(646b) Ab Initio Study into the Mechanism of the Thermal Decomposition of Acyclic Polysulfides

The mechanism of the thermal decomposition of acyclic polysulfides has been investigated using high level ab initio methods. A comparative study of experimental rate coefficients, bond dissociation energies (BDEs) and enthalpies of formation (ΔH_f) with ab initio data, calculated according to six different computational methods, pointed out that the best agreement with experimental data is obtained with the CBS-QB3 method. This method was then used to study the decomposition reactions of acyclic polysulfides and to identify the major reaction paths yielding the experimental observed decomposition products. Based on this work, a reaction network for the decomposition of dimethyldisulfide was constructed.

Polysulfides often play a critical role in the optimization of chemical processes. They are frequently used as coke inhibiting additives during the steam cracking of hydrocarbons and are also known to be efficient transfer agents during free radical polymerization. They are also more often encountered in nature then expected. Oceanic water for example contains traces of dimethyldisulfide (DMDS) produced by plankton. Also the strong flavour of garlic extract and onion oil originates from the presence of trisulfides. Despite the chemical significance of acyclic polysulfides, experimental or theoretical studies concerning their reaction behaviour are scarce (Coope and Bryce (1954), Kroto and Suffolk (1972), Braye et al. (1955), Bock and Mohmand (1977)).

Experimental rate coefficients for six reactions involving sulfur compounds were retrieved from the NIST Chemical Kinetics Database. The experimental rate coefficients were compared with ab initio results obtained with the G3, G3B3, CBS-QB3 composite methods and the BMK/cbsb7, B3LYP/cbsb7 and mPW1PW91/cbsb7 density functional theory (DFT) methods. Also the influences of the 1D hindered rotor approximation (Van Speybroeck et al. (2000)) and Eckart (1930) tunneling corrections on the rate coefficients were assessed. Best agreement with experimental rate coefficients was obtained with the CBS-QB3 method which yields an average deviation of a factor 5 between experimental and ab initio rate coefficients for the six studied reactions.

Also, as we expect polysulfides to decompose according to a radical mechanism, the accuracy of the six presented ab initio methods in predicting thermochemical data for sulfur centered radicals was assessed. To this end experimental and ab initio bond dissociation energies and enthalpies of formation for sulfur centered radicals were compared. The BDE set contained 9 S?X bonds (with X = H, C or S) while the ΔH_f set consists out of 7 sulfur centered radicals. The comparison showed that the most accurate thermochemical data are obtained with the composite methods, yielding a mean average deviation (MAD) on the BDEs of approximately 8 kJ/mol and of 5 kJ/mol on ΔH_f. The MAD is of the same order of magnitude as the uncertainty on the experimental data. The DFT based methods perform poorly as MADs amount to 20 kJ/mol. As CBS-QB3 yields most accurate rate coefficients and predicts accurate thermochemical properties, this method was selected to analyze the different possible reaction paths of the thermal decomposition of acyclic polysulfides.

Initiation reactions are often identified as the rate-determing steps of radical reaction networks. They hence play a critical role in the reaction network and accurate rate coefficients for these reactions are required to obtain reliable simulation results. As transition state theory fails in giving proper rate coefficients for this type of reactions, rate coefficient for the S?C and S?S bond scission reactions were estimated using Flexible Transistion State Theory.

Based on the CBS-QB3 calculated rate coefficients for the different decomposition reactions, a reaction network for the decomposition of DMDS was constructed (see figure). At 1000 K S?C scission in DMDS (1) proceeds more than 100 times faster than scission of an S?S bond (2) and 106 times faster than a molecular rearrangement (3). As the S?S bond strength decreases rapidly with the length of the S?S chain, trisulfides and tetrasulfides, in contrast to DMDS, preferably decompose by S?S scission. Molecular rearrangements for tri- and tetrasulfides are respectively 15 and 60 kJ/mol higher activated than S?S scission. Alkyl polysulfides hence predominantly decompose by a free radical mechanism, leading to methyl and methylperthiyl for DMDS and to sulfur centered radicals for S₃ and S₄ components.

Sulfur centered radicals are more easily formed then elemental hydrogen and carbon centered radicals. This is reflected in the BDEs for S?S and S?C bonds which are 100 to 250 kJ/mol lower then the BDEs of C?H bonds in hydrocarbons. Due to their low affinity for addition reactions and H abstractions, sulfur centered radicals mostly participate in barrierless recombination reactions leading to polysulfides. During decomposition of DMDS, large amounts of trisulfides can hence be formed due to recombination of methylperthiyl radicals with methylthiyl radicals (4). Formation of ethene starts with the addition of methyl to CH₂S with the formation of an ethylthiyl radical CH₃CH₂S^? (8). Successive internal H abstraction (9) and β-scission (10) will lead to ethene and the hydrogen sulfide radical HS^?. The formation of CS₂ was considered to occur by a reaction path with intermediate dithiiranes. Addition of sulfur centered radicals RS^? to CH₂S leads to RSCH₂S^? type of radicals (6bis) which can react in a next step to dithiirane and R^? (7). H abstraction from dithiirane (11) followed by β-scission (12) will form CS₂.

The constructed network succeeds in describing the formation of all experimentally observed products and accounts for the experimentally observed temperature dependence on the product selectivity. Higher temperatures favor the formation of dithiiranes and hence CS₂ at the expense of CH₂S and CH₃SH.

References

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2. Kroto H.W. and Suffolk R.J., Chem. Phys. Lett., 15, 545 (1972). Photoelectron Spectrum of An Unstable Species in Pyrolysis Products of Dimethyldisulfide.

3. Braye E.H., Sehon A.H. and Darwent N.d., J. Am. Chem. Soc., 77, 5283 (1955). Thermal decomposition of sulfides.

4. Bock H. and Mohmand S., Angewandte Chemie-International Edition in English, 16, 104 (1977). Unstable Intermediates in Gaseous Phase .6. Thermal-Decomposition of Alkyl Sulfides Rsnr.

5. Van Speybroeck V., Van Neck D., Waroquier M., Wauters S., Saeys M. and Marin G.B., J. Phys. Chem. A, 104, 10939 (2000). Ab initio study of radical addition reactions: Addition of a primary ethylbenzene radical to ethene (I).

6. Eckart C., Phys. Rev., 35, 1303 (1930). The penetration of a potential barrier by electrons.