Structure and Charge Distribution

The structure now universally accepted is that shown in Fig. 1, in which the two bridge hydrogen atoms lie on an axis that is perpendicularly bisected by the plane of the other six atoms. There are three mutually perpendicular twofold axes of symmetry (point group D.,), and the dimensions given are those ob-tained from a recent detailed analysis and conflation (192) of electron-diffraction data (13) with the results of high-resolution infrared spectroscopy (193), as listed in Table I (where the electron-diffraction results for deuterodiborane also appear) Even in the absence of microwave data, we can say that the parameters of diborane are now as accurately known as those for almost any other molecule of con arable size. 

X-Ray diffraction studies have yielded bond lengths 

Although nijmerous attempts have been made during the last two decades to describe the bonding along theoretical lines, none of these can be said to provide a perfect picture of the electron distribution 

The distances listed refer to the ground vibrational state. 

Parameters calculated from high-resolution infrared spectra by Lafferty, MaOci, and Coyle (Ref. 193). The boron-hydrogen distances aure values amd the boron-boron distance r.  

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Examine the structures and charge distributions of the lowest-energy anions, and draw all of the resonance contributors necessary to describe these ions. What features account for the fact that these anions are more stable than their alternatives ... 

Although even the smaller structural units of zeolites are large enough to tax the most advanced quantum chemical computational methods to their limits, nevertheless, it is now possible to determine the fundamental electronic properties of zeolite structural units. In addition to their unique geometrical (in fact, topological) properties, the electronic structure and charge distribution of zeolites are of fundamental importance in explaining their catalytic and other chemical properties. 

Since both MO and VB approaches have been shown to be equivalent (Shaik, 1981), in principle either may be used. However, the VB approach is simpler to use, more chemical in its application and well-suited to questions of transition state structure and charge distribution, intermediate formation, and mechanistic variations within a reaction family. Of particular importance is the fact that the qualitative VB procedure is far more general than the qualitative MO procedure, since it is based on simple Lewis structures. A qualitative MO treatment of elimination reactions, for example, (see Section 3, p. 161) would be too unwieldy and impractical to apply while, in contrast, a VB approach is entirely straightforward in its application. 

The AMI-calculated structure and charge distribution of radical the trication of [1-carotene have been reported and its UV absorption spectrum estimated from INDO/S methods.158 The decomposition of the ftiran radical cation proceeds by two separate pathways according to a recent theoretical study, one via formation of propene radical cation and CO, the other a lower energy process via acetylene and a ketene radical cation.159 As a result of a reflection mass spectrometric study, a likely mechanism is... 

Schnockel and coworkers have also calculated at SCF level, the molecular structures and charge distributions for H2S=S (1), Cl(H)Si=S (126) and Cl2Si=S (127) as well as those of Cl2Si=0 (128), the oxygen analogue of 127, and the results are presented in Figure 5. 

FIGURE 5. Molecular structures and charge distributions calculated at the SCF level for H2Si=S (1), ClHSi=S (126), Cl2Si=S (127) and Cl2Si=0 (128). SEN = shared electron numbers. See C. Ehahardt and R. Ahlrichs, Theor. Chim. Acta, 68, 231 (1985)... 

The mechanism of epoxide polymerisation with Zn-, Al- or other metal-based coordination catalysts containing multinuclear species can be presented schematically as in scheme (4a) (the structures and charge distribution are simplified in the scheme, without differentiation of + or and <5+ or 6 charges) [1] ... 

Ohno, T. and H. Kamimura. 1983. Band structures and charge distributions along the c-axis of higher stage graphite intercalation compounds. J. Phys. Soc. Jpn. 52 223-232. 

With equal radii, for example K+ and F, the hydration energy with 75 kcal and 121.5 kcal respectively is greater for the negative ion. This finds its basis in the more eccentric situation of the positive centres of charge of the water molecule (see also the hydrogen bond, 45). This result can also be calculated when accurate account is taken of the structure and charge distribution of the water molecule (Verwey). 

The rate constants k in the reference solvent O and in solvent S can be readily measured. Values of Ig for RX and Y are obtained as described above. Thus, by applying Eq. (5-115), Ig values for the activated complex can be calculated, providing information about the structure and charge distribution of activated complexes. Values of Ig °J[YRxe 9- (5-115) are compared with Ig values for relevant solutes, or for other well-established activated complexes, whose structure and charge distribution are such that they might act as models for the activated complex under consideration. The model with a Ig value close to Ig °Jr rj5x i ... 

Research dealing with the atomic structure and charge distribution at crystal surfaces is a modern and fast-developing field. Metal and semiconductor surfaces show many interesting features, and this is certainly also the case for electrochemical interfaces, that is, metal (or semiconductor)/(aqueous) solution interfaces. This section is not intended to give a comprehensive overview of surface and interfacial... 

Electrophoresis is a possible procedure for fractionating sequence isomers since the resistance to motion will be affected by secondary structure and charge distribution as well as by any effect due to differences in total charge. This can be seen on cellulose acetate at pH 3.5 by the separation of the tetranucleotides Gp(Gp, Ap)Up for example (Sanger et al. 1965). Electrophoresis on DEAE-paper in 7% formic acid is particularly powerful, and admirably complements electrophoresis on cellulose acetate at pH 3.5. For example, ApGpGpUp is separated from Gp(Gp, Ap)Up (Brownlee 1972). 

Topsom, R.D. (1987c). Substituent Effects on Ground-State Molecular Structures and Charge Distributions. Prog.Phys.Org.Chem., 16, 85-124. 

Fig. 1.2 X-ray structure and charge distribution of the bis(ylide)nickel catalyst [NiPh(Ph2PCHCMeO)(i-PrjPCH2)].

Hiemstra, T., Rahnemaie, R., and van Riemsdijk. W. H. (2004). Surface complexation of carbonate on goethite IR spectroscopy, structure and charge distribution. J. Colloid Interface Sci. 278, 282-290. 

This section reports about theoretical investigations of the band structures and charge distributions of B32 type Zintl phases. In the first subsection the methods used for calculating the band structure data of the corresponding phases are briefly outlined. In the remaining subsections various aspects of the chemical bonding in Zintl phases are analyzed. 

Recent computational studies [23[ on the structure and charge distribution of hydrogen bonded F2 suggest a more differentiated view on the supposedly electrophilic mechanism of direct fluorination of aliphatic and aromatic hydrocarbons. Ab-initio calculations indicate that even for the complex of Fj with the extremely strong hydrogen-bond donor F3F as a model system, the energy of complex formation is very low - only 0.38 kcal mol (MP2/6-31+G //MP2/6-31+G level of theory, ZPE and BSSE correction) [24] (Scheme 2.9). Because of the low polarizabil-... 

Kassapidou K, Jesse W, Kuil ME, Lapp A, Egelhaaf S, van der Maarel JRC. Structure and charge distribution in DNA and poly(styrenesulfonate) aqueous solutions. Macromolecules 1997 30 2671-2684. 

Gussoni M. Role of vibrational intensities in the determination of molecular structure and charge distribution. J Mol Struct 1984 113 323-340. 

Reversible, competitive enzyme inhibitors are often referred to as structural analogs, that is, they are molecules that resemble the structure and charge distribution of the natural substrate for a particular enz)mae. Because of this resemblance, the inhibitor can occupy the enzyme active site. However, no reaction can occur, and enzyme activity is inhibited. This inhibition is said to be competitive because... 

A structural analog is a molecule that has a structure and charge distribution very similar to that of the natural substrate of an enzyme. Generally they are able to bind to the enzyme active site. This inhibits enzyme activity because the normal substrate must compete with the structural analog to form an enzyme-substrate complex. 

Understanding catalytic mechanisms and designing inhibitors requires an understanding of the structure and charge distribution of the oxocarbenium ion. The resonance structures of an oxocarbenium ion imply a molecule that is somewhere between a carbocation (la) and an oxonium ion (lb). 

It should be mentioned that the structure and charge distribution of p-carotene (1) trication radical have been predicted by AMI calculations and the optical spectrum estimated based on INDO/S calculations [140],... 

J.R.C. van der Maarel, W. Jesse, M.E. Kuil and A. Lapp, Structure and charge distribution in poly(styrenesulfonate) ion exchange resins, Macromolecules, 1996, 29, 2039-2045. 

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