So far, action spectroscopy in cold ion traps has been relying on a change of the ion mass by fragmentation upon photon absorption, pre-dissociation of tagged ions or via laser induced chemical reactions. Despite the advances of these techniques they could not be applied to many important ions. Here, a new method of action spectroscopy in cold ion traps, termed leak-out spectroscopy (LOS Patent pending: DE 10 2021 127 556.3 (Universität zu Köln), 22.10.2021 is presented. LOS is based on transfer of internal energy to translational energy upon collision of the excited ion with a neutral collision partner. By detecting these accelerated ions leaving the ion trap, their spectrum is recorded. P. C. Schmid, O. Asvany, T. Salomon, S. Thorwirth, and S. Schlemmer, "Leak-Out Spectroscopy, A Universal Method of Action Spectroscopy in Cold Ion Traps", J. Phys. Chem. A, 126, 8111-8117 (2022) his method is background free and may be applied to any ion (cation/anion). Ro-vibrational spectra are recorded in high-resolution. IR-MMW double resonance is used to record rotational spectra. The method also works using pulsed lasers. Recent examples will be highlighted. Moreover, due to the trapping of a finite ensemble, all ions addressed by the excitation can be kicked out. Therefore, the content of the trap can be analyzed for isomers or other isobaric but spectroscopically distinct species. This analysis can also be used for an isomer specific preparation prior to subsequent experiments.

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Footnotes:

Patent pending: DE 10 2021 127 556.3 (Universität zu Köln), 22.10.2021) P. C. Schmid, O. Asvany, T. Salomon, S. Thorwirth, and S. Schlemmer, "Leak-Out Spectroscopy, A Universal Method of Action Spectroscopy in Cold Ion Traps", J. Phys. Chem. A, 126, 8111-8117 (2022) T

A clean sample of ortho (o) and para (p) H₃⁺ is prepared in a cryogenic 22-pole ion trap by removing one of the two nuclear spin species. This isolation is reached on time scales below one second by a selective excitation of the ν₂ vibrational mode of H₃⁺ addressing a rotational state associated with one of the two species. In subsequent collisions of the excited H₃⁺ with Helium buffer gas the vibration-to-translation (V-T) energy transfer produces fast H₃⁺ of the addressed spin configuration which then leaves the trap via a small electrostatic barrier. Rotationally inelastic collisions ensure that all species belonging to one nuclear spin state are visiting the state which is subject to laser excitation. Following this protocol the o/p-ratio of H₃⁺ coming from the ion source is determined to be 1:1, as expected. In the presence of normal hydrogen, n-H₂, as an additional collision gas in the cryogenic ion trap the o/p-ratio of H₃⁺ reaches a stationary value close to 1:3. This ratio can be rationalized by the ordering of the lowest energy states of o-H₃⁺ and p-H₃⁺ as will be discussed. This work shows that nuclear spin specific but also structural isomer specific preparation and analysis is now possible in cryogenic ion traps thanks to LOS. With this aid, action spectra of isomer mixtures can be disentangled and it will become possible to determine isomer branching ratios of chemical reactions, as well as to study state-specific reactions like for the fundamental H₃⁺ + H₂ collision system considered in this work. Using a clean sample of p-H₂ as a collision gas will allow us to even determine state-to-state rate coefficients for this reaction. These rate coefficients are most important for the o/p-ratio of H₃⁺ in space.

The photodecomposition pathways of polyatomic molecules are important processes ranging from UV photodamage of biopolymers to the transformations undergone by aerosols in the troposphere. This typically occurs in complex environments in which chromophores are embedded in electrolyte solvents. In this poster, we will present an experimental scheme in which target molecular ions are complexes with ions and solvent molecules and exposed to UV light in a temperature controlled ion trap. The nascent photoproducts are then cooled and tagged with H₂ molecules so that their structures can be established by comparison of their vibrational band patterns with those calculated for candidate structures. We demonstrate this approach on the photodecarboxylation of the conjugate base of benzoylbenzoic acid, a proxy molecule widely used to simulate radical photoinitiation by UV excitation at the air-water interface. We cool and trap the phenide carbanion photoproduct as well as a radical anion generated by loss of HCO₂ from the deprotonated scaffold. We will discuss the dependence of the branching ratios for CO₂ loss vs solvent loss in the BBA-M²⁺-(solvent)_n complexes with M=Mg and Ca and solvent = CH₃CN and H₂O. Analysis of the frequency dependence of the product branching ratios supports a model in which some of the UV photon energy remains trapped in an electronically excited state, potentially revealing formation of triplet states long invoked to rationalize their propensity to act as radical photosensitizers.

Asymmetric synthesis with high stereoselectivity and yield remains a fundamental challenge in organic chemistry. Optimization of reaction parameters requires universal methods for fast and accurate characterization of chiral reaction products and intermediates with low sample consumption. In the present work, a gas-phase method for online monitoring of enantiomeric ratios is developed. The asymmetric synthesis is performed on a microfluidic chip reactor coupled to a cryogenic ion trap triple mass spectrometer. The chiral analyte is transferred from solution to the gas phase By electrospray ionization, where it is thermalized to room temperature and then interacts with a chiral selector molecule. Diastereomeric complexes are formed, and the gas phase vibrational spectra of the mass-selected complexes are recorded in the O-H and N-H stretching region. Differences in the position and intensity of the modes allow differentiation and quantification of the enantiomers. The method is first demonstrated for mixtures of L- and D-alanine. Diastereomeric complexes are formed with one to three 2-butanol molecules. For a fast determination of the diastereomeric ratio, a smaller spectral region ( 60 cm⁻¹) of the vibrational spectrum of the complex with two 2-butanol molecules is recorded. Evaluation of the data with cosine similarity matching shows that the ratio of the diastereomeric complexes determined using this method is directly transferable to the enantiomeric ratio in solution. The method is then used to optimize the parameters of an on-chip transfer hydrogenation. The influence of reaction parameters, such as the nature of the solvent and acid, on the selectivity of the reaction is optimized using this method. The influence of varying the chiral selector is also studied.

Nonlinear infrared spectroscopy in condensed phases offers valuable structural and dynamical information, but can be challenging to interpret due to factors like spectral congestion, solvent background, and low sensitivity. Cryogenic Ion Vibrational Spectroscopy (CIVS) overcomes some of these challenges by measuring infrared spectra of isolated ions quenched into their lowest-energy configurations. Spectra must be recorded indirectly using an action response, typically the photodissociation of weakly bound “tag” molecules like N₂. Traditionally, CIVS employs tunable single-color laser sources with few-cm⁻¹bandwidth and ns pulse durations. The combination of ultrafast laser sources with the sensitivity of CIVS presents an exciting new platform to directly probe vibrational dynamics in complex molecular ion systems. Here, the instrumentation and methodology for time-resolved CIVS will be discussed, and preliminary nonlinear (pump-probe) spectra of N₂-tagged tris(acetonitrile)tricarbonyl rhenium(I) (Re(CO)₃(CH₃CN)₃⁺·N₂) will be presented. The pump-probe experiments capture the bleaching signals of the asymmetric and symmetric carbonyl stretching modes. Excited-state absorption features are not observed, consistent with the expected Feynman pathways and previous action-based nonlinear measurements. The intensities of the two carbonyl stretch transitions oscillate with pump-probe delay time with a period equal to the energy difference between the transitions, demonstrating coherent coupling between the modes. These studies show the potential of combining CIVS with ultrafast sources into a single platform. This technique is expected to be extended to two-dimensional nonlinear spectroscopy and other systems of chemical and biological interest.

Metal-bearing cyanide species (MgCN, MgNC, MgC₃N, etc) have been detected in the outer circumstellar envelope of IRC+10216. However, data on the formation of these species, as well as the kinetic rates of such reactions is lacking. In our laboratory, we are developing a versatile custom-built ion instrument, which unites a glow-discharge ion source and ion trapping. This combination leverages the advantages of both techniques in reaction kinetics study. In this work, we describe not only our unique ion instrument, but also share preliminary results, details of the upcoming experiments, and future directions of implementing cavity ringdown spectroscopy to the system and making the ion trap cryogenic for astrochemically relevant research.

Cryogenic Ion Vibrational Spectroscopy (CIVS) is a tandem mass spectrometry and infrared spectroscopy technique used to gather structural information about ionic species. When coupled with a “reaction trap”, the technique is especially powerful because ion manipulations can be performed prior to spectroscopic investigation. Species which are difficult to obtain directly the solution phase (i.e. solvated species, catalytic intermediates, etc.) can be accessed in the gas phase via ion-molecules reactions. Despite the robustness of the technique, there are still limitations in the complexity of the clusters that are achievable because only one synthetic step can be performed. Additionally, the inability to perform mass selection creates heavily cluttered spectra which can hinder the subsequent spectroscopic characterization step. Here, we present a next-generation prototype instrument with a pair of cryogenic mass-selective digital ion traps. The advantages of the new design include a smaller footprint, less expensive circuitry and modular ion trap design to create a larger number of ion manipulation and mass filtering steps. The digital ion traps run on square wave RF potentials whose frequency and duty cycle can be manipulated to create high- and low-mass pass filters, respectively. Together, frequency and duty cycle filtering allow for mass selection within each cryogenic ion trap. These RF manipulations are compatible with the high pressure and low temperature necessary for solvent clustering. We show preliminary studies highlighting the performance in creating microsolvated clusters followed by in-situ mass selection of a specific cluster size. We also show that that the preferential formation of a particular cluster size can be achieved with duty cycle manipulation during the clustering process. Finally, we show that a mass-selected cluster can be transferred to a second ion trap and undergo a second clustering step, resulting in a multi-solvent cluster. Future plans for the multi-reaction trap instrument include characterizing water networks around small peptides by inserting a D₂O as a position sensitive spectroscopic molecular probe.

Many biomolecules exhibit several structural isomers, which govern their functionality. The structure and thus the function of biomolecules are strongly influenced due to hydration with single water molecules and further by hydration shells.Laage, Chem. Rev. 2017his influence can be observed in different molecular systems e.g. in the change of the selective binding in host-guest complexes involving crown ethers Glendening, JACS 1999 in the influence on the delocalized proton in charged water complexes Tuckerman, Science 1997r in intramolecular vibrational energy redistribution after excitation by photons or collisions. Spectroscopy plays an increasingly important role in the study of structural details and thereby the function in molecular systems. Wolk, Acc. Chem. Res. 2014n this contribution an experimental setup to study the structure of small biomolecular ions and the influence of hydration by means of rotationally resolved pre-dissociation spectroscopy will be presented. The setup consists of a nano-ESI, followed by two skimmers, enabling a gentle transfer into vacuum and avoiding breaking apart of the loosely bound water molecules. Further, the ions are confined in a cryogenic 16-pole wire ion trap, reaching temperatures below 3 K Geistlinger, Rev. Sci. Instrum. 2021 leading to the binding of up to four helium atoms on protonated glycine ions.Geistlinger, J. Mol. Spectrosc. 2021html:<hr /><h3>Footnotes: Laage, Chem. Rev. 2017T Glendening, JACS 1999, Tuckerman, Science 1997o Wolk, Acc. Chem. Res. 2014I Geistlinger, Rev. Sci. Instrum. 2021, Geistlinger, J. Mol. Spectrosc. 2021

The infrared spectroscopy of cryptands containing Na⁺, K⁺, Rb⁺, Sr⁺², and Ba⁺² ions are studied both theoretically and experimentally. Specifically, [2,2,2] cryptand complexes are modified to contain a phenyl group as shown below. The complexes are formed in solution and brought into the gas phase by electrospray methods. After mass selection, the complexes are introduced into a cryo-cooled octupole ion trap and cooled to 5 K. The aromatic chromophore in the crypt provides a UV absorption that is used to record a UV photofragment spectrum that shifts to unique wavelengths depending on size and charge of the embedded ion. Infrared spectra in the alkyl CH stretch region are recorded using IR-UV double resonance. The UV wavelength is fixed on the S₀-S₁ origin of the complex, while a tunable IR laser records the IR spectrum in the ground electronic state by depleting the photofragment ion signal. The resulting spectra are investigated theoretically by first searching for low-lying conformers and subsequently modeling the CH stretch fundamentals of these conformers with local mode Hamiltonians which include anharmonic coupling between CH stretches and nearby background states. In contrast to our previous work on alkanes, the ethoxy bridges enhance both CH₂ scissor and wag vibrational frequencies, with the result that scissor overtones and combination bands are detuned from the CH stretch vibrations. The increase in the wag frequencies leads to these modes playing a significant role in the spectral patterns of the CH stretch spectral region. These spectra are used to identify those conformers that are observed experimentally and to explore the role of the central ion's size in altering the structure of the crypt and, consequently, the peak patterns in the CH stretch spectral region.

The use of quantum chemistry to compute spectra, vibrational or electronic, for use in assigning gas phase spectra and structures, is common. However, there is no accepted standard for appropriate criteria for what constitutes an acceptable match, in particular for the purposes of structure determination. This is due to challenges associated with anharmonic effects, systematic deficiencies of electronic structure methods, and a host of other idiosyncratic effects. I will discuss examples of these challenges from my own lab and propose some best practices and figures of merit. I hope this will provide a judgement-free starting point for the development of more rigorous methods for comparing experimental and computed spectra.