Planar and nonplanar forms

Figure 8.9 Rotomeric equilibrium between planar and nonplanar form of 2-(2 -hydroxyphenyl)benzotriazole UVAs. After DeBillis et al. [336]. Reprinted with permission from A.D. DeBellis et al., ACS Symposium Series, 805, 453-467. Copyright (2002), American Chemical Society...

While 4bp/-type structures can be considered as resonance structures, 4cp/is also an alternative. The importance of structure 4cp/is supported by the fact that the negative charge at the carbon atoms in the 2 and 5 positions is indeed larger than at the 3 and 4 positions. ° The common feature in all 4ap/-, 4bp7-, and 4cp/-type structures is that the lone pair of the planar tricoordinate phosphorus is fully delocalized, resulting in an enhanced bond order about the tricoordinate planar phosphorus. The d-orbital participation at phosphorus is insignificant in 4, either in the planar or nonplanar form. °... 

Our intuition derived from the per and 7r-fluoro effects suggests that fluorination of the nonplanar ethane should be accompanied by an increased ionization potential. Reference 3 corroborates this C2H6 and C2F6 have ionization potentials of 11.52 and 13.4 eV respectively. Likewise, our intuition suggests that the ionization energies of planar CH3 and planar CF3 should be comparable. From the definition of the inversion barrier, AE nv. as the energy difference of the planar and pyramidal forms and the identity for adiabatic ionization potentials (equation 3),... 

Several semiempirical two-electron MO generalizations of the PPP method were developed that apply to both planar and nonplanar molecnles. The complete neglect of differential overlap (CNDO) method was proposed by Pople, Santry, and Segal in 1965. The intermediate neglect of differential overlap (INDO) method was proposed by Pople, Beveridge, and Dobosh in 1967. Both methods treat only the valence electrons explicitly. The valence-electron Hamiltonian has a similar form as (17.1) ... 

A speculative proposal was made thirty years ago by Schmid and Krenmayr77, namely that a nitrosyl ion solvated, but not covalently bonded, by a water molecule may be involved in these systems. This hypothesis was investigated theoretically in 1984 by Nguyen and Hegarty78 who carried out ab initio SCF calculations of structure and properties employing the minimal STO-3G basis set, a split-valence basis set plus polarization functions. Optimized geometries of six planar and two nonplanar forms were studied for the nitrosoacidium ion. The lowest minimum of molecular electrostatic potential... 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

Example 14.4 4,5-Dimethylphenanthrene. Using the C NMR spectrum measured by Stothers et al. [275], we deduce A/// = 36.8 kcal/mol assuming planarity. Closely neighboring methyl groups that are separated by five bonds in the molecular skeleton, however, result in chiral nonplanar conformations [276]. Modeling, where appropriate, the CC bonds on those described for biphenyl and m-stHbene, one predicts A/// = 47.8 for the nonplanar form, in acceptable accord with the reported value [248], 46.26 + 1.46 kcal/mol, a result that is self-explanatory. 

Figure 12.14 Calculated structures of the [C2Cjlm]+ cation, showing the two different torsion conformers obtainable by rotation of the ethyl group around the C-N bond relative to the imidazolium ring. Planar (left) and nonplanar (right) rotamers are viewed perpendicular to and along the ring plane. The nonplanar form is known from x-ray structures 69. (Adapted from Umebayashi, Y., Fujimori, T, Sukizaki, T., Asada, M., Fujii, K., Kanzaki, R., and Ishiguro, S.-i., /. Phys. Chem. A, 109, 8976-8982, 2005. With permission.)...

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