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In this work, we investigated bonding features of 15 ruthenium(II) nitrile complexes of the type [Ru(tpy)(L)-(CH3CN)]n+, containing the tridentate tpy ligand (tpy = 2,2′:6′,2″-terpyridine) and various bidentate ancillary ligands L; 12 compounds originally synthesized by Loftus et al. [J. Phys. Chem. C 123, 10291–10299 (2019)] and three new complexes. We utilized local vibrational force constants derived from the local mode theory as a quantitative measure of bond strength complemented with the topological analysis of the electron density and the natural bond orbital analysis. Loftus et al. suggested that nitrile dissociation occurs after light induced singlet–triplet transition of the original complexes and they used as a measure of nitrile release efficiency quantum yields for ligand exchange in water. They observed larger quantum yields for complexes with smaller singlet–triplet energy gaps. The major goal of this work was to assess how the Ru–NC and Ru–L bond strengths in these 15 compounds relate to and explain the experimental data of Loftus et al., particularly focusing on the question whether there is a direct correlation between Ru–NC bond strength and measured quantum yield. Our study provides the interesting result that the compounds with the highest quantum yields also have the strongest Ru–NC bonds suggesting that breaking the Ru–NC bond is not the driving force for the delivery process rather than the change of the metal framework as revealed by first results of a unified reaction valley approach investigation of the mechanism. Compounds with the highest quantum yield show larger electronic structure changes upon singlet–triplet excitation, i.e., larger changes in bond strength, covalency, and difference between the singlet and triplet HOMOs, with exception of the compound 12. In summary, this work provides new insights into the interplay of local properties and experimental quantum yields forming in synergy a useful tool for fine tuning of existing and future design of new nitrile releasing ruthenium compounds. We hope that this work will bring theoretical and experimental studies closer together and serves as an incubator for future collaboration between computational chemists and their experimental colleagues. [ABSTRACT FROM AUTHOR]

The catalytic effects of iridium pincer complexes for the hydrogenation of carbon dioxide were investigated with the Unified Reaction Valley Approach (URVA), exploring the reaction mechanism along the reaction path traced out by the reacting species on the potential energy surface. Further details were obtained with the Local Mode Analysis performed at all stationary points, complemented by the Natural Bond Orbital and Bader's Quantum Atoms in Molecules analyses. Each of the five reaction paths forming the catalytic cycle were calculated at the DFT level complemented with DLPNO-CCSD(T) single point calculations at the stationary points. For comparison, the non-catalytic reaction was also investigated. URVA curvature profiles identified all important chemical events taking place in the non-catalyzed reaction and in the five reactions forming the catalytic cycle, and their contribution to the activation energy was disclosed. The non-catalytic reaction has a large unfavorable activation energy of 76.3 kcal/mol, predominately caused by HH bond cleave in the H 2 reactant. As shown by our study, the main function of the iridium pincer catalyst is to split up the one–step non-catalytic reaction into an energy efficient multistep cycle, where HH bond cleavage is replaced by the cleavage of a weaker IrH bond with a small contribution to the activation energy. The dissociation of the final product from the catalyst requires the cleavage of an IrO bond, which is also weak, and contributes only to a minor extent to the activation energy. This, in summary, leads to the substantial lowering of the overall activation barrier by about 50 kcal/mol for the catalyzed reaction. We hope that this study inspires the community to add URVA to their repertoire for the investigation of catalysis reactions. [ABSTRACT FROM AUTHOR]

The interplay between FeC and CO bonding in carboxymyoglobin (MbCO) and the role of potential hydrogen bonding between the CO moiety and the side chains of the surrounding protein amino acids have been the subject of numerous experimental and theoretical studies. In this work, we present a quantitative measure for the intrinsic FeC and CO bond strength in MbCO, as well as for CO⋯H bonding, based on the local vibrational mode analysis, originally developed by Konkoli and Cremer. We investigated a gas phase model, two models of the wild-type protein, and 17 protein mutations that change the distal polarity of the heme pocket, as well as two protein mutations of the heme porphyrin ring. Based on local mode force constants, we could quantify for the first time the suggested inverse relationship between the CO and FeC bond strength, the strength of CO⋯H bonding, and how it weakens the CO bond. Combined with the natural orbital analysis, we could also confirm the key role of π back donation between Fe and the CO moiety in determining the FeC bond strength. We further clarified that CO and FeC normal modes couple with other protein motions in the protein environment. Therefore, normal mode frequencies/force constants are not suited as bond strength descriptors and instead their local mode counterparts should be used. Our comprehensive results provide new guidelines for the fine-tuning of existing and the design of MbCO models with specific FeC, CO, and CO⋯H bond strengths. Graphical abstract [ABSTRACT FROM AUTHOR]

The relativistic Hamiltonian Normalised Elimination of the Small Component with atomic unitary transformation (NESCau) was used for the first time to calculate geometries and harmonic vibrational frequencies of actinide sandwich compounds An (C n H n) 2 (An: Th, Pa, U, Np, Pu; n = 5 − 8). In addition, the Local Vibrational Mode analysis, the Natural Bond Orbital analysis, and the Atoms in Molecules analysis were applied to quantitatively assess the intrinsic strength and the nature of the An-ring interactions. Our results show that actinide sandwich compounds prefer strong covalent interactions between actinide and carbon atoms, similar to those found for homologous ferrocene-type complexes Fe (C n H n) 2 . The metal centre rather than the ring size plays a key role in the strength of those interactions. [ABSTRACT FROM AUTHOR]

Experimentalists recently characterized the difficulty in isolating gaseous histidine in its neutral form. To understand the factors which stabilize neutral histidine, the intrinsic nature of the intramolecular hydrogen bonding networks in the four most stable histidine conformers was investigated via density functional theory combined with the local vibrational mode analysis originally introduced by Konkoli and Cremer, quantum theory of atoms in molecules, non-covalent interaction analysis, and natural bond orbital population analysis. Our results show a positive correlation between intramolecular hydrogen bond strength and structural stability, where the presence of the O - H ⋯ N α bond type is a major factor. [ABSTRACT FROM AUTHOR]

In this work hydrogen bonding in a diverse set of 36 unnatural and the three natural Watson Crick base pairs adenine (A)–thymine (T), adenine (A)–uracil (U) and guanine (G)–cytosine (C) was assessed utilizing local vibrational force constants derived from the local mode analysis, originally introduced by Konkoli and Cremer as a unique bond strength measure based on vibrational spectroscopy. The local mode analysis was complemented by the topological analysis of the electronic density and the natural bond orbital analysis. The most interesting findings of our study are that (i) hydrogen bonding in Watson Crick base pairs is not exceptionally strong and (ii) the N–H⋯N is the most favorable hydrogen bond in both unnatural and natural base pairs while O–H⋯N/O bonds are the less favorable in unnatural base pairs and not found at all in natural base pairs. In addition, the important role of non-classical C–H⋯N/O bonds for the stabilization of base pairs was revealed, especially the role of C–H⋯O bonds in Watson Crick base pairs. Hydrogen bonding in Watson Crick base pairs modeled in the DNA via a QM/MM approach showed that the DNA environment increases the strength of the central N–H⋯N bond and the C–H⋯O bonds, and at the same time decreases the strength of the N–H⋯O bond. However, the general trends observed in the gas phase calculations remain unchanged. The new methodology presented and tested in this work provides the bioengineering community with an efficient design tool to assess and predict the type and strength of hydrogen bonding in artificial base pairs. [ABSTRACT FROM AUTHOR]

Seventeen singlet excited states of ethylene have been calculated via time-dependent density functional theory (TDDFT) with the CAM-B3LYP functional and the geometries of 11 excited states were optimized successfully. The local vibrational mode theory was employed to examine the intrinsic C=C/C–H bond strengths and their change upon excitation. The natural transition orbital (NTO) analysis was used to further analyze the C=C/C–H bond strength change in excited states versus the ground state. For the first time, three excited states including π y ′ → 3s, π y ′ → 3p y and π y ′ → 3p z were identified with stronger C=C ethylene double bonds than in the ground state. [ABSTRACT FROM AUTHOR]

In this work, we investigated the strength of the H - and H 2 interaction with the Fe atom of a [NiFe] hydrogenase mimic, and how this interaction can be modulated by changing the Fe ligand in trans-position relative to H - and H 2 . We used as a quantitative measure of bond strength local vibrational force constants derived from the Konkoli–Cremer local mode analysis, complemented by the topological analysis of the electronic density and the natural bond orbital analysis. Seventeen different ligands were investigated utilizing density functional theory calculations, including σ -donor ligands such as CH 3 - , C 2 H 5 - , NH 3 , and H 2 O , π -donor ligands such as Cl - , F - , and OH - , and σ -donor/ π -acceptor ligands such as CN - and CO. According to the local mode analysis, Fe–H interactions are strengthened by σ -donor or π -donor ligands and weakened by σ -donor/ π -acceptor ligands. In contrast, the H–H bond of H 2 is weakened by σ -donor or π -donor ligands and strengthened by σ -donor/ π -acceptor ligands. We also present a new metal–ligand electronic parameter (MLEP) for Fe–H ligands which can be generally applied to evaluate the Fe–H bond strength in iron complexes and iron hydrides. These results form a valuable basis for future [NiFe] hydrogenase-based catalyst design and fine tuning, as well as for the development of efficient biomimetic catalysts for H 2 generation. [ABSTRACT FROM AUTHOR]

The intrinsic bonding nature of λ 3 -iodanes was investigated to determine where its hypervalent bonds fit along the spectrum between halogen bonding and covalent bonding. Density functional theory with an augmented Dunning valence triple zeta basis set (ω B97X-D/aug-cc-pVTZ) coupled with vibrational spectroscopy was utilized to study a diverse set of 34 hypervalent iodine compounds. This level of theory was rationalized by comparing computational and experimental data for a small set of closely-related and well-studied iodine molecules and by a comparison with CCSD(T)/aug-cc-pVTZ results for a subset of the investigated iodine compounds. Axial bonds in λ 3 -iodanes fit between the three-center four-electron bond, as observed for the trihalide species IF 2 − and the covalent FI molecule. The equatorial bonds in λ 3 -iodanes are of a covalent nature. We explored how the equatorial ligand and axial substituents affect the chemical properties of λ 3 -iodanes by analyzing natural bond orbital charges, local vibrational modes, the covalent/electrostatic character, and the three-center four-electron bonding character. In summary, our results show for the first time that there is a smooth transition between halogen bonding → 3c–4e bonding in trihalides → 3c–4e bonding in hypervalent iodine compounds → covalent bonding, opening a manifold of new avenues for the design of hypervalent iodine compounds with specific properties. [ABSTRACT FROM AUTHOR]