Spectral moments of phenylenes

In a series of publications Estrada (Estrada, E. J. Chem. Inf. Comput. Sci. 1996, 36, 844-849; 1997 37, 320-328; 1998, 38, 23-27) employed spectral mo ments of line graphs in QSPR and OSAR relationship studies of various class es of compounds. A recent paper (Markovic, S.; Gutman, I. J. Chem. Inf. Com put. Sci. 1999, 39, 289-293) reported that in QSPR and QSAR investigations of benzenoid hydrocarbons based on linear combination of spectral moments, it made no difference whether one used spectral moments of the molecular gr aph or those of the line graph. In the present work spectral moments of mol ecular graphs (M-k) and line graphs (mu (k)) of phenylenes are considered. The first few M-k's and mu (k)'s of phenylenes are dependent on identical s tructural parameters. It is proved that the two sets of moments of phenylen es are linearly dependent. It is also shown that in the case of the heat of formation of phenylenes there is no advantage in using lower spectral mome nts of line graphs instead of lower spectral moments of molecular graphs. I n this way the redundancy observed in the case of benzenoid hydrocarbons is also shown to exist in the class of phenylenes.