Crystal Structure, Thermal Behaviour and Vibrational Spectra of Tetraethylammonium Dihydrogenmonophosphate Bis Phosphoric Acid

Single crystals of the tetraethylammonium dihydrogenmonophosphate bis trihydrogenmonophosphate       3 2 2 4 3 4 4 CH CH N H PO H PO TEP    2 , were grown by slow evaporation solution technique at room temperature. The compound was characterised by IR, Raman, differential thermal analysis (TG-DTA) and single crystal X-ray diffraction. It crystallizes in the monoclinic system (space group 1 2 P c ) with the following unit cell dimensions: a = 7.765 (2) Å, b = 16.531 (4) Å, c = 14.843 (2) Å, β = 100.99 (2) ̊, 4 Z  , 3 1.67Mg m , x D    3 1.532 Mg m , Dm    λ (MoKa) = 0.71073 Å, ,T = 20 (2) ̊. The structure was solved by the direct method and refined to final R value of 0.0342 and for 3239 independent reflections. The structure consists of infinite parallel two-dimensional planes built of mutually 1 0.384 mm ,    F


Introduction
In organic-cation monophosphates, the phosphate anions generally observed are the acidic ones   2 4 HPO  or . Such anions are interconnected by strong hydrogen bonds so as to build infinite networks with various geometries: ribbons [1], chains [2,3], two-dimensional network [4][5][6], and three-dimensional network [7].These entities can be associated to organic molecules to produce compounds having a particular interest as nonlinear optical materials.
During a systematic investigation of interactions between monophosphoric acid and organic molecules containing one or more nitrogen atom different structures of monophosphate salts have been described: NH CH NH H PO [8], NH CH NH H PO H PO [9], NH CH NH HPO [2],   NH CH NH H PO [11],   , here after abbreviated to (TEP), which is a close analogue of previously ob-

Synthesis
The crystal of TEP is easily prepared by slow evaporation at room temperature of an aqueous solution of H 3 PO 4 and .Schematically the reaction is:

H O 
After some days of evaporation, colorless needle-shaped monocrystals appear in the solution.The chemical analysis of phosphorus and acidic proton has been carried out [20].
Density was measured at room temperature by flotation in toluene.The average value of density, , is agreement with that calculated, .The cell contains two formula units of

Characterizations
The infrared spectrum was recorded in the range 400-4000 cm -1 with a "Perkin Elmer FTIR-1000" spectrophotometer using a sample dispersed in a KBr pellet.Back scattering Raman spectra were obtained under microscope with a Horiba Jobin Yvon Raman spectrometer (Lab RAM HR 800 λ = 633 nm) in the 50 -4050 cm -1 range.
Setaram thermoanalyser, TG-DTA92, was used to perform thermal treatment on samples of . TG-DTA thermograms were obtained with 34.35 mg sample in an open platinum crucible, heated in air with 5˚C•min −1 heating rate, from room temperature to 275˚C, an empty crucible was used as reference.

X-Ray Single Crystal Structure Determination
Single-crystal X-ray data were collected at room temperature on a Nonius Kappa-CCD diffractometer using Mo-K α radiation λ = 0.71073 Å through the program COLLECT [21].Correction for Lorentz-polarisation effect, peak integration and background determination were carried out with the program DENZO [22].Frame scaling and unit cell parameters refinement were performed with the program  1.The crystal structure has been solved and refined in the monoclinic symmetry, space group 1 2 P c, using the WINGX environment [23] and based on SHELXS97 [24] and SHELXL97 [25] softwares.All the hydrogen positions of the diprotonated cation were placed geometrically and held in the riding mode, the C-H bonds were fixed and affined at 0.78 and 1.01Å, respectively).Interatomic distances, bond angles and the hydrogen bonds scheme are listed in Tables 2 and 3 respectively.

Structural Analysis
The asymmetric unit is composed of one   and one H PO 2 can be described as an alternation of organic One can distinguish the chains of the acid molecules and the chains of the dihydrogenphosphates anions running parallel to the Z axis in each layer (see Figure 2).
The molecules/anions in the chains are related to each  Table 3. Principal interatomic distances (Å) and bond angles (˚) and detailed of the hydrogen bonding scheme.5) 174 ( 5) 171 ( 4) 165 ( 4) 162 ( 4) 177 ( 4) 178 ( 4) 154 ( 2)  3).The dihydrogenphosphate anions and the acid 4 molecules do not form any straight contact between themselves.All their of one O-H groups are involved in the hydrogen bonds with the O(7)atoms of the acid 4 molecules and all their of 4 three O-H group are involved in the hydrogen bonds with the and of the dihydrogenphosphate anions .
 


The acid 4 molecules form strong hydrogen bonds between themselves.
Note that the atoms of the dihydrogenphosphates anions participate in two hydrogen bonds with the OH groups (O(9)-H( 9) and (O( 12)-H( 12)) of the neighbour orthophosphoric acid molecules of the same chain.
The structure is based on sheets of

O-H O 
The organic group of tetraethylammonium monocation structure is similar to that in the TESe and TEAs analogue [19,28].Each tetrapropylammonium cation makes three short contacts with trimmers of the dihydrogenarsenate through the hydrogen bonds: (see Table 3).The distance is equal to 2.37 (3) Å, respectively.The distance is equal to 3.315 (3) Å, showing that those hydrogen bonds are very weak [29].The angle is equal to 154 (2).The structure of the tetrapropylammonium cation is similar to that in the crystals of tetraethylammonium chloride, is in the tetraethylammonium chloride monohydrate [30], tetraethylammonium chloride tetrahydrate [31] and anhydrous tetraethyl-ammonium chloride [32].The lengths of the N-C bonds are in the range between 1.512 (3) (N-Cl) and 1.516 (3) Å (N-C3).The lengths of the other two N-C bonds are equal to 1.522 (3) Å.The C-N-C angles are in between 106.5(2) to 111.1(2)˚.The C-C bonds lengths are in the region between 1.509(4) and 1.517(4).Thus, they are very similar.The final coordinates and U eq or U iso of TEP are given in Table 4.

Vibrational Investigations
The FT-IR and FT-Raman spectra of the title crystal were measured for the powder sample at room temperature.The bands observed in the measured region arise from the vibrations of hydrogen bonds, vibrations of the tetrapropylammonium cation, phosphate groups, and lattice vibrations.Vibrational spectra are shown in

The Hydrogen Bond Vibrations
The valence vibrations of O-H groups interconnected by a system of hydrogen bonds in the crystal appear in IR spectrum as broad bands in the 3500 -1800 cm -1 region [15].As can be seen from Table 3, these consist of hydrogen bonds of type with length ranging from 2.521 2 to 2.617 (2) Å.X-ray data show that H atoms of OH groups generate hydrogen bonds with the oxygen atoms of the PO 4 group.
The stretching type of vibrations of hydrogen bonds νOH displays a well defined and medium intense band in IR with the AB structure in the region between 3500 and 2000 cm −1 .In IR spectrum the frequency of A band is 2750 cm −1 and B is 2460 cm −1 .For Raman experiment the broad maxima are found at 2822 and 2550 cm −1 .The intense IR bands correspond to a strong hydrogen bond with the distance in the region 2.6 Å -2.7Å [33].The in-plane OH bending mode δOH gives rise to a medium band in infrared spectrum without counterparts in Raman spectrum at 1252 cm −1 .The out-of-plane bending O ( 4) O ( 5) O ( 6) O ( 7) O ( 8) O ( 9) O ( 10) O ( 11 C ( 6) C ( 7) C ( 8) illustrated in Figure1, viewed in projection along the direction of its two-fold screw axes.

Figure 1 . 4 .Figure 2 .
Figure 1.Projection along the a axis of the atomic arrangement of 