Carbon monoxide, labelled with isotopic

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

Exchange reactions between bulk and adsorbed substances can be studied by on-line mass spectroscopy and isotope labeling. In this section the results on the interaction of methanol and carbon monoxide in solution with adsorbed methanol and carbon monoxide on platinum are reported [72], A flow cell for on-line MS measurements (Fig. 1.2) was used. 13C-labeled methanol was absorbed until the Pt surface became saturated. After solution exchange with base electrolyte a potential scan was applied. Parallel to the current-potential curve the mass intensity-potential for 13C02 was monitored. Both curves are given in Fig. 3.1a,b. A second scan was always taken to check the absence of bulk substances. 

Isotope labeling studies furnished a convincing body of evidence to indicate that even in the presence of carbon monoxide, only carbon dioxide is converted to methanol [Eq. (3.2)].394-398 In a review article371 Chinchen considers this as the dominant route, with CO merely serving as a source of C02 via the water-gas shift reaction. 

Recognition of the easier chemisorption of carbon monoxide on atoms of low CN as indicated by its preference for rough films formed at low temperature led to a study of the stepped Au(332) surface,63 although even here desorption was complete when temperature was raised to ambient. The use of isotopically labelled molecules showed that the IR band at 2120 cm-1 contained four components, although TPD revealed only two. The band at 2120 cm-1 red shifted, and the surface potential increased linearly, with increasing coverage. 

Methanol reactions have also been studied on polycrystalline wafers of UO2 [76]. Two parent desorption states existed for methanol adsorbed at 90 K. Molecularly adsorbed methanol desorbed at 110 K, and methanol generated by surface recombination of methoxides and protons desorbed at 180 K. Carbon (Is) XPS demonstrated that methanol dissociatively adsorbed on the urania surface and that methoxide was the only surface intermediate present above 150 K. Primary reaction products were methane and carbon monoxide at 480 K. Oxygen atoms not removed from the surface as CO were incorporated into the oxide surface isotopically labeled U 02 surfaces did not exchange oxygen with methoxide to produce C 0 [76]. 

Hydroformylation of ( + )-(S)-3-methyl-l-hexene gives 3% of (7 )-3-ethylhexanal (14) with 70% optical purity130 168. Interestingly, isotopic labelling reveals that no direct insertion of carbon monoxide into the C-H bond occurs, but a stereoselective double bond or metal alkyl isomerization preceeds hydroformylation with only partial loss of optical information34-122. 

Infrared spectroscopy has limited applications in the detection and assay of labelled compounds. Absorption peaks at 2193 cm for carbon monoxide- C and at 2144 cm for CO are sufficiently separated to determine the isotope ratio [139]. With C contents ranging from 1 to 20 atom%, the mean error quoted was about 2% and had a relative standard deviation of about 6%. [Car-Z on Z- C] acetophenone gives a peak in the infrared spectrum at 1685 cm whUe the (carbo/y /- Cyiabelled compound has a corresponding absorption at 1645 cm [128]. In certain cases it is therefore possible to utilise published data for a rapid assay of labelled compounds. Infrared data on C- and N-labelled compounds have been reviewed [140]. 

The incorporation of isotopic labels via organoboron chemistry has been the subject of previous communications from Kabalka and co-workers, and two further examples have been reported this year. For example, the preparation of primary alkylamines by the reaction of organoboranes with NH4OH, described in 1981, has now been used to give N-labelled primary amines, and C-labelled carboxylic acids are readily prepared from the reaction of organoboranes with C-enriched carbon monoxide. ... 

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