Ab-initio molecular orbital

A Gaussian expansion contains two parameters the coefficient and the exponent. The most flexible way to use Gaussian functions in ab initio molecular orbital calculations permits both of these parameters to vary during the calculation. Such a calculation is said to use... 

Fig. 2.7 The addition of a 3d orbital to 2p gives a distorted orbital. (Figure adapted from Hehre WJ, L Radom, p i)R Sdileycr and ] A Hehre 1986. Ab initio Molecular Orbital Theory. New York, Wiley.)...

I nple J A and D L Beveridge, 1970. Approximate Molecular Orbital Theory. New York, McGraw-Hill. Riduirds W G and D L Cooper 1983. Ab initio Molecular Orbital Calculations for Qieniists. 2nd Edition. Oxford, Clarendon Press. 

The cornerstone of semiempirical and ab initio molecular orbital methods is the Harhee equation and its extensions and variants, the Harhee-Fock and Roothaan-Hall equations. We have seen that the Hamiltonian for the hydrogen atom. 

Hehre, W.J. Radom, L. Schleyer, P.v.R. Pople, J.A.Ab Initio Molecular Orbital Theory, John Wiley and Sons, New York, 1986... 

A number of derivatives of antimonin are also known (141,142). The potential aromaticity of this ring system has aroused considerable interest and has been investigated with the aid of spectroscopy as well as ab initio molecular orbital calculations (143). There seems to be no doubt that antimonin does possess considerable aromatic character. 

Oxirane (1) and methyloxirane (3) are miscible with water, ethyloxirane is very soluble in water, while compounds such as cyclopentene oxide and higher oxiranes are essentially insoluble (B-73MI50501) (for a discussion of the solubilities of heterocycles, see (63PMH(l)l77)). Other physical properties of heterocycles, such as dipole moments and electrochemical properties, are discussed in various chapters of pmh. The optical activity of chiral oxiranes has been investigated by ab initio molecular orbital methods (8UA1023). 

W J. Hehre, L. Radom, P. v. R. Schleyer, and J. Pople, Ab Initio Molecular Orbital Theory, Wiley-lnterscience, New Y k, 1986. 

Unlike the stable molecule N2O, the sulfur analogue N2S decomposes above 160 K. In the vapour phase N2S has been detected by high-resolution mass spectrometry. The IR spectrum is dominated by a very strong band at 2040 cm [v(NN)]. The first ionization potential has been determined by photoelectron spectroscopy to be 10.6 eV. " These data indicate that N2S resembles diazomethane, CH2N2, rather than N2O. It decomposes to give N2 and diatomic sulfur, S2, and, hence, elemental sulfur, rather than monoatomic sulfur. Ab initio molecular orbital calculations of bond lengths and bond energies for linear N2S indicate that the resonance structure N =N -S is dominant. 

The [S3N3]" cation is of interest as an example of an antiaromatic eight TT-electron system (Section 4.4). Ab initio molecular orbital calculations indicate that a triplet cation, with a planar ring, is more stable than the singlet cation. The [S3N3]" cation has been obtained as the norbomene adduct 5.16, but salts of the free cation have not been isolated. ... 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

The tetramethylammonium salt [Me4N][NSO] is obtained by cation exchange between M[NSO] (M = Rb, Cs) and tetramethylammonium chloride in liquid ammonia. An X-ray structural determination reveals approximately equal bond lengths of 1.43 and 1.44 A for the S-N and S-O bonds, respectively, and a bond angle characteristic bands in the IR spectrum at ca. 1270-1280, 985-1000 and 505-530 cm , corresponding to o(S-N), o(S-O) and (5(NSO), respectively. Ab initio molecular orbital calculations, including a correlation energy correction, indicate that the [NSO] anion is more stable than the isomer [SNO] by at least 9.1 kcal mol . ... 

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