Microhydrated dihydrogen phosphate clusters probed by gas phase vibrational spectroscopy and first principles calculations

Abstract

We report infrared multiple photon dissociation (IRMPD) spectra of cryogenically-cooled H2PO4(-)(H2O)n anions (n = 2-12) in the spectral range of the stretching and bending modes of the solute anion (600-1800 cm(-1)). The spectra cannot be fully understood using the standard technique of comparison to harmonic spectra of minimum-energy structures; a satisfactory assignment requires considering anharmonic effects as well as entropy-driven hydrogen bond network fluctuations. Aided by finite temperature ab initio molecular dynamics simulations, the observed changes in the position, width and intensity of the IRMPD bands with cluster size are related to the sequence of microsolvation. Due to stronger hydrogen bonding to the two terminal P[double bond, length as m-dash]O groups, these are hydrated before the two P-OH groups. By n = 6, all four end groups are involved in the hydrogen bond network and by n = 12, the cluster spectra show similarities to the condensed phase spectrum of H2PO4(-)(aq). Our results reveal some of the microscopic details concerning the formation of the aqueous solvation environment around H2PO4(-), provide ample testing grounds for the design of model solvation potentials for this biologically relevant anion, and support a new paradigm for the interpretation of IRMPD spectra of microhydrated ions.

Probing the microsolvation of a quaternary ion complex: gas phase vibrational spectroscopy of (NaSO4−)2(H2O)n=0–6, 8

“…While band A initially appears weak, its relative intensity grows substantially for n 4 4. Similar observations were reported in our previous results on microhydrated sulfate, 27 bicarbonate, 44 bisulfate 40 and dihydrogen phosphate 45,46 anions. The positions of the sulfate bending modes (band H) also stay relatively constant.…”

“…20 Recent ab initio molecular dynamics (AIMD) simulations combined with IRMPD spectra of microhydrated H 2 PO 4 À anions show that dynamic and entropic effects have a profound effect on the spectral fingerprints observed in IRMPD spectra even when the ions are precooled to cryogenic temperatures. 45,46 Moreover, an assignment of these spectra based on 0 K harmonic spectra of minimum-energy structures may be problematic and misleading. Performing computational demanding AIMD simulations goes beyond the scope of this work, but we have taken care in not trying to over-interpret the present results.…”

“…This may be seen as an indication for the onset of water-water hydrogen bond network formation as well as an increase in the importance of dynamic effects in this size range. 45,46…”

Anharmonicities and coherent vibrational dynamics of phosphate ions in bulk H2O

“…These predictions were recently confirmed experimentally 31 . The role of the bending modes is further investigated by deuteration experiments (Fig.…”

Disentangling the Contribution of Multiple Isomers to the Infrared Spectrum of the Protonated Water Heptamer

“…23 A more reliable estimate of isomer populations and prediction of IR spectra for hydrogen-bonded clusters requires considering anharmonic effects as well as entropy-driven hydrogen-bond network fluctuations. 34,35 Such calculations go beyond the scope of the present study but will hopefully be stimulated by the present results. Summarizing, the present study resolves two important issues regarding the structure of protonated water clusters in the intermediate size range and at low internal energies.…”