School of Chemistry - Research Publications

Permanent URI for this collection

Search Results

Now showing 1 - 10 of 46
  • Item
    Thumbnail Image
    Bond dissociation energy of FeCr+ determined through threshold photodissociation in a cryogenic ion trap
    Marlton, SJP ; Liu, C ; Bieske, EJ (American Institute of Physics, 2024-01-21)
    The bond dissociation energy of FeCr+ is measured using resonance enhanced photodissociation spectroscopy in a cryogenic ion trap. The onset for FeCr+ → Fe + Cr+ photodissociation occurs well above the lowest Cr+(6S, 3d5) + Fe(5D, 3d64s2) dissociation limit. In contrast, the higher energy FeCr+ → Fe+ + Cr photodissociation process exhibits an abrupt onset at the energy of the Cr(7S, 3d54s1) + Fe+(6D, 3d64s1) limit, enabling accurate dissociation energies to be extracted: D(Fe-Cr+) = 1.655 ± 0.006 eV and D(Fe+-Cr) = 2.791 ± 0.006 eV. The measured D(Fe-Cr+) bond energy is 10%-20% larger than predictions from accompanying CAM (Coulomb Attenuated Method)-B3LYP and NEVPT2 and coupled cluster singles, doubles, and perturbative triples electronic structure calculations, which give D(Fe-Cr+) = 1.48, 1.40, and 1.35 eV, respectively. The study emphasizes that an abrupt increase in the photodissociation yield at threshold requires that the molecule possesses a dense manifold of optically accessible, coupled electronic states adjacent to the dissociation asymptote. This condition is not met for the lowest Cr+(6S, 3d5) + Fe(5D, 3d64s2) dissociation limit of FeCr+ but is satisfied for the higher energy Cr(7S, 3d54s1) + Fe+(6D, 3d64s1) dissociation limit.
  • Item
    No Preview Available
    Bond dissociation energies for Fe2+, Fe2O+, and Fe2O2+ clusters determined through threshold photodissociation in a cryogenic ion trap
    Marlton, SJP ; Liu, C ; Watkins, P ; Buntine, JT ; Bieske, EJ (AIP Publishing, 2023-07-14)
    Understanding and controlling the chemical behavior of iron and iron oxide clusters requires accurate thermochemical data, which, because of the complex electronic structure of transition metal clusters, can be difficult to calculate reliably. Here, dissociation energies for Fe2+, Fe2O+, and Fe2O2+ are measured using resonance enhanced photodissociation of clusters contained in a cryogenically cooled ion trap. The photodissociation action spectrum of each species exhibits an abrupt onset for the production of Fe+ photofragments from which bond dissociation energies are deduced for Fe2+ (2.529 ± 0.006 eV), Fe2O+ (3.503 ± 0.006 eV), and Fe2O2+ (4.104 ± 0.006 eV). Using previously measured ionization potentials and electron affinities for Fe and Fe2, bond dissociation energies are determined for Fe2 (0.93 ± 0.01 eV) and Fe2- (1.68 ± 0.01 eV). Measured dissociation energies are used to derive heats of formation ΔfH0(Fe2+) = 1344 ± 2 kJ/mol, ΔfH0(Fe2) = 737 ± 2 kJ/mol, ΔfH0(Fe2-) = 649 ± 2 kJ/mol, ΔfH0(Fe2O+) = 1094 ± 2 kJ/mol, and ΔfH0(Fe2O2+) = 853 ± 21 kJ/mol. The Fe2O2+ ions studied here are determined to have a ring structure based on drift tube ion mobility measurements prior to their confinement in the cryogenic ion trap. The photodissociation measurements significantly improve the accuracy of basic thermochemical data for these small, fundamental iron and iron oxide clusters.
  • Item
    No Preview Available
    Charge transfer transitions of the O2+-Ar and O2+-N2 complexes
    Catani, KJ ; Bartlett, NI ; Scholz, MS ; Muller, G ; Taylor, PR ; Bieske, EJ (AIP Publishing, 2023-07-14)
    Electronic transitions are observed for the O2+-Ar and O2+-N2 complexes over the 225-350 nm range. The transitions are not associated with recognized electronic band systems of the respective atomic and diatomic constituents (Ar+, Ar, O2+, O2, N2+, and N2) but rather are due to charge transfer transitions. Onsets of the O2+-Ar and O2+-N2 band systems occur at 3.68 and 3.62 eV, respectively, corresponding to the difference in the ionization potentials of Ar and O2 (3.69 eV), and N2 and of O2 (3.51 eV), suggesting the band systems arise from intramolecular charge transfer transitions to states correlating with O2(X3Σg-) + Ar+ (2Pu) and O2(X3Σg-) + N2+(X2Σg+) limits, respectively. The dominant vibronic progressions have ωe values of 1565 cm-1 for O2+-Ar and 1532 cm-1 for O2+-N2, reasonably close to the value for the neutral O2  molecule in its X3Σg- state (1580 cm-1). Higher energy band systems for O2+-Ar and O2+-N2 are assigned to transitions to states correlating with the O2 (a1Δg) + Ar+ (2Pu) and O2 (a1Δg) + N2+(X2Σg+) limits, respectively.
  • Item
    No Preview Available
    Photo-induced 6π-electrocyclisation and cycloreversion of isolated dithienylethene anions
    Buntine, JT ; Carrascosa, E ; Bull, JN ; Muller, G ; Jacovella, U ; Glasson, CR ; Vamvounis, G ; Bieske, EJ (ROYAL SOC CHEMISTRY, 2022-07-13)
    The diarylethene chromophore is commonly used in light-triggered molecular switches. The chromophore undergoes reversible 6π-electrocyclisation (ring closing) and cycloreversion (ring opening) reactions upon exposure to UV and visible light, respectively, providing bidirectional photoswitching. Here, we investigate the gas-phase photoisomerisation of meta- (m) and para- (p) substituted dithienylethene carboxylate anions (DTE-) using tandem ion mobility mass spectrometry coupled with laser excitation. The ring-closed forms of p-DTE- and m-DTE- are found to undergo cycloreversion in the gas phase with maximum responses associated with bands in the visible (λmax ≈ 600 nm) and the ultraviolet (λmax ≈ 360 nm). The ring-open p-DTE- isomer undergoes 6π-electrocyclisation in the ultraviolet region at wavelengths shorter than 350 nm, whereas no evidence is found for the corresponding electrocyclisation of ring-open m-DTE-, a situation attributed to the fact that the antiparallel geometry required for electrocyclisation of m-DTE- is energetically disfavoured. This highlights the influence of the carboxylate substitution position on the photochemical properties of DTE molecules. We find no evidence for the formation in the gas phase of the undesirable cyclic byproduct, which causes fatigue of DTE photoswitches in solution.
  • Item
    No Preview Available
    Disentangling Electronic Spectra of Linear and Cyclic Hydrogenated Carbon Cluster Cations, C2n+1H+ (n=3-10)
    Marlton, SJP ; Buntine, JT ; Liu, C ; Watkins, P ; Jacovella, U ; Carrascosa, E ; Bull, JN ; Bieske, EJ (AMER CHEMICAL SOC, 2022-09-29)
    Electronic spectra are measured for protonated carbon clusters (C2n+1H+) containing between 7 and 21 carbon atoms. Linear and cyclic C2n+1H+ isomers are separated and selected using a drift tube ion mobility stage before being mass selected and introduced into a cryogenically cooled ion trap. Spectra are measured using a two-color resonance-enhanced photodissociation strategy, monitoring C2n+1+ photofragments (H atom loss channel) as a function of excitation wavelength. The linear C7H+, C9H+, C11H+, C13H+, C15H+, and C17H+ clusters, which are predicted to have polyynic structures, possess sharp 11Σ+ ← X̃1Σ+ transitions with well-resolved vibronic progressions in C-C stretch vibrational modes. The vibronic features are reproduced by spectral simulations based on vibrational frequencies and geometries calculated with time-dependent density functional theory (ωB97X-D/cc-pVDZ level). The cyclic C15H+, C17H+, C19H+, and C21H+ clusters exhibit weak, broad transitions at a shorter wavelength compared to their linear counterparts. Wavelengths for the origin transitions of both linear and cyclic isomers shift linearly with the number of constituent carbon atoms, indicating that in both cases, the clusters possess a common structural motif.
  • Item
    Thumbnail Image
    Probing Colossal Carbon Rings
    Marlton, SJP ; Buntine, JT ; Watkins, P ; Liu, C ; Jacovella, U ; Carrascosa, E ; Bull, JN ; Bieske, EJ (AMER CHEMICAL SOC, 2023-02-09)
    Carbon aggregates containing between 10 and 30 atoms preferentially arrange themselves as planar rings. To learn more about this exotic allotrope of carbon, electronic spectra are measured for even cyclo[n]carbon radical cations (C14+-C36+) using two-color photodissociation action spectroscopy. To eliminate spectral contributions from other isomers, the target cyclo[n]carbon radical cations are isomer-selected using a drift tube ion mobility spectrometer prior to spectroscopic interrogation. The electronic spectra exhibit sharp transitions spanning the visible and near-infrared spectral regions with the main absorption band shifting progressively to longer wavelength by ≈100 nm for every additional two carbon atoms. This behavior is rationalized with a Hückel theory model describing the energies of the in-plane and out-of-plane π orbitals. Photoexcitation of smaller carbon rings leads preferentially to neutral C3 and C5 loss, whereas rings larger than C24+ tend to also decompose into two smaller rings, which, when possible, have aromatic stability. Generally, the observed charged photofragments correspond to low energy fragment pairs, as predicted by density functional theory calculations (CAM-B3LYP-D3(BJ)/cc-pVDZ). Using action spectroscopy it is confirmed that C14+ and C18+ photofragments from C28+ rings have cyclic structures.
  • Item
    Thumbnail Image
    Excited-State Barrier Controls E ? Z Photoisomerization in p-Hydroxycinnamate Biochromophores
    Ashworth, EK ; Coughlan, NJA ; Hopkins, WS ; Bieske, EJ ; Bull, JN (AMER CHEMICAL SOC, 2022-09-23)
    Molecules based on the deprotonated p-hydroxycinnamate moiety are widespread in nature, including serving as UV filters in the leaves of plants and as the biochromophore in photoactive yellow protein. The photophysical behavior of these chromophores is centered around a rapid E → Z photoisomerization by passage through a conical intersection seam. Here, we use photoisomerization and photodissociation action spectroscopies with deprotonated 4-hydroxybenzal acetone (pCK-) to characterize a wavelength-dependent bifurcation between electron autodetachment (spontaneous ejection of an electron from the S1 state because it is situated in the detachment continuum) and E → Z photoisomerization. While autodetachment occurs across the entire S1(ππ*) band (370-480 nm), E → Z photoisomerization occurs only over a blue portion of the band (370-430 nm). No E → Z photoisomerization is observed when the ketone functional group in pCK- is replaced with an ester or carboxylic acid. The wavelength-dependent bifurcation is consistent with potential energy surface calculations showing that a barrier separates the Franck-Condon region from the E → Z isomerizing conical intersection. The barrier height, which is substantially higher in the gas phase than in solution, depends on the functional group and governs whether E → Z photoisomerization occurs more rapidly than autodetachment.
  • Item
    No Preview Available
    An ion mobility mass spectrometer coupled with a cryogenic ion trap for recording electronic spectra of charged, isomer-selected clusters
    Buntine, JT ; Carrascosa, E ; Bull, JN ; Jacovella, U ; Cotter, MI ; Watkins, P ; Liu, C ; Scholz, MS ; Adamson, BD ; Marlton, SJP ; Bieske, EJ (AIP Publishing, 2022-04-01)
    Infrared and electronic spectra are indispensable for understanding the structural and energetic properties of charged molecules and clusters in the gas phase. However, the presence of isomers can potentially complicate the interpretation of spectra, even if the target molecules or clusters are mass-selected beforehand. Here, we describe an instrument for spectroscopically characterizing charged molecular clusters that have been selected according to both their isomeric form and their mass-to-charge ratio. Cluster ions generated by laser ablation of a solid sample are selected according to their collision cross sections with helium buffer gas using a drift tube ion mobility spectrometer and their mass-to-charge ratio using a quadrupole mass filter. The mobility- and mass-selected target ions are introduced into a cryogenically cooled, three-dimensional quadrupole ion trap where they are thermalized through inelastic collisions with an inert buffer gas (He or He/N2 mixture). Spectra of the molecular ions are obtained by tagging them with inert atoms or molecules (Ne and N2), which are dislodged following resonant excitation of an electronic transition, or by photodissociating the cluster itself following absorption of one or more photons. An electronic spectrum is generated by monitoring the charged photofragment yield as a function of wavelength. The capacity of the instrument is illustrated with the resonance-enhanced photodissociation action spectra of carbon clusters (Cn +) and polyacetylene cations (HC2nH+) that have been selected according to the mass-to-charge ratio and collision cross section with He buffer gas and of mass-selected Au2 + and Au2Ag+ clusters.
  • Item
    No Preview Available
    Electronic spectra of positively charged carbon clusters-C2n+ (n=6-14)
    Buntine, JT ; Cotter, M ; Jacovella, U ; Liu, C ; Watkins, P ; Carrascosa, E ; Bull, JN ; Weston, L ; Muller, G ; Scholz, MS ; Bieske, EJ (AIP Publishing, 2021-12-07)
    Electronic spectra are measured for mass-selected C2n +(n = 6-14) clusters over the visible and near-infrared spectral range through resonance enhanced photodissociation of clusters tagged with N2 molecules in a cryogenic ion trap. The carbon cluster cations are generated through laser ablation of a graphite disk and can be selected according to their collision cross section with He buffer gas and their mass prior to being trapped and spectroscopically probed. The data suggest that the C2n +(n = 6-14) clusters have monocyclic structures with bicyclic structures becoming more prevalent for C22 + and larger clusters. The C2n + electronic spectra are dominated by an origin transition that shifts linearly to a longer wavelength with the number of carbon atoms and associated progressions involving excitation of ring deformation vibrational modes. Bands for C12 +, C16 +, C20 +, C24 +, and C28 + are relatively broad, possibly due to rapid non-radiative decay from the excited state, whereas bands for C14 +, C18 +, C22 +, and C26 + are narrower, consistent with slower non-radiative deactivation.
  • Item
    No Preview Available
    Action spectroscopy of isomer-selected luciferin anions
    Kjaer, C ; Bull, JN ; Carrascosa, E ; Nielsen, SB ; Bieske, EJ (SPRINGER, 2021-03)