We compare the intensity ratio of symmetric to antisymmetric C−O stretching vibrational transitions in CO₂⁻ and MCOO⁻ (M = H, Ag and Bi) using photodissociation spectroscopy. This ratio depends strongly on the bonding partner M, caused by a dynamic change in the molecular charge distribution during vibrational motion. Density functional theory calculations indicate that such charge oscillations can occur for both the symmetric and antisymmetric C−O stretching vibrations in these systems. In the symmetric C−O stretching modes, however, they are at play only if a bonding partner is present, which acts as a reservoir for charge during CO bond compression in the symmetric stretching vibration.

La atom reactions with acetonitrile (CH₃CN) and pentanenitrile (C₄H₉CN) are carried out in a laser-vaporization supersonic molecular beam source. Metal-containing species are observed using time-of-flight mass spectrometry. In this talk, we report the mass-analyzed threshold ionization (MATI) spectroscopic characterization of two metal-containing radicals, La(CH₃CN) and La(C₄H₉CN), formed by La associations with acetonitrile and pentanenitrile, respectively. Adiabatic ionization energies of the two La-alkane nitrile species and their vibrational frequencies are measured from the MATI spectra. Metal-ligand binding modes and molecular structures are investigated by comparing the spectroscopic measurements with density functional theory calculations and spectral simulations. For both alkane nitriles, the preferred La binding site is identified to be the nitrile group with a π-bind mode, the resultant metal complexes are three-membered metallacycles. While a single isomer is observed for La(CH₃CN), two rotational conformers are identified for La(C₄H₉CN). The binding and structures of these metal-alkane nitrile radicals are different from those formed by metal ion reactions, where metal ions were reported to favor σ binding with the nitrogen atom.K. Eller, W. Zummack, H. Schwarz, L. M. Roth, B. S. Freiser, J. Am. Chem. Soc., 1991, 113, 833-839html:<hr /><h3>Footnotes: K. Eller, W. Zummack, H. Schwarz, L. M. Roth, B. S. Freiser, J. Am. Chem. Soc., 1991, 113, 833-839

Ce(C₂H₂) and Ce(C₄H₆) are produced by the Ce-mediated ethylene activation and investigated by mass-analyzed threshold ionization (MATI) spectroscopy, isotopic substitutions, and relativistic quantum chemical computations. The MATI spectrum of Ce(C₂H₂) exhibits two nearly identical band systems separated by 128 cm⁻¹, and that of Ce(C₄H₆) shows three similar band systems separated by 55 and 105 cm⁻¹. These separations are not affected by deuteration. The observed band systems for the two Ce-hydrocarbon species are attributed to the spin-orbit splitting arising from interactions of triplet and singlet states. Ce(C₂H₂) is a metallacyclopropene in C_2v symmetry, and Ce(C₄H₆) is a metallacyclopentene in C_s symmetry. The low-energy valence electron configurations of the neutral and ionic states of each species are Ce 4f¹6s¹ and Ce 4f¹, respectively. The remaining two electrons that are associated with the isolated Ce atom or ion are spin paired in a molecular orbital that is a bonding combination between a Ce 5d orbital and a hydrocarbon π* antibonding orbital.

Incorporation of the unnatural D-proline (^DP) stereoisomer into a polypeptide sequence is a typical strategy to encourage formation of β-hairpin loops because natural sequences are often unstructured in solution. Using conformation-specific IR and UV spectroscopy of cold (10 K) gas-phase ions, we probe the inherent conformational preferences of the ^DP and ^LP diastereomers in the protonated peptide [YAPAA+H]⁺, where only intramolecular interactions are possible. Consistent with the solution phase studies, one of the conformers of [YADPAA+H]⁺ is folded into a charge-stabilized β-hairpin turn. However, a second predominant conformer family containing two sequential γ-turns is also identified, with similar energetic stability. A single conformational isomer of the ^LP diastereomer, [YALPAA+H]⁺, is found and assigned to a structure that is not the anticipated “mirror image” β-turn. Instead, the ^LP stereo center promotes a cis alanine-proline amide bond. The assigned structures contain clues that the preference of the ^DP diastereomer to support a trans-amide bond and the proclivity of ^LP for a cis-amide bond is sterically driven and can be reversed by substituting glycine for alanine in position 2, forming [YGLPAA+H]⁺. These results provide a basis for understanding the residue-specific and stereo-specific alterations in the potential energy surface that underlie these changing preferences, providing insights to the origin of β-hairpin formation.

In the past decade, infrared and ultraviolet spectroscopy in cryo-cooled ion traps have become workhorse techniques to characterize the gas-phase 3D structures of biological ions. Often, multiple conformers of a single molecular ion are observed. While slow collisional cooling should result in funneling of many structures into a single minimum, recent studies show evidence for the kinetic trapping of entropically-favored structures near room temperature when these species are cooled to about 10K. In order to elucidate how the initial population fractionates during the cooling process, we use a variety of conformer specific IR-UV double resonance techniques to measure population distributions of the peptide ion [YGPAA+H]⁺ in the gas phase at 10K. Previous studies conducted in our lab show the YGPAA peptide adopts two spectroscopically distinct conformers which differ principally in the cis/trans configuration of the carboxylic acid group at the C-terminus. By using IR-UV hole filing spectroscopy (HFS) and population transfer spectroscopy (PTS) we demonstrate the ability to selectively excite and interconvert between conformations and to quantitatively measure the distribution of conformer populations within the ion trap. Experimentally, we find a 65:35 ratio for the trans:cis conformer population. These conformers are connected through a single calculated transition state, allowing intramolecular isomerization rates and equilibrium population distributions to be calculated by Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The relationship between the observed population ratio and the temperature-dependent equilibrium constant will be discussed.

The introduction of non-native D-amino acids into peptides is known to reduce conformational entropy in peptides. D-proline has been shown to promote the formation of β-hairpin loops when paired with Gly, providing a framework for building these loops with different lengths of anti-parallel beta-sheet. This study seeks to characterize and compare the conformational preferences of a model protonated pentapeptide containing DPG, [YAP^DGA+H]⁺, with its L-Pro counterpart via conformation specific cold ion spectroscopy as a foundation for future consideration of larger beta-hairpin models. The UV spectrum of YAP^DGA of the Tyr chromophore is beautifully sharp, but contains a complicated set of transitions that could arise from the presence of more than one conformer. To assess this possibility, we recorded non-conformation specific IR “gain” spectra in the hydride stretch region. The IR spectrum so obtained displays a set of five strong IR transitions that bear a close resemblance to those found in one of the conformers of its close analog, [YAP^DAA+H]⁺, signaling that a single conformer dominates the population. Two transitions at 3392 and 3464 cm-1 are slightly shifted versions of the C10 and C14 hydrogen bonds found in one of the conformers of [YAP^DAA+H]⁺, and are characteristic of formation of a β-hairpin loop. Notably, in [YAP^DGA+H]⁺, there is at most a minor second conformer with a free carboxylic acid OH, appearing weakly in the IR “gain” spectrum. As expected, the UV spectrum of YAP^LGA is more congested, which suggests the presence of multiple conformers. Further investigation into this peptide will reveal the conformational preferences of the L-pro containing molecule. Preliminary data affirms that D-proline containing peptides show reduced conformational states when compared to their natural counterparts.

Infrared signatures of three molecular ions of relevance to the interstellar medium and planetary atmospheres have been detected at the Free Electron Laser for Infrared eXperiments, FELIX, at Radboud University (Nijmegen, The Netherlands) in combination with the 4K FELion 22-pole ion trap facility. Mid-infrared vibrational modes of protonated tricarbon monoxide, HC₃O⁺, protonated vinyl cyanide, C₂H₃CNH⁺, and protonated ethyl cyanide, C₂H₅CNH⁺, were detected using resonant photodissociation of the respective Ne-complexes by monitoring the depletion of their cluster mass signal as a function of wavenumber. The infrared fingerprints compare very favorably with results from high-level quantum-chemical calculations performed at the CCSD(T) level of theory.

Molecular ions play a central role in the chemistry of the interstellar medium (ISM) and act as benchmarks for state of the art ab initio theory. The molecular ion H₃⁺ initiates a chain of ion-neutral reactions which drives chemistry in the ISM, and observing it either directly or indirectly through its isotopologues is valuable for understanding interstellar chemistry. Improving the accuracy of laboratory measurements will assist future astronomical observations. H₃⁺ is also one of a few systems whose rovibrational transitions can be predicted to spectroscopic accuracy ( < 1 cm⁻¹), and with careful treatment of adiabatic, nonadiabatic, and quantum electrodynamic corrections to the potential energy surface, predictions of low lying rovibrational states can rival the uncertainty of experimental measurementsL. G. Diniz, J. R. Mohallem, A. Alijah, M. Pavanello, L. Adamowicz, O. L. Polyansky and J. Tennyson, Phys. Rev. A (2013), 88, 032506. New experimental data will be needed to benchmark future treatment of these corrections. Previously we have reported 26 transitions within the fundamental band of H₃⁺ with MHz-level uncertaintiesJ. N. Hodges, A. J. Perry, P. A. Jenkins II, B. M. Siller, and B. J. McCall, J. Chem. Phys. (2013), 139, 164201.A. J. Perry, J. N. Hodges, C. R. Markus, G. S. Kocheril, and B. J. McCall, J. Mol. Spectrosc. (2015), 317, 71-73.A. J. Perry, C. R. Markus, J. N. Hodges, G. S. Kocheril, and B. J. McCall, 71st International Symposium on Molecular Spectroscopy (2016), MH03 With recent improvements to our overall sensitivityC. R. Markus, A. J. Perry, J. N. Hodges, and B. J. McCall, Opt. Express (2017), 25, 3709-3721. we have expanded this survey to include additional transitions within the fundamental band and the first hot band. These new data will ultimately be used to predict ground state rovibrational energy levels through combination differences which will act as benchmarks for ab initio theory and predict forbidden rotational transitions of H₃⁺. We will also discuss progress in measuring rovibrational transitions of the isotopologues H₂D⁺ and D₂H⁺, which will be used to assist in future THz astronomical observations.

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Footnotes:

L. G. Diniz, J. R. Mohallem, A. Alijah, M. Pavanello, L. Adamowicz, O. L. Polyansky and J. Tennyson, Phys. Rev. A (2013), 88, 032506.. J. N. Hodges, A. J. Perry, P. A. Jenkins II, B. M. Siller, and B. J. McCall, J. Chem. Phys. (2013), 139, 164201.

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Footnotes:

A. J. Perry, C. R. Markus, J. N. Hodges, G. S. Kocheril, and B. J. McCall, 71st International Symposium on Molecular Spectroscopy (2016), MH03. C. R. Markus, A. J. Perry, J. N. Hodges, and B. J. McCall, Opt. Express (2017), 25, 3709-3721.,

Diatomic molecular ions that contain alkaline earth atoms are of interest for experiments involving ultra-cold molecular ions. The alkaline earth atomic cations are well suited for laser cooling as they have transitions that are analogous to those of the alkali metals. Hence, Coulomb crystals are readily formed in rf traps. Reactions of these atomic ions yield diatomic products that are sympathetically cooled to low translational temperatures by the surrounding atomic ions. In principle, spectroscopic measurements may be used to probe the internal energies of the molecular ions. However, gas phase spectroscopic data for the ions of interest are lacking. In the present study we have investigated CaO⁺ using pulsed field ionization-zero kinetic energy photoelectron spectroscopy (PFI-ZEKE). Molecular constants for low energy vibrational levels for the ground state (²Π_3/2) and two electronic states (²Π_1/2 and ²Σ⁺) have been determined. These measurements also provide the first accurate value for the ionization energy of CaO. Comparisons with high-level theoretical calculations will be discussed.

Laser-produced tin plasmas are the prime candidates for the generation of extreme ultraviolet (EUV) light around 13.5 nm in nanolithographic applications. This light is generated primarily by atomic transitions in highly charged tin ions: Sn⁸⁺-Sn¹⁴⁺. Due to the electronic configurations of these charge states, thousands of atomic lines emit around 13.5 nm, clustered in a so-called unresolved transition array. As a result, accurate line identification becomes difficult in this regime. Nevertheless, this issue can be circumvented if one turns to the optical: with far fewer atomic states, only tens of transitions take place and the spectra can be resolved with far more ease. We have investigated optical emission lines in an electron-beam-ion-trap (EBIT), where we managed to charge-state resolve the spectra. Based on this technique and on a number of different ab initio techniques for calculating the level structure, the optical spectra could be assigned [1,2]. As a conclusion the assignments of EUV transitions in the literature require corrections. The EUV and optical spectra are measured simultaneously in the controlled conditions of the EBIT as well as in a droplet-based laser-produced plasma source providing information on the contribution of Sn^q+ charge states to the EUV emission. [1] A. Windberger, F. Torretti, A. Borschevsky, A. Ryabtsev, S. Dobrodey, H. Bekker, E. Eliav, U. Kaldor, W. Ubachs, R. Hoekstra, J.R. Crespo Lopez-Urrutia, O.O. Versolato, Analysis of the fine structure of Sn¹¹⁺ - Sn¹⁴⁺ ions by optical spectroscopy in an electron beam ion trap, Phys. Rev. A 94, 012506 (2016). [2] F. Torretti, A. Windberger, A. Ryabtsev, S. Dobrodey, H. Bekker, W. Ubachs, R. Hoekstra, E.V. Kahl, J.C. Berengut, J.R. Crespo Lopez-Urrutia, O.O. Versolato, Optical spectroscopy of complex open 4d-shell ions Sn⁷⁺ - Sn¹⁰⁺, arXiv:1612.00747