Experimental bond distances

Figure 4.9 Experimental bond distances for selected diatomic Croup 11 compounds (data are from Refs. [34, 159]).

Pensak and McKinney (28) [PM], using this method, have recently reported a systematic study of first-row transition metal carbonyl complexes for which experimental bond distances and angles were reliably reproduced, along with key bond dissociation energies. 

B3LYP density functional models provide somewhat better bond length results than the other density functional models, generally very close to experimental distances and to those from MP2 calculations. As with the other density functional models, the errors are largest where one (or two) second-row elements are involved. This is apparent from Figure 5-4, which compares B3LYP/6-311+G and experimental bond distances. 

A summary of mean absolute errors is provided in Table 5-5. Together with a series of plots of calculated vs. experimental bond distances, this further clarifies the performance of practical models Hartree-Fock models with STO-3G, 3-21G and 6-3IG basis sets (Figures... 

Figure 5-29 STO-3G vs. Experimental Bond Distances in Molecules Incorporating Third and Fourth-Row, Main-Group Elements...

The differences between the experimental bond distances of sulfur bases (Figs. 18 and 19), and those of the parent molecules.7 6 The differences between calculated 7r-bond orders of uracil and of its thio analogs (numbers in parentheses). 

The only monocyclic 1,2,3-triazine of which the structure has been determined by X-ray crystallographic analysis is 4,5,6-tris(4-methoxyphenyl)-l,2,3-triazine (72CB3704). The results show that the 1,2,3-triazine ring is planar, as one expects for a molecule with some degree of electron delocalization. The experimental bond distances and angles are given in Table 1. They are in reasonable agreement with calculated values. 

This enumeration gives only a selection of the published data. In Table 1 the experimental bond distances and bond angles are given and compared with calculated values. 

Table 1 Experimental Bond Distances and Angles of 1,2,4-Triazines and Calculated Values... 

Experimental bond distances (pm corresponding compounds in brackets) Ref. for the C-El bonds, Ref.13 for the Si-El bonds. 

The electronic configuration is (y2n n.y, indicating a Mo=Mo double bond. The experimental bond distance is 252.3 pm, and the molecule is a paramagnetic species. 

TABLE 30. Calculated and experimental bond distances (A) and bond angles (deg) of tin(II) acetate shown in Figure 20a... 

The first task is to get reasonable starting values for these parameters. The undeformed (ideal) bond distance r0 can be estimated from the average of known experimental bond distances of relatively undistorted, compounds. Experimental bond lengths are usually elongated by 1-10% due to steric strain (e.g., mutual repulsion of the substituents - remember the picture of ethane above). A first approximation to the force constant kb can be calculated from the fundamental vi-... 

These results for Si(OH)4 are consistent with the observation that average bond distances in solids can be reproduced quite well using molecular cluster models and ab initio Hartree-Fock techniques, at least for three- to six-coordinate central metal atoms from the first through third rows of the Periodic Table. Gibbs et al. (1987) have shown excellent agreement between calculated equilibrium bond distances in tetrahedral molecules and average experimental bond distances for tetrahedrally coordinated metal atoms. For the third-row elements, the addition of flexible d polarization functions was required to match experiment closely (in... 

All these calculations were performed for the experimental bond distances of 1.458(6) A and 1.401(10) A as determined by electron diffraction [169]. The use of slightly different bond distances as in our earlier calculations [130] gave only minor changes in the eigenvalues. The best agreement with the experimentally determined first ionization potential (7.54 0.04 eV) [170] and electron affinity (2.70 eV) [171] is obtained with the Hedin-Lundqvist exchange-correlation potential and the minimal basis set as defined above. 

Recent w-SC HMO (improved tu-technique) calculations did not change the correlation between the calculated and experimental bond distances in cytosine. 

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