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.