A HIGH-RESOLUTION SPECTROSCOPIC STUDY OF THE OPEN-SHELL COMPLEX ARNO2

Microwave and infrared spectra of the open-shell complex ArNO2 have be en recorded. The microwave spectrum (6-18 GHz) consisted solely of the a-type transitions 5(05)<--4(04), 4(04)<--3(03), 3(03)<--2(02), and 2 (02)<--1(01) involving the K-a=0 state and 5(23)<--4(22), 5(24)<--4(23 ), 4(22)<--3(21), 4(23)<--3(22), 3(21)<--2(20), 3(22)<--2(21), involvi ng the K-a similar or equal to 2 state. These transitions showed struc ture due to fine, magnetic hyperfine and electric quadrupole interacti ons. The infrared spectrum, associated with the nu(3) asymmetric vibra tional mode of the NO2 monomer, consisted of three bands (P-R(0), (R)Q (0), and (R)R(0) and both K doublets of P-P(2), (P)Q(2) and (P)R(2)) c entered around 1615 cm(-1). The data have been fitted to a semirigid H amiltonian to determine the molecular parameters. The derived paramete rs are analyzed in terms of those of the free NO2 radical. Changes in these parameters upon complexation can be caused by a geometric effect due to the rotation of the inertid axes from the monomer to the compl ex, and an electronic effect caused by a distortion of the electronic wave functions on complex formation. The electronic changes (which may give an indication of incipient chemical bond formation) are shown to be very small. The absence of odd K-a '' states in both the infrared and microwave spectra was rationalized in terms of a high frequency tu nneling motion of the NO2 within the complex. Both a dynamics calculat ion and a model potential based on atom-atom interactions provided add itional support for a nonplanar equilibrium structure with a low barri er to planarity. (C) 1996 American institute of Physics.