On the electronic structure of dihalogenophosphenium cations

The bonding properties of the dihalogenophosphenium cations, PX₂⁽⁺⁾, X = F, Cl, Br, I, are evaluated by relativistic effective core potential calculations with a DZP basis set. Our investigations include an analysis of the singlet-triplet energy separation in the cations and the determination of the relative cation and anion stabilities in the gas phase, via corresponding group transfer reactions. A comparison is made with other stabilized, previously reported low-coordinated π-bonded phosphorus cations.     

Molecular and electronic structure of benzeneselenenyl molecules and cations

Molecular and electronic structures of a select group of molecules and cations, including benzeneselenenyl chloride, benzeneselenenyl bromide, and benzeneselenol have been studied using ab initio calculations at the Hartree–Fock and MP2 levels of theory. Very few experimental data are available for this class of compounds. The properties and structures of these molecules are compared. © John Wiley & Sons, Inc.     

MNDO study of the electronic structure and potential energy surface of fluorinated singlet cyclopentadienyl and methylcyclopentadienyl cations

The electronic structure and geometry of some symmetric fluorinated singlet cyclopentadienyl and methylcyclopentadienyl cations are studied by the MNDO method. The structure of the potential energy surface (PES), which is a pseudorotation surface, is investigated. Extreme points of the PES, determining the PES barrier, correspond to structures with inverted frontier molecular orbitals. (Anti)aromaticity of fluorinated methylcyclopentadienyl cations is estimated using the Dewar-Breslow criterion. Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 6, pp. 1023–1030, November–December, 1996. Translated by L. Smolina     

Ionization energy and electronic structure of polycytidine

Ribonucleic acid (RNA) polycytidine (poly rC) homopolymer thin films were prepared on highly oriented pyrolytic graphite (HOPG) substrates. The films were grown from aqueous solution directly in a vacuum in multiple steps with use of an electrospray (ES) deposition system. Before poly rC deposition and after each deposition step the sample was characterized with X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). Evaluation of the UP-spectra sequence allowed the determination of ionization energy and highest occupied molecular orbital (HOMO) electronic structure, as well as the charge injection barriers between HOPG and poly rC. Comparison with earlier results on polyadenosine (poly rA) indicates significant differences between ionization energies (poly rC: 8.1 eV; poly rA: 6.8 eV) and orbital alignment at the graphite interface. The larger ionization energy of poly rC results in a larger hole injection barrier and a smaller electron injection barrier relative to the HOPG Fermi level.     

X-ray spectra and electronic energy structure of carboaluminides Ti-Al-C

The electronic energy structure, TiL_2,3 X-ray emission, and TiL2j XANES spectra of diamond and titanium carboaluminides Ti-Al-C are calculated by the local coherent potential method in terms of multiple scattering theory. The cluster version of the MT approximation is used to calculate the crystal potential. The electronic energy structures of diamond and the ternary and binary titanium carbide systems are compared in a single approximation. The broadening of the hybridized band in Ti-Al-C is discussed and compared with the experimental CK_α emission spectra. The specifics of chemical binding in the compounds is explained by delocalization of the carbon sp³ configuration and formation of a metal bond in carboaluminides. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 3, pp. 505-514, May–June, 2000.     

The molecular and electronic structure of radical cations derived from tetramethyltin and hexamethylditin: An SCF-MO study

Molecular geometries and energies have been calculated, using the semi-empirical MNDO method for the closed-shell species SnMe₄, Sn₂Me₆, and (SnMe₃)⁺; and using the UHF-MNDO method for the radicals (SnMe₄)⁺, (Sn₂Me₆)⁺ and SnMe₃^.. The radical cation (SnMe₄)⁺ is calculated to have C_3v skeletal symmetry, with a C_2v isomer some 15 kJ mol^?1 higher in energy. The dinuclear radical cation (Sn₂Me₆)⁺ is calculated to be a σ(SnSn) radical, of D_3d skeletal symmetry: although the calculated Sn(5s) spin density is extremely low, the tin atoms are far from planarity. Calculated spin densities are compared with experimental hyperfine couplings.     

The synthesis, characterisation and electronic structure of N-heterocyclic carbene adducts of P(I) cations

Two different syntheses of N-heterocyclic carbene complexes of P(I) cations are presented; the structural features of these phosphamethine cyanine salts, in conjunction with the results of computational investigations, provide insight into the nature of the bonding of these heavy allene analogues.     

Steric and electronic structure of complexes of pyrylium and thiopyrylium cations with borabenzene anion

The coordination of pyrylium and thiopyrylium cations with borabenzene was studied by DFT [B3LYP/6-311++G(d,p)] calculations. The structures of charge-transfer molecular complexes formed by the aromatic ions were predicted. The stabilization is due both to electron density transfer and to covalent bonding.     

The formation and structure of radical cations

ions generated from a number of different presursors have been studied by high kinetie energy ion—molecule reations. It has been shown that at least four distinct stable species oeeur, of which acetonitrile and methyl isoeyanide retain their original structure. With imidazole or pyrazole as precursors, a mixture of open thain radical cations, not identical to the above species and probably interconvertible via the 1H-azirine radocal cation, is formed. From butrynitrile, pyrrole, crotonitrile, allyl interconvertible via the and cyanocyopropane a fourth species, probably the vinylidenimine ion, is formed.     

Copper doping of small gold cluster cations: influence on geometric and electronic structure

The effect of Cu doping on the properties of small gold cluster cations is investigated in a joint experimental and theoretical study. Temperature-dependent Ar tagging of the clusters serves as a structural probe and indicates no significant alteration of the geometry of Au(n) (+) (n = 1-16) upon Cu doping. Experimental cluster-argon bond dissociation energies are derived as a function of cluster size from equilibrium mass spectra and are in the 0.10-0.25 eV range. Near-UV and visible light photodissociation spectroscopy is employed in conjunction with time-dependent density functional theory calculations to study the electronic absorption spectra of Au(4-m)Cu(m) (+) (m = 0, 1, 2) and their Ar complexes in the 2.00-3.30 eV range and to assign their fragmentation pathways. The tetramers Au(4) (+), Au(4) (+)[middle dot]Ar, Au(3)Cu(+), and Au(3)Cu(+)[middle dot]Ar exhibit distinct optical absorption features revealing a pronounced shift of electronic excitations to larger photon energies upon substitution of Au by Cu atoms. The calculated electronic excitation spectra and an analysis of the character of the optical transitions provide detailed insight into the composition-dependent evolution of the electronic structure of the clusters.     

The structure of diphosphine radical cations

When R₂NNR₂ molecules lose an electron to give (R₂NNR₂)^+· radical cations, the whole unit becomes planar, with a(π₁)²(π₂)¹ configuration. However, because R₃P molecules are far more strongly pyramidal than R₃N molecules, this flattening on electron loss is less, and phosphorous centred radical cations do not achieve planarity. This is clearly so for (R₂PPR₂)⁺ centres, whose liquid and solid state spectra analysed herein in terms of two equivalent ³¹P hyperfine couplings, show ca. 9% 3s character. This indicates considerable bending at each phosphorous centre. Furthermore, the form of the spectra, with no x — y splitting of the ‘perpendicular’ lines, suggests that each ³¹P coupling shares a common axis. This means that a trans conformation is required, as expected because this relieves steric strain and favours “π” type orbital overlap.     

Molecular and electronic structure,and stability of organic cations,dications and donor-acceptor complexes

The MNDO method with optimization of geometric parameters has been used to study the fragments of the potential energy surface for the acylium cation MeC⁺, radical dication MeCO^2+., and the (1 n) complexes of general formula MeCOCl ·nAlCl₃ (wheren=1, 2, 3). The stability of these complexes with respect to certain dissociation mechanisms has been studied. The calculations show that the the hitherto unknown RCOCl · 2AlCl₃ and RCOCl · 3AlCl₃ can actually exist. Moreover, it has been shown that there can be complexes where the charge on the strongly electrophilic carbon atom is greater than that of the carbon in the acylium cation. The result of the study provided an explanation for the superacidity of the RCOX · 2AlX₃ systems in reactions with saturated hydrocarbons.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 854–858, May, 1993.     

Molecular structure effects in intramolecular electronic energy transfer

A study of intramolecular energy transfer (intra-ET) in a series of bichromophoric molecules composed of cyclic α-diketones and para substituted benzene ring is reported. Preliminary results show that the transfer efficiency is strongly structure dependent suggesting that Dexter-type exchange interaction is responsible for intra-ET between close chromophores in a bichromophoric molecule.     

The unimolecular dissociation of electronic state-selected methyl iodide cations

Within the context of a recently proposed model, the angle-bending Schrödinger equation for linear molecules is cast in the form of the Legendre equation with an added potential. This is solved by a complete set expansion in terms of associated Legendre functions. The resulting solutions are incorporated into the existing model. Computation of the lower vibrational excitations of HCN is carried out.     

DVM Xα calculations on the electronic structure of “superalkali” cations

The electronic structure of the radicals Li₂F, Li₃O, Li₄N, Li₂Cl, Li₃S, Li₄P, Na₂F, Na₃O, Na₄P, Na₂Cl, Na₃S, Na₄P, Cs₂F, Cs₂Cl and NH₄ were calculated by the discrete variational Xα method. In all cases the electron affinity of the cation is less than that of the ligand alkali cations, thus giving rise to the name “superalkali”.     

The resonance energy of amides and their radical cations

Structural Chemistry - The resonance energy of an amide can be calculated through comparison with a model amine and a model ketone (or aldehyde) with subtraction of the “residual”...     

The electronic structures of aminopyridines and their mono- and di-protonated cations

Using a semi-empirical ASMO SCF method for valence electron systems previously proposed by the present authors, the electronic structures of aminopyridines and their mono- and di-protonated cations are calculated. It is explained that the mono-protonation occurs on the ring nitrogen atoms and the di-protonation takes place on the nitrogen atoms both in the ring and the amino group. The electron migration to the added protons are calculated to be about 0.7 e for all the cations treated. The calculated results for n – ^* and – ^* singlet transitions of aminopyridines and their protonated cations show fairly good agreement with the experimental results. The electronic structures of pyridone imines, the tautomeric isomers of aminopyridines, are also briefly discussed.

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Zusammenfassung Mittels eines semiempirischen ASMO SCF-Verfahrens wird die Elektronenstruktur von Aminopyridinen und ihrer mono- und diprotonierten Kationen berechnet. Die erste Protonierung findet am N-Atom im Ring und die zweite an der Aminogruppe statt. Für die Elektronenverschiebung zu den angelagerten Protonen ergibt sich in allen Fällen etwa 0,7 e. Die berechneten n - ^*- und - ^*- Singulettübergänge zeigen befriedigende Übereinstimmung mit dem Experiment. Schließlich wird noch die Elektronenstruktur von Pyridoniminen diskutiert.

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Résumé Calcul des structures électroniques des aminopyridines et de leurs cations mono- et di-protonés, en utilisant une méthode semi-empirique ASMO SCF pour les électrons de valence proposée précédemment par les auteurs. On explique la mono-protonation sur les atomes d'azote du cycle et la di-protonation sur les atomes d'azote du cycle et du groupe aminé. Le transfert d'électron sur les protons supplémentaires est de l'ordre de 0,7 e pour tous les cations étudiés. Les résultats calculés pour les transitions singulets n ^* et ^* des aminopyridines et de leurs cations sont en bon accord avec l'expérience. Les structures électroniques des pyridone imines, tautomères des aminopyridines, sont brièvement discutées.

     

The electronic spectrum and electronic structure of 1,5 dinitronaphthialene

The electronic absorption spectra of the n-heptane and ethanol solutions and the polarized absorption spectrum of the single crystal were measured with 1,5-dinitronaphthalene. 1,5-Dinitronaphthalene shows three absorption bands at 198.7, 230 and 323 m. The 230 m and 323 m. bands are polarized almost parallel with the long and short axes of the naphthalene ring respectively. The assignment of the bands has been made by combining the observed results with the theoretical consideration.

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Zusammenfassung Das UV-Absorptionsspektrum von 1,5-Dinitronaphthalin in Lösung von n-Heptan und Äthanol und das Polarisationsabsorptionsspektrum des Einkristalls wurde aufgenommen. Es zeigt drei Banden bei 198,7, 230 und 323 m, wobei die Polarisationsrichtungen der beiden letzteren nahezu parallel zur langen und kurzen Achse des Naphthalinringes liegen. Die Zuordnung der Banden wurde auf Grund einer Kombination experimenteller und theoretischer Gegebenheiten vorgenommen.

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Résumé Le spectre électronique d'absorption du 1,5-dinitronaphthalène a été mesuré dans l'éthanol, le n-heptane et dans le monocristal. Des bandes à 198,7, 230 et 323 m la deuxième (troisième) est polarisée presque parallèle à l'axe longue (courte) du noyau naphthalénique. La comparison aux résultats théoriques permet l'interprétation des bandes.