Challenges in predicting ΔrxnG in solution: The chelate effect.

Gibbs energies for reactions involving aqueous ions are challenging to predict due to the large solvation energies of such ions. A stringent test would be the ab initio reproduction of the aqueous-phase chelate effect, an entropic effect in reactions of very small enthalpy changes. This paper examines what is required to achieve such a reproduction for the paradigmatic reaction M(NH3)4 2+ + 2 en → M(en)2 2+ + 4 NH3 (en = 1,2-ethylenediamine), for which ΔrxnG* and ΔrxnH* are -2.3 and +1.6 kcal mol-1, respectively, if M = Zn. Explicit solvation via simulation was avoided in order to allow sufficiently accurate electronic structure models; this required the use of continuum solvation models (CSMs), and a great deal of effort was made in attempting to lower the relative errors of ΔsolvG*[M(NH3)4 2+] vs ΔsolvG*[M(en)2 2+] from the CSMs available in Gaussian software. CSMs in ADF and JDFTx software were also tested. A uniform 2.2 kcal mol-1 accuracy in ΔrxnG* for all three metal-atom choices M = {Zn, Cd, Hg} was eventually achieved, but not from any of the known CSMs tested, nor from cavity size reoptimization, nor from semicontinuum modeling: post facto solvation energy corrections [one per solute type, NH3, en, M(NH3)4 2+, M(en)2 2+] were needed. It is hoped that this study will aid (and encourage) further CSM development for coordination-complex ions.

Comment on "Local, solvation pressures and conformational changes in ethylenediamine aqueous solutions probed using Raman spectroscopy" by M. Cáceres, A. Lobato, N. J. Mendoza, L. J. Bonales and V. G. Baonza, Phys. Chem. Chem. Phys., 2016, 18, 26192.

Liquid ethylenediamine contains both trans and gauche conformers, but there are conflicting claims in the literature that the relative abundance of gauche conformers is either completely quenched (J. Mol. Struct., 1999, 482, 639-646) or enhanced (Phys. Chem. Chem. Phys., 2016, 18, 26192-26198) in 1 M aqueous solutions. Density-functional-theory spectra predictions are employed here to resolve the conflict. In both the 1999 and 2016 reports, the effects seen were misinterpreted, and are instead due to loss of direct amine-amine H-bond interaction upon dilution, which appears to be complete at ∼0.66 mole fraction of water. Both trans and gauche conformers of ethylenediamine are concluded to be present, in both liquid and aqueous phases, with as yet no solid evidence for a shift in conformer ratio.

Nitrous oxide dimer: an ab initio coupled-cluster study of isomers, interconversions, and infrared fundamental bands, and experimental observation of a new fundamental for the polar isomer.

Improved quantum chemistry (coupled-cluster) results are presented for spectroscopic parameters and the potential energy surface for the N(2)O dimer. The calculations produce three isomer structures, of which the two lowest energy forms are those observed experimentally: a nonpolar C(2h)-symmetry planar slipped-antiparallel geometry (with inward-located O atoms) and a higher-energy polar C(s)-symmetry planar slipped-parallel geometry. Harmonic vibrational frequencies and infrared intensities for these isomers are calculated. The low-frequency intermolecular vibrational mode predictions should be useful for future spectroscopic searches, and there is good agreement in the one case where an experimental value is available. The frequency shifts for the high-frequency intramolecular stretching vibrations, relative to the monomer, were calculated and used to help locate a new infrared band of the polar isomer, which corresponds to the weaker out-of-phase combination of the nu(1) antisymmetric stretch of the individual monomers. The new band was observed in the region of the monomer nu(1) fundamental for both ((14)N(2)O)(2) and ((15)N(2)O)(2) using a tunable infrared diode laser to probe a pulsed supersonic jet expansion, and results are presented.

Carbocation branching observed in a simulation.

We have observed the branching rearrangement of a straight-chain secondary carbocation (C9H19+) in an ab initio molecular dynamics (AIMD) reverse-annealing (rising-temperature) simulation. The mechanism observed is one involving closed (protonated-cyclopropane) structures, previously observed in traditional geometry optimization calculations. However, the simulations give us a better understanding of the dynamics involved, leading to two advances: a simpler description of carbenium ion structures in general and the discovery of important entropy effects.

Short-range order in liquid aluminum chloride: ab initio molecular dynamics simulations and quantum-chemical calculations.

We use ab initio molecular dynamics simulations based on density-functional theory and quantum-chemistry calculations on molecular clusters to examine the structure of liquid AlCl3. In the past, conflicting descriptions of the short-range-order in molten AlCl3, based on either edge-sharing dimers or corner-sharing oligomers, have been proposed. This liquid also poses a simulation challenge, due to the possibility of ring-like trimers which can be metastable on the order of >10 ps. Simulations which begin with monomers, either random or ordered, appear to be able to produce proper ratios of ring-trimer to dimer-plus-tail molecular structures without the need to achieve long-time scale chemical equilibrium. Single-molecule calculations lend further support to the conclusion that the liquid is composed largely of edge-sharing dimers.

Femtosecond multidimensional imaging of a molecular dissociation.

The coupled electronic and vibrational motions governing chemical processes are best viewed from the molecule's point of view-the molecular frame. Measurements made in the laboratory frame often conceal information because of the random orientations the molecule can take. We used a combination of time-resolved photoelectron spectroscopy, multidimensional coincidence imaging spectroscopy, and ab initio computation to trace a complete reactant-to-product pathway-the photodissociation of the nitric oxide dimer-from the molecule's point of view, on the femtosecond time scale. This method revealed an elusive photochemical process involving intermediate electronic configurations.