In this paper, we present a theoretical investigation of structural and vibrational properties of selected gasphase UX3 (X = F, Cl, Br, and I) and U(CH3)(3) molecules by density functional methodologies or with a post Hartree-Fock MP2 perturbative approach. Relativistic scalar corrections have been explicitly included either by a frozen core approximation with a quasi-relativistic treatment (QR) of the valence electron shells or by energy-adjusted large core quasi-relativistic effective core potential (QRECP) scheme. The influence of the size of the core (large core, LC, or small core, SC) as well as of the addition of polarization functions has also been examined on one derivative, i.e., UCl3. MP2/LC-QRECP optimized geometries and vibrational frequencies are found in good agreement with the available estimated or experimental data. Among the different DFT approaches, the best agreement is obtained for DFT/QR computations which reproduce the experimental (or estimated) C-3v molecular structures of all the selected species. In contrast, the DFT/LC-QRECP approaches provide irregular results, strongly dependent on the choice of the functional, Nevertheless, the use of a small core pseudopotential greatly reduces the margin between MP2 and DFT/QRECP calculations. At this level, both "classical" nonlocal gradient corrected and hybrid density functionals provide reasonable results. The only exception concerns the B3LYP functional that is clearly inadequate for an effective treatment of electron correlation in open-shell molecular systems.