On structural ordering and branching of acyclic saturated hydrocarbons

We consider the problem of ordering acyclic graphs corresponding to saturated hydrocarbons and the problem of defining molecular branching. In particular, we consider the recently introduced graph theoretical invariant λλ1, as the basis for the ordering of graphs and the definition of molecular branching. λλ1 is defined as the leading eigenvalue of the path matrix in which the element (i,j) represents the leading eigenvalue for the path connecting vertex i and vertex j. Among 35 nonane isomers and 75 decane isomers no two structures have the same λλ1. Normalized leading eigenvalue λλ1/n is suggested as the index of molecular branching. For smaller alkanes the new index parallels previously suggested indices of molecular branching yet it shows no degeneracy that limited the use of earlier descriptors of molecular branching.