Calculated bond angles

Molecular mechanics (also known diS force-field calculations) is a method for the calculation of conformational geometries. It is used to calculate bond angles and distances, as well as total potential energies, for each conformation of a molecule. Steric enthalpy can be calculated as well. Molecular orbital calculations (p. 34) can also give such information, but molecular mechanics is generally easier, cheaper (requires less computer time), and/or more accurate. In MO calculations, positions of the nuclei of the atoms are assumed, and the wave equations take account only of... 

Regarding the first of these factors, comparison of CISD-calculated bond angles in NH2 (102.6°) and CH3 (120.0°) indicates that the N-H bonds in NH2 have less 2s character than the C-H bonds in CH3.77 Such a difference, however, was found to account for no more than a quarter of the ca. 18 kcal/mol difference between the BDEs in Table 5 for R=R =H.77... 

Calculated bond angles in general vary with conformation more so than do bond length due to the "softer" nature of their deformation energy functions,... 

Kroto and Santy sl> have used the CNDO/2 method to calculate bond angles of a few molecules in their excited states. Again their results seem good. Subsequently 68>, they performed a far more laborious open-shell calculation which resulted in only slightly improved bond angles. 

Wyberg, in his modified CNDO/2 method 30>, calculated bond angles which are in good agreement with experiment. Del Bene and Jaffe 31> in their modified CNDO/2 method, however, were unable to reproduce satisfactory bond angles. 

Minimization of electrostatic interactions as defined by the various parameters described above, was done in order to calculate bond angles in halomethanes, -silanes and -germanes, as well as one-centre compounds that contain lone pairs and double bonds. The comparison with experimental values is such as to leave no doubt about the power of the VSEPR model. 

Table 1 Observed and calculated bond angles in some halides and hydrides of oxygen...

Boron has the structure ls22s22plx, and we know that it forms a covalent compound, boron trichloride experimental work shows that there are three equal strength B—Cl bonds in the molecule. A good explanation of this is that hybrid orbitals are formed from boron 2s, 2pv and 2pv orbitals. This is illustrated in Figure 15b. The hybrid orbitals lie in the same plane and are known as sn2 hybrids, since they are formed from one s and two p orbitals. Note that each of the hybrid orbitals will be only partly filled, since there are only three electrons of principal quantum number 2 to be allocated. The layout of the BC13 molecule is shown in Figure 15c. The calculated bond angle for sp2 hybrids is 120°, and this is confirmed experimentally. 

Ab initio calculations on the three tautomeric forms of 2//-1,2,6-thiadiazine 1,1-dioxide reveals that for the 2H- (17) and 4H- (20) tautomers the planar conformers are less stable than the nonplanar boat conformers by 2.7 and 1.2 kcal moU, respectively. For the 3,5-dimethyl- derivative there is reasonable agreement between the calculated bond angles and those obtained by x-ray crystallographic measurements <90JP0470>. In contrast, for the fully conjugated tautomer (18) theoretical calculations reveal that the planar form is the more stable and that aromatic character is to be... 

Values for the calculated bond angles in NH3 and H2O are shown in Table 3.21. There is quite reasonable agreement at the Hartree-... 

Fig. 5 List of isoelectronic molecules EL2 showing calculated bond angles and partial charges Aq of the central fragments for E=BH, C, N [89]. Experimental values of the bond angles of isolated molecules are given in parentheses...

X-ray crystal structure analyses yield information about the precise locations of the atoms contained within the crystal unit cell. From these locations, the geometry of the molecule under consideration can be derived by calculating bond angles, bond distances and torsion (dihedral) angles. 

Figure 7 Plot of calculated bond angles (deg) for XM(NH)3 versus...

SH2, i.e. in cases where the central atom remains the same. On the other hand, the trends observed on the experimental bond angles, i.e. the increase as BX E molecules are charged to is also seen on the calculated values. It is only the molecule pair NEj and OF2 which an opposite change is observed in the calculated bond angles. However, the differences are tee small to warrant detailed considerations. 

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