Infrared Spectra of Mass-Selected Mg⁺−H₂ and Mg⁺−D₂ Complexes

Abstract

Rotationally resolved infrared spectra of Mg(+)-H(2) and Mg(+)-D(2) are recorded in the H-H (4025-4080 cm(-1)) and D-D (2895-2945 cm(-1)) stretch regions by monitoring Mg(+) photofragments. The nu(HH) and nu(DD) transitions of Mg(+)-H(2) and Mg(+)-D(2) are red-shifted by 106.2 +/- 1.5 and 76.0 +/- 0.1 cm(-1) respectively from the fundamental vibrational transitions of the free H(2) and D(2) molecules. The spectra are consistent with a T-shaped equilibrium structure in which the Mg(+) ion interacts with a slightly perturbed H(2) or D(2) molecule. From the spectroscopic constants, a vibrationally averaged intermolecular separation of 2.716 A (2.687 A) is deduced for the ground state of Mg(+)-H(2) (Mg(+)-D(2)), decreasing by 0.037 A (0.026 A) when the H(2) (D(2)) subunit is vibrationally excited.

Mixing Laser Spectroscopy and Mass Spectrometry-Infrared Spectra of Metal Cation–Hydrogen Complexes

“…23,24 In contrast, recent work suggests that electrostatic, induction and orbital effects all play a role in deciding the magnitude of the red-shift with the importance of orbital interactions depending critically on the electronic structure of the metal cation. 16 Predicting the infrared spectra of the M + -H 2 complexes from first principles is a demanding, though feasible, enterprise. It should be remarked that it is not possible to reproduce the finer details of the spectra assuming that the complexes have rigid t-shaped structures.…”

“…1 these experimental energetic data have been supplemented by ab initio and Dft calculations which predict structural and energetic properties that can be compared with the experiment. [14][15][16] Standard ab initio calculations provide a rough guide to the properties of the M + -H 2 complexes. However, large amplitude vibrational motions play an important role due to the low mass of the H 2 sub-unit and the relatively weak nature of the intermolecular bond.…”

Rotationally resolved infrared spectrum of the Na+-D2 complex: An experimental and theoretical study

“…6 and 8), distinguishing them from other M + -H 2 and M + -D 2 complexes that we have investigated, all of which exhibit a vibrationally induced decrease in R 0 . [9][10][11][12][13][14] The IR spectrum of Na + -D 2 provides no direct information on the D-D bond length, which for a rigid complex would be related to the A rotational constant. For a rigid, T-shaped, planar molecule, A could be determined from B and C via the inertial defect ( = 1/C − 1/B − 1/A = 0).…”

“…Ultimately, the information derived from this study should contribute to a comprehensive picture of bonds between metal cations and dihydrogen by complementing earlier gas-phase spectroscopic and theoretical studies of other M + -H 2 and M + -D 2 complexes (M + = Li + , B + , Na + , Mg + , Al + , Cr + , Mn + , Zn + , and Ag + ). [6][7][8][9][10][11][12][13][14][15] A significant aspect of our investigations is the capacity to record rotationally resolved infrared spectra, providing quantitative structural information.…”

Spectroscopic Study of the Benchmark Mn+−H2 Complex

“…Because the H-H stretch (ν HH ) frequency of Mn + -H 2 is only slightly reduced compared with the free H 2 molecule (4161 cm -1 ), excitation of this vibrational mode is sufficient to sever the weak Mn 22 The IR photodissociation experiments were conducted using a home-built tandem mass spectrometer that has been used for studies of other similar M + -H 2 complexes (M ) Li, B, Na, Mg, and Al). [25][26][27][28][29][30] Briefly, the Mn + -H 2 complexes were generated by laser vaporization of a 4 mm diameter manganese rod adjacent to a supersonic expansion of pure H 2 gas. The central portion of the supersonic expansion was selected by a skimmer and passed into a quadrupole mass filter that was tuned to the mass of Mn + -H 2 .…”