Ab Initio and Density Functional Study of Complexes between the Methylamines and Water

We present results of ab initio and density functional studies of the interaction between the methylamines (methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA)) and one or two water molecules. The structures and energetics of the hydrogen-bonded complexes of MA and DMA with one and two water molecules and of TMA with one water molecule are reported at three levels of theory: MP2/6-31+G(d,p), B3LYP/6-31+G(d,p), and B3LYP/6-311+G(2d,p). All three levels of theory give ZPE and BSSE corrected binding energies of 20−22 kJ mol -1 for the 1:1 complexes and 32−40 kJ mol -1 for the 1:2 complexes. The calculated thermodynamics of 1:1 and 1:2 complex formation are used to estimate the adsorption thermochemistry of the amines at the air−water interface. This adsorption is best described by a “critical complex” formed of one amine molecule and two water molecules. The formation of the amine−water complexes in the atmosphere is also briefly discussed. Because of the strong hydrogen bonding possible, such complexes could act as cloud condensation nuclei.