Rotational spectra of C4N, C6N, and the isotopic species of C3N

Two new carbon chain radicals terminated with a nitrile group, C4N and C6N, have been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. In addition, at least three hyperfine-split rotational transitions of the singly-substituted isotopic species of C3N have also been observed. Both C4N and C6N are linear chains with (2)Pi electronic ground states, and both radicals have resolvable hyperfine structure and lambda-type doubling in their lowest rotational levels. At least four transitions in the lowest-energy fine structure component ((2)Pi(1/2)) were measured between 7 and 22 GHz for both molecules, and at most nine spectroscopic constants were required to reproduce the measured spectra to a few parts in 10(7). Precise sets of rotational, centrifugal distortion, spin-rotation, and hyperfine coupling constants were also determined for the isotopic species of C3N by combining the centimeter-wave measurements here with previous millimeter-wave data. The C-13 hyperfine coupling constants of isotopic C3N differ from those of the isoelectronic chain C4H, but are fairly close to those of isovalent C2H, indicating a nearly pure (2)Sigma electronic ground state for C3N. Although the strongest lines of C6N are more than five times less intense than those of C5N, owing to large differences in the ground state dipole moments, both new chains are more abundant than C5N. Searches for C7N have so far been unsuccessful. The absence of lines at the predicted frequencies implies that the product of the dipole moment times the abundance (mu.N-a) is more than 60 times smaller for C7N than for C5N, suggesting that the ground state of C7N may be (2)Pi, for which the dipole moment is calculated to be small. (C) 2003 American Institute of Physics.