• Hypodiphosphate;
  • Crystal structure;
  • IR spectroscopy;
  • Raman spectroscopy;
  • Thermal behavior


  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Single crystals of the alkali-metal hypodiphosphates K4P2O6·8H2O (1) and Na2K2P2O6·8H2O (2) could be obtained and their crystal structures determined. The compounds 1 and 2 crystallize isotypic in the orthorhombic space group Pbca (no. 61) with four formula units in the unit cell. The crystal structures are built up by discrete [P2O6]4– units in an ethane-like staggered conformation, by the corresponding alkali-metal cations and water molecules. FT-IR/FIR and FT-Raman spectra of the crystalline title compounds were recorded and a complete assignment for the [P2O6]4– modes is proposed. Raman spectra of aqueous hypodiphosphate solutions deliver additional polarization data supporting the band assignment. Compounds 1 and 2 show a complete H2O loss in case of slow heating avoiding the formation of a hydrate melt.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Until today the described main group hypodiphosphates have only academic interest with the exception of diammonium hypodiphosphate, (NH4)2H2P2O6, its ferroelectricity has been recently discovered.1 Thus, it is not surprising that especially structure related investigations are scarce and scattered in literature although hypodiphosphoric acid (H4P2O6) was described already in 1877 by Salzer.2 The first comprehensive contribution on the syntheses and properties of alkali-metal hypodiphosphates was published by Müller in 1913.3 Until now no systematic elucidation on the crystal structures of quaternary alkali-metal hypodiphosphates followed. Only the crystal structure of Na4P2O6·10H2O was determined in 1973.4 Structural data of hypodiphosphates of the heavier alkali metals are still missing.

The situation for the alkali-metal dihydrogen hypodiphosphates, mostly used as starting compounds for quaternary hypodiphosphates, is somewhat better, because the crystal structures of Na2H2P2O6·6H2O, Na3HP2O6·9H2O, and of(NH4)2H2P2O6 have been determined.57 In addition, a remarkable dioxonium dihydrogenhypodiphosphate, [(H3O)2H2P2O6], is described and characterized only by X-ray crystal structure determination.8

Possible reasons for the paucity of hypodiphosphate compounds are the relatively laborious preparation of neutral salts as well as of their acidic species as, for example, [H2P2O6]2– and [HP2O6]3– and on the other side the relative instability of [P2O6]4– compounds because they disproportionate easily according to the literature:9

H4P2O6 + H2O [rlarr2] H3PO3 + H3PO4


[P2O6]4– + H2O [rlarr2] [HPO3]2– + [HPO4]2–

In aqueous solutions hypodiphosphoric acid (H4P2O6) behaves as tetrabasic acid, which causes a pH dependency of the species present (pKS1 = 2.20; pKS2 = 2.81; pKS3 = 7.72; pKS4 = 10.03).10 The alkali-metal salts are well soluble in aqueous solutions,3,10 allowing so the substitution of cations by ion exchange, which results in obtaining of new alkali-metal hypodiphosphates.

To receive further crystal structure information and data for alkali-metal hypodiphosphates and to establish trustworthy vibrational frequency sets for [P2O6]4– compounds crystal structures and IR/FIR and Raman spectra of the title compounds were determined completely and evaluated carefully.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

The new hypodiphosphate hydrate compounds were synthesized by soft chemistry reactions. Their single crystals were grown from aqueous solutions and their crystal structures were determined by X-ray diffraction analysis. Experimental details of the structure determinations are given in Table 1. Selected interatomic distances and bond angles are listed in Table 2.

Table 1.  Crystal data and structure refinement of K4P2O6·8H2O (1) and Na2K2P2O6·8H2O (2).
Empirical formulaK4H16O14P2Na2K2H16O14P2
Crystal colorcolorlesscolorless
Formula weight /g·mol–1458.47426.25
Crystal systemorthorhombicorthorhombic
Space groupPbcaPbca
Dcalcd. /g·cm–31.9451.976
μ Mo-Kα /mm–11.4011.010
Temperature /K223(2)223(2)
DiffractometerStoe IPDS IIStoe IPDS II
2θ range /°1.00–25.021.00–25.01
Index ranges–12 ≤ h ≤ 13,–12 ≤ h ≤ 12,
 –13 ≤ k ≤ 14,–13 ≤ k ≤ 13,
 –14 ≤ l ≤ 14–13 ≤ l ≤ 13
Unique reflections13801264
Data / refined parameters1380 / 1231264 / 124
Goodness-of-fit on F21.1831.207
R1 [I ≥ 2σ(I)]0.03480.0291
wR2 [I ≥ 2σ(I)]0.06480.0760
R1 (all data)0.04710.0314
wR2 (all data)0.06740.0774
Largest diff. peak and hole peak /e·Å–30.262 / –0.3750.440 / –0.469
Table 2.  Selected internuclear distances /Å and angles /° for K4P2O6·8H2O (1) and Na2K2P2O6·8H2O (2).
K4P2O6·8H2O (1)   

The hypodiphosphate anion is common in both salt structures presented herein. In each of the title compounds the discrete ethane-like [P2O6]4– anions in staggered conformation are linked by O–M–O bonds (M = Na and K) to a three-dimensional structure. The [P2O6]4– anions are on a center of inversion, with P–P distances of 2.204 Å for compound 1 and 2.185 Å for 2. The P–P central bond links two PO3 groups with P–O distances from 1.527 to 1.533 Å and they are thus very similar to those reported for Na4P2O6·10H2O,4 Co2P2O6·12H2O,11 and Ni2P2O6·12H2O.12 A system of hydrogen bonds plays an important role for the stability of the crystal structures of 1 and 2.

K4P2O6·8H2O and Na2K2P2O6·8H2O (1 and 2)

K4P2O6·8H2O (1) and Na2K2P2O6·8H2O (2) crystallize isotypic in the orthorhombic space group Pbca with four formula units in the unit cell. Both compounds are characterized by discrete [P2O6]4– ions in staggered conformation, octahedral [MO6] (M = Na and K) units, [KO7] polyhedra, and water molecules, which are coordinated to the Na+ and K+ ions and [P2O6]4– ions and involved in a hydrogen-bond network.

As shown in the cell-packing diagram the (K2)+ ions form layers, which alternate with the layers of [MO6] octahedra (M = Na and K) and [P2O6]4– ions along the b axis (Figure 1). Within the layer each [P2O6]4– ion is connected to two edge-shared [MO6] octahedra, [(K1)O6] for 1 and [NaO6] for 2 (Figure 2).

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Figure 1. Projection of the crystal structure of Na2K2P2O6·8H2O (2) along the b axis.

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Figure 2. View of the chain of edge-shared [MO6] octahedra [M = (K1) and Na].

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In compound 1 each (K1)+ is octahedrally coordinated by five H2O molecules and one [P2O6]4– ion. The [(K1)O6] octahedra are distorted from a regular octahedron with (K1)–O bond lengths from 2.633 to 2.763 Å (Table 2). In compound 2 each (K1)+ ion is replaced by a Na+ ion with Na–O distances from 2.374 to 2.469 Å (Table 2).

The (K2)+ ions are sevenfold coordinated by two oxygen atoms of one [P2O6]4– ion and five H2O molecules. The (K2)–O bond lengths range from 2.797 to 2.967 Å for compound 1 and from 2.797 to 3.086 Å for compound 2, which are significantly longer than those to the (K1) in compound 1 (Table 2 and Figure 3).

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Figure 3. View of the edge-shared [(K1)O6] and [(K2)O7] polyhedra.

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The [(K2)O7] coordination polyhedron can be considered as a distorted monocapped trigonal prism with O5v as cap. The [MO6] octahedra [M = (K1) for 1 and Na for 2] and [(K2)O7] polyhedra are connected through common edges and corners completing the three-dimensional framework structure (Figure 2, Figure 3, and Figure 4).

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Figure 4. View of the corner-shared [(K2)O7] polyhedra.

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In the [P2O6]4– ion the phosphorus atom is surrounded tetrahedrally by three oxygen atoms and one additional phosphorus atom. The P–P distance amounts to 2.204 Å for compound 1 and 2.185 Å for 2, and P–O bond lengths range from 1.527 to 1.533 Å and they are thus very similar to those reported for Na4P2O6·10H2O,4 Co2P2O6·12H2O,11 and Ni2P2O6·12H2O.12

The oxygen atoms of the hexadiphosphate(VI) anion are connected by hydrogen bonds to the water molecules. The hydrogen-bond lengths between oxygen atoms of water molecules and those of the [P2O6]4– ions range from 2.717(1) to 3.096(1) Å and the O–H···O angles range from 163.5(1) to 177.9° (Table 3). These values agree well with those reported for Ni2P2O6·12 H2O12 indicating effective hydrogen-bridge bonding.

Table 3.  Selected hydrogen-bond lengths /Å and angles /° for compounds K4P2O6·8H2O (1) and K2Na2P2O6·8H2O (2).
D–H···AH···A /ÅD···A /ÅD–H···A /°
K4P2O6·8H2O (1)   
K2Na2P2O6·8H2O (2)   

Vibrational Spectra and Band Assignment

IR and Raman Spectra of the Crystalline Hypodiphosphates

The title compounds consist of the respective alkali-metal cations, the [P2O6]4– ions, and of the corresponding hydrate water molecules. The dominant [P2O6]4– units as well as the water molecules can be clearly detected by their vibrational frequencies. Herein, in this contribution, the vibrational behavior of the [P2O6]4– ions is the more interesting part. Therefore, the vibrational frequencies of [P2O6]4– are given in Table 4 along with the proposed assignment and in comparison to literature data.1317 The IR/FIR and Raman spectra of Na2K2P2O6·8H2O (2) are presented exemplarily in Figure 5.

Table 4.  Raman and IR/FIR frequencies /cm–1 of crystalline and dissolved K4P2O6·8H2O (1) and solid Na4P2O6·10H2O (3) along with their estimated intensities and proposed assignment as well as literature data.
K4P2O6·8H2O (1)  Na4P2O6·10H2O (2)  
  • a)

    a) D3d symmetry for [P2O6]4–.

  • b)

    b) Aqueous solution of K4P2O6·8H2O; Corresponding Raman values of Palmer:14 ν1 / A1g 1023, ν2 / A1g 666 and ν9 / Eg ca. 320 cm–1.

  • c)

    c) IR/FIR frequencies of solid Na4P2O6 of Palmer:14 ν10 / Eu 1085, ν5 / A2u 942, ν6 / A2u 568 and ν11 / Eu 493 cm–1.

  • d)

    d) Usual ν numbering according to Ref. 13, but distinct from Refs. 1416. Mode description according to Ref. 14

  • e)

    e) Estimated intensities: s: strong, m: medium, w: weak, v: very, sh: shoulder, br: broad, p: polarized. ν: stretching, δ: bending. [Crystalline Na2K2P2O6·8H2O (2): Raman (intensity): 3110 m, vbr; 1058 m-s; 1040 vvw, sh; 1021 vvs; 695 w-m; 643 vvw; 595 vvw; 508 vw, sh; 492 m; 322 sh; 303 m-s; 235 vw-w; 196 vw-w; 126 w cm–1. IR (intensity): 3431 vs, vbr; 3205 s, vbr; 1701 m, br; 1660 w, br; 1419 m, br; 1390 m, br; 1101 sh; 1055 vvs; 911 vs; 839 m-s; 748 w-m; 620 m; 524 w; sh, 506 m-s; 457 m; 242 m; 210 w-m; 195 m; 144 w cm–1. Frequency assignment in analogy to that of Na4P2O6·10H2O. [P2O64–]: Raman from Baudler:15 observed/calculated: A1g: 1068/1066, 651/663, 274/265; Eg: 1177/1252, 459/507, 403(?)/399 cm–1. IR/FIR, Raman from Loewenschuss and Marcus:16 calculated: A1g: 1062, 670, 275; A2u: 942, 562; Eg: 1168, 508, 325; Eu: 1085, 494, 200 cm–1. IR/FIR, Raman from Viste and Lunden (6-31G*/B3LYP solv.):17 calculated: A1g: 1062, 670; A2u: 942, 562 cm–1.]

MIR/FIRRaman MIR/FIRc)RamanAssignment / D3da)
 crystallinesolutionb)  mode descriptiond)
   3415 m, br  
3274 vbre)  3266 m, br3205 m, vbrH2O stretching
 3107 vbr3210 br, p3050 sh3052 w, br 
1648 m 1632 br, p1662 m H2O bending
1369 w  1358 w combination?
1114 m1140 w 1162 sh  
1060 s  1065 vvs ν10 / Eu : ν(P–O)
 1066 m  1060 m-sν7 / Eg : ν(P–O)
 1033 sh    
 1015 vs1018 vs, p 1019 vsν1 / A1g : ν(P–O)
925 s  915 s ν5 / A2u : ν(P–O)
   873 w  
717 m     
671 w668 m667 m, p(654 w)657 mν2 / A1g : δ(PO3)
   572 w  
546 s  522 m-s ν6 / A2u : δ(PO3)
 504 m487 m, br 491 mν8 / Eg : δ(PO3)
460 vs473 w 473 vs ν11 / Eu : δ(PO3)
 330 w  347 vw 
 309 m313 w-m (dp?) 311 m-sν3 / A1g : ν(P–P)
 249 w290 w (p?) 257 mν9 / Eg : δ(PO3)
234 s, br  230 m, br ν12 / Eu : δ(PO3)
 201 w 195 w, br194 vw 
187 s175 vw171 w, dp   
 136 w-m  136 wlattice vibrations
 111 w, sh  89 vw 
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Figure 5. Room-temperature FT-Raman (a) (λexc. = 1064 nm) and FT-IR/FIR (b) spectrum of crystalline Na2K2P2O6·8H2O (2). (Raman intensities in arbitrary units).

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For comparison reasons, all up to date known experimental and calculated [P2O6]4– frequencies are summarized in Table 4 some are given as table footnotes.

The [P2O6]4– ion belongs to the ethane-like structure type. Due to the possible rotation of the two –PO3 groups against each other staggered (D3d) as well as eclipsed (D3h) conformation is possible. Vibrational spectroscopy allows a decision between both possible [P2O6]4– conformations on the basis of the IR and Raman selection rules.18

The accurate symmetry of the staggered [P2O6]4– ion depends on the P–O distances. C2 symmetry results, when three P–O distances differ from each other. If only one P–O distance is different from the other two, C2h symmetry originates and in the case of equal P–O distances D3d symmetry emerges. The vibrational spectra of the title compounds as well as of the aqueous [P2O6]4– solution are best interpreted by a [P2O6]4– unit, which behaves vibrationally with D3d symmetry rather than with C2h symmetry.

A vibrational analysis for such a [P2O6]4– unit delivers:14,18,19

Γvib([P2O6]4– / D3d) = 3 A1g(RE) + A1u(ia) + 2 A2u(IR) + 3 Eg(RE) + 3 Eu(IR),

with RE = Raman, IR = infrared and ia = inactive, and with 2 A1g + Eg + A2u + Eu as stretching and A1g + 2 Eg + A2u + 2 Eu as bending modes.

[P2O6]4– with D3d symmetry has an inversion center and therefore the mutual exclusion rule is valid. For the title compounds this is verified as well as possible (cf. Table 4 and Figures S1 and S2, Supporting Information).

The proposed [P2O6]4– mode assignment is in excellent agreement with the determined crystal structures of 1 and 2. In detail, the [P2O6]4– frequencies are assigned on the basis of the respective Raman intensity, of the band shape, and the polarization measurements of aqueous K4P2O6·8H2O solutions, and of the sequence of the calculated DFT data.16,17 The A1g species are polarized, whereas the Eg species are depolarized. The Raman active A1g species ν1–ν3 are found at 1019, 657, and 311 cm–1 for Na4P2O6·10H2O (Figure S1) and 1051, 668, and 309 cm–1 for compound 1, respectively. Those [P2O6]4– Raman bands agree roughly with these given by Palmer14 for a 50 % aqueous solution of K4P2O6. The IR active A2u species ν5 at 925 and ν6 at 546 cm–1 differ from the literature values14 but they are the only possible ones in the respective frequency regions.

The Raman active Eg species are found and attributed at 1066, 504, and 249 cm–1. The published and assigned Eg values from Baudler15 as well as the calculated ones of Loewenschuss and Marcus16 and from Viste and Lunden17 disagree at all from our presented Eg data (Table 4). Raman spectra of aqueous K4P2O6 solutions at different pH values show that the published [P2O6]4– Raman frequencies from Baudler15 belong to [HP2O6]3– anions and definitely not to [P2O6]4– units. Finally, the IR active Eu species ν10–ν12 agree roughly with the measured ones (except ν12) from Palmer14 and the calculated ones of Loewenschuss and Marcus.16 Herein, ν12 is found and attributed at 230 cm–1. The vibrational modes below of about 200 cm–1 are ascribed to lattice vibrations.

Raman Spectra of Aqueous K4P2O6·8H2O Solutions

Potassium salt systems with better solubility are used for recording Raman solution spectra with polarization measurements because Na4P2O6·10H2O has low solubility in NaOH solutions. Appropriate solutions with about 20 wt.- % each were freshly prepared out of aqueous hypodiphosphoric acid (“H4P2O6”) and KOH for different pH values from 2 until 11. For the Raman spectra of [P2O6]4– ions the solutions with pH 10 and 11 can be used only. They prove clearly the existence of [P2O6]4– ions therein. In the Raman solution spectra (pH 11) the following relevant bands are determined: 1018 vs, 667 m, 313 w-m, 290 w and 171 w. Certainly the bands at 1018 and 667 cm–1 are polarized and this one at 171 cm–1 is depolarized. For the modes at 313 and 290 cm–1, the polarization state is not so clear and it seems that this one at 290 cm–1 is polarized and that one at 313 cm–1 is depolarized. Considering all mentioned vibrational information1417 the frequency assignment of the [P2O6]4– Raman modes in Table 4 (for Na4P2O6·10H2O) for the A1g species is done. The solution Raman spectra (pH 11) for the [P2O6]4– ion are given in Figure 6.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

The crystal structure determination of the heavier alkali-metal hypodiphosphates could have begun with the receipt of K4P2O6·8H2O single crystals. For the [P2O6]4– units a trustworthy set of vibrational frequencies could be obtained and their assignment proposed. In addition the Raman spectra of aqueous K4P2O6·8H2O solutions inclusively the first recorded polarization measurements support the assignment of the Raman active [P2O6]4– bands.

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Figure 6. Room-temperature FT-Raman spectra (λexc. = 1064 nm) of solid K4P2O6·8H2O) (a), for aqueous K4P2O6 solution (20 wt.- %, pH = 11) (b) and its polarization recording (c). (Raman intensities in arbitrary units).

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Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

The FT-Raman spectra of crystalline and dissolved hypodiphosphates were recorded with a Raman module FRA 106 (Nd:YAG laser, λexc. = 1064 nm) attached to a BRUKER 66 v interferometer. The FT-IR/FIR spectra of the solid hypodiphosphates were obtained from PE-hypodiphosphate pellets (100–500 cm–1) and of the solids compounds spread between CsI pellets (500–3500 cm–1), respectively, with the mentioned FT-IR interferometer. Polarization data of aqueous hypodiphosphate solutions were obtained using a rotatable half-wave plate.20

TG measurements of the title compounds were carried out in an atmosphere of flowing argon (0.3 L·min–1) from room temperature to 500 °C with a heating rate of 5 K·min–1 using a Robotherm Magnetic Suspension balance. Compounds 1 and 2 lost all water molecules in the temperature ranges 40–190 °C (1) or 60–170 °C (2), respectively, very likely combined with disproportionation (Figure S3, Supporting Information)).

Disodium dihydrogen hypodiphosphate, Na2H2P2O6·6H2O, was prepared according to Leininger and Chulski.21 For hypodiphosphoric acid solution, Na2H2P2O6·6H2O (0.5 g, 1.59 mmol) was dissolved in bidestillated H2O (20 mL). The disodium dihydrogen hypodiphosphate was passed over an acidic cation-exchange column (Dowex 50WX2 50–100). About 35–40 mL H4P2O6 solution were collected at pH values of 1.5–3.5.

K4P2O6·8 H2O (1) was prepared by addition of KOH (0.349 g, 6.24 mmol, Fluka) in an aqueous solution of hypodiphosphoric acid (25 mL, 0.310 g, 1.56 mmol). The mixture was placed in a vacuum desiccator at room temperature. After some days colorless plate-shaped crystals of compound 1 were obtained and isolated.

Na2K2P2O6·8 H2O (2) was obtained by addition of KOH (0.336 g, 6 mmol, Fluka) in an aqueous solution of disodium dihydrogen hypodiphosphate (0.738 g, 3 mmol) at 35 °C according to:


Slow cooling at room temperature yielded plate-shaped colorless crystals of compound 2 within some days.

The concentrated [P2O6]4– solutions for the Raman measurements were prepared freshly for different pH values by considering the dissociation constants of hypodiphosphoric acid.22 Starting material was K2H2P2O6·8H2O respective hypodiphosphoric acid (“H4P2O6”), and the pH adjustment was accomplished with KOH solution.

X-ray Diffraction Analysis: All data were collected with a Stoe IPDS-II single-crystal X-ray diffractometer with graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) at 223 K. Crystal structure solution by direct methods using SHELXS-9723 yielded in all cases the heavy-atom positions. Subsequent difference Fourier analyses and least-squares refinements with SHELXL-9723 allowed localization of the remaining atom positions. The hydrogen positions were determined by a final difference Fourier syntheses. Details of the X-ray structure analyses and crystallographic data for compounds 1 and 2 are presented in Table 1. Selected bond lengths and angles are gathered in Table 2. Selected hydrogen-bond lengths are summarized in Table 3. For the preparation of the structure drawings, programs DIAMOND24 and POV-Ray25 were applied.

Further details of the crystal structure investigations may be obtained from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail:, for deposited data.html) on quoting the depository numbers CSD-424363 for K4P2O6·8H2O (1), CSD-424364 for Na2K2P2O6·8H2O (2), and CSD-424365 for Na4P2O6·10H2O.

Supporting Information (see footnote on the first page of this article): Room temperature FT-Raman and FT-IR spectra of crystalline K4P2O6·8H2O and of crystalline Na4P2O6·10H2O. Thermal decomposition of crystalline Na2K2P2O6·8H2O (2) in the temperature range 25–500 °C.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

The authors are indebted to Prof. Dr. Arnold Adam for generous providing his facilities and they thank Karin Bode for recording the vibrational spectra.

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusions
  6. Experimental Section
  7. Acknowledgements
  8. Supporting Information

Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors.


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