Effects of the Nature of the C(1) Substituent on the Conformational Equilibrium of (E)-derivatives of 3-Methoxypropene: Determination of the origin of the phenomenon using energy partitioning techniques Intermolecular interactions taking place in the compounds of the general formula (E)-CH3OCH2CHCHX were analyzed in terms of one- and two-center interaction energies within the framework of SCF ab initio (STO-3G), Extended Hückel and CNDO methods. The results confirmed the important role of unoccupied orbitals in the stabilization of the different conformers.