Quantum-chemical calculations of solvent effects on the electronic structure and reactivity of molecules

Journal of Structural Chemistry -     

Quantum-chemical study of the geometric and electronic structure of the CoC2 molecule

DFT (B3LYP) calculations have been performed to study the CoC₂ molecule in its different geometric conformations and electronic states. The energies have been refined using ab initio multiconfigurational CASSCF/CASPT2 calculations. Both approaches are in a good semi-quantitative agreement between themselves and predict the symmetric triangular (C_2v) structure to be more stable than the linear (C_∞v) conformation. The ground state has been found to be a quartet, which can formally be regarded as an ionic Co²⁺–C₂²⁻ complex, resulting from a transfer of the two 4s electrons of the cobalt atom to the 3σ_g orbital of the C₂ ligand and distributing the remaining seven valence electrons over the split 3d orbitals.     

Quantum-chemical calculation of interactions of two benzene molecules

Theoretical and Experimental Chemistry -     

Quantum-chemical calculations of the electronic structure and reactivity of silylenes

The origin of the different reactivities of carbenes and silylenes has been discussed. Molecular orbital calculations have been carried out in order to estimate the effects of substituents on the singlet-triplet splittings E _ST in these molecules. The data ofab initio calculations have been used to establish the linear correlation between E _ST values for carbenes and silylenes. Both qualitative and quantitative appraisals of the applicability of the semiempirical MNDO method to calculations of E _ST values for carbenes and silylenes have been derived. E _ST values for a large number of substituted carbenes and silylenes have been computed by the PM3 method, one of the latest versions of the MNDO approach (Ref. 14).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1006–1008, June, 1994.     

On the mechanism of mechanochemical dimerization of anthracene. Quantum-chemical calculation of the electronic structure of anthracene and its dimer

The electronic structure of anthracene, its dimer, and intermediate structures composed of two anthracene molecules were calculated in the density functional theory. The calculated potential barrier to anthracene dimerization is ～55 kcal/mol; the dissociation barrier is ～45 kcal/mol. The pressure required for the reaction to reach the transition state and acting on the anthracene crystal is ～60 kbar. Lower pressures, ～10 kbar, are required for molecules to approach each other to distances of ～3 Å, at which tunnel dimerization is possible for photoexcited molecules.     

Quantum-chemical modeling of the atomic and electronic structure of thin diamond-like nanocrystallites

The structural and electronic characteristics of thin diamond-like nanocrystallites (with cross-sectional areas 5 < S < 280 Ų) were investigated by the electron density functional tight binding (DFTB) method. A new type of extended “hybrid” (sp³ + sp²) nanostructures, in the form of monolithic diamond-like sp³ crystallites encapsulated in a graphite-like sp² shell, was discovered. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 4, pp. 199–203, July–August, 2006.     

Quantum-chemical investigation of spatial and electronic structure of verdazyl and its derivatives

The unrestricted HF method in the INDO approximation with optimized geometry was used to describe the electronic structure of the hetero-ring of verdazyl and the derivatives of the latter. The results obtained are in good agreement with the experimental EPR spectra of triphenylverdazyl radical and its dipole moment. These data may be a basis for understanding and predicting the chemical properties and reactivity of verdazyl and its derivatives.     

Gas-phase ion-molecule reactions of small nitroalkanes and their deprotonated anions

Gas-phase reactions of nitromethane (1), nitroethane (2), 2-nitropropane (3), 2-methyl-2-nitropropane (4) and nitrocyclopropane (5) were studied at 300 K using the flowing afterglow technique. These nitroalkanes react with gas-phase bases HO(-), CH(3)O(-) and HOO(-) very rapidly with rate coefficients of (2.5-4.3) x 10(-9) cm(3) s(-1) and reaction efficiencies of 60-100%, for example, k = 3.2 x 10(-9) cm(3) s(-1) (86%) for 5 reacting with hydroperoxide anion. Proton transfer (PT) is the only reaction observed for 1 while elimination (E2) is the exclusive pathway for 4 yielding isobutene and NO(2)(-). Both PT and E2 reactions are observed for 2, 3 and 5, the former being the major pathway. Deprotonated anions of 1, 2, 3 and 5 were subjected to reactivity studies with CH(3)I, CO(2), CS(2) and SO(2). Nucleophilic substitution (S(N)2) reaction occurs with CH(3)I while characteristic products CS(2)O(-) and SO(3)(-) are formed from CS(2) and SO(2), respectively, along with competing adduct formation. The SN(2) rate is greater, whereas the reactivities with the triatomic reagents are smaller for deprotonated nitrocyclopropane than for the other acyclic anions. These observations strongly suggest that the reactions of nitroalkane [M - H](-) anions occur through initial attack from the terminal oxygen; the nitrocyclopropane carbanion is more strained and, thus, less stabilized by resonance [R(2)C(-) - NO2 <--> R(2)=NO(2)(-)] resulting in the greater basicity/nucleophilicy and the less negative charge on the oxygen site.     

Importance of enolate anions in the high-pressure kinetics of nitroalkanes and nitroaromatics

We review and systematize experimental evidence concerning the extreme high-pressure kinetics of condensed phase nitroalkanes and nitroaromatics. This systematization suggests that enolate aci anions of the nitroalkanes and enolate aci-quinone anions of the nitroaromatics are important in the initial high-pressure kinetics of such systems. Kinetic data from experiments which observed the effect of: (1) small amounts of chemical additives, (2) substitution for protons, (3) difference in chemical isomer, (4) exposure to ultra-violet light, and (5) application of very high pressure can all be rationalized within the framework of our hypothesis. We suggest a static high-pressure ultra-violet absorption spectroscopy experiment on nitromethane that would test the proposed hypothesis.