Vibrational spectra of adamantanes X10H16 and diamantanes X14H20 (X=C, Si, Ge, Sn): A theoretical study

Optimized geometrical parameters are obtained for adamantanes (X10H16) and diamantanes (X14H20) (X=C, Si, Ge, Sn) using B3LYP method with 6-311G** and/or LANL2DZ basis sets. For carbon compounds Hartree-Fock (HF) and MP2 methods are also used for the study. The harmonic vibrational frequencies are obtained for all the molecules at their respective optimized geometries. The algorithm for the scaled quantum mechanical (SQM) method reported from our lab is modified to include Pulay's scaling procedure. Experimental frequencies of adamantane (X=C) and four of its isotopomers with different symmetry point groups are fitted to the calculated harmonic frequencies to get suitable scale factors for the diagonal local force constants. These scale factors are used to predict the experimental vibrational frequencies of adamantane and diamantane. The assignments are proposed for all fundamentals of the title compounds based on normal coordinate analysis.