Intermolecular noncovalent interactions with carbon in solution.

One of the most familiar carbon-centered noncovalent interactions (NCIs) involving an antibonding π*-orbital situated at the Bürgi-Dunitz angle from the electron donor, mostly lone pairs of electrons, is known as n → π* interactions, and if it involves a σ* orbital in a linear fashion, then it is known as the carbon bond. These NCIs can be intra- or inter-molecular and are usually weak in strength but have a paramount effect on the structure and function of small-molecular crystals and proteins. Surprisingly, the experimental evidence of such interactions in the solution phase is scarce. It is even difficult to determine the interaction energy in the solution. Using NMR spectroscopy aided with molecular dynamics (MD) simulation and high-level quantum mechanical calculations, herein we provide the experimental evidence of intermolecular carbon-centered NCIs in solution. The challenge was to find appropriate heterodimers that could sustain room temperature thermal energy and collisions from the solvent molecules. However, after several trial model compounds, the pyridine-N-oxide:dimethyltetracyanocyclopropane (PNO-DMTCCP) complex was found to be a good candidate for the investigation. NBO analyses show that the PNO:DMTCCP complex is stabilized mainly by intermolecular n → π* interaction when a weaker carbon bond gives extra stability to the complex. From the NMR study, it is observed that the NCIs between DMTCCP and PNO are enthalpy driven with an enthalpy change of -28.12 kJ mol-1 and dimerization energy of ∼-38 kJ mol-1 is comparable to the binding energies of a conventional hydrogen-bonded dimer. This study opens up a new strategy to investigate weak intermolecular interactions such as n → π* interaction and carbon bonds in the solution phase.

Hydrogen Bonding Directed Reversal of 13 C NMR Chemical Shielding.

The deshielding or downfield 13 C NMR chemical shift of amide carbonyl carbon upon H-bonding is a widely observed phenomenon. This downfield shift is commonly used as a spectroscopic ruler for H-bonding. However, the very first observation of an upfield 13 C NMR of thiocarbonyl carbon in thioamides upon H-bonding encouraged us to explore the physical origin of the reversal of 13 C NMR chemical shielding. Careful NMR analysis shows that sulfur and selenium-centered H-bonds (S/SeCHBs) induce a shielding effect on the 13 CC=S(Se) while changing from amides to thioamides or selenoamides. In addition, natural chemical shielding (NCS) analysis shows that the σ11 and σ22 components of the isotropic shielding tensor (σ) have a crucial role in this unusual shielding.