Infrared spectra of the ammonium ion in ammonium hexavanadate (NH4)2V6O16

Spworhimica Arm. Vol. 46A. No. I I. pp. IMY-1657. Printcd in Great Britain

1990 0

05t?&R53Y/W $3.w+o.(Xl 1990 Pergamon Press plc

Infrared spectra of the ammonium ion in ammonium hexavanadate W-IM& D. DE WAAL and A. M. HEYNS* Department of Chemistry, University of Pretoria, 0002 Pretoria, South Africa

and K.-J.

RANGE*

and C. EGLMEIER

Institute of Inorganic Chemistry, University of Regensburg, Universitatstr. 31, D-8400 Regensburg, F.R.G. (Received

13 February

1990;

in final form 3 June 1990; accepted 5 June 1990)

Abstract-The infrared bands of the NH: and ND: groups in (NH&V6016 and its deuterated analogue can be assigned with a fair amount of certainty at 90 K under the space group P2,Im(C~). The N-D stretching modes of isotopically dilute NHP+ ions in the crystal are in agreement with the predicted splitting into C,, C, and C,(2) components. The frequencies, shapes and temperature dependence of these modes suggest that both normal and bifurcated hydrogen bonds are formed. The latter closely resembles corresponding bonds in NH,VO,, but the normal hydrogen bonds are not as strong as the similar bonds in NH4VOS. This can be expected as NH: ions are dynamic in character in (NH4)rV60ih and remain so down to temperatures of 90 K.

INTRODUCTION

of the ammonium ion have been reported in ammonium vanadate [l]. When this compound is heated in air in an open system the first decomposition product is ammonium hexavanadate, (NH&V60i6 [2]. The space group of (NH4)2V6016 is P2,lm with NH;, V(l), O(1) and O(2) occupying 2(e) positions with C1 symmetry and V(2), O(3), O(4) in 4(f) positions with C, symmetry [3,4]. In a recent structure analysis of the compound [5] the coordination of oxygen atoms surrounding the vanadium and nitrogen atoms were determined (Fig. 1) but the position of hydrogen atoms remained unresolved. The ammonium ion is surrounded by several oxygen atoms belonging to the hexavanadate groups, and the eight closest N-O distances are between 2.824 and 3.169 A [S]. This can be compared with the N-O distances in NHV03 which vary between 2.85 and 3.40 8, [6]. With a co-ordination number of eight the ammonium ion in (NH&V60i6 can be expected to be highly dynamic. Room temperature i.r. and Raman spectra of (NH&V60r6 recorded up to 1080 cm-’ have been reported [7], involving mainly V-O vibrations. An i.r. spectrum recorded up to 4000 cm-’ [2] revealed that room temperature N-H bands were not in agreement with the number predicted under P2Jm. Low temperature spectra of the compound could possibly reveal further splitting of bands and provide more information on the ammonium ion. Because coupling and Fermi resonance in the N-H stretching region could complicate the spectrum, a low temperature i.r. study of isotopically dilute NH3D+ ions in (NH&V60L6 was used to determine the nature of hydrogen bonds in the crystal. (NH&V6016 containing various percentages of deuterium were used to identify bands of the different isotopical species. THE INFRARED spectra

EXPERIMENTAL A method originally developed for the preparation of (NHJ2V30X was modified to obtain (NH&V6016. (NH&VnO16 was originally reported [8] to be a by-product of the reaction when V205 reacts with NH&I in the presence of metallic tin as reducing agent. It was found that by * Authors to whom correspondence should be addressed. 1649

D.

1650

DE WAAL et al.

Fig. 1. The coordination polyhedra of nitrogen, vanadium V(1) and V(2) in (NH4)2Vh0,6.

intentionally omitting tin (NHJ2V60i6 is formed [S]. Pure (NH4)rV60L6 was prepared by adding V,O, to a saturated NH&l solution and allowing the mixture to reflux between 90 and 100 “C for approximately 3 h, yielding a bright yellow product. Deuterated samples were obtained by substituting Hz0 with stoichiometric mixtures of Hz0 and DzO during preparation. Infrared spectra were recorded both at room temperature and 90 K on a Bomem Michelson-100 FAIR spectrometer with a resolution of 4 cm-‘. All samples were in the form of KBr pellets and to ensure that frequency shifts due to ion exchange did not occur the 5% deuterated sample was also recorded in the form of a CsCl pellet with identical results to that obtained with KBr. During the temperature dependence study, spectra were recorded at various temperatures between 80 and 291 K on a Bruker IFS 113 V spectrometer with a resolution of 1 cm-‘. During low temperature recordings a continuous flow cryostat (Oxford Instruments Model CF 1100) was used with liquid nitrogen as cryogen to keep samples at different temperatures between 80 K and room temperature.

RESULTS AND DISCUSSION

Infrared spectra

Under site group C,, v,(A,) of the NH: ion is expected to remain single while Q(E) should split into two bands and both I+(PJ and Y.,(PJ into three bands each. Further factor group splitting should result in the same number of bands in the i.r. and Raman spectra as half the modes under P2,lm are i.r. active and the other half Raman active: A’ splits into A,(R) + B,(i.r.) and A” into B,(R) +A,(i.r.). The N-H stretching region in the i.r. spectra of (NH&V60r6 at room temperature and 90 K are shown in Fig. 2, with the low temperature spectra of pure and variously deuterated samples of the same compound in Figs 3 and 4. A full assignment of the spectra is given in Tables 1 and 2. It was previously reported [2] that only one of the two stretching modes (Y, and Q) is observed in the room temperature i.r. spectrum of pure (NH4)2V60,6 and that even though the low C, symmetry of ammonium in this compound should cause splitting of degenerate vibrations, single modes are present for both Ye

Infrared spectra of NH; in (NH&V,O,,

I

, 3980

1

1

(

3020

WAVENUMBERS

Fig. 2. The N-H stretching region in (NH,),V,O,,

1

1651

J

2060 IN cm-’

at room temperature (top) and 90 K (bottom).

and vq. These modes were observed at 3216cm-’ (Fig. 2) and 1405 cm-’ at room temperature. At low temperatures, a tentative assignment of the N-H modes can be made as follows: 3250(1+), 3194(v,) and 3177(v3), thereby accounting for all three i.r. active components of v3. v, can be assigned to the satellite at 3115 cm-’ which shows indications of being split. This is, of course, in contradiction to the selection rules. The absorption peak at 3041 cm-’ can be assigned to v2 + v4. v2 does not split into two components, even at 90 K where it appears at 1638 cm-‘, but v4 splits into three components at 1420, 1398 and 1385 cm-’ at low temperature (Fig. 4) with the 2v4 overtone at 2814 and 2766 cm-’ (Figs 2 and 3). Extremely weak features at 1974 and 1930cm-’ probably represent the combination bands of v,+ v~, however, there are no indications of the occurrence of v4 + vs which should appear in the frequency range 1700 to 1750 cm-‘. These observations show that the NH: ions in (NH4)2V6016 are more dynamical than the ones in

1

3180

2860 WAVENUMBERS

2540

2220

1900

IN cm-’

Fig. 3. Low temperature i.r. spectra of (NH&VhO1,, and various percentage deuterated samples between 3400 and 1900 cm-‘.

1652

D. DE WAAL ef al.

1410

1320 WAVENUMBERS

1230

1140

1050

IN cm-’

Fig. 4. Low temperature ix. spectra of (N~~)*V*O,~and various percentage deuterated between 1450 and 1050 cm-‘.

samples

NH,V03 where the combination bands yi+ %, (i=2,4) could easily be identified at ambient temperatures [9]. The values of vg, calculated to be equal to 336 and 292 cm-’ in (NH&V6016 are considerably lower than the ones in NH4V03 at ambient conditions. This shows that the NH: ions in (NH&V6016 have more reorientational freedom than the ones in NI&V03, even though the N-O distances in (NH&V60,,, are shorter on average than the ones in NH4V03. Upon cooling the samples to 90 K no considerable sharpening of the combination band Y,+ Yg took place, showing that the NK ions remain dynamic even at very low temperatures. The fundamental stretching modes in particular, however, cannot be assigned unambiguously in (NH~)*V~O~6since, like in all the other NH: compounds, the frequency region where these bands occur, is also complicated by the occurrence of combinations and overtones such as v2 + Y, and 2~~that have been observed here. For this reason the i.r. spectrum of the isotopically dilute NH3D+ species was recorded since the N-D stretching frequency is not complicated by Fermi resonance between energy levels such as Q, vi, 2v4 and v2 + v4. In an ammonium of C, symmetry, substitution of one of the N-II bonds by N-D could cause the resultant NH@” species to have either C, symmetry with the N-D bond coinciding with the o-plane in the crystal, or C,Ysymmetry with the N-D bond pointing in the opposite direction but still in the o-plane of the crystal, or it can have a C, symmetry in both of two equivalent positions, fixed by the condition that two of the N-H bonds be identical in a NH: ion of C, site symmetry. In other words, the vI band of the NH3D+ species should split into C,, C, and C,(2) components.

Infrared spectra of NH: in (NH,),V,O,, Table 1. Low temperature and 2000 cm-’

i.r. modes of the deuterated species in (NH&V,O,,

Wavenumbers 100% (NH&V@u

Assignment

v&W 1, VW-ID:) v,(NH:), v,(NHD:) v,(N% , v,(NHD:)

v,(NH:), v,(NHD;) ~2 +

v4W-G

~2 +

v4W-G)

1

v,WH,W 2V4@&+)

2~40W) 2v4N-U

3250m 3194m 3177sh 3115m 3041m 2846w

-

M’JH,D+)

1

2v,(NHD;) 2v4(NH3D+) v,(ND:) vx(ND:) v,(NHD:) v,(ND:) v,(ND:) v,(NH,D+) v,(NH,D+) Zv,(NHD;) v,(NHsD+) 2v,(ND:) 2v.,(ND: ) 2v,(ND:)

1653

2814~ 2766~ -

-

5% D 3247m 3197s 3177sh 3103m

-

3050m 2917~ 2846w 2818~ 2770vw 2739vw

in cm-’

30% D

50% D

70% D

3250m 3198s 3184sh 3103m 309Osh 3079sh 291&w 2a48w 2836~ 2820w 2774~~

3247m 3188s 3120m.sh 306Om 2915~ 2846~ -

3243m 3185s 3140sh 3117m -3070sh 2916~ 2847~ 2786~ 2727~ 2482~~ 2436s 2427s 2404vs 2385vs.sh 2306~~ 2283~ 2238~ 2165~~ 2139w

-

-

2479vw -

2388sh 2356w.s~ 2345w.sp -

23f%W

2287~

-

between 3500

2359w 2351~ 2287~ 2241~~

2819vw

2474~~ 2420 2404sh 2390sh 2379w 2355~ 2355~ 2306sh 2284w 2237~~

-

s = strong, w = weak, sh = shoulder, m = medium, v = very, b = broad.

Atmospheric CO* has absorption bands in the i.r. between 2386 and 2317 cm-’ which is observed even after purging the instrument with dry nitrogen. This prevented the use of OS-l% deuterated (NHJ2V60r6 samples to study low intensity vl(NHJD+) bands in the same frequency region. At 5% deuteration NHjD+ modes are considerably stronger than those of CO2 in the same region. In Fig. 3 the N-D stretching modes in 5% D (NHJ2V60L6 are represented by three components at 2356, 2345 and 2287cm-’ at 90K and can be compared to those in NH,VO, as it also contains NH: ions of C, symmetry. In the latter, NH: ions are bonded by normal, strong hydrogen bonds on the one hand and weaker bifurcated hydrogen bonds on the other hand [9]. The N-D stretching modes of isotopically dilute NH,D+ reflect these differences in hydrogen bonding, and the three predicted components are observed at 2196, 2348 and 2370 cm-’ [l]. The latter two frequencies correspond to the ones at 2345 and 2356 cm-’ in (NH.,) 2V 60 ,6, suggesting the existence of bifurcated hydrogen bonds in this compound as well. The hydrogen bond associated with the v1(NH3D+) band at 2287 cm-’ is normal and quite strong, even though less so than the bond corresponding to the 2196 cm-’ band in NH4V03. Of the six modes predicted for v&NH3D+), only two are observed at 1261 and 1245 cm-’ at low temperature. The following NH,D: modes are present as low-intensity bands in 5% deuterated (NH&V~OM: y3 at 2917cm-’ (Fig. 3), v2 at 1559cm-’ and 1319cm-’ (Fig. 4). Weak bands at 1363 and 1176, 1160 cm-’ can be attributed to the v2 and v4 modes of NHD:, with a band at 1339 cm-’ being assigned to v,+ v6(ND;) and a weak shoulder at 2388 cm-’ to v, of the same species. At 30% deuteration even more bands become visible. A new shoulder at 3184 cm-’ is assigned to the N-H stretch, v3(NHD:). A band at 2836cm-’ reaches maximum intensity in this spectrum and is therefore assigned to v3(NH3D+) on grounds of statistical distribution of the NH,_,D: species. All other bands assigned to NH3D+ also reach

1654

D. DE WAAL et al.

maximum relative intensity at 30% D, including a weak new band at 2479 cm-’ which is assigned to 2v4(NH3D+). A shoulder at 1116 cm-’ can be attributed to vq(NH2D:). Some NH: vibrations can still be distinguished in the spectrum of 50% deuterated (NHJ2V6016 but bands representing the NH*D;, NHD: and ND: species become more prominent. ND; modes in the N-D stretching region start to overlap with those of NH3D+: v3(ND4+) occurs at 2404 cm-’ and q(ND4+) at 2379 cm-‘. One shoulder in this region can be attributed to r+(NHD:) while another at 2306cm-’ coincides with 2vb(NH3D+). In the bending region the v2 and v4 modes of NH2D: at 1583 cm-’ and 1316,1185,1116 cm-‘, respectively, reach maximum intensity here as would be expected at 50% deuteration. Two new bands in this region, at 1131 and 1094 cm-’ can be assigned to the v4 bending mode of NHD:. These bands reach maximum intensity at 70% deuterium content. From the low temperature spectrum of 70% D (NHJ2V6016 it becomes clear that the number of observed fundamentals in (NH4)2V6016 closely agrees with the theoretical predictions. Three bands in the N-D stretching region at 2436, 2427 and 2404 cm-’ can tentatively be assigned to v3, leaving a single mode at 2385 cm-’ to be attributed to vI. The three components predicted for v4 are observed at 1059, 1067 and 1083 cm-‘. In (NH4)2V6016 only one of the two bands expected for v2 becomes visible as a weak band, but in (ND4)2V60,6 two modes of considerable intensity appear at 1124 and 1109 cm-‘, thus accounting for all the fundamentals predicted for ND: under P2,lm. Overtones in the N-D stretching region are well separated from the fundamentals and occur at 2238cm-’ (2~~) and 2165, 2139cm-’ (2vJ, respectively. As opposed to undeuterated (NH4)2V6016 where only one of the two vi + v6 (i = 2,4) bands, viz. v2 + v6r was observed, both are present in (ND&V,& where v2+ vg appear as a Strong mode at 1409 Cm-’ and v4+ v6 as a weak mode at 1345 cm-‘. Both these bands occur close to other modes at 70% deuteration, one in the middle of two strong v4(N114+)components and the other between vd(NH4+) and vq(NH2D:), which is probably responsible for the enhancement of these modes in (ND4)2V6016. v6 is calculated at between 260 and 300 cm-‘. Two new bands at Table 2. Low temperature i.r. modes of the deuterated species in (NH&V60,, between 2000 and 1000 cm-’ Wavenumbers in cm-’ 100% Assignment

(NH&V@,6

v,+G’W) ~2 +

WW

v,W-G

) )

~2W%D;

1

4W) ~2 +

NW

M’W

) 1

v,WW v,(NHD:) ~4 +

v&W)

v2WH2D:) v2W-W;) v,WH,D+) vdNHG+) VW-W) VW-W) v,WHb+ v2WD:

)

1974vw 1930vw 1638b.w -

5% D

-

1420s

1653~~ 1559w 1421~s

-

-

1398s 1385s

1399vs 1375w 1363~~ 1339vw 1319vw

-

1176~~ -1160vvw

-

-1104vvw -

1261~ 1245~ -

1

v4NH2D:) v2WD:) v,PHW v,WW v,WD:) NW)

30% D

70% D

50% D

1967vw.b -

-

-

1950w.b -

1923w.b -

1588~ 1423s -

1583~ 1426m 1412sh 1397m.sp 1386sh 1370vw 1346~ 1324~ 1316~ 1260m 1245~ 1185w 1174w 1131sh -

1558w 1420sh 1409m 1402m 1385~ 1379w 1345w 1323~ 1316vw.sh 1260w 1243~ 1184m 1173m 1136m 1124s -

1398s 1345w 1322~ 1260s 1245m 1183~ 1173w 1 I lhw.sh llllw 1084vw 1068~~ -

1116~ llllw 1094sh 1085~~ 1069~~ -

1109s 1083vs 1067sh 1059s

Infrared spectra of NH: in (NH&V6016

h

0

h

1655

0 a

h

Y

d

0

50

100

150

200

250

300

TEMPERATURE IN K

Fig. 5. Temperature (NHAV~OM.

dependence

of the stretching

modes of isotopically dilute NH@+ in

2727 and 2482cm-’ can be assigned to 2v,(NHD:) and 2v4(NH3D+). In the N-H stretching region three bands that occurred at 3250, 3194 and 3177cm-’ in 100% (NHJ2V60r6 and were assigned to vS(NI-&+)shifted to 3243,3185 and 314Ocm-‘. These bands can now be attributed to the N-H stretch in NHDZ. Temperature dependence The temperature dependence of the various i.r. active ammonium bands, especially those of isotopically dilute NH,D+, can provide more information about the nature of hydrogen bonds in the (NHJ,V,O,, crystal. The behaviour of v1(NH3D+) modes of (NHJ2V60r6 between 80 K and room temperature are shown in Fig. 5. The highest frequency mode at 2356 cm-’ shows a decrease in wavenumbers with the increase in temperature at a rate of -0.03 cm-’ K-’ and can be compared to v1(NH3D+) in 1% D NH4V03 at 2370 cm-’ which shows similar behaviour by moving downwards at a rate of -0.05 cm-’ K-r [l]. A comparison between NH3D+ modes in the two compounds was

.... . 1 l

3220

7 e t

.

.

.**

-

50

100

150 200 TfHPERATlMIN K

250

MO

Fig. 6. The temperature dependence of some NH: modes in pure (NH4)2Vh0,h: two v3 (top) and two v4 bands (bottom).

D. DE WAAL et al.

1656

shown previously [l]. The vi(NH3D+) band in (NH&V60i6 at 2345 cm-’ shifts very slowly towards lower frequency (dvldT= -0.002 cm-’ K-l) and therefore remains virtually unchanged between 80 K and room temperature. From these results it can be assumed that both these bands in (NH4)2V6016 represent a N-H bond in which the hydrogen atom becomes more and more associated with a particular oxygen atom at higher tempertures with a concomitant increase in hydrogen bond strength, suggesting that the bands represent bifurcated hydrogen bonds, as the bands at 2370 and 2348 cm-’ [l] in NH.,V03 do. The third v,(NH,D+) mode in (NH4)2V6016 at 2287 cm-’ shows a blue shift upon an increase in temperature at a rate of 0.09 cm-’ K-l. It can be assumed that this band represents a N-H bond associated with normal hydrogen bonding as its behaviour shows a decrease in strength of the bond at higher temperatures. However, when compared with the strong, normal and almost straight line hydrogen bonds in NH4V03 represented in the spectrum by a band at a much lower frequency (2196 cm-‘) and of greater intensity than the one in (NH&V60i6 it becomes clear that the normal hydrogen bonds in (NH4)2V6016 are not of the same strength as that in NH.,VO+ This could be expected as it was already established that NH: ions in (NHJ2V60i6 have more reorientational freedom than the ones in NH4V03. The three bands in the N-H stretching region at 3250, 3194 and 3115 cm-’ can probably be associated with three different N-H bond lengths in (NHJ2Vb0,+ The highest frequency band that can be assumed to represent the longest N-H bond shows a decrease in wavenumber with increasing temperature at a rate of -0.07 cm-’ K-’ (Fig. 6). Similar behaviour is observed for the 3115 cm-’ band with dvldT= -0.01 cm-’ K-l. The hydrogen bonds associated with these bands increase in strength at higher temperatures, showing the typical behaviour of bifurcated hydrogen bonding which was also observed in the two higher frequency vi(NH3D+) modes. The 3194cm-’ band moves towards higher frequencies with increasing temperature which is the normal behaviour for a hydrogen bond. Similar behaviour for the different bonds is repeated for N-H and N-D stretching vibrations in 30% D (NH&V60i6 and the highest deuterated sample (Fig. 7), respectively. The two components of the bending vibration vq(NI-G) reflect the opposite behaviour of the two types of hydrogen bonding identified in (NH.J2V60r6 (Fig. 6) as the one shows a decrease in frequency at a rate of -0.07 cm-’ K-’ while the other

r-'

2440

1050. 50

100

150 200 TEMPERATURE IN K

Fig. 7. The temperature dependence of some ND;

250

300

modes in (ND&VnOIn:

v,, Y,, v2 and v+

Infrared spectra of NH: in (NH&VbO16

1657

moves towards higher frequency at 0.04 cm-’ K-‘. In (NHJ2V6016 the v2 and v4 bending modes also move in opposite directions upon an increase in temperature. Acknowledgement-The Research Development

financial support given by the Fonds der Chemischen Industrie, (Pretoria) and the University of Pretoria is gratefully acknowledged.

Foundation

for

REFERENCES D. de Waal, A. M. Heyns, K.-J. Range and C. Eglmeier, Specfrochim. Acfa 46A, 1639 (1990). K.-J. Range, R. Zintl and A. M. Heyns, Z. Naturforsch. 43B, 309 (1988). S. Block, Nature l&5(4724), 541 (1960). H. T. Evans and S. Block, Inorg. Chem. S(lO), 1808 (1966). K.-J. Range, C. Eglmeier, A. M. Heyns and D. de Waal, Z. Narurforsch. 45B, 31 (1990). F. C. Hawthorne and C. Calvo, J. Solid Slate Gem. 22, 157 (1977). L. V. Kristallov, 0. V. Koryakova, L. A. Perelyaeva and M. Tsvetkova, Russ. 1. Inorg. Chem. 32(8), 1073 (1987). (81 F. R. Thtobald, J.-G. Theobald, J. C. Vedrine, R. Clad and J. Reward, .I. Phys. Chem. Solids 45, 581 (1984). [9] A. M. Heyns, M. W. Venter and K.-J. Range, Z. Naturforsch. 42B, 843 (1987). (11 [2] [3] [4] [5] [6] [7]

U(A)

46:11-H