Systematic analysis of electronic barrier heights and widths for concerted proton transfer in cyclic hydrogen bonded clusters: (HF)n, (HCl)n and (H2O)n where n = 3, 4, 5.

The MP2 and CCSD(T) methods are paired with correlation consistent basis sets as large as aug-cc-pVQZ to optimize the structures of the cyclic minima for (HF)n, (HCl)n and (H2O)n where n = 3-5, as well as the corresponding transition states (TSs) for concerted proton transfer (CPT). MP2 and CCSD(T) harmonic vibrational frequencies confirm the nature of each minimum and TS. Both conventional and explicitly correlated CCSD(T) computations are employed to assess the electronic dissociation energies and barrier heights for CPT near the complete basis (CBS) limit for all 9 clusters. Results for (HF)n are consistent with prior studies identifying Cnh and Dnh point group symmetry for the minima and TSs, respectively. Our computations also confirm that CPT proceeds through Cs TS structures for the C1 minima of (H2O)3 and (H2O)5, whereas the process goes through a TS with D2d symmetry for the S4 global minimum of (H2O)4. This work corroborates earlier findings that the minima for (HCl)3, (HCl)4 and (HCl)5 have C3h, S4 and C1 point group symmetry, respectively, and that the Cnh structures are not minima for n = 4 and 5. Moreover, our computations show the TSs for CPT in (HCl)3, (HCl)4 and (HCl)5 have D3h, D2d, and C2 point group symmetry, respectively. At the CCSD(T) CBS limit, (HF)4 and (HF)5 have the smallest electronic barrier heights for CPT (≈15 kcal mol-1 for both), followed by the HF trimer (≈21 kcal mol-1). The barriers are appreciably higher for the other clusters (around 27 kcal mol-1 for (H2O)4 and (HCl)3; roughly 30 kcal mol-1 for (H2O)3, (H2O)5 and (HCl)4; up to 38 kcal mol-1 for (HCl)5). At the CBS limit, MP2 significantly underestimates the CCSD(T) barrier heights (e.g., by ca. 2, 4 and 7 kcal mol-1 for the pentamers of HF, H2O and HCl, respectively), whereas CCSD overestimates these barriers by roughly the same magnitude. Scaling the barrier heights and dissociation energies by the number of fragments in the cluster reveals strong linear relationships between the two quantities and with the magnitudes of the imaginary vibrational frequency for the TSs.

Dissociation Energy of the H2O···HF Dimer.

Even though (H2O)2 and (HF)2 are arguably the most thoroughly characterized prototypes for hydrogen bonding, their heterogeneous analogue H2O···HF has received relatively little attention. Here we report that the experimental dissociation energy ( D0) of this important paradigm for heterogeneous hydrogen bonding is too large by 2 kcal mol-1 or 30% relative to our computed value of 6.3 kcal mol-1. For reference, computational procedures similar to those employed here to compute D0 (large basis set CCSD(T) computations with anharmonic corrections from second-order vibrational perturbation theory) provide results within 0.1 kcal mol-1 of the experimental values for (H2O)2 and (HF)2. Near the CCSD(T) complete basis set limit, the electronic dissociation energy for H2O···HF is ∼4 kcal mol-1 larger than those for (H2O)2 and (HF)2 (∼9 kcal mol-1 for the heterogeneous dimer vs ∼5 kcal mol-1 for the homogeneous dimers). Results reported here from symmetry-adapted perturbation theory computations suggest that this large difference is primarily due to the induction contribution to the interaction energy.

Communication: Gas phase vibrational spectroscopy of the azide-water complex.

The vibrational spectra of the azide-water complex, N3 -(H2O), and its fully deuterated isotopologue are studied using infrared photodissociation (IRPD) spectroscopy (800-3800 cm-1) and high-level ab initio computations. The IRPD spectrum of the H2-tagged complex exhibits four fundamental transitions at 3705, 3084, 2003, and 1660 cm-1, which are assigned to the free OH stretching, the hydrogen-bonded O-H stretching, the antisymmetric N3 stretching, and the water bending mode, respectively. The IRPD spectrum is consistent with a planar, singly hydrogen-bonded structure according to an MP2 and CCSD(T) anharmonic analysis via generalized second-order vibrational perturbation theory. The red-shift of the hydrogen-bonded OH stretching fundamental of 623 cm-1 associated with this structure is computed within 6 cm-1 (or 1%) and is used to estimate the proton affinity of azide (1410 kJ mol-1). Born-Oppenheimer molecular dynamics simulations show that large amplitude motions are responsible for the observed band broadening at cryogenic temperature. Temperature-dependent (6-300 K) IR multiphoton dissociation spectra of the untagged complex are also presented and discussed in the context of spectral diffusion observed in the condensed phase.