Aqueous solvation of Mg(ii) and Ca(ii): A Born-Oppenheimer molecular dynamics study of microhydrated gas phase clusters

Abstract

The hydration features of [Mg(HO)] and [Ca(HO)] clusters with n = 3-6, 8, 18, and 27 were studied by means of Born-Oppenheimer molecular dynamics simulations at the B3LYP/6-31+G** level of theory. For both ions, it is energetically more favorable to have all water molecules in the first hydration shell when n ≤ 6, but stable lower coordination average structures with one water molecule not directly interacting with the ion were found for Mg at room temperature, showing signatures of proton transfer events for the smaller cation but not for the larger one. A more rigid octahedral-type structure for Mg than for Ca was observed in all simulations, with no exchange of water molecules to the second hydration shell. Significant thermal effects on the average structure of clusters were found: while static optimizations lead to compact, spherically symmetric hydration geometries, the effects introduced by finite-temperature dynamics yield more prolate configurations. The calculated vibrational spectra are in agreement with infrared spectroscopy results. Previous studies proposed an increase in the coordination number (CN) from six to eight water molecules for [Ca(HO)] clusters when n ≥ 12; however, in agreement with recent measurements of binding energies, no transition to a larger CN was found when n > 8. Moreover, the excellent agreement found between the calculated extended X-ray absorption fine structure spectroscopy spectra for the larger cluster and the experimental data of the aqueous solution supports a CN of six for Ca.

Assessing the Intrinsic Strengths of Ion–Solvent and Solvent–Solvent Interactions for Hydrated Mg2+ Clusters

“…From the k a cum values of the implicitly solvated clusters, as shown in Table 4, we observed the implicit addition of outer shells to [Mg(H 2 O)n] 2+ (n = 1-6) clusters to increase in stability alongside increasing CN, the results revealing a CN of 6 to result in the most stable configuration due to the Mg-O and HB interactions. The CN of 6, as suggested from local mode analysis, agrees with recent experimental [16,19,21,22,24] and computational works [20,32,34,39,[41][42][43][44]46,47,49,51]. In this work, via the utilization of cumulative local mode stretching force constants k a cum , we observed the divalent Mg ion to coordinate with up to six water molecules, whereas in our previous work regarding hydrated calcium clusters, the divalent Ca ion was observed to undergo coordination with up to eight water molecules [106].…”

“…The more restricted CN of the Mg 2+ has previously been attributed to the smaller ion size [39,107,108], whereas the larger size of the Ca 2+ ion has been linked to weaker inner ion-solvent interactions in retrospect to that of the hydrated Mg 2+ ion [51]. As shown by Tables 3 and 5, the k a cum values of explicitly hydrated systems, based upon inner ionsolvent interactions, are larger for [Mg(H 2 O) n ] 2+ (n = 1-6) (1-6) clusters in comparison to that of their Ca 2+ counterparts by differences of 0.601 to 1.341 mdyn/Å (i.e., 1a-6a in 2).…”

“…Numerous solid-state structures of inorganic composites have validated the existence of the six-coordinated structure ([Mg(H 2 O) 6 ] 2+ ) [31]. Theoretical investigations into the solvation of Mg 2+ span across molecular dynamic (MD) simulations [20,[32][33][34][35], Monte Carlo simulations [32,[36][37][38], Car Parrinello molecular dynamic simulations (CPMD) [39], effective fragment potential (EFP) calculations [40], ab initio molecular dynamics (AIMD) [28,[41][42][43], density functional theory (DFT) [41,[44][45][46][47][48][49][50], Møller-Plesset secondorder perturbation theory (MP2) [41,44,47,49], coupled cluster theory with singles, double, and perturbative triples (CCSD(T)) [47,49], and Born-Oppenheimer molecular dynamics (BOMD) [51]; among these studies, a CN of 6 has been reported. However, the DFT study of Pavlov and co-workers has suggested that the Mg 2+ ion forms stable configurations when coordinated to seven water molecules [52] [41,47]; Bai et al also reported that the first hydration shell influences the charge transfer between the divalent magnesium cation and outer solvent shells [41].…”

Ground-State Structures of Hydrated Calcium Ion Clusters From Comprehensive Genetic Algorithm Search

“…Using a semiempirical coupling method with a basinhopping global optimization approach, Wales and co-workers searched the low-lying structures of Ca 2+ (H 2 O) n with n 1-20 clusters, showing that Ca 2+ prefers to locate at the center of the cluster surrounded by eight adjacent water molecules (González et al, 2005). The CN of the calcium ion attained by Monte Carlo (MC) (Bernal-Uruchurtu and Ortega-Blake, 1995) and MD (Obst and Bradaczek, 1996;Tongraar et al, 1997;Todorova et al, 2008;Wanprakhon et al, 2011;León-Pimentel et al, 2018) studies varies from 6 to 10. However, the results from DFT calculations showed that there are six water molecules in the first hydration shell (Megyes et al, 2004;Peschke et al, 2000;Carl et al, 2007;Lei and Pan, 2010).…”

Manifolds of low energy structures for a magic number of hydrated sulfate: SO42−(H2O)24

“…While this permits accurate local exploration and proper account of anharmonic and temperature effects on populations and vibrational spectra, it is not yet amenable to extensive exploration of structural space. 15,[20][21][22] Global methods such as basin-hopping global optimization, [23][24][25] simulated annealing 26 and various genetic algorithms 27,28 have been developed to improve the energylandscape exploration. Approximate quantum methods have been used to extend the range of system sizes in comparison with the DFT framework.…”