The ground state of MoCr(O2CH)4 at theab initio SCF and CI levels. A symmetry adapted RHF energy functional with an artificial double minimum

Ab initio restricted Hartree-Fock (RHF) calculations carried out on the ground state of MoCr(O₂CH)₄ lead to two distinct energy minima according to the initial guess made for the set of trial vectors. It is shown that these two symmetry-adapted wavefunctions can be correlated with a twofold degenerate broken-symmetry solution previously characterized for the related system of higher symmetry Cr₂(O₂CH)₄. Complete CI expansions have been carried out from either RHF polarized wavefunction using as a basis the set of eight frontier MO's with high metal character. These expansions yield poorly resymmetrized wavefunctions. A similar CI expansion has finally been carried out from a wavefunction resymmetrized at the SCF level and corresponding to a saddle point of the RHF energy hypersurface. The total energy associated with this latter expansion is the lowest obtained in the present work. The natural orbital analysis corresponds to ()^1.86()^3.58()^1.54()^0.46()^0.42 (^*)^0.14 and shows that this resymmetrized CI expansion is in many respects similar to the correlated wavefunctions obtained for the homobinuclear parent systems.     

Femtosecond dynamics of relaxation of photoexcited meso-tetraferrocenylporphyrin in the nonprotonated and diprotonated forms (Fc4PH2 and Fc4PH42+)

The relaxation of the Q¹(—*) excited state of the nonprotonated Fc₄PH₂ and diprotonated Fc₄PH₄ ²⁺ forms of meso-tetraferrocenylporphyrin was studied by femtosecond laser absorption spectroscopy. Transition from the Q¹(—*) state to the charge-transfer state was shown to occur within 208±10 fs for Fc₄PH₂ and 9±3 ps for Fc₄PH₄ ²⁺. A fast vibrational relaxation with a characteristic time of 120—140 fs was found for both forms. The relaxation time of Fc⁺—P^– charge-transfer state for Fc₄PH₂ was 17±4 ps.     

An extension of the CNDO/2 formalism for the study of transition metal complexes

A previously described extended CNDO/2 method is used for investigating the energy level distribution and electronic structure of trifluorophosphine metal complexes: Cr(PF₃)₆, Fe(PF₃)₅, Ni(PF₃)₄. The results are compared with the few experimental data which are known for these complexes. The metal-phosphorus bonds show large (P M) and (M P) charge transfers but small total charge transfers (M P) which induce on the metal in any case a small positive charge.Such ( + ) coordination bonds seem to be generally characterized by small bond overlap populations (or small Wiberg indices).     

On the crystal chemistry of three copper(II)-arsenates: Cu3(AsO4)2-III,Na4Cu(AsO4)2, and KCu4(AsO4)3

The three copper(II)-arsenates were synthesized under hydrothermal conditions; their crystal structures were determined by single-crystal X-ray diffraction methods:Cu₃(AsO₄)₂-III:a=5.046(2) Å,b=5.417(2) Å,c=6.354(2) Å, =70.61(2)°, =86.52(2)°, =68.43(2)°,Z=1, space group ,R=0.035 for 1674 reflections with sin / 0.90 Å^–1.Na₄Cu(AsO₄)₂:a=4.882(2) Å,b=5.870(2) Å,c=6.958(3) Å, =98.51(2)°, =90.76(2)°, =105.97(2)°,Z=1, space group ,R=0.028 for 2157 reflections with sin / 0.90 Å^–1.KCu₄(AsO₄)₃:a=12.234(5) Å,b=12.438(5) Å,c=7.307(3) Å, =118.17(2)°,Z=4, space group C2/c,R=0.029 for 1896 reflections with sin / 0.80 Å^–1.Within these three compounds the Cu atoms are square planar [4], tetragonal pyramidal [4+1], and tetragonal bipyramidal [4+2] coordinated by O atoms; an exception is the Cu(2)^[4+1] atom in Cu₃(AsO₄)₂-III: the coordination polyhedron is a representative for the transition from a tetragonal pyramid towards a trigonal bipyramid. In KCu₄(AsO₄)₃ the Cu(1)^[4]O₄ square and the As(1)O₄ tetrahedron share a common O—O edge of 2.428(5) Å, resulting in distortions of both the CuO₄ square and the AsO₄ tetrahedron. The two Na atoms in Na₄Cu(AsO₄)₂ are [6] coordinated, the K atom in KCu₄(AsO₄)₃ is [8] coordinated by O atoms.Die drei Kupfer(II)-Arsenate wurden unter Hydrothermalbedingungen gezüchtet und ihre Kristallstrukturen mittels Einkristall-Röntgenbeugungsmethoden ermittelt:Cu₃(AsO₄)₂-III:a = 5.046(2) Å,b = 5.417(2) Å,c = 6.354(2) Å, = 70.61 (2)°, = 86.52(2)°, = 68.43(2)°,Z = 1, Raumgruppe ,R = 0.035 für 1674 Reflexe mit sin / 0.90 Å^–1.Na₄Cu(AsO₄)₂:a = 4.882(2) Å,b = 5.870(2) Å,c = 6.958(3) Å, = 98.51(2)°, = 90.76(2)°, = 105.97(2)°,Z = 1, Raumgruppe ,R = 0.028 für 2157 Reflexe mit sin / 0.90 Å^–1.KCu₄(AsO₄)₃:a = 12.234(5) Å,b = 12.438(5) Å,c = 7.307(3) Å, = 118.17(2)°,Z = 4, Raumgruppe C2/c,R = 0.029 für 1896 Reflexe mit sin / 0.80 Å^–1.Die Cu-Atome in diesen drei Verbindungen sind durch O-Atome quadratisch planar [4], tetragonal pyramidal [4 + 1] und tetragonal dipyramidal [4 + 2]-koordiniert; eine Ausnahme ist das Cu(2)^{[4 + 1]}-Atom in Cu₃(AsO₄)₂-III: Das Koordinationspolyeder stellt einen Vertreter des Übergangs von einer tetragonalen Pyramide zu einer trigonalen Dipyramide dar. In KCu₄(AsO₄)₃ haben das Cu(1)^[4]O₄-Quadrat und das As(1)O₄-Tetraeder eine gemeinsame O—O-Kante von 2.428(5) Å, was eine Verzerrung der beiden Koordinationsfiguren CuO₄-Quadrat und AsO₄-Tetraeder bedingt. Die zwei Na-Atome in Na₄Cu(AsO₄)₃ sind durch O-Atome [6]-koordiniert, das K-Atom in KCu₄(AsO₄)₃ ist [8]-koordiniert.

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Zur Kristallchemie dreier Kupfer (II)-Arsenate: Cu₃(AsO₄)₂-III, Na₄Cu(AsO₄)₂ und KCu₄(AsO₄)₃

     

The Effect of cis-trans Isomerism on the Electronic-vibrational Structure of the Ground and Excited States and Reactivity of 2-(2-Furyl)- and 2-(2-Thienyl)oxazole

The fluorescent properties, structure, and electronic structure of the ground and excited singlet and triplet electronic states of the cis and trans forms of 4,5-dihydro-2-(2-furyl)oxazole, 4,4-dihydro-2-(2-thienyl)oxazole, 2-(2-furyl)oxazole (FO), and 2-(2-thienyl)oxazole (TO) have been studied. The orbital nature of the lower excited singlet and triplet states has been studied by the semiempirical INDO/S (valence approximation) and PPP/S ( approximation) methods. It was shown that for FO and TO molecules the lower triplet state is of the * type, for which delocalization of the electronic excitation on atoms is characteristic. In the singlet excitation state inversion was observed of the energy levels of the delocalized * states and n* states localized over several bonds (for the free TO and FO molecules the lower excited singlet states S₁* were assigned to * and n* types respectively). Owing to the low position of the T * and T _n* levels relative to the singlet level of * type, the rate constant for intercombination conversion is greater than the rate constant for radiative decay. Consequently an efficient population of the triplet states of the molecules occurs under conditions of electronic-vibrational excitation. The direction of reactions during synthesis was compared with the localization indices in the ground state for electrophilic, nucleophilic, and radical substitution, and also with the excitation localization numbers L for a wide selection of electronically excited states. It was concluded that the change in the structure of the azole molecule on replacing an O atom by an S atom, or on changing from a partially hydrogenated to a heteroaromatic system, was the main reason for the change of all the spectral parameters characterizing the electronic-vibrational or the spin-orbital interaction of the most reactive groups of atoms in the molecular structure.     

On the modulation of electron energy distribution function in radiofrequency SiH4, SiH4−H2 bulk plasmas

Electron energy distribution functions (EDF) in SiH₄, SiH₄–H₂ radiofrequency discharges have been calculated by solving the time-dependent Boltzmann equation in the presence of a sinusoidal field. Particular emphasis is given to the modulation of EDF as a function of the applied frequency (·10⁶/p ₀ ·10⁸ sec^–1 torr^–1) and of gas composition. The results show that at /p ₀ = ·10⁶ sec^–1 torr^–1 EDF follows in a quasistationary mode the variation of the field with the exception of a small range of electric field near to the zero crossing. Still, at the higher considered frequency (/p ₀ =·10⁸ sec^–1 torr^–1), we observe some modulation of EDF. The necessity of using a time-dependent approach is tested by comparing the present results with the corresponding ones obtained by using the effective field approximation (i.e., the approximation which solves instead of the time-dependent Boltzmann equation the corresponding stationary one at the effective values of the rf field). The two sets of results can differ by orders of magnitude in the tail of EDF, the differences decreasing with increasing molar fraction of H₂ and increasing field frequency. The role of excited states (second-kind collisions) is studied by inserting in the Boltzmann equation given concentrations of vibrational and electronic states. The results show that second-kind collisions strongly affect EDF especially in pure silane. Finally a satisfactory agreement has been found between theoretical and experimental results concerning the modulation of electrons of given energy in pure silane discharges.     

Electronic interaction between the nitrogen,oxygen, and silicon atoms in the molecules of α-aziridin-1-ylalkoxy(triethyl)silanes

By the PMR method we have established the existence of a high frequency of the inversion of the nitrogen atom in the molecules of -aziridin-1-ylalkoxysilanes. This is due to the fact that the high (because of p-d bonding with the silicon atom) electronegativity of the oxygen atom in the Si-O-C-N system makes possible an interaction between the unshared electron pair of the nitrogen atom and the antibonding orbital of the C-O bond. The latter, in its turn, increases the degree of p-d bonding between the oxygen and silicon atoms (in these compounds the order of the Si-O bond is greater than in the alkoxysilanes).     

Metal complexes of allyl derivatives of C60 fullerene: molecular and electronic structure prediction from density functional calculations

The problem of existence of ³—-complexes of C₆₀ fullerene with transition metal atoms is discussed. The complexes C₆₀R₃Co(CO)₃ (R = H, F, Cl, Br), C₆₀H₃NiCp, and C₆₀H₃Fe(CO)Cp, where C₆₀R₃ is an allyl derivative of C₆₀ fullerene, were shown to be sufficiently stable. In these complexes the metal atoms are ³—-bound to the fullerene cage. In contrast to this, the metal atoms in the C₆₀H₃Li and C₆₀H₃FeCp complexes are ⁵—-coordinated to the carbon cage. Density functional calculations were carried out with the Perdew—Burke—Ernzerhof exchange-correlation potential (PBE). It was concluded that the type of bonding in the complexes of allyl derivatives of C₆₀ fullerene depends on the nature of the species attached. Among the systems studied, the maximum energy of the ³—-bond was obtained for the C₆₀H₃NiCp complex. The results obtained can be useful in the design of synthesis of new fullerene derivatives with the ³—-coordination of the transition metal atoms to the carbon cage.     

Spectroscopic investigation and thermal behavior of new carbonyl complexes [M(CO)4(N—N)(CuX)], (M = W,Mo; N—N = 2,2′-bipyridine, 1,10-phenanthroline; X = Cl,SCN)

[M(CO)₄(N—N)] reacts with CuCl to give new heterobimetallic metal carbonyls of the type [M(CO)₄(N—N)(CuCl)], M = W, Mo; N—N = 2,2-bipyridine (bipy), 1,10-phenanthroline (phen). Reactions of [M(CO)₄(N—N)(CuCl)] with NaSCN produced the series of complexes of general formula [M(CO)₄(N—N)(CuSCN)]. The i.r. spectral of all the bimetallic carbonyls exhibited the general four (CO) band patterns of the precursors. The u.v.–vis. spectral data for precursors and products showed bands associated with * (nitrogen ligands), dd (intrametal), as well as MLCT d* (nitrogen ligands) and MLCT d *(CO) transitions. The [M(CO)₄(N—N)(CuX)] (X = Cl, SCN) emission spectra showed only one band associated with the MLCT transition. The t.g. curves revealed a stepwise loss of CO groups. The initial decomposition temperatures of the [M(CO)₄(N—N)(CuX)] series suggest that the bimetallic compounds are indeed thermally less stable than their precursors, and the X-ray data showed the formation of MO₃, CuMO₄, Cu₂O and CuO as final decomposition products, M = W, Mo. The spectroscopic data suggests that the heterobimetallic compounds are polymeric.     

Organometallic complexes containing thiosemicarbazone and related ligands. Part II(1). η3-allyl dicarbonyl complexes of molybdenum(II) with attached thiosemicarbazone ligands

Summary The -allylmolybdenum(II) complexes [MoX(CO)₂-(NCMe)₂(³-C₃H₄R)] (X=Cl, Br and I; R=H and 2-Me) react either in dichloromethane or acetonitrile with thiosemicarbazones to give the new complexes [MoX-(CO)₂(RRCNNHCSNH₂)(³-C₃H₄R)] (R=H or Me; R'=Me, Et, Pr or Ph)via displacement of acetonitrile ligands.     

Charge-transfer complexes of indenophanes with π-acceptors

The charge-transfer (CT) spectra of the -complexes formed by a number of -acceptors with several indenophanes as well as indene as a model compound have been measured in methylene chloride at 20 °C. Association constants and transition energies of these complexes as well as ionization potentials of the -donors have been determined. The data obtained indicate the existence of transannular electronic interactions in the indenophane nucleus. Furthermore, the pseudo-para- andmeta[2.2]indenophane isomers (3 and4) show a large difference in their -base strength. A good linear relationship has been observed between the association constants and _max of the long wavelength CT bands for the -complexes of these -donors with both tetracyanoethylene (TCNE) and 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). All CT complexes studied have a 1:1 stoichiometry.

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Charge-transfer-Komplexe aus Indenophanen und -Akzeptoren

Zusammenfassung Die Charge-transfer-Spektren (CT-Spektren) von -Komplexen aus Indenophanen bzw. der Modellverbindung Inden und verschiedenen -Akzeptoren wurden in Methylenchlorid bei 20 °C bestimmt. Die Assoziationskonstanten und Übergangsenergien dieser Komplexe sowie die Ionisationspotentiale der -Donatoren wurden ermittelt. Die Daten sprechen für das Vorliegen transannularer elektronischer Wechselwirkungen im Indenophan-System. Die isomeren pseudo-para- und -meta[2.2]indenophane3 und4 unterscheiden sich in ihrer -Basizität deutlich. Es besteht eine gute lineare Korrelation zwischen den Assoziationskonstanten und _max der langwelligen CT-Banden der verschiedenen -Donatoren mit Tetracyanoethylen (TCNE) und 2,3-Dichlor-5,6-dicyano-p-benzochinon (DDQ). Alle untersuchten CT-Komplexe besitzen 1:1-Stöchiometrie.

     

Binding/antibinding analyses for diatomic interactions H 2 + system

The region-functional concept of electron density has been quantitatively examined for 1s_g, 2p_u, 2p_u, and 3d_g states of H ₂ ⁺ system on the basis of Berlin diagram which divides the three-dimensional molecular space into binding and antibinding regions. The electronic charge, Hellmann-Feynman (H-F) force, and stabilization energy of the system are partitioned into the binding and antibinding contributions by the regional integrations.Dynamic behaviors of the electron density (i.e. electron-cloud preceding and following) during the interaction processes are also clarified using the centers of electron density and force density.Differences in attractive and repulsive, and - and -type interactions are discussed from the force and density point of view.     

A Bridged High-Spin Complex Bis-[Ni(II)( rac-5,5,7,12,12,14-hexamethyl- 1,4,8,11-tetraazacyclotetradecane)]-2,5-pyridinedicarboxylate Diperchlorate Monohydrate

A bridged high-spin complex,bis-[Ni(II)(rac-5,5,7,l2,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)]-2,5-pyridinedicarboxylate diperchlorate monohydrate has been obtained by reaction of [Ni(II)(rac-5,5,7, 12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)](ClO₄)₂ and 2,5-pyridinedicarboxylic acid in aqueousalkaline (NH₄OH) medium. C₃₉H₇₇Cl₂N₉Ni₂O₁₃, chemical formula weight 1068.42, orthorhombic, P2₁2₁2₁, a = 11 .423(3) Å,b = 14.770(6) Å, c = 31.608(7) Å, = = =90.00°, V = 5333(3) ų, Z = 4, D_calc = 1.331 g cm^-3, _calc = 0.869 mm^-1, F(000) = 2272, T = 293(2), R = 0.0870 for 2686 observed reflections [I > 2(I)]. The complexincludes two folded [Ni(rac- Me₆[14]aneN₄)]²⁺ units havingopposite diastereomeric configuration. They are bridged through a dianion of2,5-pyridinedicarboxylic acid, with one Ni-atom coordinated to the O-atom ofthe 2-carboxylic group and the pyridine N-atom (forming a 5-membered chelatering), and with the second Ni-atom coordinated to both O-atoms of the 5-carboxylic group (forming a 4-membered chelate ring). Hydrogen bonding involving macrocyclic NH groups, both 2- and 5-carboxylic groups, perchlorate anions and water molecules gives rise to the formation of an infinite supramolecular network in the title compound's crystals.     

Halogenation of π-cyclopentadienyltricarbonylrhenium and π-cyclopentadienyl(triphenylphosphine)dicarbonylrhenium

Conclusions The following previously unknown rhenium semicoordinaticn compounds were synthesized: [-C₅H₅Re(CO)₂PPh₃Cl]⁺Cl^–, [-C₅H₅Re(CO)₃Cl]⁺Cl^–, [-C₅-H₅Re(CO)₂PPh₃Cl]⁺Be₄, [-C₅H₅Re(CO)Cl₂]⁺ [Re(CO)₄Cl₂]^–, and -C₅H₅Re(CO)₂I₂, the latter of which was separated into the cis and trans isomers.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimlcheskaya, No. 2, pp. 408–413, February, 1981.The authors express their gratitude to V. F. Sizom, P. V. Petrovskii, and B. V. Lokshin for taking the mass, IR, and PMR spectra.     

The electronic structure of molecules by a many-body approach

The vertical valence ionization potentials of cyclopropane, ethylene oxide and ethylene imine are calculated by a many-body Green's function method. For C₃H₆ the ordering of the ionization potentials is 2e(), 1e(), 2a₁(), 1a₂(), 1e(). The assignment of the 2a₁ and the 1a₂ ionization potentials which has been controversial is thus clarified. The ordering is in agreement with the result obtained via Koopmans' theorem. For ethylene oxide and ethylene imine Koopmans' theorem fails in predicting the correct order of ionic states. For C₂H₄O the ordering of the ionization potentials is 2b ₁(), 4a ₁, 1a ₂(), 2b ₂,3a ₁, 1b ₁(), 1b ₂, 2a ₁ and for C₂H₅N 6a, 5a, 3a, 2a, 4a, 3a, 1a, 2a. The agreement of the computed ionization potentials with the experimental values is very satisfactory.     

New structural aspects of 3-vinyl-1H-indoles for predicting the outcome of Diels-Alder reactions

Summary Some selected 3-vinyl-1H-indoles have been synthesised and the first¹³C-NMR studies performed; in addition He(I) photoelectron spectra and the results of perturbation MO calculations of some examples of this class of compounds are presented. The molecular characteristics obtained thereby can be used to predict the results of [4s+2s]-cycloaddition reactions with 3-vinylindoles.

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Neue strukturelle Aspekte von 3-Vinylindolen zur Vorhersage der Reaktionsergebnisse bei Diels-Alder-Reaktionen

Zusammenfassung An selektiv synthetisierten 3-Vinyl-1H-indolen werden erstmals¹³C-NMR-spektroskopische Studien durchgeführt und exemplarisch an einigen Vertretern dieser Strukturklasse He(I)-Photoelektronenspektren sowie Ergebnisse von Störungs-MO-Rechnungen vorgestellt. Die daraus gewonnenen Moleküleigenschaften können zur Vorhersage des Reaktionsausganges von [4s+2s]-Cycloadditionen mit 3-Vinylindolen verwendent werden.

     

Ru(II) Chloro-Bis(bipyridyl) Complexes with Substituted Pyridine Ligands: Interpretation of Their Electronic Absorption Spectra

A number of complexes were synthesized with the general formula cis-[Ru(Bipy)₂(L)(Cl)](BF₄), where Bipy is 2,2"-bipyridine, L is pyridyne (Py), 4-aminopyridine (4-NH₂py), 4-picoline (4-Mepy), nicotin-amide (3-CONH₂py), isonicotinamide (4-CONH₂py), 3- and 4-cyanopyridine (3-CNpy, 4-CNpy), 4,4"-bipyridine (4,4"-Bipy), trans-1,2-bis(4-pyridyl)ethylene (Bpe), 4,4"-azopyridine (Azpy), pyrazine (Pyz), imidazole (Imid), and NH₃. The semiempirical CINDO-CI method was used to calculate the energies and intensities of transitions in the electronic absorption spectra. The differences observed in the spectra of these compounds are mainly due to the positions of the charge-transfer transitions d (Ru) *(L). Depending on the positions of these transitions, ligands L can be divided into three groups: 1) transitions Ru L lie in the region of the first long-wavelength band d (Ru) *(Bipy) (L = Azpy, Pyz); 2) transitions Ru L lie between the first and second bands due to the charge transfer to Bipy (L = 3-CONH₂py, 4-CONH₂py, 4,4"-Bipy, Bpe, 4-CNpy), and 3) transitions Ru L lie in the region of the second band d (Ru) *(Bipy) (L = Py, 4-Mepy, 3-CNPy, 4-NH₂py, Imid).     

Applications of the virtual charge model to the electronic structures and spectra of benzaldehyde and acetophenone

Summary The virtual charge model (Tapia model) in conjunction with the CNDO/S-CI approximation in the frame of SCF-MO theory was employed to study the effects of solvent on the electronic structures and spectra of benzaldehyde and acetophenone molecules. The CNDO/S calculations in presence of solvation indicate a satisfactory interpretation of the medium effects on the electronic structures and spectra of the molecules investigated. The prediction of our MO calculations by means of the Tapia model concerning the solvochromic shifts of n * and * transitions are in accord with the observed trends which indicate a blue shift for the n * band and a red shift for the * band upon a change of solvent from non-polar to polar solvents.

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Anwendung des Virtual Charge-Modells auf die Elektronenstrukturen und Spektren von Benzaldehyd und Acetophenon

Zusammenfassung Das Virtual-Charge-Modell (Tapia-Modell) im Zusammenhang mit der CNDO/S-CI-Näherung im Rahmen der SCF-MO-Theorie wurde zum Studium der Lösungsmitteleffekte auf die Elektronenstrukturen und Spektren von Benzaldehyd und Acetophenon herangezogen. Die CNDO/S-Rechnungen bei Anwesenheit von Solvens erlauben eine befriedigende Interpretation der Mediumeffekte auf Elektronenstrukturen und Spektren der untersuchten Verbindungen. Die aus MO-Rechnungen folgenden Voraussagen ergeben auf Basis des Tapia-Modells solvatochrome Verschiebungen für die n *- und *-Übergänge. Die vorausgesagten Effekte stehen im Einklang mit den experimentell beobachteten Trends: Blauverschiebung für die n *-Bande und Rotverschiebung für die *-Bande beim Wechsel von nichtpolarem zu polarem Lösungsmittel.

     

The nature of the chemical bond revisited. An energy partitioning analysis of diatomic molecules E2 (E=N–Bi, F–I), CO and BF

The nature of the chemical bonds in the diatomic molecules E₂ (E=N–Bi, F–I), CO and BF has been studied with an energy partitioning analysis using gradient-corrected density functional theory calculations. The results make it possible to estimate quantitatively the strength of covalent and electrostatic attractions and the Pauli repulsion between the atoms. The data suggest that some traditional explanations regarding the strength of the molecules should be modified. The energy partitioning analysis shows that the chemical bonds in the group 15 diatomic molecules have significant electrostatic character, which increases from 30.1% in N₂ to 58.3% in Bi₂. The contribution of the electrostatic attraction to the binding interactions in Sb₂ and Bi₂ is larger than the covalent bonding. The strength of the bonding in the triply bonded dinitrogen is less than that of the bonding. The calculations indicate that E is between 32.2% (Bi₂) and 40.0% (P₂) of the total orbital interaction energy (E_orb). The much stronger bond of N₂, as compared with the heavier group 15 E₂ homologues, is not caused by a particularly strong contribution by the bonding, but rather by the relatively large interactions. The comparison of N₂ with isoelectronic CO shows that the electrostatic character in the heteroatomic molecule is slightly smaller (28.8%) than in the homoatomic molecule. The contribution of the bonding in CO is larger (49.2%) than in N₂ (34.3%). The reason why CO has a stronger bond than N₂ is the significantly weaker Pauli repulsion in CO. The electrostatic character of the bonding in BF is slightly larger (32.0%) than in CO and N₂. BF has much weaker -bonding contributions that provide only 11.2% of the covalent interactions, which is why BF has a much weaker bond than CO and N₂. The chemical bonds in the dihalogen molecules have much higher covalent than electrostatic character. The E_orb term contributes between 74.4% (Br₂) and 79.7% (F₂) to the total attractive interactions. The relatively weak bond in F₂ comes from the rather large Pauli repulsion.Contribution to the Jacopo Tomasi Honorary Issue     

A theoretical determination of the electronic spectrum of Methylenecyclopropene

The vertical electronic spectrum of methylenecyclopropene, the prototype of the nonalternant hydrocarbons known as fulvenes, has been studied using multiconfigurational second-order perturbation theory. The calculations comprise three valence states and the 3s, 3p, and 3d members of the Rydberg series converging to the first ionization limit. Vertical excitation energies to three valence states are found at 4.13, 6.12, and 6.82 eV. The second of them corresponds to an excitation from the highest occupied orbital to a ^* orbital, while the other two are ^* excitations. The third transition gives rise to the most intense feature in the electronic spectrum. The results are rationalized within the scheme of two interacting double bonds. Comparisons are made between this and the previous theoretical calculations of the electronic spectra of related systems and also between the available experimental data of methylenecyclopropene in solution.