Methanol, labelled with isotopic

Evidence in support of the mechanism shown in figure 21.4 comes from isotope-labeling experiments. When 180-labeled methanol reacts with benzoic acid, the methyl benzoate produced is found to be l80-labeled but the water produced is uniabeled. Thus, it is the C-OH bond of the carboxylic acid that is broken during the reaction rather than the CO—H bond and the RO-H bond of the alcohol that is broken rather than the R-OH bond. 

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. 

It is of interest to note that identifications and partial identifications, based on mass spectrometric data, were obtained for all the metabolic peaks detected by HPLC-RC of the original methanolic extracts. The detail and the wealth of data that were acquired in this study demonstrate (i) the value of using substrates labelled with both radio and stable isotopes (ii) the effectiveness of the purification procedures coupled with the use of HPLC-RC to monitor metabolites during fractionation and (iii) the importance of GC-MS and HPLC-MS as powerful analytical tools that facilitate the identification of free GAs and GA conjugates, respectively and which also distinguish between endogenous and metabolite pools. 

Fischer carbenes can be generated from diazomalonates in the gas phase, as reported by Beauchamp and co-workers. Suitable cations for study were obtained by electrospray of solutions of Ag or Cu salts with the diazoma-lonate 21 in methanol water acetonitrile (80 20 0.1). The further reaction steps were conducted in a quadrupole ion trap spectrometer. Complexes formed via electrospray first undergo Fischer carbene formation and subsequently Wolff rearrangement upon collisional activation. Experiments with isotopically labeled diazomalonates suggest the underlying mechanism shown in Scheme 8. 

Walker, A.P., Lambert, R.M., Nix, R.M., and Jennings, J.R. Methanol synthesis over catalysts derived from CeCu2 — transient studies with isotopically labeled reactants, uf Catal 1992,138, 694—713. 

In another study on the cofeeding of ethylene and methanol with isotopic labeling over a ZSM-5 catalyst, Ronning et al. [138] drew the conclusion that ethylene is unreactive and its methylation could not be an important part in the reaction network. They determined that the ethylene conversion is increased in the presence of methanol but only at rather low levels of conversion. 

A cleai-cut answer was provided by Irving Roberts and Harold C. Urey of Columbia University in 1938. They prepared methanol that had been enriched in the rnass-18 isotope of oxygen. When this sfflnple of methanol was esterified with benzoic acid, the methyl benzoate product contained all the label that was originally present in the methanol. 

Residues of isoxaflutole, RPA 202248 and RPA 203328 are extracted from surface water or groundwater on to an RP-102 resin solid-phase extraction (SPE) cartridge, then eluted with an acetonitrile-methanol solvent mixture. Residues are determined by liquid chromatography/tandem mass spectrometry (LC/MS/MS) on a Cg column. Quantitation of results is based on a comparison of the ratio of analyte response to isotopically labeled internal standard response versus analyte response to internal standard response for calibration standards. 

Soil samples are extracted with methanol-water (7 3, v/v) using a Soxtec extractor. After addition of an isotopically labeled internal standard (IS) and dilution to 50 mL, the extracts are analyzed by electrospray LC/MS/MS. 

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. 

A mechanism proposed 87) for the alkaline hydrolysis of tetraethyl pyrophosphate, which is markedly accelerated by HPO e ions, has been substantiated by isotopic labeling 88). The nucleophilic attack by HPOJp on the symmetrical pyrophosphate 131 is considered to lead initially to the unsymmetrical P P1-diethyl pyrophosphate dianion 132 which decomposes spontaneously under the conditions of reaction to give the diethyl phosphate anion and POf 102. The latter reacts with water to form inorganic phosphate and with alcohols suclj as methanol and ethylene glycol to produce alkyl phosphates. 

When o-, m- and p-nitroanisole with 14C-labelled at the methoxy group were irradiated under identical conditions in methanol in the presence of sodium methoxide, only m-nitroanisole underwent methoxy exchange, with the limiting quantum yield (

labelled isotope experiments support a a complex intermediate and indicate an Sjv23Ar mechanism (direct substitution in the triplet state) for this reaction (equation 12) and for 4-nitroveratroles (equation 13). Further evidence from quenching and lifetime experiments also support a direct displacement SAr2Ar mechanism for the photosubstitution reaction of nitroaryl ethers with hydroxide ions13. 

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