Microwave Structure Determinations on Tertiary Butyl Acetylene and Tertiary Butyl Cyanide

Abstract

The microwave spectra of eight isotopic species of tertiary butyl acetylene and four isotopic species of tertiary butyl cyanide have been measured and analysed. Some transitions in excited vibrational states have been assigned. Measurements of the Stark effect lead to a dipole moment of 0.661±0.004 D for (CH₃)₃CCCH and 3.95±0.05 D for (CH₃)₃CCN. From the isotope shifts in the moments of inertia the following bond distances (r_s) are obtained: r(C≡C) = 1.209±0.001 A and r(≡C–H) = 1.056±0.001 A in (CH₃)₃CCCH; r(C≡N) = 1.159±0.001 A in (CH₃)₃CCN. The tertiary butyl group appears to have very nearly the same dimensions in these two compounds as in (CH₃)₃CH and (CH₃)₃CF. The tertiary carbon atom cannot be located with high precision, but the best estimates lead to a C–C≡ distance of 1.495±0.015 A in both (CH₃)₃CCCH and (CH₃)₃CCN. This is 0.02–0.04 A longer than the normal value for this distance. There appears to be a consistent tendency for the (CH₃)₃C— group to lengthen the adjacent bond, but it is difficult to find an explanation in terms of existing theories.