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127I Solid‐State NMR Spectroscopy of Organic Periodates Featuring Halogen Bonded IO4− ⋅ ⋅ ⋅ IO4− Adducts
No full textHydroxyanions and oxyanions can overcome electrostatic repulsion between like charges to form supramolecular architectures whose formation is driven by non-covalent interactions such as hydrogen bonds and halogen bonds (HaB). We report here a 127I solid-state nuclear magnetic resonance (SSNMR) study of a series of six organic periodates including compounds which feature I ⋅ ⋅ ⋅ O HaB between pairs of IO4− anions and control samples which do not feature HaB. 127I SSNMR spectra of powdered samples acquired under stationary conditions at 9.4, 11.7, and 21.1 T are simulated using an exact diagonalization of the Zeeman-quadrupolar Hamiltonian to provide the isotropic chemical shift, 127I nuclear quadrupolar coupling constant (CQ), and quadrupolar asymmetry parameter for each compound. One of the HaB compounds, 4-(pyrrolidin-1-yl)pyridinium periodate, is characterized by the largest CQ(127I) value measured to date for a periodate anion, 52.70 MHz. Control organic periodates which do not have HaBs have CQ(127I) values that are much lower than those seen in the halogen-bonded systems, thereby easily differentiating between these two sets of compounds. The CQ(127I) values for those compounds featuring only halogen-bonded periodate anions correlate with the shear strain of the anion, which may be attributed to the influence of the HaB on the local geometry. More rigorous correlations between structure and the 127I NMR data are confounded by the presence of dynamics in some of the samples
Proton delocalization in short hydrogen bonds assembling HSeO4− anions into supramolecular adducts
No full textFour hydrogen-bonded complexes of selenic acid with N-heterocycles (pyridine, 4,4′-bipyridil, quinoline and 2,2,6,6-tetramethylpyperidine) were studied in the crystalline state by single crystal X-ray diffraction, Fourier-transform infrared spectroscopy, and density functional theory with periodic boundary conditions. In all cases short SeO-H⋯OSe hydrogen bonds (≤2.61 Å) were found, either ‘isolated’ ones or within infinite chains. The coherent quasi-adiabatic proton transfer pathways were computed providing broad asymmetric single-well or (low-barrier) double-well potentials with significantly delocalized protons. The ground state vibrationally-averaged proton positions are noticeably shifted from the equilibrium ones towards hydrogen bond centres, and for asymmetric low-barrier double wells, a proton transfer in the first vibrationally excited state is established. The computed O-H stretching frequencies lie in the range of 1600-3050 cm−1 and are in semi-quantitative agreement with experiment. Moreover, in case of low-barrier double-well potentials, a rather exotic H/D isotope effect, namely, a higher vibrational frequency for the O-D stretching than for the O-H stretching, is predicted. The mutual influence of neighboring SeO-H⋯OSe and SeO⋯H-N bonds affecting the bridging proton position in both bonds has also been addressed, indicating the vivid geometric cooperativity in complexes with ‘isolated’ SeO-H⋯OSe hydrogen bonds
Anion⋯anion self-assembly under the control of σ- and π-hole bonds
No full textThe electrostatic attraction between charges of opposite signs and the repulsion between charges of the same sign are ubiquitous and influential phenomena in recognition and self-assembly processes. However, it has been recently revealed that specific attractive forces between ions with the same sign are relatively common. These forces can be strong enough to overcome the Coulomb repulsion between ions with the same sign, leading to the formation of stable anion⋯anion and cation⋯cation adducts. Hydroden bonds (HBs) are probably the best-known interaction that can effectively direct these counterintuitive assembly processes. In this review we discuss how σ-hole and π-hole bonds can break the paradigm of electrostatic repulsion between like-charges and effectively drive the self-assembly of anions into discrete as well as one-, two-, or three-dimensional adducts. σ-Hole and π-hole bonds are the attractive forces between regions of excess electron density in molecular entities (e.g., lone pairs or π bond orbitals) and regions of depleted electron density that are localized at the outer surface of bonded atoms opposite to the σ covalent bonds formed by atoms (σ-holes) and above and below the planar portions of molecular entities (π-holes). σ- and π-holes can be present on many different elements of the p and d block of the periodic table and the self-assembly processes driven by their presence can thus involve a wide diversity of mono- and di-anions. The formed homomeric and heteromeric adducts are typically stable in the solid phase and in polar solvents but metastable or unstable in the gas phase. The pivotal role of σ- and π-hole bonds in controlling anion⋯anion self-assembly is described in key biopharmacological systems and in molecular materials endowed with useful functional properties
Halogen Bonding Assembles Anion⋅⋅⋅Anion Architectures in Non‐centrosymmetric Iodate and Bromate Crystals
No full text: Single crystal X-ray diffraction of iodate and bromate salts shows that the I and Br atoms in IO3 - and BrO3 - anions form short and linear O-I/Br⋅⋅⋅O contacts with the O atoms of nearby anions. Non-centrosymmetric systems are formed wherein anions are orderly aligned into supramolecular 1D and 2D networks. Theoretical evidences, namely the outcome of QTAIM and NCIplot studies, prove the attractive nature of these contacts and the ability of iodate and bromate anions to act as robust halogen bond (HaB) donors. The HaB is proposed as a general and effective assisting tool to control the architecture of acentric iodate salts
Upgrade of Weak σ‐Hole Bond Donors via Cr(CO)3 Complexation
No full textMolecular recognition mediated by σ-hole interactions is enhanced as the electrostatic potential at the σ-hole becomes increasingly positive. Traditional methods to strengthen σ-hole donor ability of atoms such as halogens often involve covalent modifications, such as, introducing electron-withdrawing substituents (neutral or positively charged) or electrochemical oxidation. Metal coordination, a relatively underexplored approach, offers a promising alternative. In this study, η6-coordination of Cr(CO)3 to haloarenes, a neutral system, is demonstrated to significantly increase the electrophilic character of halogen bond donors, enabling weak donors such as chloroanisole to form short and directional Cl⋅⋅⋅O halogen bonds. Structural characterization using single-crystal X-ray diffraction and computational analysis of a series of η6-Cr(CO)3-coordinated haloarenes provides evidence for this enhancement. Furthermore, the effect is shown to extend to other heteroatomic substituents on the coordinated arene, e. g., other halogen atoms as well as elements of groups 16, 15, and 14 of the periodic table, broadening the scope of this approach
Chalcogen and Hydrogen Bond Team up in Driving Anion⋅⋅⋅Anion Self-Assembly
No full text: H-selenite anions (HSeO3 - ) form in the solid unprecedented anionic supramolecular chains wherein single units are assembled via alternating short Se⋅⋅⋅O and H⋅⋅⋅O contacts. Crystallographic analyses and computational studies (the quantum theory of "atoms-in-molecules", QTAIM, and the noncovalent interaction plot, NCIPlot) consistently prove the attractive nature of these chalcogen bonds (ChBs) and hydrogen honds (HBs), the Janus-type character of HSeO3 - anions which act as both donors and acceptors of ChB and HB, and the possible stability of anion dimers in solution. The effectiveness of the ChBs herein described may lead to consider the HSeO3 - moiety as a new entry in the toolbox of crystal engineering based on ChB
Osme Bond: Geometric and Energetic Features in the Adducts between OsO4 and Lewis Bases
No full text: Adducts between OsO4 and Lewis bases exert a role in important oxidation processes such as epoxidation and dihydroxylation. It has been shown that the attractive interaction driving the formation of these adducts is a σ-hole bond involving the metal as the electrophilic species; the term Osme Bond (OmB) was proposed for designating it. Here some new adducts between OsO4 and various bases have been characterized through single crystal x-ray diffraction (XRD) and computational studies (density functional theory, DFT), confirming the existence of a robust correlation between σ-hole interaction energy and deformation of the tetrahedral geometry of OsO4. Also some adducts formed by RuO4 with nucleophiles were investigated computationally
Taxonomy of Chemical Bondings: Opportunities and Challenges
No full textThe concept of the chemical bond is fundamental to chemistry, governing atomic interactions that define all known matter. Despite this central role, the classification and most convenient naming of chemical bonds remain subjects of debate due to the diverse theoretical models and experimental observations. Modelings from quantum mechanical calculations and heuristic principles from experimental observations offer valuable and complementary insights, but sometimes the match and coalescence of these different approaches into a common terminology is not immediate. This paper describes a hierarchical categorization of noncovalent interactions based on the electrophilic atom involved, aligning with IUPAC definitions of hydrogen bonding (HB), halogen bonding (HaB), chalcogen bonding (ChB), and pnictogen bonding (PnB). The resulting taxonomy may avoid some ambiguities that arise from naming interactions based on single chemical/physical features. The proposed categorization that moves from more general and comprehensive terms to more specific and descriptive terms may ensure clarity, comprehensiveness, consistency with periodic trends, and invariancy over evolving understanding of the chemical bonds so that findings can be communicated and stored effectively via both human and machine based protocols
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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