Robin Walsh

Chemistry of carotenoid oxidation and free radical reactions

When oxygenic photosynthesis evolved, one of the key functions of carotenoids was to protect aerobic photosynthetic organisms against destruction by photodynamic sensitization. Aerobic photosynthesis would not exist without the coevolution of carotenoids alongside the chlorophylls. As carotenoids are abundant in nature, in many fruits and vegetables, they are able to react with excited states of appropriate energy and quench them, and they can react with free radicals according to their reactivity, redox potentials, and X—H bond energies. This report concerns the bimolecular reactions of carotenoids with oxygen species, such as 3 O2, 1 O2, HO · , HOO · ,O ·ˇ 2 , etc.

Anti‐ and Prooxidant Properties of Carotenoids

Carotenoids can be effective singlet oxygen quenchers and inhibit free-radical induced lipid peroxidation. A remarkable property of β-carotene (1a) is the change from an antioxidant to a prooxidant depending on oxygen pressure and concentration. In the present study a considerable number of carotenoids (1a, 2c, 2d, 2e, 3a, 4a, 5a, 6a, 7a, 8a, 8h, 8i, 8j, 9f, 10a, 11a, 12g) was investigated using two independent approaches: 1. Comparison of their effects on inhibition of the free-radical oxidation of methyl linoleate, and 2. The direct study of the effect of oxygen partial pressure upon their rates of oxidation. It is shown that some carotenoids (7a, 8a) are even more effective than the well-known compounds β-carotene (1a) and astaxanthin (5a) and are powerful antioxidants without any prooxidative property. Different carotenoids display different behaviour depending on chain length and end groups. The influence of these functional groups on the antioxidative reactivity is discussed.

Prototype Si—H insertion reaction of silylene with silane. Absolute rate constants, temperature dependence, RRKM modelling and the potential-energy surface

Time-resolved studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with monosilane, SiH4. The reaction was studied in the gas phase over the pressure range 1–100 Torr, with both Ar and SF6 as bath gases, at six temperatures in the range 298–665 K. The reaction of SiH2 with SiH4 to form disilane, Si2H6, is pressure dependent, consistent with a third-body assisted association reaction. The high-pressure rate constants, obtained by extrapolation, gave the Arrhenius equation: log(k∞/cm3 molecule–1 s–1)=(–9.91 ± 0.04)+(3.3 ± 0.3 kJ mol–1)/RT In 10. These Arrhenius parameters are consistent with a fast, nearly collision-controlled, process. RRKM modelling, based on a variational transition state, used in combination with a weak collisional deactivation model, gave good fits to the pressure-dependent curves. The step sizes (energies removed in a down collision) corresponded to collisional efficiencies (βc) of ca. 0.5 for SF6 and ca. 0.2 for Ar.The rate constants for the insertion and reverse decomposition (of Si2H6) have been combined to obtain a precise value of the equilibrium constant Kp at 552 K. Using the third-law method, a value for ΔfH°(SiH2)= 273 ± 2 kJ mol–1 is derived which represents the most precise experimental value for this quantity yet obtained. Ab initio calculations at the correlated level, reveal the presence of two weak complexes (local-energy minima) on the potential-energy surface corresponding to either direct or inverted geometry of the inserting silylene fragment. Surprisingly, the latter is the lower in energy, lying 51.5 kJ mol–1 below the unassociated reactants. These complexes rearrange to disilane with very low barriers. The implications of these findings and the nature of the insertion process are discussed.

Some comments on kinetics and mechanism in the pyrolysis of monosilane

Abstract The mechanism of pyrolysis of monosilane in the light of recent work by Robertson, Hils and Gallagher (RHG). It is argued that at reactant pressures 40 Torr, and at temperatures 650 K, the reaction is initiated homogeneously by the unimolecular process SiH 4 → SiH 2 + H 2 , as originally proposed by us, but contrary to RHGs interpretation. At low pressures, however, the reaction may well be surface initiated. In the secondary stages of decomposition we discuss possible chain branching processes involving silylenes and point out continuing mechanistic difficulties.

ROOM TEMPERATURE OBSERVATION OF GEH2 AND THE FIRST TIME-RESOLVED STUDY OF SOME OF ITS REACTIONS

Abstract Using a laser flash photolysis/laser probe technique, applied to two different gaseous precursor molecules, we report absorption signals in the wavenumber region 17109–17120 cm −1 , attributable to previously unobserved rotational transitions in the vibronic spectrum of GeH 2 . By time-resolved monitoring of GeH 2 , rate constants have been obtained for its reactions with O 2 , C 2 H 2 , i-C 4 H 8 , 1,3-C 4 H 6 , C 3 H 8 and Me 3 SiH. The measurements show that GeH 2 is unreactive towards CH bonds but inserts readily in SiH bonds, as well as reacting rapidly (close to the collision rate), with π-bonds. These results represent the first direct kinetic measurements on GeH 2 . Comparisons show that it is slightly less reactive than SiH 2 .

Absolute rate constants for the gas-phase reactions of silylene with silane, disilane and the methylsilanes

Absolute rate constants for reactions of silylene have been determined by time-resolved measurements of its decay at room temperature, following formation by pulsed-laser photolysis of phenylsilane in the presence of various added silanes. For SiH4 and Si2H6 the rate coefficients are pressure dependent and the former reaction is successfully modelled using RRKM theory. High-pressure (or pressure-independent) rate constants (in 10–10 cm3 molecule–1 s–1) are: SiH4, ca. 4.0; Si2H6, ca. 6.5; MeSiH3, 3.66 ± 0.22; Me2SiH2, 3.31 ± 0.26; and Me3SiH, 2.47 ± 0.14. These results are compared with other determinations and the rate constants for the analogous reactions of SiMe2. A model for the insertion reaction is proposed in which the nucleophilic stage of the process plays an important role.

Time-resolved studies of the reactions of gas-phase dimethylsilylene

We report rate constants for the reactions of SiMe2 with SiH4, MeSiH3, Me2SiH2, Me3SiH, Me4Si and pentamethyldisilane obtained using laser flash photolysis/laser absorption techniques. ArF exciplex laser (193 nm) photolysis of pentamethyldisilane was used to generate SiMe2 radicals in the gas phase and the transient absorption of 457.9 nm radiation from an Ar+ laser was monitored in real time. Both time-resolved and end product analysis experiments are described, confirming that the transient species observed is the SiMe2 radical. When normalised for the number of Si-H bonds in the reactant molecules, the rate constants for SiMe2 + SiH4, MeSiH3, Me2SiH2 and Me3SiH show a systematic increase with increasing methyl group substitution. We discuss possible explanations for this correlation. An improved estimate of ΔHf0(SiMe2, 298 K.) = 26 ± 2 kcal mol−1 is obtained by combining these results with those from previous kinetic studies.

Relative rate studies for silylene

Abstract The photochemical decomposition of gaseous phenylsilane has been investigated at 206 nm and at 298 K. Observation of benzene and phenyldisilane, with and without added oxygen, indicate that formation of C 6 H 6 + SiH 2 is an important primary process. Photolysis in the presence of added Me 3 SiH, C 2 H 4 , C 2 H 2 , CH 3 CCCH 3 and O 2 yielded rate cons- tants for reaction of SiH 2 with these species relative to phenylsilane. The results show SiH 2 to be a highly reactive and undiscrimi- nating species, except in its reaction with oxygen. No reaction with CH 4 was observed. The possibility of reaction with H 2 is discussed.

Thermochemistry of silicon-containing compounds. Part 1.—Silicon–halogen compounds, an evaluation

Literature data on the heats of formation of silicon–halogen compounds have been collected and reviewed. The coverage includes all tetravalent monosilicon compounds containing Si—H—X, where X is a single halogen, as well as the subhalides SiXn, where n= 1, 2 or 3. The data are critically evaluated from the standpoints of bond additivity and general chemical reactivity of the species involved as well as by detailed consideration of individual studies. Earlier compilations or reviews are discussed. A set of recommended values (with uncertainties) is proposed. For the divalent species, SiX2, a self-consistent set of lone-pair stabilisation energies is obtained.

The absorption cross section of the HCO radical at 614.59 nm and the rate constant for HCO+HCO → H2CO+CO

Abstract Using a combination of XeCl exciplex laser flash photolysis of gas-phase glyoxal and formaldehyde and time-resolved cw dye laser absorption at 614.59 nm, we have determined the ratio k 1 /σ for the reaction HCO+HCO → H 2 CO+CO (1) at 295 ±2 K. Similar studies involving the 308 nm photolysis of a variety of aldehydes combined with a determination of the absolute yields of the resulting hydrocarbon products have allowed us to deduce the initial yields of HCO radicals and hence the absorption cross section for HCO at the monitoring wavelength. We find σ=(2.3±0.6) × 10 −18 cm 2 , giving k 1 =(7.5±2.9)× 10 −11 cm 3 molecule −1 s −1 . Our values are compared with previous results.