Raman spectroscopy of likasite at 298 and 77 K

Raman spectroscopy at 298 and 77 K has been used to study the structure of likasite, a naturally occurring basic copper(II) nitrate of formula Cu₃NO₃(OH)₅·2H₂O. An intense sharp band is observed at 3522 cm⁻¹ at 298 K which splits into two bands at 3522 and 3505 cm⁻¹ at 77 K and is assigned to the OH stretching mode. The two OH stretching bands at 3522 and 3505 provide estimates of the hydrogen bond distances of these units as 2.9315 and 2.9028 Å. The significance of this result is that equivalent OH units in the 298 K spectrum become two non-equivalent OH units at 77 K suggesting a structural change by cooling to liquid nitrogen temperature. A number of broad bands are observed in the 298 K spectrum at 3452, 3338, 3281 and 3040 cm⁻¹ assigned to H₂O stretching vibrations with estimates of the hydrogen bond distances of 2.8231, 2.7639, 2.7358 and 2.6436 Å. Three sharp bands are observed at 77 K at 1052, 1050 and 1048 cm⁻¹ attributed to the ν₁ symmetric stretching mode of the NO₃ units. Only a single band at 1050 cm⁻¹ is observed at 298 K, suggesting the non-equivalence of the NO₃ units at 77 K, confirming structural changes in likasite by cooling to 77 K.