FPT-INDO calculations

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.

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 ... 

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... 

Table 4. Results of Some FPT-INDO Calculations, Using the Solvaton Model, of the Effect of the Dielectric (e) of the Medium on Some Values of lj(N=C) in Hz ...

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 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... 

Finally, in using the INDO-FPT method to obtain a and b by a least squares procedure, as in ref. 20, it would seem most reasonable to omit two-bond coupling constants in the fit. It is known that, although the INDO calculations are generally successful in predicting the correct trends for 2/(A-B), the absolute values are generally in poor agreement with experiment. (121)... 

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... 

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. 

To confirm equation (4), we used the FPT (Finite Perturbation Theory) INDO (Intermediate Neglect of Differential Overlap) method (39) to calculate the Jqjj for various values of torsion angles. A comparison of the experimental and calculated values is plotted in Figure 5. 

Calculations of spin-spin coupling constants based on the FPT method (Eq. 25), which have been the most successful in correlating conformational and substituent trends, have been performed in the semiempirical INDO approximation77 of SCF MO theory. By this approach, Eq. 25 becomes... 

In 1971, Maciel et al. (40) reported /(C-C) values calculated using the finite perturbation technique (FPT) in conjunction with molecular orbital theory at the INDO level of approximation. The calculations were carried out on more than 75 molecules and only the Fermi contact contribution was evaluated. Molecules with strained rings were not considered. Reasonable agreement with experiment was realized and it was observed that computed /(C-C) values were approximately related to Pj(C)aS(C)b- In addition, it was found that computed /(C-C) and... 

Doddrell et al. (80, 66) have examined the angular dependence of V(C-C) in a number of aliphatic and alicyclic alcohols. In contrast to the carboxylic acids, 37(C-C) is observed to be a maximum, 5-4 Hz, for a dihedral angle near 0°. For the trans arrangement, — 180°, 3/(C-C) is 3-2 Hz. At dihedral angles of 90 and 270°, 3/(C-C) is less than 0-4 Hz. Observed 3/(C-C) values for the alcohols and carboxylic acids (66, 80, 52) have been compared with values calculated (INDO-FPT) for the model compounds butane, 2-butanol, and butanoic acid. The calculated coupling constants are given in Table V. (80) Also, experimental and calculated 3J(C-C) values for the carboxylic acids are shown in Fig. 2. The calculations, which assumed only the Fermi contact... 

MO calculations on a small number of molecules for which experimental data were available at that time. Only the Fermi contact contribution was considered and although agreement with experiment is rather poor, it is interesting that no correlation between calculated /(N-C) values and Pj(N)jiC) was found. This result is in contrast to the calculated results obtained for carbon-carbon coupling constants. More recent INDO-FPT calculations on a series of amine derivatives indicate that the Fermi contact contribution to /(N-C) is related to Pj(N)s(C) in this class of structurally similar compounds. (96) The results of these calculations are illustrated in Fig. 3. (... 

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