Tethered peptides are synthetic peptides in which a chemical linkage between two remote sites in the peptide sequence bind these sites together. The Fairlie group has devised a tether that locks the pentapeptide ‘core’ into a single turn of an alpha helix, robust to large swings in pH, temperature, and denaturant. By changing the chirality of the amino acids, left- (D) and right-handed (L) helices can be exclusively formed. Catenating these sub-units leads to α-helices of greater length. This talk describes the propensity of these tethered peptides to maintain their alpha helical nature upon the removal of solvent and transition into the gas phase as protonated ions. To this end we explore the structures of three tethered peptides linked together through the formation of a lactam between the lysine and aspartic acid residues, Y[KAAAD]-NH₂, F[KAAAD]-NH₂, and YR[KAAAD]-NH₂ (tether denoted by brackets). UV photofragment spectroscopy and IR-UV double resonance methods will be carried out on the cryocooled, protonated ions to probe the hydrogen bonding patterns of these molecules with the goal of elucidating the unique spectroscopic signatures of isolated single-turn alpha helices. The effect of the protonation site and handedness of the helix on the H-bonds in the single turn helices will also be described.

Strong electrostatic interactions such as zwitterionic pairing between oppositely charged amino acids are common in the condensed phase at neutral pH and can play a large role in determining the conformational landscape of peptides and proteins. Whether such interactions are possible in the absence of solvent however, has been previously debated. Growing experimental evidence suggests that these interactions are indeed possible in isolated gas phase ions and may give rise to unique fragmentation upon UV irradiation. In this study we use a series IR-UV-IR triple resonance techniques performed at 10 K to investigate the influences of these electrostatic interactions on the electronic and structural properties of model YGRXR ( X = gly, asp) pentapeptide systems and their methyl ester counterparts. The initial electronic spectra, under single UV photon conditions, of model systems which may possess zwitterionic pairing hardly show discrete electronic transitions, rather a broad absorbtion which mainly gives rise to tyrosine side chain cleavage is observed. Upon methylation of the carboxylate functional groups, which prevents zwitterionic interactions, the cold action spectra become well resolved such that sharp electronic transitions due to the ππ* transition of the tyrosine aromatic ring are observed. Using an UV-IR double resonance scheme it is possible to enhance the tyrosine side chain cleavage after an initial UV excitation, provided that the IR laser is fixed on a vibrational. Under these conditions the Franck Condon progressions for the non-methyl esterified systems become clearly observable. These initial results suggest that local excitation of the chromophore may couple to the autoionizing state responsible for electron detachment, similar to the mechanism postulated for photoinduced electron detachment from gas phase anions. Using IR-UV-IR triple resonance, conformer specific IR spectra can be taken for zwitterionic systems despite the large “off-resonance” absorbtion. A comparison of the IR spectra reveal that the +1 charge states are more prone to form zwitterionic interactions than the +2. Harmonic-level vibrational frequency calculations will be performed on candidate structures and compared to experimental spectra such that the influences of zwitterionic ionic pairing on the 3-dimensional structure can be directly compared to conformations without such parings.

We applied the two-color resonant two-photon photoionization efficiency and mass-analyzed threshold ionization (MATI) spectroscopic techniques to record the cation spectra of p-chloroanisole. In particular, several vibronic states were used as the intermediate levels to record the MATI spectra to investigate whether a significant change in molecular geometry upon ionization and to obtain more information about the active cation vibrations. The adiabatic ionization energy of this molecule has been precisely measured to be 66 100 ± 5 cm-1. These experimental data suggest that the molecular geometry of p-chloroanisole in the cationic ground D0 state resembles that in the electronically excited neutral S1 state. Most of the observed distinct MATI bands result from the active vibrations involving in-plane ring deformation of the p-chloroanisole cation.

The phenoxide anion (C₆H₅O^–), with a large dipole moment of 4.0 D for its neutral core, can support an excited dipole-bound state (DBS) 97 cm⁻¹below the electron detachment threhold [Liu et al. Angew. Chem. Int. Ed. 52, 8976-8979 (2013)]. The vibrational mode-specific autodetachments from the vibrational levels of the DBS were first discovered in this system by high-resolution resonant photoelectron (PE) imaging. Here, we report a photodetachment spectrum of the phenoxide anion up to 2000 cm⁻¹above the detachment threshold. Several new vibrational peaks are observed due to autodetachment from combinational levels of the DBS. The corresponding resonant PE spectra show a clear peak due to the lowest-frequency bending mode, suggesting the possibility of a slight nonplanar structure for the phenoxy radical.

The photodissociation charge transfer processes of Ag⁺ cation-π complexes with small aromatics were studied using velocity map imaging. Ions formed by laser vaporization are pulse extracted and mass-selected in a linear time-of-flight mass spectrometer. The ion beam is then intersected with a UV laser causing dissociation, and the ions are detected using a fast-phosphor screen. The detector has spatial resolution which allows us to extract the total kinetic energy release of the dissociation process, which gives us information on the binding energy of the ion. Excitation with 355 nm lead to the dissociative charge transfer of Ag⁺-toluene and Ag⁺-furan, as seen previously with the Ag⁺-benzene complex.

Zinc cation is studied as a model system for single atom catalysis in the gas phase with infrared photodissociation spectroscopy. Zn(C₂H₂)_n⁺ (n = 1–5) clusters are produced via laser vaporization of zinc in a supersonic expansion of acetylene and argon. Clusters are mass-selected and studied with infrared photodissociation spectroscopy in the C–H stretching region. Smaller clusters (n = 1–3) are studied with the use of a weakly bound argon tag. These spectra are assigned with B3LYP/Def2TZVP computational studies. Zn(C₂H₂)⁺ is found to consist of a C_2v three membered metallacycle, where zinc is equidistant from both carbon atoms of acetylene. Zn(C₂H₂)₂⁺ does not contain a metallacycle, but features zinc binding more closely to one of the carbon atoms of each acetylene in a C₂ configuration. The three-coordinate cluster is predicted to be lowest in energy as a π-bound D_3h structure, with a low energy C₃ structure. When the spectrum of this cluster is measured with argon tagging the D_3h isomer is most abundant. When measured without the tag the C₃ isomer is found to be in abundance. The spectra of the four and five coordinate clusters are found to contain a feature 160 cm⁻¹ to the red of the acetylene C–H asymmetric stretch. This is attributed to a fourth acetylene ligand forming a metal vinyl radical, accompanied by formation of Zn(II). This transfer of the radical center from zinc to a ligand activates the acetylene, and could be the first step in single atom catalysis by zinc.

Vanadium cation is studied as a model system for single atom catalysis in the gas phase with infrared photodissociation spectroscopy. Intermediates in the cyclotrimerization of acetylene to form benzene are observed. V(C₂H₂)_n⁺ clusters are produced via laser vaporization of vanadium in a supersonic expansion of argon containing acetylene. Clusters of V(C₂H₂)⁺, V(C₂H₂)₂⁺, and V(C₂H₂)₃⁺ are studied with infrared photodissociation spectroscopy with the aid of argon tagging in the C–H stretching region. These spectra are assigned on the basis of B3LYP computations. V(C₂H₂)⁺ is a three membered metallacycle, where the hydrogens bend away from the vanadium. V(C₂H₂)₂⁺ is a bimetallacycle where both acetylene ligands interact with vanadium symmetrically through their π-bonds in a C_2v configuration. The structure of V(C₂H₂)₃⁺ is found to vary with the concentration of acetylene in the supersonic expansion. At low concentrations of acetylene two isomers of V(C₂H₂)₃⁺ are observed, a trimetallacycle, and a bimetallacycle which includes a five membered ring and a three membered ring. As the concentration of acetylene is increased past 5% the trimetallacycle decreases in abundance. Benzene is observed at yet higher concentration of acetylene. An expansion gas consisting of 15% acetylene in argon results in the exclusive formation of V(Bz)⁺.

Ln (Ln = La and Ce) atom reactions with ammonia are carried out in a pulsed laser vaporization supersonic molecular beam source, and metal-containing species are observed with time-of-flight mass spectrometry and characterized by mass-analyzed threshold ionization (MATI) spectroscopy. The MATI spectrum of LaNH exhibits a single vibronic band system with a strong origin band and two weak vibronic progressions, whereas the spectrum of CeNH shows two band systems separated by 80 cm⁻¹, with each being similar to the LaNH spectrum. By comparing with theoretical calculations, both LaNH and CeNH are identified as linear molecules with symmetry, and the two vibronic progressions are attributed to excitations of Ln-N stretching and LnNH bending modes in the ions. The additional band system observed for CeNH is due to spin-orbit splitting from interactions of a pair of nearly degenerate triplets and a pair of nearly degenerate singlets. The ground valence electron configurations of LaNH and CeNH are La 6s¹ and Ce 4f¹6s¹, and ionization of each species removes the Ln 6s¹ electron. The remaining two electrons that are associated with the isolated Ln atoms or ions are spin paired in a molecular orbital that is a bonding combination between a Ln 5d orbital and a N π* antibonding orbital.

The electronic structures of UN and UN⁺ are of interest for the testing and development of relativistic quantum chemistry methods. The ground state UN was probed by Matthew and Morse¹, who found that the electronic configuration (5f²7s) differed from that of the isoelectronic UO⁺ cation (5f³). In the present study we examine the ionization energy of UN and the low energy states of UN⁺ by means of pulsed-field ionization zero kinetic energy photoelectron spectroscopy (PFI-ZEKE). Resonantly enhanced two photon ionization (R2PI) coupled with a time of flight mass spectrometer was used to confirm production of the UN molecule and locate suitable electronically excited states for subsequent access to UN⁺ via high-n Rydberg states. The results will be compared to the predictions from ligand field theory and high-level ab ignition calculations. 1. D. J. Matthew and M. D. Morse, J. Chem. Phys. 138, 184303 (2013)

Diode Pumped Alkali Lasers (DPALS) are promising candidates for high-power directed energy applications including data transmission and ballistics defense. Selection of an appropriate buffer gas for the gain media requires absence of undesirable chemical reactions while still meeting the kinetic requirements of the system. Small hydrocarbons have been investigated as a potential buffer gas, and while these meet many of the kinetic requirements of the system, they produce unwanted side products, depleting the gain media. Recent measurements including Laser Induced Fluorescence (LIF), dispersed fluorescence, and ion lifetime measurements indicate that the dominant pathway to the products involves ions and highly excited alkali atoms with very long relaxation times.