RESEARCH ARTICLE Vibrational spectra of the rare‐occurring complex hydrogen arsenate minerals pharmacolite, picropharmacolite, and vladimirite: Dominance of Raman over IR spectroscopy to discriminate arsenate and hydrogen arsenate units Petre Makreski 1 | Jovica Todorov 1 | Vasil Makrievski 1 | Ljupčo Pejov 1 | Gligor Jovanovski 1,2 1 Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius University, Arhimedova 5, 1000 Skopje, Republic of Macedonia 2 Research Center for Environment and Materials, Macedonian Academy of Sciences and Arts, Bul. Krste Misirkov 2, 1000 Skopje, Republic of Macedonia Correspondence Petre Makreski and Ljupčo Pejov, Institute of Chemistry, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius University, Arhimedova 5, 1000 Skopje, Republic of Macedonia. Email: petremak@pmf.ukim.mk; ljupcop@pmf.ukim.mk Funding information Ministry of Education and Science of Macedonia; Macedonian Academy of Sci- ences and Arts Abstract This work represents the first complete experimental and theoretical study of the infrared (IR) spectra of pharmacolite, Ca(AsO 3 OH)·2H 2 O; vladimirite, Ca 4 (AsO 3 OH)(AsO 4 ) 2 ·4H 2 O; and picropharmacolite, Ca 4 Mg(AsO 3 OH) 2 (AsO 4 ) 2 ·11H 2 O. IR spectra, collected at both room and liquid nitrogen temperature, have shown similar spectral view among minerals in the region of ν 1 to ν 4 modes, surprisingly not involving complexity in vladimirite and picropharmacolite spectral view due to the presence of both AsO 3 OH and AsO 4 units compared to pharmacolite where only AsO 3 OH units appear in the structure. On the other hand, vladimirite and picropharmacolite Raman spectra in the corresponding ν 1 to ν 4 region (955–350 cm -1 ) exhibit existence of 3 sets of 2 bands (6 in total) completely reflecting the existence of 3 symmet- rically nonequivalent AsO 4 groups in the structures of vladimirite and picropharmacolite (of which one is protonated) and only one set of ν 1 to ν 4 bands from the structurally equivalent AsO 3 OH units in the pharmacolite structure. The liquid nitrogen temperature IR spectra enabled to infer impor- tant information regarding the OH vibrational bands related to the AsO 3 OH groups and to the water molecules. Essentially, all bands in the vibrational spectra were assigned and correlated with the findings for a plethora of struc- turally similar complex hydrogen arsenate minerals. To support the tentative assignment of bands in the vibrational spectra, quantum theoretical calcula- tions were performed within the framework of density functional theory. On the basis of theoretical calculations, a few reassignments of bands appearing in the OH stretching region are proposed. X‐ray powder diffraction was used to confirm authenticity of the title systems. KEYWORDS arsenate minerals, hydrogen arsenates, hydrogen bonding, quantum theoretical methods, vibrational spectroscopy Received: 5 September 2017 Revised: 16 November 2017 Accepted: 28 November 2017 DOI: 10.1002/jrs.5324 J Raman Spectrosc. 2018;1–17. Copyright © 2018 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jrs 1