Abstract:The Li atom adducts of formaldehyde (LiOCH2) and formaldimine (LiNHCH2) are produced in the gas phase by neutralization of the corresponding cations. Subsequent reionization, ca. 0.3 micros later, shows that the nominally hypervalent complexes LiXCH2 (X=O or NH) are stable, residing in potential energy minima. In the time span between the neutralization and reionization events, the LiXCH2 molecules dissociate partly into their constituents, Li + XCH2, the fragmentation extent of LiNHCH2 being more extensive. A… Show more
“…The only fragment in Scheme that is not observed in the MI spectra is CH 2 NH···Li + , which corresponds to an internal fragment from the PheGly sequence (R 2 = H) and an a 1 * fragment from the GlyPhe sequence (R 1 = H). This is attributed to the low Li + affinity of CH 2 NH (formaldimine), as compared to the Li + affinities of Phe, Gly, or PhCH 2 CHNH (imine from Phe). − 1 Dissociation of [XxxYyy + Li] + Complexes via a Mixed Anhydride Intermediate That Removes Sequence Information a a See ref . …”
Section: Resultsmentioning
confidence: 99%
“…The Li + affinities of CH 2 NH, Gly, and Phe are 165, 174, and 203 kJ/mol, respectively. The Li + affinity of PhCH 2 CHNH is unknown; it should be significantly larger than that of CH 2 NH, judging from the difference in Li + affinities between Phe and Gly.…”
The Li+ complexes of the isomeric dipeptide pairs PheGly/GlyPhe, PheAla/AlaPhe, and TrpAla/AlaTrp, namely, [Pep + Li]+, and of the corresponding lithium carboxylates, namely, [Pep - H + 2Li]+, are produced in the gas phase by desorption ionization, and their unimolecular chemistry is probed by tandem mass spectrometry experiments at various activation conditions. At low internal energies, monolithiated isomers dissociate to the same products, formed through a mixed anhydride intermediate in which the sequence information is lost. Isomerization to the mixed anhydride is less competitive at higher internal energies, which start promoting sequence-specific fragmentations. On the other hand, dilithiated isomers (they contain a permanent COO-Li+ salt bridge) do not rearrange to an anhydride and give rise to substantially different fragmentation patterns; structurally diagnostic c1- and y1-type fragments are observed at all internal energies, allowing for unequivocal sequence assignment. The mono- and dilithiated peptides undergo loss of their aromatic side chain to form distonic radical ions carrying Li+ charge(s) and one unpaired electron at an alpha-C atom of the peptide backbone. The yield of such metal-bound peptide radicals is particularly high from the dilithiated complexes, [Pep - H + 2Li]+. Upon activation, the Li+ ions become mobile and can be shuttled to the various basic sites of the dipeptides, where they may initiate backbone fragmentation or the elimination of small neutral molecules.
“…The only fragment in Scheme that is not observed in the MI spectra is CH 2 NH···Li + , which corresponds to an internal fragment from the PheGly sequence (R 2 = H) and an a 1 * fragment from the GlyPhe sequence (R 1 = H). This is attributed to the low Li + affinity of CH 2 NH (formaldimine), as compared to the Li + affinities of Phe, Gly, or PhCH 2 CHNH (imine from Phe). − 1 Dissociation of [XxxYyy + Li] + Complexes via a Mixed Anhydride Intermediate That Removes Sequence Information a a See ref . …”
Section: Resultsmentioning
confidence: 99%
“…The Li + affinities of CH 2 NH, Gly, and Phe are 165, 174, and 203 kJ/mol, respectively. The Li + affinity of PhCH 2 CHNH is unknown; it should be significantly larger than that of CH 2 NH, judging from the difference in Li + affinities between Phe and Gly.…”
The Li+ complexes of the isomeric dipeptide pairs PheGly/GlyPhe, PheAla/AlaPhe, and TrpAla/AlaTrp, namely, [Pep + Li]+, and of the corresponding lithium carboxylates, namely, [Pep - H + 2Li]+, are produced in the gas phase by desorption ionization, and their unimolecular chemistry is probed by tandem mass spectrometry experiments at various activation conditions. At low internal energies, monolithiated isomers dissociate to the same products, formed through a mixed anhydride intermediate in which the sequence information is lost. Isomerization to the mixed anhydride is less competitive at higher internal energies, which start promoting sequence-specific fragmentations. On the other hand, dilithiated isomers (they contain a permanent COO-Li+ salt bridge) do not rearrange to an anhydride and give rise to substantially different fragmentation patterns; structurally diagnostic c1- and y1-type fragments are observed at all internal energies, allowing for unequivocal sequence assignment. The mono- and dilithiated peptides undergo loss of their aromatic side chain to form distonic radical ions carrying Li+ charge(s) and one unpaired electron at an alpha-C atom of the peptide backbone. The yield of such metal-bound peptide radicals is particularly high from the dilithiated complexes, [Pep - H + 2Li]+. Upon activation, the Li+ ions become mobile and can be shuttled to the various basic sites of the dipeptides, where they may initiate backbone fragmentation or the elimination of small neutral molecules.
“…FAB of mixtures of LiCl and 18-crown-6 or LiCl and 1,4,8,11-tetraazacyclotetradecane produces Li ϩ O᎐ ᎐ CH 2 and Li ϩ NH᎐ ᎐ CH 2 . 178 NRMS experiments on each of these complexes reveal the corresponding neutral counterparts are stable for at least 3 µs in the gas phase. Ab initio calculations suggest three bound structures for both complexes: two C-centred radical ion-pair complexes, Li ϩ .…”
Section: B Reactions Of Unusual Neutralsmentioning
“…The study of metal ion‐water complexes is important from the point of view of selectivity of biological ion channels 10, 11 and various other physical phenomena 12. Metal ion–ligand complexes, e.g., metal ion–CH 3 CN and metal ionHCHO, are interesting from the point of view of charge transfer 13, 14. An experimental study of such systems is complicated because they exist in ionic form only at very low concentrations 15.…”
ABSTRACT:Electronic structures and properties of several anions, metal cations, and their complexes with neutral molecules were investigated at the HF/6-31G** and B3LYP/6-31G** levels of theory. Charges shifted from atomic sites due to atomic orbital hybridization called hybridization displacement charges (HDC) were investigated in detail. It has been found that many components of HDC are associated with each atom of ion that are shifted from the atomic sites, those associated with metal cations being shifted by large distances as found previously in electrically neutral systems. It is shown that atomic orbitals are appreciably rehybridized in going from neutral molecules to anions and cations. Molecular dipole moments and surface molecular electrostatic potentials (MEP) are obtained satisfactorily using HDC for the various types of species mentioned above. In the OH Ϫ OH 2 O complex, reversal of direction of shift of an HDC component associated with the hydrogen atom of H 2 O involved in hydrogen bonding, indicates that the hydrogen bond between OH Ϫ and H 2 O would have some covalent character. Other atomic site-based point charge models cannot provide such information about the nature of bonding.
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