FPT INDO method

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,... 

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... 

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 16.3. 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. 

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... 

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... 

A double perturbation expansion of the FC coupling term up to second order in both the magnetic and Coulomb residual interactions was carried out within the CS-INDO method in polymethyne chains. It should be noted, however, that such an expansion usually converges only slowly and, in order for the SOS approach to be equivalent to the FPT and CHF ones, some of the terms in it must be summed to infinite order with respect to the Coulomb interaction. The authors conclude that their calculations do not succeed in generating unique effective parameters 5 (0) and fo " example,... 

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... 

Blizzard and Santry (16, 17) also examined C-C coupling constants using the INDO-FPT method. They included the orbital and spin dipolar terms as well as the contact term. Of the 15 molecules considered none were cyclic hydrocarbons. The contact term was found to be the most important, followed by the orbital and spin dipolar terms. The authors reported that the contact term completely dominates in the case of C-C single bonds, while the other terms become important when the two carbons are joined by multiple bonds. 

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)... 

With one exception (see below) V(C,C) spin coupling constants in alkanes are all positive. According to a theoretical treatment by the INDO-FPT method the Fermi contact mechanism for spin coupling dominates, whereas orbital and dipolar... 

INDO FPT calculations have been performed by Pachter and WesseP to study the dependence of Vhc and Vhc couplings on a nitrogen lone-pair and carbonyl group, and the obtained relationships have been used to study the spatial relationships in a diazepam molecule. Juc couplings across a sulfur atom in two series of compounds, both including a sulfide, a sulfoxide and a sulfone have been measured and calculated by Kleinpeter and co-workers by the use of the DPT method. 

---