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Against all odds: Carbonic acid molecules were trapped from the gas phase in a solid noble‐gas matrix at <10 K and studied by IR spectroscopy. The 2H and and 13C isotopologues were also examined. Gas‐phase carbonic acid is thought to exist as a 1:10:1 mixture of two monomeric conformers and the cyclic dimer (H2CO3)2. This data is vital in the search for gas‐phase carbonic acid in astrophysical environments.

The variation with the intermolecular distance of features in hydrogen bond (HB) dimers dependent on the electron density ρ(r) are studied in four complexes representative of weak/medium HB interactions. Topological properties, energy densities and integrated atomic properties are obtained with ρ(r) of dimers at B3LYP/6-311++G(d,p) optimized structures obtained upon fully relaxing the geometry of monomers. The dependence of A–H⋯B bond properties on intermolecular R(H⋯B) distances allows to characterize the nature of the interaction as monomers move nearer from infinite separation. At long distances the interaction is only electrostatic while for separations about 1 Å larger than the equilibrium distance Req, quantum effects arising from ρ(r) begin to dominate. In the immediate neighborhood of Req the interaction is mainly led by the stabilization of the H-donor due in turn to energy lowerings in A and B atoms associated to polarization effects. The mutual penetration of electron densities of donor and acceptor monomers provokes a considerable reduction of atomic volumes for H and B atoms which reveals in the form of redistribution rather than transfer of charge. This range of distances exhibits noncovalent bond features but shortly after, when monomers approximate a few tenths of Å below Req, characteristics typical of covalent interactions begin to appear while the rate of change of all the ρ(r)-dependent properties increases rapidly.

The variation with the intermolecular distance of geometries, energies, and other properties dependent on the electron density ρ(r) are studied in three cyclic dimers linked by two hydrogen bonds: formic acid and formamide homodimers and the heterodimer formamide/formic acid complex. Topological features, energy densities and integrated atomic properties provided by AIM theory are calculated with ρ(r) obtained at B3LYP/6-311++G(d,p) optimized geometries for a number of intermonomer distances covering large separations, equilibrium, and short distances. The variation with these distances of properties studied allows to characterize the nature of the interaction in A–H⋯B (A=N, O and B=O) hydrogen bonds. Whereas at large distances the attraction is purely electrostatic, quantum effects associated with redistributions of ρ(r) mainly around H and B atoms dominate the interaction in the neighborhood of equilibrium. Mutual penetration of the electron densities of these atoms leads to considerable reductions of their atomic volumes and associated polarization effects as well as energetic stabilization of atom A. Although the interaction in this range of intermonomer separations displays noncovalent features, when the dimers move at distances shorter than equilibrium, characteristics typical of covalent interactions begin to appear while the systems leave the planar structures presented until then. This work complements our previous study [O. Galvez, P. C. Gomez, and L. F. Pacios, J. Chem. Phys. 115, 11166 (2001)] of dimers with one single hydrogen bond.

Aims. We studied the interaction between CO 2 (guest) and H 2 O (host) molecular ices. Methods. Ices of CO 2 and H 2 O are prepared by four different deposition techniques: sequential deposition (amorphous water ice followed by addition of CO 2 ), co-deposition (both gases added simultaneously), inverse sequential deposition (carbon dioxide ice followed by addition of water) and crystalline sequential deposition (crystalline water ice is prepared first and CO 2 is added afterwards). Samples are deposited at 80 K and are studied by temperature programmed desorption and transmission infrared spectroscopy. Results. Two slightly different varieties of association of CO 2 and H 2 O are revealed from the different spectroscopic properties of the asymmetric stretching band of 12 CO 2 and 13 CO 2 . The two varieties are found to co-exist in some of the samples at 80 K, whereas only the so-called internal CO 2 remains after heating at 105 K. At 80 K carbon dioxide is able to adhere to a crystalline water ice surface. Activation energies for the desorption of CO 2 from amorphous (E d = 20.7 ± 2 kJ mol −1 ) and crystalline (E d = 19.9 ± 2 kJ mol −1 ) water ice are derived from measurements of the sticking of CO 2 as a function of ice temperature. Conclusions. These findings may have implications for the study of icy bodies of the Solar System.