The infrared spectra of solid methylammonium halides—II

Abstract

The infrared spectra of solid films of methylammonium chloride, bromide and iodide were measured down to 12–15°K. No new phases were found below liquid air temperature. The measurements were extended into the far infrared and new evidence was found in favor of the assignment of a weak band near 470 cm −1 in the spectra of the γ and β-phases to the fundamental of the torsional oscillation mode. Additional arguments are given in favor of one- dimensional internal rotation in the α phases. The spectra of a complete set of compounds of type CD 3NH 3 +X −, CH 3ND 3 +X −, CD 3ND 3 +X − were measured and it is shown that an almost perfect coincidence of the bands due to the bending modes ν 3 and ν 10 obliges us to reassign these bands. From the splitting observed it can be concluded that correlation field splittings may be as large as crystal field splittings. Krimm's isotopic frequency ratios were found helpful in making assignments. It follows from the new assignments that the spectacular change in relative intensities between CH and NH bending modes which were reported earlier does not actually occur. The δ-phases are characterized by large correlation field effects. The cause of the splittings of the rocking modes ν 11 and ν 12 were studied by the method of isotopic solid solutions. It was found that for these vibrations the larger splittings are of the crystal field type while the smaller ones are of the correlation field type. The changes in the relative intensities of these two bands are explained as due to the changes upon deuteration in the relative contributions to these modes of the motions of the CH 3 and NH 3 + groups.