- Email: [email protected]
Cl adducts and their isomerization reactions
19 August 2002
Chemical Physics Letters 362 (2002) 205–209 www.elsevier.com/locate/cplett
Density functional theory study of CS2=Cl adducts and their isomerization reactions Dongqi Wang, David Lee Phillips* Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong Received 1 May 2002
Abstract Density functional theory calculations are presented for the formation and isomerization of CS2 =Cl complexes. The S@C@S Cl complex can be produced readily, but the SC(Cl)S complex is difficult to make directly with a barrier to reaction of 17.8 kcal/mol. The isomerization of S@C@S Cl into the SC(Cl)S adduct has a barrier of 13.2 kcal/mol. Therefore, the SC(Cl)S adduct probably forms via first production of the S@C@S Cl complex and subsequent isomerization to SC(Cl)S. Cl atom reaction with either the S@C@S Cl and SC(Cl)S species provides a radical initiated isomerization reaction to give the other species with much lower barriers to isomerization (about 1:4–2:6 kcal/ mol). Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The formation of CS2 =Cl complexes has been observed in both gas and solution phase environments [1–8]. These complexes are of interest in atmospheric chemistry [1–3] as well in synthetic chemistry for their intriguing selectivity in photochlorination reactions with alkanes like 2,3-dimethylbutane (DMB) [4–8]. Several theoretical studies have investigated the possible structures of the CS2 =Cl complex or adduct species and found likely structures for the Cl atom attached to either the C or the S atom of CS2 [9–11]. An elegant experimental investigation of photochlorination reactions found that the CS2 =Cl complex respon-
*
Corresponding author. Fax: +852-2857-1586. E-mail address: [email protected] (D.L. Phillips).
sible for tertiary selectivity had a strong electronic absorption band 370 nm with a shoulder 490 nm [7]. A recent transient resonance Raman study directly probed the structure of this CS2 =Cl complex and comparison of the Raman vibrational frequencies to those predicted by density functional theory calculations for the probable structures of the CS2 =Cl complex revealed that the complex involved in tertiary selective photochlorination reactions has the Cl atom attached to the S atom of CS2 to produce a loosely bound S@C@S Cl species [8]. In this Letter, we report density functional theory calculations (UB3LYP/6-311G*) done to investigate the isomerization reactions between CS2 =Cl complex isomers. We also investigated radical initiated isomerization via the reaction of the Cl atom with the CS2 =Cl complex. We found that formation of the S@C@S Cl complex
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 0 1 5 - 1
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D. Wang, D.L. Phillips / Chemical Physics Letters 362 (2002) 205–209
occurs readily with little or no barrier while formation of the SC(Cl)S complex is difficult to form with a barrier to reaction of 17.8 kcal/mol. The S@C@S Cl complex can isomerize into the SC(Cl)S complex via an isomerization reaction with a barrier of 13.2 kcal/mol. Thus, production of the SC(Cl)S adduct would likely occur via first formation of the loosely bound S@C@S Cl complex followed by isomerization to SC(Cl)S. Reaction of a Cl atom with either the S@C@S Cl complex or the SC(Cl)S adduct leads to a facile radical initiated isomerization reaction to give the other complex species with barriers of 2.6 and 1.4 kcal/mol, respectively.
2. Calculations Hybrid density functional theory (DFT) UB3LYP/6-311G* calculations [12,13] were done for the isomerization reaction of the two probable CS2 =Cl complexes as well as a Cl radical initiated isomerization reaction. All of these calculations made use of the GA U S S I A N 98W programs [14]. All of the stationary points were fully optimized and vibrational analysis was performed to verify the nature of each stationary structure and extract the zero-point energy. Intrinsic reaction coordinate (IRC) calculations were done to confirm transition states connected the relevant reactants and products [15,16].
3. Results and discussion Fig. 1 presents schematic diagrams for the optimized geometry obtained from UB3LYP/6311G* calculations for selected reactants, transition states, and products for the isomerization reactions investigated in this Letter. Selected geometry parameters from the UB3LYP/6-311G* calculations are also shown in Fig. 1. Fig. 2 shows schematic diagrams for the UB3LYP/6-311G* computed relative energies (in kcal/mol) for the reactants, transition states and products for the isomerization reactions investigated here. We first considered formation of the CS2 =Cl complex from addition of the Cl atom to CS2 . Addition of Cl to
the S atom of CS2 was very facile with little or no barrier to form the S@C@S Cl species (denoted as the S-isomer) and no transition state was found for the UB3LYP/6-311G* calculations. This is consistent with the S@C@S Cl species being a weakly bound molecular complex that involves only smaller changes in the structure and energy of the CS2 moiety. In contrast, Cl atom addition to the C atom of CS2 to form SC(Cl)S (denoted as the C-isomer) involves a large barrier to reaction of about 17.8 kcal/mol including zero-point energies (see diagram of Fig. 2a). This is consistent with the 80.2 kJ/mol barrier found previously from MP4(full)/6-311G* calculations [11]. We next examined the isomerization reaction between the SC(Cl)S and S@C@S Cl species. The transition state TS2 was confirmed by IRC calculations to connect the SC(Cl)S and S@C@S Cl species. The barrier to isomerization from S@C@S Cl to SC(Cl)S is about 13.2 kcal/ mol with the zero-point energy included for the UB3LYP/6-311G* calculations (see Fig. 2b). The reverse reaction has a barrier of about 12 kcal/mol. The DFT calculations find the S@C@S Cl and SC(Cl)S to have similar energies with the S@C@S Cl species being more stable by about 1.2 kcal/mol. Inspection of Fig. 1 reveals that the structures of TS1 and TS2 are similar to one with both involving elongation of the CS bonds, a change in the SCS angle to about 147–150° and partial formation of the CACl bond. However, TS1 and TS2 display some noticeable and interesting differences. For example, direct formation of SC(Cl)S causes TS1 to have both CS bonds which is close to that found lengthened to 1.623 A for both CS bonds in the SC(Cl)S species. In addition, the CACl bond formation is closer to that ) than for of the SC(Cl)S species for TS1 (2.070 A TS2 (2.192 A). The SCCl angle in TS1 of 106.2° is also noticeably closer to that for the SC(Cl)S species (121.4°) than that found for TS2 (91.2°) for the isomerization reaction. The optimized geometry for TS1 is very much consistent with direct formation of the SC(Cl)S species. Our UB3LYP/ 6-311G* optimized geometry for TS1 is in excellent agreement with that found previously from MP2(full)/6-311G* calculations (CACl bond , CAS bond lengths of 1.61 A , length of 2.05 A
D. Wang, D.L. Phillips / Chemical Physics Letters 362 (2002) 205–209
207
Fig. 1. Schematic diagram showing the optimized geometry for selected reactants, transition states and products for the reaction of CS2 with Cl to form the SC(Cl)S complex (a), the isomerization reaction between the S@C@S Cl and SC(Cl)S complexes (b), and the Cl radical initiated isomerization reaction between the S@C@S Cl and SC(Cl)S complexes (c). The numbers are values for selected and bond angles in degrees. key structural parameters from the UB3LYP/6-311G* calculations with bond lengths in A
SASACl angles of 105.7°, and SACAS angle of 148.6°) [11]. In contrast, the TS2 structure of the isomerization reaction involves serious disruption of only one CS bond length compared to two for TS1. Our results suggest direct addition to the C atom of CS2 to form the SC(Cl)S species has a relatively narrow cone of acceptance near a C2v transition state TS1 with a fairly high barrier to reaction due to the need to disrupt both CS bonds. We note the Cl atom addition to S atoms have a much more likely chance to occur based on the probability of orientation to approach the CS2 molecule and the low barrier to form the weak S@C@S Cl complex. The results of our calculations suggest that when Cl first reacts with CS2 it will probably first form a weak S@C@S Cl complex that may further isomerize into the SC(Cl)S adduct via the 13.2 kcal/mol barrier of TS2 which involves only large disruption of one
CS bond rather than two to directly react with CS2 to form the SC(Cl)S adduct via the 17.8 kcal/mol barrier of TS1. We next investigated a radical initiated isomerization reaction that may occur when a Cl atom approaches either of the two S@C@S Cl and SC(Cl)S complexes. The UB3LYP/6-311G* calculations indicate that this kind of isomerization reaction proceeds very easily with a small barrier to reaction of about 2.6 kcal/mol with zeropoint energy included for isomerization of S@C@S Cl into SC(Cl)S and about 1.4 kcal/mol for the reverse reaction (see Fig. 2c). This suggests a Cl atom collision with either of the CS2 =Cl complexes could cause an isomerization and this could become an important consideration at higher Cl atom concentrations. It is interesting to examine why the barrier to isomerization between the two CS2 =Cl complex species is dramatically
208
D. Wang, D.L. Phillips / Chemical Physics Letters 362 (2002) 205–209
(a)
(b)
(c) Fig. 2. Schematic diagram showing the UB3LYP/6-311G* calculated relative energies (in kcal/mol) for the reaction of CS2 with Cl to form the SC(Cl)S complex (a), the isomerization reaction between the S@C@S Cl and SC(Cl)S complexes (b), and the Cl radical initiated isomerization reaction between the S@C@S Cl and SC(Cl)S complexes (c).
reduced in the presence of another chlorine atom. The second Cl atom approaches the C atom from the side of S1 of the CS2 =Cl complex asymmetrically. This enables the reformation of the molecular orbitals to become much easier since two unpaired electrons are included in the system and the delocalization of these electrons activates the original orbitals. This enables the transition state TS3 to have a significantly lower energy than TS2. Inspection of Fig. 1 shows that TS2 and TS3 dis-
play some intriguing differences in their optimized geometry. The ClAC bond becomes a bit weaker ) compared to TS2 (2.192 A ) and in TS3 (2.241 A ) the SACl bond is a bit stronger in TS3 (2.543 A than in the S@C@S Cl species (2.636 A). These differences between TS3 and TS2 and S@C@S Cl for the CACl and SACl bonds are probably due to different interactions between the Cl atoms and the C@S bonds. In TS2, the ClACC bond formation occurs asymmetrically with the Cl interacting mainly with the CAS2 bond which near that of the SC(Cl)S species becomes 1.637 A and close to that while the CAS1 bond is 1.571 A of the parent CS2 molecule. When the second Cl atom approaches the CS2 =Cl complex, it prefers to attack the second CS bond from the opposite side of the CS2 molecule. This leads the CAS1 bond in the S@C@S Cl species to change from 1.552 to in TS3 accompanied by a CACl bond 1.619 A . At the same time the CAS2 length of 2.241 A bond in the S@C@S Cl species changes from to 1.580 A in TS3 accompanied by a 1.569 A in TS3 compared stronger S2ACl bond of 2.543 A in the S@C@S Cl species. Similarly, to 2.636 A the second Cl atom can cause the SC(Cl)S complex to 2.241 A to weaken the CACl bond from 1.752 A in TS3 as the SACl bond forms in TS3 and the in the comCAS bonds strengthen from 1.662 A for CAS1 and 1.580 A for CAS2 in plex to 1.619 A TS3. These changes in TS3 structure compared to the reactant CS2 =Cl complexes indicate that the second Cl atom prefers to attack the CAS bond that does not interact as much with the other Cl atom. The TS3 structural changes also suggest that the two Cl atoms and two CAS bonds form a large conjugation system that makes it easier to reform the CAS bonds. The dissociation of a weak SACl or CACl bond can also help compensate for the energy needed for the CAS bond reformation so as to give a lower barrier for isomerization. It would be interesting to examine if it is common for radical initiated isomerization reactions to have noticeably lower barriers to reaction compared to the normal isomerization reaction for other CS2 =halogen complexes or other types of loosely bound complexes like CS2 =OH [2,17–20]. It would also be interesting to experimentally investigate if the radical initiated isomerization
D. Wang, D.L. Phillips / Chemical Physics Letters 362 (2002) 205–209
reactions cause any noticeable effects at higher Cl atom concentrations for adduct formation in gas and solution phases. Acknowledgements This work was supported by grants from the Committee on Research and Conference Grants (CRCG), the Research Grants Council (RGC) of Hong Kong (HKU 7087/01P) and the Large Items of Equipment Allocation 1993–94 from the University of Hong Kong. References [1] D. Martin, I. Barnes, K.H. Becker, Chem. Phys. Lett. 140 (1987) 195. [2] J.M. Nicovich, C.J. Shackelford, P.H. Wine, J. Phys. Chem. 94 (1990) 2896. [3] T.J. Wallington, J.M. Andino, A.R. Potts, Chem. Phys. Lett. 176 (1991) 103.
209
[4] G.A. Russell, J. Am. Chem. Soc. 80 (1958) 4997. [5] P.S. Skell, H.N. Baxter, C.K. Taylor, J. Am. Chem. Soc. 105 (1983) 120. [6] K.U. Ingold, J. Lusztyk, K.D. Raner, Acc. Chem. Res. 23 (1990) 219. [7] J.E. Chateauneuf, J. Am. Chem. Soc. 115 (1993) 1915. [8] D. Wang, Y.-L. Li, W.S. Ho, K.H. Leung, D.L. Phillips, J. Org. Chem. 67 (2002) 747. [9] P. Marshall, J. Mol. Struct. Theochem. 236 (1991) 309. [10] M.L. McKee, Chem. Phys. Lett. 209 (1993) 195. [11] C. Wilson, D.M. Hirst, J. Chem. Soc. Faraday Trans. 93 (1997) 2831. [12] A.D. Becke, J. Chem. Phys. 98 (1993) 1372. [13] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 58 (1988) 785. [14] M.J. Frisch, et. al., GA U S S I A N 98 (Revision A.7), Gaussian, Pittsburgh, PA, USA, 1998. [15] C. Gonzalez, H.B. Schlegel, J. Chem. Phys. 90 (1989) 2154. [16] C. Gonzalez, H.B. Schlegel, J. Phys. Chem. 94 (1990) 5523. [17] T.P. Murrells, E.R. Lovejoy, A.R. Ravishankara, J. Phys. Chem. 94 (1990) 2381. [18] E.R. Lovejoy, T.P. Murrells, A.R. Ravishankara, J. Phys. Chem. 94 (1990) 2386. [19] E.W.-G. Diau, Y.-P. Lee, J. Phys. Chem. 95 (1991) 7726. [20] M.L. McKee, Chem. Phys. Lett. 201 (1993) 41.
Chemical Physics Letters 362 (2002) 205–209 www.elsevier.com/locate/cplett
Density functional theory study of CS2=Cl adducts and their isomerization reactions Dongqi Wang, David Lee Phillips* Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong Received 1 May 2002
Abstract Density functional theory calculations are presented for the formation and isomerization of CS2 =Cl complexes. The S@C@S Cl complex can be produced readily, but the SC(Cl)S complex is difficult to make directly with a barrier to reaction of 17.8 kcal/mol. The isomerization of S@C@S Cl into the SC(Cl)S adduct has a barrier of 13.2 kcal/mol. Therefore, the SC(Cl)S adduct probably forms via first production of the S@C@S Cl complex and subsequent isomerization to SC(Cl)S. Cl atom reaction with either the S@C@S Cl and SC(Cl)S species provides a radical initiated isomerization reaction to give the other species with much lower barriers to isomerization (about 1:4–2:6 kcal/ mol). Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The formation of CS2 =Cl complexes has been observed in both gas and solution phase environments [1–8]. These complexes are of interest in atmospheric chemistry [1–3] as well in synthetic chemistry for their intriguing selectivity in photochlorination reactions with alkanes like 2,3-dimethylbutane (DMB) [4–8]. Several theoretical studies have investigated the possible structures of the CS2 =Cl complex or adduct species and found likely structures for the Cl atom attached to either the C or the S atom of CS2 [9–11]. An elegant experimental investigation of photochlorination reactions found that the CS2 =Cl complex respon-
*
Corresponding author. Fax: +852-2857-1586. E-mail address: [email protected] (D.L. Phillips).
sible for tertiary selectivity had a strong electronic absorption band 370 nm with a shoulder 490 nm [7]. A recent transient resonance Raman study directly probed the structure of this CS2 =Cl complex and comparison of the Raman vibrational frequencies to those predicted by density functional theory calculations for the probable structures of the CS2 =Cl complex revealed that the complex involved in tertiary selective photochlorination reactions has the Cl atom attached to the S atom of CS2 to produce a loosely bound S@C@S Cl species [8]. In this Letter, we report density functional theory calculations (UB3LYP/6-311G*) done to investigate the isomerization reactions between CS2 =Cl complex isomers. We also investigated radical initiated isomerization via the reaction of the Cl atom with the CS2 =Cl complex. We found that formation of the S@C@S Cl complex
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 0 1 5 - 1
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D. Wang, D.L. Phillips / Chemical Physics Letters 362 (2002) 205–209
occurs readily with little or no barrier while formation of the SC(Cl)S complex is difficult to form with a barrier to reaction of 17.8 kcal/mol. The S@C@S Cl complex can isomerize into the SC(Cl)S complex via an isomerization reaction with a barrier of 13.2 kcal/mol. Thus, production of the SC(Cl)S adduct would likely occur via first formation of the loosely bound S@C@S Cl complex followed by isomerization to SC(Cl)S. Reaction of a Cl atom with either the S@C@S Cl complex or the SC(Cl)S adduct leads to a facile radical initiated isomerization reaction to give the other complex species with barriers of 2.6 and 1.4 kcal/mol, respectively.
2. Calculations Hybrid density functional theory (DFT) UB3LYP/6-311G* calculations [12,13] were done for the isomerization reaction of the two probable CS2 =Cl complexes as well as a Cl radical initiated isomerization reaction. All of these calculations made use of the GA U S S I A N 98W programs [14]. All of the stationary points were fully optimized and vibrational analysis was performed to verify the nature of each stationary structure and extract the zero-point energy. Intrinsic reaction coordinate (IRC) calculations were done to confirm transition states connected the relevant reactants and products [15,16].
3. Results and discussion Fig. 1 presents schematic diagrams for the optimized geometry obtained from UB3LYP/6311G* calculations for selected reactants, transition states, and products for the isomerization reactions investigated in this Letter. Selected geometry parameters from the UB3LYP/6-311G* calculations are also shown in Fig. 1. Fig. 2 shows schematic diagrams for the UB3LYP/6-311G* computed relative energies (in kcal/mol) for the reactants, transition states and products for the isomerization reactions investigated here. We first considered formation of the CS2 =Cl complex from addition of the Cl atom to CS2 . Addition of Cl to
the S atom of CS2 was very facile with little or no barrier to form the S@C@S Cl species (denoted as the S-isomer) and no transition state was found for the UB3LYP/6-311G* calculations. This is consistent with the S@C@S Cl species being a weakly bound molecular complex that involves only smaller changes in the structure and energy of the CS2 moiety. In contrast, Cl atom addition to the C atom of CS2 to form SC(Cl)S (denoted as the C-isomer) involves a large barrier to reaction of about 17.8 kcal/mol including zero-point energies (see diagram of Fig. 2a). This is consistent with the 80.2 kJ/mol barrier found previously from MP4(full)/6-311G* calculations [11]. We next examined the isomerization reaction between the SC(Cl)S and S@C@S Cl species. The transition state TS2 was confirmed by IRC calculations to connect the SC(Cl)S and S@C@S Cl species. The barrier to isomerization from S@C@S Cl to SC(Cl)S is about 13.2 kcal/ mol with the zero-point energy included for the UB3LYP/6-311G* calculations (see Fig. 2b). The reverse reaction has a barrier of about 12 kcal/mol. The DFT calculations find the S@C@S Cl and SC(Cl)S to have similar energies with the S@C@S Cl species being more stable by about 1.2 kcal/mol. Inspection of Fig. 1 reveals that the structures of TS1 and TS2 are similar to one with both involving elongation of the CS bonds, a change in the SCS angle to about 147–150° and partial formation of the CACl bond. However, TS1 and TS2 display some noticeable and interesting differences. For example, direct formation of SC(Cl)S causes TS1 to have both CS bonds which is close to that found lengthened to 1.623 A for both CS bonds in the SC(Cl)S species. In addition, the CACl bond formation is closer to that ) than for of the SC(Cl)S species for TS1 (2.070 A TS2 (2.192 A). The SCCl angle in TS1 of 106.2° is also noticeably closer to that for the SC(Cl)S species (121.4°) than that found for TS2 (91.2°) for the isomerization reaction. The optimized geometry for TS1 is very much consistent with direct formation of the SC(Cl)S species. Our UB3LYP/ 6-311G* optimized geometry for TS1 is in excellent agreement with that found previously from MP2(full)/6-311G* calculations (CACl bond , CAS bond lengths of 1.61 A , length of 2.05 A
D. Wang, D.L. Phillips / Chemical Physics Letters 362 (2002) 205–209
207
Fig. 1. Schematic diagram showing the optimized geometry for selected reactants, transition states and products for the reaction of CS2 with Cl to form the SC(Cl)S complex (a), the isomerization reaction between the S@C@S Cl and SC(Cl)S complexes (b), and the Cl radical initiated isomerization reaction between the S@C@S Cl and SC(Cl)S complexes (c). The numbers are values for selected and bond angles in degrees. key structural parameters from the UB3LYP/6-311G* calculations with bond lengths in A
SASACl angles of 105.7°, and SACAS angle of 148.6°) [11]. In contrast, the TS2 structure of the isomerization reaction involves serious disruption of only one CS bond length compared to two for TS1. Our results suggest direct addition to the C atom of CS2 to form the SC(Cl)S species has a relatively narrow cone of acceptance near a C2v transition state TS1 with a fairly high barrier to reaction due to the need to disrupt both CS bonds. We note the Cl atom addition to S atoms have a much more likely chance to occur based on the probability of orientation to approach the CS2 molecule and the low barrier to form the weak S@C@S Cl complex. The results of our calculations suggest that when Cl first reacts with CS2 it will probably first form a weak S@C@S Cl complex that may further isomerize into the SC(Cl)S adduct via the 13.2 kcal/mol barrier of TS2 which involves only large disruption of one
CS bond rather than two to directly react with CS2 to form the SC(Cl)S adduct via the 17.8 kcal/mol barrier of TS1. We next investigated a radical initiated isomerization reaction that may occur when a Cl atom approaches either of the two S@C@S Cl and SC(Cl)S complexes. The UB3LYP/6-311G* calculations indicate that this kind of isomerization reaction proceeds very easily with a small barrier to reaction of about 2.6 kcal/mol with zeropoint energy included for isomerization of S@C@S Cl into SC(Cl)S and about 1.4 kcal/mol for the reverse reaction (see Fig. 2c). This suggests a Cl atom collision with either of the CS2 =Cl complexes could cause an isomerization and this could become an important consideration at higher Cl atom concentrations. It is interesting to examine why the barrier to isomerization between the two CS2 =Cl complex species is dramatically
208
D. Wang, D.L. Phillips / Chemical Physics Letters 362 (2002) 205–209
(a)
(b)
(c) Fig. 2. Schematic diagram showing the UB3LYP/6-311G* calculated relative energies (in kcal/mol) for the reaction of CS2 with Cl to form the SC(Cl)S complex (a), the isomerization reaction between the S@C@S Cl and SC(Cl)S complexes (b), and the Cl radical initiated isomerization reaction between the S@C@S Cl and SC(Cl)S complexes (c).
reduced in the presence of another chlorine atom. The second Cl atom approaches the C atom from the side of S1 of the CS2 =Cl complex asymmetrically. This enables the reformation of the molecular orbitals to become much easier since two unpaired electrons are included in the system and the delocalization of these electrons activates the original orbitals. This enables the transition state TS3 to have a significantly lower energy than TS2. Inspection of Fig. 1 shows that TS2 and TS3 dis-
play some intriguing differences in their optimized geometry. The ClAC bond becomes a bit weaker ) compared to TS2 (2.192 A ) and in TS3 (2.241 A ) the SACl bond is a bit stronger in TS3 (2.543 A than in the S@C@S Cl species (2.636 A). These differences between TS3 and TS2 and S@C@S Cl for the CACl and SACl bonds are probably due to different interactions between the Cl atoms and the C@S bonds. In TS2, the ClACC bond formation occurs asymmetrically with the Cl interacting mainly with the CAS2 bond which near that of the SC(Cl)S species becomes 1.637 A and close to that while the CAS1 bond is 1.571 A of the parent CS2 molecule. When the second Cl atom approaches the CS2 =Cl complex, it prefers to attack the second CS bond from the opposite side of the CS2 molecule. This leads the CAS1 bond in the S@C@S Cl species to change from 1.552 to in TS3 accompanied by a CACl bond 1.619 A . At the same time the CAS2 length of 2.241 A bond in the S@C@S Cl species changes from to 1.580 A in TS3 accompanied by a 1.569 A in TS3 compared stronger S2ACl bond of 2.543 A in the S@C@S Cl species. Similarly, to 2.636 A the second Cl atom can cause the SC(Cl)S complex to 2.241 A to weaken the CACl bond from 1.752 A in TS3 as the SACl bond forms in TS3 and the in the comCAS bonds strengthen from 1.662 A for CAS1 and 1.580 A for CAS2 in plex to 1.619 A TS3. These changes in TS3 structure compared to the reactant CS2 =Cl complexes indicate that the second Cl atom prefers to attack the CAS bond that does not interact as much with the other Cl atom. The TS3 structural changes also suggest that the two Cl atoms and two CAS bonds form a large conjugation system that makes it easier to reform the CAS bonds. The dissociation of a weak SACl or CACl bond can also help compensate for the energy needed for the CAS bond reformation so as to give a lower barrier for isomerization. It would be interesting to examine if it is common for radical initiated isomerization reactions to have noticeably lower barriers to reaction compared to the normal isomerization reaction for other CS2 =halogen complexes or other types of loosely bound complexes like CS2 =OH [2,17–20]. It would also be interesting to experimentally investigate if the radical initiated isomerization
D. Wang, D.L. Phillips / Chemical Physics Letters 362 (2002) 205–209
reactions cause any noticeable effects at higher Cl atom concentrations for adduct formation in gas and solution phases. Acknowledgements This work was supported by grants from the Committee on Research and Conference Grants (CRCG), the Research Grants Council (RGC) of Hong Kong (HKU 7087/01P) and the Large Items of Equipment Allocation 1993–94 from the University of Hong Kong. References [1] D. Martin, I. Barnes, K.H. Becker, Chem. Phys. Lett. 140 (1987) 195. [2] J.M. Nicovich, C.J. Shackelford, P.H. Wine, J. Phys. Chem. 94 (1990) 2896. [3] T.J. Wallington, J.M. Andino, A.R. Potts, Chem. Phys. Lett. 176 (1991) 103.
209
[4] G.A. Russell, J. Am. Chem. Soc. 80 (1958) 4997. [5] P.S. Skell, H.N. Baxter, C.K. Taylor, J. Am. Chem. Soc. 105 (1983) 120. [6] K.U. Ingold, J. Lusztyk, K.D. Raner, Acc. Chem. Res. 23 (1990) 219. [7] J.E. Chateauneuf, J. Am. Chem. Soc. 115 (1993) 1915. [8] D. Wang, Y.-L. Li, W.S. Ho, K.H. Leung, D.L. Phillips, J. Org. Chem. 67 (2002) 747. [9] P. Marshall, J. Mol. Struct. Theochem. 236 (1991) 309. [10] M.L. McKee, Chem. Phys. Lett. 209 (1993) 195. [11] C. Wilson, D.M. Hirst, J. Chem. Soc. Faraday Trans. 93 (1997) 2831. [12] A.D. Becke, J. Chem. Phys. 98 (1993) 1372. [13] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 58 (1988) 785. [14] M.J. Frisch, et. al., GA U S S I A N 98 (Revision A.7), Gaussian, Pittsburgh, PA, USA, 1998. [15] C. Gonzalez, H.B. Schlegel, J. Chem. Phys. 90 (1989) 2154. [16] C. Gonzalez, H.B. Schlegel, J. Phys. Chem. 94 (1990) 5523. [17] T.P. Murrells, E.R. Lovejoy, A.R. Ravishankara, J. Phys. Chem. 94 (1990) 2381. [18] E.R. Lovejoy, T.P. Murrells, A.R. Ravishankara, J. Phys. Chem. 94 (1990) 2386. [19] E.W.-G. Diau, Y.-P. Lee, J. Phys. Chem. 95 (1991) 7726. [20] M.L. McKee, Chem. Phys. Lett. 201 (1993) 41.