HF/6-31G* calculations

The thermodynamic stabilities of three possible annular tautomers of the parent 1,2,4,5-thiatriazine 134 were compared using ab initio HF/6-31G calculations (OOJOC931). The 4H isomer 134a appears to be the most stable (it is more stable than 134b by 11.6 kcal/mol and more stable than 134c by 15.5 kcal/mol), presumably because it allows low-energy distortion from planarity and formation of the boat conformation. 

Regarding current ab initio calculations it is probably fair to say that they are not really ab initio in every respect since they incorporate many empirical parameters. For example, a standard HF/6-31G calculation would generally be called "ab initio", but all the exponents and contraction coefficients in the basis set are selected by fitting to experimental data. Some say that this feature is one of the main reasons for the success of the Pople basis sets. Because they have been fit to real data these basis sets, not surprisingly, are good at reproducing real data. This is said to occur because the basis set incorporates systematical errors that to a large extent cancel the systematical errors in the Hartree-Fock approach. These features are of course not limited to the Pople sets. Any basis set with fixed exponent and/or contraction coefficients have at some point been adjusted to fit some data. Clearly it becomes rather difficult to demarcate sharply between so-called ab initio and semi-empirical methods.4... 

In all cases, the high degree of pyramidality attributed to the presence of two electronegative oxygen atoms at nitrogen, confirms the predictions of HF/6-31G calculations.5 On the basis of geometries for all acyclic amides in the Cambridge Structural Database in 2002,117 amides 31b and 31f are the most pyramidal amides of this type.5... 

On the one hand, x-ray crystallography data could support this idea because the heterocycles are coplanar relative to the B-H bonds. Moreover, in the case of the compound in Strncture 5.5, this conformation remains in solution, according to NOE NMR measnrements. On the other hand, the solid-state CH- -HB distances are too long (< 2.65 A), and the HF/6-31G calculations carried ont for the componnd have shown, that where the heterocyclic rings are oriented orthogonally, the conformation is more stable. Thns, this question is still open. It is quite probable that the interaction between protonic hydrogens on the a-carbons and hydridic BH hydrogens is electrostatically attractive. 

In contrast, the HF/6-31G calculations by Rozas and co-workers have resulted in the dihydrogen-bonded complex L-H H-F as a stationary strucmre [2], However, when even a weak electric field is applied to this complex, the system changes to L+- H-H- F with an H H distance of 0.734 A, very close to that of molecular hydrogen. 

Ab initio HF/6-31G calculations on A-formyloxy-A-methoxyformamide (91) predicted a strongly pyramidal nitrogen with an average angle of 110.3° (Figure 16). 

Density fnnctional calculations on 215b-d at the B3P86 level have been carried ont as part of a detailed study of the HERON reaction of hydrazines (see Section VI.C). These afforded symmetrical gauche structures l(EE) as lowest energy conformers N—N (1.37 A), N—C(0) (1.38 A) and N—O (1.40 A) bond lengths were similar to those from HF/6-31G calculations. ... 

INDO and HF/6-31G calculations suggest that secondary orbital interaction of the dipolarophile and the lone pair of electrons on the dipole nitrogen would favor the endo isomer (43,93). Since this result is not typically observed experimentally. 

To study the effect of the energy gap between donor and acceptor, we carried out HF/6-31G calculations on [(GC),(AT)] [41] and compared various procedures for determining the coupling elements within the one-electron approximation. In the complex [(GC),(AT)], guanine and adenine act as donor and acceptor sites. The donor-acceptor energy gap can be modulated by ap-... 

In that study [31], we estimated the electronic coupling with the help of HF/6-31G calculations. Any attempt to expand such an investigation into a reasonably quantitative description of the variation of the electronic coupling over time would be much too costly. As noted above, one can overcome that problem by constructing a special semiempirical method (e.g., NDDO-HT) affording sufficiently accurate estimates of electronic matrix elements, or by using an approximate relation between H a and the overlap of related orbitals. 

FIGURE 23. HF/6-31G calculated geometry of the complex (9-BBN)3Si—CgH6+ (50). The two inserts on the right side indicate the dimensions of the opening above the central Si atom formed by atoms H2, H3, H4 (black atoms), and below the central Si atom by atoms H5, H6, H7 (black atoms). Distances in A. Reprinted with permission from Reference 113. Copyright (1997) American Chemical Society... 

Comparison of cyclic compounds 38 and 39 shown in equation 88 via correlated MP2/6-31G //HF/6-31G calculations indicated that the cyclic trisilylene 38 is more stable than the trisila-s-triazine structure 39 by 30 kcalmol-1 at the CCSD/6-31GV/MP2/ 6-31G ... 

The Sn reactions of cyclohexanone acetals substituted at C(2) with sulfur, iodine, or chlorine are thought to occur when the nucleophile attacks the oxocarbenium ion intermediate with the substituent on C(2) in an axial conformation.104 The most stereospecific reactions (i.e. >92% trans), were when the substituent at C(2) was sulfur. This mechanism (Scheme 26) is supported by HF/6-31G calculations that show the oxocarbenium ion (66) with the sulfur at C(2) in the axial position to be the most stable, and by the high yield of the trans-isomer (67) in the products. 

Thus, the three-center potential integrals are not retained. Ferenczy et al. found that the AMI MEP maps are able to reproduce the main characteristics of the HF/STO-3G MEP maps for the water, formaldehyde, formamide and the cytosine molecules and for the cyanate ion [40]. However, the NDDO AMI MEP maps gave deeper minima which were closer to the molecules than those in the HF/STO-3G MEP maps. The contour plots of MEP for the cytosine molecule are displayed in Fig. 2. It can be seen that the NDDO AMI MEP map correctly predicts the N3 nitrogen atom as a primary protonation center instead of the 02 oxygen atom. This finding is in agreement with the HF/STO-3G MEP map [28] and the experimental as well as the theoretical proton affinities [29, 30]. Similar results were also obtained by Luque et al. based on the quasi ab initio MNDO MEP map [37], INDO/S, HF/4-31G and HF/6-31G calculations showed an opposite order of protonation [27, 31] as discussed earlier. 

How good are ab initio dipole moments Hehre s extensive survey of practical ab initio methods [39] indicates that fairly good results are given by HF/6-31G // HF/6-31G (dipole moment from a HF/6-31G calculation on a HF/6-31G geometry) calculations, and that MP2/6-31G //MP2/6-31G calculations are... 

Fig. 12.9. The optimized structures of trehalose / 2-butene complex obtained from the HF/6-31G calculation...

FIGURE 7. HF-6-31G calculated MO energies for vinylamine (115) in several conformations 116 to 121 and those of related molecules (in hartree = 27.2117 eV)... 

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