Single-crystal X-ray diffraction analysis is the most direct and definitive technique for determining (or confirming) the geometric structure of chemical compound. In this paper, we describe the capacity of semi-empirical methods such as AM1, PM3, PM6 and NODCs for determining interatomic distances and bond angles for three compounds P1 ((1S, 3R,8R)-2,2-dichloro-3,7,7,10-tetramethyl-tricyclo [6,4,0,01,3] dodec-9-ene), P2 (1S,3R,8R,9S,11R)-2,2,10,10-tetrachloro-3,7,7,11 tetramethyltetracyclo [6,5,0,01.2,09.116 ] tridecane) and P3 (1S,3R,8R,9S,11R)-2,2,10,10-tetrabromo-3,7,7,11 tetramethyltetracyclo [6,5,0,01.2,09.116 ] tridecane) including experimental data interatomic distances and bond angles are available. The results obtained show a good agreement with experimental reference values, a few exceptions, for semi-empirical methods AM1 and PM6 appear more reliable than PM3 and NODC.

The two quantum methods Hartree-Fock HF/6-31G* (d, p) and density functional theory DFT/3-21G* (d, p) were used to calculate the equilibrium of the Si-F and Si-Cl bonds in SiH₃X compounds where X may be F^- or Cl^- ; the atomic electron affinity of chloride (Cl^-), fluoride (F^-), chlorine (Cl) and fluorine (F); entropy (S), heat capacity (C_v), total energy and reaction enthalpy of fluorosilanes, chlorosilanes and silyl radicals; and bond angles and bond lengths of SiH₃F and SiH₃Cl. Inter-atomic distances of the Si-F and Si-Cl bonds in SiH₃F and SiH₃Cl calculated using HR and DFT are in good agreement with the experimental values. The optimal distance of the Si-F bond is shorter than that of the Si-Cl bond in SiH₃X. Electron affinities calculated using HF and DFT are not in agreement with those obtained experimentally. The values of entropy (S) increase in parallel with the increase in the number of fluorine atoms in the silanes. The geometric structures of SiH₃F and SiH₃Cl both belong to the C_3v point group. Their bond angles are slightly different. SiH₃F has slightly higher energy than SiH₃Cl. This might be due to the value of the bond angle in SiH₃F, which is 109.18

β-himachalene behaves as a nucleophile while dichlorocarbene behaves as an electrophile. Equimolar condensation of β-himachalene and dichlorocarbene results in a single product: (1S,3R,8R)-2,2-dichloro-3,7,7,10-tetramethyltricyclo[6,4,0,0^1.3]dodec-9-ene, also referred to as dichlorocarbene β-himachalene ? (referred to as P₁ here), formed by reaction at the ? side of the C₆=C₇ double bond of β-himachalene. This regioselectivity is controlled by the frontier orbitals, as is the reaction mechanism. Electron density is particularly high around the C₆=C₇ double bond of the HOMO orbital. However when β-himachalene reacts with two equivalents of dichlorocarbene under the same conditions the result is two products: (1S,3R,8R,9S,11R)-2,2,10,10-tetrachloro-3,7,7,11-tetramethyltetracyclo[6,5,0,0^1.2,0^9.11]tridecane and (1S,3R,8R,9R,11S)-3,7,7,11-tetrachloro-3,7,7,11-tetramethyltetracyclo[6,5,0,0^1.2,0^9.11]tridecane (referred to here as P₂ and P₃ respectively). The same two products are also obtained when P₁ reacts with one equivalent of dichlorocarbene. The attack takes place simultaneously at the ? and β sides of the C₂=C₃ double bond. Study of the two reactions using the ab-initio quantum density functional theory method (B3LYP/6-31G(d)) shows that they are stereoselective, chemospecific, concerted and exothermic. P₃ is formed in greater quantity than P₂.