Bond increment calculation

In contrast to the point charge model, which needs atom-centered charges from an external source (because of the geometry dependence of the charge distribution they cannot be parameterized and are often pre-calculated by quantum mechanics), the relatively few different bond dipoles are parameterized. An elegant way to calculate charges is by the use of so-called bond increments (Eq. (26)), which are defined as the charge contribution of each atom j bound to atom i. 

The Values of Gibbs Potential (—AG(298 K)) (kj mol-1) for Hydrogen Bonding ArOH Calculated by Equation (15.9), for Increments see Handbook [4]... 

Bondi and Simkin presented a similar calculation for liquids, using the heat of vaporization (AHv) (241, 242). The purpose was to obtain an estimate of but in the process a value of the H bond increment, 5(OH) is derived. This increment is .. . a measure of (but not identical with) the heat of formation of the H bond,. . Values for 5(OH) are very similar to enthalpy values, though they are not intended as thermodynamic functions. Bondi gives an extensive discussion of their use with simple and polyhydric alcohols and a few other compound classes. Table 7-V compares 6(OH) and AH values. 

Considering that the calculated structures and the experimentally investigated compounds differ considerably, the agreement of the bonding increments is quite satisfying. Table 1 contains the calculated and the experimental data for the two pairs of compounds. As the 2,3-dihydro-2-methyl-... 

Tab. 3.1 Experimental and theoretically predicted lattice parameters (A) for CaO in the [ZnS]/[NaCI]/[CsCI] structure types derived from GGA calculations, ionic radii tables, bond-valence calculations, and volume increments.

The heat of combustion of Ge(C4Hg-i)4, AHcomb —2755 1.2 kcal/mol, has been compared with a calculated value (—2756.0 kcal/mol) based on bond increment data [6]. 

The heats of formation of some linear alkanes calculated by the bond increment method are shown in Table 1.4. As an example of such calculations, let us determine the AHf values for methane and ethane. For methane, there... 

Calculations are based on bond increment values in Table 1.5. 

One way to describe the extent to which heats of formation depend on bonding patterns is to consider an isodesmic reaction—a reaction in which both the reactants and the products have the same number of bonds of a given type, even though there may be changes in the relationship of one bond to another. For example, consider the hypothetical conversion of n-hexane to 2,2-dimethylbutane. Both the reactant and the product have five C-C and fourteen C-H bonds. The simple bond increment approach would calculate that the heat of the reaction should be 0, but the data in Table 1.6 indicate that the heat of the reaction should be -4.4kcal/mol. Therefore, the heat of an isodesmic reaction is an indication of deviation from the additivity of bond energies. " ... 

These examples only hint at the analysis of heats of formation of organic compounds that is possible. Benson and co-workers summarized the methods and data for calculations for the major functional groups in organic chemistry. ° In addition, the data allow calculation of heat capacities and entropies of these compounds in the same marmer in which heats of formation are determined. Heats of formation are valuable reference points in discussing the stabilities of various isomers or products of reactions, whether they are calculated by bond increments or group increments or are derived as part of a theoretical calculation. 

As indicated, the heat of formation (HFO) is taken as the sum of three terms. The first is the steric energy (E). The second is the bond enthalpy (BE), which is calculated from normal bond increments. The third is a partition function increment (PFC), which is itself the sum of three terms a conformational population increment (POP), a torsional contribution term (TOR), and a translation / rotation term (T / R). Since the molecular mechanics calculation has considered only one conformation, the POP and TOR terms are defined to be zero for this calculation (but see below). The T/R term is a molecular translational and rotational term that is always taken to be 2.4kcal/mol at room temperature, since even an anfi-butane molecule that is not rotating about the C2-C3 bond will still be undergoing translation and rotation of the entire molecule as a unit. Therefore, HFO is the total of the bond increment contributions (BE = —35.08) plus the steric energy (E = 2.17) plus the partition function increment (PFC = 2.4). The sum, -30.51 kcal/mol, is slightly lower than the literature value of -30.15 kcal/mol. ... 

We can also calculate a strainless heat of formation of anfi-butane, which is the heat of formation of an anti structure made up of hypothetical strainless bonding interactions. As shown, we add the strainless bond increments to obtain the value of -32.92 kcal/mol for the strainless bond enthalpy (SBE). This value must also be corrected for the T / R term, but the POP and TOR terms are again taken to be zero. Thus, the strainless AH°f (hfs) is the sum of —32.92 and + 2.40, which is —30.52 kcal/mol. That differs from the heat of formation calculated with normal bond increments (—30.51 kcal/mol) by 0.01 kcal/mol, which is the inherent strain energy of anti-butane. ... 

Moreover, their exaltation A. the difference between the true diamagnetic susceptibility %u and the one calculated by an additive scheme using atom and bond increments xw... 

An alternative to improving atomic/ionic refractions was to express molecular refraction through bond increments. A system of bond refractions is definitely superior to the system of atomic refractions, as it allows to account for chemical interactions explicitly. The concept of bond refraction was introduced by Bachinskii [183] who suggested that the molar refraction (as well as volumes, heats of combustion, etc) of organic compounds can be calculated of bond increments. According to Bachinskii, Rc-c =l/4f c + l/4 c. = l/47Jc + etc. This method is not quite con-... 

It was found that structures with up to 14 rotatable bonds are readily handled using the rule-based conformational analysis 2 (conjugated and double bonds), 3 (single bonds), and 4 (sp -sp bonds) rotamers per bond were calculated in the databases for such structures. The default for alpha (sp -sp ) bonds in Chem-X is a 6-point analysis, but 4 points can offer a large gain in time without necessarily a reduction in yield. The number of points is based on the increment of rotation about a particular bond and equals 360° per rotational increment. Thus a 60° increment would equal a 6-point bond. For a corporate database of 62,000 company compounds, a total of 29 million conformations were accepted by the rules from a total of 912 million considered... 

The magnetic exaltation is defined as the difference between the true mean molar magnetic susceptibility and the one calculated by a hypothetical, additive incremental scheme by use of atom and bond increments according to the... 

The CFF and MMFF force fields use charges calculated from the charge bond increments, as shown in equation (12)... 

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