wizyta

od 2020-09-20

Title: Static and MAS 35CI NMR and molecular motions of ClO ions in the various phases of multimethylammonium perchlorates

Source: Berichte der Bunsengesellschaft fur Physikalische Chemie

Year : 1986

Abstract:

Static and MAS*) 35CI NMR data of CIO−4 ions in the low-, intermediate- and high-temperature phases of trimethylammonium, dimethylammonium and monomethylammonium perchlorates are reported. The observed solid-solid phase transitions are interrelated with the motional state of the perchlorate ions. In the low-temperature phase III of trimethylammonium perchlorate there is an anisotropic motion of the ClO−4 ions with a constant quadrupole frequency of 185 kHz and an increasing value of the asymmetry parameter when approaching phase transition III-II. The activation energy for this motion is found to be 44.3 kj/mol. At the transition III-II the ClO−4 ions gain orientational degrees of freedom manifested by the change of the quadrupole frequency to 159 kHz. In phase II the perchlorate ions undergo an axial motion overcoming the energy barrier of 40.7 kj/mol. In the low-temperature phase III of dimethylammonium perchlorate there are rigid ClO−4 ions. At the phase transition III-II the ClO−4 ions gain considerable motional freedom indicated by lowering the quadrupole frequency to 119 kHz, due to an anisotropic motion. In the high-temperature phase I the anions undergo isotropic motion with the activation energy of 11.9 kj/mol. The low-temperature phase III of monomethylammonium perchlorate is characterized by rigid ClO−4 ions. At the transition III-II the ClO−4 ions gain a considerable motional freedom and undergo an axial motion in phase II. In phase I motion of the anions is isotropic. The motional behaviour of the anions along with NMR data for the cations suggests that the low-temperature ordering of the ions in phase HI is due to hydrogen-bonds N[BOND]H … O[BOND]Cl, the partial breaking of which, accompanied by the phase transition III-II, leads to an axial motion of the ions in phase II. A complete breaking of all hydrogen-bonds in phase I allows the ions to reorient isotropically. The quadrupole coupling constant of rigid ClO−4 ions amounts to about 1 MHz.

DOI: 10.1002/bbpc.19860901207   (Pobrane:  aktualizowanie)