Dennis L. Lichtenberger

Valence and core photoelectron spectroscopy of C60, buckminsterfullerene

Abstract The MgKα X-ray (XPS) and He(I)/He(II) ultraviolet (UPS) photoelectron spectra of several thin films of C 60 (one to three monolayers) prepared by vapor deposition on gold are reported. The core XPS spectrum shows a single narrow carbon 1s ionization at 285.1 eV. The valence UPS spectra show very sharp valence ionization bands, comparable to gas phase spectra, with wide separations between the lowest ionization energy features. The ionizations are consistent with theoretical calculations based on the highly symmetric truncated icosahedral structure. The first vertical ionization energy relative to the vacuum level is estimated to be 7.6 ± 0.2 eV from these solid state measurements.

Correlations of lone pair ionization energies with proton affinities of amino acids and related compounds. Site specificity of protonation

Abstract The gas phase proton affinities of amino acids are compared with their adiabatic ionization energies obtained from photoelectron spectra. Using primary aliphatic amines as reference species, a linear correlation is found between the proton affinities and the adiabatic nitrogen lone pair ionization energies for those amino acids which protonate on the amine group, even in cases where the nitrogen lone pair is not the highest occupied molecular orbital. Many of the amino acids fit the correlation well, which confirms the prediction of amine protonation from earlier studies and also corroborates the assignment of the bands in the complex photoelectron spectra of these species. Proline and safcosine, amino acids with a secondary nitrogen, deviate from this correlation and instead fit a correlation using secondary aliphatic amines as reference species. Deviations from the correlation exist for molecules, such as lysine, methionine and tryptophan, which contain an intramolecular hydrogen bond between the basic side-chain and the amine site. The gas phase proton affinities of these species are larger than predicted by the correlation. Deviations from the correlation are also predicted for very basic amino acids, such as histidine and arginine, which protonate preferentially on the side-chain instead of the amine group.

Electronic Properties of Pentacene versus Triisopropylsilylethynyl-Substituted Pentacene: Environment-Dependent Effects of the Silyl Substituent

Energy measures of the intra- and intermolecular electronic effects of triisopropylsilylethynyl substitution on pentacene have been obtained from the combination of closely related gas phase and solid phase ultraviolet photoelectron spectroscopy (UPS) measurements along with solution electrochemical measurements. The results show that the shift to lower ionization energy that is expected with this substitution and observed in the gas phase measurements becomes negligible in solution and is even reversed in the solid phase. The principles that emerge from this analysis are supported by electronic structure calculations at the density functional theory level. The relation between the gas phase and solid phase UPS measurements illustrated here provides a general approach to investigating the electronic effects acting on molecules in the condensed phase, which in this case are greater than the direct substituent electronic effects within the molecule. Electronic properties such as lower ionization energies built into the single-molecule building blocks of materials and devices may be reversed in the solid state.

One- to two-electron reduction of an [FeFe]-hydrogenase active site mimic: the critical role of fluxionality of the [2Fe2S] core.

The one- to two-electron reduction of mu-(1,2-ethanedithiolato)diironhexacarbonyl that has been observed under electrochemical conditions is dependent on scan rate and temperature, suggesting activation of a structural rearrangement. This structural rearrangement is attributed to fluxionality of the [2Fe2S] core in the initially formed anion. Computations support this assessment. Upon an initial one-electron reduction, the inherent fluxionality of the [2Fe2S] complex anion allows for a second one-electron reduction at a less negative potential to form a dianionic species. The structure of this dianion is characterized by a rotated iron center, a bridging carbonyl ligand, and, most significantly, a dissociated Fe-S bond. This fluxionality of the [2Fe2S] core upon reduction has direct implications for the chemistry of [FeFe]-hydrogenase mimics and for iron-sulfur cluster chemistry in general.

Investigation of metal-dithiolate fold angle effects: implications for molybdenum and tungsten enzymes.

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the interactions between the sulfur π-orbitals of arene dithiolates and high-valent transition metals as minimum molecular models of the active site features of pyranopterin Mo/W enzymes. The compounds (Tp*)MoO(bdt) (compound 1), Cp2Mo(bdt) (compound 2), and Cp2Ti(bdt) (compound 3) [where Tp* is hydrotris(3,5-dimethyl-1-pyrazolyl)borate, bdt is 1,2-benzenedithiolate, and Cp is η5- cyclopentadienyl] provide access to three different electronic configurations of the metal, formally d1, d2, and d0, respectively. The gas-phase photoelectron spectra show that ionizations from occupied metal and sulfur based valence orbitals are more clearly observed in compounds 2 and 3 than in compound 1. The observed ionization energies and characters compare very well with those calculated by density functional theory. A “dithiolate-folding-effect” involving an interaction of the metal in-plane and sulfur-π orbitals is proposed to be a factor in the electron transfer reactions that regenerate the active sites of molybdenum and tungsten enzymes.

The He I valence photoelectron spectrum ofC70 in the gas phase

Abstract The high-resolution He I photoelectron spectrum of C 70 in the gas phase is reported and compared with that of C 60 and theoretical calculations. Similar to C 60 , the C 70 spectrum shows two separate valence ionization band envelopes in the low energy region, but there are more separate ion states visible within the two envelopes. The first envelope of overlapping ionizations spans from about 7.3 to 8.8 eV, and the second from about 8.9 to 10.3 eV. The greater complexity of the C 70 spectrum compared to that of C 60 is expected due to the lower symmetry of C 70 and the ten additional π electrons from the additional carbon atoms. The first vertical ionization energy is 7.47 ± 0.02 eV, which is destabilized 0.17 eV from that of C 60 .

On the Molecular and Electronic Structures of AsP3 and P4

The molecular and electronic structures of AsP(3) and P(4) have been investigated. Gas-phase electron diffraction studies of AsP(3) have provided r(g) bond lengths of 2.3041(12) and 2.1949(28) A for the As-P interatomic distances and the P-P interatomic distances, respectively. The gas-phase electron diffraction structure of P(4) has been redetermined and provides an updated value of 2.1994(3) A for the P-P interatomic distances, reconciling conflicting literature values. Gas-phase photoelectron spectroscopy provides experimental values for the energies of ionizations from the valence molecular orbitals of AsP(3) and P(4) and shows that electronically AsP(3) and P(4) are quite similar. Solid-state (75)As and (31)P NMR spectroscopy demonstrate the plastic nature of AsP(3) and P(4) as solids, and an extreme upfield (75)As chemical shift has been confirmed for the As atom in AsP(3). Finally, quantum chemical gauge-including magnetically induced current calculations show that AsP(3) and P(4) can accurately be described as strongly aromatic. Together these data provide a cohesive description of the molecular and electronic properties of these two tetraatomic molecules.

Inexpensive and high‐precision digital power supply and counting interface for UPS, XPS, and Auger spectrometers

A digital data‐acquisition system has been built to interface two photoelectron spectrometers to a DEC LSI‐11/23 processor.

The Ionizations of C 60 in the Gas Phase and in Thin Solid Films.

The high-resolution He I photoelectron spectrum of C 60 in the gas phase is reported and compared with the photoelectron spectrum of C 60 as a thin film prepared by vapor deposition (one to three monolayers) on gold. The spectra show low valence ionization bands that are very sharp and well-separated for a molecule of this size, consistent with the highly symmetric truncated icosahedral structure and theoretical calculations. The total band widths of the valence ionizations from the thin film samples are comparable to those from the gas phase species, showing that the electronic interactions between the molecules and with the surface do not significantly influence these measurements of the molecular electronic structure. The gas phase photoelectron spectra also show vibrational fine structure in the first and second ionization bands with spacings that are consistent with the two totally symmetric vibrational modes of C 60 . The first vertical ionization energy relative to the vacuum level is determined to be 7.61 ± 0.02 eV from these gas phase measurements.

Ionization band profile analysis in valence photoelectron spectroscopy

Abstract Analytical descriptions of partially resolved and unresolved vibrational progressions in valence photoelectron ionization bands are discussed. The 2 T 2g valence ionization of Cr(CO) 6 is examined as an example of a partially resolved vibrational progression, and the Jahn-Teller split 2 E ′ and 2 E ″ valence ionizations of Fe(CO) 5 are examined as examples of overlapping ionizations with unresolved vibrational progressions. Asymmetric Gaussian band shapes are obtained when vibrational broadening is responsible for the overall contour of the molecular ionization. Attempts to model the valence ionizations with symmetric Gaussian peak models can lead to serious misrepresentations of the ionization bands. This is found to be particularly important for overlapping ionizations such as the Jahn-Teller split bands of Fe(CO) 5 , where attempts to model the contours with symmetric Gaussian peak shapes lead to physically unreasonable band positions, relative amplitudes, halfwidths and band areas. The treatment of ionizations with resolved vibrational fine structure is also addressed with the 2 A 1 g ionization of osmocene. A series of related Gaussian components are used to model the resolved vibrational progressions. The goodness of fit between the model and the data is discussed in each case. Advantages and limitations in the analytical representation of valence photoelectron data are addressed.