Assignment of photoelectro bands for naphthalene and anthracene by penning ionization electron spectroscopy

The Ne¹(³P₂) Penning electron spectra and the Ne I photoelectron spectra were measured in the gas phase. The observed systematic differences in their relative intensities were interpreted in terms of the electron distributions of the relevant molecular orbitals and used for assignment of the deep π bands, π₁ (12.4 eV) for naphthalene, and π₂ (11.9 eV) and π₁ (12.8 eV) for anthracene.     

An MNDO study of the structures,vibrational frequencies,and ionization energies of the first five poly-ynes

The recently developed semi-empirical SCF MO method MNDO was found to correctly predict the orderings of the π orbitals in the first four poly-ynes. The mean difference between the calculated π orbital energies and the observed PES ionization values was 0.22 eV. Among the remaining orbitals the highest was of 2p σ⁺_g symmetry in all five-poly-ynes. The ionization from the highest occupied π orbital. These are discussed in terms of the vibrational fine structure observed in the photoelectron and emission spectra.     

Valence bond descriptions of benzene and cyclobutadiene and their counterparts with localized bonds

Geometry optimizations have been performed for benzene and cyclobutadiene and for the corresponding moieties with nonresonating double bonds, viz. 1,3,5‐cyclohexatriene and 1,3‐cyclobutadiene. The calculations were done using the valence bond self‐consistent field method including orbital optimization. Both strictly local and delocalized p‐like orbitals were used for the π system, which influences the strengths of the π bonds. The calculations result in geometries and resonance and stabilization energies for benzene and cyclobutadiene, which are compared with theoretical models of aromaticity. The importance of resonance is discussed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003     

Triplet states of fluorocarbonyl compounds determined by trapped electron spectroscopy

Ion cyclotron resonance trapped electron spectra for carbonylfluoride, trifluoroacetyl fluoride, hexafluoroacetone, acetyl fluoride and I,I.I-trifluoroacetone are reported. Comparisons of data obtained for ³(π → π^*) vertical excitation energies are used in conjunction with photoelectron spectroscopic data to deduce the effects of fluorine and trifluoromethyl groups on the energies of σ, π and π^* orbitals associated with the carbonyl group.     

Photoelectron spectra and electronic structure of nitrogen-containing chelate boron complexes

A brief review of the results of studying some classes of nitrogen-containing chelate boron complexes by ultraviolet photoelectron spectroscopy and density functional theory is reported. The quantum chemical modeling of the substitution effects of a complexing agent, heteroatoms, and functional groups in α, β, and γ positions of the chelate ring allowed us to establish the features of the electronic structure of the studied complexes. It is found that the substitution of heteroatoms in the chelate ring has no substantial influence on the structure of the highest occupied molecular orbital (HOMO). In imidoylamidinate complexes, as opposed to formazanates and β-diketonates, there is no noticeable mixing of π orbitals of the chelate and benzene rings. In condensed nitrogen heterocycles the HOMO is stabilized by 0.2-0.3 eV and π orbitals of the benzene ring are stabilized by 0.8-1.2 eV. The HOMO of substituted aza-boron-dipyridomethene correlates with anthracene and acridine π₇ orbitals, which causes the fine structure of the first band. It is shown that in an energy range below 11 eV the calculated results reproduce well the energy gaps between the ionization states of the complexes.     

The consequences of σ and π conjugative interactions in mono-, di- and triacetylenes. A photoelectron spectroscopic investigation

The HeI photoelectron spectra of mono-, di-, and triacetylenes are presented. In these compounds the two-centre π-orbitals of the ethynyl groups conjugate with the π-orbitals of double bonds or benzene moieties, or with the Walsh orbitals of three-membered ring systems. Assuming the validity of Koopmans' approximation, the observed energies of the radical cation states reached by electron ejection from π-orbitals can be rationalized in terms of a simple LCBO-MO model in those cases, where the molecule is planar. The corresponding numerical results for the ionization energies are in excellent agreement with experiment, if the three parameters of the model are properly calibrated. In contrast, the bands assigned to ejection from in plane π-orbitals are shifted to lower energies by ca. 0.5 eV with respect to the expectation values derived from the above model, due to ‘through-bond’ interaction with lower lying σ-orbitals. Extensive σ/π mixing occurs in the non planar compounds for all orbitals. The assignments of the spectra of diethynylmethane, 1,4-hexadiyne, 1,2-diethynylethane and of cis- and trans-diethynylcyclopropane are backed by semiempirical SCF calculations. The spectra of the cis and trans isomers of diethynylethyleneoxide and diethynylethylenesulfide are discussed by comparison with the corresponding hydrocarbons and with oxirane and thiirane respectively. Finally, the following topics are considered in detail: (a) The effect of spin orbit coupling on the spectrum of 1-iodo-1-butyne-3-ene; (b) the effect of the essentially free internal rotation in divinylacetylene on the band shapes of its photoelectron spectrum and (c) the relationship between the conjugative properties of ethylenic π-orbitals and of the Walsh-orbitals of cyclopropane.     

The electronic structure of transition-metal carbonyl complexes of norbornadiene and mesitylene

The photoelectron spectra of several transition-metal carbonyl complexes of norbornadiene and mesitylene have been measured. The data for the norbordiene complexes have been compared with those reported previously for norbornadieneiron tricarbonyl. The shifts in energies of the π orbitals of the ligands introduced by the metal carbonyl moieties are remarkably insensitive to the choice o0f transition metal or geometry of the complex. However, the characters of the metal d-orbital ionization bands are markedly affected by the choice of ligand.     

Ab initio calculations on urea

Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the ¹A₁(π → π*) state. The ¹A₁(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.     

Ultraviolet photoelectron spectroscopy of poly(p-phenylene sulfide) (PPS)

Ultraviolet photoelectron spectra were measured for films of poly(p-phenylene sulfide) (PPS) prepared by vacuum evaporation. The threshold ionization potential was determined to be 6.0 ± 0.1 eV. The peaks in the photoelectron spectra are assigned by comparison with theoretical calculations, and the π bandwidths of PPS and related compounds are discussed.     

Non-bonded interactions in 2,2,4,4-tetramethyl-1,3-cyclobutanedithione and 2,2,4,4-tetramethyl-3-thio-1,3-cyclobutanedione

Electronic absorption and emission spectra as well as He(I) photoelectron spectra of 2,2,4,4-tetramethyl-1,3-cyclobutanedithione and 2,2,4,4-tetramethyl-1-3-thio-1,3-cyclobutanedione have been interpreted on the basis of molecular orbital calculations. The results show that the non-bonded orbital of the dithione is split owing to through-bond interaction, the magnitude of splitting being 0.4 eV. The π* orbital of the dithione appears to be split by about 0.2 eV. Electronic absorption spectra show evidence for the existence of four n—π* transitions, arising out of the splitting of the orbitals referred to above, just as in the case of 2,2,4,4-tetramethyl-1,3-cyclobutanedione. Electronic and photoelectron spectra of the thio-dione show evidence for weak interaction between the CS and C&.zdbnd;O groups, probably via π* orbitals. Infrared spectra of both the dithione and the thio-dione are consistent with the planar cyclobutane ring; the ring-puckering frequency responsible for non-bonded interactions is around 67 cm^?1 in both the dithione and the thio-dione, the value not being very different from that in the dione. The 1,3-transannular distance is also similar in the three molecules.     

Photoelectron Spektra of Azabenzenes and Azanaphthalenes: II. A Reinvestigation of Azanaphthalenes by High-Resolution Photoelectron Spectroscopy

The bands with I_v < 13 eV in the photoelectron spectra of quinoline (IX), isoquinoline (X), cinnoline (XI), quinazoline (XII), and quinoxaline (XIII) have been reassigned in a way consistent with the assignment proposed for pyridine (II), the diazines (III, IV, V), s-triazine (VI), and 1,2,4,5-tetrazine (VII). The bands corresponding to the ejection of an electron from a π-orbital have been identified by a regression calculation based on a HMO perturbation treatment. It has been found that the combined through-space and through-bond interaction of the lone pairs in III, IV, V and in their corresponding benzologues XI, XII, XIII are the same within experimental error ( ±, 0.2 eV). Our assignment is also supported by an empirical correlation of the pK_{a, 1?} values and the mean lone-pair ionization potentials of the azaderivatives I to XIII.     

Valence Photoionization of Thymine: Ionization Energies,Vibrational Structure,and Fragmentation Pathways from the Slow to the Ultrafast

The photoionization of thymine has been studied by using vacuum ultraviolet radiation and imaging photoelectron photoion coincidence spectroscopy after aerosol flash vaporization and bulk evaporation. The two evaporation techniques have been evaluated by comparison of the photoelectron spectra and breakdown diagrams. The adiabatic ionization energies for the first four electronic states were determined to be 8.922±0.008, 9.851±0.008, 10.30±0.02, and 10.82±0.01 eV. Vibrational features have been assigned for the first three electronic states with the help of Franck–Condon factor calculations based on density functional theory and wave function theory vibrational analysis within the harmonic approximation. The breakdown diagram of thymine, as supported by composite method ab initio calculations, suggests that the main fragment ions are formed in sequential HNCO-, CO-, and H-loss dissociation steps from the thymine parent ion, with the first step corresponding to a retro-Diels–Alder reaction. The dissociation rate constants were extracted from the photoion time-of-flight distributions and used together with the breakdown curves to construct a statistical model to determine 0 K appearance energies of 11.15±0.16 and 11.95±0.09 eV for the m/z 83 and 55 fragment ions, respectively. These results have allowed us to revise previously proposed fragmentation mechanisms and to propose a model for the final, nonstatistical H-loss step in the breakdown diagram, yielding the m/z 54 fragment ion at an appearance energy of 13.24 eV.     

Threshold-photoelectron spectroscopic study of methyl-substituted hydrazine compounds

The valence shell electronic structures of methylhydrazine (CH(3)NHNH(2)), 1,1-dimethylhydrazine ((CH(3))(2)NNH(2)) and tetramethylhydrazine ((CH(3))(4)N(2)) have been studied by recording threshold and conventional (kinetic energy resolved) photoelectron spectra. Ab initio calculations have been performed on ammonia and the three methyl substituted hydrazines, with the structures being optimized at the B3-LYP/6-31+G(d) level of theory. The ionization energies of the valence molecular orbitals were calculated using the Green's function method, allowing the photoelectron bands to be assigned to specific molecular orbitals. The ground-state adiabatic and vertical ionization energies, as determined from the threshold photoelectron spectra, were IE(a) = 8.02 +/- 0.16 eV and IE(v) = 9.36 +/- 0.02 eV for methylhydrazine, IE(a) = 7.78 +/- 0.16 eV and IE(v) = 8.86 +/- 0.01 eV for 1,1-dimethylhydrazine and IE(a) = 7.26 +/- 0.16 eV and IE(v) = 8.38 +/- 0.01 eV for tetramethylhydrazine. Due to the large geometry change that occurs upon ionization, these IE(a) values are all higher than the true thresholds. New features have been observed in the inner valence region and these have been compared with similar structure in the spectrum of hydrazine. The effect of resonant autoionization on the threshold photoelectron yield is discussed. New heats of formation (Delta(f)H) are proposed for the three hydrazines on the basis of G3 calculations: 107, 94, and 95 kJ/mol for methylhydrazine, 1,1-dimethyhydrazine and tetramethylhydrazine, respectively. The previously reported Delta(f)H for tetramethylhydrazine is shown to be erroneous.     

Photoelectron Spectroscopy of peri-Amino Naphthalenes

The He I photoelectron spectra of peri-amino and dimethylamino naphthalenes are presented. The differences in the ionization energies of the π-bands are interpreted by separation of the perturbation of the amino substituent into an inductive destabilization and conjugative stabilization. This affords the assignment of the photoelectron bands of ionization energies below 11 eV and an estimation of the dihedral angle in the peri-dimethylamino derivatives. The data on the peri-amino naphthalenes indicate some angular distortion in contrast to 2-aminonaphthalene.     

The Electronic Structure of Azuleno[1,2,3-cd]phenalene and Azuleno[5,6,7-cd]phenalene,a Comparison

The photoelectron (PE.) spectra of azuleno[l, 2, 3-cd]phenalene ( 1 ) and azuleno- [5,6,7-cd]phenalene( 2 ) have been recorded. The first five bands of both compounds could be assigned to transitions corresponding to removal of electrons from 4a₂, 6b₁, 5b₁, 3a₂ and 4b_l orbitals. This assignment is based mainly on a comparison between the observed ionization potentials and orbital energies calculated in a HMO and a PPP model. The UV./VIS. polarized absorption spectrum of 1 in the region 10000–45000 cm^?1 has been measured by means of the stretched film technique. The measurements were performed in polyethylene sheets at 77°K. Several bands could be assigned to π* ← π transitions calculated by a PPP-CI method. A comparison between the electronic structures of 1 and 2 is made by means of a simple HMO diagram.     

Threshold photoelectron study of naphthalene, anthracene, pyrene, 1,2-dihydronaphthalene, and 9,10-dihydroanthracene

Threshold photoelectron spectra (TPESs) were obtained for naphthalene, anthracene, pyrene, 1,2-dihydronaphthalene, and 9,10-dihydroanthracene using imaging photoelectron photoion coincidence spectroscopy, from threshold to a photon energy of ～20 eV. Outer valence Green's function calculations at the OVGF∕cc-pVTZ level of theory were used to assign molecular orbitals to the observed TPES features. There is generally good agreement between the predicted and observed bands. Threshold regions for each molecule exhibit vibrational structure which is readily assigned based on previous PES studies. While the measured adiabatic ionization energies (IE(a)) for naphthalene, anthracene, and pyrene are in good agreement with previous works, new values are reported for the two dihydro species (1,2-dihydronaphthalene, 8.010 ± 0.010 eV and 9,10-dihydroanthracene, 8.335 ± 0.010 eV). A comparison is also made with the G3∕∕B3LYP composite method, which consistently overestimates the IE values by 0.06-0.09 eV. The double ionization energies for anthracene and pyrene have been measured to be 19.3 ± 0.2 and 19.8 ± 0.2 eV, respectively.     

New Convenient Methods for the Preparation of Pendant Chain Cyclobutadiene Tricarbonyliron Complexes

Simple, economical, and more practical procedures have been developed for the synthesis of precursors to pendant chain cyclobutadiene metal complexes. Facile reduction of the mono and disubstituted 1,2-cyclobutenediones gave cyclobutenediols which were converted to their corresponding trans dibromides with PBr₃. Reduction and complexation using Fe₂(CO)₉ formed the pendant chain cyclobutadiene tricarbonyliron complexes in good yields.     

Ab initio MO Calculations of benzene + TCNE and naphthalene + TCNE complexes with STO-3G π-split basis set

Ab initio SCF MO calculations have been carried out on benzene + TCNE (tetracyanoethylene) and naphthalene + TCNE complexes with the STO -3G, STO -3G π-split (STO -3G for π orbitals and a split basis for π orbitals), and 4–31G basis sets. The interaction energy, gross charges, dipole moment, and the electron density in the middle plane of the complexes have also been evaluated. The STO -3G π-split basis set is appropriate for the calculation of large π–π stacking complexes from two points of view, production of reliable results and ease of computations. The approximation scheme based on the semiorthogonalized orgitals is revealed to be very efficient to save CPU time and storage in such calculations. The stable conformation and the charge-transfer interaction of the two complexes are discussed on the basis of the calculated quantities.     

Are Unsaturated Isocyanides so Different from the Corresponding Nitriles?

Simple unsaturated and cyclopropylic isocyanides are synthesized by an efficient and simple approach. These compounds with gradually increasing distance between the unsaturated moiety and the isonitrile group are studied by UV photoelectron spectroscopy and quantum chemical calculations, and also compared to the corresponding nitriles. The first photoelectron band of the unsaturated compounds is linked to removal of an electron from the HOMO, which corresponds to CC multiple-bond ionization in antibonding interaction with the π-isocyanide bond (in the same plane) for conjugated systems, or in antibonding interaction with the pseudo-π-CH(2) group for isolated systems. For the 1-ethenyl derivatives, both cyano and isocyano groups act as a π-electron acceptor from the vinyl group, but the isocyano π system is much more strongly destabilized (ionization energies (IEs) shift to smaller values) by vinyl (3.12 eV) than the cyano π system is (2.70 eV). In comparison with the 1-ethynyl derivatives, a less pronounced destabilization (2.69 eV) of π(NC) by the ethynyl system (1.86 eV for π(CN)), and nearly the same order of magnitude of the energetic gap between the total antibonding (π(CC)-π(NC)) and the total bonding (π(CC)+π(NC)) IEs for ethenyl and ethynyl compounds are noted. The huge values of these last-named data for H(2)C=CH-NC (3.85 eV) and for HC≡C-NC (4.04 eV) reflect the strong interaction between the unsaturated carbon-carbon moiety and the isocyanide group, and thus more efficient conjugation than for the corresponding nitriles.     

Excited electronic states of the fluorobenzenes by variable angle electron impact spectroscopy

Electron-impact excitation spectra of benzene, fluorobenzene, o-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3,4-tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene have been measured at impact energies of 50 eV and either 25 eV or 30 eV, and scattering angles from 5° to 80°. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet → triplet, π → π^*, transition. In benzene, fluorobenzene, o-difluorobenzene, and 1,3,5-trifluorobenzene, an additional singlet → triplet transition was detected at about 5.6 eV. Three singlet → singlet transitions analogous to the 4.90, 6.20, and 6.95 eV transitions in benzene are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet → singlet transition that is most clearly observed in the hexafluorobenzene spectrum with a peak at 5.32 eV.