Electronic and vibrational spectroscopies of aromatic clusters with He in a supersonic jet: The case of neutral and cationic phenol-Hen (n = 1 and 2).

Van der Waals clusters composed of He and aromatic molecules provide fundamental information about intermolecular interactions in weakly bound systems. In this study, phenol-helium clusters (PhOH-Hen with n ≤ 2) are characterized for the first time by UV and IR spectroscopies. The S1 ← S0 origin and ionization energy both show small but additive shifts, suggesting π-bound structures of these clusters, a conclusion supported by rotational contour analyses of the S1 origin bands. The OH stretching vibrations of the PhOH moiety in the clusters match with those of bare PhOH in both the S0 and D0 states, illustrating the negligible perturbation of the He atoms on the molecular vibration. Matrix shifts induced by He attachment are discussed based on the observed band positions with the help of complementary quantum chemical calculations. For comparison, the UV and ionization spectra of PhOH-Ne are reported as well.

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

Structures and ionization-induced solvation dynamics of phenol-(argon)n clusters, PhOH-Arn (n ≤ 5), were studied by using a variety of isomer-selective photoionization and vibrational spectroscopic techniques. Several higher-energy isomers were found and assigned for the first time by systematically controlling the experimental conditions of the supersonic expansion. This behavior is also confirmed for the PhOH-Kr2 cluster. Solvation structures are elucidated by evaluating systematic shifts in the S1 ← S0 origin and ionization energies obtained by resonance-enhanced photoionization, in addition to the OH stretching frequency obtained by IR photodissociation. Isomer-selective picosecond time-resolved IR spectroscopy for the n = 2 clusters revealed that the dynamics for the ionization-induced intermolecular π → H site-switching reaction strongly depends on the initial isomeric structure. In particular, the reaction time for the (1|1) isomer is 7 ps, while that for (2|0) is <3 ps. This difference shows that the switching time is determined by the distance of the reaction coordinate between the initial π-site and the final OH-site.

Alkali and Alkaline Earth Metal Ions Complexes with a Partial Peptide of the Selectivity Filter in K+ Channels Studied by a Cold Ion Trap Infrared Spectroscopy.

The front cover artwork is provided by Takashi Tsujino (Science Graphics Co., Ltd.) . The image shows the efficacy of a bottom-up approach to ion selectivity of K+ channels. The GYG-K+ complex, which replicates the local portion of K+ channels, has three conformations with an equivalent distribution. Read the full text of the Article at 10.1002/cphc.202000033.

Alkali and Alkaline Earth Metal Ions Complexes with a Partial Peptide of the Selectivity Filter in K+ Channels Studied by a Cold Ion Trap Infrared Spectroscopy.

The infrared (IR) spectra of alkali and alkaline earth metal ion complexes with the Ac-Tyr-NHMe (GYG) peptide have been measured by laser photodissociation in a cold ion trap coupled with an electrospray mass spectrometer. The GYG peptide corresponds to a portion of the ion selectivity filter in the KcsA K+ channel that allows K+ to pass, but blocks Na+ even though it has a smaller ionic radius than K+ . This current study extends a previous investigation on Na+ and K+ to the entire set of alkali metaI ions and alkaline earth dications. IR-IR hole-burning (IR2 dip) spectroscopy has established the coexistence of several conformers of the GYG-metal ion complexes. The structures of the conformers were assigned by comparison between the isomer-selected IR spectra and theoretical IR spectra obtained from quantum chemical calculations. It was found that the structure of the dominant conformer correlates with the ability of the ion to permeate through the K+ channel.

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

Infrared (IR) spectra of resorcinol (Rs)-Arn clusters (n = 1 and 2) have been measured in the neutral and cationic ground states (S0 and D0) by IR dip and resonance-enhanced multiphoton ionization (REMPI)-IR spectroscopy. The OH stretching vibrations in S0 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 S0. In the D0 state, the IR spectra of Rs+-Arn 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+-Ar2 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+-Ar2 and RsII+-Ar2. This result is in sharp contrast to phenol+-Ar2, in which one of the Ar atoms migrates to the OH site with 100% yield. The mechanism leading to the nonunity yield in Rs+-Ar2 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+-Ar2 and its Rs+-Ar fragments is discussed in terms of the Ar binding energies estimated from the photoionization and photodissociation efficiency spectra.