Strategy for Modeling the Infrared Spectra of Ion-Containing Water Drops

Abstract

Hydrated ions are ubiquitous in environmental and biological media. Understanding the perturbation exerted by an ion on the water hydrogen bond network is possible in the nanodrop regime by recording vibrational spectra in the O-H bond stretching region. This has been achieved experimentally in recent years by forming gaseous ions containing tens to hundreds of water molecules and recording their infrared photodissociation spectra. In this paper, we demonstrate the capabilities of a modeling strategy based on an extension of the AMOEBA polarizable force field to implement water atomic charge fluctuations along with those of intramolecular structure along the dynamics. This supplementary flexibility of nonbonded interactions improves the description of the hydrogen bond network and, therefore, the spectroscopic response. Finite temperature IR spectra are obtained from molecular dynamics simulations by computing the Fourier transform of the dipole moment autocorrelation function. Simulations of 1-2 ns are required for extensive sampling in order to reproduce the experimental spectra. Furthermore, bands are assigned with the driven molecular dynamics approach. This method package is shown to compare successfully with experimental spectra for 11 ions in water drops containing 36-100 water molecules. In particular, band frequency shifts of the free O-H stretching modes at the cluster surface are well reproduced as a function of both ion charge and drop size.

Infrared Spectra of Deprotonated Dicarboxylic Acids: IRMPD Spectroscopy and Empirical Valence‐Bond Modeling

“…The AMOEBA polarizable force field, already known to provide an accurate computational framework for vibrational spectroscopy in the gas phase, [48][49][50] was extended here to account for possible intramolecular proton transfer using a two-state empirical valence bond approach, [10] with the aim of describing deprotonated dicarboxylic acids HO 2 CÀ (CH 2 ) n À CO À 2 . IRMPD spectra measured for n = 2 to 4 reveal signatures of proton sharing that are accounted for neither by static quantum chemistry calculations in the harmonic approximation, nor by finite temperature anharmonic spectra based on the Fourier transform of the dipole moment time autocorrelation function using the nonreactive AMOEBA potential.…”

“…AMOEBA has been successfully used to model the structural and energetic properties of biomolecules in the gas and condensed phases,,, as well as thermodynamic properties of solvated ions or protein‐ligand complexes . Its application to IR spectroscopy is more recent but allowed anharmonic and temperature effects to be correctly accounted for in gas phase molecules …”

“…[44][45][46][47] Its application to IR spectroscopy is more recent but allowed anharmonic and temperature effects to be correctly accounted for in gas phase molecules. [48][49][50] Several ingredients of the AMOEBA model require parametrization, in particular the atomic charges, dipoles and quadrupoles of the multipolar expansion. In the original spirit 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 of this ab initio force field, these parameters were derived from dedicated quantum chemistry calculations, using the distributed multipole analysis (DMA) developed by Stone.…”

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

“…56 The AMOEBA14 parameter set for water 57 was used, however vibrational spectra were found to be greatly improved with the use of fluctuating atomic charges. 44 The charges on H are allowed to change in a −0.05 : +0.06] range in response to distance and angle fluctuations, while the charge on O adapts to maintain overall neutrality. The intramolecular OHO bending force constant was also modified from 600 to 400 kcal.mol −1 .Å −2 .…”

“…42 In the last years, we have extended the AMOEBA force field to the finite temperature calculation of infrared spectra of gas phase molecules. [43][44][45] This force field has been used to study the zinc coordination number in water as a function of cluster size and temperature 46 and to identify the low-energy structures of SO 2− 4 (H 2 O)n, n=9-13. 8 The latter work included discussion of the especially stable structure at n=12.…”