Résumé : The effects of substitution of X = C by Si or Ge in X(CH3) 3 moieties attached to the formal double bond of 3,3- dimethylcyclopropene are examined. Regularities in observed trends of vibrational frequencies implicating the moieties containing the X atom, as the X atomic mass is increased, are extrapolated to X = Sn. The results of this extrapolation made it possible to assign the known experimental vibrational frequencies of 3,3-dimethyl-1-(trimethylstannyl)cyclopropene and 3,3-dimethyl-1,2-bis(trimethylstannyl)cyclopropene. © 2004 Elsevier B.V. All rights reserved.