Infrared spectrum of chlorine in concentrated matrices

In studying reactions of chlorine and fluorine and their compounds in inert matrices (1-3), we have observed infrared absorptions near 892 -i cm and 546 cm -I which we can only attribute to molecular fluorine and chlorine, respectively. These frequencies can be compared with the corresponding gas phase Raman shifts of 892 and 557 cm -1 (4). These absorptions are clearly visible only when fluorine or chlorine is present at a concentration of about 3% or greater. Infrared spectra for several other homonuclear diatomic molecules have been observed in condensed systems, including liquid fluorine (5). Thus, while one can normally ignore homonuclear diatomic species in the interpretation of infrared spectra, these species should be expected to absorb in a sufficiently concentrated matrix. The spectrum of CI 2 in Ar (1:20) at 17°K is shown in Fig. l; it shows the expected isotopic splitting arising from 35CI and 37CI in natural relative abundance of ~3:1. A vibration involving two equivalent chlorine atoms should exhibit an infrared spectrum of three lines with absorbances in the ratio of approximately 9.4:6.2:1. In our spectrum, -i -i only two lines are visible, centered at 546.3 cm and 539.4 cm The absorbance of the higher frequency line is 1.48 times that of the second, very close to the ratio of 1.52 expected for the two strong lines