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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Molecular Spectroscopy

Background:

  • Harmonic vibrational frequencies are crucial molecular properties.
  • Relativistic effects can influence electronic structure and molecular properties, particularly for heavy elements.
  • Density Functional Theory (DFT) is a widely used method for electronic structure calculations.

Purpose of the Study:

  • To investigate the impact of relativistic effects on harmonic vibrational frequencies of various hydrides.
  • To assess the accuracy and robustness of DFT calculations using the Dirac-Coulomb Hamiltonian for vibrational frequency predictions.
  • To quantify the contribution of relativistic effects, including spin-orbit interactions, to vibrational frequencies.

Main Methods:

  • Four-component Dirac-Coulomb Hamiltonian within Density Functional Theory (DFT).
  • Finite difference method for calculating vibrational frequencies from molecular energies.
  • Systematic testing of basis sets, exchange-correlation functionals, and numerical differentiation step lengths.
  • Calculations performed on 15 hydrides (H2X, XH3, XH4) and HC≡CPbH3.

Main Results:

  • Relativistic effects were found to be noticeable for heavier hydrides (e.g., H2Te, H2Po, SbH3, BiH3, SnH4, PbH4).
  • The influence of relativity on vibrational frequencies increases with the mass of the central atom and is more significant for higher frequency modes.
  • Spin-orbit effects were a minor contributor to total relativistic effects, except for H2Te and H2Po.
  • In HC≡CPbH3, relativity primarily affected modes with substantial lead atom displacements.

Conclusions:

  • Relativistic quantum chemical calculations are essential for accurate prediction of harmonic vibrational frequencies in heavy element compounds.
  • The employed DFT method is robust and reliable for studying relativistic effects on molecular vibrations.
  • Understanding these relativistic contributions is vital for interpreting spectroscopic data and predicting chemical behavior of heavy element hydrides.