molecular orbital studies (hardness, chemical potential and

Middle-East Journal of Scientific Research 18 (5): 597-608, 2013ISSN 1990-9233© IDOSI Publications, 2013DOI: 10.5829/idosi.mejsr.2013.18.5.11723

Corresponding Autor: Adeoye Idowu Olatunbosun, Department of Pure and Applied Chemistry, Ladoke Akintola Universityof technology, P.M.B. 4000, Ogbomoso, Oyo State, Nigeria.

597

Molecular Orbital Studies (Hardness, Chemical Potential andElectrophilicity) and Solvent Effect on 3-Mehtyl-,

4-Methyl and 4-Phenylpicolinic Acid: Density Functional Theory (DFT)

Adeoye Idowu Olatunbosun and Semire Banjo

Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology, P.M.B. 4000, Ogbomoso, Oyo State, Nigeria

Abstract: Ab initio and density functional theory methods were used to study chemical shifts and globalreactivity index of 3- methylpicolinic acid (3-MPA), 4-methylpicolinic acid (4-MPA) and 4-phenylpicolinc acid(4-PPA). All calculated chemical shifts for 3-MPA at both ab initio and DFT correlated well with the experimentaldata. Chemical shifts calculated at HF/6-31G** fitted most with R = 0.999. The hydroxyl hydrogen atom2

experienced downfield resonance in 3-MPA acid by 0.32 ppm as compared to 4-MPA. Global reactivity indexes(hardness, chemical potential and electrophilicity) revealed that 4-PPA should be a better nucleophile than4-MPA and 3-MPA.

Key words: Substituted picolinic acid Molecular orbital study DFT

INTRODUCTION Ag(I) were recently reported [6-9]. The 3-methylpicolinic

Picolinic acid, an isomer of nicotinic acid, is one of complexes were synthesized and characterized bythe most important chelating agents present in the human spectroscopic techniques and thermal characterization inbody. Liver and kidneys biosynthesized picolinic acid the solid state [10].during the catabolism of the essential amino acid In our recent work, DFT/B3LYP was used to studytryptophan and transported to the pancreas [1]. It is also solvents effect on geometry and electronic properties ofsecreted during digestion in the intestine which is used as picolinic acid which showed that there was a greatera complexing agent in the absorption of essential metals shielding/de-shielding on carbon nuclei around nitrogensuch as chromium, zinc, manganese, copper, iron and and oxygen atoms of picolinic acid especially the polarmolybdenum [2, 3]. Picolinic acid and its isomers played solvents [11]. The electron pushing effect of methoxylroles as diverse as reaction partners in industrial (OCH ) substituent on 4-methoxypicolinic acid wasprocesses [4] and as building blocks in photovoltaic noticed which resulted into upfield resonance asdevices [5]. compared to 4-nitropicolinic acid in which nitro (NO )

The vibrational frequencies, UV-Visible and NMR group brought about downfield in HNMR [12]. Thespectra of piconilic acid and its substituted derivatives present study is to investigate the performance ofwith its metal complexes of Fe(III), Ni(II), Cu(II), Zn(II) and computational methods, ab initio and density functional

acid and 6-methylpicolinic acid with their cobalt (II)

3

21

Scheme 1: Schematic structures of the studied molecules

Middle-East J. Sci. Res., 18 (5): 597-608, 2013

598

theory are used to correlate experimental NMR of experimentally observed values are 133.13 and 18.85 ppm3-Methylpicolinic acid. Furthermore, molecular for C2 and C7 respectively. The effect of 3-methyl onorbital studies on 3- methylpicolinic acid (3-MPA), chemical shifts for C1, C2 C3, C4 and C6 are -0.83, 7.28,4-methylpicolinic acid (4-MPA) and 4-phenylpicolinc acid 2.18, 2.65 and 1.22 ppm respectively as compared to(4-PPA) are carried out. Solvents effect on molecular un-substituted picolinic acid [15]. Similar substituentstructures and electronic properties of these molecules as effect is observed when calculated chemical shifts forshown in scheme 1 are also explored using acetone, 3-MPA are compared to the calculated chemical shifts ofethanol, diethyl ether, N,N-dimethylformamide (DMF) and un-substituted picolinic acid [12].tetrahydrofuran (THF). The H3(H5) chemical shifts calculated at ab initio are

MATERIALS AND METHODS HF/6-31G** and HF/6-311G** respectively. These are

The studies substituted picolinic acids were modeled ppm for B3LYP/6-31G*, B3LYP/6-31G** and B3LYP/6-and equilibrium geometry calculations were performed at 311G** respectively; these are observed experimentally atdensity functional theory (Beckes’s three-parameter 7.76 and 8.45 ppm for H3 and H5 respectively. In order tohybrid functional [13] employing the Lee, Yang and Parr compare the experimental and theoretical chemical shiftscorrelation functional B3LYP [14]). Ab initio and DFT ( H and C NMR) for 3-MPA, correlation graphics basedwere used to calculate vibration frequencies and chemical on the calculations have been presented in Figure 1.shift for these molecules. Three different basis sets were The chemical shifts correlation values are found to beused (6–31G*, 6-31G** and 6-311G**) to study the 0.998 for both ab initio and DFT except correlation grapheffect of basis sets on electronic properties and NMR. for HF/6-31G** with 0.999 fitting factor; thus chemicalThe unscaled vibrational frequencies calculated at shifts predicted at HF/6-31G** are closer to theB3LYP/6-311++G** level for the molecules were experimental values.compared. The absorption transitions were calculatedfrom the optimized geometry in the ground state S 4-MPA and 4-PPA Chemical Shifts: The optimized0

using TD-B3LYP/6-31G* in five different solvents. molecular structures for 4-MPA and 4-PPA using B3LYPThe convergence criteria for the energy calculations and method with 6-31G*, 6-31G** and 6-311G** basis set wasgeometry optimizations used in the density functional used to calculate chemical shifts as shown in Table 2.methods were default parameters in the Spartan 06 In the absence of experimental results the calculatedprogram. chemical shifts using DFT method can provide reasonable

RESULT AND DISCUSSION [16, 17] in which chemical shifts calculated for 3-MPA

Chemical Shifts for 3-MPA: The experimental chemical 3-MPA, 4-MPA and 4-PPA are compared as it gives bettershifts (NMR) data for 3-MPA [10] are compared to the agreement with the experimental values than plaintheoretical calculated values obtained using ab initio and calculations using optimized geometry [18]. The C NMRDFT methods as listed in Table 1. The C1(C6) chemical values are greater than 100 ppm which is the typicalshifts calculated at ab initio are 148.38 (157.94), chemical shifts for organic molecules [19]. The average149.91(159.46) and 154.129(164.71) ppm for HF/6-31G*, chemical shifts calculated for C1, C4, C5 and C6 areHF/6-31G** and HF/6-311G** respectively. The C1(C6) 148.214, 126.283, 151.163 and 161.666 ppm for 4-MPA,chemical shifts at DFT are 152.24(166.58), 145.00(156.98) 150.184, 124.519, 148.332 and 163.739 ppm for 3-MPA; andand 153.34(167.66) ppm for B3LYP/6-31G*, B3LYP/6- 142.821, 124.129, 151.761 and 161.629 ppm for 4-PPA31G** and B3LYP/6-311G** respectively, however the respectively. The substituent effect on C1, C4, C5 and C6experimental values chemical shifts for these two carbon in the picolinic ring are -1.044, 0.976, -0.164 and 0.276 ppmatoms are 148.62 and 167.47 ppm for C1 and C6 for 4-mehtyl substituted, 0.927, -0.788, -2.995 and 2.532respectively. The C2(C7) chemical shifts are 130.09(19.99), ppm for 3-methyl substituted and -6.436, -1.178, 0.434 and131.58(20.09) and 137.25(20.36) for HF/6-31G*, HF/6-31G** 0.239 ppm for 4-phenyl substituted respectively asand HF/6-311G** respectively. For DFT, they are compared to un-substituted picolinic acid [11]. Therefore,137.62(23.33), 133.10(23.50) and 143.27(24.04) ppm. The C1 and C5 experienced shielding effect and C4 and C6

7.83(8.94), 7.86(8.92) and 7.88(8.90) ppm for HF/6-31G*,

calculated at DFT to be 7.30(8.63), 8.47(9.79) and 7.51(8.82

1 13

information that can assist in structural elucidation

attested to, thus average chemical shifts calculated for

13


599

Table 1: Experimental and calculated C NMR and H NMR chemical shifts for 3-methylpicolinic acid13 1

Atom DFT/6-31G* DFT/6-31G** DFT/6-311G** Averg. HF/6-31G* HF/6-31G** HF/6-311G** Expt. [10]C1 152.239 144.995 153.317 150.184 148.375 149.914 154.129 148.62C2 137.624 133.103 143.266 137.998 130.090 131.588 137.248 133.13C3 138.155 133.052 143.195 138.134 138.061 138.916 145.068 139.69C4 124.451 119.792 129.314 124.519 122.063 123.059 128.336 125.80C5 150.678 142.377 151.942 148.332 144.963 145.844 151.327 146.26C6 166.575 156.982 167.660 163.739 157.936 159.46 164.706 167.47C7 23.327 23.501 24.937 23.922 19.985 20.086 20.364 18.85H1 5.949 7.135 5.843 6.309 5.664 5.818 5.399 -H3 7.296 8.473 7.516 7.762 7.834 7.856 7.883 7.76H4 7.017 8.226 7.188 7.477 7.403 7.474 7.426 7.46H5 8.628 9.791 8.816 9.078 8.944 8.922 8.895 8.45*H7 2.39 3.32 2.63 2.78 . 2472 2.453 2.586 2.45*average chemical shifts for CH hydrogen atoms3

Fig. 1: Correlation between experimental and calculated NMR at ab initio and DFT levels

experienced are de-shielded for 4-MPA, whereas C4 and The average proton chemical shifts calculated for H1,C5 are shielded and C1 and C6 are de-shielded for 3-MPA. H4 and H5 are 5.979, 7.417 and 9.148 ppm for 4-MPA,For 4-PPA, C1 and C4 are shielded and C5 and C6 and 6.309, 7.477 and 9.078 ppm for 3-MPA and 6.051, 7.679,de-shielded. CI chemical shift strongly is shielded in 9.286 ppm for 4-PPA respectively. The H1/H5 is de-4-PPA as compared to 4-MPA. shielded/shielded in all the molecules and H4 is only


600

Table 2: C NMR and H NMR (ppm) calculated at DFT level with three different basis sets for 4-methylpicolinic acid and 4-phenylpicolini acid13 1

4-methyl- 4-phenyl---------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------

Atom DFT/6-31G* DFT/6-31G** DFT/6-311G** Averg. DFT/6-31G* DFT/6-31G** DFT/6-311G** Averg.C1 151.022 141.005 152.616 148.214 151.706 141.681 135.075 142.821C2 128.521 123.053 131.980 127.851 126.592 121.134 130.242 125.989C3 146.807 142.164 153.125 147.365 150.281 145.803 156.977 151.020C4 126.713 121.434 130.702 126.283 124.504 119.264 128.618 124.129C5 153.505 145.184 154.814 151.168 154.135 145.824 155.323 151.761C6 164.310 154.824 165.666 161.600 164.356 154.868 165.662 161.629C7 21.360 21.606 22.581 21.849 139.347 135.241 146.721 140.436C8 - - - - 127.652 122.318 132.387 127.452C9 - - - - 129.115 123.812 133.659 128.862C10 - - - - 128.969 123.609 133.708 128.762C11 - - - - 129.242 123.930 133.795 128.989C12 - - - - 127.960 122.609 132.762 127.777H1 5.609 6.777 5.550 5.979 5.679 6.855 5.620 6.051H2 7.852 9.110 8.060 8.341 8.132 9.402 8.321 8.618H4 6.955 8.166 7.131 7.417 7.214 8.439 7.383 7.679H5 8.704 9.858 8.882 9.148 8.829 9.993 9.036 9.286*H7 2.318 3.345 2.467 2.710 - - - -H8 - - - - 7.416 8.621 7.587 7.88H9 - - - - 7.383 8.567 7.588 7.846H10 - - - - 7.334 8.517 7.551 7.801H11 - - - - 7.401 8.585 7.609 7.865H12 - - - - 7.476 8.678 7.643 7.932*average chemical shifts for CH hydrogen atoms3

shielded in 3-MPA and 4-MPA. Comparing the position DMF and THF respectively. For 3-MPA and 4-MPA, C1of methyl group on hydroxyl hydrogen atom (H1) and C7 are shielded whereas in 4-PPA, C2, C3 C4, C6 andchemical shifts, the hydroxyl hydrogen atom (H) C7 are de-shielded in solvents.experienced downfield resonance in 3-MPA by 0.32 ppm The effects of solvents on HNMR chemical shifts are(Table 2). more on H1, H2/H3 and H4 in 4-MPA/3-MPA. All

Solvents Effect on NMR of Studied Substituted Picolinic H7 and H5 (for ethanol solvent) in 3-MPA and H12Acid: Solvent effect is estimated by comparing calculated (for diethyl ether, DMF and THF solvents) in 4-PPA. Thechemical shifts in solution and gas phase at a particular differences in chemical shifts in solvents as compared tolevel of calculation. In this work, the CNMR and HNMR gas phase calculation could be explained in terms of13 1

calculated at DFT/6-31G* are used to estimate effect of changes in free energy hypersurface of the nuclei [20].solvents on chemical shifts as shown in Figure 2. Theeffect of solvents on C NMR chemical shifts for 4-MPA Vibration Frequencies: The vibration frequencies13

and 4-PPA is pronounced on C1, C3, C6 and C7 whereas calculated at B3LYP/6-311++G** level for 3-MPA, 4-MPAfor 3-MPA, it is on C1, C2, C6 and C7. For instance, the and 4-PPA are shown in Table 3. The ring carbonestimated solvents effect on C1(C6) of the picolinic ring in hydrogen stretching vibrations are in the region 3221,3-MPA are -2.893(6.273), -4.349(9.052), -2.027(4.653), - 3196, 3192, 3188, 3181, 3173, 3167 and 3154 cm for2.846(6.720) and -2.504(5.393) ppm for acetone, ethanol, 4-PPA. These are calculated at 3205, 3178 and 3152 cmdiethyl ether, DMF and THF respectively as compared to for 4MPA; and 3192, 3168 and 3158 cm for 3-MPA. Thegas phase. In 4-MPA molecule, the solvents effect on CH carbon hydrogen stretching vibrations are 3115, 3188C1(C6) of the picolinic ring are -3.912(6.086), -4.863(9.226), and 3030 cm for 4-MPA; and 3111, 3102 and 3043 cm-2.934(4.461), -4.125(6.350) and -3.467(5.231) ppm for for 3-MPA. The C=O stretching vibration for 4-PPA, 4-acetone, ethanol, diethyl ether, DMF and THF MPA and 3-MPA are at 1819, 1818 and 1815 cmrespectively. The effect of solvents on C1(C6) in 4-PPA respectively. The position and electron donatingare-4.271(6.436),-4.712(7.942), -3.182(3.708), -4.113(4.961) ability of the substituent affect the C=O stretchingand -3.854(4.295) ppm for acetone, ethanol, diethyl ether, vibration intensity. The C=O stretching vibration for

1

hydrogen atoms are de-shielded except H7 in 4-MPA,

1

1

1

31 1

1


601

Fig. 2: Solvents effect on C NMR and H NMR in ppm calculated at DFT/6-31G* level.13 1

4-Methoxypicolinic acid and 4-Nitropicolinic acid at the calculated at 1427 and 1415 cm for 4-Methoxypicolinicsame level of calculations were 1821 and 1826 cm acid (4-MOPA) and 4-Nitropicolinic acid (4-NPA)1

respectively [12], this showed that electron donating respectively [12]. The C-H in plane deformation ( CH) oflower C=O stretching vibration. The OH stretching CH are 1492 and 1414 cm for 4-MPA; and 1486 andvibration is calculated at 3773 cm for 4-PPA, 3770 cm 1421 cm for 3-MPA. The C-O stretching coupled with1 1

for 4-MPA and 3751 cm for 3-MPA with vibration higher OH in plane deformation ( OH) is calculated at 1360,1

intensity in 4-substituted ones; thus para-substitution 1356 and 1357 cm for 4-PPA, 4-MPA and 3-MPAenhanced the COOH stretching vibration intensity respectively. This has been calculated at 1355, 1360 and(Table 3). 1358 cm for 4-MOPA, 4-NPA and un-substituted

The C=C stretching coupled with C=N are calculated picolinic acid respectively [11, 12].at 1648, 1642, 1628 and 1568 cm for 4-PPA; 1633 and The C-H out-of-plane deformations ( CH) are1

1605 for 4-MPA and 1629 and 1600 cm for 3-MPA. calculated at 1009, 996, 990, 948, 934, 878 and 859 cm for1

The 4-MPA experienced bathochromic shifts as compared 4-PPA. These are calculated at 995, 933, 865 and 800 cmto 3-MPA. The C=C stretching coupled with C-H in plane for 4-MPA; and 1000, 952, 829 and 798 cm for 3-MPA.deformation ( CH) are 1477 and 1418 cm for 4-PPA, 1426 The CH out-of-plane deformation is 1020 and 1015 cm1

cm for 4-MPA and 1454 cm for 3-MPA. This has been for 4-MPA and 3-MPA respectively. The OH out-of-plane1 1

1

31

1

1

1

1

1

1

31

( )2 2

HOMO LUMOE EE IP EAv rN

+∂ + = ≈ − ≈ − ∂

2

2=


602

Table 3: Some selected theoretical (DFT/6-311++G**) and experimental vibrational frequencies of 4-Phenylpicolinic acid and 4-Methylpicolinic acid

Theoretical Assignment Theoretical Assignment

4-PPA 4-MPA 3-MPA

3773 (102.93) OH 3770 (97.16) 3751 OH3221, 3196 CH 3205, 3178 3192, 3168, CH of pico.3192, 3188 CH 3152 3158 CH of pico.3181, 3173 CH 3115, 3088 3111, 3102 CH3

3167, 3154 CH 3030 3043 CH3

1819 (402.42) C=O 1818 (362.81) 1815 (354.28) C=O1642, 1648 C=C 1633 1629 C=C1628 C=C 1605 1600 C=C1588, 1530 C=C + C=N1503 C=N + CH 1507 1499 C=N + CH1477 C=C + CH 1492 1486 CH3

1418 C=C + CH 1489 1479 C=C + CH3

1426 1454 C=C + CH1414 1421 CH3

1360 (61.68) C-O + OH 1356 1357 C-O + OH1357 C=C+ CH 1323 1302 C=C+ CH1302 C-Ph + OH 1259 1235 C-CH + OH3

1280 C=C + C-N 1299 1280 C=C + C-N1207, 1184 CH 1170 1192 CH + OH3

1146 C-C + C-O 1145 1140 CH + C=C1105 C=C-C 1109 1114 C-O + C-CH3

1098 C-C + C-O 1064 1064 CH3

1076 CH 1020 1015 CH3

1045, 1017 1013 845Ring Ring

10111009, 996 CH 995, 933 1000, 957 CH990, 948 CH 865, 800 829, 798 CH934, 878, 859 CH850 756 748, 729Ring Ring

629 Ring

617 OH 579 599 OH566 OH 506 547 Ring

33 CO-OH 77 128 CH3

, stretching; , in-plane bending; , out plane bending; , rocking, , breathing

deformation ( OH) are 617 and 566 cm for 4-PPA, (1)1

579 cm for 4-MPA and 599 cm for 3-MPA. The CH1 13

rocking vibration was at 77 cm for 4MPA and 128 cm1 1

for 3-MPA respectively.

Global Reactivity Descriptors and Electronic Properties:The ionization potential (IP) and electron affinity (EA)are determined from the energy difference between the (3)energy of the compound derived from electron transferwhich is approximated as; IP -E and EA -EHOMO LUMO

respectively based on Koopman’s theorem [21]. The The electrophilicity index has been used as structuralchemical hardness ( ), chemical potential (µ), softness depictor for the analysis of the chemical reactivity of(1/2 ) and electrophilicity index ( ) of a molecule are molecules [26-28]. It measures the propensity of a speciesdeduced form IP and EA values [22-25] as shown in the to accept electrons. Domingo et al. [29] proposed that thefollowing equations 1, 2 and 3. high nucleophility and electrophility of heterocycles

(2)


603

Table 4: UV ( nm), HOMO (eV), LUMO (eV), Dipole moment (Debye), salvation energy (kJ/mol) and global nucleophilicity index calculated at DFT/ 6-31G* in the gas phase and solvents for 4-PPA, 4-MPA and 3-MPA

4-Methylpicolinic acid Gas phase Ethanol Acetone Diethyl ether DMF THFDFT/6-31G* HOMO -7.13 -7.30 -7.22 -7.18 -7.21 -7.20

LUMO -1.52 -1.60 -1.52 -1.52 -1.53 -1.52H-L 5.61 5.70 5.70 5.66 5.68 5.68D.M 4.35 5.98 5.72 5.35 5.74 5.53Sol. E -40.22 -53.02 -48.48 -41.25 -41.98 -43.58µ -4.33 -4.45 -4.37 -4.35 -4.37 -4.36

2.805 2.85 2.85 2.83 2.84 2.84S 0.178 0.175 0.175 0.177 0.176 0.176

3.342 3.474 3.350 3.344 3.362 4.346UV 204.22 207.84 207.62 206.65 207.98 207.19

3-Methylpicolinic acidDFT/6-31G* HOMO -7.07 -7.13 -7.05 -7.06 -7.04 -7.05

LUMO -1.43 -1.49 -1.38 -1.38 -1.37 -1.38H-L 5.64 5.64 5.67 5.68 5.67 5.67D.M 3.78 5.25 4.99 4.65 5.01 4.82Sol. E -39.71 -52.06 -47.57 -40.55 -47.07 -42.80µ -4.25 -4.31 -4.215 -4.22 -4.205 -4.215

2.82 2.82 2.835 2.84 2.835 2.835S 0.177 0.177 0.176 0.176 0.176 0.176

3.203 3.294 3.133 3.135 3.119 3.133UV 207.27 208.75 208.46 207.89 208.69 208.24

Expt. [10] UV 215 nm4-Phenylpicolinic acid

HOMO -6.77 -6.47 -6.57 -6.62 -6.59 -7.01LUMO -1.69 -1.69 -1.69 -1.69 -1.69 -1.69H-L 5.08 4.78 4.88 4.93 4.90 5.32D.M 4.64 7.47 5.87 5.54 6.15 5.89Sol. E -39.22 -68.04 -61.24 -53.99 -54.43 -56.12µ -4.23 -4.08 -4.13 -4.16 -4.14 -4.35

2.54 2.39 2.44 2.465 2.45 2.66S 0.197 0.209 0.205 0.203 0.204 0.188

3.522 4.483 3.495 3.510 3.498 3.557UV 242.72 246.83 246.33 245.25 246.76 246.24

corresponds to opposite extremes of the scale of global 4-MPA are 2.81, -4.33, 0.178 and 3.342 eV respectively andreactivity indexes. A good, more reactive, nucleophile is that of 3-MPA are 2.82, -4.25, 0.177 and 3.203 eVcharacterized by a lower value of µ, and in opposite a respectively. For 4-PPA, the chemical hardness, chemicalgood electrophile is characterized by a high value of µ, . potential, softness and electrophilicity index in gas phaseThe electronegativity and hardness are used extensively are 2.54, -4.23, 0.197 and 3.522 eV. Therefore, 4-PPAto predict the chemical behavior to explain aromaticity in should be a better nucleophile than 4-MPA and 3-MPA,organic compounds [30]. A hard molecule has a large similarly 4-MPA is a better molecule to be involved inHOMO–LUMO gap and a soft molecule has a small interaction with electrophiles as compared to 3-MPA.HOMO–LUMO. The LUMO represents electron(s) Among other things, dipole moment is important propertyaccepting ability and HOMO as electron donating ability when considering the interactions of molecules inof a molecule. solvents. The higher the value of dipole moment the

The energy of salvation, energy band gap, HOMO, stronger the intermolecular interactions would beLUMO, dipole moment, softness, chemical potential, expected, however the orientation of the dipole momentelectrophilicity/nucleophilicity index and UV-Vis vector is also an important parameter [11, 12]. Theadsorption maximum calculated are displaced in Tables 4. calculated dipole moment values for 4-PPA, 4-MPA andThe values of chemical hardness, chemical potential, 3-MPA in gas phase are 4.64, 4.35 and 3.78 Debyesoftness and electrophilicity index in gas phase for respectively.


604

Fig. 3: Local ionization potential maps of 4-Methylpicolinic acid in various solvents

The HOMO and LUMO energies calculated in 4-MPA and 3-MPA calculated in different solvents aresolvents revealed that HOMOs experienced stabilization displayed in Figures 3 and 4. The interactions of 4-MPAin 4-PPA and destabilization in both 4-MPA and 4-PPA and 3-MPA molecules with solvents increased the local(except in ethanol). The solvation energy calculated in the ionization potential energy as compared to the gas phase;solvents is ordered as ethanol > acetone ˜ DMF > THF > this shows that solvent has effect on ionization potentialDiethyl ether for 4-PPA and 4-MPA. For 3-MPA, it is of the molecules. The minimum energy required to removeordered as ethanol > acetone > DMF > THF > diethyl an electron from 4-MPA molecular surface is higher thanether. The calculated absorption maxima in solvents are that in 3-MPA. Therefore, the minimum energy required toshifted to longer wavelengths as compared to gas phase remove an electron in different solvents could be arranged[11, 12]. as ethanol > acetone > DMF > Diethyl ether ˜ THF > gas

The local ionization potential energy surface is an phase for both 4-MPA and 3-MPA. The minimum energyoverlaying of the energy of electron removal (ionization) required to remove an electron from 3-MPA is lower byonto the electron density; this has been another indicator 0.05 kJ/mol in ethanol to 0.031 kJ/mol in THF asof electrophilic attraction apart of the electrostatic compared to 4-MPA. The hydrogen atom of thepotential. The regions with red color represent regions in hydroxyl group (-OH) which is main center forthe molecular surface where electron removal goes electrophilic interactions revealed that hydroxyl hydrogen(with minimal energy) most easily, therefore easy of atom on 3-MPA is more acidic than that of 4-MPA inelectron removal. The local ionization potential energy for solvents (Figures 3 and 4).


605

Fig. 4: Local ionization potential maps of 3-Methylpicolinic acid in various solvents

Thermodynamic Properties: The thermodynamic 3-MPAfunctions (heat capacity (C °), enthalpy (H °) and C ° = -5.00 x 10 T + 0.122T + 0.902; R = 0.999 p,m m

entropy (S °)) for 4-MPA, 3-MPA and 4-PPA obtained H ° = 3.00 x 10 T + 0.014T + 81.33; R = 0.999 m

from theoretical harmonic frequencies are shown in S ° = -4.00 x 10 T + 0.117T + 55.03; R = 0.999 Figure 5. All the thermodynamic functions increase withthe increase in temperature from 100 to 900 K which is due 4-PPAto the enhancement of molecular vibrations with C ° = -8.00 x 10 T + 0.185T + 1.374; R = 0.999 increasing in temperature at one atmospheric pressure. H ° = 4.00 x 10 T + 0.016T + 123.3; R = 0.999The correlations between these thermodynamic S ° = -3.00 x 10 T + 0.143T + 59.76; R = 0.999parameters and temperature (T) are plotted and fitted byquadratic equations. The fitting factor (R ) for C °, H ° All the thermodynamic calculations are performed in2

p,m m

and S ° for 4-MPA, 3-MPA and 4-PPA is found to be the gas phase; therefore scale factors have beenm

0.999 as shown below. recommended for better accurate prediction [31].

4-MPA providing information for further study of the two isomersC ° = -5.00 x 10 T + 0.119T + 1.752; R = 0.999 which can be useful to determine the directions ofp,m

5 2 2

H ° = 3.00 x 10 T + 0.013T + 81.33; R = 0.999 chemical reactions according to the second law ofm5 2 2

S ° = -3.00 x 10 T + 0.113T + 56.60; R = 0.999 thermodynamics.m5 2 2

p,m5 2 2

m5 2 2

m5 2 2

p,m5 2 2

m5 2 2

m5 2 2

All these thermodynamic data would be helpful in


606

Fig. 5: The relationship graphs of thermodynamic functions and temperature calculated at B3LYP/6-311++G**

CONCLUSION REFERENCES

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