The electronic and molecular structure of radicals with a C=C core

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Acetylene in the lowest triplet state (T_1) has three local minima at cis, trans, and vinylidene configurations according to theoretical calculations. Isomerization reaction among those configurations has been paid a lot of attentions. We have observed an electronic transition between the T_2 - T_1 potential surface at the cis-bent configuration around 7400 cm^<-1> region by using near-IR diode laser kinetic spectroscopy combined with a pulsed mercury photo-sensitized reaction. By now 0-0 band of C_2H_2, C_2HD and C_2D_2 have been rotationally analyzed and sever inhomogeneous rotational perturbations have been recognized in some specific Ka states in the lowest triplet state of C_2H_2, but no perturbations were observed in C_2HD and C_2D_2. The perturber for the Ka=1 state of vibrational ground state in the lowest triplet state is a mystery. One candidate for this perturbation is an interaction among the other isomers. Some high level ab initio calculations predicted that there is a po … More tential minimum at the trans-isomer configuration with a small potential barrier in both the T_2 and T_1 surfaces. In case of T_2, the trans minimum is predicted to be lower than the cis. Therefore, an interaction between the cis and trans should be expected, especially when some vibrational modes are excited. Recently we have observed 1-0 band of C_2H_2 and C_2D_2 in 8170 and 7970 cm^<-1> region, respectively, in which the cis-bent vibrational mode is excited. After the rotational analysis based on the combination difference, more sever perturbations have been recognized. Moreover, the rotational constant around the C=C axis is abnormally enlarged by the vibrational excitation. This strong Coriolis type interaction may suggest a possibility of the isomerization reaction because the rotational quantum number of Ka, which is the angular momentum along the C=C axis, can be a motion of constant in the isomerization including the linear configuration. In order to discuss such an interaction between different point groups as cis (C_<2v>) and trans (C_<2h>) configurations, Complete Nuclear Permutation Inversion group G_8 should be introduced as a common symmetrical platform. Less