Oxygen atom lone pairs

More recently, Jin (92MI2) investigated several mono- and dimethylated cycloheptazol-8-ones. The similarity of the electronic spectra of pyrazolo-tropones 85, 86a, and 87a is explained by the very similar 10-77 systems present. However, the spectra of isoxazoles 90 and 91 and of oxazole 114b are different from those of the pyrazoles. This difference was attributed to the small contribution of the oxygen-atom lone-pair electrons to the conjugation, which minimizes the contribution of the 10-77 system. 

Experimental investigation of the electronic charge density in pyroelectric lithium sulphate monohydrate was reported in [65, 66]. In these experiments lithium sulphate monohydrate single crystal was studied by neutron [65] and X-ray diffraction [66] at different temperatures. Static deformation and charge density model refinements allowed an estimation of the differences in electron densities between 80 K and 298 K. Changes in the sulphate oxygen atom lone-pair deformation densities, caused by contraction in S04 - Li" contacts between 298 K and 80 K were also found. 

Preferred geometry of the benzene oxide-oxepin system can be predicted by molecular orbital methods. Thus benzene oxide la is predicted to be markedly non-planar (with the epoxide ring at an angle of 73° to the benzene ring), while the oxepin lb has been predicted to prefer a shallow boat structure (MINDO/3) or a planar structure ab initio) As previously mentioned, the proportion of each tautomer present at equilibrium is both temperature and solvent-dependent. Molecular orbital calculations have been used to rationalize the solvent effects, both in terms of the more polar character of the arene oxide that is favored in polar solvents and the strengthening of the oxirane C-C bond upon coordination of the oxygen atom lone pair in polar solvents. Thus values in the range 1.5-2.0 D and 0.76-1.36 D for the dipole moments of arene oxide la and oxepin lb have been calculated. 

The uv absorption spectrum of simple aliphatic nitrocompounds consists of an extended band centered at 2700-2800 A and is most probably associated with an n 7t transition involving oxygen-atom lone-pair electrons . Both the photochemical and thermal decompositions of nitrocompounds are extremely complex, in part owing to secondary reactions between products and substrate although the overall mechanisms are still unclear in many cases, the primary modes of decomposition of some molecules have been established. 

It is also tempting to propose a stabilizing interaction between the electron cloud associated with the oxygen atom lone pairs and the electron-depleted carbon atom of the iminium unit. This suggestion in this case is supported by single-crystal X-ray analysis (Fig. 5.6), although other catalysts of this general... 

Fig. 15.9 The gas-phase structure of H2O2 showing the oxygen atom lone pairs. The angle shown as 111° is the internal dihedral angle, the angle between the planes containing each OOH-unit see Table 15.3 for other bond parameters.

Water leaves and a carbocation forms. Step 2 occurs readily for two reasons. First, the OH group is protonated to form water, a far better leaving group than hydroxide ion. Second, the oxygen atoms lone pair electrons resonance stabilize the carbocation formed in this Sj,j 1 reaction. 

The Lewis structure shows that methyl methaciylate has the formula C5 Hg O2, with 40 valence electrons. You should be able to verily that the two CH3 groups have. s -hybridized carbons, the inner oxygen atom is s hybridized, the outer oxygen atom uses 2 p atomic orbitals, and the three double-bonded carbons are s p hybridized. These assignments lead to the following inventory of a bonds and inner-atom lone pairs ... 

The n—7r stabilizing interaction which obtains in each conformation is listed above. Since the oxygen 2p lone pair AO is a better intrinsic donor orbital than the oxygen hybrid sp2 lone pair AO, we conclude that n—tt interactions favor the conformation in which all atoms are contained in the same plane. 

It is in fact a general rule that compounds of the type R2c(or)2, having two oxygen atoms singly bonded to the same carbon atom, are Unstable in acid solution. A reason for this is that both oxygens have lone pairs of... 

Molecular orbital calculations have also provided theoretical justification for these stereoelectronic effects in tetracovalent and pentacovalent phosphorus species (2-7). As has been shown in molecular orbital calculations on the X -P-X2 (X = 0,N) structural fragments, the X.-P bond is strengthened (as indicated by an increase in the Mulliken overlap population) while the P-X3 bond is weakened when the X atom lone pair is app to the P-X3 bond. Thus, in the g,t conformation of dimethyl phosphate (Structure ll the overlap population for the trans P-0 bond is. 017 electron lower than the overlap population for the gauche P-0 bond. As shown for g,t dimethyl phosphate one lone pair (shaded in 1) on the gauche bond oxygen is app to the trans bond, while no lone pairs on the trans bond oxygen are app to the gauche bond. Thus, the weakest X.-P bond has one app lone pair and no lone pairs on X. app to the P=X2 bond. 1... 

In H2O, the two H atoms and two lone pairs around O point to the comers of a tetrahedron. The H- O- H bond angle of 105° is close to the theoretical tetrahedral bond angle of 109.5°. Water has a bent molecular shape, because two of the groups around oxygen are lone pairs of electrons. 

Protonation can occur on any lone pair of the three oxygen atoms. Protonation on the tertiary alcohol oxygen is fine, but it returns us to a previous structure. Protonation on the primary alcohol oxygen is also fine, but we know from the 0 study that its methylene is not a site for attack. What is left is to protonate one of the ether oxygen s lone pairs. 

From the perspective of Molecular Orbital Theory, the energy of the lowest unoccupied antibonding orbital (LUMO) has been estimated at 3.8 eV, indicates the high electron affinity with respect to the central carbon atom, hence it is susceptible to attack by nucleophile and to the reduction while that of highest occupied molecular orbital (HOMO) is susceptible to attack by electrophile due to its high localized electron density as oxygen inplane lone pairs. It also interacts weakly with Lewis and Bronsted acids [21, 22a]. 

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