Molecules ab initio calculations for

There are only a few studies of these molecules (30, 36,124). They have been summarized in a review (30) together with those of unsaturated members of the CgHg family. All but one of the semiempirical quantum chemical studies quoted in Table 1 of the review yielded cubane (which is [4] prismane) as the most strained member of the family. Except in ref 36, the existence of inverted carbon atoms (the bridgehead ones in the bicyclobutane units) in the octabisvalene molecule has been overlooked. There is no agreement in the literature concerning the relative stability of cuneane and octabisvalene. As mentioned earlier, the experimental determination and ab initio calculation have been carried out only for 1 thus, more experimental measurements and further ab initio calculations for molecules 76a and 83 are necessary. 

The kernel-neighborhood fragment calculation affords the opportunity to make ab initio calculations for molecules whose complexity exceeds the capacity of available computer programs, particularly since the increase in computer time is essentially a linear function of the complexity. In addition, the fragment calculation may save time when employed as an alternative to a calculation made on a complex, full molecule. 

Choose the region (single or multiple molecules) of interest for an ab initio calculation from the total molecular system. HyperChem performs the ab initio calculation for the selected region using the perturbation of an electrostatic potential arising from the net charges on the atoms of the un selected part. (For further details of this electrostatic potential perturbation implemented in HyperChem, please see the second part of this book. Theory and Methods). 

For small molecules, the accuracy of solutions to the Schrodinger equation competes with the accuracy of experimental results. However, these accurate ab initio calculations require enormous computation and are only suitable for the molecular systems with small or medium size. Ab initio calculations for very large molecules are beyond the realm of current computers, so HyperChem also supports semi-empirical quantum mechanics methods. Semi-empirical approximate solutions are appropriate and allow extensive chemical exploration. The inaccuracy of the approximations made in semi-empirical methods is offset to a degree by recourse to experimental data in defining the parameters of the method. Indeed, semi-empirical methods can sometimes be more accurate than some poorer ab initio methods, which require much longer computation times. 

It seems that the substituents play an essential role in the kinetic stabilization of the molecule 201 ab initio calculations for the formally anti-aromatic unsubstituted indacene shows that the minimum energy corresponds to a Dzh symmetry and a completely delocalized 12-jr-electron system.202... 

The 72 kJ mol-1 discrepancy observed for Z)//°[(CO)3Cr-C6H6] alerts us to two important issues (1) Stepwise bond dissociation enthalpies are often very different from their mean (2) bond dissociation enthalpy values may not be transferable from one molecule to another. In other words, how can we assess the assumption that 3(Z)//°)(Cr-CO) in Cr(CO)6 is similar to Z)//j°(Cr-CO) + Z)// (Cr-CO) + DH (Cr-CO) in Cr(CO)3(C6H6) This question will be discussed in section 5.3. With regard to the former issue, the data in figure 5.4 provide further illustration for some first-row hydrides. The values shown in the plot are results from ab initio calculations for the species involved, in excellent agreement with the most accurate experimental data [78]. 

Molecular dynamic studies used in the interpretation of experiments, such as collision processes, require reliable potential energy surfaces (PES) of polyatomic molecules. Ab initio calculations are often not able to provide such PES, at least not for the whole range of nuclear configurations. On the other hand, these surfaces can be constructed to sufficiently good accuracy with semi-empirical models built from carefully chosen diatomic quantities. The electric dipole polarizability tensor is one of the crucial parameters for the construction of such potential energy curves (PEC) or surfaces [23-25]. The dependence of static dipole properties on the internuclear distance in diatomic molecules can be predicted from semi-empirical models [25,26]. However, the results of ab initio calculations for selected values of the internuclear distance are still needed in order to test and justify the reliability of the models. Actually, this work was initiated by F. Pirani, who pointed out the need for ab initio curves of the static dipole polarizability of diatomic molecules for a wide range of internuclear distances. 

Odelius and co-workers reported some time ago an important study involving a combined quantum chemistry and molecular dynamics (MD) simulation of the ZFS fluctuations in aqueous Ni(II) (128). The ab initio calculations for hexa-aquo Ni(II) complex were used to generate an expression for the ZFS as a function of the distortions of the idealized 7), symmetry of the complex along the normal modes of Eg and T2s symmetries. An MD simulation provided a 200 ps trajectory of motion of a system consisting of a Ni(II) ion and 255 water molecules, which was analyzed in terms of the structure and dynamics of the first solvation shell of the ion. The fluctuations of the structure could be converted in the time variation of the ZFS. The distribution of eigenvalues of ZFS tensor was found to be consistent with the rhombic, rather than axial, symmetry of the tensor, which prompted the development of the analytical theory mentioned above (89). The time-correlation... 

In addition, ab initio calculations for the unknown imsubstituted sydnone molecule (1, R = R = H) and the tautomeric species 2H-l,2,3-oxadiazol-5-one (442) and 5-hydroxy-1,2,3-oxadiazoIe (443) have been reported. The meso-ionic tautomer (1) is calculated to be less stable... 

Studies of larger species are more complex and the difficulty in the evaluation of their potential surfaces increases with their size. Up to now accurate potentials have been obtained by inversion of spectroscopic data or through high level ab initio calculations " for several triatomic vdW systems. Thus, the interactions for such clusters are available with satisfactory accuracy, which permits the testing of various models of nonadditivity for their ability to reproduce a number of experimental observations. These facts made complexes composed of two rare-gas atoms and a dihalogen molecule especially attractive targets for the study of nonadditive forces. The first attempt to extract information on nonadditive interactions from... 

Ab initio Calculations on Molecules containing Five or Six Atoms Table 6 Computed energies and barriers to rotation for N204... 

Correction factors improve the fit between semiempirically calculated and experimentally measured spectra, but the agreement does not become as good as does the fit of corrected ab initio to experimental spectra. This is because deviations from experiment are less systematic for semiempirical than for ab initio methods (a characteristic that has been noted for errors in semiempirical energies [98]). For AMI calculations, correction factors of 0.9235 [99] and 0.9532 [100], and for PM3, factors of 0.9451 [99] and 0.9761 [100], have been recommended. A factor of 0.86 has been recommended for SAMI for non-H stretches [101]. However, the variation of the correction factor with the kind of frequency is bigger for semiempirical than for ab initio calculations for example, for correcting carbonyl stretching frequencies, examination of a few molecules indicated (author s work) that (at least for C, H, O compounds) correction factors of 0.83 (AMI) and 0.86 (PM3) give a much better fit to experiment. 

Electron scattering from molecules is receiving increasing attention, and theoretically it can be treated by calculation of the static potential (the interaction potential of an electron with the unperturbed charge distribution). Ab initio calculations for N2 using wavefunctions varying between minimal-basis and near-HF quality have been reported by Truhlar et a/.,237 and compared with semi-empirical INDO calculations. The anisotropy of the potential is only correctly described if tf-functions are included in the basis set. 

Ground State of CO. The CO molecule has been extensively studied, both experimentally and theoretically. Table 14 compares ground-state 02+) spectroscopic constants calculated by the Hartree-Fock method109 and by the density functional approach99 with experiment.104 In addition to these spectroscopic constants, the polar nature of the molecule provides a further measurable quantity, the dipole moment. Since the intensities of infrared vibration-rotation bands allow the dipole moment to be determined as a function of C-O separation this provides a useful comparison with the results of ab initio calculations. For example, the positive sign obtained from the equilibrium dipole moment by Hartree-Fock calculations was viewed as a reason to question the negative value found experimentally, whereas the current view is that the positive sign is a defect of the Hartree-Fock method. 

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