Colin F. Poole

Descriptors compatible with the calibration models for the WSU-2025 compound descriptor database are assigned for 24 organosilanes of general structure R4Si, 15 linear organosiloxanes of general structure R3SiO(R2SiO)mSiR3 with m = 0-4, 11 cyclic organosiloxaes of general structure (R2SiO)m with m = 3-6, hexamethyldisilazane, hexamethylcyclotriazane, hexamethyldisilthiane, and heptamethylcyclotetrasiloxanol. The R group for organosiloxanes includes hydrogen, chlorine, methyl, vinyl, ethyl, octyl, and trimethylsiloxy. For organosilanes as well as the above substituents the R group includes acetate, methoxy, ethoxy, phenyl, nanofluorohexyl, 3-aminopropyl, 3-isocyantopropyl, 3-glycidoxypropyl, and 3-methacryloxypropyl. Descriptors were assigned using the Solver method from experimental measurements of gas chromatographic retention factors at several temperatures on up to 12 varied stationary phases and liquid-liquid partition constants in a minimum of three totally organic biphasic systems (n-hexane-acetonitrile, n-heptane-N,N-dimethylformamide, and n-heptane-2,2,2-trifluoroethanol). Organosilicon compounds are characterized by weak dispersive interactions in the form of often negative E and small L descriptors compared with organic compounds of a similar size, weak dipole type interactions, S descriptor, except for organosilanes with polar functional groups incorporated in alkyl substituents, an absence of hydrogen-bond acidity, A = 0, except for compounds with -NH-, -NH2 or -OH functional groups (Si-H bond is not hydrogen-bond acidic), and generally relatively large but variable hydrogen-bond basicity, B descriptor, moderated for Si-O compounds by the type of substituents co-bonded to the silicon atom. The solvation parameter model provides mixed results for the estimation of the octanol-water, octanol-air, and water-air partition constants for six volatile organosiloxanes, likely resulting from the uncertainty in their experimental values since the partition constants fall outside the range for convenient laboratory measurements.