CNDO/2—»-molecular orbital calculation

Figure 3.7 Model substrates with a zigzag face on graphite for CNDO molecular orbital calculation. (Adapted from ref. 10.)...

CNDO molecular orbital calculations [11] were performed to calculate the 7t charge distribution in substituted anthraquinones. Geometry optimization through energy minimization was prohibited by the size of the anthraquinones, and thus standard molecular bond distances and angles [12] were employed. CNDO calculations were also performed on tautomeric structures of hydroxy-substituted anthraquinones and resonance structures of amino-substituted anthraquinones. 

Semiempirical molecular orbital calculations have gone through many stages of refinement and elaboration since Pople s 1965 papers on CNDO. Programs like PM3, which is widely used in contemporary research, are the cumulative achievement of numerous authors including Michael Dewar (1977), Walter Thiel (1998), James Stewart (1990), and their coworkers. 

HyperChem currently supports one first-principle method ab initio theory), one independent-electron method (extended Hiickel theory), and eight semi-empirical SCFmethods (CNDO, INDO, MINDO/3, MNDO, AMI, PM3, ZINDO/1, and ZINDO/S). This section gives sufficient details on each method to serve as an introduction to approximate molecular orbital calculations. For further details, the original papers on each method should be consulted, as well as other research literature. References appear in the following sections. 

Geometry-optimized CNDO/2 molecular orbital calculations are carried out on poly(5,5 -bibenzoxazole-2,2 -diyl-1,4-phenylene)- and poly(2,5-benzoxazole)-model compounds to determine conformational energies as a function of rotation about each type of rotationable bond within the repeat units. 

The latter three, (4)—(6), are utilized for qualitative elucidation of the observed ICD bands in signs, and the former three, (1)—(3), are successfully applicable for quantitative calculations of the observed ICD bands in both signs and magnitudes. The ICD of P-cyclodextrin complexes with benzene derivatives or azanaphthalenes162) has been analyzed by a molecular orbital calculation, using an approximation of PPP-type, which has been compared with the theoretical spectra calculated by using the CNDO/S-CI method on the basis of the MCD spectra 16S). 

The experimental observations on the mechanism(s) of electroreduction of 2-thio-pyrimidines have been interpreted on the basis of their electronic structures as calculated with the aid of the CNDO/2 and Huckel procedures l42). The energies of LUMO (lowest unoccupied molecular orbitals), calculated for pyrimidine and its 2-oxo-and 2-thioderivatives, were compared with the reduction half-wave potentials (Table V). These show that the presence of a carbonyl or thione substituent at C2 enhances the electron acceptor properties of the molecule, which are correlated with formation of a dimer susceptible to photooxidation. 

Nonetheless, some useful information regarding methylation has been obtained from molecular orbital calculations. One CNDO/2 study on the allyl and pentadienyl anions indicated that in all cases the methyl group would serve to withdraw net electron density from the anions196. For the allyl anion, a 1-methyl substituent would withdraw a substantial 0.102 electrons, while at the formally uncharged 2 position the effect was consider-... 

The above values were selected, in the case of polyacetylene, from averages taken from available experimental and theoretical structural data. For the halogenated chains, appropriate values were selected from the results of both ab initio and CNDO/2 molecular orbital calculations and from structural data available on small-molecule analogues. Due to the unavailability of structural data for the silylated analogues, values were taken from those used for polyacetylene (except, of course, for those bonds and angles associated with an X atom) so as to deduce the effect of the substitution alone without inclusion of concomitant structural modifications along the polymer backbone resulting from such substitutions. 

Lasaga, A. (1982). Optimization of CNDO for molecular orbital calculations on silicates. Phys. Chem. Mineral. 8, 36-46. 

Properties such as photoconduc.tivityl l t l and photoluminescence of silicon polymers have been reported because of their wider optical band gap compared with crystalline silicon. Theoretical investigations of silicon polymers have been also reported ll2l-[21] Xakeda, Matsumoto and Fukuchi calculated the electronic structure of polysilane chains using the semi-empirical approach called the Complete Neglect of Differential Overlaps (CNDO) Molecular-Orbital (MO) method They discussed the dependence of the size and... 

Local density functional (LDF) quantum mechanical calculations for materials science. deMon for density functional calculations. Turbomole for Hartree-Fock and MP2 ab initio calculations. ZINDO for extended Fliickel, PPP, CNDO, and INDO semiempirical molecular orbital calculations and prediction of electronic spectra. Plane Wave for band structures of semiconductors. ESOCS for electronic structure of solids. Silicon Graphics and IBM workstation versions. 

Molecular orbital calculations show that electronic distributions at the positions a and p to nitrogen are similar to those at the corresponding positions of quinoxalines. Molecular orbital parameters obtained using CNDO functions a + n calculated electronic densities) are shown, and bond orders are given in Table 1. ... 

A theoretical treatment of 1,2-oxathietane indicates planarity with aS-0 bond length of 1.669 A and a C-S-0 angle of 100.6°. The electronic spectrum was calculated. The character of the HOMO is largely that of the sulfur 3p orbital. A CNDO molecular orbital study of the retrocycloaddition of 1,2-oxathietane 2-oxide to sulfur dioxide and ethylene shows that strong heteroatom asymmetry lifts the stereoelectronic requirement that the thermal fragmentation occur by a suprafacial-antarafacial path. ... 

Molecular orbital calculations (CNDO/2) on cyclic SN3 species, SN3, SN3, indicate the cation is more stable than the anion and that c -orbitals play an insignificant role. Several tetrathiatetraphosphatricyclooctanes contain four-membered rings with three phosphorus atoms and one sulfur atom. " ... 

FIGiJflE 3.22 Drawing of the top view and transition-moment vectors at 365 and 532 nm of tiie closed form of DE obtained by CNDO/S AMI MORAC molecular orbital calculations.The 532 nm is parallel to the long axis of the molecule. After reference 42, redrawn by permission of ACS. 

A long-standing difficulty of most semiempirical molecular orbital calculations (HMO, EHT, PPP, CNDO, INDO, MINDO, etc.) has been the... 

In the cyclopropenyl radical with three Ti-electrons, the third electron has to be accommodated in one of the two degenerate molecular orbitals causing the Jahn-Teller distortion, which results in a change in symmetry from Dj, to C2,.. The ab initio (STO-3G 4-31G), as well as semi-empirical (CNDO/2), molecular orbital calculations demonstrate that the odd electron is incorporated in the pyramidalized rr-type orbital giving rise to loss of resonance stabilization in the three-membered ring. Thus, the cyclopropenyl radical is considered to be a highly unstable, short-lived species. 

True Hiickel molecular orbital calculations, among other limitations, only address TT-electrons. All atom, semiempirical calculations that include both n- and a-electrons were developed originally by Pople and coworkers [17] implementing the method of complete neglect of differential overlap (CNDO), followed by the increasingly rigorous method of intermediate neglect of differential overlap (INDO) [18]. 

---