Surface acceptor

Superbasic Surface Centres with Ionic Character. - As mentioned earlier reactions between alkali metal atoms and surface acceptor centres such as hydroxyl groups or holes near cationic vacancies lead to the creation of centres of higher basicity. In the first of these examples the reason is the replacement of a hydrogen atom by a more electropositive element, such as an alkali metal atom in the second example it is the result of introduction of an electron from the alkali metal to the hole trapped on the O " anion, the vacancy being filled by a univalent cation. It should be noted that both of the surface configurations so formed can cause strong one-electron or two-electron donor activity. Closer physico-chemical examination has shown that these centres tend to be electron pair rather than one-electron donating in character. They mostly occur on surfaces on which alkali-metal vapours have reacted with oxides heated at the lower temperatures, e.g., MgO calcined... 

The quantity of alkali metal retained on the MgO surface and the concentration of the newly created ionic superbasic centres depends on the position of metal in the Periodic Table. The greater the electropositivity in the sequence sodium, potassium, caesium, the greater is the reactivity with surface acceptor centres of MgO surfaces. It is possible that metals having lower ionization energy, such as potassium or caesium (Table 1), react with these surface centres of MgO, which are not affected by sodium atoms. In consequence an oxide surface that has been heated to a particular temperature is able to bind more caesium than sodium atoms. The increase of the quantity of metal retained on MgO surfaces is not followed by a simultaneous increase in the number of newly created ionic superbasic centres. The largest quantity of such centres is formed on MgO surfaces doped with potassium. It is interesting to note that in the case of MgO-K and MgO-Cs systems two types of superbasic centres occur, one with a basic strength of 33 < H < 35, the second one with H > 35 (Table 1). ... 

D. J. Leary, J. O. Barnes, and A. G. Jordan, Calculation of carrier concentration in polycrystalline films as a function of surface acceptor state density Application for ZnO gas sensors, J. Electrochem. Soc. 129 (1982) 1382-1386. 

The present design of our MAS probe is clearly amenable to variable temperature operation. Hence, one can study site exchange processes as a function of temperature and possibly separate rotational motion from translational motion of the molecule on the surface. Anisotropic rotational motion can be studied by nmr, e.g. deuterated pyridine and n-butyl amine. The ease of the experiment suggested that nmr with enriched samples would be equally as straightforward. In the near future, the Al, Ag, Rh nmr spectroscopy of the surface acceptor can also be studied. 

For surface acids a distinction is made between protic (Bronsted centers) and non-protic (Lewis centers). Bronsted centers can release surface protons, while Lewis centers represent surface acceptor sites for electron pairs and thus bind nucleophiles. 

It is then possible to solve (2.1) numerically to obtain the band bending Ug. A graphical solution (valid in the absence of an external field) is sketched in Fig. 5.2-65 for an intrinsic semiconductor ( b = 0) with a single surface acceptor level and various values of Uj and N y For extrinsic semiconductors ( b 0) the procedure is similar, provided the origin is translated to Ug = — b and the appropriate function for Qsc is chosen. The same procedure can also be used in the presence of an external field by translating the origin of the coordinates. 

E, energy of the top of valence band E, band gap E, surface acceptor level cp work function X, electron affinity , ionization energy... 

Fig. 2.7 Initiation efficiency of DDNP as a function of density and specific surface (acceptor 0.35 g of TNT compressed by 76.5 MPa with reinforcing cap), by kind permission of Dr. Stmad [9]...

A study of the incorporation of 2-amino-2-deoxy-D-glucose, UDP-2-acetamido-2-deoxy-D-glucose, and CMP-sialic acid into Ehrlich ascites cells suggested that the surface acceptor for CMP-sialic acid added exogenously is not identical to that for the surface sialoglycopeptide synthesized endogenously. ... 

In the case of the interaction with surface acceptor states, not related to oxygen adsorption, one can proceed as in the case of the first mechanism proposed by Kohl. In the case of an interaction with oxygen adsorbates, one can consider that the dissociation of oxygen ions is increased and examine the implications. 

N GaAs(llO) 60...300 Kelvin oxygen induced surface acceptors... 

Sialyltransferase activities in normal and virally transformed mouse 3T3 cells have also been measured with specific endogenous acceptors (Bosmann, 1972a). The acceptors were prepared by incubating the intact cells with neuraminidase and then trypsin. The desialylated material released from the cells was used to measure sialyltransferase activity in crude detergent extracts of the cells. Cells transformed by MSV, RSV, or Py had more acceptor and transferase activity than normal 3T3 cells. Surface sialyltransferase activity as measured by the ability of intact cells to transfer sialic acid- C from CMP-NANA- C to undefined surface acceptors was also higher in the transformed lines (Bosmann, 1972 ). 

From a bond-valence perspective, at the point of zero charge of a surface, there is a minimum in the number of strong Lewis acids and Lewis bases (i.e., highly charged terminations) on the surface, which results in low bond-valence transfer between surface acceptors and donators, and aqueous species. Thus the lowest interaction between a face and the ambient aqueous solution occurs where the pH of the solution is equal to the point of zero charge of that face, and hence the crystal has very low growth and dissolution perpendicular to that face. 

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