The NMR parameters (1H and 13C chemical shifts and coupling constants) for a series of naturally occurring molecules have been calculated mostly with DFT methods, and their spectra compared with available experimental ones. The comparison includes strychnine as a test case, as well as some examples of recently isolated natural products (corianlactone, daphnipaxinin, boletunone B) featuring unusual and/or crowded structures and, in the case of boletunone B, being the subject of a recent revision. Whenever experimental spectra were obtained in polar solvents, the calculation of NMR parameters was also carried out with the Integral Equation-Formalism Polarizable Continuum Model (IEF-PCM) continuum method. The computed results generally show a good agreement with experiment, as judged not only by statistical parameters but also by visual comparison of line spectra. The origin of the remaining discrepancies is attributed to the incomplete modeling of conformational and specific solvent effects.

Nuclear shieldings, including the Fermi contact and pseudocontact terms, have been calculated with DFT methods in a variety of open-shell molecules (nitroxides, aryloxyl and various transition-metal complexes), thereby predicting (1)H and (13)C chemical shifts. In general, when experimental data are reliable a good agreement with experimental values is observed, thus demonstrating the predictive power of DFT also in this context. However, the general accuracy is lower than that for closed-shell species. A few inconsistencies in literature values are reconciled by reassigning some shifts. Structural, magnetic, and dynamic parameters have also been put into the Solomon-Bloembergen equations to predict signal line shapes, in particular those of signals that are difficult to locate or are undetectable. Guidelines are provided to predict the order of magnitude of relaxation rates. It is shown that DFT-predicted paramagnetic shifts can greatly assist in obtaining and understanding the NMR spectra of paramagnetic molecules, which generally require different experimental strategies and exhibit problems in detection and assignment.

The decatungstate anion [W10O32]4– is a widely used photocatalyst for promoting hydrogen atom transfer (HAT) reactions. The mechanism implicated in the activation of organic substrates, however, still needs to be clarified and has been claimed to involve an unknown relaxed excited state of triplet multiplicity, tagged wO. A subpicosecond investigation allowed us to follow early events leading to the chemically reactive species. A hot singlet excited state (S1 HOT) has been individuated through pump–probe experiments, yielding S1 by ultrafast decay (<1 ps). The reactive species wO arises from S1 in competition with decay to S0 (efficiency ca. 0.5) and has been detected spectroscopically by flash photolysis experiments, with peculiar absorption bands in the near-UV (370 nm) and visible (600–800 nm) regions. TD-DFT calculations demonstrated that excitation to S1 occurs through a ligand to metal charge transfer (LMCT) transition, involving a displacement of electron density from dicoordinated (bridging) oxygen to tungsten atoms. Population of wO ensues and involves a reorganization of the singly occupied orbital centered on oxygen (not tungsten) atoms. As a result, monocoordinated O centers acquire a partial radical character that well explains the known chemistry, essentially hydrogen atom transfer (HAT), and highlights the similarity with nπ* carbonyl triplets. This rationalization may help in devising other photocatalysts able to promote HAT processes from unactivated precursors.

C chemical shifts and n J CH coupling constants have been determined both experimentally (by means of J-resolved NMR spectroscopy) and theoretically (by DFT calculations) for a series of organic molecules. With the exception of halogen-bonded carbon nuclei, a good correlation is observed between experimental and calculated data. The magnitude of the most important contributions to the spin-spin coupling constant (Fermi-contact, diamagnetic, and paramagnetic spin-orbit contributions) has been determined. The spinorbit terms are negligible or cancel out ( 1 J CH and 3 J CH ), thus leaving the contact term as the only relevant contribution, but become important for 2 J CH in aromatic (but not in aliphatic) compounds. Relativistic effects on the 13 C chemical shift of carbon bonded to a fairly heavy atom (bromine) have also been investigated. Finally, conformational effects on the long-range n J CH coupling constants has been investigated in a model alkane derivative (n-butyl chloride). The implications to structure prediction and determination by NMR are discussed.