Hydrogen bond
Hydrogen bonds can vary in strength from weak (1–2 kJ mol) to strong (161.5 kJ mol in the ion ). Typical enthalpies in vapor include:
Hydrogen-bridged cations
The most significant trait of bridging hydrogens in NMR spectra is their upfield shift. Due to the increased number of bonds that the hydrogen atom is part of, the higher electron density around hydrogen shields the H nucleus, causing its chemical shift to appear at negative ppm. Characterization of the dimethylcyclodecyl ion by Sorensen in 1978 relied on low-temperature NMR, as the ion slowly decomposes at above -70 °C, and showed the bridging hydrogen at δ -3.9. McMurry's characterization of the more stable "in"-bicyclo[4,4,4]-tetradecyl cation showed two broad 12 proton peaks at δ 2.5 and 1.9, and a broad proton singlet at δ -3.46. Depending on the length of bridging chains in a similar bridged bicyclo[3,3,1]-nonyl cations the upfield shift of the bridging hydrogen can vary from δ -6.50 to 0.07. This is actually due to a change in C-H-C bond angle, where change in bond angle affects the bonding nature of the 3 center-2 electron bond.