Bond length Computed

Fig. 17.4 Differences between experimental and computational S-N bond lengths (Computational methods are listed in Table 17.1)...

R. Shepard, G. S. Kedziora, H. Lischka, I. Shavitt, T. Miiller, P. G. Szalay, M. Kallay, and M. Seth, The Accuracy of Molecular Bond Lengths Computed by Multireference Electronic Stmcture Methods, Chem. Phys. 349, 37-57 (2008). 

At both SCF and Cl levels the asymmetric structure (A) is found to lie higher than the symmetric structure (S) the barrier is calculated to be 10-11 kcalmoP at the SCF level and is lowered slightly with the Cl treatment 9.8-10 kcal mol ). This low barrier may be rationalized in terms of aromatic stabilization of the n-electron transition state. This suggestion is also confirmed by the values of the C-C (1.401 A) and C-O (1.288 A) bond lengths computed at the STO-3G level, which are in accord with the description of this as an aromatic transition state. 

Ca-Ca bond lengths computed though DFT can be used as an index to predict the hydraulic activity of P-C2S and other cement clinker compounds. 

For each pair of interacting atoms (/r is their reduced mass), three parameters are needed D, (depth of the potential energy minimum, k (force constant of the par-tictilar bond), and l(, (reference bond length). The Morse ftinction will correctly allow the bond to dissociate, but has the disadvantage that it is computationally very expensive. Moreover, force fields arc normally not parameterized to handle bond dissociation. To circumvent these disadvantages, the Morse function is replaced by a simple harmonic potential, which describes bond stretching by Hooke s law (Eq. (20)). 

The amount of computation necessary to try many conformers can be greatly reduced if a portion of the structure is known. One way to determine a portion of the structure experimentally is to obtain some of the internuclear distances from two-dimensional NMR experiments, as predicted by the nuclear Over-hauser effect (NOE). Once a set of distances are determined, they can be used as constraints within a conformation search. This has been particularly effective for predicting protein structure since it is very difficult to obtain crystallographic structures of proteins. It is also possible to define distance constraints based on the average bond lengths and angles, if we assume these are fairly rigid while all conformations are accessible. 

It was found that computed bond lengths were much too short in CNDO/1 because of poor balance between the attractive V b... 

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