Photodissociation spectroscopy of MgCH+4

Abstract

The photodissociation spectroscopy of MgCH 4 ϩ has been studied in a reflectron time-of-flight mass spectrometer. MgCH 4 ϩ molecular absorption bands are observed to the red of the Mg ϩ (3 2 P J ←3 2 S 1/2 ) atomic ion resonance lines. The photofragmentation action spectrum consists of a broad structureless continuum ranging from 310 nm to 342 nm, and peaking near 325 nm. In this spectral region, both the nonreactive ͑Mg ϩ ͒, and two reactive fragmentation products ͑MgH ϩ and MgCH 3 ϩ ͒ are observed, all with similar action spectra. The product branching is independent of wavelength, Mg ϩ :MgCH 3 ϩ :MgH ϩ ϳ60:33:7. The absorption is assigned to the transition (1 2 E←1 2 A 1 ) in C 3v symmetry ͑with 3 coordination͒, followed by a geometrical relaxation of the complex toward states of 2 B 1 and 2 B 2 symmetry in C 2v geometry ͑with 2 coordination͒.Dissociation requires a nonadiabatic transition to the ground electronic surface. Analysis of broadening in the photofragment flight time profile shows the nonreactive Mg ϩ product angular distribution to be isotropic, with an average translational energy release which increases slightly from E t ϳ370Ϯ150 cm Ϫ1 at 332.5 nm to E t ϳ520Ϯ180 cm Ϫ1 at 315 nm. These values are less than 2% of the available energy and are well below statistical expectations. Analogous experiments on MgCD 4 ϩ show the kinetic energy release in the nonreactive channel to be significantly larger for the CD 4 case, ranging from E t ϳ540Ϯ180 cm Ϫ1 at 332.5 nm to E t ϳ830Ϯ200 cm Ϫ1 . These results clearly demonstrate that the dissociation is nonstatistical. Preliminary ab initio potential surface calculations suggest a possible dynamical mechanism to explain these unusual results.

Photodissociation spectroscopy of Mg+–C6H5X (X=H, F, Cl, Br)

“…Such complexes have been examined with equilibrium mass spectrometry, 2 collisioninduced dissociation techniques, 3 radiative association methods, 4 fixed-frequency photodissociation, 5,6 and more recently, with tunable laser photoionization and photodissociation spectroscopy. [7][8][9][10][11][12][13][14][15][16][17][18][19] Among the myriad of metal ion-molecule complexes, special attention has been directed to the so-called complexes, in which a metal cation interacts with the electrons of a multiple bond or an aromatic ring. This type of metalligand binding is ubiquitous in organometallic complexes, 1 metal transport through cell membranes, 20 substrate binding at enzyme active sites, 21 and cationic polymerization.…”

Photodissociation spectroscopy of Ca+(C2H4)

“…3,4 For Mg ϩ ͑CH 4 ͒, the broad continuum Mg ϩ (3s→3 p) absorption spectrum is consistent with a large geometry change on excitation and fast dissociation. 3 We observe both nonreactive (Mg ϩ ) and reactive fragmentation products ͑MgH ϩ and MgCH 3 ϩ ͒. The dominant reaction product is the methyl fragment, MgCH 3 ϩ , showing that insertive geometries are important in the reaction.…”

“…3 We observe both nonreactive (Mg ϩ ) and reactive fragmentation products ͑MgH ϩ and MgCH 3 ϩ ͒. The dominant reaction product is the methyl fragment, MgCH 3 ϩ , showing that insertive geometries are important in the reaction. Measurement of the nonreactive (Mg ϩ ) product translational energy release suggests that Mg ϩ kicks impulsively off the extended H-CH 3 bond so that, even when C-H bond breaking does not occur, the nascent CH 4 product is left highly vibrationally and rotationally excited.…”

“…[1][2][3][4][5][6] For example, we have investigated excited state chemical dynamics in weakly bound bimolecular complexes M ϩ ͑CH 4 ͒ ͑with MϭMg and Ca͒. 3,4 For Mg ϩ ͑CH 4 ͒, the broad continuum Mg ϩ (3s→3 p) absorption spectrum is consistent with a large geometry change on excitation and fast dissociation. 3 We observe both nonreactive (Mg ϩ ) and reactive fragmentation products ͑MgH ϩ and MgCH 3 ϩ ͒.…”

Size-dependent reactivity in open shell metal-ion polar solvent clusters: spectroscopic probes of electronic-vibration coupling, oxidation and ionization

“…To our knowledge, ab initio calculations to probe important regions of the potential surfaces for the Sr þ -based reactions have not been carried out. However, Kleiber and co-workers have performed experimental and theoretical studies on related Mg þ systems that probe the nature of the key interactions [124][125][126][127]. Their work shows clearly that conical intersections between excited states and the ground state facilitate dissociation and resultant internal excitation of the fragments.…”