatic and vertical ionization energies of methanesulfenic acid are calculated at the G2(MP2) level of theory as fiHf,298 = -1 4 1.8 kJ mol-', IE, = 8.7 1 eV, and I& = 9.17 eV. The 0-H and C-H bond dissociation energies (BDE) in CH3SOH differ with BDE(0-H) = 287.4 kJ mol-' being lower than BDE(C-H) = 397 kJ mol-'. The heats of formation and ionization energies of CH3SO' and 'CH2SOH are calculated at fiHf,298(CH$O') = -77.4 kJ mol-', AHf,298('CHzSOH) = 32.2 kJ mol-', IEa(CH3SO') = 8.57 eV, and IE,('CH2SOH) = 6.80 eV. The 'CH2SH=O isomer is calculated to be 205 kJ mol-' less stable than CH3SO'.
Introduction The reaction of dimethylsulfide(DMS) with hydroxyl radical has been a paradigm for oxidations of organic sulfides in the gas phase' and solutiona2The gas-phase reaction represents the first step in the oxidationcascade whereby DMS is converted to sulfuric acid, methanesulfonic acid, and dimethylsulfone in the troposphere.3 Kinetic modeling of the DMS oxidation with photolytically generated hydroxyl radicals assumes that the first step consists of competitive attacks of OH' at the carbon and sulfur atoms in DMS (eq l).3a Attack at carbon produces water and the
+
CH3SCH3 *OH
/cH3:H2*
4.
'
H20
\
2
3
(methy1thio)methyl radical (l), a stable species that was characterized re~ently.~The competitive attack at sulfur is thought to produce.an adduct, dimethylhydroxylsulfuranylradical (2), which dissociatesfurther to form presumably methanesulfenic acid (3).3 Thelatter is a reactive but intrinsicallystable molecule that has been studied previ~usly.~ Oxidation of DMS in aqueous solution served as a model for the reactions of hydroxyl radicals formed in body tissue by highenergy radiation. The radicals formed are thought to be scavenged by methionine-containing peptides296 in reactions similar to that of DMS and OH'. The mechanism suggested for the latter reaction in aqueous solution6gpresumes predominant formation of stable 2, which then eliminateswater to form 1(eq 2). Evidence CHPCH,
+ 'OH
-
(CH3),S'-OH 2
-
CH3SCH2' 1
+ HZO (2)
for the transient formation of 2 rests on the observation of a UV absorption band at 340 nm that appears in pulse radiolysis of aqueous DMS solutions saturated with NzO and irradiated at 25 OC.&J Under these conditions 2 decays within 1 P S . ~ ' *Abstract published in Advance ACS Abstracts, March 15, 1994.
0022-365419412098-3701$04.50/0
In both the solution and the gas-phase mechanisms, 2 is presumed to exist as a bound structure, as shown in eqs 1 and 2, with the S-0 bond dissociation energy estimated at 54 kJ mol-l.38 In contrast to this, 2 formed by femtosecond reduction of its stable cation 2+ in the gas phase dissociates exothermically by $0,S-C, and H - O bond cleavages, yielding DMS, 3, and dimethylsulfoxide, respectively, that were identified by mass ~pectrometry.~ In keeping with this experimentalresult, ab initio calculations carried out at the UHF/6-31G(d) level indicated that onvertical electron transfer 2+ forms 2 on a repulsive potential energy surface, such that 2 dissociates without a barrier by simple bond cleavage^.'^ On the basis of the gas-phase dissociations and ab initio calculations, we concluded that the hypervalent radical 2 does not represent a bound str~cture.~b The question of the stability of 2 and its dissociation energetics is of critical importance for understanding the mechanism of DMS oxidation. If 2 exists in a potential energy minimum, it could dissociate on a manifold of competitive reactions whose kinetics and product branching ratios will be determined by the corresponding activation energies. By contrast, if 2 is thermodynamically unstable, it can represent at best a transition state for a single reaction path leading to only one product.* This would imply that the other products observed in the gas-phaselJ and s o l u t i ~ nreactions ~.~ should be formed by different and yet unknown mechanisms not involving 2. In this work we report the results of ab initio calculations concerning the DMS-hydroxyl radical reaction. We focus on locatingthe potential energy minimum for 2 and investigateseveral points on the reaction path leading to 1and water. Also reported are the enthalpies of formation and ionization energies of 3 and its dissociation products CH3S0, 'CHZSOH,and *CH2SH=O, for which there have been incomplete or conflicting experimental data.sc,d Calculations. Standard ab initio calculations were carried out using the Gaussian 92 set of programs9 Geometries were first optimized with the 6-31G(d) or 6-31+G(d) basis sets to obtain potential energy minima (all frequencies real) and first-order saddle points (1 imaginary frequency). Zero-point vibrational energies and 298 K enthalpies and entropies were obtained from the harmonic vibrational frequencies calculated with the above basis sets and scaled by 0.89.10 Theunscaled harmonic frequencies are given in the Appendix. Further geometry refinement was obtained by optimization at the SCF level with the larger 0 1994 American Chemical Society
TureEek
3702 The Journal of Physical Chemistry, Vol. 98, No. 14, 1994 TABLE 1: MP2/6-31 species
+ C(a) and GZ(MP2) Total Energies' MP2( full) / QCISD(T)/ 6-31 + G ( q b 6-311G(d,p) -512.997 585 -512.685 207 -512.687 604
CHiSOH (3) CHsSOH'+ (P+)
-513.129 126 -512.819 198 -512.805 109
CHsSOH'+ (N)" CH3SCHs CH2SCH2' (1)
477.146 769
-477.288 378
CH3SO' (5)
-512.386 -512.390 -512.349 -512.352 -76.212 -75.531 -75.533 -39.676 -39.678
892 788 181 348 295 991 698 728 623
-512.503 813
-552.685 -552.687 -552.671 -552.678
316 016 819 830
'CH2SOH (6) HzO OH' CH3'
H' --
(CH3)2S***HO'(2)
TS(HO*.*H--CH2SCH3)(4)
-512.471 633 -76.275 769 -75.589 179 -39.732 256
MP2/ 6-31 1 + G ( 3 d J 2 ~ ) ~ -513.210 -512.892 -512.895 -512.874 -512.876 477.325
ZPVE'
236 113 203 213 990 884
132.2 130.5
-512.592 920 -512.598 175 -512.552090 -512.555 996 -76.317 971 -75.616 370 -75.619 352 -39.731 353 -39.733 331
100.7
132.2 191.1 155
96.2 53.7 21.3 72
G2(MP2)b -513.294 217 -512.973 456 -512.974 210 -512.956 611 -512.957 252 -477.398 976 -476.749 397' -476.750 OOOI -512.686 074 -512.687 766 -512.645 606 -512.646 376 -76.334 942 -75.646 214 -75.646 371 -39.749 138 -39.749 180 -0.500 ooo
215.2 204.8
0 Hartree; 1 hartree = 2625.5 kJ mol-I. For open-shell species the upper line gives spin-unprojected (UMP2) and the lower line spin-projected (PMP2, (S2) = 0.75) energies. From RHF/6-31G(d) and UHF/6-31 + G(d) harmonic frequencies scaled by 0.89. dSingle-pointcalculation on optimized neutral geometry. Reference 4c; H298 - HO= 15.6 kJ mol-'.
TABLE 2
Total and Relative Energies of (C, Hh 0, S) Isomers
species
HF/ 6-311G(d,p)
CH3SO'
-51 1.988 285
'CH2SOH
-511.961 939
'CHzSH=O
-511.909 481
CHoSO+ CH3SO+ (N)d +CH2SOH +CH2SOH (N)"
-511.655 992 -511.639667 -511.705 213 -511.675 177
relative energy
MP2 (PMP2)"
MP4 (PMP4)"
ZPVEb
-512.450 -512.453 -512.420 -512.423 -512.372 -512.374 -512.141 -512.141 -512.173 -512.143
-512.500 772 -512.502 536 -512.467 542 -512.469 361 -512.419 971 -512.421 480 -512.188 130 -512.190 308 -512.220930 -512.195 091
100.7
0
96.2
83.5
277 771 507 260 208 580 277 836 100 114
91.9 101.6 (100.7) 101.7 (96.2)
205 827 (8.57) 820 (8.50) 741 (6.80) 804 (7.46)
After annihilationof higher spin states. From HF/6-31G(d) harmonic frequencies scaled by 0.89; units of kJ mol-I. From MP4/6-31 lG(d,p) Ionization energies (eV) in parentheses. Single-point calculations on UHF/6-31 lG(d,p) optimized neutral
+ ZPVE + H298;units of kJ mol-'.
geometries.
6-3 1lG(d,p) basis set," and with the 6-3 l+G(d) basis set including Mprller-Plesset perturbational treatment of electron correlation effects truncated at second order (MP2(FULL)).9 With 2, standard SCF iteration proceduresgfailed to converge, but satisfactory results were obtained by using the quadratic convergence method.13 The 6-31lG(d,p) geometries were employed for single-point energy calculations using the MprllerPlesset theory12 (frozen core) truncated at fourth order with singlet, doublet, and triplet excitations (MP4(SDTQ)). Singlepoint calculations were also carried out on the MP2(FULL)/ 6-3 l+G(d) optimized geometries at the G2(MP2) level,1° including quadratic configuration interaction (QCISD(T))14 calculationswith the 6-3 1 lG(d,p)basis set, and M P 2 calculations with the large 6-311+G(3df,Zp) basis set." The G2(MP2) method has been shown recently to successfully substitute full G2 calculationsls that, for open-shell species, exceeded our computational resources. The G2(MP2) total energiesare given in Table 1; the MP4(SDTQ)/6-31 lG(d,p) total energies are summarized in Table 2.
Results and Discussion Complex of DMS with OH. Full geometry optimization of 2 with the 6-31+G(d) basis set was started at the S O bond length of 1.620 A, which is slightly shorter than the optimized S-0 bond length in CH3SOH. The calculations show a large negative energy gradient along the S-O coordinate (-307 kJ mol-' A-I) at the
initial S-0 bond length. Optimization leads to a significant increase in the S-O distance to reach an energy minimum at 3.35 A. The potential energy surface at this S-0 separation is extremely flat, as a further S-0 distance increase to 3.6 A results in only a 1.4 kJ mol-' energy increase, whereas shortening to 3.1 A requires
