Pearson Electronics

Wide-band current transformer (Pearson Electronics, Palo Alto, CA, USA) to control chamber current. 

Note The bandwidth and current capability of the current probes used for noise measurements are important. Popular choices for such probes are from Pearson Electronics and Fischer Custom Communications at www.pearsonelectronics.com and www.fischercc.com, respectively. For very high currents (up to thousands of amperes if necessary), a possible choice are current probes based on the Rogowski principle. This type of probe is available from several manufacturers, for example Power... 

Fig. 1.8. Ionization energy (/) and electron affinity (A) gaps in eV for radicals. [Adapted from R. G. Pearson, J. Am. Chem. Soc. 110 7684 (1988).]...

Efforts to establish a theoretical explanation of the reactivity of nucleophilic reagents have centered on correlations with intrinsic electron-donor properties which are the fundamental basis of nucleophilicity. According to Edwards and Pearson, in general, such properties include basicity, polarizability, and the presence of unshared electron pairs on the atom adjacent to the nucleophilic atom of the reagent. When only the first two of these properties are operative, Eq. (8), which was proposed by Edwards, has proved successful in... 

Metallation and oxymetallation reactions have been observed with the salts of only a few metals, namely mercury(II) (66, 67), thallium(III) (66,67), lead(IV) (66, 67), palladium(II) (100), gold(III) (63), and platinum-(II) (29). These facts correlate well with what Chatt (1) has termed class b, and Pearson (130) has called "soft acid character. Soft acids are characterized by low charge, large size, and, often, d electrons in their outer shell. No class b metal is known, in fact, which contains fewer than five d... 

Ahrland et al. (1958) classified a number of Lewis acids as of (a) or (b) type based on the relative affinities for various ions of the ligand atoms. The sequence of stability of complexes is different for classes (a) and (b). With acceptor metal ions of class (a), the affinities of the halide ions lie in the sequence F > Cl > Br > I , whereas with class (b), the sequence is F < Cl" < Br < I . Pearson (1963, 1968) classified acids and bases as hard (class (a)), soft (class (b)) and borderline (Table 1.23). Class (a) acids prefer to link with hard bases, whereas class (b) acids prefer soft bases. Yamada and Tanaka (1975) proposed a softness parameter of metal ions, on the basis of the parameters En (electron donor constant) and H (basicity constant) given by Edwards (1954) (Table 1.24). The softness parameter a is given by a/ a - - P), where a and p are constants characteristic of metal ions. 

Experimental data as well as density functional theory show that the ground-state properties of solids depend primarily on the densities of the valence electrons. Therefore, pE may be considered to be the electronic chemical potential (Pearson, 1997). Since pE denotes the energy per mole of... 

Another property that is related to chemical hardness is polarizability (Pearson, 1997). Polarizability, a, has the dimensions of volume polarizability (Brinck, Murray, and Politzer, 1993). It requires that an electron be excited from the valence to the conduction band (i.e., across the band gap) in order to change the symmetry of the wave function(s) from spherical to uniaxial. An approximate expression for the polarizability is a = p (N/A2) where p is a constant, N is the number of participating electrons, and A is the excitation gap (Atkins, 1983). The constant, p = (qh)/(2n 2m) with q = electron charge, m = electron mass, and h = Planck s constant. Then, if N = 1, (1/a) is proportional to A2, and elastic shear stiffness is proportional to (1/a). 

It is shown that the stabilities of solids can be related to Parr s physical hardness parameter for solids, and that this is proportional to Pearson s chemical hardness parameter for molecules. For sp-bonded metals, the bulk moduli correlate with the chemical hardness density (CffD), and for covalently bonded crystals, the octahedral shear moduli correlate with CHD. By analogy with molecules, the chemical hardness is related to the gap in the spectrum of bonding energies. This is verified for the Group IV elements and the isoelec-tronic III-V compounds. Since polarization requires excitation of the valence electrons, polarizability is related to band-gaps, and thence to chemical hardness and elastic moduli. Another measure of stability is indentation hardness, and it is shown that this correlates linearly with reciprocal polarizability. Finally, it is shown that theoretical values of critical transformation pressures correlate linearly with indentation hardness numbers, so the latter are a good measure of phase stability. 

Perepichka DF, Bryce MR, Pearson C, Petty MC, Mclnnes EJL, Zhao JP (2003) A covalent tetrathiafulvalene-tetracyanoquinodimethane diad extremely low HOMO-LUMO gap, thermoexcited electron transfer, and high-quality Langmuir-Blodgett films. Angew Chem Int Ed 42 4636 1639... 

Fig. 6. Raman spectra of localized vibrations due to nB and 10B in Si before (control) and after passivation by hydrogen or deuterium. [ Reprinted with permission from The Materials Research Society, Stutzmann, M. and Herrero, C.P. (1988). Defects in Electronic Materials, MRS Proceedings 104 (eds. M. Stavola, S.J. Pearson and G. Davies), p. 271. Also from Stutzmann and Herrero, 1988. And with permission from The American Physical Society, Herrero, C.P. and Stutzmann, M. (1988). Phys. Rev. B 38, 12668.]...

Absolute rates for the addition of the methyl radical and the trifluoromethyl radical to dienes and a number of smaller alkenes have been collected by Tedder (Table l)3. Comparison of the rate data for the apolai4 methyl radical and the electrophilic trifluoromethyl radical clearly show the electron-rich nature of butadiene in comparison to ethylene or propene. This is also borne out by several studies, in which relative rates have been determined for the reaction of small alkyl radicals with alkenes. An extensive list of relative rates for the reaction of the trifluoromethyl radical has been measured by Pearson and Szwarc5,6. Relative rates have been obtained in these studies by competition with hydrogen... 

R. Wu, J. S. Schumm D. L Pearson, J. M. Tour, Convergent Synthetic Routes to Orthogonally Fused Conjugated Oligomers Directed Toward Molecular Scale Electronic Device Applications, I. Org. Chem. 1996, 61, 6906-6921. 

The next step is the identification of the concept of chemical hardness, 17, with the second derivative of the energy with respect to the number of electrons, formulated by Parr and Pearson [14]... 

This concept was introduced qualitatively in the late 1950s and early 1960s by Pearson, in the framework of his classification of Lewis acids and bases, leading to the introduction of the hard and soft acids and bases (HSAB) principle [19-21]. This principle states that hard acids prefer to bond to hard bases and soft acids to soft bases. In many contributions, the factor of 1/2 is omitted. The inverse of the hardness was introduced as the softness S=l/rj [22]. A third quantity, which can be expressed as a derivative with respect to the number of electrons is the Fukui function, was introduced by Parr and Yang [23,24] ... 

HF M. S. Banna and D. A. Shirley. J. Chem. Phys. 63 1975, 4759 H20 K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, and S. Iwata, HandbookofHelPhotoelectron Spectra of Fundamental Organic Compounds, Japan Scientific Society Press, Tokyo, 1981 All electron affinities R.G. Pearson. Inorg. Chem. 27 1988, 734, except for F2 and Cl2 which are taken from J. A. Ayala, W. E. Wentworth and E. C. M. Chen, J. Phys. Chem 85,768,1981. 

As mentioned in the Introduction, no structural information on these species was available for more than 40 years after the discovery of the first Zintl metal cluster anions, since no pure crystalline phases could be isolated and characterized structurally. Nevertheless, early efforts to rationalize the observed formulas and chemical bonding of these intermetallics and related molecules utilized the Zintl-Klemm concept [75, 76] and the Mooser-Pearson [77] extended (8 — N) rule. In this rule N refers to the number of valence electrons of the more electronegative metal (and thus anionic metal) in the intermetallic phases. 

This did not mean there was much sympathy with Brodie s algebraic alternative for molecular models or with Pearson s more sophisticated attempt to introduce the mathematics of ether squirts into chemistry. Nor were chemists ready to give up the periodic table and pictorial theories in the daily work of the laboratory. But, like Robinson, who said that he considered his electronic theory of reaction mechanisms his "most important contribution to knowledge," many chemists considered theory, not chemical fact or chemical production, to be the highest aim of science. 34 And many agreed with Coulson that you cannot have deep theory without mathematics. 

The fact that the electron concentration appears to play some role in the control of this family of structures was noted by Nowotny (Schwomma et al. 1964a, b, Jeitschko and Parthe 1967, Flieher et al. 1968a, b, Parthe 1969) and reported by Pearson (1972). The second rule, discernible in the electron diffraction of the T Xm Nowotny chimney-ladder phases, is the appearance, along the c-axis of a... 

We underline these results and the implied concepts quoting from a comprehensive review on this subject (Simon 1983). We remember indeed that, ever since it was experimentally possible to determine atomic distances in molecules and crystals, efforts have been made to draw conclusions about the nature of the chemical bonding, and to compare interatomic distances (dimensions) in the compounds with those in the chemical elements. Distances between atoms in an element can be measured with high precision. As such, however, they cannot be simply used in predicting interatomic distances in the compounds. In a rational procedure, reference values (atomic radii) have to be extracted from the individual (interatomic distances) measured values. Various functions have been suggested for this purpose. In the specific case of the metals it has been pointed out that interatomic distances depend primarily on the number of ligands and on the number of valence electrons of the atoms (Pearson 1972). 

Phase conventional name and symbol, Pearson symbol Example and composition range (Okamoto 2000) c/a axial ratio ( ) Villars etal. (1995) Me/ at s,p electrons njnits,p,d electrons... 

The principle of hard and soft Lewis acids and bases, proposed by Pearson (1963), is useful to describe these reactions. A Lewis acid is any chemical species that employs an empty electronic orbital available for reaction, while a Lewis base is any chemical species that employs a doubly occupied electronic orbital in a reaction. Lewis acids and bases can be neutral molecules, simple or complex ions, or neutral or charged macromolecules. The proton and all metal cations of interest in subsurface aqueous solutions are Lewis acids. Lewis bases include H, O, oxyanions, and organic N, S, and P electron donors. A list of selected hard and soft Lewis acids and bases found in soil solutions is presented in Table 6.1. 

---