Thermodynamic stability and optimized structures of hypervalent molecules M2CN (M = Li, Na, K)

Abstract

Abstract The hypervalent M2CN (M = Li, Na, K) molecules have been detected in the vapor at elevated temperatures over mixtures of alkali metals and sodium cyanide by means of Knudsen-effusion mass spectrometry. The experimentally obtained ionization energies were IP (Li2CN+) = 5.4 ± 0.2 eV, IP(Na2CN+) = 4.9 ± 0.2 eV and IP (K2CN+) = 4.0 ± 0.2 eV, agreeing well with the theoretical values 5.13, 4.66 and 3.7 eV, respectively, calculated for the vertical ionization. The energies of dissociation into MCN and M were D°0 (LiNCLi) = 137 ± 14 kJ/mol, D°0 (NaCNNa) = 104 ± 14 kJ/mol, and D°0 (KCNK) = 81 ± 8 kJ/mol. Although the theoretically calculated values were slightly lower (103.8 kJ/mol for Li2CN, 72.8 kJ/mol for Na2CN, and 74.5 kJ/mol for K2CN), the dissociation energies indicate moderately strong bonding of alkali metals to the alkali cyanides (MCN) of octet molecules. The agreement between experiment and theory confirms the existence of the M2CN species, which are the hypervalent molecules with more than one electronegative atom. The theoretically optimized structures of M2CN are quite similar to each other, with four isomers (two planar structures with Cs symmetry and two linear structure with C∞v symmetry), although there are differences in bond length. The favored structure of M2CN has Cs symmetry and is best described as a complex of the CN− anion with the M+2 cation.