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 sp(2) carbon (ß 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.

Rotational Doppler Effect: A Probe for Molecular Orbitals Anisotropy.

The vibrationally resolved X-ray photoelectron spectra of X2Σg+(3σg−1) and B2Σu+(2σu−1) states of N2+ were recorded for different photon energies and orientations of the polarization vector. Clear dependencies of the spectral line widths on the X-ray polarization as well as on the symmetry of the final electronic states are observed. Contrary to the translational Doppler, the rotational Doppler broadening is sensitive to the photoelectron emission anisotropy. On the basis of theoretical modeling, we suggest that the different rotational Doppler broadenings observed for gerade and ungerade final states result from a Young's double-slit interference phenomenon.

Intensity oscillations in the carbon 1s ionization cross sections of 2-butyne.

Carbon 1s photoelectron spectra for 2-butyne (CH3C≡CCH3) measured in the photon energy range from threshold to 150 eV above threshold show oscillations in the intensity ratio C2,3/C1,4. Similar oscillations have been seen in chloroethanes, where the effect has been attributed to EXAFS-type scattering from the substituent chlorine atoms. In 2-butyne, however, there is no high-Z atom to provide a scattering center and, hence, oscillations of the magnitude observed are surprising. The results have been analyzed in terms of two different theoretical models: a density-functional model with B-spline atom-centered functions to represent the continuum electrons and a multiple-scattering model using muffin-tin potentials to represent the scattering centers. Both methods give a reasonable description of the energy dependence of the intensity ratios.

Conformations and CH/π 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 ± 5% and 70 ± 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 γ-CH/π 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.

Chemical reactivity of alkenes and alkynes as seen from activation energies, enthalpies of protonation, and carbon 1s ionization energies.

Electrophilic addition to multiple carbon-carbon 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.

Single and double core-hole ionization energies in molecules.

The recently demonstrated ability to measure double-hole core-ionization energies in first-row elements has led to a renewed interest in the use of such energies to investigate the effects of initial-state charge distribution and final-state charge rearrangement on the energies of chemical processes that involve addition of charge to a molecule. With theoretical calculations for the molecules CH(4-n)X(n), X = F, Cl, and for C(CH(3))(4) as a basis, the relationships between one-hole and two-hole ionization energies, on one hand, and initial-state and final-state effects, on the other, are reviewed. It is shown that higher-order corrections to the traditionally used relationships are quantitatively significant but do not lead to qualitatively different conclusions. The role of the Wagner plot as a way to display the relationships among the various quantities of interest is discussed, and a generalized Wagner plot for displaying two-site double-hole ionization energies is presented. Some possible applications of measurements of double-hole ionization energies to the investigation of molecular conformation and molecular fragmentation are discussed.

Accuracy of Calculated Chemical Shifts in Carbon 1s Ionization Energies from Single-Reference ab Initio Methods and Density Functional Theory.

A database of 77 adiabatic carbon 1s ionization energies has been prepared, covering linear and cyclic alkanes and alkenes, linear alkynes, and methyl- or fluoro-substituted benzenes. Individual entries are believed to carry uncertainties of less than 30 meV in ionization energies and less than 20 meV for shifts in ionization energies. The database provides an unprecedented opportunity for assessing the accuracy of theoretical schemes for computing inner-shell ionization energies and their corresponding chemical shifts. Chemical shifts in carbon 1s ionization energies have been computed for all molecules in the database using Hartree-Fock, Møller-Plesset (MP) many-body perturbation theory of order 2 and 3 as well as various approximations to full MP4, and the coupled-cluster approximation with single- and double-excitation operators (CCSD) and also including a perturbational estimate of the energy effect of triple-excitation operators (CCSD(T)). Moreover, a wide range of contemporary density functional theory (DFT) methods are also evaluated with respect to computing experimental shifts in C1s ionization energies. Whereas the top ab initio methods reproduce the observed shifts almost to within the experimental uncertainty, even the best-performing DFT approaches meet with twice the root-mean-squared error and thrice the maximum error compared to CCSD(T). However, a number of different density energy functionals still afford sufficient accuracy to become tools in the analysis of complex C1s photoelectron spectra.

Additivity of substituent effects. Core-ionization energies and substituent effects in fluoromethylbenzenes.

Carbon 1s ionization energies have been measured for all of the carbon atoms in eight fluoromethylbenzenes. Enthalpies of protonation have been calculated for protonation at all of the ring carbons in the same molecules. These data together with previously reported data on fluorobenzenes and methylbenzenes provide the basis for studying the additivity of substituent effects and the correlation between enthalpies of protonation with core-ionization energies. Although a linear additivity model accounts reasonably well for both the ionization energies and the enthalpies of protonation, a better description, especially for the enthalpies, is obtained by inclusion of nonlinear terms that account for interactions between two substituents on the same molecule. There are families of nearly parallel correlation lines between enthalpies of protonation and core-ionization energies. The existence of several families can be primarily understood in terms of the linear additivity picture and more completely understood when the nonlinear terms are taken into account. The role of the methyl group as a polarizible pi-electron donor is contrasted with the role of fluorine, which is a substituent of low polarizibility that acts to withdraw electrons from the adjacent carbon and to donate electrons through resonance to the ring. The role of the hydrogen atoms as pi-electron acceptors in the protonated species is illustrated.

Recoil excitation of vibrational structure in the carbon 1s photoelectron spectrum of CF4.

The carbon 1s photoelectron spectrum of CF4 measured at photon energies from 330 to 1500 eV shows significant contributions from nonsymmetric vibrational modes. These increase linearly as the photon energy increases. The excitation of these modes, which is not predicted in the usual Franck-Condon point of view, arises from the recoil momentum imparted to the carbon atom in the ionization process. A theory is presented for quantitative prediction of the recoil effect; the predictions of this theory are in agreement to the measurements. The experiments also yield the vibrational frequencies of the symmetric and asymmetric stretching modes in core-ionized CF4, the change in CF bond length upon ionization, -0.61 pm, and the Lorentzian linewidth of the carbon 1s hole, 67 meV.

The substituent effect of the methyl group. Carbon 1s ionization energies, proton affinities, and reactivities of the methylbenzenes.

High-resolution carbon 1s photoelectron spectra have been measured for methyl-substituted benzenes. By using these data together with molecular structure calculations to predict the vibrational profiles expected in the spectra, it has been possible for the first time to assign 1s ionization energies to each of the inequivalent carbon atoms in these molecules. There exist linear correlations between the ionization energies and the energy changes for other chemical processes, such as enthalpies of protonation and activation energies for hydrogen exchange and protodesilylation. There are deviations from these correlations for sites in which hyperconjugation plays a role in the process. These can be understood by recognizing that the core-ionization energies reflect primarily the Hammett parameter sigma whereas the other energies reflect sigma+. The ionization and reaction energies can be summarized compactly with a linear model in which the total effect of the substituents is equal to the sum of the effects of the individual substituents. A slightly better description is obtained with a quadratic model, which allows for interaction between the substituents.

Boron 1s photoelectron spectrum of 11BF3: vibrational structure and linewidth.

The boron 1s photoelectron spectrum of (11)BF(3) has been measured at a photon energy of 400 eV and a resolution of about 55 meV. The pronounced vibrational structure seen in the spectrum has been analyzed to give the harmonic and anharmonic vibrational frequencies of the symmetric stretching mode, 128.1 and 0.15 meV, as well as the change in equilibrium BF bond length upon ionization, -5.83 pm. A similar change in bond length has been observed for PF(3) and SiF(4), but a much smaller change for CF(4). Theoretical calculations for BF(3) that include the effects of electron correlation give results that are in reasonable accord with the experimental values. The Lorentzian (lifetime) width of the boron 1s core hole in BF(3) is found to be 72 meV, comparable to the value of 77 meV that has been reported for CF(4).

Fluorine as a pi donor. Carbon 1s photoelectron spectroscopy and proton affinities of fluorobenzenes.

The carbon 1s ionization energies for all of the carbon atoms in 10 fluorine-substituted benzene molecules have been measured by high-resolution photoelectron spectroscopy. A total of 30 ionization energies can be accurately described by an additivity model with four parameters that describe the effect of a fluorine that is ipso, ortho, meta, or para to the site of ionization. A similar additivity relationship describes the enthalpies of protonation. The additivity parameters reflect the role of fluorine as an electron-withdrawing group and as a pi-electron donating group. The ionization energies and proton affinities correlate linearly, but there are four different correlations depending on whether there are 0, 1, 2, or 3 fluorines ortho or para to the site of ionization or protonation. That there are four correlation lines can be understood in terms of the ability of the hydrogens at the site of protonation to act as a pi-electron acceptor. A comparison of the ionization energies and proton affinities, together with the results of electronic structure calculations, gives insight into the effects of fluorine as an electron-withdrawing group and as a pi donor, both in the neutral molecule and in response to an added positive charge.

Reactivity and core-ionization energies in conjugated dienes. Carbon 1s photoelectron spectroscopy of 1,3-pentadiene.

The high-resolution carbon 1s photoelectron spectrum of trans-1,3-pentadiene has been resolved into contributions from the five inequivalent carbon atoms, and carbon 1s ionization energies have been assigned to each of these atoms. Spectra have also been measured for propene and 1,3-butadiene at better resolution than has previously been available. The ionization energies for the sp2 carbons are found to correlate well with activation energies for electrophilic addition and with proton affinities. Comparing the results for 1,3-pentadiene with those for ethene, propene, and 1,3-butadiene as well as with results of theoretical calculations makes it is possible to assess the effect of the terminal methyl group in 1,3-pentadiene. As in propene, the methyl group contributes electrons to the beta carbon through the pi system. In addition, there is a significant (though smaller) contribution from the methyl group to the terminal (delta) CH2 carbon, also through the pi system. Most of the effect of the methyl group is present in the ground-state molecule. There are only relatively small contributions from the methyl group to the ionization energies from redistribution of charge in the pi system in response to the removal of a core electron. In addition to these specific effects, there is an overall decrease in average ionization energy as the size of the molecule increases as well as effects that are specific to the conjugated systems in 1,3-butadiene and 1,3-pentadiene. The results provide insight into the reactivity and regioselectivity of conjugated dienes.

Gas-phase structure, conformation, and sulfur 2p photoelectron spectroscopy of pentafluorosulfur fluorosulfonate, SF5OSO2F.

The structure of SF(5)OSO(2)F has been investigated using gas-phase electron diffraction and quantum-chemical calculations. It is found to exist primarily in the gauche form (SF(5) group gauche relative to the S-F bond of the SO(2)F group with phi(S-O-S-F = 71(7) degrees ). A small contribution of the trans conformer cannot be excluded. Photoelectron spectroscopy gives ionization energies for the sulfur 2p electrons that reflect the relative electronegativities of fluorine and oxygen. The widths of the peaks in the photoelectron spectra indicate that there is considerable vibrational excitation associated with the core ionization of the sulfur atoms.

Electronegativities from core-ionization energies: electronegativities of SF5 and CF3.

Core-ionization energies have been measured for SF(6) (S 2p), SF(5)SF(5) (S 2p), SF(5)Br (S 2p and Br 3d), and SF(5)CF(3) (S 2p and C 1s). These results, together with others that establish correlations between core-ionization energies and Pauling electronegativities, make it possible to assign group electronegativities to SF(5) and CF(3). This method gives electronegativities for these groups comparable to that of bromine, whereas analysis of the effect of these groups on acidity indicates electronegativities comparable to that of fluorine. Other methods of estimating electronegativity fall between these extremes. These disparities can be understood in part as reflecting the effects of polarizibility of the substituent, which tends to lower both the core-ionization energy and the deprotonation energy, making the electronegativity appear to be less in one case and more in the other. In addition, and possibly more important, the core-ionization energies presented here reflect the effect of the group on an adjacent atom, whereas the acidity reflects the effect on a remote atom. It appears that fluorine has a large effect on an adjacent atom but a relatively small effect on a remote atom. By contrast SF(5) and CF(3) have a relatively small effect on an adjacent atom, but this effect falls off only slowly with distance from the substituent. Thus, the effective electronegativities of CF(3) and SF(5) relative to those of the halogens depend on the site at which the molecule is probed as well as on the process that is under consideration.