Probing chemical dynamics with negative ions

Experiments are reviewed in which key problems in chemical dynamics are probed by experiments based on photodetachment and/or photoexcitation of negative ions. Examples include transition state spectroscopy of biomolecular reactions, spectroscopy of open shell van der Waals complexes, photodissociation of free radicals, and time-resolved dynamics in clusters. The experimental methods used in these investigations are described along with representative systems that have been studied.     

The interpretation of the stability of aromatic hydrocarbon ions by valence bond theory

Hückel theory predicts the experimentally observed stabilities of the cyclic polyene ions C _nH_n ^–, C_nH_n ⁺, n=3, 5, 7 (C ₃H₃ ^–stable, C ₃H₃ ^–unstable, etc.) correctly. This result can also be obtained by a simple valence bond calculation which takes into account only the lowest energy resonance structures. E. g. it turns out that C ₃H₃ ⁺should be a singlet ground state while C ₃H₃ ^– should be a triplet ground state. However, the stability cannot be explained by just counting the number of resonance structures.

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Zusammenfassung Es wird gezeigt, daß die Stabilitätsverhältnisse der Cyclopolyenionen C _nH_n ^–, C _nH_n ⁺, n=3, 5, 7 (C ₃H₃ ⁺stabil, C ₃H₃ ^– instabil, usw.), die nach der Hückeltheorie richtig erklärt werden, auch nach der VB-Methode vorhergesagt werden können. Dabei ist es ausreichend, nur die energetisch niedrigsten Resonanzstrukturen mit zu berücksichtigen. Es ergibt sich dann z. B., daß C ₃H₃ ⁺einen Singulett-Grundzustand besitzt, C ₃H₃ ^–dagegen einen Triplett-Grundzustand. Die Anzahl der möglichen Resonanzstrukturen erlaubt jedoch keinen Rückschluß auf die Stabilitätsverhältnisse.

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Résumé La théorie de Hückel prédit correctement les stabilités expérimentales des ions des cyciopolyènes C _nH_n ^–, C _nH_n ⁺, n=3, 5, 7 (C ₃H₃ ⁺stable, C ₃H₃ ^–instable, etc.). Ce résultat peut être obtenu aussi par un simple calcul au cadre de la méthode de la mésomérie, ne comprenant que les formules de plus basse énergie. Il en résulte, par exemple, que l'état fondamental du C ₃H₃ ⁺devrait être un singulet, celui de C ₃H₃ ^–un triplet. Cependant, on ne peut pas expliquer la stabilité en comptant le nombre des formules mésomères.

     

Program for predicting interatomic distances in crystals by the bond valence method

The program is a computer realization of the bond valence method (BVM), used to predict bond lengths in crystal structures from topological data. Using object-oriented programming made it possible to analyze structures of any complexity (up to 6480 crystal chemical positions). Hardware and software requirements: i486DX processor, operating system Windows 3.1/95, 4M RAM, and about 2M disk space. The program was developed at the Crystallography and Crystal Chemistry Department, Geological Faculty, Moscow State University. Translated fromZhumal Strukturnoi Khimii, Vol. 39, No. 4, pp. 708–713, July–August, 1998.     

The electronic structure of inorganic benzenes: valence bond and ring-current descriptions

Valence bond (VB) theory and ring-current maps have been used to study the electronic structure of inorganic benzene analogues X(6)H(6) (X = C (1), Si (2)), X(6) (X = N (3), P (4)), X(3)Y(3)H(6) (X,Y = B,N (5), B,P (6), Al,N (7), Al,P (8)), and B(3)Y(3)H(3) (Y = O (9), S (10)). It is shown that the homonuclear compounds possess benzene-like character, with resonance between two Kekulé-like structures and induced diatropic ring currents. Heteronuclear compounds typically show localization of the lone pairs on the electronegative atoms; Kekulé-like structures do not contribute. Of the heteronuclear compounds, only B(3)P(3)H(6) (6) has some benzene-like features with a significant contribution of two Kekulé-like structures to its VB wave function, an appreciable resonance energy, and a discernible diatropic ring current in planar geometry. However, relaxation of 6 to the optimal nonplanar chair conformation is accompanied by the onset of localization of the ring current.     

The valence bond description of xylylenes

The ground states or ortho-, meta- and para-xylylenes and low lying excited states of meta-xylylenes are investigated by the valence-bond approach. Weights of structural formulas are calculated. A criterion for biradical character is defined as the sum of the weights of biradical structures. It is found that meta-xylylene is best described as a benzene ring relatively unperturbed by the two adjacent méthylène radicals, and that ortho- and para-xylylene are unequal mixtures of localized Kékulé structures and aromatic biradical structures. Surprisingly, low lying excited states of meta-xylylene deviate from the zwitterionic picture expected for singlet excited states of biradicals.     

Some considerations about equations correlating valence and length of a chemical bond

It is confirmed that the Zachariasen equation is more accurate than the Brown-Shannon equation. Moreover, it is shown that, given the value of the parameter R₁ relative to the MoO bond in MoO₆ octahedra, the calculated valence of molybdenum in Mo(V)O₆ octahedra remains very close to 5 v.u. by varying the other bond parameter B in a wide range. For other oxidation states of molybdenum in MoO₆ octahedra, the difference between the calculated valence and the formal oxidation state shows different types of dependence on B. These behaviours could explain some errors in valence data of the literature. The parameters for the MoO bond in MoO₆ octahedra are recalculated as R₁=1.8788 Å and B=0.3046 Å for all molybdenum oxidation states from +3 to +6.     

The orthogonality problem in valence bond calculations

The energy of the π-electron system in benzene has been obtained by means of valence bond calculations using orthogonal and nonorthogonal basis orbitals at different levels of configuration interaction, which shows the influence of the orthogonalization procedures.     

Calculation of the bulk modulus of simple and complex crystals with the chemical bond method

The relation between the lattice energies and the bulk moduli on binary inorganic crystals was studied, and the concept of lattice energy density is introduced. We find that the lattice energy densities are in good linear relation with the bulk moduli in the same type of crystals, the slopes of fitting lines for various types of crystals are related to the valence and coordination number of cations of crystals, and the empirical expression of calculated slope is obtained. From crystal structure, the calculated results are in very good agreement with the experimental values. At the same time, by means of the dielectric theory of the chemical bond and the calculating method of the lattice energy of complex crystals, the estimative method of the bulk modulus of complex crystals was established reasonably, and the calculated results are in very good agreement with the experimental values.     

Role of sublattices in the formation of the chemical bond in ion-covalent crystals

Crystals of ZnGeAs₂ and AgGaTe₂ are used as examples to show the results of applying the sublattice method. The latter enables one to visualize the formation of valence band structure, electron density and chemical bond as a result of interactions between the atoms in various sublattices of the crystal.     

Electronic structure and chemical bond in KN<Subscript>3</Subscript> and KSCN crystals

Crystal structure parameters, band spectra, distribution maps of the toal and partial electron densities of KN₃ and KSCN are calculated from the first principles using the density functional method and the general properties and distinctions in the band spectra are analyzed. The groups of balance subbands are qualitatively similar; in the upper valence band of KN₃ there is weak hybridization of p states of the complex anion and metal, which is not observed in KSCN. KSCN crystals are direct band gap materials and KN₃ are indirect band gap materials. The existence of weak covalent bonding of complex anions with metal is found from the electron density distribution maps in both crystals and also between metal atoms in KSCN, which is absent in KN₃.