Umbrella vibration, hydrogen-bonded

The second event in simulation C is a second proton transfer, observed at 60 fs, and is due to an intramolecular 0-H attraction. The fast vibrations of the methyl group induce an umbrella inversion type motion that, along with a stretch in a N-O bond, leads to the hydrogen transfer from the methyl to the nitro group. This reaction yields the aci acid form of nitromethane. The process of bond rearrangement is illustrated as ... 

Introduction of a second adsorbed ammonia molecule in the neighboring position generates a weak potential barrier in this rotation (broken line in Fig. 7b), which decreases with increase of the distance between the two adsorbed species. The most influenced intramolecular vibration after the adsorption is the umbrella, connected with molecular inversion of the three N-H bonds in gas-phase ammonia. Stronger attraction of the nitrogen atom to the siuface and repulsion of the hydrogens converts the symmetric double-well inversion potential of the gas molecule into a distorted single well (Fig. 8). This change in the potential ciuve modifies the distances between the vibrational levels for this mode. 

---