An open-shell RHF CNDO/S (CI) analysis of the trans-stilbene radical cation: Optical and photoelectron spectra

Comparison of open-shell RHF CNDO/S (CI) computations with existing optical and photoelectron spectra of the trans-stilbene radical cation indicates a significant structural relaxation subsequent to ionization.     

Ab initio CI study and vibronic analysis of the photoelectron spectra of formaldoxime

The photoelectron spectrum of formaldoximie, CH₂NOH, has been re-investigated with higher resolution and interpreted by, means of ab initio SCF Cl calculations. Calculations have confirmed that the states increase in energy as π₁ < n₁ < π₂ < n₂ and have shown the existence of a shake-up peak at ≈15 eV. The calculation of Franck-Condon factors allowed the interpretation of the observed vibrational structure.     

Raman spectra and structure of biphenyl isoatomers (the S0, S1, T1 states and the cation and anion radicals)

The Raman and transient resonance Raman spectra of biphenyl (BP) and its perdeuterated analogue (BP-d₁₀) in three different electronic states (S₀, S₁ and T₁) and in two different ionized states (cation and anion) have been recorded in solution. The S₀ Raman spectra have also been measured for the crystalline state. The obtained set of spectra are analysed on the basis of the established vibrational assignments for the ground state of the planar (crystal) and the non-planar (solution) structures. The analysis suggests that BP in solution exists as a twisted structure in the S₁ state, but that it takes planar or nearly planar structures in the T₁, the cationic and the anionic states.     

Non-koopmans contributions to the photoelectron spectra of p-quinodimethane, 3,7-dimethyl-p-quinodimethane and tetracyano-p- quinodimethane: A limited Cl description

The low-lying doublet states of the p-quinodimethane (1) 3,7-dimethyl-p-quinodimethane (2) and tetracyano-p-quinodimethane (TCNQ) (3) cations are addressed within the semi-empirical open-shell RHF CNDO/S(Cl) formalism. Results indicate that the reduced intensity observed experimentally for the third ionization in both 1 and 2 arises from a pronounced configuration mixing between the parent one-electron Koopmans state and the non-Koopmans HOMO → LUMO valence excitation. Similar contributions are found to account for the relative intensifies of the first and second ionization bands (1.0 : 1.425) in 3.     

An Isolation and liquid phase EPR spectra of the cation radical salts of N-methyl and N-ethylphenothiazinium perchlorate

N-Methyl and N-ethylphenothiazines were oxidized with bromine and sodium perchlorate to give stable cation radical salts. Their well-resolved epr spectra were obtained in nitromethane solution. In order to assign the hfs constants, the cation radicals of N-deuteriomethyl and N-deuterioethylphenothiazines were prepared with lead tetraacetate and trifluoroacetic acid in solutions.     

AM1/CI, CNDO/S and ZINDO/S computations of absorption bands and their intensities in the UV spectra of some 4(3H)-quinazolinones

A detailed analysis of both frontier MOs and electronic transitions in UV spectra of 16 4-quinazolinone derivatives has been carried out in MO terms, by semiempirical methods AM1/CI, CNDO/S and ZINDO/S. On the basis of experimental and theoretical investigations by the ZINDO/S and CNDO/S methods the long-wavelength bands of 4(3H)-quinazolinone and its derivatives have been assigned to n-->pi(*) transition of the CO fragment and to the transition caused by intramolecular charge transfer from Ph and NCN fragments to CO group. It was shown that theoretically obtained electronic transitions applying method AM1/CI are not in agreement with experimental data observed for the 4(3H)-quinazolinone and 2,4(1H,3H)-quinazolinedione. Good correlation of theoretical and experimental data has been obtained by the method ZINDO/S for the wavelengths and the molar extinction coefficients of the compounds studied. Satisfactory correlation of theoretical and experimental data has also been obtained by the method CNDO/S with singly and doubly excited configurations, for the wavelengths only. Such correlations on experimental and theoretical wavelength and molar absorption coefficients of 4-quinazolinone derivatives are carried out for the first time.     

A valence bond analysis of electronic degeneracies in Jahn-Teller systems: low-lying states of the cyclopentadienyl radical and cation

The lowest doublet electronic state of the cyclopentadienyl radical (CPDR) and the lowest singlet state of the cyclopentadienyl cation (CPDC) are distorted from the highly symmetric D(5h) structure due to the Jahn-Teller effect. A valence bond analysis based on the phase-change rule of Longuet-Higgins reveals that in both cases the distortion is due to the first-order Jahn-Teller effect. It is shown that, while for the radical an isolated Jahn-Teller degeneracy is expected, in the case of the cation the main Jahn-Teller degeneracy is accompanied by five satellite degeneracies. The method offers a chemically oriented way for identifying the distortive coordinates.     

Xα SW study of bonding,photoelectron spectra and photoionization transition states of (cyclobutadiene)iron tricarbonyl

We report Xα SW MO calculations on (η²-C₄H₄)Fe(CO)₃ and its five transition states, formed by removing a half of an electron from each of the five uppermost filled MO_s. Eigenvalues in the transition states depend only slightly upon the MO from which the “high-ionization” occurs. The MOs are more polarized in the transition states than in the ground state. The C₄H₄-Fe bonding is highly covalent.     

Ab initio scf and CI study of the ground and excited states of the HN2 radical

Non-empirical SCF and CI calculations are reported for the HN₂, free radical in various low-lying electronic states. The nature of the angular and N-N and N-H stretching potential curves of each of these species is investigated, including a study of the dissociative behavior of such states. The ground state is found to be only very slightly bound with respect to NH stretch, in contrast to what is observed for isoelectronic HCO, The vertical electronic spectrum of HN₂, appears to be marked by a single long wavelength transition (1.95 eV) from the bent (124°) ‘A’ ground state to the linear ²Π excited species, but at least four other intra-valence and an additional n → 3s Rydberg species are indicated in the 5.5–8.0 eV absorbing region.     

The E.S.R. Spectrum of the tetra(1-adamantyl)cyclobutadiene radical cation

A revised interpretation is proposed for the ¹H and ¹³C hyperfine coupling in the e.s.r. spectrum of the tetra-1-adamantylcyclobutadiene radical cation, and assumes that free rotation occurs about the 1-adamantyl bond.     

A CNDO/S study of core hole satellites in the photoelectron spectra of para-, meta- and ortho-nitroanilines

Core-level photoelectron spectra of geometrical isomers of nitroanilines are studied by a semi-empirical method. Energy results alone reveal the presence of satellites for the ionization from the NO₂ group in all three isomers, in contrast to some experimental observations. Intensity calculations, using HOMOLUMO overlap, agree with the calculation using full π-overlap for para-nitroaniline, but differ considerably for meta- and ortho-nitroanilines.     

Poly(thiophenes) derivatized with linear and macrocyclic polyethers: from cation detection to molecular actuation

The association of linear or macrocyclic polyethers with the electronic properties of the π-conjugated polythiophene backbone leads to functional conducting polymers that exhibit metal cation dependent electronic properties. Based on this concept, various classes of cation sensors have been proposed and investigated for almost two decades. The interactions of metal cations with linear or macrocyclic polyether functional groups lead to modifications of the electronic properties of the π-conjugated backbone through various mechanisms including direct electronic effects on a single conjugated chain, collective electrochemical processes, or conformational changes. Conjugated polymers and oligomers representative of these various processes are discussed with an emphasis on recent examples of derivatized conjugated systems in which the interactions between metal cations and polyether groups serve as driving force to create molecular motion in conjugated systems.     

The He(I) photoelectron spectra of OsO4, and RuO4

The photoelectron spectra of ruthenium tetroxide and osmium tetroxide excited by He(I) radiation are reported. From their interpretation it follows that the first two strong low energy transitions in the electronic absorption spectra of OsO₄, RuO₄, TcO^?₄, ReO^?₄, MoO^2?₄ and WO^2?₄ can be assigned to t₁ → 2e and 3t₂ → 2e respectively.     

On the simulation of photoelectron spectra complicated by conical intersections: higher-order effects and hot bands in the photoelectron spectrum of triazolide (CH)2N3(-)

We report simulated photoelectron spectra for 1,2,3-triazolide (CH)(2)N(3)(-), which reveal the vibronic energy levels of the neutral radical 1,2,3-triazolyl, (CH)(2)N(3). The spectral simulation using a quasidiabatic Hamiltonian H(d) comprised of polynomials through 4th order (thereby extending conventional quadratic expansions), is compared to both the experimental spectrum and a standard Franck-Condon (adiabatic) simulation. The quartic H(d) is far superior to the quadratic H(d), reproducing the main features of the experimental spectrum and allowing for their subsequent assignment. The contributions from excited anion states successfully reproduce the observed vibronic transitions to the red of the assigned band origin of the neutral species. The algorithmic extensions required for the determination of these hot band contributions to the total spectrum are discussed. Convergence of the spectral envelope with respect to the vibronic basis, including both the principal and hot bands, required more than 10(9) terms.     

Calculation of excited singlet and triplet state polarizabilities using the CNDO/S CI method. An application to naphthalene

A procedure is outlined for the calculation of molecular static electric polarizabilities in excited singlet and triplet states using the ”finite perturbation theory“ in conjunction with the CNDO/S CI method. Numerical results for the ground and the three lowest excited singlet and triplet states of naphthalene are presented. It turns out that the generalized Hellmann-Feynman theorem is approximately valid for the CNDO/S CI wavefunctions and that triplet and singlet state polarizabilities in states of the same symmetry may strongly differ.     

Ab initio optical rotatory dispersion and electronic circular dichroism spectra of (S)-2-chloropropionitrile

Coupled-cluster and density-functional methods have been used to determine specific rotations and electronic circular dichroism (ECD) rotational strengths for (S)-2-chloropropionitrile. Coupled-cluster specific rotations using both the length- and velocity-gauge representations of the electric-dipole operator, computed with basis sets of triple-zeta quality containing up to 326 functions, compare very well with recently reported gas-phase cavity-ring-down polarimetry data. ECD rotational strengths for the six lowest-lying excited states are found to vary in sign, and the second excited state, which has a larger rotational strength than the first by a factor of 4, was found to yield a much larger contribution (by a factor of 10) to the overall negative specific rotation observed both experimentally and theoretically. However, both valence and Rydberg states appear to make substantial contributions to the total rotation, often of opposite sign from the converged/linear-response result. Furthermore, the sum-over-states approach was found to be inadequate for reproducing the specific rotations derived from the linear-response approach, even when 100 excited states (well beyond the estimated ionization limit) were included in the summation. Density-functional specific rotations using the B3LYP functional with basis sets of quadruple-zeta quality containing up to 588 functions are found to be too large compared to experiment by approximately a factor of 2. This error appears to be related to both the underestimation of the electronic excitation energies, as well as concomitant overestimation of the corresponding ECD rotational strengths. Although earlier studies reported good agreement between density-functional specific rotations and experiment when electric-field-dependent functions were used in conjunction with a double-zeta-quality basis set, the results reported here, which are near the basis-set limit, suggest that this agreement may be fortuitous.     

Angular- and light-polarization-dependent valence UV photoelectron spectra of the hexatriacontane (n-C36H74) crystal

Valence-band angle-resolved photoelectron spectra of oriented polycrystalline films of hexatriacontane (n-C₃₆H₇₄) were measured by He II (40.8 cV) light. Significant dependence of the spectra on photoemissive angle and light-incidence angle was observed. The advantages and problems of angle-resolved UV photoelectron spectroscopy to the study of electronic structures of organic crystals are discussed.     

The He(II) photoelectron spectra of the fluorosubstituted ethylenes and their analysis by the Green's function method

The 30.4 nm He(II) photoelectron spectra of the fluorosubstituted ethylenes have been recorded. The assignment of all main bands is obtained from many-body Green's function calculations. The results from the semi-empirical HAM/3 method lead to a nearly identical assignment. For the mono- and difluoroethylenes, an unambiguous interpretation of the spectra can be established from empirical considerations alone. The set of spectra has been reexamined with respect to the “perfluoro-effect” rules. Also discussed are the ionisation energies as a function of the geminal FCF bonding angle and the similarity of the spectra of the cis and trans isomers of 1,2-difluoroethylene. Additional weak bands were detected in the energy region 21–24 eV in all spectra and were attributed to “shake-up” transitions on the basis of 2ph-Tamm-Dancoff Green's function calculations. The orbital model of ionisation breaks down for the ionisation out of the F(2s) and C(2s) orbitals in general. The calculations reveal a charge transfer character of the excitations accompanying ionisation from the C(2s) orbitals.     

X-ray photoelectron spectra of inorganic molecules: XXX. Organometallic derivatives of tungsten(O) and tungsten(II)

The W 4f binding energies of various carbonyl-containing complexes of tungsten are reported. Comparisons between W^II complexes containing the [(η³-allyl)W(CO)₂] moiety and neutral derivatives of the type W(CO)_6-xL_x are made. The absence of a clear-cut correlation between the W 4f binding energies and the value of x in W(CO)_6-xL_x is discussed and these results compared with those obtained for the analogous molybdenum complexes.