Modeling the configuration about the nitrogen atom in methyl- and silyl-substituted amines (original) (raw)

1988, The Journal of Physical Chemistry

Bond lengths, angles, energies, and electron density maps were calculated for a substituted series of four amines, N(CH3)"(SiH3)>,, (n = 0, 1, 2, 3). When n = 0, 1, or 3, the amine crystallizes at about 115 K as a monomer. However, when n = 2, while the molecules in the gas phase are monomeric, at room temperature, the molecules in the crystal phase are pentamers. In the monomeric form for each amine in the series, the average calculated values of LSiNSi, LSiNC, and LCNC tend to be preserved at 120.0°, 117.8', and 112.2O, respectively. The nonbonded cation-cation distances also tend to be preserved. Reaction energies for six double-replacement reactions within the series indicate that the pentamer-forming compound occurs on the less stable side of the calculated reactions. The electronegativity of the nitrogen increases as methyl is substituted for silyl. The lengthening of the bonds associated with the change in electronegativity is ascribed to the relatively large buildup of electron density in the neighborhood of the nitrogen in N(SiH3)3. A unique angle, 0, was defined in terms of line LN making the same angle with each bond such that, for a given amine NRR'R", 0 = LLNR = LLNR' = LLNR". From the deformation maps, each lone pair lies along LN. Finally, the maps show that the bonding electron density peak is located interior to the bond for all nonplanar configurations. The molecule that forms the pentamer shows the largest displacement of the peak off the SiN bond, perhaps representing a pathway for nucleophilic attack.