Spectroscopic Identification of Oxonium and Carbenium Ions of Protonated Phenol in the Gas Phase:  IR Spectra of Weakly Bound C₆H₇O⁺−L Dimers (L = Ne, Ar, N₂)

Abstract

Structural isomers of isolated protonated phenol (C(6)H(7)O(+)) are characterized by infrared (IR) photodissociation spectroscopy of their weakly bound complexes with neutral ligands L (L = Ne, Ar, N(2)). IR spectra of C(6)H(7)O(+)-L recorded in the vicinity of the O-H and C-H stretch fundamentals carry unambiguous signatures of at least two C(6)H(7)O(+) isomers: the identified protonation sites of phenol include the O atom (oxonium ion, O-C(6)H(7)O(+)) and the C atoms of the aromatic ring in the ortho and/or para position (carbenium ions, o/p-C(6)H(7)O(+)). In contrast, protonation at the meta and ipso positions is not observed. The most stable C(6)H(7)O(+)-L dimer structures feature intermolecular H-bonds between L and the OH groups of O-C(6)H(7)O(+) and o/p-C(6)H(7)O(+). Extrapolation to zero solvation interaction yields reliable experimental vibrational frequencies of bare O-C(6)H(7)O(+) and o/p-C(6)H(7)O(+). The interpretation of the C(6)H(7)O(+)-L spectra, as well as the extrapolated monomer frequencies, is supported by B3LYP and MP2 calculations using the 6-311G(2df,2pd) basis. The spectroscopic and theoretical results elucidate the effect of protonation on the structural properties of phenol and provide a sensitive probe of the activating and ortho/para directing nature of the OH group observed in electrophilic aromatic substitution reactions.

Infrared spectra of protonated polycyclic aromatic hydrocarbon molecules: Azulene

“…If this were the case, they would be present in the AzuH + ion cloud assayed in the ICR by IRMPD. In fact, such highlying isomers have unambiguously been detected for related protonated aromatic molecules by IRPD spectroscopy of ions generated in an electron impact cluster ion source, such as phenol, 19 para-fluorophenol, 26 and fluorobenzene. 20 For this reason, the IRMPD spectrum is compared to the IR spectra calculated for all six available AzuH + isomers in Fig.…”

“…13 The first technique employs modern, relatively low-intensity, optical parametric oscillator laser systems in the frequency range 800-4000 cm −1 to drive one-photon IRPD of AH + -L n cluster ions. 12 This approach is based on the evaporation of one or more of the weakly bound ligands upon resonant absorption of a single photon ͑messenger technique͒, 14 and has been applied to a variety of AH + -L n cluster ions, including A = benzene, [15][16][17][18] phenol, 12,19 fluorobenzene, 20 para-halogenated phenols, 21 toluene, 17 pyridine, 18 aniline, 22 imidazole, 23 various amino acids and peptides, 24 and neurotransmitters. 25 The IRPD method can also be applied to break weak chemical bonds in certain AH + isomers, 11,26 but usually fails to dissociate common AH + ions because the energy of a single IR photon is insufficient to break strong covalent bonds.…”

Understanding the Nucleophilic Character and Stability of the Carbanions and Alkoxides of 1-(9-Anthryl)ethanol and Derivatives

“…The thermodynamic stability of the carbanions and alkoxides of 1a-2g was obtained from the calculated CH and OH absolute gas-phase acidities [32]. Gas phase studies are required to separate intrinsic molecular properties from interfering solvation effects [33]. The absolute gas-phase acidity Δ acid G° is given by Δ acid G° = G°(anion) + G°(H+) − G°(alcohol) [34].…”

Laboratory infrared spectroscopy of PAHs

“…Lee, it was to behave as a spy (Okumura et al 1990) -and hence, the use of rare gas atoms rather than small molecules became popular. Messenger atom spectroscopy was for instance used to record the IR spectra of several ionized aromatic systems, particularly benzene and various of its ring-substituted derivatives (Solcà & Dopfer 2004).…”