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 ∆G _r MUEs for msRRHO, sRRHO(100) and sRRHO schemes are 1.2, 3.6 and 3.1 kcal mol ^-1 .
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