I have recently read about the Z topological index found by Hosoya. The papers/sources The Z index is the sum of the coefficients of the matching polynomials (no. of ways to choose an independent edge from the set of edges in the graph - if we let each carbon atom be a vertex and each bond between a pair of carbon atoms is an edge.). The paper states that in particular for alkanes or saturated hydrocarbons, there is a correlation found between the Z index (which measures the extent of branching - in what way??) and the boiling point (bp) of these alkanes.
I know that the bp of alkanes increases with its size (due to strong force of attractions). However, I am confused about how the Z index measures the extent of branching, and any underlying reasons that the Z index will correlate with the bp and other physiochemical properties.
The papers I have read so far fail to enlighten me on these questions above. Please help. Thank you.
Some ideas I have so far: Since Z index measures branching (or at least correlates to branching since more compacted graph will have a smaller sum of matching coefficient?), and branching correlates to the bp of alkanes (due to less surface area sometimes if you have more branching), perhaps thats why the Z index correlates to bp...