Ionization-Induced π → H Site Switching in Resorcinol–Ar_n (n = 1 and 2) Clusters Probed by Infrared Spectroscopy

Abstract

Infrared (IR) spectra of resorcinol (Rs)−Ar n clusters (n = 1 and 2) have been measured in the neutral and cationic ground states (S 0 and D 0 ) by IR dip and resonanceenhanced multiphoton ionization (REMPI)-IR spectroscopy. The OH stretching vibrations in S 0 keep their frequency regardless of the number of Ar atoms and the conformation of the OH groups in Rs (rotamers RsI and RsII), demonstrating that the Ar atoms are attached to the aromatic π-ring (π-bound structure) in S 0 . In the D 0 state, the IR spectra of Rs + −Ar n reflect the difference in the Rs conformations (RsI + and RsII + ). For n = 1, the IR spectra of both rotamers are almost the same as those of the corresponding monomer cations, indicating that Ar ligands essentially remain π-bonded after ionization. In contrast, the IR spectra of Rs + −Ar 2 show hydrogen-bonded and free OH stretching vibrations, demonstrating that for a significant fraction of the clusters, the Ar atoms migrate from the π-bound site to the OH groups. The ionization-induced π → H migration yields are not unity for both rotamers RsI + −Ar 2 and RsII + −Ar 2 . This result is in sharp contrast to phenol + −Ar 2 , in which one of the Ar atoms migrates to the OH site with 100% yield. The mechanism leading to the nonunity yield in Rs + −Ar 2 is discussed in terms of the number of OH binding sites and Franck−Condon factors. The ionization excess energy dependence of the IR spectra of Rs + −Ar 2 and its Rs + −Ar fragments is discussed in terms of the Ar binding energies estimated from the photoionization and photodissociation efficiency spectra.

Torsional IR spectra of three conformers of the resorcinol molecule

“…According to calculations, the first excited torsional state of conformer A of the RES molecule is split due to tunneling by 2*10 -6 cm -1 . This is comparable to the calculated value of the tunneling splitting of the first excited torsional state of the CTL molecule [34] (from 1.1*10 -5 to 6.1*10 -6 cm -1 , experimental value 1.04*10 -5 cm -1 [33]). It can be assumed that the tunneling splitting of the first excited torsion state of conformer A of the RES molecule can also be resolved experimentally.…”

“…A comparison of the experimental frequencies of torsional vibrations of a number of para-and meta-substituted phenols in cyclohexane with the corresponding frequencies in the gas phase showed very good agreement [26], which indicates a very weak influence of cyclohexane on the values of the frequencies of torsional vibrations in para-and meta-substituted phenols. In addition, the assignment of the band at 411 cm -1 to the torsional vibration of the O-H bond in the CTL molecule [26] is in good agreement with the results of [33,75] (415 cm -1 ). Therefore, we think the assignment in [26] of the band at 318 cm -1 to the torsional vibration of O-H bonds in the RES molecule to be very reliable.…”

“…Today, these assignments for the free RES molecule look very doubtful. Indeed, according to [33,34,75], the torsional vibration of the hydrogen-bonded O-H group in the free CTL molecule appears at a frequency of 415 cm -1 . Since, in addition to the barrier to overcoming planarity, an additional contribution that prevents exit from the plane is made by the hydrogen bond, these two effects provide a significant increase in the frequency of torsional vibration in comparison with H-bond free the phenol molecule -309.2 cm -1 [76].…”

“…The authors note the presence of only A and B conformers in the samples, and also that torsional splitting of lines remained unresolved for all nondeuterated RES conformers. Infrared (IR) photodissociation spectra RES+Ar and RES+2Ar were registered in the works of Patzer [32] and Miyazaki [33], however, the authors analyzed only the region of O-H stretching vibrations. The work of Schneider [34], although it concerns mainly 1,3 dimethoxybenzenes, however, provides important arguments explaining why in some cases, when recording RES spectra, only two and not three conformers of the molecule are observed.…”

Isomer-Selective Spectroscopy and Dynamics of Phenol–Arn (n ≤ 5) Clusters

“…This hydrogen-bond-like interaction (OH + ···Ar) causes the ionization induced π → H binding site switching. Such site switching phenomena have been reported not only for substituted acidic arenes (e.g., PhNH 2 , resorcinol, , aminobenzonitrile, , and alkylanilides , ) but also in heteroaromatic systems such as (substituted) indole , and pyrrole . This site switching can be detected sensitively by IR spectroscopy of the OH (or NH) stretching vibration through the large spectral shifts induced by H-bonding of Ar atoms (the OH stretching in free PhOH + (3536 cm –1 ) is red-shifted to 3436 cm –1 by H-bonding with Ar). ,, …”