title: Quantum-chemical studies of methyl and silyl torsional frequencies and the structures of disilylsulphide and trisilylphosphine.
abstract: Torsional frequencies of methyl and silyl groups occurring in a range of molecules have been calculated by HF, B3LYP and MP2 methods using several basis sets. Linear correlations between calculated and observed values are derived and used to predict unobserved or dubious frequencies. The current experimental value for the E torsion in trimethylphosphine is questioned. The relative merits of the B3LYP and MP2 methods are explored. MP2 calculations can show wide variation with respect to basis set. In cases where two or more silyl groups are attached to a common atom, as in disilylsulphide (SiH3)2S, disilylmethane (SiH3)2CH2, trisilylmethane (SiH3)3CH and tetrasilylmethane (SiH3)4C, marked differences occur between B3LYP and MP2 estimates. These may be linked to concomitant differences in conformation or potential barrier restraining internal rotation. In disilylmethane the B3LYP results agree much better with experiment than those from the MP2 method. HF and B3LYP calculations for disilylsulphide and trisilylphosphine give normal C2v and C3v equilibrium structures, respectively, but in MP2 structures the silyl groups are twisted through 6-13 degrees yielding C2 and C3 configurations. It may be possible to distinguish between these structures through the observation of isolated SiH stretching frequencies in the spectra of fully deuterated materials. Dispersion forces could contribute to the twisting calculated by the MP2 method. Further studies of the microwave and vibrational spectra of disilylsulphide and trisilylphosphine isotopomers are needed.
authors.affiliation: School of Chermistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UK. dmckean@staffmail.ed.ac.uk
authors.forename: Donald C
authors.initials: DC
authors.lastname: McKean
mesh_terms: D004583:Electrons; D006859:Hydrogen / Q000737:chemistry; D008872:Microwaves; D008968:Molecular Conformation; D010720:Phosphines / Q000737:chemistry*; D011789:Quantum Theory; D012821:Silanes / Q000737:chemistry*; D012825:Silicon / Q000737:chemistry; D013053:Spectrophotometry / Q000379:methods*; D013055:Spectrophotometry, Infrared; D013059:Spectrum Analysis, Raman; D013440:Sulfides / Q000737:chemistry*; D013696:Temperature
chemical_list: D010720:Phosphines; D012821:Silanes; D013440:Sulfides; C503292:disilylsulphide; C503293:trisilylphosphine; D006859:Hydrogen; D012825:Silicon