Solvation of magnesium and singly ionized magnesium atoms in NH3 clusters: Theory and experiment

Abstract

The solvation of group II metals has not been studied in great details as compared to alkali metals and we show that due to the closed shell nature of these metals, the clusters possess unique features. Besides clusters allow the observation of singly ionised solvated ions that do not exist in solution and the characterisation of their properties is important since many of these singly ionised systems are reactive. The ionisation thresholds of magnesium atom solvated by ammonia have been investigated by tuneable laser ionisation of clusters prepared in a simple pickup source. The ionisation potential IP of clusters Mg(NH3)n have been measured for n =1 up to 35. The solvation of the magnesium ion is more efficient than for the closed shell neutral atoms resulting in a steep decrease of the ionisation energy of clusters with ammonia of increasing sizes (1.3 eV for n=1, 2.4 eV for n=2 and 3.2 eV for n=3 etc...). The effect becomes smaller large clusters (n¿20)for higher order clusters but the potential continues to decrease and the asymptote does not appear to be reached at n=35 , corresponding to increased electrostatic stabilisation of the ion cluster with these sizes. Quantum chemical calculations have been performed which reveal the unique features of the solvation of the neutral closed shell metal, ns2 and the metal ion ns1 by ammonia molecules. The formation of neutral clusters are dominated by singly coordinated cyclic sub-units containing 3 ammonia, while the ion is best stabilised in triply coordinated magnesium. The experimental ionisation energies are in excellent agreement with calculated values for singly coordinated, neutral cluster ionised to locally adiabatic states, doubly coordinated with ammonia.

Photodissociation studies of microsolvated metal cation complexes Mg+(NCCH3)n (n=1–4)

“…Recently, Maître et al investigated the solvation of Mg and Mg ϩ by NH 3 molecules both theoretically and experimentally. 16 The solvation of Mg ϩ by (NH 3 ) n was found to be more efficient than that of Mg, resulting in a steep decrease of the ionization energy of the ammonia clusters with increasing size. The transition energies to the low-lying excited states of Mg ϩ -(H 2 O) n , 6 Sr ϩ -͑NH 3 ) n , 7 and Mg ϩ -͑FCH 3 ) n , 9 were also calculated, and the results are in reasonable agreement with experiments.…”

Photodissociation and ab initio studies of Mg+(NH3)n, n=1–4: Electronic structure and photoinduced reaction

“…33 Its total binding energy, ⌬E 0 , is calculated to be Ϫ1.67 eV. The most stable nϭ2 cluster ͑2a͒ is a C 2 symmetry structure with two ammonia molecules staggered, whose ⌬E 0 is Ϫ2.96 eV.…”

“…Maitre and co-workers have conducted photoionization experiments of Mg͑NH 3 ) n (nϭ1 -37) and analyzed the ionization potentials with the aid of ab initio calculations. 33 Recently, Farrar and co-workers 34 have reported the mass spectra of Sr ϩ -ammonia clusters generated by laser vaporization and observed chemical species such as Sr ϩ (ND 3 ) n D x (xϭ0 -4). 34 In the Mg ϩ -ammonia system, we have also found similar cluster ions containing up to two hydrogen atoms such as Mg ϩ (NH 3 ) n (H) m (mϭ1,2) in addition to Mg ϩ (NH 3 ) n .…”

Zero energy kinetic electron and mass analyzed threshold ionization spectroscopy of Na⋅(NH3)n (n=1, 2, and 4) complexes

“…12 Of central interest in gas phase studies is the dependence of ionization potential, dissociation energy, and geometrical structure on the solvation number. [13][14][15][16][17][18][19] Among the complexes reported, alkali and alkali earth metal atoms solvated in water or ammonia are the most widely studied, partially because of their importance in biological systems. 20,21 On the theoretical front, Greer et al carried out an ab initio calculation on the smallest complexes ͑solvation num-ber nϭ1 and 2͒ formed by sodium and ammonia molecules.…”