Lone pairs s character

Photoelectron spectroscopic studies show that the first ionization potential (lone pair electrons) for cyclic amines falls in the order aziridine (9.85 eV) > azetidine (9.04) > pyrrolidine (8.77) > piperidine (8.64), reflecting a decrease in lone pair s-character in the series. This correlates well with the relative vapour phase basicities determined by ion cyclotron resonance, but not with basicity in aqueous solution, where azetidine (pKa 11.29) appears more basic than pyrrolidine (11.27) or piperidine (11.22). Clearly, solvation effects influence basicity (74JA288). 

When only one of both coupled nuclei bears one lone pair, the absolute value of of the FC term strongly depends on the lone pair s character, which increases rapidly along the periodic table. That is why it is expected that AT(Sn,H) < 0 in while for lighter A atoms bearing only one LP... 

Values of Jnn> ost of which are directly bonded couplings, range from 4-21 Hz, but relationships with structure have not been clearly defined. In hydrazines, a dependence on both dihedral angle and lone pair s character has been predicted the latter is expected to make a negative contribution to because of the negative i (42). Qualitatively, appears to... 

The 1 1 solid adduct of tetrathiafulvalene (TTF) and tetracyanoquin-odimethane (TCNQ) is the first-discovered molecular metal, which consists of alternate stacks each composed of molecules of the same type (Fig. 4.1.3). A charge transfer of 0.69 electron per molecule from the HOMO (mainly S atom s lone pair in character) of TTF to the LUMO of TCNQ results in two partially filled bands, which account for the electrical conductivity of TTF TCNQ. 

MO s are mainly 0 2p lone pair in character, by running preliminary calculations on the model U(OH) li) I.E. s have been explicitly calculated using the Slater transition state formalism only for a few selected ground state MO s. The choice has been made according to the dominant atomic population (mainly ligand, metal 6d, and metal 5d based MO s) to evaluate the effects of differential relaxation energies (Table 2). It turned out, however, that these effects are comparable despite the difference in the atomic compositions of various orbitals (see Section 1.2). The most relevant Xa results are reported in Tables 3 and 4. 

The proton affinities (gas phase) of thiirane and other three-membered heterocycles have been determined azirane (902.5), thiirane (819.2), phosphirane (815.0), oxirane (793.3 kJ moF ) (80JA5151). Increasing s character in the lone electron pairs decreases proton affinities. Data derived from NMR chemical shifts in chloroform indicate the order of decreasing basicity is azirane > oxirane > thiirane (73CR(B)(276)335). The base strengths of four-, five- and six-membered cyclic sulfides are greater than that of thiirane. 

These interactions are most commonly observed for divalent chalcogen atoms and the nitrogen atom (the electron donor D) lies within the X-E-Y (E = S, Se, Te) plane, preferably along the extension of one of the covalent bonds as in 15.3. This anisotropy is a clear indication that these short E N contacts have some bonding character, i.e., they are subject to the geometric restrictions of orbital overlap. Eor example, in the diselenide 15.4 the nitrogen lone pairs are clearly oriented towards the Se-Se linkage. ... 

As discussed in Sections 5.1-5.3, arenediazonium ions are Lewis acids in which the (3-nitrogen forms the center of electrophilic character. This was demonstrated by the addition of hydroxide ions and water molecules. Other nucleophiles can also be added and, in principle, these reactions display the same mechanistic characteristics as those with OH and H20. According to the nature of the atom of the nucleophile that provides the lone pair of electrons, O-, S-, Se-, N-, P-, or C-coupling can occur. With N- and C-coupling, important and large groups of compounds are formed, namely azo compounds (mainly important as azo dyes) and triazenes, respectively. These compounds will be discussed in Chapters 12 and 13, respectively. 

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