Force constants have been evaluated by the WILSON group theoretical method for various ij3 molecules possessing a planar trigonal symmetry. Mean-square amplitude quantities and mean amplitudes of vibration for the bonded as well as nonbonded atom pairs have been computed at the temperatures T=0 and T=298 °K for C3 , Si3 , and XeF2 by the CYVIN method. The molar thermodynamic functions from spectroscopic data have been calculated for Si3 and XeF2 on the basis of a rigid rotator, harmonic oscillator model. BASTIANSEN-MORINO shrinkage effects have been calculated for various linear asymmetrical ijk, linear symmetrical ij2 and tetrahedral ij4 molecules at the temperatures T=0 and T=298 °K.