Environmental effects in the MO treatment of the trimethylamine-trimethylboron addition reaction

An attempt has been made to estimate charge redistribution effects in the Lipscomb MO treatment of the trimethylamine-trimethylboron addition reaction. The zeroth-order results which show substantial electron shifts and features suggesting charge imbalance in the amine and alkylboron accompanying reaction are modified by a self-consistent, linear -upon-orbital charge adjustment method. The form and parametrization are derived from known atom behavior upon charging and from attempts at separation of charge effects in limited SCF calculations for simpler molecules. Whereas the charge imbalances appear to be satisfactorily remedied, the method as presently parametrized does not simultaneously improve correlations with the limited set of experimental observables for this reaction system.

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Zusammenfassung Ein Versuch zur AbschÄtzung der Ladungsumverteilung bei der Dimethylamin-Trimethylboran Addition wurde mittels des MO-Verfahrens nach Lipscomb gemacht. Die Ergebnisse der nullten Ordnung, die wesentliche Elektronenverschiebungen zeigen und darauf hindeuten, da\ die Reaktion zu unwahrscheinlichen Ladungsverteilungen im Amin und Alkylboran führt, werden durch eine lineare Beziehung zwischen und der Orbitalladung modifiziert. Form und Parameter erhÄlt man aus dem Verhalten von Atomen gegenüber Zusatzladungen und aus Separationsversuchen von Ladungseffekten bei einfachen SCF-Rechnungen an kleineren Molekülen. Zwar verschwindet die problematische Ladungsverteilung, aber der Grad der übereinstimmung mit einigen anderen experimentell beobachteten Grö\en bleibt bei den gegenwÄrtigen Parametern unverÄndert.

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Résumé On a tenté l'estimation des effets de redistribution de charge dans le traitement O. M. de Lipscomb pour la réaction d'addition triméthylamine-triméthylbore. Les résultats d'ordre zéro qui révèlent des déplacements électroniques importants et suggèrent le non équilibre des charges sont modifiés par une méthode self-consistante d'ajustement linéaire des charges. La forme et les paramètres sont déduits de la connaissance du comportement atomique à la charge et de tentatives pour séparer les effets de charge dans des calculs SCF limités pour des molécules plus simples. Alors que les déséquilibres de charge sont compensés d'une manière satisfaisante, la méthode ainsi paramétrisée ne premet pas d'améliorer simultanément les corrélations avec l'ensemble limité d'observables expérimentales du système réagisant.

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Research performed under the auspices of the U.S. Atomic Energy Commission     

Structural and electronic effects on the reactivity of carbamoylsilanes towards acrylonitrile addition

The effect of substituents on Si and N on t_1/2 values in the addition of carbamoylsilanes to acrylonitrile was explored. After examination of steric and structural parameters, the best correlation was found to be that rates increased with a decrease in the ionization potential of the carbamoylsilane.     

Bonded exciplex formation: electronic and stereoelectronic effects

As recently proposed, the singlet-excited states of several cyanoaromatics react with pyridine via bonded-exciplex formation, a novel concept in photochemical charge transfer reactions. Presented here are electronic and steric effects on the quenching rate constants, which provide valuable support for the model. Additionally, excited-state quenching in poly(vinylpyridine) is strongly inhibited both relative to that in neat pyridine and also to conventional exciplex formation in polymers, consistent with a restrictive orientational requirement for the formation of bonded exciplexes. Examples of competing reactions to form both conventional and bonded exciplexes are presented, which illustrate the delicate balance between these two processes when their reaction energetics are similar. Experimental and computational evidence is provided for the formation of a bonded exciplex in the reaction of the singlet excited state of 2,6,9,10-tetracyanoanthracene (TCA) with an oxygen-substituted donor, dioxane, thus expanding the scope of bonded exciplexes.     

Theory of photoelectric complex formation titrations using metal indicators

The general principles of photoelectric complex formation titrations using metal indicators are given and the mathematical equations necessary for the exact determination of the equivalence point are derived for titrations with one-colour and two-colour indicators.A method for preparing a comparison solution of exactly the right composition is suggested.The influence of various factors on the titrations is discussed and it is shown that the theoretically attainable precision and accuracy are extremely high.Some titrations of copper and magnesium with EDTA are described which illustrate the accuracy attainable.     

The electronic spectrum of the triethylamine-perfluoro-tert-butanol complex

The far-ultraviolet absorption spectrum of the triethylamine-perfluoro-tert-butanol complex has been measured in the gas phase. The photoelectron band of lowest energy has also been determined. It is about 13000 cm^?1 higher than for free triethylamine. Up to at least 60000 cm^?1, the UV spectrum is readily interpreted as a triethylamine spectrum shifted to higher frequencies by about 13000 cm^?1. The bands of lowest frequency are the 3s and 3p Rydberg bands which follow the ionization potential. Up to 60000 cm^?1, no charge transfer type band has been found.     

Solvent effects in the aza-Michael addition of anilines

The aza-Michael addition of functionally substituted anilines to enoates is significantly affected by the hydrogen bond donor ability of the solvent. Very polar protic fluorinated alcohols (hexafluoroisopropanol and trifluoroethanol) favor the addition of weak nucleophiles, whereas these solvents are no longer required for anilines bearing additional OH or NH₂ group.     

Molecular modeling of the influence of complex formation on the conformation and electronic absorption spectra of crown-containing styryl dyes

The conformation and electronic absorption spectra of crown-containing styryl dyes and their complexes with metal salts have been calculated. The long-wave absorption bands have been assigned. It was shown that the observed large hypsochromic shift of the absorption spectra during the formation of complexes of the cis-form of the dyes under consideration with alkaline earth ions is due to a change in the conformation of the chromophore and the resultant approximate prohibition with respect to symmetry of the long-wave electronic transition.Institute of Chemical Physics, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 39–45, March–April, 1993.     

Polyelectrolyte effects on the charge transfer complex formation between FMN and indole derivatives

The complex formation of flavin mononucleotide (FMN) with tryptamine, indoleacetate, and trytophan was investigated in the presence of polyelectrolytes; that is, sodium polyethylene sulfonate (NaPES), sodium polystyrene sulfonate (NaPSS), and a copolymer of diethyldiallylammonium chloride and sulfur dioxide (DECS). The complexation of FMN and tryptamine was strongly retarded by the macrocations and macroanions, whereas that of FMN and indoleacetate was enhanced by the macrocations. Furthermore, the equilibrium constants of FMN-tryptophan complex were insensitive to the addition of polyelectrolytes. These results suggest that the complexation of FMN was strongly influenced by the electrostatic interactions between the reactant ions and macroions, in addition to those between the reactant ions, in conformity with the secondary salt effect.     

Calculated electronic transitions of the water ammonia complex

We have calculated vertical excitation energies and oscillator strengths of the low lying electronic transitions in H2O, NH3, and H2ONH3 using a hierarchy of coupled cluster response functions [coupled cluster singles (CCS), second order approximate coupled cluster singles and doubles (CC2), coupled cluster singles and doubles (CCSD), and third order approximate coupled cluster singles, doubles, and triples (CC3)] and correlation consistent basis functions (n-aug-cc-pVXZ, where n=s,d,t and X=D,T,Q). Our calculations indicate that significant changes in the absorption spectra of the photodissociative states of H2O and NH3 monomers occur upon complexation. In particular, we find that the electronic transitions originating from NH3 are blueshifted, whereas the electronic transitions originating from H2O are redshifted.     

Reinvestigation of the electronic spectroscopy of the Au-Ar complex

The Au-Ar complex is reinvestigated employing resonance-enhanced multiphoton ionization spectroscopy. Spectra are reported, corresponding to the atomic transition Au(6p<--6s). This electronic excitation yields (2)Pi and (2)Sigma(+) states of Au-Ar, which interact under the influence of spin-orbit coupling. The spectra are consistent with strong sigma-pi mixing induced by the large spin-orbit coupling of Au, leading to strong interaction of the two Omega=12 states, which arise from the Ar((1)S(0))+Au((2)P(12,32)) asymptotes, and the consequent formation of a "shelf" on the outer wall of the lowest Omega=12 state. In addition, high-level ab initio calculations are reported on the ground electronic state, X (2)Sigma(+), including extrapolation to the basis set limit.     

Modifications of hydrophobic cavity and their effects on the complex formation with a hydrophobic substrate

The derivatives of CP44 ($\text{1}$a) were designed and synthesized to modify the natures of its hydrophobic cavity, and the effects on the complex formation with a hydrophobic substrate in aqueous solution were examined.     

Theory of electronic circular dichroism lineshapes

A microscopic, quantum field theory of lineshapes for electronic circular dichroism spectra is presented. A simple, model Hamiltonian for a single impurity in a crystal is considered. In this formalism, electron-photon coupling terms contribute directly to the magnetic transition dipole moment. Lineshape functions for absorbance and circular dichroism spectra are derived. Electronic circular dichroism spectra contain vibronic contributions which do not appear in absorbance spectra. This treatment does not require perturbation theory to obtain the vibrational contribution to the circular dichroism lineshape.     

Protonation thermodynamics of 1,10-phenanthroline in aqueous solution. Salt effects and weak complex formation

The protonation of 1,10-phenanthroline has been studied potentiometrically in different aqueous salt media (LiCl, NaCl, KCl, Me₄NCl, Et₄NI, MgCl₂, CaCl₂, SrCl₂, and BaCl₂) in the ionic strength range 0≤1≤1 M. This ligand forms two protonated species, [H(phen)]⁺ and [H(phen)₂]⁺; the monoligand species shows protonation constants strongly dependent on the medium. Medium effects were explained by the formation of some weak species: [H(phen)Cl]⁰, [M(phen)]²⁺ (M=alkaline earth metal cations). Formation thermodynamic parameters are reported.     

Azide addition to give a tetra-azazirconacycle complex

Addition of the dilithium salt, ortho-(Me3SiNLi)2C6H4, to ZrCl4 affords a base-free, D2d-symmetric complex Zr(IV)[ortho-(Me3SiN)2C6H4]2 (2), with rigorously planar ortho-phenylenediamine ligands. Lewis acidic 2 readily coordinates donor ligands such as NHEt2 to give the five-coordinate complex, Zr(IV)(NHEt2)[ortho-(Me3SiN)2C6H4]2 (3), which is also accessible by the reaction of Zr(NEt2)4 with 2 equiv of ortho-(Me3SiNH)2C6H4. Aryl azides react with 2 and 3 to give an unusual tetra-azametallacycle complex, 4, via 1,2-addition of a ligand N-Si bond to the organic azide. An X-ray crystal structure of 4 reveals a planar, five-membered metallacycle comprising the zirconium atom, one nitrogen atom of the ortho-(Me3SiN)2C6H4 ligand, and all three nitrogen atoms of the aryl azide.     

Application of the generalized standard addition method to correction for the effects of peak overlap and intermetallic compound formation in potentiometric stripping analysis

Applications of the generalized standard addition method in potentiometric stripping analysis are presented. Accurate quantitative results can be obtained, even when the analytical signals are strongly affected by peak overlaps or by the formation of intermetallic compounds. The possibilities and limitations of the approach are discussed. An optimized experimental procedure for correction of the copper-zinc intermetallic interference is introduced.