Sequential Ligation of Mg⁺, Fe⁺, (c-C₅H₅)Mg⁺, and (c-C₅H₅)Fe⁺ with Ammonia in the Gas Phase:  Transition from Coordination to Solvation in the Sequential Ligation of Mg⁺

Abstract

Experimental results are reported that track the kinetics of the sequential ligation of Mg+, Fe+, (c-C5H5)Mg+, and (c-C5H5)Fe+ with ammonia in the gas phase as a function of the number of ligands added. The energetics of the sequential ligation of Mg+ with ammonia has also been followed theoretically. Molecular orbital calculations with density functional theory (DFT) performed at the B3LYP/6-31+G(d) level have been used to compute the binding energies for direct coordination of the ammonia molecules with Mg+ and for solvation that involves N−H···N interactions. Single-point calculations were also done using the optimized geometries from B3LYP/6-31+G(d) at B3LYP/6-311++G(2df,p) and MP4SDTQ(fc)/6-311++G(2df,p). Relative binding energies and standard enthalpies of formation (ΔH°f,298) have been calculated at all three levels of theory investigated. The experiments were performed with a selected-ion flow tube (SIFT) apparatus in helium buffer gas at 0.35 ± 0.01 Torr and 294 ± 3 K. The measured rate coefficients for ligation of both atomic metal ions exhibit a maximum for the second addition of ammonia, which disappears in the presence of a c-C5H5 substituent as the rate of ligation of Mg+ and Fe+ is enhanced by about 2 orders of magnitude. These trends have been interpreted in terms of the dependence of the lifetime of the collision intermediate on its degrees of freedom and the depth of its potential energy well. For the ligation of Mg+, we propose that the precipitous drop in the observed rate of ligation of the fourth ligand and the emergence of a weakly bonded population of ligated ions in measured multicollision-induced dissociation spectra for Mg(NH3)3 + and Mg(NH3)4 + may signify a change in the nature of the bonding from direct bonding to hydrogen bonding in a second coordination shell with the addition of the third ligand. The observed variations in onset energy for multicollision-induced dissociation generally are consistent with a negative trend in the ligation energy with increasing ligation for the four ligated systems investigated.

Hydration of gas-phase ytterbium ion complexes studied by experiment and theory

“…The calculations indicate that for the cations studied, pentahydrates with four inner-shell water molecules and one outer-shell water molecule are energetically more favorable than pentahydrates with five inner-shell water molecules. The absence of outer-sphere pentahydrates is in accord with previous studies by Bohme and co-workers [47][48][49] and can be attributed to rapid dehydration of the outer-sphere water molecule and a steric hindrance to addition of a fifth inner-shell water molecule.…”

“…Referring to the computed energetics for these hydrations (Table 4), the first hydration is more energetically favorable, as evaluated by both exothermicity (DH) and exoergicity (DG). Similar results-slower first addition than second addition-were reported and rationalized by Bohme and co-workers for NH 3 and D 2 O association with bare metal cations [47][48][49].…”

“…Á(H 2 O) 1-3 complexes produced by ESI is attributed to the degrees of freedom introduced by the coordinated water molecule. In the NH 3 and D 2 O association reactions mentioned earlier [47][48][49], the observation that the first associations were slower than the second for bare metal cations was similarly attributed to the fewer degrees of by Bohme and co-workers, the third association reactions also exhibited slower kinetics than the second association reactions; this trend was attributed to the decrease in binding energies [48,49]; we adopt this convincing interpretation to understand the trends observed in our hydration kinetics. Referring to Table 4, it is notable that the entropy change for the third hydration of Yb(OH) 2 ?…”

“…The general ordering of hydration rates for the biligated cations was 1st \ 2nd [ 3rd [ 4th ()5th). The computed energetics do not fully account for this ordering and we invoke the rationale put forth by Bohme and co-workers [47][48][49]: the first hydration is inefficient due to fragmentation of the hot association complex with few degrees of freedom; the efficiencies of subsequent hydrations are dominated by the binding energies; the great inefficiency of the fifth hydration reflects a shift to outer-shell coordination.…”

Inductively coupled plasma mass spectrometer with axial field in a quadrupole reaction cell

“…Signals for the ions that form metal-ammonia clusters were found to maximize at higher V AFT than those for nonreactive ions. Fe ϩ is reported to cluster with ammonia forming Fe ϩ (NH 3 ) n , n ϭ 1-3, with the total rate of clustering of (1.7 Ϯ 0.5) ϫ 10 Ϫ11 cm 3 molecule Ϫ1 s Ϫ1 at 0.35 torr He buffer [29]. These clustering reactions can be readily observed in the DRC if the mass bandpass is set to a value allowing stability of both the measured cluster and its precursor (Fe ϩ ) in the quadrupolar field.…”