Lone-pair orbitals oxygen

Sr) and the nonbonding orbital formed by the Oxygen 2s and 2pz orbitals eombining to form the "lone pair" orbital direeted along the z-axis away from the two Hydrogen atoms). 

The synclinal conformation (sc) is appropriate for overlap of an oxygen nonbonded pair with the a C—Cl orbital. The preferred ap relationship, requires an antiperiplanar alignment of a lone-pair orbital with the bond to the electronegative substituent. Because of the donor-acceptor nature of the interaction it is enhanced in the order F < O < N for the donor (D) atom and N < O < F for the acceptor (A) atom. 

Even molecules as simple as dimethoxymethane give evidence of anomeric effects. The preferred conformation of dimethoxymethane aligns each C—O bond with a lone-pair orbital of the adjacent oxygen. ... 

For conjugated carbonyl compounds, such as a,) -enones, the orbital diagram would be similar, except for the recognition that the HOMO of the ground state is ij/2 of the enone system, rather than the oxygen lone-pair orbital. The excited states can sometimes be usefully represented as dipolar or diradical intermediates ... 

The FMOs of acrolein to the left in Fig. 8.2 are basically slightly perturbed butadiene orbitals, while the FMOs of protonated acrolein resemble those of an allyl cation mixed in with a lone-pair orbital on the oxygen atom (Fig. 8.2, right). Based on the FMOs of protonated acrolein, Houk et al. [2] argued that the predominant interaction in a normal electron-demand carbo-Diels-Alder reaction is between the dienophile LUMO and diene HOMO (Fig. 8.1, left). This interaction is greatly... 

In several previous Sections (1.15, 1.19) we considered the interaction of lone-pair orbitals with orbitals localized in other regions of the molecules. In some cases an atom (ether oxygen or thioether sulfur, halogens) may carry several lone pairs. These lone pairs are generally described as localized orbitals pointing in tetrahedral directions on the atom to which they belong as in Fig. 47. The elee-... 

It is expected from simple Ihermochemical considerations that adjacent n-, 0-or lone pair orbitals should have a significant influence over the facility of atom transfer reactions. Thus, the finding that /-buloxy radicals show a marked preference for abstracting hydrogens a to ether oxygens (Figure 1.9) is not... 

Short nonbonded Si—Si distances have been observed in four membered metallacycles (Scheme 12b) with a Pt, Ir, W, or Nb atom [138-142] in place of one of the oxygen (nitrogen) atoms of 1,3-cyclodisilazanes (1,3-cyclodisilazanes) and in U-silylene-bridged dinuclear platinum complexes (Scheme 12c) [143, 144]. Electron donating occupied orbitals are expected to be on the platinum atoms like lone pair orbitals on the oxygen atoms in cyclodisiloxanes. 

The square cell is convenient for a model of water because water is quadrivalent in a hydrogen-bonded network (Figure 3.2). Each face of a cell can model the presence of a lone-pair orbital on an oxygen atom or a hydrogen atom. Kier and Cheng have adopted this platform in studies of water and solution phenomena [5]. In most of those studies, the faces of a cell modeling water were undifferentiated, that is no distinction was made as to which face was a lone pair and which was a hydrogen atom. The reactivity of each water cell was modeled as a consequence of a uniform distribution of structural features around the cell. 

The optical absorption spectra of sulfonyl radicals have been measured by using modulation spectroscopy s, flash photolysis and pulse radiolysis s techniques. These spectra show broad absorption bands in the 280-600 nm region, with well-defined maxima at ca. 340 nm. All the available data are summarized in Table 3. Multiple Scattering X, calculations s successfully reproduce the experimental UV-visible spectra of MeSO 2 and PhSO 2 radicals, indicating that the most important transition observed in this region is due to transfer of electrons from the lone pair orbitals of the oxygen atoms to... 

The Hiickel energy levels and p basis orbital coefficients for formaldehyde and ethylene are shown in Figure 4.9. If we examine the orbital interactions for case 1, we can see that in the orientation such that the lone pair on oxygen is oriented in a way that a bond can begin to form between the oxygen lone pair and the carbon ir orbital, there are two possible orbital interactions. 

The relevant orbitals in the dimerization of RNO are the usual four 7r-orbitals, plus the nonbonding lone pairs on nitrogen, that correlate with the a and a levels, thus giving rise to the crossing that makes the reaction symmetry-forbidden. The unperturbed lone pairs of oxygen are not included. 

The spectra and calculations all led to the conclusion that there is an usually large interaction between both the 7T and lone-pair orbitals in the carbonyl portion of the molecule with the n and a orbitals of the olefin portion. The first ionization potential (9.57 eV) involves ionization of an electron from the oxygen lone pair, whereas the second (11.19 eV) involves ionization of an electron from the olefin rr-bond. The most vertical ionization is from the 7 ax MO (16.11 eV), the second lone-pair orbital on oxygen. 

The CHO radical is a o-radical and the main qualitative feature of the comparison between Tables 2 and 3 is the tendency for the a GHOs to be less contracted in CHO than is found in CH20. In particular, the sp2 hybrid on carbon which nominally contains the unpaired electron is considerably expanded (exponent 1.5923 compared to 1.8660 in CH20). The hydrogen orbital and the aoc orbital are also noticeably expanded while the lone pairs on oxygen are largely unaffected. 

Inspection of delocalization effects shows the interaction of one lone pair at oxygen (localized perpendicular to the molecule plane) with the antibonding jT -bond orbital of the CN bond (estimated energy gain ca 84 kJmoU ) corresponding to the resonance hybrid shown in Figure 5. 

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