Molecular structure of tris(cyclopropylsilyl)amine as determined by gas electron diffraction and quantum-chemical calculations

The molecular structure and conformation of tris(cyclopropylsilyl)amine (TCPSA) has been studied by means of gas-phase electron diffraction at 338 K and quantum-chemical calculations. A total of 12 relatively stable conformations of TCPSA molecule were considered. According to the experimental results and the DFT calculations the most stable conformer corresponds to a configuration (according to the Prelog–Klyne notation) of the type (−ac)(−ac)(+ac)-(−ac)(−ac)(+ac), where the first three parentheses describe the three different Si–N–Si–C torsional angles and the latter ones depict the rotation of the three cyclopropyl groups about the Cring–Si axes, respectively. The quantum-mechanical calculations were performed using various density functional (B3LYP, X3LYP and O3LYP) and perturbation MP2 methods in combination with double- and triple-ζ basis sets plus polarization and diffuse functions. The most important experimental geometrical parameters of TCPSA (ra Å, ∠h1 degrees) are: (Si–N)av = 1.741(3), (Si–C)av = 1.866(4), (C–C)av = 1.510(3), (C–C(Si))av = 1.535(3), (N–Si–C)av = 115.1(18)°. For the purpose of comparison and searching for reasons leading to the planarity of the Si3N moiety in trisilylated amines we carried out NBO analysis and optimized the geometries of numerous silylamines. Among these compounds was tris(allylsilyl)amine (TASA), which is isovalent and isoelectronic to TCPSA. Utilizing the structural results we obtained we could show that Si+⋯Si+ electrostatic repulsive interaction is predominantly responsible for the planarity of the Si3N skeleton in TCPSA and in all other trisilylamines we considered. We also found that regardless the size and partial charges of the substituents the Si–N–Si bond angle in various disilylamines amounts to 130 ± 2°.