Quantum chemical energy calculation

Quantum chemical DFT calculations at the B3LYP/6-31G(d) level have been used to study the enantioselective lithiation/deprotonation of O -alkyl and O-alk-2-enyl carbamates in the presence of (—)-sparteine and (—)-(f )-isosparteine.7 Complete geometry optimization of the precomplexes consisting of the carbamate, the chiral ligand, and the base (/-PrLi), for the transition states of the proton-transfer reaction, and for the resulting lithio carbamates have been performed in order to quantify activation barriers and reaction energies. 

Results of nonempirical quantum chemical energy and geometry calculations are available in the literature (117,118) for hundreds of molecules. 

The simplest model of an amide bond is found in formamide, and several features of protonated formamide are highly relevant to the cleavage of protonated peptides into b and y ions. Amides are bidentate bases, and it has been demonstrated from correlations between core electron energies and proton affinities [213] and from quantum chemical calculations [214] that the carbonyl oxygen is more basic than the amide nitrogen. As demonstrated by FT-ICR, metastable ion dissociation, and RRKM and quantum chemical model calculations [214], the unimolecular dissociation of a protonated formamide molecule depends on which site the proton is attached to ... 

The second crucial choice for a quantum chemical DFT calculation is the basis set. The valence shell of the TMs has s and d orbitals. As a minimum requirement for useful calculations it is necessary to have at least a double-zeta quality for the n(s) and (n - l)d valence orbitals. The status of the lowest-lying empty n p) orbitals is at present controversial (36). However, it has been shown that the basis set should have at least one function that describes the empty n p) orbital of the TM (4,37). Extra/-type polarization functions improve the accuracy particularly of the calculated energies, but it seems that they are less important for the TMs than d-polarization functions for main-group elements. 

Decide whether entropic effects are likely to be important (for example if charged species are released to the solvent) and, if so, decide on whether a quantum chemical approach (calculating the partition function within a harmonic-oscillator approximation) may be used or whether a molecular dynamics-based approach (e.g., free-energy perturbation theory) should be used to properly sample phase space. 

The activation of C-H bonds for different hydrocarbons can occur both at Zn + and ZnOZn + sites. We will first discuss hydrocarbon activation by Zn +. The results presented here are based on quantum-chemical cluster calculations. The reaction energies involved in the overall catalytic cycle for the activation of ethane over a Zn + cation and a ZnOZn " " oxycation adsorbed on a representative cluster chosen to model the ZSM-5 adsorption site are compared in Fig. 4.22. 

Scheme of the gas-phase photochemistry of psoralen based on quantum-chemical CASPT2 calculations. Energies are referred to (Sti)min... 

Table 11 Zn-C bond distances in dimethylzinc, diethylzinc, di-iso-propylzinc, and di-tert-butylzinc determined by gas electron diffraction and quantum chemical DFT calculations mean Zn-C bond dissociation energies derived from experimental thermochemical data and from calculated energies of isodesmic reactions combined with the experimental bond dissociation energy of Mc2Zn and net atomic charges of Zn and a-C atoms obtained by Mulliken population analysis...

So, within the limitations of the single-detenninant, frozen-orbital model, the ionization potentials (IPs) and electron affinities (EAs) are given as the negative of the occupied and virtual spin-orbital energies, respectively. This statement is referred to as Koopmans theorem [47] it is used extensively in quantum chemical calculations as a means for estimating IPs and EAs and often yields results drat are qualitatively correct (i.e., 0.5 eV). 

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