title: (HCl)2 and (HF)2 in small helium clusters: quantum solvation of hydrogen-bonded dimers.
abstract: We present a rigorous theoretical study of the solvation of (HCl)(2) and (HF)(2) by small ((4)He)(n) clusters, with n=1-14 and 30. Pairwise-additive potential-energy surfaces of He(n)(HX)(2) (X=Cl and F) clusters are constructed from highly accurate four-dimensional (rigid monomer) HX-HX and two-dimensional (rigid monomer) He-HX potentials and a one-dimensional He-He potential. The minimum-energy geometries of these clusters, for n=1-6 in the case of (HCl)(2) and n=1-5 for (HF)(2), correspond to the He atoms in a ring perpendicular to and bisecting the HX-HX axis. The quantum-mechanical ground-state energies and vibrationally averaged structures of He(n)(HCl)(2) (n=1-14 and 30) and He(n)(HF)(2) (n=1-10) clusters are calculated exactly using the diffusion Monte Carlo (DMC) method. In addition, the interchange-tunneling splittings of He(n)(HCl)(2) clusters with n=1-14 are determined using the fixed-node DMC approach, which was employed by us previously to calculate the tunneling splittings for He(n)(HF)(2) clusters, n=1-10 [A. Sarsa et al., Phys. Rev. Lett. 88, 123401 (2002)]. The vibrationally averaged structures of He(n)(HX)(2) clusters with n=1-6 for (HCl)(2) and n=1-5 for (HF)(2) have the helium density localized in an effectively one-dimensional ring, or doughnut, perpendicular to and at the midpoint of the HX-HX axis. The rigidity of the solvent ring varies with n and reaches its maximum for the cluster size at which the ring is filled, n=6 and n=5 for (HCl)(2) and (HF)(2), respectively. Once the equatorial ring is full, the helium density spreads along the HX-HX axis, eventually solvating the entire HX dimer. The interchange-tunneling splitting of He(n)(HCl)(2) clusters hardly varies at all over the cluster size range considered, n=1-14, and is virtually identical to that of the free HCl dimer. This absence of the solvent effect is in sharp contrast with our earlier results for He(n)(HF)(2) clusters, which show a approximately 30% reduction of the tunneling splitting for n=4. A tentative explanation for this difference is proposed. The implications of our results for the interchange-tunneling dynamics of (HCl)(2) in helium nanodroplets are discussed.
authors.affiliation: Department of Chemistry, New York University, New York, NY 10003, USA.
authors.forename: Hao
authors.initials: H
authors.lastname: Jiang
authors.forename: Antonio
authors.initials: A
authors.lastname: Sarsa
authors.forename: Garold
authors.initials: G
authors.lastname: Murdachaew
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Szalewicz
authors.forename: Zlatko
authors.initials: Z
authors.lastname: Bacić
mesh_terms: D002627:Chemistry, Physical / Q000379:methods*; D016000:Cluster Analysis; D019281:Dimerization; D006371:Helium / Q000737:chemistry*; D006851:Hydrochloric Acid / Q000737:chemistry; D006858:Hydrofluoric Acid / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D015233:Models, Statistical; D008968:Molecular Conformation; D009010:Monte Carlo Method; D011336:Probability; D011789:Quantum Theory; D012984:Software; D012997:Solvents / Q000737:chemistry
chemical_list: D012997:Solvents; D006371:Helium; D006859:Hydrogen; D006851:Hydrochloric Acid; D006858:Hydrofluoric Acid