Are there structurally relevant attractive interactions between nitrogen atoms and group 14 elements in their aminomethyl compounds?

The three compounds H3SiCH2NMe2, Me8GeCH2NMe2, and Me3SnCH2NMe2 have been p repared for the purpose of the determination of their molecular structures. The gas-phase structure of H3SiCH2NMe2 has been established by electron di ffraction (GED) and ab initio calculations up to the MP2/6-311G** level of theory. The geometry of Me3GeCH2NMe2 could be studied only by theoretical m ethods, while the structure of Me3SnCH2NMe2 has been determined by single-c rystal X-ray diffraction (XRD) and computational methods. The E-C-N units ( E = Si, Ge, Sn) in all three compounds adopt angles that are larger than th e tetrahedral angle (H3SiCH2NMe2 GED 114.7(3)degrees, MP2/6-311G** 111.4 de grees; Me3GeCH2NMe2 SCF/6-31G* 116.1 degrees; Me3SnCH2NMe2 XRD 113.0(2)degr ees, SCF/dzp 115.4 degrees), and all three compounds adopt gauche conformat ions; that is, the lone pairs at nitrogen are declined away from the plane E-C-N. These facts are interpreted in terms of the absence of an earlier po stulated alpha-effect, which should lead to an attractive interaction betwe en E and N centers. The structures, energies, and orbital interactions in t he natural bond orbital (NBO) model for three possible conformations of the SiCH2NC2 skeleton (lone pairs at N vs Si atom) of H3SiCH2NMe2 have been ca lculated ab initio and show the gauche conformer to be the ground state and the syn form (+6.6 kJ mol(-1)) as well as the anti form (+10.2 kJ mol(-1)) to be transition states to rotation about the N-C(H-2) bond. The NBO calcu lations do not confirm the earlier postulated lp(N) --> d(E), sigma(NC) --> d(E), or lp(N) --> sigma*(EX) interactions as important contributors to el ectron delocalization. Calculations on the model system FH2SiCH2NMe2 predic t a tendency of SiCN units to adopt small bond angles if very electronegati ve groups are bound to silicon. The conformational preferences of the unsub stituted H3SiCH2NMe2 are overridden in favor of a conformation bringing the lone pair of electrons closer to the electrophilic Si center. This compoun d is predicted to have significant lp(N) --> sigma*(SiF) interactions.