Ab-initio calculation of energy levels

Ab initio calculation of energy changes accompanying distortions of the dioxathiaazapentalene cation (8) showed structure (8) not to have the lowest energy but to be rather a transition state between the two valence bond structures (9) and (10). This process corresponds to an in-plane bend parallel to the 3c,4e-hypervalent bond at sulfur. An out-of-plane bend about the SN bond also leads to an energy minimum but this is at a much higher level and is therefore less realistic <91PS(55)35>. 

Bouma, Poppinger and Radom" made a detailed analysis of the possible structures of the ions of n- and isobutane (among other hydrocarbons) using ab initio calculations of different levels. They reviewed also older calculations. The HOMO orbital of n-butane is the 7ag[C2h] orbital. It is sufficiently separated from the next lower MO, such that one can assume that the electron is eliminated from the HOMO. The corresponding ionized state does not correspond to a minimum and the authors calculate a difference between the vertical and the adiabatic ionization energy of 1.0 eV, compared with an experimentally determined value of The HOMO of n-butane includes... 

Ab initio calculation of Diels-Alder reactions of a series of 5-heteroatom substituted cyclopentadienes Cp-X (65 X = NH, 50 X = NH, 64 X = NH3, 67 X = O", 54 X = OH, 68 X = OH3% 69 X = PH, 51 X = PH, 70 X = PH3% 71 X = S, 55 X = SH, 72 X = SH/) with ethylene at HF/6-31++G(d)//HF/6-31-i i-G(d) level by BumeU and coworkers [37] provided counterexamples of the Cieplak effect. The calculation showed that ionization of substituents has a profound effect on the n facial selectivity deprotonation enhances syn addition and protonation enhances anti addition. The transition states for syn addition to the deprotonated dienes are stabilized relative to those of the neutral dienes, while those for anti addition are destabilized relative to those of the neutral dienes. On the other hand, activation energies for syn addition to the protonated dienes are similar to those of the neutral dienes, but those for anti addition are very much lowered relative to neutral dienes (Table 6). 

The homolytic bond dissociation energies (BDEs) of phenohc O—H bonds has been the subject of a computational study focusing on substituent effects by ab initio and density functional theory (DFT) methods.6 Consistent overestimation of the BDEs by MP2 and MP4 calculations was associated with spin contamination in the reference UHF wave functions, whilst the DFT calculations (particularly the B3LYP/6-31G level of theory) were relatively unaffected. Ab initio calculations of the photosensitized C—C BDEs of /f-phenethyl ethers has revealed a significant configurational... 

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