Photoionization-induced π ↔ H site switching dynamics in phenol⁺–Rg (Rg = Ar, Kr) dimers probed by picosecond time-resolved infrared spectroscopy

Abstract

The ionization-induced π↔ H site switching reaction in phenol(+)-Rg (PhOH(+)-Rg) dimers with Rg = Ar and Kr is traced in real time by picosecond time-resolved infrared (ps-TRIR) spectroscopy. The ps-TRIR spectra show the prompt appearance of the non-vanishing free OH stretching band upon resonant photoionization of the π-bound neutral clusters, and the delayed appearance of the hydrogen-bonded (H-bonded) OH stretching band. This result directly proves that the Rg ligand switches from the π-bound site on the aromatic ring to the H-bonded site at the OH group by ionization. The subsequent H →π back reaction converges the dimer to a π↔ H equilibrium. This result is in sharp contrast to the single-step π→ H forward reaction in the PhOH(+)-Ar2 trimer with 100% yield. The reaction mechanism and yield strongly depend on intracluster vibrational energy redistribution. A classical rate equation analysis for the time evolutions of the band intensities of the two vibrations results in similar estimates for the time constants of the π→ H forward reaction of τ+ = 122 and 73 ps and the H →π back reaction of τ- = 155 and 188 ps for PhOH(+)-Ar and PhOH(+)-Kr, respectively. The one order of magnitude slower time constant in comparison to the PhOH(+)-Ar2 trimer (τ+ = 7 ps) is attributed to the decrease in density of states due to the absence of the second Ar in the dimer. The similar time constants for both PhOH(+)-Rg dimers are well rationalized by a classical interpretation based on the comparable potential energy surfaces, reaction pathways, and density of states arising from their similar intermolecular vibrational frequencies.

Deciphering environment effects in peptide bond solvation dynamics by experiment and theory

“…One possibility to overcome these limitations is the generation of size-and isomer-selected (micro-)hydrated peptides in the gas phase and study the dynamics of a single or several selected solvent molecules with time-resolved pumpprobe techniques, such as time-resolved photoelectron spectroscopy [26][27][28][29] or the recently developed picosecond time-resolved infrared (ps-TRIR) spectroscopy 25,30 . The latter one has successfully been applied to monitor in real time the ionizationinduced π →H site-switching dynamics of rare gas ligands attached to the phenol molecule [30][31][32][33][34] and the water migration dynamics in the 4-aminobenzonitrile-water 35 and trans-acetanilidewater (AA + − W) 36 cluster cations. Combined with theoretical simulations, such experiments provide a detailed atomistic picture of the dynamics of individual solvent molecules.…”

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

“…76 Due to π → H site-switching, the spectra show a hydrogen-bonded OH stretching vibration (ν OH HB ) at ∼3473 cm −1 except for the n = 1 cluster, in which the free OH stretching vibration (ν OH f = 3536 cm −1 ) and a weakly hydrogen-bonded OH stretching vibration (∼3500 cm −1 ) appear due to a large-amplitude motion of the Ar atom. 42,77 The ν OH HB band is broader and blue-shifted from that of the cold H-bound structure (3465 cm −1 ) 30,33,34 due to excess energy available from the exothermic π → H site-switching reaction. [36][37][38]62,63,66 The shifts from IE 0 of n = 0 are indicated in parentheses.…”

“…30,33,34 It should be noted that the ν OH HB peak in the n = 1 cluster occurs at higher frequency than those because of partial H-bonding due to the pendulum motion. 41,42 This observation means that photoionization results in excitation of intermolecular vibrations, leading to a weakening of the H-bond between the OH group and Ar. However, the ν OH HB frequency and bandwidth of the H-bound structures are almost independent of the cluster size and isomer structures in the initial S 0 states prior to ionization.…”

“…For the n = 1 cluster, the spectrum reproduces previous results. 32,42,43 The band at 36296 cm −1 , red-shifted from the monomer by −53 cm −1 , is assigned to the S 1 origin band of the (1|0) structure. For n = 2, the spectrum shows similar features as those of PhOH−Ar 2 .…”

“…Pioneering work in this field was performed by our group for the ionization-induced π → H site switching dynamics of PhOH−Ar n≤3 clusters by structurally sensitive picosecond time-resolved infrared (ps-TRIR) spectroscopy. 22,34,40,42,50,51 Because the solvation dynamics occurs on the picosecond time scale (related to the involved low-frequency intermolecular modes), this approach provides sufficient time and spectral resolution to monitor both the time and structural evolution of the intermolecular rearrangement reactions. In the neutral electronic states (S 0 , S 1 ) of PhOH−Ar n , it has been established that Ar solvates aromatic solutes at the π-electron cloud through its relatively large polarizability leading to dominating dispersion forces.…”