Kinetic isotope effects and tunneling in cyclic double and triple proton transfer between acetic acid and methanol in tetrahydrofuran studied by dynamic /sup 1/H and /sup 2/H NMR spectroscopy

Journal Article · Wed Feb 22 00:00:00 EST 1984 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00316a007· OSTI ID:6313519

Gerritzen, D; Limbach, H H

Previous studies of proton exchange and hydrogen bonding between acetic acid (A) and methanol (B) in tetrahydrofuran-d/sub 8/ have been extended to the study of the primary kinetic H/D isotope effects of the exchange. For this purpose a new combination of dynamic /sup 1/H and /sup 2/H NMR spectroscopy has been used to perform an ''NMR proton inventory''. The following rate law was obtained at deuterium fractions D=0 and D=1 of the exchangeable protonic sites: NU=k/sup LL/ C/sub A/C/sub B/+k/sub LLL/C/sub A//sup 2/C/sub B/ (L=H,D). This was attributed to a superposition of cyclic double and triple proton exchange involving one and two molecules of acetic acid and one molecule of methanol. Additional experiments were carried out at intermediate deuterium fractions. Thus, the kinetic HH/HD/DD and HHH/HHD/DDD isotope effects of the exchange as a function of the temperature was measured successfully. This has been achieved for the first time for well-defined intermolecular multiple-proton-transfer reactions. The possibility of determining the number of protons transferred in a chemical reaction by performing an NMR proton inventory is discussed. The rule of the geometric mean (RGM) is fulfilled for the kinetic isotope effects of the LLL process, which are almost independent of temperature within the margin of error. By contrast, the RGM is not fullfilled for the LL process, and the kinetic isotope effects depend strongly on the temperature. The energies of activation and frequency factors fit Bell's criteria of tunneling. The kinetic results are in good agreement with predictions of transition-state theory but can be explained by an intermolecular tunneling model. The results are proof that acetic acid and methanol form cyclic hydrogen-bonded 1:1 and 2:1 complexes which have very low concentration in tetrahydrofuran.

• Research Organization: Universitaet Freiburg, Germany
• OSTI ID: 6313519
• Journal Information: J. Am. Chem. Soc.; (United States), Vol. 106:4
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ACETIC ACID
NMR SPECTRA
PROTON TRANSPORT
DEUTERIUM
ISOTOPE EFFECTS
METHANOL
EXPERIMENTAL DATA
MATHEMATICAL MODELS
TEMPERATURE EFFECTS
TETRAHYDROFURAN
ALCOHOLS
CARBOXYLIC ACIDS
CHARGED-PARTICLE TRANSPORT
DATA
FURANS
HETEROCYCLIC COMPOUNDS
HYDROGEN ISOTOPES
HYDROXY COMPOUNDS
INFORMATION
ISOTOPES
LIGHT NUCLEI
MONOCARBOXYLIC ACIDS
NUCLEI
NUMERICAL DATA
ODD-ODD NUCLEI
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
RADIATION TRANSPORT
SPECTRA
STABLE ISOTOPES
400301* - Organic Chemistry- Chemical & Physicochemical Properties- (-1987)
400302 - Organic Chemistry- Isotope Effects- (-1987)