First experimental characterization of aminocarbene

Directed Gas-Phase Formation of Aminosilylene (HSiNH2; 1A'): The Simplest Silicon Analogue of an Aminocarbene, under Single-Collision Conditions

The aminosilylene molecule (HSiNH₂, X¹A')-the simplest representative of an unsaturated nitrogen-silylene-has been formed under single collision conditions via the gas phase elementary reaction involving the silylidyne radical (SiH) and ammonia (NH₃). The reaction is initiated by the barrierless addition of the silylidyne radical to the nonbonding electron pair of nitrogen forming an HSiNH₃ collision complex, which then undergoes unimolecular decomposition to aminosilylene (HSiNH₂) via atomic hydrogen loss from the nitrogen atom. Compared to the isovalent aminomethylene carbene (HCNH₂, X¹A'), by replacing a single carbon atom with silicon, a profound effect on the stability and chemical bonding of the isovalent methanimine (H₂CNH)-aminomethylene (HNCH₂) and aminosilylene (HSiNH₂)-silanimine (H₂SiNH) isomer pairs is shown; i.e., thermodynamical stabilities of the carbene versus silylene are reversed by 220 kJ mol^-1. Hence, the isovalency of the main group XIV element silicon was found to exhibit little similarities with the atomic carbon revealing a remarkable effect not only on the reactivity but also on the thermochemistry and chemical bonding.

Ammonia elimination from protonated nucleobases and related synthetic substrates

The results are reported of mass-spectrometric studies of the nucleobases adenine 1h (1, R = H), guanine 2h, and cytosine 3h. The protonated nucleobases are generated by electrospray ionization of adenosine 1r (1, R = ribose), guanosine 2r, and deoxycytidine 3d (3, R = deoxyribose) and their fragmentations were studied with tandem mass spectrometry. In contrast to previous EI-MS studies of the nucleobases, NH(3) elimination does present a major path for the fragmentations of the ions [1h + H](+), [2h + H](+), and [3h + H](+). The ion [2h + H - NH(3)](+) also was generated from the acyclic precursor 5-cyanoamino-4-oxomethylene-dihydroimidazole 13h and from the thioether derivative 14h of 2h (NH(2) replaced by MeS). The analyses of the modes of initial fragmentation is supported by density functional theoretical studies. Conjugate acids 15-55 were studied to determine site preferences for the protonations of 1h, 2h, 3h, 13h, and 14h. The proton affinity of the amino group hardly ever is the substrate's best protonation site, and possible mechanisms for NH(3) elimination are discussed in which the amino group serves as the dissociative protonation site. The results provide semi-direct experimental evidence for the existence of the pyrimidine ring-opened cations that we had proposed on the basis of theoretical studies as intermediates in nitrosative nucleobase deamination.