Analysis of FTIR spectra of CH 2 79 Br 35 Cl; the m 4 and m 5 fundamentals and their hot-bands m 4 þ m 6  m 6 and m 5 þ m 6  m 6 Marcel Snels * Istituto di Scienze dell’Atmosfera e del Clima, Sezione di Roma, CNR Via Fosso del Cavaliere 100, 00133 Rome, Italy Received 25 July 2003; in revised form 17 November 2003 Abstract The infrared spectrum of isotopically pure CH 2 79 BrCl has been recorded at a resolution of 0.0023 cm 1 (FWHM) in the range 550–800 cm 1 with a Bruker IFS 120 HR Fourier transform spectrometer in Wuppertal. Here we report the full rotational analysis of the m 4 and m 5 fundamentals and of the hot-bands m 4 þ m 6  m 6 and m 5 þ m 6  m 6 . Ground state combination differences were constructed for all bands, yielding improved ground state constants, up to quartic terms, as well as reliable rotational constants for the m 4 , m 5 , and m 6 states. Ó 2003 Elsevier Inc. All rights reserved. Keywords: FTIR; Halons 1. Introduction All fundamental modes of bromochloromethane (Halon 1011) have been identified from medium reso- lution infrared gas phase spectra, most recently by Gi- orgianni et al. [1]. Ground state rotational constants for all isotopic chlorine and bromine species have been de- termined by Niide and Ohkoshi [2] from microwave spectra. No high resolution infrared spectra of CH 2 BrCl have been published, as far as we know, except for a partial analysis of the m 1 band, measured at a resolution of 0.008 cm 1 [3]. Here we present the first full rotational analysis of the A-type fundamentals m 4 and m 5 and of the hot-bands m 4 þ m 6  m 6 and m 5 þ m 6  m 6 . By using ground state combination differences improved ground state constants could be determined as well as rotational constants for the m 6 state. 2. Experimental details The high resolution infrared spectra (0.0023 cm 1 ) of CH 2 79 BrCl were recorded in the wavenumber range 550–800 cm 1 with a Bruker 120 HR interferometer in Wuppertal. The spectrometer was equipped with a Globar source, a KBr beam splitter, and a HgCdTe detector. An optical filter eliminating radiation above 1200 cm 1 was used. In addition, electronic filtering was applied. A sample of isotopically pure CH 2 79 BrCl was prepared in Wuppertal from Na 79 Br and CH 2 ICl. The sample pressure was 150 Pa (1.5 mbar) in a 28 cm cell equipped with KBr windows. The Doppler width at room temperature is about 0.0008 cm 1 . The spectrum has been calibrated with lines of residual CO 2 vapor in the interferometer [4]. A correction of +1.4  10 3 cm 1 had to be applied to the experimental data. The wave- number precision is of the order of 4  10 4 cm 1 , the accuracy circa 6  10 4 cm 1 . 3. Results and discussion Bromochloromethane occurs in four isotopic varie- ties in nature, CH 2 79 Br 35 Cl (38.22%), CH 2 81 Br 35 Cl (37.31%), CH 2 79 Br 37 Cl (12.38%), and CH 2 81 Br 37 Cl (12.09%). The spectra presented in this work, were re- corded for an isotopically pure sample of 79 Br, with relative abundances of CH 2 79 Br 35 Cl (75.53%) and CH 2 79 Br 37 Cl (24.47%). All molecules possess one plane * Fax:+39-06-49-93-42-91. E-mail address: m.snels@isac.cnr.it. 0022-2852/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2003.12.003 Journal of Molecular Spectroscopy 224 (2004) 13–17 www.elsevier.com/locate/jms