Molecular lanthanide compounds are a computationally addressable guidepost whose insight is critical to deconvoluting the structure-property relationships that inform the design of novel materials. Of the many ways to study molecular lanthanide compounds, one underexplored approach is to examine their properties as adsorbates on surfaces. In this work, we present a combined experimental and computational study of the topography and electronic structure of sub-monolayer europium tris(2,2,6,6-tetramethyl-3,5-heptanedionato), Eu(THD)3, molecules adsorbed on Cu(111) and Au(111) using Scanning Tunneling Microscopy (STM), differential conductance (dI/dV) spectroscopy and mapping, and Density Functional Theory (DFT) calculations. STM topographic images of the adsorbed complex revealed adsorbates whose morphology resembles a 5-petal flower. DFT calculations showed the complex undergoes drastic structural rearrangement when adsorbed on the surfaces and corroborated the 5-petal flower on-surface structure. Calculations also demonstrated a metal-centered reduction of the europium complex from Eu(III) to Eu(II) when adsorbed on Cu(111), contrasting with retention of the trivalent state on Au(111). These differences in oxidation state are reflected in experimental dI/dV spectra, which contain features that are prescribed as having 4f character based on calculated partial density of states (PDOS) for the 4f orbitals. Finally, dI/dV mapping showed ligand states that are in good agreement with theory. This work marks the first example of surface-dependent valence tautomerism for an adsorbed lanthanide compound.

ABSTRACT Valence tautomerizations are prototypical pericyclic rearrangements with broad relevance in (bio)organic chemistry that can exhibit significant contributions from quantum mechanical tunneling (QMT), as demonstrated previously e.g., for the oxepin—benzene oxide equilibrium. Such contributions were evaluated computationally for the valence tautomerizations of heteroepins C 6 H 6 X (X = S, Se, and Te) and thiepin oxides (X = SO or SO 2 ). Ring contraction of the heteroepins becomes increasingly exothermic from X = O to X = Te; however, as an exception to the Bell–Evans–Polanyi principle, the associated activation barriers also increase. Analysis of the intrinsic reaction coordinates reveals that the critical C1–C6 distance in the heteroepins strongly widens within this series, resulting in increasingly wider and higher barriers despite the growing thermodynamic driving force. Consequently, estimated tunneling probabilities decrease dramatically across the series, ruling out tunneling as a significant contributor to the experimentally observed instability of thiepin, selenepin, and tellurepin. In contrast, the valence tautomerizations of thiepin‐ S ‐oxide and thiepin‐ S,S ‐dioxide follow conventional Bell–Evans–Polanyi behavior. The equatorial conformer of thiepin‐ S ‐oxide is expected to not exhibit any appreciable tunneling reactivity, while its axial conformer is projected to undergo rapid ring contraction by heavy‐atom tunneling. The bicyclic tautomer of C 6 H 6 SO 2 is predicted to ring‐expand to the less strained thiepin‐ S,S ‐dioxide with a half‐life suitable for matrix isolation spectroscopy experiments. Overall, this study of the potential role of QMT in the valence tautomerizations of heteroepins and thiepin oxides provides broader insights into how characteristic molecular features influence the intrinsic barrier width of a reaction.

Pure methanol desorbs from pristine Ag(111) surfaces under ultrahigh vacuum conditions at 160 K, limiting its availability in catalysis on this surface. To combat this, we devise how to adsorb methanol at room temperature without additives. The methanol adsorption near ambient pressures yields monosized trimers. We use low-temperature scanning tunneling microscopy and single-molecule manipulation to investigate the binding pattern of these clusters and compare them to hexamers formed at low pressure below the methanol desorption temperature. Supported by density functional theory, we unravel that the low-temperature, low-pressure clusters are hydrogen bonded cyclic hexamers, while the room temperature, high-pressure clusters are stabilized by stronger hydrogen bonds of three methanol molecules to a central chemisorbed oxygen atom. Our study shows that trace amounts of oxygen adatoms formed from methanol at high pressure on the Ag(111) surface may aid heterogeneous methanol chemistry under ambient conditions by increasing the residence time on a catalyst surface.

The high relevance of electron solvation in several branches of physics, chemistry, and environmental science arises from its efficient electron transfer mechanism. The effect of solvated electrons on solvent structure has been considered local and transient due to a lack of real-space studies. An experiment was designed to study the impact of solvated electrons on the ammonia structure while adsorbed to Cu(110) using low-temperature scanning tunneling microscopy with an adjoined femtosecond laser. The enhanced molecular kinetics induced by the solvated electrons are explained using density functional theory and first-principles molecular dynamics. The electrons have a substantially different impact on the kinetics of ammonia within clusters below and above a cluster size threshold, reflecting hydrogen bond rearrangement (mass transport) and hydrogen bond cleavage (desorption), respectively. This size-dependent effect has implications on the efficiency of processes that involve solvated electrons. Altering the solvent structure more than transiently demands the subsequent solvation of two electrons.

The binding and transport of the highly hydrophilic sulfate ion pose a significant challenge. Receptors capable of both strong sulfate binding and efficient transport are rare. Macrocycles c5 , featuring a constrained aromatic amide backbone that convergently locks multiple amide NH and phenyl NH donors, along with five large amide dipoles, create a preorganized, electropositive cavity that binds sulfate in a 1:1 stoichiometry, with affinities ( K a ’s) from 10 8 to 10 13 M −1 in solvents of low to high polarity. The water‐soluble c5c binds sulfate, forming a 4:1 complex in aqueous solution. Macrocycles c5a and c5b with hydrophobic sidechains serve as carriers that mediate efficient sulfate transport across both a bulk chloroform phase and lipid bilayers. With their synthetic tunability, macrocycles c5 can be tailored to media of different properties, yielding effective sulfate binders and carriers with broad applicability.

Zirconium alloys such as zircaloy-4 are used as tritium (T) getter materials in tritium-producing burnable absorber rods (TPBARs) due to their ability to capture T, thereby forming metal hydrides. Developing an understanding of T adsorption onto zircaloy prior to diffusion into the subsurface is relevant for rational tritium getter and TPBAR design, to improve material properties for nuclear applications. Herein, density functional theory calculations revealed the preferred binding sites for T adsorption on Zr(001) and Zr(100). The energy barriers of T transfer, along the surface and from the surface to the subsurface were computed. The adsorption properties of Zr(001) were found to be superior to those of Zr(100). Surface tin impurities were found to strongly repel T. The presence of subsurface and surface tin resulted in higher absorption energy barriers for both the forward and reverse processes. Based on the calculated energy barriers, a surface to surface T diffusion coefficient of 9.53 × 10-10 m2 s-1 is expected for pristine Zr(001). A surface to subsurface T diffusion coefficient on the order of 10-13 m2 s-1 is predicted in pristine Zr, decreasing to 10-19 m2 s-1 for the transfer with a subsurface tin impurity.

Helical aromatic oligoamide foldamers (1a-c) with tunable lengths were computationally examined for their ability to bind selected sugars and sugar alcohols. These helices feature cylindrically shaped inner cavities lined with multiple inward-facing amide carbonyl oxygens acting as hydrogen-bond acceptors, enabling sugar binding via hydrogen bonding. Each of the helical foldamers has an overall dipole moment that increases with the length of the helix. The binding of a guest typically results in a reduction of the overall helix dipole moment within the complex, although there are several exceptions. The strength of host-guest interactions correlated positively with the number of hydrogen bonds formed. Longer helix 1c showed stronger interaction energies (up to -84.45 kcal mol-1), particularly with disaccharides, while shorter helix 1a bound sugars more weakly due to fewer established hydrogen bonds. The helical hosts exhibit structural adaptibility upon binding guests, with host distortion upon binding decreased with increasing helix length. Despite reduced binding energies, the complexes retained binding capability in aqueous environments, demonstrating their viability for aqueous-phase applications. This study underscores the critical roles of helical length and dipole alignment in optimizing sugar binding, providing a theoretical foundation for designing synthetic receptors for sugars and sugar alcohols based on aromatic oligoamide foldamers.

The Cover Feature shows the organic radical molecule α,γ-bisdiphenylene-β-phenylallyl (BDPA), which exhibits long-term stability due in part to its large number of resonance forms. The singly occupied molecular orbital (SOMO, top middle) of the molecule extends only partly into the lone phenyl ring. A radical frontier density (RFD, top right) map highlights the regions with greatest radical density in red. The bottom portion depicts BDPA adsorbed on Cu(100), where it can be deposited at room temperature as single molecules. More information can be found in the Research Article by J. D. Teeter and co-workers (DOI: 10.1002/cphc.202400852).

The on-surface synthesis of various organic compounds relies on the self-assembly and subsequent dissociation of halogen-substituted organic molecules for polymerization and functionalization. Here, we demonstrate that the photolytic disassembly and dissociation of bromobenzene molecules within magic-sized tetramer nanoclusters are influenced by halogen bonding on the Cu(111) surface. We explain this phenomenon using a combination of two-photon photoemission spectroscopy, scanning tunneling microscopy, and density functional theory computations. The interactions that determine the preferred cluster sizes of trimers to pentamers arise from a combination of halogen bonding and weak hydrogen bonding. Surface adsorption enhances halogen bonding while weakening the weak hydrogen bonds in the nanoclusters. The most stable tetramers are constructed from a trimer foundation that employs halogen-3 synthons with an exterior fourth molecule. The exterior bromobenzene in this tetramer may detach from the trimer core cluster or undergo dehalogenation before the other bromobenzene molecules under irradiation. The work function of the Cu(111) surface is significantly decreased by the presence of a tetramer. This reduction facilitates the photodissociation of bromobenzene by allowing electrons from the surface to occupy the antibonding molecular orbitals associated with the C-Br bond. The work function increases steadily as smaller clusters and dissociated bromobenzene (phenyl and Br) are formed photolytically. The molecules of the trimers are not photodissociated because the energy levels of the C-Br antibonding orbitals in the trimer core are notably higher in energy than those of the exterior molecule in the tetramer. Our study highlights the potential of weak noncovalent interactions to guide selective photolytic reactions on surfaces.

The adsorption of ions on metallic surfaces is a powerful method to alter their electronic structure and thus tune their reactivity. A prominent example is chlorine on Ag(111). We investigate chlorine created by the room-temperature adsorption of chloroform on Ag(111) at supersaturation and the structures it forms from individual monomers to a full layer by using low-temperature scanning tunneling microscopy. The data is supplemented by temperature-programmed desorption and X-ray photoelectron spectroscopy after low-temperature adsorption under ultrahigh-vacuum conditions. Data interpretation is supported by density functional theory (DFT) calculations that account for dispersion forces. At low chlorine coverages, each chlorine locally alters the electronic structure of the surface. The adsorbed chlorine-induced local environment modification thereby creates preferential adsorption sites for other chlorines in their vicinity, stabilizing extended chlorine structures on Ag(111). Oligomer formation leads to distance-dependent cooperative effects of the charge transfer and thus impacts the electronic structure of the surface beyond the change by individual chlorines. At intermediate chlorine coverage, chlorine forms meandering chains with atoms adsorbed in alternating hcp and fcc hollow sites at distinct chlorine-chlorine distances. The one-dimensional structures convert to an open network at intermediate coverages and a two-dimensional hexagonal superstructure at saturation coverage. The DFT calculations suggest that the charge density extracted from the surface into the chlorines and the interaction between chlorine and silver atoms is improved as chlorines are adjoined closer at intermediate and high coverages.

Tetrakis(tetrahydroborato)zirconium(IV), Zr(BH4)(4), is a volatile compound that has been widely used as a single-source precursor to grow carbon-free thin films of zirconium diboride by chemical vapor deposition (CVD) on a wide range of substrates that include oxides, semiconductors, and metals. However, the basic surface chemistry of the compound that underlies the initial stages of the CVD process is largely unknown for any substrate. We studied the adsorption and decomposition of Zr(BH4)(4) on a Pd(111) surface with the experimental techniques of reflection absorption infrared spectroscopy (RAIRS), temperature-programmed reaction spectroscopy (TPRS), and X-ray photoelectron spectroscopy (XPS) and with density functional theory and first-principles molecular dynamics simulations. After exposing Pd(111) at 90 K to Zr(BH4)(4) (g), a RAIR spectrum was obtained that closely matched that of the pure compound, indicating that it adsorbs without dissociation at 90 K. However, upon heating to 200 K the RAIR spectrum undergoes dramatic changes indicating that a new surface species is formed that retains both terminal B-H bonds and bridging metal-H-B bonds, as indicated by B-H stretches in the ranges of 2563-2540 and 2143-2135 cm(-1), respectively. Hydrogen desorption is first observed at around 178 K and the presence of a stable hydrogen-containing surface intermediate is revealed by additional H-2 desorption peaks at 330 and 426 K. A combination of theoretical methods reveals that BH adsorbed at a 3-fold hollow site on the Pd(111) surface is the most stable species, but once formed two BH molecules can dimerize to form HBBH. A stable configuration for B2H2 is achieved through formation of B-H---Zr bridge bonds with a Zr atom located at one or both ends of a B2H2 molecule. Calculated vibrational frequencies and intensities provide an excellent match with the experimental RAIR spectra.

The adsorption of the radical α,ɣ-bisdiphenylene-β-phenylallyl (BDPA) molecule to the Cu(100) surface was studied using scanning tunnelling microscopy (STM), scanning tunnelling spectroscopy (STS), and density functional theory (DFT) calculations accounting for dispersion forces. BDPA on Cu(100) was observed to align preferentially along ⟨ 01 1 ‾ ⟩ ${\langle 01\bar{1}\rangle }$ directions due to weak Cu-C chemisorption between fluorenyl carbons with the underlying copper atoms. The curved shape of the BDPA molecule on Cu(100) can be ascribed to the lack of molecular orbital character on the phenyl substituent. A Kondo-like feature from differential conductance (dI/dV) measurements centered close to the Fermi energy ( E F ${{E}_{{\rm F}}}$ ) suggests the retention of an electron spin-1/2 state, which is corroborated by hybrid DFT calculations that place the SOMO (singly occupied molecular orbital) below and SUMO (singly unoccupied molecular orbital) above E F ${{E}_{{\rm F}}}$ for BDPA adsorbed to Cu(100).

In this article, we describe a fully computational laboratory exercise that results in an increase of students' understanding of what quantum chemical geometry optimization calculations are doing to find minimum energy structures. This laboratory exercise was conducted several times over multiple years at a small private undergraduate institution, St. Bonaventure University. Through this experiment, physical chemistry undergraduate students are exposed to chemical problems for which computations provide a necessary supplement to chemical intuition, thus cementing the importance of computational work in contemporary chemistry. Students apply their understanding of geometry optimizations to problems of complex 3-D molecular structures that stretch their intuition, including the geometries and isomers of closo-carboranes and of the hexamer of the cocatalyst methylaluminoxane. Students are also exposed to vibrational frequency calculations as a diagnostic tool for determining whether structures represent energetic minima or transition states, and they are exposed to the vibrational zero-point energy correction.

The convergent positioning of functional groups in biomacromolecules leads to good binding, catalytic and transport capabilities. Synthetic frameworks capable of convergently locking functional groups with minimized conformational uncertainty-leading to similar properties-are highly desirable but rare. Here we report C5-symmetric aromatic pentaamide macrocycles synthesized in one pot from the corresponding monomers. Their crystal structures reveal a star-shaped, fully constrained backbone that causes ten alternating NH/CH hydrogen-bond donors and five large amide dipoles to orient towards the centre of the macrocycle. With a highly electropositive cavity in a high-energy unbound state, the macrocycles bind anions in a 1:1 stoichiometry in solution, with high affinity for halides and very high affinity for oxoanions. We demonstrate that such macrocycles are able to transport anions across lipid bilayers with a high chloride selectivity and restore the depleted airway surface liquid of cystic fibrosis airway cell cultures.

A computational experiment investigating common organic chemistry mechanisms has been developed and implemented in a junior/senior-level physical chemistry laboratory course at two institutions. Students investigated various reactions that proceed via S(N)1, S(N)2, E1, and E2 mechanisms using hybrid indicate that students at both institutions were able to better visualize and interpret the 3D representation of transition states, stepwise reaction mechanisms, and reaction coordinate diagrams of the aforementioned reactions.

The precise spatial positioning of functional groups in biomacromolecules leads to astonishing binding, catalytic, and transport capabilities. In contrast, synthetic frameworks capable of convergently locking functional groups with minimized conformational uncertainty are highly desirable but rare. Here we report C5-symmetric aromatic pentaamide macrocycles c5a-c synthesized in one pot from the corresponding monomers. The crystal structure of c5c reveals a fully constrained backbone that enforces ten alternating NH/CH hydrogen-bond donors and five large amide dipoles to point to the center of the macrocycle. With a highly electropositive cavity in a high-energy unbound state, macrocycles c5 bind anions in a 1:1 stoichiometry in solution, with Ka values up to 106 M-1 for halides and 108 M-1 for oxoanions. Macrocycle c5a was able to selectively transport chloride ions across lipid bilayers. The extraordinary binding of phosphate-related ions by c5 also enabled catalytic ester amidation owing to the stabilization of the corresponding transition states.

The adsorption orientation of molecules on surfaces influences their reactivity, but it is still challenging to tailor the interactions that govern their orientation. Here, we investigate how the substituent and the surface structure alter the adsorption orientation of halogenated benzene molecules from parallel to tilted relative to the surface plane. The deviation of the parallel orientation of bromo-, chloro-, and fluorobenzene molecules adsorbed on Cu(111) and Cu(110) surfaces is determined, utilising the surface selection rule in reflection-absorption infrared spectroscopy. On Cu(111), all three halogenated molecules are adsorbed with their molecular plane almost parallel to the surface at low coverages. However, they are tilted at higher coverages; yet, the threshold coverages differ. On Cu(110), merely bromo- and chlorobenzene follow this trend, albeit with a lower threshold for both. In contrast, fluorobenzene molecules are tilted already at low coverages. The substantial influence of the halogen atom and the surface structure on the adsorption orientation, resulting from an interplay of molecule-molecule and molecule-surface interactions, is highly relevant for reactivity confined to two dimensions.

Cooperativity plays a critical role in self-assembly and molecular recognition. A rigid aromatic oligoamide macrocycle with a cyclodirectional backbone binds with DABCO-based cationic guests in a 2 : 1 ratio in high affinities ( K total ≈10 13  M −2 ) in the highly polar DMF. The host–guest binding also exhibits exceptionally strong positive cooperativity quantified by interaction factors α that are among the largest for synthetic host–guest systems. The unusually strong positive cooperativity, revealed by isothermal titration calorimetry (ITC) and fully corroborated by mass spectrometry, NMR and computational studies, is driven by guest-induced stacking of the macrocycles and stabilization from the alkyl end chains of the guests, interactions that appear upon binding the second macrocycle. With its tight binding driven by extraordinary positive cooperativity, this host–guest system provides a tunable platform for studying molecular interactions and for constructing stable supramolecular assemblies.