Structures of protonated methanol clusters and temperature effects

Abstract

The accurate evaluation of pKa's, or solvation energies of the proton in methanol at a given temperature is subject to the determination of the most favored structures of various isomers of protonated (H(+)(MeOH)n) and neutral ((MeOH)n) methanol clusters in the gas phase and in methanol at that temperature. Solvation energies of the proton in a given medium, at a given temperature may help in the determination of proton affinities and proton dissociation energies related to the deprotonation process in that medium and at that temperature. pKa's are related to numerous properties of drugs. In this work, we were interested in the determination of the most favored structures of various isomers of protonated methanol clusters in the gas phase and in methanol, at a given temperature. For this aim, the M062X/6-31++G(d,p) and B3LYP/6-31++G(d,p) levels of theory were used to perform geometries optimizations and frequency calculations on various isomers of (H(+)(MeOH)n) in both phases. Thermal effects were retrieved using our homemade FORTRAN code. Thus, we accessed the relative populations of various isomers of protonated methanol clusters, in both phases for temperatures ranging from 0 to 400 K. As results, in the gas phase, linear structures are entropically more favorable at high temperatures, while more compact ones are energetically more favorable at lower temperatures. The trend is somewhat different when bulk effects are taken into account. At high temperatures, the linear structure only dominates the population for n ≤ 6, while it is dominated by the cyclic structure for larger cluster sizes. At lower temperatures, compact structures still dominate the population, but with an order different from the one established in the gas phase. Hence, temperature effects dominate solvent effects in small cluster sizes (n ≤ 6), while the reverse trend is noted for larger cluster sizes.

Hydrogen-bonded ring closing and opening of protonated methanol clusters H+(CH3OH)n (n = 4–8) with the inert gas tagging

“…In our previous study on H + (MeOH) 7 , we have found C and Ct share similar temperature region of thermal stability. 45 However, the isomer switching temperature is very different in their calculations, and part of the isomer population behavior clearly conflicts with the present result (for example, in n = 5, the L type isomer is predicted to be most favored in all the temperature and the population switching from C to L does not occur in their calculation). We can find that L type isomers dominate the whole temperature range for n = 4.…”

“…[16][17][18][19]25 Temperature or vibrational energy dependence of structures has been reported particularly for charged clusters including water molecules. 7,[37][38][39][40][41][42][43][44][45] One of the important features of the H-bond networks of H + (MeOH) n is their simplicity. 4,[30][31][32][33][34][35][36] Methanol is also a typical protic solvent, and hydrogen-bond (H-bond) network structures of protonated methanol clusters, H + (MeOH) n , have been studied by infrared (IR) spectroscopy and density functional theory (DFT) calculations.…”

“…44,45 These previous studies suggest that protonated methanol clusters are a very suitable model system to examine temperature dependence of microscopic H-bonded network structures, though their experimental confirmation has not yet been systematically performed. 44,45 These previous studies suggest that protonated methanol clusters are a very suitable model system to examine temperature dependence of microscopic H-bonded network structures, though their experimental confirmation has not yet been systematically performed.…”

Structures, binding energies, temperature effects, infrared spectroscopy of [Mg(NH3)n = 1−10]+ clusters from DFT and MP2 investigations

“…The basis set superposition error (BSSE) corrections have been obtained employing the counterpoise method as implemented in Gaussian 09 code. The revised version of Tempo [46] code has been used to derive the temperature effects on the relative population of the various isomers for each n cluster size. For a given temperature, their relative stabilities are evaluated through their canonical probabilities defined as:…”

Binding energies and isomer distribution of neutral acetonitrile clusters

“…Tempo code were first published in our work on the structures of protonated methanol clusters. 34 The corrected version of this program has been applied successfully to ammonia clusters 35,36 and protonated ammonia clusters. [37][38][39] It should be noted that the free energies G n k T ð Þ can be also calculated using the module freqchk of Gaussian 16.…”