Ortho-to-para nuclear spin ratio of cyclopropenylidene (
c-C
3H
2) in TMC-1 cores was investigated using the 45 m radio telescope at Nobeyama Radio Observatory. We have observed four
ortho lines (J
KaKc=2
12-1
01, 3
12-3
03, 3
21-3
12, and 3
30-3
21) and four
para lines (J
KaKc=2
02-1
11, 2
11-2
02, 2
20-2
11, and 3
22-3
13) of
c-C
3H
2 in six cores in TMC-1. A statistical equilibrium analysis of the oserved intensities revealed clear correlation between the
ortho-to-para ratio of c-C3H2 and chemical evolution of the molecular cloud; the
ortho-to-para ratio in younger cores is less than 2, while that in older cores is close to the statistical ratio of 3. A simple chemical model calculation indicates that the correlation is ascribed to the conservation of nuclear spin angular moment in chemical reactions relevant to the production, destruction, and re-production of
c-C3H2.
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