Angular distributions in the photoelectron spectroscopy of furan,thiophene, and pyrrole

The photoelectron angular distributions of furan, thiophene, and pyrrole are reported. The trends in the value of the asymmetry parameter β and in the shapes and positions of bands in the spectra are used to assign the peaks that correspond to ionization from the π orbitals in these molecules.     

Theoretical investigation of photoelectron spectra of furan,pyrrole, thiophene,and selenole

The ionization spectra of furan, pyrrole, thiophene, and selenophene have been calculated within the framework of the nonempirical quantum-chemical method with the Green's one-particle function in the approximation of the third order algebraic diagram construction [ADC(3)]. The calculated energies and the intensity of vertical transitions pertaining to the ionization of outer and inner shells are compared with the newest experimental data. The good agreement of theoretical and experimental results enabled a detailed assignment and interpretation of the observed photoelectron spectra to be carried out. Problems of disturbing the picture of orbital ionization are considered; the mechanism of formation of low-lying photoelectron satellites is explained. Certain general rules and trends of the behavior of the spectra of the systems studied are considered. Dedicated to Academician of the Russian Academy of Sciences B. A. Trofimov on his 70^th jubilee. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1366–1379. September, 2008.     

Inversion vibration of PH3+(X2A2") studied by zero kinetic energy photoelectron spectroscopy

We report the first rotationally resolved spectroscopic studies on PH3+(X2A2") using zero kinetic energy photoelectron spectroscopy and coherent VUV radiation. The spectra about 8000 cm(-1) above the ground vibrational state of PH3+(X2A2") have been recorded. We observed the vibrational energy level splittings of PH3+(X2A2") due to the tunneling effect in the inversion (symmetric bending) vibration (nu2+). The energy splitting for the first inversion vibrational state (0+/0-) is 5.8 cm(-1). The inversion vibrational energy levels, rotational constants, and adiabatic ionization energies (IEs) for nu2+ = 0-16 have been determined. The bond angles between the neighboring P-H bonds and the P-H bond lengths are also obtained using the experimentally determined rotational constants. With the increasing of the inversion vibrational excitations (nu2+), the bond lengths (P-H) increase a little and the bond angles (H-P-H) decrease a lot. The inversion vibrational energy levels have also been calculated by using one dimensional potential model and the results are in good agreement with the experimental data for the first several vibrational levels. In addition to inversion vibration, we also observed firstly the other two vibrational modes: the symmetric P-H stretching vibration (nu1+) and the degenerate bending vibration (nu4+). The fundamental frequencies for nu1+ and nu4+ are 2461.6 (+/-2) and 1043.9 (+/-2) cm(-1), respectively. The first IE for PH3 was determined as 79670.9 (+/-1) cm(-1).     

Reactivity and regioselectivity of the pyrrole,furan, and thiophene radical cations in polymerization reactions

Within the framework of the delocalization model, based on the perturbation theory, the rate constants of the reactions of the pyrrole, furan, and thiophene radical cations with the initial compounds were compared. It was estabished that attack of a cation in the 2-position of the heterocycle is more probable in comparison with the 3-position. The results of a comparison of the reactivity of the radical cations and the corresponding molecules in the triplet state in these reactions are discussed in connection with the experimental data.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, Nos. 5–6, pp. 483–487, September–December, 1992.     

Triplet states of furan,thiophene, and pyrrole

Low-lying triplet electronic states have been detected in furan, thiophene, and pyrrole by the method of variable-angle, electron-impact spectroscopy. Singlet → triplet transitions occur with maximum intensity at 3.99 eV and 5.22 eV in furan, 3.75 eV and 4.62 eV in thiophene, and 4.21 eV in pyrrole. A weak transition at 5.22 eV in pyrrole is assigned as the lowest observed singlet → singlet excitation in that molecule.     

Resonantly enhanced multiphoton ionization and zero kinetic energy photoelectron spectroscopy of 2-indanol conformers

We report studies of a supersonically cooled 2-indanol using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. In the REMPI experiment, we have identified three conformers of 2-indanol and assigned the vibrational structures of the first electronically excited state for the two major conformers. Conformer Ia contains an intramolecular hydrogen bond between the -OH group and the phenyl ring, while conformer IIb has the -OH group in the equatorial position. We have further investigated the vibrational spectroscopy of the cation for the two major conformers using the ZEKE spectroscopy. The two conformers display dramatically different vibrational distributions. The ZEKE spectrum of conformer Ia shows an extensive progression in the puckering mode of the five member ring, indicating a significant geometry change upon ionization. The ZEKE spectra of conformer IIb are dominated by single vibronic transitions, and the intensity of the ZEKE signal is much stronger than that of conformer Ia. These results indicate an invariance of the molecular frame during ionization for conformer IIb. We have performed ab initio and density functional theory calculations to obtain potential energy surfaces along the dihedral angle involving the -OH group for all three electronic states. In addition, we have also calculated the vibrational distribution of the ZEKE spectrum for the puckering mode of the five member ring. Not only the vibrational frequencies but also the intensity distributions for both conformers have been reproduced satisfactorily. The adiabatic ionization energies have been determined to be 68 593+/-5 cm(-1) for conformer Ia and 68 981+/-5 cm(-1) for conformer IIb.     

Renner-Teller effect in C2H2+(X2Pi u) studied by rotationally resolved zero kinetic energy photoelectron spectroscopy

The Renner-Teller effect in C(2)H(2)(+)(X(2)Pi(u)) has been studied by using zero kinetic energy (ZEKE) photoelectron spectroscopy and coherent extreme ultraviolet (XUV) radiation. The rotationally resolved vibronic spectra have been recorded for energies up to 2000 cm(-1) above the ground vibrational state. The C triple bond C symmetric stretching (upsilon(2)), the CCH trans bending (upsilon(4)), and the CCH cis bending (upsilon(5)) vibrational excitations have been observed. The assigned vibronic bands are 4(1)(1)(kappa(2)Sigma(u)(+))(hot band), 4(1)(0)(mu/kappa(2)Sigma (u)(-/+)), 5(1)(0)(mu/kappa(2)Sigma (g)(+/-)), and 4(2)(0)(mu(2)Pi(u)), 4(2)(0)(kappa(2)Pi(u)), 4(1)(0)5(1)(0) (mu(2)Pi(g)), 0(0)(0)(X(2)Pi(u)), and 2(1)(0)(X(2)Pi(u)). The Renner-Teller parameters, the harmonic frequencies, the spin-orbit coupling constants, and the rotational constants for the corresponding vibronic bands have been determined by fitting the spectra with energy eigenvalues from the Hamiltonian that considers simultaneously Renner-Teller coupling, vibrational energies, rotational energies, and spin-orbit coupling interaction.     

Colored electrically conducting polymers from furan,pyrrole, and thiophene

Furan, pyrrole, and thiophene were polymerized under catalysis by trichloro- and trifluoroacetic acid to produce colored polymers which were characterized by various methods, including electronic and proton magnetic resonance spectra and electrical conductances of deeply colored trichloroacetic acid adducts. The predominant repeat units of these polymers are of the same type as those of deeply colored cyclopentadiene polymers, except that a CH₂ group is replaced by O, NH, or S.     

Vibrational structure, spin-orbit splitting, and bond dissociation energy of Cl2+(X2 Pi g) studied by zero kinetic energy photoelectron spectroscopy and ion-pair formation imaging method

The isotopomer-resolved vibrational and spin-orbit energy structures of Cl(2) (+)(X (2)Pi(g)) have been studied by one-photon zero kinetic energy photoelectron spectroscopy. The spin-orbit energy splitting for the ground vibrational state is determined as 717.7+/-1.5 cm(-1), which greatly improves on the accuracy of the previously reported data. This value is found to be in good agreement with the ab initio quantum chemical calculation taking account of the inner shell electron correlation. The first adiabatic ionization energy (IE) of Cl(2) is determined as 92 645.9+/-1.0 cm(-1). Using the ion-pair formation imaging method to discriminate signals of Cl(+)((1)D(2)) from those of Cl(+)((3)P(j)), the threshold for ion-pair (E(tipp)) production, Cl(+)((1)D(2))+Cl(-)((1)S(0))<--Cl(2)(X (1)Sigma(g) (+)), is determined as 107 096(-2) (+8) cm(-1). By using the determined IE and E(tipp) for Cl(2) and also the reported IE and electronic affinity for chlorine atom, the bond dissociation energies of Cl(2)(X (1)Sigma(g) (+)) and Cl(2) (+)(X (2)Pi(g)) have been determined as 19 990(-2) (+8) and 31 935.1(-2) (+8), respectively.     

Quantum-chemical interpretation of the mass spectra of pyrrole,furan, and thiophene

A simple quantum-chemical interpretation of the mass-spectrometric fragmentation of organic molecules in which the probability of the cleavage of the bonds under the influence of electron impact is related to their self-consistent -electron orders and the possible rearrangement processes are described on the basis of the long-range bond orders is proposed. This approach was tested in the case of pyrrole, furan, and thiophene.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 620–624, May, 1987.     

Resonantly enhanced multiphoton ionization and zero kinetic energy photoelectron spectroscopy of benzo[g,h,i]perylene

We report zero kinetic energy (ZEKE) photoelectron spectroscopy of benzo[g,h,i]perylene (BghiP) via resonantly enhanced multiphoton ionization (REMPI). Our analysis concentrates on the vibrational modes of both the first electronically excited state and the ground cationic state. Extensive vibronic coupling due to a nearby electronically excited state manifests through strong Franck-Condon (FC) forbidden bands, which are stronger than even the FC allowed bands in the REMPI spectrum. Theoretical calculations using Gaussian are problematic in identifying the electronic configurations of the excited electronic states and predicting the transition energies. However, by setting the keyword for the second excited electronic state, both density functional theory and configuration interaction methods can reproduce the observed spectrum qualitatively. The general agreement significantly helps with the vibrational assignment. The ZEKE spectra demonstrate propensity in preserving the vibrational excitation of the intermediate electronic state. In addition, almost all ZEKE spectra exhibit a similar vibrational distribution, and the distribution can be reproduced by an FC calculation from the vibronic origin of the first excited electronic state to the cationic state using Gaussian 09. These results suggest a remarkable structural stability of BghiP in accommodating the additional charge. All observed vibrational bands of the cation are IR active, establishing the role of ZEKE spectroscopy in mapping out far-infrared bands for astrophysical applications.     

Triplet π→π* transitions in thiophene,furan and pyrrole by low-energy electron-impact spectroscopy

Low-energy (0–3 V above threshold) electron-impact excitation spectra and transmission spectra are presented for thiophene, furan and pyrrole. For each molecule two low-lying triplet states are reported. An additional new transition has been observed in thiophene at 6.5 eV. Assignments are suggested for some of the observed direct and resonance excitation processes.     

Resonantly enhanced multiphoton ionization and zero kinetic energy photoelectron spectroscopy of chrysene: a comparison with tetracene

We report the electronic and vibrational spectroscopy of chrysene using resonantly enhanced multiphoton ionization (REMPI) and zero kinetic energy (ZEKE) photoelectron spectroscopy. As an isomer of tetracene, chrysene contains a kink in the middle of the four fused hexagonal rings, which complicates not just the symmetry but, more importantly, the molecular orbitals and hence vibronic transitions. Incidentally, the two nearby electronically excited states of chrysene have the same symmetry, and vibronic coupling introduces no out-of-plane vibrational modes. As a result, the REMPI spectrum of chrysene contains essentially only in-plane ring deformation modes, similar to that of tetracene. However, density functional calculations using gaussian even after the inclusion of vibronic coupling can only duplicate the observed REMPI spectrum in a qualitative sense, and the agreement is considerably worse than our recent work on a few pericondensed polycyclic aromatic hydrocarbons and on tetracene. The ZEKE spectrum of chrysene via the origin band of the intermediate electronic state S(1), however, can be qualitatively reproduced by a straightforward Franck-Condon calculation. The ZEKE spectra from vibrationally excited states of the S(1), on the other hand, demonstrate some degree of mode selectivity: the overall intensity of the ZEKE spectrum can vary by an order of magnitude depending on the vibrational mode of the intermediate state. A scaling factor in the theoretical vibrational frequency for the cation is also needed to compare with the experimental result, unlike tetracene and pentacene.     

Theoretical fine spectroscopy with symmetry-adapted-cluster configuration-interaction method: outer- and inner-valence ionization spectra of furan, pyrrole, and thiophene

Theoretical fine spectroscopy has been performed for the valence ionization spectra of furan, pyrrole, and thiophene with the symmetry-adapted-cluster configuration-interaction general-R method. The present method described that the pi(1) state interacts with the pi(3) (-2)pi*, pi(2) (-2)pi*, and pi(2) (-1)pi(3) (-1)pi* shake-up states providing the split peaks and the outer-valence satellites, both of which are in agreement with the experiments. The intensity distributions were analyzed in detail for the inner-valence region. In particular, for furan, theoretical intensities were successfully compared with the intensity measured by the electron momentum spectroscopy. The interactions of the 3b(2) and 5a(1) states with the shake-up states were remarkable for furan and pyrrole, while the 4b(2) state of thiophene had relatively large intensity.     

Observation of rotamers of m-aminobenzoic acid: zero kinetic energy photoelectron and hole-burning resonantly enhanced multiphoton ionization spectroscopy

We report studies of supersonically cooled m-aminobenzoic acid using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. Two conformers have been identified and characterized using the hole-burning method in the REMPI experiment. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S(1)) of the neutral species and those of the ground state cation (D(0)) have been assigned, and the adiabatic ionization potentials have been determined for both conformers. The REMPI spectra are dominated by in-plane motions of the substituents and ring deformation modes. A propensity of Deltav=0, where Deltav is the change in vibrational quantum number from the S(1) to the D(0) state, is observed in the ZEKE spectra. The origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. Comparisons of these results with those of p-aminobenzoic acid will be analyzed.     

Zero kinetic energy photoelectron spectroscopy of p-amino benzoic acid

We report studies of supersonically cooled p-amino benzoic acid using one-color resonantly enhanced multiphoton ionization and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state S(1) of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 64 540+/-5 cm(-1). A common pattern involving the activation of five vibrational modes of the cation is recognizable among all the ZEKE spectra. A propensity of Deltav=0, where v is the vibrational quantum number of the intermediate vibronic state from S(1), is confirmed, and the origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. A puzzling observation is the doublet splitting of 37 cm(-1) in the ZEKE spectrum obtained via the inversion mode of the S(1) state. This splitting cannot be explained from our density functional calculations.     

The C-H bond dissociation energy of furan: photoelectron spectroscopy of the furanide anion

Using photoelectron spectroscopy, we interrogate the cyclic furanide anion (C(4)H(3)O(-)) to determine the electron affinity and vibrational structure of the neutral furanyl radical and the term energy of its first excited electronic state. We present the 364-nm photoelectron spectrum of the furanide anion and measure the electron affinity of the X?(2)A(') ground state of the α-furanyl radical to be 1.853(4) eV. A Franck-Condon analysis of the well-resolved spectrum allows determination of the harmonic frequencies of three of the most active vibrational modes upon X?(2)A(') ← X?(1)A(') photodetachment: 855(25), 1064(25), and 1307(40) cm(-1). These modes are ring deformation vibrations, consistent with the intuitive picture of furanide anion photodetachment, where the excess electron is strongly localized on the α-carbon atom. In addition, the A?(2)A(') excited state of the α-furanyl radical is observed 0.68(7) eV higher in energy than the X?(2)A(') ground state. Through a thermochemical cycle involving the known gas-phase acidity of furan, the electron affinity of the furanyl radical yields the first experimental determination of the C-H(α) bond dissociation energy of furan (DH(298)(C(4)H(3)O-H(α))): 119.8(2) kcal mol(-1).     

Complete vapor phase assignment for the fundamental vibrations of furan,pyrrole and thiophene

Vibrational spectroscopic studies, including IR vapor, Raman vapor and Raman liquid spectra, have been made to obtain the complete set of fundamental vibrational frequencies in the vapor and liquid states for furan, pyrrole and thiophene. For furan, vapor values have been determined for the two previously ambiguous fundamentals, ν₁₁ and ν₁₈. Also determined is the vapor frequncy of two fundamentals of furan for which only the liquid value had been known. The fundamental vibrational frequencies of pyrrole have been completely determined in the gas and liquid states. The thiophene results confirm the assignment of Rico et al. [Spectrochim. Acta 21, 689 (1965)], although for several of the fundamental modes the vapor frequency is now measured. The Raman vapor spectra are conclusive concerning the refinements in vibrational assignment for furan and pyrrole, where virtually every binary combination band involving the out-of-plane fundamentals that yield an A₁ transition is observed. The Raman vapor results establish two significant Fermi resonances affecting fundamental vibration levels in pyrrole. Also, ¹³C and ³⁴S isotopomers are identified in the Q-branches of the Raman vapor spectra at natural abundance. A comparison of the spectroscopic and calorimetric ideal-gas thermodynamic properties is made. The differences are negligible in the region where the calorimetric data are most reliable.     

Electronics structures of Rb,Cs and some of their metallic oxides studied by photoelectron spectroscopy

Photoemission measurements with He and Ne resonance lines and Al Kα radiation are reported on bulk samples of the alkali metals Rb, Cs, their suboxides Cs₇O, Cs₁₁O₃ and (Cs₁₁O₃)Rb₇. For comparison, the Hel spectrum of the “normal” oxide Cs₂O is added. The occurrence of ionic clusters in a metallic matrix is typical for the suboxides. Binding energies, Auger transitions, and electron concentrations are discussed. The spectra of the suboxides show a narrow non-bonding oxygen 2p band at 2.7 eV. Different binding energies are found for Cs atoms in the clusters and for the atoms in the metallic regions of (Cs₁₁O₃)Cs₁₀. The compound Cs₁₁O₃ consists of ionic [Cs₁₁O₃]^5? clusters, which are bound by 5 free electrons in accordance with the chemical bond model.