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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 102; no. 48; pp. 9803 - 9810
• Main Authors: Milburn, Rebecca K, Baranov, Vladimir I, Hopkinson, Alan C, Bohme, Diethard K
• Format: Journal Article
• Language: English
• Published: American Chemical Society 26.11.1998
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp981914z

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.