Larisa G. Shpinkova

A study of the spectroscopic and thermodynamic properties of furan by means of photoabsorption, photoelectron and photoion spectroscopy

Abstract Three experimental techniques (photoabsorption, photoelectron, and photoion spectroscopy) have been used to study the spectroscopic and thermodynamic properties of furan. The absolute photoabsorption cross-sections of furan-h4 and furan-d4 have been measured using a double ion chamber and a new Rydberg series converging onto the G 2 A 1 ionisation threshold has been observed. HeI-excited photoelectron spectra of the X 2 A 2 , the A 2 B 1 and the G 2 A 1 states of furan-d4 have been recorded. Vibrational structure has been observed in all three bands and has allowed the energies of the ν3, ν4, ν6 and ν8 vibrational modes to be determined. Time-of-flight mass spectra have been recorded using monochromatic synchrotron radiation, and appearance energies have been measured for 19 fragment ions and the doubly charged parent ion. The fragmentation processes leading to the production of several high-intensity fragment ions have been modelled using ab initio and semi-empirical methods.

A study of the unimolecular decomposition of internal-energy-selected furan molecular ions by threshold-photoelectron-photoion coincidence spectroscopy

Abstract The unimolecular decomposition of internal-energy-selected furan molecular ions has been studied by means of threshold-photoelectron–photoion coincidence spectroscopy. Monochromatic synchrotron radiation was used as the ionisation source, and the molecular ion internal energy was established through the detection of a threshold electron. A pulsed electric field was applied to extract the ions from the interaction region and direct them towards a time-of-flight mass spectrometer. Breakdown curves were measured for photon energies up to 30 eV, and these have allowed appearance energies for a wide range of fragment ions to be determined. In the threshold region the breakdown curves have been measured for various ion residence times by introducing electronic delays between the detection of the threshold electron and the application of the ion extraction field. The breakdown curves have been modelled using the RRKM (Rice, Ramsperger, Kassel and Marcus)/QET (quasi-equilibrium theory) approach, and this has allowed activation energies and transition state geometries to be deduced. The threshold photoelectron spectra of furan-h 4 and furan-d 4 have been measured from the ionisation threshold to 28 eV, and vibrational structure has been observed and assigned in the bands due to the X 2 A 2 , the A 2 B 1 and the G 2 A 1 states. Vibrational progressions discernible between 16.2 and 17.3 eV have been attributed to autoionisation from a p-type Rydberg series converging onto the G 2 A 1 state ionisation threshold.

Time-of-flight mass spectrometry study of the fragmentation of valence shell ionised nitrobenzene

Abstract A time-of-flight mass spectrometry study has been carried out to investigate the fragmentation processes occurring in nitrobenzene as a result of valence shell photoionisation. Synchrotron radiation has been used to record spectra in the photon energy range 11–31 eV, and appearance energies have been determined for 18 fragment ions. These have enabled, previously unknown, heats of formation to be estimated for C 5 H 2 + and C 6 H 3 + . The fragment ion appearance energies have been compared to similar data for benzene and toluene in order to highlight the influence of the substituent on the fragmentation patterns. The time-of-flight spectra show that the peak associated with the NO + fragment changes shape as a function of excitation energy, and at high photon energy the peak consists of two components, one of which is narrow and the other broad. The latter component is due to fragments possessing substantial initial kinetic energy. In contrast, the peak associated with the NO 2 + fragment always appears broad. This behaviour is discussed in relation to the initial formation of a doubly charged ion and a subsequent coulomb repulsion. In a separate experiment the absolute photoabsorption cross section of nitrobenzene has been measured between the ionisation threshold and 35 eV using a double ion chamber. Some of the broad features have been attributed, tentatively, to valence shell excitations into π∗ orbitals.

The design and performance of a threshold-photoelectron-photoion coincidence spectrometer for the study of unimolecular decomposition in polyatomic ions.

The design, construction and performance of a threshold-photoelectron–photoion coincidence (TPEPICO) spectrometer for the study of unimolecular decomposition in polyatomic ions is described. The spectrometer incorporates a hemispherical electrostatic energy analyser and a time-of-flight (TOF) mass spectrometer. The entrance lens to the hemispherical analyser has been designed to have a high collection efficiency for low energy electrons but to discriminate strongly against energetic electrons. This arrangement has resulted in a resolution of about 3.5 meV being achieved for the threshold electron peak recorded at the krypton 2P3/2 ionisation limit. A pulsed electric field is used to extract the ions from the interaction region and propel them towards the TOF analyser. Computer modelling has been used to trace the electron and ion trajectories through their respective analysers. These simulations have enabled the effective interaction volume to be defined, and this has allowed the transmission efficiency of energetic fragment ions, formed through a process which also yielded a threshold electron, to be quantified. The ion TOF peak shape has been examined as a function of initial kinetic energy and as a function of ion residence time. The contribution of energetic fragments, having specific initial spatial and directional properties, to the TOF peak shape has been determined by tracing the paths of individual ions. The actual performance of the spectrometer is illustrated by a TPEPICO spectrum of the krypton isotopes. Experimental breakdown curves for furan are presented as an example of the use of the apparatus to study unimolecular decomposition in polyatomic ions. By introducing a delay between the detection of the threshold electron and the application of the ion extraction field, breakdown curves can be recorded as a function of ion residence time in the source region. The procedure for analysing the data is described, and the experimental factors that need to be taken into account to obtain a meaningful comparison with theoretical predictions are discussed.

Does Tetrahydrofuran Ring Open upon Ionization and Dissociation? A TPES and TPEPICO Investigation

The threshold photoelectron spectrum (TPES) of tetrahydrofuran (THF) is compared to that of the unsaturated furan molecule. In general, there is a similarity in the orbital ionization profile for the two species, though unlike furan, THF exhibits (modest) vibrational detail only in the (9b)(-1) X (2)B band. An adiabatic ionization energy of 9.445 +/- 0.010 eV has been derived from the onset of the TPES spectrum. Threshold photoelectron photoion coincidence spectroscopy was used to explore the loss of a hydrogen atom from ionized THF over the photon energy range of 9.9-10.4 eV. RRKM fitting of the resulting breakdown curves yields an E(0) of 0.85 +/- 0.03 eV (82 +/- 3 kJ mol(-1)) (AE = 10.30 +/- 0.04 eV). If the G3 IE of 9.48 eV is used to convert the experimental data from photon energy to THF ion internal energy, E(0) = 0.81 +/- 0.01 eV (78 +/- 1 kJ mol(-1)). The latter value is closer to the G3 E(0) of 72 kJ mol(-1) for the formation of the cyclic ion 1. A variety of ring-opening reactions were also probed at the B3-LYP/6-31+G(d) and G3 levels of theory. The distonic isomer (*)CH(2)CH(2)CH(2)OCH(2)(+) lies 70 kJ mol(-1) higher than ionized THF, which places it within 1 kJ mol(-1) of the threshold for the dissociation to 1. All of the probed H-loss products from the distonic isomer (which includes singlet and triplet species) lie significantly higher in energy than ion 1, eliminating the possibility that ionized THF dissociates to m/z 71 via a ring-opening reaction in the present experiment. The derived Delta(double dagger)S value for the dissociation, 8 +/- 5 J K(-1) mol(-1), is also consistent with the formation of 1. The experimentally derived E(0) values can be used to derive the Delta(f)H(o)(0) for ion 1. Together with the Delta(f)H(o)(0) values for the THF ion (752.0 +/- 2 kJ mol(-1), derived from the neutral Delta(f)H(o)(0) of -154.9 +/- 0.7 kJ mol(-1) and experimental IE of 9.445 +/- 0.010 eV) and H atom (218.5 kJ mol(-1)) our E(0) of 82 +/- 3 kJ mol(-1) yields a Delta(f)H(o)(0) for ion 1 of 620 +/- 4 kJ mol(-1) (Delta(f)H(o)(298) = 594 +/- 4 kJ mol(-1)), in good agreement with the G3 Delta(f)H(o)(0) of 621 kJ mol(-1). Appearance energies for all fragment ions up to photon energies of 34 eV are also reported and discussed in comparison with the available literature.

The influence of Jahn—Teller interaction, vibronic coupling and spin—orbit splitting on the Hel excited photoelectron spectra of the boron trihalides

Hel excited photoelectron spectra of the boron trihalides have been recorded with improved resolution, and several new vibrational progressions have been observed in the spectra of BCl3, BBr3 and BI3. The spectra illustrate, as would be expected, that spin—orbit coupling becomes important in the heavier trihalides, and affects the photoelectron band structure strongly. Progressions involving excitation of the symmetric stretching vibrational mode are observed in all four molecules. Some of the new structure revealed in the photoelectron spectra of BCl3, BBr3 and BI3 is assigned, and the molecular orbital configurations in BBr3 and BI3 are discussed, taking into account existing theoretical predictions and the present experimental results.

Long Bonds and Short Barriers: Ionization and Isomerization of Alkyl Nitriles

Ab initio molecular orbital calculations demonstrate that ionizing alkyl nitriles produces a dramatic geometry change involving lengthening of a C-CH(2)CN bond. The experimental determination of the adiabatic ionization energy of these species is thus very difficult. In addition, there are generally low barriers for 1,2-H shift reactions in the molecular ions leading to RCHCHN(+*) and RCHCNH(+*) isomers, which makes generating pure ionized alkyl nitrile in a mass spectrometer a challenge. Threshold photoelectron spectroscopy and threshold photoelecton photoion coincidence spectroscopy were employed to study the ionization and dissociation of two alkyl nitriles, in particular, pentanenitrile and 2,2-dimethylpropanenitrile. Threshold ionization is shown to result not in the respective molecular ions, but rather in isomeric forms, resulting in dissociation thresholds that lie below the calculated adiabatic ionization energies of the two molecules. Appearance energies for all observed fragment ions are reported and compared to available literature values. Charge separation in the dissociation of doubly ionized 2,2-dimethylpropanenitrile is observed as fragment-ion time-of-flight peak broadening at high photon energies.

An experimental and theoretical investigation of the valence shell photoelectron spectrum of cyanogen chloride

The valence shell photoelectron spectrum of cyanogen chloride has been studied using HeI and synchrotron radiation. In the outer valence region the molecular orbital model of ionization holds, and the main bands can be associated with single-hole states. However, in the inner valence region electron correlation effects become important, and these result in complex satellite structure being observed. Vertical ionization energies and spectral intensities have been computed using the Greens function approach, and the results have facilitated an interpretation of the experimental spectra. Photoelectron angular distributions and branching ratios have been measured and have been used to assess the bonding characteristics of the outer valence molecular orbitals. The experimental data for the 8σ orbital display an energy dependence which suggests that photoionization from this orbital may be influenced by the chlorine 3p Cooper minimum. The extent to which the 8σ orbital can be considered as a chlorine atom lone-pair is discussed. Vibrational structure has been observed in the [Xtilde] 2Π, Ã 2Σ+ and [Btilde] 2Π photoelectron bands recorded with HeI radiation, and has been assigned to progressions involving the ν+ 1 and ν+ 3 modes.

Methyl t-Butyl Ether and Methyl Trimethylsilyl Ether Ions Dissociate near Their Ionization Thresholds: A TPES, TPEPICO, RRKM, and G3 Investigation

The threshold photoelectron spectra and threshold photoelectron photoion coincidence (TPEPICO) mass spectra of methyl t-butyl ether, (CH(3))(3)COCH(3) (MTBE), and methyl trimethylsilyl ether, (CH(3))(3)SiOCH(3) (MTMSE), have been measured using synchrotron radiation. The effect of silicon substitution on the unimolecular dissociation processes and the threshold photoelectron spectrum has been investigated. Both molecular ions dissociate at low internal energies. For ionized MTBE, the parent ion is no longer observed at an internal energy of only 0.2 eV. For this reason, it was not possible to fit the TPEPICO data to extract reliable thermochemical information. G3 level calculations place the molecular ion 5 kJ mol(-1) above the lowest-energy dissociation products, (CH(3))(2)COCH(3)(+) + (*)CH(3), suggesting the participation of an isomer, potentially the distonic ion (*)CH(2)(CH(3))(2)CO(+)(H)CH(3), in the dissociation. However, the calculations are not considered accurate enough to reliably determine the role this isomer plays, if any. RRKM modeling of the threshold region of the TPEPICO breakdown curves for ionized MTMSE leads to an E(0) for methyl loss of 63 +/- 2 kJ mol(-1), in good agreement with the G3 value of 66 kJ mol(-1). The resulting Delta(f)H(0) for (CH(3))(2)SiOCH(3)(+) of 384 +/- 10 kJ mol(-1) (Delta(f)H(298) = 361 +/- 10 kJ mol(-1)) is 28 kJ mol(-1) lower than the G3 value of 412 kJ mol(-1) due to the G3 Delta(f)H(0) for neutral MTMSE being 16 kJ mol(-1) higher than the previously reported value and the fact that the experimental IE(a) is 6 kJ mol(-1) lower than the G3 estimate. Appearance energy values for higher-energy fragmentation channels up to 36 (for MTBE) and 32 eV (for MTMSE) are reported and compared to literature values. An investigation of fragment ion peak broadening at high internal energy indicated that the two doubly charged molecular ions are not stable on the microsecond time scale. Each was found to dissociate into two singly charged ions along one or more neutral species.

A fluorescence polarisation study of the CS2+ and the transitions

Abstract The fluorescence yields and polarisations of the CS 2 + A 2 Π u → X 2 Π g and the B 2 Σ u + → X 2 Π g transitions have been measured between their thresholds and 30 eV using synchrotron radiation. In the outer valence region the fluorescence yields display prominent structure due to autoionising Rydberg states. New features, associated with multi-electron transitions, have been observed between 16 and 30 eV, and most have been assigned through reference to the inner valence shell photoelectron spectrum. Some of the new features exhibit vibrational progressions involving excitation of the ν 1 ′ mode. The influence of shape resonances on the photoionisation partial cross-sections has been considered. The polarisation measurements for the A 2 Π u → X 2 Π g and the B 2 Σ u + → X 2 Π g transitions have been compared with predictions derived from theoretical photoionisation partial cross-sections. The predictions for the B 2 Σ u + → X 2 Π g fluorescence polarisation display a rapid increase in the threshold region, in agreement with the experimental results. Resonant excitation and decay processes produce significant variations in the fluorescence polarisation, and these changes can be used to deduce the symmetry of the excited state. This information has allowed the assignments of some of the Rydberg series converging onto the B 2 Σ u + or the C 2 Σ g + ionisation thresholds to be clarified.