Authors: Getachew Kebede, Peter Broqvist, Anders Eriksson, and Kersti Hermansson
We propose that crystalline ionic hydrates constitute a valuable resource for benchmarking theoretical methods for aqueous ionic systems. Many such structures are known from the experimental literature, and they contain a large variety of water–water and ion–water structural motifs. Here we have collected a data set (CRYSTALWATER50) of 50 structurally unique “in-crystal” water molecules, involved in close to 100 nonequivalent O–H···O hydrogen bonds. A dozen well-known DFT functionals were benchmarked with respect to their ability to describe these experimental structures and their OH vibrational frequencies. We find that the PBE, RPBE-D3, and optPBE-vdW methods give the best H-bond distances and that anharmonic OH frequencies generated from B3LYP//optPBE-vdW energy scans outperform the other methods, i.e., here we performed B3LYP energy scans along the OH stretching coordinate while the rest of the structure was kept fixed at the optPBE-vdW-optimized positions
J. Chem. Theory Comput. 15, p. 584, 2019
DOI: 10.1021/acs.jctc.8b00423
We present periodic plane-wave density functional theory (DFT) Perdew–
We present experimental and calculated IR spectra of the water molecules in crystalline aluminium nitrate nonahydrate and a method to generate a realistic and well resolved isotope-isolated spectrum from periodic DFT calculations. Our sample crystal contains 18 structurally different OH groups and is a perfect benchmark compound to validate vibrational models and the structure–property relationship of bound water molecules. FTIR spectra (ATR technique) were recorded for the Al(NO3)3·9H2O crystal at 138 and 298 K, and due to a multitude of OH contributions and couplings, they are naturally poorly resolved and yield a broad OH band in the range 3500 to 2700 cm−1 at both temperatures. Isotope-isolated IR spectra have the clear advantage over non-deuterated spectra that they are better resolved and easier to interpret – here we have extended the experimental study by simulating the isotope-isolated IR spectrum, using PBE-D2 and auxiliary B3LYP calculations and an anharmonic OH vibrational model. We find excellent agreement between the shapes and frequency ranges of the experimental and calculated OH spectral bands. We make use of four different vibrational models: (i) a harmonic lattice-dynamical model for the isotope-isolated crystal with 1 H among 71 D, (ii) a harmonic lattice-dynamical model for the normal undeuterated crystal involving all the vibrational couplings, (iii) a harmonic 1-dimensional uncoupled OH vibrational model, and (iv) the anharmonic variant of the previous model, which yields the final spectrum. We also use the individual frequencies, resolved by the calculations, to quantify new or extended relationships involving OH frequencies versus local electric fields and H-bond distances. We explore the correlation between OH frequency and molecular dipole moment for bound water molecules.
O hydrogen bonds) in the Al(NO3)3·9H2O crystal have been calculated from DFT calculations. We combine the advantage of studying different water environments using one and the same compound and method (instead of comparing a set of independent experiments, each with its own set of errors) with the advantage of knowing the exact atomic positions, and the advantage of calculating properties that are difficult to extract from experiment. We find very large Wannier dipole moments for H2O molecules surrounding the cations: 4.0–4.3 D (compared to our calculated value of 1.83 D in the gas phase). These are induced by the ions and the H-bonds, while other water interactions and the relaxation of the internal water geometry in fact decrease the dipole moments. We find a good correlation between the water dipole moment and the O