The presence of ions and their electric fields is known to affect the structure and function of biomolecules. For example, the catalytic function of enzymes is highly dependent on the local electric fields around the active site. Another example is the well-known Hofmeister series that ranks the solubility of proteins in aqueous solution as a function of the nature of ions present. To gain a molecular understanding of the interactions between ions, biomolecules, and solvent water, we have studied microsolvated metal ion-peptide complexes using cryogenic ion vibrational spectroscopy. As a starting point, we examined the vibrational spectra of complexes of Li⁺, Na⁺, and K⁺ with simple dipeptides and tripeptides. The use of conformer-specific IR–IR spectroscopy allows us to unambiguously distinguish between the energetically lowest conformers and quantify their relative population. We show that the formation of an intramolecular hydrogen bond by the terminal amine is favored over an additional metal ion coordination site, in agreement with prediction from DFT and ab initio calculations. The different metal ions induce small frequency shifts due to vibrational Stark effects which provide quantitative information about the strength of the electric field. Interestingly, we find that the polarization of the amide group by the metal ion increases the strength of the hydrogen bond, highlighting that the metal ion can have remote effects on the peptide. We also studied the sequential solvation of the metal ion – peptide complexes with water molecules to probe the interaction with solvent. The spectral assignment is greatly aided by using H₂O/D₂O substitution, which enables us to clearly distinguish bands belonging to the peptides from those of the water molecules. We find that the addition of water reduces the electric field strength experienced by the peptide by increasing the metal ion - peptide distance. This also decreased the strength of the intramolecular hydrogen bond in the peptide and promotes the formation of a new peptide conformation upon solvation. Comparison with DFT computations is a lot more difficult for the microsolvated complexes due to the overestimation of the strength of the H-bonding interactions. This leads to large discrepancies between the computed and experimental spectra. The observed structures are also often not amongst the predicted lowest energy conformers. Potential strategies to alleviate these issues and enable the study of larger clusters will be discussed.

In order to light up their emblematic stretch-bend 1:2 Fermi resonance, water molecules have to engage in rather strong isolated or cooperative [1] hydrogen bonds, because the wavenumber gap to be closed between the symmetric OH stretching fundamental (OHb) and the OH bending overtone (b2) in isolated water exceeds 500 cm⁻¹. It is easier for downshifted OHb to reach 3-quantum combination states involving b2 + some intermolecular vibration of the hydrogen-bonded water (e.g. libration (lib) or dimer stretch). It was recently shown [2] that ketone monohydrates develop a systematic and rather sizeable (effective coupling matrix element W ≈ 10 cm⁻¹) resonance between OHb and b2+lib around 3500 cm⁻¹, whereas tertiary amines show preliminary evidence [3] for a similarly systematic and even stronger (W ≈ 20 cm⁻¹) resonance between OHb and b2+dimer stretch around 3300 cm⁻¹. This jet vibrational spectroscopy evidence is put under scrutiny by chemical and isotope substitution. Consequences for future HyDRA blind tests [4] on the theoretical predictability of hydrogen-bonded water stretching wavenumbers are discussed. [1] K. E. Otto, Z. Xue, P. Zielke, M. A. Suhm, Phys. Chem. Chem. Phys. 16 (2014) 9849–9858 [2] T. L. Fischer, T. Wagner, H. C. Gottschalk, A. Nejad, M. A. Suhm, J. Phys. Chem. Lett. 12 (2021) 138–144 [3] E. Lwin, T. L. Fischer, M. A. Suhm, J. Phys. Chem. Lett. 14 (2023) 10194–10199 [4] T. L. Fischer, M. Bödecker, S. M. Schweer et al., Phys. Chem. Chem. Phys. 25 (2023) 22089–22102

In 2023, the Leutwyler group published the last in a series of benchmark studies involving 1-naphthol. The intermolecular dissociation energies of 59 different weakly-bound complexes with 1-naphthol were measured using the stimulated-emission-pumping/resonant 2-photon ionization technique.R. Knochenmuss, R. K. Sinha, S. Maity, S. Leutwyler, Intermolecular Dissociation Energies of 1-Naphthol Complexes with Ethene and Chloroethenes. ChemRxiv. 2023. https://doi.org/10.26434/chemrxiv-2023-djdhzR. Knochenmuss, R. K. Sinha, and S. Leutwyler, Benchmark Experimental Gas-Phase Intermolecular Dissociation Energies by the SEP-R2PI Method. Annu. Rev. Phys. Chem., Vol. 71:189-211.. The authors then compared the experimentally determined energies to those predicted by DFT, creating a rather large data set to benchmark dissociation energies. More recent studies by both computational and rotational spectroscopy groups have found some interesting new results with the Leutwyler data set, obtained from comparing the predicted energies to more rigorous computational methods and when the target structures are confirmed by rotational spectroscopy. Often, the agreement is good, but not always. Obtaining the experimental rotational structures is a time-intensive process. This talk will summarize the combined findings and ultimately suggest a broader collaborative effort from the spectroscopy community to complement the Leutwyler experimental data set. The effort would also benefit from new contributions from the spectroscopic community, such as OH stretching frequency shifts obtained for the 1-naphthol complexes. R. Knochenmuss, R. K. Sinha, S. Maity, S. Leutwyler, Intermolecular Dissociation Energies of 1-Naphthol Complexes with Ethene and Chloroethenes. ChemRxiv. 2023. https://doi.org/10.26434/chemrxiv-2023-djdhz

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Footnotes:

Near-infrared (NIR) spectra of H₃O^+...X_n (X =Ar, N₂, and CO, n = 1 – 3) in the first overtone region of OH-stretching vibrations (4800 – 7000 cm⁻¹) were measured. Not only overtones of stretch/bend but also several high-quanta bands are observed, and assignments of these observed bands are not obvious at a glance. High-precision anharmonic vibrational simulations based on the discrete variable representation approach were performed. Decent agreements demonstrate that the present system can be a benchmark for high precision anharmonic vibrational computations of NIR spectra. Furthermore, it is interesting to compare IRPD spectra of the same systems in mid-IR to examine the differences and similarities in the vibrational coupling schemes in mid- and near-IR.

Infrared Pre-dissociation (IRPD) spectroscopy of DH₂O^+...Ar shows a rather complex series of vibrational features between 1400 – 1800 cm⁻¹and 2300 – 3400 cm⁻¹. IR-IR double-resonance spectra confirm the two isotopomers are responsible for all the peculiar vibrational features. While some of the bands have been previously identified, for example, it is known that the association band (combination of hindered rotation and bending modes) appears in the lower energy region and bending overtone and combination bands (O-H and hydrogen-bond stretch modes) tend to split the OH stretch fundamental in the higher energy region, there are far more peaks in the observed spectra that need to be accounted for. Anharmonic vibrational calculations based on potential surface at CCSD(T)/aug-cc-pvTZ were performed to elucidate the interplay of different anharmonic schemes.

Ion selectivity is a fundamental property behind many key biological or chemical processes. Crown ethers provide a valuable benchmark as a model system to understand these phenomena. The most remarkable feature of this molecule is its ability to selectively bind certain ions, depending on its ring size. 18-Crown-6 (18C6), for example, is known to selectively capture K⁺ in aqueous solution. However, gas phase studies show that 18C6 binds more strongly to smaller ions (Li⁺ and Na⁺) than K⁺. Hence, this suggests that water molecules play an important role in the selective behavior of 18C6. Although numerous studies have investigated such hydration effects applying laser spectroscopy to molecular clusters, the underlying mechanism of ion selectivity remains unclear at the molecular level. This is largely attributed to the difficulty in analyzing the backbone structure of crown ethers from their vibrational spectra. Here we present a combined experimental and theoretical study of the M⁺18C6(H₂O)_n (M=Li, Na, and K) clusters using cryogenic double ion trap IR spectroscopy[1] to focus on the CH stretch region. The CH stretch band is greatly perturbed by the Fermi resonance with CH₂ bending overtones, making spectral analysis difficult. However, our measurements and theoretical calculations incorporating anharmonic couplings successfully resolved this problem, demonstrating that the CH stretch vibrations are sensitive to the structure of 18C6, and the hydration significantly affects the encapsulated structures of the ion-18C6 complexes. [1] Y. Suzuki, K. Hirata, J. M. Lisy, S. Ishiuchi, M. Fujii, J. Phys. Chem. A. 125, 9609 (2021).

Gas-phase cyclobutyl radical (c-C₄H₇) is generated at a rotational temperature of T_rot = 26(1) K in a slit-jet discharge mixture of 70% Ne/30% He and 0.5-0.6% cyclobromobutane (c-C₄H₇Br). A fully rovibrationally resolved absorption spectrum of the α-CH stretch fundamental band between 3062.9 cm⁻¹ to 3075.7 cm⁻¹ is obtained and analyzed, yielding first precision structural and dynamical information for this novel radical species. Of particular dynamical interest, a one-dimensional potential energy surface with respect to the ring puckering coordinate computed at CCSD(T)/ANO2 level of theory reveals a double minimum C_s puckered geometry, separated by an exceedingly shallow planar C_2v transition state barrier of ∆E_barr = 1.01 cm⁻¹ (2.89 ×10⁻³ kcal/mol). Eigenvalues and eigenfunctions for this double minimum potential are solved using Numerov-Cooley methods, yielding zero-point energies for the ground state (E₀ ≈ 27 cm⁻¹) far above the interconversion barrier. Intensity alternation in the experimental spectrum due to nuclear spin statistics upon exchanging 3 pairs of identical fermionic hydrogen atoms (I_H = 1/2) matches the K_a + K_c = even \colon odd = 36 \colon 28 prediction, implying the unpaired electron in the radical center lies in an out-of-plane p_π orbital. Thus, high-resolution infrared spectroscopy provides the first experimental confirmation of a shallow double minimum ring puckering potential and a delocalized ground state wave function peaked at C_2v transition state structure for the cyclobutyl radical.

Interatomic Coulombic decay (ICD) is an important energy transfer process, where the excess energy from the de-excitation of the excited atom (or ion) within a cluster may be transferred to the neighboring atom (or molecule) and induces its ionization. It has been widely accepted that ICD-induced molecular fragmentation occurs through a two-step process, involving ICD as the first step and dissociative-electron-attachment (DEA) as the second step. However, the theoretical calculations recently proposed a one-step mechanism, where ICD directly causes the dissociation of a molecule with high efficiencyL. S. Cederbaum, J. Phys. Chem. Lett., 2020, 11: 8964. To identify the theoretical prediction, we select the ArCH₄ dimer as a prototype system and perform the fragmentation experiment by employing a transversal reaction microscope. By detecting two ions and one electron in coincidence, we can reconstruct the kinetic energy release (KER) and electron energy spectrum (EES) after ArCH₄ dissociation, and then investigate the corresponding fragmentation dynamics. In our experiment, besides the well-known ionization pathways induced by ICD producing the Ar⁺/CH₄⁺ ion pair(channel A), a coincident island (channel B) representing the Ar⁺/CH₃⁺ ion pair is also observed. Through the comparison of the KER and EES of channel B with that of channel A, we demonstrate that channel B is from the ionization dissociation reaction induced by ICD. More details, after the Ar atom is simultaneously ionized and excited, its excess energy is transferred to CH₄, populating the CH₄⁺ into excited vibrational states. The molecular ion dissociates subsequently into the CH₃⁺/H ion pair. The yield of channel B is 1.7 times larger than that of channel A, indicating it is of high efficiencyS. Yan, et al. Physical Review Letters, 2023, 131: 253001.This new decay pathway offers a novel approach to directly break covalent bonds in DNA molecules, bypassing the processes involving DEA. It has potential applications in developing new radiotherapy. L. S. Cederbaum, J. Phys. Chem. Lett., 2020, 11: 8964.. S. Yan, et al. Physical Review Letters, 2023, 131: 253001..

Metal-doped boron clusters are ideal model systems to investigate the metal-boron bonds and provide opportunities to discover novel boron nanostructures and interesting phenomena. In this talk, I will present recent studies on metal-doped boron cluster, PbB₈⁻ using photoelectron spectroscopy. Well-resolved photoelectron spectra are obtained for PbB₈⁻ and used to compare with theoretical calculations to elucidate the structures and chemical bonding properties. By adding an electron to the C_7v PbB₈ cluster, which consists of a planar B₈ disk with the Pb atom situated along the C₇ axis, the Pb atom spontaneously moves to the off-axis position in the PbB₈⁻ anion. Photoelectron spectroscopy of PbB₈⁻ reveals a broad ground state transition and a large energy gap, suggesting a highly stable closed-shell PbB₈ borozene complex and a significant geometry change upon electron detachment. Quantum chemistry calculations indicate that the LUMO of the C_7v PbB₈ cluster is a degenerate π orbital mainly consisting of the Pb 6p_x and 6p_y atomic orbitals. Occupation of one of the 6p orbitals spontaneously breaks the C_7v symmetry in the anion due to the Jahn-Teller effect. The large amplitude of the position change of Pb in PbB₈⁻ relative to PbB₈ is surprising, owing to the interactions between the Pb 6p orbital with the π orbital of the B₈ borozene.