Deuterium isotope effects on 15N, 13C and 1H chemical shifts of proton sponges

Autor: J. Nowicka-Scheibe, Wojciech Schilf, Jens Abildgaard, Eugeniusz Grech, J. Klimkiewicz, Valery A. Ozeryanskii, Mariusz Pietrzak, Simon Bolvig, Poul Erik Hansen, A.F Pozharski
Rok vydání: 2002
Předmět:
Zdroj: Journal of Molecular Structure. 615:121-140
ISSN: 0022-2860
DOI: 10.1016/s0022-2860(02)00216-8
Popis: Deuterium isotope effects on 15N, 13C and 1H chemical shifts have been measured in the protonated forms of DMAN, 4-bromo, 4-picryl, 4-nitro and 2- and 2,7-chloro derivatives. Structures have been geometry optimised using BPW91/6-31(d)G density functional theory (DFT) methods and show good correspondence to experimental X-ray data. 15N, 13C and 1H chemical shifts are calculated using GIAO/DFT methods. Chemical shifts for the two tautomers are calculated. Equilibrium constants are determined from 1J(N,H) coupling constants. A comparison of 1J(N,H) and 1J(N,D) couplings for the non-symmetrical derivatives show that upon deuteriation the equilibrium is shifted towards the dominant form. The experimental deuterium isotope effect on chemical shifts of DMANH+ shows only small values despite the apparently strong intramolecular hydrogen bonds as judged from primary isotope effects. The counter ion has little effect on the one-bond isotope effect. Secondary isotope effects on 15N and 13C chemical shifts for the non-symmetrical derivatives are divided into intrinsic and equilibrium contributions. The one-bond deuterium intrinsic isotope effect on 15N chemical shifts is close to 0.3 ppm and is related to charge delocalisation and the close proximity of the positive charge. The equilibrium contributions to the isotope effects at the nitrogens and at carbons are seen to increase throughout the series in parallel with the increase of the equilibrium constant. For 1H resonances only equilibrium contributions are found at the aromatic hydrogens. An important feature in the analysis of the protonated DMANs is the multi-isotope effect approach.
Databáze: OpenAIRE
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