The photoelectron (PE) spectra of some 3-monosubstituted 2-methylpropenes H2C=C(CH3)CH2X IX = Cl, Br, I, OH, OMe, OEt, SH, SMe, SEt, N(Me)2 and N(Et)21 have been recorded. A preliminary analysis is presented indicating some trends in the ionization potentials associated with application of OVGF method and NBO analysis from MP2/6-31G(d,p) and cc-pVDZ level of theory indicating that the more effective hyperconjugation effect leads to the most stable conformers. The sensitivity of the outermost ionization energies of selected molecules with respect to the level of theory was analyzed. Application of the CASPT2 method with ANO basis set and geometries from MP2 calculations provided results in excellent agreement with the experimental data. (c) 2008 Elsevier B.V. All rights reserved.34941699263268