A model system for the investigation of intramolecular halogen bonds is introduced. Two molecules capable of intramolecular halogen bonding have been studied in comparison with eight control compounds by 15N, 13C, and 19F NMR spectroscopy. Iodine‐ and bromine‐centered halogen bonds are indicated by decreases in the 15N NMR chemical shifts of the halogen bond acceptor atom of approximately 6 and 1 ppm, respectively. 13C NMR chemical shifts of the alkynyl carbons in 2‐ethynylpyridine systems are good indicators of halogen bonding, with differences of up to 2.4 ppm between halogen‐bonded and related control compounds. Halogen bond strengths in different solvents, as indicated by 19F NMR chemical shifts, decrease in the following order: Cyclohexane > toluene > benzene > dichloromethane > acetone > pyridine. Chemical shift effects associated with the structural and electronic properties of intramolecular halogen‐bonded systems are modeled well by calculations at the B3LYP/6‐311+G(2d,p) level of theory.
The electronic properties of a pyrazine-containing arylene ethynylene unit are influenced by hydrogen bond and halogen bond donors that are held in proximity of the pyrazine rotor. These interactions are evident with iodine- and bromine-centered halogen bonds and O-H- and C-H-based hydrogen bonds. Bathochromic shifts of UV-vis and fluorescence signals are the best indicators of this intramolecular attraction. The effects can be attenuated in solvents that are less favorable for intramolecular halogen or hydrogen bonding, such as 2-propanol, and amplified in solvents that are supportive, such as toluene. Intramolecular attractions promote planarity in the pyrazine ethynylene system, likely increasing the effective conjugation of the unsaturated backbone. Additionally, computations at the B3LYP and M062X levels of theory using 6-311++G(2d,p) and aug-cc-pVTZ basis sets suggest that the Lewis acidity of the halogen and hydrogen atoms influences electronic behavior even in the absence of conformational constraints.
The cover picture shows two triangular structures capable of intramolecular halogen bonding. By holding halogen bond donor and acceptor groups in close proximity, the rigid arylethynyl backbone eliminates the entropic challenges typically associated with the study of halogen bonding in solution. By using this template, the effects of halogen bonding on 13C, 15N and 19F NMR chemical shifts could be studied in dilute solutions with a variety of solvents and a range of temperatures. The results are supported by comparisons to eight model compounds and by computational predictions at the B3LYP/6‐311+G(2d,p) level of theory. Details are discussed in the article by N. P. Bowling et al. on . The authors are grateful to Julie A. Sittler, MFA Graphic Design, for her contribution of the cover graphic.
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