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

Abstract

The gas phase infrared spectrum (3250 to 3810 cm1) of the singly hydrated ammonium ion, NH4+(H2O), has been recorded by consequence spectroscopy of mass selected and isolated ions. The obtained four bands are assigned to N-H stretching modes and O-H stretching modes, respectively. The observed N-H stretching modes 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 observed O-H stretching modes 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) cm1 (NH4+) and 29(3) cm1 (H2O) deviate systematically from the corresponding values of the free molecules, a fact which is rationalized in terms of a symmetric top analysis. The recorded relative band intensities compare favorably with predictions of high level ab initio calculations except for the 3(H2O) band for which the observed value is about 20 times weaker than the calculated one. This long standing puzzle motivated us to examine the a 3(H2O)/1(H2O) intensity ratios from other published action spectra in other cationic complexes. These suggest that the 3(H2O) intensities become smallermore » 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 suggested that the coupling of the 3(H2O) and 1(H2O) modes to other cluster modes indeed varies by orders of magnitude. These findings altogether render the 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 single and double with 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. Our extrapolated values at the CCSD(T) complete basis set (CBS) limit are H(0 K) (NH4+(H2O)) = 85.40(± 0.24) kJ/mol and H(298 K) (NH4+H2O)) = 78.1(± 0.3) kJ/mol, in which double standard deviations are indicated in parenthesis. This work was supported by the Office of Basic Energy Sciences of the US Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Departmetn of Energy.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
902675
Report Number(s):
PNNL-SA-52932
Journal ID: ISSN 0021-9606; JCPSA6; KC0301020; TRN: US200718%%54
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 126; Journal Issue: 7
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ABSORPTION SPECTRA; BINDING ENERGY; ELECTRONIC STRUCTURE; FRAGMENTATION; INFRARED SPECTRA; AMMONIA; HYDRATES; gas phase infrared spectrum; singly hydrated ammonium ion; spectroscopy

Citation Formats

Pankewitz, Tobias, Lagutschenkov, Anita, Niedner-schatteburg, Gereon, Xantheas, Sotiris S, and Lee, Yuan-Tseh. Infrared spectrum of NH4+(H2O): Evidence for mode specific fragmentation. United States: N. p., 2007. Web. doi:10.1063/1.2435352.
Pankewitz, Tobias, Lagutschenkov, Anita, Niedner-schatteburg, Gereon, Xantheas, Sotiris S, & Lee, Yuan-Tseh. Infrared spectrum of NH4+(H2O): Evidence for mode specific fragmentation. United States. doi:10.1063/1.2435352.
Pankewitz, Tobias, Lagutschenkov, Anita, Niedner-schatteburg, Gereon, Xantheas, Sotiris S, and Lee, Yuan-Tseh. Wed . "Infrared spectrum of NH4+(H2O): Evidence for mode specific fragmentation". United States. doi:10.1063/1.2435352.
@article{osti_902675,
title = {Infrared spectrum of NH4+(H2O): Evidence for mode specific fragmentation},
author = {Pankewitz, Tobias and Lagutschenkov, Anita and Niedner-schatteburg, Gereon and Xantheas, Sotiris S and Lee, Yuan-Tseh},
abstractNote = {The gas phase infrared spectrum (3250 to 3810 cm1) of the singly hydrated ammonium ion, NH4+(H2O), has been recorded by consequence spectroscopy of mass selected and isolated ions. The obtained four bands are assigned to N-H stretching modes and O-H stretching modes, respectively. The observed N-H stretching modes 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 observed O-H stretching modes 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) cm1 (NH4+) and 29(3) cm1 (H2O) deviate systematically from the corresponding values of the free molecules, a fact which is rationalized in terms of a symmetric top analysis. The recorded relative band intensities compare favorably with predictions of high level ab initio calculations except for the 3(H2O) band for which the observed value is about 20 times weaker than the calculated one. This long standing puzzle motivated us to examine the a 3(H2O)/1(H2O) intensity ratios from other published action spectra in other cationic complexes. These suggest that the 3(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 suggested that the coupling of the 3(H2O) and 1(H2O) modes to other cluster modes indeed varies by orders of magnitude. These findings altogether render the 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 single and double with 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. Our extrapolated values at the CCSD(T) complete basis set (CBS) limit are H(0 K) (NH4+(H2O)) = 85.40(± 0.24) kJ/mol and H(298 K) (NH4+H2O)) = 78.1(± 0.3) kJ/mol, in which double standard deviations are indicated in parenthesis. This work was supported by the Office of Basic Energy Sciences of the US Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Departmetn of Energy.},
doi = {10.1063/1.2435352},
journal = {Journal of Chemical Physics},
number = 7,
volume = 126,
place = {United States},
year = {Wed Feb 21 00:00:00 EST 2007},
month = {Wed Feb 21 00:00:00 EST 2007}
}