STO-3G calculations

The input file for an STO-3G calculation of the bond distances, energies, and other molecular properties of the isolated water molecule in the gaseous state at 0 kelvins is... 

The diagrams below give STO-3G calculated charge densities for the betu l cation and its a-formyl and a-cyano derivatives. Analyze die effect of these substituents on... 

STO-3G calculations find the corresponding transition state to be more stable than other possible conformations by several kilocalories per raole. The origin of the preference for this transition-state conformation is believed to be a stabilization of the C=0 LUMO by the a orbital of the perpendicularly oriented substituent. 

MO (STO-3G) calculations on the gas-phase hydration reaction of formaldehyde suggest a concerted process involving two water molecules as a low-energy mechanism for hydration. 

Fig. 10.4. Total 7i-electron density for some substituted benzenes. [From STO-3G calculations as reported by W. J. Hehre, L. Radom, and J. A. Pople, J. Am. Chem. Soc. 94 1496 (1972).]...

Consider then a HF/STO-3G calculation on aspirin (acetylsalicylic acid) ... 

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). 

Ab initio (3-21G( )//STO-3G) calculations by Chandrasekhar and Schleyer163 on 1,4-disilabenzene 58, its Dewar benzene isomer 59, and a silylene isomer 60 showed that all three species exhibited approximately similar stabilities, the silylene 60 being 9.9 kcal mol-1 more stable than the planar aromatic form 58, which was 5.9 kcal mol-1 more stable than the Dewar benzene form 59. 

The bond angles are also satisfactorily reproduced in most of the cases. This is also true for dihedral angles which are sometimes more accurately predicted than by the use of the larger, double-zeta sets. For instance, experimental evidence favors the gauche structure for 1,2 difluoro ethane. STO-3G calculations lead to a gauche structure, while 4-31G calculations predict a trans structure. 

FIGURE 22.2 The changes of hond order and free valence indices as well as the energy during the dissociation of the H2 molecule obtained in an UHF/STO-3G calculation. 

According to STO-3G calculations, the structural stability of the planar 67r-electron four-membered ring is achieved in thiacyclobutadiene (77T3061), which has a cyclic ylide-type bonding. 

The MP2/4-31G/STO-3G calculations show (83TL1863) the tropylium cation to be 112.7 kcal/mol more stable than the hypothetical bicyclo [4.1.0]hepta-2,4-dien-7-yl cation. This difference, attributed to the effects of aromatic stabilization, should be compared with the value of AE (172) - (124) = 44.8 kcal/mol (at the same computational level). Therefore, the energy of aromatic stabilization of borepin comes to about 40% of that of the tropylium cation. The 6-31G /6-31G calculated AE (172) -(124) = 37.6 kcal/mol, whereas nonaromatic cycloheptatriene is a mere... 

Errors are defined relative to the experimental value. Time is defined relative to the STO 3G calculation. 

Figure 5.1. Verification of Eqs. (5.1) and (5.2) by means of Mulliken charges deduced from STO-3G calculations involving complete optimizations of geometry and orbital exponents. The net charge q is of (5.1) for the lower line and qa of Eq. (5.2) for R—H [44].

At a quite different level of approximation, this class of compounds was investigated by means of STO-3G calculations involving a detailed optimization of all the geometric and exponent parameters [42]. The Mulhken net atomic charges and the... 

This section is about diaUcylethers, aldehydes, ketones and alcohols. Oxygen net charges deduced from standard STO-3G calculations are indicated in Table 6.7 (see also Fig. 6.9), along with their NMR shifts (ppm from water) for dialkylethers [140] and carbonyl compounds [140], as well as selected ionization potentials [147]. 

Bond lengths from STO-3G calculations show considerable scatter over the full range of distances. The 3-2IG model leads to much improved results, and the 6-3IG model to small additional improvements. Note that both 3-2IG and 6-3 IG calculations lead to bond distances which are generally shorter than experimental lengths where only first-row elements are involved (distances < 1.6A), and to distances which are generally longer than experimental lengths where second-row elements are involved (distances > 1.6A). This is the same... 

Except for systems where the difference in energy between the conformers is very small, even the STO-3G Hartree-Fock model properly assigns ground-state conformation. However, conformational energy differences from STO-3G calculations show large errors in some cases. Results from 3-2IG calculations are generally even worse, and the simplest Hartree-Fock model to provide a reliable (and for the most part quantitative) account of conformational energy differences is the 6-3IG model. Except for formic acid and methyl... 

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. 

Choose a small molecule and carry out a HF/STO-3G calculation, including a... 

---