13 research outputs found
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 (4ÎŁ final state), and 544.338(17) and 544.423(17) eV for O2 (2ÎŁ 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 540â543âŻeV. These proposed standards should provide optimal internal calibration for a wide range of oxygen-containing compounds.publishedVersio
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Electronic Properties of Chlorine, Methyl, and Chloromethyl as Substituents to the Ethylene Group-Viewed from the Core of Carbon
âSubstituent effectsâ 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 CâC 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 (Îą 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 Ď system. Moreover, the adopted approach is extended to include substituentâsubstituent interaction parameters
Electronic properties of chlorine, methyl, and chloromethyl as substituents to the ethylene group-viewed from the core of carbon
âSubstituent effectsâ 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 CâC 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 (Îą 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 Ď system. Moreover, the adopted approach is extended to include substituentâsubstituent interaction parameters
Vibrational Structure and Vibronic Coupling in the Carbon 1s Photoelectron Spectra of Ethane and Deuteroethane
The carbon 1s photoelectron spectrum of ethane, C2H6, has been measured at a photon energy of 329 eV and an instrumental resolution of 70 meV. The spectrum shows a rich vibrational structure which is resolved using least-squares fits to the data. Only C-H stretching and CCH bending modes contribute significantly to the spectrum. The lack of excitation of the C-C stretching mode is explained in terms of changes in hybridization at the spectator carbon. To investigate the possibility of incomplete localization of the core hole, the spectra of C2H6 and C2D6 were measured at higher experimental resolution (35 meV). The spectra are accurately fit by a model based on ab initio calculations of the vibrational energies and the geometry changes following ionization, and including vibronic coupling of the two degenerate, localized hole states. A small splitting on the order of 10-20 meV is found for the 2A2u and 2A1g core-ionized states
Chemical Insights from High-Resolution X-ray Photoelectron Spectroscopy and ab Initio Theory: Propyne, Trifluoropropyne, and Ethynylsulfur Pentafluoride
High-resolution carbon 1s photoelectron spectroscopy of propyne (HC=CH3) shows a spectrum in which the contributions from the three chemically inequivalent carbons are clearly resolved and marked by distinct vibrational structure. This structure is well accounted for by ab initio theory. For 3,3,3-trifluoropropyne (HC=CF3) and ethynylsulfur pentafluoride (HC=SF5), the ethynyl carbons show only a broad structure and have energies that differ only slightly from one another. The core-ionization energies can be qualitatively understood in terms of conventional resonance structures; the vibrational broadening for the fluorinated compounds can be understood in terms of the effects of the electronegative fluorines on the charge distribution. Combining the experimental results with gas-phase acidities and with ab initio calculations provides insights into the effects of initial-state charge distribution and final-state charge redistribution on ionization energies and acidities. In particular, these considerations make it possible to understand the apparent paradox that SF5 and CF3 have much larger electronegativity effects on acidity than they have on carbon 1s ionization energies
Conformations and CH/Ď Interactions in Aliphatic Alkynes and Alkenes
The carbon 1s photoelectron
spectra of a series of aliphatic alkynes
and alkenes that have the possibility of possessing two or more conformers
have been recorded with high resolution. The two conformers of 2-hexyne
and 4-methyl-1-pentyne, anti and gauche, have been identified and
quantified from an analysis of their carbon 1s photoelectron spectra,
yielding 30 Âą 5% and 70 Âą 6% anti conformers, respectively.
In the case of 1-hexyne, the photoelectron spectrum is shown to provide
partial information on the distribution of conformers. Central to
these analyses is a pronounced ability of the C1s photoemission process
to distinguish between conformers that display weak Îł-CH/Ď
hydrogen bonding and those that do not. For the corresponding alkene
analogs, similar analyses of their C1s photoelectron spectra do not
lead to conclusive information on the conformational equilibria, mainly
because of significantly smaller chemical shifts and higher number
of conformers compared with the alkynes
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ZahlElectronicPropertiesChlorineSupplementaryInformation.pdf
âSubstituent effectsâ 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 CâC 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 (Îą 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 Ď system. Moreover, the adopted approach is extended to include substituentâsubstituent interaction parameters
Chemical Reactivity of Alkenes and Alkynes As Seen from Activation Energies, Enthalpies of Protonation, and Carbon 1s Ionization Energies
Electrophilic addition to multiple carbonâcarbon
bonds has
been investigated for a series of twelve aliphatic and aromatic alkenes
and the corresponding alkynes. For all molecules, enthalpies of protonation
and activation energies for HCl addition across the multiple bonds
have been calculated. Considering the protonation process as a cationic
limiting case of electrophilic addition, the sets of protonation enthalpies
and gas-phase activation energies allow for direct comparison between
double- and triple-bond reactivities in both ionic and dipolar electrophilic
reactions. The results from these model reactions show that the alkenes
have similar or slightly lower enthalpies of protonation, but have
consistently lower activation energies than do the alkynes. These
findings are compared with results from high resolution carbon 1s
photoelectron spectra measured in the gas phase, where the contribution
from carbons of the unsaturated bonds are identified. Linear correlations
are found for both protonation and activation energies as functions
of carbon 1s energies. However, there are deviations from the lines
that reflect differences between the three processes. Finally, substituent
effects for alkenes and alkynes are compared using both activation
and carbon 1s ionization energies