2.3.1 Benson Rules

The key to a Benson rule is the simple description of an atom and the bonding structure around it. The general form is:

X-(Y1)(Y2)(Y3)...(Yn)

Where X is the center atom and the Y's are a description of the atoms (including multiplicity) bonded to the center atom. Associated with each Benson rule is a set of thermodynamic quantities, namely standard enthalpy, standard entropy and heat capacities at several temperatures. The Benson rule represents how the center atom contributes to the thermodynamics of the molecule as a whole. The contributions of all the center atoms (all the atoms in the molecule except for hydrogens) are added to give the total thermodynamics of the molecule. It is the heart of the additivity concept.

The atom descriptions of the center atoms and the connected atoms can more than just the simple atomic description of the molecule. The descriptions can also describe the valency of the atom. For example, the carbon atom descriptions can be C_s, C_d, C_t or C_a for carbons which are singly, doubly, triply or aromatic bonded, respectively. In additional, a meta-atom can be defined representing a functional group such as, for example, carboxyl represented as a single atom called co. For example, an aldehyde carbon bonded to a singly bonded carbon (and the hydrogen of the aldehyde) is represented as:

CO-(Cs)(H)

The description of the Y's in equation 1 have also this extended description supplemented by how many times (a slash followed by the multiplicity) that atom is bonded to the center atom. For example the Benson structure for a primary carbon is represented as:

Cs-(C)-(H)/3

As described previously applying Benson rules consists of identifying all the center atoms (in the form shown in equation 1) and adding up all their contributions. The JTHERGAS system has a library consisting of 366 Benson rules, the majority of which correspond to the tables in Benson's book:

1. Table A.1: Hydrocarbons

2. Table A.2 Oxygen-containing Compounds

3. Table A.3 Nitrogen-containing Compounds

4. Table A.4 Halogen-containing Compounds

5. Table A.5 Sulfur-containing Compounds

6. Table A.6 Organometallic, Organophosphorous, Compounds

In order to implement these rules in a general way, two types of meta-atoms have been defined. The first have to do with the types of carbon bonding, namely, double bonded carbon (C_d), triple bonded carbon (C_t), carbon on a benzene ring (C_B), allenic carbon (C_a) and carbon on a fused ring (C_bf). The second type of meta-atom have multiple atoms, namely, CO, NO₂ , CN, SO, SO₂ , CS, and PO. Associated with each of these atom and meta-atom types is a molecular structure with wild-card atoms. Before the application of the Benson rule, the set of Benson rule meta-atoms are substituted into the structure. The atom type of the atom changes. For example, for double-bonded carbon, the atom type is changed from simple C to C_d.

For multiple atom meta-atoms, the entire group is substituted. For example, the meta-atom CO is bonded to two ligands at the C. Upon substitution, the CO atoms are eliminated and the two ligands are connected with a new atom type labeled 'CO'. In general, within the meta-atom, the distinctly specified atoms are eliminated leaving the bonding to the wild-card atoms.

These meta-atoms are defined within the database. New meta-atom substitutions can be easily implemented by updating the database (for example within the graphical interface). The core JAVA library does not have to be modified by a software specialist.