Photoelectron spectrum and quantum chemical analysis of the structure of bis(1,3,6-trimethyluracilyl-5)methane

Photoelectron spectra of bis(1,3,6-trimethyluracilyl-5)methane (I) and 1,3,6-trimethyluracil (II) were studied; AM1 optimization of geometric characteristics was carried out. The total energy minimum and the best agreement between the values of IP_m and -ɛ_m were obtained for conformations with nearly orthogonal location of uracilyl fragments. In such conformations, the highest occupied orbitals are pseudodegenerate. To interpret the photoelectron spectra, we employed ab initio calculations in STO-3G and 4-31G basis sets. For uracil and its derivatives, all methods give the π, π, n⁻, n⁺, π sequence of the highest orbitals. A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences. Translated fromZhurnal Struktumoi Khimii, Vol. 36, No. 1, pp. 102–107, January–February, 1995. Translated by L. Smolina     

Synthesis,characterization, and quantum chemical calculational studies on 3-<Emphasis Type="Italic">p</Emphasis>-methylphenyl-4-amino- 1, 2, 4-triazole-5-thione

The title compound of 3-p-methylphenyl-4-amino-1, 2, 4-triazole-5-thione was synthesized and characterized by elemental analysis, IR, electronic spectra, and X-ray single crystal diffraction. Quantum chemical calculations of the structure, natural bond orbital, and thermodynamic functions of the title compound were performed by using B3LYP/6-311G** and HF-6-311G** methods. Both the methods can well simulate the molecular structure. Vibrational frequencies were predicted, assigned and compared with the experimental values, and B3LYP/6-311G** method is superior to HF/6-311G** method to predict the vibrational frequencies. Electronic absorption spectra calculated by B3LYP/6-311G** method have some red shifts compared with the experimental ones and natural bond orbitals analyses indicate that the two absorption bands are mainly derived from the contribution of n → π^* and π → π^* transitions. On the basis of vibrational analyses, the thermodynamic properties of the title compound at different temperatures have been calculated, revealing the correlations between C ⁰ _p,m , S ⁰ _m , H ⁰ _m , and temperatures.     

Orbital phase control of the conformations of α- and β-substituted enamines and vinyl ethers

The conformational stabilities of the α- and β-substituted enamines and vinyl ethers were predicted by orbital phase theory and confirmed by ab initio molecular orbital calculations. Cyclic interaction significantly occurs among the nonbonding orbital n _Y for the lone pair on the hetero atom Y (N in the enamines or O in the ethers), the π and π* orbitals of the CC bond, and the σ_C-H or σ*_C-X orbitals on the substituent CH₂X. The cyclic -n _Y-π-σ_C-H-π*- interaction is favored by the orbital phase continuity in the α-substituted molecules, while the cyclic -n _Y-π-σ*_C-X-π*- interaction is favored in the β-substituted molecules. The most stable conformation was then predicted to be synperiplanar or (pseudo)equatorial in the α-substituted molecules and anticlinical or (pseudo)axial in the β-substituted molecules. Received: 8 May 1998 / Accepted: 30 July 1998 / Published online: 16 November 1998     

Electronic structure of monosubstituted benzenes and X-ray emission spectroscopy

The electronic structure of the phenol molecule in the gas phase was studied by X-ray emission spectroscopy (using the O-Kα and C-Kα spectra). MNDO calculations were performed, which made it possible to construct theoretical spectra and interpret experimental spectra. The structure of the molecular orbitals of phenol was compared with those of benzene and water. The π-interaction of the phenyl fragment with the oxygen-containing substituent was investigated. The contribution of the 2p atomic orbital of the oxygen atom to the π-HOMO of phenol is considerably less than that to lower-lying orbitals. For Part 3, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2187–2193, December, 1997.     

Electronic structure of monosubstituted benzenes and X-ray emission spectroscopy

The electronic structure of fluorobenzene was investigated by X-ray emission spectroscopy (using the F−Kα- and C−Kα-spectra) and quantum-chemical MNDO calculations. Molecular orbitals of fluorobenzene were compared with those of benzene and hydrogen fluoride. The P_π−p_π-interaction between the phenyl ring and the fluorine atom in the fluorobenzene molecule is weak for both the outer and inner π levels. For Part 2, see Ref. 1. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1454–1460, August, 1997.     

Joint X-ray spectroscopic and quantum-chemical study of the electronic structure of pentafluorophenylalkyl ethers

The high resolution X-ray emission O-Kα spectra of pentafluorophenylalkyl ethers C₆F₅OR (R=Et, Prⁱ, and Bu^t) exhibit differences related to a change in the electronic structure of the compounds as R is varied. The search for stable conformers was performed by the semiempirical PM3 method. The most probable structures of C₆F₅OR were determined by the comparison of the experimental and theoretical X-ray spectra plotted for each conformer usingab initio calculations in the 6–31 G basis set. Substituent R in pentafluorophenylalkyl ethers is situated outside of the ring plane. The fluorination of the benzene ring changes the energy level of the lone electron pair of oxygen relative to the levels of orbitals of the ring and substituent R and leads to an increase in the efficiency of interactions in the σ-system. Deceased. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2443–2450, December, 1998.     

X-ray spectra and electronic structure of K x C60

Highly resolved CK_α spectra of the compounds K₃C₆₀ and K₆C₆₀ synthesized by thermal decomposition of potassium azide in a vacuum chamber of an X-ray tube were obtained. The possibility of monitoring the extent of intercalation of fullerenes by the CK_α spectra is demonstrated. The results are compared with the MNDO calculations for K₆C₆₀. The data are used to construct a diagram of the structure of the highest occupied and C1s levels in the series C₆₀−K₃C₆₀−K₆C₆₀. Deceased. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 3, pp. 514–519, May–June, 1996.     

Positional selectivity in electrophilic substitution reactions of π-excessive heterocycles (review)

Existing experimental data on positional selectivity in electrophilic substitution reactions of π-excessive heterocycles are classified. These data are discussed basing on the results of the authors' quantum-chemical calculations [RHF/6-31G(d), MP2/6-31G(d), and B3LYP/6-31G(d)] of the σ-complexes formed during attack of electrophiles such as H⁺, Me⁺, Me₃Si⁺, Br⁺, NO₂ ⁺, MeCO⁺, and SO₃ at the α- and β-positions of furan, thiophene, selenophene, pyrrole and its N-substituted derivatives, N-R-pyrroles (R = Me, t-Bu, SiMe₃, Si(i-Pr)₃, C₆H₄(p-NO₂), SO₂Ph, CHO, CO₂Me), and the corresponding α- and β-substituted electrophilic substitution products. The differences in energies of the α-and β-isomers of the σ-complexes characterize the preferred direction of electrophilic attack, while the differences in the energies of the isomeric products make it possible to assess the energy preference of one of them. Analysis of the obtained data demonstrates the effects of the studied heterocycles' structure, the nature of the electrophile, and the thermal and steric factors on the positional selectivity (α/β ratio) in electrophilic substitution reactions of π-excessive five-membered heteroaromatic compounds.     

Density functional quantum chemical calculation of the X-ray fluorescence spectra of dimeric manganese carbonyl Mn<Subscript>2</Subscript>(CO)<Subscript>10</Subscript>

Based on the density functional quantum chemical calculations, the electronic structure of binuclear manganese decacarbonyl Mn₂(CO)₁₀ is analyzed. The calculation results are used to interpret the CK _α , OK _α, MnL _α, and $ MnK_{\beta _5 } $ MnK_{\beta _5 } X-ray fluorescence spectra of Mn₂(CO)₁₀. The theoretical fluorescence spectra constructed using these calculations are in good agreement with the experiment.     

Electronic structure and OK α spectra of carbonyl-containing pentafluorobenzene derivatives

The OK spectra have been obtained for several pentafluorobenzene derivatives, using acetone as a reference molecule. The MNDO calculations have been performed for all the compounds under study, and simulated spectra have been plotted. The -interaction between the carbonyl group and the pentafluorophenyl ring has been shown to occur mostly in the system of inner molecular orbitals.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 314–318, February, 1993.     

Solvent Effect on Protonation Constants of 5, 10, 15, 20-Tetrakis(4-sulfonatophenyl)porphyrin in Different Aqueous Solutions of Methanol and Ethanol

The protonation constants of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, H₂tpps⁴⁻, were determined in water–ethanol and water–methanol mixed solvents, using a combination of spectrophotometric and potentiometric methods at 20 °C and 0.1 mol⋅dm⁻³ sodium perchlorate as supporting electrolyte. Two protonation constants, K ₁ and K ₂, were characterized and were analyzed in various media in terms of the Kamlet, Abboud and Taft (KAT) parameters. Single-parameter correlations of the protonation constant K ₁ versus α (hydrogen-bond donor acidity) and π ^* (dipolarity/polarizability) are poor in all solutions, but dual-parameter (α and π ^*) correlation represents a significant improvement with regard to the single- and multi-parameter models. However, the single-parameter correlation of log ₁₀ K ₂ in terms of β (hydrogen-bond acceptor basicity) shows a better result than dual- and multi-parameter correlations. Linear correlation is observed when the experimental log ₁₀ K ₁ and log ₁₀ K ₂ values are plotted versus the calculated ones when the KAT parameters are considered. To evaluate the protonation constants of H₂tpps⁴⁻, the Yasuda-Shedlovsky extrapolation is used to obtain the log ₁₀ K ₁ and log ₁₀ K ₂ values at zero percent organic solvent. Finally, the results are discussed in terms of the effect of the solvent on protonation.     

Quantum chemical and experimental studies on the structure and vibrational spectra of substituted 2-pyranones

A systematic study on the structural characteristics of the 2-pyranone ring containing molecules with bromine, nitrile, and amide substituents at the C-3 position in the ring is conducted in the electronic ground (S ₀) state by DFT calculations using the B3LYP/6-311++G** method. The geometrical structure of the bromine substituted compound, which shows potent hepatoprotective activity, is studied both in the ground (S ₀) and first excited singlet (S ₁) states using RHF/6-311++G** and CIS/6-311++G** methods respectively. The molecules are found to exist in two isomeric forms gauche and trans that have the enthalpy difference of less than 3.32 kcal/mol; the latter is the preferred orientation in the gaseous phase. The S ₁ state is a ¹(π,π*) state that arises π-electron transfer from the region of a double bond in the pyranone ring to the region of the internuclear bond connecting the 2-pyranone and benzene rings. A complete vibrational analysis is conducted for the 3-bromo-6-(4-chlorophenyl)-4-thiomethyl-2H-pyran-2-one molecule based on the experimental infrared spectra in the 50–4000 cm⁻¹ region and DFT/6- 311++G** computations of vibrational frequencies for the gauche and trans isomeric forms. Spectral assignments based on the potential energy distribution along the internal coordinates confirm the nonplanar structure of the molecule.     

Theoretical analysis of the internal rotation in aminoborane and borylphosphine

Using a recently proposed orbital deletion procedure and the block-localized wavefunction method, the rotational barriers in H₂BNH₂ and H₂BPH₂ are analyzed in terms of conjugation, hyperconjugation, steric effect and pyramidalization. With the zero-point energy corrections, the π-binding strengths in the planar H₂BNH₂ and H₂BPH₂ are both around 20 kcal/mol at the HF level using the 6-311+G** basis set. With the deactivation of the π atomic orbitals on the boron atom and the evolution from a planar structure to a 90°-twisted structure, the steric repulsion between the B‐H and the N‐H or P‐H is relieved and moreover, the negative hyperconjugation from the lone electron pair or pairs on the nitrogen or phosphorus atoms to the antibonding orbital ^χ* _B H ₂ of the BH₂ group stabilizes the twisted structure by 7.4(8.8) or 4.0(5.0) kcal/mol at the HF/6-31G*(6-311+G**) level. However, the repulsive interaction between the lone pair(s) and the two BH σ bonds is so prominent that the overall steric effect contributes 20.3(22.9) and 19.3(19.8) kcal/mol to the rotational barriers in H₂BNH₂ and H₂BPH₂ with the 6-31G*(6-311+G**) basis set. The present techniques and analyses may also give some clues to justify the parameterization in the empirical molecular mechanics methods. Received: 17 April 1998 / Accepted: 17 September 1998 / Published online: 1 February 1999     

Quantum chemical analysis of the structure and spectroscopic properties of aryl vinyl ethers

A series of aromatic vinyl ethers and some compounds close to them in structure are studied by DFT (B3LYP/6-311+G(2d,p)) and MP2(full)/6-311+G(2d,p) methods. Measurements of Raman spectra are also used. The calculation of vibrational spectra of aryl vinyl ether (AVE) isomers shows that stretching vibrations ν(C=C) are most conformation sensitive. The calculated value of I(C=C) for vinyl phenyl ether more than twice exceeds the corresponding value for vinyl methyl ether. The calculated and experimental values of I(C=C) are consistent with the hypothesis about the presence of a common conjugated π-system in the molecules of substituted AVEs. Here the bridging oxygen atom provides the π,p,π-interaction.     

On quantum-chemical estimates of constants of isotropic hyperfine coupling with protons in free radicals

A scheme of consitent quantum-chemical calculations of constants of isotropic hyperfine coupling (IHFC) with protons (a _H ^iso ) in free radicals is considered for the case where the spin populations ρ_s ^H are determined in the basic set of symmetrically orthogonalized atomic orbitals taking model σ-and π-electron fragments as an example. The competence of using two coefficients of proportionalityK(H) when estimating the proton IHFC constants by semiempirical quantum-chemical methods was demonstrated. Theoretical substantiation of empirical values of the above coefficients previously used is revealed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 245–249, February, 1999.     

Examining the impact of ancillary ligand basicity on copper(I)–ethylene binding interactions: a DFT study

A theoretical investigation at the density functional theory level (B3LYP) has been conducted to elucidate the impact of ligand basicity on the binding interactions between ethylene and copper(I) ions in [Cu(η ²-C₂H₄)]⁺ and a series of [Cu(L)(η ²-C₂H₄)]⁺ complexes, where L = substituted 1,10-phenanthroline ligands. Molecular orbital analysis shows that binding in [Cu(η ²-C₂H₄)]⁺ primarily involves interaction between the filled ethylene π-bonding orbital and the empty Cu(4s) and Cu(4p) orbitals, with less interaction observed between the low energy Cu(3d) orbitals and the empty ethylene π*-orbital. The presence of electron-donating ligands in the [Cu(L)(η ²-C₂H₄)]⁺ complexes destabilizes the predominantly Cu(3d)-character filled frontier orbital of the [Cu(L)]⁺ fragment, promoting better overlap with the vacant ethylene π*-orbital and increasing Cu → ethylene π-backbonding. Moreover, the energy of the filled [Cu(L)]⁺ frontier orbital and mixing with the ethylene π*-orbital increase with increasing pK _a of the 1,10-phenanthroline ligand. Natural bond orbital analysis reveals an increase in Cu → ethylene electron donation with addition of ligands to [Cu(η ²-C₂H₄)]⁺ and an increase in backbonding with increasing ligand pK _a in the [Cu(L)(η ²-C₂H₄)]⁺ complexes. Energy decomposition analysis (ALMO-EDA) calculations show that, while Cu → ethylene charge transfer (CT) increases with more basic ligands, ethylene → Cu CT and non-CT frozen density and polarization effects become less favorable, yielding little change in copper(I)–ethylene binding energy with ligand pK _a. ALMO-EDA calculations on related [Cu(L)(NCCH₃)]⁺ complexes and calculated free energy changes for the displacement of acetonitrile by ethylene reveal a direct correlation between increasing ligand pK _a and the favorability of ethylene binding, consistent with experimental observations.     

Chemical effects on K and L shell production cross sections and transfer probabilities in Nb compounds

In this study, the chemical effects on σ_Ki (i = α, β), σ_Lα cross sections, K_β/K_α X-ray intensity ratios and vacancy transfer probabilities from K to L (η _KL) for pure Nb and Nb compounds were investigated. The samples were excited by 59.5 keV γ-rays from ²⁴¹Am and 5.96 keV photon energy from a ⁵⁵Fe annular radioactive sources. K and L X-rays emitted by samples were counted by an Ultra-LEGe detector with a resolution of 150 eV at 5.9 keV. While it was observed that the chemical bonding had an effect on the σ_Kβ, σ_Lα cross sections and K_β/K_α X-ray intensity ratios for compounds, it was almost negligible for σ_Kα cross section because K_α transitions (2P_3/2,1/2→1S_1/2) occurred in inner shells. It is well known that interactions between central element atom and ligands come into existence in valence state, so outer energy levels are sensitive to the chemical environment. The experimental values of σ_Kα cross section and η _KL are in good agreement with theoretically calculated and other experimental values of pure niobium, but the experimental values of the σ_Kβ, σ_Lα cross sections and K_β/K_α X-ray intensity ratios have differences for some compounds because valence electrons have different bond distances and binding energies in different compounds.     

An ab initio simulation of the UV/Visible spectra of substituted chalcones

The electronic absorption spectra of 29 phenyl-ring substituted chalcones have been investigated with the time-dependent density functional theory (TD-DFT) and polarizable continuum TD-DFT (PCM-TD-DFT). It turns out that the hybrid PBE1PBE functional with the 6-31G basis set provide reliable λ_max when the solvent effects are included in the model. Comparisons with experimental values lead to a mean absolute error of 12 nm (0.136 eV). Moreover, the observed substituent effects are reproduced by calculation qualitatively. The λ_max of substituted chalcone in phenyl ring A is less sensitive to substitution than that in ring B. The linear correlation of Hammett’s substituent constants (σ_P) with LUMO energies is better with respect to HOMO energies. The calculation reveals that the maximum absorption band mainly results from the π→π* transition from HOMO to LUMO. The analysis of the electron density plots of frontier molecular orbitals show that most transitions should be of valence excitation nature.      

Fluorescence and photochemical properties of crystalline boron difluorides β-diketonato

The fluorescence and photochemical properties of crystalline β-diketonatoboron difluorides (DBD) RCOCHCOR¹BF₂ were studied. These compounds are characterized by relatively high photochemical stability. The introduction of electron-donating groups into the aromatic α-substituent of the chelate ring increases and the introduction of electron-withdrawing groups decreases the fluorescence intensity of DBD. Anisoylbenzoylmethanotoboron difluoride was found to exhibit the highest fluorescence intensity. The substituents were shown to influence the relative arrangement of singlet and triplet ηπ^* and ππ^* levels and luminescence properties of compounds. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1030–1033, June, 2000.     

Electronic structures of monosubstituted benzenes and X-ray emission spectroscopy 5. Nitrobenzene

The electronic structure of nitrobenzene was studied by X-ray emission spectroscopy. The O-Kα, N-Kα, and C-Kα spectra of the title compound in the gas and solid phases were obtained. Based on the results of quantum chemical MNDO calculations, theoretical spectra were constructed. An interpretation of the experimental spectra is given. π-Interaction between phenyl fragment and nitro group in nitrobenzene is weak. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1297–1300, August, 2006.