The Na+–H2 cation complex: Rotationally resolved infrared spectrum, potential energy surface, and rovibrational calculations

Electronic Spectrum and Photodissociation Chemistry of the 1-Butyn-3-yl Cation, H3CCHCCH. J Phys Chem A 2020;124:2366-2371. [PMID: 32119779 DOI: 10.1021/acs.jpca.9b11810]

The B̃¹A' ← X̃¹A' electronic spectra of the 1-butyn-3-yl cation (H₃CCHCCH⁺) and the H₃CCHCCH⁺-Ne and H₃CCHCCH⁺-Ar complexes are measured using resonance enhanced photodissociation over the 245-285 nm range, with origin transitions occurring at 35936, 35930, and 35928 cm^-1, respectively. Vibronic bands are assigned based on quantum chemical calculations and comparison of the spectra with those of the related linear methyl propargyl (H₃C₄H₂⁺) and propargyl (H₂C₃H⁺) cations. The photofragment ions are C₂H₃⁺ (major) and C₄H₃⁺ (minor), with the preference for C₂H₃⁺ consistent with master equation simulations for a mechanism that involves rapid electronic deactivation and dissociation on the ground state potential energy surface.

Complexes of Alkali Metal Cations and Molecular Hydrogen: Potential Energy Surfaces and Bound States

Complexes between metal cations and molecular hydrogen are systems quite amenable for precise spectroscopic and theoretical studies, and at the same time, they are relevant for applications in hydrogen storage and astrochemistry. In this work, we report new intermolecular potential energy surfaces and rovibrational states calculations for complexes involving molecular hydrogen and alkaline metal cations, M⁺-H₂ (M⁺ = Na⁺, K⁺, Rb⁺, Cs⁺). The intermolecular potentials, formulated in an internally consistent way to emphasize differences in the properties of the systems, are represented by simple analytical expressions whose parameters have been optimized from comparison with accurate ab initio calculations. Properties of the low-lying bound states-binding energies, frequencies, and rotational constants-are compared with previous measurements or computations and an overall good agreement is achieved, supporting the reliability of the present formulation. Variations of these properties as a function of the cation size and isotopic substitution, with a proper sequence of ortho and para rotational levels, are also discussed.

Nuclear motion in the σ-bound regime of metal-H₂ complexes: [Mg(H₂)(n=1-6)]²⁺

The dynamic, quantum structure of [Mg(H2)(n=1-6)](2+)complexes is investigated via ab initio path integral molecular dynamics simulations. These complexes represent the strong, σ-complex regime of metal-H2 interactions and are representative of bonding motifs found in metal-organic frameworks. Significant nuclear motion within the coordination sphere is observed, even though the ligands remain largely intact. Quantum effects are found to be important in the H-H and metal-H2 stretch coordinates, but the remaining motion in the molecule is well represented by classical simulations. Nearly free rotation of the dihydrogen moiety is observed in all complexes. Statistical averages and distributions of structural parameters are found to deviate nontrivially from the same parameters in static, equilibrium structures.

Test of the interaction potential energy for Na⁺-H₂ by gaseous ion transport data

Transport properties of Na(+) ions in gaseous hydrogen are calculated using the recently developed "beyond Monchick-Mason" (BMM) approximation and an ab initio Na(+)-H2 potential energy surface. Good agreement with the experimental data on the reduced mobility and longitudinal diffusion coefficient proves the accuracy of the surface and the adequacy of the BMM method, allowing for its optimal parameterization.

Attaching molecular hydrogen to metal cations: perspectives from gas-phase infrared spectroscopy

In this perspective article we describe recent infrared spectroscopic investigations of mass-selected M(+)-H(2) and M(+)-D(2) complexes in the gas-phase, with targets that include Li(+)-H(2), B(+)-H(2), Na(+)-H(2), Mg(+)-H(2), Al(+)-H(2), Cr(+)-D(2), Mn(+)-H(2), Zn(+)-D(2) and Ag(+)-H(2). Interactions between molecular hydrogen and metal cations play a key role in several contexts, including in the storage of molecular hydrogen in zeolites, metal-organic frameworks, and doped carbon nanostructures. Arguably, the clearest view of the interaction between dihydrogen and a metal cation can be obtained by probing M(+)-H(2) complexes in the gas phase, free from the complicating influences of solvents or substrates. Infrared spectra of the complexes in the H-H and D-D stretch regions are obtained by monitoring M(+) photofragments as the excitation wavelength is scanned. The spectra, which feature full rotational resolution, confirm that the M(+)-H(2) complexes share a common T-shaped equilibrium structure, consisting essentially of a perturbed H(2) molecule attached to the metal cation, but that the structural and vibrational parameters vary over a considerable range, depending on the size and electronic structure of the metal cation. Correlations are established between intermolecular bond lengths, dissociation energies, and frequency shifts of the H-H stretch vibrational mode. Ultimately, the M(+)-H(2) and M(+)-D(2) infrared spectra provide a comprehensive set of benchmarks for modelling and understanding the M(+)···H(2) interaction.

Mixing laser spectroscopy and mass spectrometry-infrared spectra of metal cation-hydrogen complexes

We describe recent experiments in which mass spectrometry and laser spectroscopy are combined to characterize Li(+)-H(2), Na(+)-H(2), B(+)-H(2), and Al(+)-H(2) complexes in the gas-phase. The infrared spectra, which feature full resolution of rotational sub-structure, are recorded by monitoring M(+) photo fragments as the infrared wavelength is scanned. The spectra deliver detailed information on the way in which a hydrogen molecule is attached to a metal cation including the intermolecular separation, the force constant for the intermolecular bond and the H-H stretching frequency. The complexes all possess T-shaped equilibrium geometries and display a clear correlation between the length and force constant of the intermolecular bond and the dissociation energy. In contrast, the data do not support any straight forward correlation between the frequency shift for the H-H stretch mode and the dissociation energy.

Spectroscopic study of the benchmark Mn+-H2 complex

We have recorded the rotationally resolved infrared spectrum of the weakly bound Mn+-H2 complex in the H-H stretch region (4022-4078 cm(-1)) by monitoring Mn+ photodissociation products. The band center of Mn+-H2, the H-H stretch transition, is shifted by -111.8 cm(-1) from the transition of the free H2 molecule. The spectroscopic data suggest that the Mn+-H2 complex consists of a slightly perturbed H2 molecule attached to the Mn+ ion in a T-shaped configuration with a vibrationally averaged intermolecular separation of 2.73 A. Together with the measured Mn+...H2 binding energy of 7.9 kJ/mol (Weis, P.; et al. J. Phys. Chem. A 1997, 101, 2809.), the spectroscopic parameters establish Mn+-H2 as the most thoroughly characterized transition-metal cation-dihydrogen complex and a benchmark for calibrating quantum chemical calculations on noncovalent systems involving open d-shell configurations. Such systems are of possible importance for hydrogen storage applications.

Experimental and theoretical study of rotationally inelastic collisions of CN(A2pi) with N2

Optical-optical double resonance was employed to study rotational energy transfer in collisions of selected rotational/fine-structure levels of CN(A2pi, v = 3) with N2. The CN radical was generated by 193 nm photolysis of BrCN in a slow flow of N2 at total pressures of 0.2-1.4 Torr. Specific fine-structure lambda-doublet levels of CN(A2pi, v = 3) were prepared by pulsed dye laser excitation on isolated lines in the CN A-X (3,0) band, while the initially excited and collisionally populated levels were observed after a short delay by laser-induced fluorescence in the B-A (3,3) band. Total removal rate constants for specified rotational/fine-structure levels involving total angular momentum J from 4.5 to 12.5 were determined. These rate constants decrease with increasing J, with no obvious dependence on the fine-structure/lambda-doublet label. State-to-state relative rate constants were determined for several initial levels and show a strikingly strong collisional propensity to conserve the fine-structure/lambda-doublet label. Comparison is made with the results of quantum scattering calculations based on potential energy surfaces averaged over the orientation of the N2 molecule. Reasonable agreement is found with experimentally determined total removal rate constants. However, the computed state-to-state rate constants show a stronger propensity for fine-structure and lambda-doublet changing transitions. These differences between experiment and theory could be due to the neglect of the N2 orientation and the correlation of the CN and N2 angular motions.

Infrared spectra of mass-selected Mg+-H2 and Mg+-D2 complexes

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.