Ozone-Activated Cataluminescence Sensor System for Dichloroalkanes Based on Silica Nanospheres

The detection and monitoring of dichloroalkanes, which are typical chlorinated volatile organic compounds (CVOCs) with obvious biological toxicity, is of significance for environmental pollution and public health. Herein, a novel ozone-activated cataluminescence (CTL) sensor system based on silica nanospheres was developed for highly sensitive and fast quantification of dichloroalkanes. A typical CTL system coupled with a plasma-ozone-assist unit was designed for promoting the CTL response of dichloroalkanes. The ozone generated by plasma provides a new pathway of catalytic oxidation process, which accompanied by the CTL signal amplification of dichloroalkanes results in an enhanced CTL sensor system with improved limit of detection (1,2-dichloroethane: 0.04 μg mL^-1, 1,2-dichloropropane: 0.03 μg mL^-1) and benign selective performance under the interference of CO₂, H₂O, NO, NO₂, SO₂, CS₂, and other common CVOCs. Moreover, a segmented CTL mechanism including co-adsorption of ozone and dichloroalkanes, thermal elimination, the ozonation route, and a luminous step was ratiocinated based on multiple characterizations and discussion. The proposed methodology and theory open up an attractive perspective for the analysis of less active volatile organic compounds.

The Unimolecular Reactions of CF3CHF2 Studied by Chemical Activation: Assignment of Rate Constants and Threshold Energies to the 1,2-H Atom Transfer, 1,1-HF and 1,2-HF Elimination Reactions, and the Dependence of Threshold Energies on the Number of F-Atom Substituents in the Fluoroethane Molecules

The recombination of CF₃ and CHF₂ radicals in a room-temperature bath gas was used to prepare vibrationally excited CF₃CHF₂* molecules with 101 kcal mol^-1 of vibrational energy. The subsequent 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions were observed as a function of bath gas pressure by following the CHF₃, CF₃(F)C: and C₂F₄ product concentrations by gas chromatography using a mass spectrometer as the detector. The singlet CF₃(F)C: concentration was measured by trapping the carbene with trans-2-butene. The experimental rate constants are 3.6 × 10⁴, 4.7 × 10⁴, and 1.1 × 10⁴ s^-1 for the 1,2-H atom transfer and 1,1-HF and 1,2-HF elimination reactions, respectively. These experimental rate constants were matched to statistical RRKM calculated rate constants to assign threshold energies (E₀) of 88 ± 2, 88 ± 2, and 87 ± 2 kcal mol^-1 to the three reactions. Pentafluoroethane is the only fluoroethane that has a competitive H atom transfer decomposition reaction, and it is the only example with 1,1-HF elimination being more important than 1,2-HF elimination. The trend of increasing threshold energies for both 1,1-HF and 1,2-HF processes with the number of F atoms in the fluoroethane molecule is summarized and investigated with electronic-structure calculations. Examination of the intrinsic reaction coordinate associated with the 1,1-HF elimination reaction found an adduct between CF₃(F)C: and HF in the exit channel with a dissociation energy of ∼5 kcal mol^-1. Hydrogen-bonded complexes between HF and the H atom migration transition state of CH₃(F)C: and the F atom migration transition state of CF₃(F)C: also were found by the calculations. The role that these carbene-HF complexes could play in 1,1-HF elimination reactions is discussed.

Experimental and Quantum Mechanical Study of Nucleophilic Substitution Reactions of meta- and para-Substituted Benzyl Bromides with Benzylamine in Methanol: Synergy Between Experiment and Theory

This work involves the experimental and theoretical study of the nucleophilic substitution of meta- and para-substituted benzyl bromides with benzylamine. Conductometric rate experiments confirm the applicability of the Hammett linear free-energy relationship to this system. To gain a deep understanding of the physical chemistry at play, a quantum mechanical study of the reaction is also conducted. The quantum mechanical calculations not only reproduce the experimental free energy of activation, but also provide greater insights at the molecular and atomic level. Isolation of the calculated transition state structure and application of the Hammett equation to its electronic, structural, and energetic properties are studied.

Characterization of the 1,1-HCl Elimination Reaction of Vibrationally Excited CD3CHFCl Molecules and Assignment of Threshold Energies for 1,1-HCl and 1,2-DCl plus 1,1-HF and 1,2-DF Elimination Reactions

Vibrationally excited CD3CHFCl molecules with 96 kcal mol(-1) of energy were generated by the recombination of CD3 and CHFCl radicals in a room-temperature bath gas. The four competing unimolecular decomposition reactions, namely, 1,1-HCl and 1,2-DCl elimination and 1,1-HF and 1,2-DF elimination, were observed, and the individual rate constants were measured. The product branching fractions are 0.60, 0.27, 0.09, and 0.04 for 1,2-DCl, 1,1-HCl, 1,2-DF, and 1,1-HF elimination, respectively. Electronic structure calculations were used to define models of the four transition states. The statistical rate constants calculated from these models were compared to the experimental rate constants. The assigned threshold energies with ±2 kcal mol(-1) uncertainty are 60, 72, 65, and 74 kcal mol(-1) for the 1,2-DCl, 1,1-HCl, 1,2-DF, and 1,1-HF reactions, respectively. The loose structure of the 1,1-HX transition states, which is exemplified by the order of magnitude larger pre-exponential factor relative to the 1,2-HX elimination reactions, compensates for the high threshold energy; thus, the 1,1-HX elimination reaction rates can compete with the 1,2-HX elimination reactions for high levels of vibrational excitation in CD3CHFCl. The 1,1-HCl and 1,1-HF reactions are observed via the CD2═CDF and CD2═CDCl products formed from isomerization of the CD3CF and CD3CCl carbenes. These D-atom migration reactions are discussed, and the possibility of tunneling is evaluated. The transition states developed from the 1,1-HCl and 1,1-HF reactions of CD3CHFCl are compared to models for the HCl and HF elimination reactions of CHF2Cl, CHFCl2, and CH2FCl.

QTAIM analysis of the HF, HCl, HBr, and HOH elimination reactions of halohydrocarbons and halohydroalcohols

The 1,2-HX elimination reaction (where X = F, Cl, Br, OH) has been established as an important reaction in the degradation of compounds introduced into the upper atmosphere, including common CFC replacement compounds. By analyzing the electron densities of the transition state geometries of these reactions using QTAIM, we see that we can divide these reactions into two types. For HF and HOH elimination, the transition state is a complete ring of bonds, and neither the C-H nor the C-X bonds have been broken at the maximum of energy. There is very little accumulation of electron density on the X atom, with the majority of charge being lost by the hydrogen atom undergoing elimination, being transferred on to the two carbon atoms. In HCl and HBr elimination, a similar loss of electron density of the hydrogen atom is accompanied by significant accumulation of electron density on the X atom and a smaller change in electron density on the carbon atoms. The C-X bond is broken in the transition state geometry, with no ring critical point being present. This may explain the relative stabilities of halohydrocarbons and haloalcohols with respect to loss of H-X.

Isomerization of neopentyl chloride and neopentyl bromide by a 1,2-interchange of a halogen atom and a methyl group

The recombination of chloromethyl and t-butyl radicals at room temperature was used to generate neopentyl chloride molecules with 89 kcal mol(-1) of internal energy. The observed unimolecular reactions, which give 2-methyl-2-butene and 2-methyl-1-butene plus HCl, as products, are explained by a mechanism that involves the interchange of a methyl group and the chlorine atom to yield 2-chloro-2-methylbutane, which subsequently eliminates hydrogen chloride by the usual four-centered mechanism to give the observed products. The interchange isomerization process is the rate-limiting step. Similar experiments were done with CD(2)Cl and C(CH(3))(3) radicals to measure the kinetic-isotope effect to help corroborate the proposed mechanism. Density functional theory was employed at the B3PW91/6-31G(d',p') level to verify the Cl/CH(3) interchange mechanism and to characterize the interchange transition state. These calculations, which provide vibrational frequencies and moments of inertia of the molecule and transition state, were used to evaluate the statistical unimolecular rate constants. Matching the calculated and experimental rate constants, gave 62 ± 2 kcal mol(-1) as the threshold energy for interchange of the Cl atom and a methyl group. The calculated models also were used to reinterpret the thermal unimolecular reactions of neopentyl chloride and neopentyl bromide. The previously assumed Wagner-Meerwein rearrangement mechanism for these reactions can be replaced by a mechanism that involves the interchange of the halogen atom and a methyl group followed by HCl or HBr elimination from 2-chloro-2-methylbutane and 2-bromo-2-methylbutane. Electronic structure calculations also were done to find threshold energies for several related molecules, including 2-chloro-3,3-dimethylbutane, 1-chloro-2-methyl-2-phenylpropane, and 1-chloro-2-methyl-2-vinylpropane, to demonstrate the generality of the interchange reaction involving a methyl, or other hydrocarbon groups, and a chlorine atom. The interchange of a halogen atom and a methyl group located on adjacent carbon atoms can be viewed as an extension of the halogen atom interchange mechanisms that is common in 1,2-dihaloalkanes.

Reactions of CF3CH2I+O(3P): competing mechanisms of HF elimination

Ab initio density functional and molecular orbital calculations provide singlet and triplet electronic potential energy surfaces for the reactions of CF3CH2I+O(3P) leading to OI and HF eliminations, reactions which have been the subject of recent experimental studies. A barrier to OI formation occurs on the triplet potential energy surface; there is no reverse barrier to OI formation on the singlet pathway. Findings suggest that two competing pathways may form HF. One is an addition-insertion-elimination process involving insertion of O into the C-I bond. The alternate path involves OI elimination, addition of an O atom to CF3CH2, and subsequent HF elimination. The computed reactant pathways and energetics are discussed in relation to recent experiments.

Unimolecular Rate Constants, Kinetic Isotope Effects and Threshold Energies for FH and FD Elimination from CF3CHFCH3 and CF3CHFCD3

The combination of CF(3)CHF and CH(3) or CD(3) radicals was used to prepare vibrationally excited CF(3)CHFCH(3) or CF(3)CHFCD(3) molecules with 97 kcal mol(-1) of internal energy. The experimental unimolecular rate constants were 3.7 x 10(6) s(-1) for 2,3-FH elimination from CF(3)CHFCH(3) and 1.3 x 10(6) s(-1) for 2,3-DF elimination from CF(3)CHFCD(3). Unimolecular rate constants for 1,2-FH elimination reaction were approximately 230 and 98 times smaller for CF(3)CHFCH(3) and CF(3)CHFCD(3), respectively, than the corresponding rate constants for 2,3-FH elimination. Density functional theory (DFT) was used to calculate the structures and vibrational frequencies of the molecules and transition states; this information was subsequently employed for calculations of RRKM rate constants. Comparison of the experimental and calculated rate constants gave a threshold energy of 73 +/- 2 kcal mol(-1) for the 1,2-FH elimination process and 60.5 +/- 1.5 kcal mol(-1) for the 2,3-FH elimination reaction from CF(3)CHFCH(3). The calculated kinetic-isotope effects agree with the experimental results. The experimentally derived threshold energies for 1,2-FH and 2,3-FH elimination reactions from several fluoropropanes and fluorochloropropanes are summarized and compared to those from DFT calculations.

Rate Constants and Kinetic Isotope Effects for Unimolecular 1,2-HX or DX (X = F or Cl) Elimination from Chemically Activated CF3CFClCH3-d0, -d1, -d2, and -d3

Chemically activated CF(3)CFClCH(3), CF(3)CFClCD(3), CF(3)CFClCH(2)D, and CF(3)CFClCHD(2) molecules with 94 kcal mol(-1) of internal energy were formed by the combination of CF(3)CFCl radicals with CH(3), CD(3), CH(2)D, and CHD(2) radicals, which were generated from UV photolysis of CF(3)CFClI and CH(3)I, CD(3)I, CH(2)DI, or CHD(2)I. The total (HF + HCl) elimination rate constants for CF(3)CFClCH(3) and CF(3)CFClCD(3) were 5.3 x 10(6) and 1.7 x 10(6) s(-1) with product branching ratios of 8.7 +/- 0.6 in favor of HCl (or DCl). The intermolecular kinetic isotope effects were 3.22 and 3.18 for the HCl and HF channels, respectively. The product branching ratios were 10.3 +/- 1.9 and 11.8 +/- 1.8 (10.8 +/- 3.8 and 11.6 +/- 1.7) for HCl/HF and DCl/DF, respectively, from CF(3)CFClCH(2)D (CF(3)CFClCHD(2)). The intramolecular kinetic-isotope effects (without correction for reaction path degeneracy) for HCl/DCl and HF/DF elimination from CF(3)CFClCH(2)D (CF(3)CFClCHD(2)) were 2.78 +/- 0.16 and 2.98 +/- 0.12 (0.82 +/- 0.04 and 0.91 +/- 0.03), respectively. Density function theory at the B3PW91/6-311+G(2d,p) and B3PW91/6-31G(d',p') levels was investigated, and the latter was chosen to calculate frequencies and moments of inertia for the molecules and transition states. Rate constants, branching ratios and kinetic-isotope effects then were calculated using RRKM theory with torsional motions treated as hindered internal rotations. Threshold energies for HF and HCl elimination from CF(3)CFClCH(3) were assigned as 61.3 +/- 1.5 and 58.5 +/- 1.5 kcal mol(-1), respectively. The threshold energy for Cl-F interchange was estimated as 67 kcal mol(-1). The difference between the transition states for HCl and HF elimination is discussed.

Unimolecular Rate Constants for HX or DX Elimination (X = F, Cl) from Chemically Activated CF3CH2CH2Cl, C2H5CH2Cl, and C2D5CH2Cl: Threshold Energies for HF and HCl Elimination

Vibrationally activated CF(3)CH(2)CH(2)Cl molecules were prepared with 94 kcal mol(-1) of vibrational energy by the combination of CF(3)CH(2) and CH(2)Cl radicals and with 101 kcal mol(-1) of energy by the combination of CF(3) and CH(2)CH(2)Cl radicals at room temperature. The unimolecular rate constants for elimination of HCl from CF(3)CH(2)CH(2)Cl were 1.2 x 10(7) and 0.24 x 10(7) s(-1) with 101 and 94 kcal mol(-1), respectively. The product branching ratio, k(HCl)/k(HF), was 80 +/- 25. Activated CH(3)CH(2)CH(2)Cl and CD(3)CD(2)CH(2)Cl molecules with 90 kcal mol(-1) of energy were prepared by recombination of C(2)H(5) (or C(2)D(5)) radicals with CH(2)Cl radicals. The unimolecular rate constant for HCl elimination was 8.7 x 10(7) s(-1), and the kinetic isotope effect was 4.0. Unified transition-state models obtained from density-functional theory calculations, with treatment of torsions as hindered internal rotors for the molecules and the transition states, were employed in the calculation of the RRKM rate constants for CF(3)CH(2)CH(2)Cl and CH(3)CH(2)CH(2)Cl. Fitting the calculated rate constants from RRKM theory to the experimental values provided threshold energies, E(0), of 58 and 71 kcal mol(-1) for the elimination of HCl or HF, respectively, from CF(3)CH(2)CH(2)Cl and 54 kcal mol(-1) for HCl elimination from CH(3)CH(2)CH(2)Cl. Using the hindered-rotor model, threshold energies for HF elimination also were reassigned from previously published chemical activation data for CF(3)CH(2)CH(3,) CF(3)CH(2)CF(3), CH(3)CH(2)CH(2)F, CH(3)CHFCH(3), and CH(3)CF(2)CH(3). In an appendix, the method used to assign threshold energies was tested and verified using the combined thermal and chemical activation data for C(2)H(5)Cl, C(2)H(5)F, and CH(3)CF(3).

Ab initiodirect dynamics trajectory simulation of C2H5F→C2H4+HF product energy partitioning

Direct dynamics classical trajectory simulations were performed to study product energy partitioning in C(2)H(5)F-->C(2)H(4)+HF dissociation. The intrinsic reaction coordinate potential energy curve, reaction energetics, and transition state (TS) properties were calculated for this reaction at different levels of electronic structure theory, and MP2/6-31G( *) was chosen as a meaningful and practical method for performing the direct dynamics. The trajectories show that the HF bond, uncoupled from the other degrees of freedom, is formed within the first 10 fs as the system moves from the TS towards products. The populations of the HF vibration states, determined from the simulations, decrease monotonically as found from experiments. However, the simulation's populations for the low and high energy vibration states are larger and smaller, respectively, than the experimental results. The HF rotational temperature found from the simulations is in agreement with experiment. Increasing the TS's excess energy gives higher rotational temperatures for both C(2)H(4) and HF. Energy is partitioned to the products from both the excess energy in the TS and the potential energy release in the exit channel. Partitioning from these two energy sources is distinguished by varying the TS's excess energy. An analysis of the simulation's energy disposal shows that the fractions of the excess energy partitioned to relative translation, C(2)H(4) vibration, C(2)H(4) rotation, HF vibration, and HF rotation, are 0.17, 0.64, 0.076, 0.067, and 0.046, respectively, and are in good agreement with previous simulations on empirical potentials and experiments. The partitioning found for the potential energy release is 81%, <0.05%, 5%, 11%, and 3% to relative translation, C(2)H(4) vibration, C(2)H(4) rotation, HF vibration, and HF rotation. This result is substantially different than the deduction from experiments, which summarizes the partitioning as 20%, 45%, 24%, and <12% to relative translation, C(2)H(4) vibration+rotation, HF vibration, and HF rotation. Possible origins of the difference between the simulations and experiments in the release of the potential energy is discussed.