Microwave, infrared and Raman spectra and structure of 2-nitropropane

The microwave spectrum of 2-nitropropane, C 3 H 7 NO 2 , has been investigated from 12.5 to 40.0 GHz. Only α-type transitions were observed and R -branch assignments are made for the ground vibrational state. The effective rotational constants are found to have the following values: A = 5132 ± 12, B = 2895.70 ± 0.14, and C = 2706.51 ± 0.12 MHz. It is shown that these rotational constants are only consistent for the conformer where one NO bond eclipses the CH bond. From a diagnostic least-squares adjustment to fit these three rotational constants along with reasonably assumed structural parameters for the isopropyl moiety, excluding the CC distance, the following parameters are obtained: r( C  C ) = 1.533 ± 0.006 A ̊ , r( C  N ) = 1.508 ± 0.018 A ̊ , r( N  O ) = 1.218 ± 0.015 A ̊ , ∩ CNO = 116.8 ± 1.5°, and ∩ CCN = 108.9 ± 1.7° . From a temperature study of the microwave spectrum, it appears that the NO 2 group is nearly freely rotating. The IR (3500−80 cm −1 ) and Raman (3500-10 cm −1 ) spectra have been recorded for both the gaseous and solid phases of 2-nitropropane. The Raman spectrum of the liquid has also been recorded and qualitative depolarization values obtained. All of the normal modes are assigned based on band contours, depolarization values, and group frequencies. The vibrational data are also consistent with a nearly freely rotating NO 2 group. These results are compared to the similar quantities for several related molecules.