He(I) and He(II) photoelectron spectra of 1-aza-1,3-butadiene-tricarbonyliron complexes: [Fe(CO)3(R1NCHCHCHR2)]

He(I) and He(II) photoelectron spectra are reported for the 1-aza-1,3-butadienes (R¹NCHCHCHR² denoted by R¹,R²-ABD) t-Bu,Me-ABD and i-Pr,Ph-ABD and their tricarbonyliron complexes [Fe(CO)₃(R¹,R²-ABD)]. Assignments of ionizations from the iron d and ligand orbitals have been made with the aid of He(I)/He(II) intensity ratios and some semi-empirical molecular orbital calculations on the model ligand Me,H-ABD (MNDO) and on the model complex [Fe(CO)₃(H,H-ABD)] (CNDO/S).A remarkable feature is the lowering of the ionization energy from the Fe d_xz/yz² orbital with respect to the other d orbitals (d_xy/d_x²–y²/d_z²)⁶ by about 0.9 eV, an effect which has not been found for the related [Fe(CO)₃(1,3-butadiene)] complexes. The involvement of the nitrogen lone pair in the bonding between the R¹,R²-ABD and Fe(CO)₃ moieties is discussed.     

He I photoelectron spectroscopic study of closo-dicarbadodecaboranes and their 9-halogeno derivatives

He I photoelectron spectra of the closo-carboranes 1,2-C₂B₁₀H₁₂ and 1,7-C₂B₁₀H₁₂, and their 9-halogeno derivatives are reported. The low ionization energy bands are gathered into two groups. The MNDO quantum chemical method provides the same picture and indicates that these bands correspond predominantly to the BH (CH) bonding molecular orbitals. Bromine and iodine bring about an almost uniform increase in ionization energies whereas the shape of the low ionization energy features of the chloro derivatives differ from those of the parent compounds. This effect is interpreted by the conjugative interaction of cluster π-type orbitals with the chlorine non-bonding orbitals.     

UV photoelectron spectra of some transition metal(II) acetylacetonates

The He(I) photoelectron spectra of acetylacetone (HAA) and its metallo complexes, M(II)(AA)₂ (M(II) = Mn, Co, Ni, Cu and Zn), have been measured. These spectra show characteristic metal-dependence, from which the assignment is made. The order of the orbital energy level, d > π₃ > n_? > n₊, holds for all the complexes reported here. The splitting of these orbitals is found to depend on the central metal ion specifically.     

Ultraviolet photoelectron spectroscopic study of MS σ bonding energy and of spiroconjugation effects in group IVA thiospiranes

He(I) and He(II) ultraviolet photoelectron spectra of tetrathiometallaspiranes

(M = C, Si, Ge, Sn) yield data for quantitative characterization of several structural effects. Spiroconjugation of the lone pair orbitals of the sulfur atoms is indicated by the splitting patterns of the n_s ionization bands in the low IE region (8–9_eV); quantitative evaluation confirms that there is a marked decrease of the spiroconjugation effects with increasing size of M. Energy values are assigned by UP spectral analysis to MS σ bond orbitals which show a small variation with M between ca. 10 and 11 eV, consistent with the small electronegativity variation, and to CS bond orbitals in the spirane rings, fairly constant around 13.5 eV. Ionizations of the quasi-valence 4d orbitals in the Sn derivative are identified at 34.38 and 35.42 eV, and suggest, by comparison with other known Sn compounds, a considerably high overall electronegativity of the ligands in the present compound.     

Non-bonded interactions in 2,2,4,4-tetramethyl-1,3-cyclobutanedithione and 2,2,4,4-tetramethyl-3-thio-1,3-cyclobutanedione

Electronic absorption and emission spectra as well as He(I) photoelectron spectra of 2,2,4,4-tetramethyl-1,3-cyclobutanedithione and 2,2,4,4-tetramethyl-1-3-thio-1,3-cyclobutanedione have been interpreted on the basis of molecular orbital calculations. The results show that the non-bonded orbital of the dithione is split owing to through-bond interaction, the magnitude of splitting being 0.4 eV. The π* orbital of the dithione appears to be split by about 0.2 eV. Electronic absorption spectra show evidence for the existence of four n—π* transitions, arising out of the splitting of the orbitals referred to above, just as in the case of 2,2,4,4-tetramethyl-1,3-cyclobutanedione. Electronic and photoelectron spectra of the thio-dione show evidence for weak interaction between the CS and C&.zdbnd;O groups, probably via π* orbitals. Infrared spectra of both the dithione and the thio-dione are consistent with the planar cyclobutane ring; the ring-puckering frequency responsible for non-bonded interactions is around 67 cm^?1 in both the dithione and the thio-dione, the value not being very different from that in the dione. The 1,3-transannular distance is also similar in the three molecules.     

The electronic structure of flavin derivatives: Part I. Ab initio calculations for 1H-alloxazine and 10H-isoalloxazine,their reduced derivatives and related compounds; assignments of photoelectron spectra

Ab initio calculations with a (7s3p) basis set are performed on uracil, lumazine, isoalloxazine, its 10-methyl derivative and 1,5-dihydroisoalloxazine. He(I) and He(II) photoelectron spectra of methylated derivatives of these compounds are reported.Comparison of photoelectron spectra with ab initio calculations leads to a new assignment of the uracil photoelectron spectrum. The photoelectron spectra of lumazine and the (iso)-alloxazines are analyzed with the aid of earlier spectra of pyrazine, quinoxaline, o-phenylenediamine and the results for uracil. Combined with the ab initio results the analysis leads to a consistent assignment of the photoelectron spectra.     

Photoelectron spectra of the gauche and trans conformers of 1,2-dibromoethane

The He I photoelectron spectra of the gauche and trans conformers of 1,2-dibromo-ethane have been obtained. Time-averaged spectra of 1,2-dibromoethane recorded at different temperatures have been analyzed by a spectrum-stripping procedure to give resulting spectra which represent each of the two stable conformers. The correlation of the ionization potentials of the two conformers is consistent with data obtained from MO calculations. In comparison with the case of 1,2-dichloroethane, 1,2-dibromoethane shows different non-bonded Br—Br interactions and generally larger resonance splittings of the ethane orbitals.     

Photoelectron spectra of potassium salts of hydrotris(pyrazol-1-yl)borates: electronic structure of the electron withdrawing scorpionates Tp(CF3)2, Tp*Cl and comparison to Tp* and Tp

The electronic structures of the potassium salts of the homoscorpionates hydrotris(3,5-dimethylpyrazol-1-yl)borate (Tp*, 1), hydrotris(4-chloro-3,5-dimethylpyrazol-1-yl)borate (Tp*^Cl, 2) and hydrotris(3,5-bis(trifluoromethyl)pyrazol-1-yl)borate (Tp^(CF₃)₂, 3) are compared using gas-phase photoelectron spectroscopy and density functional theory (DFT). DFT calculations also are reported for the generic scorpionate potassium (hydrotris(pyrazol-1-yl)borate) (KTp). This is the first such experimental probe of the electronic structure of halogen containing scorpionate ligands and subtle differences in the ionizations from the frontier orbitals in the photoelectron spectra of 1 and 3 are observed that give insight into the influence of substituents upon metal–scorpionate bonding. Distinct assignments of the ionizations from the nitrogen σ-donor orbitals (σ_N) and σ_BH molecular orbitals are possible experimentally by the use of variable (He I and He II) excitation energies. The experimentally observed first ionization energy of 3 is stabilized by ∼2.0 eV relative to 1 due to the strong electron withdrawing effect of the trifluoromethyl groups. The photoelectron spectroscopic studies of NaTp^(CF₃)₂ further confirm the assignments of ionizations from σ_N orbitals for 3 associated with the a and e sets in C₃ symmetry. The X-ray crystal structure of 2 as the (μ-aqua)₃(potassium hydrotris(4-chloro-3,5-dimethylpyrazol-1-yl)borate)₂ dimer is also reported.     

Electronic structure of UC and UN and its comparison with photoemission spectra

Molecular orbital calculations for UC and UN are carried out by the Xα discreet variational method, considering octahedral UC₆²⁰⁻ and UN₆¹⁵⁻ clusters. The main features of photoemission spectra are well described by the calculations. Whereas the highest occupied levels are composed mainly of 5f U orbitals, the levels, responsible for chemical bonding, contain important admixtures of 5f, 6d U orbitals with 2p (C(N) orbitals.     

The electronic structure of some heterocycles with bridgehead nitrogen: photoelectron spectra and ab initio molecular orbital calculations

He(I) and He(II) photoelectron spectra are reported for the cycl[3,3,3]azine (1), cycl[3,2,2]azine (2), indolizine (6) and imidazo[1,2-a] pyridine (7), as well as He(I) spectra for related compounds (3–5). Ab initio molecular orbital calculations have been used to assign the spectra of 1, 2, 3, 6 and 7, and to give information about the nature of the π-electron energy levels. The first IP for 1 is singularly low (5.86 eV), and this has been interpreted in terms of occupancy of the 1a₁'' orbital which is normally vacant in related compounds. In the cyclazines, the nitrogen lone pair seems to be split into two π-levels.     

Photoelectron He(I) Spectra of small silanes

The photoelectron He(I) spectra of methyl-, dimethyl-, trimethyl- and ethylsiliane are reported and assigned with the help of CNDO/2 calculations.For each silane the ordering of the valence MO's obtained from calculations is, very close to that of the corresponding alkane, and is in agreement with spectral evidence. Participation of silicon d orbitals in bonding is not substantial but is more important in the outermost SiC and SiH MO's than in the π MO's mainly localized on the alkyl groups.     

Uv photoelectron spectra of (cyclopentadienyl)-(cycloheptatrienyl)-zirconium, -niobium and -molybdenum

He(I) photoelectron spectra of the compounds (C₅H₅)M(C₇H₇) (M = Zr, Nb, Mo) are reported. The results are compared with those for the corresponding 3d transition metal compounds and discussed in terms of qualitative MO considerations.     

The Electronic Structure of Cubane (C8H8) as Revealed by Photoelectron Spectroscopy

High resolution He (Iα) and He (IIα) photoelectron spectra of cubane are reported. The assignments of the bands to different states of the cubane radical cation are made on the basis of ab initio STO-3G and MINDO/3 calculations, using geometries optimized within each treatment. The vibrational fine-structure observed supports the proposed assignment. An open shell MINDO/3 model for ground state cubane radical cation suggests that the Jahn-Teller distorted system fluctuates between twelve equivalent structures of C_2v-symmetry. Localized molecular orbitals derived from the STO-3G model of cubane indicate that the major feature which discriminates this molecule with respect to other hydrocarbons is the large interaction matrix element between the opposed CC-σ-orbitals of each face.     

The He(Iα) Photoelectron Spectra of Substituted 1,2-Dithietes

The He(Iα) photoelectron (PE.) spectra of a series of substituted 1,2-dithietes have been recorded and assigned with respect to the orbital sequence derived from an STO-3G model calculation and by correlation with the PE. spectra of related compounds. The results provide additional support for the presence of a closed, four-membered ring moiety in all the 1,2-dithietes investigated. In all cases the two highest occupied molecular orbitals are b₂(π)= HOMO, a₂(π), with exception of 3,4-bis(trifluoromethyl)-1,2-dithiete where the sequence b₂(π), a₂(π) or a₂(π), b₂(π) is uncertain.     

Photoelectron spectroscopy study of the triphenyl derivatives of the group IV elements

The photoelectron He(I) spectra of several phenyl derivatives of the Group IV elements are reported, and the first few bands are assigned to the corresponding MO's. A sizable interaction among the π orbitals of the rings has been found for the carbon derivatives HCPh₃, HC(mesityl)₃ and H₂CPh₂, but not for the Si, Ge and Sn triphenyl derivatives. The exceptional behaviour of the carbon compounds has been attributed to the short central atom—ring distance, by analogy with findings for the Group V triphenyl derivatives.The charge transfer from the rings towards the Si atom is substantially reduced compared with that in H₃SiPh.     

The electronic structure of aromatic molecules: ab inito molecular orbital studies and photoelectron spectra for the aza-derivatives of indole,benzofuran and benzothiophen

The He(I) photoelectron spectra for the aza-derivatives of benzofuran (4 and 7), benzothiophen (8) and indole (3a, 3b, 6a, 9b), and combined He(I) and He(II) spectra of the 1,2-benzisothiazole (5), 1,2,3-benzothiadiazole (11) and benzotriazole (9a) have been obtained and assigned by a combination of heteroatom substituent effects and ab initio molecular orbital calculations. The variations in lone pair levels (LP_NLP_O, LP_S) and π-levels, between these and the monocyclic compounds are discussed.     

Electronic structure,UV spectra and reactivity of non-benzenoid systems by the INDO/S-CI method: Part II. Methylenecyclopropene and its derivatives

The electronic spectra, frontier molecular orbitals, charge distribution and dipole moments of nine compounds containing the cyclopropenyl ring have been studied by the all-valence-electrons INDO/S-CI method. The theoretical results agree fairly well with the experimental indications, where available. The σ-contribution to the net charge density on the ring as well as the μ_σ and s—p polarization contributions to the dipole moments are higher than in the fulvene derivatives, indicating greater participation of the smaller-ring system. The UV spectra of compounds containing two coupled rings correlate with those of the component frameworks, in agreement with previous semi-empirical calculations.     

UV photoelectron and ab initio quantum mechanical characterization of nucleotides: The valence electronic structure of anionic 2′-deoxyadenosine-5′-phosphate

He(I) ultraviolet (UV) photoelectron spectroscopy and ab initio, self-consistent field (SCF) calculations with the 6-31G basis set have been employed to characterize the valence electronic structures of anionic 2′-deoxyadenosine-5′-phosphate (5′-dAMP⁻). Theoretical ionization potentials (IPs) of 5'-dAMP^-, of the neutral model compounds 9-methyladenine (9-MeA) and 3-hydroxytetrahydrofuran (3-OH-THF), and of the model anion CH₃HPO₄⁻ have been obtained by applying Koopmans' theorem to ab initio SCF results. The ionization potentials predicted from the SCF calculations have been compared to He(I) photoelectron spectra of 9-MeA and 3-OH-THF. The SCF calculations predict a value (8.45 eV), for the highest occupied π orbital in 9-MeA which agrees well with the experimental vertical IP (8.39 eV). However, IPs for the highest occupied lone-pair orbitals in 3-OH-THF are predicted to be more than 1.52 eV higher than the experimental IPs. Results from recently reported [H. S. Kim and P. R. LeBreton, Proc. Natl. Sci. USA 91, 3725–3729 (1994), and N. S. Kim and P. R. LeBreton, J. Am. Chem. Soc., 118, 3694 (1996)] second-order Møller-Plesset perturbation (MP2) calculations and configuration interaction calculations using the configuration interaction singles (CIS) method indicate that configuration interaction effects strongly influence the energies of the first five ionization events arising from removal of electrons from the closed-shell model anion CH₃HPO₄⁻. Results from the 6-31G SCF calculations of 5′-dAMP⁻, 9-MeA, 3-OH-THF, and CH₃HPO₄⁻ indicates that valence orbital electron distributions in the nucleotide and in the model compounds and anion are similar. The correspondence between the orbital structure of the nucleotide, and the model compounds and anion makes it possible, employing experimental photoelectron data and MP2/CIS computational results for the model compounds and anion, to individually correct IPs calculated for the nucleotide at the 6-31G SCF level. Here, this approach has provided values for the 13 lowest IPs of 5′-dAMP⁻ and indicates that the first IPs of the base, sugar, and phosphate groups are 6.1, 7.8, and 5.5 eV, respectively. © 1996 John Wiley & Sons, Inc.     

Allylic interactions in organometallics: probing Electronic structure in (η5-C5H5)Fe(CO)2R,R = CH3, η1-C3H5, η1-C5H5.

The He(I) photoelectron spectra of (η⁵-C₅H₅)Fe(CO)₂R, where R = CH₃, η¹-C₃H₅ and η¹-C₅H₅, have been recorded. The lowest lying ion states result from ionization of molecular orbitals with large Fe 3d character; these move to lower anergy when R places double bonds in an allylic relationship to the metal atom. The cyclic voltammetric oxidation potential correlates well with the energies of the lowest ion states. A significant interaction between olefin π orbitals and the allylic metal center is proposed.     

LiPN<Subscript>2</Subscript> and NaPN<Subscript>2</Subscript> crystals: Structural features and chemical bonding

The band structure spectra, densities of states, and valence and difference densities of LiPN₂ and NaPN₂ crystals were obtained by DFT self-consistent calculations using the nonlocal pseudopotentials and the localized pseudoorbital basis. Crystal-chemical analysis of these compounds shows that they occupy an intermediate position between the ideal structures of β-cristobalite and chalcopyrite, which manifests itself in the peculiarities of the electronic structure and chemical bonding. The valence band consists of three allowed subbands and differs radically from the typical valence band of chalcopyrite crystals in both subband structure and contributions of the s, p, and d atomic orbitals to the crystal orbitals.