Analysis of line positions of the ₆ band of ³⁵ClNO₂ around 411 cm^-1

The first investigation of the nu(6) band of the (ClNO2)-Cl-35 isotopic species of nitryl chloride, located at 410.11824 (+/- 0.00007) cm(-1) has been performed using a high resolution (0.00102 cm(-1)) Fourier transform spectrum recorded at the SOLEIL synchrotron source. This B-type band is rather weak but a long optical path was used and, during the spectrum recording, the nitryl chloride sample was kept at low temperature (221 K) in the optical cell. In this way, the rather strong nu(2)-nu(3) difference band located at 422.6 cm(-1), observed at 296 K during a previous investigation of the far infrared region [Orphal J, Morillon-Chapey M, Klee S, Mellau GC, Winnewisser M. J Mol Spectrosc 1998;190:101-6], and interfering with observation, could be strongly reduced. This first assignment of the nu(6) band of (ClNO2)-Cl-35 was pursued up to high J and K-a quantum number values, J = 79 and K-a = 37. The nu(6) band (expected to be of B-type) has a clearly unusual line intensity pattern, since the P branch is about twice as strong as the R-branch. Since the (weak) nu(6) band is located close by to the strong A-type nu(3) band located at 370 cm(-1) [Anantharajah A, Kwabia Tchana F, Manceron L, Orphal J, Flaud JM. J Quant Spectrosc Radiat Transf 2020; 253:107,078], one could reasonably expect that the nu(6) band borrows part of its intensity through the existence of a C-type Coriolis resonance that couples together the 61 double left right arrow 3(1) interacting energy levels. However, during the 6(1) energy level computation, we could not evidence such resonance, and only a classical Watson's type A-type reduced rotational Hamiltonian, involving a single upper state, was used for this calculation.