By J. D. Kemp and K. S. Pitzer. Journal of the American Chemical Society 59(2), 276–279 (1 February 1937). DOI doi.org/10.1021/ja01281a014
Original publisher title, byline, date and page range verified. The article links ethane entropy and the third law with hindered methyl-group rotation; no numerical barrier, calorimeter specification or full-text result attributed here.