The cyclacene tubes T(R, r) consisting of the Huckel type intermolecularly unioned (r = 6) cyclacenes having R arenoid rings (R = 3 - 6) and a single silaannulenic periphery are considered for AM1 type semiempirical molecular orbital calculations. The cases in which the silaannulenic periphery occupies different locations (layers A-D from top to middle) along the tube axis are investigated. It has been found that doping of silicon atoms in the top (or bottom) periphery of the cyclacene tubes has a stabilizing effect irrespective of 4m or 4m + 2 type character and thus no cryptoannulenic effect is exhibited. The silicon doping in every case is characterized with endothermic heats of formation with local minima for tubes with R = 4. (C) 2000 Elsevier Science Ltd. All rights reserved.