Trans-bent triplet acetylene

The excited electronic states of acetylene have been discussed extensively, both experi-mentally h and theoretically the latter also review the measured data. All the low-lying excited states of acetylene, both triplet and singlet, are either known or suspected of being either cis or trans bent to some degree. In the discussion here of the acetylene and... 

The acetylene radical anion was generated by an electron transfer to the solute acetylene ( CH= CH) in y-ray irradiated 2-MTHF matrix at 77 K. The ESR spectrum observed immediately after y-irradiation consists of a sharp central singlet at g = 2.0023 due to a trapped electron (et ) and a broad septet with ca. 1.9 mT /t/splitting due to the matrix 2-MTHF radical. By illumination with L > 580 nm light the singlet due to ef disappeared and concomitantly a spectrum due to the solute radical became visible (Fig. 5.8(a)). Upon successive illumination with X > 330 nm the solute radical spectrum completely disappeared and only the matrix radical spectrum was observed (Fig. 5.8(b)). By subtracting spectrum (b) from (a) the solute radical spectrum was separately observed as shown in Fig. 5.8(c). Thus an anisotropic triplet spectrum was clearly revealed and attributed to the /i/splitting due to two H atoms of the solute acetylene radical anion with a trans-bent structure (frans- CH= CH ) based on the comparison of the experimental ESR spectrum with the theoretical one as will be described below. 

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