Formation of N-heterocyclic, donor-stabilized borenium ions

Cationic and zwitterionic boryl bromide species and a borenium-boryl bromide cation have been synthesised which represent new N-donor stabilised cationic boron compounds with β-diketiminate ligands. The unexpected borenium-boryl bromide results from a head-to-tail dimerisation of the corresponding zwitterionic boryl bromide accompanied by proton migration. The electronic nature of these new species was studied by DFT calculations.     

Stereochemistry of retro-diel-salder fragmentation in gas-phase ions formed by chemical ionization

Retro Diels Alder fragmentation is highly stereospecific in the diones 1 under chemical ionization conditions, both with methane and isobutane as the reagent gases. Only the cis-isomers yield abundant protonated diene and quinone ions. The isotope effect indicates preferential protonation on a CO oxygen, and a subsequent H-migration prior to the formation of the protonated diene cations in the cis isomers.     

Quantum dynamic calculation for gas-phase nucleophilic substitution (SN2) reactions

According to experimental data, gas-phase S_N2 reactions have low efficiency characterized by the ratio k(T)/k_c, where k_c is the collision rate constant. The energy profile of the reaction pathway is a double-well curve, and the top of the central barrier may be located either above or below the energy level of the reagents. Nonempirical calculations of the potential surfaces for S_N2 reactions have shown that their dynamics may be described in the collinear collision approximation. The probability of a reactive collision is determined by the interference of direct processes and processes of decay of quasibound states arising upon formation of the pre-reaction complex. The reflection probability in the direct process is determined by the degree of excitation of the reactive system in channels that are closed at the central barrier point, which depends on the curvature of the reaction pathway and the depth of the potential well for the pre-reaction complex. Depending on these factors, the direct process may correspond either to total transmission or to total reflection (low reaction efficiency). In the first case, interference of direct processes with decay processes decreases the reaction probability; in the second case, such interference increases the reaction probability. Calculations of the thermal reaction rate constants k(T) have shown good agreement with experimental data.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 4, pp. 435–443, July–August, 1986.The author acknowledges M. V. Bazilevskii, A. M. Berezhkovskii, V. A. Tikhomirov, and G. É. Chudinov for useful discussions.     

Quantum chemical definition and calculation of oxidation number

A new quantum chemical definition of oxidation number is proposed, in the present paper, as a direct generalization of the corresponding classical definition. According to the proposed general definition, the oxidation number can be calculated by use of molecular orbital data and a population analysis method or by use of other quantum chemical methods. For the practical calculation, we present a corresponding concrete calculation procedure within the framework of the maximum overlap population principle, which is very simple and very easy to use. The calculated numerical results are, on the whole, in good agreement with chemists' intuitive concepts of chemical bonding.     

Quantum chemical calculation of force constants and vibrational spectra

As a result of the development of direct derivative methods and improved computational facilities, ab initio quantum chemical calculations have become an increasingly important source of information for the determination of molecular force constants. Within the Hartree-Fock (H-F) SCF model and using moderate size basis sets such calculations are now economically feasible for molecules of up to 2o–3o atoms. At this level of theory, harmonic diagonal force constants are overestimated by 1o–3o%, corresponding to 5–15% in the frequencies. However, the largely systematic errors can be accounted for by simple empirical corrections. The resulting SQM (Scaled Quantum Mechanical) force fields are probably the most reliable ones available at present for larger molecules. Calculated infrared intensities are semi-quantitatively correct. Beyond the H-F model, large scale calculations including electron correlation give great improvements in the force constants, but there are still residual errors of a few percent.     

Quantum chemical calculation for the inhibitory effect of compounds

The effects of the molecular structure on the corrosion inhibition efficiency are investigated by nine methods of calculations. The selected thio compounds were previously identified as corrosion inhibitors for mild steel in the 1.0 M HCl solution. The electronic properties such as highest occupied molecular orbital (EHOMO) energy, lowest unoccupied molecular orbital (ELUMO) energy, dipole moment (μ), and Fukui indices are calculated and discussed. Results show that the corrosion inhibition efficiency increase with the increase in both EHOMO and μ values, respectively, and decrease in ELUMO. QSAR approach is utilized in this study; a good relationship is found between the experimental corrosion inhibition efficiency (IE_exp, %) and the theoretical corrosion inhibition efficiency (IE_theor, %). The calculated inhibition efficiency is found closer to the experimental inhibition efficiency with a coefficient of correlation (R ²) of 0.875.     

Quantum chemical calculation of hydration for alkali metal salts

Calculations using the GAUSSIAN 03 program package were performed to determine the hydrated structures of lithium, potassium, LiCl, NaCl, and sulfonic cation exchanger in the form of alkali metal ions. For ions with positive hydration, the distance between the radii of the first and second spheres of hydration water is larger than in pure water; for ions with negative hydration, this distance is smaller. Salts with different types of hydration for cation and anion have hydrated structures similar to those of ions with negative hydration.     

Quantum chemical calculation of phosphorescence microwave double resonance spectra

Lifetimes of individual spin sublevels, vibronic intensities of phosphorescence, zero field splitting, hyperfine and nuclear quadruple tensors for the lowest triplet state, g factors, and dipole nature of microwave transitions have been calculated on the bases of CNDO and INDO methods taking into account spin–orbit, spin–spin, vibronic, and hyperfine interactions by perturbation theory. The results are in qualitative agreement with phosphorescence microwave double resonance data. Influence of intermolecular interaction on the zero field splitting are also investigated.     

Quantum chemical model of solvation for calculation of electrode potentials of redox processes involving ferrocene,cobaltocene, and their ions

Quantum chemical calculations of solvation energy for ferrocene and cobaltocene molecules and their ionic forms in water, acetonitrile, methanol, and acetone are performed in terms of the B3LYP density functional method by taking into account solvation effects and using the polarized continuum model (PCM). Standard electrode potentials of the corresponding redox pairs, the effect of solvent on them, and the overall energy of the transfer of cobaltocene cation and anion between two solvents are calculated. The calculation results well agree with the available experimental data. The present study provides sufficiently reliable grounds for the application of an ion—metallocene molecule redox pair as a pilot system for the comparison of electrode potentials and solvation energies in different solvents.     

取代基咪唑啉与铁原子化学吸附作用能的量子化学计算

用量子化学方法计算了６个咪唑啉型化合物及其与铁原子的化学吸附作用能,探讨了这种作用能与缓蚀性能的关系。得到咪唑啉环上氮与铁的配位键长、双原子作用能以及重叠集居数。研究发现具有ｐ－π共轭体系的咪唑啉以及在环上引入供电子基团或取代芳烃,能增强氮与铁原子的化学吸附作用力。计算结果可为设计性能较好的新型咪唑啉缓蚀剂提供有用的信息。     

Quantum chemical calculation of nickel and copper atomic valencies in crystalline oxides

A new quantum chemical definition is proposed of the full atomic valence, V_A, taking into account both the covalent and ionic parts of the chemical bonds formed by atoms in molecules and crystals. The full atomic valencies, covalencies, and charges on atoms are calculated for nickel–oxygen crystalline compounds in the CNDO band theory approximation. A comparison of the chemical bonding in nickel and copper crystalline oxides is given. © 1994 John Wiley & Sons, Inc.     

Quantum chemical calculation of cation interaction with water molecules and ethylene glycol

A computer modeling is carried out for the structure and IR spectra of ethylene glycol-9 water molecules and ethylene glycol-9 water molecules-M⁺ systems, where M⁺ = Na⁺, K⁺. The presence of cations changes the structure of the glycol hydrate shell, which leads to a decrease in the activation energy of glycol self-diffusion in the solution with the addition of salts in comparison with its value in the water-glycol solution.     

Quantum dynamics of gas-phase SN2 reactions.

Understanding the state-resolved dynamics of elementary chemical reactions involving polyatomic molecules, such as the well-known reaction mechanism of nucleophilic bimolecular substitution (SN2), is one of the principal goals in chemistry. In this Review, the progress in the quantum mechanical treatment of SN2 reactions in the gas phase is reviewed. The potential energy profile of this class of reactions is characterized by two relatively deep wells, which correspond to pre- and post-reaction chargedipole complexes. As a consequence, the complex-forming reaction is dominated by Feshbach resonances. Calculations in the energetic continuum constitute a major challenge because the high density of resonance states imposes considerable requirements on the convergence and the energetic resolution of the scattering data. However, the effort is rewarding because new insights into the details of multimode quantum dynamics of elementary chemical reactions can be obtained.     

Borinium, borenium, and boronium ions: synthesis, reactivity, and applications

Boron cations are elusive and highly electrophilic species that play a key role in the chemistry of boron. Despite early interest in the chemistry of boron cations, until recently they have largely remained chemical curiosities. However, hints at harnessing their potential as potent electrophiles have begun to appear and developments in weakly coordinating anion technology suggest that this is an area of research that is ripe for exploration. It has been nearly 20 years since the last major review on boron cations; herein we summarize the progress in the area since that time.     

Hydration of gas-phase ions formed by electrospray ionization

The hydration of gas-phase ions produced by electrospray ionization was investigated. Evidence that the hydrated ions are formed by two mechanisms is presented. First, solvent condensation during the expansion inside the electrospray source clearly occurs. Second, some solvent evaporation from more extensively solvated ions or droplets is apparent. To the extent that these highly solvated ions have solution-phase structures, then the final isolated gas-phase structure of the ion will be determined by the solvent evaporation process. This process was investigated for hydrated gramicidin S in a Fourier-transform mass spectrometer. Unimolecular dissociation rate constants of isolated gramicidin S ions with between 2 and 14 associated water molecules were measured. These rate constants increased from 16 to 230 s-1 with increasing hydration, with smaller values corresponding to magic numbers.     

Electronic fluorescence spectra of gas-phase positive molecular ions

A new technique for observing electronic spectra of gas-phase positive molecular ions is described. The ions are produced by electron impact on a supersonic molecular beam, and the resulting fluorescent radiation is dispersed. Some results on N₂, N₂O and CO₂ are presented. They suggest that this technique could be most productive for studying large polyatomic cations.     

Quantum chemical calculation of vibrational spectra of large molecules--Raman and IR spectra for Buckminsterfullerene

In this work we demonstrate how different modern quantum chemical methods can be efficiently combined and applied for the calculation of the vibrational modes and spectra of large molecules. We are aiming at harmonic force fields, and infrared as well as Raman intensities within the double harmonic approximation, because consideration of higher order terms is only feasible for small molecules. In particular, density functional methods have evolved to a powerful quantum chemical tool for the determination of the electronic structure of molecules in the last decade. Underlying theoretical concepts for the calculation of intensities are reviewed, emphasizing necessary approximations and formal aspects of the introduced quantities, which are often not explicated in detail in elementary treatments of this topic. It is shown how complex quantum chemistry program packages can be interfaced to new programs in order to calculate IR and Raman spectra. The advantages of numerical differentiation of analytical gradients, dipole moments, and static, as well as dynamic polarizabilities, are pointed out. We carefully investigate the influence of the basis set size on polarizabilities and their spatial derivatives. This leads us to the construction of a hybrid basis set, which is equally well suited for the calculation of vibrational frequencies and Raman intensities. The efficiency is demonstrated for the highly symmetric C(60), for which we present the first all-electron density functional calculation of its Raman spectrum.