The dynamics of chlorine-atom reactions with polyatomic organic molecules

Chlorine atoms react with a variety of organic molecules by abstraction of an H atom, making HCl and a radical co-product, and investigations of these reactions provide a large and valuable body of data for improved fundamental understanding of the mechanisms of reactions involving polyatomic molecules. The kinetics and dynamics of reactions of Cl atoms with alkanes have been extensively studied both by experimental and computational methods, and the key outcomes and conclusions are reviewed. These reactions serve as benchmarks for the interpretation of recent experimental data on the dynamics of reactions of Cl atoms with heteroatom functionalized organic molecules such as alcohols, ethers, amines, alkyl halides and thiols. Although bearing many similarities to the dynamics of the alkane reactions, significant differences are found: in particular, the extent of HCl rotational excitation from reactions of Cl atoms with the functionalized molecules is much greater than the very cold rotational distributions obtained for H-atom abstraction from simple alkanes such as methane, ethane, propane and butane. These observations and the scattering dynamics are discussed in terms of reaction energetics, barriers and transition state geometries, and evidence is presented for post-transition-state interactions between the separating HCl and polar organic radical for which the HCl rotation appears to be a sensitive probe

Photodissociation dynamics of the A (2)Sigma(+) state of SH and SD radicals

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Imaging the quantum-state specific differential cross sections of HCl formed from reactions of chlorine atoms with methanol and dimethyl ether

Center-of-mass frame scattering angle distributions obtained directly from crossed molecular beam velocity map images are reported for HCl formed in different rotational levels of its vibrational ground state by reaction of Cl atoms with CH<sub>3</sub>OH and CH<sub>3</sub>OCH<sub>3</sub>. Products are observed to scatter over all angles, with peaks in the distribution in the forward and backward directions (θ = 0 and 180° with respect to the relative velocity vectors of the Cl atoms). Products of both reactions exhibit differential cross sections that vary with the rotational quantum number of the HCl, with a greater propensity for forward scatter for J = 2, shifting to more pronounced backward scatter for J = 5. This trend is, however, more evident for reaction of dimethyl ether than for methanol. The mean fractions of the available energy channeled into product kinetic energy vary with scattering angle, but the angle-averaged fractions are, respectively, 0.37 and 0.42 for the methanol and dimethyl ether reactions. On average, 46% or more of the available energy of the reactions becomes internal energy of the radical co-product. Results are interpreted with the aid of computed energies of transition states and molecular complexes along the reaction pathways, and comparisons are drawn with recent measurements of the scattering distributions and energy release for reactions of Cl atoms with small alkanes

Nonadiabatic dynamics in the CH₃+HCl→CH₄+Cl(2P_J) reaction

Nonadiabatic dynamics in the title reaction have been investigated by 2+1 REMPI detection of the Cl (P 32 2) and Cl* (P 12 2) products. Reaction was initiated by photodissociation of C H3 I at 266 nm within a single expansion of a dilute mixture of C H3 I and HCl in argon, giving a mean collision energy of 7800 cm-1 in the center-of-mass frame. Significant production of Cl* was observed, with careful checks made to ensure that no additional photochemical or inelastic scattering sources of Cl* perturbed the measurements. The fraction of the total yield of Cl (PJ2) atoms formed in the J= 1 2 level at this collision energy was 0.150±0.024, and must arise from nonadiabatic dynamics because the ground potential energy surface correlates to C H4 +Cl (P 32 2) products

Imaging the quantum-state specific differential cross sections of HCl formed from reactions of chlorine atoms with methanol and dimethyl ether

Center-of-mass frame scattering angle distributions obtained directly from crossed molecular beam velocity map images are reported for HCl formed in different rotational levels of its vibrational ground state by reaction of Cl atoms with CH₃OH and CH₃OCH₃. Products are observed to scatter over all angles, with peaks in the distribution in the forward and backward directions (θ = 0 and 180° with respect to the relative velocity vectors of the Cl atoms). Products of both reactions exhibit differential cross sections that vary with the rotational quantum number of the HCl, with a greater propensity for forward scatter for J = 2, shifting to more pronounced backward scatter for J = 5. This trend is, however, more evident for reaction of dimethyl ether than for methanol. The mean fractions of the available energy channeled into product kinetic energy vary with scattering angle, but the angle-averaged fractions are, respectively, 0.37 and 0.42 for the methanol and dimethyl ether reactions. On average, 46% or more of the available energy of the reactions becomes internal energy of the radical co-product. Results are interpreted with the aid of computed energies of transition states and molecular complexes along the reaction pathways, and comparisons are drawn with recent measurements of the scattering distributions and energy release for reactions of Cl atoms with small alkanes

Imaging the dynamics of reactions of chlorine atoms with methyl halides

The dynamics of H-atom abstraction reactions of Cl atoms with CH<sub>3</sub>Cl and CH<sub>3</sub>Br have been studied using velocity map imaging to obtain center-of-mass frame differential cross sections for HCl formed in its vibrational ground state and rotational levels with J = 2−5. The HCl products are preferentially backward scattered in both reactions, with broad angular distributions of velocities that extend to sideways and forward scatter, and which show almost no variation with J. The products are formed with a wide range of kinetic energies, and the mean fractions of the total available energy becoming product translational energy are 0.25 and 0.23 for the respective reactions. The CH<sub>2</sub>Cl and CH<sub>2</sub>Br are formed with considerable internal excitation, estimated to correspond, on average, to fractions of the total available energy of 0.69 and 0.72, respectively. The scattering dynamics are interpreted with the aid of ab initio calculations of the minimum energy pathways for the two reactions, which exhibit low barriers late on the reaction coordinate

Reaction optimization and mechanism in maleimide [5 + 2] photocycloaddition: a dual approach using tunable UV lasers and time-dependent DFT

An in-depth study of the intramolecular [5 + 2] photocycloaddition of maleimides using tunable UV lasers has demonstrated that the peak in quantum yield and rate both occur at wavelengths some 50 nm red shifted from the maxima observed in the UV spectra. A detailed explanation for these findings using time-dependent DFT calculations is presented, and the implications for a previously adopted mechanism are discussed