MAGNETIC FINE STRUCTURE IN THE INFRARED SPECTRUM OF $NO_{2}$

Author Institution: Research Division, Polaroid Corporation; National Bureau of Standards, Washington 25, D.C.; Department of Chemistry, Tufts UniversityWe report the first observation of magnetic fine structure in the infrared spectrum of a polyatomic free radical. The combination band $\nu_{1} + \nu_{3} (a_{1})$ of $NO_{2}$ at $2906.10 cm^{-1}$ has been examined at $.02 cm^{-1}$ resolution, and roughly half of the strong rotational transitions of this band are seen as clearly resolved single lines. P and R branch lines with $K/J>^{1}/_{2}$ seem at first to be missing from the spectrum, but are found to be present as doublets on closer examination. The observed splitting is associated with coupling of the odd electron spin to rotation about the ''A'' axis. The coupling constants for the ground and excited vibrational states are roughly equal, and consistent with the ground state microwave value for $\epsilon_{3a}$. The coupling constants for the B and C axes are much smaller, and do not produce observable effects in the infrared spectrum. A very detailed and accurate rotational analysis of this band was required before the magnetic structure could be treated with any certainly. In the first stage of the rotational analysis, a standard least-square polynomial curve-fitting program was used with an I.B.M. 1620 computer to identify and fit the P and R sub-branches of identical $K_{-1}$. This type of program is very flexible, and permits the testing of lines of doubtful identification. The ''empirical constants'' from the curve-fitting calculation were then converted to molecular constants, and the entire band calculated on an I.B.M. 7090 computer, using a program prepared by us to work with Schwendeman's deck of prolate top expansion coefficients and with ''A'' selection rules. This type of magnetic analysis is likely to prove useful for many other free radicals. While $ClO_{2}$ and $NO_{2}$ have been analyzed in great detail by microwave spectroscopy, there will be many other free radicals (particularly those with several nuclei of $I\geq ^{1}/_{2}$) which will resist microwave analysis. In these cases, infrared analysis under very high resolution will supply valuable information on the magnetic coupling of the odd electron