Getrennter spektroskopischer Nachweis von Carbenium‐ und Fluoronium‐Isomeren von protoniertem Fluorbenzol

Abstract

Bloß eine Frage der Technik! Carbenium‐ und Fluoronium‐Isomere von protoniertem Fluorbenzol wurden erstmals in der Gasphase mit spektroskopischen Methoden zweifelsfrei identifiziert (siehe Schema; blau: C‐C6H6F+, rot: F‐C6H6F+). Die Carbenium‐Ionen wurden mithilfe von IR‐Photodissoziation (IRPD) schwach gebundener C6H6F+⋅(N2)2‐Komplexe nachgewiesen und die F‐C6H6F+‐Ionen selektiv durch IRPD von isoliertem C6H6F+ detektiert.

Anharmonic Vibrational Spectroscopy Calculations for Proton-Bound Amino Acid Dimers

“…There has been great progress in recent years in experimental studies of biological molecules in the gas phase. Mass spectrometric techniques and vibrational spectroscopy, often in a combined framework, have been the main experimental tools in this area. − These experiments have provided unprecedented insight into the mechanisms underlying fundamental biological processes such as protein folding, enzyme substrate binding, nucleic acid tautomerization and base pairing. − Experimental spectroscopic techniques that have been recently applied to the study of fairly large biological molecules include electrospray ionization techniques (ESI) and matrix assisted laser desorption ionization (MALDI), infrared photo dissociation techniques (IRPD). − The application of sensitive IRPD to isolated and microsolvated protonated peptides and proton bound dimers of amino acids has provided useful information on the dissociation behavior, preferred protonation sites and the influence of solvation in nonpolar environments. − Similarly, the development and appreciation of the ESI methodology makes possible the efficient ionization of biomolecules as large as ubiquitin and has thus provided unique information on noncovalent bonding of OH and NH groups especially when applied microsolvated proton clusters such as (H 2 O) 2 -8H + and water bound ammonium clusters such as (H 2 O) 3 -6(NH 4 + ). − …”

Interaction of the Benzenium Ion with Inert Ligands: IR Spectra of C6H7+Ln Cluster Cations (L=Ar, N2, CH4, H2O)

“…In the past two years, the application of two sensitive IR spectroscopic methods has provided the first structural characterization of basic AH + ions in the gas phase under controlled microsolvation conditions 3–5. The first technique involves IR photodissociation (IRPD) spectroscopy of size‐selected AH + L n complexes, in which AH + is solvated by a well‐defined number of inert ligands L (e.g., L=Ar or N 2 ) 3, 4, 5a. The IR spectrum of AH + is obtained by monitoring the laser‐induced evaporation of the weakly bound ligand(s) through vibrational predissociation.…”

“…The modest influence of the weak intermolecular interaction between AH + and L may either be controlled by the variation of L (i.e., the strength of the interaction) or determined by quantum chemical calculations 3, 4, 6, 10, 11. So far, this strategy has been applied to AH + L n complexes of protonated benzene (C 6 H 7 + L, L=Ar and N 2 ),3 protonated phenol (C 6 H 7 O + Ar n , n =1 and 2),4 and protonated fluorobenzene (C 6 H 6 F + (N 2 ) 2 ) 5a. IRPD spectra of C 6 H 7 + L in the CH stretch range have unambiguously shown for the first time that the σ complex corresponds to the most stable structure of isolated C 6 H 7 + in agreement with condensed‐phase data and ab initio calculations 3.…”

“…Protonation at the less favorable meta and ipso positions is not observed. As an alternative to IRPD of weakly bound AH + L n clusters,3, 4, 5a the second approach utilizes IRPD of the bare AH + monomer. The latter technique enables the spectroscopic characterization of AH + completely free from any solvation effect (i.e., without messenger) and has so far been applied only to protonated fluorobenzene, C 6 H 6 F + 5b.…”

“…The more stable carbenium isomers of C 6 H 6 F + escaped spectroscopic detection, because HF elimination requires much more energy for these ions. Very recently, several carbenium isomers of C 6 H 6 F + have been selectively identified through the application of the messenger approach to C 6 H 6 F + (N 2 ) 2 5a…”