ABSTRACT The experimental and theoretical chemistry of small, endothermic molecules is an area that has seen considerable renaissance over the past 10 years. This article is not exhaustive in scope but rather focuses on highlights and certain aspects such as the description of the chemical bond in this field. To give a more balanced approach to the subject, considerable attention is given to the use of Pauling 3-electron bonds and increased valence structures. The increased valence- bond concept is introduced briefly. The combined application of diffraction methods (X-ray and electron diffraction), microwave and Raman spectroscopy together with quantum chemical approaches (ab initio methods and density functional computations) have led in the last few years to an improved understanding of molecular properties of numerous molecules which are highly explosive, endothermic or even unknown. This interaction of theory and experiment has greatly enhanced the development of small molecule chemistry and has led to realistic expectations for the synthesis of as yet unknown molecules. The article includes discussion of bond properties (S2N2, NF5) and dissociation reaction paths of “unstable” molecules such as O2NNCO and OCN-NCO.
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