Pairing definition

A somewhat more subtle ion pair definition was introduced by Ebeling (Eb) [210-212]. Ebeling s definition of the association constant ensures consistency up to the level of the second ionic virial coefficient between the... 

G. N. Lewis, Valence and the Structure of Atoms and Molecules, American Chemical Society Monograph, The Chemical Catalog Co., New York, 1923. Lewis also gave a definition equivalent to that of Bronsted at this time, but he considered the electron-pair definition to be more general. 

Atom-type designations in atom-pairs have constitutional, topological, and electronic character - atoms of the same type share atomic identity, the same number of non-hydrogen bonding partners, and the same number of bonding n electrons (Fig. 13.1-4(b)). Because of this representation, molecules tend to have many, fewer than the theoretical maximum possible, atom-pairs (1/2 [n (n—l)]2 for a molecule with n atoms), both by virtue of having multiple atoms of the same type, and because the order of the two atoms appearance within an atom-pair is not important (Fig. 13.1-4(c)). A very significant aspect of the definition of atom-types is that Carhart et al. provided both a distance metric and a normalized similarity score for molecules based on the atom-pair definition (Fig. 13.1-4(d)). Formally, such a provision is a requirement for any metric descriptor space intended to afford a basis for comparison of molecular... 

It is essential to stress that our present theoretical knowledge about the electronic properties of the nucleic acids and their constituents arises from many calculations carried out by different techniques, and that although the exact values of the calculated electronic indices depend, of course, on the particular method utilized, the comparison of the different results clearly shows that they all lead to substantially the same conelusions in so far as the general aspects of the electronic structure are concerned. In particular, the calculations make possible, naturally, the determination of the sites at which the different indices have their most significant values. This is, frequently, one of the most important aspects of the results, because this determination enables one to locate the essential sites associated with the eorresponding physico-chemical properties of the nucleic acids. Now, all the methods so far utilized for quantum-mechanical calculations on the nucleic bases or base pairs definitely lead to practically the same conclusions in this respect. These... 

It is generally believed that the bonding in O2 is correctly described in terms of molecular orbitals. However, this method, as traditionally formulated, argues the involvement of px, Py and p orbitals, which have no physical meaning, and the bond order of dioxygen remains a mystery. This is the most glaring, but by no means the only, failure of the electron-pair definition of bond order. 

The purpose of permutation is the same as with the EFT, namely the bisect of the data sequence progressively until data pairs are reached. By definition whenA = 2... 

By using the determinant fomi of the electronic wave functions, it is readily shown that a phase-inverting reaction is one in which an even number of election pairs are exchanged, while in a phase-preserving reaction, an odd number of electron pairs are exchanged. This holds for Htickel-type reactions, and is demonstrated in Appendix A. For a definition of Hilckel and Mbbius-type reactions, see Section III. 

Finally, the distinction between Huckel and Mobius systems is considered. The above definitions are valid for Hiickel-type reactions. For aromatic Mobius-type reations, the reverse holds An ATS is formed when an even number of electron pairs is re-paired. 

We have seen that a base can be defined as combining with a proton and, therefore, requires at least one lone pair of electrons. A more general definition of acids and bases, due to G. N. Lewis, describes a base as any species (atom, ion or molecule) which can donate an electron pair, and an acid as any species which can accept an electron pair— more simply, a base is an electron-pair donor, an acid an electron-pair acceptor. Some examples of Lewis acids and bases are ... 

Two other atomic properties have been used in the definition of atom type, thereby increasing its fuzziness relative to that in the ap and tt descriptors - atomic log P contribution (yielding hydrophobic pairs, hps, and torsions, hts) and partial atomic charges (charge pairs, cps, and charge torsions, cts). 

Carhart R E, D H Smith and R Venkataraghavan 1985. Atom Pairs as Molecular Features in Structure Activity Studies Definition and Applications. Journal of Chemical Information and Computer Scienc 25 64-73. 

The Lewis definitions of acids and bases provide for a more general view of acid-base reactions than either the Arrhenius or Br0nsted-Lowry pic ture A Lewis acid is an electron pair acceptor A Lewis base is an electron pair donor The Lewis approach incorporates the Br0nsted-Lowry approach as a subcategory m which the atom that accepts the electron pair m the Lewis acid is a proton... 

Achiral (Section 7 1) Opposite of chiral An achiral object is supenmposable on its mirror image Acid According to the Arrhenius definition (Section 1 12) a substance that ionizes in water to produce protons Accord mg to the Br0nsted-Lowry definition (Section 1 13) a sub stance that donates a proton to some other substance According to the Lewis definition (Section 1 17) an electron pair acceptor... 

On the assumption that the pairs of electrons in the valency shell of a bonded atom in a molecule are arranged in a definite way which depends on the number of electron pairs (coordination number), the geometrical arrangement or shape of molecules may be predicted. A multiple bond is regarded as equivalent to a single bond as far as molecular shape is concerned. 

Equation 54 implies that U is a function of S and P, a choice of variables that is not always convenient. Alternative fundamental property relations may be formulated in which other pairs of variables appear. They are found systematically through Legendre transformations (1,2), which lead to the following definitions for the enthalpy, H, Hehnholt2 energy,, and Gibbs energy, G ... 

NMR data for 4-methyloxazole have been compared with those of 4-methylthiazole the data clearly show that the ring protons in each are shielded. In a comprehensive study of a range of oxazoles. Brown and Ghosh also reported NMR data but based a discussion of resonance stabilization on pK and UV spectral data (69JCS(B)270). The weak basicity of oxazole (pX a 0.8) relative to 1-methylimidazole (pK 7.44) and thiazole (pK 2.44) demonstrates that delocalization of the oxygen lone pair, which would have a base-strengthening effect on the nitrogen atom, is not extensive. It must be concluded that not only the experimental measurement but also the very definition of aromaticity in the azole series is as yet poorly quantified. Nevertheless, its importance in the interpretation of reactivity is enormous. 

Another fundamental property of chemical bonds is polarity. In general, it is to be expected that the distribution of the pair of electrons in a covalent bond will favor one of the two atoms. The tendency of an atom to attract electrons is called electronegativity. There are a number of different approaches to assigning electronegativity, and most are numerically scaled to a definition originally proposed by Pauling. Part A of Table 1.6... 

The first two are very soluble in water but the last is less so. Weaker bases inelude ammonium hydroxide where X is NH4. In faet every aeid ean generate a base by loss of a proton and the definition now ineludes any eompound eapable of donating eleetron pairs, e.g. amines. Bases turn litmus paper blue and show eharaeteristie effeets on other indieators. They are soluble in water, tarnish in air, and in eoneentrated form are eorrosive to the toueh. Common examples are given in Table 3.5. ... 

According to Eddy, as quoted by Small, the analgesic action of neopine, n omorphine, 6-acetylneomorphine or 3 6-diacetylneomorphine (p. 218) is definitely less than that of morphine and its corresponding analogues. The first two are about half as toxic as codeine and morphine respectively, and the second pair are more toxic than their morphine analogues. None of the four shows the Straub reaction and the convulsant action is less marked. 

Although the word balanced is ambiguous and not definitive, the common meaning for a balanced laminate is a laminate in which all equal-thickness laminae at angles 0 other than 0° and 90° to the reference axis occur only in 0 pairs. The individual -n O and - 0 layers are not necessarily adjacent to each other. Note also that balanced laminates are required to be symmetric about the laminate middle surface, so there must be two + Q laminae and two - 0 laminae for each 0 pair. The behavioral characteristics of a balanced laminate are that shear-... 

To illustrate the relationship between the microscopic structure and experimentally accessible information, we compute pseudo-experimental solvation-force curves F h)/R [see Eq. (22)] as they would be determined in SEA experiments from computer-simulation data for T z [see Eqs. (93), (94), (97)]. Numerical values indicated by an asterisk are given in the customary dimensionless (i.e., reduced) units (see [33,75,78] for definitions in various model systems). Results are correlated with the microscopic structure of a thin film confined between plane parallel substrates separated by a distance = h. Here the focus is specifically on a simple fluid in which the interaction between a pair of film molecules is governed by the Lennard-Jones (12,6) potential [33,58,59,77,79-84]. A confined simple fluid serves as a suitable model for approximately spherical OMCTS molecules confined... 

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