High-temperature collisional energy transfer in highly vibrationally excited molecules II: Isotope effects in isopropyl bromide systems

Values for 〈ΔE_down〉, the average downward energy transferred from the reactant to the bath gas upon collision, have been obtained for highly vibrationally excited undeuterated and per-deuterated isopropyl bromide with the bath gases Ne, Xe, C₂H₄, and C₂D₄, at ca. 870 K. The technique of pressure-dependent very low-pressure pyrolysis (VLPP) was used to obtain the data. For C₃H₇Br, the 〈ΔE_down〉 values (cm^?1) are 490 (Ne), 540 (Xe), 820 (C₂H₄), and 740 (C₂D₄), and for C₃D₇Br, 440 (Ne), 570 (Xe), 730 (C₂H₄), and 810 (C₂D₄). The uncertainties in these values are ca. ±10%. The 〈ΔE_down〉 values for the inert bath gases Ne and Xe show excellent agreement with the theoretical predictions of the semi-empirical biased random walk model for monatomic/substrate collisional energy exchange [J. Chem. Phys., 80 , 5501 (1984)]. The relative effects of deuteration of the reactant molecule on 〈ΔE_down〉 also compare favorably with the predictions of this theoretical model. Extrapolated high-pressure rate coefficients (s^?1) for the thermal decomposition of reactant are 10^13.6±0.3 exp(?200 ± 8 kJ mol^?1/RT) for C₃H₇Br and 10^13.9±0.3 exp(?207 ± 8 kJ mol^±1/RT) for C₃D₇Br, which are consistent with previous studies and the expected isotope effect.     

High temperature collisional energy transfer in highly vibrationally excited molecules: Isotope effects in tert-butyl chloride systems

The average downward collisional energy transfer (<ΔE_down>) is obtained for highly vibrationally excited tert-butyl chloride, both undeuterated and per-deuterated, with Kr, N₂, CO₂, and C₂H₄ bath gases, at ca. 760 K. Data are obtained using the technique of pressure-dependent very low-pressure pyrolysis. Reactant internal energies to which the data are sensitive are in the range 200–250 kJ mol^?1. For C₄H₉Cl, the <ΔE_down> values (cm^?1) are 255 (Kr), 265 (N₂), 440 (CO₂), and 585 (C₂H₄), and for C₄D₉Cl, 245 (N₂), 370 (CO₂), and 540 (C₂H₄). The uncertainties in these values are ca. 20% (40% for Kr); the uncertainties in the deuteration ratios are 10–15%. The value for Kr is in agreement with theoretical predictions of a biased random walk model for internal energy change in monatomic/substrate collisions. The effect of deuteration of <ΔE_down> is also in accord with that predicted by a modification of the theory. Extrapolated highpressure rate coefficients for the thermal decomposition of reactant are 10^13.6 exp(-187 kJ mol^?1/RT) s^?1 (C₄H₉Cl) and 10^14.2 exp(?196 kJ mol^?1/RT) s^?1 (C₄D₉Cl), in accord with other studies and the expected isotope effect.     

Collisional energy transfer in highly vibrationally excited molecules: A very low-pressure pyrolysis study of acetyl chloride

The average downward energy transfer (〈Δ E_down〉) is obtained for highly vibrationally excited acetyl chloride with Ne and C₂H₄ bath gases at ca. 870 K. Data are obtained by the technique of very low-pressure pyrolysis (VLPP). Fitting these data by solution of the appropriate reaction-diffusion integrodifferential master equation yields the gas/gas collisional energy transfer parameters: 〈Δ E_down〉 values are 220 ± 10 cm^?1 (Ne bath gas) and 330 ± 20 cm^?1 (C₂H₄). These energy transfer quantities are much less than those predicted by statistical theories, or those observed for similar sized molecules such as CH₃CH₂Cl. These results are explained by the qualitative predictions of the biased random walk model wherein the fundamental mechanism of energy transfer is the multiple interactions between the bath gas and the individual atoms of the reactant molecule, during the course of the collision event. The charge distribution of acetyl chloride decreases the number of such interactions, thereby reducing the amount of energy transferred per collision.     

Vibrational relaxation and dissociation in the perfluoromethyl halides,CF3Cl,CF3Br,andCF3I

The processes of vibrational relaxation and unimolecular dissociation of the perfluoromethyl halides CF₃Cl, CF₃Br, and CF₃I have been studied in the shock tube with the laser-schlieren technique. Vibrational relaxation was resolved in pure CF₃Cl and CF₃Br (400–484 K and 400–500 K, respectively), and in the mixtures; 2% CF₃Cl/Kr (500–1000 K), 10% CF₃Cl/Kr (440–670 K), 4% CF₃Br/Kr (450–850 K), and 2% CF₃I/Kr (620–860 K). Relaxation in the pure gases is extremely rapid, but shows a well-resolved, accurately exponential decay which provides very precise relaxation times in close agreement with ultrasonic results. Relaxation times as short as 0.1 μs-atm can be resolved, showing the method has a resolution within a factor 2–3 of the best ultrasonic methods. Relaxation dilute in rare gas shows a complex double exponential behavior consistent with a two-stage series process. Rates of CF₃(SINGLEBOND)X fission in these mixtures were measured over 1800–3000 K, P<0.55 atm, for CF₃Cl; 1600–2500 K, P<0.55 atm, in CF₃Br; and 1260–2100 K, P<0.34 atm, in CF₃I. Rates for dissociation were derived from a full profile modeling using a secondary mechanism of six CF₃ reactions. RRKM analysis showed all dissociations to lie near the low pressure limit. Using literature barriers, these rates are best fit with (ΔE)_all=−270 cm⁻¹ for CF₃Cl, 〈ΔE〉_down=0.3 T for CF₃Br, and 〈ΔE〉_down=800 cm⁻¹ for CF₃F. All these transfers are on the large side, similar to those found in other halogenated methanes. © 1997 John Wiley & Sons, Inc.     

Thermal decomposition of CF3Br using Br-atom absorption

Br-atom atomic resonance absorption spectrometry (ARAS) has been developed and applied to measure thermal decomposition rate constants for CF₃Br (+ Kr)→CF₃+Br (+ Kr) over the temperature range, 1222–1624 K. The Br-atom curve-of-growth (145<λ<163 nm) was determined using this reaction. For [Br]≤1×10¹² molecules cm⁻³, absorbance, (ABS)=1.410×10⁻¹³ [Br], yielding σ=1.419×10⁻¹⁴ cm². The curve-of-growth was then used to convert (ABS) to Br-atom profiles which were then analyzed to give measured rate constants. These can be expressed in second-order by k₁=8.147×10⁻⁹ exp(−24488 K/T) cm³ molecule⁻¹ s⁻¹ (±33%, 1222≤T≤1624 K). A unimolecular theoretical approach was used to rationalize the data. Theory indicates that the dissociation rates are closer to second- than to first-order, i.e., the magnitudes are 30–53% of the low-pressure-limit rate constants over 1222–1624 K and 123–757 torr. With the known, E₀=ΔH₀⁰=70.1 kcal mole⁻¹, the optimized theoretical fit to the ARAS data requires 〈ΔE〉_down=550 cm⁻¹. These conclusions are consistent with recently published data and theory from Kiefer and Sathyanarayana. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 859–867, 1998     

Thermal decomposition of CH3I using I-atom absorption

The recently developed I-atom atomic resonance absorption spectrometric (ARAS) technique has been used to study the thermal decomposition kinetics of CH₃I over the temperature range, 1052–1820 K. Measured rate constants for CH₃I(+Kr)→CH₃+I(+Kr) between 1052 and 1616 K are best expressed by k(±36%)=4.36×10⁻⁹ exp(−19858 K/T) cm³ molecule⁻¹ s⁻¹. Two unimolecular theoretical approaches were used to rationalize the data. The more extensive method, RRKM analysis, indicates that the dissociation rates are effectively second-order, i.e., the magnitude is 61–82% of the low-pressure-limit rate constants over 1052–1616 K and 102–828 torr. With the known E₀=ΔH₀⁰=55.5 kcal mole ⁻¹, the optimized RRKM fit to the ARAS data requires (ΔE)_down=590 cm⁻¹. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 535–543, 1997.     

Cyanogen pyrolysis and the CN + NO reaction behind incident shock waves

The reaction chemistry of C₂N₂? Ar and C₂N₂? NO? Ar mixtures has been investigated behind incident shock waves. Progress of the reaction was monitored by observing the cyano radical (CN) in absorption at 388.3 nm. A quantitative spectroscopic model was used to determine concentration histories of CN. From initial slopes of CN concentration during cyanogen pyrolysis, the rate constant for C₂N₂ + M → 2CN + M (1) was determined to be k₁ = (4.11 ± 1.8) × 10¹⁶ exp(?47,070 ± 1400/T) cm³/mol · s. A reaction sequence for the C₂N₂? NO system was developed, and CN profiles were computed. By comparison with experimental CN profiles the rate constant for the reaction CN + NO → NCO + N (3) was determined to be k₃ = 10^{(14.0 ± 0.3)} exp(?21,190 ± 1500/T) cm³/mol · s. In addition, the rate of the four-centered reaction CN + NO → N₂ + CO (2) was estimated to be approximately three orders of magnitude below collision frequency.     

Stereoelectronic control of the retro diels–alder reaction in 8-(N-pyrrolidyl)bicyclo[4.3.0]nona-3,7-diene isomers

The cis- and trans-annulated isomers of 8-(N-pyrrolidyl)bicyclo[4.3.0]nona-3,7-diene show different propensities for the retro Diels–Alder fragmentation following electron impact ionization. Molecular ions of the cis-annulated isomer decompose predominantly via the retro Diels–Alder reaction to give [C₉H₁₃N] ⁺· fragments of the appearance energy (AE)=8.45±0.05eV and critical energy E_c=133±8kJ mol^?1. The trans-annulated isomer gives abundant [M–H]⁺ (AE=9.34±0.08eV) and [M–C₆H₆]⁺· fragments, in addition to [C₉H₁₃N]⁺· ions of AE=8.98±0.05eV and E_c=181±8kJ mol^?1. The ionization energies (IE) were determined as IE_cis=7.07±0.05 eV and IE_trans=7.10±0.06eV. The stereochemical information is much less pronounced in unimolecular decompositions of long-lived (metastable) molecular ions which show very similar fragmentation patterns for both geometrical isomers. Nevertheless, the isomers exhibit different kinetic energy release values in the retro Diels–Alder fragmentation; T_0.5=3.8±0.3 and 4.8±0.2 kJ mol^?1 for the cis and trans isomer respectively. Topological molecular orbital calculations indicate that the retro Diels–Alder reaction prefers a two-step path, with a subsequent cleavage of the C(5)? C(6) and C(1)? C(2) bonds. The open-ring distonic intermediate represents the absolute minimum on the reaction energy hypersurface. The cleavage of the C(1)? C(2) bond is the rate-determining step in the decomposition of the cis isomer, with the critical energy calculated as 137 kJ mol^?1. The cleavage of the C(5)? C(6) bond becomes the rate-determining step in the trans-annulated isomer because of stereoelectronic control. The difference in the energy barriers to this cleavage in the isomers (ΔE=95k Jmol^?1) provides a quantitative estimate of the magnitude of the stereoelectronic effect in cation radicals.     

Incubation in cyclohexene decomposition at high temperatures

A detailed master equation simulation has been carried out for the thermal unimolecular decomposition of C₆H₁₀ in a shock tube. At the highest temperatures studied experimentally [J. H. Kiefer and J. N. Shah, J. Phys. Chem., 91, 3024 (1987)], the average thermal vibrational energy is greater than the reaction threshold and therefore 〈ΔE〉 (up and down steps) is positive for molecules at that energy, rather than negative; the converse is true at lower temperatures. The calculated incubation time, in which the decomposition rate constant rises to 1/e of its steady state value, is found to be only weakly dependent on temperature (at constant pressure) between 1500 K and 2000 K and to depend almost exclusively on 〈ΔE〉_d (down steps, only), and not on collision probability model. Simulations of the experimental data show the magnitude of 〈ΔE〉_d depends weakly on assumed collision probability model, but is nearly independent of temperature. The second moment 〈ΔE〉^½ is found to be independent of both temperature and transition probability model. The experimental data are not very sensitive to the possible energy-dependence of 〈ΔE〉_d for a wide range of assumptions. It is concluded that the observed experimental “delay times” probably can be identified with the incubation time; further experiments are desirable to test this possibility and obtain more direct measures of the incubation time.     

The unimolecular dissociation of 3,4-dihydro-2H-pyran over a wide temperature range

The thermal decomposition of 3,4-dihydro-2H-pyran (DHP, C₅H₈O) has been investigated by two methods: in shock waves with the laser-schlieren technique using mixtures of 5 and 10% DHP in krypton over 900–1500 K, 110–560 torr; in a flow tube having a reaction pressure 0.5 torr above atmospheric using the decomposition of allylethyl ether as an internal standard, and covering 663–773 K. The retro-Diels-Alder dissociation to the stable acrolein and ethylene is the dominant channel for all conditions. Precise rate constants (rms deviation of 10%) were obtained for this process over the indicated temperature ranges. Unimolecular falloff is evident in the shock-tube results, and RRKM calculations also predict a slight falloff at the lower temperatures. These RRKM calculations use a routine vibration model transition state and agree closely with the high-temperature data when 〈ΔE〉_down is a fixed 400 cm^?1. Arrhenius expressions for k_∞ derived from the two measurements are in close accord and also consistent with most previous studies of this reaction. © 1995 John Wiley & Sons, Inc.     

The relaxation of HCN(011) by V-T,R and V-V energy transfer

Tuned output from an optical parametric oscillator has been used to excite HCN directly to its (011) level. By careful use of a “cold-gas filter”, it has proved possible to distinguish between the time-resolved fluorescence from HCN(011) and that from HCN(001) formed during collisional relaxation. Rate constants for relaxation from both levels have been obtained for the partners: (i) He, Ne, Ar and Kr, and (ii) HCN, CO₂, N₂O, OCS, CS₂, C₂H₂ and C₂D₂. With the rare gases, HCN(011) is relaxed to (001) by V-T,R energy transfer, with rate constants (cm³ molecule^?1 s^?1) at 298 ± 4 K of: k_He⁰¹¹ = (7.9 ± 1.05) × 10^?13; k_Ne⁰¹¹ = (1.56 ± 0.12) × 10^?13; k_Ar⁰¹¹ = (1.20 ± 0.17) × 10^?13; k_Kr⁰¹¹ = (6.7 ± 0.65) × 10^?14. The molecular collision partners also transfer HCN(011) to (001). The rates are much greater and clearly near-resonant V-V energy exchange is important. The results are compared to first-order Sharma-Brau theory, with fair agreement where near-resonant channels exist.     

Rate constants for the reactions of Br atoms with a series of alkanes,alkenes, and alkynes in the presence of O2

Rate constants of Br atom reactions have been determined using a relative kinetic method in a 20 l reaction chamber at total pressures between 25 and 760 torr in N₂ + O₂ diluent over the temperature range 293–355 K. The measured rate constants for the reactions with alkynes and alkenes showed dependence upon temperature, total pressure, and the concentration of O₂ present in the reaction system. Values of (6.8 ± 1.4) × 10^?15, (3.6 ± 0.7) × 10^?14, (1.5 ± 0.3) × 10^?12, (1.6 ± 0.3) × 10^?13, (2.7 ± 0.5) × 10^?12, (3.4 ± 0.7) × 10^?12, and (7.5 ± 1.5) × 10^?12 (units: cm³ s^?1) have been obtained as rate constants for the reactions of Br with 2,2,4-trimethylpentane, acetylene, propyne, ethene, propene, 1-butene, and trans-2-butene, respectively, in 760 torr of synthetic air at 298 K with respect to acetaldehyde as reference, k = 3.6 × 10^?12 cm³ s^?1. Formyl bromide and glyoxal were observed as primary products in the reaction of Br with acetylene in air which further react to form CO, HBr, HOBr, and H₂O₂. Bromoacetaldehyde was observed as an primary product in the reaction of Br with ethene. Other observed products included CO, CO₂, HBr, HOBr, BrCHO, bromoethanol, and probably bromoacetic acid.     

Electrochemical behaviour and determination of rimsulfuron herbicide by square wave voltammetry

The voltammeric behaviour of rimsulfuron herbicide has been studied by square wave stripping voltammetry on static hanging mercury drop electrode. It exhibited a well-defined peak within the pH range of 1.0–6.0, having a maximum peak response at ?600 mV (vs.Ag/AgCl) at pH 3.0. The factors such as accumulation potential (E_acc), accumulation time (t_acc), frequency (f), pulse amplitude (ΔE) and step potential (ΔE_s) have been optimised. The calibration plot was a straight line in the range of 4.4–134.4 μg L^?1 with a detection limit of 1.3 μg L^?1. The validity of the method was assessed from the recoveries of spiked lake water, tomato juice and agrochemical formulation of Doncep^®. The results of the experiments conducted for five recoveries were 48.8 ± 1.7 and 49.7 ± 1.0 μg L^?1, which are very close to the rimsulfuron spiked to lake water and tomato juice (50 μg L^?1), with a relative error of –2.4% and ?0.6%, respectively. The electrode reaction mechanism was also postulated.     

Collisional de-excitation of the metastableD-states of Ba+ by He,Ne, N2 and H2

The rate constants 〈σ · υ〉 for collisional de-excitation of the metastable 5D states of Ba⁺ ions have been determined in an ion trap experiment. TheD-states are selectively populated by pulsed laser excitation of the 6P _1/2 or 6P _3/2 state and the decay at different background pressures is monitored by the change in fluorescence intensity of the excited ions. From the pressure dependence of the decay constants we calculate the de-excitation rate constants for different collision partners, averaged over the velocity distribution of the trapped ion cloud. For He, Ne, H₂ and N₂ we obtain in the c.m. energy range of 0.1–0.5 eV: 〈σ·υ〉 (He)=3.0±0.2·10^?13cm³/s, 〈σ·υ〉 (Ne)=5.1±0.4·10^?13cm³/s, 〈σ·υ〉 (H₂)=3.7±0.3·10^?11cm³/s, 〈σ·υ〉 (N₂)=4.4±0.3·10^?11cm³/s. The results can be understood qualitatively by a consideration of the ion-atom and ion-molecules interaction potential.     

Study of the reactions of oxygen atoms with carbonothioicdichloride,carbonothioicdifluoride, and tetrafluoro-1,3-dithietane

The reactions of ground-state oxygen atoms with carbonothioicdichloride, carbonothioicdifluoride, and tetrafluoro-1,3-dithietane have been studied in a crossed molecular jet reactor in order to determine the initial reaction products and in a fast-flow reactor in order to determine their overall rate constants at temperatures between 250 and 500 K. These rate constants are??(O + C₂CS) =(3.09 ± 0.54) × 10^?11 exp(+115 ± 106 cal/mol/RT),??(O + F₂CS) = (1.22 ± 0.19) × 10^?11 exp(-747 ± 95 cal/mol/RT), and??(O + F₄C₂S₂) = (2.36 ± 0.52) × 10^?11 exp(-1700 ± 128 cal/mol/RT) cm³/molec˙sec. The detected reaction products and their rate constants indicate that the primary reaction mechanism is the electrophilic addition of the oxygen atom to the sulfur atom contained in the reactant molecule to form an energy-rich adduct which then decomposes by C-S bond cleavage.     

Elastic modulus of the crystalline phase of poly(cis 1,4-isoprene)

The crystalline elastic modulus of poly(cis 1,4-isoprene) has been measured by x rays and calculated by molecular mechanics. The experimental modulus is E = (2.3 ± 0.3) × 10⁹N m^?2. The calculated modulus is E = (8.8 ± 0.1) × 10⁹N m^?2 for chains in S⁺ cis S^?T conformation and E = (6.1 ± 0.1) × 10⁹N m^?2 for chains in S⁺ cis S+T conformation. The modulus calculated for a statistical structure including both conformation is E = (6.7 ± 0.1) × 10⁹N m^?2. The experimental modulus is thought to be a lower limit because of partial crystallinity of the sample. The chief mechanism of deformation is rotation around single bonds adjacent to the double bond.     

Gas phase thermal diffusivities by a thermal lens technique

A simplified design of thermal lens apparatus is presented in which a chopped cw argon laser beam produces a transient thermal lens in a cylindrical gas cell. The axial intensity variation of a cw helium-neon laser probing this lens is analysed to yield the thermal diffusivities and thus the thermal conductivity coefficients of Kr, CO₂, CH₄, C₂H₆, C₃H₈, C₃H₆ and C₄H₁₀ as 9.4 × 10^?3 ± 4%, 1.6 × 10^?2 ± 3%, 2.98 × 10^?2 ± 4%, 2.03 × 10^?2 ± 4%, 2.05 × 10^?2 ± 7%, 1.6 × 10^?2 ± 8% and 1.9 × 10^?2 ± 8% respectively in W m^?1 K^?1 at 300 K. The method is rapid, requiring only that the sample be transparent at both laser frequencies used. A simplified mathematical analysis is shown to be adequate for this system. For the conditions specified, self-lensing of the argon laser beam is shown to be compensated by using an effective laser beam diameter.     

Impact of the anion and chalcogen on the crystal structure and properties of 4,6‐dimethyl‐2‐pyrimido(thio)nium halides

By the reaction of urea or thiourea, acetylacetone and hydrogen halide (HF, HBr or HI), we have obtained seven new 4,6‐dimethyl‐2‐pyrimido(thio)nium salts, which were characterized by single‐crystal X‐ray diffraction, namely, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium bifluoride, C₆H₉N₂O⁺·HF₂^? or (dmpH)F₂H, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium bromide, C₆H₉N₂O⁺·Br^? or (dmpH)Br, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium iodide, C₆H₉N₂O⁺·I^? or (dmpH)I, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium iodide–urea (1/1), C₆H₉N₂O⁺·I^?·CH₄N₂O or (dmpH)I·ur, 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium bifluoride–thiourea (1/1), C₆H₉N₂S⁺·HF₂^?·CH₄N₂S or (dmptH)F₂H·tu, 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium bromide, C₆H₉N₂S⁺·Br^? or (dmptH)Br, and 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium iodide, C₆H₉N₂S⁺·I^? or (dmptH)I. Three HCl derivatives were described previously in the literature, namely, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium chloride, (dmpH)Cl, 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium chloride monohydrate, (dmptH)Cl·H₂O, and 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium chloride–thiourea (1/1), (dmptH)Cl·tu. Structural analysis shows that in 9 out of 10 of these compounds, the ions form one‐dimensional chains or ribbons stabilized by hydrogen bonds. Only in one compound are parallel planes present. In all the structures, there are charge‐assisted N⁺—H…X^? hydrogen bonds, as well as weaker C_Ar⁺—H…X^? and π⁺…X^? interactions. The structures can be divided into five types according to their hydrogen‐bond patterns. All the compounds undergo thermal decomposition at relatively high temperatures (150–300 °C) without melting. Four oxopyrimidinium salts containing a π⁺…X^?…π⁺ sandwich‐like structural motif exhibit luminescent properties.     

The production and subsequent relaxation of vibrationally excited OH in the reaction of atomic oxygen with HBr

A fast discharge flow apparatus equipped for EPR detection of radicals has been used to investigate the reaction O + HBr → OH + Br. At 295°K, measurements showed that more than 97% of all OH produced in this reaction was formed initially in its first vibrationally excited state. Rate constants for physical deactivation of OH(v = 1) by O(³P), Br(²P_3/2), H₂O, and HBr were measured as (1.45 ± 0.25) × 10^?10, (6.4 ± 2.4) × 10^?11, (1.35 ± 0.50) × 10^?11, and < 10^?12 cm³/molec·sec, respectively.