Direct observation of the primary and secondary C-Br bond cleavages from the 1,2-dibromopropane photodissociation at 234 and 265 nm using the velocity map ion imaging technique

Photodissociation dynamics of 1,2-dibromopropane has been investigated at 234 and 265 nm by using the velocity map ion imaging method. At both pump energies, a single Gaussian-shaped speed distribution is observed for the Br*((2)P(1/2)) fragment, whereas at least three velocity components are found to be existent for the Br((2)P(3/2)) product. The secondary C-Br bond cleavage of the bromopropyl radical which is energized from the ultrafast primary C-Br bond rupture should be responsible for the multicomponent translational energy distribution at the low kinetic energy region of Br((2)P(3/2)). The recoil anisotropy parameter (beta) of the fragment from the primary C-Br bond dissociation is measured to be 0.53 (0.49) and 1.26 (1.73) for Br((2)P(3/2)) and Br*((2)P(1/2)), respectively, at 234 (265) nm. The beta value of Br((2)P(3/2)) from the secondary C-Br bond dissociation event at 265 nm is found to be 0.87, reflecting the fact that the corresponding Br((2)P(3/2)) fragment carried the initial vector component of the bromopropyl radical produced from the primary bond dissociation event. Density functional theory has been used to calculate energetics involved both in the primary and in the secondary C-Br bond dissociation dynamics.     

A measurement of vibrational excitation in NO produced in the photodissociatton of NOCl and NOBr at 193nm

Infrared emission has been observed from the vibrational state v_n (n from 1 to ? 16) of the nascent NO(X²Π) photo-fragment produced in the photodissociation of NOCl and NOBr at 193 nm. The photodissociation was observed to be a single-photon process. Models and mechanisms of photodissociation are discussed in view of experimental observations.     

State and velocity distributions of Cl atoms produced in the photodissociation of ICI at 237 nm

Photofragment spectroscopy of ICI molecules photodissociated at 237 nm is studied by 2 + 1 resonance-enhanced multi-photon ionization and time of flight techniques. Doppler profiles of the chlorine atom fragments in two spin—orbit states show that chlorine atoms in the ground state, ²P_3/2, are produced from a perpendicular dissociative transition, and chlorine atoms in the excited state, ²P , arise from a parallel transition. The possible electronically excited states leading to dissociation in both the perpendicular and parallel cases are considered.     

Nonadiabatic dynamics in the photodissociation of ICH2CN at 266 and 304 nm studied by the velocity map imaging

Photodissociation dynamics of iodoacetonitrile (ICH2CN) have been investigated at pump wavelengths of 266 and 304 nm using a photofragment ion image velocity mapping technique. At both wavelengths, the prompt C-I bond rupture takes place on the repulsive excited states to give I(2P3/2) and I*(2P1/2), and their speed and spatial distributions are simultaneously measured. The recoil anisotropy parameter (beta) at 266 nm is determined to be 1.10 and 1.60 for I and I*, respectively, while it is found to be much higher at 304 nm to give beta=1.70 and 1.90 for I and I*, respectively. The branching ratios for I*I channels are measured to be 0.724 and 0.136 at 266 and 304 nm, respectively, giving insights on nonadiabatic transition phenomena and relative oscillator strengths of optically accessible transitions of ICH2CN. Accordingly, relative oscillator strengths of parallel/perpendicular transitions and nonadiabatic transitions among the excited states are quantitatively characterized. A large portion of the available energy (41%-48%) goes into the internal energy of the CH2CN fragment. A modified impulsive model in which the CH2CN fragment is assumed to be rigid predicts the energy disposal quite well. Delocalization of an unpaired electron of the CH2CN radical during the C-I bond cleavage, leading to a large structural change of the CH2CN moiety, may be responsible for internally hot fragments.     

Intramolecular electron density redistribution upon hydrogen bond formation in the anion methyl orange at the water/1,2-dichloroethane interface probed by phase interference second harmonic generation

Surface second harmonic generation (SSHG) studies of the azobenzene derivative p-dimethylaminoazobenzene sulfonate, often referred as Methyl Orange (MO), at the neat water/1,2-dichloroethane (DCE) interface is reported. The two forms of the anionic MO dye, which are usually observed in bulk solution, with one form being hydrogen bonded to a water molecule through the azo nitrogens (MO/H2O) and the other form not being hydrogen bonded (MO) have also been observed at the water/DCE interface. Their equilibrium constant has been compared with the corresponding bulk solution and found to be identical. The adsorption equilibrium of the two forms has been determined and the Gibbs energy of adsorption measured to be -30 kJmol(-1) for both forms. From a light polarisation analysis of the SH signal, the angle of orientation of the MO transition dipole moment was found to be 34 +/- 2 degrees for MO and 43 +/- 2 degrees for MO/H2O under the assumption of a Dirac delta function for the angle distribution, a difference explained by the different solvation properties of the two forms. Furthermore, the wavelength dependence analysis of these data revealed an interference pattern resulting from the electronic density redistribution within the hydrated anionic form occurring upon the formation of the hydrogen bond with a water molecule. This interference pattern was clearly evidenced with the use of another dye at the interface in order to define a phase reference to both forms of Methyl Orange.     

Vector properties of the O(1D2) fragment produced from the photolysis of ozone in the wavelength range of 298 to 320 nm

The speed averaged translational anisotropy and electronic angular momentum polarization of the O(1D2) atomic fragment formed from the photodissociation of ozone in the atmospherically important long wavelength region of the Hartley band (298 to 320 nm) have been measured using resonance enhanced multiphoton ionization time of flight mass spectrometry. The translational anisotropy parameter, beta, is found to decline from 1.1 for photolysis at 300 nm to a minimum value of 0 at 310 nm which is the threshold for production of O(1D2) in conjunction with the O2(a 1Deltag v = 0) molecular cofragment. For photolysis wavelengths greater than 310 nm, O(1D2) is formed from the dissociation of internally excited ozone molecules. The corresponding beta parameters are markedly lower than for atomic fragments produced with the same speed from the photolysis of ground state ozone molecules. This result is consistent with two different pathways contributing to the photolysis of internally excited ozone at the longest wavelengths studied corresponding to initial internal excitation either in the symmetric or asymmetric stretching vibration. In addition, the polarization of the atomic angular momentum has been determined with the incoherent polarization parameters a0(2)(||) and a0(2)(_|) increasing from values of -0.53 and -0.62 at 300 nm to -0.37 and -0.19 at 317 nm, consistent with the increasing contribution from the photolysis of internally excited ozone as the dissociation wavelength lengthens. Evaluation of these alignment parameters allows the populations of the magnetic substrates, mj, to be determined. For example, for a photolysis wavelength of 303 nm the populations of mj = 0, +/- 1, +/- 2 are in the ratio of 0.36: 0.56: 0.08 and this ratio is essentially independent of the photolysis wavelength. The coherent contribution to the atomic polarization is quantified by the Re{a1(2)(||, _|)} and Im{a1(1)(||, _|)} parameters and these are found to vary from -0.21 and 0.21 at 300 nm to -0.04 and 0.24 at 313 nm, respectively.     

Chloroacetone photodissociation at 193 nm and the subsequent dynamics of the CH3C(O)CH2 radical--an intermediate formed in the OH + allene reaction en route to CH3 + ketene

We use a combination of crossed laser-molecular beam experiments and velocity map imaging experiments to investigate the primary photofission channels of chloroacetone at 193 nm; we also probe the dissociation dynamics of the nascent CH(3)C(O)CH(2) radicals formed from C-Cl bond fission. In addition to the C-Cl bond fission primary photodissociation channel, the data evidence another photodissociation channel of the precursor, C-C bond fission to produce CH(3)CO and CH(2)Cl. The CH(3)C(O)CH(2) radical formed from C-Cl bond fission is one of the intermediates in the OH + allene reaction en route to CH(3) + ketene. The 193 nm photodissociation laser allows us to produce these CH(3)C(O)CH(2) radicals with enough internal energy to span the dissociation barrier leading to the CH(3) + ketene asymptote. Therefore, some of the vibrationally excited CH(3)C(O)CH(2) radicals undergo subsequent dissociation to CH(3) + ketene products; we are able to measure the velocities of these products using both the imaging and scattering apparatuses. The results rule out the presence of a significant contribution from a C-C bond photofission channel that produces CH(3) and COCH(2)Cl fragments. The CH(3)C(O)CH(2) radicals are formed with a considerable amount of energy partitioned into rotation; we use an impulsive model to explicitly characterize the internal energy distribution. The data are better fit by using the C-Cl bond fission transition state on the S(1) surface of chloroacetone as the geometry at which the impulsive force acts, not the Franck-Condon geometry. Our data suggest that, even under atmospheric conditions, the reaction of OH with allene could produce a small branching to CH(3) + ketene products, rather than solely producing inelastically stabilized adducts. This additional channel offers a different pathway for the OH-initiated oxidation of such unsaturated volatile organic compounds, those containing a C=C=C moiety, than is currently included in atmospheric models.     

Excited-state kinetics in the photolysis of azoethane and hexafluoroazomethane at 366 nm,up to one atmosphere pressure,and from room temperature to 150°C

The photochemistry of azoethane and hexafluoroazomethane at 366 nm has been reinvestigated up to 1 atm pressure, and over a range of temperature from 27 to 150°C. The Stern-Volmer type quenching plots primarily demonstrate the decomposition of a single electronic and vibrationally excited state for azoethane, but competitive photodissociation from two different electronic and vibrationally excited states, which was previously postulated for hexafluoroazomethane and azoisopropane, is confirmed for hexafluoroazomethane. It is concluded, however, that two different electronic and vibrationally excited photodissociating states are present in azoethane photolysis, but that one of them is difficult to detect, at least by the present approach Photosensitization with biacetyl at 436 nm also causes the dissociation of azoethane, and this is probably from the vibrationally equilibrated first triplet state. The energy barrier for this process was found to be 5.0 kcal/mol.     

Volumetric properties and viscosities of the methyl butanoate + n-heptane + cyclo-octane ternary system at 283.15 and 313.15 K and its binary constituents in the temperature range from 283.15 to 313.15 K

The density and kinematic viscosity of the systems methyl butanoate+cyclo-octane and n-heptane+cyclo-octane were determined at four temperatures in the range 283.15–313.15 K over the whole concentration range. The densities and viscosities of the ternary system methyl butanoate+n-heptane+cyclo-octane were determined at 283.15 and 313.15 K. For the binary systems, the dependence of V^E on composition and temperature was obtained in order to calculate other mixture properties, such as the isobaric thermal expansion coefficients, the temperature coefficients of the molar excess volume and the pressure coefficients of the molar excess enthalpy. In the case of the system n-heptane+cyclo-octane the values of these properties and have been compared with those predicted using the group-contribution model by Nitta et al. in combination with a parameters set available in the literature. Experimental binary and ternary viscosities were correlated for comparison, by means of several empirical and semi-empirical models. Kinematic viscosities were also used to test the predictive capability of the group-contribution model UNIFAC-VISCO. In addition, several empirical equations for predicting ternary properties from only binary results have also been applied.     

Solutions of alkali soaps and water in fatty acids. XII. Correlations between the acid sodium octanoates in the isotropic liquidL 2-phase at water contents from 82% down to 10% and those in adjacent liquid crystalline phases

A close connection exists between the isotropic L₂-phase and the liquid crystalline lamellar D-phase. This connection is evidenced both with respect to similar compositions of their acid sodium octanoates and concerning the structure of the phases.The two-phase zone between these phases is very narrow, from 82 % down to about 55 % of water, it then widens slowly to about 40 % and thereafter very rapidly to 22 % of water; below the latter value it remains broad. The border of the L₂-phase is in the whole region from 82 % to 22 % of water located at molar ratios between octanoic acid and sodium octanoate of somewhat above 2.0; the border of the D-phase lies at the same values down to a water content of 40 %. The acid octanoate 1 NaC₈ 2 HC₈ x H₂O plays thus a considerable role in the phase transition all the way down to a water content of 22 %. As long as free bulk-water exists in both phases the structures in them remain alike; this similarity is greatest as long as the amount of bulk-water is so large that it suffices to form coherent domains. At contents of water below 40 %, that is after the disappearance of the bulk-water and when a decrease in the content of water begins to affect the hydration of the acid octanoates, the structural similarity between the phases disappears rapidly.