The relationship between electron densities and the pKa values in a series of methylpyrazines

Based on the literature values, the pK₁ and pK₂ for pyrazine and its six methyl derivatives, the correlation with electron density on the nitrogen atoms N₁ and N₄ was found. The effect of methyl group was proved to be additive. The total electron density, π-electron density, and dipole moments for compounds under study were determined by the CNDO /2-MO method.     

Calculation of molecular crystals within the PCILOCC framework

Stabilization energies and equilibrium distances of one-dimensional (HF)_n and (H₂O)_n chains have been calculated by means of the PCILO method for finite chain length and by the PCILOCC method for infinite chain length. Both types of calculation are compared with corresponding CNDO /2-MO and CNDO /2-CO calculations. Further we have performed an analysis of the individual contributions of the stabilization energies per monomer of the PCILO and PCILOCC calculations. The results show that the PCILOCC method is well suited for the calculation of molecular associations with translational symmetry.     

CNDO-MO calculation of NnFm molecules

The s-p separation model CNDO -MO method has been introduced and developed for both the open- and closed-shell systems since 1975. This method has two chief advantages over most of the other CNDO methods. (1) The ns and np bases of the same atom may be considered independent to infer different bonding behavior. (2) Pariser–Parr and Nishimoto–Mataga approximations are applied to the Coulombic repulsion integrals, which not only simplifies the calculation and saves computer time but also gives reliable computational results. A series of nitrogen fluoride molecules such as NF, NF₂, NF₃, cis-N₂F₂, trans-N₂F₂, and N₂F₂ have been selected for this MO calculation. In each case, both ionization potential and dipole moment were calculated. The results are closer to the observed values than those reported in other works.     

Studies on pyrazinyl-pyrazylidene tautomerism of pyrazine-acetonitrile derivatives

The equilibria between the pyrazinyl and 2(1H)-pyrazylidene forms of pyrazineacetonitrile derivatives were studied by IR,¹H-NMR and UV spectroscopy. The excess charges and dipole moments for the two tautomers were calculated by the CNDO/2-MO method.

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Untersuchungen über die Pyrazinyl-Pyraziliden-Tautomerie von Pyrazinacetonitril-Derivaten

Zusammenfassung Die Gleichgewichte zwischen Pyrazinyl- und 2(1H)-Pyrazyliden-Formen von Pyrazinacetonitril-Derivaten wurden mittels IR-, PMR- und UV-Spektroskopie untersucht. Die Überschußladungen und Dipolmomente der zwei Tautomeren wurden nach dem CNDO/2-MO-Verfahren berechnet.

     

Semi-quantitative and semi-empirical versions in the quasi-relativistic SCF-MO-LCAO methods: Numerical calculations for (PtCl6)2−

Quasi-relativistic (QR ) versions of the CNDO and Mulliken–Wolfsberg–Helmholz (MWH) semiempirical methods based on the SCF -QR -MO -LCAO method given earlier are worked out. For the CNDO method only the basic formulas and matrix element expressions are given, while for the MWH one, the parametrization as well as the basis functions and group spinor overlap integral calculation were discussed. (PtCl₆)^2? complex was chosen for a test calculation. The energy level values and LCAO coefficients were obtained and compared with the nonrelativistic calculations. One of the results was the occurrence of a very strong reduction of the spin-orbit interaction due to covalency. The calculation proves the semi-empirical versions of the QR -MO -LCAO method to be quite realizable in practice.     

Fabrication of a high thermal-stable methacrylate-silicate hybrid nanocomposite: hydrolytic versus non-hydrolytic sol–gel synthesis of methacryl-oligosiloxanes

A high thermal-stable methacrylate-silicate hybrid nanocomposite (hybrimer) was fabricated based on UV-induced polymerization of a highly-condensed methacryl-oligosiloxane resin (Hy-MO). The Hy-MO resin was newly synthesized via an acid-catalyzed hydrolytic sol–gel reaction of 3-(methacryloxy) propyl trimethoxysilane (MPTS) and diphenyldimethoxysilane. The basic properties of Hy-MO were compared with a conventional resin prepared by a Ba(OH)₂·H₂O-catalyzed non-hydrolytic sol–gel reaction of MPTS and diphenylsilanediol (N-MO). It was found that Hy-MO have a higher siloxane condensation degree, larger molecular size and weight than the N-MO due to the highly mobile proton ions that effectively catalyze the overall sol–gel reaction. The resultant Hy-MO hybrimer showed a higher thermal stability than N-MO hybrimer.     

Calculation of excited-state properties of some small molecules: Comparative study of the CNDO/S and CNDO/2-vn−1 potential methods

The transition energy and geometry of the lowest excited (nπ*) singlet and triplet states of CO, CS, HNO, H₂CO, HFCO, and F₂CO molecules are calculated by CNDO /S and CNDO /2-V_N?1 potential methods, and the results are compared with those of experimental and ab initio theoretical studies, wherever available. In the calculation of the vertical transition energy, the performance of the CNDO /S method is seen to be generally more satisfactory than that of the CNDO /2-V_N?1 potential method, while the reverse is true for the excited-state geometry. The CNDO /S method as such fails to describe the geometry of the excited state, but a combined version (CNDO /S-2) of CNDO /S and CNDO /2, as well as the CNDO /2-V_N?1 potential method is fairly successful in this regard.     

Torsional isomerization of biologically active bicyclic molecules

Rotational isomerization of bipyridines C₅H₄N? C₅H₄N was studied by CNDO /2, PPP /CI , and CNDO /CI methods. It is shown that CNDO /2 overestimates the angle of rotation ? between the pyridine rings ca. two times. The angle ? was determined for 2,2′-bipyridine by means of correlation of the theoretical (CNDO /2) and experimental dipole moment. It was also found from the correlation between the theoretical and experimental UV spectra. It is shown that there is an explicit dependence of the results upon the distance between heteroatoms (PPP /CI ). It has been found that the CNDO /CI method correctly predicts the value of the rotational angles and their sequence in bipyridines.     

CNDO/2 calculations of some polyazaindenes

The theoretically estimated dipole moments of indolizine, imidazo[1,2-a]pyridine, imidazo-[1,5-a]pyridine and of pyrazolo[ 1,5-a]pyridine obtained by the CNDO/2 approximation have been compared with the experimental values. The bond angles and bond distances for these polyazaindenes have been estimated.     

A comparative study of the structure and bonding of HOO,HOS, HSO,and HSS radicals by CNDO/2 and INDO methods

Equilibrium geometries, force constants, barriers to linearity, charge distributions, dipole moments, and electron spin density of HOO, HOS, HSO, and HSS radicals are calculated by CNDO/2 and INDO methods using respectively the original and some recently introduced scheme of parametrization. Three sets of calculations, namely, CNDO/2(sp), CNDO/2(spd), and INDO, are performed, and the results are compared with the ab initio and experimental values, wherever available. A good agreement is obtained for geometry in the case of CNDO/2 (sp) and INDO calculations. The performance of CNDO/2 (spd) calculations in this regard is quite unreliable. The stretching force constants are considerably overestimated by all the methods, while the bending force constants are in reasonable agreement with the ab initio values. With respect to dipole moments, the CNDO/2 values are in better agreement with the ab initio results than the INDO values. In all the cases, the dipole moment vector directions are in complete disagreement with the ab initio predictions.     

Charge-Transfer Complexes between Substituted Pyridine N-Oxides and 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone

Spectrophotometry was used to study charge-transfer complexes between substituted pyridine N-oxides and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. Vertical ionization potentials of pyridine N-oxides in the gas phase and in solution were calculated by the CNDO/S3' method. The calculated ionization potentials in chloroform nicely correlate with the experimental charge-transfer wavelengths of the complexes.     

All-valence-electron scf-mo calculations on the electronic structure and reactivities of some halogenated benzene and azabenzene derivatives

All-valence-electron CNDO/2 SCF-MO calculations have been performed on some fluoro, chloro and methoxy derivatives of benzene, pyridine and the diazines. The computed charge distributions and the dipole moments are discussed and compared with available experimental data. The relative basicities of the derivatives of pyridine and the diazines have been investigated and an attempt made to correlate the results with known pK_a values. Localisation energies for the nucleophilic substitution of halogen by methoxide ion have also been calculated and the predicted relative reactivities compared with experimental observations, where these were available.     

Calculation of the in-plane force constants and vibrational spectra of pyridine and its deuterated derivatives by the CNDO/2 force method

The in-plane vibrations of pyridine and its deuterated derivatives (4-d₁, 2,6-d₂, 3,5-d₂, and d₅) have been studied with the CNDO/2 force method. The calculated values of the force field scaled by five empirical parameters yield good agreement with the experimental spectra. The results seem to permit the reassignment of some bands.     

Vergleich der Reaktivität von Triethyltrithiophosphit und Trialkylphosphit bei der Synthese von P?N-Verbindungen

Comparison of the Reactivity of Triethyl Phosphorotrithioite and Triethylphosphite by Synthesis of P? N Compounds Triethyl phosphorotrithioite, (EtS)₃P, reacts unlike triethylphosphite with carbon tetrachloride and nitrogen containing nucleophiles. No reaction occurs with diester amides of phosphoric acid. When (EtS)₃P is allowed to react with primary aliphatic amines not amides of phosphoric acid are obtained, but (EtS)₂PCCl₃ and EtSNHEt are yielded. In the reaction of (EtS)₃P with (EtO)₂P(O)H, CCl₄, and NaN₃ results the expected monophosphazene (EtO)₂P(O)? N?P(SEt)₃ only. The different nucleophilicity of phosphorus in (EtS)₂P and (EtO)₃P follows from CNDO/2-MO calculations, too.     

CNDO/2 calculations of relaxation and reconstruction of diamond and silicon [111] surfaces

CNDO /2 calculations have been performed on the clusters X₄H₉ and X₄Y₉ modeling the [111] diamond and silicon surfaces. The X is either carbon or silicon atom and the Y is a pseudoatom containing one sp³ hybrid orbital. It is shown that in the CNDO /2 approximation in the foregoing pseudoatom models, the charge distribution of the cluster is better than the hydrogen atom, because the electronegativity of the hydrogen differs significantly from the electronegativity of the sp² orbital of the silicon atom. Using the CNDO /2 parametrization, the electronegativity of the hydrogen is very near to the electronegativity of the sp³ orbital of the carbon atom, thus the hydrogen can be used for the saturation of the carbon clusters.     

Interstitial and lattice substituted models of an expanded ice-I hlattice

An ice-I _hlattice (O?O=0.286 nm) with small molecules placed interstitially and with some placed at lattice positions was investigated by the CNDO/2-MO technique. The interstitial molecules included H₂, N₂, O₂, HF, CO₂, H₂O, NH₃, CH₄ and CH₂O, whereas those involved in lattice substitution included HF, NH₃ and H₂O. From the calculations it is found that all interstitial and lattice substituted systems are stabler than the sum of the components, the enhanced stability depending on the system. Generally, lattice substituted systems are stabler than the corresponding interstitial models. Charges residing on the atomic positions reflect the amount of interaction with the matrix as well as indicating how the change in charge would facilitate other interactions with the solvent.     

The most stable CNDO/2 water dimer

The most stable CNDO/2 water dimer is a planarC _2h species with a partial O-O bond. STO-3G calculations indicate that the stability of this structure is due to an artifact of the CNDO/2 method.     

CONFORMATIONAL PREFERENCES ABOUT Cpy-S BONDS IN DI-2-PYRIDYL DISULFIDE

Abstract

The total energy, dipole moment and electron densities for each possible rotational conformation about the C_py-S bonds of di-2-pyridyl disulfide were evaluated by using the semi-quantitative CNDO/2 method. The conformations in which the pyridine rings are coplanar with the valency plane of the bonded sulfur atom (cis-cis, cis-trans and trans-trans) were predicted to be the most favored ones.

Results of the theoretical study, when compared to some experimental determinations such as dipole moment and variable temperature pmr spectra, provided evidence that easy interconversion between these conformations can occur.     

Theoretical description of the 2A′' and 2A′ states of the peroxyformyl radical

The equilibrium geometries and spin density distributions for some phosphoranyl radicals and the radical anion PH₃O^? were calculated by the UHF CNDO/2 method. Barriers to permutational isomerization in PF₄ and PH₃OH were estimated.