Abstract
Explicitly correlated coupled cluster theory at the CCSD(T)-F12x level (T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys.127, 221106, 2007) has been employed to study structures and vibrations of complexes of type c-C(3)H(3)(+)·L and H(2)C(3)H(+)·L (L = Ne, Ar, N(2), CO(2), and O(2)). Both cations have different binding sites, allowing for the formation of weak to moderately strong hydrogen bonds as well as "C-bound" or "π-bound" structures. In contrast to previous expectations, the energetically most favourable structures of all H(2)C(3)H(+)·L complexes investigated are "C-bound", with the ligand bound to the methylenic carbon atom. The theoretical predictions enable a more detailed interpretation of infrared photodissociation (IRPD) spectra than was possible hitherto. In particular, the bands observed in the range 3238-3245 cm(-1) (D. Roth and O. Dopfer, Phys. Chem. Chem. Phys.4, 4855, 2002) are assigned to essentially free acetylenic CH stretching vibrations of the propargyl cation in "C-bound" H(2)C(3)H(+)·L complexes.
Highlights
Complexes between a cation containing hydrogen atoms and an atomic or molecular ligand may form hydrogen bonds (H-bonds) with vastly varying binding energies
A well-known example of very strong H-bonds is provided by the ‘‘Zundel-cation’’, H5O+ 2, where the proton is shared between two water molecules[1,2] and the equilibrium dissociation energy De for dissociation into fragments H3O+ and H2O is as high as 143 kJ molÀ1.3 On the other hand, much weaker H-bonds exist between CnH+ m ions and ligands without permanent electric dipole moments such as rare-gas (Rg) atoms or important atmospheric molecules like N2, O2, or CO2
At Rinter = 3.0 A, already 55% or 47% of the equilibrium dissociation energies are obtained for L = N2 or L = CO2, respectively
Summary
Complexes between a cation containing hydrogen atoms and an atomic or molecular ligand may form hydrogen bonds (H-bonds) with vastly varying binding energies. A well-known example of very strong H-bonds is provided by the ‘‘Zundel-cation’’, H5O+ 2 , where the proton is shared between two water molecules[1,2] and the equilibrium dissociation energy De for dissociation into fragments H3O+ and H2O is as high as 143 kJ molÀ1.3 On the other hand, much weaker H-bonds exist between CnH+ m ions and ligands without permanent electric dipole moments such as rare-gas (Rg) atoms or important atmospheric molecules like N2, O2, or CO2. Already dimers composed of simple carbocations and neutral atomic or molecular ligands (CnH+ m ÁL) may exhibit complex potential energy surfaces (PESs) with multiple minima of comparable energies. Experimental work by Lossing[9] found it to be 1.1 Æ 0.1 eV higher in energy than c-C3H+ 3
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