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The [FHCL]- Molecular Anion: Structural Aspects, Global Surface, and Vibrational Eigenspectrum
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| Journal of Chemical Physics |
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The [FHCl]- molecular anion has been investigated in detail by means of state-of-the-art ab initio electronic structure methods, including restricted Hartree-Fock (RHF), Møller-Plesset perturbation theory (MP2-MP4), and coupled-cluster and Brueckner methods incorporating various degrees of excitation [CCSD,CCSD(T), BD, BD(T), and BD(TQ)]. The one-particle Gaussian basis sets ranged in quality from F[6s4p2d], Cl[10s7p2d], and H[4s2p] tp F[18s13p6d4f], Cl[20s14p7d5f], and H[8s3p2d1f]. The first phase of the investigation focused on the prediction of thermochemical, spectroscopic, and bonding properties of [FHCl]- and the chemical interpretation thereof. The final proposals for the geometric structure and binding energy of the complex and r sub e (H-F) = 0.963 plus or minus 0.000e Å, R sub e (H-Cl) + 1.925 plus or minus 0.015 Å, and D sub 0 (HF+Cl-) + 21.8 plus or minus 0.4 kcal mol superscript -1. A Morokuma decomposition of the ion-molecule bonding gave the following electrostatic (ES), polarization (PL), exchange repulsion (EX), dispersion (DISP), and charge-transfer plus higher-order mixing (CT+MIX components of the vibrationless complexation energy: -27.3 (ES), -5.2 (PL), +18.3 (EX), -4.5 (DISP), and -5.0 (CT+MIX) kcal mol-1. The second phase of the work involved the construction of a CCSD global surface from 208 and 228 energy points for linear and bent conformations, respectively, these being fit to rsm errors of only 3.9 and 9.3 cm -1, respectively, below 8000 cm-1. The surface was represented by a flexible analytic form which reproduces the quartic force field at equilibrium, exhibits the proper asymptotic properties, and is generally applicable to ion-molecule systems. The final phase of the study entailed the determination of converged J=0 and J=1 variational eigenstates of the [FHCl]- surface to near the HF+Cl- dissociation threshold by employing Jacobi coordinates and vibrational configuration interaction expansions in terms of natural models. The fundamental vibrational frequencies given by the analysis were v1 = 247, v2 = 876, and v3= 2884 cm-1. The complete vibrational eigenspectrum was then analyzed in terms of several contemporary dynamical issues, including vibrational adiabaticity, anharmonic resonances, densities of high-lying states, and signatures of quantum ergodicity.
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