We exploit gas-phase cluster ion techniques to provide insight into the local interactions underlying divalent metal ion-driven changes in the spectra of carboxylic acids at the air–water interface. This information clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentrations of Ca2+ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca2+·RCO2−]+·(H2O)n=0to12 and RCO2−·(H2O)n=0to14 cluster ions toward the features observed at the air–water interface. Surprisingly, not only does stepwise hydration of the RCO2− anion and the [Ca2+·RCO2−]+ contact ion pair yield solvatochromic responses in opposite directions, but in both cases, the responses of the 2 (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directions. Simulations of the [Ca2+·RCO2−]+·(H2O)n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C–C bond that connects the head group to the hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends.

We report the vibrational spectra of the hydronium and methylammonium ions captured in the C 3v binding pocket of the 18-crown-6 ether ionophore. Although the NH stretching bands of the CH 3 NH 3 + ion are consistent with harmonic expectations, the OH stretching bands of H 3 O + are surprisingly broad, appearing as a diffuse background absorption with little intensity modulation over 800 cm −1 with an onset ∼400 cm −1 below the harmonic prediction. This structure persists even when only a single OH group is present in the HD 2 O + isotopologue, while the OD stretching region displays a regular progression involving a soft mode at about 85 cm −1. These results are rationalized in a vibrationally adiabatic (VA) model in which the motion of the H 3 O + ion in the crown pocket is strongly coupled with its OH stretches. In this picture, H 3 O + resides in the center of the crown in the vibrational zeropoint level, while the minima in the VA potentials associated with the excited OH vibrational states are shifted away from the symmetrical configuration displayed by the ground state. Infrared excitation between these strongly H/D isotope-dependent VA potentials then accounts for most of the broadening in the OH stretching manifold. Specifically, low-frequency motions involving concerted motions of the crown scaffold and the H 3 O + ion are driven by a Franck-Condon-like mechanism. In essence, vibrational spectroscopy of these systems can be viewed from the perspective of photochemical interconversion between transient, isomeric forms of the complexes corresponding to the initial stage of intermolecular proton transfer.vibrational spectroscopy | hydrogen bonding | vibrationally adiabatic | proton transfer

Studies of large proteins, protein complexes, and membrane protein complexes pose new challenges, most notably the need for increased ion mobility (IM) and mass spectrometry (MS) resolution. This review covers evolutionary developments in IM‐MS in the authors' and key collaborators' laboratories with specific focus on developments that enhance the utility of IM‐MS for structural analysis. IM‐MS measurements are performed on gas phase ions, thus “structural IM‐MS” appears paradoxical—do gas phase ions retain their solution phase structure? There is growing evidence to support the notion that solution phase structure(s) can be retained by the gas phase ions. It should not go unnoticed that we use “structures” in this statement because an important feature of IM‐MS is the ability to deal with conformationally heterogeneous systems, thus providing a direct measure of conformational entropy. The extension of this work to large proteins and protein complexes has motivated our development of Fourier‐transform IM‐MS instruments, a strategy first described by Hill and coworkers in 1985 (Anal Chem, 1985, 57, pp. 402–406) that has proved to be a game‐changer in our quest to merge drift tube (DT) and ion mobility and the high mass resolution orbitrap MS instruments. DT‐IMS is the only method that allows first‐principles determinations of rotationally averaged collision cross sections (CSS), which is essential for studies of biomolecules where the conformational diversities of the molecule precludes the use of CCS calibration approaches. The Fourier transform‐IM‐orbitrap instrument described here also incorporates the full suite of native MS/IM‐MS capabilities that are currently employed in the most advanced native MS/IM‐MS instruments. © 2020 John Wiley & Sons Ltd. Mass Spec Rev

The reduction of carbon dioxide to oxalate has been studied by experimental Collisionally Induced Dissociation (CID) and vibrational characterization of the alkali metal oxalates, supplemented by theoretical electronic structure calculations.

The heterogeneous reaction of NO with sea spray aerosols yields the ClNO molecule, which is postulated to occur through water-mediated charge separation into NO and NO ions followed by association with Cl. Here we address an alternative mechanism where the attack by a halide ion can yield XNO by direct insertion in the presence of water. This was accomplished by reacting X(DO) (X = Cl, Br, I) cluster ions with NO to produce ions with stoichiometry [XNO]. These species were cooled in a 20 K ion trap and structurally characterized by vibrational spectroscopy using the D messenger tagging technique. Analysis of the resulting band patterns with DFT calculations indicates that they all correspond to exit channel ion-molecule complexes based on the association of NO with XNO, with the NO constituent increasingly perturbed in the order I > Br > Cl. These results establish that XNO can be generated even when more exoergic reaction pathways involving hydrolysis are available and demonstrate the role of the intermediate [XNO] in the formation of XNO.

We report the binding geometries of the isomers that are formed when the hydrogen oxalate ((CO2)2H=HOx) anion attaches to dinuclear coinage metal phosphine complexes of the form [M1M2dcpm2(HOx)]+ with M=Cu, Ag and dcpm=bis(dicyclohexylphosphino)methane, abbreviated [MM]+. These structures are established by comparison of isomer‐selective experimental vibrational band patterns displayed by the cryogenically cooled and N2‐tagged cations with DFT calculations of the predicted spectra for various local minima. Two isomeric classes are identified that feature either attachment of the carboxylate oxygen atoms to the two metal centers (end‐on docking) or attachment of oxygen atoms on different carbon atoms asymmetrically to the metal ions (side‐on docking). Within each class, there are additional isomeric variations according to the orientation of the OH group. This behavior indicates that HOx undergoes strong and directional coordination to [CuCu]+ but adopts a more flexible coordination to [AgAg]+. Infrared spectra of the bare ions, fragmentation thresholds and ion mobility measurements are reported to explore the behaviors of the complexes at ambient temperature.

We report the structural evolutions of water networks and solvatochromic response of the CH3NO2 – radical anion in the OH and CH stretching regions by analysis of the vibrational spectra displayed by cryogenically cooled CH3NO2 –·(H2O) n=1–6 clusters. The OH stretching bands evolve with a surprisingly large discontinuity at n = 6, which features the emergence of an intense, strongly red-shifted band along with a weaker feature that appears in the region assigned to a free OH fundamental. Very similar behavior is displayed by the perdeuterated carboxylate clusters, RCO2 –·(H2O) n=5–7 (R = CD3CD2), indicating that this behavior is a general feature in the microhydration of the triatomic anionic domain and not associated with CH oscillators. Electronic structure calculations trace this behavior to the formation of a “book” isomer of the water hexamer that adopts a configuration in which one of the water molecules resides in an acceptor–acceptor–donor (AAD) (A = acceptor, D = donor) H-bonding site. Excitation of the bound OH in the AAD site explores the local network topology best suited to stabilize an incipient −XO2H–OH–(H2O)2 intracluster proton-transfer reaction. These systems thus provide particularly clear examples where the network shape controls the potential energy landscape that governs water network-mediated, intracluster proton transfer. The CH stretching bands of the CH3NO2 –·(H2O) n=1–6 clusters also exhibit strong solvatochromic shifts, but in this case, they smoothly blue-shift with increasing hydration with no discontinuity at n = 6. This behavior is analyzed in the context of the solute-ion polarizability response and partial charge transfer to the water networks.

N 2 O 5 is an atmospheric species which serves as night-time sink for NO x species. Its reconversion to NO x products occurs through solvation in atmospheric aerosols. Detection of N 2 O 5 and NO 3 − fragmentation products in such aerosols has previously utilized chemical ionization featuring halides (of which chlorine is ubiquitous in sea-spray aerosols). We examine the solvation behavior of N 2 O 5 and the critical number of water molecules to form HNO 3 from N 2 O 5 and water. We have been able to generate and spectroscopically characterize N 2 O 5 -halide ions formed from halide-water clusters. We observe X − N 2 O 5 species whose spectra best correspond to a calculated (O 2 NX)(ONO 2 − ) species. a a Funding for this work was provided by the NSF's Center for Aerosol Impacts on Climate and the Environment.