Electron spin resonance spectra of solvated alkali metal atoms in frozen solutions of sodium, potassium, rubidium, and caesium in hexamethylphosphoramide

Abstract

Abstract Low temperature glasses formed by rapid freezing of potassium-, rubidium-, and caesium-hexamethylphosphoramide (HMPA) solutions gave electron spin resonance (ESR) spectra which consisted of a central singlet (Gaussian) resonance and a multiplet of lines characteristic of hyperfine coupling to a single metal nucleus. The species giving rise to the resolved metal hyperfine coupling have been identified as solvated alkali metal atoms with metal n s orbital occupation in the range 60–70% of the free-atom value. The species responsible for the central singlet have been identified as large Bohr radius states with associated low (≲ 1% of the free-atom value) unpaired electron spin density at the metal nucleus. In sodium-HMPA glasses, a strong absorption from collodial sodium metal effectively obscured the resonance from the low atomic character state, but signals attributed to spin transitions of the solvated sodium atom were observed. An estimate of matrix pertubations in A iso in terms of an atomic-like interaction potential for lithium, potassium and caesium atoms trapped in a HMPA matrix predicts shifts in A iso (from the free-atom value) an order-of-magnitude below those observed experimentally. The observed deviations in A iso and g e from free atom values have been interpreted in terms of a molecular orbital picture originally proposed to explain the magnetic properties of silver atoms in various polar matrices. A comparison of magnetic parameters ( A iso, g e) for the states identified in frozen M-HMPA solutions with those for fluid solutions of the alkali metals in amines and ethers show strong similarities between results for the two phases. Our results are used to distinguish between current models for the fluid solutions. It is proposed that the observed hyperfine splitting in fluid amine solutions arises from a dynamic interconversion between two (or more) paramagnetic species, one with a very low (≲ 1%) occupancy of the metal n s orbital and one with approximately 60–70% occupancy.