Communication: Water activation and splitting by single metal-atom anions.

We report experimental and computational results pertaining to the activation and splitting of single water molecules by single atomic platinum anions. The anion photoelectron spectra of [Pt(H2O)]-, formed under different conditions, exhibit spectral features that are due to the anion-molecule complex, Pt-(H2O), and to the reaction intermediates, HPtOH- and H2PtO-, in which one and two O-H bonds have been broken, respectively. Additionally, the observations of PtO- and H2 + in mass spectra strongly imply that water splitting via the reaction Pt- + H2O → PtO- + H2 has occurred. Extending these studies to nickel and palladium shows that they too are able to activate single water molecules, as evidenced by the formation of the reaction intermediates, HNiOH- and HPdOH-. Computations at the coupled cluster singles and doubles with perturbatively connected triples level of theory provide structures and vertical detachment energies (VDEs) for both HMOH- and H2MO- intermediates. The calculated and measured VDE values are in good agreement and thus support their identification.

High-Resolution Absorption and Electronic Circular Dichroism Spectra of (R)-(+)-1-Phenylethanol. Confident Interpretation Based on the Synergy between Experiments and Computations.

Using density functional theory and its time-dependent extension for excited states, the S0 →S1 high-resolution vibronic absorption and electronic circular dichroism spectra of (R)-(+)-1-phenylethanol are computed and compared to experimental spectra measured in jet-cooled conditions in the region within 1000 cm-1 of the 0-0 transition. The agreement between theory and computation is satisfactory and allows a confident assignment of several experimental bands in terms of fundamentals of different modes. Cases are documented for which the analysis of optical anisotropy factors, owing to their signed nature, remarkably enhances the possibility of a robust assignment of the experimental absorption bands. Computational analysis shows that the experimental spectra are dominated by Herzberg-Teller contributions and that the electronic circular dichroism spectrum and the anisotropy factors are also strongly modulated by the effect of Duschinsky mixings.

Chiroptical inversion for isolated vibronic transitions of supersonic beam-cooled molecules.

Circular dichroism-resonance-enhanced multiphoton ionization (CD-REMPI) was used for CD measurements on several single vibronic transitions of supersonic beam-cooled (R)-(+)-1-phenylethanol. Due to the low molecular densities within a supersonic beam and the expected small anisotropy factor of 1-phenylethanol in the permille region, the precision of the experimental method had to be significantly improved. Therefore, a single laser pulse evaluation combined with a twin-peak technique enabled within the used supersonic beam setup is presented. For the electronic transition S0 → S1 of (R)-(+)-1-phenylethanol (π → π* transition of the phenyl ring at 266 nm) ten different vibrational modes as well as the 0-transition were investigated with one-color (1 + 1) CD-REMPI. The results deliver new experimental insights on the influence of molecular vibrations on the anisotropy factor. TD-DFT theoretical predictions show how the angle between the electronic and magnetic transition dipole moments of the electronic transition can be modified by different vibrational modes, making even a flip of the sign of the anisotropy factor possible.

Time-of-flight mass spectrometry: Introduction to the basics.

The intention of this tutorial is to introduce into the basic concepts of time-of-flight mass spectrometry, beginning with the most simple single-stage ion source with linear field-free drift region and continuing with two-stage ion sources combined with field-free drift regions and ion reflectors-the so-called reflectrons. Basic formulas are presented and discussed with the focus on understanding the physical relations of geometric and electric parameters, initial distribution of ionic parameters, ion flight times, and ion flight time incertitude. This tutorial is aimed to help the applicant to identify sources of flight time broadening which limit good mass resolution and sources of ion losses which limit sensitivity; it is aimed to stimulate creativity for new experimental approaches by discussing a choice of instrumental options and to encourage those who toy with the idea to build an own time-of-flight mass spectrometer. Large parts of mathematics are shifted into a separate chapter in order not to overburden the text with too many mathematical deviations. Rather, thumb-rule formulas are supplied for first estimations of geometry and potentials when designing a home-built instrument, planning experiments, or searching for sources of flight time broadening. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 36:86-109, 2017.

Mass-Selected Circular Dichroism of Supersonic-Beam-Cooled [D4 ]-(R)-(+)-3-Methylcyclopentanone.

UV spectroscopy and electronic circular dichroism (ECD) experiments on supersonic-beam-cooled deuterated (R)-(+)-3-methylcyclopentanone ([D4 ]-(R)-(+)-3-MCP) have been performed by using a laser mass spectrometer. The spectral resolution not only allowed excitation and CD measurements for single vibronic transitions but also for the rotational P, Q, and R branches of these transitions. The investigated n→π*18042501 transition showed the largest anisotropy factor ever observed for chiral molecules in the gas phase, which, due to residual saturation of the excited transition, represents only a lower limit for the real anisotropy factor. Furthermore, one-color (1+1+1) and two-color (1+1') resonance-enhanced multiphoton ionization (REMPI) measurements were performed and the effusive-beam (room temperature) and supersonic-beam results for [D4 ]-(R)-(+)-3-MCP were compared. These results allowed a differentiation between single-step ECD (comparable to conventional ECD) and cumulative ECD (only possible in multiphoton excitation) under supersonic-beam conditions.

Mass-Selective Chiral Analysis.

Three ways of realizing mass-selective chiral analysis are reviewed. The first is based on the formation of diastereomers that are of homo- and hetero- type with respect to the enantiomers of involved chiral molecules. This way is quite well-established with numerous applications. The other two ways are more recent developments, both based on circular dichroism (CD). In one, conventional or nonlinear electronic CD is linked to mass spectrometry (MS) by resonance-enhanced multiphoton ionization. The other is based on CD in the angular distribution of photoelectrons, which is measured in combination with MS via photoion photoelectron coincidence. Among the many important applications of mass-selective chiral analysis, this review focuses on its use as an analytical tool for the development of heterogeneous enantioselective chemical catalysis. There exist other approaches to combine chiral analysis and mass-selective detection, such as chiral chromatography MS, which are not discussed here.

Laser mass spectrometry with circularly polarized light: circular dichroism of cold molecules in a supersonic gas beam.

An experiment on chiral molecules that combines circular dichroism (CD) spectroscopy, mass-selective detection by laser mass spectrometry (MS), and cooling of molecules by using a supersonic beam is presented. The combination of the former two techniques (CD-laser-MS) is a new method to investigate chiral molecules and is now used by several research groups. Cooling in a supersonic beam supplies a substantial increase in spectroscopic resolution, a feature that has not yet been used in CD spectroscopy. In the experiments reported herein, a large variation in the electronic CD of carbonyl 3-methylcyclopentanone was observed depending on the excited vibrational modes in the n → π* transition. This finding should be of interest for the detection of chiral molecules and for the theoretical understanding of the CD of vibronic bands. It is expected that this effect will show up in other chiral carbonyls because the n → π* transition is typical for the carbonyl group.

Dynamic Oil Consumption Measurement of Internal Combustion Engines using Laser Spectroscopy.

A new approach has been developed to measure dynamic consumption of lubricant oil in an internal combustion engine. It is based on the already known technique where sulfur is used as a natural tracer of the engine oil. Since ejection of motor oil in gaseous form into the exhaust is by far the main source of engine oil consumption, detection of sulfur in the exhaust emission is a valuable way to measure engine oil consumption in a dynamic way. In earlier approaches, this is done by converting all sulfur containing chemical components into SO2 by thermal pyrolysis in a high temperature furnace at atmospheric pressure. The so-formed SO2 then is detected by broadband-UV-induced fluorescence or mass spectrometric methods. The challenge is to reach the necessary detection limit of 50 ppb. The new approach presented here includes sulfur conversion in a low-pressure discharge cell and laser-induced fluorescence with wavelength and fluorescence lifetime selection. A limit of detection down to 10 ppb at a temporal resolution in the time scale of few seconds is reached. Extensive, promising studies have been performed at a real engine test bench. Future developments of a compact, mobile device based on these improvements are discussed.

Communication: In search of four-atom chiral metal clusters.

A combined study utilizing anion photoelectron spectroscopy and density functional theory was conducted to search for four-atom, chiral, metal, and mostly metal clusters. The clusters considered were AuCoMnBi(-/0), AlAuMnO(-/0), AgMnOAl(-/0), and AuAlPtAg(-/0), where the superscripts, (-/0), refer to anionic and neutral cluster species, respectively. Based on the agreement of experimentally and theoretically determined values of both electron affinities and vertical detachment energies, the calculated cluster geometries were validated and examined for chirality. Among both anionic and neutral clusters, five structures were identified as being chiral.

Resonance-enhanced multiphoton ionization with circularly polarized light: chiral carbonyls.

An introduction to the principle and possibilities of the new method of circular dichroism laser mass spectrometry is given and its state of development is reviewed. This method allows enantiosensitive, mass-selective probing of chiral molecules. It is based on the combination of resonance-enhanced multiphoton ionization with circularly polarized light and specially modified time-of-flight mass spectrometry. As an example, application to carbonyls is presented.

Laser mass spectrometry with circularly polarized light: circular dichroism of molecular ions.

In recent experiments of resonance-enhanced laser ionization, large differences between circular dichroism measured for molecular and fragment ions were found by several research groups for different molecular systems. In the case of 3-methylcyclopentanone (3-MCP) we attributed this effect to a large circular dichroism of the molecular ion. In the work presented here, this effect in 3-MCP is studied by ion spectroscopy, by varying the neutral intermediate excited state involved in resonance-enhanced multiphoton ionization (REMPI) and by performing REMPI-induced measurements of circular dichroism at different laser pulse energies. It turns out that the dynamics of structural changes in the ionic ground state strongly influences the observed ionic circular dichroism.

Laser mass spectrometry with circularly polarized light: two-photon circular dichroism.

Extensive studies of two-photon circular dichroism (TPCD) on 3-methylcyclopentanone are presented following the first TPCD experiment of gas phase molecules by Compton et al. [J. Chem. Phys.125 (2006) 144304]. (2 + 1)-Multiphoton ionization in a specially designed time-of-flight mass analyzer has been used to perform these studies. CD of two-photon transitions from the molecular ground state to low lying Rydberg states is strongly enhanced with respect to corresponding one-photon transitions in good agreement with Compton. Differences between CD values determined via the molecular ion and via fragment ions indicate strong molecular ion CD effects. This would be the first time that circular dichroism of isolated molecular ions has been measured.

Multiphoton ionization and circular dichroism: new experimental approach and application to natural products.

Enantio-sensitive laser mass spectrometry is the combination of multiphoton ionization by circularly polarized laser light with mass spectrometric detection of ions. The method has been developed as a tool for the fast investigation of chiral molecules in sample mixtures without any preceding separation and offers many new experimental possibilities. The main difficulties of the detection of circular dichroism in this way arise from systematic and statistical deviations. Herein, we report the newest approach to overcome these problems using a so-called twin-peak ion source, back-reflection of the laser light, and reference substances. By these means, the detection limit for circular dichroism can be lowered from the percent to the per-mill range. The capabilities of the new setup are demonstrated by the investigation of several natural products.

Anion ZEKE-spectroscopy of the weakly bound iodine water complex.

Zero kinetic electron energy photodetachment spectroscopy of I(-)·H(2)O and I(-)·D(2)O has been performed from 27 660 to 28 500 cm(-1) and from 27 660 to 35 900 cm(-1), respectively. The I(-)·D(2)O spectral data and theoretical studies resulted in a reassignment of earlier anion-ZEKE spectra of iodide water ( Bässmann , C. ; et al. Int. J. Mass Spectrom. Ion Processes 1996 , 159 , 153 ). In opposite to the I(-)·H(2)O, the I(-)·D(2)O spectrum reveals a regular progression of the iodine-water van der Waals stretching mode and a short progression of even quanta of the van der Waals rocking mode. A rough estimation delivers dissociation thresholds of the anionic and of the lower and the upper spin-orbit component of the neutral van der Waals complex. A high resolution ZEKE spectrum of the van der Waals stretching mode (v = 1) reveals significant fine structure, which is found again in a former photodissociation spectrum of the anionic complex ( Ayotte , P. ; et al. J. Phys. Chem. A 1998 , 102 , 3067 ). Our assignments are supported by theoretical calculations of molecular structures and vibrational motions. Vibrational frequencies and isotope effects are reproduced very satisfyingly by these calculations.

Photodetachment-photoelectron spectroscopy of HS- x H2S and DS- x D2S: the transition states of the SH + H2S and SD + D2S reactions.

The transition state region for neutral hydrogen transfer reactions can be accessed by photodetachment of a stable negative ion with a geometry similar to that of the neutral transition state. In this work the SH + H(2)S and SD + D(2)S reactions are investigated by photodetachment-photoelectron spectroscopy of HS(-) x H(2)S and DS(-) x D(2)S. The spectra exhibit vibrational structure which is attributed to the antisymmetric stretching mode (H-atom motion) of the neutral transitions state for H-atom transfer. The spectra are compared to one-dimensional simulations performed using a wave packet propagation scheme. Electronic structure calculations of the anionic, neutral and transition-state geometries and calculations of the vertical detachment energy at different levels of theory are used to support the analysis of the spectra. A vertical detachment energy VDE of 3.06 eV has been determined.

Progress in circular dichroism laser mass spectrometry.

Circular dichroism in ion yield has promising new potentials for chiral analysis. Our progress of its development is described here. Circular dichroism in ion yield is achieved by resonance-enhanced multiphoton ionization. The feasibility of circular dichroism spectroscopy and quantitative determination of circular dichroism by this method is demonstrated. Several excitation schemes have been applied using different types of lasers, which vary in wavelength and repetition rate. Progress to improve the statistical error and thus the lower limit of measurable circular dichroism is described. This is achieved by adding achiral compounds or racemic mixtures of chiral compounds to the sample gas as reference substances and ionizing them by the same laser pulse. Therefore, in the mass spectrum of every single laser pulse, ion signals of sample and reference species appear both being subject to the same kind of instrumental fluctuations (in particular of laser pulse energy). In another approach, a laser repetition rate of 200 Hz allowed averaging of large numbers of laser pulses.

Photodetachment-photoelectron spectroscopy of disulfanide: the ground and first excited electronic state of HS2 and DS2.

Photodetachment-photoelectron spectra of HS(2)(-) and DS(2)(-) are presented. The obtained electron affinities (EA (HS(2)) = 1.916 +/- 0.015 eV and EA (DS(2)) = 1.918 +/- 0.015 eV) are in good agreement with earlier results (S. Moran and G. B. Ellison, J. Phys. Chem., 1988, 92, 1794). Photodetachment into the neutral ground state of the radicals HS(2) and DS(2) leads to excitation of the S-S stretching mode, whereas photodetachment into the first excited state causes a S-S-H bending and a weak S-H stretching motion. Additionally, weak peaks of S(2) were observed in the spectra of HS(2) and DS(2), leading to the conclusion that photodetachment with 2.77 eV (448 nm) causes dissociation of HS(2)(-) and DS(2)(-) into S(2)(-) and H/D. Taking 2.77 eV as an upper limit for D(0)(S(2)(-)-H) a lower limit for Delta(f)H(0)(HS(2 (g))) of 100.8 kJ mol(-1) is obtained which is an improvement on the literature value of 107.145 +/- 10.46 kJ mol(-1).

The weakly-bound complex Cl.N2O studied by high resolution photodetachment photoelectron spectroscopy.

Zero kinetic energy photoelectron spectroscopy of the weakly bound complex Cl(-).N(2)O is presented. Adiabatic detachment energies between the anionic complex and the neutral complex states X(1/2), I(3/2), and II(1/2) as well as frequencies of van der Waals modes are supplied. Supported by theoretical simulations, these results correlate with a T-shaped structure of the anionic and an L-shaped structure of the neutral molecular complex. The weakly bound neutral complex Cl.N(2)O has been discussed as a prereactive state of the chemical reaction Cl + N(2)O --> ClO + N(2) [A. Lesar, M. Hodoscek, M. Senegacnik, J. Chem. Phys. 1996, 105, 917].

The electron affinity of phenanthrene.

Phenanthrene is studied by photodetachment-photoelectron spectroscopy. Due to the absence of a parent ion peak in the anion mass spectrum the electron affinity could not be determined directly. However, this absence is the first indication that this molecule has a negative electron affinity. The first three water complexes of phenanthrene were studied, supplying insights into its microsolvation property. Moreover, the electron affinity of the bare molecule could be determined to be -0.01+/-0.04 eV by an extrapolation method using the water cluster data. The experimental work is supported by ab initio calculations for determining the structure of the water complexes. Finally a correlation between the electron affinity and the reduction potential of polycyclic aromatic hydrocarbons is investigated.

Mass-selected resonance-enhanced multiphoton ionisation spectra of laser-desorbed molecules for environmental analysis: 16 representative polycyclic aromatic compounds.

Mass-selected gas-phase UV spectra of laser-desorbed molecules at room temperature have been measured via resonance-enhanced multiphoton ionisation (REMPI) and time-of-flight mass selection. The wavelength range of 260 to 320 nm is optimal for detection of polycyclic aromatic hydrocarbons (PAHs) using REMPI-mass spectrometry. A new laser desorption/laser ionisation source has been used which features a compact size and thermal equilibrium of the desorbed molecules. 16 PAHs have been investigated which have been selected by the US Environmental Protection Agency (EPA). These 16 EPA-PAHs are commonly used world-wide to characterise the PAH-load of environmental samples.