Spectroscopy of Reactive Complexes and Solvated Clusters: A Bottom

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Spectroscopy of Reactive Complexes and Solvated Clusters: A Bottom-Up Approach Using Cryogenic Ion Traps Etienne Garand J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05712 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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The Journal of Physical Chemistry

Spectroscopy of Reactive Complexes and Solvated Clusters: A Bottom-Up Approach Using Cryogenic Ion Traps Etienne Garand*

Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, United States

*Author to whom correspondence should be addressed email: [email protected]

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Abstract In this paper, applications of cryogenic ion traps for forming reaction intermediates and solvated clusters from precursor ions generated by electrospray ionization are presented and discussed. These studies are motivated by the aim of spectroscopically probing isolated complexes that exhibit higher levels of complexity in chemical compositions and intermolecular interactions, which make them more closely resemble the systems existing in real-world environments. Illustrative examples are provided to highlight the current capabilities, showcase the detailed information available in the spectroscopic results, and outline general future directions.


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1. Introduction Studying isolated molecular ions and clusters in the gas phase has many advantages,1-3 one of them being the ability to access molecular objects with well-defined structures and internal energies under a controlled environment. This can yield a more straightforward comparison between experimental and theoretical results, leading to a positive feedback loop in which theory can help to interpret experimental results and experiments can help to benchmark theories. Another crucial advantage is the ability to access species or structures that are difficult to isolated otherwise, such as reactive intermediates that do not buildup sufficient concentrations to be directly observed in the condensed phase. Moreover, gas phase manipulations, such as photoexcitation or temperature-controlled collisional reaction, can further afford access to different parts of the potential energy surface, allowing piecemeal studies to come together to build a comprehensive picture. The results from all of these studies can contribute to the understanding of condensed phase observations, where the environment is more heterogeneous and interpretation more ambiguous. However, making such connections from isolated gas phase molecules and clusters to “real-world” systems is not always straightforward. Hence, there are increasing efforts directed toward studying gas phase systems exhibiting higher levels of complexity, in chemical compositions or intermolecular interactions, that approach those existing in real-world environments, all the while still maintaining the simplicity and control that enable the detailed and precise interpretation of the experimental results. The field of mass spectrometry (MS) has implemented various approaches for bringing large molecular species into the gas phase. The most commonly used one is the electrospray ionization (ESI)4-5 source, which allows for a diverse array of solution-phase ions to be gently transferred into the vacuum chambers of a mass spectrometer. However, the atmospheric pressure nature of this ion source imposes certain restrictions on the reactivity and fragility of the produced ions, i.e., the vast majority of species produced by ESI are representative of stable ionic species in solution or strongly interacting ion-molecule complexes. In contrast, supersonic expansion sources can rapidly cool and transfer ions into the vacuum, affording the capability to produce interesting species such as highly reactive intermediates or cold clusters consisting of very weakly interacting partners.6-8 The limitations of supersonic expansion source lie in the requirement of relatively robust molecular precursors with significant vapor pressure, and the



size of the clusters produced can be further capped by the limited cooling capacity of the expansion. Of course, there are a myriad of modifications that can variously expand the applicability of both types of ion sources, such as coupling laser ablation7, 9 or laser desorption10 to a pulsed supersonic expansion. In this paper, we present the application of cryogenic ion traps, used as post-processing vessels for ions produced by ESI, to access reactive intermediates and solvated clusters. The advantages of the cryogenic ion trap setup lie in its broad applicability and its relatively straightforward incorporation into a typical ESI-MS setup. Moreover, it is generally compatible with cryogenic ion spectroscopy, allowing for direct spectroscopic characterization of the produced species. Cryogenic ion traps were first utilized to thermalize ions and study low-temperature ionmolecule reactions.11-13 Their combination with ESI sources is a more recent event, with a focus mostly directed at cooling large ions before spectroscopic characterization. Such an approach for quenching the internal motions in an ESI ion goes a long way towards producing well-resolved vibrational and electronic spectra that are easier to interpret and convey more detailed structural information.14-16 Extending these developments, infrared action spectroscopy coupled to ESIMS was made more versatile by forming weakly-bound “tagged” adducts in cryogenic ion traps.17-18 These tags serve as messengers for single photon infrared absorption,19-22 making it possible to acquire IR spectra of mass-selected ESI ions with tabletop lasers. A few years ago, we introduced a dual cryogenic ion trap setup23 with the aim of overcoming limitations of the ESI source while maintaining the ability to form tagged adducts. In our instrument, one of the temperature-controlled ion traps is used to process ESI ions to construct the desired species, either by condensing solvent molecules onto an ion or performing specific ion-molecule reactions. These complexes are then cooled and tagged in a second ion trap. The temperatures of these two sequential ion traps are maintained independently to allow for optimal conditions in both, affording greater versatility and control. Thus, we are able to study gradually more complex systems while still having access to the full power of theoretical comparisons. Below, after a brief discussion of the experimental approach and instrumentation, examples are presented that illustrate various applications in accessing and characterizing reactive intermediates and weakly bound clusters. The possible challenges involved in studying


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well-defined but ever more complex gas phase systems as well as some strategies that go toward addressing them are also discussed. 2. General Experimental Concerns 2a. Infrared Predissociation (IRPD) Spectroscopy of Tagged Ions The low number density (typically 105-107 ions/cm3) and total number of ions (103-105) present in mass-selective experiments require the use of action spectroscopy to obtain spectroscopic information. For vibrational spectroscopy, the commonly used schemes are infrared multiple photon dissociation (IRMPD)24-30 and IR-UV double resonance,16, 31-35 both of which provide sufficient energy to dissociate a covalent bond. Another approach is to form an adduct containing a weak non-covalent bond to an inert “tag” that can be broken with the energy of a single IR photon.18-19, 21-22, 36-41 These IRPD experiments can be performed using moderate power table-top lasers and are not limited to molecules with suitable UV chromophores. The tradeoff is that the formation of these weakly bound adducts is not always easy and requires cryogenic conditions. Moreover, the tag is not completely innocent and can influence the structure and vibrational spectrum of the ion.36, 42-44 Efficient adduct formation necessitates the internal energy of the complex to be close to or below the binding energy of the tag. For ESI ions, internal energy can be removed by collisions with buffer gas inside of a cryogenically cooled ion trap. The exact temperature that the ion trap is held at depends on the choice of tagging species. Figure 1. Estimated vapor pressures as a function of temperature for various tagging species in the 0-100 K range. A typical binding energy of each tag to a singly charged positive ion is indicated at the top of each curves.

Figure 1 shows the estimated vapor pressure45 of a few possible tags, and highlights why hydrogen is our

preferred tag species. Its vapor pressure is high even at 10 K, such that condensation onto the 5 ACS Paragon Plus Environment


trap electrodes can be avoided while achieving ion internal temperatures of

Spectroscopy of Reactive Complexes and Solvated Clusters: A Bottom

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