Acetylene Radical Anions Trans-Bent Structure

3 Acetylene Radical Anions Trans-Bent Structure 

Acetylene is one of the simplest hydrocarbons and a fundamentally important chemical in organic chemistry. The electronic structure of acetylene and its related compounds have been extensively studied both experimentally and theoretically, and a number of interesting observations have been reported. For example, a trans-bent structure has been reported for acetylene in an excited state [42, 43] and also in a radical anion form [34, 44]. The interaction between acetylene and metal atoms such as A1 and Li has been studied by inert gas matrix isolation (MI) ESR and IR methods [45-48]. Here we present our ESR study on the structural distortion in acetylene radical anion in the glassy 2-MTHE matrix, which is another example showing a mixing of the k and higher-lying a orbitals at the C=C carbons similar to that at the C=C carbons for the perfluoroalkene radical anions. In addition photo-induced isomerization reaction of the acetylene radical anion in the matrix is briefly presented. 

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. 

When CD= CD was used instead of CH= CH, the triplet was changed into an anisotropic quintet due to two magnetically equivalent D atoms, as shown in Fig. 5.9(a) the /t/coupling decreased, as expected, by a factor of 6.514, the ratio of the and H(D) nuclear g-factors. Thus, the triplet observed for the irradiated CH=CH/2-MTHF system was attributable to the two magnetically equivalent protons belonging to the solute molecule. Three isomer radical anions of acetylene, trans-CR=CH, cm-CH=CH , and CH2=C, are candidate species responsible for the triplet because they have two magnetically equivalent H atoms. 

The observation of couplings is very important to clarify the electronic structure of because a large portion of the unpaired electron is 

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