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

• Main Authors: Valeriia V. Mulloyarova, Daria O. Ustimchuk, Aleksander Filarowski, Peter M. Tolstoy
• Format: Article
• Language: English
• Published: MDPI AG 2020-04-01
• Series: Molecules
• Subjects: phosphinic acids; phosphoric acids; hydrogen bond; heterodimers; heterotrimers; heterotetramers
• Online Access: https://www.mdpi.com/1420-3049/25/8/1907

Hydrogen-bonded heterocomplexes formed by POOH-containing acids (diphenylphosphoric <b>1</b>, dimethylphosphoric <b>2</b>, diphenylphosphinic <b>3</b>, and dimethylphosphinic <b>4</b>) 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 <b>1</b> and <b>4</b> (or <b>1</b> and <b>3</b>) 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 <b>1</b> and <b>4</b> the structure has alternating molecules of <b>1</b> and <b>4</b> in the cycle, while in case of acids <b>1</b> and <b>3</b> the cycle has two molecules of <b>1</b> followed by two molecules of <b>3</b>.