Rotational Isomerization of Carbon-Carbon Single Bonds in Ethyl Radical Derivatives in a Room-Temperature Solution

The rotational isomerization of 1,2-disubstituted ethyl radical derivatives, reaction intermediates often found in the reaction of 1,2-disubstituted ethane derivatives, has never been measured because of their short lifetime and ultrafast rotation. However, the rotational time constant is critical for understanding the detailed reaction mechanism involving these radicals, which determine the stereoisomers of compounds produced via the intermediates. Using time-resolved infrared spectroscopy, we found that the CF₂BrCF₂ radical in a CCl₄ solution rotationally isomerizes with a time constant of 47 ± 5 ps at 280 ± 2 K. From this value and the rotational barrier heights of related compounds, CH₃CH₂ and CH₃CH₂CHCH₃ radicals in CCl₄ were estimated to rotationally isomerize within 1 ps at 298 K, considerably faster than ethane and n-butane, which rotationally isomerize with time constants of 1.8 and 81 ps, respectively. The time constant for the rotational isomerization was similar to that calculated using transition state theory with a transmission coefficient of 0.75.