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| 2. |
- Hakkert, A, et al.
(författare)
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Choosing a system to control level crossing signaling devices
- 1996
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Ingår i: Proceedings of Road safety in Europe and Strategic Highway Research Program (SHRP). Conference in Prague, the Czech Republic, September 20-22, 1995. - Linköping : Statens väg- och transportforskningsinstitut. ; , s. 51-56
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 3. |
- Hakkert, Sebastiaan B., et al.
(författare)
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Halogen bond symmetry: The N-X-N bond
- 2015
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Ingår i: Journal of Physical Organic Chemistry. - : Wiley. - 0894-3230 .- 1099-1395. ; 28:3, s. 226-233
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Tidskriftsartikel (refereegranskat)abstract
- Halogen bonding is the attractive interaction of an electrophilic region of a halogen with a nucleophile. Its geometry, energy, and dominantly electrostatic nature resemble that of the hydrogen bond. Halogen bond strength critically depends on the electron deficiency of the halogen. Accordingly, halonium ions that bear a formal positive charge are the strongest halogen bond donors known so far. Halonium ions, similar to H+, are capable of simultaneously interacting with two Lewis bases. The structure and properties of the resulting three-center halogen bonds are reviewed herein, based on their spectroscopic and computational data, with the emphasis being placed on one representative example, the [N-X-N]+ interaction. The iodine and bromine-centered systems form symmetric three-center-four-electron halogen bonds that have comparable orbital and electrostatic contributions and a large extent of charge delocalization. Their formation yields significant stabilization. The [N-Cl-N]+ bond is computationally predicted to resemble the symmetry of the three-center bond of the heavier halogens, whereas the fluorine-centered halogen bond is predicted to be asymmetric, and thus behave more similar to the analogous, asymmetric three-center [N-H-N]+ hydrogen bond. Experimental confirmation of the latter predictions remains to be accomplished, along with studies of the possible influence of the disorder of the local environment on halogen bond symmetry, and investigations in the gaseous phase. The electronic structure and the geometry of three-center halogen bonds are of fundamental interest, and also of practical importance, for example due to the occurrence of three-centered halogen bonds in synthetic reagents.
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| 4. |
- Hakkert, Sebastiaan B., et al.
(författare)
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The N-15 NMR chemical shift in the characterization of weak halogen bonding in solution
- 2017
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Ingår i: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1359-6640 .- 1364-5498. ; 203, s. 333-346
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Tidskriftsartikel (refereegranskat)abstract
- © The Royal Society of Chemistry. We have studied the applicability of 15 N NMR spectroscopy in the characterization of the very weak halogen bonds of nonfluorinated halogen bond donors with a nitrogenous Lewis base in solution. The ability of the technique to detect the relative strength of iodine-, bromine- and chlorine-centered halogen bonds, as well as solvent and substituent effects was evaluated. Whereas computations on the DFT level indicate that 15 N NMR chemical shifts reflect the diamagnetic deshielding associated with the formation of a weak halogen bond, the experimentally observed chemical shift differences were on the edge of detectability due to the low molar fraction of halogen-bonded complexes in solution. The formation of the analogous yet stronger hydrogen bond of phenols have induced approximately ten times larger chemical shift changes, and could be detected and correlated to the electronic properties of substituents of the hydrogen bond donors. Overall, 15 N NMR is shown to be a suitable tool for the characterization of comparably strong secondary interactions in solution, but not sufficiently accurate for the detection of the formation of thermodynamically labile, weak halogen bonded complexes.
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