Metal-stabilized phenoxyl radicals appear to be important intermediates in a variety of enzymatic oxidations. We report that transition metal coordination also supports an aminyl radical, resulting in a stable crystalline complex: [Rh(I)(trop2N.)(bipy)]+OTf- (where trop is 5-H-dibenzo[a,d]cycloheptene-5-yl, bipy is 2,2'-bipyridyl, OTf- is trifluorosulfonate). It is accessible under mild conditions by one-electron oxidation of the amide complex [Rh(I)(trop2N)(bipy)], at a potential of -0.55 volt versus ferrocene/ferrocenium. Both electron paramagnetic resonance spectroscopy and density functional theory support 57% localization of the unpaired spin at N. In reactions with H-atom donors, the Rh-coordinated aminyl behaves as a nucleophilic radical.

Electron capture dissociation (ECD) was studied with doubly charged dipeptide ions that were tagged with fixed-charge tris-(2,4,6-trimethoxyphenyl)phosphonium-methylenecarboxamido (TMPP-ac) groups. Dipeptides GK, KG, AK, KA, and GR were each selectively tagged with one TMPP-ac group at the N-terminal amino group while the other charge was introduced by protonation at the lysine or arginine side-chain groups to give (TMPP-ac-peptide ϩ H) 2ϩ ions by electrospray ionization. Doubly tagged peptide derivatives were also prepared from GK, KG, AK, and KA in which the fixed-charge TMPP-ac groups were attached to the N-terminal and lysine side-chain amino groups to give (TMPP-ac-peptide-ac-TMPP) 2ϩ dications by electrospray. ECD of (TMPP-ac-peptide ϩ H) 2ϩ resulted in 72% to 84% conversion to singly charged dissociation products while no intact charge-reduced (TMPP-ac-dipeptide ϩ H) ϩ• ions were detected. The dissociations involved loss of H, formation of (TMPP ϩ H) ϩ , and N-C ␣ bond cleavages giving TMPP-CH 2 CONH 2 ϩ (c 0 ) and c 1 fragments. In contrast, ECD of (TMPP-ac-peptide-ac-TMPP) 2 g g ϩ resulted in 31% to 40% conversion to dissociation products due to loss of neutral TMPP molecules and 2,4,6-trimethoxyphenyl radicals. No peptide backbone cleavages were observed for the doubly tagged peptide ions. Ab initio and density functional theory calculations for (Ph 3 P-ac-GK ϩ H) 2ϩ and (H 3 P-ac-GK ϩ H) 2ϩ analogs indicated that the doubly charged ions contained the lysine side-chain NH 3 ϩ group internally solvated by the COOH group. The distance between the charge-carrying phosphonium and ammonium atoms was calculated to be 13.1-13.2 Å in the most stable dication conformers. The intrinsic g p g y gp p recombination energies of the TMPP ϩ -ac and (GK ϩ H) ϩ moieties, 2.7 and 3.15 eV, respectively, indicated that upon electron capture the ground electronic states of the (TMPPac-peptide ϩ H) ϩ• ions retained the charge in the TMPP group. Ground electronic state (TMPP-ac-GK ϩ H) ϩ• ions were calculated to spontaneously isomerize by lysine H-atom transfer to the COOH group to form dihydroxycarbinyl radical intermediates with the retention of the charged TMPP group. These can trigger cleavages of the adjacent N-C ␣ bonds to give rise to the c 1 fragment ions. However, the calculated transition-state energies for GK and GGK models suggested that the ground-state potential energy surface was not favorable for the formation of the abundant c 0 fragment ions. This pointed to the involvement of excited electronic states according to the Utah-Washington mechanism of ECD. (J Am Soc Mass Spectrom 2007, 18, 2146 -2161) © 2007 American Society for Mass Spectrometry E lectron-based methods of gas-phase ion chemistry rely on partial or complete neutralization of gas-phase ions to produce transient species with open electronic shells that undergo various dissociations [1]. Amongst the several electron-based methods in current use, electron capture dissociation (ECD) [2] has received much attention lately because of its potential for ...

Carbenes derived from five-membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N-heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug-cc-pVTZ level. The examined parameters include the energies of the σ-lone pair at Ccarbene and the π-HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet-triplet energy gap was used as a measure of the stability of the N-heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the pπ population and the natural charge at Ccarbene with NHC structure. Additionally, the transition metalNHC bond in L-AuCl and L-TiCl4 and the nature of the orbital interactions between the NHC and the transition-metal fragment were analysed in detail by the extended transition state-natural orbitals for chemical valence (ETS-NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHCgold and the NHCtitanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC.