Temperature-Dependent Kinetic Study of the Reactions of Hydrogen Atoms with H2S and C2H4S

A discharge-flow reactor combined with modulated molecular beam mass spectrometry technique was employed to determine the rate constants of H-atom reactions with hydrogen sulfide and thiirane. The rate constants for both reactions were determined at a total pressure of 2 Torr from 220 to 950 K under pseudo-first-order conditions by monitoring either consumption of H atoms in excess of H2S (C4H4S) or the molecular species in excess of atomic hydrogen. For H + H2S reaction, a suggested previously strong curvature of the Arrhenius plot was confirmed: kl = 8.7 × 10−13 × (T/298)2.87 × exp(−125/T) cm3 molecule−1 s−1 with a conservative uncertainty of 15% at all temperatures. Non-Arrhenius behavior was also observed for the reaction of H-atom with C2H4S, with the experimental rate constant data being best fitted to a sum of two exponential functions: k2 = 1.85 × 10−10 exp(−1410/T) + 4.17 × 10−12 exp(−242/T) cm3 molecule−1 s−1 with an independent of temperature uncertainty of 15%.


Introduction
The present work reports temperature-dependent measurements of rate constants for two elementary reactions involving hydrogen atoms, with H 2 S and C 2 H 4 S.The H-atom reaction with H 2 S is of importance in combustion chemistry and industrial processes, and is included in a detailed chemical mechanism to describe reactions in the H 2 -S 2 -H 2 S system [1]: The temperature dependence of the rate constant of reaction (1), which proceeds through the abstraction of a hydrogen atom, is also of theoretical interest, in particular for assessing the effect of quantum tunneling [2,3].Reaction (1) has been intensively studied over the past few decades, both experimentally [4][5][6][7][8][9][10][11][12][13][14][15] and theoretically [2,3,13,15,16].Although there is now some experimental and theoretical evidence for the curvature of the Arrhenius plot for reaction (1), it has never actually been observed experimentally in any single study conducted over a sufficiently wide temperature range.All previous experimental work was carried out over a limited temperature range, and "unmodified" Arrhenius expressions were reported for the reaction rate constant.One of the aims of this work was to provide experimental evidence of the curvature of the Arrhenius dependence of k 1 within the framework of one study through rate constant measurements over an extended temperature range, T = 220-950 K.
It is known that reactions of C 2 H 4 S (thiirane, ethylene sulfide) with various atoms are rapid and proceed through S-atom abstraction, leading to almost stoichiometric production of ethylene and sulfur-containing radicals [17][18][19][20].This allows the desulfurization reactions of thiirane to be used in laboratory research both as sources of radicals and as scavengers of unwanted active species.The reaction of C 2 H 4 S with hydrogen atoms leads to the formation of SH (an important intermediate in atmospheric and combustion chemistry [21,22]), and can be used as an alternative source of SH radicals to those involving H 2 S [23]. (2) The kinetic information on reaction (2) is rather scarce and uncertain.Only two measurements of the reaction rate constant were reported, by Yokota et al. [24] at T = 300-425 K using the relative rate method, and in absolute measurements by Lee et al. from 223 to 423 K [20].The activation energies reported in two studies are very close; however, the absolute values of the reaction rate constant differ by a factor of 3. The objective of the present work was to provide new measurements of the rate constant over an extended temperature range, T = 220-950 K.

Reaction H + H 2 S 2.1.1. Measurements of the Reaction Rate Constant
All measurements were carried out under flow conditions at nearly 2 Torr of total pressure of Helium, and with detection of the gas phase species involved using mass spectrometry [23].
The reaction rate constant was determined under pseudo-first-order conditions either from H 2 S decays ([H 2 S] 0 ≤ 4 × 10 11 molecule cm −3 ) in excess of hydrogen atoms, or from the kinetics of H-atom consumption ([H] 0 ≤ 2 × 10 11 molecule cm −3 ) monitored in excess of hydrogen sulfide.The concentrations of the excess reactants were varied between 0.18 and 6.64 × 10 13 molecule cm −3 for H-atom, and between 0.85 and 44.7 × 10 13 molecule cm −3 for [H 2 S] (see Table 1).The linear flow velocity in the reactor was in the range of 1025-3400 cm s −1 .Examples of H 2 S decays observed at different excess concentrations of hydrogen atoms are shown in Figure 1.The consumption of the excess reactant, H atoms, along the reaction zone did not exceed a few percent (mainly due to the wall loss, k w < 10 s −1 ).In all cases, the average concentration of H was used in the calculations.The concentration of H 2 S decays exponentially, [ Examples of H2S decays observed at different excess concentrations of hydrogen atoms are shown in Figure 1.The consumption of the excess reactant, H atoms, along the reaction zone did not exceed a few percent (mainly due to the wall loss, kw < 10 s −1 ).In all cases, the average concentration of H was used in the calculations.The concentration of H2S decays exponentially, [H2S] = [H2S]0 × exp(−k1 ′ × t), where k1 ′ = k1[H] is the pseudo-firstorder rate constant of H2S loss.The pseudo-first-order rate constants, k1 ′ , determined from the slopes of the straight lines like those in Figure 1, are shown in Figure 2, as a function of H-atom concentration.The pseudo-first-order rate constants, k 1 , determined from the slopes of the straight lines like those in Figure 1, are shown in Figure 2, as a function of H-atom concentration.One can note that the intercepts in Figure 2 are close to zero, which is consistent with the observed lack of H2S consumption in the absence of hydrogen atoms in the reactor.Diffusion corrections were applied to all the measured values of k1 ′ in order to take into account the axial and radial gradient of H2S concentration in the flow tube [25].Corrections were generally less than 3%, and only at T = 950 K were they somewhat higher, reaching 7%.The slopes of the straight lines in Figure 2 provide the bimolecular rate constants at respective temperatures, which are summarized in Table 1.
In the second series of experiments, the kinetics of H-atom consumption in an excess of hydrogen sulfide was recorded.Figure 3   One can note that the intercepts in Figure 2 are close to zero, which is consistent with the observed lack of H 2 S consumption in the absence of hydrogen atoms in the reactor.Diffusion corrections were applied to all the measured values of k 1 in order to take into account the axial and radial gradient of H 2 S concentration in the flow tube [25].Corrections were generally less than 3%, and only at T = 950 K were they somewhat higher, reaching 7%.The slopes of the straight lines in Figure 2 provide the bimolecular rate constants at respective temperatures, which are summarized in Table 1.
In the second series of experiments, the kinetics of H-atom consumption in an excess of hydrogen sulfide was recorded.Figure 3  the observed lack of H2S consumption in the absence of hydrogen atoms in the reactor.Diffusion corrections were applied to all the measured values of k1 ′ in order to take into account the axial and radial gradient of H2S concentration in the flow tube [25].Corrections were generally less than 3%, and only at T = 950 K were they somewhat higher, reaching 7%.The slopes of the straight lines in Figure 2 provide the bimolecular rate constants at respective temperatures, which are summarized in Table 1.
In the second series of experiments, the kinetics of H-atom consumption in an excess of hydrogen sulfide was recorded.Figure 3 1.The combined uncertainty on k 1 was estimated to be around 15% for both series of experiments by adding in quadrature statistical error (≤2%) and those on the measurements of the absolute concentration of H 2 S (≈7%), (H) (≈10%), flows (3%), pressure (2%), and temperature (1%).Reaction (1) produces SH radicals, and H-atom kinetics can be potentially impacted by the fast secondary reaction: The rate constant of reaction ( 3) is not well known.Measurements of k 3 available in the literature are scattered between 1.1 × 10 −11 and 4.15 × 10 −11 cm 3 molecule −1 s −1 at room temperature [4,8,10,26].Although this secondary reaction is quite fast, its impact on the measurements of k 1 can be considered negligible due to the low initial concentrations of H atoms used ([H] 0 ≤ 2 × 10 11 molecule cm −3 ).

Comparison with Previous Data
Figure 5 shows the temperature-dependent data available for k 1 .As noted in the Introduction section and as can be seen in Figure 5, the rate constant for reaction (1) is relatively well established.The most reliable measurements of k1 at room temperature [8,[10][11][12] (not shown in Figure 5 for clarity) are in good agreement with each other.Bradley et al. [8] measured k1 in a flow reactor using electron spin resonance for detection of hydrogen atoms: kl = 8.3 × 10 −13 cm 3 molecule −1 s −1 .The same value was reported for k1 by Nicholas et al. [10] who monitored the concentrations of SH and S2 intermediates upon decomposition of H2S in a pulsed radio-frequency discharge and derived k1 from computer simulation of SH and S2 profiles.Husain and Slater [11] reported an absolute value of kl = (8.6 ± 0.5) × 10 −13 cm 3 molecule −1 s −1 , determined via the pulsed photolysis resonance fluorescence method.Finally, Clyne and Ono [12] measured kl = (7.41± 0.39) × 10 −13 cm 3 molecule −1 s −1 , employing resonance fluorescence method in a discharge flow system.It can be noted that the present value of kl measured around room temperature, e.g., (6.0 ± 0.9) × 10 −13 cm 3 molecule −1 s −1 at T = 295 K, is somewhat lower than these previous measurements.
The first temperature-dependent measurement of the rate constant for reaction (1) was conducted by Mihelcic and Schindler [5], who used a discharge flow system combined with ESR detection of hydrogen atoms.The most reliable measurements of k 1 at room temperature [8,[10][11][12] (not shown in Figure 5 for clarity) are in good agreement with each other.Bradley et al. [8] measured k 1 in a flow reactor using electron spin resonance for detection of hydrogen atoms: k l = 8.3 × 10 −13 cm 3 molecule −1 s −1 .The same value was reported for k 1 by Nicholas et al. [10] who monitored the concentrations of SH and S 2 intermediates upon decomposition of H 2 S in a pulsed radio-frequency discharge and derived k 1 from computer simulation of SH and S 2 profiles.Husain and Slater [11] reported an absolute value of k l = (8.6 ± 0.5) × 10 −13 cm 3 molecule −1 s −1 , determined via the pulsed photolysis resonance fluorescence method.Finally, Clyne and Ono [12] measured k l = (7.41± 0.39) × 10 −13 cm 3 molecule −1 s −1 , employing resonance fluorescence method in a discharge flow system.It can be noted that the present value of k l measured around room temperature, e.g., (6.0 ± 0.9) × 10 −13 cm 3 molecule −1 s −1 at T = 295 K, is somewhat lower than these previous measurements.
The present measurements of k l are in very good agreement with those of Kurylo et al. [6], Pratt and Rogers [9], and, in the range of experimental uncertainty, with the data from Peng et al. [15].The k l values measured by Mihelcic and Schindler [5] are a bit higher than others.Rommel and Schiff [7] suggested that the k l values reported by Michelcic and Schindler [5] may have been overestimated.In fact, the values of k l were derived from decays of H atoms under experimental conditions, where initial H 2 S concentration was comparable to that of hydrogen atoms.In addition, secondary reaction (3), which should operate under such conditions, was not taken into account.
The present data for k l obtained in a wide temperature range clearly show a wellpronounced curvature of the Arrhenius plot.The fit of the current data with the modified Arrhenius expression (the continuous black line in Figure 5) yields This expression for k l is recommended from the present study in the temperature range 220-950 K, with a conservative (independent of temperature) overall 2σ uncertainty of 15%.The black dashed lines in Figure 5 represent the deviations from this equation by a factor of 1.3, showing that practically all existing data (except at the highest temperatures) fall into this range.
It should be noted that the curvature of the Arrhenius plot is observed experimentally for the first time, although it was suggested in some previous studies.All previous experimental work was carried out over a limited temperature range, and resulted in "unmodified" Arrhenius expressions for k l .Yoshimura et al. [13] based on their measurements of k l at room and high temperatures (Figure 5) suggested a strong non-Arrhenius dependence of k l on temperature, which was explained by a conventional transition-state theory combined with ab initio calculations.Peng et al. [15], combining their experimental data (T = 298-598 K) with those available in the literature, reported k l = 5.8 × 10 −17 × T 1.94 exp(−455/T) cm 3 molecule −1 s −1 (blue dotted line in Figure 5).This expression reasonably (within 25%) describes the k l data from the present work.The theoretical analysis carried out by the authors also supported the significant curvature of the Arrhenius plot.The experimentally observed non-Arrhenius temperature dependence of k l was also reasonably reproduced by the recent quantum dynamics calculations of Qi et al. [2], concluding that the non-Arrhenius behavior is caused by the pronounced quantum tunneling.The calculations of Qi et al. [2] showed that both abstraction and exchange mechanisms are important for the H + H 2 S reaction.Unfortunately, the available experimental data do not make it possible to define the contribution of these two mechanisms as a function of temperature.

Reaction H + C 2 H 4 S 2.2.1. Measurements of the Reaction Rate Constant
A similar experimental protocol described above for reaction (1) was also applied to measure the rate constant of the reaction of hydrogen atoms with thiirane.As in the previous case, the rate constant of reaction (2) was determined under pseudo-first order conditions by two methods: from kinetics of thiirane consumption ([C 2 H 4 S] 0 ≤ 3.6 × 10 11 molecule cm −3 ) in excess of hydrogen atoms and from H-atom decays ([H] 0 ≤ 2.1 × 10 11 molecule cm −3 ) recorded in excess of C 2 H 4 S.The concentrations of the corresponding excess reactants are shown in Table 2.The flow velocity in the reactor was in the range (1045-3250) cm s −1 .Examples of second-order plots obtained from C 2 H 4 S and H-atom decays are shown in Figures 6 and 7, respectively.The similarity of the results obtained with different initial concentrations of C 2 H 4 S (Figure 6, T = 220 K) and hydrogen atoms (Figure 7) indicates a minor influence of possible secondary chemistry on the measurements of k 2 .The final values of k 2 determined at different temperatures from the slopes of the straight lines like those in Figures 6 and 7 are shown in Table 2.

Comparison with Previous Data
The rate constant of the H + C 2 H 4 S reaction is displayed as a function of temperature in Figure 8.The present data on the temperature dependence of k 2 are best represented by the sum of two exponential functions (the continuous black line in Figure 8):  a Units of 10 13 molecule cm −3 .b units of 10 −12 cm 3 molecule −1 s −1 ; statistical 2σ uncertainty is given; total estimated uncertainty is 15%.c HW: halocarbon wax; Q: quartz.d k2 derived from C2H4S (C2H4S kinetics) or H-atom (H kinetics) decay monitored in excess of H and C2H4S, respectively.

Comparison with Previous Data
The rate constant of the H + C2H4S reaction is displayed as a function of temperature in Figure 8.The present data on the temperature dependence of k2 are best represented by the sum of two exponential functions (the continuous black line in Figure 8):  To our knowledge, only two studies of the reaction are available in the literature.Yokota et al. [24] studied the reaction of thiirane with hydrogen atoms produced from the mercury photosensitization of H2.The reaction rate constant was determined relative to that of the H-atom reaction with H2S at three temperatures between 300 and 425 K: k2 = (9.5 ± 1.2) × 10 −11 exp[-(980 ± 88)/T] cm 3 molecule −1 s −1 .The relative rate data were placed on To our knowledge, only two studies of the reaction are available in the literature.Yokota et al. [24] studied the reaction of thiirane with hydrogen atoms produced from the mercury photosensitization of H 2 .The reaction rate constant was determined relative to that of the H-atom reaction with H 2 S at three temperatures between 300 and 425 K: The relative rate data were placed on an absolute basis with the values of k 1 reported by Kurylo et al. [6], which can be considered valid to date (see Figure 5).Lee et al. [20] carried out absolute measurements of k 2 using the flash photolysis-resonance fluorescence technique, and reported k 2 = (2.87 ± 0.12) × 10 −11 exp[−(945 ± 12)/T] cm 3 molecule −1 s −1 over a temperature range from 223 to 423 K.The present data for k 2 clearly support the results of the relative rate measurements of Yokota et al. [24], being higher by a factor of 3-4 compared with the absolute measurements of Lee et al. [20] (Figure 8).The reason for such a large discrepancy is difficult to determine at this stage.Perhaps the point is in determining the absolute concentrations of thiirane, which (i) was photolyzed in the study of Lee at al. [20] to generate H atoms, and (ii) is known to decompose during storage.
In the study of Yokota et al. [24], C 2 H 4 , H 2 S, and elemental sulfur were detected as reaction products in accordance with the formation of SH radical and C 2 H 4 in the primary step, followed by the production of H 2 S and S in the self-reaction of SH radicals: In this work, we also observed the formation of SH, S and S 2 under specific conditions (high concentrations of both reactants); however, quantitative measurements were quite difficult, and were not performed.

Materials and Methods
All experiments were performed in a conventional discharge fast-flow reactor coupled with a modulated molecular beam-sampling quadrupole mass spectrometer for the detection of the gas phase species [23].Two different flow reactors (45 cm length and 2.4 cm i.d.) available in the laboratory were used.The low-temperature flow reactor, a Pyrex tube coated with halocarbon wax with a jacket for the circulation of thermostatically controlled ethanol (Figure 9), covered a temperature range between 220 and 330 K.The high-temperature flow reactor was employed over a temperature range of 295−950 K and consisted of an electrically heated uncoated Quartz tube with water-cooled attachments.
Molecules 2023, 28, 7883 1 an absolute basis with the values of k1 reported by Kurylo et al. [6], which can be c ered valid to date (see Figure 5).Lee et al. [20] carried out absolute measurement using the flash photolysis-resonance fluorescence technique, and reported k2 = (2.87 × 10 −11 exp[−(945 ± 12)/T] cm 3 molecule −1 s −1 over a temperature range from 223 to The present data for k2 clearly support the results of the relative rate measureme Yokota et al. [24], being higher by a factor of 3-4 compared with the absolute me ments of Lee et al. [20] (Figure 8).The reason for such a large discrepancy is diffi determine at this stage.Perhaps the point is in determining the absolute concentrati thiirane, which (i) was photolyzed in the study of Lee at al. [20] to generate H atom (ii) is known to decompose during storage.
In the study of Yokota et al. [24], C2H4, H2S, and elemental sulfur were detec reaction products in accordance with the formation of SH radical and C2H4 in the pr step, followed by the production of H2S and S in the self-reaction of SH radicals: In this work, we also observed the formation of SH, S and S2 under specific cond (high concentrations of both reactants); however, quantitative measurements were difficult, and were not performed.

Materials and Methods
All experiments were performed in a conventional discharge fast-flow reacto pled with a modulated molecular beam-sampling quadrupole mass spectrometer f detection of the gas phase species [23].Two different flow reactors (45 cm length a cm i.d.) available in the laboratory were used.The low-temperature flow reactor, a tube coated with halocarbon wax with a jacket for the circulation of thermostaticall trolled ethanol (Figure 9), covered a temperature range between 220 and 330 K.The temperature flow reactor was employed over a temperature range of 295−950 K an sisted of an electrically heated uncoated Quartz tube with water-cooled attachment Hydrogen atoms were produced using two methods.The first one employed t crowave discharge of H2/He mixtures.In the second method, H atoms were formed fast reaction of F atoms with H2, F + H2 → H + HF k6 = 1.24 × 10 −10 exp(−507/T) cm 3 molecule −1 s −1 (T = 220-960 K) [27].In this case, the fluorine atoms were generated in the microwave discharge of F2/H tures and titrated with an excess of H2 in the movable injector (Figure 9).The frac F2 dissociated in the microwave discharge exceeded 95%.Ensuring that there w Hydrogen atoms were produced using two methods.The first one employed the microwave discharge of H 2 /He mixtures.In the second method, H atoms were formed in the fast reaction of F atoms with H 2 , F + H 2 → H + HF (6) k 6 = 1.24 × 10 −10 exp(−507/T) cm 3 molecule −1 s −1 (T = 220-960 K) [27].
In this case, the fluorine atoms were generated in the microwave discharge of F 2 /He mixtures and titrated with an excess of H 2 in the movable injector (Figure 9).The fraction of F 2 dissociated in the microwave discharge exceeded 95%.Ensuring that there was no molecular fluorine in the reactor was important in the presence of thiirane, since it was observed that these two stable compounds show high reactivity towards each other.
In experiments with high concentrations of hydrogen atoms (kinetics of H 2 S, and C 2 H 4 S consumption in excess of H), they were detected at m/z = 80/82 (HBr + ) after being transformed to HBr in reaction (9).
The absolute calibration of the mass spectrometer for HOBr was accomplished using reaction (8) in excess of Br 2 , and relating the consumed fraction of Br 2 to the concentration of HOBr produced: [HOBr] = ∆[Br 2 ].Absolute HBr concentrations were determined using two methods.In the first method, the absolute concentration of HBr was calculated from the flow rate of a manometrically prepared HBr/He mixture.The second method was the chemical transformation of the H atom into HBr in reaction (9), linking the consumed fraction of Br 2 and the concentration of HBr formed: [HBr] = ∆[Br 2 ].The results of the two methods coincided within a few percent.The absolute concentrations of other stable species (Br 2 , NO 2 , F 2 , H 2 , H 2 S and C 2 H 4 S) in the reactor were calculated from their flow rates, obtained from the measurements of the pressure drop of their monometrically prepared mixtures in He stored in calibrated volume flasks.

Conclusions
In this work, we have investigated the kinetics of the reactions of H atoms with hydrogen sulfide and thiirane, using a discharge flow reactor combined with mass spectrometry.For the H + H 2 S reaction, previously assumed on the basis of experimental and theoretical data, the strong curvature of the Arrhenius plot was confirmed via measurements of the reaction rate constant over an extended temperature range (220-950 K).Similar curved Arrhenius dependence was also observed for reaction of H atoms with C 2 H 4 S, which was studied at T = 220-950 K (and for the first time at T > 425 K).The reaction was found to be fast enough to be used as an alternative (free of H 2 S) source of SH radicals (an important intermediate in combustion and atmospheric chemistry) in laboratory studies.

Figure 1 .
Figure 1.Reaction H + H2S: example of H2S decays observed with different concentrations of H atoms at T = 575 K.

Figure 1 .
Figure 1.Reaction H + H 2 S: example of H 2 S decays observed with different concentrations of H atoms at T = 575 K.

Figure 2 .
Figure 2. Reaction H + H2S: dependence of the pseudo-first-order rate constant, k1 ′ = k1[H], on the concentration of H atoms at different temperatures.

Figure 2 .
Figure 2. Reaction H + H 2 S: dependence of the pseudo-first-order rate constant, k 1 = k 1 [H], on the concentration of H atoms at different temperatures.
displays typical exponential decays of [H] with time, [H] = [H] 0 × exp(−k 1 ×t), where k 1 = k 1 [H 2 S] + k w is the pseudo-first-order rate constant, with k w representing the heterogeneous loss of H atoms.

Figure 3 .
Figure 3. Reaction H + H2S: typical H-atom decays observed in the presence of different concentrations of H2S at T = 325 K.Figure 3. Reaction H + H 2 S: typical H-atom decays observed in the presence of different concentrations of H 2 S at T = 325 K.

Figure 3 .
Figure 3. Reaction H + H2S: typical H-atom decays observed in the presence of different concentrations of H2S at T = 325 K.Figure 3. Reaction H + H 2 S: typical H-atom decays observed in the presence of different concentrations of H 2 S at T = 325 K. Examples of the typical second-order plots observed at different temperatures are shown in Figure 4.The low intercept values of the plots in Figure 4 are in agreement with the wall loss rate of H atoms, k w < 10 s −1 , measured in the absence of H 2 S in the reactor.Diffusion corrections applied to the measured values of k 1 did not exceed 10%.Final values of k 1 determined in this series of experiments are presented in Table1.The combined uncertainty on k 1 was estimated to be around 15% for both series of experiments by adding in quadrature statistical error (≤2%) and those on the measurements of the absolute concentration of H 2 S (≈7%), (H) (≈10%), flows (3%), pressure (2%), and temperature (1%).

Figure 4 .
Figure 4. Reaction H + H 2 S: dependence of the pseudo-first order rate constant, k 1 = k 1 [H 2 S] + k w , on the concentration of H 2 S at different temperatures.

Molecules 2023, 28 , 7883 6 of 13 Figure 5 .
Figure 5. Temperature dependence of the rate constant of reaction (1).Uncertainties shown for the present measurements of k1 correspond to estimated total uncertainty of 15%.

Figure 5 .
Figure 5. Temperature dependence of the rate constant of reaction (1).Uncertainties shown for the present measurements of k 1 correspond to estimated total uncertainty of 15%.

Figure 6 .
Figure 6.Reaction H + C2H4S: pseudo-first-order rate constant, k2 ′ = k2[H], as a function of the concentration of H atoms at different temperatures.Figure 6. Reaction H + C 2 H 4 S: pseudo-first-order rate constant, k 2 = k 2 [H], as a function of the concentration of H atoms at different temperatures.

Figure 6 .
Figure 6.Reaction H + C2H4S: pseudo-first-order rate constant, k2 ′ = k2[H], as a function of the concentration of H atoms at different temperatures.Figure 6. Reaction H + C 2 H 4 S: pseudo-first-order rate constant, k 2 = k 2 [H], as a function of the concentration of H atoms at different temperatures.

Figure 6 .
Figure 6.Reaction H + C2H4S: pseudo-first-order rate constant, k2 ′ = k2[H], as a function of the concentration of H atoms at different temperatures.

Figure 7 .
Figure 7. Reaction H + C2H4S: dependence of the pseudo-first-order rate constant, k2 ′ = k2[C2H4S] + kw, on the concentration of C2H4S at T = 325 K measured with two different initial concentrations of H atoms.

Figure 7 .
Figure 7. Reaction H + C 2 H 4 S: dependence of the pseudo-first-order rate constant, k 2 = k 2 [C 2 H 4 S] + k w , on the concentration of C 2 H 4 S at T = 325 K measured with two different initial concentrations of H atoms.

Figure 8 .
Figure 8. Summary of the measurements of k2.Uncertainties shown for selected present measurements of k2 correspond to the estimated total uncertainty of 15%.

Figure 8 .
Figure 8. Summary of the measurements of k 2 .Uncertainties shown for selected present measurements of k 2 correspond to the estimated total uncertainty of 15%.

Figure 9 .
Figure 9. Low-temperature flow reactor: configuration used in the measurements of the ra stants of reactions (1) and (2).

Figure 9 .
Figure 9. Low-temperature flow reactor: configuration used in the measurements of the rate constants of reactions (1) and (2).

Table 1 .
Experimental conditions and results of the measurements of the rate constant of reaction (1).
a Units of 10 13 molecule cm −3 .b units of 10 −12 cm 3 molecule −1 s −1 ; statistical 2σ uncertainty is given, total estimated uncertainty is 15%.c HW: halocarbon wax; Q: quartz.d k 1 derived from H 2 S (H 2 S kinetics) or H-atom (H kinetics) decays monitored in excess of H and H 2 S, respectively.

Table 2 .
Experimental conditions and results of the measurements of the rate constant of reaction (2).

Table 2 .
Experimental conditions and results of the measurements of the rate constant of reaction (2).T (K) [Excess Reactant]