Corrections, bond length

Torsional parameters and VdW parameters for internal hydrogen bonds in the N—C—N moiety were obtained by fitting the ab initio rotational profiles of methylenediamine (MDA, 15) and /V-methylmelliylenediamine (NMMDA, 16). A comparison of relative conformational energies between ab initio and MM2 results for 15 and 16 is provided in Table 6. Bond length correction terms for inner and outer C—N bonds (K, K2 and... 

G. J. Arai, Reel. Trav. Chim. Pays-Bas, 1962, 81, 307. Bond lengths corrected in ref. 7. 

The librational motion can cause one atom to move in an arc relative to another. This is particularly so for hydrogen atoms and affects both the X-H and H- A bond lengths. Corrections are made by assuming that the motion is harmonic and that the molecules librate as a rigid body about their center of mass... 

Craven BM, Swaminathan S (1984, publ. 1985) Neutron diffraction carbon-hydrogen bond lengths corrected for harmonic and anharmonic thermal vibration. Thins Am Cryst Assoc 20 133-135... 

The first index was later called radius-corrected connectivity index and the second one bond-length-corrected connectivity index [Sun, Huang et al, 1996]. 

It is rare indeed that all conditions of gathering intensity data are optimized in all of the ways indicated above. Where they are not, possibilities of systematic error in the results may creep in. Failure adequately to determine the parameters for absorption correction throws into serious question the value of anisotropic temperature factors for analyzing the thermal motion of molecules and of atoms in molecules so as to make bond-length corrections. The same effect can result from loss of intensity due to crystal decomposition, while data collection is following a digital sequence along a particular direction, if not compensated with the aid of repeated measurements on standard reflections. 

In the left column of Figure 7.2 are shown partial structures. The MM4 numbers are given, followed by the MP2/BC numbers in parentheses. Bond lengths are all given in terms of r. The standard bond length corrections were used to obtain the values from the MP2/BC values (C-H, +0.0056 A, C-Br, -0.0124, C-C, +0.002, and C=0,... 

In this particular case, because the molecule is nonpolar, basis set extension beyond cc-pVTZ hardly affects the bond angle. Use of empirical bond length corrections results in bond lengths that lie all within about 0.001 A of each other (Table 5). 

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