Abstract
We examine the interaction between an open-shell chlorine atom and a para-H2 molecule in the region of configuration space that corresponds to a weakly bound Cl–para-H2 van der Waals dimer. By constructing and diagonalizing the Hamiltonian matrix that represents the coupled Cl atom electronic and H2 rotational degrees of freedom, we obtain one-dimensional energy curves for the Cl–para-H2 system in this region of configuration space. We find that the dimer exhibits fairly strong electronic-rotational coupling when the Cl–H2 distance R is close to ; however, this coupling does not modify substantially the positions and depths of the van der Waals wells in the dimer’s curves. An approximation in which the para-H2 fragment is treated in the strict limit thus appears to yield an accurate representation of those states of the weakly bound Cl–para-H2 dimer that correlate with H2 in the limit.
Highlights
Experimental studies [1] of the infrared absorption spectra of solid para-H2 matrices that contain chlorine atoms as substitutional impurities indicate that Cl–H2 interactions raise the transition energy associated with the 2P1/2← 2P3/2 spin-orbit (SO) transitions of the Cl impurities
In the para-H2 matrix, the Cl atom’s SO transition is blue shifted by about 60 cm−1 by Cl–H2 interactions [1], which amounts to a shift of about 5 cm−1 for each of the twelve H2 molecules in the Cl atom’s first solvation shell
For the Ω = 3/2 curve that correlates with the lower SO level of the Cl atom, the pure j = 0 approximation is completely incapable of reproducing the true Cl–H2 interaction for R values below R = 5.5a0; this is the region of configuration space where strong electronic-rotational coupling leads to the avoided crossings shown in Figure 10, 0.14
Summary
Experimental studies [1] of the infrared absorption spectra of solid para-H2 matrices that contain chlorine atoms as substitutional impurities indicate that Cl–H2 interactions raise the transition energy associated with the 2P1/2← 2P3/2 spin-orbit (SO) transitions of the Cl impurities. Similar behavior in the Cl–H2 dimer might raise concerns that the effective Cl–H2 potential energy functions obtained from a pure j = 0 approximation for the H2 molecule would be insufficiently accurate for a quantitative study of the matrix-induced blue shift of the Cl SO transition In such a case, more sophisticated treatments [8] of the H2 molecules’ rotational degrees of freedom might be needed. We find some evidence for moderately strong electronic-rotational coupling in the low-energy repulsive region of the Cl–H2 potential energy surface This coupling does not, substantially change the positions or depths of the van der Waals minima for dimers composed of a Cl atom and a para-H2 molecule.
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