The potential energy curve of the proton and the dissociation energy of the OH− ion in Mg(OH)2

The overtone and combination bands, Eon, of the infrared-active (vu) and Raman active (vg) OH stretching vibrations of Mg(OH)2 are measured (values for Mg(OD)2 in brackets). Fundamentals E01 vu at 3697 (2720) cm−1 and vg at 3655 cm−1; first overtone E02: vu + vg at 7154 (5305) cm−1; second overtone E03; 3vu at 10480 (7830) cm−1; third overtone E04: 3vu + vg at 13640 (10250) cm−1; fourth overtone E05: 5vu or 3vu + 2vg at 16610 cm−1; fifth overtone E06 at 19320 cm−1. The isotope shift factor (theoretical 1.37) decreases from 1.36 for the fundamentals E01 to 1.33 for E04. The potential barrier between proton sites in adjacent OH− groups is of the order of 2.6 eV (60 kcal mol−1). The dissociation energy of the OH− groups in the Mg(OH)2 lattice, 4.70 eV (108.3 kcal mol−1), agrees well with the thermodynamic value, 4.71 eV (108.6 keal mol−1), derived for the free OH− ion.