Photofragment Translational Spectroscopy Studies of H Atom Loss Following Ultraviolet Photoexcitation of Methimazole in the Gas Phase

UV Photodissociation Dynamics of Organic Hydroperoxides: Experiment and Theory

The UV photodissociation dynamics of three organic hydroperoxides (ROOH, R = tert-butyl, cyclopentyl, and cyclohexyl) are examined experimentally at 282 nm utilizing velocity map imaging of the OH X2Π3/2 (v″ = 0, J″) products. The three systems have similar O-O bond dissociation energies based on W1BD calculations and thus similar energy release to products. In each case, the experimental total kinetic energy release (TKER) distributions are bimodal, composed of narrow low and broad high TKER components extending over the available energy. The associated angular distributions of the OH X2Π products are isotropic, differing dramatically from those predicted for direct photodissociation. Complementary theoretical calculations map the relaxed potential energy profile for each ROOH along the steeply repulsive excited state (S1) potential leading to RO + OH products. Low CCOO torsional barriers predicted along the ROOH dissociation pathway enable the OH products to recoil in many different directions, yielding isotropic angular distributions. Simple models of photodissociation suggest that the low TKER component arises from internal conversion to the ground state (S0) potential, leading to a common RO + OH product asymptote. A simple impulsive model for dissociation captures some aspects of the high TKER component but neglects significant geometric changes in the alkyl substituent from ROOH to the RO product. This study provides new insight into the solar photolysis of organic hydroperoxides and the regeneration of OH radicals in atmospheric oxidation cycles.

Effects of Ring Fluorination on the Ultraviolet Photodissociation Dynamics of Phenol

The dynamics of photoinduced O-H bond fission in five fluorinated phenols (2-fluorophenol, 3-fluorophenol, 2,6-difluorophenol, 3,4,5-trifluorophenol, and pentafluorophenol) have been investigated by H Rydberg atom photofragment translational spectroscopy following excitation at many wavelengths in the range 220 ≤ λ ≤ 275 nm. The presence of multiple fluorine substituents reduces the efficiency of O-H bond fission (by tunneling) from the first excited (1¹ππ*) electronic state, whereas all bar the perfluorinated species undergo O-H bond fission when excited at shorter wavelengths (to the 2¹ππ* state). As in bare phenol, O-H bond fission is deduced to occur by non-adiabatic coupling at conical intersections between the photoprepared "bright" ππ* states and the 1¹πσ* potential energy surface. In all cases, the fluorophenoxyl photoproducts are found to be formed in a range of vibrational levels, all of which include an odd number of quanta (typically one) in an out-of-plane (a″) vibrational mode; this product vibration is viewed as a legacy of the parent out-of-plane motions that promote non-adiabatic coupling to the dissociative 1¹πσ* potential. The radical products also show activity in in-plane vibrations involving coupled (both in- and out-of-phase) C-O and C-F wagging motions, which can be traced to the impulse between the recoiling O and H atoms and, in detail, are sensitive to the presence (or not) of an intramolecular F···H-O hydrogen bond. Upper limit values for the O-H bond dissociation energies are reported for all molecules studied apart from pentafluorophenol.

Chiral Differentiation of Non-Covalent Diastereomers Based on Multichannel Dissociation Induced by 213-nm Ultraviolet Photodissociation

Here we present the implementation of 213-nm ultraviolet photodissociation (UVPD) in a FT-ICR mass spectrometer for chiral differentiation in the gas phase. The L/D amino acid-substituted serine octamer ions were selected as examples of diastereoisomers for chiral analysis. Several kinds of fragment ions were observed in these experiments, including fragment ions that are similar to the ones observed in corresponding collision-activated dissociation (CAD) experiments, fragment ions generated with different protonation sites by only destroying non-covalent bonds, and unique non-covalent cluster radical ions. The latter two kinds of fragment ions are found to be more sensitive to the chirality of the substituted units. Further experiments suggest that the formation of radical ions is mainly affected by chromophores on side chains of the substituted units and micro surroundings of the characterized non-covalent diastereoisomers. A comparing experiment performed by only changing the wavelength of UV laser to 266 nm shows that the 213-nm UV laser has the priority in the diversity of fragmentation pathways and potential of further application in chiral differentiation experiments.