CONFORMATIONAL DYNAMICS OF THE CO-CO2 COMPLEX IN ARGON MATRICES .1. THERMODYNAMICAL CONSIDERATIONS DERIVED FROM THE OBSERVED KINETICS

a previous work, experimental evidence Tor different conformations of the weakly bonded CO:CO2 complex has been given in argon matrices, fro m the analysis of FTIR absorption spectra. A spontaneous interconversi on occurs between a high frequency form (HF A and B lines: The CO freq uency is shifted toward high frequencies upon complexation) and a low frequency form (LF line: The CO frequency is shifted toward low freque ncies upon complexation) which has been characterized at different tem peratures between 5 and 12, K. Above 12 K, the HF doubler only remains , Is an attempt to explain tile observed phenomena, concentration, mat rix, and isotopic effects are studied, as well as the reverse conversi on which is tell times faster than the direct one. A thermodynamical a nalysis of the conversion is performed. From the temperature behavior of the equilibrium constant ii, we can suppose that the conversion exh ibits two regimes: 4 low temperature one below 8 K and a high temperat ure: one between 8 and 12 K. A reasonable explanation could be that th ree conformations are involved: A more stable one corresponding to the narrow low frequency line (probably tile T shape observed in molecula r beam experiments) and two closely related less stable conformations which give rise to the A and B lines of the high frequency doublet. Th e energy differences between these conformations are small: Delta H = -140 J mol(-1) and -337 J mol-l between the LF form and the HF B and i t forms, respectively. A subtle mechanism, involving a 3 concerted exc hange between the CO molecule of the complex and a nearest argon atom, is proposed to explain the experimental observations. (C) 1996 Americ an Institute of Physics.