No description available.

Non-covalent bonds are an important class of intermolecular interactions, which result in the ordering of atoms and molecules on the supramolecular scale. One such type of interaction is brought about by the bond formation between a region of positive electrostatic potential (σ-hole) interacts and a Lewis base. Previously, the halogen bond has been extensively studied as an example of a σ-hole interaction, where the halogen atom acts as the bond donor. Similarly, carbon, and the other tetrel elements can participate in σ-hole bonds. This thesis explores the nature of the carbon tetrel bond through the use of computational chemistry and solid state nuclear magnetic resonance (NMR) spectroscopy. The results of calculations of interaction energies and NMR parameters are reported for a series of model compounds exhibiting tetrel bonding from a methyl carbon to the oxygen and nitrogen atoms in a range of functional groups. The ¹³C chemical shift (𝛿iso) and the ¹ᶜ𝐽(¹³C,¹⁷O/¹⁵N) coupling across the tetrel bond are recorded as a function of geometry. The sensitivity of the NMR parameters to the non-covalent interaction is demonstrated via an increase in 𝛿iso and in |¹ᶜ𝐽(¹³C,¹⁷O/¹⁵N)| as the tetrel bond strengthens. There is no direct correlation between the NMR trends and the interaction energy curves; the energy minimum does not appear to correspond to a maximum or minimum chemical shift or J-coupling value. Gauge-including projector-augmented wave density functional theory (DFT) calculations of 𝛿iso are reported for crystals which exhibit tetrel bonding in the solid state. Experimental 𝛿iso values for sarcosine, betaine and caffeine and their tetrel-bonded salts generally corroborate the computational findings. This work offers new insights into tetrel bonding and facilitates the incorporation of tetrel bonds as restraints in NMR crystallographic structure refinement.

No description available.

No description available.

The concept of noncovalent bonding has evolved over the last number of years to include a very interesting class of interactions that is analogous to hydrogen bonding, called σ-hole interactions. These result from the depletion of electrostatic charge on the opposite end of a covalent bond between an electron-withdrawing substituent and a bond donor atom, which resides in groups 14-17 of the periodic table. One of these interactions is the tetrel bond (TB), whereby the bond donor is a group 14 element (T=C, Si, Ge, Sn, Pb). This thesis's primary goal is to explore the solid-state NMR parameters arising from the formation of tetrel bonds. To this end, combined density functional theory (DFT) and experimental multinuclear solid-state NMR spectroscopic investigations are carried out on complexes featuring carbon, Pb(II) and tin tetrel bonds. Firstly, solid-state NMR and computational approaches are used to examine a series of cocrystals formed from either caffeine or theophylline and several other small organic acceptor molecules. It is shown that the NMR response due to tetrel bond formation is detectible, but it can be hidden by other effects, including those of crystal packing. Careful analysis of NMR data alongside DFT calculations can reveal that the weak tetrel bond in these sorts of complexes increases the ¹³C chemical shift by 3-5 ppm. Next, a study of five Pb(II) centres hemidirectionally coordinated by isonicotinoyl hydrazone ligands demonstrates that the ²⁰⁷Pb NMR response is highly sensitive to the Pb(II) coordination environment. The NMR data indicate that a tetrel bond can induce an NMR response corresponding to a coordination environment between hemidirectional and holodirectional character. Finally, a series of organotin chloride donor molecules complexed with N-oxides and carboxylates, which feature short and linear tetrel bonds, are subjected to magic angle spinning (MAS) NMR experiments. The recorded data gives rise to a correlation between the tetrel bond length and both the experimental chemical shift and the ¹J(¹¹⁹Sn-³⁵Cl) coupling. Throughout this thesis, it is demonstrated that the isotropic chemical shift, the principal components of the chemical shift tensor, and indirect spin-spin coupling can be used to probe and gain insights into the electronic environment at the tetrel bond. More importantly, this work is fundamental to rationalize NMR data while refining crystal structure data in NMR crystallographic approaches for compounds featuring tetrel bonds.

La mesure de couplage dipolaire permet d’accéder à la structure tridimensionnelle d’un composé solide. Cependant, en présence d’une forte densité de spins couplés, le phénomène de troncature dipolaire rend difficile l’obtention de ces informations par RMN du solide. Ce problème peut être affranchi par l’étude de spins rares en abondance naturelle. En effet, avec une abondance naturelle de 1.1 %, la probabilité que trois 13C soient couplés, et avec elle la troncature dipolaire, devient négligeable. Une méthodologie basée sur la séquence de recouplage dipolaire POST-C7 permet d’accéder à des informations structurales d’échantillons en abondance naturelle sensibles à la fois à la conformation moléculaire et à l’empilement cristallin par mesure de couplages dipolaires 13C-13C. La sensibilité de détection des signaux RMN 13C est augmentée à l’aide la polarisation dynamique nucléaire ce qui permet de réduire considérablement les temps d’expériences. De plus, la séquence de recouplage R20_9_2 aidée de supercycles s’est montrée être plus robustes que POST-C7 face à de fortes anisotropies de déplacement chimique ou de forts couplages hétéronucléaires 1H-13C. La seconde problématique abordée concerne l’attribution de signaux 13C. En effet, il existe seulement quelques exemples de détermination de connectivités 13C -13C en abondance naturelle. Nous montrons ici que des spectres de corrélations dipolaires 13C-13C peuvent être obtenus en quelques jours à l’aide de la séquence de recouplage R20_9_2. Contrairement aux méthodologies basées sur le couplage J, notre séquence requiert un temps d’excitation DQ plus court ce qui la rend adaptée à l’étude de solides désordonnés. / Measurment of dipolar coupling provides 3D structural information of powder samples. However, in practice, the high density of spins in organic compounds prevents the measurements of long-range dipolar couplings in solid-state NMR by the so-called dipolar truncation effect. The study of rare spins on natural abundance allows to overcome this problem. In fact, with a natural abundance of 1.1 %, the probability for three 13C to be coupled is negligible. We developed a methodology based either on the dipolar recoupling NMR pulse sequence POST-C7 or on the dramatic increase in sensitivity provided by dynamic nuclear polarization. We demonstrated that its methodology provides a measure of 13C-13C dipolar couplings in natural abundance powder samples and that the so-obtained distance information is sensitive to both molecular conformation and crystal packing of powder samples. Moreover, we show that the recoupling pulse sequence R20_9_2 is more robust to strong chemical shift anisotropy and also to strong 1H-13C heteronuclear dipolar couplings than POST-C7. The second challenge involves 13C signal assignment for natural abundance. In fact, there are only a few examples of 13C-13C correlation spectra obtained for natural abundance samples. Here, we show that 13C-13C correlation spectra sequence based on the reintroduction of 13C−13C dipolar couplings can be obtained with standard MAS probe and within few days using R20_9_2 pulse sequence. Contrary to pulse sequences based on 13C-13C J coupling, our pulse sequence requires shorter DQ excitation time and hence, is more suitable for samples having short T2 relaxation times such as amorphous solids.