Gas-phase thermochemistry of noncovalent ligand-alkali metal ion clusters: An impact of low frequencies

The experimental gas-phase thermochemistry of reactions: M+(S)(n-1) + S? M+(S)(n) and M+ + nS? M+(S)(n), where M is an alkali metal and S is acetonitrile/ammonia, is reproduced. Three approximations are tested: (1) scaled rigid-rotor-harmonic-oscillator (sRRHO); (2) the sRRHO(100) identical to (1), but with all vibrational frequencies smaller than 100 cm(-1) replaced with 100 cm(-1); (3) Grimme's modified scaled RRHO (msRRHO) (Grimme, Chem. Eur. J., 2012, 18, 9955-9964). The msRRHO approach provides the most accurate reaction entropies with the mean unsigned error (MUE) below 5.5 cal mol(-1) K-1 followed by sRRHO(100) and sRRHO with MUEs of 7.2 and 16.9 cal mol(-1) K-1. For the first time, we propose using the msRRHO scheme to calculate the enthalpy contribution that is further utilized to arrive at reaction Gibbs free energies (? G(r)) ensuring the internal consistency. The final increment G(r) MUEs for msRRHO, sRRHO(100) and sRRHO schemes are 1.2, 3.6 and 3.1 kcal mol(-1).