Comparisons bond lengths

Results from local density models and BP, BLYP and EDF 1 density functional models are, broadly speaking, comparable to those from 6-3IG models, consistent with similarity in mean absolute errors. As with bond length comparisons, BLYP models stand out as inferior to the other non-local models. Both B3LYP/6-31G and MP2/6-31G models provide superior results, and either would appear to be a suitable choice where improved quality is required. 

Consistent with earlier remarks made for bond length comparisons, little if any improvement results in moving from the 6-3IG to the 6-311+G basis set for Hartree-Fock, local density and density functional models, but significant improvement results for MP2 models. 

Fig. 31. Molecular structure of Ba5(thd)9(H20)3(0H) showing the pyramidal Ba5 unit (t-Bu groups have been omitted for clarity). In this formulation 0(10-0(13) are assigned as HiO ligands and 0(10) as an OH ligand. Assignment of hydrogen atom locations was based on bond length comparisons. (Redrawn from Ref. 157.)...

Table 4.2. Effect of offset corrections on agreement between theoretical and experimental bond lengths. Comparison of N-N and C-N bond lengths in 1,2,4-triazole, between HF/3-21G theoretical calculations and single crystal neutron diffraction analysis at 15 K. [199]. (For atomic notation see Fig. 3.2)...

Search the literature for the experimental results for the H—O bond lengths and the H—O—H bond angle, and include a discussion of the comparison in your report. 

We shall examine the simplest possible molecular orbital problem, calculation of the bond energy and bond length of the hydrogen molecule ion Hj. Although of no practical significance, is of theoretical importance because the complete quantum mechanical calculation of its bond energy can be canied out by both exact and approximate methods. This pemiits comparison of the exact quantum mechanical solution with the solution obtained by various approximate techniques so that a judgment can be made as to the efficacy of the approximate methods. Exact quantum mechanical calculations cannot be carried out on more complicated molecular systems, hence the importance of the one exact molecular solution we do have. We wish to have a three-way comparison i) exact theoretical, ii) experimental, and iii) approximate theoretical. 

The pyrimidine ring is virtually flat. Its corrected bond lengths, as determined by a least-squares analysis of the crystal structure data for a unit cell of four molecules, are shown in formula (2) (60AX80), and the bond angles derived from these data show good agreement with those (3) derived by other means (63JCS5893) for comparison, each bond... 

Table 2 Comparison of Bond Lengths and Angles of Monoheterocycles...

Table 4 Comparison of Bond Lengths and. Angles of Dibenzo Heterocycles...

Table 5 Comparison of C—X Bond Lengths for Parent Heterocycles and their Dibenzo Derivatives...

For the antiaromatic three-membered heterocycles, experimental data are available only for thiirenes (and there is some doubt about the true antiaromaticity of thiirenes). Bond lengths have been calculated, however, for these antiaromatic 47r-systems (80PAC1623). In comparison with the corresponding saturated heterocycles, the C—X bond lengths are increased by 0.05 to 0.17 A and the C—C bond length is decreased by 0.2 A. 

Figure 1.25 A comparison of bond lengths in the complexes RuCl2(PPh3)3 and RuHCl(PPh3)3.

Figure 2.91 compares bond lengths for a range of these complexes [169]. Comparison of Ir—P bond lengths (e.g. for compounds I, II and V) shows the shortest Ir—P bonds trans to Cl and the longest for P trans to H. A similar effect is seen on Ir—Cl bond lengths by comparing the same compounds. 

Careful comparison of Pt-P bond lengths for the series trany-Pt(Pcy3)2X2 (X = H, Cl, Br, I) with those for p-any-Pt(PR3)2X2 (PR3 = PMe3 or PEt3) shows a more definite increase in Pt-P with anion size for the cyclohexyl-phosphine complexes (Table 3.16) believed to be owing to intermolecular X- H and X- C non-bonded interactions arising from overcrowding [151]. 

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