Isotopic Consequences of Host–Guest Interactions; Noncovalent Chlorine Isotope Effects

Abstract

Although weak intermolecular interactions are the essence of most processes of key importance in medicine, industry, environment, and life cycles, their characterization is still not sufficient. Enzymatic dehalogenations that involve chloride anion interaction within a host–guest framework is one of the many examples. Recently published experimental results on host–guest systems provided us with models suitable to assess isotopic consequences of these noncovalent interactions. Herein, we report the influence of environmental and structural variations on chlorine isotope effects. We show that these effects, although small, may obscure mechanistic interpretations, as well as analytical protocols of dehalogenation processes.

Deuterium equilibrium isotope effects in a supramolecular receptor for the hydrochalcogenide and halide anions

“…To deconvolute and better understand the role of both C-H/D hydrogen bond donor polarity and solvent on the DEIE of Cl À binding in these receptors, a systematic study of these two variables would be required, similar to those previously reported, which we intend to pursue in future work. 14,27,28 Analysis of the data for competitive titrations of 2 H/D with Br À revealed no DEIE at any of the tracked 13 C NMR signals; however, each calculated DEIE has a relatively large percent error (0.99-4.64%, compared to 0.20% for the DEIE of Cl À binding), which could potentially obscure small DEIEs. We attribute these large percent errors to a limitation in the Perrin method that assumes that the hosts are fully bound by guest at saturation.…”

“…binding in these hosts. 13 Because isotope effects, both equilibrium and kinetic, originate solely from changes in the vibrational energy of the isotopically labelled bond, the EIE arising from this study came from changes in the vibrational energies of the bonds in the supramolecular hosts when participating in hydrogen bonding with Cl À isotopes. Indeed, a linear relationship was observed between the hydrogen bond donor (D) D-H bond lengths in the host-guest complex and the computed 35/ 37 Cl EIE.…”

“…In addition, 13 C NMR spectroscopy is sensitive to isotopic labelling and can show changes in chemical shis between isotopomers. We were able to differentiate between the 13 C NMR signals for C ab , C 1 and C 2 for free and bound 2 H and 2 D (Fig.…”

“…In an effort to decrease reactivity of HS À with 2 H/D and DMSO over long periods of time and decrease oxygen and water contaminations, some titrations with HS À were performed by splitting the host solution of 2 H/D between four Jyoung NMR tubes. For each point in the competitive titration, TBASH was added to a new solution of 2 H/D inside an N 2 -lled glovebox shortly before obtaining a 13 C NMR spectra (Titration method B in ESI †). The C ab , C 1 and C 2 13 C NMR signals were tracked for 2 H and 2 D in each titration for each anion.…”

“…In addition, we were unable to triplicate any DEIE from the data which showed a good linear t (R 2 > 0.99). We hypothesize that the high nucleophilicity and air and water sensitivity of HS À made it incompatible with the long experiment times needed for 13 C NMR spectroscopy titrations. In addition it is important to note that HS À is the only protic guest investigated in these studies, and it is also possible that vibrational coupling between the S-H motif and the receptor may further complicate the measurement of these small EIEs.…”