FPT INDO

Comparison of the calculated and experimental values shows that the FPT-INDO method reproduces in a satisfactory way the coupling constants measured on rigid model compounds (2). The coefficients A,... 

Figure 5. Relationship between Jqu the C-O-C-H dihedral angle represented by equation (4) (full line). Squares represent FPT-INDO calculations, stars represent experimental values from Ref. 5.

Figure 7. A plot of the calculated FPT-INDO results for the one-bond coupling constant Jpr, in 2-methoxytetrahydropyran as a function of torsion angle around the aglycon C-0 bond with the methoxy group in axial (curve labeled with squares) and equatorial (stars) positions.

Fig.2 The calculated 13C chemical shift map of the Cp carbon in N-acetyl-N -methyl-L-alanine amide obtaining using the FPT INDO method. The chemical shifts were calculated at 15° intervals for the dihedral angles(< >,i /).

So far, we have investigated higher-order structure of polypeptides by solid-state high-resolution NMR not only using experimental but also theoretical methods[2-4]. The chem cal shifts can be characterized by variations in the electronic states of the local conformation as defined by the dihedral angles(4>,W). Ando et al. have calculated contour map for the Cp carbons of an alanine dipeptide by using the FPT INDO method within the semi-empirical MO framework. The calculated map reasonably predicts the experimental version. This shows that the chemical shift behavior of the L-alanine residue Cp-carbonyl carbons in the... 

Intermolecular solvent-solute interactions influence the charge distribution on a carbohydrate molecule. Subtle electronic changes that occur as a result of these interactions are responsible for the solvent dependence of carbon -proton coupling constants. The general aspects of solvent effects on NMR parameters have been reviewed,78-79 and consequently, only a very brief outline of the theoretical model within FPT INDO SCF MO formalism is considered here. 

The non-bonded, through-space transmission of spin information, known as the Barfield transmission effect, has been investigated theoretically for five-membered rings by the FPT-INDO-SCF-MO method.50 An early observation of this effect was the non-equivalence of /endo-endo and exo-exo in norbomanes,51,52 which was also... 

The pendant hydroxymethyl group in sugars has been a popular vehicle for experimental and theoretical studies of coupling constants, and the application of FPT-INDO calculations to both anomers of the eight aldohexopyranoses yielded an equation for the dependence of 3/c-4,h-6 ... 

Fig. 19. (a) Observed 15N chemical shift diagram of poly(L-alanine) in the solid state, (b) Calculated 15N shielding diagram of A-acetyl-r.-alanine methylamide (taking hydrogen bonds with two formamide molecules), as a dipeptide model of poly(L-alanine), by means of the FPT-INDO method. 

NMR CHEMICAL SHIFT AND ELECTRONIC STRUCTURE Table 1.1. Calculated C chemical shieldings of hydrocarbons by FPT INDO method... 

Most recently, ab initio calculations for the NMR chemical shifts have become available for medium-size molecules because of the remarkable power capabilities of modernworkstations, personal computers and supercomputers [32]. This leads to a quantitative discussion on the chemical shift behaviors. For example, the ab initio MO calculation with the 4-31G basis set using the GIAO-CHF (gauge independent atomic orbital-coupled Har-tree-Fock) method on A-acetyl-A -methyl-L-alanineamide which is the same model molecule as the case of the above FPT INDO calculation will be... 

The isotropic NMR chemical shifts calculated by the finite perturbation theory (FPT)-INDO method for the aromatic forms are listed in Table 16.2, and those for the quinoid forms are listed in Table 16.3. 

Table 16.2. Calculated N shielding constants for the aromatic models by the FPT-INDO...

Table 16.3. Calculated shielding constants for the quinoid models by the FPT-INDO method...

To obtain a deeper understanding between the experimental finding that the isotropic chemical shift and the principal value of 633 depend upon the hydrogen-bond length, theoretical calculations of chemical shifts have been carried out by the FPT-INDO method. A decrease of Rn o leads to a decrease of the calculated isotropic shielding in agreement with the experimental results. Therefore, such a relationship suggests that the isotropic chemical shift value can be used in the estimation of Rn - o- This is similar to the case of the carbonyl chemical shift described above. 

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