Detection of protonated vinyl cyanide, CH2CHCNH+, a prototypical branched nitrile cation

The rotational spectrum of protonated vinyl cyanide, CH2CHCNH(+), a prototypical branched nitrile species and likely intermediate in astronomical sources and in the planetary atmosphere of Titan, has been detected in a pulsed-discharge supersonic molecular beam by means of Fourier transform microwave spectroscopy. Fifteen lines arising from 11 a-type rotational transitions have been observed between 9 and 46 GHz, several with partially resolved nitrogen hyperfine structure. From this data set, the leading spectroscopic constants, including all three rotational constants, have been determined to high accuracy. The agreement between experimental rotational constants and those calculated at the CCSD(T) level of theory is of order 0.1%. An even better estimate was obtained through empirical scaling using calculated and experimental rotational constants of isoelectronic vinyl acetylene. Measurement of a small nitrogen quadrupole coupling constant in protonated vinyl cyanide is consistent with a quadruply bound nitrogen atom and a H(+)-N≡C-R type structure. Because vinyl cyanide is abundant in molecule-rich astronomical sources and possesses a high proton affinity, and because protonated vinyl cyanide is unreactive with hydrogen and other well-known interstellar species, this cation is an excellent candidate for astronomical detection. The present work suggests that other organic molecules containing the nitrile group and closely related species such as protonated vinyl acetylene can probably be detected with the same instrumentation.