The calculation of valence angles for H2O and NH3 using the maximum overlap principle

A new approximate method for the determination of the valence bond angles for MX **** _k -type molecules, based on the maximum overlap principle, is proposed. The valence bond angles are determined from the conditions of maxima of the total bonds strength. The method has been used to calculate the valence angles and the hybrid orbitals for the water, and the ammonia molecule, respectively. The calculated valence bond angles for both molecules are about 2 too high with respect to experimental equilibrium values. Also the best hybrid orbitals using the Golebiewski simplified method were calculated.

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Zusammenfassung Eine neue NÄherungsmethode für die Bestimmung der Valenzwinkel für Moleküle vom Typ MX **** _k wird vorgeschlagen, die sich auf das Prinzip der maximalen überlappung gründet. Die Valenzwinkel werden aus der Forderung nach einem Maximum der GesamtbindungsstÄrke abgeleitet. Die Methode wird zur Berechnung der Valenzwinkel und der Hybridorbitale von H₂O und NH₃ angewendet. Die berechneten Valenzwinkel sind für beide Moleküle etwa 2 zu hoch gegenüber den experimentellen Gleichgewichtswerten. Die besten Hybridorbitale werden entsprechend der vereinfachten Methode von Golebiewski berechnet.

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Résumé On propose, sur la base du principe du recouvrement maximum, une nouvelle méthode approchée pour la détermination des angles des liaisons de valence des molécules de type MX **** _k . Ces angles sont déterminés à partir des conditions de maxima de la «force totale de liaison». La méthode a été utilisée pour calculer les angles de valence et les orbitales hybrides de l'eau et de l'ammoniac. Les valeurs obtenues sont supérieures d'environ 2 aux valeurs expérimentales. Les meilleures orbitales hybrides de ces deux molécules ont été aussi calculées par la méthode simplifiée de Golebiewski.

     

The time evolution of the sol-gel process:29Si NMR study of hydrolysis and condensation reactions of tetramethoxysilane

The time evolution of the structural intermediate formation obtained during hydrolysis and condensation reactions in tetramethoxysilane (TMOS), methanol, H₂O, HCl (molar ratio 1:5:2: 0.001) sol-gel reaction mixture was investigated by²⁹Si NMR spectroscopy at ?75°C. It is obvious that the substitution of the Si methoxy group (OCH₃) by a hydroxyl (OH) shifts the resonance lines downfield. The dimer peaks are located about 9–10 ppm upfield from the corresponding monomers, and the Si located in the middle of the trimers appeared at the chemical shift of 18–19 ppm upfield from the corresponding monomers. The peaks of trimers, cyclic tetramers and the other higher oligomers are relatively broad and weak, and exhibit a multiplet structure. The time dependence of the concentrations of the individual types of silicon in the reaction mixture were evaluated from the integral intensity of the corresponding Si atoms from the²⁹Si NMR spectra. The results obtained represent input data to the various kinetic models of the complex hydrolysis and condensation reactions.     

Construction of the best hybrid orbitals for a central atom and ligands

A new appropriate maximum overlap method for the construction of the best hybrid orbitals proposed by Murrell and simplified by Golebiewski has been extended simultaneously to the central atom and its ligands. Simple and explicit matrix formulae for the best hybrids are given and, as a particular example, chlorine trifluoride is considered explicitly.     

Investigation of 3,5-dichlorosalicylate-copper(II)-(3-pyridylmethanol or N,N′-diethylnicotinamide) complex systems by EPR spectroscopy

Copper(II) complex systems, containing 3,5-dichlorosalicylic acid (3,5-Cl₂salH) and different copper salts (Cu(ac)₂ or CuSO₄), with varying concentrations of 3-pyridylmethanol (ronicol, ron) or N,N′-diethylnicotinamide (denia) as ligands, were prepared. The effects of Cu(II) salts containing anions of different basicity and two different N-donor ligands with varying ligand-to-metal ratio (x) on the formation of the resultant complexes in the water/methanol solutions were studied by EPR spectroscopy at low temperature of 98 K. When ligand concentration was increased, the resolution and signal intensity of the ¹⁴N perpendicular super-hyperfine splitting patterns increased (for ron more progressively), becoming saturated at higher ligand concentrations. In comparing the ronicoland denia-containing systems, both g-factors (g _⊥, g _‖) are systematically higher and the parallel hyperfine splitting (A _‖) systematically lower in denia-containing systems for both Cu(II) salts.     

Maximum overlap approximation calculations on polyatomic molecules: IV. Saturated haloalkanes

The extended maximum overlap approximation (EMOA) method has been applied to a series of fluorine and chlorine derivatives of some alkanes in the present communication. Strictly localized molecular orbitais for 34 molecules have been constructed explicitly from hybrid atomic orbitals on IEHT level of approximation. The calculated heats of formation and dipole moments are in remarkable agreement with experimental data. Results are comparable with MENDO/3 and CNDO/2 calculations, respectively. Charge distributions show that the electron withdrawing effect of the halogen extends throughout the molecular skeleton with decreasing intensity; the inductive effects of the groups +I [CH₃—] < +I [CH₃CH₂-] < +I [CH₃CH₂CH₂-] < +I [(CH₃)₂CH—] < +I [(CH₃)₃C—] (positive) and —I [F—] > —I [Cl—] (negative) are perfectly reproduced. The s-electron density on carbon and proton in C—H bond correlates with the experimental carbon-13—proton nuclear spin-spin coupling constant in the series of some chloroethanes.     

Copper(II) complexes with derivatives of salen and tetrahydrosalen: a spectroscopic, electrochemical and structural study

Salen type complexes, CuL, the corresponding tetrahydrosalen type complexes, Cu[H₄]L, and N,N′-dimethylated tetrahydrosalen type complexes, Cu[H₂Me₂]L, were investigated using cyclic voltammetry, and electronic and ESR spectroscopy. In addition, the analogous copper(II) complexes with a derivative of the tetradentate ligand ‘salphen’ [salphen=H₂salphen=N,N′-disalicylidene-1,2-diaminobenzene] were studied. Solutions of CuL, Cu[H₄]L and Cu[H₂Me₂]L are air-stable at ambient temperature, except for the complex Cu(^tBu, Me)[H₄]salphen [H₂(^tBu, Me)[H₄]salphen=N,N′-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-1,2-diaminobenzene]. Cu(^tBu, Me)[H₄]salphen interacts with dioxygen and the ligand is oxidatively dehydrogenated (–CH₂–NH–→–C=N–) to form Cu(^tBu, Me)[H₂]salphen and finally, in the presence of base, Cu(^tBu, Me)salphen. X-ray structure analysis of Cu(^tBu, Me)[H₂Me₂]salen confirms a slightly tetrahedrally distorted planar geometry of the CuN₂O₂ coordination core. The complexes were subjected to spectrophotometric titration with pyridine, to determine the equilibrium constants for adduct formation. It was found that the metal center in the complexes studied is only of weak Lewis acidity. In dichlormethane, the oxidation Cu(II)/Cu(III) is quasireversible for the CuL type complexes, but irreversible for the Cu[H₄]L and Cu[H₂Me₂]L type. A poorly defined wave was observed for the irreversible reduction Cu(II)/Cu(I) at potentials less than −1.0 V. The ESR spectra of CuL at both 77 K and room temperature reveal that very well resolved lines can be attributed to the interaction of an unpaired electron spin with the copper nuclear spin, ¹⁴N donor nuclei and to a distant interaction with two equivalent protons [A^Cu(iso)≈253 MHz, A^N(iso)≈43 MHz, A^N(iso)≈20 MHz]. These protons are attached to the carbon atoms adjacent to the ¹⁴N nuclei. In contrast to CuL, the number of lines in the spectra of the complexes Cu[H₄]L and Cu[H₂Me₂]L is greatly reduced. At room temperature, only a quintet with a considerably smaller nitrogen shf splitting constant [A^N(iso)≈27 MHz] is observed. Both factors, planarity and conjugation, are thus essential for the observation of distant hydrogen shf splitting in CuL. Due to the C=N bond hydrogenation, the coordination polyhedra of the complexes Cu[H₄]L and Cu[H₂Me₂]L is more flexible and more sensitive to ligand modification than that of CuL. The electron-withdrawing effect of the phenyl ring of the phenylenediamine bridge is reflected in a reduction of the copper hyperfine coupling constants in Cu(^tBu, Me)[H₄]salphen and Cu(^tBu, Me)[H₂Me₂]salphen complexes [A^Cu(iso)≈215 MHz].     

Synthesis,structural characterization,EPR spectroscopy and Hirshfeld surface analysis of a novel Cu2+‐doped 3,14‐diethyl‐2,13‐diaza‐6,17‐diazoniatricyclo[16.4.0.07,12]docosane bis(perchlorate)

Cyclam derivatives and their metal complexes have been found to exhibit an anti‐HIV effect and stimulate the activity of stem cells from bone marrow. The strength of their binding to the CXCR4 receptor correlates with anti‐HIV and stem‐cell activities. Knowledge of the conformation and crystal packing of various macrocyclic metal complexes has become important in developing new effective anti‐HIV drugs. The synthesis and preparation of single crystals of a new Cu²⁺‐doped macrocyclic compound, (3,14‐diethyl‐2,6,13,17‐tetraazatricyclo[16.4.0.0^7,12]docosane)copper(II) bis(perchlorate)–3,14‐diethyl‐2,13‐diaza‐6,17‐diazoniatricyclo[16.4.0.0^7,12]docosane bis(perchlorate) (0.69/0.31), {[Cu(C₂₂H₄₄N₄)](ClO₄)₂}_0.69·(C₂₂H₄₆N₄²⁺·2ClO₄^?)_0.31, is reported. Characterization by X‐ray diffraction analysis shows that the asymmetric unit contains half of a centrosymmetric molecule. The macrocyclic ligand in the compound adopts the most stable trans‐III conformation. The Cu—N distances of 2.015 (3) and 2.047 (3) Å are normal, but the long axial Cu—O bond of 2.795 (3) Å may be due to a combination of the Jahn–Teller effect and the strong in‐plane ligand field. The crystal structure is stabilized by hydrogen bonding between secondary N—H groups, the N atoms of the macrocycle and the O atoms of the perchlorate anions. Hirshfeld surface analysis with 2D (two‐dimensional) fingerprint plots indicates that the main contributions to the crystal packing are from H…H (58.0%) and H…O/O…H (41.9%) interactions. Electron paramagnetic resonance (EPR) properties are also described.     

A total synthesis of chelidonine

Condensation of homophthalic anhydride 3 with the Schiff base 4 is exploited as the key step in a total synthesis of the benzophenanthridine alkaloid (±)-chelidonine (8).