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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2016)

Molecular crystals occur in a variety of chemically relevant problems, including pharmaceuticals and organic semi-conductor materials. There has been much interest in developing computational models which can predict crystal structures and properties accurately and with reasonable computational expense. One such model, hybrid many-body interaction (HMBI), fragments a chemical system into monomer, dimer, and many-body interactions, each of which may be handled using a different level of theory. HMBI has been used to predict crystal structures, lattice energies, and relative polymorph stability, particularly in cases where other methods such as periodic density functional theory (DFT) have struggled.

This dissertation extends the HMBI model in two important ways. First, the computational cost of these calculations is significantly reduced by the development and implementation of an algorithm to exploit space group symmetry. This algorithm reduces the number of monomer and dimers calculations that need to be performed by eliminating symmetrically equivalent ones. Exploitation of space group symmetry provides additional computational savings during a crystal geometry optimization by reducing the number of degrees of freedom that need to be optimized, which tends to decrease the number of optimization steps required to reach convergence.

Second, the ability to predict molecular crystal structures and properties at finite temperature is developed by coupling the HMBI model with the quasi-harmonic approximation. Traditional approaches either neglect temperature or approximate it with a harmonic vibrational model. However, molecular crystals expand appreciably with temperature and this expansion has significant impacts on crystal properties. Typically, as crystals expand, the lattice energy weakens and the phonon modes soften. Neglecting this expansion causes thermochemical properties such as enthalpy and entropy to be overestimated near room temperature. The quasi-harmonic HMBI model is demonstrated to predict temperature-dependent molar volumes, thermochemistry, and mechanical properties in excellent agreement with experiment for several small-molecule crystals---carbon dioxide, ice, acetic acid, and imidazole. These developments also pave the way toward computational prediction of molecular crystal phase diagrams as a function of temperature and pressure. Preliminary results examining the high-pressure phase diagram of carbon dioxide are presented.

    Cover page: Predicting Structures and Thermodynamic Properties for Molecular Crystals at Finite Temperatures
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2016)
    Molecular crystals expand appreciably upon heating due to both zero-point and thermal vibrational motion, yet this expansion is often neglected in molecular crystal modeling studies. Here, a quasi-harmonic approximation is coupled with fragment-based hybrid many-body interaction calculations to predict thermal expansion and finite-temperature thermochemical properties in crystalline carbon dioxide, ice Ih, acetic acid and imidazole. Fragment-based second-order Möller-Plesset perturbation theory (MP2) and coupled cluster theory with singles, doubles and perturbative triples [CCSD(T)] predict the thermal expansion and the temperature dependence of the enthalpies, entropies and Gibbs free energies of sublimation in good agreement with experiment. The errors introduced by neglecting thermal expansion in the enthalpy and entropy cancel somewhat in the Gibbs free energy. The resulting ∼ 1-2 kJ mol(-1) errors in the free energy near room temperature are comparable to or smaller than the errors expected from the electronic structure treatment, but they may be sufficiently large to affect free-energy rankings among energetically close polymorphs.
      Cover page: How important is thermal expansion for predicting molecular crystal structures and thermochemistry at finite temperatures?
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2016)
      Molecular crystal structures, thermodynamics, and mechanical properties can vary substantially with temperature, and predicting these temperature-dependencies correctly is important for many practical applications in the pharmaceutical industry and other fields. However, most electronic structure predictions of molecular crystal properties neglect temperature and/or thermal expansion, leading to potentially erroneous results. Here, we demonstrate that by combining large basis set second-order Møller-Plesset (MP2) or even coupled cluster singles, doubles, and perturbative triples (CCSD(T)) electronic structure calculations with a quasiharmonic treatment of thermal expansion, experimentally observable properties such as the unit cell volume, heat capacity, enthalpy, entropy, sublimation point and bulk modulus of phase I crystalline carbon dioxide can be predicted in excellent agreement with experiment over a broad range of temperatures. These results point toward a promising future for ab initio prediction of molecular crystal properties at real-world temperatures and pressures.
        • Article
        • Peer Reviewed
        UC Riverside Previously Published Works (2017)
        Solid carbon dioxide exhibits a rich phase diagram at high pressures. Metastable phase III is formed by compressing dry ice above ∼10-12 GPa. Phase VII occurs at similar pressures but higher temperatures, and its stability region is disconnected from III on the phase diagram. Comparison of large-basis-set quasi-harmonic second-order Møller-Plesset perturbation theory calculations and experiment suggests that the long-accepted structure of phase III is problematic. The experimental phase III and VII structures both relax to the same phase VII structure. Furthermore, Raman spectra predicted for phase VII are in good agreement with those observed experimentally for both phase III and VII, while those for the purported phase III structure agree poorly with experimental observations. Crystal structure prediction is employed to search for other potential structures which might account for phase III, but none are found. Together, these results suggest that phases III and VII are likely identical.
          Cover page: Theoretical predictions suggest carbon dioxide phases III and VII are identical
          • Article
          • Peer Reviewed
          Chemical Sciences (2021)
          C-term magnetic circular dichroism (MCD) spectroscopy is a powerful method for probing d-d and f-f transitions in paramagnetic metal complexes. However, this technique remains underdeveloped both experimentally and theoretically for studies of U(v) complexes of Oh symmetry, which have been of longstanding interest for probing electronic structure, bonding, and covalency in 5f systems. In this study, C-term NIR MCD of the Laporte forbidden f-f transitions of [UCl6]- and [UF6]- are reported, demonstrating the significant fine structure resolution possible with this technique including for the low energy Γ7 → Γ8 transitions in [UF6]-. The experimental NIR MCD studies were further extended to [U(OC6F5)6]-, [U(CH2SiMe3)6]-, and [U(NC(tBu)(Ph))6]- to evaluate the effects of ligand-type on the f-f MCD fine structure features. Theoretical calculations were conducted to determine the Laporte forbidden f-f transitions and their MCD intensity experimentally observed in the NIR spectra of the U(v) hexahalide complexes, via the inclusion of vibronic coupling, to better understand the underlying spectral fine structure features for these complexes. These spectra and simulations provide an important platform for the application of MCD spectroscopy to this widely studied class of U(v) complexes and identify areas for continued theoretical development.
            Cover page: Near-infrared C -term MCD spectroscopy of octahedral uranium( v ) complexes
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