A version of butyl-cyanide (2-cyanobutane or C4H9CN)

A chirped-pulse broadband spectrum of 2-cyanobutane is shown in the Figure below. The spectrum is composed of three measured spectra with local oscillator (LO) frequencies of 12.2 GHz, 14.5 GHZ and 15.7 GHz. The LO signal, marked with ”LO” (purple), is suppressed by two diplexers after the IQ mixer, as visible in the noise level dip at 30 MHz left and right of the LO frequency. The receiver sensitivity is somewhat reduced more than 500 MHz apart from the LO, visible in the reduced and sloped noise level at around 13.3 GHz and above 16.2 GHz. The top spectrum shows the full dynamic range, while the bottom spectrum is ten times zoomed-in vertically to show the details of noise and lines.
Lines marked with a plus (+, blue) are identified as belonging to 2-cyano-anti-butane. Lines marked with an asterisk (*, red) identify 2-cyano-gauche(-)-butane lines and lines marked with a circle (o, green) refer to 2-cyano-gauche(+)-butane respectively. Assigned lines which have been used in fitting molecular constants are labelled with their J; Ka; Kc values as well as their respective color. Lines and features in the spectrum, that are unidentified or known electronic interferences, are noted with an exclamation mark. This work has been used for a first fit of vibrationally excited conformers of 2-cyanobutane. This work helps a lot in finding butyl-cyanide in space.

Broadband CP-FT spectrum of 2-cyanobutane

Hyperfine Structure of the lowest energy conformer

The transition J;Ka;Kc = 2;2;1–1;1;0 of 2-cyano-anti-butane, is shown in the  spectrum below. As expected the intensity of this low J-value transition in the cooled molecular beam reslts in a reasonably high signal-to-noise ratio. For the FT a 25.6 s (or 80000 points) long section of the FID signal was taken. Here the hyperfine splitting due to the 14N nucleus is obvious. Consequently, the transition is splitted into five components; the theoretical intensities are depicted by the blue lines. The F-quantum numbers are assigned as shown.