Motional Dynamics of Halogen‐Bonded Complexes Probed by Low‐Field NMR Relaxometry and Overhauser Dynamic Nuclear Polarization

Abstract

Halogen bonding is asubject of considerable interest owing to wide-ranging chemical, materials and biological applications. The motional dynamics of halogenbonded complexes play ap ivotalr ole in comprehending the nature of the halogen-bonding interaction. However, not many attempts appear to have been made to shed light on the dynamical characteristics of halogen-bonded species. For the first time, we demonstrateh ere that the combination of low-field NMR relaxometry andO verhauser dynamic nuclear polarization (ODNP) makes it possible to obtain ac ogentp icture of the motional dynamics of halogen-bonded species. We discussh ere the advantages of this combined approach. Low-field relaxometry allows us to infer the hydrodynamic radius and rotational correlation time, whereas ODNP probest he molecular translational correlation times (involving the substrate as well as the organic radical) with high sensitivity at low field. Noncovalent interactions are among the most discussed and debated areas in contemporaryc hemistry and biology.S everal experimentala nd theoretical efforts have been made to characterizea nd classify these interactions.A mong several such interactions H-bonding, halogenb onding, etc. play an important role in chemistry. [1] Halogen bonding (XB) has been defined as an attractive interaction between an electrophilicr egion aroundahalogen atom (X) in am olecule or moiety,a nd an ucleophilic region (B) in another molecule or moiety.S tructurally,t he electrophilic region at the halogen atom has been associated with a" s hole". In recent times, halogen bondingh as impacted diverse fields including supramolecular chemistry,o rganic synthesis, catalysis, separation chemistry,b iology,e tc. [2] In adducing evidencef or halogen bonding av arietyo ft heoretical studies have been reported, besidese xperimental structural studies in solution and solid state. [3] NMR [4a-m] andE SR [5] investigations are among major experimentalt echniques employed in recent times. In particular, one may mention NMR chemicals hift titration, [4a] heteronuclear Overhauser effect (NOE) measurements, [4g] as well as NMR self-diffusion measurements [4h] on halogen bondedc omplexes. ESR studies have typically involved an itroxide free radicalt hat doubles up as the nucleophile. [5] In our view,m ost such studies reported thus far do not directly establish the motional dynamics of "halogen bonded" species, but principally infer the presence of some form of interaction, for example, either by NMR chemical shift changes or by changes in self-diffusion coefficients measured by NMR. The NMR chemical shift( 1 H/ 19 F) titrationa pproach runs into difficulties when small or negligible chemical shift change is observed, and the stoichiometry of the adduct is unknown.Interpretations of NOE 4g measurements are often vitiated by employing at wo-spin model well beyond its region of approximate validity in am ultispin system, i.e.,b eyond the initial build-up rate regime. NOE'sa lso becomev ery weak on the rotational model if the s...