Cl Adduct Research Articles

Non-covalent C–Cl…π interaction in acetylene-carbon tetrachloride adducts: Matrix isolation infrared and ab initio computational studies

Non-covalent halogen-bonding interactions between π cloud of acetylene (C2H2) and chlorine atom of carbon tetrachloride (CCl4) have been investigated using matrix isolation infrared spectroscopy and quantum chemical computations. The structure and the energies of the 1:1 C2H2–CCl4 adducts were computed at the B3LYP, MP2 and M05-2X levels of theory using 6–311++G(d,p) basis set. The computations indicated two minima for the 1:1 C2H2–CCl4 adducts; with the C–Cl…π adduct being the global minimum, where π cloud of C2H2 is the electron donor. The second minimum corresponded to a C–H…Cl adduct, in which C2H2 is the proton donor. The interaction energies for the adducts A and B were found to be nearly identical. Experimentally, both C–Cl…π and C–H…Cl adducts were generated in Ar and N2 matrixes and characterized using infrared spectroscopy. This is the first report on halogen bonded adduct, stabilized through C–Cl…π interaction being identified at low temperatures using matrix isolation infrared spectroscopy. Atoms in Molecules (AIM) and Natural Bond Orbital (NBO) analyses were performed to support the experimental results. The structures of 2:1 ((C2H2)2–CCl4) and 1:2 (C2H2–(CCl4)2) multimers and their identification in the low temperature matrixes were also discussed.

Halogen effect on mixed-valence state of biosmocenium(II, IV) salts

The mixed-valence biosmocenium(II, IV)–Cl adduct [(C5H5)OsII(μ2-η5:η5-C10H8) (C5H5)OsIVCl]Y(1a:YPF6, 1b:YBF4) and biosmocenium-Br adduct [(C5H5)OsII(μ2-η5:η5-C10H8) (C5H5)OsIVBr]PF6(2) were prepared. The structures of 1b and 2 were determined by single crystal X-ray analysis, and it was found that Os-X bond was fixed on cis position to the other half of osmocenyl moiety. Although 1b and 2 had the trapped-valence state in solid state, valence-detrapping depending on temperature was observed in solution state by using variable-temperature 1H NMR spectroscopy. It was found that intramolecular electron exchange reaction OsII–OIV ⇄ OsIV–OsII occurred accompanied by halogen migration. The activation energy of the reaction was determined. The energy decreased in the order A > 2 > 1a depending on a kind of halogen ligand bonded to Os center, and the tendency can be explained by HSAB theory.

Understanding the Mechanism of Polymerization of ε-Caprolactone Catalyzed by Aluminum Salen Complexes

Studies of the kinetics of polymerization of ε-caprolactone (CL) by salen-aluminum catalysts comprising ligands with similar steric profiles but different electron donating characteristics (R = OMe, Br, or NO2) were performed using high initial monomer concentrations (2 M < [CL]0 < 2.6 M) in toluene-d8 at temperatures ranging from 20 to 90 °C. Saturation behavior was observed, enabling determination of monomer equilibrium constants (Keq) and catalytic rate constants (k2) as a function of R and temperature. While Keq varied only slightly with the electron donating properties of R (Hammett ρ = +0.16(8)), k2 showed a more significant dependence reflected by ρ = +1.4(1). Thermodynamic parameters ΔG° (associated with Keq) and ΔG(‡) (associated with k2) were determined, with the former being ∼0 kcal/mol for all catalysts and the latter exhibiting the trend R = OMe > Br > NO2. Density functional theory (DFT) calculations were performed to characterize mechanistic pathways at a microscopic level of detail. Lowest energy transition-state structures feature incipient bonding of the nucleophile to the lactone carbonyl that is approaching the metal ion, but a distinct CL adduct is not an energy minimum on the reaction pathway, arguing against Keq being associated with coordination of monomer according to the typical coordination-insertion mechanism. An alternative hypothesis is presented associating Keq with "nonproductive" coordination of substrate in a manner that inhibits the polymerization reaction at high substrate concentrations.

ChemInform Abstract: Formation of Amine, Phosphine, and Thioether Adducts of Chlorotriborane(7).

The chlorotriborane(7) (B 3 H 6 Cl) adduct of N(CH 3 ) 3 was formed by the reaction of B 4 H 8 •N(CH 3 ) 3 with HCl in dichloromethane or with HgCl 2 in chloroform. The reaction of B 3 H 7 •N(CH 3 ) 3 with BCl 3 in dichloromethane was found to be a better preparative method for B 3 H 6 Cl•N(CH 3 ) 3 . The BCl 3 treatment was employed to convert the N(CH 3 ) 2 H, N(CH 3 )H 2 , NH 3 , and S(CH 3 ) 2 adducts of B 3 H 7 into the corresponding adducts of B 3 H 6 Cl

A Kinetic and Spectroscopic Study of the CH3I−Cl and ICH2I−Cl Adducts

Laser-induced fluorescence from the CH3I-Cl and ICH2I-Cl adducts formed in association reactions between chlorine atoms and CH3I and CH2I2 has been observed for the first time. The LIF excitation and dispersed fluorescence spectra have been measured in the range 345-375 nm and 380-480 nm, respectively, at 204 and 296 K. The excitation spectra exhibit vibrational fine structure, and a semiquantitative analysis of the spectra yields a similar binding energy for both adducts of approximately 60 kJ mol(-1). The adduct fluorescence is efficiently quenched by N2 and exhibits a zero-pressure lifetime of approximately 25-30 ns. Using LIF excited from the CH3I-Cl and ICH2I-Cl adducts, the kinetics of the reactions of atomic chlorine with methyl iodide and diiodomethane have been investigated, the results showing that both reactions proceed via two independent channels, an association reaction to form the adduct and a bimolecular abstraction reaction. At T approximately 200 K, the association reaction is predominant, and CH3I-Cl formation is irreversible, with rate coefficients for adduct formation found to be pressure-dependent and in reasonable agreement with the literature. At approximately 200 K, removal of the adduct is dominated by reaction with radical species (CH3 and ClSO) and by self-reaction, which proceed at close to the gas kinetic limit. At 296 K, CH3I-Cl formation is reversible, and the equilibrium constant, K(p) = (70.9 +/- 27.4) x 10(3) atm(-1), was determined, which is in excellent agreement with the literature, and the adduct does not significantly react with CH3I. The uncertainty is at the 95% confidence level (2sigma) and includes systematic errors. At approximately 200 K, the ICH2I-Cl adduct is again stabilized, with pressure-dependent rate coefficients reaching the high pressure limit at lower pressures than for the Cl + CH3I reaction. At room temperature, the ICH2I-Cl adduct is removed via an additional unimolecular decomposition channel, which dominates over the reversible decomposition channel to reform Cl + CH2I2. Neither adduct was observed to undergo significant reaction with molecular oxygen at approximately 200 or 296 K, with an upper limit rate coefficient determined as k < 10(-16) cm(3) molecule(-1) s(-1).

Absolute Rate Coefficient Determination and Reaction Mechanism Investigation for the Reaction of Cl Atoms with CH2I2 and the Oxidation Mechanism of CH2I Radicals

The gas-phase reaction of atomic chlorine with diiodomethane was studied over the temperature range 273-363 K with the very low-pressure reactor (VLPR) technique. The reaction takes place in a Knudsen reactor at pressures below 3 mTorr, where the steady-state concentration of both reactants and stable products is continuously measured by electron-impact mass spectrometry. The absolute rate coefficient as a function of temperature was given by k = (4.70 +/- 0.65) x 10-11 exp[-(241 +/- 33)/T] cm3molecule-1s-1, in the low-pressure regime. The quoted uncertainties are given at a 95% level of confidence (2sigma) and include systematic errors. The reaction occurs via two pathways: the abstraction of a hydrogen atom leading to HCl and the abstraction of an iodine atom leading to ICl. The HCl yield was measured to be ca. 55 +/- 10%. The results suggest that the reaction proceeds via the intermediate CH2I2-Cl adduct formation, with a I-Cl bond strength of 51.9 +/- 15 kJ mol-1, calculated at the B3P86/aug-cc-pVTZ-PP level of theory. Furthermore, the oxidation reactions of CHI2 and CH2I radicals were studied by introducing an excess of molecular oxygen in the Knudsen reactor. HCHO and HCOOH were the primary oxidation products indicating that the reactions with O2 proceed via the intermediate peroxy radical formation and the subsequent elimination of either IO radical or I atom. HCHO and HCOOH were also detected by FT-IR, as the reaction products of photolytically generated CH2I radicals with O2 in a static cell, which supports the proposed oxidation mechanism. Since the photolysis of CH2I2 is about 3 orders of magnitude faster than its reactive loss by Cl atoms, the title reaction does not constitute an important tropospheric sink for CH2I2.

Ionic and excited species in irradiated poly(vinyl chloride) doped with aromatic admixtures: low temperature studies

The influence of temperature (77–290 K) on the fate of dopant radical ions and respective excited states in irradiated poly(vinyl chloride) (PVC) matrix, doped with pyrene, (Py) and tris(2-ethylhexyl) trimellitate (TOTM) is described. At 77 K dopant radical ions start to recombinevia tunneling charge transfer, leading to weak isothermal luminescence (ITL). The wavelength-selected radiothermoluminescence (WS RTL) broad maxima observed for doped PVC in the temperature range 95–110 K have a similar origin, i.e., recombination of dopant radical ionsvia tunneling. Apart from the peaks representing the absorption of dopant radical ions the absorption maximum at 411 nm found for the PVC−Py system is attributed to Py−Cl adduct generated in Py•++Cl− reaction. The mechanisms involved in these processes are discussed.

Spectroscopic and kinetic study of the gas-phase CS 2[sbnd]Cl adduct

Time-resolved UV–vis absorption spectroscopy (TRUVVAS) has been coupled with 248 nm laser flash photolysis (LFP) of Cl 2CO in the presence of CS 2 (and in some cases O 2, NO or NO 2) to generate the CS 2 Cl radical adduct in the gas phase and study the spectroscopy and kinetics of this species. CS 2 Cl is found to possess a strong absorption band at λ max ≈ 365 nm with σ max = (2.3 ± 0.7) × 10 −17 cm 2 molecule −1 (base e) and a weaker band at λ max ≈ 480 nm; the gas-phase spectrum of CS 2 Cl is very similar to the previously reported liquid-phase spectrum in CCl 4 solvent. Reaction of CS 2 Cl with O 2 is found to be very slow; our data suggest that the rate coefficient for this reaction at 240 K is less than 5.0 × 10 −18 cm 3 molecule −1 s −1. Rate coefficients for CS 2 Cl reactions with CS 2 Cl ( k 3), NO ( k 5), and NO 2 ( k 6) were measured at 240 K and, in units of 10 −11 cm 3 molecule −1 s −1, were found to be 2 k 3 = 15 ± 6, k 5 = 2.2 ± 0.5, and k 6 = 1.3 ± 0.4, where the uncertainties are estimates of accuracy at the 95% confidence level.

Rates and Mechanisms for the Reactions of Chlorine Atoms with Iodoethane and 2-Iodopropane

The reaction of Cl atoms with iodoethane has been studied via a combination of laser flash photolysis/resonance fluorescence (LFP-RF), environmental chamber/Fourier transform (FT)IR, and quantum chemical techniques. Above 330 K, the flash photolysis data indicate that the reaction proceeds predominantly via hydrogen abstraction. The following Arrhenius expressions (in units of cm3 molecule(-1) s(-1)) apply over the temperature range 334-434 K for reaction of Cl with CH3CH2I (k4(H)) and CD3CD2I (k4(D)): k4(H) = (6.53 +/- 3.40) x 10(-11) exp[-(428 +/- 206)/T] and k4(D) = (2.21 +/- 0.44) x 10(-11) exp[-(317 +/- 76)/T]. At room temperature and below, the reaction proceeds both via hydrogen abstraction and via reversible formation of an iodoethane/Cl adduct. Analysis of the LFP-RF data yields a binding enthalpy (0 K) for CD3CD2I x Cl of 57 +/- 10 kJ mol(-1). Calculations using density functional theory show that the adduct is characterized by a C-I-Cl bond angle of 84.5 degrees; theoretical binding enthalpies of 38.2 kJ/mol, G2'[ECP(S)], and 59.0 kJ mol(-1), B3LYP/ECP, are reasonably consistent with the experimentally derived result. Product studies conducted in the environmental chamber show that hydrogen abstraction from both the -CH2I and -CH3 groups occur to a significant extent and also provide evidence for a reaction of the CH3CH2I x Cl adduct with CH3CH2I, leading to CH3CH2Cl formation. Complementary environmental chamber studies of the reaction of Cl atoms with 2-iodopropane, CH3CHICH3, are also presented. As determined by relative rate methods, the reaction proceeds with an effective rate coefficient, k6, of (5.0 +/- 0.6) x 10(-11) cm3 molecule(-1) s(-1) at 298 K. Product studies indicate that this reaction also occurs via two abstraction channels (from the CH3 groups and from the -CHI- group) and via reversible adduct formation.

Kinetics Studies of Aqueous Phase Reactions of Cl Atoms and Cl2- Radicals with Organic Sulfur Compounds of Atmospheric Interest

A laser flash photolysis-long path UV-visible absorption technique has been employed to investigate the kinetics of aqueous phase reactions of chlorine atoms (Cl) and dichloride radicals (Cl2(-)) with four organic sulfur compounds of atmospheric interest, dimethyl sulfoxide (DMSO; CH3S(O)CH3), dimethyl sulfone (DMSO2; CH3(O)S(O)CH3), methanesulfinate (MSI; CH3S(O)O-), and methanesulfonate (MS; CH3(O)S(O)O-). Measured rate coefficients at T = 295 +/- 1 K (in units of M(-1) s(-1)) are as follows: Cl + DMSO, (6.3 +/- 0.6) x 10(9); Cl2(-) + DMSO, (1.6 +/- 0.8) x 10(7); Cl + DMSO2, (8.2 +/- 1.6) x 10(5); Cl2(-) + DMSO2, (8.2 +/- 5.5) x 10(3); Cl2(-) + MSI, (8.0 +/- 1.0) x 10(8); Cl + MS, (4.9 +/- 0.6) x 10(5); Cl2(-) + MS, (3.9 +/- 0.7) x 10(3). Reported uncertainties are estimates of accuracy at the 95% confidence level and the rate coefficients for MSI and MS reactions with Cl2(-) are corrected to the zero ionic strength limit. The absorption spectrum of the DMSO-Cl adduct is reported; peak absorbance is observed at 390 nm and the peak extinction coefficient is found to be 5760 M(-1) cm(-1) with a 2sigma uncertainty of +/-30%. Some implications of the new kinetics results for understanding the atmospheric sulfur cycle are discussed.

Observation of Adducts in the Reaction of Cl Atoms with XCH2I (X = H, CH3, Cl, Br, I) Using Cavity Ring-Down Spectroscopy

The reactions of Cl atoms with XCH2I (X = H, CH3, Cl, Br, I) have been studied using cavity ring-down spectroscopy in 25-125 Torr total pressure of N2 diluent at 250 K. Formation of the XCH2I-Cl adduct is the dominant channel in all reactions. The visible absorption spectrum of the XCH2I-Cl adduct was recorded at 405-632 nm. Absorption cross-sections at 435 nm are as follows (in units of 10(-18) cm2 molecule(-1)): 12 for CH3I, 21 for CH3CH2I, 3.7 for CH2ICl, 7.1 for CH2IBr, and 3.7 for CH2I2. Rate constants for the reaction of Cl with CH3I were determined from rise profiles of the CH3I-Cl adduct. k(Cl + CH3I) increases from (0.4 +/- 0.1) x 10(-11) at 25 Torr to (2.0 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) at 125 Torr of N2 diluent. There is no discernible reaction of the CH3I-Cl adduct with 5-10 Torr of O2. Evidence for the formation of an adduct following the reaction of Cl atoms with CF3I and CH3Br was sought but not found. Absorption attributable to the formation of the XCH2I-Cl adduct following the reaction of Cl atoms with XCH2I (X = H, CH3, Br, I) was measured as a function of temperature over the range 250-320 K.

Nucleophilic Acyl Substitutions of Anhydrides with Protic Nucleophiles Catalyzed by Amphoteric, Oxomolybdenum Species

[reaction: see text] Among six different group VIb oxometallic species examined, dioxomolybdenum dichloride and oxomolybdenum tetrachloride were the most efficient catalysts to facilitate nucleophilic acyl substitution (NAS) of anhydrides with a myriad array of alcohols, amines, and thiols in high yields and high chemoselectivity. In contrast to the well-recognized redox chemical behaviors associated with oxomolybdenum(VI) species, the catalytic NAS was unprecedented and tolerates virtually all kinds of functional groups. By using benzoic anhydride as a mediator for in situ generation of an incipient mixed anhydride-MoO(2)Cl(2) adduct with a given functional alkanoic acid, one can achieve oleate, dipeptide, diphenylmethyl, N-Fmoc-alpha-amino, pyruvic, and tert-butylthio ester, N-tert-butylamide, and trityl methacrylate syntheses with appropriate protic nucleophiles. The amphoteric character of the Mo=O unit in oxomolybdenum chlorides was found to be responsible for the catalytic NAS profile as supported by a control NAS reaction of using an authentic adduct-MoOCl(2)(O(2)CBu(t)())(2) between pivalic anhydride and MoO(2)Cl(2) as the catalyst.

The Atmospheric Reaction between DMSO and the Chlorine Radical

The reaction between dimethyl sulfoxide (DMSO) and chlorine was studied theoretically, using ab initio calculation in a high correlated level. A systematic study of reaction paths was conducted to address the role of the (CH3)2SO−Cl adduct formation. The channels that lead to CH3SOCH2 + HCl and CH3SOCl + CH3 will be relevant, and the global atmospheric rate constant was determined to be 1.2 × 10-10 cm3 molecule-1 s-1, at 298.15 K and 1 atm. The atmospheric implications of this reaction to DMSO chemistry also are discussed.

Kinetics and Mechanism of the Gas-Phase Reaction of Cl Atoms and OH Radicals with Fluorobenzene at 296 K

Smog chamber/FTIR techniques were used to study the kinetics and mechanism of the reaction of Cl atoms and OH radicals with fluorobenzene, C6H5F, in 700 Torr of N2 or air diluent at 296 K. Reaction of Cl atoms with C6H5F proceeds via two pathways: H-atom abstraction to give HCl and the C6H4F radical and adduct formation to give the C6H5F−Cl adduct. At 296 K the rate constant for the abstraction channel is k5a(Cl + C6H5F) = (1.1 ± 0.1) × 10-17 cm3 molecule-1 s-1. The C6H5F−Cl adduct undergoes rapid (k ∼ 108 s-1) decomposition to reform C6H5F and Cl atoms and reaction with Cl atoms via a mechanism which, at least in part, leads neither to production of C6H5Cl nor to reformation of C6H5F. As the steady-state Cl atom concentration is increased, the fraction of the C6H5F−Cl adduct undergoing reaction with Cl atoms increases causing an increase in the effective rate constant for the reaction of C6H5F with Cl atoms. The equilibrium between Cl atoms, C6H5F, and the C6H5F−Cl adduct is established rapidly and has an equilibrium constant estimated to be K5b=[C6H5F−Cl]/[C6H5F][Cl] = (3.2 ± 2.4) × 10-18 cm3 molecule-1. An upper limit of k9 < 6 × 10-17 cm3 molecule-1 s-1 was established for the reaction of the C6H5F−Cl adduct with O2. The reaction of OH radicals with C6H5F was studied and a rate constant of k(OH + C6H5F) = (7.9 ± 2.2) × 10-13 cm3 molecule-1 s-1 was determined. The results are discussed with respect to the available literature concerning reaction of Cl atoms and OH radicals with aromatic compounds. As part of this work, rate constants for reaction of OH radicals with 2-, 3-, and 4-fluorophenol of (6.3 ± 1.3) × 10-12, (2.3 ± 0.5) × 10-11, and (2.5 ± 0.5) × 10-11 cm3 molecule-1 s-1 were determined.

Photoinduced Cl transfer reaction between biphenyl and carbon tetrachloride studied by nanosecond time-resolved infrared spectroscopy and picosecond time-resolved fluorescence spectroscopy

Photochemical reaction between biphenyl and carbon tetrachloride is studied with nanosecond time-resolved infrared spectroscopy and picosecond time-resolved fluorescence spectroscopy. Fluorescence lifetime of biphenyl in carbon tetrachloride solutions is measured with a picosecond time-resolved fluorescence spectrometer. The recorded lifetime is 3.8 ps, shorter than fluorescence lifetime in cyclohexane by a factor of four thousand. After the photoexcitation of biphenyl in carbon tetrachloride, the trichloromethyl (CCl 3) radical is formed as a reaction intermediate, which is confirmed by the detection of the radical's 896 cm −1 infrared band with nanosecond time-resolved infrared spectroscopy. One of the Cl atoms in solvent carbon tetrachloride is transferred to biphenyl to form the CCl 3 radical and a biphenyl–Cl adduct. Together with the already reported spectroscopic detection of the CCl 3 radical as an intermediate in photochemical reactions of trans-stilbene or anthracene with carbon tetrachloride, it is likely that this series of reactions are common for aromatic molecules.

Time‐resolved study of the photochemical reaction of 4‐dimethylaminobenzonitrile with carbon tetrachloride

AbstractThe mechanism of 4‐dimethylaminobenzonitrile (DMABN) fluorescence quenching by carbon tetrachloride was studied using time‐resolved fluorescence, absorption and Raman techniques. Picosecond and nanosecond time‐resolved Raman spectra of DMABN and DMABN‐d6 were obtained in resonance with different electronic states to identify intermediates, their structure and time‐dependent dynamic changes. The quenching is attributed to intermolecular electron transfer between B‐state DMABN and carbon tetrachloride with a time constant of ∼500 fs leading to the formation of the DMABN radical cation as the initial intermediate followed by a ∼13 ps process yielding a DMABN Cl adduct as product with a lifetime longer than 50 ns. It is likely that the chlorine atom is located at the amino para‐ring position of the highly reactive DMABN radical cation. It is suggested that this reaction mechanism is general for other donor–aromatic–acceptor type DMABN derivatives. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Electrospray ionization tandem mass spectrometric studies of competitive pyridine loss from platinum(II) ethylenediamine complexes by the kinetic method.

The competition between pyridine ligand loss in square planar Pt(II) complexes has been examined using the doubly and singly charged ions of complexes consisting of platinum(ethylenediamine) coordinated to two different substituted pyridines. Collision induced dissociation (CID) of [Pt(en)Py(1)Py(2)](2+) (where Py(1) = one of ten different substituted pyridines and Py(2) = pyridine) results in loss of the protonated pyridines to yield the singly charged platinum ions [Pt(en)Py(1)-H](+) and [Pt(en)Py(2)-H](+). In contrast, fragmentation of [Pt(en)Py(1)Py(2)-H](+) results in neutral pyridine loss to yield the ions [Pt(en)Py(1)-H](+) and [Pt(en)Py(2)-H](+). In the latter case, the correlation between relative losses of each pyridine compared to their gas-phase proton affinities is poor. A novel chloride ion abstraction reaction occurs for the fragmentation of [Pt(en)Py(1)Py(2)](2+) when Py(1) = o-C(5)H(4)CIN and Py(2) = C(5)H(5)N, to yield the [Pt(en)(Cl)Py(2)](+) and [o-C(5)H(4)N](+) pair of ions. In order to model this process the competition between nitrogen and chlorine binding in [Pt(NH(3))(3)(o-NC(5)H(4)Cl)](2+) has been examined using density functional theory (DFT) calculations at the B3LYP/LANL2DZ level of theory. Both adducts are minima with the N adduct being more stable than the Cl adduct by 22.7 kcal mol(-1). Furthermore, the Cl adduct exhibits a significant stretching of the C-Cl bond (to 1.935 A), consistent with the observed chloride ion abstraction reaction, which is endothermic by 9.0 kcal mol(-1) (relative to the N adduct).

Mass Spectrometric Identification of Increased C16 Ceramide Levels During Apoptosis

A variety of molecular changes occur during the process of apoptosis. Much of the recent work has focused on changes in critical cellular proteins, proteins necessary for the initiation and continuation of the apoptotic process. Given the fact that numerous membrane changes occur throughout the apoptotic process, we initiated an investigation aimed at determining the major lipid changes that occurred during programmed cell death. When ionizing radiation was used to initiate the apoptotic process in Jurkat cells, one of the major changes that occurred within 24 h was an increase in a species with a m/z of 572 as determined by negative ion electrospray mass spectrometry. This particular mass ion displayed high performance liquid chromatography characteristics of a neutral lipid species. Further analysis by collision-induced-dissociation tandem mass spectrometry indicated only one daughter species indicative of a Cl adduct and therefore a parental mass of 537. Comparison to a commercial C16 ceramide yielded identical spectra by mass spectrometry (MS) and MS/MS analysis in the negative ion mode. Increases in C16 ceramide levels occurred 2 h after initiation of apoptosis by ionizing radiation, and its accumulation paralleled apoptosis as determined by cellular morphology. Interestingly, radiation-sensitive Jurkat cells displayed increased levels of long term C16 ceramide accumulation, whereas radiation-resistant K562 cells did not. These findings were supported by increases in caspase-3 activity in Jurkat cells, whereas caspase-3 activity in K562 cells remained unchanged. C16 ceramide accumulation and sensitivity to ionizing radiation was investigated further in a melanoma cell line. Only those cells that were radiation sensitive (approximately 70-75%) displayed increases in long term ceramide accumulation. Taken together, these results indicated a correlation between increases in C16 ceramide accumulation and radiation sensitivity. Increases in long term C16 ceramide accumulation were also seen in Fas-induced apoptosis, which occurred at time points greater than 2 h. Analysis of mitochondrial modifications using the mitochondrial probe nonyl acridine orange (NAO) indicated that initial increases in C16 ceramide levels closely paralleled the decrease in mitochondrial mass during Fas or radiation-induced apoptosis. Taken together, these results support a role for C16 ceramide in the effector (mitochondrial) phase of apoptosis.

Kinetics and Mechanism of the Gas-Phase Reaction of Cl Atoms with Benzene

The gas-phase reaction of Cl atoms with benzene has been studied using both experimental and computational methods. The bulk of the kinetic data were obtained using steady-state photolysis of mixtures containing Cl2, C6H6, and a reference compound in 120−700 Torr of N2 diluent at 296 K. Reaction of Cl atoms with C6H6 proceeds via two pathways; (a) H-atom abstraction and (b) adduct formation. At 296 K the rate constant for the abstraction channel is k1a = (1.3 ± 1.0) × 10-16 cm3 molecule-1 s-1. Phenyl radicals produced via H-atom abstraction from C6H6 react with Cl2 to give chlorobenzene. The main fate of the C6H6−Cl adduct is decomposition to reform C6H6 and Cl atoms. A small fraction of the C6H6−Cl adduct undergoes reaction with Cl atoms via a mechanism which does not lead to the production of C6H5Cl, or the reformation of C6H6. As the steady-state Cl atom concentration is increased, the fraction of the C6H6−Cl adduct undergoing reaction with Cl atoms increases causing an increase in the effective rate c...

Atmospheric Chemistry of CH3I: Reaction with Atomic Chlorine at 1−700 Torr Total Pressure and 295 K

The title reaction was studied using long path length FTIR spectroscopy coupled to a smog chamber. In 1 Torr of N2 diluent at 295 K a large kinetic isotope effect was observed, kH/kD = 6, suggesting that the reaction of Cl atoms with CH3I proceeds via H atom abstraction to give CH2I radicals and HCl with a rate constant of (9.0 ± 1.8) × 10-13 cm3 molecule-1 s-1. In 700 Torr total pressure of N2, O2, and air diluents the reactivity of Cl toward CH3I was 50% greater than that at 1 Torr. The increase in reactivity is ascribed to the formation of the CH3I−Cl adduct. In 700 Torr of O2 the majority (≈98%) of the adduct decomposes back to reactants, and a small fraction (≈2%) is lost via one or more processes that do not regenerate reactants. In 1 atm of air at 295 K the reaction of Cl atoms with CH3I produces less than 20% yield of CH3Cl. The potential role of the title reaction in atmospheric chemistry is discussed.