H-bond and electric field correlations for water in highly hydrated crystals

• We present periodic plane-wave density functional theory (DFT) Perdew-Burke-Ernzerhof (PBE-D2) calculations for four highly hydrated crystals, Na2CO3 center dot 10H(2)O, MgSO4 center dot 7H(2)O, MgSO4 center dot 11H(2)O, and Al(NO3)(3)center dot 9H(2)O, containing 37 structurally unique water molecules and 74 unique hydrogen bonds. The calculated R(H center dot center dot center dot O) distances lie in the range 1.60-2.05 angstrom, the anharmonic OH frequencies in the range 2570-3425 cm(-1), and the water dipole moments lie in the range 2.9-4.3 Debye, as calculated from the Wannier function centers and the nuclei. We present the following findings. (i) Our optimized intramolecular r(OH) distances are always larger than the gasphase value and thus more accurate than those derived from neutron diffraction experiments; (ii) The local in situ electric field over the molecule, calculated from the positions of the nuclei and the Wannier centers in the surrounding crystal, appears to be a good descriptor of the pertturbation from the water molecule's surroundings as the internal molecular properties (r(e), nu, mu) are found to correlate well with the crystal-generated electric field; (iii) We have added DFT-calculated data points to the well-known experimental 'OH frequency versus R(H center dot center dot center dot O) correlation curve in a region where the experimental data points are scarce; (iv) For all 37 water molecules, the Wannier centers located in the lone-pair region, and those located in the OH bonds, displace about equally much due to the polarizing environment. Finally, we propose that our resulting 'OH frequency versus Wannier-type electric field' correlation curve may constitute a useful tool for predicting OH vibrational frequencies from snapshots from PBE-D2-based ab initio molecular dynamics simulations of water-containing systems.

Ämnesord

NATURVETENSKAP  -- Kemi -- Materialkemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Materials Chemistry (hsv//eng)

Nyckelord

water dipole moments; crystalline hydrates; hydrogen bonding; density functional theory; OH vibrations

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