Isotopically labeled molecules

When a molecule adsorbs to a surface, it can remain intact or it may dissociate. Dissociative chemisorption is conmion for many types of molecules, particularly if all of the electrons in the molecule are tied up so that there are no electrons available for bonding to the surface without dissociation. Often, a molecule will dissociate upon adsorption, and then recombine and desorb intact when the sample is heated. In this case, dissociative chemisorption can be detected with TPD by employing isotopically labelled molecules. If mixing occurs during the adsorption/desorption sequence, it indicates that the mitial adsorption was dissociative. 

The study of the infrared spectrum of thiazole under various physical states (solid, liquid, vapor, in solution) by Sbrana et al. (202) and a similar study, extended to isotopically labeled molecules, by Davidovics et al. (203, 204), gave the symmetry properties of the main vibrations of the thiazole molecule. More recently, the calculation of the normal modes of vibration of the molecule defined a force field for it and confirmed quantitatively the preceeding assignments (205, 206). 

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. 

CCR is easily measured by heteronuclear NMR experiments of isotopically labeled molecules. The information extracted from these experiments will significantly improve the resolution of NMR structures, especially bound conformations of weakly bound ligands, since /-couplings cannot be used in this case. The reason is the fact that the nonbound conformation significantly contributes to the averaged values of the coupling constant. 

The acidity probes discussed above are the most commonly used. However, the use of many different probes has been reported in the literature. This list includes nitriles, alkanes, amines, water, di-hydrogen, deuterium, isotopically labeled molecules, benzene, etc. Probe molecules can also be used to measure basicity on zeohtes. In this case, weakly acidic molecules such as CO2, pyrrole, acetic acid and halogenated light paraffins have been used. Space does not permit discussion of these in any detail, but information about these probes and their applications can be found in the following references [87, 127-130]. 

The possibility of deriving useful information on the conformation of cyclodipeptides from the coupling constants 15N— H, l3C— H, and l3C—15N in isotope-labeled molecules has been discussed (82MI1). 

In practice, two possibilities exist the use of a stable isotope-labeled molecule or a radiolabeled IS. The most commonly used stable isotope is deuterium, which can be incorporated relatively easily into molecules. 15N, 13C, and 180 are used too, but to a lesser extent. Considerable efforts have been made to increase the availability and range of stable isotope-labeled compounds. In the United States, the National Stable Isotope Resource Center has been established at the Los Alamos Scientific Laboratories to help meet the need for stable isotope-labeled compounds. Funds have been made available for the synthesis of many labeled compounds in sufficient... 

If the reactants are in the gas phase, the reactor may be either static or dynamic. Small static reactors are convenient for basic research where either the reactants are expensive (e.g. isotopically labelled molecules) or the reaction slow. Dynamic reactors, where reactants flow through the catalyst bed, provide a better simulation of practical use in a fixed bed reactor the catalyst remains in place, and it is in the form of large particles or pellets. 

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. 

There is one obvious difficulty in determining intramolecular kinetic isotope effects with the present PIPECO techniques for metastable ions (Sect. 3.2.1), which is that peaks for product ions from isotopically labelled molecules will overlap. Indeed, peaks for ions separated by only one mass unit will typically be largely superimposed. The time-of-flight technique is, in effect, measuring velocity and what is required is measurement of mass and translational energy in separate analysers. The problem... 

The energy and the frequency of the molecular vibrational modes depend on the mass of the atoms directly involved as well as on the bond energy. Consequently, the position of the absorption peaks are shifted when isotopically labeled molecules are used. This is a useful property for the confirmation of the assignment of vibrational modes when needed. The adsorption of heavy water allows investiga-... 

The use of a stable-isotope labelled molecule is probably the closest approach to an ideal internal standard, because of the nearly identical physical and chemical properties between the molecules. Under most gas chromatographic systems, the compound and it deuterat-... 

Valyon and Hall [13] employed isotopically labeled molecules such as 02 and N 0 to study the very interesting issue of the release of molecular oxygen during... 

There are two levels of assumption in the internal reference method. The more basic is that there is not complete assurance that the rate coefficients of the reference reaction are the same in the differently isotopically labelled molecules. This comment is particularly pertinent in the case of metastable ions, where it has been shown that secondary isotope effects can be very large (> 10) (see Sect. 7.5.4). 

The complex-formation mechanism was demonstrated quite conclusively by some experiments with isotopically labeled molecules in which 0( 2>) was quenched by C 0. The absolute rate of formation of C 0 from... 

The Stark effect is related to a change in energy levels of atoms and molecules in the presence of a strong external electric field and is observed as a shift and splitting of the spectral rotational lines. The applicability of the method is restricted to the gas phase and more complex compounds require the use of isotopically labeled molecules [43,44,50-52]. 

Our discussion so far has considered the impurities to be fixed in the ice lattice but this is obviously not exactly true for they can move slowly by solid-state diffusion. Similarly individual water molecules migrate in a self-diffusion process which can be followed by using isotopically labelled molecules. The structure and mass of these are very little different from those of ordinary water molecules, so that a study of their diffusion gives information about self-diffusion in the ice crystal. The only case where this is not obviously true is for the isotopes of hydrogen, where the mass ratio to the proton is considerable, but the nature of the experimental results enables us to sidestep this difficulty. 

Laser-MS has also been used to resolve optical spectra of naturally occurring isotopic species. In this application, a peak in the MS due to an ion containing the isotope of interest (e.g. C-13 in benzene [25] or other molecules [26]) is selected. The intensity of the mass peak is then monitored as a function of the laser, producing the excitation spectrum of the isotopically labeled molecule. 

The resulting spectrum [35] (Figure 3) is dramatically simplified by comparison to that obtained following electron bombardment ionization [36]. The vibrational excitation in the ion can be varied by changing the wavelength of the first, ionization laser to be resonant with a intermediate Rydberg level of differing v. This results in spectra with different vibrational activity and these results combined with the same spectra obtained for isotopically labeled molecules can be used to provide a detailed vibrational analyis [37] for this band system which was previously only subject to qualitative interpretation. 

RIA employs isotopically labeled molecules and permits measurements of extremely small amounts of antigen, antibody, or antigen-antibody complexes. It can be used to measure any substance that will serve as an antigen or hapten. The concentration of labeled molecules is determined by measuring their radioactivity. This highly sensitive analytical method has many applications in hormone assays as well as for... 

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