Processes in the long range interaction domain: Astro- and Surface Chemistry
Résumé
In recent theoretical studies on the H2/Cu system [1] it has been shown that the chemisorption of H2 can be satisfactorily reproduced using an embedding approach of a cluster model consisting of 22 Cu atoms. This suggests that maybe such treatment can also be applied to the more delicate problem of the description of the physisorption domain. Since a good test of the accuracy of the potential energy function is provided by the comparison of experimental and calculated spectroscopic data, we have calculated the ro-vibrational energy levels of a H2 molecule physisorbed on a metallic surface, Ag or Cu(100), using a 5D or 2D potential energy function determined at different levels of approximation and compared them with EELS experiments data[2]. Highly correlated electronic calculations on the 22 Cu cluster model have been performed with the 2012.1 version of the MOLPRO code [3] using the coupled cluster CCSD(T) method. In a second step, periodic calculations have been performed with the VASP code [4] and various functionals and finally the embedding method has been applied within the ONIOM approach. At each step, the comparison is made with experimental data, and we show [5] that only the embedded corrected potential can provide accurate spectroscopic data for the ro-vibrational transitions.
To describe the reactivity of the CN- anion in its 1+ electronic ground state with neutral atoms that can be present in the interstellar media (H, F, Cl, and O, S), we have used highly correlated ab initio wave functions within the UCCSD(T)-F12 approach, to map the potential energy surfaces (PESs). With H atom, the reaction [CN-(1+) + H(2S)] proceeds along the 2+ electronic ground state of HCN- (or HNC-) until the crossing with the X1+ electronic ground state of the neutral form HCN (or HNC), where electron detachment occurs. The process is rather similar with the two halogen atoms F and Cl, with some differences due to the larger electronic affinity of the halogens, making possible the existence of negative FCN- and ClCN- in bent geometries. The reaction of CN- with O and S atoms proceeds via a different process since the lowest electronic state at long distance, the 3 state, does not correlate with the stable ground state of the XCN- anion (X=O or S). This 3 state and its bent components are crossing at medium distance the X1+ ground state of XCN- or XNC-, and at shorter distance, the X2 state of the neutral XCN or XNC where the extra electron can detach. With both O and S atoms, it is shown that the spin-orbit couplings can efficiently lead the [CN- (1+) + O/S (3P)] reaction towards the stable X1+ ground state of XCN- and XNC-. [6] Consequently the CN- anion can be a generator of more complex molecular anions in astrochemistry.