Reactivity and structure of CF3I on Ru(001)
A variety of surface-sensitive techniques are used to elucidate the reaction pathways, as well as adsorbate structures, associated with thermal activation of CF{sub 3}J following adsorption on Ru(001) at 100 K. XPS shows that the C-I bond of CF{sub 3}I dissociates below 200 K to form CF{sub 3}(ad) and I(ad); the subsequent reactions of CF{sub 2} are best viewed as being regulated by the availability of surface sites. CF{sub 3}(ad) dissociates to CF{sub 2}(ad) below 200 K. Further CF{sub 3} dissociation, some of which is activated by H(ad), occurs between 200 and 400 K until all available sites are filled. Desorption of the remaining CF{sub 3}, peaking at 705 K, once again opens surface sites for decomposition. This is followed by recombination of the products to form CF{sub 3}(g). No evidence for CF(ad) is ever observed. Hydrogen coadsorption studies explain interesting features associated with fluorine evolution. HREELS and ESDIAD results indicate that CF{sub 3} adopts a tilted configuration on Ru(001). 53 refs., 11 figs., 3 tabs.
Thermal chemistry of CF3I on Ag(111) : a TPD and RAIRS study
Temperature programmed desorption and reflection absorption infrared spectroscopy have been used to study the thermal chemistry of CF3I on Ag(111). At coverages ≤0.03 CF3IAg, asorption occurs with ICF3 bond dissociation at T = 86 K, while at higher coverages CF3I adsorbs molecularly. The CI bond direction of the adsorbed molecules depends on the coverage. When molecular adsorption occurs at coverages ≤0.15 CF3IAg, the CI bond direction is parallel to the surface. Above this coverage the adsorbed molecules reorder to a form in which the molecular axis is titled away from the surface plane. The first layer saturates at 0.3 CF3IAg coverage. Multiayers form readily at 86 K. When the CF3IAg(111) system is heated, the multilayers desorb at ∼ 100 K, and some of the CF3I thermally dissociates into CF3 and I at ∼ 110 K. The fate of the products depends on the molecular orientation prior to the fragmentation. When the coverage is high and the CI axis is tilted from the surface, the CF3 radicals desorb during dissociation at ∼ 110 K, leaving only iodine atoms on the surface. At low coverage, when the CI axis is parallel to the surface, both the CF3 and the iodine remain bound. The non-dissociated CF3I desorbs molecularly at T ≤ 150 K. The CF3 that remains after CI bond scission desorbs as radicals at ∼ 300 K, and the adsorbed iodine desorbs atomically at ∼ 800 K. The relationship of molecular orientation to previous results on CF3IAg(111) thermal and photochemistry is discussed.
Interaction of Diclofop-Methyl and MCPA on Wild Oats (Avena fatua)
Diclofop-methyl {methyl 2-[4-(2,4-dichlorophenoxy)phenoxy] propanoate} loses part of its activity on wild oats (Avena fatua L.) when it is applied in combination with ester or amine formulations of MCPA {[4-chloro-o-tolyl)oxy] acetic acid.} Uptake of diclofop-methyl is reduced in the presence of commercial formulations of MCPA, particularly amine formulations, which are only partly compatible with the diclofop-methyl formulation. With added MCPA, regardless of formulation, hydrolysis of diclofop-methyl to the acid diclofop is slowed down, both in intact wild oat plants and in a crude leaf extract. At the same time, conversion of any acid formed to inactive conjugates is accelerated in the presence of MCPA. It is concluded that the loss of activity of diclofop-methyl by MCPA can be accounted for by the effects of MCPA on uptake and metabolic conversion of diclofop-methyl.