ENTHALPIES OF SUBLIMATION AND FORMATION OF POLYCYCLIC AROMATIC-HYDROCARBONS (PAHS) DERIVED FROM COMPARATIVE MOLECULAR-FIELD ANALYSIS (COMFA) - APPLICATION OF MOMENT OF INERTIA FOR MOLECULAR ALIGNMENT

A Quantitative Structure-Property Relationship (QSPR) is developed fro m the 3D-QSAR method CoMFA to estimate the sublimation enthalpy (Delta (sub)H(m)(0) (298.15 K)) and formation enthalpy (Delta(f)H(m)(0) (298. 15 K)) of a series of polycyclic aromatic hydrocarbons (PAHs). A uniqu e feature of the present analysis is the application of moment of iner tia as the basis for aligning the PAH molecules in the data set, which included nonplanar structures. Partial least-squares (PLS) regression with cross validation was employed to correlate each of the two therm odynamic properties with the steric and electrostatic fields surroundi ng each PAH molecule. The resulting CoMFA model for Delta(sub)H(m)(0) (298.15 K) yielded r(2) values of 0.817 (cross-validated) and 0.948 (c onventional) using a training set of 30 PAHs, while that for Delta(f)H (m)(0) (298.15 K) yielded r(2) values of 0.617 (cross-validated) and 0 .998 (conventional) using a training set of 16 PAHs. CoMFA-predicted v alues of Delta(sub)H(m)(0) (298.15 K) for a test set of 5 PAHs agreed within an average 5% of the corresponding experimental values. Contrib utions to the CoMFA models from the 3D steric and electrostatic fields were 93% steric and 7% electrostatic for Delta(sub)H(m)(0) (298.15 K) and 66% steric and 34% electrostatic for Delta(f)H(m)(0) (298.15 K), reflecting the strong correlation between steric bulk as encoded in th e CoMFA steric fields and both of these thermodynamic properties. The present CoMFA models were used to predict the Delta(sub)H(m)(0) (298.1 5 K) and Delta(sub)H(m)(0) (298.15 K) values of several PAHs for which the corresponding experimental values are unavailable.