THE CO-CO2 COMPLEX IN ARGON MATRICES - EXPERIMENTAL-EVIDENCE FOR 2 CONFORMATIONS WITH SPONTANEOUS INTERCONVERSION

The CO:CO2 complex in argon matrices is identified near the CO absorption which appears at 2138.49 cm-1, slightly shifted from the pure argon value of 2138.64 cm-1, because of the presence Of CO 2. It exhibits two features on each side of the CO frequency: a doublet at 2143.34 and 2143.01 cm-1 (HF A and B lines) and a narrow line at 2135.38 cm-1 (LF line); these small shifts indicate weakly complexed C-O stretching modes. When the temperature is raised from 5 to 12 K, the LF line decreases progressively and disappears at the benefit of the HF A and B lines, the total absorption intensity remaining unchanged. This effect is perfectly reversible and can be explained by an exchange between two different conformations of the CO:CO2 complex, with complexed C-O bonding shifted either towards a higher frequency (HF form), or towards a lower one (LF form). Furthermore, after a fast cooling down to any given temperature below 12 K, the intensity ratio between the two forms is time dependent; the high temperature form (HF form) converts into the LF form within a few minutes. The equilibrium value of the intensity ratio LF/HF and the rate constants for the conversion are temperature dependent. We have checked that this conversion, occurring without any temperature change, is not photoinduced. © 1995 American Institute of Physics.