Ionized Phenol and Its Isomers in the Gas Phase

Abstract

Protonated 4-bromophenol and 4-bromoanisole produced by methane chemical ionization are found to easily be dehalogenated upon high (8 keV) or low (20-30 eV) energy collisional activation giving essentially phenol and anisole radical cations, respectively. Under similar conditions, protonated unsubstituted anisole is also readily demethylated generating the phenol ion but not cyclohexadienone ions. Other nonconventional isomers of ionized phenol are only detected by MS/MS/MS experiments performed on [M-CO] •+ ions from salicylaldehyde. (U)B3LYP/6-311++G(d,p) and CASPT2/6-31G(d,p) calculations indicate the higher stability of the phenol radical cation with respect to the other six-membered-ring isomers. The least energy demanding fragmentation, namely, the decarbonylation, is shown to involve the intermediacy of six-membered ketones, open-chain ketenes, and five-membered cyclopentadiene isomeric ions. The rate determining step corresponds to the enol-keto interconversion with an energy barrier of about 276 kJ/mol relative to the phenol ion, which is markedly smaller than that required for hydrogen atom loss, deprotonation, or CO loss from an open-chain form. This suggests a crucial role played by the solvent in the readiness of the deprotonation of phenol ions in nonpolar media. The adiabatic ionization energy of phenol is evaluated as IE a (C 6 H 5 O) ) 8.35 ( 0.2 eV (exptl: 8.49 eV), and the proton affinity of the phenoxy radical is evaluated as PA(C 6 H 5 O) ) 863 ( 10 kJ/mol (exptl: 860 kJ/mol), PA(phenol) ) 826 ( 10 kJ/mol (exptl: 818 kJ/mol), and PA(anisole) ) 848 ( 10 kJ/mol.

Isomeric recognition by ion/molecule reactions: The ionized phenol-cyclohexadienone case

“…The close similarity between both CA spectra can readily be explained on the basis of the potential energy surface connecting ion 1 •؉ , ion 2 •؉ , and their fragments [8]. Indeed, it was theoretically demonstrated (see Scheme 1) that (i) decarbonylation represents the least energy requirement fragmentation of ion 1 •؉ and ion 2 •؉ , (ii) that the loss of carbon monoxide from 1 •؉ occurs by the intermediacy of 2 •؉ , and (iii) that the corresponding transition structure is lower in energy than the loss of hydrogen atom from 1 •؉ by no less than 100 kJ mol Ϫ1 [8].…”

“…• (m/z 140) are radical processes, it is reasonable to believe that the 3 •؉ distonic ion (see Scheme 1) represents the reactive species rather than the cyclic counterpart, ionized cyclohexadienone 2 •؉ . Actually, the isomerization process 2 •؉ ¡ 3 •؉ is likely to spontaneously occur in the ion source because, as indicated earlier in Scheme 1, the corresponding critical energy is negligible, about 8 kJ mol Ϫ1 above 2 •؉ , and because the distonic ion 3 •؉ represents the most stable isomer, being 26 kJ mol Ϫ1 more stable than the cyclic species 2 •؉ [8]. Such a reactivity was already demonstrated in the case of electron ionization of cyclobutanone.…”

“…Indeed, it was theoretically demonstrated (see Scheme 1) that (i) decarbonylation represents the least energy requirement fragmentation of ion 1 •؉ and ion 2 •؉ , (ii) that the loss of carbon monoxide from 1 •؉ occurs by the intermediacy of 2 •؉ , and (iii) that the corresponding transition structure is lower in energy than the loss of hydrogen atom from 1 •؉ by no less than 100 kJ mol Ϫ1 [8]. In other words, post-collisional isomerization processes are likely to occur between activated 1 •؉ and 2 •؉ .…”

“…This excess of internal energy brought by the reactive ions could induce endothermic processes. The observation, among the ion/molecule reaction products, of the m/z 66 radical cations coming from the metastable decomposition of ionized phenol fixes the extent of internal energy excess in the reactive ions to 276 kJ mol Ϫ1 that is the energy barrier on the way to the decarbonylation of ionized phenol [8] (see Scheme 1 and Figure 3). This threshold being by far higher than the energy required by the endothermic process clearly rationalizes the observation of the m/z 93 ions.…”

“…Although ionized phenol (1 •ϩ ) and its cyclohexadienone isomer (2 •ϩ ) were already abundantly studied [7], we recently experimentally and theoretically reinvestigated the C 6 H 6 O •ϩ system to, inter alia, shed more light on the energetics of the mechanism of decarbonylation, the least energy demanding dissociation of ionized phenol [8]. This reaction was shown to involve the intermediacy of ionized cyclohexadienone (2 •ϩ ) and two distonic ions (3 •ϩ and 4 •ϩ ), as shown in Scheme 1 (values indicated below the formula correspond to relative energies in kJ mol Ϫ1 ) [8]. The rate-determining step of the overall process corresponds to the enolketone isomerization 1 •ϩ ¡ 2 •ϩ with an energy barrier of about 276 kJ mol Ϫ1 above ionized phenol.…”