Liquid state 1H and 19F NMR experiments in the temperature range between 110 and 150 K have been performed on mixtures of tetrabutylammonium fluoride with HF dissolved in a 1:2 mixture of CDF3 and CDF2Cl. Under these conditions hydrogen bonded complexes between F− and a varying number of HF molecules were observed in the slow proton and hydrogen bond exchange regime. At low HF concentrations the well known hydrogen bifluoride ion [FHF]− is observed, exhibiting a strong symmetric H‐bond. At higher HF concentrations the species [F(HF)2]−, [F(HF)3]− are formed and a species to which we assign the structure [F(HF)4]−. The spectra indicate a central fluoride anion which forms multiple hydrogen bonds to HF. With increasing number of HF units the hydrogen bond protons shift towards the terminal fluorine's. The optimized gas‐phase geometries of [F(HF)n]−, n = 1 to 4, calculated using ab initio methods confirm the D∞h, C2v, D3h and Td symmetries of these ions. For the first time, both one‐bond couplings between a hydrogen bond proton and the two heavy atoms of a hydrogen bridge, here 1JHF and 1JHF where |1JHF|≥|1JHF'|, as well as a two‐bond coupling between the heavy atoms, here 2JFF, have been observed. The analysis of the differential width of various multiplet components gives evidence for the signs of these constants, i.e. 1JHF and 2JSF>0, and 1JHF|. <0. Ab initio calculations of NMR chemical shifts and the scalar coupling constants using the Density Functional formalism and the Multi‐configuration Complete Active Space method show a reasonable agreement with the experimental parameters and confirm the covalent character of the hydrogen bonds studied.

NMR studies have been performed on the association of 3′,5′-di-O-acetyl-2′-deoxyuridine with a 2′-deoxyadenosine derivative in solution. Using a CDClF 2 /CDF 3 solvent mixture, measurements at very low temperatures allowed the observation of individual complexes in the slow-exchange regime. From 2D NOE connectivities of the imino proton resonances at 125 K the predominant species could be assigned to a Watson-Crick geometry. However, concentration-dependent 15 N chemical shifts of specifically labeled uridine and adenosine nucleosides indicate a considerable population of Hoogsteen base pairs at ambient temperatures. These results together with a more downfield chemical shift of the Watson-Crick bound imino proton point to a stronger hydrogen bond in Watson-Crick when compared to Hoogsteen base pairs.

Association constants for the dimerization of acylated uridine and two analogues, 4-thiouridine and 6-oxadihydrouridine, have been determined in chloroform solution. At 293 K, self-association constants K were found to decrease in the order uridine b 6-oxadihydrouridine b 4-thiouridine. Lowtemperature 1D and 2D NMR measurements in a deuterated freon mixture allowed the unambiguous assignment of the different dimeric species formed. For homodimers of uridine, the 2-and 4-carbonyl groups act as H-bond acceptors with equal frequency, whereas for the 6-oxa analogue the 2-carbonyl is strongly favored over the 4-carbonyl group. No hydrogen bonding of the 4-S atom is observed in the 4-thio analogue. In agreement with the more negative atomic charge on the O4 oxygen predicted from ab initio calculations, 1 H chemical shifts of the H-bonded proton in uridine dimers indicate a stronger hydrogen bond in the 4-as compared to the 2-position.

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