Theoretical calculations Subject

Indazoles have been subjected to certain theoretical calculations. Kamiya (70BCJ3344) has used the semiempirical Pariser-Parr-Pople method with configuration interaction for calculation of the electronic spectrum, ionization energy, tt-electron distribution and total 7T-energy of indazole (36) and isoindazole (37). The tt-densities and bond orders are collected in Figure 5 the molecular diagrams for the lowest (77,77 ) singlet and (77,77 ) triplet states have also been calculated they show that the isomerization (36) -> (37) is easier in the excited state. 

H2S2 (hydrogenpersulfide), the smallest member of the polysulfane series [15], has been studied extensively by molecular spectroscopy and theoretical calculations [16] (and references therein). By now, accurate knowledge of its structure, torsional potential and vibrational modes has been established. Ab initio calculations readily reproduce these properties [17]. The value of the SSH angle in hydrogen disulfide was a subject of controversies for some time. However, recent experiments led to a different value which is in favour of the ab initio calculated value [17]. 

Theoretical calculations have been carried out on a number of zinc-containing enzymatic systems. For example, calculations on the mechanism of the Cu/Zn enzyme show the importance of the full protein environment to get an accurate description of the copper redox process, i.e., including the electronic effects of the zinc ion.989 Transition structures at the active site of carbonic anhydrase have been the subject of ab initio calculations, in particular [ZnOHC02]+, [ZnHC03H20]+, and [Zn(NH3)3HC03]+.990... 

A thiepin is formally isoelectronic with the 8ic-electron 1,3,5,7-cyclooctatetraene and 1,3,5-cycloheptatrienide ion and, if planar, may actually be antiaromatic. Recently, the question of the antiaromaticity of thiepin has been the subject of interest for both synthetic and theoretical chemists. The apparent instability of the thiepin ring system is in good agreement with theoretical calculations. Dewar and Trinajstic 68) have reported that the thiepin is considered to be weakly antiaromatic (RE = — 1.45 kcal mol-1) based on PPP SCF MO calculations. On the other hand, Hess Jr. and Schaad 69) have found it to be substantially antiaromatic (RE = —0.232 J) by using the Huckel MO method. This result was also supported by a graph-theoretical treatment by Aihara 70). 

One point worth noting is that the theoretical calculation of chemical shifts predicts absolute chemical shifts, whereas experimental values are reported with respect to some convenient but otherwise arbitrary reference. Although theoretical chemical shift trends in a series of compounds may be compared with experimental trends, it can be useful to also calculate the chemical shift of a solid reference compound for comparison, while realizing that such a calculation will also be subject to some degree of inaccuracy. 

The second fixation reaction that has been the subject of theoretical study is the nickel(0)-catalyzed coupling reaction of C02 with acetylene (equation 1), which was theoretically investigated with the ab initio SD-CI method [25]. The theoretical calculations clearly showed that if the nickel(O) moiety was eliminated from the reaction system, the activation barrier increased very much, and that the C-C bond formation between C02 and acetylene was accelerated by the charge-transfer from the nickel(O) d orbital to the orbital resulting from the combination between n orbitals of acetylene and CO2. Actually, the HOMO contour map of Ni(PH3)(C02)(C2H2) clearly displays this orbital mixing in the transition state. 

Theoretical calculations have been fundamental in solving the controversy on the mechanism for the dihydroxylation of double bonds by transition metal oxo complexes. Nowadays, this topic which was the subject of a controversy just a few years ago seems to be solved in favor of the [3+2] pathway, at least in a vast majority of the cases. Despite this spectacular success there are still a number of open issues for this particular reaction which have not been solved, and which continue to be a challenge for computational chemists. Among this, one can mention the correlation between the nature of the substrate and its reactivity with permanganate, and the mechanisms leading to the proportion of products experimentally observed when CrC Cb is applied. Hopefully, these issues will be solved in the future with the help of theoretical calculations. 

Gas-phase solvation has so far given only very indirect evidence concerning the structure and details of molecular interactions in solvation complexes. Complex geometries and force constants, which are frequently subjects of theoretical calculations, must therefore be compared with solution properties, however, the relevant results are obscured by influences arising from changes in the bulk liquid or by the dynamic nature of the solvation shells. With few exceptions, structural information from solutions cannot be adequately resolved to yield more than a semiquantitative picture of individual molecular interactions. The concepts used to convert the complex experimental results to information for structural models are often those of solvation numbers 33>, and of structure-making or structure-... 

The ethylene bromonium and 1-bromoethyl cations and their neutral and anionic counterparts have been the subject of a tandem mass spectrometric study of dissociation and gas-phase redox reactions. IR and Raman studies of the bioactive bromonium cation (19), as its hydrogensulfate salt, agree with the results of an X-ray structure determination, and theoretical calculations are also in agreement, except for the details of the NO2 groups. The azaallenium ion (22) is an intermediate in the photolysis of (20) (21) and (22) could both be seen. Flash photolysis of (23) leads to (24), (25), and (26), all of which could be trapped by nucleophiles (27) was not an intermediate. NMR lineshape analysis of the spectmm of (28) leads to reaction rate constants of formation for both the intimate ion pair (29) and the solvent-separated ion pair (30). ... 

Recent developments in rotational spectroscopy [19] have provided an accurate study of a number of hydrogen bonded dimers in the gas phase. Experimental information is thus available nowadays on the molecular characteristics of such systems. Due to major advances in the field of computational chemistry, a number of theoretical calculations were performed (for an overview on the subject see references [1,20,21]). 

Since theoretical calculations can be carried out best with small molecules, it is not surprising that oxetane and some of its derivatives have been the subject of most types of theoretical methods. 

There are considerable difficulties in comparing theory and experiment even in such model experiments. The theoretical calculations are subject to the approximations inherent in the method, and also to uncertainties in the pupil function used to characterize the lens and in the two parameters used to characterize the crack. The experiments are subject to the difficulties of making a crack that is straight and flat to a fraction of the acoustic wavelength used, over the length measured by the line-focus-beam lens, and to the sensitivity of the results in some cases to small changes in x or z. Nevertheless, when all these considerations are taken into account it does seem... 

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