Water-Catalyzed Dehalogenation Reactions of Isobromoform and Its Reaction Products

MnO2/TiO2-Catalyzed ozonolysis: enhancing Pentachlorophenol degradation and understanding intermediates

Pentachlorophenol is a pesticide widely known for its harmful effects on sewage, causing harm to the environment. In previous studies, our group identified adsorption as a crucial factor in catalytic ozonation processes, and subsequent observations revealed the catalyst's role in reducing toxicity during degradation. In this research, we quantified organochlorine intermediates and low molecular weight organic acids generated under optimal pH conditions (pH 9), with and without the catalyst. Additionally, we assessed the reactivity of these intermediates through theoretical calculations. Our findings indicate that the catalyst reduces the duration of intermediates. Additionally, the presence of CO2 suggests enhanced mineralization of pentachlorophenol, a process notably facilitated by the catalyst. Theoretical calculations, such as Fukui analysis, offer insights into potential pathways for the dechlorination of aromatic molecules by radicals like OH, indicating the significance of this pathway.

Coexisting oxidation and reduction of chloroacetaldehydes in water by UV/VUV irradiation

Haloacetaldehydes (HALs) are the third largest disinfection by-product (DBP) ubiquitously detected in finished drinking water and have relatively higher toxicity than currently regulated DBPs. To efficiently alleviate them, this study investigated a green, chemical-free technology by using ultraviolet/vacuum ultraviolet (UV/VUV) on degrading three refractory chlorinated HALs (Cl-HALs). The results indicate that the rates of Cl-HALs decomposition in tap water irradiated by UV/VUV were 23-70 times higher than those irradiated by UV, proving that VUV instead of UV played the key role in degrading Cl-HALs. Increasing Cl-HALs dosage, pH, and dissolved oxygen (DO) all decreased the Cl-HALs degradations significantly, and the rates in tap water were apparently lower than those in ultrapure water. Unlike previous studies, this study proved that both oxidation and reduction were present during the VUV process. Photooxidation via oxidative radicals like •OH mineralized Cl-HALs, leading to substantial drops of total organic carbon; photoreduction via reductive radicals like •H dehalogenated Cl-HALs, resulting in formation of considerable intermediate organics (e.g., formic acid and acetic acid). No matter what pathway, the mass balances of chlorine were always maintained, meaning that dehalogenation occurred instantaneously rather than sequentially. Although the overall photodegradation rates dropped with rising pH and DO, photoreduction was increased with rising pH while photooxidation was elevated with rising DO. The results hence provide insights to better understand the VUV technology in controlling micropollutants in water.

Insights into unexpected photoisomerization from photooxidation of tribromoacetic acid in aqueous environment using ultrafast spectroscopy

Haloacetic acids are carcinogenic disinfection by-products (DPBs) and their photo-decomposition pathways, especially for those containing bromine and iodine, are not fully understood. In this study, femtosecond transient absorption (fs-TA) spectroscopy experiments were introduced for the first time to investigate the photochemistry of tribromoacetic acid. The fs-TA experiments showed that a photoisomerization intermediate species HOOCCBr₂-Br (iso-TBAA) was formed within several picoseconds after the excitation of TBAA. The absorption wavelength of the iso-TBAA was supported by time-dependent density calculations. With the Second-order Møller-Plesset perturbation theory, the structures and thermodynamics of the OH-insertion reactions of iso-TBAA were elucidated when water molecules were involved in the reaction complex. The calculations also revealed that the isomer species were able to react with water with its reaction dynamics dramatically catalyzed by the hydrogen bonding network. The proposed water catalyzed OH-insertion/HBr elimination mechanism predicted three major photoproducts, namely, HBr, CO and CO₂, which was consistent with the photolysis experiments with firstly reported CO formation rate and mass conversion yield as 0.096 min^-1 and 0.75 ± 0.1 respectively. The spectroscopic technique, numerical tool and disclosed mechanisms provided insights on photodecomposition and subsequent reactions of polyhalo-DPBs contain heavy atom(s) (e.g., Br, I) with water, aliphatic alcohols or other nucleophiles.

Treating disinfection byproducts with UV or solar irradiation and in UV advanced oxidation processes: A review

This review focuses on the degradation kinetics and mechanisms of disinfection byproducts (DBPs) under UV and solar irradiation and in UV-based advanced oxidation processes (AOPs). A total of 59 such compounds are discussed. The processes evaluated are low pressure, medium pressure and vacuum UV irradiation, solar irradiation together with UV/hydrogen peroxide, UV/persulfate and UV/chlorine AOPs. Under UV and solar irradiation, the photodegradation rates of N-nitrosamines are much higher than those of halogenated DBPs. Among halogenated DBPs, those containing iodine are photodegraded more rapidly than those containing bromine or chlorine. This is due to differences in their bond energies (E_N-N < E_C-I < E_C-Br < E_C-Cl). Molar absorption coefficients at 254 nm and energy gaps can be used to predict the photodegradation rates of DBPs under low pressure UV irradiation. But many DBPs of interest cannot be degraded to half their original concentration with less than a 500 mJ cm^-2 dose of low pressure UV light. HO^• generally contributes to less than 30% of the degradation of DBPs except iodo-DBPs in UV/H₂O₂ AOPs. Reaction mechanisms under UV irradiation and in HO^•-mediated oxidation are also summarized. N-N bond cleavage initiates their direct UV photolysis of N-nitrosamines as C-X cleavage does among halogenated compounds. HO^• generally initiates degradation via single electron transfer, addition and hydrogen abstraction pathways. Information on the reaction rate constants of SO₄^•- and halogen radicals with DBPs is rather limited, and little information is available about their reaction pathways. Overall, this review provides improved understanding of UV, solar and AOPs.

Solvent-dependent complex reaction pathways of bromoform revealed by time-resolved X-ray solution scattering and X-ray transient absorption spectroscopy

The photochemical reaction pathways of CHBr3 in solution were unveiled using two complementary X-ray techniques, time-resolved X-ray solution scattering (TRXSS) and X-ray transient absorption spectroscopy, in a wide temporal range from 100 ps to tens of microseconds. By performing comparative measurements in protic (methanol) and aprotic (methylcyclohexane) solvents, we found that the reaction pathways depend significantly on the solvent properties. In methanol, the major photoproducts are CH3OCHBr2 and HBr generated by rapid solvolysis of iso-CHBr2-Br, an isomer of CHBr3. In contrast, in methylcyclohexane, iso-CHBr2-Br returns to CHBr3 without solvolysis. In both solvents, the formation of CHBr2 and Br is a competing reaction channel. From the structural analysis of TRXSS data, we determined the structures of key intermediate species, CH3OCHBr2 and iso-CHBr2-Br in methanol and methylcyclohexane, respectively, which are consistent with the structures from density functional theory calculations.

Direct measurement of electrostatic interactions between poly(methyl methacrylate) microspheres with optical laser tweezers

In this study, we measured the force of electrostatic interactions between poly(methyl methacrylate) (PMMA) particles dispersed in organic solvent mixtures of cyclohexyl bromide (CHB) and n-decane. Optical laser tweezers were employed to directly measure interactive forces between paired PMMA particles in a CHB medium that contained n-decane in various volume ratios. CHB, having a moderate dielectric constant, provided an environment with a high charge storage capacity. The addition of n-decane lowered the effective refractive index of the medium, which increased the optical trapping efficiency. We also fabricated microscope flow cells with a commonly used UV-curable adhesive and quantified the effects of dissolved adhesive compounds through interactive force measurements and nuclear magnetic resonance analysis. In addition, we studied the impact of CHB dissociation into H⁺ and Br^- ions, which could screen electrostatic interactions.

Competing Molecular and Radical Pathways in the Dissociation of Halons via Direct Chemical Dynamics Simulations

A great deal of attention has been given to the decomposition chemistry of halons (halomethanes) due to their role in stratospheric ozone depletion. Knowledge of certain aspects of dissociation of halons such as the competition between radical and molecular pathways and their mechanistic details is limited. Halon molecules can isomerize to an iso form containing a halogen-halogen bond and such iso-halon forms have been identified as intermediates in condensed phase chemistry. Recently, a quantum chemistry study of role of iso-halons in the gas phase decomposition of halomethanes has been reported. In the present work, we have investigated the ground state dissociation chemistry of select halon molecules - CF₂Cl₂, CF₂Br₂, CHBr₃, and CH₂BrCl using electronic structure theory calculations and direct chemical dynamics simulations. Classical trajectories were generated on-the-fly using density functional PBE0/6-31G* level of theory at a fixed total energy. Simulation results showed that molecular products, in general, were dominant for all the four molecules at the chosen energy. A variety of mechanisms such as direct dissociation via multicenter transition states, decomposition via isomerization, radical recombinations, and roaming pathways contributed to the formation of molecular products. Atomic level mechanisms are presented, and the role of iso-halons in the gas phase chemistry of halomethanes is clearly established.

Degradation of iodinated disinfection byproducts by VUV/UV process based on a mini-fluidic VUV/UV photoreaction system

This study investigated the VUV/UV photodegradation of three iodinated disinfection byproducts (I-DBPs), namely, triiodomethane (TIM), diiodoacetamide (DIAcAm) and triiodoacetic acid (TIAA), based on a mini-fluidic VUV/UV photoreaction system (MVPS). The pseudo-first-order rate constants (k) of TIM, DIAcAm and TIAA under VUV/UV irradiation (1769, 1301 and 1174 m² einstein^-1, respectively) were higher than those under UV irradiation (1003, 832 and 766 m² einstein^-1, respectively). The enhancement of photodegradation could be attributed to the indirect HO^• oxidation rather than the direct VUV photolysis. As a whole, compared with the indirect HO^• oxidation, the direct UV photolysis had a relatively greater contribution to the degradation of the three I-DBPs, especially DIAcAm and TIAA (ca. 57% and 60%, respectively). The two electron-withdrawing groups in DIAcAm and TIAA (i.e., acylamino and carboxyl) decreased the electron density of the C-I bonds, thus weakening the electrophilic attack of HO^•. The iodine in the three I-DBPs was released to form I^- and a small fraction (< 6%) of I^- was oxidized to IO₃^-, indicating that HO^• preferred to break the C-I bonds rather than oxidize I^-. The direct UV photolysis proceeded via H₂O-catalyzed deiodination reactions, while the indirect HO^• oxidation proceeded via deiodination reactions along with HO^• addition. The VUV/UV photodegradation of the three I-DBPs was more favored at an acidic pH but inhibited by the water matrix components (i.e., NOM, Cl^- and alkalinity) to different extents.

Impact of UV irradiation on Chlorella sp. damage and disinfection byproducts formation during subsequent chlorination of algal organic matter

The frequent occurrence of algal blooms in surface water has attracted more and more attention, which caused many water quality problems, including disinfection byproducts (DBPs). Algal organic matter (AOM) including intracellular organic matter (IOM) and extracellular organic matter (EOM), was a well-known precursor to DBPs formation in drinking water. This study evaluated the effect of ultraviolet (UV) irradiation on the cell integrity, IOM release and DBPs formation during subsequent chlorination of Chlorella sp. Results showed the damage rates of algal cells increased to 40.1% after the high UV irradiation of 528 mJ/cm², which contributed to the release of IOM. In addition, UV irradiation was effective in reducing the formation of haloacetic acids (HAAs) both in AOM and IOM, but promoted the formation of nitrogenous DBPs (N-DBPs) from AOM in subsequent chlorination. Furthermore, neutral pH exerted a positive effect on the formation of DBPs. UV irradiation decreased the bromine substitution factor (BSF) value of AOM at a high bromide level. The BSF values increased with increasing of the concentration of bromide. Moreover, more amino acids and low molecular weight precursors were produced after UV irradiation in filtered supernatant, which contributed to the formation of N-DBPs with algal chlorination. Overall, this information demonstrated pre-oxidation of UV irradiation could be used to treat the algal-rich drinking water.

Regulation, formation, exposure, and treatment of disinfection by-products (DBPs) in swimming pool waters: A critical review

The microbial safety of swimming pool waters (SPWs) becomes increasingly important with the popularity of swimming activities. Disinfection aiming at killing microbes in SPWs produces disinfection by-products (DBPs), which has attracted considerable public attentions due to their high frequency of occurrence, considerable concentrations and potent toxicity. We reviewed the latest research progress within the last four decades on the regulation, formation, exposure, and treatment of DBPs in the context of SPWs. This paper specifically discussed DBP regulations in different regions, formation mechanisms related with disinfectants, precursors and other various conditions, human exposure assessment reflected by biomarkers or epidemiological evidence, and the control and treatment of DBPs. Compared to drinking water with natural organic matter as the main organic precursor of DBPs, the additional human inputs (i.e., body fluids and personal care products) to SPWs make the water matrix more complicated and lead to the formation of more types and greater concentrations of DBPs. Dermal absorption and inhalation are two main exposure pathways for trihalomethanes while ingestion for haloacetic acids, reflected by DBP occurrence in human matrices including exhaled air, urine, blood, and plasma. Studies show that membrane filtration, advanced oxidation processes, biodegradation, thermal degradation, chemical reduction, and some hybrid processes are the potential DBP treatment technologies. The removal efficiency, possible mechanisms and future challenges of these DBP treatment methods are summarized in this review, which may facilitate their full-scale applications and provide potential directions for further research extension.

Transient isomers in the photodissociation of bromoiodomethane

The photochemistry of halomethanes is fascinating for the complex cascade reactions toward either the parent or newly synthesized molecules. Here, we address the structural rearrangement of photodissociated CH₂IBr in methanol and cyclohexane, probed by time-resolved X-ray scattering in liquid solution. Upon selective laser cleavage of the C-I bond, we follow the reaction cascade of the two geminate geometrical isomers, CH₂I-Br and CH₂Br-I. Both meta-stable isomers decay on different time scales, mediated by solvent interaction, toward the original parent molecule. We observe the internal rearrangement of CH₂Br-I to CH₂I-Br in cyclohexane by extending the time window up to 3 μs. We track the photoproduct kinetics of CH₂Br-I in methanol solution where only one isomer is observed. The effect of the polarity of solvent on the geminate recombination pathways is discussed.

Singlet versus Triplet Excited State Mediated Photoinduced Dehalogenation Reactions of Itraconazole in Acetonitrile and Aqueous Solutions

Photoinduced dehalogenation of the antifungal drug itraconazole (ITR) in acetonitrile (ACN) and ACN/water mixed solutions was investigated using femtosecond and nanosecond time-resolved transient absorption (fs-TA and ns-TA, respectively) and nanosecond time-resolved resonance Raman spectroscopy (ns-TR³) experiments. An excited resonance energy transfer is found to take place from the 4-phenyl-4,5-dihydro-3H-1,2,4-triazol-3-one part of the molecule to the 1,3-dichlorobenzene part of the molecule when ITR is excited by ultraviolet light. This photoexcitation is followed by a fast carbon-halogen bond cleavage that leads to the generation of radical intermediates via either triplet and/or singlet excited states. It is found that the singlet excited state-mediated carbon-halogen cleavage is the predominant dehalogenation process in ACN solvent, whereas a triplet state-mediated carbon-halogen cleavage prefers to occur in the ACN/water mixed solutions. The singlet-to-triplet energy gap is decreased in the ACN/water mixed solvents and this helps facilitate an intersystem crossing process, and thus, the carbon-halogen bond cleavage happens mostly through an excited triplet state in the aqueous solutions examined. The ns-TA and ns-TR³ results also provide some evidence that radical intermediates are generated through a homolytic carbon-halogen bond cleavage via predominantly the singlet excited state pathway in ACN but via mainly the triplet state pathway in the aqueous solutions. In strong acidic solutions, protonation at the oxygen and/or nitrogen atoms of the 1,2,4-triazole-3-one group appears to hinder the dehalogenation reactions. This may offer the possibility that the phototoxicity of ITR due to the generation of aryl or halogen radicals can be reduced by protonation of certain moieties in suitably designed ITR halogen-containing derivatives.

A theoretical study on the catalytic role of water in methanol steam reforming on PdZn(111)

The catalytic role of water in the methanol steam reforming process on the PdZn(111) surface is explored theoretically.

Photodegradation of iodinated trihalomethanes in aqueous solution by UV 254 irradiation

Photodegradation of 6 iodinated trihalomethanes (ITHMs) under UV irradiation at 254 nm was investigated in this study. ITHMs underwent a rapid photodegradation process through cleavage of carbon-halogen bond with first-order rate constants in the range of 0.1-0.6 min(-1). The effects of matrix species including nitrate, humic acid (HA), bicarbonate, sulfate, and chloride were evaluated. The degradation rate increased slightly in the presence of nitrate possibly due to generation of HO at a low quantum yield via direct photolysis of nitrate, while HA lowered the photodegradation rate of ITHMs due to its competitive UV absorption. Moreover, bicarbonate, sulfate, and chloride had no significant effect on photodegradation kinetics, as there is no UV absorption for these 3 species. In the study using surface water, treated water, and secondary effluent from a wastewater treatment plant, high turbidity and natural organic matters present in the water inhibited the photodegradation of ITHMs. The degradation rates of 6 ITHMs in UV/H2O2 system were rather comparable and significantly higher than those achieved in the UV system without H2O2. To develop a quantitative structure-reactivity relationship (QSAR) model, the logarithm of measured first-order rate constants was correlated with a number of molecular descriptors. The best correlation was obtained with a combination of 3 molecular descriptors, namely the bond strength of carbon-halogen to be broken in the rate-determining step, steric and electronic effects of all substituents to the carbon center.

Solvent-dependent photochemistry of 2,2,2-tribromoethyl-(2'-phenylacetate)

Photolysis (254 nm) of the title compound 1 produces a variety of stable products, which vary significantly with the nature of the solvent. Solvents that serve as efficient H atom donors (methanol, ethanol, isopropyl alcohol) favor products arising from a net reduction of one or more of the C-Br bonds. These include 2,2-dibromoethyl-(2'-phenylacetate) 2 and 2-bromoethyl-(2'-phenylacetate) 3. In the presence of nucleophiles, products such as 2-(2'-phenylacetoxy)acetic acid 5a and/or its ester derivatives are produced. Phenylacetic acid 6 is formed in some cases but under the conditions studied appears to be a minor product. The results are interpreted in terms of a general mechanism that features formation of an iso-tribromo intermediate 9 and/or a geminate radical-atom pair.

Simultaneous degradation of disinfection byproducts and earthy-musty odorants by the UV/H2O2 advanced oxidation process

Advanced treatment technologies that control multiple contaminants are beneficial to drinking water treatment. This research applied UV/H(2)O(2) for the simultaneous degradation of geosmin, 2-methylisoborneol, four trihalomethanes and six haloacetic acids. Experiments were conducted in de-ionized water at 24 ± 1.0 °C with ng/L amounts of odorants and μg/L amounts of disinfection byproducts. UV was applied with and without 6 mg/L H(2)O(2.) The results demonstrated that brominated trihalomethanes and brominated haloacetic acids were degraded to a greater extent than geosmin and 2-methylisoborneol. Tribromomethane and dibromochloromethane were degraded by 99% and 80% respectively at the UV dose of 1200 mJ/cm(2) with 6 mg/L H(2)O(2), whereas 90% of the geosmin and 60% of the 2-methylisoborneol were removed. Tribromoacetic acid and dibromoacetic acid were degraded by 99% and 80% respectively under the same conditions. Concentrations of trichloromethane and chlorinated haloacetic acids were not substantially reduced under these conditions and were not effectively removed at doses designed to remove geosmin and 2-methylisoborneol. Brominated compounds were degraded primarily by direct photolysis and cleavage of the C-Br bond with pseudo first order rate constants ranging from 10(-3) to 10(-2) s(-1). Geosmin and 2-methylisoborneol were primarily degraded by reaction with hydroxyl radical with direct photolysis as a minor factor. Perchlorinated disinfection byproducts were degraded by reaction with hydroxyl radicals. These results indicate that the UV/H(2)O(2) can be applied to effectively control both odorants and brominated disinfection byproducts.

Induction of apoptosis by UV-irradiated chlorinated bisphenol A in Jurkat cells

Chlorinated derivatives of bisphenol A (ClBPAs) have been detected in wastewater from waste paper recycling plants. We previously reported that bisphenol A (BPA) and ClBPAs [3-chlorobisphenol A, 3,3'-dichlorobisphenol A, and 3,3',5-trichlorobisphenol A] irradiated with ultraviolet (UV) B or UVC (not with UVA) induced inhibition of cell growth, and that 3-hydroxybisphenol A (3-OHBPA) was detected in the photoproducts [Mutou, Y., Ibuki, Y., Terao, Y., Kojima, S., Goto, R., 2006b. Chemical change of chlorinated bisphenol A by ultraviolet irradiation and cytotoxicity of their products on Jurkat cells. Environmental Toxicology and Pharmacology, 21, 283-289]. The formation of hydroxylated BPAs by UV irradiation might contribute to the inhibition of cell growth, but the mechanism of the growth inhibition is not clarified. In this study, we investigated whether BPA and ClBPAs exposed to UVA, UVB, or UVC, and 3-OHBPA could induce the death of Jurkat cells and whether the pattern of cell death was apoptosis. ClBPAs exposed to UVB and UVC induced significant cell death, but those exposed to UVA and BPA did not. The cell death was apoptosis because chromatin condensation and DNA fragmentation were detected. Activation of caspase-3, -8, and -9 and cytochrome c release indicated that ClBPAs exposed to UVB or UVC induced apoptosis via typical apoptotic pathways. In addition, 3-OHBPA induced apoptosis similar to UVB- or UVC-irradiated ClBPA. These results suggested that the photoproducts of ClBPAs generated by UV irradiation, containing 3-OHBPA, contributed to the induction of apoptosis.

Change of estrogenic activity and release of chloride ion in chlorinated bisphenol a after exposure to ultraviolet B

Bisphenol A (BPA) and chlorinated bisphenol A (ClBPAs) were detected in wastewater from waste paper recycling plants. In previous study, we showed the acute cytotoxicity of oxidized products of BPA and ClBPAs generated by ultraviolet (UV) irradiation. However, estrogenic activities of these photoproducts have not been studied. Therefore, we investigated change of estrogenic activities of BPA and ClBPAs [3-chlorobisphenol A (3-ClBPA), 3,3'-dichlorobisphenol A (3,3'-diClBPA) and 3,3',5-trichlorobisphenol A (3,3',5-triClBPA)] after UVB irradiation using yeast two-hybrid assay. The agonist activities of ClBPAs were higher than that of BPA in the absence of S9. ClBPAs irradiated with UVB lost agonist activities. The addition of S9 also completely erased the activity. The antagonist activities of BPA and ClBPAs with or without UVB irradiation were not detected both in the absence or presence of S9. UVB irradiation (0-100 J/cm2) decreased the agonist activity of 3,3'-diClBPA in proportion to increase of released chloride ion. The agonist activity was completely lost at 50 J/cm2 of UVB, of which dose could dissociated almost all chlorine. These findings suggested that UVB irradiation could decrease the estrogenic activity of chlorinated compounds, which was due to the selective release of chloride ion.

Triplet MLCT Photosensitization of the Ring-Closing Reaction of Diarylethenes by Design and Synthesis of a Photochromic Rhenium(I) Complex of a Diarylethene-Containing 1,10-Phenanthroline Ligand

Synthesis of the diarylethene-containing ligand L1 based on Suzuki cross-coupling reaction between thienyl boronic acid and the dibromophenanthroline ligand is reported. On coordination to the rhenium(I) tricarbonyl complex system, the photochromism of L1 could be photosensitized and consequently extended from intraligand excitation at lambda< or =340 nm in the free ligand to metal-to-ligand charge-transfer (MLCT) excitation at lambda< or =480 nm in the complex. The photochromic reactions were studied by (1)H NMR, UV/Vis, and steady-state emission spectroscopy. Photosensitization was further probed by ultrafast transient absorption and time-resolved emission spectroscopy. The results provided direct evidence that the formation of the closed form by the MLCT-sensitized photochromic process was derived from the (3)MLCT excited state. This supports the photosensitization mechanism, which involves an intramolecular energy-transfer process from the (3)MLCT to the (3)IL(L1) state that initiated the ring-closure reaction. The photophysical and electrochemical properties of the complex were also investigated.

Modeling SN2 Reactions in Methanol Solution by ab Initio Calculation of Nucleophile Solvent−Substrate Clusters

[Structure: see text]. Ab initio calculations were used to study the S(N)2 reactions of the CH3OCH2I molecule with a methoxide ion (CH3O-) and a methanol molecule by systematically building up the reaction system with explicit incorporation of the methanol solvent molecules. For the reaction of CH3OCH2I with a methoxide ion, the explicit incorporation of the methanol molecules to better solvate the methoxide ion led to an increase in the barrier to reaction. For the reaction of CH3OCH2I with a methanol molecule, the explicit incorporation of the methanol molecules led to a decrease in the barrier to reaction because of an inclination of this reaction to proceed with the nucleophilic displacements accompanied by proton transfer through the H-bonding chain. The H-bonding chain served as both acid and base catalysts for the displacement reaction. A ca. 10(15)-fold acceleration of the methanol tetramer incorporated S(N)2 reaction was predicted relative to the corresponding methanol monomer reaction. The properties of the reactions examined are discussed briefly.

A Theoretical Study of the Mechanism of the Water-Catalyzed HCl Elimination Reactions of CHXCl(OH) (X = H, Cl) and HClCO in the Gas Phase and in Aqueous Solution

A systematic ab initio investigation of the water-assisted decomposition of chloromethanol, dichloromethanol, and formyl chloride as a function of the number of water molecules (up to six) building up the solvation shell is presented. The decomposition reactions of the chlorinated methanols and formyl chloride are accelerated substantially as the reaction system involves additional explicit coordination of water molecules. Rate constants for the decomposition of chlorinated methanols and formyl chloride were found to be in reasonable agreement with previous experimental observations of aqueous phase decomposition reactions of dichloromethanol [CHCl(2)(OH)] and formyl chloride. For example, using the calculated activation free energies in conjunction with the stabilization free energies from the ab initio calculations, the rate constant was predicted to be 1.2-1.5 x 10(4) s(-1) for the decomposition of formyl chloride in aqueous solution. This is in good agreement with the experimental rate constant of about 10(4) s(-1) reported in the literature. The mechanism for the water catalysis of the decomposition reactions as well as probable implications for the decomposition of these chlorinated methanol compounds and formaldehydes in the natural environment and as intermediates in advanced oxidation processes are briefly discussed.

Surface Photochemistry of Bromoform on Ice: Cross Section and Competing Reaction Pathways

The 266 nm photodissociation of bromoform adsorbed on an amorphous solid water (ASW) layer has been investigated for the first time under well-defined ultrahigh vacuum conditions. Time-of-flight (TOF) measurements indicate direct release of gas-phase Br, CHBr2, Br2, and CHBr species, with potential implications for stratospheric chemistry. Furthermore, new, ice-surface-mediated C-C (C2H2Br2) and C-O (CHBrO, CO) species are revealed in postirradiation temperature programmed desorption (TPD) and reflection absorption infrared (RAIR) spectra. A cross section of approximately 5 x 10(-20) cm2 is determined for bromoform photodissociation at 266 nm based on the integrated area of both the TOF spectra of Br and Br2 and the postirradiation TPD curves of CHBr3. The involvement of the free, non-hydrogen-bonded water groups at the ASW surface in the formation of the photoproducts is evident from the RAIRS results.

Water-Catalyzed Dehalogenation Reactions of the Isomer of CBr4 and Its Reaction Products and a Comparison to Analogous Reactions of the Isomers of Di- and Trihalomethanes

A combined experimental and theoretical study of the UV photolysis of a typical tetrahalomethane, CBr4, in water and acetonitrile/water was performed. Ultraviolet photolysis of low concentrations of CBr4 in water mostly leads to the production of four HBr leaving groups and CO2. Picosecond time-resolved resonance Raman (Ps-TR3) experiments and ab initio calculations indicate that water-catalyzed O-H insertion/HBr elimination of the isomer of CBr4 and subsequent reactions of its products lead to the formation of these products. The UV photolyses of di-, tri-, and tetrahalomethanes at low concentrations in water-solvated environments are compared to one another. This comparison enables a general reaction scheme to be deduced that can account for the different products produced by UV photolysis of low concentrations of di-, tri-, and tetrahalomethanes in water. The fate of the (halo)formaldehyde intermediate in the chemical reaction mechanism is the key to determining how many strong acid leaving groups are produced and which carbon atom final product is likely formed by UV photolysis of a polyhalomethane at low concentrations in a water-solvated environment.

Comparison of the dehalogenation of polyhalomethanes and production of strong acids in aqueous and salt (NaCl) water environments: Ultraviolet photolysis of CH[sub 2]I[sub 2]

The ultraviolet photolysis of CH(2)I(2) was studied in water and salt water solutions using photochemistry and picosecond time-resolved resonance Raman spectroscopy. Photolysis in both types of environments produces mainly CH(2)(OH)(2) and HI products. However, photolysis of CH(2)I(2) in salt water leads to the formation of different products/intermediates (CH(2)ICl and Cl(2) (-)) not observed in the absence of salt in aqueous solutions. The amount of CH(2)(OH)(2) and HI products appears to decrease after photolysis of CH(2)I(2) in salt water compared to pure water. We briefly discuss possible implications of these results for photolysis of CH(2)I(2) and other polyhalomethanes in sea water and other salt aqueous environments compared to nonsalt water solvated environments.