The nmr spectra of 2,3-dihydro-1,4-diazepinium salts are discussed. From the magnitude of the coupling constants around the conjugated part of the molecules, a planar partial structure with complete electron delocalisation is indicated. The molecule as a whole adopts a half-chair configuration which rapidly inverts at room temperature. The effect of ring substitution on this exchange process is correlated with the size of the substituents. The mass spectra of a variety of 2,3-dihydro-1,4-diazepinium salts are reported. The molecular ion of the cation is only rarely observable due to thermal dissociation of the salt. The 1 2 major breakdown process involves the loss of the (N1-C2) fragment but other pathways are also considered. The factors governing some anomalous fragmentation patterns are discussed. The rates of bromination in methanol of a number of 2,3-dihydro-1,4-diazepinium salts were measured under first-order conditions, normally by stopped flow methods. Differences in rate of factors of up to 10000 between dihydrodiazepines with only alkyl or aryl substituents, are correlated with steric effects in the region of the reaction site, 5,6,7-Unsubstituted 2,3-dihydro-1,4-diazepinium salts are tedious to make by standard methods, Their facile preparation from l,5-diaryl-1,5-diazapentadienium salts under high dilution conditions, is discussed, and the scope of the synthesis was explored. The mechanism of the reaction was investigated using mono-amine model compounds. Attempts to prepare 5,7-dimethyl-dihydrodiazepinium salts by this method generally resulted in the formation of 2-methylimidazolines. The parent 2,3-dihydro-1,4-diazepinium perchlorate was obtainable by this high dilution synthesis in sufficient quantities for a study of its properties to be made. it is active towards electrophiles, giving 6-substitution products, and reacts with-nucleophiles at the 5(7) position. In contrast to 5,7" substituted dihydrodiazepines, the 6-halogeno derivatives are inert towards nucleophiles at the 6-positiono A series of 1,2-dihydropyrimidines was prepared, and the chemical and spectroscopic properties of these heterocycles are compared with those of the isoelectronic 2,3-dihydro-1,4-diazepines. Dihydropyrimidines show meneidic reactions with electrophiles at the 5-position, taut only 6-unsubstituted derivatives react with nucleophiles at the 6-position. A convenient synthetic route from 4-methyldihydropyrimidines to 5-methyldihydrodiazepinium salts is described. The effect of electronic perturbation on the 1,5-diazapentadienium system was investigated using series of 1,3-dimethyl-1,2-dihydro-2-oxo- and 2-thiopyrimidinium salts. Substitution reactions of 4,6-unsutastituted derivatives with electrophiles have been shown to proceed by alternative mechanisms; those pyrimidines with 4(6)-methyl substituents also show substitution reactions in the methyl groups. Stable adducts are formed by reaction of the 4,6-unsubstituted compounds with bases. The structure and chemical properties of these pseudobases are discussed. The electronic structure of 2,3-dihydro-1,4-diazepinium perchlorates and the other 1,5-diazapentadienium salts considered in this thesis, was investigated by 13C nmr spectroscopy.