Lone-pair ionizations

The three highest occupied orbitals of sulfoxides are the lone pairs ns and n0, as well as the 7iso bond210. The 1,3-dithietane 1-oxide adds a lone-pair ionization and destabilizes the n0 and nso radical-cation states compared with thietane oxide. According to a hyperconjugative MO model, the ns+ combination in 1,3-dithietane is destabilized by about leV relative to the basis orbital energy a(ns) due to the combination with the... 

Figure 17. Camphor PECD from the HOMO (carbonyl oxygen lone-pair) ionization. Experimental data are (S)-camphor, (R)-camphor (both from Ref. [36]) (R)- and (S)-enantiomer data from Ref [64] O data from Ref [65]. Also shown as curves are CMS-Xa calculations (Ref. [36]) and B-spline calculations (Ref [57]).

The F and R parameters are qualitatively analogous to the field and resonance parameters, 5 and SR, of Swain and Lupton19 that is, they measure the a and 7r-electronegativities, respectively, of substituents. However, the F and R values are more appropriate for correlating processes in which a localized positive charge develops than are the S and 91 values. Hence the F and R values correlate lone pair ionization potentials and proton affinities better than the corresponding and 91 values do. 

Table 3. Lone pair ionization potentials in CH3X and HX model systems...

Typical lone pair ionization potential data have been presented before and AO overlap integral data in support of these ideas are given in Table 8. Ab initio results are shown in Table 9 and support the notions that the pi donating ability of heteroatoms is energy gap controlled in CH2X but matrix element controlled in CH2=CHX. 

Another situation where the aforementioned condition is met is along a series of H2 A isoelectronic molecules where A varies along a row of the Periodic Table. Here, Hpq remains roughly constant and, thus, the bending force increases as AEpq decreases, a trend which obtains as the lone pair ionization potential of the central atom decreases. Typical data are shown in Scheme 8. 

In piperidine the electron lone-pair can occupy either an axial or an equatorial position in 1-methylpiperidine the axial orientation (lb) is favoured by 99 1 over the equatorial (la). PE spectra and ab initio calculations on methylpiperidines indicate that axial 2-methyl substituents lower the amine lone-pair ionization potential by about 0.26 eV, while equatorial 2-methyl substituents as well as methyl groups on carbon atoms 3 and 4 lower the lone-pair IP by less than 0.1 eV63. This establishes the mechanism of stabilization of the amine radical cation as hyperconjugative electron release, which is larger for CC bonds than for CH bonds. The anti-periplanar orientation of the nitrogen lone-pair and the vicinal C—Me bond (lc) is much more favourable for this type of interaction than the synclinal orientation (Id). 

FIGURE 3. Regression of lone-pair ionization energies of 4-substituted quinuclidines vs Taft s a values of the substituents. Reproduced with permission from Reference 12... 

When using azole derivatives the experimental determination of lone-pair ionization potentials by photoelectron spectroscopy suffers from considerable band overlap involving the ionization of the 7r-system (69MI1 74PMH1 79JOC2093 80JHC689). 

The lone-pair ionization potentials calculated according to Nakajima and Pullman.281... 

The photoelectron spectrum of pyrrolidine in the low-energy (8-13 eV) region has been explored as part of a study of lone-pair ionization processes and an IP of 8.82 eV recorded for ionization from the N lone-pair. In A-methyl-3-pyrrolidinone the bands at 8.83, 9.53 and 12.24 eV have been assigned to ionization from the N lone-pair, the O lone-pair and the carbonyl 7r-system respectively. In 2-pyrrolidinone the first two bands are overlapped (at 9.53 and 9.76 eV), presumably a result of the amide resonance interaction, and on the basis of band appearance a reversed assignment to O lone-pair and N lone-pair ionization respectively has been proposed the 7rco ionization band is at 11.91 eV. Data are also available in the same paper for succinimide and its A-chloro and A-bromo derivatives (78MI30407). 

Table 9 Lone Pair Ionization Potentials <72ja5599)...

Finally, we mention the radical cation of oxetane. Without any doubt, this is a 7t species produced by oxygen lone pair ionization [327, 329]. It has sizeable hfcs in the P and y positions (afl = 65.5 G aY = 10.8 G) and is characterized by a highly anisotropic and positively shifted g-factor (g" = 2.0046, g = 2.0135). The substantial difference between the p hfcs and g-factors of the oxetane ion and that... 

Block et al.194 examined the effects of trimethylsilyl substitution on the first vertical ionization potentials by photoelectron and Penning ionization electron spectroscopy studies of a range of cyclic and noncyclic sulfides and ethers. It was shown that substitution of oxirane 218 with a trimethylsilyl substituent as in 219 lowered the ionization potential by 0.90 eV (20.8 kcal/mol), while similar substitution of dimethyl ether 220 in 221 lowered the ionization potential by 0.64 eV (14.8 kcal/mol). By comparison, the effects of silyl substitution on sulfur lone-pair ionization potentials was found to be smaller thus the ionization potential of dimethyl sulfide 222 is lowered by 0.37 eV upon trimethylsilyl substitution in 223, and the trimethylsilyl-substituted thiirane 225 is lowered by 0.59 eV relative to thiirane 224. The raising of the energy of the sulfur lone-pair electrons in the thiirane 225 is also apparent from its UV spectrum, where there is a bathochromic shift in the absorption maximum compared to the parent 224. 

Oxygen lone pair ionization energies of siloxanes. 208... 

Photoelectron spectra of the methyl halides have also been studied in the condensed phase (zinc matrix)78. The ionization bands are broad relative to the gas and are shifted ca 1 eV towards lower ionization energies. This polarization shift is slightly larger for a ionizations than for halogen lone-pair ionizations. 

The PES studies of alkyl dihalides have been mostly concerned with the transfer of interhalogen interactions through the unbranched alkane chain98 99. The halogen lone-pair ionizations are easily identified, and provide a ready probe for these studies. Measurements of the spin-orbit (SO) splitting, vibrational fine structure and lone-pair bandwidths produced the following conclusions ... 

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