The contrasting geometrical features between organic and inorganic counterparts of amines and oxanes are explained in terms of an offset between attractive (donor–acceptor) and repulsive (donor–donor) interactions. Natural bond orbital (NBO) calculations carried out at the density functional theory level of theory reveal that hyperconjugative effects in the organic amines and ethers are overcome by repulsive interactions occurring between the lone pair on the nitrogen/oxygen atom and the adjacent σ­(C–R) bond orbitals. Although displaying lower energies than in the corresponding organic derivatives, the LP­(X) → σ*­(E–R) (X = N, O; E = Si, Ge, Sn) interactions in heavier counterparts overcome the LP­(X)···σ­(E–R) repulsions, impacting thus their structural behavior. In addition, NBO deletion optimizations emphasize that among hyperconjugations, back-bonding effects of the LP­(X) → d­(E) type dictate to a lesser extent the anomalous structures of the inorganic amines and oxanes.

Nuclear magnetization storage, once limited by longitudinal and transverse relaxation lifetimes, T 1 and T 2, can be prolonged by symmetry-adapted nuclear spin order, i.e. long-lived states (LLS) and long-lived coherences (LLC), which have significantly extended relaxation time constants compared to T 1 and T 2, respectively. Excitation and/or detection of LLS currently involves pulses covering wide frequency ranges in high-magnetic-field spectrometers. This leads to excitation of unwanted signals that may overlap and interfere with the resonances of interest. Herein, we present a new pulse sequence that converts longitudinal magnetization to LLS and further to detectable magnetization using only frequency-selective pulses. We demonstrate the suitability of this sequence for different J-coupled spin pairs in dipeptide AlaGly and protein Ubiquitin. The newly developed method is adapted for investigations of LLS in complex systems such as proteins and mixtures of metabolites where selected molecular groups are to be investigated separately.

Solution-state distance restraints for protein structure determination with Ångström-level resolution rely on through-space transfer of magnetization between nuclear spins. Such magnetization transfers, named Overhauser effects, occur via dipolar magnetic couplings. We demonstrate improvements in magnetization transfer using long-lived coherences (LLCs)—singlet-triplet superpositions that are antisymmetric with respect to spin-permutation within pairs of coupled magnetic nuclei—as the magnetization source. Magnetization transfers in the presence of radio-frequency irradiation, known as ‘rotating-frame’ Overhauser effects (ROEs), are predicted by theory to improve by the use of LLCs; calculations are matched by preliminary experiments herein. The LLC-ROE transfers were compared to the transmission of magnetization via classical transverse routes. Long-lived coherences accumulate magnetization on an external third proton, K, with transfer rates that depended on the tumbling regime. transfers in the LLC configuration for (I,S) are anticipated to match, and then overcome, the same transfer rates in the classical configuration as the molecular rotational correlation times increase. Experimentally, we measured the LLC-ROE transfer in dipeptide AlaGly between aliphatic protons in different residues K = Ala − Hα and (I,S) = Gly − Hα1,2 over a distance = 2.3 Å. Based on spin dynamics calculations, we anticipate that, for such distances, a superior transfer of magnetization occurs using LLC-ROE compared to classical ROE at correlation times above . The LLC-ROE effect shows potential for improving structural studies of large proteins and offering constraints of increased precision for high-affinity protein-ligand complexes in slow tumbling in the liquid state.