Vacant MO Research Articles

ДИССОЦИАТИВНЫЙ ЗАХВАТ ЭЛЕКТРОНОВ МОЛЕКУЛАМИ ДИГИДРОКУМАРИНА И 5,6-МЕТИЛЕНДИОКСИ-1-ИНДАНОНА

The molecules of dihydrocoumarin and 5,6-methylenedioxy-1-indanone were studied by the negative ion resonant electron capture mass spectrometry for slow (0-15 eV) electrons. Such studies of the structure of vacant molecular orbitals (VMO) and dissociative electron attachment (DEA) for isolated molecules of prospective organic semiconducting materials, which are of great interest in terms of use as elementary components of organic nanoelectronics devices, are the most promising, since a correspondence between the energies of vacant MO isolated molecules and the maxima of the density of states in the conduction band of thin films was revealed earlier. Also, there is an idea developing in modern science, that the results of studies of vacant orbitals of individual molecules in the gas phase, in particular, using the DEA method, are necessary to identify the features of the energy structure of the conduction band of molecular organic condensates, starting from energies below the vacuum level, by the amount of binding energy of molecules in the film. In particular, it was shown that the substances studied in this work cannot be used as monomeric compounds in the design of electrically conductive polymers in the future, since they do not form negative molecular ions in the gas phase at energies close to thermal.

Scandium 2-mercaptobenzothiazolate: Synthesis, structure and electroluminescent properties

The 2-mercaptobenzothiazolate scandium complex Sc(mbt) 3(THF) was obtained by the reaction of Sc[N(SiMe 3) 2] 3 with 2-mercaptobenzothiazole in THF. The X-ray diffraction analysis has shown that three flat mbt ligands coordinated to the metal center in η 2 fashion via N and exocyclic S atoms. To investigate the electronic structure and to interpret its electronic absorption and emission spectra, quantum chemical calculations of the complex at the B3LYP/TZVP DFT level have been performed with use of the Gaussian03 package. It was found that three higher occupied MO’s of the complex are ligand-localized while lower vacant MO’s contain a significant metal d contribution. Sc(III) complex was used as the emitter in the two-layer device with the configuration of indium tin oxide/ N, N′-bis(3-methylphenyl)- N, N′-diphenylbenzidine/Sc(mbt) 3/Yb, which exhibits EL emission at 550 nm.

Heteroatom influence in substituted benzenes and pyridine-like heterocycles in terms of direct and indirect intramolecular interactions

The general expression for the one-electron density matrix obtained previously for saturated organic molecules (V. Gineityte, J. Mol. Struct. (Theochem), 343 (1995) 183) is shown to be applicable also to substituted benzenes and pyridine-like heterocycles. On this basis, a new interpretation of the influence of a heteroatom (substituent) upon the remaining fragment of an aromatic molecule is suggested. To this end, the occupation number of a 2p z AO of the aromatic ring has been expressed as a sum of five terms, two of them describing the intramolecular charge transfer and the remaining ones representing the secondary (induced) dipole moments arising within the ring under the influence of heteroatom, viz. the so-called ipso–ortho ( para–meta), para–ipso and ortho–meta dipole moments. Just the latter two moments proved to play the principal role in the formation of the observed picture of the electron density distribution, viz. of an increase (reduction) of occupation numbers of 2p z AOs in the ortho and para positions after introducing an electron-donating (accepting) substituent. For pyridine-like heterocycles and substituted benzenes, these dipole moments are determined mostly by the direct and the indirect interactions, respectively, between the highest occupied and the lowest vacant MO of benzene. Orbitals of the heteroatom (substituent) play the role of mediators in the above-mentioned indirect interaction.

Resonance capture of electrons by molecules of substituted pyrans

Negative-ion mass spectrometry in the mode of resonance capture of electrons and photoelectron spectroscopy in combination with quantum-chemical calculations showed that the formation of the resonance states of negative molecular ions in the reaction of electrons with molecules of the mechanism of intershell Feshbach resonance with the consecutive excitation of an electron from several higher occupied MO to one vacant MO. In a low-energy region, the resonance at 1.4 eV is a resonance of form and the resonance at 3–4 eV is the usual electron exciting Feshbach resonance with a parent triplet state (π.π*)3. The one and the same vacant π*CC MO is “active” in all the resonances mentioned.

Interpretation of resonance states of the molecular negative perfluoropropylene ion

An algorithm for interpreting resonance states of molecular negative ions has been suggested. This algorithm, using quantum-chemical calculations, allows one to select vacant MO that can be excited in a series of resonances, the energy gap between which coincides with the difference between the corresponding ionization energies. The resonance states of the molecular negative perfluoropropylene ion at energies of electrons from 0.5 to 12.0 eV are formed according to the mechanism of electron-excited Feschbach resonance. Spectral parameters of these resonances have been established.

Quantum-chemical study of allylic compounds with two bonded heavy atoms

It has been shown by the CNDO method that the bathochromic shift of the long-wave absorption band in the transition from allylstannane to compounds of the type C=C-C-Sn-X and C=C-Sn-X (where X is a heavy atom) is connected with the formation of a low-energy vacant σ*S-X orbital, localized mainly in the region of the Sn-X chemical bond, and of an occupied σSn-X orbital, the energy of which is somewhat higher than of the σC-Sn orbital. The dependence of the position of the long-wave absorbance region on conformation is related to the fact that, in planar and nonplanar conformers, the long-wave transitions are of a different type (σ → σ* and π → σ*, respectively); the\(E_{\sigma \to \sigma ^ * } trans---E_{\pi \to \sigma ^ * } \) bathochromic shift is determined to a large degree by the difference in the energies of the highest occupied MO (ɛπ - ɛσ) in the s-trans form. In the nonplanar conformers the heavy atom orbitals interact with the π-orbital of the ethylene moiety through the bridge group; this leads to a significant delocalization of the HOMO and to a considerable change in its energy. On the other hand, their interaction with the π*-orbital in compounds of the C=C-C-Sn-X type is very low and does not favor the delocalization of lower vacant MO. In vinyldistannane the π*-orbital is noticeably delocalized, due to the interaction with the π*Sn-Sn orbital in planar and with the σ*Sn-Sn orbital in nonplanar conformers.

Resonant electron trapping and photoelectron spectroscopy for substituted fluorinated nitrobenzenes

Mass spectra have been recorded for dissociative electron capture DEC negative ions together with photoelectron spectra for the molecules of fluorinated nitrobenzenes. Nitrobenzene DEC is governed in the main by orbitals having appreciable contributions from the nitro group atoms. Resonant states are formed in the molecular negative fluoronitrobenzene ions via the Feshbach electronically excited resonance mechanism, which involves the excitation of an electron from a series of filled MO to several vacant MO.

Semiempirical structure analysis for long-wave X-ray emission satellites. 2. Period II and III transition-element hydrides

Comparison with OCM calculations shows that the Rydberg states and states in the continuum are very important in configuration interaction for the hydrides, which is due to the very small numbers of vacant MO included in the CI basis (only one each for HF and HCl). Therefore, the hydrides are the most inconvenient substances for LWS structure calculations for x-ray spectra by this method. Nevertheless, here again the role of the vacant MO in the CI is important. One naturally expects that the role of those MO will increase considerably as their number increases (see in particular [17]).

Quantum-chemical investigation of allyl compounds of group IV elements. 2. Electron absorption spectra

1. A breach is coplanarity in allyl compounds of silicon, germanium, tin, and lead results in a modification and mixing of the upper occupied MO of the π and σ types, and to an increase in the energy gap between them. The same situation occurs with two lower vacant MO. The energy of quasilocal electron transitions, π → π* and σ → σ*, remains practically unchanged in this case, and the energy of the transition with charge transfer π-σ* decreases significantly. The maximum longwave displacement of this absorption band takes place in the gauche conformer. 2. The root cause of the intensification degree of the modification of MO and of electron absorption spectra when passing to allyl compounds with heavier atoms is the closer approach in the position on the scale of energies of boundary a and σ* MO of the CH2-XH3 group.

Stability of Hartree-Fock solutions and potential-energy surfaces in a reaction involving inversion of the filled and vacant MO of reagent and product

There may be a region in the potential-energy surface (PES) a reaction forbidden by symmetry in the ground electronic state but occurring with inversion of the filled and vacant MO of the reagent and product in which the corresponding solutions obtained in the restricted Hartree-Fock approximation are singlet-unstable. There are then singlet-stable solutions with lower energies, but which correspond to configurations of lower symmetry. As a result, the PES may have reaction channels of reduced symmetry. An example is considered of the cyclization of a linear D/sub infinity h/ structure for CO/sub 2/ to the C/sub 2V/ structure forbidden by symmetry for the ground electronic state. MINDO/3 calculations show that a curve with two wells applies for the region in the PES of the ground singlet state for which the RHF solutions are singlet-unstable, where the section of the minimum-energy path along the distortion coordinate leads to a geometrical configuration of lower symmetry, i.e., there are two degenerate reaction channels in this region of the PES, which correspond to geometrical configurations of symmetry C/sub 1/.

ChemInform Abstract: Isomers of C2B3H5 and the Diamond‐Square‐Diamond (DSD) Rearrangement Mechanism.

AbstractA theoretical study of trigonal‐bipyramidal C2B3H5 shows that DSD rearrangements are blocked by crossing of filled and vacant MO′s indicating that these processes face very high activation barriers.

Magnetic Circular Dichroism and Molecular Orbital Studies on the Intramolecular Charge‐Transfer Transitions in the Bridged Tropylium Cations

AbstractThe magnetic circular dichroism (MCD) spectra of the bridged tropylium cations have been measured at room temperature. Very weak MCD bands have been observed in the longest wavelength region and assigned to the intramolecular charge‐transfer (CT) transitions arising from the electronic excitations from the highest occupied MO(HOMO) of the remote chromophore to the lowest vacant MO(LVMO) of the tropylium cation. In the shorter wavelength region, four distinct MCD bands with alternating sign have been detected, clarifying four π→π* transitions in the perturbed tropylium moiety.

Quantum-chemical calculations of transition states and activation energies of molecular 1,2-elimination (1,2-addition) of HNO2

1. Quantum-chemical calculations of the reactants and the transition states in the reaction of 1,2-addition (1,2-elimination) of HNO2 to a double bond have been carried out. The orbital interactions of the reactants were analyzed, and it was shown that in the course of addition electron density is transferred from HNO2 to the olefin, i.e., HNO2 acts as a nucleophile. 2. The regioselectivity of the addition reaction of HNO2 is determined by the character of the lower vacant MO of the olefin. The addition to nitroethylene should proceed with a lower energy of activation than in the reaction with ethylene, and in opposition to the Markownikoff rule, while the addition to alkyl-substituted ethylene is according to the Markownikoff rule. 3. The introduction of an additional nitro group to the molecule of nitroethane and also replacement of the methyl group in nitroethane by an amino group lead to a considerable decrease in the activation energy of the reaction of the 1,2-elimination of HNO2. 4. The dipole moment of the transition state of the 1,2-elimination of HNO2 is much smaller than in the initial nitroethane, which is explained by mutual compensation of the component due to the polarization of the HNO2-ethylene system and the dipole moment of the HNO2 molecule itself.

Correlation of Polarographic Half Wave Potentials with Lowest Vacant MO Energy of Substituted Benzal-p-Chloranilines

Correlation of Polarographic Half Wave Potentials with Lowest Vacant MO Energy of Substituted Benzal-<i>p</i>-Chloranilines

Perturbative ab initio calculations of intermolecular energies. II. The He ··· He problem

AbstractThe ground state energy of the He2 system is calculated according to the techniques described in Paper I around the van der Waals equilibrium, without assuming the constancy of the intraatomic correlation energies. The second‐order results do not present an attractive region; the S2 decrease of the intraatomic correlation correlation corrections is larger than the attractive interatomic correlation corrections. The further orders reverse progressively the situation and finally give a qualitatively correct potential curve after the fifth order. Reaching almost exact solutions in the considered basis, one can demonstrate that the intermolecular calculation of the system A + B in the union of the basis for A and B involves an energy decrease which simply represents the effect of the extension of the basis set; and that a proper intermolecular calculation must compare the A + B energy with the A and B energies calculated in conveniently extended basis sets including the vacant MO's of the partner.