Use of Probe Molecules

In analogy to infrared spectroscopy, probe molecules can be used to study Lewis acid sites. In the case of NMR spectroscopy, however, due to the much smaller resonance frequencies (which correspond to a much longer time scale) exchange effects may lead to an average line instead of the expected separate lines due to molecules adsorbed on the various adsorption sites. From Heisenberg s uncertainty relation  

For r tcrit the two lines merge into a single line while for t to-it they can be 

If one denotes by vj, pj and rj, respectively, the resonance frequency, the relative concentration and the mean residence time of a nucleus at an adsorption site , then the shape of the NMR spectrum for a system with two different sorts of adsorption sites j = a, b with pa + pb = 1 strongly depends on the quantity  

---

Lavalley, J.C. (1991) Use of probe molecules for the characterization of the surface basicity of divided metal oxides, Trends Phys. Chem., 2, 305. 

Use of Probe Molecules on Metallic Surfaces as Evidence of Coordination and Organometallic Chemistry at Metal Surfaces... 

Materials can be characterized directly by PL if they exhibit intrinsic photoluminescent properties as described in Section 2 or, if this is not the case, via the use of probe molecules. 

The use of probe molecules that will sorb on specific acid sites has also been used to differentiate Brpnsted and Lewis sites. Studies on H-ZSM-5 and H-ZSM-8 suggest that pyridine cannot access the Lewis sites and thus essentially measures Brpnsted acidity 2,6-Dimethylpyridine has been reported to sorb preferentially on the Brpnsted sites of a variety of zeolites. Various amines have also been used to measure surface acidity. Since amines are weaker bases than either ammonia or pyridine, they preferentially sorb on the stronger acid sites. Both n-butyl amine and t-butyl amine have been used as adsorbates in TPD studies of zeolites. Comparison of the TPD spectra with butene isomerization has shown that butyl amines measure only the strong acid sites. 

Physical methods for size determination are mainly related to the use of X-ray based diffraction, scattering and absorption techniques, microscopy, and magnetic measurements. Physical and chemical methods may be combined, for example, in the use of infrared spectroscopy coupled to the use of probe molecules such as CO to determine the fraction of exposed metal atoms. However, as Chp 3 has already dealt with characterisation of supported metal systems by X-ray absorption and infrared spectroscopies in some detail, they will not be included here. 

A common experimental means of characterizing the acidity of zeolites is the use of probe molecules. IR spectra leave no doubt that ammonium ions are formed upon adsorption of ammonia in zeolites, the OH band characteristic for bridging Si-0(H)-Al sites disappears and NH4+ bending bands appear [12]. The energy of ammonia desorption. 

There is no doubt that zeolite electronegativity plays a predominant role in determining the final acidity of the zeolite Brpnsted sites. When, however, different structures with similar framework compositions were compared, different acidity, as measured by use of probe molecules, was found. For example, the binding energy of NH3 on H-Mordenite and H-ZSM-5 is 160 and 145 kJ moff respectively... 

Three TS-1 zeolites were prepared by three different methods. In order to determine that all of them are "well manufactured", DRX, IR, SEM and DRS were used. The jointly use of probe molecules (H2O2 eind CeH6) and DRS allowed us to detect differences in the population of the named "closed" and "open" Ti" sites and in their geometries in the three zeolites. These differences lead to distinct catalytic behavior when these solids are tested in the oxidation of benzene with H2O2. 

In an analogous way, the existence and properties of hydroxy groups of meso-porous materials were studied by transmission IR spectroscopy with and without the use of probe molecules such as pyridine (cf., e.g., in Sect. 53.2.6.2 [753, 754]) For the evaluation of experimental data reported (e.g., in [754] of... 

Usually, zeolitic materials are synthesised as sodium or potassium zeolites. If the starting zeolites, however, are in such a cationic form, the use of probe molecules (pjrridine [28], acetonitrile [29], carbon monoxide [30] etc.) is indispensable for IR spectroscopy to confirm solid-state ion exchange. An example is demonstrated by Figure 3. 

Using qualitalive and quantitative models, including both short and bng range effects, progress has been made in understanding acidity in zeolites, but limitations that still exist in measuring that acidity by use of probe molecules. 

Most of the techniques described here cannot establish whether a metal carbonyl cluster is present on the surface of a zeolite particle or within the intracrystalline cages. Chemical treatments have been used to determine whether the clusters are inside or outside, for example by the use of probe molecules that are too large to fit in the pores and those that do fit. 

However, if it is known that the surface hydroxyl groups under study are free, i.e. that hydrogen bonding can be excluded, the measurement of activated (unloaded) samples would be preferred in any case since the error of the deprotonation energy calculated by the use of Eq. 8 is considerably smaller than the error of both methods based on the use of probe molecules. 

In general, it is accepted [140,141] that Lewis acidity of zeolites and related catalysts (sUica-alumina, y-alumina) is related to the presence of aluminium species on the smface. Therefore, two possibUities exist for the study of Lewis acidity by NMR (i) an analysis of highly resolved Al NMR spectra similar to the MAS NMR spectroscopy described in Sect. 2.1 and (ii) the use of probe molecules such as pyridine or others. 

Hedgehog (Hh) signaling has a rich history in chemical biology with a wide variety of small-molecule probes acting on numerous signaling nodes [38]. Hh inhibition in development leads to cyclopia in vertebrates, and its dysregulation in adult humans causes basal cell carcinoma (BCG), medulloblastoma, pancreatic cancer, and others. Thus, the available tools for its perturbation can serve as a useful case study for the development and use of probe molecules in biology. 

---