The cluster-Bethe-lattice method is used to investigate SiH bond-bending modes of theHSi-(OSi)3group ina-SiO2:Halloys. When theHSi-(OSi)3group hasC3symmetry, the SiH bond-bending mode has a double degenerate frequency at 876cm−1as found by experiments. But when theHSi-(OSi)3group does not haveC3symmetry, the nonsymmetrical coupling between SiH and its local bonding environment may cause the SiH bond-bending mode to split off and have an additional feature in the spectrum that is not revealed by experiments. ©1997 American Institute of Physics.