The study of polynitrogen pentazolate salts， which are usually achieved from the precursors of N₅ covalent compounds， is the hotspot of research in the field of new energetic materials. In most cases， the stability of N₅ covalent compounds will significantly affect the possibility of successful preparations of pentazolate salts. Herein， the calculations of dissociation energy （E_BD） of the bonds in the straight side chain and activation energy （E_a） of the N₅ ring were carried out for the selected eighteen non-aryl substituted N₅ compounds （R—N₅ or N₅—R—N₅） by using the B3LYP/6-31G** method of density functional theory， and meanwhile the influence of the side chain on their stabilities and pyrolysis mechanism were investigated. When R is the hydroxyl or amino group， the side chain and the N₅ ring are more prone to break， making it difficult to obtain the N₅^- ring. When R is alkyl， the E_a of the N₅ ring cleavage is relatively larger， making it more likely to produce the N₅^- ring， and the stability of the side chain′s C—N bond as well as the N₅ ring will be little affected by the length of the alkyl chain. The sequential cleavage of two N₅ rings occurs in the bicyclic molecular structures and the energy barrier of the second ring is higher than that of the first one， resulting in the formation of N₂ and azide. The C—C bond on the side chain of the molecule will be broken before the break of C—N bond， which may significantly reduce the E_BD of the C—N bond but have little effect on the E_a of the N₅ ring. Therefore， for the preparations of pentazolate salts from covalent pentazoles， cutting off the C—C bond first may be more conducive to obtaining N₅^- ring.