Search

Experimental evidence for an effect of molecular conformation on inner-shell ionization energies has been observed for the first time. Examples are seen in the carbon 1s spectra of butyronitrile, 1-fluoropropane, and propanal, and other similar molecules. At room temperature these exist in two different conformations, with different distances and, hence, different Coulombic interactions between the negatively charged electronegative group and the methyl carbon. The experimental results are in accord with theoretical predictions with respect to both ionization energies and populations of the different conformers.

Theoretical models of lineshapes in Ar2p photoionization spectra have been calculated for free, neutral argon clusters of different sizes. The lineshape models are fitted to experimental spectra and used to estimate the mean cluster size realized in the experiment. The results indicate that size estimators working from stagnation conditions [R. Karnbach, M. Joppien, J. Stapelfeldt, J. Wörmer and T.Möller, Rev. Sci. Instrum., 1993, 64, 2838] may underestimate the mean cluster size.

The carbon 1s photoelectron spectra of cyclohexane, cyclohexene, 1,3-cyclohexadiene, and 1,4-cyclohexadiene have been measured. These have been analyzed in terms of a theoretical model of the vibronic structure that includes the effects of the changes in equilibrium geometry that accompany core ionization as well as those of vibronic coupling in the core-ionized molecule. This theoretical approach gives an excellent description of the vibronic structure and makes it possible to identify the contributions of the inequivalent carbon atoms to the spectra. The carbon 1s ionization energies are reported and used in the analysis of photoelectron spectra of adsorbed molecules.

Vibrationally resolved C1s photoelectron spectra of benzene and d6-benzene have been recorded using monochromated synchrotron radiation at photon energies of 330 eV. The spectrum of normal benzene displays considerable vibrational structure. Particularly noteworthy is the strong excitation of a combined CCH-bending and CC-stretching mode which splits the main peak into two well-defined maxima. In d6-benzene, the vibrational energy levels are less well separated and the vibrational structure is reduced to strong asymmetry of the main peak and a broad tail extending toward higher ionization energy. The recorded spectra are analyzed using first-principle and curve-fitting procedures. A theoretical model that allows for incomplete localization of the core hole, results in very good fits to the experimental spectra of both benzene and d6-benzene.

Carbon 1s ionization energies for ethene, propene and 2-methylpropene have been measured in the gas phase at high resolution using synchrotron radiation and analysed by means of ab initio calculations. For the first time, resolution is good enough to assign ionization energies to the inequivalent carbons in propene and 2-methylpropene. A linear correlation is found between the ionization energies and activation energies for addition of the electrophiles HF, HCl, HBr and HI to these molecules. The correlation shows that both reactivity and regiospecificity are quantitatively related to core-ionization energies. Theoretical analysis of the core-ionization energies shows that the differences between ionization energies for the doubly bonded carbons are due to the charge distribution in the unionized molecule. Theoretical analysis of the transition state for addition of HCl to propene and 2-methylpropene indicates that a significant portion of the difference between Markovnikov and anti-Markovnikov addition is also due to the charge distribution in the initial state and not to different ability of the molecules to delocalize the added charge in the transition state. The increase in reactivity with the number of methyl groups is also strongly influenced by the initial-state charge distribution.

Mixed neutral molecular clusters have been produced by co-expansion of chloroform and methanol and characterized by carbon 1s photoelectron spectroscopy and theoretical modeling. The produced clusters are in the range 50–150 molecules and the clusters are not homogeneously mixed: chloroform is enriched in the bulk and methanol is found on or close to the surface. This is based on evidence from photoelectron depth profiling and molecular dynamics simulations of mixed clusters. The simulations suggest that methanol forms cyclic or linear oligomers in the surface layers of the mixed clusters. From thermodynamic models, the enrichment of methanol on the cluster surface can be rationalized from the difference in surface tension between the two pure components, which is connected to the qualitative differences in the respective bonding patterns.