Gas-phase Photoelectron Spectroscopy Research Articles

On the Molecular and Electronic Structures of AsP3and 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.

Optimization of Spin-Unrestricted Density Functional Theory for Redox Properties of Rubredoxin Redox Site Analogues

Quantum chemical calculations of metal clusters in proteins for redox studies require both computational feasibility as well as accuracies of at least ∼50 mV for redox energies but only ∼0.05 Å for bond lengths. Thus, optimization of spin-unrestricted density functional theory (DFT) methods, especially the hybrid generalized gradient approximation functionals, for energies while maintaining good geometries is essential. Here, different DFT functionals with effective core potential (ECP) and full core basis sets for [Fe(SCH(3))(4)](2-/1-) and [Fe(SCH(3))(3)](1-/0), which are analogs of the iron-sulfur protein rubredoxin, are investigated in comparison to experiment as well as other more computationally intensive electron correlation methods. In particular, redox energies are calibrated against gas-phase photoelectron spectroscopy data so no approximations for the environment are needed. B3LYP gives the best balance of accuracy in energy and geometry compared B97gga1 and BHandH and is better for energies than Møller-Plesset perturbation theory series (MP2, MP3, MP4SDQ) and comparable to coupled cluster [CCSD, CCSD(T)] methods. Of the full core basis sets tested, the 6-31G** basis sets give good geometries, and addition of diffuse functions to only the sulfur significantly improves the energies. Moreover, a basis set with an ECP on only the iron gives a less accurate but still reasonable geometries and energies.

Probing Ligand Effects on the Redox Energies of [4Fe−4S] Clusters Using Broken-Symmetry Density Functional Theory

A central issue in understanding redox properties of iron-sulfur proteins is determining the factors that tune the reduction potentials of the Fe-S clusters. Recently, Solomon and coworkers have shown that the Fe-S bond covalency of protein analogs measured by %L, the percent ligand character of the Fe 3d orbitals, from ligand K-edge X-ray absorption spectroscopy (XAS) correlates with the electrochemical redox potentials. Also, Wang and coworkers have measured electron detachment energies for iron-sulfur clusters without environmental perturbations by gas-phase photoelectron spectroscopy (PES). Here the correlations of the ligand character with redox energy and %L character are examined in [Fe(4)S(4)L(4)](2-) clusters with different ligands by broken symmetry density functional theory (BS-DFT) calculations using the B3LYP functional together with PES and XAS experimental results. These gas-phase studies assess ligand effects independently of environmental perturbations and thus provide essential information for computational studies of iron-sulfur proteins. The B3LYP oxidation energies agree well with PES data, and the %L character obtained from natural bond orbital analysis correlates with XAS values, although it systematically underestimates them because of basis set effects. The results show that stronger electron-donating terminal ligands increase %L(t), the percent ligand character from terminal ligands, but decrease %S(b), the percent ligand character from the bridging sulfurs. Because the oxidized orbital has significant Fe-L(t) antibonding character, the oxidation energy correlates well with %L(t). However, because the reduced orbital has varying contributions of both Fe-L(t) and Fe-S(b) antibonding character, the reduction energy does not correlate with either %L(t) or %S(b). Overall, BS-DFT calculations together with XAS and PES experiments can unravel the complex underlying factors in the redox energy and chemical bonding of the [4Fe-4S] clusters in iron-sulfur proteins.

Theoretical and Spectroscopic Investigations of the Bonding and Reactivity of (RO)3M≡N Molecules, where M = Cr, Mo, and W

The electronic structures of the molecules ((t)BuO)(3)M[triple bond]N (M = Cr, Mo, W) have been investigated with gas phase photoelectron spectroscopy and density functional calculations. It is found that the alkoxide orbitals mix strongly with the M[triple bond]N triple bond orbitals and contribute substantially to the valence electronic structure. The first ionization of ((t)BuO)(3)Cr[triple bond]N is from an orbital of a(2)(C(3v)) symmetry that is oxygen based and contains no metal or nitrogen character by symmetry. In contrast, the first ionizations of the molybdenum and tungsten analogues are from orbitals of a(1) and e symmetry that derive from the highest occupied M[triple bond]N sigma and pi orbitals mixed with the appropriate symmetry combinations of the oxygen p orbitals. In this a(1) orbital, the oxygen p orbitals mix with the highest occupied M[triple bond]N orbital of sigma symmetry. This mixing reduces the metal character, consequently reducing the metal-nitrogen overlap interaction in this orbital. From computational modeling, the polarity of the M[triple bond]N bond increases down the group such that W[triple bond]N has the highest charge separation. In addition to investigation of the effects of the metals, the electronic influences of substitution at the alkoxide ligands have been examined for the molecules (RO)(3)Mo[triple bond]N (R = C(CH(3))(2)H, C(CH(3))(3), and C(CH(3))(2)CF(3)). The introduction of CF(3) groups stabilizes the molecular orbital energies and increases the measured ionization energies, but does not alter the overall electronic structure. The bonding characteristics of the ((t)BuO)(3)M[triple bond]N series are compared with those of organic nitriles.

Electron Transfer Parameters of Triisopropylsilylethynyl-Substituted Oligoacenes

Understanding the electronic properties and electron transfer characteristics of functionalized oligoacenes is of great importance for the fabrication of organic electronic devices. Charge transfer parameters of bis-triisopropylsilylethynyl-substituted anthracene, tetracene, and pentacene have been investigated based on the analysis of their ionization energies and radical cation reorganization energies. High-resolution gas-phase photoelectron spectroscopy measurements and first-principles quantum-mechanical calculations at the density functional theory level have been performed. The results indicate that the ionization energies in the gas phase and the inner-sphere reorganization energies are sensitive to the number of fused rings and the substitution by the triisopropylsilylethynyl (TIPS) group. This TIPS functional group shifts the first ionization band of these molecules to lower energy in the gas phase due to mixing between the frontier occupied orbitals of the TIPS group with the highest occupied ac...

Solvation free energies of molecules. The most stable anionic tautomers of uracil.

Anionic states of nucleic acid bases are suspected to play a role in the radiation damage processes of DNA. Our recent studies suggested that the excess electron attachment to the nucleic acid bases can stabilize some rare tautomers, i.e. imine-enamine tautomers and other tautomers with a proton being transferred from nitrogen sites to carbon sites (with respect to the canonical tautomer). So far, these new anionic tautomers have been characterized by the gas-phase electronic structure calculations and photoelectron spectroscopy experiments. In the current contribution we explore the effect of water solvation on the stability of the new anionic tautomers of uracil. The accurate free energies of solvation are calculated in a two step approach. The major contribution was calculated using the classical free-energy perturbation adiabatic-charging approach, where it is assumed that the solvated molecule has the charge distribution given by the polarizable continuum model. In the second step the free energy of solvation is refined by taking into account the real, average solvent charge distribution. This is done using our accelerated QM/MM simulations, where the QM energy of the solute is calculated in the mean potential averaged over many MD steps. We found that in water solution three of the recently identified anionic tautomers are 6.5-3.6 kcal mol(-1) more stable than the anion of the canonical tautomer.

Photoelectron Spectroscopy and Electronic Structure Calculations of d1 Vanadocene Compounds with Chelated Dithiolate Ligands: Implications for Pyranopterin Mo/W Enzymes

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the electronic structures of open-shell bent vanadocene compounds with chelating dithiolate ligands, which are minimum molecular models of the active sites of pyranopterin Mo/W enzymes. The compounds Cp2V(dithiolate) [where dithiolate is 1,2-ethenedithiolate (S2C2H2) or 1,2-benzenedithiolate (bdt), and Cp is cyclopentadienyl] provide access to a 17-electron, d1 electron configuration at the metal center. Comparison with previously studied Cp2M(dithiolate) complexes, where M is Ti and Mo (respectively d0 and d2 electron configurations), allows evaluation of d0, d1, and d2 electronic configurations of the metal center that are analogues for the metal oxidation states present throughout the catalytic cycle of these enzymes. A "dithiolate-folding effect" that involves an interaction between the vanadium d orbitals and sulfur p orbitals is shown to stabilize the d1 metal center, allowing the d1 electron configuration and geometry to act as a low-energy electron pathway intermediate between the d0 and d2 electron configurations of the enzyme.

2-amino-1-propanol versus 1-amino-2-propanol: Valence band and C 1s core-level photoelectron spectra

Valence band and C 1s core-level photoelectron spectra of S-(+)-2-amino-1-propanol (alaninol) and S-(+)-1-amino-2-propanol (isopropanolamine) have been studied by means of synchrotron radiation photoelectron spectroscopy in gas phase. The alaninol, the reduced derivative of the alanine, is a good test system of amino acid-like structures. The isopropanolamine, presenting the inversion of the two functional groups of the alaninol at the chiral carbon, offers the opportunity to study the effect of -OH and -NH(2) structural position on the photoelectron spectra. The influence of the conformational contribution on the electronic structure and the photoelectron spectra has been interpreted using density functional and ab initio theoretical calculations. Agreement has been achieved by taking into account the presence, in gas phase, of two conformers with different population ratios in both chiral systems. The C 1s core-level spectra of alaninol and isopropanolamine are reported and the peak positions of the three carbon atoms of the molecules are assigned.

Interaction of C−Si, C−Sn, and Si−Si σ-Bonds with Chalcogen Lone Pairs

The ability of neighboring C-Si, C-Sn, and Si-Si groups in conformationally constrained cyclic molecules to reduce the lowest ionization energies of sulfur, selenium, and tellurium compounds has been determined by charge-transfer spectroscopy of complexes with tetracyanoethylene. For selected compounds, ionization energies were determined by gas-phase photoelectron spectroscopy. The lowest ionization energies measured by photoelectron spectroscopy, with one exception, correlate with the charge-transfer spectroscopic data. In addition, theoretical analysis has provided insight into the photoelectron spectra and the geometry-dependent interaction between C-Si or C-Sn bonds and chalcogen lone pairs. Substantial lowering of ionization energies is found which is anticipated to have important consequences in the chemistry of these and related species.

Cerium(III) and Cerium(IV) Bis(η8-pentalene) Sandwich Complexes: Synthetic, Structural, Spectroscopic, and Theoretical Studies

The Ce(III) anionic bis(pentalene) sandwich complex K[Ce{C 8H4(SiiPr3-1,4)2} 2] (1) has been prepared by treatment of CeCl3 with K 2[C8H4(SiiPr3-1,4) 2] and crystallographically characterized as its 18-crown-6 complex. Oxidation of 1 with Ag[BPh4] affords the neutral, formally Ce(IV) sandwich complex [Ce{C8H4(SiiPr 3-1,4)2)2] (2), whose molecular structure has also been determined. The electronic structure of 2 has been investigated in detail by a combination of magnetic studies, K-edge XANES measurements, gas-phase photoelectron spectroscopy, and density functional calculations.

Planar Tetracoordinate Carbon Atoms Centered in Bare Four-membered Rings of Late Transition Metals

Planar tetracoordinate carbons (ptC's) can be stabilized by four-membered ring perimeters composed of four bare transition metal atoms. DFT analyses of the molecular orbitals, electronic structures, energies, and magnetic properties of these CM4 species (where M represents isoelectronic combinations of Cu, Ni, Ag, and Pd) reveal striking similarities with main group metal ptC analogues (e.g., CAl2Si2, CAl4Na-, and C5Li2). While the CCu4(2+), CAg4(2+), and CNiCu3+ ions have the largest HOMO-LUMO separations, CCu4(2+) is the best candidate for detection by gas-phase photoelectron spectroscopy.

Studies of Potential Inversion in an Extended Tetrathiafulvalene

Electrochemical oxidation of the extended tetrathiafulvalene 9,10-bis(1,3-dithiole-2-ylidene)-9,10-dihydroanthracene (2) was studied in N,N-dimethylformamide. A single, two-electron oxidation peak occurs, and on the return sweep of a cyclic voltammogram, a two-electron reduction peak is seen. The oxidation of 2 to its cation radical and dication occurs with potential inversion (i.e., removal of the second electron occurs more easily than removal of the first). The extent of potential inversion was estimated by cyclic voltammetry to be 0.28 V by analysis of the process in terms of concerted structural change and electron transfer. Failure to detect the cation radical by EPR of an equimolar mixture of neutral 2 and the dication is consistent with this value. The inner reorganization energy of the cation radical was determined by gas-phase photoelectron spectroscopy (PES) to be 0.31-0.35 eV. Calculations, consistent with earlier experimental data, show rather large changes in structure associated with the oxidation processes. These large structural changes contrast with the relatively small inner reorganization energy found by PES. This observation prompted an analysis of voltammetry in terms of two-step processes, with structural change either preceding or following electron transfer. Agreement of simulations based on this mechanism with experimental voltammograms was equally as good as with the concerted mechanism. Notably, the two-step mechanism produced more realistic values of the transfer coefficient and electron-transfer rate constant for the first step of oxidation.

Vibronic Coupling in the Ground and Excited States of Oligoacene Cations

The vibrational coupling in the ground and excited states of positively charged naphthalene, anthracene, tetracene, and pentacene molecules is studied on the basis of a joint experimental and theoretical study of ionization spectra using high-resolution gas-phase photoelectron spectroscopy and first-principles correlated quantum-mechanical calculations. Our theoretical and experimental results reveal that, while the main contribution to relaxation energy in the ground state of oligoacene systems comes from high-energy vibrations, the excited-state relaxation energies show a significant redistribution toward lower-frequency vibrations. A direct correlation is found between the nature of the vibronic interaction and the pattern of the electronic state structure.

Probing Charge Transport in π-Stacked Fluorene-Based Systems

The molecular parameters governing charge transport along a pi-stacked fluorene chain in poly(dibenzofulvene) are studied by a joint experimental and theoretical approach involving high-resolution gas-phase photoelectron spectroscopy and quantum-mechanical methods. We specifically investigate the electronic couplings between fluorene moieties as well as the intramolecular reorganization energies, for both holes and electrons. Our results indicate that a pi-stacked fluorene chain favors hole transport over electron transport. The values for electronic couplings and reorganization energies estimated here are compared with those derived recently for pentacene.

Studies of the Inner Reorganization Energies of the Cation Radicals of 1,4-Bis(dimethylamino)benzene, 9,10-Bis(dimethylamino)anthracene, and 3,6-Bis(dimethylamino)durene by Photoelectron Spectroscopy and Reinterpretation of the Mechanism of the Electrochemical Oxidation of the Parent Diamines

The inner reorganization energy of the cation radical of 1,4-bis(dimethylamino)benzene, 1, has been determined to be 0.72 +/- 0.02 eV by means of gas-phase photoelectron spectroscopy (PES). PES studies of 9,10-bis(dimethylamino)anthracene, 2, and 3,6-bis(dimethylamino)durene, 3, demonstrate that their reorganization energies are smaller than that of 1. The effect of lowering the inner reorganization energy on the rate constant for an electrochemical electron-transfer reaction is to increase the electron-transfer rate constant, k(s). However, voltammetric studies of the two-electron oxidation of 2 and 3 indicate that the values of k(s) for each step are smaller than those for 1, in contradistinction to the measured differences in reorganization energies. The voltammetric studies of 2 and 3 were reinterpreted according to a mechanism in which each step of oxidation was written as a two-step process, electron transfer with a small inner reorganization energy plus a chemical step of structural change. The agreement of simulations according to this mechanism with the experimental data was excellent. The new reaction scheme eliminated some suspicious features previously obtained with an analysis where electron transfer and structural change were considered to be concerted. In particular, all electron-transfer coefficients (alpha) were close to one-half, whereas the earlier treatment produced values of alpha much larger or smaller than one-half.

Vibronic Coupling in Organic Semiconductors: The Case of Fused Polycyclic Benzene–Thiophene Structures

The nature of vibronic coupling in fused polycyclic benzene-thiophene structures has been studied using an approach that combines high-resolution gas-phase photoelectron spectroscopy measurements with first-principles quantum-mechanical calculations. The results indicate that in general the electron-vibrational coupling is stronger than the hole-vibrational coupling. In acenedithiophenes, the main contributions to the hole-vibrational coupling arise from medium- and high-frequency vibrations. In thienobisbenzothiophenes, however, the interaction of holes with low-frequency vibrations becomes significant and is larger than the corresponding electron-vibrational interaction. This finding is in striking contrast with the characteristic pattern in oligoacenes and acenedithiophenes in which the low-frequency vibrations contribute substantially only to the electron-vibrational coupling. The impact of isomerism has been studied as well.

Synthesis, Gas-Phase Photoelectron Spectroscopic, and Theoretical Studies of Stannylated Dinuclear Iron Dithiolates

Stannylated dinuclear iron dithiolates (mu-SSnMe(2)CH(2)S)[Fe(CO)(3)](2), (mu-SCH(2)SnMe(2)CH(2)S) [Fe(CO)(3)](2), and (mu-SCH(2)SnMe(3))(2)[Fe(CO)(3)](2), which are structurally similar to the active site of iron-only hydrogenase, were synthesized and studied by gas-phase photoelectron spectroscopy. The orbital origins of ionizations were assigned by comparison of He I and He II photoelectron spectra and with the aid of hybrid density functional electronic structure calculations. Stannylation lowers the ionization energy of sulfur lone pair orbitals in these systems owing to a geometry-dependent interaction. The Fe-Fe sigma bond, which is the HOMO in all these systems, is also substantially destabilized by stannylation due to a previously unrecognized geometry-dependent interaction between axial sulfur lone pair orbitals and the Fe-Fe sigma bond. Since cleaving the Fe-Fe sigma bond is a key step in the mechanism of action of iron-only hydrogenase, these newly recognized geometry-dependent interactions may be utilized in designing biologically inspired hydrogenase catalysts.

Titanium Imido Complexes of Cyclooctatetraenyl Ligands

Reaction of [Ti(NR)Cl2(py)3] (R=tBu or 2,6-iPr2C6H3) with K(2)[COT] (COT=C8H8) or Li2[COT''] (COT''=1,4-C8H6(SiMe3)2) gave the monomeric complexes [Ti(NR)(eta8-COT)] or [Ti(NR)(eta8-COT'')], respectively. The pseudo-two coordinate, "pogo stick" geometry for these complexes is unique in both early transition-metal and cyclooctatetraenyl ligand chemistry. In contrast, reaction of [Ti(N-2,6-Me2C6H3)Cl2(py)3] with K2[COT] gave the mu-imido-bridged dimer [Ti2(mu-N-2,6-Me2C6H3)2(eta8-COT)2]. It appears that as the steric bulk of the imido and C8 ring substituents are decreased, dimerisation becomes more favourable. Aryl imido COT complexes were also prepared by imido ligand exchange reactions between anilines and [Ti(NtBu)(eta(8)-COT)] or [Ti(NtBu)(eta(8)-COT'')]. The complexes [Ti(NtBu)(eta(8)-COT)], [Ti(N-2,6-iPr2C6H3)2(eta8-COT)] and [Ti2(mu-N-2,6-Me2C6H3)2(eta8-COT)2] have been crystallographically characterised. The electronic structures of both the monomeric and dimeric complexes have been investigated by using density functional theory (DFT) calculations and gas-phase photoelectron spectroscopy. The most striking aspect of the bonding is that binding to the imido nitrogen atom is primarily through sigma and pi interactions, whereas that to the COT or COT'' ring is almost exclusively through delta symmetry orbitals. A DFT-based comparison between the bonding in [Ti(NtBu)(eta8-COT)] and the bonding in the previously reported late transition-metal "pogo stick"complexes [Os(NtBu)(eta6-C6Me6)], [Ir(NtBu)(eta5-C5Me5)] and [Ni(NO)(eta5-C5H5)] has also been undertaken.

Electronic Structure of Bent Titanocene Complexes with Chelated Dithiolate Ligands

Gas-phase photoelectron spectroscopy and density functional theory have been utilized to investigate the interactions between the p orbitals of dithiolate ligands and d orbitals of titanium in bent titanocene complexes as minimum molecular models of active site features of pyranopterin Mo/W enzymes. The compounds Cp(2)Ti(S-S) [where (S-S) is 1,2-ethenedithiolate (S(2)C(2)H(2)), 1, 1,2-benzenedithiolate (bdt), 2, or 1,3-propanedithiolate (pdt), 3, and Cp(-) is cyclopentadienyl] provide access to a formal 16-electron d(0) electronic configuration at the metal. A "dithiolate-folding-effect" involving an interaction of metal and sulfur orbitals is demonstrated in complexes with arene- and enedithiolates. This effect is not observed for the alkanedithiolate in complex 3.

Electrochemistry and Computations of Stable Silylenes and Germylenes

Stable silylenes, 1 and 2, and germylenes, 3 and 4, have been studied computationally and by cyclic voltammetry (CV). Electron affinities (EA) and ionization potentials (IP) were calculated; the latter agree well with available gas phase photoelectron spectroscopy data. All CV measurements in THF−Bu4NClO4 exhibit irreversible waves for all compounds, for both oxidation and reduction processes. However, in 1,2-dichlorobenzene−Bu4NClO4, the germylene derivative 3 shows a single reversible oxidation couple, whereas 4 exhibits a reversible couple at its second oxidation step, the first one being irreversible. In both cases, the redox couples become irreversible by replacing the electrolyte with either Bu4NBF4 or Bu4NHSO4, indicating the importance of the counteranion on the stability of the intermediates. The CV results also show that the unsaturated 1 and 3 are more easily oxidized than their saturated counterparts 2 and 4, and oxidation is easier for the silylenes 1 and 2 than for their germanium analogues. However the measured oxidation and reduction potentials do not correlate with the calculated IP's and EA's, probably due to kinetic and surface effects.