Vibrational spectroscopic studies of Isoleucine by quantum chemical calculations

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 365–374

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Vibrational spectroscopic studies of Isoleucine by quantum chemical calculations P.P. Moorthi a, S. Gunasekaran b, G.R. Ramkumaar a,⇑ a b

PG and Research Department of Physics, Pachaiyappa’s College, Chennai 600030, TN, India Research and Development, St. Peter’s Institute of Higher Education and Research, St. Peter’s University, Avadi, Chennai 600054, TN, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 FT-IR, FT-Raman and UV–visible

spectra of Isoleucine in the solid phase were recorded and analyzed.  The optimized geometry and vibrational wavenumbers were computed using MP2 and DFT (B3LYP) methods.  Vibrational assignment made by PED calculation by VEDA program.  Natural atomic analysis explained the intramolecular hydrogen bonding.  Chemical shift of the title compound were found.

a r t i c l e

i n f o

Article history: Received 30 October 2013 Received in revised form 5 January 2014 Accepted 10 January 2014 Available online 24 January 2014 Keywords: MP2 DFT Isoleucine FTIR FT-Raman

a b s t r a c t In this work, we reported a combined experimental and theoretical study on molecular structure, vibrational spectra and NBO analysis of Isoleucine (2-Amino-3-methylpentanoic acid). The optimized molecular structure, vibrational frequencies, corresponding vibrational assignments, thermodynamics properties, NBO analyses, NMR chemical shifts and ultraviolet–visible spectral interpretation of Isoleucine have been studied by performing MP2 and DFT/cc-pVDZ level of theory. The FTIR, FT-Raman spectra were recorded in the region 4000–400 cm1 and 3500–50 cm1 respectively. The UV–visible absorption spectra of the compound were recorded in the range of 200–800 nm. Computational calculations at MP2 and B3LYP level with basis set of cc-pVDZ is employed in complete assignments of Isoleucine molecule on the basis of the potential energy distribution (PED) of the vibrational modes, calculated using VEDA-4 program. The calculated wavenumbers are compared with the experimental values. The difference between the observed and calculated wavenumber values of most of the fundamentals is very small. 13 C and 1H nuclear magnetic resonance chemical shifts of the molecule were calculated using the gauge independent atomic orbital (GIAO) method and compared with experimental results. The formation of hydrogen bond was investigated in terms of the charge density by the NBO calculations. Based on the UV spectra and TD-DFT calculations, the electronic structure and the assignments of the absorption bands were carried out. Besides, molecular electrostatic potential (MEP) were investigated using theoretical calculations. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Isoleucine (2-Amino-3-methylpentanoic acid) is one of the branched-chain aliphatic amino acids (BCAAs) that are essential ⇑ Corresponding author. Tel.: +91 9884351008. E-mail address: [email protected] (G.R. Ramkumaar). 1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2014.01.067

substrates for protein synthesis in all organisms. Isoleucine is especially important to serious athletes and body builders because of its role in boosting energy production and assisting the body recover from strenuous physical activity. Branched-chainamino-acids promote muscle recovery after physical exercise and it is needed for the formation of hemoglobin as well as assisting with regulation of blood sugar levels. It is also involved in blood-clot

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formation. Therapeutic doses of Isoleucine may be very helpful in preventing muscle wasting and promoting tissue repair after surgery. Isoleucine is also converted in the liver to blood sugar; therefore, it can be helpful in maintaining proper blood glucose levels. Due to the hydrophobic character of Isoleucine, it is mostly found in the interior of proteins [1]. Isoleucine plays an important role in the therapy of burn vitamins [2] and directs the protein synthesis in skeletal muscles [3,4]. The conformational behavior of Isoleucine has been investigated by Lesarri et al. [5]. In my present work MP2 and DFT-B3LYP calculations based on cc-pVDZ basis set has been found to interpret the vibrational spectra of Isoleucine. The DFT is cost effective procedures for exploring the physical properties of titled Isoleucine molecule [6]. Quantum chemical computational methods are advanced technique with most significant contribution towards interpreting and predicting the vibrational spectra to resolving the structure, functional groups and orbital interactions of the molecules [7]. Density Functional theory (DFT) incorporates the electron correlation and it is an important tool for the prediction of the molecular structure, IR intensities and Raman activities of the molecule [8]. However the spectroscopic measurement and detailed theoretical studies based on quantum chemical calculations for Isoleucine have not reported so for.

Results and discussion Geometrical parameters The molecular structure of Isoleucine belongs to C1 point group symmetry with 22 atoms composing the structure. Geometrical structure of the titled molecule along with numbering of atom scheme was shown in Fig. 1. The optimized geometrical parameters were obtained by MP2 and B3LYP with cc-pVDZ basis set. The comparative optimized values of bond lengths, bond angles and dihedral angles were presented in Table 1. The calculated geometrical parameters (bond lengths and bond angles) were compared with available experimental data [15]. The distance between the atoms C2–C3 was calculated as in the range of 1.540 Å and 1.550 Å by using MP2 and B3LYP methods respectively. All the C–H bonds were shown almost similar bond length values in both MP2 and B3LYP methods. The oxygen–hydrogen atoms (O7–H17) stands with the bond distance about 0.97 Å. N9–H21 and N9–H22 stand in the bond distance of 1.02 Å range by MP2 and B3LYP methods. The C1–O6 bond length calculated at 1.21 Å by both MP2 and B3LYP method is in good agreement with the literature data 1.208 Å [16–18]. The bond distance between C1–O7 is calculated about 1.36 Å in both methods, which is excellent agreement with literature data 1.36 Å [19]. HOMO and LUMO analysis

Experimental details The compound under the investigation namely 2-Amino3-methylpentanoic acid (Isoleucine) is procured from the reputed pharmaceutical company, Chennai, India, which is a spectroscopic grade and hence used for recording the spectra without further purification. The room temperature FT-IR spectrum of Isoleucine was recorded in the region 4000–400 cm1 on BRUKER IFS 66 V spectrometer using KBr pellet with spectral resolution of ±1 cm1. The FT-Raman spectrum was also recorded in the region 3500– 50 cm1 with BRUKER IFS 100/s Raman molecule equipped with Nd:YAG laser source operating at 1064 nm line width 150 mW power. The spectra were recorded with scanning speed of 50 cm1 min1 of spectral width 4 cm1. The reported wave numbers are believed to be accurate with in ±1 cm1. The UV–visible spectral measurements were carried out using a Varian carries 5E-UV-NIR spectrophotometer at sophisticated instrumentation Analysis Facility, IIT Madras, India.

Computational details The optimized molecular structure, vibrational frequencies, thermodynamic properties, charge analysis, HOMO–LUMO energy, UV–visible and NMR spectra of the titled compound were calculated by ab initio-MP2 and DFT method adopting Becke3– Lee–Yang–Parr (B3LYP) combined with cc-pVDZ basis set using GAUSSIAN 09W program Package with molecular visualization program [9] on the personal computer at MP2 and B3LYP/cc-pVDZ calculation level [10–13]. The optimized geometrical parameters were used in the vibrational frequency calculation at MP2 and DFT levels to characterize all stationary points as minima, finally the calculated normal modes of vibrational frequencies provide thermodynamic properties through statistical mechanics. The calculation of the potential energy distribution (PED) and the prediction of IR were analyzed by VEDA-4 program [14].

Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are very important parameters in quantum chemistry to determine the interaction of molecule with other species; thus, they are termed frontier orbitals. HOMO can be through the outermost orbital containing electrons tends to give these electrons such as an electron donor. On the other hand, LUMO can be through the innermost orbital containing free places to accept electron [20]. Owing to the interaction between HOMO and LUMO orbital of a structure, transition state transition of p–p* type is observed with regard to the molecular orbital theory [21,22]. Therefore, while the energy of the HOMO is directly related to the ionization potential, LUMO energy is directly related to the electron affinity. Energy difference between HOMO and LUMO orbital is called as energy gap that is an important stability for structures [23]. 3D plots of highest occupied molecular orbitals (HOMOs) and lowest an unoccupied molecular orbitals (LUMOs) with the energy value are presented in Fig. 2. As can be seen from the figure, the energy band gap |(DE)| of the optimized geometry was calculated to be about 0.24 au at B3LYP/cc-pVDZ level of theory. The highest occupied molecular orbitals are localized mainly in amine groups. On the other hand, the lowest unoccupied molecular orbitals are also localized mainly in carboxylic acid, 1-carboxyl and aliphatic groups.

Fig. 1. Molecular structure of Isoleucine along with numbering of atoms.

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P.P. Moorthi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 365–374 Table 1 Optimized geometrical parameters (bond lengths, bond angle, and dihedral angle) of Isoleucine. Parameters

Isoleucine MP2/cc-pVDZ

Bond length (A) C1–C2 C1–O6 C1–O7 C2–C3 C2–N9 C2–H10 C3–C4 C3–C8 C3–H11 C4–C5 C4–H12 C4–H13 C5–H14 C5–H15 C5–H16 O7–H17 C8–H18 C8–H19 C8–H20 N9–H21 N9–H22

1.52 1.21 136 1.54 1.47 1.11 1.53 1.53 1.11 1.53 1.10 1.10 1.10 1.10 1.10 0.97 1.10 1.10 1.10 1.02 1.02

Bond angle (deg.) C2–C1–O6 C2–C1–O7 O6–C1–O7 C1–C2–C3 C1–C2–N9 C1–C2–H10 C3–C2–N9 C3–C2–H10 N9–C2–H10 C2–C3–C4 C2–C3–C8 C2–C3–H11 C4–C3–C8 C4–C3–H11 C8–C3–H11 C3–C4–C5 C3–C4–H12 C3–C4–H13 C5–C4–H12 C5–C4–H13 H12–C4–H13 C4–C5–H14 C4–C5–H15 C4–C5–H16 H14–C5–H15 H14–C5–H16 H15–C5–H16 C1–O7–H17 C3–C8–H18 C3–C8–H19 C3–C8–H20 H18–C8–H9 H18–C8–H20 H19–C8–H20 C2–N9–H21 C2–N9–H22 H21–N9–H22

127.26 110.48 122.18 113.56 108.67 104.30 109.46 108.34 112.47 114.34 111.86 104.37 111.11 106.72 107.846 114.89 109.12 106.97 109.96 108.83 106.07 110.72 111.27 112.03 107.64 107.08 107.87 104.59 110.19 110.72 110.37 108.79 108.05 108.63 107.31 108.38 105.35

Parameters

Dihedral angle (deg.) O6–C1–C2–C3 O6–C1–C2–N9 O6–C1–C2–N9 O7–C1–C2–C3 O7–C1–C2–N9 O7–C1–C2–H10 C2–C1–O7–H17 O6–C1–O7–H17

Exp [15] B3LYP/cc-pVDZ 1.52 1.21 1.358 1.55 1.469 1.11 1.54 1.53 1.108 1.53 1.10 1.10 1.10 1.10 1.10 0.97 1.10 1.09 1.10 1.02 1.02 126.73 111.61 121.56 113.79 109.89 103.14 109.60 107.84 112.44 114.49 111.82 104.04 111.53 106.77 107.50 115.42 109.06 107.04 110.02 108.49 106.36 110.74 111.58 112.46 107.41 106.82 107.53 105.72 110.54 110.51 110.9 108.44 107.83 108.50 108.62 109.66 106.27

1.527 1.253 1.269 1.570 1.499 1.548 1.549 1.554

117.4 118.0 124.5 112.1 109.6 110.0

109.4 112.6

C1–C2–C3–C4 C1–C2–C3–C8 C1–C2–C3–H11 N9–C2–C3–C4 N9–C2–C3–C8 N9–C2–C3–H11 H10–C2–C3–C4 H10–C2–C3–C8 H10–C2–C3–H11 C1–C2–N9–H21 C1–C2–N9–H22 C3–C2–N9–H21 C3–C2–N9–H22 H10–C2–N9–H21 H10–C2–N9–H22 C2–C3–C4–C5 C2–C3–C4–H12 C2–C3–C4–H13 C8–C3–C4–C5 C8–C3–C4–H12 C8–C3–C4–H13 H11–C3–C4–C5 H11–C3–C4–H12 H11–C3–C4–H13 C2–C3–C8–H18 C2–C3–C8–H19 C2–C3–C8–H20 C4–C3–C8–H18 C4–C3–C8–H19 C4–C3–C8–H20 H11–C3–C8–H18 H11–C3–C8–H19 H11–C3–C8–H20 C3–C4–C5–H14 C3–C4–C5–H15 C3–C4–C5–H16 H12–C4–C5–H14 H12–C4–C5–H15 H12–C4–C5–H16 H13–C4–C5–H14 H13–C4–C5–H15 H13–C4–C5–H16

66.15 61.26 177.61 172.20 60.37 55.96 49.22 176.64 67.01 171.55 58.23 63.89 177.22 56.59 56.72 55.17 68.83 176.08 177.02 58.97 56.11 59.70 176.29 61.21 170.55 50.14 70.18 60.31 179.28 58.95 56.32 64.07 175.59 71.36 51.68 69.71 65.08 175.23 54.38 51.46 68.21 170.93

69.39 58.71 174.44 167.10 64.79 50.94 44.38 172.48 71.78 167.64 51.88 66.59 177.64 53.35 62.40 62.16 62.26 176.96 169.58 65.98 48.71 52.41 176.84 68.46 168.26 48.19 72.18 62.07 177.85 57.48 54.66 65.40 174.21 74.24 54.63 66.31 61.83 178.55 57.61 54.15 65.45 173.60

111.9

114.3

Natural population analysis Atomic charges of Isoleucine (2-Amino-3-methylpentanoic acid), calculated by natural population analysis at the B3LYP/ cc-pVDZ level of theory, are given in Table 2. In quantum chemistry, a natural bond orbital or NBO is a calculated bonding orbital with maximum electron density. The NBOs are one of a sequence of natural localized orbital sets that include ‘‘natural atomic orbitals’’ (NAO), ‘‘natural hybrid orbitals’’ (NHO), ‘‘natural bonding orbitals’’ (NBO) and ‘‘natural (semi-)localized molecular orbitals’’ (NLMO). These natural localized sets are intermediate between basis atomic orbitals (AO) and molecular orbitals (MO): Atomic orbital ! NAO ! NHO ! NBO ! NLMO ! Molecular orbital

Isoleucine MP2/cc-pVDZ

B3LYP/cc-pVDZ

16.08 138.17 101.65 166.61 44.52 75.64 178.76 1.29

24.52 147.86 92.02 158.90 35.55 84.54 177.75 0.97

Natural (localized) orbitals are used in computational chemistry to calculate the distribution of electron density in atoms and in bonds between atoms. They have the ‘‘maximum-occupancy character’’ in localized 1-center and 2-center regions of the molecule. Natural bond orbitals (NBOs) include the highest possible percentage of the electron density. As can be seen from Table 2, the magnitudes of the carbon atomic charges, were noted to change from 0.63 to 0.87. The magnitudes of the carbon atom attached to the oxygen atoms (O6, O7) were corresponding of positive value, whereas the carbon atoms attached to the hydrogen atoms (H14, H15, H16) calculated to be generally negative. In fact, C1 connected to oxygen atoms (O6, O7) have the maximum charge magnitude (0.87 at B3LYP/ cc-pVDZ calculation level). The magnitudes of charges calculated

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charge magnitude was found to be for H21 which is connected to nitrogen atom while the minimum charge magnitudes were found for H18. The maximum negative charge values of O7 (0.714) which imposes to give high positive value to the neighboring hydrogen atom H17. Thermodynamic properties On the basis of vibrational analysis at B3LYP/cc-pVDZ level, the standard statistical thermodynamic functions: heat capacity (C 0p:m ), entropy (S0m ), and enthalpy changes (DH0m ) for the title compound were obtained from the theoretical harmonic frequencies and listed in Table 3. From Table 3, it can be observed that these thermodynamic functions are increasing with temperature ranging from 100 to 1000 K due to the fact that the molecular vibrational intensities increase with temperature [24,25]. The correlation equations between heat capacities, entropies, enthalpy changes and temperature are fitted by quadratic formulas. The corresponding fitting factors (R2) for these thermodynamic properties are 0.9998, 0.9995 and 0.9996, respectively. The standard deviation is very least in the calculation of enthalpy change. The corresponding fitting equations are as follows and the correlation graphics of those shows in Figs. 3–5.

C 0p:m ¼ 21:88212 ¼ 0:58024T  2:14948E  4T 2

ðR2 ¼ 0:9998Þ

S0m ¼ 232:51093 þ 0:69518T  1:66351E  4T 2

ðR2 ¼ 0:9995Þ

Table 3 Thermodynamic properties at different temperatures at the B3LYP/cc-pVDZ level for Isoleucine. Fig. 2. HOMO–LUMO of Isoleucine.

Table 2 Natural bond analysis of Isoleucine. Atoms

C1 C2 C3 C4 C5 O6 O7 C8 N9 H10 H11 H12 H13 H14 H15 H16 H17 H18 H19 H20 H21 H22

Isoleucine MP2/cc-pVDZ

B3LYP/cc-pVDZ

1.03219 0.07289 0.23631 0.38855 0.58009 0.72212 0.80016 0.56771 0.96897 0.20787 0.21765 0.18478 0.20442 0.19969 0.19913 0.18655 0.51522 0.16240 0.22076 0.21717 0.39542 0.39354

0.87336 0.11681 0.25776 0.42514 0.63385 0.61863 0.71491 0.62134 0.92656 0.22431 0.23633 0.20307 0.22159 0.21799 0.21604 0.20443 0.49568 0.18417 0.23262 0.22915 0.38888 0.38739

for oxygen atoms (O6, O7) were only found to be negative values 0.618 and 0.714 respectively. In addition, the magnitude of the nitrogen atom was recorded to be more negative value of about 0.926. Moreover, the magnitudes of the hydrogen atomic charges were arranged in an order from 0.184 to 0.388. The maximum

Temperature (K)

S0m (cal mol1 K1)

(cal mol1 K1)

DH0m (K cal mol1)

100.00 200.00 298.15 300.00 400.00 500.00 600.00 700.00 800.00 900.00 1000.00

296.38 367.57 426.97 428.04 484.35 537.84 588.46 636.15 681.02 723.26 763.09

81.13 128.12 172.82 173.68 219.50 260.41 294.90 323.76 348.14 368.98 386.93

5.57 16.11 30.85 31.17 50.85 74.90 102.72 133.70 167.32 203.21 241.02

C 0p:m

Fig. 3. Correlation graph of entropy and temperature for Isoleucine.

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P.P. Moorthi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 365–374 Table 4 Thermodynamic parameters of Isoleucine.

350

Parameters

B3LYP/cc-pVDZ

MP2/cc-pVDZ

Self consistent field energy (au)

441.571

438.999

120.399

122.494

Rotational constant (GHz)

2.046 1.137 1.037

1.996 1.192 1.065

Rotational temperature (K)

0.098 0.054 0.049

0.095 0.057 0.051

127.181 0.889 0.889 125.404

129.171 0.889 0.889 127.394

Specific heat capacity at constant volume (cal/mol K) Total 39.317 Translational 2.981 Rotational 2.981 Vibrational 33.355

38.770 2.981 2.981 32.808

Zero point vibrational energy (K cal/mol)

300 250

0

-1

-1

Heat capacity C p.m (cal mol K )

400

200 150

Thermal energy (K cal/mol) Total Translational Rotational Vibrational

100 50 0

200

400

600

800

1000

Temperature (K)

250

Entropy (cal/mol K) Total Translational Rotational Vibrational

200

Dipole moment (Debye)

150

lx ly lz ltotal

Enthalpy H

0 m

-1

(K cal mol )

Fig. 4. Correlation graph of heat capacity and temperature for Isoleucine.

102.022 40.525 29.278 32.219

101.248 40.525 29.229 31.493

0.6419 0.1009 2.0396 2.1406

0.4228 0.0408 2.3801 2.4178

100

term, the molecular partition function (i.e., a sum over the molecular energy states) is given by

50

q¼

0

X

eðEele þEv ib þErot þEtrans Þ

=kT

¼

X X X X ebEel ebEv ib ebErot ebEtrans el

0

200

400

600

800

1000

Temperature (K) Fig. 5. Correlation graph of enthalpy and temperature for Isoleucine.

v ib

rot

trans

Here, the summation is over the electronic, vibrational and rotational states can be done separately since they are assumed to be independent. Therefore,

q ¼ qel qv ib qrot qtrans

DH0m

¼ 5:82552 þ 0:07587T þ 1:7304E  4T

2

2

ðR ¼ 0:9996Þ

All the thermodynamic data supply helpful information for the study on the Isoleucine. They can be used to compute the other thermodynamic energies according to relationship of thermodynamic functions and estimate directions of chemical reactions according to the second law of thermodynamics. Notice: all thermodynamic calculations were done in gas phase and they could not be used in solution. The calculated values of some thermodynamic parameters such as zero point vibrational energy (ZPVE), thermal energy, specific heat capacity, rotational constants, rotational temperature and entropy are listed in Table 4. The electronic energy levels are generally very widely separated in energy compared to the thermal energy kT at room temperature. In each electronic level, there are several vibrational levels and for each vibrational level, there are several rotational states. This is a simplified and useful model to start with. The total energy is a sum of all these energies and is given by

Etotal ¼ Eel þ Ev ib þ Erot þ Etrans þ Eothers The term Eothers includes nuclear spin energy levels and may also be used later to include the interactions between the first four. Assuming the first three to be independent and neglecting the last

The molecular partition q function is written as the product of electronic, vibrational, rotational and partition functions. The partition function is a sum over states (of course with the Boltzmann factor b multiplying the energy in the exponent) and is a number. Larger the value of q, larger the number of states which are available for the molecular system to occupy. Since Eel > Evib > Erot > Etrans, there are far too many translational states available compared to the rotational, vibrational and electronic states. qel is very nearly unity, qvib and qrot are in the range of 1–100 while qtrans can be much in excess of 1020. We shall calculate the values of these qs and indicate how these qs are useful in calculating the equilibrium constants. The biggest value of zero point energy of Isoleucine is 122.494 kcal/mol obtained at MP2/cc-pVDZ whereas the smallest value is 120.399 kcal/mol obtained at B3LYP/ cc-pVDZ. The dipole moment of the molecule was also calculated by B3LYP method with two basis sets. Dipole moment reflects the molecular charge distribution and is given as a vector in three dimensions. Therefore, it can be used as descriptor to depict the charge movement across the molecule depending upon the centers of positive and negative charges. For charged systems, dipole moment value depends on the choice of origin and molecular orientation. As a result of B3LYP calculations the highest dipole moments were observed for MP2/cc-pVDZ whereas the smallest one was observed for B3LYP/cc-pVDZ in Isoleucine.

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UV–visible spectral analysis

Electrostatic potential

On the basis of fully optimized ground-state structure, TD-DFT/ B3LYP/cc-pVDZ calculations have been performed to determine the low-lying excited states of Isoleucine. The calculated result involving the vertical excitation energies, oscillator strength (f) and wavelength are carried out and compared with measured experimental wavelength also listed in Table 5. Typically, according to Frank–Condon principle, the maximum absorption peak (max) corresponds in an UV–visible spectrum to vertical excitation. In the experimental UV–visible spectrum of Isoleucine, there are three absorption bands with a maximum of 298 nm. The strong absorption band at 298 nm is caused by the n ? p*, and the other two calculated values of moderately intense bands are due to p ? p* transitions. The p ? p* transitions are expected to occur relatively at lower wavelength. TD-DFT/B3LYP/cc-pVDZ method predicts one intense electronic transition at 4.9826 eV (248.83 nm) with an oscillator strength f = 0.0352, in good agreement with the measured experimental data in water (exp = 247 nm) shown in Fig. 6. This electronic absorption corresponds to the transition from the ground to the first excited state and is mainly described by one electron excitation from HOMO to LUMO. The HOMO–LUMO energy gap of Isoleucine was calculated at the B3LYP/cc-pVDZ level and their values shown below reveals that the energy gap reflect the chemical activity of the molecule. The HOMO is located over the OH and CH3 groups; the transition between HOMO and LUMO implies an electron density transfer from NH2 to OH and CC groups. Moreover, these orbital significantly overlap in their position for Isoleucine. The atomic orbital compositions of the frontier molecular orbitals are sketched in HOMO–LUMO diagram.

Molecular electrostatic potential (ESP) at a point in the space around a molecule gives an indication of the net electrostatic effect produced at that point by the total charge distribution (electron + nuclei) of the molecule and correlates with dipole moments, electro negativity, partial charges and chemical reactivity of the molecule. It provides a visual method to understand the relative polarity of the molecule. An electron density is surface mapped with electrostatic potential surface depicts the size, shape, charge density and reactive sites of the molecules. The different values of the electrostatic potential represented by different colors; red represents the regions of the most negative electrostatic potential, blue represents the regions of the most positive electrostatic potential and green represents the region of zero potential. Potential increases in the order red < orange < yellow < green < blue. Such mapped electrostatic potential surfaces have been plotted of title molecule in B3LYP/cc-pVDZ basis set using the computer software Gauss view. Projections of these surfaces along the molecular plane and a perpendicular plane are given in Fig. 7. This figure provides a visual representation of the chemically active sites and comparative reactivity of atoms.

Table 5 Experimental and calculated absorption wavelength (k), excitation (E), oscillator strength (f) and frontier orbital energies of Isoleucine by TD-DFT method. k (cal) Excitation Oscillator Assignment EHomo k energies, E strengths (exp) nm (eV) (f) (eV) nm 298 247 203

317.61 3.9037 284.57 4.3569 248.83 4.9826

0.0001 0.0035 0.0352

n ? p* p ? p* p ? p*

ELumo (eV)

Energy gap

6.1977 0.8490 5.3487

Fig. 6. UV–visible spectrum of Isoleucine.

13

C NMR chemical shift assignment

The 13C and 1H NMR spectrum theoretically with the aid of Gaussian and facio software program is shown in Figs. 8 and 9. Table 6 presents the predicted chemical shift values of Isoleucine obtained by the DFT, Experimental and ChemDraw Ultra 8.0 software package along with the shielding values. In general, highly shielded electrons, appear at downfield (lower chemical shift) and vice versa. The predicted chemical shift values by the ChemDraw Ultra software program are in fairly agreement with experimental [26] and theoretical values. The carbon atom C1 appearing at very higher chemical shift value (173.1 ppm) due to double bond of oxygen atom and hence the shielding is very small and appears up field (see Table 6). In DFT-calculated atomic charges revealed that the more electron-rich atoms are C2, C3, C4, C5, and C8 they are highly shielded atoms and appear at downfield (lower chemical shift). In this study, a good correlation between atomic charges and chemical shift was made. Table 6 gives the 1H NMR predicted chemical shift values obtained by the DFT methods, experimental and ChemDraw Ultra 10.0 software. The predicted shielding values for each atom in the Isoleucine molecule by DFT-B3LYP is given in Table 6. The

Fig. 7. The contour map of molecular electrostatic potential surface of Isoleucine.

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P.P. Moorthi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 365–374 Table 6 The calculated

Fig. 8. Theoretical 13C NMR spectrum of with the aid of Gaussian and facio software program.

13

C and 1H NMR chemical shifts of Isoleucine.

Atom position

B3LYP/cc-pVDZ Absolute shielding

Chemical shift

C1 C2 C3 C4 C5 C8 H10 H11 H12 H13 H14 H15 H16 H17 H18 H19 H20 H21 H22

26.9 130.4 134.5 145.5 168.2 153.8 27.2 29.7 29.7 29.9 29.9 29.9 30.0 25.8 29.7 25.9 26.4 30.8 29.9

173.1 69.6 65.5 54.5 31.8 46.2 5.4 2.9 2.9 2.7 2.7 2.7 2.6 6.8 2.9 6.7 6.2 1.8 2.7

ChemDraw Ultra

Exp

174.7 59.1 36.0 25.1 11.3 15.0 5.1 1.6 1.6 1.8 1.8 1.8 1.1 11.0 1.8 11.0 4.3 0.9 1.1

176.8 62.4 38.6 27.1 17.4 13.8 3.6 1.9 1.9 1.9 1.9 1.9 1.9 3.7 1.9 2.0 2.0 1.9 1.9

theoretical calculations, Isoleucine has a non-planar structure of C1 point group symmetry. The molecule has 22 atoms and 60 normal modes of vibration active in both IR and Raman. It is convenient to discuss the vibrational spectra of Isoleucine molecule in terms of spectral region as described below: C–H vibrations The theoretically calculated vibrations corresponding to C–H stretching show good agreement with the experimentally observed vibrations at 2966 cm1 in FT-IR spectrum and 2991 cm1 in FTRaman spectrum of Isoleucine. The different trend reflects in the Isoleucine molecule with slightly changes in the theoretical values as well as in the experimental frequencies. The bands are appeared in very weak and medium intensities. Substitution sensitive C–H in plane bending vibrations observed in the region 1417–1421 cm1 [28,29]. In Isoleucine very weak FT-IR band at 1417 cm1 and FT-Raman band at 1421 cm1 with very weak intensity are assigned to C–H in-plane bending vibrations. Bands involving the C–H out-of-plane bending vibrations appear in the range 1000–675 cm1 [28]. The FT-Raman bands are at 1327, 1309 and 991 cm1 in Isoleucine. The bands are observed with very weak, medium and very strong intensities. Fig. 9. Theoretical 1H NMR spectrum of Isoleucine with the aid of Gaussian and facio software program.

predicted chemical shift values by the DFT theoretical methods slightly deviates from the ChemDraw Ultra values and experimental values. The spectrum of Isoleucine molecule showed a singlet at 5.4 ppm for the proton (H10) group, which is in good agreement with experimental [27] and ChemDraw Ultra value. There is no higher absolute shielding for our compound. DFT method predict that all the hydrogen atoms (H10 to H22) absolute shielding values is almost 30. In Chemical shift values, the ChemDraw Ultra and experimental values are fairly agrees but slightly deviates with theoretical values. Vibrational assignments The observed and calculated vibrational frequencies along with assignments have been summarized in Table 7. The experimental FT-IR, calculated (B3LYP, MP2) and FT-Raman spectra for Isoleucine are shown in Figs. 10 and 11. According to the

O–H vibrations The very weak FT-IR band at 3770 cm1 is assigned to the O–H stretching vibrations. Normally free O–H stretching vibrations appeared around 3600 cm1 for phenol [30]. Since the bands have been downshifted apparently caused by the O–H  O bonding, we look for the second signature; the in-plane O–H bending vibrational frequency is increased by about 30 cm1 [28]. The similar phenomenon has been observed in leucine [31] too, where the appearance of the two bands at 3203 and 3154 cm1 have been attributed to the O–H  O bonding. The strong band observed at 1187 cm1 in the FT-IR spectrum is assigned to O–H in-plane bending vibration. The theoretically computed frequencies for O–H in-plane bending vibration by B3LYP level with cc-pVDZ basis set show excellent agreement with recorded FT-IR spectrum. The observed value at 675 cm1 are assigned to the O–H out-of-plane bending vibration. NH2 vibrations The NH2 group gives rise to the six internal modes of vibrations such as: the symmetric stretching, the anti-symmetric, the

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symmetric deformation (or) the scissoring, the rocking and the wagging. The NH2 group has two (N–H) stretching vibrations, one bending asymmetric and other symmetric. The frequency of asymmetric vibration is higher than that of symmetric one. The absorption bands in the frequency range 3280–3381 cm1 are attributed to the NH2 vibration of the amine group of the organic base [32].

In our title molecule Isoleucine the NH2 symmetric and asymmetric stretching modes are respectively in the range of 3472– 3555 cm1 by B3LYP/cc-pVDZ level. The above assignment is good agreement with literature data [32]. The computed NH2 scissoring is in the range 1631 (DFT) and 1637 cm1 (MP2) using cc-pVDZ basis set. For Isoleucine the rocking mode predicted at 282, 258 and 253 cm1 (B3LYP) and 275, 263 and 235 cm1 (MP2).

Table 7 Observed and theoretical vibrational assignments of Isoleucine. Experimental FTIR

Theoretical FT-Raman

3770

2966

2991 2942

1607 1575 1513 1464

1417 1395

1447 1421 1395

1353 1328 1308

1354 1327 1309

1271 1187 1152

1087 1044 963 920 872

1032 1020 991

852

710 675 537

B3LYP

IR

MP2

IR

3701 3555 3472 3125 3107 3102 3096 3079 3037 3026 3021 2976 2924 1826 1631 1486 1478 1471 1468 1461 1429 1399 1385 1381 1364 1329 1318 1306 1277 1237 1191 1164 1138 1114 1087 1045 1020 972 930 917 853 811 785 719 650 570 520 493 426 384 322 282 258 253 228 205 190 120 49 29

62 1 1 13 44 14 40 12 34 16 35 35 44 224 38 5 6 5 5 0 21 2 12 2 14 11 13 1 0 2 3 220 56 27 5 4 3 5 32 36 12 39 23 53 61 7 22 12 4 1 0 10 21 9 0 0 2 0 0 0

3758 3608 3508 3202 3181 3176 3170 3154 3087 3082 3074 3030 3008 1838 1637 1512 1504 1496 1491 1483 1455 1413 1399 1392 1384 1347 1331 1317 1289 1250 1215 1176 1151 1137 1109 1075 1033 978 969 947 871 821 790 729 654 590 521 495 430 380 319 298 275 263 235 210 201 134 44 31

79 2 1 9 29 26 26 1 35 12 29 34 33 188 37 4 6 5 6 8 36 1 11

10 14 1 0 0 1 235 24 39 7 6 0 22 53 10 11 28 15 50 57 19 25 10 3 1 0 21 17 6 0 0 2 1 0 0

Vib. no

Assignmentsa

v1 v2 v3 v4 v5 v6 v7 v8 v9 v10 v11 v12 v13 v14 v15 v16 v17 v18 v19 v20 v21 v22 v23 v24 v25 v26 v27 v28 v29 v30 v31 v32 v33 v34 v35 v36 v37 v38 v39 v40 v41 v42 v43 v44 v45 v46 v47 v48 v49 v50 v51 v52 v53 v54 v55 v56 v57 v58 v59 v60

tOH(100) tasymNH2(100) tsymNH2(99) tasymCH2(98) tasymCH2(77) tasymCH3(78) tasymCH2(95) tasymCH3(65) tasymCH3(94) tsymCH2(82) tsymCH3(97) tsymCH3(97) tCH(98) tOC(86) dHNH(71) + s(HNCC)2(26)

t – stretching; d – bending; s – torsion. c – out of plane bending, S – scissoring, l – twisting, x – wagging. a

Potential energy distribution (PED), asym – asymmetric, sym – symmetric.

S(HCH)2(59) S(HCH)2(78) + sHCCC(10) d(HCH)2(65) + sHCCC(11) d(HCH)3(60) d(HCH)4(75) dHCN(13) + dHNC(14) + tCC(10) x(HCH)3(79) d(HCH)3(77) d(HCC)2(33) + dCCCH(10) dHCC(18) + sHCCC(17) + dCCCH(12) dHOC(24) + sHCCO(17) + dCCCH(11) d(HCN)2 + dHOC(11) s(HCCC)2(28) + c(CCCH)2(50) l(HCH)2(10) + sHCCC(12) dHNC(11) + sHCCO(17) tCC(14) + dHOC(26) tOC(40) + dHOC(26) dHNC(10) + sHCCC(11) tCC(17) + tNC(10) tNC(35) tCC(57) tCC(12) + sHCCC(18) dHCC(10) + s(HCCC)2(26) t(CC)2(26) + sHCCO(11) sHNCC(29) sHCCC(11) + t(CC)2(30) tCC(11) + sHCCC(11) + cOCOC(12) sHCCC(11) + cOCOC(13) dOCO(19) + dOCOC(16) sHOCC(53) + dOCO(15) tNC(14) + dOCC(11) tCC(17) + dOCO(30) sHOCC(12) + dNCC(15) + dCCC(22) tCC(14) + cCCCC(25) dCCC(10) dOCC(31) + dCCN(24) sHNCC(10) + dNCC(11) s(HNCC)2(39) sHNCC(17) s(HCCC)2(31) + cCCNC(16) dCCC(24) + dCCCC(14) dCCN(13) + dCCC(11) + dNCC(13) + cCCNC(22) sNCCC(53) + sCCCC(41) sNCCC(37) + sCCCC(41) sOCCC(80)

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C@O vibrations Ketones, aldehydes, carboxylic acid, carboxylic esters, lactones, acid halides, anhydrides, amides and lactams show a strong C@O stretching absorption band in the region of 1870–1540 cm1. The C@O stretching of the computed molecule was calculated in the range of 1826 and 1838 cm1 by B3LYP and MP2 respectively.

Fig. 10. Experimental and theoretical spectra of Isoleucine.

CH3 vibrations The CH methyl group stretching vibrations are generally observed in the range of 3000–2800 cm1 [35,36]. For the assignments of CH3 group frequencies one can expect nine fundamentals viz., namely the symmetrical stretching in CH3 (CH3 sym. stretch.), asymmetrical stretching (CH3 asy. stretch.), in-plane CH3 rocking, CH3 wagging and CH3 twisting modes. The recorded FT-IR and FT-Raman spectra of Isoleucine have strong and very weak intensity bands at 2966 and 2991 cm1 and they are assigned to CH3 stretching vibrations of Isoleucine. The symmetric stretching vibration of CH3 observed only in theoretically at 3021 and 3074 cm1 by B3LYP and MP2 respectively. The torsion vibration of CH3 observed only in theoretically at 1471 and 1496 cm1 by B3LYP and MP2 respectively. CH3 wagging observed theoretically at 1399 and 1413 cm1 by B3LYP and MP2 respectively. Which is good agreement with FT-IR (1395 cm1), FT-Raman (1395 cm1) frequencies. Conclusion

Fig. 11. FT-Raman spectra of Isoleucine.

CH2 vibrations The stretching modes of the CH2 group of carboxylic acids were recorded in the region 2950–2860 cm1 [32]. The major coincidence of theoretical values with that of experimental values is found in the symmetric and asymmetric vibration of the methylene (–CH2–) moiety. The CH2 stretching vibration frequency assigned about 3100–2990 cm1 in cyclohexane and other alkenes [33,34]. The CH2 stretching frequency has computed in the range 3026 cm1 (symmetric), 3125, 3096 cm1 (asymmetric) by B3LYP method and 3082 cm1 (symmetric), 3202, 3170 cm1 (asymmetric) by MP2 method. The computed CH2 asymmetric and symmetric vibrations mode show good agreement with literature as well as recorded spectrum. The computed CH2 scissoring modes are assigned in the range of 1486–1478 cm1 and 1512–1504 cm1 by B3LYP and MP2 respectively. This is in agreement with experimental value of 1575–1513 cm1 (FT-IR) CH2 scissoring. The frequency 1271 cm1 (FT-IR) is assigned to CH2 twisting vibration in Isoleucine molecule. The computed frequency range 1277 cm1 (B3LYP) and 1289 cm1 (MP2) are assigned to CH2 twisting. The experimental CH2 twisting value shows good agreement with theoretical values.

MP2, density functional calculations and vibrational spectroscopy have been applied to the investigation of the Isoleucine. The theoretically calculated values of both bond lengths and bond angles of the molecular structure with minimum energy were then compared with computed available data. This investigation shows that a better agreement between the experimental and computed data is obtained using the DFT method B3LYP with the basis set for Isoleucine. The energies of important MO’s, absorption wavelength (kmax), oscillator strength and excitation energies of the compound were also determined from TD-DFT methods and are compared with the experimental values. This study enlightens on the molecular geometry, vibrational wave numbers, 13C and 1H NMR chemical shifts for Isoleucine could be successfully elucidated by the DFT-B3LYP methods using Gaussian program. The electrostatic potential surface (ESP) together with complete analysis of the vibrational spectra, both IR and Raman and electronic spectra help to identify the structural and symmetry properties of the titled molecule. NBO analysis provides an efficient method for studying inter and intra molecular interaction in molecular system. Therefore, this study says beyond doubts that the theoretical calculation of the vibrational wave numbers for Isoleucine is quite useful for determining the vibrational assignment and for predicting new vibrational wave numbers. The calculated normal-mode vibrational wave numbers provide thermodynamic properties by way of statistical mechanics. Finally, the calculated HOMO and LUMO energies show that charge transfer occur in the molecule. References [1] E.N. Baker, T.L. Blundell, J.F. Cutfield, S.M. Cutfield, E.J. Dodson, G.G. Dodson, Crowfoot D.M. Hodgkin, R.E. Hubbard, N.W. Isaacs, C.D. Reynolds, Philos. Trans. R. Soc. Lond. 319 (1988) 369–456. [2] F. Jahoor, R.E. Shangraw, Miyoshi, H. Wallfish, N. Herndon, R. Wolfen, Am. J. Physiol. 257 (1989) 323–331. [3] E. Blomstrand, J. Eliasson, H.K.R. Karlsson, R.J. Kohnke, Nutrition 136 (2006) 269S–273S. [4] H.K.R. Karlsson, P.A. Nilsson, J. Nilsson, A.V. Chibalin, J.R. Zierath, E. Blomstrand, Am. J. Physiol. Endocrinol. METAB 287 (2004) E1–E7. [5] A. Lesarri, R. Sanchez, E.J. Cocinero, J.C. Lopez, J.L. Alonso, J. Am. Chem. soc. 127 (2005) 12952–12956.

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[6] C.K. Chu, J.W. Beach, L.S. Jeong, B.G. Choi, F.I. Comer, A.J. Alves, R.F. Schinazi, J. Org. Chem. 56 (1991) 6500–6503. [7] C. James, G.R. Pettit, O.PF. Nielsen, V.S. Jayakumar, I.H. Joe, Spectrochim. Acta A 70 (2008) 1208–1216. [8] C. James, V.S. Jayakumar, I.H. Joe, J. Mol. Struct. 784 (2006) 32–46. [9] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian 09, Revision A.O2, Gaussion Inc., Wallingford, CT, 2009. [10] A. Saiardi, H. Erdjument-Bromage, A.M. Snowman, P. Tempst, S.H. Snyder, Curr. Biol. 9 (1999) 1323–1326. [11] S.M. Voglmaier, M.E. Bembenek, A.E. Kaplin, G. Dorman, J.D. Olszewski, G.D. Prestwich, S.H. Snyder, Proc. Natl. Acad. Sci. USA 93 (1996) 4305–4310. [12] A. Saiardi, E. Nagata, H.R. Luo, A.M. Snowman, S.H. Snyder, J. Biol. Chem. 276 (2001) 39179–39185. [13] M.J. Schell, A.J. Letcher, C.A. Brearley, J. Biber, H. Murer, R.F. Irvine, FEBS Lett. 461 (1999) 169–172. [14] M.H. Jamroz, Vibrational Energy Distribution Analysis: VEDA 4 Program, Warsam, Poland, 2004. [15] K. Torii, Y. Iitaka, Acta Cryst. B27 (1971) 2237–2246. [16] A.C. Dros, R.W.J. Zijlstra, P.T. Van Duijren, A.L. Spek, H. Kooijman, R.M. Kellogga, Tetrahedron 54 (1998) 7787–7812. [17] F. Takusagawa, A. Shimada, Acta Crystallogr. 3213 (1976) 1925–1930. [18] A. Kutoglu, C. Scheringer, Acta Crystallogr. Sect. C (Cryst. Struct. Comm.) 39 (1983) 232–234. [19] M. Kandasamy, G. Velraj, S. Kalaichelvan, Spectrochim. Acta 105A (2013) 176– 183. [20] G. Gece, Corros. Sci. 50 (2008) 2981–2992.

[21] K. Fukui, Theory of Orientation and Stereoselection, Springer-Verlag, Berlin, 1975. [22] K. Fukui, Science 218 (1987) 747–754. [23] D.F.V. Lewis, C. Ioannides, D.V. Parke, Xenobiotica 24 (1994) 401–408. [24] J. BevanOtt, J. Boerio-Goates, Calculations from Statistical Thermodynamics, Academic Press, 2000. [25] D. Sajan, L. Josepha, N. Vijayan, M. Karabacak, Spectrochim. Acta A 81 (2011) 85–98. [26] http://www.ymdb.ca/spectras/1093. [27] http://www.ymdb.ca/spectras/1095. [28] G. Socrates, Infrared Characteristic Group Frequencies, John Wiley and Sons, Middlesser, 1980. [29] V. Balachandran, K. Parimala, J. Mol. Struct. 1007 (2012) 136–145. [30] H. Lampert, W. Mikenda, A. Karpten, J. Phys. Chem. A 101 (1997) 2254– 2263. [31] S. Dokmaisrijan, V. Sanghiran Lee, P. Nimmanpipug, J. Mol. Struct. THEOCHEM 953 (2010) 28–38. [32] I. Matulkova, I. Nermec, K. Teubuer, P. Nemec, Z. Micka, J. Mol. Struct. 873 (2008) 46–60. [33] K.B. Wiberg, A. Shrake, Spectrochim. Acta A 29 (1973) 583–594. [34] L.J. Bellamy, The Infrared Spectra of Complex Molecule, Chapman & Hall, London, 1975 (Chapters 2, 5, 12). [35] F.R. Dollish, W.G. Fateley, F.F. Bentely, Characteristic Raman Frequencies on Organic Compounds, John Wiley, New York, 1997. [36] R.M. Silverstein, G. Clayton Bassler, T.C. Morril, Spectroscopic Identification of Organic Compounds, John Wiley, New York, 1991.