Vibrational structure of vinyl chloride cation studied by using one-photon zero-kinetic energy photoelectron spectroscopy

The vibrational structure of vinyl chloride cation, CH(2)CHCl+ (X(2)A' '), has been studied by vacuum ultraviolet (VUV) zero-kinetic energy (ZEKE) photoelectron spectroscopy. Among nine symmetric vibrational modes, the fundamental frequencies of six modes have been determined. The first overtone of the out-of-plane CH(2) twist vibrational mode has been also measured. In addition to these, the combination and overtone bands of the above vibrational modes about 4500 cm(-1) above the ground state have been observed in the ZEKE spectrum. The vibrational band intensities of the ZEKE spectrum can be described approximately by the Franck-Condon factors with harmonic approximation. The ZEKE spectrum has been assigned based on the harmonic frequencies and Franck-Condon factors from theoretical calculations. The ionization energy (IE) of CH(2)CHCl is determined as 80705.5 +/- 2.5 (cm(-1)) or 10.0062 +/- 0.0003 (eV).     

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.     

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.     

Zero kinetic energy photoelectron study of SO2+(X2A1) using coherent extreme ultraviolet radiation

Using our newly built extreme ultraviolet (XUV) photoelectron and photoion spectrometer, we have obtained the pulsed field ionization zero kinetic energy (ZEKE) photoelectron spectra of SO2+(X2A1)<--SO2(X1A1) by coherent XUV radiation in the energy range of 12.29-12.82 eV. The adiabatic ionization potential (IP) of SO2 is 12.3458+/-0.0002 (eV), which was determined by comparing the partially resolved rotational branch contour with the simulated one. Besides the bending vibrational mode (upsilon2) which was found to be exclusive in the photoelectron spectra (PE) reported previously, we also observed the other two modes: the symmetric stretching (upsilon1) and the antisymmetric stretching (upsilon3) vibrations. The fundamental of the symmetric stretching (upsilon(1)) is 1057 cm(-1) and the overtone of the antisymmetric stretching (2upsilon(3)) is 2494 cm(-1). The new vibrational progressions (upsilon(1)00)+, (1upsilon(2)0)+, (2upsilon(2)0)+, and (0upsilon(2)2)+ have also been observed, and these new observations suggested that the irregular structure of (0upsilon(2)0)+ assigned to the previous PE spectra should be reconsidered. The comparison of the intensities of these vibrational bands with the calculated Franck-Condon factors with harmonic approximation was also made.     

Zero kinetic energy photoelectron spectroscopy of jet cooled benzo[a]pyrene from resonantly enhanced multiphoton ionization

We report zero kinetic energy (ZEKE) photoelectron spectroscopy of benzo[a]pyrene (BaP) via resonantly enhanced multiphoton ionization (REMPI). Our analysis concentrates on the vibrational modes of the first excited state (S(1)) and those of the ground cationic state (D(0)). Similar to pyrene, another peri-condensed polycyclic aromatic hydrocarbon we have investigated, the first two electronically excited states of BaP exhibit extensive configuration interactions. However, the two electronic states are of the same symmetry, hence vibronic coupling does not introduce any out-of-plane modes in the REMPI spectrum, and Franck-Condon analysis is qualitatively satisfactory. The ZEKE spectra from the in-plane modes observed in the REMPI spectrum demonstrate strong propensity in preserving the vibrational excitation of the intermediate state. Although several additional bands in combination with the vibrational mode of the intermediate state are identifiable, they are much lower in intensity. This observation implies that the molecular structure of BaP has a tremendous capability to accommodate changes in charge density. All observed bands of the cation are IR active, establishing the role of ZEKE spectroscopy in mapping out far infrared bands for astrophysical applications.     

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.     

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.     

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.     

Theoretical investigation of the (hyper)polarizabilities of pyrrole homologues C4H4XH (X = N, P, As, Sb, Bi). A coupled-cluster and density functional theory study

Geometries, inversion barriers, static and dynamic electronic and vibrational dipole polarizability (alpha), and first (beta) and second (gamma) hyperpolarizability of the pyrrole homologues C(4)H(4)XH (X = N, P, As, Sb, Bi) have been calculated by Hartree-Fock, M?ller-Plesset second-order perturbation theory, coupled-cluster theory accounting for singles, doubles, and noniterative triple excitations methods, as well as density functional theory using B3LYP and PBE1PBE functionals and Sadlej's Pol and 6-311G basis sets. Relativistic effects on the heavier homologues stibole and bismole have been taken into account within effective core potential approximation. The results show that the electronic (hyper)polarizabilities monotonically increase with the atomic number of the heteroatom, consistent with the decrease in the molecular hardness. Ring planarization reduces the carbon-carbon bond length alternation of the cis-butadienic unit, enhancing the electronic polarizability values (alpha(e)) by 4-12% and the (hyper)polarizability values (and gamma(e)) by 30-90%. Pure vibrational and zero-point vibrational average contributions to the (hyper)polarizabilities have been determined within the clamped nucleus approach. In the static limit, the pure vibrational hyperpolarizabilities have a major contribution. Anharmonic corrections dominate the pure vibrational hyperpolarizabilities of pyrrole, while they are less important for the heavier homologues. Static and dynamic electronic response properties of the pyrrole homologues are comparable to or larger than the corresponding properties of the furan and cyclopentadiene homologue series.     

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.     

Resonantly enhanced two photon ionization and zero kinetic energy spectroscopy of jet-cooled 4-aminopyridine

We report studies of supersonically cooled 4-aminopyridine (4-AP) using two-color resonantly enhanced multiphoton ionization (REMPI) 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 (S1) of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 62291+/-6 cm(-1). The REMPI spectrum of the S1 state is dominated by ring deformation modes and the inversion mode of the amino group, while the ZEKE spectra demonstrate a strong propensity of Deltav=0, where v is the vibrational quantum number of the intermediate vibronic state from S1. In addition, the ZEKE spectra obtained via different vibrational levels of the S1 state contain four common features, corresponding to the activation of four different vibrational modes of the cation. These observations are explained in terms of the structural changes from the ground state to S1 and further to the cation. The vibrational mode distributions in both the REMPI and the ZEKE spectra, the excitation energy of the S1 state, and the ionization potential of 4-AP, are remarkably similar to those of aniline, suggesting that the electronic activity is centered on the ring.     

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.     

Fluorescence spectrum of the benzyl radical in methylcyclohexane at 4.2 K

The fluorescence spectrum of the benzyl radical (BR) is characterized by participation of many totally symmetric vibrational modes with less intensity activeness. To make this point more clear, the fluorescence spectra of the benzyl radical and its deuterated compounds in methylcyclohexane (MCH) have been observed at 4.2 K and detailed vibrational analysis has been carried out. Two non-totally symmetric modes such as nu6b and nu8b play a role of Herzberg-Teller type vibronic coupling together with one totally symmetric mode nu6a. There are only a few progression bands with very weak intensity. These are nu12, nu1 and nu(phi-CH2) vibrational modes.     

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.     

Vibrational spectra of furan,pyrrole, and thiophene from a density functional theory anharmonic force field

Quartic force fields for furan, pyrrole, and thiophene have been generated using density functional theory (DFT). These were used to evaluate vibrational levels by second-order perturbation theory (PT) and also by the variational method. The results for the fundamental frequencies are in very good agreement with observation. The accuracy of overtones and combination transitions is also good, for those cases where assignments can be made. Second-order PT combines speed and quality whereas the variational approach is inherently more reliable, but converges rather slowly, requiring significant computational effort.     

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.     

Bis[bis-(4-alkoxyphenyl)amino] derivatives of dithienylethene, bithiophene, dithienothiophene and dithienopyrrole: palladium-catalysed synthesis and highly delocalised radical cations

Five diamines with thiophene-based bridges--(E)-1,2-bis{5-[bis(4-butoxyphenyl)amino]-2-thienyl}ethylene (1), 5,5'-bis[bis(4-methoxyphenyl)amino]-2,2'-bithiophene (2), 2,6-bis[bis(4-butoxyphenyl)amino]dithieno[3,2-b:2',3'-d]thiophene (3), N-(4-tert-butylphenyl)-2,6-bis[bis(4-methoxyphenyl)amino]dithieno[3,2-b:2',3'-d]pyrrole (4 a) and N-tert-butyl-2,6-bis[bis(4-methoxyphenyl)amino]dithieno[3,2-b:2',3'-d]pyrrole (4 b)--have been synthesised. The syntheses make use of the palladium(0)-catalysed coupling of brominated thiophene species with diarylamines, in some cases accelerated by microwave irradiation. The molecules all undergo facile oxidation, 4 b being the most readily oxidised at about -0.4 V versus ferrocenium/ferrocene, and solutions of the corresponding radical cations were generated by addition of tris(4-bromophenyl)aminium hexachloroantimonate to the neutral species. The near-IR spectra of the radical cations show absorptions characteristic of symmetrical delocalised species (that is, class III mixed-valence species); analysis of these absorptions in the framework of Hush theory indicates strong coupling between the two amine redox centres, stronger than that observed in species with phenylene-based bridging groups of comparable length. The strong coupling can be attributed to high-lying orbitals of the thiophene-based bridging units. ESR spectroscopy indicates that the coupling constant to the amino nitrogen atoms is somewhat reduced relative to that in a stilbene-bridged analogue. The neutral species and the corresponding radical cations have been studied with the aid of density functional theory and time-dependent density functional theory. The DFT-calculated ESR parameters are in good agreement with experiment, while calculated spin densities suggest increased bridge character to the oxidation in these species relative to that in comparable species with phenylene-based bridges.     

Etudes photochimiques—XIII : Reactions de photoaddition du benzo[b]thiophene avec des amines et avec le pyrrole

The photochemical behavior of benzo[b]thiophene is different from that of thiophene. The former does not undergo photoisomerization and is not converted into an indole when irradiated in the presence of a primary amine; its photochemical behavior resembles that of naphthalene. Thus photoexcited benzo[b]thiophene gives adducts with primary amines (1(H)), secondary amines (2), and pyrrole (3(H)): an exciplex is an intermediate. The existence of the exciplex is supported by the formation of aldehyde 7 when benzo[b]thiophene is irradiated in aqueous propylamine, the finding that photoexcited benzo[b]thiophene does not react with an alcohol and that photoexcited indole benzo[b]furan, 2-methylbenzo[b]thiophene, and 3-methylbenzo[b]thiophene do not react with propylamine or with piperidine. The results are interpreted in terms of spin densities calculated for the anion radicals of the compounds under study.     

The energetics of resonant dissociative electron attachment to molecules of five-membered heterocyclic compounds

The thermochemistry of resonant dissociative electron attachment processes for furan, thiophene, selenophene, and pyrrole molecules has been studied. The structures of the dissociation products originating from negative molecular ions at energies ranging from 2 to 6 eV have been established using the measured appearance energies of fragment ions and the known thermodynamic functions of radical and molecular dissociation products. Heats of formation and electron affinities for some radicals and molecules have been assessed by calculations and estimated experimentally. It has been concluded that the majority of the fragment ions are formedvia rearrangement processes in molecular or fragment ions.

     

A combined zero electronic kinetic energy spectroscopy and ion-pair dissociation imaging study of the F2 + (X 2Pi g) structure

Rotationally resolved pulsed field ionization and zero electronic kinetic energy photoelectron spectra for the transition F(2) (+)(X (2)Pi(g))<--F(2)(X (1)Sigma(g) (+)) have been recorded using the extreme ultraviolet coherence radiation. The vibrational energy spacings, rotational constants, and spin orbit coupling constants for the first three vibrational states of F(2) (+)(X (2)Pi(g)) have been determined accurately. The first adiabatic ionization potential (IP) of F(2) is determined as IP(F(2))=126 585.7+/-0.5 cm(-1). To determine the threshold E(tipp) for ion-pair production of F(2), the images of F(-)((1)S(0)) in the velocity mapping conditions have also been recorded at the photon energy of 126 751 cm(-1). Taking the Stark effect into account, the E(tipp) is determined as E(tipp)(F(2))=126 045+/-8 cm(-1) (15.628+/-0.001 eV). By combing the IP(F(2)) and the E(tipp)(F(2)) determined in this work and together with the reported ionization potential and electronic affinity of the F atom, the bond dissociation energies of F(2) and F(2) (+) are determined as D(0)(F(2))=1.606+/-0.001 eV and D(0)(F(2) (+))=3.334+/-0.001 eV, respectively.