Bond energies theoretical calculations

The chapter is based on our systematic theoretical studies of organo-metallic compounds using effective core potentials. - We want to point out that in these investigations only mononuclear low-spin (mainly closed-shell) transition metal compounds have been considered. The review is limited to the calculation of stable transition metal compounds that is, molecules that usually obey the 18-electron (sometimes 16-electron) rule (although the prediction of bond energies requires calculations on the respective unstable fragments, which have also been carried out). 

We shall examine the simplest possible molecular orbital problem, calculation of the bond energy and bond length of the hydrogen molecule ion Hj. Although of no practical significance, is of theoretical importance because the complete quantum mechanical calculation of its bond energy can be canied out by both exact and approximate methods. This pemiits comparison of the exact quantum mechanical solution with the solution obtained by various approximate techniques so that a judgment can be made as to the efficacy of the approximate methods. Exact quantum mechanical calculations cannot be carried out on more complicated molecular systems, hence the importance of the one exact molecular solution we do have. We wish to have a three-way comparison i) exact theoretical, ii) experimental, and iii) approximate theoretical. 

One may wonder why it is important to distinguish between and keep track of these two energies and Dq, when it seems that one would do. Actually, both are important. The bond energy Dg dominates theoretical comparisons and the dissociation energy Dq, which is the ground state of the real molecule, is used in practical applications like calculating thermodynamic properties and reaction kinetics. 

A modified definition of resonance energy has been introduced by Dewar (66T(S8)75, 69JA6321) in which the reference point is the corresponding open-chain polyene. In principle this overcomes the difficulties inherent in comparing observed stability with that of an idealized molecule with pure single and double bonds, as thermochemical data for the reference acyclic polyenes are capable of direct experimental determination. In practice, as the required data were not available, recourse was made to theoretical calculations using a semiempirical SCF-MO method. The pertinent Dewar Resonance Energies are listed in Table 30. 

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. 

Somewhat surprisingly perhaps, it has been found that [l.l.l]propellane is considerably less reactive than [2.2.1]propellane. Use the theoretically calculated enthalpy data below to estimate the bond dissociation energy of the central bond in each of the three propellanes shown. How might this explain the relative reactivity of the [1-1.1]- and [2.2. Ijpropellanes ... 

Table 12.4. Substituent Effects on Radical Stability from Measurements of Bond Dissociation Energies and Theoretical Calculations of Radical Stabilization Energies...

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. 

Does a Silylene-Complex exist This rhetorical question is the title of a theoretical paper published in 1983 [84], As a result of an ab-initio calculation, the authors came to the conclusion that a moderately positive answer can be given. However, silylene complexes are thermodynamically less stable than carbene complexes (the MSi bond energy for the hypothetical complex (OC)5Cr = Si(OH)H is 29.6 kcal/mol, the bond energy of the MC bond in (OC)5Cr = C(OH)H is 44.4 kcal/mol) [85], and therefore silylene complexes should be difficult to isolate. 

Theoretical energy curves for one-electron bonds between two atoms are calculated for bond orbitals formed by hybridization of 2s and 2p orbitals, 35 and 3 orbitals, and 35, 3p, and 3d orbitals, the same radial part being used for the orbitals in a set. It is found that for s-p hybridization the bond energy is closely proportional to S3, with 5 the magnitude of the angular part of the bond orbital in the bond direction. This relation is less satisfactorily approximated in the case of s-p-d hybridization. 

X = CO2R or CN). Theoretical calculation at B3LYP/6-31G //HF/STO-3G level showed that the Si-H bond dissociation energies of H-Si(l 11) and (MesSifsSi-H are very similar, which further justifies the use of the well-established radical-based reactivity of (MesSifsSiH as a model for surface reactions. 

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