Formation and characterization of gaseous adducts of carbon dioxide to magnesium, (CO2)MgX- (X=OH, Cl, Br).

A good fix: the structure and chemical reactivity of a reduced form of CO(2) bonded to magnesium, XMg(η(2)-O(2)C)(-), is reported. Upon reaction with water it loses CO, while it adds CH(3) upon reaction with alkyl halides, thereby signifying nucleophilicity of the carbon atom in XMg(η(2)-O(2)C)(-) in S(N)2 reactions.

Comprehensive vacuum ultraviolet photoionization study of the CF3(●) trifluoromethyl radical using synchrotron radiation.

The trifluoromethyl radical, CF(3)(●), is studied for the first time by means of threshold photoelectron spectroscopy (TPES). The radical is produced in the gas phase using the flash-pyrolysis technique from hexafluoroethane as a precursor. CF(3)(+) total ion yield and mass-selected TPES of the radical are recorded using a spectrometer based upon velocity map imaging and Wiley-McLaren time-of-flight coupled to the synchrotron radiation. The high resolution of the instrument and of the photons allows the observation of rich vibrational progressions in the TPES of CF(3)(●). By using Franck-Condon factors computed by Bowman and coworkers, we have been able to simulate the TPES. The initial vibrational temperature of the radical beam has been evaluated at 350 ± 70 K. The structures have been identified as transitions between (n(1),n(2)) and (n(1)(+),n(2)(+)) vibrational levels of CF(3) and CF(3)(+) with small excitation of the breathing mode, ν(1)(+) (,) and large excitation (n(2)(+) = 10-26) of the umbrella mode, ν(2)(+), in the cation. From the energy separation between the two resolved peaks of each band, a value of 994 ± 16 cm(-1) has been derived for the ν(1)(+) breathing frequency of CF(3)(+). For the high-lying n(2)(+) levels, the apparent ν(2)(+) umbrella spacing, 820 ± 14 cm(-1), is fairly constant. Taking into account the ν(2)(+) anharmonicity calculated by Bowman and coworkers, we have deduced ν(2)(+) = 809 ± 14 cm(-1), and semi-empirical estimations of the adiabatic ionization energy IE(ad.)(CF(3)(●)) are proposed in good agreement with most of previous works. A value of the vertical ionization potential, IE(vert.)(CF(3)(●)) = 11.02 eV, has been derived from the observation of a photoelectron spectrum recorded at a fixed photon energy of 12 eV.

Photoionization of propargyl and bromopropargyl radicals: a threshold photoelectron spectroscopic study.

In this Article, we present mass-selected threshold photoelectron spectra of propargyl as well as the 1- and 3-bromopropargyl radicals. The reactive intermediates were produced by flash pyrolysis of suitable precursors and ionized by VUV synchrotron radiation. The TPES of the propargyl radical was simulated using data from a recent high-level computational study. An ionization energy (IE) of 8.71 ± 0.02 eV was obtained, in excellent agreement with computations, but slightly above previous experimental IEs. The pyrolysis of 1,3-dibromopropyne delivers both 1- and 3-bromopropargyl radicals that can be distinguished by their different ionization energies (8.34 and 8.16 eV). To explain the vibrational structure, a Franck-Condon simulation was performed, based on DFT calculations, which can account for all major spectral features. Bromopropargyl photoionizes dissociatively beginning at around 10.1 eV. Cationic excited states of 1- and 3-bromopropargyl were tentatively identified. The dissociative photoionization of the precursor (1,3-dibromopropyne) was also examined, delivering an AE(0K) (C(3)H(2)Br(+)/C(3)H(2)Br(2)) of 10.6 eV.

Photoionization of epichlorohydrin enantiomers and clusters studied with circularly polarized vacuum ultraviolet radiation.

The photoionization of enantiomerically pure epichlorohydrin (C(3)H(5)OCl) has been studied using linearly and circularly polarized vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum was recorded and the first three bands assigned using molecular orbital calculations for the expected conformers, although uncertain experimental conformer populations and an anticipated breakdown in Koopmans' theorem leave some ambiguity. Measurements of the photoelectron circular dichroism (PECD) were obtained across a range of photon energies for each of these bands, using electron velocity map imaging to record the angular distributions, during which a record PECD chiral asymmetry factor of 32% was observed. A comparison with calculated PECD curves clarifies the assignment achieved using ionization energies alone and further suggests a likely relative population of the conformers. Threshold photoelectron-photoion coincidence methods were used to study the ionic fragmentation of epichlorohydrin. Fragment ion appearance energies show nonstatistical behavior with clear indications that the cationic epoxide ring is unstable and lower energy decay channels proceeding via ring breaking are generally open. Extensive neutral homochiral clusters of epichlorohydrin may be formed in supersonic molecular beam expansions seeded in Ar. Electron angular distribution measurements made in coincidence with dimer and trimer ions are used to effect an examination of the PECD associated with ionization of size-selected neutral cluster species, and these results differ clearly from PECD of the neutral monomer. The shifted ionization thresholds of the n-mers (n = 2, ..., 7) are shown to follow a simple linear relationship, but under intense beam expansion conditions the monomer deviates from this relationship, and the monomer electron spectra tail to below the expected monomer adiabatic ionization potential (IP). PECD measurements made in coincidence with monomer ions obtained under different beam expansion conditions were used to identify unambiguously a contribution from dissociative photoionization of larger clusters to the monomer parent mass ion yield above and below its adiabatic IP.

Threshold photoelectron spectroscopy of cyclopropenylidene, chlorocyclopropenylidene, and their deuterated isotopomeres.

Cyclopropenylidene (c-C(3)H(2)), chlorocyclopropenylidene (c-C(3)HCl), and their deuterated isotopomers were studied by the threshold photoelectron-photoion coincidence (TPEPICO) technique using VUV synchrotron radiation. The carbenes were generated via flash pyrolysis. In all species a change in geometry is visible upon ionization, with significant activity in the C═C, C-C-stretching mode and, in the case of c-C(3)H(2)/D(2), the C-H-bending mode. The electron is removed from an sp(2) like hybrid orbital centered on the carbene C atom. The mass selected threshold photoelectron (TPE) spectra were fitted by a Franck-Condon simulation, yielding the equilibrium geometry of the cation ground state ((1)A(1)). The adiabatic ionization energy IE(ad) of c-C(3)H(2) was determined to be 9.17 eV, in good agreement with calculations and literature values. Two vibrational wavenumbers of the cation were determined experimentally (ν(3)(+) = 1150 cm(-1) and ν(2)(+) = 1530 cm(-1)). Chlorocyclopropenylidene was also studied by TPE spectroscopy and has a similar IE(ad) of 9.17 eV. The spectrum also shows a vibrational progression that corresponds to the C═C- and C-C-stretching modes of the cation. The equilibrium geometry was also determined by a Franck-Condon fit. The IE(ad) of the deuterated isotopomers, c-C(3)D(2) and c-C(3)DCl, were also determined to be 9.17 eV. The spectra confirm the assignments for the nondeuterated species.

Threshold photoelectron spectroscopy of the methyl radical isotopomers, CH3, CH2D, CHD2 and CD3: synergy between VUV synchrotron radiation experiments and explicitly correlated coupled cluster calculations.

Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH(3), CH(2)D, CHD(2), and CD(3)) have been recorded in the 9.5-10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7-11 meV resolution. A threshold photoelectron-photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley-McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CH(x)D(y)NO(2), x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrational frequencies for the four radical isotopomers and the corresponding cations. Agreement with data from high-resolution IR spectroscopy is very good and a large number of predictions is made. In particular, the calculated wavenumbers for the out-of-plane bending vibrations, nu(2)(CH(3)(+)) = 1404 cm(-1), nu(4)(CH(2)D(+)) = 1308 cm(-1), nu(4)(CHD(2)(+)) = 1205 cm(-1), and nu(2)(CD(3)(+)) = 1090 cm(-1), should be accurate to ca. 2 cm(-1). Additionally, computed Franck-Condon factors are used to estimate the importance of autoionization relative to direct ionization. The chosen models globally account for the observed transitions, but in contrast to PES spectroscopy, evidence for rotational and vibrational autoionization is found. It is shown that state-selected methyl cations can be produced by TPEPICO spectroscopy for ion-molecule reaction studies, which are very important for the understanding of the planetary ionosphere chemistry.

Determination of the absolute photoionization cross sections of CH3 and I produced from a pyrolysis source, by combined synchrotron and vacuum ultraviolet laser studies.

A pyrolysis source coupled to a supersonic expansion has been used to produce the CH3 radical from two precursors, iodomethane CH3I and nitromethane CH3NO2. The relative ionization yield of CH3 has been recorded at the SOLEIL Synchrotron Radiation source in the range 9.0-11.6 eV, and its ionization threshold has been modeled by taking into account the vibrational and rotational temperature of the radical in the molecular beam. The relative photoionization yield has been normalized to an absolute cross section scale at a fixed wavelength (118.2 nm, sigma(i)(CH3) = 6.7(-1.8)(+2.4) Mb, 95% confidence interval) in an independent laboratory experiment using the same pyrolysis source, a vacuum ultraviolet (VUV) laser, and a carefully calibrated detection chain. The resulting absolute cross section curve is in good agreement with the recently published measurements by Taatjes et al., although with an improved signal-to-noise ratio. The absolute photoionization cross section of CH3I at 118.2 nm has also been measured to be sigma(i)(CH3I) = (48.2 +/- 7.9) Mb, in good agreement with previous electron impact measurements. Finally, the photoionization yield of the iodine atom in its ground state 2P(3/2) has been recorded using the synchrotron source and calibrated for the first time on an absolute cross section scale from our fixed 118.2 nm laser measurement, sigma(i)(I2P(3/2)) = 74(-23)(+33) Mb (95% confidence interval). The ionization curve of atomic iodine is in good agreement, although with slight variations, with the earlier relative ionization yield measured by Berkowitz et al. and is also compared to an earlier calculation of the iodine cross section by Robicheaux and Greene. It is demonstrated that, in the range of pyrolysis temperature used in this work, all the ionization cross sections are temperature-independent. Systematic care has been taken to include all uncertainty sources contributing to the final confidence intervals for the reported results.

Photoelectron circular dichroism spectroscopy in an orbitally congested system: the terpene endoborneol.

We have measured the photoelectron circular dichroism (PECD) of single enantiomers of endoborneol in the photon region from 9.9 to 23.6 eV by combining circularly polarized synchrotron radiation and a velocity map imaging technique. A photoelectron spectrum and the state-selected fragmentation curves of this terpene were also recorded. Unlike previous case studies, the broad featureless transitions encountered here preclude an orbital by orbital analysis of the PECD, although semiquantitative features of the highest-occupied molecular orbital PECD are identified and compared to full calculations. Despite our inability to further identify individual orbitals experimentally, we show that we are able to unambiguously assign the absolute configuration by comparison with realistic simulated PECD spectra. Furthermore, the calculations predict that for electron kinetic energies above 5 eV; the contributions of individual conformers to the PECD are nearly identical. Should this observation apply to bigger biological systems, the analysis could be greatly simplified by recording high kinetic energy electrons. On the other hand the contributions of the different conformers to the slow electron PECD seem to vary more significantly, and we deduce, within the theoretical limitations, a plausible 1:1:1 distribution of the three identified conformers.

The photoionisation of two phenylcarbenes and their diazirine precursors investigated using synchrotron radiation.

Two phenylcarbenes, chlorophenylcarbene (CPC) and trifluoromethylphenylcarbene (TFPC), were generated by jet flash pyrolysis of diazirine precursors. Their photoionisation was studied by photoelectron-photoion coincidence spectroscopy using synchrotron radiation. For CPC we determined an adiabatic ionisation energy (IE) of 8.15 eV and a vertical IE of 9.3 eV. For TFPC we obtained an adiabatic IE of 8.47 eV and a vertical IE of 8.95 eV. The photoelectron spectra are broad and unstructured due to a large increase in the angle between the phenyl group, carbene centre and the substituent (Cl or CF(3)). The geometry change upon ionisation is more pronounced for CPC. Being a singlet arylcarbene, CPC is more strongly bent in the neutral ground state than the triplet TFPC. In addition, the bond between the carbenic centre and the chlorine atom shortens upon ionisation, because the radical cation is stabilised by the non-bonding electrons of the Cl through a mesomeric effect. The photoionisation and dissociative photoionisation of the diazirine precursors are also explored. The CPC precursor, 3-chloro-3-phenyldiazirine, undergoes complete dissociative photoionisation and only the CPC(+) fragment is observed above 8.8 eV. For 3-trifluoromethyl-3-phenyldiazirine some molecular ions can be observed above 9.05 eV. However, dissociative photoionisation also sets in right at the ionisation threshold. A fit to the data yields an appearance energy of AE (0 K) approximately 9.27 eV.

A versatile electron-ion coincidence spectrometer for photoelectron momentum imaging and threshold spectroscopy on mass selected ions using synchrotron radiation.

We present a photoelectron-photoion coincidence (PEPICO) spectrometer named DELICIOUS II which combines a velocity map imaging apparatus with a Wiley-McLaren time of flight analyzer for the study of gas phase samples in interaction with the synchrotron radiation (SR). This versatile system is capable of providing photoelectron images on mass-selected compounds with kinetic energy resolutions of DeltaE/E=5% and a 17 eV bandwidth, as well as threshold photoelectron spectra with a measured resolution of 0.8 meV, as demonstrated on the 3p(-1) ionization of argon. This instrument is also employed for threshold PEPICO experiments, allowing the selection of the parent ion's internal state with sub-meV resolution for light masses (<40 amu) and with typically 2 meV resolution for a mass of 100 amu and with a mass resolving power above 200. The continuous operation of the extraction fields and the independence from the electron's time of flight are well adapted to the quasicontinuous multibunch mode of the SR. This, together with the high transmission of both the electron and ion detection, allows a high coincidence counting rate and facilitates the subtraction of false coincidences. We illustrate the spectrometer's coincidence principle of operation with examples from the valence photoionization of an Ar+Xe mixture and of CF(4).