Paper-numerical Simulation Carboxylic Acids

The Asian J. Exp. Chem, 1&2, 2, 2007, 5-9

NUMERICAL SIMULATION OF THE IR SPECTRA OF CARBOXYLIC ACIDS Y.P.Singha a

Department of Physics, Govt. Women’s Polytechnic College, Sagar (MP), INDIA 470001.E-mail: [email protected]

Abstract Our present work reports the IR spectra of Carboxylic acid’s Formic, Acetic and Benzoic Acid’s Monomers recorded by FTIR spectrometer and also simulated theoretically. The simulation were performed at AM1 level using the MOPAC and G F matrix 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 benzoic acid/ acetic acid/ formic acid/ FTIR spectra/ MOPAC/ vibrational spectra

Introduction Carboxylic acids (RCOOH) are a common and important functional group and provide the point of success to the carboxylic acids derivatives (acetyl chlorides, esters, amides etc.). Carboxylic acids are the most acidic of the common organic functional groups. Molecular structures and inter/intra molecular interactions have a direct influence on the type of structural framework that biomolecules can adopt. Understanding of fundamental processes, dynamics, molecular-orbital studies and force constants calculations are, thus, main objectives of spectroscopists. Intramolecular force field helps us by identifying fundamental frequencies, assigning fundamental frequencies to correct mode of vibrations, determining reliable force constants and designing the drug as input parameters and to predict vibrational frequencies of related molecules. Benzoic acid is the simplest aromatic carboxylic acid containing carboxyl group bonded directly to benzene ring1 . It naturally occurs in many plants and resins. It is also detected in animals.

Carboxylic monomers and dimmers are the simplest models for studying hydrogen bonded systems4,5 and they are of utmost important as doubly bonded hydrogen atoms are abundanent in nucleic acid base pairs holding together the double stranded helices in DNA6. In present study, we compared experimental results with calculated frequencies of acids using force matrix method and MOPAC method using AM1 precise. This method was 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 groups.

O H O

H3 C

H

Formic Acid Monomer

O

O

O H

Acetic Acid Monomer

O

H

Benzoic Acid Monomer

Theoretical Calculation In noncomplex molecules, the G F Matrix 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

Experimental Spectroscopic measurements were recorded using Perkin-Elmer spectrometer Model 397 using neat liquid films between Kbr windows. Preparation of Kbr Pallets: A small amount of finally grounded solid sample was intimately mixed with about 100 times or more than its weight of Potassium bromide powder. The finally grounded mixture was than pressed under very high pressure in a press (about 10/cm2) to form a small pallet ( about 1-2 mm thick and 1cm in diameter).

Results and Discussion The vibrational analysis was performed by computing the MOPAC obtained frequencies, calculated G.F. Matrix9 and force constants for all normal modes. Results of FTIR measurements and the calculated frequencies for all symmetry species are summarized in table 1,2,3,4 and 5. The results are compared with GF Matrix and AM1. MONOMER Our results for carboxylic acids monomer are summarized in table 1,2 and 3. OH Stretch: Experimental OH stretch band frequency for AA, FA and BA are 3567 cm −1 , 3557 cm −1 and 3507 cm −1 respectively.. Ibrahim et al10 observed it for acetic acid at 3583 cm −1 and Florio et al16 got this for formic acid at 3569 cm −1 . Antony et al6 observed this frequency for benzoic acid at 3602 cm −1 which is higher than those observed by others. Theoretically calculated frequencies by GF Matrix method and AM1 methods are (3529.2 cm −1 & 3328.7 cm −1 ), (3511.3 cm −1 & 3429.9 cm −1 ) and (3532.2 cm −1 &3427.3 cm −1 ) for AA, FA and BA respectively. CH3 s-stretch: Experimental CH3 s-stretch frequency for AA, FA and BA are 2842 cm −1 , 2911 cm −1 and 2987 cm −1 respectively. Ibrahim et al10 observed it for acetic acid at 3186 cm −1 and Florio et al16 got this for formic acid at 2943 cm −1 . Antony et al6 observed this frequency for benzoic acid at 2943 cm −1 . Thus these frequencies observed by us are lower than other’s observations. Theoretically calculated frequencies by GF Matrix method and AM1 methods are (2811.3 cm −1 & 2614.9 cm −1 ), (2926.4 cm −1 & 3187.7 cm −1 ) and (3111.5 cm −1 &3182.2 cm −1 ) for AA, FA and BA respectively. C=O stretch: experimental observed frequencies for this bands are 1801 cm −1 , 1815 cm −1 and 1823 cm −1 for for AA, FA and BA. Ibrahim et al10 observed it for acetic acid at 1788 cm −1 and Florio et al16 got this for formic acid at 1804 cm −1 . Antony et al6 observed this frequency for benzoic acid at 1752 cm −1 . For AA, FA and BA theoretically calculated frequencies by GF Matrix method and AM1 methods are (1819.6 cm −1 & 1896.74 cm −1 ), (1806.2 cm −1 & 2049.8 cm −1 ) and (1818.1 cm −1 &1821.9 cm −1 ) .

O-H Bend: Our observations for this bend are 1298 , 1313 and 1328 for AA, FA and BA. Ibrahim et al10 observed it for AA at 1355 cm −1 and Florio et al16 got this for FA at 1223cm −1 . Antony et al6 observed this frequency for benzoic acid at 1381 cm −1 . Theoretically calculated frequencies by GF Matrix method and AM1 methods are (1273.5 cm −1 & 1298.3 cm −1 ), (1326.8 cm −1 & 1437.7 cm −1 ) and (1277.3 cm −1 &1359.0 cm −1 ) for AA, FA and BA respectively. C-O stretch: Experimental

frequency for AA, FA and BA are 1235 cm −1 , 1206 cm −1 and 1228 cm −1

respectively. Ibrahim et al10 observed it for AA at 1182 cm −1 and Florio et al16 got this for FA at 1105 cm −1 . Antony et al6 did not observed this frequency for benzoic acid. Thus frequencies observed by us are in good agreement with each othsr and are higher than observed by others. By GF Matrix method and AM1 methods we get frequencies as (1215.8 cm −1 & 1114.0 cm −1 ), (1219.9 cm −1 & 1231.9 cm −1 ) and (1341.5 cm −1 &1541.9 cm −1 ) for AA, FA and BA respectively. O-C-O Deformation: We observed this bend frequencies at 657 cm −1 , 613 cm −1 and 668 cm −1 for AA, FA and BA respectively which is comparative to others6,10. Theoretically calculated frequencies by GF Matrix method and AM1 methods are (662.5 cm −1 & 528.2 cm −1 ), (601.5 cm −1 & 603.8 cm −1 ) and (652.7 cm −1 &646.3 cm −1 ) for AA, FA and BA respectively. Torsion: Experimental observed frequency for this bend for AA, FA and BA are 534cm −1 , 619 cm −1 and 591 cm −1 respectively. . Ibrahim et al10 observed it for AA at 542 cm −1 and Florio et al16 got this for FA at 642 cm −1

. Antony et al6 observed this frequency for benzoic acid at 444 cm −1 . FA has got more value because here

torsion is of OH. For AA, Fa and BA theoretically calculated frequencies by GF Matrix method and AM1 methods are (541.8 cm −1 & 510.2 cm −1 ), (610.6 cm −1 & 603.9 cm −1 ) and (593.4 cm −1 &371.9 cm −1 ) .

Table 1 Experimental and calculated frequencies and potential distribution in acetic acid monomer Assignment

Experimental frequencies (in cm −1 )

G F matrix Frequencies (in cm −1 )

1 2 3 4 5 6 7 8 9 10 11 12

3567 3010 2842 1801 1457 1465 1298 1235 1006 801 657 568

3529.2 2905.7 2811.3 1819.6 1513.2 1449.4 1273.5 1215.8 983.6 810.7 662.5 561.9

13 14 15 16 17 18

2996 1430 1048 642 534

2945.3 1419.6 1037.2 638.1 541.8 139.5

Potential energy distribution and mode

Species a’ OH str CH3 d-str CH3 s-str C=O str CH3 d-deform CH3 s-deform OH bend C-O str CH3 rock CC str OCO deform CCO deform Species a” CH3 d-str CH3 d-deform CH3 rock C=O op-bend C-O torsion CH3 torsion

AM1calculation Frequencies (in cm −1 )

3328.69 3069.34 2614.90 1896.74 1512.42 1388.75 1298.27 1114.03 915.73 843.61 528.20 375.99 1973.32 1107.59 797.55 531.88 510.25 134.93

Table 2 Experimental and calculated frequencies and potential distribution in formic acid monomer Assignment

Experimental frequencies (in cm −1 )

G F matrix Frequencies (in cm −1 )

1 2 3 4 5 6 7

3557 2911 1815 1409 1313 1206 613

3511.3 2926.4 1806.2 1421.7 1326.8 1219.9 601.5

8 9

1008 619

992.6 610.6

Potential energy distribution and mode

Species a’ OH str CH str C=O str CH bend OH bend C-O str OCO deform Species a” CH bend Torsion

AM1calculation Frequencies (in cm −1 )

3429.97 3187.76 2049.86 1489.68 1437.70 1231.94 603.89 988.24 603.95

Table 3 Experimental and calculated frequencies and potential distribution in benzoic acid monomer Assignment

Fxperimental frequencies (in cm −1 )

G F matrix Frequencies (in cm −1 )

AM1

Potential energy distribution and mode

Species a’ OH str CH str CH str CH str CH str CH str C=O str CC ring deformation C-C str COH bnding CCH bending OH bending C-O str CCh bending O-H bend CH ib COH bending Ring id+C-O str Ring CCH bending Ring id + CC str CH od CH od OCO deform Ring CCC bending COH bending C-O bending C=O bending Species a” CC wagging CC wagging

Frequencies (in cm −1 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

3507 3217 3130 3100 3087 2987 1823 1696 1585 1499 1456 1328 1228 1292 1280 1186 1179 1129 1074 1029 1000 808 668 600 548 420

3532.2 3210.6 3118.8 3111.5 3072.4 3012.9 1716.5 1818.1 1561.4 1518.7 1443.8 1341.5 1211.8 1312.3 1277.3 1171.4 1192.5 1134.6 1063.1 1011.1 1013.9 801.3 652.7 587.3 525.6 412.1 310.5

3427.3 3198.2 3190.2 3182.2 3175.7 3172.0 2076.2 1821.9 1765.5 1638.6 1572.6 1541.9 1435.7 1378.9 1359.0 1314.7 1229.4 1198.3 1177.5 1168.0 1089.4 796.5 646.3 536.8 509.9 409.6 200.0

28 29

980 970

971.6 967.3

30

935

929.5

rocking

971.8

31

935

937.6

CC wagging

894.5

32

850

844.1

Ring CCH

886.3

33

812

801.9

Ring CCH bend

825.9

34

709

719.8

C=O o.p.bend

723.6

35

664

657.1

Ring CCH bend

610.2

36

613

609.3

torsion

411.9

37

591

1013.2 995.6

593.4

torsion

371.9

38

199.4

wagging

150.6

39

57.6

twisting

44.0

ACKNOWLEDGEMENTS

The authors are greatful to Director, Directorate of Technical Education-Madhya Pradesh, Bhopal and Head, Department of Physics, Dr. H.S.Gour University, Sagar (MP), India and National Institute of Advanced Industrial Science and Technology, Japan for IR spectra

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