Calibration of oxygen 1s ionization energies. Accurate energies for CO2, H2O, CO, and O2

Access to accurate reference data is a prerequisite in order to translate chemical shifts to an absolute scale for inner-shell ionization energies. Calibration standards for oxygen 1s (O 1s) ionization energies are less well established than, for instance, for carbon 1s. To improve upon this situation, adiabatic and vertical O 1s ionization energies for gaseous carbon dioxide (CO2) are critically reviewed and used to establish the most accurate values currently available: 541.085(17) and 541.253(17) eV, respectively. Combining these with new precise measurements of shifts in O 1s ionization energies for H2O, CO, and O2 allows us to establish equally accurate absolute ionization energies for these molecules as for CO2. The resulting adiabatic and vertical energies are 539.728(17) and 539.827(17) eV for H2O, 542.439(17) and 542.495(17) eV for CO, 543.285(17) and 543.294(17) eV for O2 (4Σ final state), and 544.338(17) and 544.423(17) eV for O2 (2Σ final state). It is proposed that O 1s in CO2 be adopted as a standard of higher precedence, and that H2O, CO, and O2 be used also. The O 1s ionization energies in these molecules occur in the range 540–543 eV. These proposed standards should provide optimal internal calibration for a wide range of oxygen-containing compounds.publishedVersio

Electronic Properties of Chlorine, Methyl, and Chloromethyl as Substituents to the Ethylene Group-Viewed from the Core of Carbon

“Substituent effects” is an important and useful concept in organic chemistry. Although there are many approaches to parametrizing the electronic and steric effects of substituents, the physical basis for the parameters is often unclear. The purpose of the present work is to explore the properties of chemical shifts in carbon 1s energies as a well-defined basis for characterizing substituents to an ethylene C═C moiety. To this end, high-resolution carbon 1s photoelectron spectra of six chloro-substituted ethenes and seven chloro-substituted propenes have been measured in the gas phase. Site-specific adiabatic ionization energies have been determined from the spectra using theoretical ab initio calculations to predict the vibrational structures. For two molecules, 3-chloropropene and 2,3-dichloropropene, the spectral analyses give quantitative results for the conformer populations. The observed shifts have been analyzed in terms of initial-state (potential) and relaxation effects, and charge relaxation has also been analyzed by means of natural resonance theory. On the basis of core-level spectroscopy and models, chlorine, methyl, and chloromethyl have been characterized in terms of their effect on the carbon to which they are attached (α site) as well as the neighboring sp² carbon (β site). The derived spectroscopic substituent parameters are characterized by both inductive (electronegativity) effects and the ability of each substituent to engage in electron delocalization via the π system. Moreover, the adopted approach is extended to include substituent–substituent interaction parameters

Formation and Growth of Clusters of Sulfur Dioxide

Formation and growth of neutral SO2 clusters is investigated in an adiabatic-expansion setup by means of sulfur 2p (S2p) photoelectron spectroscopy and theoretical modeling. The shift in S2p ionization energy between the cluster and a single molecule, i.e., IE(cluster)-IE(monomer), is recorded and used to monitor the mean cluster size over a wide range of expansion conditions. The produced clusters were shown to fall into two different size regimes. Comparison between theoretical simulations and experimental observations suggests that while the smallest clusters belong to the ultrafine particle mode and have a liquid-like structure, the larger clusters belong to the accumulation mode of fine particles and possibly have a frozen cluster core. The transition between the two size/structure regimes occurs over a narrow interval in expansion conditions and may possibly reflect a change in growth mechanism from monomer addition to growth by cluster-cluster collisions

Carbon 1s Photoelectron Spectroscopy of Halomethanes. Effects of Electronegativity, Hardness, Charge Distribution, and Relaxation

Carbon 1s ionization energies have been measured for 12 halomethanes. These together with earlier measurements provide 27 compounds for investigating the relationship between core-ionization energies and the electronegativity and hardness of the halogens. The ionization energies correlate nearly linearly with the sum of the electronegativities of the halogens attached to the central carbon. Both electronegativity and hardness play important roles in determining the ionization energy, and it is found that the linear relationship between ionization energy and electronegativity arises from an interplay of the electronegativity and hardness of the halogens and the length and ionicity of the carbon-halogen bond