Vibrational correction factors to the partition function of isobutene: 

-----------------------------------------------------------------------------

TEMPERATURE  -  Vib. Corr. Factor
    300.000      2.278
    225.000      1.787
    150.000      1.351
     75.000      1.046
     37.500      1.001
     18.750      1.000
     15.000      1.000
      9.375      1.000
      5.000      1.000
      2.725      1.000

-----------------------------------------------------------------------------