Ab-initio studies

Bundgen P, Grein F and Thakkar A J 1995 Dipole and quadrupole moments of small molecules. An ab initio study using perturbatively corrected, multi-reference, configuration interaction wavefunctions J. Mol. Struct. (Theochem) 334 7... 

Loettgers A, Untch A, Keller H-M, Schinke R, Werner H-J, Bauer C and Rosmus P 1997 Ab initio study of the photodlssoclatlon of HCO In the first absorption band three-dimensional wave packet... 

Suzuki T, Maruyama Y, Kato T, Kikuohi K, Aohiba Y, Kobayashi Kand Nagase S 1995 Eleotroohemistry and ab-initio study of the dimetallofullerene La2 Cgg A/rgew. Chem. Int. Edn. Engl. 34 1094-6... 

Xantheas S S 1994 Ab initio studies of cyclic water clusters (HjO), n=1-6. 2. Analysis of many-body interactions J. Chem. Phys. 100 7523-34... 

HCR and co-workers carried out a number of studies by employing 3D potential energy surfaces calculated by means of highly sophisticated ab initio approaches [88,91-101]. The results of these computations are in impressive agreement with the corresponding experimental findings. The discrepancies in the order of 100 wavenumbers, as in early ab initio studies [16,17], have been reduced in the HCR studies to only a few wavenumbers. In conclusion of their paper on the ( H ) system of NH2, Gabriel et al. state We believe... 

The excellent agreement of the results of HCR ab initio studies with the corresponding experimental findings clearly shows that the strongest influence on the numerical accuracy of the vibronic levels have effects outside of the R-T effect, that is, primarly the replacement of the effective bending approaches employed in previous works by a full 3D treatment of the vibrational motions (for an analysis of this matter see, e.g., [17]). Let us note, however, that such a... 

Figure 11. Left Vibronic structure of the X Tl state of HCCS derived by Tang and Saito from experimental findings (Fig. 3 of [139]). Right Corresponding results of the ab initio study presented in [152].

These can be determined experimentally to very high accuracy from the Stark effect and molecular beam studies. The experimental accuracy is far beyond the capabilities of ab initio studies. At the other extreme, the original route to these quantities was through studies of the dielectric polarization of species in solution, and there is currently interest in collision-induced dipole moments. In either case, the quantities deduced depend critically on the model used to interpret the experiment. 

A theoretical ab initio study of the gas-phase basicities of methyldiazoles (90JA1303) included a discussion of the 4(5)-methylimidazole tautomer-ism. The RHF/4-31G calculations led to the conclusion that the 4-methyl tautomeric form 14a (R = Me, R = R = H) is 5.2 kJ moP more stable than its 5-methyl counterpart 14b. It was emphasized that this result is to be considered as basic-set dependent. However, a recent theoretical study [94JST(T)45] showed that, starting from the RHF/6-31G level, all the more accurate approximations indicate a higher intrinsic stability for the 4-methyl tautomer. At the MP2/6-31G level, the total energy of the 4-methyl tautomer is 0.7 kJ mol lower than that of the 5-methyl tautomer. Inclusion of solvation effects can, thus, strongly affect the position of the tautomeric equilibrium 14a 14b. Recently, a systematic theoretical study... 

FtG. 2. Relative energy of the excited states of 2-methylfuraii and of some reactive intermediates. [Reprinted with permission from M. D Auria. Ab initio study on the photochemical isomerization of furan derivatives. J. Org. Chem. 65, 2494-2498 (2000)]. 

DFT and post-Hartree-Fock ab initio studies on the different tautomers and rotamers of 2-hydroxy- (94) and 2,3-dihydroxypyrazine (95) indicate that the former species is stabilized by about -3 kcal/mol with respect to its keto tautomer in the gas phase [99JST229]. In solution the opposite appears to be true. For 95, the hydroxyoxo 95b and diketo tautomers 95c are most stable in the gas phase (Scheme 61). 

The carbo-Diels-Alder reaction of acrolein with butadiene (Scheme 8.1) has been the standard reaction studied by theoretical calculations in order to investigate the influence of Lewis acids on the reaction course and several papers deal with this reaction. As an extension of an ab-initio study of the carbo-Diels-Alder reaction of butadiene with acrolein [5], Houk et al. investigated the transition-state structures and the origins of selectivity of Lewis acid-catalyzed carbo-Diels-Alder reactions [6]. Four different transition-state structures were considered (Fig. 8.4). Acrolein can add either endo (N) or exo (X), in either s-cis (C) or s-trans (T), and the Lewis acid coordinates to the carbonyl in the molecular plane, either syn or anti to the alkene. 

AB INITIO STUDY OF PHASE STABILITY AND EQUATION OF STATE OF TITANIUM DIOXIDE... 

Ab-initio studies of surface segregation in alloys are based on the Ising-type Hamiltonian, whose parameters are the effective cluster interactions (ECI). The ECIs for alloy surfaces can be determined by various methods, e.g., by the Connolly-Williams inversion scheme , or by the generalized perturbation method (GPM) . The GPM relies on the force theorem , according to which only the band term is mapped onto the Ising Hamiltonian in the bulk case. The case of macroscopically inhomogeneous systems, like disordered surfaces is more complex. The ECIs can be determined on two levels of sophistication ... 

As the most notable contribution of ab initio studies, it was revealed that the different modes of molecular deformation (i.e. bond stretching, valence angle bending and internal rotation) are excited simultaneously and not sequentially at different levels of stress. Intuitive arguments, implied by molecular mechanics and other semi-empirical procedures, lead to the erroneous assumption that the relative extent of deformation under stress of covalent bonds, valence angles and internal rotation angles (Ar A0 AO) should be inversely proportional to the relative stiffness of the deformation modes which, for a typical polyolefin, are 100 10 1 [15]. A completly different picture emerged from the Hartree-Fock calculations where the determined values of Ar A0 AO actually vary in the ratio of 1 2.4 9 [91]. 

A new period in theoretical work on arenediazonium ions began with Vincent and Radom s paper in 1978. This was the first ab initio study of the methane- and benzenediazonium ions, and was carried out with a minimal (STO-3G) basis set, subject only to some (specified) symmetry constraints and a fixed CH bond length (108.3 pm). The optimized structure of the benzenediazonium ion is given in Figure 4-2. 

Following Vincent and Radom s comprehensive ab initio study (1978), Escudero et al. (1985) also performed ab initio calculations at the STO-3G level for seven 4-substituted benzenediazonium ions. For the unsubstituted benzenediazonium ion, the optimized geometries are the same as in Figure 4-1 (I). For charge density calculations another population analysis method was used (Escudero and Yanez, 1982), and this gave results different from those of Vincent and Radom. However,... 

A crystallographic scale of acidity has been developed. Measuring the mean C—H O distances in crystal structures correlated well with conventional P a(DMSO) values. An ab initio study was able to correlate ring strain in strained hydrocarbons with hydrogen-bond acidity. ... 

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