Ab-initio STO-3G calculations

AHCS1, p. 528). Ab initio (STO-3G) calculations of the geometry, eharge distribution, and gas-phase basieity of azaindoles eoneluded that the strueture of five-membered ring is almost unaffeeted by the position of the aza-N atom (83T2851). 

An X-ray analysis 34a) of ( ),( ),( )-l,5,9-cyclododecatriene (38) suggested that this molecule is most comfortable in a twisted chiral conformation of D3 symmetry, and a dynamic NMR study and a force-field calculation Mb) estimated AG = 8.6 kcal/mol for the racemization process between two enantiomeric D3 conformers. A pseudochair conformation of similar D3 symmetry was also suggested35) for 1,5,9-cyclodo-decatriyne (39), following an ab initio STO-3G calculation and photoelectron spectroscopy. 

Similarly, the CH bond of methane was stretched using ab initio STO-3G calculations the results are... 

Fully planar cis and trans conformations with a preference for cis were found for 2-(2-furyl)pyrrole (122) and 2-(2-thienyl)pyrrole (123) by a combination of dipole-moment studies and ab- initio STO-3G calculations (81JCS(P2)127). 

Our computations indicate that there is a large activation barrier for this reaction27. Both MINDO/3 and ab initio STO-3G calculations were used in preliminary investigations, and the results of a surface scan by the former in which D3h sym-... 

Ab initio STO-3G calculations were performed on the supermolecules formed by a nucleophile and a chiral substrate (2-chioropropanal or 2-methylbutanal). The nucleophile, simulated by H , is located at 1.5 A from the carbon atom, in a direction perpendicular to the carbonyl axis. Hence, to each reaction correspond two diastereoisomeric supermolecules resulting from attack on one face or the other of the TT system. These molecules are thus models of the diastereoisomeric transition states. In Fig III and IV, their relative energies are plotted as a function of their conformations. The solid (dashed) lines correspond to the transition states of type 5... 

The MO calculations of the conformational equilibria of 2-chloro-oxane and 2-fluoro-oxane have been carried out. The ab initio STO-3G calculations for 2-chloro-oxane confirmed the stabilization of the a position of the Cl atom relative to the e one. An energy difference of 5.0 kJ.mol" is found if the C - Cl bond-length 177 pm is assumed for both conformers, and... 

The UV absorption spectrum of 2,6-diaminopyridine in ethanol/isooctane mixed solvent with variable temperature or ethanol concentration is consistent with the formation of the hydrogen-bonded complex of (/i)-6-amino-2( l//)-pyridinimine with two molecules of ethanol. The formation of the imino form may be restricted to the 2,6-diaminopyridine and its methyl derivatives, in which each amino group at the 2- and 6-positions has at least one hydrogen atom. Ab initio STO-3G calculations... 

A stable pseudo-chair conformation of D3 symmetry was also indicated for 1,5,9-cyclododecatriyne (26) by means of an ab initio STO-3G calculation, as well as by photoelectron spectral analysis (52). 

In order to propose a kinetic anomeric effect in nucleophilic substitution reactions at phosphorus, it is necessary (i) to extend the observations of a generalized anomeric effect in the TBP intermediate and the tetrahedral ground state to the transition state, and (ii) to show that the effect is amplified in the transition state. Consequently, Gorenstein et al. (1979) constructed models of the transition states for hydrolysis of dimethyl phosphate monoanion and subjected these to similar ab initio STO-3G calculations. The reactive intermediates along this hypothetical reaction coordinate are derived from the dimethoxy hydroxyphosphoranes [56] [58]. 

Within the last decade the development of ab initio computations has tended to overshadow the usefulness of semiempirical procedures. And also in this contribution results of ab initio STO-3G calculations are taken into consideration (especially in connection with spin-spin coupling constants). However, the advantage of semiempirical methods mainly lies in the possibility of a semiquantitative classification of electronic ground state and excited states properties within classes of molecules which also include larger chemically relevant compounds. 

On the other hand, it has turned out that both the one-bond carbon-proton coupling constants of monosubstituted allenes are correlated with the Mulliken overlap populations Qscsh obtained from ab initio STO-3G calculations (r = 0.9330). Correlation (129) includes first- and second-row substituents (54f ). 

Indeed, in the case of cyclopropane derivatives (a three-membered cycle) the calculations [64] using the extended Hiickel method point to the preferability of the reaction path providing for retention of configuration. In actual practice, the SN2-type reactions for the series of cyclopropane derivatives proceed at very low rates and no reliable confirmation of the retention of configuration at the carbon being attacked at the stage of kinetical control has so far been produced [60]. For the cyclobutane derivatives (a four-membered cycle), ab initio (STO-3G) calculations [65] of alternative reaction paths bear witness to the preferability of a rear-side approach with inversion of configuration. 

Fig. 5.6a-c. Pathway of formaldehyde hydration reaction CH2O + 2H2O, ab initio (STO-3G) calculated [102] a hydrate complex CH20 (H20)2 b transition state of the reaction (components of the transition vector corresponding to imaginary vibration frequency = i 547 cm" are shown) c hydrated methanediol CH2(OH)2 H20 is the reaction product. (Adapted from Ref. [102] with permission from the American Chemical Society)... 

Irrespective of how the cr-complex is formed from the adducts XLIV or XLIVb, it is not clear whether the anomalous selectivity phenomenon can be fully understood on the basis of the Olah hypothesis. Ab initio STO-3G calculations [155] on the structures LII and LIII, which model, respectively, the late and the early transition states of electrophilic substitution, i.e., of the (7- and 7c-complexes in the vicinity of those structures, have shown that both approximations lead to identical predictions in regard to positional selectivity. The structure LII is based on optimized geometry of the cyclohexadienyl cation, and in the structure LIII the hydroxonium cation has been selected as an electrophilic particle ... 

Table 5.6. Relative energies of type LII, LIII transition states according to ab initio STO-3G calculations [125] in kcal/mol...

The relative merits of the various methods have been discussed somewhat in the literature. In general, it can be stated that the ab initio type calculations will be more reliable but the semiempirical calculations are considerably faster in terms of computer time. Roughly speaking, there is an increase of a factor of about 100 in going from CNDO type to ab initio STO-3G calculations. A choice of methods is normally made on the basis of prior evidence that the computational method is adequate for the problem at hand and, whenever possible, critical evaluation of the calculated results against the best experimental data which are available. 

To illustrate the performance of the method relative to a full ab initio calculation, RS6000-370 CPU times (PRDDO/M, PRDDO/M/FCP, and Gaussian 92 STO-3G) for a series of DNA sequences are presented in Table 2. It can be seen from the table that FCP calculations take approximately half the time of an equivalent all-electron calculation, which in turn are 10-20 times faster than ab initio STO-3G calculations. To... 

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