Free-electron-pair states

The typical properties of water arise from the ability of the water molecule to participate in four hydrogen bonds due to its two protons and its two lone electron pairs (2s)2 (2pz)2 which act as proton acceptors. In the condensed state, the angle between the 2px and the 2py orbital of oxygen is enlarged by hydridisation to a mixture of s- and p-state to 109°. Because both of the free electron pairs are situated in a plane... 

RG12 Valence electrons are transformed from a free electron pair into bonding electrons and a change in the valence state of atom X occurs. Before this scheme is applied in EROS, a table of valence states for each atom is scanned to determine whether this change in the valence for atom X is allowed. The scheme has importance in representing oxidations at atom X as exemplified with the change Sn - SIV (Fig. 13). 

In which the terms Ha refers to the number of free electron pairs, MW is the molecular weight, and clogP is the computed lipophilicity. While this method could be stated to be "partially in silico" because it utilizes some chemical descriptors, the need for in vivo animal data and their dominance in the individual terms really makes this approach more of an animal-human correlation than an in silico method. Finally, in the same report, the authors describe a regression based solely in animal data. Overall, the performance of these... 

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

In the pure state, a potential electrolyte such as oxalic acid (HOOCCOOH) consists of uncharged molecules. A true electrolyte such as NaCl in the pure state consists of two separate ions, Na and CP. The proton is a bare nucleus it has no electrons. It is chemically unstable as an isolated entity because of its affinity for electrons. As a result, the proton reacts with the free electron pair of oxygen in the H2O molecule. 

The simplest carbene CH2 ( methylene, Figure 3.14, center) is a bent molecule with an H,C,H bond angle of 135° and has a triplet ground state. The singlet CH2 is less stable by 8 kcal/mol. Its free electron pair occupies the sp AO (because in this orbital it is nearer to the nucleus and therefore more stabilized than in the 2pz AO), and the H,C,H bond angle amounts to 105° (two electrons in the sp AO as compared to one electron in the sp AO of triplet CH2 cf. the discussion of VSEPR theory in Section 1.1.1). 

At the stage of the betain 11, a reaction with the alcohol involving attachment of a proton (deuterium) to the free electron pair occurs (17). The resulting unsolvated alkoxide anion of the complex now abstracts a proton from the carbon atom bearing the deuterium whereby a betain with conformation 18 is formed. To comply with the stereochemical conditions o the Elcb-transition state, the free... 

This interaction with a free electron pair will have the consequence that in various cases of unsaturated organic molecules certain configurations, which contribute little to the stationary state of the free molecule, are of much greater importance in the complex formed, as a result of which many reactions can proceed under the catalytic action of these sextet substances ( 36). With the proton, thus in OH3+ and also in HF, the interaction with the organic molecules is entirely electrostatic... 

It seems not improbable that the formation of the bond between these molecules is similar to that between molecules with a sextet configuration, such as BF3, with molecules with a free electron pair. In the molecules of the first group, such as the nitro compounds, the sextet configuration, however, contributes only to a limited extent to the stationary state of the free molecule. If the energy of this sextet configuration is lowered sufficiently compared with that of the normal octet configuration by complex formation with a suitable partner, complex formation is possible. 

It has been theoretically and experimentally well established that silylenes have a singlet ground state [1]. Such species posses a free electron pair in a o-orbital and an empty orbital of Jt-symmetry therefore, they are a priori ambiphilic compounds, which can react either as an electrophile or as a nucleophile towards appropriate substrates. However, most silylenes have revealed a distinctive "electrophilic character". Dimethylsilyene, e g., adds to olefins and alkynes in the gas phase via a rate-controlling step that is accelerated by electron-donating substituents [2] these experimental results are in good agreement with a theoretical study of the reaction of SiH2 with ethylene, which shows that this cycloaddition proceeds via an initial electrophilic phase in which the silylene LUMO interacts with the 7t-electron system of the double bond [3]. Up to now, only some stable silylenes, such as recently described 1 [4] or silicocene 2 [5] have shown nucleophilic reactivity. 

Spectrophotometric methods of identification and determination of substances are based on the existence of relationships between the position and intensity of absorption bands of electromagnetic radiation, on the one hand, and molecular structure on the other. Electronic spectra result from changes in the energy states of electrons [o, ti, and free electron pairs (n)] in a molecule as a result of absorption in the UV-VIS region. The changes depend on the probability of electronic transitions between the individual energy states of the molecule. The number of absorption bands, and their positions, intensities and shapes are the spectral parameters utilized in qualitative and quantitative chemical analyses [1-3]. 

The features of the absorption spectra change if the so-called auxochromes (e.g., -NH2, -NR2, -SH, -OH, -OR) are introduced into the molecules. The presence of free electron pairs in the auxochromic group, that interact with electrons of the chromophoric group (e.g., the free electron pair at nitrogen in the -NH2 group) leads to a state of conjugation which may result in formation of a new absorption band in the spectrum. 

Metals, like many of the elements discussed in previous chapters, can exist in nature in several different oxidation states. When bonded to other elements, metal ions are almost always assigned a positive oxidation number and are somewhat electrophilic. Because of this, they are stabilized by association with electron-rich atoms. In particular, atoms that have a free electron pair can "donate" some of their electron density to the metal to form a bond. The most common and environmentally important donor atoms are oxygen, nitrogen, and sulfur. The bonds they form with metal ions range in strength from relatively weak associations such as those between a dissolved metal ion and water to very strong covalent bonds. These types of bonds are significant in both aqueous phase reactions and in the formation of insoluble compounds. 

In their organic derivatives arsenic, antimony, and bismuth occur in the valence state +3 and +5. The derivatives of the trivalent metal have a free electron pair, and some of them thus react extremely violently with electron acceptors. 

Note that we have demonstrated recently by in situ Pt-XANES that the interaction between the free electron pairs at the oj gen of various mqrgen containing unsaturated molecules and the unoccupied electronic states above the Fermi level of Pt is very small [19]. This may indeed explain the need of polarity... 

The second situation has already been dealt with in the section on the Molecular dynamics of three-electron bond formation . Whenever both heteroatoms provide a free electron pair in the ground state (e.g., for the above example in basic solution) there will be lone pair - lone pair interaction already prior to oxidation, and the latter will then take place from the joint used, doubly occupied o level. The question at which heteroatom the initial oxidation occurs becomes, therefore, irrelevant. 

It is known that complex formation (the coordination of a free electron pair of a donor atom of the ligand to the acceptor) changes the electronic structures, energy states and symmetry conditions of both coordinating ligand and the acceptor ion or molecule this results in change in their vibrational spectra, in the force constants determined from these, etc. Thus, in the event of the donor-acceptor interaction of a solvent and solute, the infrared or Raman bands of both the solute and the solvent may provide information on this process. 

An atom s valence is the sum of the multiplicities of its covalent bonds. In terms of the molecular graph drawing, it is the number of lines ending in a node. For example, the usual valences of H, 0, N, eind C atoms in their ground states are 1, 2,3, eind 4, respectively. There are chemical elements that have more than one possible valence, i.e. they can have more than one ground state. For example, phosphorus can have a valence of 3 or 5, while sulfiir can have a valence of 2, 4, and 6. Such variations correspond to variations in the number of free electron pairs. 

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