Lone Pair Ionization Energies Research Articles

Photoelectron spectroscopy of 5,12-diazatetracene

The electronic structure of 5,12-diazatetracene (DAT) has been studied in the gas phase by UV photoelectron spectroscopy and quantum chemical calculations. The comparison of DAT spectrum with the of spectrum tetracene (TC) shows the shift of outermost π-ionizations towards higher ionization energies and the appearance of nitrogen lone pair ionizations in the second and third spectral bands. The difference between nitrogen lone pair ionization energies is smaller in DAT compared to diaza analogues containing up to three fused rings. The implications for semiconductor properties are discussed.

Electronic structure of two catalytically important hafnocenes

The electronic structures of trichloro[(1,2,3,3a,7a-η)-1H-inden-1-yl]-hafnium and bis(cyclopentadienyl)hafnium dichloride have been investigated by HeI and HeII UV photoelectron spectroscopy and DFT/OVGF calculations. The chlorine lone pair ionization energies are reduced considerably on going from HfCl4 to metallocene hafnium chlorides. The electronic structure results have been related to the mechanism of olefin polymerization catalyzed by hafnocenes.

Chemistry of Mixed Sulfur-, Selenium-, or Tellurium- and Silicon-, or Tin-Containing Heterocycles

Lone pair ionization energies for 1,5-dichalcogenocanes containing endocyclic 3,7-R2Si groups and the parent 1,5-dichalcogenocanes, estimated from the TCNE charge-transfer wavelength maxima, are in good agreement with ionization energies directly obtained from photoelectron spectroscopy. These data indicate the occurrence of substantial intra-annular interaction between the chalcogen atoms and silyl groups, consistent with the well-known β -effect of silicon and the novel β-disilyl effect, when R (in R 2 Si) is Me 3 Si.

Substituent effects of halopyridines

The electronic structures of a number of halopyridines were measured by HeI photoelectron (PE) spectroscopy. Combined the empirical arguments and theoretical methods, the observed PE bands were interpreted. The careful analysis of measured π-orbital and halogen lone pair ionization energies enable us to describe substituent effects in terms of inductive, resonance, and spin–orbit coupling interactions. The Natural Population Analysis (NPA) has been also used to illuminate the substituent effects between halogen atoms with the parent ring.

Photoelectron spectroscopy and substituent effects in halonaphthalenes

The electronic structure of two isomeric dibromonaphthalenes (C 10H 6Br 2) has been investigated by HeI/HeII photoelectron spectroscopy. The spectra were assigned by Green's functions calculations and comparison with the spectra of related dibromobenzenes (C 6H 4Br 2). The analysis of π-orbital and halogen lone pair ionization energies, enabled us to determine the magnitude of bromine-bromine intramolecular interactions and distinguish between through-bond and through-space type interactions. We also discuss the halogen–halogen interactions in other polynuclear aromatics.

Photoelectron spectroscopy of trisubstituted benzenes: Dichloroiodobenzenes

The electronic structure of isomeric dichloroiodobenzenes (C 6H 3ICl 2) has been investigated by HeI/HeII photoelectron spectroscopy. The spectra were assigned by Green’s functions calculations and comparison with the spectra of related dichlorobenzenes (C 6H 4Cl 2). The careful analysis of π-orbital and halogen lone pair ionization energies, enabled us to analyze substituent effects in detail, describing them in terms of resonance, inductive and spin–orbit coupling interactions. The use of several descriptors simultaneously, rather than a single one, appears to be necessary to unravel the effects in polysubstituted benzenes. Our method of analysis is capable of describing substituent effect in detail even in polysubstituted benzenes where kinetic or equilibrium methods become impractical.

Halogens in Competition: Electronic Structure of Mixed Dihalobenzenes

The electronic structure of all isomeric dihalobenzenes C6H4XY (X, Y = Cl, Br, I) has been investigated by HeI/HeII photoelectron spectroscopy, Green's functions calculations, and comparison with the spectra of related dihalobenzenes C6H4X2 (X = Cl, Br, I). The careful analysis of measured pi orbital and halogen lone pair ionization energies enabled us to describe substituent effects in terms of resonance, inductive, steric, and spin-orbit coupling interactions.

UV photoelectron spectroscopic study of substituent effects in quinoline derivatives.

The molecular and electronic structure of eight substituted quinolines has been investigated by HeI/HeII photoelectron spectroscopy, Green's function calculations, and comparison with the spectra of related compounds. The correlation between nitrogen lone pair ionization energies and basicity in 18 substituted quinolines is discussed. The influence of different substituents has been quantified via the scheme that is based on experimental energy shifts. The relationships between nitrogen ionization energies, pK(a) values, and medicinal activity are also discussed.

Study of the planarization of the tricordinate phosphorus in phospholes; photoelectron spectra and structure of partially planarized phospholes

The gradual flattening of the tricoordinate phosphorus in phosphole by the increasing steric bulk of the substituent group is shown on the HF/6-31G* optimized geometries of alkylarylphospholes. By the decreasing pyramidality, aromaticity indices show increase, as a result of the increased conjugation. The aromaticity of 1-(2,4,6-tri-tertiarybutyl)-phosphole is similar to that of furan according to the geometrical indices. In the photoelectron spectra of the alkylaryl substituted phospholes the phosphorus lone pair ionization energy also decreases along the decreasing pyramidality. 1-(2,4,6-tri-Tertiarybutyl-phenyl)-3-methylphosphole has the lowest ionization energy value ever reported for a phosphole.

Heterocyclic pentaatomic molecules containing thio- and seleno-amido groups: linear relationship between nSe/nSe lone pair ionization energies and 1:1 adduct formation constants with diiodine

Heterocyclic pentaatomic molecules containing thio- and seleno-amido groups: linear relationship between nSe/nSe lone pair ionization energies and 1:1 adduct formation constants with diiodine

Proton Affinities and Photoelectron Spectra of Phenylalanine and N-Methyl- and N,N-Dimethylphenylalanine. Correlation of Lone Pair Ionization Energies with Proton Affinities and Implications for N-Methylation as a Method to Effect Site Specific Protonation of Peptides

A Fourier transform ion cyclotron resonance (FT-ICR) technique for measuring gas-phase proton affinities is presented which utilizes collisional dissociation of proton-bound clusters by off-resonance translational excitation. A simplified RRKM analysis relates unimolecular dissociation rates to proton affinities. This technique is used to measure values for the proton affinities of phenylalanine and N-methyl- and N,N-dimethylphenylalanine of 220.3, 223.6, and 224.5 kcal/mol, respectively (relative to the proton affinity of NH_3 = 204.0 kcal/mol). The proton affinity measured for phenylalanine is in excellent agreement with reported literature values. The photoelectron spectra of these three molecules are also presented and analyzed. Assignments of bands to specific ionization processes are aided by comparison with model compounds such as methyl-substituted amines and 2-phenylethylamines. These data are employed to examine the correlation of adiabatic nitrogen lone pair ionization energies with gas-phase proton affinities for phenylalanine, N-methylphenylalanine, and N,N-dimethylphenylalanine in comparison to correlations for other amino acids and selected aliphatic amines. Although amine nitrogen methylation increases the potential for localizing charge at the amine terminus of protonated peptides by increasing the gas-phase proton affinity, the present study establishes that the increase is not sufficient to compete with protonation of some of the more basic side chains in peptides.

Photoelectron spectrum and reactivity of silylalkyl sulphides. Stabilization of radical cations by the β-silyl effect

The effect of a β-silyl group on sulphur lone pair ionization energies has been re-investigated by use of ab initio and semi-empirical MNDO quantum-chemical calculations. The effect of the interaction between n s and σ SiC★ orbitals and that of electron reorganization have been shown to be unimportant in the case of silyl-methyl-sulphides. To interpret the observed features of the ionization energies and reactivity, the electron release by the β-silyl group and hyperconjugative interactions have been considered. Stabilization brought about by β-substitution for the ground state of the radical cation is ca. 10 kcal mol −1 for alkyl (10.11 kcal mol −1) and silyl (9.48 kcal mol −1) substituents at the MP2 6-31G★★//HF 6-31G★★ level of theory. The effects of the substituents are smaller on the neutral ground state of the molecules (6.80 and 0.50 kcal mol −1 for methyl and silyl groups, respectively) at the same level of theory. The observed low ionization energy of the β-silyl-substituted sulphides must be attributed to differences in the stabilization of the neutral ground states of the silyl- and alkyl-substituted derivatives rather than to the stabilization of the radical cationic ground state in the case of compounds of the type R 3SiCH 2SH.

Correlations of lone pair ionization energies with proton affinities of amino acids and related compounds. Site specificity of protonation

The gas phase proton affinities of amino acids are compared with their adiabatic ionization energies obtained from photoelectron spectra. Using primary aliphatic amines as reference species, a linear correlation is found between the proton affinities and the adiabatic nitrogen lone pair ionization energies for those amino acids which protonate on the amine group, even in cases where the nitrogen lone pair is not the highest occupied molecular orbital. Many of the amino acids fit the correlation well, which confirms the prediction of amine protonation from earlier studies and also corroborates the assignment of the bands in the complex photoelectron spectra of these species. Proline and safcosine, amino acids with a secondary nitrogen, deviate from this correlation and instead fit a correlation using secondary aliphatic amines as reference species. Deviations from the correlation exist for molecules, such as lysine, methionine and tryptophan, which contain an intramolecular hydrogen bond between the basic side-chain and the amine site. The gas phase proton affinities of these species are larger than predicted by the correlation. Deviations from the correlation are also predicted for very basic amino acids, such as histidine and arginine, which protonate preferentially on the side-chain instead of the amine group.

Through-bond interactions in the lone-pair ionization of p-benzoquinone

The photoelectron spectrum of p-benzoquinone for the most loosely bound electrons is calculated using the CASSCF and related methods with localization of the hole state. The results are in satisfactory agreement with the experiments and support a through-bond interaction model for the splitting of the lone-pair ionization energies where the two major contributions have different signs and tend to cancel.

Electronic structures of RN(PX 2) 2 (RCH 3 or C 6H 5; XCl, OCH 3 or OC 6H 5) ligands: an ultraviolet photoelectron spectroscopic study

Vapour phase HeI photoelectron spectra of the diphosphazane ligands RN (PX 2) 2 (RCH 3 or C 6H 5, XCl, OCH 3 or OC 6H 5) are presented along with the assignments of bands on the basis of molecular orbital calculations. The spectra are in accordance with the structural studies which indicate multiple bonding in the PNP segment. The phosphorus lone pair ionization energy appears to follow the σ donor ability of the ligands.

Experimental electronic structures of sulfur dioxide complexes: an electron spectroscopic study

He I photoelectron spectra of gas-phase complexes formed by SO{sub 2} with the electron donors trimethylamine, triethylamine, diethyl ether, and diethyl sulfide have been recorded and bands assigned to the different orbitals based on MO calculations. The shift in the ionization energy of the lone pair orbital of the donor in these complexes is shown to vary proportionally with the dissociation energy as well as the magnitude of charge transfer to SO{sub 2}. Electron energy loss spectra of the donor-SO{sub 2} complexes have been recorded to characterize the electronic transitions in the vacuum UV region. Only the amine-SO{sub 2} complex shows a band ascribable to a charge-transfer transition. In order to investigate situations where SO{sub 2} acts as an electron donor, He I spectra of complexes with BF{sub 3} and HCl have been measured. The decrease in the Mulliken population of SO{sub 2} as well as the dissociation energy of these complexes varies parallel to the shift in the lone pair ionization energy of SO{sub 2}.

Charge distribution in icosahedral carboranes: A UV photoelectron spectroscopic study

The UV photoelectron (PE) investigation of dicarba- closo-dodecaboranes 1,12-C 2B 10H 12, 1-SH-1,12-C 2B 10H 11, 2-SH-1,12-C 2B 10H 11, 1-X-1,2-C 2B 10H 11 (X=SH, phenyl), 3-phenyl-1,2-C 2B 10H 11, 4-X-1,2-C 2B 10H 11, 8-X-1,2-C 2B 10H 11, 9-X-1,2-C 2B 10H 11, 9-X-1,7-C 2B 10H 11, (X = SH,CH 3S) and 1-SH-1,7-C 2B 10H 11 is reported. The first two band systems in the PE spectrum of 1, 12-C 2B 10H 11 are assigned to cluster π-bonding orbitals and σBH(CH)-bonding orbitals, respectively. The interaction between exo-substituent and cluster π-orbitals is weak and the sulphur lone pair ionization energy depends simply on the mutual position of substituent and carbon atoms. The electron density at particular vertices in 1,2-C 2B 10H 12 decreases in the order 9 > 8 > 4 > 3 > 1.

Ultraviolet photoelectron spectroscopy of diphenyltrimethylsilyl-amine, -phosphine, -arsine, and -stibine

The He I photoelectron spectra of the Ph 2ESiMe 3 (E = N, P, As, and Sb) derivatives have been assigned with the aid of those of related simpler molecules. The substitution of a trimethylsilyl group for a hydrogen atom in Ph 2EH to give the title compounds results in a significant reduction of the lone pair ionization energy of the fifth group atom when E ≠ N. When E = N, a slight stabilization is observed. This behaviour is probably attributable to the fact that the structure around the nitrogen atom is much further from planarity in the silyl derivative than in Ph 2NH.

An ultraviolet photoelectron spectroscopic study of the electron states of iodine complexes with electron donors

HeI photoelectron spectra of the vapour phase complexes of diethylether and diethylsulphide with iodine have been investigated for the first time. The iodine orbital ionization energy decreases on complexation while the donor lone-pair orbital ionization energy increases markedly; the shifts are considerably larger in the sulphide complex as expected on the basis of enthalpy considerations.

Photoelectron spectra and basicity of tertiary phosphines PMe nPh 3−n

A study of the UV photoelectron spectra of the tertiary phosphines PMe nPh 3−n has been made, and an SCF-Xα-SW calculation for PMe 3 has been carried out. The calculation indicates that the HOMO for PMe 3 is the predominantly non-bonding line pair on phosphorus having 80% P 3 p character and 10% P 3 s character, rather than the predominant 3 s character usually predicted. The lone pair ionization energies are PMe 3, 8.62 eV; PMe 2Ph, 8.32 eV; PMePh 2, 8.28 eV; PPh 3, 7.80 eV, the opposite of the trend predicted from electronegativity consideration. It is argued that the anomalous trend arises from a mixture of steric effects and Ph πP π bonding effects, and that the usual assumption that PMe 3 is the strongest and PPh 3 the weakest Lewis base in this series of phosphines is probably wrong.