  A theoretical model of nuclear spin conversion in molecules controlled by an
external electromagnetic radiation resonant to rotational transition has been
developed. It has been shown that one can produce an enrichment of spin isomers
and influence their conversion rates in two ways, through coherences and
through level population change induced by radiation. Influence of conversion
is ranged from significant speed up to almost complete inhibition of the
process by proper choice of frequency and intensity of the external field.
