2020-03-22 21:12:19

责任者: Hao Jiang;Sarsa, A.;Murdachaew, G.;Szalewicz, K.;Bacic, Z. 单位: Dept. of Chem., New York Univ., NY, USA 来源出处: Journal of Chemical Physics(J. Chem. Phys. (USA)),2005/12/08,123(22):224313-1 摘要: We present a rigorous theoretical study of the solvation of (HCl)2 and (HF)2 by small (4He)n clusters, with n=1-14 and 30. Pairwise-additive potential-energy surfaces of Hen(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 Hen(HCl)2 (n=1-14 and 30) and Hen(HF)2 (n=1-10) clusters are calculated exactly using the diffusion Monte Carlo (DMC) method. In addition, the interchange-tunneling splittings of Hen(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 Hen(HF)2 clusters, n=1-10 [A. Sarsa et al., Phys. Rev. Lett. 88, 123401 (2002)]. The vibrationally averaged structures of Hen(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 Hen(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 Hen(HF)2 clusters, which show a ~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 关键词: atomic clusters;ground states;helium neutral molecules;hydrogen bonds;hydrogen compounds;molecular configurations;Monte Carlo methods;potential energy surfaces;solvation;tunnelling;vibrational states;(HCl)2;(HF)2;small helium clusters;quantum solvation;hydrogen-bonded dimers;small (4He)n clusters;pairwise-additive potential-energy surfaces;Hen(HCl)2 clusters;Hen(HF)2 clusters;4D potentials;2D potentials;1D He-He potential;minimum-energy geometries;He atoms;quantum-mechanical ground-state energies;vibrationally averaged structures;diffusion Monte Carlo method;DMC method;interchange-tunneling splitting;fixed-node DMC approach;localized helium density;solvent ring rigidity;cluster size;equatorial ring;helium density;dimer solvation;free HCl dimer;solvent effect;interchange-tunneling dynamics;helium nanodroplets;(4He)n;Hen(HCl)2;Hen(HF)2