PE Spectroscopy Research Articles

Electronic and Molecular Structures of the CeB6 Monomer.

The electronic and molecular structure of the CeB6 molecular unit has been probed by anion PE spectroscopy and DFT calculations to gain insight into structural and electronic relaxation on edge and corner sites of this ionic material. While boron in bulk lanthanide hexaboride materials assumes octahedral B63- units, the monomer assumes a less compact structure to delocalize the charge. Two competitive molecular structures were identified for the anion and neutral species, which include a boat-like structure and a planar or near-planar teardrop structure. Ce adopts different orbital occupancies in the two isomers; the boat-like structure has a 4f superconfiguration while the teardrop favors a 4f 6s occupancy. The B6 ligand in these structures carries a charge of -4 and -3, respectively. The teardrop structure, which was calculated to be isoenergetic with the boat structure, was most consistent with the experimental spectrum. B6-local orbitals crowd the energy window between the Ce 4f and 6s (HOMO) orbitals. A low-lying transition from the B-based orbitals is observed slightly less than 1 eV above the ground state. The results suggest that edge and corner conductivity involves stabilized, highly diffuse 6s orbitals or bands rather than the bulk-favored 5d band. High-spin and open-shell low-spin states were calculated to be very close in energy for both the anion and neutral, a characteristic that reflects how decoupled the 4f electron is from the B6 2p- and Ce 6s-based molecular orbitals.

Quantum Coherence and Disorder-Specific Effects in Simulations of 2D Optical Spectra of One-Dimensional J-aggregates

Time-resolved two-dimensional photon-echo (2D PE) spectra of linear J-aggregates containing four molecules per unit cell--a model system for a concentrated water solution of the pseudo-isocyanine dye--is theoretically considered. Analysis of a single unit cell and the full-sized aggregate is consistently carried out. Spectral features of a single unit cell are sorted out by assigning them to specific double-sided Feynman diagrams. Two different approaches of disorder are employed, the resulting differences in the spectra are discussed, and the conditions for their utilization are described. Special attention is paid to quantum coherence dynamics of a single unit cell and the full-sized J-aggregate. Possibilities of probing quantum coherences, as well as performing quantum control via two-color 2D PE spectroscopy, are discussed.

Photoelectron Spectra, Electronic Structures, and Conformational Properties of Substituted 2-Styrylpyrroles

The UV photoelectron spectra of three pairs of cis/trans-isomers of 2-(2-methylstyryl)pyrroles (1-6) have been recorded and analysed, making use of density functional theory (DFT) calculations at the B3LYP level. Compounds 1 and 2 have no further substituents in the pyrrole ring. In the other compounds the pyrrole ring is substituted by a methyl group in the 1-(3,4) or 5-position (5,6). Twisted conformations were calculated for 1-6 in which the toluene ring is generally much more distorted from the plane of the central C=C double bond than the pyrrole ring. In the cis-isomers (2, 4, 6) the torsion angle of the toluene groups amounts to about 50 o, while in the trans-isomers (1, 3, 5) the torsional angle is only about half as large. The energy difference delta IP values of the ionization bands associated with the molecular orbitals (MOs) pi7 and pi3 is related to torsion. Calculated delta IP values are generally 5-10 % larger than the experimental values, indicating that torsional vibrations contribute to the apparent distortion of the molecules. The results clearly indicate that PE spectroscopy - in combination with high-level quantum chemical calculations - is a verasatile method to distinguish E/Z isomers of heterocyclic stilbene-like molecules.

Photoelectron Spectra, Electronic Structures, and Conformational Properties of (E)-Stilbene, Styrylthiophenes, and (Thienylethenyl)pyridines

The UV photoelectron spectra of two isomeric styrylthiophenes (5, 6) and six isomeric (thienylethenyl)pyridines (7−12) have been recorded and analyzed, making use of DFT Becke3LYP calculations. The spectra were compared with those of (E)-stilbene (1e) and the isomeric styrylpyridines (2−4), which are π-isoelectronic with 5−12. Using quantum chemical analyses, planar molecular structures were obtained for all these compounds. However, from the separation ΔIP of the ionization bands associated with the π7 and π3 MOs, it is possible to make a distinction between planar and twisted molecular structures. Accordingly in these compounds, 2-substituted pyridine rings and 3-substituted thiophene rings are nearly untwisted, whereas phenyl rings, 3- and 4-substituted pyridine, and 2-substituted thiophene rings are twisted to an extent similar to that in 1e. The apparent distortion of the molecules is probably caused by torsional vibrations, so that twisted average geometries correspond to planar equilibrium structures. The B3LYP data permit detailed conclusions to be drawn with regard to the conformational preferences of 2- and 3-substituted thiophene and pyridine rings in heterocyclic analogues of 1e, as well as the to relative stabilities of isomers 7−12. The results clearly indicate that PE spectroscopy is a powerful method for analysis of conformational properties of stilbene-like molecules.

Study of the Gd/GaAs(110) system: interface and Schottky barrier formation at low and room temperatures

The interface formation upon deposition of Gd overlayers onto n-type GaAs(110) substrates was investigated at room and low (20 K, 100 K) temperatures by PE spectroscopy using synchrotron radiation, ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED) techniques. Binding energy shifts of Ga 3 d and As 3 d levels at 300 K and 100 K were observed starting from the minimal Gd coverage (∼0.03 ML). It was found that band bending saturates only after metallization of the interface at 1–2 ML. It is argued that two different mechanisms are involved in the Schottky barrier formation: Fermi level pinning by defect states and metal-induced gap states. The Gd overlayers growth at low and room temperatures leads to reacted components showing up in Ga 3 d and As 3 d photoemission spectra at coverages ≥0.03 ML. All components (reacted and unreacted) move to lower binding energies upon deposition of Gd. LEED studies at T=20–300 K showed disordered interface structures at submonolayer and high coverage ranges. Temperature induced changes in the interface formation were observed as well. At 300 K the overlayer is cluster-like even at 0.03 ML while at 100 K and 20 K it is of Stranski–Krastanov and layer-by-layer types, respectively.

Phanes with Three‐ and Four‐Membered Rings as Building Elements

AbstractThe systematic integration of the small 2π‐ and 4π‐electron systems cyclopropenylium ion, cyclopropenone, oxo‐cyclobutenylium ion, and cyclobutadiene into phane chemistry was initiated only a few years ago. [n2]Cyclopropenylophanes, [n2]cyclopropenonophanes, metal‐capped [n4]cyclobutadienosuperphanes, and other new families of double‐decker phane species became available from cycloalkydiynes through special methods of double [2+1]cycloaddition with carbenes and metal complex induced dimerization by double [2+2]cycloaddition. Phane‐specific structural features were elucidated by X‐ray structural analyses. Cyclic voltammetry and PE spectroscopy as well as MO calculations reealed considerable interactions between closely spaced π‐electron systems. Decapping cyclobutadienosuperphanes formally extended the synthetic principle to threefold [2+2]cycloadditions resulting in [n4]‐bridged tricyclo‐[4.2.0.03, 5]octa‐3,7‐dienes, which represent a new type of cage compounds. Moreover, completion to fourfold [2+2]addition was achieved with the photoinduced transformation of [34]bridged tricyclo[4.2.0.03, 5]octa‐3,7‐diene into propella[34]cubane.

On the Homoconjugation Between Donor and Acceptor Fragments. Synthesis and Properties of Bishomosquaric Acid Derivatives

AbstractThe tetraketone 3,3,6,6‐tetramethylcyclohexane‐1,2,4,5‐tetrone (11) and its congener bicyclo[3.2.2]nonane‐6,7,8,9‐tetrone (17) were synthesized by conventional procedures. The photoreaction of 11 and 17 with tetramethylethylene yields 3,3,4,4,7,7,10,10‐octamethyl‐2,5‐dioxabicyclo[4.4.0]dec‐1(6)‐ene‐8,9‐dione (22) and 4,4,5,5‐tetramethyl‐3,6‐dioxatricyclo‐ [6.3.2.02;7]tridec‐2(7)‐ene‐12,13‐dione (24), respectively. Both are formally diesters of a bishomosquaric acid. The interactions between the two nonconjugated diketone moities in 11 and 17 as well as between the donor and acceptor fragments in 22 and 24 were studied. X‐ray investigations of 11 and 17 show no deviation from the expected bond lengths and angles. In the case of 24 the distance between the carbon atoms of the donor and acceptor units is 2.39 Å, this is 0.11 Å shorter than the corresponding distance between the two acceptor units in 17. The PE‐spectroscopic investigations of 11 and 17 show a considerable interaction between the two acceptor units. This is concluded from the splitting of the PE bands (ca. 0.5 eV) of the four lone‐pair combinations. For 22 and 24 no interaction between the donor and acceptor part was detected by means of PE spectroscopy. The 13C‐NMR chemical data show a high‐field shift for the carbonyl carbon atoms in 11 and 17. These findings were substantiated by the investigation of the corresponding phenazine derivatives obtained from 11 and 17 by the reaction with o‐phenylenediamine. In the case of 22 and 24 a high‐field shift of the signals of the olefinic double bond was observed. The electronic absorption spectrum of 22 shows for the higher excited states a charge‐transfer contribution, whereas in the case of 24 a charge‐transfer contribution was already detected in the first excited state.

PE‐spectroscopic investigation of nonconjugated π,π interactions in (seco)dodecahedradienes — on the route to an experimental ethylene/ethylene cycloaddition reaction coordinate

AbstractNature and extent of nonconjugated π,π interactions in (bis)‐secododecahedradienes and (pentagonal, non‐pentagonal) dodecahedradienes with transannular π,π distances ranging from ca. 2.8 to ca. 3.5 Å – representing well defined points on the D2h [π2 + π2] reaction coordinate – were studied by PE spectroscopy. The π,π splitting of 0.68 eV measured at the largest π,π distance of ca. 3.5 Å (1,16‐dodecahedradiene 3a) was attributed to ca. 50% each of through‐space and through‐bond conjugation (analysis of precanonical MOs).

Structural chemistry of polycyclic heteroaromatic compounds. Part 4. Electronic structures of angular dithienopyridines

The Hel photoelectron spectra of nine isomeric [b,d]-annellated dithienopyridines (5–13) are reported and discussed including those of some closely related compounds such as phenanthridine (2), dithieno[3,4-a: 3′,4′-c]benzene (4) and its tetrahydro derivative 3. The ionization potentials are assigned to molecular orbitals using the results of MNDO, AM1 and PM3 calculations. The ability of PE spectroscopy as an analytical method in the analysis of the isomers 5–13 is investigated. From the spectra as well as from the theoretical results, the isomers can be divided into two groups, depending on the mode of annellation. The crystal and molecular structures of one member of each group (isomers 9 and 12) are determined by X-ray diffraction. The mode of annellation is also reflected in the bond lengths of the pyridine and thiophene rings.

Strukturen und Moleküleigenschaften sterisch überfüllter Moleküle, 37. Hexakis(trimethylsilylmethyl)benzol C6[CH2Si(CH3)3]6

Structures and Molecular Properties of Sterically Overcrowded Molecules, 37[1]. — Hexakis(trimethylsilylmethyl)benzene C6[CH2Si(CH3)3]6Both the low first vertical ionization energy of only 7.40 eV, determined by PE spectroscopy for C6[CH2Si(CH3)3]6, and the blocked substituent rotation in its radical cation as proven by ESR/ENDOR spectroscopy suggest considerable steric over‐crowdedness. To further substantiate this interpretation of the molecular properties, the crystal structure (hexagonal R3, Z = 2) has been determined at 100 K. Accordingly, the bulky trimethylsilyl groups are arranged alternatingly above or below the C6‐ring plane, out of which the methylene C centers are twisted by ±9°. Due to the angles CCSi widened to 115°, however, the intramolecular distances C(H)(H)C between non‐neighboured methyl groups of 443 and 452 pm even slightly exceed their van der Waals radii sum of 400 pm and thus avoid severe spatial congestion.

Computational and ultraviolet photoelectron spectroscopic studies of 5-substituted cyclopentadienes. Evidence that n–π orbital mixing in the HOMO is not the source of the syn π-facial selectivity in Diels–Alder reactions

A series of C5 monomethyl (1a–1f) and pentamethylcyclopentadienes (2a–2f) bearing stereogenic C5 heteroatom substituents (-NH2, -OH, -OCH3, and -SCH3) have been studied computationally (abinitio and AM1) and with ultraviolet photoelectron (pe) spectroscopy. In the case of compounds 1d–1f and 2d–2f, the conformations 3 and 4 of Cs symmetry with the lone paris anti to the cyclopentadiene ring are computed to be the most stable geometries. On the other hand, the twisted-anti conformers 5a and 5b are the most stable geometries of 1c and 2c. Analysis of the computed MO eigenvalues (orbital energies), MO eigenvectors (orbital coefficients), and the pe spectra of cyclopentadienes 2a–2f established that n–π orbital mixing is not important in the HOMO's of 1c–1d or 2c–2d. That the ionization energy of the HOMO is found to be virtually independent of the substituent at C5 in the series 2a–2f, provides support for the computational results. Because 2c–2d undergo Diels–Alder reactions selectively, syn to the heteroatom substituent, n–π orbital mixing in the HOMO cannot be the source of the π-facial selectivity observed for these compounds.

Computational and ultraviolet photoelectron spectroscopic evidence that (Z)-2-methyl-1,3-pentadiene prefers twisted s-cis conformers in the gas phase

A redetermination of the ultraviolet photoelectron (pe) spectrum of (Z)-2-methyl-1,3-pentadiene has led to a correction of the published spectrum. By studying (Z)-2-methyl-1,3,-pentadiene (1a) and (E)-2-methyl-1,3-pentadiene (1b) with MMX, MNDO, AM1, and abinitio MO computational methods and pe spectroscopy, we have shown that a combination of these methods provides useful insights on the conformational behaviour of methyl-substituted 1,3-dienes in the gas phase. Synthetic pe spectra, derived from the computed potential energy surfaces and angle-dependent orbital energies, are in good agreement with experiment. Thus, the E isomer prefers the s-trans conformer but the Z isomer prefers twisted s-cis conformations in the gas phase.

ChemInform Abstract: Electronic Structure and Conformational Properties of the Amide Linkage. Part 9. Geometrical and Electronic Structures of N‐ Alkenyllactams as Determined by PE Spectroscopy and Semiempirical Quantum Chemical Calculations.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Small and Medium Rings, 781) Cage Dimers of Norbornadiene with Perpendicular Arrangement of Subchromophores: Orbital Interaction in the Heptacyclo[6.6.0.02,6.03,13.04,11.05,9.010,14]tetradecane System

AbstractThe syntheses of functionalized norbornadiene dimers 1 are described. These cage compounds feature remarkably high D2d symmetry as regards the dimethylene and dioxo derivatives 1d and 1e. On the other hand, the mixed enone compound 11 behaves differently: πcc and lone‐pair nco orbitals mix strongly with each other, albeit only by means of the intervening five‐bond σ system. All theoretical conclusions have been verified by using PE spectroscopy. A comparison is made between systems 1 and the geometrically similar adamantanes 2, on the one hand, and the attractive but still unknown derivatives of garudane 3 on the other hand. The latter exhibit the interesting parallel topology of suitable chromophores, thus making these polycycles 3 desirable candidates for transmission studies.

Small and Medium Rings, 771) Nonconjugated π,π Interactions Approaching Classical π,π Conjugation – PE‐Spectroscopical Investigation of the 7,8;12,19‐Diseco‐1,16‐dodecahedradiene System

Transannular (orbital) interactions in compounds 1–9 are studied by means of He(Iα)‐PE spectroscopy. Unusually strong through‐space‐dominated π,π interaction is found to be operative in the nonconjugated 1,6‐dienes 1, 2, 7, and 8. The π,π splitting of 1.9 eV for disecododecahedradiene 1 is even surpassed by the record value of 2.2 eV for disecodienedione 2, which almost equals that in 1,3‐butadiene (2.46 eV). The experimental results are explained by computational methods, especially by analyses of the precanonical MOs.

ChemInform Abstract: Conformational Analysis and Structure Elucidation of 2,3‐Dimethyl‐ and 2,4‐Dimethyl‐2,4‐hexadienes by AM1 and He(I) UV PE Spectroscopy

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Chromophore Systeme, 1. Konformation und Lichtabsorption von Hexa‐1,5‐dien‐3,4‐dion (Divinylglyoxal)

Chromophoric Systems, 1. — Conformation and Absorption of Light in Hexa‐1,5‐diene‐3,4‐dione (Divinylglyoxal)Divinylglyoxal (1) is investigated by several spectroscopic and theoretical methods in order to set up a correlation between the CO/CO dihedral angle and the transition energies in unsaturated α‐diketones. According to the experimental (UV, IR, 13C‐NMR, PE spectroscopy and dipole moment) and theoretical (ab initio and molecular mechanics) results, 1 exists predominantly as a C2h conformer (Θco/co=180°, ΘVinyl/co=0°). The dependence of transition energies on CO/CO dihedral angles is similar in shape to that for saturated α‐diketones, but is significantly shifted to lower transition energies. 13C chemical shifts respond sensitively to CO/CO dihedral angles, and the ab initio calculated curve is supported by a few experimental results.

Small and Medium Rings, 75. Syntheses, Photoelectron Spectra, and Photoreactivity of Polycyclic 1,5‐Diketones: Transannular Interaction in the Cyclooctane‐1,5‐dione Fragment

AbstractFive polycyclic 1,5‐diketones 1 – 4 and 6 are synthesized and studied by means of He(Iα) PE spectroscopy. Comparison of the spectra indicates significant differences in Orbital Interaction Through Bond, which is explained by computational methods, especially by analyses of the precanonical MOs. The lone pair splitting observed in diketone 1, ΔI(n)=0.75 eV, seems to be the largest one ever found in a 1,5‐diketone, obviously due to a favourable zigzag topology of the σ frame. In addition, coupling reactions and photoreactivity are studied in compound 6, which seemed to be well suited for Orbital Interaction Through Space. The crystal structure of 6 and its photoproduct 21 is solved by X‐ray crystallography.

ChemInform Abstract: Gas-Phase Thermolysis of 4-Substituted 3,5-Dihydro-3,3,5,5-tetramethyl-4H-pyrazoles Studied by PE Spectroscopy and Flash Vacuum Pyrolysis.

ChemInform Abstract: Gas-Phase Thermolysis of 4-Substituted 3,5-Dihydro-3,3,5,5-tetramethyl-4H-pyrazoles Studied by PE Spectroscopy and Flash Vacuum Pyrolysis.

Bonding character and s-p hybridization of orbitals of hydride molecules according to photoelectron spectroscopy data

In consideration of the electron structure of the molecules in terms of canonical many-centered orbitals by s-p hybridization we mean mixture of the ns and np orbitals of an atom into one molecular orbital. The PE spectra of the valence levels of the molecules give direct information on the influence of s-p hybridization on the bonding character and energies of the levels (1, 3). In this article we discuss the influence of hybridization on the bonding character of the MO of the isoelectronic series A/sup 7/H - A/sup 6/H/sub 2/ - A/sup 5/H/sub 2/ - A/sup 4/H/sub 4/ according to the results of PE spectroscopy. To simplify the discussion we adopt the approximation of Kupmans theorem IP/sub i/ = /minus//var epsilon//sub i/.