Effect of local coordination environment on molecule vibration in N2-molecule solid

The vibration feature in a molecule solid is an important character of its structure. The different vibration frequencies of isolated nitrogen molecule (N2) and nitrogen molecule in the solid state are explored. Five solid-cluster models with the different numbers of nitrogen molecules (N46, N60, N76, N100, and N126) are constructed on the basis of -N2 crystal structures. The density functional theory is used to calculate the vibration frequencies of nitrogen molecules. The calculated infrared spectra and average vibration frequencies (AVFs) of the optimized structures for the five models are compared with each other. The results indicate that the AVF of nitrogen molecule in solid model is higher than that of isolated nitrogen molecule due to the collective effect. It is found that the AVF increases with increasing the number of molecules. The AVF of the inner molecules is always higher than that of surface molecules in the solid. On a whole, the vibration frequencies are ordered as vinner vsurface visolated for each case. The local coordination environment is believed to be mainly responsible for the differences in frequency among the isolated, surface and inner molecules. The bond length of molecule in solid is shorter than that in an isolated molecule, thus resulting in a stronger bond force and a higher vibration frequency. Similarly, due to a smaller number of molecules interacting with surface molecules, the bond force between molecules in the solid surface is weaker, thus resulting in a lower vibration frequency than in the inner region of solid.