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Journal of Molecular Structure 872 (2008) 87–92

Infrared and Raman spectra of magnesium ammoniumphosphate hexahydrate (struvite) and its isomorphous analogues.

V. Spectra of protiated and partially deuterated magnesiumammonium arsenate hexahydrate (arsenstruvite)

V. Stefov a,*, B. Soptrajanov a,b, M. Najdoski a, B. Engelen a, H.D. Lutz c

a Institut za hemija, PMF, Univerzitet ‘‘Sv. Kiril i Metodij’’, P.O. Box 162, 1001 Skopje, Macedoniab Makedonska akademija na naukite i umetnostite, Skopje, Macedoniac Anorganische Chemie, Universitat Siegen, 57068 Siegen, Deutschland

Received 10 January 2007; accepted 6 February 2007Available online 20 February 2007

Abstract

The Fourier transform infrared and Raman spectra of magnesium ammonium arsenate hexahydrate, MgNH4AsO4 Æ 6H2O (arsenstru-vite) and of its deuterated analogues were recorded at room temperature (RT) and the boiling temperature of liquid nitrogen (LNT). Notsurprisingly, the spectra show pronounced similarities with the corresponding spectra of the previously studied magnesium potassiumphosphate hexahydrate and magnesium ammonium phosphate hexahydrate with the expected differences in the regions of the arsenatevibrations. The main contribution to the intensity of the complex feature in the X–H stretching region (X being O or N) comes from thestretching vibrations of the water molecules, whereas the vibrations of the ammonium ions are less important as long as the intensity isconcerned. This is due not only to the fact that four crystallographically different water molecules of crystallization exist in the structure(as compared with a single type of ammonium ions) but also because the hydrogen bonds formed by the water molecules are much stron-ger than those in which the ammonium ions take part. Difference infrared spectra were obtained by subtracting the properly normalizedspectrum of the protiated compound from the spectrum of a deuterated analogue with low deuterium content. As evidenced by the spec-tra of the partially deuterated analogues and by the difference spectra, vibrational interactions are present in the crystal. Probably themost dramatic is the result of such an interaction producing a deep Evans-type hole in the stretching region of the difference spectrum butadditional cases of vibrational mixing are found in the low-frequency region.� 2008 Published by Elsevier B.V.

Keywords: Magnesium ammonium arsenate hexahydrate; Arsenstruvite; FT infrared spectra; FT Raman spectra

1. Introduction

Continuing our work on the vibrational spectra of phos-phate compounds [1–5] especially the struvite-type com-pounds [6–9], the results of the analysis of the vibrational(FT-IR and FT Raman) spectra of arsenstruvite (magne-sium ammonium arsenate hexahydrate, MgNH4

AsO4 Æ 6H2O or MNA) and a series of its deuterated

0022-2860/$ - see front matter � 2008 Published by Elsevier B.V.

doi:10.1016/j.molstruc.2007.02.017

* Corresponding author. Tel.: +389 2 3117 055; fax: +389 2 3226 865.E-mail addresses: viktorst@iunona.pmf.ukim.edu.mk (V. Stefov),

bojan@manu.edu.mk (B. Soptrajanov).

analogues are presented here. Magnesium ammonium arse-nate is isostructural with the well-known biomineralMgNH4PO4 Æ 6H2O (MNP), often referred to by its miner-alogical name struvite, whose major biological importanceis related to its presence in human urinary sediments andvesical and renal calculi [10]. Our interest in the analysisof the spectra of this compound is to compare them tothe spectra of the other members of the struvite family[6–9], with the only significant difference that the phosphategroup is substituted by an arsenate group. The practicalpoint in studying the struvite family systems is in the factthat the MIMIIEO4 Æ 6H2O (MI = NH4, K, Rb, Cs, Tl;

4000 3400 2800 2200 1600 1000 400

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Fig. 1. Infrared spectra of MgNH4AsO4 Æ 6H2O recorded at RT (lowercurve) and at LNT (upper curve).

1 More details of the factor-group analysis for the compounds of thestruvite family are given in our previous work [6,8].

88 V. Stefov et al. / Journal of Molecular Structure 872 (2008) 87–92

MII = Mg, Ni; E = P, As) compounds can be considered aspotential protonic conductors due to the quite strong and,hence, easily polarizible O–H � � � O bonds present in theinvestigated struvite family systems as can be seen fromtheir vibrational (infrared and Raman) spectra.

As determined by X-ray diffraction [11], MgNH4

AsO4 Æ 6H2O crystallizes in the orthorhombic space groupPmn21 ðC2v

7Þ with Z = 2. It was found that Mg2+, NH4þ,

AsO43� and two of the four crystallographically different

water molecules of crystallization occupy special positionswith Cs symmetry, while the other two are in general posi-tions. The water molecules form donor hydrogen bondsbelonging to the shortest ever found in crystalline hydrates[12] and are very similar to those of MgNH4PO4 Æ 6H2Oand MgKPO4 Æ 6H2O (MKP) [13,14]. The six Ow � � � Ohydrogen bonds have distances ranging from 261.9 to269.8 pm, whereas the seventh such distance is 314.9 pmlong and is considered to represent a weak hydrogen bond.Each NH4

þ ion is surrounded by three oxygen atoms, twofrom water molecules and one from the arsenate ion.The hydrogen bonds formed by the ammonium ionwith the water molecules are very weak, but that formedwith the arsenate oxygen atom is rather strong (theN � � � O distance is 279.4 pm). To the best of our knowl-edge, the vibrational spectra of arsenstruvite and its deuter-ated analogues have not been systematically studied as yetexcept that recently [7] we published the results of ourstudy of the O–H(D)/N–H(D) stretching region of thespectra of MNP, MKP, and MNA. In the current paper,we present a detailed analysis of the Fourier transforminfrared and Raman spectra of the struvite-type arsenatecompound MgNH4AsO4 Æ 6H2O and of its partially deuter-ated analogues, recorded from room temperature (RT)down to the boiling temperature of liquid nitrogen (LNT).

2. Experimental

Arsenstruvite was synthesized according to the methoddescribed by De Schulten [15]. The synthesis of the par-tially deuterated analogues was accomplished analogously,using as solvents H2O–D2O mixtures with appropriatecompositions. The highest content of deuterium in the sam-ples was achieved using pure D2O.

The spectra were recorded, from both pressed KBr disksand mulls, at room and liquid nitrogen temperature (RTand LNT, respectively). The infrared spectra were recordedusing a Perkin-Elmer System 2000 infrared interferometer.The variable-temperature cell P/N 21525 (Graseby Specac)with KBr windows was used for the low-temperature mea-surements. For obtaining a good signal-to-noise ratio, 64scans were collected and averaged at LNT, whereas 32scans appeared to be enough at RT. The resolution ofthe instrument was 4 cm�1. GRAMS ANALYST 2000[16] and GRAMS 32 [17] packages were used for spectraacquisition and management. The FT Raman spectra wererecorded (with a resolution of 2 cm�1) on a Brucker RFS100ns FT Raman equipped with an Nd: YAG laser emit-

ting at 1064 nm. For a good signal-to-noise ratio, 500 scanswere accumulated and averaged. All Raman spectra wererecorded under identical experimental conditions.

3. Results and discussion

The FT-IR and Raman spectra of MgNH4AsO4 Æ 6H2Orecorded at room temperature (RT) and at the boilingtemperature of liquid nitrogen (LNT) are shown in Figs.1 and 2.

3.1. Group-theoretical arguments

As already mentioned, four types of H2O moleculesand one type of ammonium ions are present in thestructure of MNA [11], an analogous structural picturebeing found in the structure of struvite (MNP) [13].Accordingly, twelve H2O internal modes and fourNH4

þ internal modes should give rise to vibrationalbands if the H2O molecules and the NH4

þ ions aretreated as ‘‘free’’ entities. The group-theoretical approachshows that, because of the static field, three infrared andthree Raman active bands are expected from each typeof H2O molecules and nine such active bands shouldappear from the ammonium ions. Clearly, the presenceof the correlation field can cause a considerable increasein the number of bands1.

3.2. Internal vibrations of the water molecules and

ammonium ions

3.2.1. Stretching vibrations of the water molecules and

ammonium ionsIn the infrared and Raman spectra in the region of the

X–H stretching of the water molecules and ammoniumions, a broad, asymmetric, structured and deuteration-sen-sitive feature appears (Figs. 3 and 4) in line with the exis-tence of strong hydrogen bonds in the crystal structure of

3400 2800 2200 1600 1000 400

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Fig. 2. Fourier transform Raman spectra of MgNH4AsO4 Æ 6H2Orecorded at RT (lower curve) and at LNT (upper curve).

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Fig. 3. Infrared spectra of MgNH4AsO4 Æ 6H2O (lower curve) and itsalmost completely deuterated analogue (upper curve) recorded at LNT inthe region of the OH and OD stretching vibrations.

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Fig. 4. Fourier transform Raman spectra of MgNH4AsO4 Æ 6H2O (lowercurve) and its almost completely deuterated analogue (upper curve)recorded at LNT in the region of the OH and OD stretching vibrations.

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Fig. 5. Deconvoluted difference infrared spectrum recorded at LNT in theregion of the m(OD) vibrations, obtained by subtracting the spectrum ofthe protiated compound MgNH4AsO4 Æ 6H2O from the spectrum of theanalogue with low deuterium content (�2% D).

V. Stefov et al. / Journal of Molecular Structure 872 (2008) 87–92 89

MNA. It should be noted that this region of the spectra isvery similar to the corresponding region in the MNP andMKP spectra [6–8].

In an attempt to simplify the spectral picture, the infra-red difference spectrum2 of the analogue with �2% deute-rium content was studied, the same procedure beingpreviously applied in the case of KMP and NMP [6–8].As seen from Fig. 5, at least ten bands appear between2570 and 2040 cm�1 in the OD/ND stretching region ofthe LNT difference spectrum. It is easy to assign thesebands to the vibrations of isotopically isolated HDO mol-ecules and NH3D+ ions, but there is still a somewhat puz-zling aspect that deserves attention.

Namely, the intensities of the bands in the O–H/N–Hstretching region of the difference spectrum of NMA aremarkedly different from those in the analogous spectra of

2 The difference spectrum was obtained by subtracting the properlynormalized spectrum of protiated MgNH4AsO4 Æ 6H2O from that of itsslightly deuterated analogue.

the two other compounds, KMP and NMP [6–8]. Thus,whereas only one strong doublet of overlapped bands isseen in the deconvoluted difference spectra of the potas-sium analogue of struvite [6] and of struvite itself [8], twoalmost equally intense and structured features separatedby a deep ‘‘well’’ exist in the corresponding spectrum ofarsenstruvite (see Fig. 5). Such an appearance is indicativeof the existence of strong Fermi-resonance interactionsleading to a redistribution of the intensities and to theappearance of Evans holes. Since such a picture is presentonly in the spectrum of arsenstruvite (and not in those ofthe phosphate analogues), it is reasonable to assume thatthe partners of one or more X–D stretching vibrationsare second-order transitions involving the stretchingmode(s) of the AsO4

3� ions the frequencies of which theseare around 800 cm�1. The remaining contributor to thesummation frequency must then be one of the ammoniumbending vibrations with a frequency close to 1400 cm�1 (cf.

1320 1300 1280 1260 1240 1220

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Fig. 7. Deconvoluted difference IR spectrum of the slightly deuterated(�2% D) MgNH4AsO4Æ 6H2O recorded at RT (lower curve) and LNT(upper curve) in the region of the ND bending vibrations of isotopicallyisolated NH3D+ ions.

90 V. Stefov et al. / Journal of Molecular Structure 872 (2008) 87–92

Fig. 1). In this way the combination frequency would beclose to 2200 cm�1 and could interact with one of the O–D or (less likely) N–D stretching modes to give rise tothe observed Evans hole.

The weak band centered around 2270 cm�1 (in the mid-dle of the Evans hole) is, most probably, due to an N–Dstretch of the isotopically isolated NH3D+ ions notinvolved in Fermi resonance although the possibility thatthis is not a true band but a result of multiple interactionsof the Fermi-resonance type could not be entirely ruledout.

Taken all together, it seems reasonable to conclude thatthe intensity of the features in the O–H/N–H and O–D/N–D stretching regions is due mainly to modes of the watermolecules whereas the N–H(D) stretches contribute to alesser degree to the shape and intensity of the complexbands. In view of the fact that the hydrogen bonds formedby the water molecules are stronger than those in which theammonium ions take part and thus the O–H � � � O bondsare easier to polarize, the above results finds a logicalexplanation. It is well-known, namely, that the strengthof the X–H � � � Y hydrogen bonds and the intensity of theX–H stretching bands are proportional, the stronger thehydrogen bond, the greater is the expected intensity ofthe m(X–H) bands.

3.2.2. Bending vibrations of the water molecules and

ammonium ionsSeveral bands appear in the vibrational spectra of the

arsenstruvite, in the region of the bending vibrations ofthe water molecules and ammonium ion, the spectral pat-tern being practically identical to that in the correspondingstruvite spectra [8]. Taking into consideration the analysisof the struvite spectra, in the LNT Raman spectrum ofarsenstruvite the bands at 1700 and 1683 cm�1 are safelyattributed to the m2(NH4) modes, doubly degenerate underTd symmetry, whereas the bands at 1477 and 1444 cm�1

certainly arise from the m4(NH4) modes, triply degenerate

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Fig. 6. Infrared (lower curve) and Raman (upper curve) spectra ofMgNH4AsO4 Æ 6H2O recorded at LNT in the region of the bendingvibrations of the water molecules and ammonium ions.

for the ideal Td symmetry (Fig. 6). In the LNT infraredspectrum, the corresponding bands appear at 1702, 1686,1475, 1451, and 1440 cm�1. The bands present around1600 cm�1 in both the infrared and Raman spectra canpositively be attributed to the bending HOH vibrations.

In the RT difference infrared spectrum of the analoguewith a small deuterium content (�2% D), two bands (ataround 1290 cm�1 and 1255 cm�1) are observed in theregion of the ND bending vibrations of isotopically iso-lated NH3D+ ions, whereas four bands (at 1303, 1295,1266, 1250 cm�1) can be seen in the corresponding LNTspectrum (Fig. 7). This may imply, as in the case of struvite[8], the existence of some kind of disorder of the ammo-nium ions, at least at subambient temperatures.

3.3. 1200–400 cm�1 region

In the 1200–400 cm�1 region, the AsO43� stretching and

bending vibrations are expected to appear, as well as thelibrational modes of the water molecules. In fact, the libra-tions of the ammonium ions should also appear in the men-tioned region, but our experience with struvite (thephosphate analogue of magnesium ammonium arsenatehexahydrate) [8] shows that the bands from these vibra-tions are unlikely to give rise to bands of appreciable inten-sity. Thus, only the bands from the arsenate antisymmetricstretching mode m3 and the bending m4 vibration3 as well asthose of the librational modes of the water molecules areexpected in the infrared spectra while the m1 mode shouldgive rise to a strong Raman band.

Since the effective symmetry of the arsenate anions isquite high, the infrared band originating from the m3 mode

3 The symmetric stretching vibration m1 and the bending mode m2 areinfrared inactive under the Td symmetry of a tetrahedral entity.

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Fig. 9. Infrared spectra of partially deuterated analogues of MgNH4

AsO4 Æ 6H2O recorded at LNT in the region of the HOH and AsO4

external vibrations (the content of deuterium increases from bottom to topspectrum).

*

V. Stefov et al. / Journal of Molecular Structure 872 (2008) 87–92 91

is not likely to be appreciably split, despite the alreadymentioned low site symmetry of the AsO4 entities. On theother hand, the low site symmetry of both the arsenate ions(Cs) and the water molecules makes possible vibrationalinteractions between the arsenate vibrations and some ofthe water librations if they fall in the same spectral region.

In view of the above, it is advantageous to studytogether the bands in the 1200–400 cm�1 region rather thandiscuss separately the external vibrations of the water mol-ecules and the internal modes of the arsenate ions. This iscontrary to the manner in which the corresponding modeswere treated in our previous work on struvite [8] and itspotassium analogue [6].

As seen in Fig. 8, between 1100 and 370 cm�1, at leasttwelve bands are present in the LNT infrared spectrum ofarsenstruvite, of which some are obviously due to the arse-nate stretches that may or may not be involved in vibra-tional interactions with the water librations.

The location of the arsenate bands is somewhat easier inthe spectra of the practically completely deuterated com-pound (Figs. 9 and 10). As seen in Fig. 9, in the infraredspectrum of MgNH4AsO4 Æ 6D2O there is a strong andbarely split band in the region where the components ofthe m3 are expected to appear. The frequency of the mainmaximum is 811 cm�1 and the two barely resolved shoul-ders on the high-frequency side appear at 829 and845 cm�1. The corresponding bands in the infrared spec-trum of the protiated compound are visible at 798, 822and 840 cm�1. Since the region around 800 cm�1 has beencleared of bands due to H2O librations (see below) there isno serious objection to the assignment of the whole feature

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Fig. 8. Infrared spectra of MgNH4AsO4 Æ 6H2O recorded at RT (lowercurve) at LNT (upper curve) in the region of the HOH and AsO4 externalvibrations.

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Fig. 10. Fourier transform Raman spectra of partially deuteratedanalogues of MgNH4AsO4 Æ 6H2O recorded at LNT in the region of theHOH and AsO4 external vibrations (the content of deuterium increasesfrom bottom to top spectrum). * – the spike at around 540 cm�1 is due to‘‘event’’.

to the components of the arsenate antisymmetricstretching.

The symmetric stretching vibration m1 is easily locatedin the Raman spectrum of the highly deuterated

92 V. Stefov et al. / Journal of Molecular Structure 872 (2008) 87–92

analogue (Fig. 10) in which the frequency of the mainmaximum is 827 cm�1. It is somewhat of a surprise thatthis frequency is higher than that of the main maximumof 818 cm�1 in the spectrum of the protiated compound.Since under the site-group approximation two of thecomponents of the m3 arsenate vibration and the symmet-ric stretching vibration m1 are of the same symmetry (A 0),it is not unconceivable that some mixing of the stretchingmodes takes place and that the weak bands at the low-frequency side of the main Raman band results fromsuch an interaction. It is also possible that coupling ofthe m1(AsO4) modes and some of the librational watermodes takes place.

The spectral picture in the infrared spectrum of the pro-tiated compound is much more complicated, with a broadand structured feature extending from 1100 cm�1 to600 cm�1 being dominant. The submaxima (true or appar-ent) are all temperature-dependent (Fig. 8), exhibiting ablue shift and gaining (perhaps only apparently) in inten-sity. Whereas this is a behavior characteristic for waterlibrations, the temperature sensitivity even of the mainmaximum (attributed above to the antisymmetric arsenatestretch) is a strong indication of a vibrational interactioninvolving some of the H2O librations and the antisymmet-ric stretching vibration of the AsO4

3� ions4 taking place.As the deuterium content in the sample is increased, the

picture around 800 cm�1 becomes simpler, with the major-ity of the maxima (those with frequencies 980, 935, 910,776, 752, and 711 cm�1) gradually disappearing whilenew bands appear at lower frequencies (at 663, 624, 572,561, and 522 cm�1). Such a behavior clearly qualifies thesebands as due to more or less pure water librations. Thehigh frequencies of some H2O librational bands stem fromthe considerable strength of the hydrogen bonds formed bythe water molecules and the fact that in the studied infraredspectra of MNA, MNP, and MKP, the highest frequencybands due to librations are found in the spectrum ofarsenstruvite.

In the LNT Raman spectra (Fig. 10) of MNA, the weakbands around 900, 765, and 750 cm�1 (not visible in thespectrum of the almost completely deuterated analogue)can be attributed to librations of the water molecules.

Returning now to the arsenate vibrations, it is practi-cally certain that in the infrared and Raman spectra ofthe almost completely deuterated analogue the peak at413 cm�1 originates from the bending m4 AsO4 bendingmode. Not surprisingly, the above frequency is lower than

4 Interactions between the modes of the anions and the librations of thewater molecules are not exceptional and have been previously observed byus [18–20].

that of the corresponding PO4 mode which is deuteration-sensitive [6,8]. It is possible that the situation may be sim-ilar in the present case too, the partner in the potentialinteraction being some librational mode of deuteratedwater molecules (Fig. 10).

In the region from 550 to 350 cm�1 (Fig. 10) in the infra-red and Raman spectra, temperature-sensitive bands areobserved that may be attributed to the already discussedm4(AsO4) vibration as well as to the m(Mg–Ow) modes andperhaps to the m2(AsO4) modes.

Acknowledgment

The financial support of the Ministry of Education andScience of the Republic of Macedonia is gratefullyacknowledged.

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