Characterisation of monomeric methylsodium and methylpotassium: infrared spectra of the matrix-isolated molecules

Abstract

Co-condensation of CH₃l with sodium or potassium vapour and an excess of N₂ at ca. 20 K gives rise to the respective methylmetal monomer CH₃Na or CH₃K which is trapped in the resulting matrix. The i.r. spectrum of the matrix in the region 200–4 000 cm^–1 discloses all six vibrational fundamentals characterising each of these molecules in C_3v symmetry, the assignments being verified by measurements taking in the fully deuteriated as well as the undeuteriated isotopomer. How closely the molecules approximate to the formulation of CH₃^–M⁺(M = Na or K) is revealed through fundamentals which are lower in energy than those of any other homoleptic methylmetal compound reported to date. Normal co-ordinate analysis underlines this aspect with M–C stretching force constants =ca. 48 N m^–1 and CH₃ rocking force constants = 10.3 (M = Na) and 7.6 N m rad^–2(M = K) which set new standards by their diminutive values.