H/D Isotope Effects on 1H-NMR Chemical Shifts in Cyclic Heterodimers and Heterotrimers of Phosphinic and Phosphoric Acids

Abstract

Hydrogen-bonded heterocomplexes formed by POOH-containing acids (diphenylphosphoric 1, dimethylphosphoric 2, diphenylphosphinic 3, and dimethylphosphinic 4) are studied by the low-temperature (100 K) 1H-NMR and 31P-NMR using liquefied gases CDF3/CDF2Cl as a solvent. Formation of cyclic dimers and cyclic trimers consisting of molecules of two different acids is confirmed by the analysis of vicinal H/D isotope effects (changes in the bridging proton chemical shift, δH, after the deuteration of a neighboring H-bond). Acids 1 and 4 (or 1 and 3) form heterotrimers with very strong (short) H-bonds (δH ca. 17 ppm). While in the case of all heterotrimers the H-bonds are cyclically arranged head-to-tail, ···O=P–O–H···O=P–O–H···, and thus their cooperative coupling is expected, the signs of vicinal H/D isotope effects indicate an effective anticooperativity, presumably due to steric factors: when one of the H-bonds is elongated upon deuteration, the structure of the heterotrimer adjusts by shortening the neighboring hydrogen bonds. We also demonstrate the formation of cyclic tetramers: in the case of acids 1 and 4 the structure has alternating molecules of 1 and 4 in the cycle, while in case of acids 1 and 3 the cycle has two molecules of 1 followed by two molecules of 3.

Inter- vs. Intramolecular Hydrogen Bond Patterns and Proton Dynamics in Nitrophthalic Acid Associates

“…This problem can be solved using a low-freezing solvent [ 89 ]. In this solvent complex, hydrogen-bonded systems can be characterized in great detail [ 90 , 91 , 92 ]. However, such experiments are not without their problems.…”

Self-Assembly of Hydrogen-Bonded Cage Tetramers of Phosphonic Acid

“…The entropy loss can be small if the molecules retain high conformational mobility relative to each other within the complex [15], or the entropy loss could be compensated by the high complexation enthalpy. Previously, hydrogen-bound cyclic (ring) tetramers were found in crystals [1,[65][66][67] or in solution [33,68], and tetramers were also considered experimentally (IR) [69] and theoretically [70] for methanol, but in the latter case, the structure with tetrahedral symmetry (with the equivalence of all four molecules) was obtained only dynamically, as a result of successive proton transfers and rearrangements of hydrogen bonds in the cluster. As far as we know, experimental proof of the existence of stable hydrogen-bonded "tetrahedral" tetramers in an aprotic medium is presented here for the first time.…”

“…Anions like P(O − )(OH) 2 are able to form linear oligomers as well [32]. Besides, recently we also studied hetero-complexes formed by molecules of two different POOH-containing acids (namely, diphenylphosphoric, dimethylphosphoric, diphenylphosphinic, and dimethylphosphinic ones), using low-temperature NMR spectroscopy and the partial hydrogen/deuterium (H/D) substitution in the mobile proton sites [33]. For such systems we showed the formation of cyclic hetero-tetramers, in addition to hetero-dimers and hetero-trimers.…”

Solid State NMR for Nonexperts: An Overview of Simple but General Practical Methods

“…The 31 P isotope is present in 100% natural abundance, has a spin quantum number of 1/2 and a chemical shift range of more than 400 ppm. 31 P NMR can be routinely used in the evaluation of the structure of organic complexes in solution [90,91] and solids [92][93][94] including organometallic compounds [26,[95][96][97][98]. Molecules containing phosphorus are very convenient NMR probes for studying mobility at interfaces [99][100][101].…”