Separation ofortho- andpara-Hydrogen in Van der Waals Complex Formation

Strong ortho/para effects in the vibrational spectrum of Cl-(H2)

The predissociation spectrum of the Cl-35(H₂) complex is measured between 450 and 800 cm^-1 in a multipole radiofrequency ion trap at different temperatures using the FELIX infrared free electron laser. Above a certain temperature, the removal of the Cl^-(p-H₂) para nuclear spin isomer by ligand exchange to the Cl^-(o-H₂) ortho isomer is suppressed effectively, thereby making it possible to detect the spectrum of this more weakly bound complex. At trap temperatures of 30.5 and 41.5 K, we detect two vibrational bands of Cl^-(p-H₂) at 510(1) and 606(1) cm^-1. Using accurate quantum calculations, these bands are assigned to transitions to the inter-monomer vibrational modes (v₁,v₂ ^l₂ ) = (0, 2⁰) and (1, 2⁰), respectively.

Predissociation spectra of the 35Cl-(H2) complex and its isotopologue 35Cl-(D2)

The predissociation spectra of the 35Cl-(H2) and 35Cl-(D2) complexes are determined within an accurate quantum approach and compared to those recently measured in an ionic trap at 8 K and 22 K. The calculations are performed using an existing three-dimensional potential energy surface. A variational approach is used for the accurate quantum calculations of the rovibrational bound states. Several methods are compared for the search and the characterization of the resonant states. A good agreement between the calculated and measured spectra is obtained, despite a slight shift to the red of the calculated spectra. The comparison shows that only the ortho or para contribution is observed in the measured 35Cl-(H2) or 35Cl-(D2) spectrum, respectively. Quantum numbers are assigned to the rovibrational resonant states. It demonstrates that the main features observed in the measured predissociation spectra correspond to a progression in the intermonomer vibrational stretching mode.

Lifetime of Parahydrogen in Aqueous Solutions and Human Blood

Molecular hydrogen has unique nuclear spin properties. Its nuclear spin isomer, parahydrogen (pH2 ), was instrumental in the early days of quantum mechanics and allows to boost the NMR signal by several orders of magnitude. pH2- induced polarization (PHIP) is based on the survival of pH2 spin order in solution, yet its lifetime has not been investigated in aqueous or biological media required for in vivo applications. Herein, we report longitudinal relaxation times (T1 ) and lifetimes of pH2 ( τ P O C ) in methanol and water, with or without O2 , NaCl, rhodium-catalyst or human blood. Furthermore, we present a relaxation model that uses T1 and τ P O C for more precise theoretical predictions of the H2 spin state in PHIP experiments. All measured T1 values were in the range of 1.4-2 s and τ P O C values were of the order of 10-300 minutes. These relatively long lifetimes hold great promise for emerging in vivo implementations and applications of PHIP.

Ab Initio Characterization of the Electrostatic Complexes Formed by H2 Molecule and Cr(+), Mn(+), Cu(+), and Zn(+) Cations

Equilibrium structures, dissociation energies, and rovibrational energy levels of the electrostatic complexes formed by molecular hydrogen and first-row S-state transition metal cations Cr(+), Mn(+), Cu(+), and Zn(+) are investigated ab initio. Extensive testing of the CCSD(T)-based approaches for equilibrium structures provides an optimal scheme for the potential energy surface calculations. These surfaces are calculated in two dimensions by keeping the H-H internuclear distance fixed at its equilibrium value in the complex. Subsequent variational calculations of the rovibrational energy levels permits direct comparison with data obtained from equilibrium thermochemical and spectroscopic measurements. Overall accuracy within 2-3% is achieved. Theoretical results are used to examine trends in hydrogen activation, vibrational anharmonicity, and rotational structure along the sequence of four electrostatic complexes covering the range from a relatively floppy van der Waals system (Mn(+)···H2) to an almost a rigid molecular ion (Cu(+)···H2).

Hydrogen storage by physisorption on dodecahydro-closo-dodecaboranes

Hydrogen physisorption on dodecahydro-closo-dodecaborane units is studied using ab initio quantum chemical calculations based on Møller-Plesset perturbation theory. After adding zero-point energy corrections, the adsorption energy due to the charge-quadrupole and the charge-induced dipole interaction is somewhat larger than the more common dispersion interaction with spacer molecules in molecular framework compounds. Furthermore, the energy landscape on the surface of the near-spherical B12H12(2-) permits considerable residual dynamics with corresponding configurational entropy that releases partly the requirements on the magnitude of the adsorption energy. If it can be made fully accessible in an open architecture the system promises an enormous storage capacity. An experimental test for Cs2B12H12 dispersed in the cages of a dealuminated faujasite zeolite amends the theoretical study.

Longitudinal Nuclear Spin Relaxation of Ortho- and Para-Hydrogen Dissolved in Organic Solvents

The longitudinal relaxation time of ortho-hydrogen (the spin isomer directly observable by NMR) has been measured in various organic solvents as a function of temperature. Experimental data are perfectly interpreted by postulating two mechanisms, namely intramolecular dipolar interaction and spin-rotation, with activation energies specific to these two mechanisms and to the solvent in which hydrogen is dissolved. This permits a clear separation of the two contributions at any temperature. Contrary to the self-diffusion coefficients at a given temperature, the rotational correlation times extracted from the dipolar relaxation contribution do not exhibit any definite trend with respect to solvent viscosity. Likewise, the spin-rotation correlation time obeys Hubbard's relation only in the case of hydrogen dissolved in acetone-d6, yielding in that case a spin-rotation constant in agreement with literature data. Concerning para-hydrogen, which is NMR-silent, the only feasible approach is to dissolve para-enriched hydrogen in these solvents and to follow the back-conversion of the para-isomer into the ortho-isomer. Experimentally, this conversion has been observed to be exponential, with a time constant assumed to be the relaxation time of the singlet state (the spin state of the para-isomer). A theory, based on intermolecular dipolar interactions, has been worked out for explaining the very large values of these relaxation times which appear to be solvent-dependent.