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1. Preferential N–H⋯:C hydrogen bonding involving ditopic NH -containing systems and N -heterocyclic carbenes

   https://doi.org/10.1039/d0ra08490e  

   Kinney, Zacharias J. ; Rheingold, Arnold L. ; Protasiewicz, John D. ( November 2020 , RSC Advances)

   null (Ed.)

   Hydrogen bonding plays a critical role in maintaining order and structure in complex biological and synthetic systems. N -heterocyclic carbenes (NHCs) represent one of the most versatile tools in the synthetic chemistry toolbox, yet their potential as neutral carbon hydrogen bond acceptors remains underexplored. This report investigates this capability in a strategic manner, wherein carbene-based hydrogen bonding can be assessed by use of ditopic NH -containing molecules. N–H bonds are unique as there are three established reaction modes with carbenes: non-traditional hydrogen bonding adducts (X–H⋯:C), salts arising from proton transfer ([H–C] + [X] − ), or amines from insertion of the carbene into the N–H bond. Yet, there are no established rules to predict product distributions or the strength of these associations. Here we seek to correlate the hydrogen bond strength of symmetric and asymmetric ditopic secondary amines with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene ( IPr , a representative NHC). In symmetric and asymmetric ditopic amine adducts both the solid-state (hydrogen bond lengths, NHC interior angles) and solution-state ( 1 H Δ δ of NH signals, 13 C signals of carbenic carbon) can be related to the p K a of the parent amine. 

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2. π-Complexation and C—H hydrogen bonding in the formation of colored cocrystals

   https://doi.org/10.1107/S2053229623002231  

   Bosch, Eric ; Moreno, Bryce S. ; Bowling, Nathan P. ( April 2023 , Acta Crystallographica Section C Structural Chemistry)

   The present study evaluates the potential combination of charge-transfer electron-donor–acceptor π–π complexation and C—H hydrogen bonding to form colored cocrystals. The crystal structures of the red 1:1 cocrystals formed from the isomeric pyridines 4- and 3-{2-[4-(dimethylamino)phenyl]ethynyl}pyridine with 1-[2-(3,5-dinitrophenyl)ethynyl]-2,3,5,6-tetrafluorobenzene, both C 14 H 4 F 4 N 2 O 4 ·C 15 H 14 N 2 , are reported. Intermolecular interaction energy calculations confirm that π-stacking interactions dominate the intermolecular interactions within each crystal structure. The close contacts revealed by Hirshfeld surface calculations are predominantly C—H interactions with N, O, and F atoms. 

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3. Synthesis of novel copper-rare earth BINOLate frameworks from a hydrogen bonding DBU-H rare earth BINOLate complex

   https://doi.org/10.1039/c8dt03335h  

   Panetti, Grace B. ; Robinson, Jerome R. ; Carroll, Patrick J. ; Gau, Michael R. ; Manor, Brain C. ; Walsh, Patrick J. ; Schelter, Eric J. ( October 2018 , Dalton Transactions)

   The preparation of a novel H-bonding DBU-H + BINOLate Rare Earth Metal complex enabled the synthesis of the first copper-Rare Earth Metal BINOLate complex (CuDBU-REMB). CuDBU-REMB was compared to the analogous Li complex using X-ray crystallography and Exchange NMR spectroscopy (EXSY). The results provide insight into the role of the secondary metal cation in the framework's stabilization. 

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4. C–H⋯S hydrogen bonding interactions

   https://doi.org/10.1039/D1CS00838B  

   Fargher, Hazel A. ; Sherbow, Tobias J. ; Haley, Michael M. ; Johnson, Darren W. ; Pluth, Michael D. ( February 2022 , Chemical Society Reviews)

   The short C–H⋯S contacts found in available structural data for both small molecules and larger biomolecular systems suggest that such contacts are an often overlooked yet important stabilizing interaction. Moreover, many of these short C–H⋯S contacts meet the definition of a hydrogen bonding interaction. Using available structural data from the Cambridge Structural Database (CSD), as well as selected examples from the literature in which important C–H⋯S contacts may have been overlooked, we highlight the generality of C–H⋯S hydrogen bonding as an important stabilizing interaction. To uncover and establish the generality of these interactions, we compare C–H⋯S contacts with other traditional hydrogen bond donors and acceptors as well as investigate how coordination number and metal bonding affect the preferred geometry of interactions in the solid state. This work establishes that the C–H⋯S bond meets the definition of a hydrogen bond and serves as a guide to identify C–H⋯S hydrogen bonds in diverse systems. 

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5. A comparison between hydrogen and halogen bonding: the hypohalous acid–water dimers, HOX⋯H 2 O (X = F, Cl, Br)

   https://doi.org/10.1039/C9CP00422J  

   Wolf, Mark E. ; Zhang, Boyi ; Turney, Justin M. ; Schaefer, Henry F. ( March 2019 , Physical Chemistry Chemical Physics)

   Hypohalous acids (HOX) are a class of molecules that play a key role in the atmospheric seasonal depletion of ozone and have the ability to form both hydrogen and halogen bonds. The interactions between the HOX monomers (X = F, Cl, Br) and water have been studied at the CCSD(T)/aug-cc-pVTZ level of theory with the spin free X2C-1e method to account for scalar relativistic effects. Focal point analysis was used to determine CCSDT(Q)/CBS dissociation energies. The anti hydrogen bonded dimers were found with interaction energies of −5.62 kcal mol −1 , −5.56 kcal mol −1 , and −4.97 kcal mol −1 for X = F, Cl, and Br, respectively. The weaker halogen bonded dimers were found to have interaction energies of −1.71 kcal mol −1 and −3.03 kcal mol −1 for X = Cl and Br, respectively. Natural bond orbital analysis and symmetry adapted perturbation theory were used to discern the nature of the halogen and hydrogen bonds and trends due to halogen substitution. The halogen bonds were determined to be weaker than the analogous hydrogen bonds in all cases but close enough in energy to be relevant, significantly more so with increasing halogen size. 

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