Additivity bond contribution

Enthalpies and other thermodynamic properties can be estimated by generalized methods based on the theory of corresponding states or additive bond contributions."... 

Therefore, rr-acceptor versus (T-donor properties of CS and CSe change more drastically than those of CO. In thiocarbonyl complexes additional bonding contribution comes from In orbitals. 

Atomistically detailed models account for all atoms. The force field contains additive contributions specified in tenns of bond lengtlis, bond angles, torsional angles and possible crosstenns. It also includes non-bonded contributions as tire sum of van der Waals interactions, often described by Lennard-Jones potentials, and Coulomb interactions. Atomistic simulations are successfully used to predict tire transport properties of small molecules in glassy polymers, to calculate elastic moduli and to study plastic defonnation and local motion in quasi-static simulations [fy7, ( ]. The atomistic models are also useful to interiDret scattering data [fyl] and NMR measurements [70] in tenns of local order. 

The next higher order of approximation, the first-order approximation, is obtained by estimating molecular properties by the additivity of bond contributions. In the following, we will concentrate on thermochemical properties only. 

In order to develop a quantitative interpretation of the effects contributing to heats of atomization, we will introduce other schemes that have been advocated for estimating heats of formation and heats of atomization. We will discuss two schemes and illustrate them with the example of alkanes. Laidler [11] modified a bond additivity scheme by using different bond contributions for C-H bonds, depending on whether hydrogen is bonded to a primary (F(C-H)p), secondary ( (C-H)g), or tertiary ( (C-H)t) carbon atom. Thus, in effect, Laidler also used four different kinds of structure elements to estimate heats of formation of alkanes, in agreement with the four different groups used by Benson. 

The accuracy of an additivity scheme can be increased by going from atomic contributions through bond contributions to group contributions. 

In contrast to chloride compounds, niobium oxides have a VEC of 14 electrons, due to an overall anti-bonding character of the a2u state, caused by a stronger Nb-O anti-bonding contribution. In some cases, the VEC cannot be determined unambiguously due to the uncertainty in the electron distribution between the clusters and additional niobium atoms present in the majority of the structures. The 14-electron compounds exhibit semiconducting properties and weak temperature-independent paramagnetism. Unlike niobium chlorides, the oxides do not exhibit a correlation between the electronic configuration and intra-cluster bond distances. 

Extended anionic partial structures of mercury occur in some high melting amalgams (MHg2 and related examples) with medium Hg content. Significant ionic bonding contributions between M and Hg in addition to covalent Hg-Hg contributions can be assumed to be responsible for the properties of these solids. 

The bond additivity rule is the first-order formalism to estimate thermochemistry of stable molecules. Bond contributions to thermochemical properties for some select bonds are given in Table II. The application of this... 

Since then, groups have been derived for heats of formation of solid nitroaromatics (Ref 20) and for solid and liquid nitroalkanes (Ref 22) Group Additivity. The basic idea behind Group Additivity is that chemical thermodynamic properties of molecules consist of contributions from the individual groups that make up the molecule. Group Additivity is therefore an extension of the series atom additivity, bond additivity,. . . , and turns out to be an excellent compromise between simplicity and accuracy... 

If the radical were restricted to resonance between the KekulA structures A and B, with the free valence on the methyl carbon, resonance would stabilize the radicals to just the same extent as the undissociated molecules, which would then have only the same tendency to dissociate as a hexaalkylethane. But actually the five structures A, B, C, D and E (each with three double bonds) contribute about equally to the structure of the radical, which thus resonates among five structures instead of two and is correspondingly stabilized by the additional resonance energy. 

As an approximation the different structures may be assumed to have the same energy, and hence to contribute equally to the normal state of the molecule. In going from structure A to structure for example, one 3d electron is promoted o a 4p arbitral, and an additional bond is formed tiro Fe—C bond formed, one C=C converted to C—C) the bond energy roughly cancels the promotion energy. We may therefore evaluate the properties of the molecule by averaging over the 560 structures A to K, with equal weights. 

For alkanes, the logarithm of viscosity has been correlated with atomic and with bond contributions to estimate tj at 0 and 20°C [13]. Considering a broader range of structural variety, neither the viscosity nor its logarithm is a constitutionally additive property. Application of the group contribution approach is based on additive parameters that allow viscosity estimations in combination with other experimental data such as density or vapor pressure. The viscosity-constitutional constant, /vc, is such an additive parameter ... 

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