Microwave spectrum and barrier to internal rotation of 2-methyl-2-nitropropane, Me3CNO2

Abstract

The microwave spectrum of 2-methyl-2-nitropropane, Me₃CNO₂, has been investigated in the region 18.0–40.0 GHz. From the spectral analysis the first value for the V₆ barrier to internal rotation of a t-butyl group has been determined as 202.5 ± 2 cal mol^–1. This barrier, substantially larger than that in MeNO₂ and CF₃NO₂, is the highest sixfold barrier so far reported.

Rotational constants for Me₃CNO₂ are: B= 2506.28 ± 0.14 MHz, C= 2112.96 ± 0.01 MHz and A(CMe₃)= 4533 ± 15 MHz, assuming A(NO₂)= 13 342 ± 100 MHz: A unique feature of the analysis is the direct determination of the A rotational constant for the t-butyl group. These constants, together with data for the ¹⁵N isotopic species lead to a C—N bond length of 1.53 ± 0.02 Å, significantly longer than that of MeNO₂. This result is consistent with the lengthening obtained in other MeX compounds upon substitution of a t-butyl group for a methyl group.

Stark effect measurements give the dipole moment µ as 3.74 ± 0.04 D.