Phenomenological exponential model sometimes used in spin chemistry is substantiated with the exactly solvable quantum problem as an example. Thus it is shown that there is no complete analogy between the influence of quantum measurement and that of chemical reaction on the system evolution. It is noted that Zeno chemical effect, i.e., chemical process influence on singlet-triplet evolution was known in spin chemistry. However, some aspects of this phenomenon, for example, connection with the problem of quantum measurements have been insufficiently studied. (C) 2010 Elsevier B.V. All rights reserved.