We report a systematic solid-state (17)O NMR study of free nucleic acid bases: thymine (T), uracil (U), cytosine (C), and guanine (G). Site-specifically (17)O-enriched samples were synthesized:[2-(17)O]thymine (1), [4-(17)O]thymine (2), [2-(17)O]uracil (3), [4-(17)O]uracil (4), [2-(17)O]cytosine (5), and [6-(17)O]guanine monohydrate (6). Magic-angle-spinning (MAS) and static (17)O NMR spectra were acquired at 11.75 T for compounds 1-6, from which information about the (17)O chemical shift and electric field gradient tensors was obtained. Extensive quantum chemical calculations were performed at the B3LYP/6-311++G(d,p) level of theory for (17)O NMR properties in various molecular models. The calculated (17)O NMR tensors are highly sensitive to the description of intermolecular hydrogen-bonding interactions at the target oxygen atom. A reasonably good agreement between experimental solid-state (17)O NMR data and B3LYP/6-311++G(d,p) calculations is achievable only in molecular cluster models where a complete hydrogen-bond network is considered. Using this theoretical approach, we also investigated the (17)O NMR tensors in two unusual structures: guanine- and uracil-quartets.

We report the first experimental determination of the carbonyl 17O electric-field-gradient (EFG) tensor and chemical-shift (CS) tensor of a urea-type functional group, R1NHC(O)NHR2. Analysis of magic-angle spinning (MAS) and stationary 17O NMR spectra of crystalline [17O]urea yields not only the principal components of the carbonyl 17O EFG and CS tensors, but also their relative orientations. The carbonyl 17O quadrupole coupling constant (QCC) and the asymmetry parameter (η) in crystalline urea were found to be 7.24 ± 0.01 MHz and 0.92, respectively. The principal components of the 17O CS tensor were determined:  δ11 = 300 ± 5, δ22 = 280 ± 5 and δ33 = 20 ± 5 ppm. The direction with the least shielding, δ11, is perpendicular to the CO bond and the principal component corresponding to the largest shielding, δ33, is perpendicular to the NC(O)N plane. The observed 17O CS tensor suggests that, in crystalline urea, the 17O paramagnetic shielding contributions from the σ → π* and π → σ* mixing are greater than that from the n → π* mixing. Quantum chemical calculations revealed very large intermolecular H-bonding effects on the 17O NMR tensors. It is demonstrated that inclusion of a complete intermolecular H-bonding network is necessary in order to obtain reliable 17O EFG and CS tensors. B3LYP/D95** and B3LYP/6-311++G** calculations with a molecular cluster containing 7 urea molecules yielded 17O NMR tensors in reasonably good agreement with the experimental data.

No abstract

We report solid-state 17O NMR determinations of the oxygen chemical shift (CS) and electric field gradient (EFG) tensors for a series of 17O-enriched organic compounds containing various functional groups. In several cases, analysis of the n O magic-angle-spinning (MAS) and static NMR spectra yields both the magnitude and relative orientations of the 17O CS and EFG tensors. We also demonstrate the feasibility of solid-state 17O NMR as a potentially useful technique for studying molecular structure and hydrogen bonding.

We report a solid-state 17 O NMR study of the 17 O electric field gradient (EFG) and chemical shielding (CS) tensors for the oxonium ion, H 3 O + , in p-toluenesulfonic acid monohydrate (TAM). Both the 17 O EFG and CS tensors of the H 3 O + ion are axially symmetric within the experimental errors. The 17 O quadrupole coupling constant (QCC) is found to be 7.05 ( 0.02 MHz, and the 17 O chemical shift anisotropy (CSA) is 87 ( 5 ppm. Experimental results are compared with extensive quantum chemical calculations using restricted HartreeFock approach (RHF), second-order Møller-Plesset perturbation theory (MP2), and density functional theory (DFT). The calculations showed that the strong hydrogen-bonding environment around the H 3 O + ion in TAM is responsible for a reduction of approximately 3 MHz in the 17 O QCC compared to that of an isolated H 3 O + ion. The effective 17 O quadrupole moment is calibrated at the B3LYP/cc-pVTZ level, Q ) -2.400 fm 2 . Using this value, we obtained the best calculated 17 O QCC for the "bound" H 3 O + ion, +7.382 MHz, which is in reasonably good agreement with the observed value. The 17 O chemical shielding tensor is also calculated using the GIAO (gauge-including atomic orbital) approach. Although the calculated isotropic 17 O chemical shifts are in excellent agreement with the experimental data, the calculations with all the basis sets employed in the present study invariably underestimated 17 O CSAs by approximately 20 ppm.