RESUMEN
State-specific reactions of the potent greenhouse gas SF5CF3 with Cu(+) were carried out in a selected ion drift cell apparatus. Copper ions were prepared in a glow discharge utilizing Ne as the working gas. Analysis of these ions using ion mobility mass spectrometry (IMS) indicated the presence of both Cu(+)(3d(10)) and Cu(+)(3d(9)4s(1)) configurations. Subsequent analysis indicates that the 3d(10) configuration consists of Cu(+)((1)S) exclusively whereas the 3d(9)4s(1)configuration is composed primarily of Cu(+)((3)D) with small contributions from Cu(+)((1)D). State-specific product formation in reactions of these ions with SF5CF3 was determined using IMS along with the known energetic requirements for product formation. These experiments reveal that Cu(+) excited states initiate fragmentation of SF5CF3 to yield SF2(+), SF3(+), SF5(+), and CF3(+), where SF3(+) represents the largest branching fraction at 90% of the total bimolecular product formation. The energetics associated with the formation of these ions suggest that molecular Cu-containing products must also be formed in all cases, indicating that the governing reaction mechanisms are more complicated than simple dissociative charge transfer. Production of SF2(+) and SF3(+) are shown to proceed via Cu(+)((3)D) and can be rationalized with a two-step mechanism proceeding through the common intermediate SF3CF3(+). Production of CF3(+) can be explained using this same mechanism but is also energetically possible from Cu(+)((1)D) in a more direct process. Energetic requirements indicate that Cu(+)((1)D) is the sole source of SF5(+) with concomitant formation of CuCF3. Cu(+)((1)S) exhibits adduct formation exclusively, but IMS spectra of the resulting Cu(+)·SF5CF3 suggest that as many as three association structures are formed.
RESUMEN
The effect of chelating phosphines was tested on the structure and pH-dependent stability of nickel-cysteine binding. (1,2-Bis(diphenylphosphino)ethane (dppe) and 1,1,1-tris[(diphenylphosphino)methyl]ethane (triphos) were used with three different cysteine derivatives (L-cysteine, Cys; L-cysteine ethyl ester, CysEt; cystamine, CysAm) to prepare complexes of the form (dppe)NiCysR(n+) and (triphos)NiCysR(n+) (n = 0 for Cys; n = 1 for CysEt and CysAm). Similar 31P {1H} NMR spectra for all (dppe)NiCysRn+ confirmed their square-planar P2NiSN coordination spheres. The structure of [(dppe)NiCysAm]PF6 was also confirmed by single-crystal X-ray diffraction methods. The (triphos)NiCysAm+ and (triphos)NiCysEt+ complexes were fluxional at room temperature by 31P NMR. Upon cooling to -80 degrees C, all gave spectra consistent with a P2NiSN coordination sphere with the third phosphorus uncoordinated. Temperature-dependent 31P NMR spectra showed that a trans P-Ni-S pi interaction controlled the scrambling of the coordinated triphos. In aqueous media, (dppe)NiCys was protonated at pH approximately 4-5, leading to possible formation of a nickel-cysteinethiol and eventual cysteine loss at pH < 3. The importance of N-terminus cysteine in such complexes was demonstrated by preparing (dppe)NiCys-bead and trigonal-bipyramidal Tp*NiCys-bead complexes, where Cys-bead represents cysteine anchored to polystyrene synthesis beads and Tp*- = hydrotris(3,5-dimethylpyrazolyl)borate. Importantly, results with these heterogeneous systems demonstrated the selectivity of these nickel centers for cysteine over methionine and serine and most specifically for N-terminus cysteine. The role of Ni-S pi bonding in nickel-cysteine geometries will be discussed, including how these results suggest a mechanism for the movement of electron density from nickel onto the backbone of coordinated cysteine.