Photoelectron spectroscopic assignment of symmetry to the ground state and first excited state of the 1,4-cyclohexadiene radical cation

The empirical correlation of the photoelectron spectra of 1,4-cyclohexadiene (molecule 4), 1, 4, 5, 8-tetrahydronaphthalene (molecule 5), 1, 4, 5, 6, 9, 1 0-hexahydroanthracene (molecule 6), and 1, 4, 5, 6, 7, 10, 11, 12-octahydronaphthacene (molecules 6) proves that the electronic ground state of these molecules is ²B_1u, assuming that they have D_2h symmetry. In particular this confirms previous predictions for 1,4-cyclohexadiene (molecule 4), for which the “inverted” orbital sequence 2b_1u(π) above lb_3g(π) had been proposed under the assumption that hyperconjugative “through-bond” interaction dominates the “through-space” interaction of the two semi-localized π-orbitals.     

Photoelectron (He(I), He(II) and X-ray) spectroscopy of γ-pyrone and its related sulphur derivatives: valence and core ionization energies and shake-up satellites

The photoelectron He(I) and He(II) spectra of 4H-pyran-4-one (1), 4H-thiopyran-4-one (2), 4H-pyran-4-thione (3) and 4H-thiopyran-4-thione (4) are reported. The assignments are based on experimental evidence, taking into account the results of theoretical calculations. The outermost orbital sequences proposed (X = O, S) are n_X (σ), 3b₁ (π), 1a₂(π) for 1, 2 and 4; and 3b₁(π), n_X(σ) and 1a₂(π) for 3. The shifts of the core-ionization energy values for 1–4 are ascribed to a drift of π-charge from the intracyclic heteroatom towards the carbonyl or thiocarbonyl group. Low-energy shake-up satellites (up to 25% with respect to the main line) are observed in the various energy regions of the XPS spectra. They are qualitatively reproduced by CNDO/2 calculations. The most important satellites derive from the transition in the ion corresponding to the 3b₁ (π) → 4b₁(π*) transition in the UV-visible spectra of the neutral molecules. Charge rearrangements accompanying this transition lead to charge depletion of the core-ionized atom.     

The photoelectron spectra of methyl-substituted allenes and of tetramethyl-bisallenyl

The split Δ = 0.6 eV of the first band at 10.5 eV in the photoelectron spectrum of allene (1) has been ascribed to a Jahn—Teller distortion by Baker and Turner. The influence of alkyl substitution on Δ and on the band position has been investigated by recording the photoelectron spectra of all methyl-substituted allenes 2 to 6, of 1,1-di-tert. butyl-allene (7) and of 1,2-cyclononadiene (8). It is shown that the dependence of Δ on substitution can be rationalized by assuming that it is due to a (pseudo) Jahn—Teller effect in 1 and in the symmetrically substituted derivatives and to the combined influence of this effect and a differential, inductive destabilization in the other cases.The photoelectron spectrum of tetramethyl-bisallenyl 9 can be explained on the basis of known parameters for semi-localized π-orbitals and for their through-space and through-bond coupling.     

The photoelectron spectra of methyl-substituted allenes and of tetramethyl-bisallenyl

The split Δ = 0.6 eV of the first band at 10.5 eV in the photoelectron spectrum of allene (1) has been ascribed to a Jahn—Teller distortion by Baker and Turner. The influence of alkyl substitution on Δ and on the band position has been investigated by recording the photoelectron spectra of all methyl-substituted allenes 2 to 6, of 1,1-di-tert. butyl-allene (7) and of 1,2-cyclononadiene (8). It is shown that the dependence of Δ on substitution can be rationalized by assuming that it is due to a (pseudo) Jahn—Teller effect in 1 and in the symmetrically substituted derivatives and to the combined influence of this effect and a differential, inductive destabilization in the other cases.The photoelectron spectrum of tetramethyl-bisallenyl 9 can be explained on the basis of known parameters for semi-localized π-orbitals and for their through-space and through-bond coupling.     

Etude par spectrometrie de photoelectrons de la structure electronique des enynes conjugues

We have measured by UV excited photoelectron spectroscopy the ionization potentials of 22 conjugated alkynes, in particular, those associated with three π orbitals (π_, π_, π₊. The corresponding bands appear in this order in the spectrum. We have also calculated the orbital energies of these compounds by two semi-empirical quantum mechanical methods (INDO and SPINDO).Discussion of the ionization potential variations in terms of polar effects confirms the spatial distribution of the orbitals predicted by calculation.The vibrational structure of the photoelectron bands confirms their attribution and the role predicted for the orbitals associated with the chemical bonds of these compounds.     

Photoelectron spectra of 1,4-dithiin and related compounds

The photoelectron (He(I)) spectra of 1,4-dithiin (1), 1,4-thiaselenin (2), 1,4-thiatellurin (3) and 4H-thiopyran (4) were assigned by comparison with published spectra of similar compounds and with the help of ab initio and semi-empirical calculations. The proposed orbital sequences for the outermost orbitals are π₄, π₃, π₂, σ for 1, 2 and 3, and π₄, π₂, σ, π₃ for 4. π₄ and π₃ correspond to the symmetric and antisymmetric combinations, respectively, of the heteroatom lone pairs, while π₂ is the antisymmetric combination of the π_eth orbitals.     

The photoelectron spectra of tricyclo[3.3.0.02,6]Octene and tricyclo[3.3.0.02,6]Octane

The photoelectron spectra of tricyclo[3.3.0.0^2,6]octene (1) and tricyclo[3.3.0.0^2,6]octane (2) have been recorded. The first bands in these spectra are correlated with orbitals which are linear combinations of the Walsh orbitals and of the olefinic π-orbital. This assignment is based on a zero differential overlap molecular orbital model as well as on the results of extended Hückel calculations. The resonance integral <π|H|W > for 1 is found to be ?1.34 eV.     

Angle-resolved polarization-dependent photoemission spectra of benzenethiol adsorbed on Cu(110)

Angle-resolved photoelectron spectra of benzenethiol chemisorbed on the Cu(110) face have been obtained with p- and s-polarized HeI (21.2 eV) radiation at room temperature. Comparison with the gas-phase spectrum and molecular-orbital correlation diagrams indicates that the benzenethiol bonds to the copper by way of the sulfur, rather than phenyl ring π-orbitals, and that the adsorption is dissociative, yielding a phenyl sulfide (mercaptide) surface species. An analysis of the polarization/angular dependence of the band intensities and comparison to symmetry-allowed transitions confirms that the molecule has the plane of the phenyl ring perpendicular to the surface. The assumed C_2v symmetry together with packing considerations further allows the orientation of the molecule and nature of the bonding to be suggested.     

The He(I) photoelectron spectra of some γ-substituted Co(III) acetylacetonate complexes

The He(I) photoelectron spectra of the complexes of Co(III) with acetylacetone and several derivatives are reported. The ionization potentials of metal-localized d-electrons and ligand π-orbitals are used to probe the extent of metal-to-ligand π-bonding and possible aromatic character in the chelate ring.     

Novel contrast mechanisms in photoelectron microscopy

Two previously undiscussed contrast mechanisms in Auger and photoelectron microscopy, namely photoelectron diffraction contrast and molecular orbital orientation contrast, are presented. The former contrast mechanism is based on the phenomenon of photoelectron diffraction and forward focusing of Auger and photoelectrons, the latter is based on near-edge absorption fine structure (NEXAFS) spectroscopy and stems from the dependence of the NEXAFS resonance intensity on the orientation of the electric field vector of the incoming linearly polarized light with respect to the molecular orbital associated with this resonance. The applicability of both contrast mechanisms was demonstrated using a nickel polycrystal and a monolayer of benzoic acid (BA) on this polycrystal as test systems. Within the photoelectron diffraction approach well-resolved images of the individual microcrystallites on the surface of Ni polycrystal were obtained by using the photoelectrons from both the localized core level (Ni 2p_3/2) and the free-electron-like valence band. Within the molecular orbital orientation approach the well-resolved images of the azimuthally aligned BA molecules on the surface of the (110) microcrystallites incorporated into a Ni polycrystal were acquired. In these experiments the photon energy was tuned to the excitation energy of the π* orbital of the phenyl ring, which is a constituent of a BA molecule, and the C1s → π* excitation was monitored by the carbon KLL Auger electrons. The distinction between photoelectron diffraction and molecular orbital orientation contrast mechanisms within an imaging experiment is discussed.     

The mass-shell and gauge-fixing conditions for the free relativistic massive quantum particle

Working in the context of the Weyl group, which describes off-mass-shell relativistic particles, we impose “gauge-fixing” constraints involvingR ⁰,R ⁺, andD as matrix element conditions to be satisfied by the on-mass-shell states of a massive particle. We evaluate the matrix elements inp-space using five sets of co-ordinates: (p ²,p), (p ²,p ⁺,p _T ), (p ²,p ^?,p _T ), (p ²,π), and (p ²,π ⁺,π _T ) where \(\pi ^\mu \equiv p^\mu /(p^2 )^{\tfrac{1}{2}} \) . We find that, only in the case ofR ⁰ with (p ²,p) coordinates,R ⁺ with (p ²,p ⁺,p _T ) coordinates, andD with (p ², π) or (p ²,π ⁺,π _T ) coordinates, can the condition be satisfied by arbitrary on-mass-shell states. In all other cases, the condition can be satisfied only by states belonging to a subset of subspaces of the on-mass-shell Hilbert space, i.e it forces a violation of the superposition principle. These results constitute thep-space quantum version of Shanmugadhasan's theorem for constrained classical systems which states that there exists, at least locally in phase space, a canonical transformation to a set of variables in which the second-class constraints become canonical pairs equal to zero with the other canonical coordinates independent of the second-class constraints.     

Comparative analysis of the energy levels of planar and core-twisted perylene bisimides in solution and solid state by UV/VIS, CV, and UPS/IPES

The frontier orbital energies of four different functionalized perylene bisimide derivatives, PBI-Ph(iPr)₂, PBI-H₄, PBI-F₂ and PBI-Cl₄, were directly determined by UV-photo electron spectroscopy (UPS) and inverse photo electron spectroscopy (IPES) and are compared to the results from cyclic voltammetry (CV) and optical absorption spectroscopy (UV/VIS). The optical spectra reveal significant differences between monomeric species in solution and assembled molecules in the condensed state for the nearly planar PBI-H₄ and PBI-F₂, which are attributed to significant π–π stacking interactions in the condensed phase. In contrast, for PBIs with bulky substituents or twisted core, i.e. 2,6-isopropylphenyl substituents at the imide positions (PBI-Ph(iPr)₂) or four chlorine substituents at perylene bay positions (PBI-Cl₄), similar spectra are observed in solution and in the condensed state, which suggests the absence of strong intermolecular π–π stacking interactions. An entirely different result is obtained for the HOMO/LUMO energy values obtained from UPS/IPES and CV measurements which do not reveal a significant impact of intermolecular π–π stacking interactions. When comparing CV and UPS/IPES results, an accentuated deviation was observed for the perylene bisimide derivatives as compared to correlations found in the literature.     

Carbon K-shell excitation spectra of linear and branched alkanes

The electron energy loss spectra of ethane, propane, n-butane, n-pentane, n-hexane, isobutane, isopentane and neopentane in the region of carbon K-shell excitation have been recorded under dipole-dominated conditions (2.8 ke V impact energy, small angle). The spectra are dominated by transitions to unoccupied valence π¹(CH₂, CH₃) and σ¹(C-C) levels. Additional weak features are assigned to Rydberg transitions. The position of the main continuum feature in each spectrum is consistent with the predictions of an empirical relationship with bond length. Systematic variations of spectral intensities are observed which support our assignments. The dominant feature in the K-shell spectrum of ethane, which was previously assigned to C 1s → 3p Rydberg transitions, is reassigned to excitation to a 3p/π¹(CH₃ ), mixed Rydberg/valence orbital (of antibonding σ-¹(C-H) character), in comparison to the other alkane spectra. An improved calibration value of 290.74(5) eV for the energy of the C 1s → π¹ transition in CO₂ is also obtained.     

Sensitive and Selective PET-Based π-expanded Phenanthrimidazole Luminophore for Zn2+ Ion

The novel photoinduced electron transfer (PET) chemosensor, 1-(1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)naphthalen-2-ol [MPPN] and its zinc complex were synthesised and characterized by electronic spectral and Frontier molecular orbital energy analysis. MPPN becomes efficient fluorescent chemosensor upon binding with metal ions and shows a strong preference toward Zn²⁺ ion. Density Functional theory (DFT) calculations reveal that luminescence of free MPPN originates from its orbital structure in which two π-orbitals (HOMO and HOMO-1) of the imidazole ring are situated between two π-orbitals (HOMO-2 and LUMO) of the naphthyl fragment. Therefore the absorption and emission processes occur between the two π- orbitals (HOMO-2 and LUMO). The two higher energy imidazole orbitals (HOMO and HOMO-1 ) serve as quenchers for the excited state of the molecule through nonradiative processes. Upon binding with Zn²⁺ ion, MPPN becomes a highly luminescent with λ_emi???421 nm. The significant enhancement of luminescence upon binding with Zn²⁺ ion is attributed to the stabilization of HOMO-2 and HOMO-1 π-orbitals of imidazole ring upon their engagement in new bonds with Zn²⁺ ion. The affinity of MPPN to zinc ion is found to be very high [K?=?6?×?10⁶ M^?1] when compared with other metals ions. The nonlinear absorption coefficient γ for MPPN is 1.9?×?10^?12 m/W and 3.9?×?10^?11 m/W for MPPN-Zn complex.     

Highly resolved electronic spectra and electronic structure of benzo[f]quinoxaline

Vibrational analysis of highly resolved phosphorescence (P), flourescence (F) and absorption spectra of benzo[f]quinoxaline (BQ) in Shpolskii matrices, at 77 K, was carried out. The spectra were dominated by phenanthrene-type fundamental vibrations. The appearance of several out-of-plane (op) modes in the P spectrum as well as the P lifetime, much shortened relative to that of the parent hydrocarbon, point to the proximity of the lowest n, π¹ and π, π¹ states of the BQ molecule. High activity of op modes in the F spectrum and complex structure of the absorption spectrum onset have been explained in terms of the pseudo-Jahn-Teller interaction between close-lying S₁ (π, π¹)and S₂(n, π¹) states.According to calculations of the BQ electronic structure, performed using a modified INDO CI method, the T₁(π, π¹)-T₂(n, π¹) and S₁(π, π¹-S₂(n, π¹) energy gaps are about 1200 and 700 cm^-1, respectively.     

A study of the centrally produced π+π−π0 system formed in the reactionpp→p f (π+π−π0)p s at 300 GeV/c

The reactionpp→p _f (π⁺π^?π⁰)p _s , where the π⁺π^?π⁰ system is centrally produced, has been studied at 300 GeV/c. The π⁺π^?π⁰ mass spectrum shows evidence for the η, ω anda ₂ (1320) as well as an enhancement in thea ₁ (1260) region. A Dalitz plot analysis of the π⁺π^?π⁰ system has been performed. Thea ₁ (1260) parameters coming from the fit of the 1⁺ S wave arem=1208±15 MeV and Γ=430±50 MeV. No evidence is found for theh ₁(1170) orh ₁(1380).     

Study of the π+π+π-π- system centrally 0 produced by incident π+ andp beams at 85 GeV/c

The reactions π⁺ p→π⁺(π⁺π⁺π^-π^-)p andpp→p(π⁺π⁺π^-π^-)p, where the (π⁺ π⁺ π^- π^-) system is centrally produced have been studied at 85 GeV/c. The π⁺π⁺π^-π^- mass spectrum shows evidence for thef ₁ (1285) meson with a mass of 1278±2 MeV and width 41±12 MeV which decays mainly through ρ⁰(770)π⁺π^-. Thef ₁(1285) is also observed in the ηπ⁺π^- channel. There is no significant evidence for ηππ or 4π decay modes of thef ₁(1420). The ρ⁰ρ⁰ production is found to be small. A Dalitz plot analysis of the 3π system shows evidence fora ₂ (1320) production and for a large contribution of theJ ^PC=1⁺⁺ ρπ wave.