Infrared spectrum of NH4+(H2O): Evidence for mode specific fragmentation

Abstract

The gas phase infrared spectrum (3250-3810 cm-1) of the singly hydrated ammonium ion, NH4+(H2O), has been recorded by action spectroscopy of mass selected and isolated ions. The four bands obtained are assigned to N-H stretching modes and to O-H stretching modes. The N-H stretching modes observed are blueshifted with respect to the corresponding modes of the free NH4+ ion, whereas a redshift is observed with respect to the modes of the free NH3 molecule. The O-H stretching modes observed are redshifted when compared to the free H2O molecule. The asymmetric stretching modes give rise to rotationally resolved perpendicular transitions. The K-type equidistant rotational spacings of 11.1(2) cm-1 (NH4+) and 29(3) cm-1 (H2O) deviate systematically from the corresponding values of the free molecules, a fact which is rationalized in terms of a symmetric top analysis. The relative band intensities recorded compare favorably with predictions of high level ab initio calculations, except on the nu3(H2O) band for which the observed value is about 20 times weaker than the calculated one. The nu3(H2O)/nu1(H2O) intensity ratios from other published action spectra in other cationic complexes vary such that the nu3(H2O) intensities become smaller the stronger the complexes are bound. The recorded ratios vary, in particular, among the data collected from action spectra that were recorded with and without rare gas tagging. The calculated anharmonic coupling constants in NH4+(H2O) further suggest that the coupling of the nu3(H2O) and nu1(H2O) modes to other cluster modes indeed varies by orders of magnitude. These findings together render a picture of a mode specific fragmentation dynamic that modulates band intensities in action spectra with respect to absorption spectra. Additional high level electronic structure calculations at the coupled-cluster singles and doubles with a perturbative treatment of triple excitations [CCSD(T)] level of theory with large basis sets allow for the determination of an accurate binding energy and enthalpy of the NH4+(H2O) cluster. The authors' extrapolated values at the CCSD(T) complete basis set limit are De [NH4+-(H2O)]=-85.40(+/-0.24) kJ/mol and DeltaH(298 K) [NH4+-(H2O)]=-78.3(+/-0.3) kJ/mol (CC2), in which double standard deviations are indicated in parentheses.

Vibrational Spectroscopy of Bisulfate/Sulfuric Acid/Water Clusters: Structure, Stability, and Infrared Multiple-Photon Dissociation Intensities

“…The two types of experiments, with and without messenger-tagging, can lead to substantially different IR spectra, typically with certain IR-active modes missing from the IRMPD spectrum. 25,31,32 We find here that comparison of the IRMPD spectra to the corresponding single-photon IRPD spectra aids in identifying the mechanism for these missing peaks.…”

“…A mechanism involving (i) has been explored in the OH stretch region by Pankewitz et al 32 for the NH 4 + (H 2 O) cation and used to analyze a slew of previous experimental results on singly hydrated cationic systems. When the cluster dissociation energy is high, the IRMPD spectra show unexpectedly low peak intensities for the antisymmetric water OH stretch compared to those for the symmetric stretch.…”

Hydration of the Calcium Dication: Direct Evidence for Second Shell Formation from Infrared Spectroscopy

“…One strategy to gain a detailed understanding of how ions and water molecules interact is to build up a solution, one molecule of water at a time. [1] Vibrational spectroscopy is a powerful structural tool for investigating hydrated ions, and has been used to investigate the structures of hydrated monovalent cations, including Ag + , [2] H + , [3][4][5][6] Cs + , [7,8] Cu + , [2] K + , [9] Mg + , [10] NH 4 + , [11,12] and Ni + , [13] and hydrated anions, including Cl À , [14,15] e À , [16] F À , [15,17] HO À , [18] and SO 4 2À . [19] Despite serving critical regulatory and structural roles in biology, experimental studies of hydrated, multiply charged ions have lagged behind those of singly charged ions, owing to the great difficulty in producing them in significant abundances; for example, formation of doubly hydrated, divalent calcium by condensation results in rapid dissociation aided by Coulomb repulsion [Eq.…”

“…[11] Niedner-Schatteburg and co-workers proposed that weak intramolecular vibrational relaxation (IVR) coupling for u asym reduces the photodissociation efficiency for this mode for NH 4 + A C H T U N G T R E N N U N G (H 2 O). [11] Although many of the effects of slow IVR should be significantly reduced in these experiments as a result of the 100 ms time period between laser pulses and the presence of multiple water molecules, radiative emission from u asym should be fast and this may be a competitive relaxation mechanism. [49] Alternatively, the absorption of multiple photons is more important for the smaller clusters because of higher dissociation energies for the loss of a water molecule.…”

Perspective: Water cluster mediated atmospheric chemistry

“…145,146 Elegant experiments made it possible to follow the evolution of vibrational spectra and of reactivity with cluster size, allowing for molecular level descriptions of hydrated clusters. [147][148][149][150] Important and general messages emerged from these studies about the role of the hydration structure around the ionic core in determining spectroscopic and chemical properties of these species. 151 Computational studies predict red shifts in the vibrational spectra of hydrates and emphasize the importance and necessity for anharmonic effects to be treated appropriately to describe their properties.…”