Gronert Model
An alternative model has been developed by S. Gronert based not on breaking molecules into fragments but based on 1,2 and 1,3 interactions
The Gronert equation reads: 
The pentanes are now calculated as:
- n-pentane = 4CC + 12CH + 9HCH + 18HCC + 3CCC + (5C + 12H) = - 35.1 kcal/mol
- isopentane = 4CC + 12CH + 10HCH + 16HCC + 4CCC + (5C + 12H) = - 36.7 kcal/mol
- neopentane = 4CC + 12CH + 12HCH + 12HCC + 6CCC + (5C + 12H) = -40.1 kcal/mol
Key in this treatment is the introduction of 1,3-repulsive and destabilizing interactions and this type of steric hindrance should exist considering the molecular geometry of simple alkanes. In methane the distance between the hydrogen atoms is 1.8 angstrom but the combined van der Waals radii of hydrogen are 2.4 angstrom implying steric hindrance. Also in propane the methyl to methyl distance is 2.5 angstrom whereas the combined van der Waals radii are much larger (4 angstrom).
In the Gronert model these repulsive 1,3 interactions account for trends in bond dissociation energies which for example decrease going from methane to ethane to isopropane to neopentane. In this model the homolysis of a C-H bond releases strain energy in the alkane. In traditional bonding models the driving force is the ability of alkyl groups to donate electrons to the newly formed free radical carbon.
Read more about this topic: Heat Of Formation Group Additivity
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