Electron pairs nonbonding

Nonbonded electron pair donors (w-donors) are expectedly readily protonated (or coordinated) with superacids. Remarkably, this includes even xenon, long considered an inert gas. The protonation of some 7T-, (T- and -bases and their subsequent ionization to carbocations or onium ions is depicted as follows ... 

In superacidic systems, water is completely protonated and no equilibrium containing free water is indicated. However, the nonbonded electron pair of H30 is still a potential electron donor and at very high acidities can be further protonated (however limited the equilibrium with H30 may be). Thus the acidity of such superacidic systems can exceed that of H30 and the leveling ont is by that of H40 . We found that similar situations exist with other electrophiles, raising their electrophilic nature (electrophilicity) substantially. 

Heterocyclic cations in which the bonding number is increased by a process other than the addition of a proton (or equivalent) present a special situation. Structures of this type usually result from the participation of an originally nonbonding electron pair or a heteroatom in the formation of a ring or a multiple bond. 

The preferred conformation is D because it maximizes the number of antiperiplanar relationships between nonbonded electron pairs and C—O bonds while avoiding the R -R van der Waals repulsions in conformations E and F. 

It might also be expected that enamines would be less basic than the corresponding saturated tertiary amines as a consequence of the delocalization of the nonbonded electron pair of the nitrogen. The older literature (/—/), which involved measurements in aqueous or partly aqueous solution, led to the opposite conclusion. This unexpected increase in basicity was rationalized in terms of an equilibrium between the enamine and the quaternary iminium hydroxide ... 

The structural chemistry of the Group 14 elements affords abundant illustrations of the trends to be expected from increasing atomic size, increasing electropositivity and increasing tendency to form compounds, and these will become clear during the more detailed treatment of the chemistry in the succeeding sections. The often complicated stereochemistry of compounds (which arises from the presence of a nonbonding electron-pair on the metal) is... 

Since nucleophilic atoms have nonbonding electrons they can be identified by inspection of Lewis structures. Draw Lewis structures of triraethylamine, methyl fluoride, and phenol. Draw all nonbonding electron pairs and identify all nucleophilic atoms. 

Backside attack may be favored in order to facilitate transfer of nonbonding electrons from the nucleophile into the electrophile s lowest-unoccupied molecular orbital (LUMO). Efficient electron transfer requires maximal overlap of the LUMO and the donor orbital (usually a nonbonded electron pair on the nucleophile). Examine the LUMO of methyl bromide. How would a nucleophile have to approach in order to obtain the best overlap Is your answer more consistent with preferential backside or frontside attack ... 

What is the preferred geometry about the radical center in free radicals Carbocation centers are characterized by a vacant orbital and are known to be planar, while carbanion centers incorporate a nonbonded electron pair and are typically pyramidal (see Chapter 1, Problem 9). 

The same equatorial preference is also manifested in the 3,3-disubstituted thietane oxides66,194. Thus, the NMR spectra of 5e,f contain two Me singlets at 1.23 and 1.30 ppm and two methylene multiplets at 3.03 and 3.53 ppm (in CDC13). The large difference in the chemical shifts of the axial and equatorial a-methylene hydrogens is characteristic of an axial nonbonded electron pair on sulfur (conformation 5e equation 73). This conformational preference is corroborated by the small differences in the chemical shifts of the two methyl groups, and fits the contention that 1,3-diaxial interactions are responsible for this ultimate result. Certainly, these interactions are greater in the conformer 5f. 

The N—>P dative bonds are weak and different in lengths (1.800 A on average), and the triflate anions are effectively extended to consider interaction with the counter ion. Again the phosphorus atom is strongly pyramidalized and features the aspects of an inert nonbonding electron pair. 

When heated, azodicarbonamide breaks apart into gaseous carbon monoxide, nitrogen, and ammonia. Azodicarbonamide is used as a foaming agent in the polymer indushy. (a) Add nonbonding electron pairs and multiple bonds as required to complete the Lewis stmcture of this molecule, (b) Determine the geometry around each inner atom. 

The Lewis stmcture of moiecuiar oxygen shows the two atoms connected by a double bond, with two nonbonding electron pairs on each oxygen atom. Both atoms in O2 are outer atoms, so there are no constraining bond angles and no need for hybridization. Atomic valence 2. S and 2 p orbitals can be used to describe the bonding in this molecule. 

As Figure 1.5 suggests, the lowest energy transition for molecules with essentially nonbonding electron pairs is of n->n character. These transitions are common for compounds containing oxygen, nitrogen, and sulfur. 

Molecules of this type are often called donor-acceptor complexes or sometimes charge transfer complexes (because charge is transferred from the donor to the acceptor as the nonbonding electron pair of the donor atom is shared with the acceptor atom). In other words, there is a formal transfer of one electron, which is evident in the formal charges on the atoms in the complex. Once formed, however, the bond is simply a covalent bond consisting of a pair of shared electrons, whose origin is irrelevant to the nature of the... 

Figure 4.12 Representation of the bonding and nonbonding electron pair domains in the ammonia molecule, an AX3E molecule.

The nonequivalence in the size and shape of bonding and nonbonding electron pair domains can alternatively be expressed in terms of the relative magnitude of their mutual Pauli repulsions, which decrease in the following order ... 

According to the VSEPR model the T-shaped CIF3 molecule has a trigonal bipyramidal arrangement of three bonding electron pairs and two nonbonding electron pairs in the valence shell of the central Cl atom. 

Maxima are always observed in the VSCC of an atom in a molecule corresponding in number and geometrical arrangement to the nonbonding electron pair domains of the VSEPR model. 

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