Sentence:  We are reporting these investigations here briefly because of their relevancy to problems of the study of apparently simple exchange reactions of chlorine and because the results furnish some information on the <WSD>activation</WSD> energy for abstraction of chlorine atoms from carbon tetrachloride . 
Meaning of the word marked with <WSD> tags:  the energy that an atomic system must acquire before a process (such as an emission or reaction) can occur