In:
Journal of Physical Organic Chemistry, Wiley, Vol. 18, No. 3 ( 2005-03), p. 240-249
Abstract:
The restricted rotation about the conjoining bond in a series of 4″‐substituted bi‐imidazole nucleosides {5‐amino‐4‐[4″‐R‐imidazol‐2″‐yl]‐1‐(β‐ d ‐ribofuranosyl)‐1 H ‐imidazole; where R=H, methyl, hydroxymethyl, oxalo, formyl} due to intramolecular hydrogen bonding between N‐3″ and the N‐6 protons concomitant with prototropic tautomerism has been examined using multinuclear ( 1 H, 13 C and 15 N) experimental NMR. Substitution at the 4″ position causes the interconversion, whilst still an intramolecular process, to yield complex spectra as the dynamic process consists of a two‐site exchange between non‐degenerate tautomeric forms (asymmetric sites). The preferred tautomers were identified experimentally in each case and compared with theoretically determined structures geometry optimized using density functional theory (DFT) at the B3LYP/6–31G(d,p) level of theory on which gauge‐independent atomic orbital‐DFT (GIAO‐DFT) computations at the B3LYP/cc‐pVTZ level of theory were applied to calculate the chemical shifts of the 1 H, 13 C and 15 N nuclei. Both the site and the extent of protonation of the bi‐imidazole nucleosides were also similarly ascertained using the same methodology. Protonation at the pyridine‐type nitrogen (N‐3″) of the outer imidazole ring, the principle site of protonation, effectively eliminated the barrier to rotation about the conjoining bond yielding time‐averaged spectra experimentally. Copyright © 2004 John Wiley & Sons, Ltd.
Type of Medium:
Online Resource
ISSN:
0894-3230
,
1099-1395
Language:
English
Publisher:
Wiley
Publication Date:
2005
detail.hit.zdb_id:
1475025-9
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