Infrared Spectroscopy of Protonated Phenol–Water Clusters

Abstract

To explore the microhydration structures of protonated phenol, size-selective infrared spectroscopy of protonated phenol-(water) clusters ( n = 1-5) was performed. The protonation of phenol can occur either at the phenyl ring or at the hydroxy group. The coexistence of the two isomer types separated by the high isomerization barrier was reconfirmed for bare protonated phenol. Preferential hydration of the hydroxy group initially occurs in both the two isomer types of protonated phenol. Development of the water hydrogen-bond network is localized around the hydroxy group up to n = 2. Intracluster proton transfer from the phenol moiety to the water moiety was observed in n ≥ 3-4. The water moiety with the HO ion core resides on the phenyl ring, and the water moiety is bound to the phenyl ring with a π-hydrogen bond. Such a structure is in striking contrast to those of phenol-(water) radical cation clusters, in which the water moiety is located away from the phenyl ring even when intracluster proton transfer occurs.

IRMPD Spectra of Protonated Hydroxybenzaldehydes: Evidence of Torsional Barriers in Carboxonium Ions

“…The visual inspection shows a best fit with the spectrum computed for the global minimum, namely the para protonated isomer, p‐ PhH + , in agreement with previous evidence pointing to predominant protonation at the para position . Vibrational modes can be assigned as described in Table , which lists the experimental IRMPD absorptions together with the calculated IR transitions of the p ‐PhH + .…”

“…Protonated phenol has in fact never been yet assayed in the fingerprint range by IR action spectroscopy. The IRMPD spectrum of protonated phenol, PhH + is presented in Figure (blue trace, bottom panel) and compared with the calculated spectra for all possible isomers, already illustrated in previous studies reporting the IRPD spectrum in the O−H stretching range …”

Microhydration of protonated 5-hydroxyindole revealed by infrared spectroscopy

“…11,12 In general, the topology of the potential energy surface of hydrated clusters strongly depends on their charge or protonation state. 5,[13][14][15] The microhydration of the protonated phenol (PhH + ) subunit of 5HI has been studied by quantum chemistry and IRPD spectroscopy. 14,16 While the theoretical study focused on the electronic structure of the carbenium ion, 16 the IRPD study indicates the coexistence of ortho/para C-protonated carbenium Furthermore, the acidity of the NH and OH functional groups is found to strongly depend on the protonation site.…”

“…5,[13][14][15] The microhydration of the protonated phenol (PhH + ) subunit of 5HI has been studied by quantum chemistry and IRPD spectroscopy. 14,16 While the theoretical study focused on the electronic structure of the carbenium ion, 16 the IRPD study indicates the coexistence of ortho/para C-protonated carbenium Furthermore, the acidity of the NH and OH functional groups is found to strongly depend on the protonation site. 17 Comparison with PhH + indicates an increase in the acidity of the OH group from 5HIH + to PhH + .…”

“…This study reveals the preference of OHÁ Á ÁW over NHÁ Á ÁW H-bonds in the neutral ground state, in line with spectroscopic data. 14 Both, OHÁ Á ÁW and NHÁ Á ÁW H-bonds are significantly strengthened upon protonation, 5 whereas the increase in the acidity of the NH group is more pronounced than that of the OH group. As a result, the topology of the hydration shell is changed upon protonation and NHÁ Á ÁW bonds are preferred.…”