OMTS

The working method used by the ad hoe group is the Object Modelling Technique (OMT). a recognized object modelling approach. 

Hydrolysis of both neutral sugars from tylosin yields S-O-mycaminosyltylonolide (OMT) (51, R = CHO, R = R" = OH) (238). However, this compound is more conveniendy obtained by hydrolysis of mycarose from the fermentation-derived demycinosyltylosin. Analogous hydrolyses of mycarose from DMOT, GS-77-3, and GS-77-1 yield, respectively, 23-deoxy-5-0-mycaminosyltylonohde (DOMT), 20-deoxo-5-O-mycaminosyltylonohde (GS-77-4), and 5-0-mycaminosyltylactone (GS-77-2), all given in Table 9 (246,250,280). 

I o document by video or photography what is being visualized is oiteri very useful. This makes it possible to analyze more carefully what has been documented. It is not possible to see all details the first time and the omt analysis of the results by all involved persons w/ill many times be valiiai)ie. 

Figure 5 shows a conformational model of OMTS for discussion the solid state spectra. At the left a tilted crown type conformation is shown, where the carbons oc-... 

While the comparison of the OMTS and the (CH2)12 spectra helped to learn something about the kind of information solid state chemical shifts can provide, we can obtain much more detailed data about the correlation of chemical shifts and the rotational isomeric states from the spectra of larger cycloalkanes. Usually conformational shift variations are discussed by (i) the so called y-gauche effect and (ii) the vicinal gauche effect, Vg 15) ... 

Many azamacrocycles, like cyclam [2] for instance, contain more than one nitrogen atom and therefore possess more than one pK. Therefore, in the discussion of the basicity of these compounds, it is not only the in- and OMt-orientation of the lone pair or the proton, respectively, that has to be considered. There is also an influence of the first on the second, third, etc. protonation. The developing Coulomb repulsions are very sensitive to the charge separation. In 15-membered rings this phenomenon has been investigated in detail (Arnaud-Neu et al., 1979). 

For larger cryptands [6] (Cox et al., 1978), the protonation/deprotonation kinetics have also been measured. Table 4 lists the kinetic and the equilibrium data for such cryptands. When compared to the neutralization of protonated tertiary amines by OH, the reaction of the second smallest protonated cryptand [2.1.1] H is 10 to 10 times slower (Cox et al., 1978), indicating a strong shielding and possibly an i -orientation of the proton. For the [2.2.1] cryptand, no k and k-i values could be calculated, probably due to a fast pre-equilibrium between in,in- and m,OMt-conformations. 

Elastic tunneling spectroscopy is discussed in the context of processes involving molecular ionization and electron affinity states, a technique we call orbital mediated tunneling spectroscopy, or OMTS. OMTS can be applied readily to M-I-A-M and M-I-A-I -M systems, but application to M-A-M junctions is problematic. Spectra can be obtained from single molecules. Ionization state results correlate well with UPS spectra obtained from the same systems in the same environment. Both ionization and affinity levels measured by OMTS can usually be correlated with one electron oxidation and reduction potentials for the molecular species in solution. OMTS can be identified by peaks in dl/dV vs bias voltage plots that do not occur at the same position in either bias polarity. Because of the intrinsic... 

A qualitative understanding of OMTS may be obtained with reference to Fig. 9. In this example we consider the simplest junction configuration, wherein a molecule is in electronic equilibrium with one metal electrode, and a relatively large insulating gap separates it from the other electrode. We will consider more complex cases later. 

When the sample is biased positively (Ub > 0) with respect to the tip, as in Fig. 9c, and assuming that the molecular potential is essentially that of the substrate [85], only the normal elastic current flows at low bias (<1.5 V). As the bias increases, electrons at the Fermi surface of the tip approach, and eventually surpass, the absolute energy of an unoccupied molecular orbital (the LUMO at +1.78 V in Fig. 9c). OMT through the LUMO at — 1.78 V below the vacuum level produces a peak in dl/dV, seen in the actual STM based OMTS data for nickel(II) octaethyl-porphyrin (NiOEP). If the bias is increased further, higher unoccupied orbitals produce additional peaks in the OMTS. Thus, the positive sample bias portion of the OMTS is associated with electron affinity levels (transient reductions). In reverse (opposite) bias, as in Fig. 9b, the LUMO never comes into resonance with the Fermi energy, and no peak due to unoccupied orbitals is seen. However, occupied orbitals are probed in reverse bias. In the NiOEP case, the HOMO at... 

Fig. 9 OMT bands for NiOEP, associated with transient reduction (1.78 V) and transient oxidation (—1.18 V). Data obtained from a single molecule in a UHV STM. The ultraviolet photoelectron spectrum is also shown, with the energy origin shifted (by the work function of the sample, as discussed in [25]) in order to allow direct comparison. The highest occupied molecular orbital, n, and the lowest unoccupied molecular orbital, %, are shown at their correct energy, relative to the Fermi level of the substrate. As in previous diagrams,

A different view of the OMT process is that the molecule, M, is fully reduced, M , or oxidized, M+, during the tunneling process [25, 26, 92-95]. In this picture a fully relaxed ion is formed in the junction. The absorption of a phonon (the creation of a vibrational excitation) then induces the ion to decay back to the neutral molecule with emission (or absorption) of an electron - which then completes tunneling through the barrier. For simplicity, the reduction case will be discussed in detail however, the oxidation arguments are similar. A transition of the type M + e —> M is conventionally described as formation of an electron affinity level. The most commonly used measure of condensed-phase electron affinity is the halfwave reduction potential measured in non-aqueous solvents, Ey2. Often these values are tabulated relative to the saturated calomel electrode (SCE). In order to correlate OMTS data with electrochemical potentials, we need them referenced to an electron in the vacuum state. That is, we need the potential for the half reaction ... 

Fig. 10 Electrochemical energy level model for orbital mediated tunneling. Ap and Ac are the gas-and crystalline-phase electron affinities, 1/2(SCE) is the electrochemical potential referenced to the saturated calomel electrode, and provides the solution-phase electron affinity. Ev, is the Fermi level of the substrate (Au here). The corresponding positions in the OMT spectrum are shown by Ar and A0 and correspond to the electron affinity and ionization potential of the adsorbate film modified by interaction with the supporting metal, At. The spectrum is that of nickel(II) tetraphenyl-porphyrin on Au (111). (Reprinted with permission from [26])...

Redox potentials in the solid state are expected to differ from those in solution [97]. Moreover, there will be shifts in the potentials of a thin film, relative to that of a solid, due to interactions with the metal support and counter electrode, including image-charge effects. There may be an opposite signed shift due to the absence of a covering layer of adsorbed molecules [99]. Another complication is the fact that electrochemical potentials are equilibrium values, and therefore reflect the energy associated with the formation of an ion in its equilibrium geometry. OMTS transitions, as discussed above, may occur so rapidly that the ion is formed in an excited... 

Fig. 11 Correlation between electrochemical potentials and OMTS bands for more than ten compounds including polyacenes, phthalo-cyanines, and porphyrins. OMTS data were acquired both from tunnel junctions and STM measurements. The standard potential relative to the normal hydrogen electrode associated with the half reaction M(solution) + e-(vac) —> M-(solution) is the y axis. The three outliers are assigned to the ring oxidation of porphyrins. (Reprinted with permission from [26])...

Two final concerns must be addressed surface oxidation state and temperature dependence. Whenever one deposits a redox-active species on a metal surface, the oxidation state of the adsorbate (and therefore the OMTS bands) may change. One example is the adsorption of a biaxially substituted dicyano cobalt phthalocy-anine salt, MCoPc(CN)2 (where M = K or Cs), on gold to form the reduced species CoPc [111]. A second example is provided by the adsorption of TCNE on gold, silver, and copper. In that order, the charge state of TCNE on the surface ranges from 0 to 3, and the OMTS reflects these changes. 

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