Zhuochao Tenga,
Xianwei Zhaoa,
Hetong Wanga,
Ying Lia,
Yanan Hana,
Yanhui Sunb and
Fei Xu*ac
aEnvironment Research Institute, Shandong University, Qingdao 266237, P. R. China. E-mail: xufei@sdu.edu.cn; Tel: +86-532-58631992
bCollege of Environment and Safety Engineering, Qingdao University of Science & Technology, Qingdao 266042, P. R. China
cShenzhen Research Institute of Shandong University, Shenzhen 518057, P. R. China
First published on 14th May 2021
Polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) and polychlorinated dibenzothiophenes/thianthrenes (PCDT/TAs) are two groups of dioxin-like compounds with oxygen and sulfur substitution, respectively. Chlorophenols (CPs) and chlorothiophenols (CTPs) are direct precursors in PCDD/F and PCDT/TA formation. The formation of chlorophenoxy radicals (CPRs) and chlorothiophenoxy radicals (CTPRs) from chlorophenols (CPs) and chlorothiophenols (CTPs) with O(3P) is an important initial step for the formation of PCDD/Fs and PCDT/TAs, respectively. In this paper, the formation of CPRs/CTPRs from the complete series reactions of 19 CP/CTP congeners with O(3P) was studied using the density functional theory (DFT) method. The rate constants of each reaction were calculated using canonical variational transition state (CVT) theory along with a small-curvature tunneling (SCT) contribution over a wide temperature range of 600–1200 K. The effect of the chlorine substitution pattern on the structural parameters, thermochemical properties and rate constants in both CPs and CTPs was discussed. This study shows that the reactions between CPs and O(3P) can be affected by the chlorine substitution at the para-position, and the reactions between CTPs and O(3P) are mostly influenced by both ortho-substitutions. The thiophenoxyl-hydrogen abstraction from CTPs by O(3P) is more likely to occur than the phenoxyl-hydrogen abstraction from CPs by O(3P). Comparison of the reactivity of CP/CTPs with O(3P) with our previous work on CP/CTPs with H and OH shows that the order for phenoxyl-hydrogen abstraction potential is CP + OH > CP + O(3P) > CP + H, and the order for thiophenoxyl-hydrogen abstraction potential is CTP + O(3P) > CTP + H > CTP + OH.
Due to the similarity in structure, source, toxicity and concentration of PCTA/DTs and PCDD/DFs in the environment, they have similar formation mechanisms under pyrolysis or combustion conditions.13 Chlorophenols (CPs) and chlorothiophenols (CTPs) are widely recognized to be the main precursors and intermediates for PCDT/F and PCDT/TA formation, respectively.20–22 CPs are widely used as intermediate in herbicide, pesticide, bactericide, drug production and dye production.23 CTPs are mainly from direct application as dyes, pesticides, inks, pharmaceuticals and poly vinyl chloride (PVC).24 CPs are carcinogenic and mutagenic even at trace levels,8,25 which have been included in the priority control list of hazardous substances by the U.S. Toxic Substances Administration and the Disease Registry.26 CTPs have slightly lower toxicity than CPs, which were reported to have strong stimulation to eyes, respiratory mucosa and skin of organisms.27 Under pyrolysis or combustion conditions, CPs and CTPs can readily form chlorophenoxy radicals (CPRs) and chlorotriophenoxy radicals (CTPRs) by abandoning the phenoxyl-H and sulfydryl-H, respectively, via unimolecular reaction, bimolecular reaction or other possible low-energy pathways (including heterogeneous reactions). Unimolecular reactions include the decomposition of CPs/CTPs and the breaking of O–H/S–H bonds. The bimolecular reactions include the abstraction reaction by H, OH, O(3P) or Cl under high temperature.
Under high temperature conditions, PCDD/F and PCTA/DT formation from CPs and CTPs as precursors contains radical–radical coupling and radical–molecule coupling.28–35 Previous studies presented that radical/radical couplings are thermodynamically comparable to radical/molecule recombination for the PCDD/DF and PCTA/DT formation.28–35 Therefore, following the radical/radical routes, a series of theoretical studies on PCTA/DT formation mechanisms from the coupling of 2-CTPRs, 2,4,5-TCTPRs, 2,4-DCTPRs, and 2,4,6-TCPRs were carried out.28,29,36,37 These studies showed that the formation of PCDDs and PCTAs was easier than that of PCDFs and PCDTs, due to the fact that the PCDDs and PCTAs can form via one elementary step less than PCDFs and PCDTs and the potential barrier of the rate-determining step of PCDDs/PCTAs are lower than that of PCDFs/PCDTs.28,29,36,37 However, the existing radical/radical mechanism cannot make a full explanation on the experimental observations that PCDFs rather than PCDDs are favored products in the gas-phase PCDD/DF formation from CPs.38–40 In addition, this conclusion is in stark contrast to the much higher concentration of PCDTs than that of PCTAs under the pyrolysis or combustion environments.38–40 Therefore, the radical/molecule mechanism was reignited, which can give large contribution to the high PCDF-to-PCDD ratio and PCDT-to-PCTA ratio.41,42 In both radical–radical and radical–molecule couplings, the formation of CPRs/CTPRs from CPs/CTPs is the initial and most important step in the PCDD/F and PCTA/DT formations.28–35 Thus, the formation of CPR/CTPRs from CP/CTPs are need to be first studied.
In recent studies conducted from this laboratory, we have performed quantum chemistry to investigate the reaction mechanisms and kinetic properties of the complete series reactions of CPs/CTPs with H and OH radicals.43–45 On the basis of these studies, we complemented and expanded on our previous work in this field to continue to study the formation of CPRs/CTPRs from the complete series reactions of CP/CTP congeners with O(3P) using the direct density functional theory (DFT) method. The rate constants were evaluated by the canonical variational transition-state (CVT) theory with the small curvature tunneling (SCT) contribution at 600–1200 K.46 The effect of the chlorine substitution pattern on the structural parameters, thermochemical properties and rate constants in both CPs and CTPs were examined. The formation potential of C(T)PRs from C(T)Ps with O(3P) are compared with that of C(T)PRs from C(T)Ps with H and OH, respectively. The results, as well as the values in our previous studies in this field, can be input into environmental dioxin prediction and control models as important parameters to predict dioxin formation mechanisms and products, reduce dioxin emissions, health risk and establish dioxin control strategies.
The rate constants of the elementary reaction involved in this paper were calculated in the temperature range of 600–1200 K using the canonical variational transition-state theory (CVT) with the small curvature tunneling contribution (SCT).46 The CVT theory was used to obtain the minimum reaction rate constant at a given temperature (T) by changing the position of the generalized transition state segmentation plane (that is, by changing the reaction coordinates).43 The separation surface in CVT theory can be obtained by
(1) |
(2) |
In Fig. 2, for a given number of chlorine substitutions of CPs, the O–H bonds in para-substituted transition states are shorter than those without chlorine substitution at para-position. For example, the O–H bond lengths of TS(2,3-DCP), TS(2,5-DCP), TS(2,6-DCP) and TS(3,5-DCP) are 1.094, 1.096, 1.088 and 1.095 Å, respectively, which are larger than those of TS(2,4-DCP) and TS(3,4-DCP) (1.083 Å and 1.081 Å). The increase in length of the O–H bond being broken and the elongation of the O–H bond being formed with respect to its equilibrium value in the reactants are the most important aspect of the geometric structure of the transition state. As shown in Fig. S3,† the O–H growth ratios from para-substituted transition states are consistently larger than those from other structures. For example, the O–H bonds in TS(2,3-DCP), TS(2,5-DCP), TS(2,6-DCP) and TS(3,5-DCP) are 14.1%, 14.3%, 13.5% and 14.7% longer than the corresponding equilibrium values of O–H bonds in CPs, and the O–H bonds are elongated by of 12.9% and 13.2% in TS(2,4-DCP) and TS(3,4-DCP).
In Fig. 3, for a given number of chlorine substitutions of CPs, the S–H bonds in the transition states with two ortho substitutions are longer than those without ortho-substitution or with one ortho-substitution. For example, the S–H bond length in TS(2,6-DCTP) has relative longer distance (1.428 Å) compared to the values of 1.404–1.425 Å from TS(2,3-DCTP), TS(2.4-DCTP), TS(2,5-DCTP), TS(3,4-DCTP) and TS(3,5-DCTP). The S–H bonds in TS(2,3-DCP), TS(2,4-DCP), TS(2,5-DCP), TS(3,4-DCP), and TS(3,5-DCP) are starched by 5.33–6.98%, and the S–H bond is 7.13% longer than the corresponding equilibrium values of S–H bond in 2,6-CPs.
ΔE | ΔH | ν | |
---|---|---|---|
Phenol | 5.45 | −13.31 | 2272i |
2-CP | 7.52 | −11.34 | 2550i |
3-CP | 8.20 | −12.26 | 2424i |
4-CP | 5.11 | −14.50 | 2273i |
2,3-DCP | 8.26 | −10.57 | 2576i |
2,4-DCP | 6.97 | −12.70 | 2496i |
2,5-DCP | 8.25 | −10.48 | 2613i |
2,6-DCP | 7.59 | −12.66 | 2551i |
3,4-DCP | 5.95 | −13.50 | 2425i |
3,5-DCP | 6.03 | −11.14 | 2513i |
2,3,4-TCP | 7.42 | −11.90 | 2569i |
2,3,5-TCP | 9.06 | −10.74 | 2691i |
2,3,6-TCP | 8.11 | −12.15 | 2645i |
2,4,5-TCP | 7.62 | −11.90 | 2652i |
2,4,6-TCP | 6.90 | −13.96 | 2599i |
3,4,5-TCP | 8.54 | −12.47 | 2500i |
2,3,4,5-TeCP | 8.29 | −11.10 | 2712i |
2,3,4,6-TeCP | 7.68 | −11.93 | 2669i |
2,3,5,6-TeCP | 8.50 | −12.90 | 2710i |
PCP | 8.00 | −12.88 | 2700i |
ΔE | ΔH | ν | |
---|---|---|---|
Thiophenol | 1.63 | −12.12 | 506i |
2-CTP | 2.51 | −12.39 | 531i |
3-CTP | 2.28 | −11.90 | 576i |
4-CTP | 1.52 | −21.46 | 523i |
2,3-DCTP | 2.42 | −13.15 | 1192i |
2,4-DCTP | 2.15 | −19.70 | 1046i |
2,5-DCTP | 2.47 | −11.03 | 1199i |
2,6-DCTP | 3.78 | −13.64 | 1292i |
3,4-DCTP | 2.84 | −20.65 | 586i |
3,5-DCTP | 2.76 | −11.71 | 651i |
2,3,4-TCTP | 4.38 | −19.06 | 1136i |
2,3,5-TCTP | 2.66 | −13.06 | 1227i |
2,3,6-TCTP | 3.32 | −10.99 | 1320i |
2,4,5-TCTP | 2.81 | −18.90 | 1053i |
2,4,6-TCTP | 3.62 | −18.08 | 1256i |
3,4,5-TCTP | 3.17 | −20.09 | 642i |
2,3,4,5-TeCTP | 3.39 | −18.53 | 1155i |
2,3,4,6-TeCTP | 3.65 | −17.61 | 1307i |
2,3,5,6-TeCTP | 4.28 | −14.44 | 1366i |
PCTP | 4.63 | −16.94 | 1327i |
From Table 1, the potential barriers are significantly correlated with the position of the chlorine substitution at the phenolic ring, but not with the number of chlorine substituents. For example, for monochlorophenols, the potential barriers of phenoxyl-hydrogen abstraction from 2-CP (7.52 kcal mol−1) and 3-CP (8.20 kcal mol−1) are higher than that from 4-CP (5.11 kcal mol−1). For trichlorophenols, the potential barriers of the phenoxyl-hydrogen abstraction from 2,3,5-TCTP (9.06 kcal mol−1) and 2,3,6-TCTP (8.11 kcal mol−1) are higher than those from 2,3,4-TCTP (7.42 kcal mol−1), 2,4,5-TCTP (7.62 kcal mol−1) and 2,4,6-TCTP (6.90 kcal mol−1), except for 3,4,5-TCTP (8.54 kcal mol−1). Obviously, for a given number of chlorine substitutions, the potential barriers for the phenoxyl-hydrogen abstraction from the para-substituted CPs are consistently higher than those for other structural conformers. The chlorine substitution at the para-position can lower the barrier heights of phenoxyl-hydrogen abstraction from CPs by O(3P). The conclusion is different from the result in our previous study of CPs with H that phenoxyl-hydrogen abstraction from CPs by H are strongly dominated by the chlorine substitution at the ortho-position of CPs, and intramolecular hydrogen bonding appears to stabilize the CTPs and reduce the reactivity of O–H bonds in CPs with the ortho-substitution.44 All the transition states have one and only one imaginary frequency. The values of the imaginary frequencies of transition states from reactions of CPs with O(3P) are shown in Table 1. From Table 1, for a given number of chlorine substitutions, the imaginary frequencies for the para-substituted transition states are smaller than those without para substitution. For trichlorophenols, the imaginary frequencies of the transition states for 2,3,4-TCP, 2,4,5-TCP, 2,4,6-TCP and 3,4,5-TCP are 2569i cm−1, 2652i cm−1, 2599i cm−1 and 2500i cm−1, respectively, whereas the values for 2,3,5-TCP and 2,3,6-TCP are 2691i cm−1 and 2645i cm−1, respectively. The chlorine substitution at the para-position can lower the value of the imaginary frequency, that is, the value change of the imaginary frequency has the same trend with the value of potential barrier.
From the reactions of CTPs with O(3P) in Table 2, for a given number of chlorine substitutions, the potential barriers for the thiophenoxyl-hydrogen abstraction from CTPs with both ortho-substitutions (substitutions at two and six points) by O(3P) consistently are higher than those from CTPs without ortho-substitution or with only one ortho-substitution by O(3P). For example, for dithiochlorophenols, the potential barriers of the thiophenoxyl-hydrogen abstraction from 2,6-DCTP (3.78 kcal mol−1) is higher than those from 2,3-DCTP (2.42 kcal mol−1), 2,4-DCTP (2.15 kcal mol−1), 2,5-DCTP (2.47 kcal mol−1), 3,4-DCTP (2.84 kcal mol−1) and 3,5-DCTP (2.76 kcal mol−1). For tetrathiochlorophenols, the potential barriers of the thiophenoxyl-hydrogen abstraction from 2,3,4,6-TeCTP (3.65 kcal mol−1) and 2,3,5,6-TeCTP (4.28 kcal mol−1) are higher than those from 2,3,4,5-TeCTP (3.39 kcal mol−1). This indicates that the chlorine substitutions at both ortho positions of CTPs increase the strength of the S–H bonds and decrease its reactivity. The imaginary frequency analysis can reconfirm the conclusion above. For example, for tetrathiochlorophenols, the imaginary frequencies of the transition states for 2,3-DCTP, 2,4-DCTP, 2,5-DCTP, 3,4-DCTP and 3,5-DCTP are 1192i cm−1, 1046i cm−1, 1199i cm−1, 586i cm−1 and 861i cm−1, respectively, which is smaller than that for 2,6-DCTP of 1292i cm−1. The CTP transition states from chlorine substitutions at both ortho positions can enhance the imaginary frequencies.
It is necessary to compare the reactions of CPs and O(3P) with the reactions of CTPs by O(3P). For a given CP and CTP, the potential barriers for the phenoxyl-hydrogen abstraction from CP by O(3P) are about 3.04–6.40 kcal mol−1 higher than thiophenoxyl-hydrogen abstraction from corresponding CTP by O(3P). In addition, the thiophenoxyl-hydrogen abstractions by O(3P) are about 0.55–7.62 kcal mol−1 more exothermic than the phenoxyl-hydrogen abstractions by O(3P), except for 3-C(T)P and 2,3,6-C(T)P. This means that CTPRs formation from CTPs with O(3P) is more likely to occur than the CPRs formation from CPs with O(3P).
It is interesting to compare the reaction mechanism of CPs/CTPs and O(3P) with the reactions of CPs/CTPs and H/OH radicals.43–45 The histograms of potential barriers (ΔE) for the phenoxyl-hydrogen abstraction from phenol and CPs and CTPs by O(3P)/H/OH are vividly displayed in Fig. 4 and 5, respectively. For the phenoxyl-hydrogen abstraction from CPs with H/OH/O(3P) in Fig. 4, for a given CP, the potential barriers for the phenoxyl-hydrogen abstraction from CP by O(3P) are about 3.86–6.96 kcal mol−1 lower than those from CP by H, and about 4.12–8.54 kcal mol−1 higher than those from CP by OH.44,45 This indicates that the order for phenoxyl-hydrogen abstraction potential is CP + OH > CP + O(3P) > CP + H.44,45 In Fig. 5, for a given CTP, the potential barrier for the thiophenoxyl-hydrogen abstractions from most CTP by O(3P) are 0.24–1.29 kcal mol−1 lower than those from CP by H radical (except for 3,4-DCTP, 2,3,4-TCTP, 3,4,5-TCTP, 2,3,4,5-TeCTP and PCTP) and 4.49–7.16 kcal mol−1 lower than those from CTPs by OH.43 This means that the order for thiophenoxyl-hydrogen abstraction potential is CTP + O(3P) > CTP + H > CTP + OH.43
Fig. 4 Histograms of potential barriers ΔE‡ (in kcal mol−1, including ZPE correction) for the phenoxyl-hydrogen abstraction from phenol and CPs by O(3P)/H/OH. For comparison, ΔE‡ of CPR formation form CPs with H44 and OH45 are provided by ref. 44 and 45 respectively. |
Fig. 5 Histograms of potential barriers ΔE‡ (in kcal mol−1, including ZPE correction) for the thiophenoxyl-hydrogen abstraction from thiophenol and CTPs by O(3P)/H/OH. For comparison, ΔE‡ of CTPR formations from CTPs with H43 and OH43 are provided by ref. 43. |
Reactions | Arrhenius formulas |
---|---|
C6H5OH + O(3P) → C6H5O + OH | k(T) = (8.20 × 10−12)exp(−3266.25/T) |
2-CP + O(3P) → 2-CPR + OH | k(T) = (1.08 × 10−11)exp(−5669.24/T) |
3-CP + O(3P) → 3-CPR + OH | k(T) = (2.01 × 10−12)exp(−5422.00/T) |
4-CP + O(3P) → 4-CPR + OH | k(T) = (3.58 × 10−12)exp(−4113.82/T) |
2,3-DCP + O(3P) → 2,3-DCPR + OH | k(T) = (1.47 × 10−12)exp(−5737.14/T) |
2,4-DCP + O(3P) → 2,4-DCPR + OH | k(T) = (8.20 × 10−13)exp(−5178.98/T) |
2,5-DCP + O(3P) → 2,5-DCPR + OH | k(T) = (7.43 × 10−13)exp(−6000.54/T) |
2,6-DCP + O(3P) → 2,6-DCPR + OH | k(T) = (3.97 × 10−12)exp(−5201.50/T) |
3,4-DCP + O(3P) → 3,4-DCPR + OH | k(T) = (1.59 × 10−12)exp(−4310.06/T) |
3,5-DCP + O(3P) → 3,5-DCPR + OH | k(T) = (4.41 × 10−15)exp(−4572.52/T) |
2,3,4-TCP + O(3P) → 2,3,4-TCPR + OH | k(T) = (9.81 × 10−12)exp(−5870.61/T) |
2,3,5-TCP + O(3P) → 2,3,5-TCPR + OH | k(T) = (2.04 × 10−13)exp(−6226.97/T) |
2,3,6-TCP + O(3P) → 2,3,6-TCPR + OH | k(T) = (1.60 × 10−12)exp(−5673.37/T) |
2,4,5-TCP + O(3P) → 2,4,5-TCPR + OH | k(T) = (4.01 × 10−12)exp(−5920.33/T) |
2,4,6-TCP + O(3P) → 2,4,6-TCPR + OH | k(T) = (7.12 × 10−12)exp(−5659.22/T) |
3,4,5-TCP + O(3P) → 3,4,5-TCPR + OH | k(T) = (1.78 × 10−12)exp(−5527.17/T) |
2,3,4,5-TeCP + O(3P) → 2,3,4,5-TeCPR + OH | k(T) = (9.08 × 10−15)exp(−5934.21/T) |
2,3,4,6-TeCP + O(3P) → 2,3,4,6-TeCPR + OH | k(T) = (7.46 × 10−13)exp(−5404.86/T) |
2,3,5,6-TeCP + O(3P) → 2,3,5,6-TeCPR + OH | k(T) = (1.16 × 10−13)exp(−5324.11/T) |
PCP + O(3P) → PCPR + OH | k(T) = (1.50 × 10−14)exp(−5733.80/T) |
Reactions | Arrhenius formulas |
---|---|
C6H5OH + O(3P) → C6H5O + OH | k(T) = (2.41 × 10−14)exp(−4858.87/T) |
2-CTP + O(3P) → 2-CTPR + OH | k(T) = (3.51 × 10−12)exp(−2809.95/T) |
3-CTP + O(3P) → 3-CTPR + OH | k(T) = (7.12 × 10−13)exp(−5178.30/T) |
4-CTP + O(3P) → 4-CTPR + OH | k(T) = (1.50 × 10−12)exp(−4887.12/T) |
2,3-DCTP + O(3P) → 2,3-DCTPR + OH | k(T) = (1.73 × 10−11)exp(−3132.24/T) |
2,4-DCTP + O(3P) → 2,4-DCTPR + OH | k(T) = (6.34 × 10−12)exp(−2778.06/T) |
2,5-DCTP + O(3P) → 2,5-DCTPR + OH | k(T) = (3.84 × 10−10)exp(−3574.35/T) |
2,6-DCTP + O(3P) → 2,6-DCTPR + OH | k(T) = (1.25 × 10−11)exp(−3897.24/T) |
3,4-DCTP + O(3P) → 3,4-DCTPR + OH | k(T) = (1.41 × 10−11)exp(−1010.34/T) |
3,5-DCTP + O(3P) → 3,5-DCTPR + OH | k(T) = (6.33 × 10−11)exp(−2724.57/T) |
2,3,4-TCTP + O(3P) → 2,3,4-TCTPR + OH | k(T) = (2.85 × 10−12)exp(−3643.04/T) |
2,3,5-TCTP + O(3P) → 2,3,5-TCTPR + OH | k(T) = (1.41 × 10−11)exp(−3031.77/T) |
2,3,6-TCTP + O(3P) → 2,4,6-TCTPR + OH | k(T) = (1.86 × 10−14)exp(−7705.67/T) |
2,4,5-TCTP + O(3P) → 2,4,5-TCTPR + OH | k(T) = (3.36 × 10−14)exp(−2920.48/T) |
2,4,6-TCTP + O(3P) → 2,4,6-TCTPR + OH | k(T) = (8.58 × 10−14)exp(−3290.82/T) |
3,4,5-TCTP + O(3P) → 3,4,5-TCTPR + OH | k(T) = (3.11 × 10−13)exp(−8033.26/T) |
2,3,4,5-TeCTP + O(3P) → 2,3,4,5-TeCTPR + OH | k(T) = (2.08 × 10−12)exp(−2062.79/T) |
2,3,4,6-TeCTP + O(3P) → 2,3,4,6-TeCTPR + OH | k(T) = (1.70 × 10−13)exp(−3182.45/T) |
2,3,5,6-TeCTP + O(3P) → 2,3,5,6-TeCTPR + OH | k(T) = (8.19 × 10−12)exp(−3662.76/T) |
PCTP + O(3P) → PCTPR + OH | k(T) = (7.80 × 10−13)exp(−3272.48/T) |
From Table S5 of ESI,† for a given temperature, the CVT/SCT rate constants for the phenoxyl-hydrogen abstraction from the para-substituted CPs by O(3P) are higher than those from other CPs by O(3P). For example, at 600 K, the rate constants for the phenoxyl-hydrogen abstraction from 2,3-TCP, 2,5-TCP and 2,6-TCP are 1.11 × 10−16, 3.62 × 10−17 and 7.27 × 10−16 cm3 per molecule per s, while the rate constants are 1.57 × 10−16 and 1.29 × 10−15 cm3 per molecule per s for 2,4-TCP and 3,4-TCP. This is agreement with the thermodynamic analysis that the chlorine substitution at the para-position of CPs can decrease the O–H bond and increase the reaction activity of CPs with O(3P).
For the reactions of CTPs with O(3P) of Table S6 of ESI,† at a given temperature, the CVT/SCT rate constants of the thiophenoxyl-hydrogen abstraction from CTPs with both ortho-substitutions by O(3P) are lower than those from CTPs without ortho-substitution or with only one ortho-substitution by O(3P). For example, at 1000 K, the rate constant is 7.76 × 10−14 cm3 per molecule per s for the thiophenoxyl-hydrogen abstraction from 2,3,4,6-TeCTP by O(3P), which is higher than the values of 2.64 × 10−13 and 2.99 × 10−13 cm3 per molecule per s for the thiophenoxyl-hydrogen abstraction from 2,3,4,5-TeCTP and 2,3,5,6-TeCTP by O(3P), respectively. This perfectly matches the structural and thermodynamic analysis above that the chlorine substitutions at both ortho positions of CTPs can enhance the potential barrier of CTPs with O(3P). For a given chlorine substitution, the CVT/SCT rate constants for phenoxyl-hydrogen abstraction from CPs by O(3P) are noticeably smaller than those thiophenoxyl-hydrogen abstraction of CTPs by O(3P) over the whole studied temperature range. For example, at 1000 K, the CVT/SCT rate constant of the thiophenoxyl-hydrogen abstraction from 2,3,4-TCP by O(3P) is 2.69 × 10−14 cm3 per molecule per s, whereas the value is 7.29 × 10−14 cm3 per molecule per s for the thiophenoxyl-hydrogen abstraction from 2,3,4-TCTP by O(3P). This is consistent with thermodynamic analysis that thiophenoxyl-hydrogen abstraction from CTPs by O(3P) is more efficient than the phenoxyl-hydrogen abstraction from CPs by O(3P).
(1) The potential barriers of CP/CTPs with O(3P) are significantly correlated with the position of the chlorine substitution at the phenolic ring, but not with the number of chlorine substituents. The chlorine substitution at the para-position of CPs can decrease the O–H bond and increase the reaction activity of CPs with O(3P). The chlorine substitutions at both ortho positions of CTPs increase the strength of the S–H bonds and decrease its reactivity.
(2) The thiophenoxyl-hydrogen abstraction from CTPs by O(3P) is more likely to occur than that phenoxyl-hydrogen abstraction from CPs by O(3P).
(3) The formation potential of C(T)PRs from C(T)Ps with O(3P) are compared with that of C(T)PRs from C(T)Ps with H and OH, respectively. The order for phenoxyl-hydrogen abstraction potential is CP + OH > CP + O(3P) > CP + H, and the order for thiophenoxyl-hydrogen abstraction potential is CTP + O(3P) > CTP + H > CTP + OH.
Footnote |
† Electronic supplementary information (ESI) available: The structure and bond length of chlorophenols, chlorophenoxy radicals, chlorothiophenols and chlorothiophenoxy radicals. The imaginary frequencies, zero-point energies, total energies and bond length change values for the transition states involved in the formation of CPRs/CPRs from CPs/CTPs precursor. Rate constants and cartesian coordinates for the transition states involved in CPRs/CPR formation from CPs/CTPs. See DOI: 10.1039/d1ra02407h |
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