Molecular orbital calculations conjugation

Molecular orbital calculations predict that oxirane forms the cyclic conjugate acid (39), which is 30 kJ moF stabler than the open carbocation (40) and must surmount a barrier of 105kJmoF to isomerize to (40) (78MI50500). The proton affinity of oxirane was calculated (78JA1398) to be 807 kJ mol (cf. the experimental values of 773 kJ moF for oxirane and 777-823 kJ moF for dimethyl ether (80MI50503)). The basicity of cyclic ethers is discussed in (B-67MI50504). 

Acetylenes XCCY with n conjugated substituents, X and Y, on both carbon atoms have planar or perpendicular conformations. The substituents can be electron-donating or -accepting. The planar conformers are linear conjugate D-TI-D, D-IT-A, or A-IT-A systems whereas the perpendicular conformers are composed of ri-D and IT-A not in conjugation with each other. The orbital phase is continuous only in the planar conformations of D-TI-A (Scheme 23, cf. Scheme 4). The acetylenes with X=D (OR, NR ) and Y=A (RCO, ROCO) prefer planar conformations. When both substituents are electron-donating or accepting, the phase is discontinuous. The acetylenes then prefer perpendicular conformations. The predicted conformational preference was confirmed by ab initio molecular orbital calculations [32]. Diacetylenic molecules show similar conformational preference, which is, however, reduced as expected [32]. 

In the case of mercuration (soft electrophile), attack at the 2-position is favoured. These observations accord with predictions based on molecular orbital calculations, that hard electrophiles (nitronium ions) should attack at C-4 and soft electrophiles (e.g. HgS04) at C-2 (68JA223). Furthermore, very hard electrophiles (e.g. S03) are predicted to attack at C-3. This is hard to verify because pyridine 1-oxide reacts at C-3 as its conjugate acid (or... 

Before concluding this section, it is noted that a fairly user-friendly SHMO (simple Hiickel molecular orbital) calculator is now available on the Internet, http //www.chem.ucalgary.ca/shmo/. With this calculator, the Hiickel energies and wavefunctions of planar conjugated molecules can be obtained on the fly. ... 

This has been proposed by Imoto and Otsu (33, 34) and confirmed by the molecular orbital calculations of Tazuke et al. (76, 77). A similar explanation has been advanced for the increased reactivity of vinylpyri-dines and vinylimidazoles in the presence of zinc salts (76, 77). Super-delocalizability as a result of conjugation with the metal salt results in spontaneous thermal polymerization. 

Dienamines differ from simple enamines in that (i) there is an additional nucleophilic site at the -position and (ii) an equilibrium mixture of three isomeric dienamines is frequently formed, consisting of the linear s-trans isomer 1, the linear s-cis isomer 2 and the cross-conjugated isomer 31 (Scheme 1). A variety of factors influence the outcome of electrophilic attack on such an equilibrium system. Hiickel molecular orbital calculations indicate a significantly higher electron density at the -positions compared to the -positions of the dienamine system2 (Figure 1). 

Molecular orbital calculations have suggested that cyclopentenone is intrinsically more x one to conjugate reduction than cyclohexenone " and thus is a good substrate on which to test new 1,2-selective reagents. The selectivity of reduction of both these enones with the best of the new reagents together with the results for 9-BBN-H, the previous reagent of choice, are tabulated for comparison (Table 2). 

For reviews of molecular-orbital calculations of nonbenzenoid cycUc conjugated hydrocarbons, see Nakajima, T. Pure Appl. Chem. 1971, 28, 219 Fortschr. Chem. Forsch. 1972, 32, 1. 

One of the most important factors affecting the wavelength of UV absorp tion by a molecule is the extent of conjugation. Molecular orbital calculation show that the energy difference between HOMO and LUMO decreases as th ... 

This rule is derived from the Hiickel MO (molecular orbital) calculation on planar monocyclic conjugated hydrocarbons (CH)m where m is an integer equal to or greater than 3, according to which (An + 2) n-electrons are contained in a closed-shell system. Examples of systems that obey the Hiickel rule include ... 

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