Non-covalent interactions
A hydrogen bond (H-bond), is a specific type of interaction that involves dipole-dipole attraction between a partially positive hydrogen atom and a highly electronegative, partially negative oxygen, nitrogen, sulfur, or fluorine atom (not covalently bound to said hydrogen atom). It is not a covalent bond, but instead is classified as a strong non-covalent interaction. It is responsible for why water is a liquid at room temperature and not a gas (given water's low molecular weight). Most commonly, the strength of hydrogen bonds lies between 0 - 4 kcal/mol, but can sometimes be as strong as 40 kcal/mol In solvents such as chloroform or carbontetrachloride one observes e.g. for the interaction between amides additive values of about 5 kJ/mol. According to L. Pauling the strength of a hydrogen bond is essentially determined by the electrostatic charges. Measurements of thousands complexes in chloroform or carbontetrachloride have led to additive free energy increments for all kind of donor-acceptor combinations.
1.1.1-Propellane
[1.1.1]Propellane is a highly strained molecule. The bonds of the two central carbon atoms have an inverted tetrahedral geometry, and the length of the central bond is 160 pm. The strength of that bond is disputed; estimates vary from 59–65 kcal/mol to no strength at all. The energy of the biradical state (with no central bond at all) is calculated to be 80 kcal/mol higher. The compound is highly unstable, and at 114 °C it will spontaneously isomerize to 3-methylidenecyclobutene with a half-life of 5 minutes. Its strain energy is estimated to be 102 kcal/mol (427 kJ/mol). Surprisingly, [1.1.1]propellane is persistent at room temperature and is somewhat less susceptible to thermal decomposition than the less strained (90 kcal/mol) [2.2.2]propellane system, which has an estimated half-life of only about 1 h at 25 °C.