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

Abstract

A 193 nm laser photofragmentation time-of-flight mass spectrometric study of chloroiodomethane Photodissociation and ab initio studies of Mg + ( NH 3 ) n , n=1-4: Electronic structure and photoinduced reaction Acetonitrile-solvated Mg ϩ complexes Mg ϩ ͑NCCH 3 ͒ n were produced in a pick-up source. The Mg ϩ ͑NCCH 3 ͒ n clusters were subjected to photodissociation in a time-of-flight reflectron mass spectrometer. Except for a minor reactive product Mg ϩ -NC at short wavelengths, only nonreactive ͑or evaporation͒ fragments were observed from photodissociation of Mg ϩ ͑NCCH 3 ͒ n (nϭ1 -4). Action spectra of Mg ϩ ͑NCCH 3 ͒ n (nϭ1 -4) were recorded in the spectral region of 230-560 nm, which provide insight into the structure and photodissociation dynamics of the complexes. For all the complexes we studied, the ligands are believed to be in the first solvation shell. Both the evaporation and reaction processes occur on the ground state surfaces of the complexes. By using the DFT/B3LYP and CIS methods, the most stable ground-state structures and the excitation spectra for Mg ϩ ͑NCCH 3 ͒ 1 -3 were obtained. The ab initio predictions appear to be consistent with the experimental results.

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

“…In fact, the calculated successive binding energies ⌬E n,nϪ1 of Mg ϩ (NH 3 ) n are 1.67͑1a͒, 1.29͑2a͒, 1.08͑3a͒, and 0.54 ͑4b͒ eV, which agree reasonably well with the values reduced from the CID experiments such as 1.60, 1.27, 0.99, and 0.45 eV for nϭ1 -4, respectively. [41][42][43] On the other hand, the successive binding energies of the first excited state can be estimated to be 2.73, 1.88, 1.24, and 1.88 ͑4b͒ eV for nϭ1 -4, respectively, by using the calculated ⌬E n,nϪ1 and the observed excitation energies. An anomalously large value for nϭ4 is due to the growth of the ionic nature in the excited-state cluster as discussed below.…”

Solvation of barium atoms and singly charged cations in acetonitrile clusters

“…The present observations for Ba + (CH 3 CN) n clusters have not been reported in previous studies on the Mg + (CH 3 CN) [1][2][3][4][5][6][7] cluster size distributions, as produced in pick-up molecular beam sources under similar conditions as here. 15 The theoretical structures of the lowest-lying isomers of Mg + (CH 3 CN) 1-3 clusters suggest that the ligands are both located in the first solvation shell and equivalently positioned around the Mg + ion, which results in a steady decrease of the SBEs with increasing cluster size. More recently, the magic numbers at n = 3, 6, 9, and 14 have been derived from Mg + (CH 3 CN) n≤18 cluster distributions, as produced running the pick-up sources at backing pressures of the pulsed valve in the range of 3-8 bars, though the origin of this behavior remains unclear.…”

“…Such conclusions encompass the interaction of M + = Mg, Ca, Sr, and Ba with a variety of polar and protic/aprotic solvents, including H 2 O, NH 3 , CH 3 OH, and CH 3 CN. [5][6][7][8][9][10][11][12][13][14][15][16] The rationalization of photodissociation experiments on Sr + (solvent) n (solvent = H 2 O, NH 3 ) clusters, specially the assignment of linear O− −Sr− −O and N− −Sr− −N backbones for n = 2, required further consideration of s-d σ hybridization of the metal atom to reduce the metal-ligand repulsion. 7 The s-d hybridization is also thought to influence the relative ligand binding energies of a number of transition metal ion-solvent complexes, to the extent that their first two binding energies are comparable and much larger than the nearly equal third and fourth binding energies.…”

“…pairs of acetonitrile molecules at N− −Ba− −N angles of 127.7 • and a N− −Ba− −N angle of 76.7 • between two closest CH 3 CN molecules.An assessment of previous results for the Mg + (CH 3 CN) 1-3 system can help to rationalize the theoretical structures of the lowest-lying isomers of Ba + (CH 3 CN) 1-4 clusters. The Mg + (CH 3 CN) n structures for n = 1, 2, and 3, respectively, as computed at the B3LYP/6-31+G * * level,15 feature C 3v , C 2v , and C 3v symmetries, with all of the acetonitrile molecules in the first solvent shell. These geometries can be interpreted in terms of 3s-3p σ hybridization of the Mg + ion, along with the location of the valence unpaired electron in one of the s-p σ orbitals 8.…”

4 Photodissociation in the gas phase

“…Apart from the persistent product Mg 1 , Mg 1 OCN and C 2 H 5 1 were obtained as photoproducts. Acetonitrile clusters (Mg 1 (NCCH 3 ) n (n ~1-4), were investigated by Yang's group 103 in the 230-560 nm range. The photodissociation products were mostly Mg 1 and nonreactive evaporation fragments.…”