Quantum mechanical studies of the kinetics, mechanisms and thermodynamics of gas-phase thermal decomposition of ethyl dithiocarbonate (xanthate)

Peer review under responsibility of Taibah University.

Figures & data

Table 2 Selected bond angles in degrees for the pyrolysis of O-ethyl, S-methyl xanthate (dithiocarbonates).

Bond angle (°)	State	DFT/6-31+G*	DFT/6-31G**	HF/3-12G*	DFT/MP2	DFT/CCSD
H1–C2–H3	GS	108.539	108.519	108.672	108.539	108.539
	TS	103.52	104.910	93.940	103.52	103.52
	PROD	–	–	–	–	–
H1–C2–C5	GS	110.868	110.752	109.743	110.868	110.868
	TS	103.980	103.980	99.94	103.980	103.980
	PROD	–	–	–	–	–
C2–C5–C6	GS	111.616	111.616	1110.829	111.616	111.616
	TS	121.046	120.963	122.260	121.046	121.046
	PROD	121.791	121.871	121.896	121.791	121.791
C2–C5–O8	GS	106.951	106.823	105.017	106.951	106.951
	TS	109.830	109.830	110.750	109.830	109.830
	PROD	–	–	–	–	–
C5–O8–C9	GS	123.054	122.770	125.709	123.054	123.054
	TS	119.080	119.080	118.030	119.080	119.080
	PROD	–	–	–	–	–
H6–C5–O8	GS	108.804	109.049	109.530	108.804	108.804
	TS	86.110	86.110	80.530	86.110	86.110
	PROD	–	–	–	–	–
O8–C9=S10	GS	119.933	120.013	120.270	119.933	119.933
	TS	124.530	125.018	124.230	124.530	124.530
	PROD	124.526	124.474	123.611	124.526	124.526
O8–C9–S11	GS	114.453	114.547	114.097	114.453	114.453
	TS	120.221	119.949	122.180	120.221	120.221
	PROD	123.818	123.987	123.468	123.818	123.818
C9–S11–C12	GS	102.198	101.938	103.143	102.198	102.198
	TS	102.913	102.517	99.160	102.913	102.913
	PROD	93.218	98.140	93.937	93.218	93.218
S10=C9–S11	GS	125.613	125.440	125.639	125.613	125.613
	TS	115.219	115.015	113.590	115.219	115.219
	PROD	111.656	111.539	112.921	111.656	111.656

Table 3 Selected dihedral angles in degrees for the pyrolysis of O-ethyl S-methyl xanthate (dithiocarbonate).

Dihedral angle (°)	State	DFT/6-31G*	DFT/6-31G**	HF/3-21G*	DFT/6-31+G*	DFT/MP2	DFT/CCSD
H1–C2–C5–O8	GS	59.918	59.948	59.675	60.063	59.948	59.675
	TS	5.770	6.180	13.050	6.200	6.180	13.050
	PROD	–	–	–	–	–	–
C2–C5–O8–C9	GS	179.026	178.960	−179.957	178.908	178.960	−179.957
	TS	15.600	15.110	13.050	15.880	15.110	13.050
	PROD	–	–	–	–	–	–
H5–O8–C9–S10	GS	179.817	179.827	179.949	179.898	179.827	179.949
	TS	−42.990	−43.140	−101.30	−45.160	−43.140	−101.30
	PROD	–	–	–	–	–	–
C5–O8–C9–S11	GS	−60.226	−0.211	−0.062	−0.136	−0.211	−0.062
	TS	−106.380	135.270	139.460	132.730	135.270	139.460
	PROD	–		–	–		–
H6–C5–O8–C9	GS	−60.226	−60.272	−59.863	−60.392	−60.272	−59.863
	TS	−106.380	−106.920	−101.300	−106.070	−106.920	−101.300
	PROD	–	–	–	–	–	–
H7–C5–O8–C9	GS	58.347	58.262	59.949	58.285	58.262	59.949
	TS	137.700	137.280	139.460	137.86	137.280	139.460
	PROD	–	–	–	–	–	–
O8–C9–S11–C12	GS	−179.899	−179.912	180.000	−179.859	−179.912	180.000
	TS	5.150	5.141	1.600	6.487	5.141	1.600
	PROD	−0.000	−0.000	−0.000	−0.000	−0.000	−0.000
C9–S11–C12–H13	GS	−61.093	−61.078	−60.771	−60.209	−61.078	−60.771
	TS	−122.220	−122.766	−119.51	−125.035	−122.766	−119.51
	PROD	−60.751	−60.781	−60.561	−61.307	−60.781	−60.561
S10=C9–S11–C12	GS	0.056	0.047	−0.011	0.104	0.047	−0.011
	TS	−176.270	−176.294	−177.910	−175.430	−176.294	−177.910
	PROD	180.000	180.000	180.000	180.000	180.000	180.000
H3–C2–C5–O8	GS	−60.485	−60.482	−59.674	−60.535	−60.482	−59.674
	TS	−108.420	−108.190	−88.040	−108.260	−108.190	−88.040
	PROD	–	–	–	–	–	–
O8=C9–S10–H1	GS	–	–	–	–	–	–
	TS	–	–	–	–	–	–
	PROD	−0.000	−0.000	−0.000	−0.000	−0.000	−0.000
H1–S10–C9–S11	GS	–	–	–	–	–	–
	TS	–	–	–	–	–	–
	PROD	180.000	180.000	180.000	180.000	180.000	180.000

Scheme 1 Two proposed pathways for the pyrolysis of xanthates.

Scheme 2 Different conformers of O-ethyl, S-methyl xanthates (dithiocarbonates).

Scheme 3 Reactant, transition and product state.

Scheme 4 Reaction path coordinate of O-ethyl, S-methyl xanthates (dithiocarbonates).

Fig. 1 Reaction path calculation.

Table 1 Selected bond lengths (Å) for the pyrolysis of O-ethyl, S-methyl xanthate (dithiocarbonate). Δd = (TS − GS).

Bond length (Å)	State	DFT/631G*	DFT/6-31G**	DFT/6-31+G*	HF/3-21G*	DFT/MP2	DFT/CCSD
H1–C2	GS	1.09	1.09	1.09	1.08	1.09	1.09
	TS	1.25	1.20	1.20	1.59	1.24	1.20
	PROD	–	–	–	–	–	–
	Δd	+0.79	+0.10	+0.10	+0.50	+0.15	+0.10
C2–C5	GS	1.52	1.52	1.52	1.52	1.53	1.52
	TS	1.42	1.42	1.42	1.31	1.43	1.42
	PROD	1.33	1.33	1.33	1.32	1.33	1.34
	Δd	−0.10	−0.10	−0.10	−0.21	−0.10	−0.10
C5–O8	GS	1.45	1.45	1.45	1.47	1.46	1.45
	TS	2.10	2.10	2.10	2.42	2.12	2.10
	PROD	–	–	–	–	–	–
	Δd	+0.65	+0.63	+0.64	+0.95	+0.66	+0.64
O8–C9	GS	1.34	1.34	1.33	1.35	1.33	1.33
	TS	1.26	1.26	1.26	1.21	1.26	1.27
	PROD	1.20	1.20	1.21	1.21	1.21	1.21
	Δd	−0.08	−0.08	−0.08	−0.13	−0.07	−0.08
C9=S10	GS	1.65	1.65	1.65	1.63	1.65	1.65
	TS	1.73	1.72	1.72	1.78	1.74	1.72
	PROD	1.79	1.79	1.79	1.76	1.77	1.79
	Δd	+0.08	+0.08	+0.08	+0.15	+0.09	+0.08
C9–S11	GS	1.79	1.79	1.79	1.76	1.77	1.79
	TS	1.80	1.80	1.80	1.76	1.81	1.81
	PROD	1.79	1.79	1.81	1.78	1.77	1.84
	Δd	+0.01	+0.01	+0.01	−0.00	+0.04	+0.01
S10–H1	GS	4.99	5.00	5.00	4.90	5.00	5.00
	TS	1.88	1.88	1.89	1.31	1.87	1.89
	PROD	1.35	1.35	1.34	1.33	1.34	1.34
	Δd	−3.11	−3.12	−3.11	−3.59	−3.13	−3.11

Table 4 Selected atomic charges (Mulliken). Δq = (TS − GS) for the pyrolysis of O-ethyl-S-methyl xanthate.

Molecule	state	DFT/6-3IG*	DFT/6-3IG**	DFT/6-3I+G*	HF/3-2IG*	DFT/MP2	DFT/CCSD
H1	GS	+0.174	+0.221	+0.221	+0.237	+0.056	+0.221
	TS	+0.140	+0.207	+0.207	+0.168	+0.247	+0.207
	PROD	+0.129	+0.134	+0.134	+0.1669	+0.015	+0.134
	Δq	−0.034	−0.014	−0.014	−0.069	+0.191	−0.014
C1	GS	−0.458	−0.785	−0.785	−0.600	−0.121	−0.785
	TS	−0.480	−0.524	−0.524	−0.429	−0.415	−0.524
	PROD	−0.285	−0.395	−0.395	−0.425	−0.153	−0.395
	Δq	−0.938	+0.261	+0.261	+0.171	−0.294	+0.261
C2	GS	−0.071	−0.031	−0.031	−0.124	+0.026	−0.031
	TS	−0.123	−0.194	−0.194	−0.454	+0.224	−0.194
	PROD	−0.285	−0.395	−0.395	−0.425	−0.153	−0.395
	Δq	−0.011	−0.163	−0.163	−0.328	+0.198	−0.163
O1	GS	−0.387	−0.267	−0.267	−0.664	−0.132	−0.267
	TS	−0.437	−0.498	−0.498	−0.564	−0.346	−0.498
	PROD	+0.389	−0.432	−0.432	−0.561	−0.280	−0.432
	Δq	−0.050	−0.231	−0.281	+0.100	−0.214	−0.281
C3	GS	+0.064	+0.159	−0.100	+0.124	−0.040	−0.100
	TS	+0.082	+0.184	+0.217	+0.172	+0.094	+0.217
	PRD	+0.098	+0.179	+0.167	+0.167	+0.134	+0.167
	Δq	+0.018	+0.025	+0.317	+0.048	+0.054	+0.317
S1	GS	−0.186	−0.216	−0.193	−0.132	−0.160	−0.193
	TS	−0.116	−0.245	−0.369	+0.024	−0.245	−0.369
	PROD	+0.158	+0.167	+0.190	+0.229	+0.038	+0.190
	Δq	+0.07	−0.229	−0.1759	+0.156	−0.085	−0.1759
C4	GS	−0.594	−0.505	−0.810	−0.789	−0.217	−0.810
	TS	−0.618	−0.526	−0.734	−0.779	+0.271	−0.734
	PROD	−0.587	−0.492	−0.718	−0.778	−0.196	−0.718
	Δq	−0.024	−0.021	−0.076	−0.010	−0.054	−0.076

Table 5 Heat of formation for the pyrolysis of O-ethyl, S-methyl xanthate (dithiocarbonates).

	GS (kJ/mol)	TS (kJ/mol	PROD (kJ/mol)
DFT/6-31G*	−2,700,810.370	−2,700,530.990	−2,700,634.640
DFT/6-31G**	−2,700,797.560	−2,700,564.825	−2,700,670.875
DFT/6-31+G*	−2,700,804.320	−2,700,560.625	−2,700,676.125
HF/3-21G*	−2,690,184.325	−2,680,127.321	−2,680,117.094
DFT/MP2	−2,700,797.560	−2,700,564.825	−2,700,670.875
DFT/CCSD	−2,700,797.560	−2,700,564.825	−2,700,670.875

Table 6 Arrhenius parameters for O-ethyl S-methyl xanthate (dithiocarbonates).

	ΔS (J/mol/K)	ΔG (kJ/mol)	ΔH (kJ/mol)	Ea (kJ/mol)	A	k (S^−1)
EXPERIMENTAL	−28.000	143.560	161.000	166.200	4.70 × 10^11	1.4 × 10^−2
DFT/6-31G*	−29.842	181.491	162.808	167.988	3.56 × 10^11	2.93 × 10^−3
DFT/6-31G**	−28.480	131.164	113.720	118.897	4.20 × 10^11	4.45 × 10^1
DFT/6-31+G*	−31.013	135.217	115.281	120.457	3.09 × 10^11	2.43 × 10^1
HF/3-21G*	−36.308	177.491	157.817	158.994	1.64 × 10^11	7.61 × 10^−3
DFT/MP2	−30.014	142.217	151.481	160.457	4.09 × 10^11	1.43 × 10^1
CCSD	−29.590	161.164	163.720	164.567	2.90 × 10^11	3.43 × 10^1

Table 7 Variation of rate of reaction (S⁻¹) with temperature (K).

Temp (K)	DFT/6-31G*	DFT/6-31G**	DFT/6-31+G**	HF/3-21G*
373	1.56 × 10^−10	1.23 × 10^−6	4.56 × 10^−7	6.78 × 10^−10
398	2.37 × 10^−9	3.45 × 10^−5	3.56 × 10^−6	4.34 × 10^−9
423	8.11 × 10^−9	4.51 × 10^−4	9.57 × 10^−4	8.23 × 10^−9
448	1.11 × 10^−8	2.99 × 10^−3	6.167 × 10^−3	6.56 × 10^−8
498	9.16 × 10^−7	7.39 × 10^−2	1.46 × 10^−1	2.46 × 10^−7
523	6.24 × 10^−6	2.93 × 10^−1	5.72 × 10^−1	6.00 × 10^−6
548	3.54 × 10^−5	1.034	1.966	1.23 × 10^−5
573	1.74 × 10^−4	3.257	6.130	3.17 × 10^−4
598	7.57 × 10^−4	9.371	17.400	2.15 × 10^−3
623	2.93 × 10^−3	24.30	44.5	3.46 × 10^−2

Table 8 B3LYP/6-31G* calculated Wiberg bond indices, B_i, for the reactants, transition states and products: percentage of evolution through the chemical process of the bond indices at the transition states %Ev, degrees of advancement of the transition states δB_av, and absolute synchronicities Sy for pathway A for the pyrolysis of O-ethyl, S-methyl dithiocarbonates.

Compound	State	C2–C5	C2–H1	H1–S10	S10–C5	C9–O8	O8–C5
O-ethyl S-methyl Xanthate	$B_i^{\text{R}}$	1.026	0.944	0.000	1.663	1.081	0.830
	$B_i^{\text{TS}}$	1.348	0.534	0.422	1.258	1.505	0.279
	$B_i^{\text{P}}$	2.042	0.000	0.928	1.006	1.807	0.000
	δBi	0.317	0.444	0.454	0.616	0.584	0.664
	%Ev	31.7	44.4	45.4	61.6	58.4	66.4
	δBav	0.5132
	A	0.00039
	Sy	0.9912