H-1 NMR studies of proton transfer equilibria in hydrogen bonds - the roleof the entropy

Citation
R. Langner et G. Zundel, H-1 NMR studies of proton transfer equilibria in hydrogen bonds - the roleof the entropy, CAN J CHEM, 79(9), 2001, pp. 1376-1380
Citations number
19
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry
Journal title
CANADIAN JOURNAL OF CHEMISTRY-REVUE CANADIENNE DE CHIMIE
ISSN journal
0008-4042 → ACNP
Volume
79
Issue
9
Year of publication
2001
Pages
1376 - 1380
Database
ISI
SICI code
0008-4042(200109)79:9<1376:HNSOPT>2.0.ZU;2-3
Abstract
The influence of the entropy on the proton transfer equilibria AH . . .B re versible arrow A(-). . .H+B in the dimethylphosphinic acid (DMP)-N-base and in the methanesulfonic acid (MSA)-N-oxide hydrogen bonds is studied by H-1 NMR spectroscopy as a function of the Delta pK(a), i.e., the pK(a) of the base minus the pK(a) of the acid. It is shown that with increasing Delta pK (a) the proton transfers to the N-oxide acceptor. In the DMP + N-base famil y of systems, first the double-minimum proton potentials (DeltaH = 0) occur s and only with further increasing Delta pK(a) do the POH . . .N reversible arrow PO-. . .H+N equilibria become symmetrical (DeltaG = 0). DeltaG and D eltaH are connected by the Gibbs-Helmholtz equation, DeltaG = DeltaH -T Del taS. Hence, the reason of this effect is the large negative interaction ent ropy term (DeltaS(I)) arising from the large order around the polar structu re, which shifts the equilibria strongly to the left-hand side. With the MS A-N-oxide hydrogen bonds a single-minimum proton potential was found. The m inimum shifts with increasing Delta pK(a) from the donor to the acceptor of the O . . .H+. . .N hydrogen bonds. It is shown that with increasing Delta pK(a) the proton potential becomes on average symmetrical, whereas the pro ton is still largely on the left-hand side. This result is also explained b y the large negative interaction entropy due to the order of the environmen t if the proton is at the acceptor B.