The hypothesis that the cyanomethyl anion CH _ { 2 } CN ^ { - } Â is responsible for the relatively narrow diffuse interstellar band ( DIB ) at 8037.8 \pm 0.15 Â Ã Â is examined with reference to new observational data .
The 0 _ { 0 } ^ { 0 } absorption band arising from the ^ { 1 } { B } _ { 1 } - \tilde { X } ~ { } ^ { 1 } { A } ^ { \prime } Â transition from the electronic ground state to the first dipole-bound state of the anion is calculated for a rotational temperature of 2.7Â K using literature spectroscopic parameters and results in a rotational contour with a peak wavelength of 8037.78Â Ã .
By comparison with diffuse band and atomic line absorption spectra of eight heavily-reddened Galactic sightlines , CH _ { 2 } CN ^ { - }  is found to be a plausible carrier of the \lambda 8037 diffuse interstellar band provided the rotational contour is Doppler-broadened with a b parameter between 16 and 33 km s ^ { -1 }  that depends on the specific sightline .
Convolution of the calculated CH _ { 2 } CN ^ { - } Â transitions with the optical depth profile of interstellar Ti ii results in a good match with the profile of the narrow \lambda 8037 DIB observed towards HD 183143 , HD 168112 and Cyg OB2 8a .
The rotational level populations may be influenced by nuclear spin statistics , resulting in the appearance of additional transitions from K _ { a } = 1 of ortho CH _ { 2 } CN ^ { - } Â near 8025 and 8050 Â Ã Â that are not seen in currently available interstellar spectra .
For CH _ { 2 } CN ^ { - } Â to be the carrier of the \lambda 8037 diffuse interstellar band , either a ) there must be mechanisms that convert CH _ { 2 } CN ^ { - } Â from the ortho to the para form , or b ) the chemistry that forms CH _ { 2 } CN ^ { - } Â must result in a population of K _ { a } ^ { \prime \prime } levels approaching a Boltzmann distribution near 3Â K .