Quantum-chemical calculations proton affinities

To verify the mechanism presented, the quantum-chemical calculations of proton affinity, Aa, were carried out for modifiers, since the corresponding experimental data are quite rare. The calculations were performed for isolated molecules, since the properties of species in the interlayer space are probably closer to the gas phase rather than the liquid. The values of Ah were calculated as a difference in the total energy between the initial and protonated forms of the modifier. Energies were calculated using the TZV(2df, 2p) basis and MP2 electron correlation correction. Preliminarily, geometries were fully optimized in the framework of the MP2/6-31G(d, p) calculation. The GAMESS suite of ah initio programs was employed [10]. Comparison between the calculated at 0 K proton affinities for water (7.46 eV) and dioxane (8.50 eV) and the experimental data 7.50 eV and 8.42 eV at 298 K, respectively (see [11]), demonstrates a sufficient accuracy of the calculation. 

Note that 1,5-diketones 29 (Scheme 2) are cyclodehydrated as soon as they are formed, and this process evidently occurs via protonation of the carbonyl group. The proton affinity in this case is probably determined by the substituent at each carbonyl group in 29. So far, there are no synthetic investigations of the problem, whereas quantum chemical calculations reveal the equal possibility for proton addition to both oxygen atoms in homophthalic dialdehyde (86KGS460). 

The quantum-chemical calculation of charge-transfer states as possible intermediates in electrophilic aromatic substitution reactions, making allowance for solvation effects, has been reviewed.6 It has been shown that a simple scaled Hartree-Fock ab initio model describes the ring proton affinity of some polysubstituted benzenes, naphthalenes, biphenylenes, and large alternant aromatics, in agreement with experimental values. The simple additivity rule observed previously in smaller... 

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 ... 

Summary Reactions of free methyl cations with diisobutylamine (1), isobutylaminotrimethylsilane (2) and hexamethyldisilazane (3) were studied by the radiochemical method. It was shown that in all cases studied, the proton transfer is a predominant channel. Probabilities for the reaction to enter this channel are 0.87 for 1, 0.67 for 2 and 0.93 for 3. The last value contradicts the well-known dependence of the lowering of the proton affinity of amines upon the CHa/SiHa ratio. However, quantum chemical calculations have shown that the interaction of the silazane HOMO with the CHa LUMO is symmetry forbidden. This fact may result in the lowering of the methyl cation affinity for silazanes and preference for the proton transfer channel. 

A quantum chemical calculation by the MNDO method yielded a negative proton affinity (the calculated AHf(NF3H ) value was too high) [4]. 

Quantum-chemical ab initio calculations have been conducted to determine the proton affinities of tripyrrolidinyl-and l,4,7-trimethyl-l,4,7-triazacyclononane (8 and 9, respectively). Their proton affinities have been found to be up to 20 kcal mol-1 higher than the values of noncyclic tertiary aliphatic amines due to an effective stabilization of the ammonium cations <2005T12371>. 

Meyer, N.C., Bolm, C., Raabe, G. and Kolle, U. (2005) Proton affinities and relative basicities of two 1,4,7-triazacyclononanes, Me3TACN and TP-TACN. Quantum-chemical ab initio calculations, solution measurements, and the structure of [TP-TACN.2H] in the solid state. Tetrahedron, 61, 12371-12376. 

A variety of ab initio quantum chemical methodologies has been used to compute the internal energy variation, AE°, and the zero-point correction energies AZPE, needed to calculate gas-phase acidities and proton affinities. 

Pople and coworkers [47] have first realized the benefit of evaluating quantum chemical methods by benchmarking them against accurate experimental measurements. Their work mainly focused on atomization energies, which were used to calculate the heats of formation for around 150 molecules having weU-estabUshed enthalpies of formation at 298K and were summarized in the so-called G2/97 benchmark test set [48] and later enhanced to the benchmark versions G3/99 [49] and G3/05 [50], where electron and proton affinities and ionization potentials of small molecules played an additional minor role. 

Calculation of proton affinity in a solvent by quantum chemical methods is a very difficult problem, if at all solvable. Suppose two calculations are done, one where the proton is placed near A, and one where it is placed near one of the water molecules (Figure 9.2). The difference between the two ground state energies should be taken. In both calculations, the geometry should be optimized. But geometry optimization can only lead to the lowest enthalpy situation. The geometry of the other state is not well-defined. 

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