Contribution of Infrared Spectroscopy to the Vibrational Study of Ethylenediammonium Chloride Thiocyanate: (C 2 H 10 N 2 ) (Cl NCS)

The C 2 H 10 N 2 Cl NCS (EDCT) compound is characterized by using infrared spectroscopy. The infrared spectrum of the title compound was recorded (400– 4000 cm − 1 ) at room temperature and discussed, essentially in terms of vibrational modes of [C 2 H 10 N 2 ] 2+ cations and [SCN] − and [Cl] − anions. Ethylenediammonium thiocyanate chloride crystallizes, at room temperature, in the triclinic system, space group P1 (C i ). The entities [C 2 H 10 N 2 ] 2+ , [SCN] − and [Cl] − occupy sites of symmetry (C 1 ). Several ground state thermodynamic parameters were calculated using the ab initio Hartree-Fock (HF) and DFT (B3LYP) methods with 6-31++G (d, p) and 6-311++G (d, p) basic sets such as vibration frequencies, rotation constants, and optimized molecular geometry. The comparison between the theoretical and experimental infrared spectrum showed good agreement. comparison between the calculated vibrational frequencies and the experimental values indicates that both methods can predict the FT-IR spectra of the title compound. The results of DFT-B3LYP method indicate better fit to experimental ones than ab initio HF upon evaluation of vibrational frequencies. Several thermodynamic parameters of the title molecule are comparatively discussed. The observed and the calculated wavenumbers are found to be in good agreement with majority modes.


Introduction
This chapter is devoted to the characterization of C 2 H 10 N 2 Cl NCS by infrared vibrational spectroscopy. These studies make it possible to highlight the structural analogies and to possibly provide some additional information to those obtained by X-ray diffraction. In this chapter, we used group theory; indeed, this valuable tool allows both to count the normal vibration modes of vibration of a crystal and to describe these vibrations in symmetrical coordinate terms. In addition, an attempt is made to assign the various modes of vibration to all the bands that have appeared. It is based on predictions theories and previous work carried out on similar compounds. Infrared is a research tool can also provide exquisite structural insights into the molecule and characterizes the vibrational modes of the molecules and has enfolded within it much information on chemical structure [1]. The combined use of FT-IR spectroscopy extracts most of the obtainable information and these are the popular tools in the chemist and physicist. Amine, amino acid and Schiff bases [2][3][4][5][6] have recently been the focus of coordination chemists due to their preparative accessibilities, structural varieties, and varied denticities. With those purposes, first, the EDCT was synthesized [7] and then characterized it by Infrared Spectroscopy. Simultaneously, to obtain the ground state optimized geometries and the vibrational wavenumbers of the different normal modes, we carried out the ab initio HF and DFT calculations. Here, the hybrid B3LYP method was used together with the 6-31++G (d, p) and 6-311++G (d, p) basis sets [8].

Experimental details 2.1 Synthesis
The title compound has been obtained by mixing, in stoichiometric proportions, a solution of ethylenediamine, a freshly prepared solution of thiocyanic acid HSCN and a solution of potassium halide. KX (X = Cl) [7].

IR spectroscopy
Infrared absorption spectrum was recorded at room temperature in the 400-4000 cm −1 frequency range on a Perkin-Elmer spectrometer equipped with a Universal ATR Accessory (UATR).
In the ground state the molecular structure of the C 2 H 10 N 2 Cl NCS (EDCT) phase calculated was optimized by the use of the DFT/B3LYP methods with the methods 6-31++G (d, p) and 6-311++G (d, p) base set, and the calculated optimized structure was used in vibrational frequency calculations. The calculated harmonic vibratory frequencies and the minimal energy of the geometric structure were scaled by (B3LYP) with the base set 6-31++G (d, p) and 6-311++G (d, p). HF/DFT calculations for EDCT are performed using GAUSSIAN 03W program [13,14]. On the other hand, the energies of the frontier orbital's were used to calculate the gap energy values and some interesting descriptors in order to predict their reactivities an behaviors at the same level of theory [14][15][16][17].

Molecular geometry
The structure of the EDCT belongs to C i point group symmetry and its molecular structure is obtained from GAUSSAN 03W and GAUSSVIEW programs are shown in Figure 1. The molecule contains one diprotonated ethylenediammonium cation, one Cl − and one SCN − anions. The comparative optimized structural parameters such as bond lengths and bond angles are presented in Table 1. The comparative graphs of bond lengths and bond angles of ethylenediammonium chloride thiocyanate for two sets are presented in Figures 2 and 3 respectively. Most of the optimized bond lengths are slightly higher than the experimental values, depending on the theoretical values, because the theoretical calculations belong to isolated molecules in the gas phase and the experimental results to solid state molecules. The  angles and binding lengths of B3LYP are compared with those of HF, the formers are generally larger than later and the values calculated by B3LYP are well correlated with the experimental data. The parameters (the vibration frequencies and the thermodynamic properties) represent a good approximation. The data presented in Table 1 show that the theoretical HF and DFT levels (B3LYP/6-311++G (d, p)) generally estimate the same values for some link lengths and angles.

Theoretical analysis of C 2 H 10 N 2 Cl NCS vibrations
The factor group method of classifying fundamental vibrational modes of crystals, as developed by Bhagavantam and Venkatarayudu [18], is certainly the most powerful method of treating C 2 H 10 N 2 Cl NCS crystal structure. The unit cell of C 2 H 10 N 2 Cl NCS contains 18 atoms which correspond to 54 degrees of vibrational freedom. To simplify the discussion of the IR data, the vibrational modes will be considered in two groups: the internal modes of SCN − anions and (C 2 H 10 N 2 ) 2+ cations. Ethylenediammonium thiocyanate chloride crystallizes, at room temperature,

Counting by the factor group method
The number of normal modes of vibration of the group SCN − isolated of ideal symmetry C ∞v is given by the representation: While that of an isolated group [C 2 H 10 N 2 ] 2+ of symmetry C 2v is given by the irreducible representation: The correlation diagram is given in Table 2. The counting of the main vibrations of this compound by the factor group method leads to the following results: • Overall vibration representation: Г (ni) = 54Ag + 54Au • Translation modes: Г (T') = 9Ag + 9Au • Rotation mode: Г (R') = 5Ag + 5Au • The representation of the internal vibrations is: The analysis in terms of internal vibrations, rotation R' and translation T' , is given in Table 3 with their activities in IR.

Enumeration by the site group method
This method was used in order to have a detailed description of the symmetry and the nature of the internal vibrations (deformation in the plane or out of the plane, symmetrical or asymmetrical elongation, torsion, etc.).

Vibrations of [C 2 H 10 N 2 ] 2+ in group (C 1 )
To describe the vibrations of the organic cation, we considered separately the vibrations of the groups (-NH 3 ) and (-CH 2 -) and the skeleton (C 2 N 2 ) 2+ .
a. Description of the normal modes of vibration of the grouping (▬NH 3 ) The group (▬NH 3 ) supposed free, has the symmetry 3 m (C 3v ), it presents nine internal vibrations schematized in Figure 4.
Each group (▬NH 3 ) occupies a site (C 1 ) in the cation. The use of correlation tables allows us to describe the symmetry of these vibrations in the molecular group of the cation ( Table 4). The result is: The group (▬CH 2 ▬) supposed free, has the symmetry mm2 (C 2v ), it has six internal vibrations schematized in Figure 5.
The (▬CH 2 ▬) groups occupy E(C 1 ) sites in the cation, the correlation method allows us to determine their vibrational symmetry in the C1 molecular group of the cation ( Table 5). The result is:    The number of coordinates is 6 = 3 N-6 (N: number of atoms in the backbone, here N = 4). Using the transforms of each coordinate under the symmetry operations of the point group C s corresponding to the cation, six symmetrized coordinates were calculated ( Table 6). At each coordinate a vibration mode has been assigned. These vibrations are shown schematically in Figure 6. The description of the normal modes of the NC 2 N backbone and their activities in IR are shown in Table 7. The irreducible representation of the internal vibration modes of the skeleton in C i is: d. Description of the vibration modes of SCN − The internal vibrations of the SCN − anion have already been studied [2], they are described in terms of symmetrized coordinates as a function of the internal coordinates. These modes are divided in the group C ∞v as follows:
Internal modes of (-CH 2 -) in (C i ).  These vibrations are shown schematically in Figure 7. The vibrational analysis in terms of internal vibrations is given in Table 8. The distribution of normal SCN group modes and their IR activity are shown in Table 9. The irreducible representation of the internal vibration modes of SCN − in C i is:

Group theory analysis
The comparisons of the experimental infrared spectra for EDCT, by using HF/6-311++G (d, p), (B3LYP) 311++G (d, p) and (B3LYP)/6-31++G (d, p) theory level, with the corresponding average predicted demonstrate good correlations as observed in Figure 8. Using the split triple valence base as well as the diffuse and polarization functions for computed harmonic vibratory frequencies of EDCT, 6-31++G (d, p) and 6-311++G (d, p), the frequencies FT-IR observed for various vibration modes were presented in Table 10. The comparative values of IR intensities activities are presented in Table 11 and their corresponding graph given in Figure 9. The comparative graph of vibratory frequencies calculated by the HF and  DFT methods to HF/6-311++G (d, p), B3LYP/6-31++G (d, p) and B3LYP/6-311++G (d, p). The basic sets for the EDCT are shown in Figure 10. It appears from the figure that the frequencies calculated by B3LYP with 6-31++G (d, p) of basis sets are closer to the experimental frequencies as HF method with 6-311++G (d, p) base set.

NH 3 modes
The asymmetric stretching ν as (NH 3 ) of symmetries (Ag + Au) are observed in IR at 3325 and 3326 cm −1 . The symmetric stretching ν s (NH 3 ) of symmetries (Ag + Au) are observed in IR at 3210 cm −1 . The asymmetric deformation δ as (NH 3 ) of symmetry (Ag + Au) observed IR at 1500 and 1570 cm −1 . The symmetric deformation δ s (NH 3 ) of symmetries (Ag + Au) are observed in IR at 1467 cm −1 . The Rocking δ ρ (NH 3 ) of symmetries (Au) are observed only in IR at 493 and 498 cm −1 . The torsion δ τ (NH 3 ) of symmetries (Au) observed in IR at 483 cm −1 . The rocking and twisting modes are assigned as predicted by the calculations and in accordance with the expected regions for similar compounds [7,8,19,20], as observed in Table 10.

CH 2 modes
By comparison with previous works reported on similar compounds containing [C 2 H 10 N 2 ] 2+ [21], we have attributed the bands observed in IR at 3222 and 2427 cm −1 to asymmetric stretching ν as (CH 2 ) and symmetric ν s (CH 2 ) of symmetries (Ag + Au), respectively. The asymmetric deformation δ as (CH 2 ) and symmetric δ s (CH 2 ) is observed at 1452 and 1200 cm −1 in IR spectrum at 1341 and 1454 cm −1 . The calculated frequencies of B3LYP/6-31++G (d, p) and B3LYP/6-311++G (d, p) methods for CH 2 asymmetric and as asymmetric vibrations showed excellent agreement with recorded spectrum as well as literature data. The Rocking δ ρ (CH 2 ) of symmetries (Ag + Au) are observed in IR at 1000 cm −1 . The torsion δ τ (CH 2 ) of symmetry (Ag + Au) observed in IR at 1124 cm −1 . The rocking and twisting modes are assigned as predicted by calculations, as indicated in Table 10.

Skeletal modes
The NCCN skeleton gives six normal modes of vibration that may be described as three skeleton stretching (2ν CN + 1ν CC ), two NCCN deformation modes and one torsional mode around the C▬C bond. The symmetrical elongations of the symmetry skeleton ν s (CC) of symmetries (Ag + Au) appear in IR at 750 cm −1 . The asymmetric stretching ν as (CN) of symmetries (Ag + Au) observed in IR at 544 cm −1 . The symmetric stretching ν s (CN) of symmetries (Au) observed in IR at 532 cm −1 . The asymmetric deformation δ as (CCN) of symmetry (Ag + Au), is observed in IR at 435 cm −1 . The symmetric deformation δ s (CCN) of symmetry (Au) observed only in IR at 430 cm −1 .

Other molecular properties
Several calculated thermodynamic parameters are presented in Table 12. Scale factors have been recommended [29] for an accurate prediction in determining the  zero-point vibration energies, and the entropy. It can be seen that the total energies decrease with the increase of the size of the basic set. Changes in the total entropy of EDCT at room temperature and in different basic sets are only marginal.

Conclusions
The present document attempts to define the appropriate frequency assignments for the thiocyanate ethylenediammonium chloride compound from the FT-IR spectrum. Vibrational frequencies and infrared intensities are calculated and analyzed by the theoretical HF and DFT (B3LYP) levels, using the 6-31++G (d, p) and 6-311++G (d, p)base sets.. The comparison between the calculated vibrational frequencies and the experimental values indicates that both methods can predict the FT-IR spectra of the title compound. The results of DFT-B3LYP method indicate better fit to experimental ones than ab initio HF upon evaluation of vibrational frequencies. Several thermodynamic parameters of the title molecule are comparatively discussed. The observed and the calculated wavenumbers are found to be in good agreement with majority modes.

Author details
Sahel Karoui* and Slaheddine Kamoun Laboratory of Material Engineering and Environment (LR11ES46), National School of Engineering, Sfax, Tunisia *Address all correspondence to: karouisahel@yahoo.fr