Barreiras de energia para os movimentos de inversão e rotação das moléculas metilamina, dimetilamina e trimetilamina foram calculadas usando a metodologia CCSD(T)//B3LYP com os conjuntos de funções de base de Dunning, cc-pVTZ, cc-pVQZ e cc-pCVTZ. O procedimento de extrapolação para o conjunto de base completa (complete basis set, CBS) e os efeitos da correlação dos elétrons do caroço também foram incluídos nos cálculos das barreiras de energia. Nossos melhores resultados estão em excelente concordâncias com os dados experimentais, indicando que a metodologia utilizada é recomendada para a predição exata de propriedades estruturais e de barreiras de energia de outros sistemas moleculares.Barrier heights of the internal rotation and inversion motions of methylamine, dimethylamine and trimethylamine molecules were calculated with the CCSD(T)//B3LYP methodology in combination with the cc-pVTZ, cc-pVQZ, and cc-pCVTZ basis sets of Dunning. The complete basis set (CBS) extrapolation procedure and core-valence (CV) correlation effects are also examined to the barrier heights. Our best estimate results (CCSD(T)/CBS+CV//B3LYP/cc-pVQZ) are in very good agreement with the experimental data, indicating the use of this methodology to provide accurate predictions of structures and barrier heights for other systems. Keywords: energy barriers, methylamine, dimethylamine, trimethylamine, CCSD//B3LYP IntroductionThe methyl-substituted amines, methylamine (CH 3 NH 2 ), dimethylamine [(CH 3 ) 2 NH], and trimethylamine [(CH 3 ) 3 N] form an interesting group of molecules which are important in organic syntheses and biological processes, as well as they are efficient corrosion inhibitors of aluminum. Beyond their technological importance, these species are rich in structural features presenting large amplitude internal motions, rotation of the methyl groups and internal inversion of the amino group.Methylamine has been extensively studied by experimentalists in which its internal rotation and inversion barrier heights were determined using microwave (MW), infrared (IR) and electron diffraction (ED) experiments. [1][2][3][4][5][6][7] Methylamine is considered a small molecule from the theoretical point of view, therefore it is appropriate to be studied using high level theoretical quantum chemical methodologies, having received many contributions. [7][8][9][10][11][12][13][14] Lee et al. 8 investigated the origin of the structural stability of methylamine concluding that the stereo electronic effect is the major interaction force affecting its stability. Csonka and Sztraka 9 using density functional and post-HartreeFock methodologies showed that the density functional BP86/6-311G(d) method provides reliable results for geometry and vibrational frequencies, as compared with the experimental results. Smeyers and Villa 10 discussed the influence of the zero-point energy correction to the rotation and inversion motions based on the MP2 results, and During and Zheng 7 calculated these motions barriers using B3LYP and MP2 methods. Recently,...