IRMPD spectroscopy of anionic group II metal nitrate cluster ions

Abstract

Anionic group II metal nitrate clusters of the formula [M 2 (NO 3 ) 5 ] -, where M 2 ϭ Mg 2 , MgCa, Ca 2 , and Sr 2 , are investigated by infrared multiple photon dissociation (IRMPD) spectroscopy to obtain vibrational spectra in the mid-IR region. The IR spectra are dominated by the symmetric and the antisymmetric nitrate stretches, with the latter split into high and low-frequency components due to the distortion of nitrate anion symmetry by interactions with the cation. Density functional theory (DFT) is used to predict geometries and vibrational spectra for comparison to the experimental spectra. Calculations yield two stable isomers: the first one contains two terminal nitrate anions on each cation and a single bridging nitrate ("mono-bridging"), while the second structure features a single terminal nitrate on each cation with three bridging nitrate ligands ("tri-bridging"). The tri-bridging isomer is calculated to be lower in energy than the mono-bridging one for all species. Theoretical spectra of the tri-bridging structure provide a better qualitative match to the experimental infrared spectra of T he combination of ion-trapping mass spectrometers and free-electron lasers tunable through the mid-infrared region of the electromagnetic spectrum have provided invaluable access to the vibrational spectra of discrete, gas-phase metal complexes [1][2][3][4][5][6][7][8][9][10]. It is generally difficult to acquire a conventional linear absorption spectrum of species commonly investigated by mass spectrometry because of the low ion concentrations. However, use of an action spectroscopy approach allows photon absorption to be monitored by measuring fragmentation yields that result from wavelength-specific infrared multiple-photon dissociation (IRMPD) [11][12][13].Using the uranyl (UO 2 2ϩ ) and the europium (Eu 3ϩ ) cations, our group produced the first vibrational spectra of gas-phase metal nitrate anions of the formula [UO 2 (NO 3 ) 3 ] -and [Eu(NO 3 ) 4 ] - [14]. These experiments were carried out at the FOM Institute for Plasma Physics in Nieuwegein, The Netherlands, using the free electron laser for infrared experiments (FELIX). FELIX provides a tunable, high-intensity beam of photons across the mid-IR range to a high-resolution mass spectrometer. The IRMPD spectra of both the uranyl and europium nitrate anions are dominated by the antisymmetric nitrate stretch ( 3 ), which is split into low and high-frequency components corresponding to the O-N-O (O-atoms involved in bonding with the metal), and (non-coordinating) NϭO stretches respectively. Previous studies have shown that the splitting of the 3 (⌬ 3 ) is proportional to the interaction strength between the metal and the nitrate ligands [15,16]. In the uranyl and europium nitrate anions, ⌬ 3 was found to be 264 cm -1 and 273 cm -1 respectively [14].In a later study, IRMPD spectroscopy was used to record vibrational spectra of anionic group II metal nitrate complexes of the formula [M(NO 3 ) 3 ] -where M ϭ Mg 2ϩ , Ca 2ϩ , Sr 2ϩ , and Ba 2ϩ [17]. By observ...

Infrared multiple photon dissociation spectroscopy of sodium and potassium chlorate anions

“…It is difficult to assess whether the change in splitting predicted by DFT reproduces any difference in the experiment because of the broad, featureless IRMPD spectrum for [K(ClO 3 ) 2 ] − . However, the predicted decrease in splitting is consistent with expected weaker interaction between ClO and K + , relative to Na + , and with trends predicted and observed in IRMPD studies of group II metal‐nitrate anions 19, 33…”

IRPD spectroscopy and ensemble measurements: Effects of different data acquisition and analysis methods

“…Because the PDY depends on a ratio of correlated abundances, it does not depend on fluctuations in initial precursor population from scan to scan. This is the method currently used for the majority of IRPD spectra obtained using FELIX [31,32,[35][36][37][38][39][40], including many tightly-bound complexes for which the absorption of many mid-infrared photons is typically necessary to induce fragmentation. An action spectrum can also be obtained by measuring the extent of depletion of the precursor ion or appearance of a fragment ion.…”

Infrared spectrum of potassium‐cationized triethylphosphate generated using tandem mass spectrometry and infrared multiple photon dissociation

“…It is known that an accurate prediction of the liquid‐phase spectrum requires consideration of and accounting for condensed‐phase effects such as dipole‐dipole interactions. Therefore, the vibrational frequencies for neutral TEP were re‐calculated at the B3LYP/6‐311++G(3df,2pd) level of theory using the Integral‐Equation‐Formalism Polarizable Continuum Model (IEFPCM) method to account for solvent effects 28, 29. In the IEFPCM model the environment surrounding the TEP molecule is treated macroscopically as a continuous medium characterized by its dielectric permittivity.…”

“…The vibrational frequencies were left unscaled for comparison with IRMPD spectra. This DFT approach was used here because of its success in the prediction of vibrational features for gas‐phase actinide17–21 and other metal ion complexes in our previous IRMPD work 27, 28. A comprehensive survey of the vibrational spectra predicted using other functionals and basis sets is currently underway and will be reported at a later date.…”