Helium atoms can attach to molecular cations via ternary collision processes forming weakly bound ( ≈ 1 kcal/mol) He-M⁺ complexes. We developed a novel sensitive action spectroscopic scheme for molecular ions based on an observed rotational state dependency of the He attachment process [1]. A detailed account of the underlying kinetics will be presented on the example of the CD⁺ ion, where our studies indicate a decrease of around 50% for the rotational state dependent ternary He attachment rate coefficient of the J=1 level with respect to the J=0 level. Experiments are performed on mass-selected ions stored in a temperature-variable (T ≥ 3.9 K) cryogenic rf 22-pole ion trap in the presence of a high number density of He ( ≈ 10¹⁵ cm⁻³) [2]. Rotational spectra of the bare ions are recorded by measuring the change in the number of formed He-M⁺ complexes after a certain storage time as a function of excitation wavelength. Here we will also present the first measurements of the rotational ground state transitions of CF⁺ (J=1−0, hfs resolved) and NH₃D⁺ (J_K=1₀−0₀), recorded in this way.
[1] Brünken et al., ApJL 783, L4 (2014)
[2] Asvany et al., Applied Physics B 114, 203 (2014)

In the customary approach to the theoretical description of the nuclear motion in molecules, the molecule is seen as a near-static structure rotating in space. Vibrational motion causing small structural deformations induces a perturbative treatment of the rotation-vibration interaction, which fails in fluxional molecules, where all vibrational motions are large compared to the linear extension of the molecule. An example is protonated methane (CH₅⁺) P. Kumar and D. Marx, Physical Chemistry Chemical Physics 8, 573 (2006); Z. Jin, B. J. Braams, and J. M. Bowman, The Journal of Physical Chemistry A 110, 1569 (2006); A. S. Petit, J. E. Ford, and A. B. McCoy, The Journal of Physical Chemistry A 118, 7206 (2014). For this molecule, customary theory fails to simulate reliably even the low-energy spectrum. Within the traditional view of rotation and vibration being near-separable, rotational and vibrational wavefunctions can be symmetry classified separately in the molecular symmetry (MS) group P.R. Bunker and P. Jensen, Molecular Symmetry and Spectroscopy (NRC Research Press, Ottawa, Canada, 1998). In the present contribution we discuss a fundamental group theoretical approach to the problem of determining the symmetries of molecular rotation-vibration states. We will show that all MS groups discussed so far are subgroups of the special orthogonal group in three dimensions SO(3)Being precise, we must include O(3) and SU(2), but our theory can be easily extended to these two groups. This leads to a group theoretical foundation of the technique of equivalent rotations H. Longuet-Higgins, Molecular Physics 6, 445 (1963). The MS group of protonated methane (G₂₄₀) represents, to the best of our knowledge, the first example of an MS group which is not a subgroup of SO(3) (nor of O(3) nor of SU(2)). Because of this, a separate symmetry classification of vibrational and rotational wavefunctions becomes impossible in this MS group, consistent with the fact that a decoupling of vibrational and rotational motion is impossible. We want to discuss the consequences of this. In conclusion, we show that the prototypical floppy molecule CH₅⁺ represents a new class of molecules, where usual group theoretical methods for determining selection rules and spectral assignments fail so that new methods have to be developed.

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Footnotes:

P. Kumar and D. Marx, Physical Chemistry Chemical Physics 8, 573 (2006); Z. Jin, B. J. Braams, and J. M. Bowman, The Journal of Physical Chemistry A 110, 1569 (2006); A. S. Petit, J. E. Ford, and A. B. McCoy, The Journal of Physical Chemistry A 118, 7206 (2014).. P.R. Bunker and P. Jensen, Molecular Symmetry and Spectroscopy (NRC Research Press, Ottawa, Canada, 1998).. Being precise, we must include O(3) and SU(2), but our theory can be easily extended to these two groups.. H. Longuet-Higgins, Molecular Physics 6, 445 (1963)..

H₅⁺ is a highly fluxional intermediate found in interstellar clouds. The rotational/torsional couplings in this molecule are of great interest due to the unusually large coupling between these modes. However, theoretical studies of highly fluxional molecules like H₅⁺ are challenging due to the lack of a good zero-order model. In order to better understand the rotation/vibration interaction, a method has been developed to model the rotational/torsional motions. This method is based upon diffusion Monte Carlo (DMC). In this approach, the vibrational contribution to the wavefunction is modeled using standard DMC approaches, while the rotational/torsional contribution is treated as a set of coefficients that are assigned to the various rotational/torsional state vectors. The potential portion of the Hamiltonian is expressed as a low-order expansion in terms of the torsion angle between the two outer H₂ units. The expansion coefficients are evaluated at each time step for each walker and depend on the 3N−7 other internal coordinates. The transition frequencies obtained from this method for J ≤ 1 agree well with results obtained using other methods such as fixed-node diffusion Monte Carlo.Sarka, J.; Fábri, C.; Szidarovszky, T.; Császár, A.G.; Lin Z.; McCoy, A.B., “Modeling Rotations, Vibrations, and Rovibrational Couplings in Astructural Molecules - A Case Study Based on the H₅⁺ Molecular Ion.”, accepted by Mol. Phys.his new method is advantageous over the fixed-node approach because it allows for multiple state calculations at once which saves on computation time.

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Footnotes:

Sarka, J.; Fábri, C.; Szidarovszky, T.; Császár, A.G.; Lin Z.; McCoy, A.B., “Modeling Rotations, Vibrations, and Rovibrational Couplings in Astructural Molecules - A Case Study Based on the H₅⁺ Molecular Ion.”, accepted by Mol. Phys.T

Frequencies of high-J rotational lines of HCO⁺ and its isotopologues have been measured precisely by using an Evenson-type spectrometer in Toyama. The tunable far-infrared spectrometer (TuFIR in short) is based on synthesizing terahertz radiation from two mid-infrared CO₂ laser lines and one microwave source. Study of the isotopologues containing H or D, ¹²C, and ¹⁶O were reported last year. In the present work, isotopologues of H or D, ¹³C, and ¹⁶O have been studied. The HCO⁺ ions are produced by discharging a ¹³CO, H₂ (or D₂), and Ar mixture in an extended negative glow discharge cell cooled with liquid nitrogen. Because the low-J rotational lines have been investigated by other groups, our present study was focussed mainly to the measurements of higher-J rotational lines. Currently we have observed the lines J + 1 ← J(J=11, 13-21) for H¹³CO⁺, and J + 1 ← J (J=13-18, 20-22, 24-25) for D¹³CO⁺. Molecular contstants for these isotopologues (B, D, H, L) have been modified. From the analysis of the intensity of each rotational line, we estimate the rotational temperature to be as low as 140K. This low temperature makes it difficult to measure yet higher-J lines. Measurement of other isotopogues such as those containing oxygen isotopes is now in preparation.

A novel method for double-resonance photofragmentation spectroscopy in a cold quadrupole ion trap has been developed and utilized to differentiate the structures of a cold protonated adenine dimer. A burn laser generates a population hole of a certain conformer of the dimer stored in a cold quadrupole ion trap, and an auxiliary dipolar RF ejects the photofragments by the burn laser from the trap. A probe laser detects depletion of a certain conformer by the burn laser, and a conformer-specific UV or IR spectrum of a cold ion is obtained by scanning the wavelength of the burn or the probe laser. This simple and versatile method is applicable to any type of double-resonance photofragmentation spectroscopy in a cold quadrupole ion trap. To demonstrate its capability, it was applied to UV-UV hole-burning spectroscopy of a protonated adenine dimer. It is proved that a cold protonated adenine dimer has at least two hydrogen-bonding geometries and each has multiple electronically excited states with significantly different spectral bandwidths, possibly due to different excited state dynamics.

The splitting of H₂O into H₂ and O₂ is an attractive option for alternative energy, but the oxygen evolution step poses a significant challenge. A decades-long effort to produce a suitable water oxidation catalyst (WOC) has made progress on this front, but the precise reaction mechanism of these catalysts is still not well understood. One of the most extensively studied WOCs is [Ru(tpy)(bpy)(H₂O)]²⁺ (tpy = 2,2’:6,2”-terpyridine, bpy = 2,2’-bipyridine). Presented here are gas-phase infrared spectra of water clusters of [Ru(tpy)(bpy)(OH₂)]²⁺ and the first intermediate of the catalytic cycle, [Ru(tpy)(bpy)(OH)]²⁺. In particular, the O-H stretches are used as a probe of solvation strength, and trends in their spectral shifts are examined as a function of cluster size. With the aid of density functional theory (DFT) calculations, these spectra reveal structural changes induced by solvation that provide clear evidence for proton-coupled electron transfer (PCET), in support of proposed mechanisms.

The solvation of metal cations, a process that dictates chemistry in both catalytic and biological systems, has been well studied using gas-phase spectroscopy. However, until recently the solvation of cation-anion pairs has been poorly explored. Here we present gas-phase spectra of Ni(H₂O)_m²⁺ (m=4-10) and NiOH(H₂O)_n⁺ (n=2-5) obtained via cryogenic ion vibrational spectroscopy (CIVS). Our results indicate that as cluster size decreases, the NiOH(H₂O)_n⁺ moiety becomes more favorable over the Ni(H₂O)_m²⁺ moiety. Analysis of the spectral data in conjunction with density functional theory calculations shows that both species have a 1^st solvation shell consisting of six lingands. However, the NiOH(H₂O)_n⁺ clusters show evidence of strong interactions between a first solvation shell water ligand and the OH⁻ group of the metal, similar to the interactions previously observed in CaOH(H₂O)_n⁺ and MgOH(H₂O)_n⁺.

Cryogenic ion vibrational predissociation (CIVP) spectroscopy was used to examine the onset of solvation upon the incremental addition of water molecules to the Mg₂SO₄²⁺(H₂O)_n cation (n = 4 – 11). D₂ predissociation spectra are reported for each cluster over the range 1000-3800 cm⁻¹. Initially, the Mg²⁺ atoms each interact with two oxygen atoms on the sulfate anion in a bifurcated arrangement. The breaking of this motif occurs upon addition of the eighth water molecule as evidenced by splitting of the water bend, and broad absorption in the 3000-3400 cm⁻¹ range indicative of hydrogen bonding between the water molecules and sulfate ion.

The catalyzed reduction of CO₂ is an important step in the conversion of this small molecule into liquid fuels. Nickel 1,4,8,11-tetraazacyclotetradecane, Ni(cyclam), is a well-known catalyst for the reduction of CO₂ in solution. Cryogenic ion vibrational predissociation (CIVP) spectroscopy of CO₂-messenger tagged ions cooled in a temperature controlled ion trap was used to study the starting Ni²⁺(cyclam) reactant, and possible reaction intermediates and products in the gas phase. Additionally, parental CO₂ reduction was observed on the Ni(I) species [Ni(bipyridine-(NMe)₂)]₂(diphenyldiacetylene).

The reaction between La atom and isoprene (CH₂=CHC(CH₃)=CH₂) was investigated in a supersonic molecular source. La(C₂H₂), La(C₃H₄), and La(C₅H₈) were observed by time-of-flight mass spectrometry, and their structures and electronic states were characterized by mass-analyzed threshold ionization spectroscopy. Both La(C₂H₂) and La(C₃H₄) are three-membered metallacycles formed by the C-C bond cleavage and hydrogen migration. La(C₂H₂) has a C_2v structure, whereas La(C₃H₄) has a C_s structure. La(C₅H₈) was identified as lanthano-methylcyclobutene (La(CH₂C(CH₃)=CHCH₂)) (C₁) formed by association and double-bond migration. All three complexes have a doublet ground state with the highest occupied molecular orbital being largely a La 6s character. Ionization removes the metal based electron, and the resultant ion has a similar structure to the neutral complex.

Ce(C₂H₂) and Ce(C₄H₆) complexes were observed in the reaction of Ce atom with ethylene in a supersonic molecular beam source and investigated by mass-analyzed threshold ionization spectroscopy (MATI) and theoretical calculations. Preliminary data analysis shows that Ce(C₂H₂) has a triangle structure (C_2v) with Ce binding to C₂H₂ in a two-fold mode and Ce(C₄H₆) has a five-membered metallacyclic structure (C_s) with Ce binding to the two terminal carbon atoms of butadiene. The ground states of both species are triplets with a 4f¹6s¹ Ce-based electron configuration and those of the corresponding ions are doublets from the removal of the 6s¹ electron. The Ce(C₂H₂) complex is formed by ethylene dehydrogenation, whereas Ce(C₄H₆) by ethylene dehydrogenation and carbon-carbon bond coupling. The MATI spectra of Ce(C₂H₂) and Ce(C₄H₆) are rather similar to those of the corresponding La complexes previously observed by our group, except that the spectra of the Ce complexes exhibit two electronic transitions with almost identical vibrational intervals. This observation suggests that the existence of a 4f electron results in an increased complexity of the electronic spectra and states of the lanthanide hydrocarbons.

To gain a better understanding of the binding mechanism and assist in the optimization of relevant drug and chemical probe design, both experimental and theoretical studies were performed on a series of amino acid-linked cisplatin derivatives, including glycine-, lysine-, and ornithine-linked cisplatin, Gplatin, Kplatin, and Oplatin, respectively. Cisplatin, the first FDA-approved platinum-based anticancer drug, has been widely used in cancer chemotherapy. Its pharmacological mechanism has been identified as its ability to coordinate to genomic DNA, and guanine is its major target. In previous reports, cisplatin was successfully utilized as a chemical probe to detect solvent accessible sites in ribosomal RNA (rRNA). Among the amino-acid-linked cisplatin derivatives, Oplatin exhibits preference for adenine over guanine. The mechanism behind its different selectivity compared to cisplatin may relate to its potential of forming a hydrogen bond between the carboxylate group in Pt (II) complex and the 6-amino moiety of adenosine stabilizes A-Oplatin products. Tandem mass spectrometry analysis also indicates that different coordination sites of Oplatin on adenosine affect glycosidic bond stability. Infrared multiple photon dissociation (IRMPD) action spectroscopy experiments were performed on all three amino acid-linked cisplatin to characterize their structures. An extensive theoretical study has been performed on Gplatin to guide the selection of the most effective theory and basis set based on its geometric information. The results for Gplatin provide the foundation for characterization of the more complex amino acid-linked cisplatin derivatives, Oplatin and Kplatin. Structural and energetic information elucidated for these compounds, particularly Oplatin reveal the reason for its alternative selectivity compared to cisplatin.

Modified nucleosides, both naturally occurring and synthetic play an important role in understanding and manipulating RNA and DNA. Naturally occurring modified nucleosides are commonly found in functionally important regions of RNA and also affect antibiotic resistance or sensitivity. Synthetic modifications of nucleosides such as fluorinated and arabinosyl nucleosides have found uses as anti-virals and chemotherapy agents. Understanding the effect that modifications have on structure and glycosidic bond stability may lend insight into the functions of these modified nucleosides. Modifications such as the naturally occurring 2′-O-methylation and the synthetic 2′-fluorination are believed to help stabilize the nucleoside through the glycosidic bond stability and intramolecular hydrogen bonding. Changing the sugar from ribose to arabinose alters the stereochemistry at the 2′ position and thus shifts the 3D orientation of the 2′-hydroxyl group, which also affects intramolecular hydrogen bonding and glycosidic bond stability. The structures of 2′-deoxy-2′-fluorocytidine, 2′-O-methylcytidine and cytosine arabinoside are examined in the current work by measuring the infrared spectra in the IR fingerprint region using infrared multiple photon dissociation (IRMPD) action spectroscopy. The structures accessed in the experiments were determined via comparison of the measured IRMPD action spectra to the theoretical linear IR spectra determined by density functional theory and molecular modeling for the stable low-energy structures. Although glycosidic bond stability cannot be quantitatively determined from this data, complementary TCID studies will establish the effect of these modifications. Comparison of these modified nucleosides with their RNA and DNA analogues will help elucidate differences in their intrinsic chemistry.

In living systems, the local structures of DNA and RNA are influenced by protonation, deprotonation and noncovalent binding interactions with cations. In order to determine the effects of Na⁺ cationization on the gas-phase structures of 2′-deoxyguanosine, [dGuo+Na]⁺, and guanosine, [Guo+Na]⁺, infrared multiple photon dissociation (IRMPD) action spectra of these two sodium cationized DNA and RNA mononucleosides are measured over the range extending from  500 to  1850 cm⁻¹ using the FELIX free electron laser. Complementary electronic structure calculations are performed to determine the stable low-energy conformations of these complexes. Geometry optimizations and frequency analyses of these species are performed at the B3LYP/6-31G* level of theory, whereas single-point energies are calculated at the B3LYP/6-311+G(2d,2p) level of theory to determine the relative stabilities of these conformations. Comparison of the measure IRMPD action spectra and computed linear IR spectra enable the conformations accessed in the experiments to be elucidated. In both cases, preferential binding of the Na⁺ cation to O6 and N7 positions of the nucleobase is observed. Present results for the sodium cationized nucleosides are compared to results for the analogous protonated forms of these nucleosides to elucidate the effects of multiple chelating interactions with the sodium cation to hydrogen bonding interactions in the protonated systems on the structures and stabilities of these nucleosides.

Depending upon the number and location of nitrogen atoms in a N-heterocyclic azabenzene, the addition of a single water molecule can result in a positive electron affinity. The transfer of a proton from a solvating water azine base can be induced by excess electron attachment. Here we explore this phenomenon through the use of photoelectron spectroscopy and electronic structure theory. Carefully calibrated density functional theory (DFT) computations indicate that the excess electron predominantly resides in a π* orbital of the heterocycle.