Mössbauer investigation ofI129in Te-thiourea compounds and the quadrupole moment of the first excited state ofTe125

The quadrupole interaction at I in Te-thiourea compounds containing ${}_{}{}^{129}{}_{}{}^{}\mathrm{Te}_{}^m{}_{}{}^{}$ is studied by ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ Mössbauer spectroscopy. Pure $\sigma$ bonds are observed, in sharp contrast to the pure $\pi$ bonding at Te in these compounds. The $\sigma$ bond of I is presumably derived from one of the former Te $\pi$ bonds, and involves a reorientation through 90° of the electric field gradient. From the ratio of the ${}_{}{}^{125}{}_{}{}^{}\mathrm{Te}$ to the ${}_{}{}^{129}{}_{}{}^{}\mathrm{I}$ quadrupole splittings, a new value $Q=-0.31\mathrm{}$ barn is derived for the quadrupole moment of the first excited state in ${}_{}{}^{125}{}_{}{}^{}\mathrm{Te}$.