Gas phase dissociation energies of saturated AH_n^.+ radical cations and AH_n neutrals (A = Li-F, Na-Cl): Dehydrogenation, deprotonation, and formation of AH_n-2^.+ - H₂ complexes

The dissociation energies corresponding to the two possible A-H cleavages of AHn.+ (A = Li-F and Na-Cl) radical cations (loss of a H+ and loss of a H.) have been computed at the CCSD(T)/ 6-311++G(3df,2pd) level of theory and compared to those of their neutral precursors. Removing an electron from AHn decreases dramatically its deprotonation energy, especially for the AH4.+ molecules (CH4.+ and SiH4.+), which become one of the most acidic species of the row, their acid character being only exceeded by FH.+ and ClH.+, respectively. However, dehydrogenation energies only decrease for the systems on the left side of the row (up to CH4.+ and SiH4.+) for which the electron is removed from a A-H bonding orbital. Nevertheless, the loss of hydrogen is the more favorable cleavage in all cases except FH.+. Ionization of SiH4 leads to a Jahn-Teller distorted structure that corresponds to a SiH2.+ - H2 complex. Other AHn-2.+ - η2H2 complexes in the doublet spin state have also been found to be stable for A = Be, Mg, Al, and P, the hydrogen molecule complexes being more stable than their corresponding AHn.+ radical cations, for Be, Mg, and Al.