A Combined Experimental and Theoretical Investigation on The Molecular Structure, Ft-Ir Spectra of 4-((1h-Indol-3-Yl) Methylene Amino) Phenol

: This study involves the synthesis of the title molecule 1H-IMAP, characterized by using FT-IR spectrum and theoretical calculation of the optimized geometrical parameters of the most stable structure of the title molecule 1H-IMAP and computation of its vibrational frequencies using Hartree-Fock(HF) method of basis sets 6-31G(d,p), HF/6-311G(d,p) and Density functional theory (DFT) method of basis set B3LYP/6-31G(d,p) has been made by using Gaussion 03 software and compared both theoretical and experimental values. Non-Linear optical (NLO) properties of the 1H-IMAP was studied by determining the electric dipole moment, polarizability, and hyperpolarizability with the aid of the above mentioned basis sets.


Introduction
Indole derivatives as dual-effective medicines for neurodegenerative disease treatment The hydroxyl derivative of indole is important in the central nervous system, and it has been shown to cause long-term serotonin depletion in brain tissues [1][2][3][4]. The synthesis technique, experimental and theoretical FT-IR studies of 4-((1H-indol-3yl) methylene amino) phenol are described in this paper. To the best of our knowledge, a complete theoretical study of 1H-IMAP have not been reported so for. In this study HF and DFT level of theories were utilized to determine the optimized geometry, vibrational wave numbers, molecular parameters viz., dipole moment, polarizability (α), hyperpolarizability (β), chemical potential, hardness (), and electrophilicity index ().

Experimental Section
A mixture of Indole-3-Carboldehyde (72g, 1.0mol) is added dropwise and with stirring during 2hrs to p-hydroxy aniline (73g, 1.0mol). During this addition the temperature rises from 25 to 40 0 C and an aqueous layer separates nearer at the end of the addition. The organic layer is treated with anhydrous potassium carbonate (15g), stirred at 25 0 C for 17 hrs, and then decanted onto barium oxide (12g). After the mixture has been stirred for 10 hrs, it is filtered and the organic filterate is distilled to separate the imine as a colourless liquid. The compound 4-((1H-indol-3yl) methylene amino) phenol in the solid form was synthesized. The FT-IR spectrum of this compound was recorded in the range of 400-4000cm -1 on PERKIN-ELMER 360 model IR double beam spectrophotometer using KBr pellet technique with 4 At the optimized structure of 1H-IMAP, no imaginary wave number modes were observed, indicating that a real minima on the potential surface was discovered. The best shape was determined by reducing energy with all geometrical parameters without imposing molecular symmetry requirements. The fundamental modes are overestimated by the HF and DFT hybrid B3LYP functional methods. Density functional theory calculations have been claimed to offer excellent vibrational frequencies of molecular compounds when the measured frequencies are scaled to correct for the expected treatment of electron correlation, basis inadequacies, and anharmonicity [6,7,8,9]. As a result, scaling factors must be applied to achieve a much greater agreement with experimental results.
Thus the scaling factors: 0.8992, 0.9051 and 0.9550 [10] have been uniformly applied to the HF and B3LYP methods respectively. The observed discrepancy between theoretical and experimental frequencies of any particular vibration of the molecule could be a consequence of basis set incompleteness, neglect of anharmonicity and electron correlation. The general tendency of quantum chemical method is overestimated the force constant at the exact equilibrium geometry of the molecule [11]. The assignments of the calculated wave numbers are aided by Gauss view program [12]. Combining the result of Gauss view program and the vibrational frequency assignments were made with a high degree of precision, taking into account symmetry.

Results and Discussion Molecular Geometry
In accordance with the atomic numbering scheme shown in Figure 1, the optimised structural parameters of 1H-1MAP determined by ab inito HF level with 6-31G(d,p), 6-311G(d,p), and DFT/B3LYP level with 6-31G(d,p) basis sets are described in Table 1 .

Figure1.Optimized molecular structure of 1H-1MAP
Since the exact crystal structure of the title compound is not available till now, the optimized structure can only be compared with other similar systems for which the crystal structures have been solved. Therefore optimized geometrical parameters of 1H-1MAP are compared to those of *N2'-(1H-indol-3ylmethylene) carbonic dihydrazide. The optimized bond distance of all the C-H bands is greater than the experimental value (0.95 Å) of *N2'-(1H-indol-3ylmethylene) carbonic dihydrazide.
For the optimized indole(PhI) ring of the title compound, it has been observed that the optimized C3-H9, C4-H10, C5-H11, C6-H12 and C7-H14 bond distances are larger than the experimental C-H bond distance 0.9500Å of the reference molecule [13].
Computed bond distances of C1-C2, C1-C6, C2-C3, C2-C8, C3-C4, C4-C5, C5-C6 and C7-C8 bond distances in indole ring(PhI) of the title compound by DFT method show closely related to experimental data of reference molecule [13]. But at same time C-C bond distance computed in HF method is found to be basis set sensitive and hence the value computed by HF/6-311G(d,p) coincides very well with experimental C-C bond length of reference molecule [13].
The experimental bond distance of C=N is 1.2862 Å and the computed C16=N28 bond distance is 1.2866 Å in B3LYP/6-31G(d,p), 1.2589 Å in HF/6-31G(d,p) 1.2559 Å in HF/6-311G(d,p). The B3LYP/6-31G(d,p) method renders values that are very much coincide with the experimental values of reference molecule and better than other two methods.
The optimized (PhI) C1-N15 and C7-N15 bond distances of five membered ring of indole moiety are computed. The B3LYP/6-31G(d,p) method shows good agreement with the experimental value of reference compound 1.3784 Å. The optimized C19-H22, C20-H24, C21-H26 and C23-H27 bond distances in phenyl ring (Ph) moiety values are larger than the experimental C-H bond distance 0.9500Å of the reference molecule [13].
The optimized (Ph) C18-N28 bond distance for title compound computed by HF/6-31G (d,p), HF/6-311G(d,p) and B3LYP/6-31G(d,p), methods are 1.4078, 1.4084 and 1.4049Å respectively. On comparison with C-N bond distance 1.3712Å of experimental XRD value of reference compound and it has been observed that the computed values are remarkably greater than the experimental value.
The experimental bond distance of O-H is 0.8400Å in [13]. The computed O29-H30 bond distances for the title compound in HF and DFT methods show anomalously different from that of experimental bond distance of reference molecule.

VIBRATIONAL ASSIGNMENT
The title molecule consists of 30 atoms so they have 84 normal vibrational modes which are IR active. According to theoretical calculation, the optimized geometry of the title molecule 1H-IMAP has C1 point group symmetry. Optimized ground state geometries and vibrational modes for studied molecule were obtained by HF and DFT (B3LYP) methods. The fundamental vibrational frequencies calculated for IH-IMAP at HF and B3LYP levels using the triple split valence basis set along with polarization function, 6-31G(d,p) and HF/6-311G(d,p) and observed FT-IR frequencies for various modes of vibrations have been collected in table (3).
Comparison of the frequencies calculated at HF and B3LYP with experimental values table (3) reveals the overestimation of the calculated vibrational modes due to neglect of anharmonicity in real system. Inclusion of electron correlation in Density Functional Theory to certain extends makes the frequency values smaller in comparison with the HF frequency data. Reduction in the computed harmonic vibrations, though basis set sensitive are only marginal as observed in the DFT values using 6-31G(d,p). Anyway not with standing the level of calculations, it is to scale down the calculated harmonic frequencies in order to experimental.
The calculated frequencies were scaled by factors 0.8992, 0.9051 and 0.955 for HF and DFT computation respectively. The resulting vibrational frequencies for the optimized geometries, the proposed vibrational assignments and available experimental Infrared frequencies are also given in table (3).
Vibrational modes are numbered from smallest to largest frequency. In the last columns are given a detailed description of the normal modes. The combined experimental and theoretical spectrum is given in figure(4).

C=N Stretching Vibration
The identification of C=N stretching vibration is not the easiest task, since the mixing of the several bonds are possible in this region Silverstein et al assigned C=N stretching absorption in the region 1689-1471 cm -1 [16]. In the present work, mode (72) have been assigned to C=N stretching vibrations are 1702 cm -1 for HF/6-31G(d,p), 1698 cm -1 for HF/6-311G(d,p) and 1614 cm -1 for B3LYP/6-31G(d,p) method respectively.