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1. Using DFT to understand the surface-adsorbate interaction between N-Heterocyclic carbenes and Pd/Cu(111) single atom alloys

   Simpson, S (August 2025, American Chemical Society)

   We explore the charge transfer events four different classes of N-heterocyclic carbenes (NHCs) with a Pd/Cu(111) single atom alloy surface. We provide a frontier molecular orbital approach to understand similarities and differences between these systems. We demonstrate that this approach can be applied to the PDOS, COHP, and molecular charges to better understand the surface-adsorbate system. Density of This work emphasizes the composition- and geometry-dependent nature of NHC adsorption on SAAs and provides insights into tailoring these systems for catalytic and storage applications. 

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2. The Search for Molecular Corks Beyond Carbon Monoxide: A Quantum Mechanical Study of N-Heterocyclic Carbene Adsorption on Pd/Cu(111) and Pt/Cu(111)

   Simpson, Scott (March 2022, ACS Spring 2022)

   Periodic Density Functional Theory calculations reveal the potential application of 10 imidazole based N-heterocyclic carbenes (NHCs) to behave as “molecular corks” for hydrogen storage on single atom alloys, comprised of Pd/Cu(111) or Pt/Cu(111). Calculations show that functionalizing the NHC with different electron withdrawing/donating functional groups results in different binding energies of the NHC with the alloy surfaces. The results are compared to DFT calculations of carbon monoxide bound to these alloys. The Huynh electronic parameter (HEP) is calculated for several simple imidazole NHCs to gauge σ-donor ability, while Se-NMR and P-NMR calculations of selenourea derivatives and carbene-phosphinidene adducts, respectively, have been utilized to gauge π-acidity of the NHCs. It is demonstrated that consideration of both σ and π donating/accepting ability must be considered when predicting the surface-adsorbate binding energy. It was found that electron withdrawing groups tend to weaken the NHC-surface interaction while electron donating substituents tend to strengthen the interaction. 

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3. Search for molecular corks beyond carbon monoxide: A quantum mechanical study of N-heterocyclic carbene adsorption on Pd/Cu(111) and Pt/Cu(111) single atom alloys

   Simpson, Scott (December 2021, Pacifichem 2021)

   Periodic Density Functional Theory calculations reveal the potential application of 10 imidazole based N-heterocyclic carbenes to behave as “molecular corks” for hydrogen storage on single atom alloys, comprised of Pd/Cu(111) or Pt/Cu(111). Calculations show that functionalizing the NHC with different electron withdrawing/donating functional groups results in different binding energies of the NHC with the alloy surfaces. The results are compared to DFT calculations of carbon monoxide bound to these alloys. The Huynh electronic parameter (is calculated for several simple imidazole NHCs to gauge σ-donor ability, while Se-NMR of and P-NMR calculations of selenourea derivatives and carbene-phosphinidene adducts, respectively, have been utilized to gauge π-acidity of the NHCs. It is demonstrated that consideration of both σ and π donating/accepting ability must be considered when predicting the surface-adsorbate binding energy. It was found that electron withdrawing groups tend to weaken the NHC-surface interaction while electron withdrawing substituents tend to strengthen the interaction. 

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4. Understanding bonding of N-heterocyclic carbenes on Pd/Cu(111) single atom alloys via non-local density functional theory to store hydrogen via the molecular corking effect

   Simpson, S. (March 2024, ACS Spring 2024)

   N-heterocyclic carbenes(NHCs) have garnered the attention of material scientists and chemists for their tunable electronic properties. NHCs anchored to surfaces have attractive features and may provide new applications that traditional self-assembled monolayers (SAMs) have yet to be employed. In-fact, NHCs have been utilized to functionalize surfaces to tune reactivity and/or selectivity. However, the underlying mechanisms to control the surface-adsorbate interaction is still in its infancy, especially for SAAs. Herein we utilize periodic non-local density functional theory (DFT) calculations to better understand how changing the NHC backbone influences the bonding between the surface and the adsorbate with the end goal to utilize a relatively new mechanism to store hydrogen. 

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5. Tools for Understanding Molecular Orbitals Interactions of Molecules on Surfaces – Density Functional Theory Calculations of H2 Adsorbed on Cu(111) and Pd/Cu(111)

   https://doi.org/10.26434/chemrxiv-2025-k0znj  

   Simpson, Scott; Mulvey, Devin (December 2025, ChemRxiv)

   We provide a summary of contemporary computational tools utilized in the study of adsorbate interactions with solid-state materials from the perspective of a quantum chemist. This work contains a focused theoretical primer of interactions between the molecular orbitals of an adsorbate and the electronic bands of a solid as well as a review of the promising methodologies for disentangling these contributions. We apply these tools in a methodological fashion to density functional theory (DFT) calculations of molecular hydrogen (H2), H2 adsorbed to the pristine Cu(111) surface, and H2 adsorbed to a single atom alloy comprised of palladium and copper (Pd/Cu(111)) to provide chemically intuitive explanations of bonding in these systems. 

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