Structures ab initio calculations

Table /. X-ray and Ab Initio Calculated Structures of Selected Cycloproparenes ... 

Table 5. Ab initio Calculated Structures of Benzocyclopropenyl Cation (273) and its 1 -Fluoro Derivative 282 ...

The structural data reported from the X-ray analysis are very similar to the ab initio calculated structural data for silyl-substituted arenium cations. The calculations of the p-trimethylsilyl (319) (Figure 9) and p-triethylsilyl toluenium ions (318) [HF/6-31G(d)] and the silylbenzenium ion 53 [MP2(fc)6-31G(d)] all show similar geometries and the characteristic geometric data resemble closely the main features found in the crystal structure. 

Fig. 17 Left-hand side variation of global instability index G of 48 ab initio structure models of Li ion conducting oxides versus the relative difference between the volume of the ab initio structure models and the experimentally determined unit cell volume for the same phase. Right-hand side linear correlation between the (unsealed) activatirai energy barriers derived by the B VSE method for experimental and for ab initio calculated structure models. In both graphs the six cases for which the ab initio model overestimates the unit cell volume by more than 8% are marked as open symbols. These less reliable data points are excluded liom the calculation of the r.h.s. linear correlation...

Dykstra C E 1988 Ab initio Calculation of the Structures and Properties of Molecules (Amsterdam Elsevier)... 

Ab-initio calculations are particularly usefiil for the prediction of chemical shifts of unusual species". In this context unusual species" means chemical entities that are not frequently found in the available large databases of chemical shifts, e.g., charged intermediates of reactions, radicals, and structures containing elements other than H, C, O, N, S, P, halogens, and a few common metals. 

Fig. 5.35 Geometry predicted by CASSCF ab initio calculations of the two possible transition structure geometries for the Diels-Alder reaction between ethene and butadiene. (Figure adapted from Houk KN, J Gonzalez and Y Li 1995. Pericyclic Reaction Transition States Passions and Punctilios 1935-1995. Accounts of Chemical Research 28 81-90.)...

With Lammerstma and Simonetta in 1982, we studied the parent six-coordinate diprotonated methane (CH/ ), which has two 2e-3c bonding interactions in its minimum-energy structure (Cid- On the basis of ab initio calculations, with Rasul we more recently found that the seven-coordinate triprotonated methane (CHy ) is also an energy minimum and has three 2e-3c bonding interactions in its minimum-energy structure 3 ). These results indicate the general importance of 2e-3c bonding in protonated alkanes. 

Atextbook describing the theory associated with calculations of the electronic structure of molecular systems. While the book focuses on ab initio calculations, much of the information is also relevant to semi-empirical methods. The sections on the Hartree-Fock and Configuration Interactions methods, in particular, apply to HyperChem. The self-paced exercises are useful for the beginning computational chemist. 

Not all iterative semi-empirical or ab initio calculations converge for all cases. For SCF calculations of electronic structure, systems with a small energy gap between the highest occupied orbital and the lowest unoccupied orbital may not converge or may converge slowly. (They are generally poorly described by the Hartree-Fock method.)... 

This method is available for all semi-empirical methods except Extended Hiickel, and for ab initio calculations. This algorithm may be used if the structure is far from a minimum. 

The success of simple theoretical models in determining the properties of stable molecules may not carry over into reaction pathways. Therefore, ab initio calculations with larger basis sets may be more successful in locating transition structures than semi-empir-ical methods, or even methods using minimal or small basis sets. 

Theoretical methods ranging from the now obsolete HMO studies to ab initio calculations have been used extensively on pyrazoles. Although not emphasized in earlier reviews (66AHC(6)347,67HC(22)l), the most recent publications (B-76MI40402,79RCR289) contain several references to theoretical studies. Some publications related to structural studies are to be found in the following sections, especially in connection with NMR spectroscopy (Section 4.04.1.3.4), UV spectroscopy (Section 4.04.1.3.6), PE spectroscopy (Section 4.04.1.3.9) and tautomerism (Section 4.04.1.5). 

Later there was an attempt by ab initio calculation to fit the electron structure of diazirine into the Walsh model of cyclopropane (69MI50800). According to these SCF-LCAO-MO calculations three MOs add to the description of the lone electron pairs, all of which also contribute to some extent to ring bonding. As to strain, 7r-character and conjugative effect, the term pseudo-rr-character was used. 

The first step for any structure elucidation is the assignment of the frequencies (chemical shifts) of the protons and other NMR-active nuclei ( C, N). Although the frequencies of the nuclei in the magnetic field depend on the local electronic environment produced by the three-dimensional structure, a direct correlation to structure is very complicated. The application of chemical shift in structure calculation has been limited to final structure refinements, using empirical relations [14,15] for proton and chemical shifts and ab initio calculation for chemical shifts of certain residues [16]. 

We recently proposed a new method referred to as RISM-SCF/MCSCF based on the ab initio electronic structure theory and the integral equation theory of molecular liquids (RISM). Ten-no et al. [12,13] proposed the original RISM-SCF method in 1993. The basic idea of the method is to replace the reaction field in the continuum models with a microscopic expression in terms of the site-site radial distribution functions between solute and solvent, which can be calculated from the RISM theory. Exploiting the microscopic reaction field, the Fock operator of a molecule in solution can be expressed by... 

These reactions are believed to proceed through a complex of the alkene with a singlet excited state of the aromatic compound (an exciplex). The alkene and aromatic ring are presumed to be oriented in such a manner that the alkene n system reacts with p orbitals on 1,3-carbons of the aromatic. The structure of the excited-state species has been probed in more detail using CAS-SCF ab initio calculations. ... 

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