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

Abstract

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-harmonicoscillator (sRRHO); (2) the sRRHO(100) identical to (1), but with all vibrational frequencies smaller than 100 cm À1 replaced with 100 cm À1 ; (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 À1 K À1 followed by sRRHO(100) and sRRHO with MUEs of 7.2 and 16.9 cal mol À1 K À1 . 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 (ΔG r ) ensuring the internal consistency. The final ΔG r MUEs for msRRHO, sRRHO(100) and sRRHO schemes are 1.2, 3.6 and 3.1 kcal mol À1 .

Kinetics of direct and water‐mediated tautomerization reactions of five‐membered cyclic amides or lactams

“…The hindered internal rotations of the O–H group in the enols are well‐behaved with rotation barriers of 20–45 kJ mol −1 with cis enols $${\rm N\bond C\bond O\bond H}$$ typically more stable than trans enols. Both sets of species have very few low frequency modes, $$\bar{\nu } < 100\ {\rm cm}^{-1}$$, which would probably necessitate quasi‐rigid rotor harmonic oscillator treatments but given that those that are found are ring puckering modes such a complication is not deemed necessary 37,38 …”