Knowledge Bank

Band series from three spectroscopically distinct species are observed in the low resolution microwave (LRMW) Spectra of t-butylacetaldehyde, t-butylacetyl fluoride and t-butylacetyl cyanide. Two of the series from each compound are readily assignable to the syn ($\tau OCCC=0\circ$) and skew ($\tau OCCC=120\circ$) conformations. The third band series from each compound is due to a species with average energy about 1.5 Kcal/mol higher than the approximately equally stable syn and skew forms and has a rotational constant B+C value corresponding to $\tau -90\circ$. This species is not a third stable rotamer but is composed of those species freely rotating about $\tau OCCC$ above the internal rotation barrier. The torsional barrier in each compound is essentially 3-fold with a height of 500 (+400, -200) cal/mol. t-Butylactyl chloride only displays bands from a syn rotamer. The skew minimum and low internal rotation barrier appear to be ellminated by the bulky chloride substituent.