6-31G basis set

Schematic representation of some of the lower frequencies in the ion-dipole complex for the Cl + MeCl m and the imaginary frequency of the transition structure, calculated using a 6-31G basis set. 

Even more recent calculations using STO-3G and 6-31G basis sets could not safely predict diazomethane as the more stable compound in comparison with diazirine, although there is an experimental energy difference of 125 kJ moP 79JST(52)275). 

So far, the only polarized basis set we ve used is 6-31G(d). Its name indicates that it is the 6-31G basis set with d functions added to heavy atoms. This basis set is becoming very common for calculations involving up to medium-sized systems. This basis set is also known as 6-31G. Another popular polarized basis set is 6-31G(d,p), also known as 6-31G, which adds p functions to hydrogen atoms in addition to the d functions on heavy atoms. 

The values in red are within O.OlA of the experimental value. Using the 6-31G basis set, including diffuse functions on the hydrogen atom, improves the result over that obtained with diffuse functions only on the fluorine atom, although the best result with this basis set is obtained with no diffuse functions at all. 

Determine the effect of basis set on the predicted chemical shifts for benzene. Compute the NMR properties for both compounds at the B3LYP/6-31G(d) geometries we computed previously. Use the HF method for your NMR calculations, with whatever form(s) of the 6-31G basis set you deem appropriate. Compare your results to those of the HF/6-311+G(2d,p) job we ran in the earlier exercise. How does the basis set effect the accuracy of the computed chemical shift for benzene ... 

Note that functions 1 through 9 form the heart of the 6-31G basis set three sets of functions formed from six, three and one primitive gaussian. 

Yamamoto et al. have developed a catalytic enantioselective carbo-Diels-Alder reaction of acetylenic aldehydes 7 with dienes catalyzed by chiral boron complexes (Fig. 8.10) [23]. This carbo-Diels-Alder reaction proceeds with up to 95% ee and high yield of 8 using the BLA catalyst. The reaction was also investigated from a theoretical point of view using ab-initio calculations at a RHF/6-31G basis set. 

These calculations have been conducted on the basis of RHF optimized geometries, considering the 6-31G basis set for the n-alkane compounds (11), and the 6-31G basis set for the polyacene series (12). In both cases, the basis set contention has been checked by comparison with more thorough investigations on small compounds, such as ADC[3] calculations (11a) on n-butane based on the 6-31IG, 6-31G and 6-31G basis, or the MRSDCI ionization spectrum of ethylene as obtained by Murray and Davidson (33) using a 196-CGTO basis set. 

The optimization of the geometry leads to a good agreement with experiment for AIH (Rcxp = 1.648 A [25] ). ForMgH a previously calculated internuclear distance [25) using a 6-31G basis set, is equal to 1.863 A which is not very close to our value for this molecule. Note, however, that in the particular case of MgH, the energy presents a flat minimum between 1.79 A and 1.82 A, its variation beeing of the order of lO a.u in this interval. 

The mechanistic interpretation of the pH-rate profiles for quinone methide disappearance relied on Hartree-Fock calculations with 6-31G basis sets as well as on product studies. In Fig. 7.20, we show the potential-density maps of the protonated and neutral pyridoindole quinone methide with negative charge density colored red and positive charge density colored blue. Inspection of the methide... 

F(r) was also computed from ab initio wave functions in the framework of the HF/SCF method using 3-21G and 6-31G basis sets due to the large size ofLR-B/081, the calculation has as yet been performed on isolated molecular fragments, adopting a geometry based on molecular dimensions from X-ray diffraction studies. 

The first study, by Ismail et al. [153], used the CASSCF method with a 6-31G basis set and an active space of 14 electrons in 10 orbitals to locate conical intersections and pathways connecting them to the Franck Condon region. Two such conical intersections were identified in that work, the ci2 and ci3, as defined above. In that work the barrier leading to ci2 was calculated to be 10 kcal/mol, too high to make this conical intersection relevant. But the barrier leading to ci3 was found to be much smaller, 3.6 kcal/mol, and it was concluded that ci3 is involved in the dominant decay path. Reaching this intersection requires first a conical intersection between the nn state, which is vertically the Si state, and the non state, which is vertically the S2 state. Merchan and Serrano-Andres followed up this study [140] using a method... 

For most molecules studied, modest Hartree-Fock calculations yield remarkably accurate barriers that allow confident prediction of the lowest energy conformer in the S0 and D0 states. The simplest level of theory that predicts barriers in good agreement with experiment is HF/6-31G for the closed-shell S0 state (Hartree-Fock theory) and UHF/6-31G for the open-shell D0 state (unrestricted Hartree-Fock theory). The 6-31G basis set has double-zeta quality, with split valence plus d-type polarization on heavy atoms. This is quite modest by current standards. Nevertheless, such calculations reproduce experimental barrier heights within 10%. 

In order to gather more information about this problem, it was deemed worthwhile to follow the energetics of the alkylation reaction of water by methyl-, ethyl-, and fluo-roethyldiazonium ions. The main goal of these calculations was to establish whether transition-state calculations can provide information about hard versus soft electrophilic character of these species.12 Computations at Hartree-Fock and MP2 level were performed using the 6-31G basis set. It was found that both at the Hartree-Fock level and when correlation energy affects were included, the ethyl and fluoroethyl species do not show the presence of a transition state, while the methyl species show a small transition state. It was concluded that transition state computations cannot shed light on the characters of these species. 

Another study16 investigates the effect of benzene ring fusion on the reactivity of 1,2-Oxathietane. Ab initio calculations were performed using the 3-21G and the 6-31G basis sets, at Hartree-Fock and MP2 calculational levels. It was found that the allowed (8s + 2s) cycloreversion is unfavorable energetically. A subsequent experimental and theoretical study17 favors biradical intermediates in the valence tautomerism of benzoxathiete to monothio-o-benzoquinone. 

The basis set used for calculations were the STO-3G, the 3-21G, and the 6-31G basis sets, as implemented by the Gaussian-88 computer program. The energies of interaction were computed by using the supermolecule approach where the sum of the energies of the subsystems are substracted from the energy of the complex. All the species were geometry optimized. 

The molecules were subjected to complete geometry optimization via ab initio calculations, using the 6-31G basis set. The energies of the molecules, as well as the net atomic charges, were calculated. 

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