Molecular-orbital calculations protonation

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). 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

Thus, Segura replaced ct with a scaled energy change, obtained by a molecular orbital calculation, for this proton transfer ... 

Molecular orbital theory indicates that there is little difference between the stability of the two tautomers of purine, 42 and 43. Molecular orbital calculations indicate that purine forms a monocation by protonation at N-3 or at A precise X-ray crystal-... 

For the series PhmMe3 mGe the ring proton hyperfine couplings increase as m decreases (Table 2), consistent with an increase in the unpaired spin density at the ring carbon atoms. Hilckel molecular orbital calculations bear this out8,14. Overall, however, the small values of the ring proton hyperfine couplings reveal that there is only a small Ge 4p-C 2p overlap in these radicals. 

Ab initio molecular orbital calculations at the G2 level of theory is found31 consistently to reproduce experimental proton affinities to an accuracy of 10 kJmol-1 for a range of bases with PA spanning ca 500 kJmol-1. 

The new treatment had its origins partly in ab initio molecular orbital calculations of substituent effects and partly in extensive studies of gas-phase proton transfer reactions from about 1980 (Section V.A). Various aspects of this work essentially drew attention to the importance of substituent polarizability. In 1986 Taft, Topsom and their colleagues252 developed a scale of directional substituent polarizability parameters , oa, by ab initio calculations of directional electrostatic polarization potentials at the 3-21G//3-31G level for a large set of CH3X molecules. The oa values were shown to be useful in the correlation analysis of gas-phase acidities of several series of substrates252, and such work has subsequently been extended by Taft and Topsom151. 

These authors conclude that the problem of internal solvation is still an experimental and theoretical challenge GB measurements for this type of molecules of low volatility are not always in good agreement194. Molecular orbital calculations may help to solve the difficult experimental problems, but they have to take into account conformational isomerisms and the prototropic tautomerisms of the amidine and guanidine moieties. In light of the above discussion, the proton affinities deduced from the experimental GB values should be based on accurate estimations of the entropy of cyclization 86. 

The a-methylbenzyl cation (1) can be approached from the alcohol dehydration direction or the alkene protonation direction, as shown, and both of these processes have been the subject of ab initio molecular orbital calculations. It was found that the alcohol dehydration has a transition state about half way between the two stmctures shown, with the transition state and the carbocation having about the same amount of 7T-orbital overlap. However, the alkene protonation has an earlier transition state with less effective 7r-orbital overlap than that in the cation. This is held to explain the different Yukawa-Tsuno r+ values found for the two processes, 0.7-1.1 for alkene... 

Stabilized with respect to the amide structure, is some 30 kcal mole less stable than the O-protonated form, therefore refers to gas phase protonation. Molecular orbital calculations by Hopkinson and Csizmadia (1973) put the same difference at only 6-2 kcal mole , again ignoring solvation. 

The phosphorus analogue of pyrrole, phosphole, has a degree of aromatic character, according to molecular orbital calculations and nmr spectra (Brown, 1962 Chuchman et al., 1971). 1-Methyl-phosphole has a p/fg-value of 0-5 (Quin et al., 1969), much higher than that of pyrrole. It polymerizes rapidly in aqueous acid. The site of protonation of 1,2,5-triphenylphosphole is phosphorus according to the infrared spectra of some of its stable salts (Chuchman et al., 1971). 

Ultraviolet spectra of benzoic acid in sulphuric acid solutions, published by Hosoya and Nagakura (1961), show a considerable medium effect on the spectrum of the unprotonated acid, but a much smaller one in concentrated acid. The former is probably connected with a hydrogen-bonding interaction of benzoic acid with sulphuric acid which is believed to be responsible for a peculiarity in the activity coefficient behaviour of unprotonated benzoic acid in these solutions (see Liler, 1971, pp. 62 and 129). The absence of a pronounced medium effect on the spectra in >85% acid is consistent with dominant carbonyl oxygen protonation. In accordance with this, Raman spectra show the disappearance in concentrated sulphuric acid of the carbonyl stretching vibration at 1650 cm (Hosoya and Nagakura, 1961). Molecular orbital calculations on the structure of the carbonyl protonated benzoic acid have also been carried out (Hosoya and Nagakura, 1964). 

The protonation of ketene, CH2 =C=0, in superacid solution leads exclusively to the acetyl cation, CH3. CO (Olah et al., 1972), but initial 0-protonation is not ruled out. Molecular orbital calculations confirm that the cations resulting from protonation of ketene at the carbon atom are more stable than the O-protonated ones, the a-C protonated cations being the least stable (Hopkinson, 1973). 

The gas phase basicities at both the a- and (3-positions in five-membered heterocycles have been studied by ion cyclotron resonance equilibrium and bracketing experiments on deuteriated substrates. a-Protonation is preferred by 2.8-4.6 kcal mol-1 for both furan and thiophene as compared to 0-2.9 kcal mol 1 for pyrrole (81NJC505) and heteroatom protonation is much less favored than a-protonation. Semiempirical (MNDO) molecular orbital calculations have provided quantitative confirmation of the above conclusions. 

Molecular orbital calculations indicate that the lowest triplet state of benzene and its derivatives should be appreciably more basic than the ground states of the molecules.211 Furthermore, it is predicted that the preferential sites of protonation may be different in the excited states than they are in the ground states. 

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