Persistence of Dual Free Internal Rotation in the Helium “Tagged” NH4+(H2O)∙Hen=0-3 Ion-Molecule Complexes: Expanding the Case for Quantum Delocalization in He Tagging
To explore the extent of the molecular cation perturbation induced by complexation with He atoms required for the application of cryogenic ion vibrational predissociation (CIVP) spectroscopy, we compare the spectra of a bare NH4+(H2O) ion (obtained using infrared multiple photon dissociation (IRMPD)) with the one-photon CIVP spectra of the NH4+(H2O)·He1-3 clusters. Not only are the vibrational band origins minimally perturbed, but the rotational fine structures on the NH and OH asymmetric stretching vibrations, which arise from the free internal rotation of the −OH2 and −NH3 groups, also remain intact in the adducts. To establish the location and the quantum mechanical delocalization of the He atoms, we carried out diffusion Monte Carlo (DMC) calculations of the vibrational zero point wave function, which indicate that the barriers between the three equivalent minima for the He attachment are so small that the He atom wave function is delocalized over the entire −NH3 rotor, effectively restoring C3 symmetry for the embedded −NH3 group.
Thermodynamics of Water Dimer Dissocation in the Primary Hydration Shell of the Iodide Ion with Temperature-Dependent Vibrational Predissociation Spectroscopy
The strong temperature dependence of the I–·(H2O)2 vibrational predissociation spectrum is traced to the intracluster dissociation of the ion-bound water dimer into independent water monomers that remain tethered to the ion. The thermodynamics of this process is determined using van’t Hoff analysis of key features that quantify the relative populations of H-bonded and independent water molecules. The dissociation enthalpy of the isolated water dimer is thus observed to be reduced by roughly a factor of three upon attachment to the ion. The cause of this reduction is explored with electronic structure calculations of the potential energy profile for dissociation of the dimer, which suggest that both reduction of the intrinsic binding energy and vibrational zero-point effects act to weaken the intermolecular interaction between the water molecules in the first hydration shell. Additional insights are obtained by analyzing how classical trajectories of the I–·(H2O)2 system sample the extended potential energy surface with increasing temperature.
Understanding the Ionic Liquid [NC4111][NTf2] from Individual Building Blocks: An IR-Spectroscopic Study
This study explores at the molecular level the interactions underlying the IR spectra of the ionic liquid [NC4111][NTf2] and its deuterated isotopomer [d9-NC4111][NTf2] by first isolating the spectra of charged ionic building blocks using mass-selective CIVP spectroscopy and then following the evolution of these bands upon sequential assembly of the ionic constituents. The spectra of the (1,1) and (2,2) neutral ion pairs are recorded using superfluid helium droplets as well as a solid neon matrix, while those of the larger charged aggregates are again obtained with CIVP. In general, the cluster spectra are similar to that of the bulk, with the (2,2) system displaying the closest resemblance. Analysis of the polarization-dependent band intensities of the neutral ion pairs in liquid droplets as a function of external electric field yields dipole moments of the neutral aggregates. This information allows a coarse assessment of the packing structure of the neutral pairs to be antiparallel at 0.37 K, in contrast to the parallel arrangement found for the assembly of small, high-dipole neutral molecules with large rotational constants (e.g., HCN). The role of an extra anion or cation attached to both the (1,1) and the (2,2) ion pairs to form the charged clusters is discussed in the context of an additional remote, more unfavorable binding site intrinsic to the nature of the ionic IL clusters and as such not anticipated in the bulk phase. Whereas for the anion itself only the lowest energy trans conformer was observed, the higher clusters showed an additional population of the cis conformer. The interactions are found to be consistent with a minimal role of hydrogen bonding
Comparison of the local binding motifs in the imidazolium-based ionic liquids [EMIM][BF4] and [EMMIM][BF4] through cryogenic ion vibrational predissociation spectroscopy: Unraveling the roles of anharmonicity and intermolecular interactions
We clarify the role of the critical imidazolium C(2)H position (the central C between N atoms in the heterocycle) in the assembly motif of the [EMIM][BF4] ionic liquid by analyzing the vibrational spectra of the bare EMIM+ ion as well as that of the cationic [EMIM]2[BF4]+ (EMIM+ = 1-ethyl-3-methylimidazolium, C6H11N2+) cluster. Vibrational spectra of the cold, mass-selected ions are obtained using cryogenic ion vibrational predissociation (CIVP) of weakly bound D2 molecules formed in a 10 K ion trap. The C(2)H behavior is isolated by following the evolution of key vibrational features when the C(2) hydrogen, the proposed binding location of the anion to the imidazolium ring, is replaced by either deuterium or a methyl group (i.e., in the EMMIM+ analogue). Strong features in the ring CH stretching region of the bare ion are traced to Fermi resonances with overtones of lower frequency modes. Upon incorporation into the EMIM+∙∙∙BF4‒∙∙∙EMIM+ ternary complex, the C(2)H oscillator strength is dramatically increased, accounting for the much more complicated patterns derived from the EMIM+ ring CH stretches in the light isotopomer that are strongly suppressed in the deuterated analogue. Further changes in the spectra that occur when the C(2)H is replaced by a methyl group are consistent with BF4⁻ attachment directly to the imidazolium ring in an arrangement that maximizes the electrostatic interaction between the molecular ions.
Site-Specific Vibrational Spectral Signatures of Water Molecules in the "Magic" H3O+(H2O)20 and Cs+(H2O)20 Clusters
Theoretical models of proton hydration with tens of water molecules indicate that the excess proton is embedded on the surface of clathrate-like cage structures with one or two water molecules in the interior. The evidence for these structures has been indirect, however, because the experimental spectra in the critical H-bonding region of the OH stretching vibrations have been too diffuse to provide band patterns that distinguish between candidate structures predicted theoretically. Here we exploit the slow cooling afforded by cryogenic ion trapping, along with isotopic substitution, to quench water clusters attached to the H3O+ and Cs+ ions into structures that yield well resolved vibrational bands over the entire 215 – 3800 cm-1 range. The “magic” H3O+(H2O)20 cluster yields particularly clear spectral signatures that can, with the aid of ab initio predictions, be traced to specific classes of network sites in the predicted pentagonal dodecahedron H-bonded cage with the hydronium ion residing on the surface.
Microhydration of Contact Ion Pairs in M2+OH-(H2O)n=1-5 (M=Mg, Ca) Clusters: Spectral Manifestation of a Mobile Proton Defect in the First Hydration Shell
Vibrational predissociation spectra of D2-“tagged” Mg2+OH–(H2O)n=1–6 and Ca2+OH–(H2O)n=1–5 clusters are reported to explore how the M2+ OH– contact ion pairs respond to stepwise formation of the first hydration shell. In both cases, the hydroxide stretching frequency is found to red-shift strongly starting with addition of the third water molecule, quickly becoming indistinguishable from nonbonded OH groups associated with solvent water molecules by n= 5. A remarkably broad feature centered around 3200 cm–1 and spanning up to1000 cm–1 appears for the n≥ 4 clusters that we assign to a single-donor ionic hydrogen bond between a proximal first solvent shell water molecule and the embedded hydroxide ion. The extreme broadening is rationalized with a theoretical model that evaluates the range of local OH stretching frequencies predicted for the heavy particle configurations available in the zero-point vibrational wave function describing the low-frequency modes. The implication of this treatment is that extreme broadening in the vibrational spectrum need not arise from thermal fluctuations in the ion ensemble, but can rather reflect combination bands based on the OH stretching fundamental that involve many quanta of low-frequency modes whose displacements strongly modulate the OH stretching frequency.
Vibrational spectral signature of the proton defect in the three-dimensional H+(H2O)21 cluster
The way in which a three-dimensional network of water molecules accommodates an excess proton is hard to discern from the broad vibrational spectra of dilute acids. The sharper bands displayed by cold gas phase clusters, H+(H2O)n, are therefore useful because they encode the network-dependent speciation of the proton defect and yet are small enough to be accurately treated with electronic structure theory. Here we identify the previously elusive spectral signature of the proton defect in the three-dimensional cage structure adopted by the particularly stable H+(H2O)21 cluster. Cryogenically cooling the ion and tagging it with loosely bound deuterium (D2) enabled detection of its vibrational spectrum over the 600 to 4000 cm-1 range. The excess charge is consistent with a tri-coordinated H3O+ moiety embedded on the surface of a clathrate-like cage.
Communication: He-tagged vibrational spectra of the SarGlyH+ and H+(H2O)2,3 ions: Quantifying Tag Effects in Cryogenic Ion Vibrational Predissociation (CIVP) spectroscopy
To assess the degree to which more perturbative, but widely used “tag” species (Ar, H2, Ne) affect the intrinsic band patterns of the isolated ions, we describe the extension of mass-selective, cryogenic ion vibrational spectroscopy to the very weakly interacting helium complexes of three archetypal ions: the dipeptide SarGlyH+ and the small protonated water clusters: H+(H2O)2,3, including the H5O2+ “Zundel” ion. He adducts were generated in a 4.5 K octopole ion trap interfaced to a double-focusing, tandem time-of-flight photofragmentation mass spectrometer to record mass-selected vibrational predissociation spectra. The H2 tag-induced shift (relative to that by He) on the tag-bound NH stretch of the SarGlyH+ spectrum is quite small (12 cm-1), while the effect on the floppy H5O2+ ion is more dramatic (125 cm-1) in going from Ar (or H2) to Ne. The shifts from Ne to He, on the other hand, while quantitatively significant (maximum of 10 cm-1), display the same basic H5O2+ band structure, indicating that the He-tagged H5O2+ spectrum accurately represents the delocalized nature of the vibrational zero-point level. Interestingly, the He-tagged spectrum of H+(H2O)3 reveals the location of the non-bonded OH group on the central H3O+ ion to fall between the collective non-bonded OH stretches on the flanking water molecules in a position typically associated with a neutral OH group.
Ion Mobility Spectrometry, Infrared Dissociation Spectroscopy and ab initio Computations towards Structural Characterization of the Deprotonated Leucine Enkephalin Peptide Anion in the Gas-Phase
Although the sequencing of protonated proteins and peptides with tandem mass spectrometry has blossomed into a powerful means of characterizing the proteome, much less effort has been directed at their deprotonated analogues, which can offer complementary sequence information. We present a unified approach to characterize the structure and intermolecular interactions present in the gas-phase pentapeptide leucine-enkephalin anion by several vibrational spectroscopy schemes as well as by ion-mobility spectrometry, all of which are analyzed with the help of quantum-chemical computations. The picture emerging from this study is that deprotonation takes place at the C terminus. In this configuration, the excess charge is stabilized by strong intramolecular hydrogen bonds to two backbone amide groups and thus provides a detailed picture of a potentially common charge accommodation motif in peptide anions.
On the Character of the Cyclic Ionic H-Bond in Cryogenically Cooled Deprotonated Cysteine
Modes of Activation of Organometallic Iridium Complexes for Catalytic Water and C-H Oxidation
Cryogenic Ion Chemistry and Spectroscopy
The use of mass spectrometry in macromolecular analysis is an incredibly important technique, and has allowed efficient identification of secondary and tertiary protein structures.Over twenty years ago, Chemistry Nobelist John Fenn and co-workers revolutionized themethod by developing ways to non-destructively extract large molecules directly from solutioninto the gas phase. This advance, in turn, enabled rapid sequencing of biopolymers through tandem mass spectrometry at the heart off the burgeoning field of proteomics. In this Account, we discuss how cryogenic cooling, mass selection, and reactive processing together provide a powerful way to characterize ion structures as well as rationally synthesize labile reaction intermediates. We can accomplish this by first cooling the ions close to 10K, then condensing onto them weakly bound, chemically inert small molecules or rare gas atoms. We can use this assembly as a medium in which to quench reactive encounters by rapid evaporation of the adducts, as well as to provide a universal means for acquiring highly resolved vibrational action spectra of the embedded species by photoinduced mass-loss. Moreover, we can obtain the spectroscopic measurements with readily available, broadly tunable pulsedinfrared lasers because absorption of a single photon is sufficient to induce evaporation. We discuss the implementation of these methods with a new type of hybrid photofragmentation mass spectrometer involving two stages of mass selection with two laser excitation regionsinterfaced to the cryogenic ion source.
We illustrate several capabilities of the cryogenic ion spectrometer by presenting recentapplications to peptides, a biomimetic catalyst, a large antibiotic molecule (vancomycin), and reaction intermediates pertinent to the chemistry of the ionosphere. First, we demonstrate how site-specific isotopic substitution can be used to identify bands due to local functional groups in a protonated tripeptide designed to stereoselectively catalyze bromination of biaryl substrates. This procedure directly reveals the particular H-bond donor and acceptor groups that enforce the folded structure of the bare ion as well as provide contact points for non-covalent interaction with substrates. We then show how photochemical hole-burning involving only vibrational excitations can be used in a double-resonance mode to systematically disentangle overlapping spectra that arise when several conformers of a dipeptide are prepared in the ion source. Finally, we highlight our ability to systematically capture reaction intermediates and spectroscopically characterize their structures. Through this method, we can identify the pathway for water-network-mediated, proton coupled transformation of nitrosonium, NO+ to HONO, a key reaction controlling the cations present in the ionosphere.
Through this work, we reveal the critical role played by water molecules occupying the second solvation shell around the ion, where they stabilize the emergent product ion in a fashion reminiscent of the solvent coordinate responsible for the barrier to charge transfer in solution. Looking to the future, we predict that the capture and characterization of fleeting intermediate complexes in the homogeneous catalytic activation of small molecules like water, alkanes, and CO2 is a likely avenue rich with opportunity.
Ionic liquids from the bottom up: Local assembly motifs in [EMIM][BF4] through cryogenic ion spectroscopy
To clarify the intramolecular distortions suffered by the complementary ions in an archetypal ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4], we report the vibrational spectra of the isolated ionic constituents and small aggregates cooled to about 10 K. Deuteration of bare EMIM+ at the C(2) position, the putative hydrogen bond donating group, establishes that the bulk shift is too small (<10 cm-1) for hydrogen bonding to be a dominant structural feature. We then analyze how the vibrational patterns evolve with increasing size to identify the spectral signatures of well-defined structural motifs in the growing assembly. Surprisingly, the main features of the bulk spectrum are already developed in the cluster with a single BF4- anion sandwiched between just two EMIM+ cations, suggesting that this local motif, while not strongly hydrogen bonded, nonetheless induces considerable intensity in the C(2)H stretches and is a robust feature in the local molecular structure of the liquid.
Hiding in Plain Sight: Unmasking the Diffuse Spectral Signatures of the Protonated N-Terminus in Simple Peptides
Survey vibrational spectra of several representative protonated peptides reveal very diffuse absorptions near 2500 cm-1 that are traced to pentagonal cyclic ionic hydrogen bonds (C5 interactions) involving the excess charge centers. This broadening occurs despite the fact that the ions are cooled close to their vibrational zero-point levels and their spectra are obtained by predissociation of weakly bound adducts (H2, N2, CO2) prepared in a cryogenic ion trap. The C5 band assignments are based on H/D isotopic substitution, chemical derivatization, solvation behavior, and calculated spectra. We evaluate the extent to which this broadening is caused by cubic coupling terms in the normal mode expansion of the potential energy surface, and find that the discrete mixed states extend over the range of the observed features. While this qualitatively reproduces the deuterated isotopologue of GlyGlyH+, the essentially continuous nature of the higher energy band in the light isotopologue points to extensive mixing with background vibrational states.
Integration of cryogenic ion vibrational predissociation spectroscopy with a mass spectrometric interface to an electrochemical cell
Cryogenic ion vibrational predissociation (CIVP) spectroscopy is used to structurally characterize electrochemically (EC) generated oxidation products of the benchmark compound reserpine. Ionic products were isolated using EC-ESI coupled to a 25 K ion trap prior to injection into a double focusing, tandem time-of-flight photofragmentation mass spectrometer. Vibrational predissociation spectroscopy was carried out by photoevaporation of weakly bound N2 adducts over the range 800-3800 cm-1 in a linear (i.e. single photon) action regime, thus enabling direct comparison of the experimental vibrational pattern with harmonic calculations. The locations of the NH and OH stretching fundamentals are most consistent with formation of 9‑hydroxyreserpine, which is a different isomer than considered previously. This approach thus provides a powerful structural dimension for the analysis of electrochemical processes detected with the sensitivity of mass spectrometry.