Ab initio cluster calculations of silica surface sites

Silica surface sites, which can be formed in cleavage processes, and their hydrolyzed counterparts are investigated with ab initio cluster calculations. Natural Bond Orbital (NBO) theory is used to characterize bonding around silica surface sites. Higher energy lone pairs of electrons on oxygen atoms either hyperconjugate to vicinal silanol/siloxane antibonding orbitals or backdonate electron density via donor–acceptor π-type bonding with participation of pd or p hybrids on silicon atoms. Upon substitution of hydroxyl groups of orthosilicic acid with silica monomers the strength of siloxane and silanol Si–O bonding increases as energies of bonding orbitals and contributions from p-orbitals decrease. Silanone sites and a complementary pair of silyl/siloxy radical sites are found to be the most stable geminal and single non-hydrolyzed sites, respectively. Atomic charges based on natural wavefunctions and on fitting to electrostatic potential, and characteristic bands of IR spectra associated with siloxane and silanol stretching vibrations of silica surface sites are reported.