Semiempirical CNDO calculations

Normally, you would expects all 2p orbitals in a given first row atom to be identical, regardless of their occupancy. This is only true when you perform calculations using Extended Hiickel. The orbitals derived from SCE calculations depend sensitively on their occupation. Eor example, the 2px, 2py, and 2pz orbitals are not degenerate for a CNDO calculation of atomic oxygen. This is especially important when you look at d orbital splittings in transition metals. To see a clear delineation between t2u and eg levels you must use EHT, rather than other semiempirical methods. 

A method which is similar to the Pariser-Parr-Pople method for the n electron system and is applicable to common, saturated molecules has been proposed by Pople 28>. This method is called the CNDO complete neglect of differential overlap) SCF calculation. Katagiri and Sandorfy 29> and Imamura et al. °) have used hybridized orbitals as basis of the Pariser-Parr-Pople type semiempirical SCF calculation. 

Semiempirical (CNDO, MNDO, ZINDO, AMI, PM3, PM3(tm) and others) methods based on the Hartree-Fock self-consistent field (HF-SCF) model, which treats valence electrons only and contains approximations to simplify (and shorten the time of) calculations. Semiempirical methods are parameterized to fit experimental results, and the PM3(tm) method treats transition metals. Treats systems of up to 200 atoms. 

Table 24. Semiempirical (CNDO/II) calculations of anion-solvent interaction...

For 3-methyl-anhydro-5-hydroxy-l,2,3,4-oxatriazolium hydroxide (4 R = Me) electron populations, charge densities, and orbital energies, and dipole moment have been calculated by using different semiempirical CNDO/2 methods and ab initio calculations (PEMCSCF) at different levels <891JQ465>. The TT-electrons are found to be highly delocalized and the compound is highly polar. 

Limited knowledge exists with respect to detailed molecular structures of simple pyrans and heteropyrans. The semiempirical CNDO/2 calculations, using a full geometry optimization procedure, predict 4 and 5 to exhibit small deviations of the heterocyclic rings from the planar conformations.34 Unfortunately, similar calculated data for thiopyrans 6, 7, and 9 were not presented.45 Accounts of several X-ray diffraction investigations on compounds possessing pyran or thiopyran fragments are available.46 51... 

Theoretical calculations using the semiempirical CNDO/2 method including d- orbital participation in (Id X = O, NH, S Y = S) predicted singlet ground states for these systems as well as substantial n- bond character between the S—C bonds arising from dir-pTr overlap, and very little double bond character in the central C—C bond (74JA1817). There is also considerable contribution to the electronic structure from dipolar canonical forms in (Id X = O, NH Y = S) compared to thieno[3,4-c]thiophene (9) as evidenced by the respective calculated dipole moments of 0.15 and 3.21 D for the former and 0.00 D for the latter. 

Finally, semiempirical complete neglect of differential overlap (CNDO) calculations [145] again emphasize N2 binding to the trigonal cavity, but now with one nitrogen atom at the center of the cavity and the other on the outside of the cluster as shown in Figure 25. 

Good agreement between the CNDO/S semiempirical HAB calculation and the experimental k j for the Ru/Ru-DNA duplex is found. Of course, this comparison requires use of Eq. (4) and a specified value of (0.9 eV) in addition to the measured driving force of 0.7 eV. Combining these data yields a calculated kB1 = 7.1 x 106 s 1 compared to the experimental k j = 1.6 x 106 s 1. Extensive use of the same ruthenium complexes as D/A groups in protein studies means that there is not much uncertainty in X (ca. 0.2 eV). 

Table 4.5. Compositions of molecular orbitals of SiF4 from semiempirical molecular orbital (CNDO) calculations and MS-SCF-Ya calculations, together with comparison of calculated x-ray photoelectron transition intensities with experimental values...

We have used semiempirical CNDO/S calculations for estimating the magnitude and nature of shifts accompanying the transition from a molecule to its vdW dimer. (Nj), (HF). HCl... HF, (H O) and (CO ) were studied. Qualitatively speaking, the results agree with those obtained by the ... 

FSSF is isomeric with SSF (3). Qualitative evidence has been presented (1 ) which indicates that SSFg is the more stable isomer. From our estimated free energy data, we predict a Gibbs energy change for the isomerization reaction FSSF(g) = SSF (g) of -15.4 kcal mol at 298.15 K, indicating that SSF is substantially more stable than FSSF. This conflicts with relative stability predictions based on results from two Independent MO studies (5, 6). These semiempirical (CNDO/2) calculations indicate that the instability of FSSF relative to SSF amounts to only about 2.0 kcal mol". If we assume that these MO calculations are correct, we calculate that Dq (FS-SF) > Dq (S=SF2) by roughly 7.0 kcal mol". This seems very unlikely in view of what is known (1 ) about the... 

Most of the early theoretical studies of Lewis acid catbonyl interactions focused on alkali metal cations such as Li and Na, partly due to the significance of such interactions in biological systems and partly due to computational limitations. In 1973 some of the first calculations on the formaldehyde/Li system were performed. Using both ab initio (SCF-LCAO-MO) and semiempirical (CNDO/2) methods the researchers found that in the ground state the complex possesses Civ symmetry in which Li lies on the axis of the carbonyl C—O bond with an O— Li distance of 1.77 A. 

The lowest energy absorption band is forbidden when the cubane has high symmetry but can be weakly allowed when it has low symmetry. Semiempirical CNDO/S calculation shows that the spectrum shape in the low energy region depends strongly on substituents (39). Among tert-... 

A good illustration of this type of discrepancy is presented in Tables I and II. In the case of 2-oxopyridine, semiempirical CNDO/2 and MNDO methods and ab initio HF/STO-3G calculations predict the lactim tautomer of the molecule to be more stable than the lactam one, while the MINDO/3 and HF/3-21G calculations give a reverse result. In view of the large uncertainty in the experimental value -1.3 10.5 kJ mole-1 (probably overestimated), it is not possible to draw any other conclusion than that the experimental values and the theoretical values are spanning the... 

Semiempirical INDO calculations of the species 36—38 indicate the relative energies 36 >37 >38 the allene 38 can be destabilized by benzannelation in positions which decrease the number of Kekul6 structures However, the calculated energy differences are small, and the reverse result is obtained with the CNDO/2, Extended Hiickel, and MINDO/2 methods 56,57,68) ... 

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