The conformational behaviour of the some methallyl compounds (H2C=C(CH3)CH2X; X=F, Cl, Br, I, OH, OMe, OEt, SH, SMe, SEt, NMe2 and NEt2) has been investigated through MP2 and B3LYP levels of theory using the 6-31G(d,p) and/or 6-311 + G(d,p) basis sets. For certain set of compounds the 3-21G and CEP-31G basis set were used. The calculations indicate the existence of s-cis and gauche conformers, considering the C=C-C-X dihedral angle, and the skew and trans forms with respect to the C-C-X-R dihedral angle. Good agreement is obtained among the relative energies calculated at different levels of theory, except for the oxygen derivatives, and with respect to experimental data from the literature for some compounds. Similar conformational behavior is observed among the analogous methallyl, allyl and propanones derivatives. (C) 2003 Elsevier B.V. All rights reserved.6374354