Infrared spectra of partially deuterated water dimers in the fundamental O-D stretch region

Spectra of mixed H/D water dimers are studied in the mid-infrared region of the O-D stretch fundamental (2630-2800 cm-1) using a pulsed supersonic slit jet and a tunable optical parametric oscillator infrared source. Over 30 bands, belonging to nine of the ten possible isotopologues (only H2O-HOD is missed), are observed and analyzed. The implications for excited state tunneling splittings, lifetime effects, and vibrational shifts are discussed. These are the first significant new experimental results on (gas phase) mixed water dimers in over 25 years, and they are valuable for testing water dimer theoretical calculations, a field which continues to be of lively current interest.

Dihydrogen Bonding-Seen through the Eyes of Vibrational Spectroscopy

In this work, we analyzed five groups of different dihydrogen bonding interactions and hydrogen clusters with an H3+ kernel utilizing the local vibrational mode theory, developed by our group, complemented with the Quantum Theory of Atoms-in-Molecules analysis to assess the strength and nature of the dihydrogen bonds in these systems. We could show that the intrinsic strength of the dihydrogen bonds investigated is primarily related to the protonic bond as opposed to the hydridic bond; thus, this should be the region of focus when designing dihydrogen bonded complexes with a particular strength. We could also show that the popular discussion of the blue/red shifts of dihydrogen bonding based on the normal mode frequencies is hampered from mode-mode coupling and that a blue/red shift discussion based on local mode frequencies is more meaningful. Based on the bond analysis of the H3+(H2)n systems, we conclude that the bond strength in these crystal-like structures makes them interesting for potential hydrogen storage applications.

Mechanistic Details of the Sharpless Epoxidation of Allylic Alcohols—A Combined URVA and Local Mode Study

In this work, we investigated the catalytic effects of a Sharpless dimeric titanium (IV)–tartrate–diester catalyst on the epoxidation of allylalcohol with methyl–hydroperoxide considering four different orientations of the reacting species coordinated at the titanium atom (reactions R1–R4) as well as a model for the non-catalyzed reaction (reaction R0). As major analysis tools, we applied the URVA (Unified Reaction Valley Approach) and LMA (Local Mode Analysis), both being based on vibrational spectroscopy and complemented by a QTAIM analysis of the electron density calculated at the DFT level of theory. The energetics of each reaction were recalculated at the DLPNO-CCSD(T) level of theory. The URVA curvature profiles identified the important chemical events of all five reactions as peroxide OO bond cleavage taking place before the TS (i.e., accounting for the energy barrier) and epoxide CO bond formation together with rehybridization of the carbon atoms of the targeted CC double bond after the TS. The energy decomposition into reaction phase contribution phases showed that the major effect of the catalyst is the weakening of the OO bond to be broken and replacement of OH bond breakage in the non-catalyzed reaction by an energetically more favorable TiO bond breakage. LMA performed at all stationary points rounded up the investigation (i) quantifying OO bond weakening of the oxidizing peroxide upon coordination at the metal atom, (ii) showing that a more synchronous formation of the new CO epoxide bonds correlates with smaller bond strength differences between these bonds, and (iii) elucidating the different roles of the three TiO bonds formed between catalyst and reactants and their interplay as orchestrated by the Sharpless catalyst. We hope that this article will inspire the computational community to use URVA complemented with LMA in the future as an efficient mechanistic tool for the optimization and fine-tuning of current Sharpless catalysts and for the design new of catalysts for epoxidation reactions.

Uranium: The Nuclear Fuel Cycle and Beyond

This review summarizes the recent developments regarding the use of uranium as nuclear fuel, including recycling and health aspects, elucidated from a chemical point of view, i.e., emphasizing the rich uranium coordination chemistry, which has also raised interest in using uranium compounds in synthesis and catalysis. A number of novel uranium coordination features are addressed, such the emerging number of U(II) complexes and uranium nitride complexes as a promising class of materials for more efficient and safer nuclear fuels. The current discussion about uranium triple bonds is addressed by quantum chemical investigations using local vibrational mode force constants as quantitative bond strength descriptors based on vibrational spectroscopy. The local mode analysis of selected uranium nitrides, N≡U≡N, U≡N, N≡U=NH and N≡U=O, could confirm and quantify, for the first time, that these molecules exhibit a UN triple bond as hypothesized in the literature. We hope that this review will inspire the community interested in uranium chemistry and will serve as an incubator for fruitful collaborations between theory and experimentation in exploring the wealth of uranium chemistry.

Capturing Individual Hydrogen Bond Strengths in Ices via Periodic Local Vibrational Mode Theory: Beyond the Lattice Energy Picture

Local stretching force constants derived from periodic local vibrational modes at the vdW-DF2 density functional level have been employed to quantify the intrinsic hydrogen bond strength of 16 ice polymorphs, ices Ih, II, III, IV, V, VI, VII, VIII, IX, XI, XII, XIII, XIV, XV, XVII, and XIX, that are stable under ambient to elevated pressures. Based on this characterization on 1820 hydrogen bonds, relationships between local stretching force constants and structural parameters such as hydrogen bond length and angle were identified. Moreover, different bond strength distributions, from uniform to inhomogeneous, were observed for the 16 ices and could be explained in relation to different local structural elements within ices, that is, rings, that consist of different hydrogen bond types. In addition, criteria for the classification of hydrogen bonds as strong, intermediate, and weak were introduced. The latter was used to explore a different dimension of the water-ice phase diagram. These findings will provide important guidelines for assessing the credibility of new ice structures.

Exploring the Mechanism of Catalysis with the Unified Reaction Valley Approach (URVA)—A Review

The unified reaction valley approach (URVA) differs from mainstream mechanistic studies, as it describes a chemical reaction via the reaction path and the surrounding reaction valley on the potential energy surface from the van der Waals region to the transition state and far out into the exit channel, where the products are located. The key feature of URVA is the focus on the curving of the reaction path. Moving along the reaction path, any electronic structure change of the reacting molecules is registered by a change in their normal vibrational modes and their coupling with the path, which recovers the curvature of the reaction path. This leads to a unique curvature profile for each chemical reaction with curvature minima reflecting minimal change and curvature maxima, the location of important chemical events such as bond breaking/forming, charge polarization and transfer, rehybridization, etc. A unique decomposition of the path curvature into internal coordinate components provides comprehensive insights into the origins of the chemical changes taking place. After presenting the theoretical background of URVA, we discuss its application to four diverse catalytic processes: (i) the Rh catalyzed methanol carbonylation—the Monsanto process; (ii) the Sharpless epoxidation of allylic alcohols—transition to heterogenous catalysis; (iii) Au(I) assisted [3,3]-sigmatropic rearrangement of allyl acetate; and (iv) the Bacillus subtilis chorismate mutase catalyzed Claisen rearrangement—and show how URVA leads to a new protocol for fine-tuning of existing catalysts and the design of new efficient and eco-friendly catalysts. At the end of this article the pURVA software is introduced. The overall goal of this article is to introduce to the chemical community a new protocol for fine-tuning existing catalytic reactions while aiding in the design of modern and environmentally friendly catalysts.

Interplay of Ring Puckering and Hydrogen Bonding in Deoxyribonucleosides

The Cremer-Pople ring puckering analysis and the Konkoli-Cremer local mode analysis supported by the topological analysis of the electron density were applied for the first comprehensive analysis of the interplay between deoxyribose ring puckering and intramolecular H-bonding in 2'-deoxycytidine, 2'-deoxyadenosine, 2'-deoxythymidine, and 2'-deoxyguanosine. We mapped for each deoxyribonucleoside the complete conformational energy surface and the corresponding pseudorotation path. We found only incomplete pseudorotation cycles, caused by ring inversion, which we coined as pseudolibration paths. On each pseudolibration path a global and a local minimum separated by a transition state were identified. The investigation of H-bond free deoxyribonucleoside analogs revealed that removal of the H-bond does not restore the full conformational flexibility of the sugar ring. Our work showed that ring puckering predominantly determines the conformational energy; the larger the puckering amplitude, the lower the conformational energy. In contrast no direct correlation between conformational energy and H-bond strength was found. The longest and weakest H-bonds are located in the local minimum region, whereas the shortest and strongest H-bonds are located outside the global and local minimum regions at the turning points of the pseudolibration paths, i.e., H-bonding determines the shape and length of the pseudolibration paths. In addition to the H-bond strength, we evaluated the covalent/electrostatic character of the H-bonds applying the Cremer-Kraka criterion of covalent bonding. H-bonding in the puric bases has a more covalent character whereas in the pyrimidic bases the H-bond character is more electrostatic. We investigated how the mutual orientation of the CH₂OH group and the base influences H-bond formation via two geometrical parameters describing the rotation of the substituents perpendicular to the sugar ring and their tilting relative to the ring center. According to our results, rotation is more important for H-bond formation. In addition we assessed the influence of the H-bond acceptor, the lone pair (N, respectively O), via the delocalization energy. We found larger delocalization energies corresponding to stronger H-bonds for the puric bases. The global minimum conformation of 2'-deoxyguanosine has the strongest H-bond of all conformers investigated in this work with a bond strength of 0.436 which is even stronger than the H-bond in the water dimer (0.360). The application of our new analysis to DNA deoxyribonucleotides and to unnatural base pairs, which have recently drawn a lot of attention, is in progress.

Quantitative Assessment of Halogen Bonding Utilizing Vibrational Spectroscopy

A total of 202 halogen-bonded complexes have been studied using a dual-level approach: ωB97XD/aug-cc-pVTZ was used to determine geometries, natural bond order charges, charge transfer, dipole moments, electron and energy density distributions, vibrational frequencies, local stretching force constants, and relative bond strength orders n. The accuracy of these calculations was checked for a subset of complexes at the CCSD(T)/aug-cc-pVTZ level of theory. Apart from this, all binding energies were verified at the CCSD(T) level. A total of 10 different electronic effects have been identified that contribute to halogen bonding and explain the variation in its intrinsic strength. Strong halogen bonds are found for systems with three-center-four-electron (3c-4e) bonding such as chlorine donors in interaction with substituted phosphines. If halogen bonding is supported by hydrogen bonding, genuine 3c-4e bonding can be realized. Perfluorinated diiodobenzenes form relatively strong halogen bonds with alkylamines as they gain stability due to increased electrostatic interactions.

Heavy snow: IR spectroscopy of isotope mixed crystalline water ice

Temperature and isotopic dependence of simulated and experimental spectra shed light on the vibrational modes of crystalline water ice.

Re-evaluation of the bond length-bond strength rule: The stronger bond is not always the shorter bond

A set of 42 molecules with N-F, O-F, N-Cl, P-F, and As-F bonds has been investigated in the search for potential bond anomalies, which lead to reverse bond length-bond strength (BLBS) relationships. The intrinsic strength of each bond investigated has been determined by the local stretching force constant obtained at the CCSD(T)/aug-cc-pVTZ level of theory. N-F or O-F bond anomalies were found for fluoro amine radicals, fluoro amines, and fluoro oxides, respectively. A rationale for the deviation from the normal Badger-type inverse BLBS relation is given and it is shown that electron withdrawal accompanied by strong orbital contraction and bond shortening is one of the prerequisites for a bond anomaly. In the case of short electron-rich bonds such as N-F or O-F, anomeric delocalization of lone pair electrons in connection with lone pair repulsion are decisive whether a bond anomaly can be observed. This is quantitatively assessed with the help of the CCSD(T) local stretching force constants, CCSD(T) charge distributions, and G4 bond dissociation energies. Bond anomalies are not found for fluoro phosphines and fluoro arsines because the bond weakening effects are no longer decisive.

Hidden Bond Anomalies: The Peculiar Case of the Fluorinated Amine Chalcogenides

Bond anomalies have been investigated for a set of 53 molecules with either N-F, Ti-P, Cr-H, Pb-C, or Pb-F bonds for which reverse rather than inverse bond length-bond strength relationships have been previously claimed. The intrinsic strength of each bond investigated was determined utilizing the associated local stretching force constant obtained at the CCSD(T)/aug-cc-pVTZ level of theory. For the metal containing molecules, LC-ωPBE calculations with the aug-cc-pVTZ (Cr, Pb) and the 6-31++G(d,p) basis set (Ti) were carried out. For bonds containing a metal atom, any bond anomaly could not be confirmed. Previously reported results were due to ill-defined bond strength descriptors or lacking accuracy. In the case of the fluoro amines, methyl fluoro amines, and the fluoro amine oxides, direct or hidden bond anomalies were detected, which result from two or more opposing electronic effects: a dominant bond shortening effect due to electron withdrawal and a bond weakening due to lone pair repulsion or hybridization defects. Bond anomalies can be disguised by a complex interplay of electronic effects. These hidden bond anomalies could be identified in this work for the fluoro amine chalcogenides.

6th Heron Island Conference on Reactive Intermediates and Unusual Molecules

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