Ultra Science, 19(2), 2007, 185-188NUMERICAL SIMULATION OF THE INTERNAL VIBRATIONS OF OH GROUP IN AMINO-SALICYLIC ACIDS

Y.P.SINGH*, RATNESH DASb and R.A.SINGHc

*Department of Physics, Govt. Women’s Polytechnic College, Sagar (MP),470001 E-mail: Y_P_S_2k@Yahoo.com bDepartment of Chemistry, Dr. H.S.Gour University, Sagar (MP), INDIA, 470001, E-mail: ratnesh_das@breakthru.comc Department of Physics, Dr. H.S. Gour University , Sagar (MP),470003

Abstract

Our present work reports the IR spectra of amino substituted salicyclic Acids recorded by FTIR spectrometer and also simulated theoretically. The simulation were performed using GF matrix and AM1, PM3, DFT method. In this work following steps were taken: optimizing the geometry, computing the IR spectra and comparing it with experimental spectra. Assuming Cs point symmetry, vibrational assignments for the observed frequencies have been proposed. The spectra exhibit distinct features originating from low frequency vibrational modes caused by intra-molecular motion.

Keywords

3-amino salicylic acid, 4-amino salicylic acid, 5-amino salicylic acid, OH group, FTIR spectra, vibrational spectra, AM1, PM3, DFT,G-F Matrix

Introduction

Salicylic acid contains both a hydroxyl and a carboxyl group, which react with either an acid

or an alcohol. The presence of the carboxyl group appears to enhance the antiseptic property. Many

hair tonics and remedies for athlete’s foot, corns and warts employ the keratolytic action of salicylic

acid 1.

Volvo, Colombo and Furic 2 calculated normal coordinates for salicylic acid molecule and

proposed the assignment of the observed Raman and IR spectra.

Boczar Marck et al 3 studied theoretical simulation of the s stretching band for salicylic acid

takes into account adiabatic couplings between the high-frequency O-H stretching and the low-

frequency intermolecular O.......O stretching modes. Jadrijevic et al 4 analyzed the structures and the

specral properties of salicylic acid and benzoic acids by means of FT-IR and NMR spectroscopies.

With the availability of powerful computers and the advent of efficient density functional

theory (DFT) methods implemented in standard codes, structure and dynamics of systems containing

a few tens of atoms (or even more) are now within reach. Vibrational spectra of small molecules of

biological or pharmaceutical relevance are routinely treated combing DFT electronic structure

calculations with a harmonic analysis 5.

We compared experimental results with calculated frequencies of OH group of amino-

salicylic acids using force matrix method and AM1, PM3 and DFT method. These methods were able

to account breadth of spectrum as well as description of vibrational modes to encourage the

application of a similar procedure to a larger and more complex group.

COMPUTATIONAL AND THEORETICAL DETAILS

In noncomplex molecules, the G F Matrix 13 is given by:-

G tt’= Σ3Ni=1 (B ti B t’i’) 1/ mi where t, t’ = 1,2,3,……, 3N-6

In which m i is the mass of the atom to which the subscript I refers and B ti , Bt’i’ are constants

determined by geometry of molecule.

Internal coordinate St are related with Cartesian displacement coordinate ξi as :

St = Σ 3Ni=1 Bti ξi where t = 1,2,3….., 3N-6

On solving, G.F. matrix for any atom α is obtained as:

G tt’= Σ3N α =1 μ α St α . St’ α

Where dot represents the scalar product of two vectors and μ α = 1/m α , the reciprocal of the mass

of atom α

The AM1 and PM3 semi empirical approaches were performed as implemented in MOPAC

program 6 and the PRECISE keywords were used. DFT calculations were performed using HYPER

CHEM program 7 at the B3LYP 8 levels of theory with 6-31G* basis set 9 . The vibrational IR spectra

were calculated at the B3LYP/ 6-31G* levels of theory. We have transformed the harmonic force

fields, determined initially in the Cartesian coordinates, were transformed to the force fields in the

internal local coordinates. The force fields obtained were used to calculate the potential energy

distribution (PED)10. Contributions greater than 10% are given.

RESULTS and DISCUSSIONS

We can get information from computational vibrational spectra only when we compare

calculated frequencies with experimental spectrum. Our results are given in table 1. Due to

anharmonicity, the harmonic vibrational frequencies were found to be lowered by1 to 3% in almost

all cases except AM1 method.

The number of internal vibrations for a group is given by 3m-3, where m is the number of

atoms in the group. Thus OH has 3 modes of vibrations. Out of these, 2 modes belong to a’ and one

mode to a”. The a’ modes are O-H stretching & in-plane-bending COH and mode for a” is out-of-

plane wagging mode. Volovsek et al2 studied salicylic acid and they got OH stretching at 2928 cm ,

in-plane-bending at 1448 cm and wagging at 573 cm . Zerbi and Sandroni 13 also got similar

result. A.K.Tiwari 14 observed OH stretching at 3610 cm , in-plane bending OH at 1195 cm and

wagging at 650 cm . Chad C. Trout et al15 observed OH stretching at 3419 cm , OH in-plane

bending at 1207 cm , but he didn’t study low frequency spectra, similarly M. Jadrijevic et al4

observed OH stretching at 3374 cm and OH Bending at 1461 cm .Humbert et al 16 attributed the

1301cm-1 peak to a benzene ring stretch coupled to OH in-plane bending In present work we get

frequencies for them as :-

Table 1 Internal vibrations of OH group

Mode of Vibration 3-Amino-Salicyclic Acid (cm-1)

4-Amino-Salicyclic Acid (cm-1)

5-Amino-Salicyclic Acid (cm-1)

a’

O-H Stretching 3396 (E)3369.1(AM1)3373.1(PM3)3379.1(GF)3381.3(DFT)

3389(E)3341.2(AM1)3338.7(PM3)3371.5(GF)3394.6(DFT)

3394(E)3358.2(AM1)3338.9(PM3)3380.4(GF)3384.7(DFT)

Bending C-OH 1475(E)1512.6(AM1)1519.4(PM3)1493.8(GF)1481.5(DFT)

1489(E)1515.7(AM1)1523.1(PM3)1501.3(GF)1499.4(DFT)

1498(E)1532.7(AM1)1525.9(PM3)1515.8(GF)1505.6(DFT)

a” Wagging 310(E)327.1(AM1)328.3(PM3)318.3(GF)317.5(DFT)

305(E)321.6(AM1)328.79PM3)317.4(GF)311.4(DFT)

315(E)329.1(AM1)331.6(PM3)322(GF)320.6(DFT)

E :- Experimental frequenciesGF:- Theoretical frequencies calculated by GF Matrix methodAM1:- Theoretical frequencies calculated by quantum mechanical AM1 MOPAC methodPM3:- Theoretical frequencies calculated by quantum mechanical PM3 MOPAC methodDFT:- Theoretical calculations were performed using HYPER CHEM program at the B3LYP levels of theory with 6-31G* basis set

CONCLUSIONS

Theoretical semi-empirical quantum mechanical AM1, PM3, DFT and GF matrix calculations

of the geometry and vibrational frequencies of the –OH group of o-,m-,p- amino salicylic acids are

presented in this paper and compared with infrared spectra. The calculated geometries and

frequencies agree well ( for DFT and G.F. Matrix) with the experimental ones, but there are some

differences between frequencies mainly due to intermolecular interactions, anharmonicity and

computational basis set. The results indicate that, the exchange functional proposed by Becke and the

correlation functional of Lee, Yang and Parr with 6-31G* basis set is the optimal model for studying

o-,m-,p- salicylic acids.

ACKNOWLEDGEMENTS

The authors are grateful to Director, Directorate of Technical Education-Madhya Pradesh,

Bhopal and Head, Department of Physics, Dr. H.S.Gour University, Sagar (MP), India, National

Institute of Advanced Industrial Science and Technology, Japan for IR spectra and Hypercube Inc for

providing Hyperchem Package 7 for molecular modeling.

Figure 1: FT-IR Spectra of 3-amino salicylic acid

Figure 2: FT-IR Spectra of 4-amino salicylic acid

Figure 3: FT-IR Spectra of 5-amino salicylic acid

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