Basicity B3LYP

The nitrido and phosphido complexes of TMs have been the subjects of intensive experimental studies in the recent years. Of particular interest has been the issue of Lewis basicity of the nitrogen and phosphorus atoms in the TM=N and TM=P groups. Table 7.17 lists the BDEs calculated at the MP2/II, B3LYP/B and CCSD(T)/B levels of theory for LnMN-X and LnMP-X, where X is a group-13 Lewis acid or a chalcogen atom [86, 87]. 

TABLE 6. Values of /s.min relevant to the acidities or basicities of hydroxylamine (5), dimethyl-hydroxylamine (6), acetoxime (7) and acetohydroxamic acid (8). The Is min were computed at the HF/6-31GV/B3LYP/6-31G level and are in eV"... 

Although water oxide, once formed, proved to be a remarkably efficient oxygen atom donor, it is quite likely that it is simply too high in energy to be a viable oxidant. A second problem with the preliminary calculations by Bach and coworkers" is that NH3 is not a realistic nucleophile for such studies it is what was feasible at that time. This inadequacy has only recently been ameliorated by Ottolina and Carrea , who used the more nucleophilic trimethylamine (TMA) as the attacking nucleophile and got an entirely different and much more chemically realistic hypersurface for the HO—OH oxidation of amines. The prior results at the MP2 level compared favorably with the results of the B3LYP/6-31-I-G method, so the basic problem was in the nucleophile and not with the... 

In an earlier report Bach and coworkers suggested that MesN and MesP had nearly the same gas-phase barriers with peroxyformic acid (AA 5 = 0.5 kcalmol", MP4//MP2/6-31G ) and that their relative reactivity in protic solvent is better attributed to a much greater ground-state solvation of the more basic tertiary amine than the polarizability of the phosphorus. In a more recent study, a 3.3 kcalmol" barrier difference was observed with HCO3H in favor of the P nucleophile at B3LYP/6-31- -G(d,p) (3.0 versus —0.3 kcalmol". Table 12). Thus, the less reactive f-BuO—OH is a more discriminating oxidant with a later TS and the difference in barrier heights for N versus P oxidation widens. These data also point out that rate differences should be compared for a common set of nucleophiles and that when you use oxidants of different reactivity you should... 

Table 6-5 compares absolute basicities obtained from Hartree-Fock STO-3G, 3-21G, 6-31G and 6-311+G models, local density models, BP, BLYP, EDFl and B3LYP density frmctional models and MP2 models, all with 6-3IG and 6-311+G basis sets and MNDO, AMI andPM3 semi-empirical models, with experimental enthalpies. All reactions are highly endothermic because they lead to a free proton. ... 

Density functional models and MP2 models show more consistent behavior. With the 6-311+G basis set, calculated basicities are generally very close to experimental values. The corresponding results with the 6-3IG basis set are generally not as good, although the differences are not that great. In terms of mean absolute errors, local density models perform the worst, and B3LYP/6-311+G and MP2/ 6-311+G models perform the best. 

A special but important case of relative acidity/basicity comparisons involves reactions which differ only by remote (from the reaction site ) substitution. The example here relates to acidities ofp-substituted benzoic acids to that of the parent compound. Data are provided in Table 6-19. Results from STO-3G, 3-21G and 6-31G Hartree-Fock models, local density models, BP, BLYP, EDFl and B3LYP density functional models and MP2 models all with the 6-3IG basis-set, and MNDO, AMI and PM3 semi-empirical models are provided. 

C=C stretching frequencies experimentally range from 1570 cm in cyclobutene to 1872 cm in tetrafluoroethylene (see appropriate tables in Appendix A7). All levels of calculation reproduce the basic trend in frequencies but, on the basis of mean absolute errors, show widely different performance (Table 7-5). Local density and MP2 models with the 6-311+G basis set perform best and semi-empirical models and density functional models (except the B3LYP model) with the 6-3IG basis set perform worst. Hartree-Fock models with the 3-2IG and larger basis sets also turn in good performance. 

The gas-phase basicity (GB) of 3-thio-5-oxo 1, 5-thio-3-oxo 2, and 3,5-dithio 4 derivatives of 2,7-dimethyl-[l,2,4]-triazepine (Figure 1) has been measured by means of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry and complemented with theoretical calculations. The experimental FTICR results are discussed in Section 13.14.4.1.l(i). The structures and vibrational frequencies of all stable protonated tautomers and all transition states connecting them have been obtained by means of the B3LPY density functional method, together with 6-31G basis set expansion. The final energies were obtained at the B3LYP/6-311 + G(3df,-2p) level (2002JPC7383). 

B3LYP/TZVP geometries of all species shown in Scheme 1, followed by MP4(sdtq)/6-311+G single-point energy calculations. The theoretical proton affinities are shown for comparison, the experimental PA for pyridine is 222 kcal/mol. It would appear that olefins that protonate on a zeolite to yield persistent carbenium ions are very basic indeed. 

Like 8d [109], the X-ray structure of 8a [108] exhibited a distorted square pyramidal phosphorus. The structural parameters were similar to those obtained by DFT calculations (B3LYP/6-31G ) for 8a and 8b. Calculations on smaller model systems with different substituent pattern, including the tetracoordinate phos-phirenium cation (see Scheme 11), allowed elucidation of the bonding, which was described by a n - o delocalization [108], This result is basically in agreement with the proposal of Regitz and coworkers [109], who used resonance structures to account for delocalization. 

The gas-phase and aqueous basicities of six 1,2,3-triazoles (71-76), have been determined, the former by FTICR and the latter by spectrophotometry and II NMR. The gas-phase experiments are in good agreement with the Gibbs free energies calculated at the B3LYP/6-31G level. In contrast, only semiquantitative ascertainments are possible when basicities in the gas phase and in solution are compared. The equilibria involved are complex due to tautomerism [87],... 

The thermodynamic parameters characterizing the basicity of the tetrazole ring with respect to lithium cation in the gas phase have been obtained experimentally (ion cyclotron resonance) and by calculations G2, G2(MP2), B3LYP/6-31+G )) <2000PCA2824, 2004JCI1727, 2004PCA4812>. 

Another method which takes into account electron correlation is density functional theory (DFT), which is implemented in Gaussian with the label B3LYP. The basic idea of DFT is to calculate the energy of an atom or molecule in terms of electron density rather than the wavefunction tf/ The method has become increasingly popular because it can do reasonably accurate calculations on large molecules in significantly less time than that for the HF method. 

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