Sequential ligation of Mg+, Fe+, (c-C5H5)Mg+, and (c-C5H5)Fe+ with ammoniain the gas phase: Transition from coordination to solvation in the sequential ligation of Mg+
Rk. Milburn et al., Sequential ligation of Mg+, Fe+, (c-C5H5)Mg+, and (c-C5H5)Fe+ with ammoniain the gas phase: Transition from coordination to solvation in the sequential ligation of Mg+, J PHYS CH A, 102(48), 1998, pp. 9803-9810
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 theoreticall
y. Molecular orbital calculations with density functional theory (DFT) perf
ormed at the B3LYP/6-31+G(d) level have been used to compute the binding en
ergies for direct coordination of the ammonia molecules with Mg+ and for so
lvation that involves N-H ... N interactions. Single-point calculations wer
e also done using the optimized geometries from B3LYP/6-31+G(d) at B3LYP/6-
311++G(2df,p) and MP4SDTQ(fc)/G-311++G(2df,p). Relative binding energies an
d standard enthalpies of formation (Delta H degrees(f,298)) have been calcu
lated at all three levels of theory investigated. The experiments were perf
ormed with a selected-ion flow tube (SIFT) apparatus in helium buffer gas a
t 0.35 +/- 0.01 Torr and 294 +/- 3 K. The measured rate coefficients for li
gation 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 Life
time 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 mul
ticollision-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 th
ird ligand. The observed variations in onset energy for multicollision-indu
ced dissociation generally are consistent with a negative trend in the liga
tion energy with increasing ligation for the four ligated systems investiga
ted.