Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 412–422

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Quantum chemical density functional theory studies on the molecular structure and vibrational spectra of mannitol P.P. Moorthi a,d, S. Gunasekaran b, S. Swaminathan c, G.R. Ramkumaar d,⇑ a

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 c Department of Textile Technology, Anna University, Chennai 600025, TN, India d Department of Physics, C. Kandaswami Naidu College for Men in Anna Nagar East, Chennai 600102, TN, India b

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–Vis spectra

of mannitol was examined.  The optimized geometry and

vibrational wavenumbers were computed using ab initio and DFT(B3LYP) methods.  Vibrational assignment made by TED calculation by VEDA program.  Natural atomic analysis explained the intramolecular hydrogen bonding.  First hyperpolarizability and HOMO, LUMO energy gap were theoretically predicted.

a r t i c l e

i n f o

Article history: Received 29 June 2014 Received in revised form 17 August 2014 Accepted 23 August 2014 Available online 30 August 2014 Keywords: FT-IR DFT Vibrational analysis MEP surface

a b s t r a c t A collective experimental and theoretical study was conducted on the molecular structure and vibrational spectra of mannitol. The FT-IR and FT-Raman spectra of mannitol were recorded in the solid phase. The molecular geometry, vibrational frequencies, thermodynamic functions and atomic charges of mannitol in the ground state have been calculated by using the ab initio HF (Hartree–Fock) and density functional methods (B3LYP) invoking cc-pVDZ basis set. The complete vibrational assignments were performed on the basis of Total Energy Distribution (TED) of the vibrational modes. The UV absorption spectra of the title compound dissolved in water. Natural bond orbital analysis has been carried out to explain the charge transfer or delocalization of charge due to the intra-molecular interactions. The 1H and 13C nuclear magnetic resonance (NMR) chemical shifts of the molecule were calculated by GIAO methods. The first order hyperpolarizability (b0) of this novel molecular system and related properties (b, a0 and Da) of mannitol are calculated using B3LYP/cc-pVDZ and HF/cc-pVDZ methods on the finitefield approach. By using TD-DFT calculation, electronic absorption spectra of the title compound have been predicted and a good agreement with experimental one is established. In addition, the molecular electrostatic potential (MEP) have been investigated using theoretical calculations, the calculated HOMO and LUMO energies shows that the charge transfer within the molecule. Ó 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 9884351008. E-mail address: [email protected] (G.R. Ramkumaar). http://dx.doi.org/10.1016/j.saa.2014.08.066 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

P.P. Moorthi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 412–422

Introduction Mannitol, chemically (2R,3R,4R,5R)-Hexan-1,2,3,4,5,6-hexol [C6H14O6], is a polyol (sugar alcohol) considered as an important sweetener which is naturally found in variety of plants, fungi, marine algae and fresh mushrooms [1]. Polyols have low caloric values and have the ability to metabolize with nominal increase in the blood sugar which makes them important for food as well as pharmaceutical industry. D-Mannitol is white crystalline powder with granular form soluble in water, functionally used as sweetener, humectants, texturizer, stabilizer and bulking agent in food and has wide range of applications in pharmaceutical industries as inert osmotic control substance and as a facilitator for the transportation of medicines [2,3]. It is diuretic, used to force urine production in people with acute kidney failure. Increase the urine production and prevent the kidney from shutting down, and also speeds up elimination of certain toxic substances in the body. Mannitol reduces the swelling and pressure inside the eye around the brain. It is an osmotic agent that causes a dehydrating effect on the airway epithelium leading to release of inflammatory mediators from inflammatory cells in the bronchial mucosa, and ultimately leading to airway smooth muscle contraction [4]. Mannitol has been promoted as a renal protective agent in patients at high risk of developing renal failure, such as those undergoing cardiac and vascular surgery, renal transplantation, and in patients with jaundice and rhabdomyolysis. It also acts as a free-radical scavenger and reduces the harmful effects of free radicals during ischemia–reperfusion injury. Smith and associates [5] showed that mannitol clearance closely reflected glomerular filtration rate in man, there has been clinical attention in mannitol. With normal kidney function, after a single intravenous dose, the half-life of mannitol in the circulating plasma is 15 min [6]. However, the detailed theoretical studies based on DFT methods for mannitol have not been reported so far. This work deals with spectroscopic characterization and DFT studies of mannitol. Experimental details The compound under the investigation namely (2R,3R,4R,5R)Hexan-1,2,3,4,5,6-hexol [mannitol] was procured from the reputed pharmaceutical company, Chennai, Tamil Nadu, INDIA, and which was used without further purification. The FT-IR spectrum of the compound was recorded in the 4000–400 cm1 region in evacuation mode on Bruker IFS 66 V spectrophotometer using KBr pellet technique (solid phase) with 4.0 cm1 resolution. The FT-Raman spectrum of mannitol was recorded on a BRUKER RFS 100/S model interferometer equipped with an FRA-106 FT-Raman accessory in the 4000–400 cm1 stokes region using the 1064 nm line of Nd: YAG laser for excitation, operating at 150 mW powers. The reported wavenumbers are believed to be accurate within ± 4 cm1. The UV–Vis spectral measurements were carried out using a Varian Cary 5E-UVNIR spectrophotometer at Sophisticated Instrumentation Analysis Facility, IIT Madras, India. (1)H and (13)C NMR spectra have been recorded using BRUKER AVANCE III 500 MHZ NMR at SAIF, IIT Madras, India.

mannitol. The calculations of geometrical parameters in the ground state were performed using the Gaussian 09 W program [7]. DFT calculations were carried out with Becke’s three-parameter hybrid model using the Lee–Yang–Parr correlation functional (B3LYP) method. The geometry generated from standard geometrical parameters at (B3LYP) method by adopting split-valence polarized cc-pVDZ basis set. The ‘cc-p’, stands for ‘correlation-consistent polarized’ and the ‘V’ indicates they are valence-only basis sets. They include successively larger shells of polarization (correlating) functions (d, f, g, etc.). More recently these ‘correlation-consistent polarized’ basis sets have become widely used and are the current state of the art for correlated or post-Hartree–Fock calculations. Example of these are: cc-pVDZ – Double-zeta. For that we go with basis set of cc-PVDZ. The optimal geometry was determined by minimizing molecular symmetry constraints. Harmonic vibrational wavenumbers were calculated using analytic second derivatives to confirm the convergence with minima in the potential surface. At the optimized structure of examined species, no imaginary wavenumber modes were obtained, proving that a true minimum on the potential surface was found. The relative intensity of the most intense line appears to be theoretically overvalued by comparison with experimental IR and Raman bands. The vibrational modes were assigned on the basis of TED analysis using VEDA 4 program [8]. The total energy distribution corresponding to each of the observed wavenumbers shows the reliability and accuracy of the spectral analysis. The electronic properties such as HOMO and LUMO energies were determined by time-dependent DFT (TD-DFT) approach, while taking solvent effect into account. Results and discussion Geometrical parameters The bond length between the atoms in mannitol molecule was theoretically calculated by both B3LYP and HF methods. The mannitol molecular structure comprised with 26 atoms and fitted to C1 point group symmetry. Geometrical structure of the title molecule along with numbering of atom scheme was shown in Fig. 1. The optimized geometrical parameters were obtained by HF and B3YLP 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, bond angles and dihedral angles) were compared with available experimental data [9,10]. In the mannitol molecule all C–C and C–H bonds shows bond length 1.5 Å and 1.11 Å respectively on both B3LYP and HF methods, which is due to the attachment of electron withdrawing OH group on all carbon atoms. All C–O bonds were explored same bond length as 1.4 Å on both B3LYP and HF. The oxygen–hydrogen bonds were also provided similar bond

Computational details The combination of spectroscopic methods with DFT calculations are important tools for understanding the fundamental vibrational properties and the electronic structure of the compounds. The DFT–B3LYP correlation functional calculations have been carried out to provide complete information about the structural characteristics and the fundamental vibrational modes of

413

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

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Table 1 Optimized geometrical parameters (bond lengths, bond angles, and dihedral angles) of mannitol. Parameters

Bond angle (deg.) C2–C1–O7 C2–C1–H13 C2–C1–H14 O7–C1–H13 O7–C1–H14 H13–C1–H14 C1–C2–C3 C1–C2–O8 C1–C2–H15 C3–C2–O8 C3–C2–H15 O8–C2–H15 C2–C3–C4 C2–C3–O9 C2–C3–H16 C4–C3–O9 C4–C3–H16 O9–C3–H16 C3–C4–C5 C3–C4–O10 C3–C4–H17 C5–C4–O10 C5–C4–H17 O10–C4–H17 C4–C5–C6 C4–C5–O11 C4–C5–H18 C6–C5–O11 C6–C5–H18 O11–C5–H18 C5–C6–O12 C5–C6–H19 C5–C6–H20 O12–C6–H19 O12–C6–H20 H19–C6–H20 C1–O7–H21 C2–O8–H22 C3–O9–H23

Parameters

Mannitol B3LYP/cc-pVDZ

Bond length (Å) C1–C2 C1–O7 C1–H13 C1–H14 C2–C3 C2–O8 C2–H15 C3–C4 C3–O9 C3–H16 C4–C5 C4–O10 C4–H17 C5–C6 C5–O11 C5–H18 C6–O12 C6–H19 C6–H20 O7–H21 O8–H22 O9–H23 O10–H24 O11–H16 O11–H25 O12–H26

Table 1 (continued)

HF/cc-pVDZ

1.52 1.42 1.10 1.10 1.54 1.41 1.11 1.54 1.42 1.10 1.53 1.44 1.10 1.52 1.43 1.10 1.42 1.10 1.10 0.96 0.97 0.97 0.96 0.67 0.97 0.96

1.52 1.41 1.09 1.09 1.53 1.39 1.09 1.53 1.41 1.09 1.52 1.40 1.09 1.52 1.41 1.09 1.40 1.08 1.09 0.94 0.94 0.94 0.94 0.46 0.94 0.94

109.25 107.41 108.34 111.04 111.58 109.07 110.66 107.64 106.54 112.32 107.95 111.56 106.00 106.96 107.21 107.52 108.30 110.83 116.06 104.19 108.67 110.15 107.65 110.02 113.44 107.36 110.00 108.05 108.08 109.83 105.14 110.20 109.71 111.89 110.81 109.00 107.42 107.42 104.58

109.86 107.46 109.71 110.98 109.99 108.76 111.79 107.93 106.57 110.59 109.60 110.25 115.87 111.12 107.24 103.92 108.69 109.89 115.94 108.66 108.08 106.07 107.84 110.16 111.79 108.61 109.60 108.99 108.08 109.72 105.90 110.14 109.70 111.54 110.65 108.85 109.69 107.57 107.53

Exp. 1.512 1.421 1.033 0.927 1.519 1.423 1.017 1.525 1.432 0.972 1.517 1.427 0.971 1.522 1.431 0.906 1.415 0.970 1.074 0.927 0.920 0.837 0.825 0.675 0.873 0.778 111.6 111.7 113.5 112.1 108.7 98.5 113.0 110.6 114.8 109.7 111.5 95.9 113.4 109.3 106.6 110.3 110.9 106.0 113.4 109.5 102.5 107.2 111.0 113.3 113.6 109.6 111.3 110.2 101.9 110.0 111.1 108.8 106.5 113.1 110.2 106.9 103.8 107.7 104.5

C4–O10–H24 C5–O11–H25 C6–O12–H26

Mannitol B3LYP/cc-pVDZ

HF/cc-pVDZ

Exp.

108.21 104.81 108.81

106.94 107.22 109.92

113.4 111.0 101.9

Dihedral angle (deg.)

B3LYP/cc-pVDZ

HF/cc-pVDZ

O7–C1–C2–C3 O7–C1–C2–O8 O7–C1–C2–H15 H13–C1–C2–C3 H13–C1–C2–O8 H13–C1–C2–H15 H14–C1–C2–C3 H14–C1–C2–O8 H14–C1–C2–H15 C2–C1–O7–H21 H13–C1–O7–H21 H14–C1–O7–H21 C1–C2–C3–C4 C1–C2–C3–O9 C1–C2–C3–H16 O8–C2–C3–C4 O8–C2–C3–O9 O8–C2–C3–H16 H15–C2–C3–C4 H15–C2–C3–O9 H15–C2–C3–H16 C1–C2–O8–H22 C3–C2–O8–H22 H15–C2–O8–H22 C2–C3–C4–C5 C2–C3–C4–O10 C2–C3–C4–H17 O9–C3–C4–C5 O9–C3–C4–O10 O9–C3–C4–H17 H16–C3–C4–C5 H16–C3–C4–O10 H16–C3–C4–H17 C2–C3–O9–H23 C4–C3–O9–H23 H16–C3–O9–H23 C3–C4–C5–C6 C3–C4–C5–O11 C3–C4–C5–H18 O10–C4–C5–C6 O10–C4–C5–O11 O10–C4–C5–H18 H17–C4–C5–C6 H17–C4–C5–O11 H17–C4–C5–H18 C3–C4–O10–H24 C5–C4–O10–H24 H17–C4–O10–H24 C4–C5–C6–O12 C4–C5–C6–H19 C4–C5–C6–H20 O11–C5–C6–O12 O11–C5–C6–H19 O11–C5–C6–H20 H18–C5–C6–O12 H18–C5–C6–H19 H18–C5–C6–H20 C4–C5–O11–H25 C6–C5–O11–H25 H18–C5–O11–H25 C5–C6–O12–H26 H19–C6–O12–H26 H20–C6–O12–H26

57.65 65.43 174.76 178.20 55.11 64.68 64.09 172.80 53.01 161.16 80.53 41.38 174.09 54.15 64.77 65.57 174.48 55.56 57.85 62.08 178.98 167.73 70.19 51.17 64.02 174.69 57.41 176.34 55.05 62.22 56.52 64.76 177.96 173.02 47.77 70.42 156.29 84.39 35.09 38.24 157.55 82.95 81.72 37.58 157.07 144.78 90.07 28.45 171.55 67.70 52.33 52.63 173.38 66.57 66.17 54.56 174.60 166.02 43.31 74.37 169.10 71.26 50.62

41.48 80.35 161.22 162.34 40.50 77.91 79.55 158.60 40.18 162.98 44.27 76.13 156.57 38.27 81.85 83.11 158.57 38.45 38.64 79.66 160.21 162.88 74.52 46.85 65.33 175.36 55.82 172.46 53.15 66.38 55.46 63.83 176.62 71.75 162.94 46.78 168.25 71.43 48.43 47.54 167.85 72.27 70.46 49.84 169.71 43.11 168.42 75.11 171.52 67.67 52.41 51.78 172.52 67.67 67.42 53.31 173.12 163.00 40.98 77.19 175.38 64.78 56.56

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distance of 0.97 Å in the both methods. The bond angles of C–C–C were observed in the range of 110.6–113.44 on both B3LYP and HF methods. C–C–H provided similar bond angle values in both methods with the range of 107.6–111.0. C–O–C shows low bond angle values, which is due to the electro negative oxygen containing lone pair electrons. The higher bond angle values were obtained for O–C–H, which was may be some satiric repulsion of atoms in the molecule. All other bond lengths fall within the expected ranges. The variation in bond angle depends on the electro negativity of the central atom, the presence of lone pair of electrons and the conjugation of the double bonds. If the electro negativity of the central atom decreases, the bond angle decreases. Further the results of our calculations, the experimental and calculated geometric parameters agree well with remaining geometrical parameters. The small deviations observed are probably due to the intermolecular interactions in the crystalline state of the molecule. The comparative graphs of bond lengths, bond angles and dihedral angles of title molecule are presented in Figs. S1–S3 (Supporting materials) respectively. NBO analysis Natural bond orbital analysis originated as a technique for studying hybridization and covalency properties in polyatomic wave functions, based on local block Eigen vectors of the oneparticle density matrix. NBOs would correspond to the picture of localized bonds and lone pairs as basic units of molecular structure. The electron density of atoms in the mannitol ((2R,3R,4R,5R)-Hexan-1,2,3,4,5,6-hexol) molecule evaluated by natural population analysis at the B3LYP/cc-pVDZ level of theory were shown in Table 2. The more substituted terminal carbon atoms C1 and C6 having slightly negative charge than that of other carbon atoms, which are having low electron crowd on itself. Generally the oxygen atoms are electron rich in nature and itself having lone pair electrons, which are exhibited higher values of negative charge by both B3LYP and HF methods. Normally the H atoms having positive charge, moreover the H atoms connected

Table 2 Natural bond analysis of mannitol by B3LYP, HF methods with cc-pVDZ basis set. Atoms

C1 C2 C3 C4 C5 C6 O7 O8 O9 O10 O11 O12 H13 H14 H15 H16 H17 H18 H19 H20 H21 H22 H23 H24 H25 H26

Mannitol B3LYP/cc-pVDZ

HF/cc-pVDZ

0.06473 0.10873 0.06687 0.09948 0.07906 0.05652 0.74640 0.76193 0.75295 0.76153 0.77579 0.76583 0.18194 0.19498 0.18139 0.25000 0.17840 0.21591 0.17033 0.17459 0.45641 0.45911 0.46750 0.46468 0.47263 0.46365

0.01322 0.16853 0.12233 0.15806 0.13651 0.01914 0.79628 0.81363 0.80556 0.81225 0.83039 0.81855 0.16297 0.17847 0.15906 0.24236 0.16001 0.20096 0.15268 0.15931 0.46736 0.46782 0.47837 0.47272 0.48153 0.47525

Fig. 2. Comparison of NBO atomic charges by B3LYP and HF methods for mannitol.

to oxygen atoms were showed highly positive magnitude due to electro negativity of oxygen atoms. The graphical representation of charge density on mannitol molecular structure through NBO analysis was shown in Fig. 2. Thermodynamic properties The thermodynamic quantities such as heat capacity at constant pressure (C0pm), entropy (S0m), enthalpy (DH0m), for various ranges (100–1000 K) of temperatures are determined on the basis of vibrational analysis at B3LYP/cc-pVDZ level and presented in Table 3. All the thermodynamic data were provide supportive information on the mannitol molecular structure for the further more additional study. From Table 3, it can be observed that the thermodynamic parameters are increasing with temperature ranging from 100 to 1000 K, due to the fact that the vibrational intensities of molecule with temperature [11,12]. The following quadratic equations are used to predict approximately the values of heat capacity at constant pressure, entropy and enthalpy changes with temperature. The correlation graphics of those parameters shows in Fig. 3.

C 0pm ¼ 30:71394 þ 0:75146T  3:11788E  4T 2 ðR2 ¼ 0:99975Þ S0m ¼ 237:62937 þ 0:91658T  2:41854E  4T 2 ðR2 ¼ 0:99977Þ

DH0m ¼ 9:64449 þ 0:11082T þ 2:04051E  4T 2 ðR2 ¼ 0:99949Þ All the thermodynamic data’s 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 in thermochemical field. The total energy of a molecule is the sum of translational, rotational, vibrational and electronic energies, i.e., E = Et + Er + Ev + Ee. Table 3 Thermodynamic properties at different temperatures at the B3LYP/cc-pVDZ level for mannitol. T (K)

S0m (cal mol1 K1)

C0pm (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

321.71 414.33 492.09 493.49 566.39 634.83 698.86 758.57 814.25 866.26 915.00

103.85 168.16 225.17 226.24 282.47 331.18 371.02 403.45 430.27 452.86 472.17

6.55 20.29 39.59 40.00 65.48 96.24 131.42 170.20 211.92 256.11 302.39

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rotational constants; rotational temperature and entropy of the molecule calculated by DFT method (B3LYP level) and HF method were presented 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 Evib þ Erot þ Etrans þ Eothers The term Eothers includes nuclear spin energy levels and may also be use later to include the interactions between the first four. Assuming the first three to be independent and neglecting the last term, the molecular partition function (i.e., a sum over the molecular energy states) is given by:

q¼

X

eðEele þEvib þErot þEtrans Þ

=kT

¼

X X X X ebEel ebEvib ebErot ebEtrans el

Fig. 3. Correlation graph between entropy, heat capacity and enthalpy with temperature.

vib

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 qvib qrot qtrans Table 4 Thermodynamical parameters of mannitol. Parameters

B3LYP/cc-pVDZ

HF/cc-pVDZ

Self consistent field energy (a.u.)

688.304

684.454

Zero point vibrational energy (K cal/mol)

136.345

148.670

Rotational constant (GHz)

1.522 0.526 0.431

1.714 0.518 0.422

Rotational temperature (K)

0.073 0.025 0.020

0.082 0.024 0.020

145.214 0.889 0.889 143.437

156.781 0.889 0.889 155.004

Specific heat capacity at constant volume (cal/mol K) Total 51.829 Translational 2.981 Rotational 2.981 Vibrational 45.868

48.135 2.981 2.981 42.174

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

Entropy (cal/mol K) Total Translational Rotational Vibrational

117.586 41.504 31.209 44.873

111.168 41.504 31.127 38.536

4.209 0.474 0.976 4.347

4.627 0.711 1.503 4.917

62.003 67.640 65.650

63.339 68.689 65.396

0.772 7.900 16.966

0.791 7.141 15.328

Dipole moment (Debye)

lx ly lz ltotal Quadrupole moment (Debye-Ang)

lxx lyy lzz Octupole moment (Debye-Ang)

lxxx lyyy lzzz

The statistical thermochemical analysis of mannitol is carried out considering the molecule to be at room temperature and one atmospheric pressure. Thermodynamic parameters such as zero point vibrational energy (ZPVE), thermal energy, specific heat capacity,

The molecular partition q function is written as the product of electronic, vibrational, rotational and partition functions. The largest value of zero point energy of mannitol is 148.670 kcal/mol obtained at HF/cc-pVDZ whereas the smallest value is 136.345 kcal/mol obtained at B3LYP/cc-pVDZ. The dipole moment of the molecule was also calculated by HF and B3LYP method with basis set. 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 HF/cc-pVDZ whereas the smallest one was observed for B3LYP/cc-pVDZ in mannitol. UV–Vis spectral analysis The UV–Vis electronic spectrum of compound in water solvent was recorded within 200–800 nm range is shown in Fig. 4. To support experimental observations, the theoretical electronic excitation energies, absorption wavelengths and oscillator strengths were calculated by the TD-DFT with in GAUSSIAN 09W program. All calculations were performed assuming the title compound was in the liquid phase and with water solvent. The experimental and calculated results of UV–Vis spectral data were compared in Table 5. The experimentally measured UV–Vis data at 196.65 nm, 194.40 nm and 189.51 nm showed good agreement with theoretically computed data at 197.54 nm, 193.30 nm and 185.74 nm respectively, which was obtained by TD-B3LYP/cc-p VDZ method. These excitations corresponds to p–p⁄, p–p⁄ and n ? r⁄ and electronic transitions. The analysis of the wave function indicates that the electron absorption corresponds to the transition from the ground to the first excited state. It is mainly described by an electron excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The HOMO energy characterizes the ability of electron donating, LUMO characterizes the ability of electron accepting, and the gap between HOMO and LUMO characterizes the molecular chemical stability [13]. The HOMO is located over the entire carbon chain and LUMO transudation implies an electron density transfer to the electronegative

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P.P. Moorthi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 412–422 Table 5 Experimental and calculated absorption wavelength (k), excitation (E), oscillator strength (f) and frontier orbital energies of mannitol by DFT method. k (Exp. nm)

k (Cal. nm)

Excitation energies E (eV)

Oscillator strengths (f)

Assignment

EHOMO (eV)

ELUMO (eV)

Energy gap

196.65 194.40 189.51

197.14 193.30 185.74

6.2891 6.4142 6.6752

0.0027 0.0042 0.0006

p ? p⁄ p ? p⁄ n ? r⁄

5.86927

1.14098

7.0102

Fig. 4. UV–Vis spectrum of mannitol.

hydroxyl group from carbon chain. The HOMO and LUMO surfaces are sketched in Fig. 5. According to the B3LYP/cc-pVDZ calculation, the energy gap (DE) between HOMO (5.86927 eV) and LUMO (1.14098 eV) of the molecule is about 7.0102 eV. This energy gap between HOMO and LUMO explains the ultimate charge transfer interactions within the molecule. HOMO and LUMO analysis Numerous organic molecules that containing conjugated p electrons are characterized and analyzed by means of vibrational spectroscopy [14,15]. In most cases, even in the absence of inversion symmetry, the strongest bands in the Raman spectrum are weak in the IR spectrum and vice versa. But the intermolecular charge transfer from the donor to accepter group thorough a

single-double bond conjugated path can induce large variations of both the molecular dipole moment and the molecular polarizability, making IR and Raman activity strong at the same time. The experimental spectroscopic actions described above is very well accounted by ab initio calculations in p conjugated systems that predict remarkably large Raman and infrared intensities for the same normal modes [16]. It is also observed in jot title molecule the bands in FT-IR spectrum have their counterparts in Raman shows that the relative intensities in IR and Raman spectra are comparable resulting from the electron cloud movement through p conjugated frame work from electron donor to electron acceptor groups. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are very significant parameters for quantum chemistry. Using these parameters we can determine the way, in which molecule interacts with other species; hence, they are called the frontiers orbital. HOMO can be thought the outermost orbital containing donor electrons and energy of the HOMO is directly related to the ionization potential. On the other hand LUMO can be thought the innermost orbital containing free places to accept electrons and their energy is directly related to the electron affinity [17]. Energy gap between HOMO and LUMO orbital describe the chemical reactivity and kinetic stability of molecule [18,19]. Recently, the energy gap between HOMO and LUMO has been used to prove the bioactivity from intermolecular charge transfer (ICT) [20,21]. The graphical structures of interaction between HOMO and LUMO in mannitol, with its energy by B3LYP/cc-pVDZ method are shown in Fig. 5. The HOMO and LUMO energy gap of mannitol calculated by HF/cc-pVDZ and B3LYP/ccpVDZ methods are presented in Table 5. According to B3LYP/ccpVDZ calculation, the energy band gap (DE) (translation from HOMO to LUMO) of the molecule is about 7.01025 eV.

HOMO energy ðB3LYP=cc-pVDZÞ ¼ 5:86927 eV LUMO energy ðB3LYP=cc-pVDZÞ ¼ 1:14098 eV HOMO—LUMO energyðB3LYP=cc-pVDZÞ ¼ 7:01025 eV

Fig. 5. HOMO–LUMO of mannitol at B3LYP/cc-pVDZ.

Fig. 6. The contour map of molecular electrostatic potential surface of mannitol at B3LYP/cc-pVDZ.

418 Table 6 The calculated

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13

C and 1H NMR chemical shifts of mannitol.

Atom position

C1 C2 C3 C4 C5 C6 H13 H14 H15 H16 H17 H18 H19 H20 H21 H22 H23 H24 H25 H26

B3LYP/cc-pVDZ

HF/cc-pVDZ

Absolute shielding

Chemical shift

Absolute shielding

Chemical shift

Exp.

128.5 105.4 122.8 105.1 113.4 108.9 27.2 26.7 28.0 25.2 27.7 28.0 26.9 27.5 32.9 31.9 32.0 31.5 31.4 32.6

71.48 94.62 77.20 94.85 86.57 91.10 5.42 5.89 4.65 7.36 4.94 4.64 5.71 5.12 0.28 0.74 0.55 1.08 1.17 0.01

144.6 124.1 140.0 123.0 131.1 127.0 28.1 27.7 28.9 25.9 28.6 28.5 27.9 28.3 32.6 32.1 32.0 31.8 31.7 32.5

55.37 75.91 59.98 77.02 68.91 72.97 4.55 4.91 3.69 6.72 4.02 4.11 4.67 4.31 0.01 0.52 0.64 0.81 0.95 0.07

60.62 75.92 60.76 75.92 69.62 72.78 3.74 3.79 3.66 5.10 3.70 3.66 3.65 3.62 3.22 3.30 3.29 3.15 3.12 3.09

Fig. 7. (a)

13

C NMR spectra of mannitol. (b) 1H NMR spectra of mannitol.

Electrostatic potential In mannitol compound, various hydrogen-bonding interactions are manifested and play an important role in determining stability of the compound. The lone pair electrons, which provide stabilization to the molecular structure, enhance its bioactivity. Hence, it is of importance to study the electrostatic potential distribution of the compound. MESP is a property that the electron and nuclei of a compound create the electrostatic potential surface (ESP) at each point in the surrounding space [22]. Serves as a useful quantity to explain hydrogen bonding, reactivity and structure activity relationship of the compound and correlates with dipole moment, electro negativity, partial charges and site of chemical reactivity of the compound, it provides a visual method to understand the relative polarity of a compound. The ESP is a physical property of a compound related to how a compound is first ‘‘seen’’ or ‘‘felt’’ by another approaching species, a portion of a compound that

has a negative electrostatic potential is susceptible to an electrophonic attack-the more negative the better. The ESP, which is relegated to the electro negativity and the partial charges on the different atoms of the compound, when plotted on the isodensity surface of the compound is termed as MESP. The MESP is an important parameter, and their study leads to a better understanding of complex biological processes involving the charge–dipole, dipole– dipole, and quadrupole–dipole interactions, as seen from Fig. 6, the MESP surface of mannitol depicts a uniform distribution, red1 areas in the MESP map refer to the regions of negative potentials and correspond to the electron-rich and electron-poor regions, respectively, whereas the green color signifies the neutral electrostatic potential. The surface provides necessary information about the reactive sites, the electron total density onto which the electrostatic potential sur1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.

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face has been mapped is shown in Fig. 6. It is visible that the red represents region of the most negative MESP, which indicates the delocalization of p-electrons. NMR chemical shift assignment The 1H and 13C theoretical and experimental chemical shifts, isotropic shielding tensors and the assignments of mannitol are presented in Table 6. The 1H and 13C NMR spectra of mannitol are presented in Fig. 7a and b aliphatic carbons give signals in overlapped areas of the spectrum with chemical shift values from 100 to 150 ppm [23,24]. Each peak identifies a carbon atom in a different environment within the molecule. In mannitol, six different carbon environments are found and the carbon atoms C1, C3, C5 were showed more shielding than the carbon atoms C2, C4, C6 due to (–I) effect of OH group. The applied magnetic field experienced by the carbon nuclei is affected by the electro negatively of the atoms attached to them. The chemical shift value of mannitol was observed in the range of 71–94 ppm and 55–77 ppm on both B3LYP and HF respectively. While C2 and C5 has been attached with C1H14H13, C6H19H20 (CH2) group respectively which having same value of 5 ppm. The highest chemical shift values 94.62, 94.85 and 91.10 ppm were obtained in B3LYP, which are associated with C2, C4, and C6 respectively and also in HF method the highest chemical shift values associated with C2, C4, and C6 were 75.91, 77.02 and 72.97 ppm. Table 6 gives the 1H NMR predicted theoretical chemical shift values obtained by the HF, B3LYP methods and experimental values. The predicted shielding values for each atom in the mannitol molecule by DFT–B3LYP are also given in Table 6. The predicted chemical shift values by the theoretical methods slights deviates from the experimental values. The Fig. 7b shows 1H NMR spectrum of mannitol. From the figure we can assign that a singlet at 5.10 ppm for two hydrogen atoms, triplet at 3.12 ppm for one hydrogen atom and two multiples observed in the range of 3.56– 3.79 and 3.29–3.42 ppm.

Table 7 The B3LYP and HF calculated electric dipole moments (Debye), polarizability (in a.u.), dipole moments compound, b components and btot value of mannitol. Parameters

B3LYP

HF

lx ly lz l axx axy ayy axz ayz azz a0 a Da

4.209 0.474 0.976 4.347 97.744 6.603 79.006 5.033 1.261 83.677 86.809 60.153 120.306 161.274 31.72 66.649 29.878 21.056 39.309 43.819 42.892 42.541 9.492 2.95386  1030

4.627 0.711 1.505 4.917 87.887 5.236 73.037 3.697 0.628 76.877 79.267 54.026 108.052 77.185 29.213 44.942 31.096 21.112 24.228 33.992 22.727 28.934 12.053 1.75500  1030

bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byzz bzzz btot (esu)

anisotropy of the polarizability Da and the mean first hyperpolarizability bo using the x, y, z components they are defined as: 1=2

l ¼ ðl2x þ l2y þ lz2 Þ ao ¼ axxþ ayyþ azz =3 h

a ¼ 21=2 ðaxx  ayy Þ2 þ ðaxx  ayy Þ2 þ ðaxx  ayy Þ2 þ 6a2xx bo ¼ ðb2x þ b2y þ b2z Þ

i1=2

1=2

Hyperpolarizability calculations

bx ¼ bxxx þ bxyy þ bxyz The polarizability a, the hyperpolarizability b and electric dipole moment l of the mannitol are calculated by finite field method using B3LYP/cc-pVDZ basis set available in DFT package. To calculate all the electric dipole moments and the first hyperpolarizabilities for the isolated molecule, the origin of the Cartesian coordinate system (x, y, z) = (0, 0, 0) was chosen at own center of mass of mannitol. The first hyperpolarizability (b0) of this novel molecular system and related properties (b, a0, Da) of mannitol are calculated and it is based on the finite-field approach. In the presence of an applied electric field, the energy of a system is a function of the electric field. First hyperpolarizability is a third rank tensor that can be described by a 3  3  3 matrixes is a tetrahedral. The 26 components of the 3D matrix can be reduced to 10 components due to the Kleinman symmetry [25]. It can be given in the lower tetrahedral format. It is obvious that the lower part of the 3  3  3 matrixes is a tetrahedral. The components of b are defined as the coefficients in the Taylor series expansion of the energy in the external electric field. When the external electric field is weak and homogeneous, this expansion becomes:

E ¼ E0  la F a  1=2aab F a F b  1=6babc F a F b F c þ . . . : where E0 is the energy of the unperturbed molecules, Fa is the field at the origin la , aab and babc is the components of dipole moment, polarizability and the first hyperpolarizability, respectively. The total static dipole moment l, the mean polarizability ao, the

by ¼ byyy þ bxxy þ byzz bz ¼ bzzz þ bxxz þ byyz The first hyperpolarizability of mannitol calculated by DFT/ccpVDZ (2.95386  1030 esu) and HF/cc-pVDZ (1.75500  1030 esu), are shown in Table 7. Vibrational assignments The observed and calculated vibrational frequencies along with assignments have been summarized in Table 8. For visual comparison, the observed and calculated (simulated) FT-IR and FT-Raman spectra of the title compound were presented in Figs. 8 and 9, which is convenient to discuss the vibrational spectra of mannitol molecule in terms of spectral region as described below: C–H vibrations Aliphatic compounds commonly exhibit multiple weak bands in the region 3000–2850 cm1 due to C–H stretching vibrations. The bands due to CH in plane bending vibrations, interacting somewhat with C–C stretching vibrations, are observed as a number of medium-weak intensity sharp bands in the region 1300–1000 cm1 the CH out-of-plane bending vibrations are strongly complied vibration and occur in the region 900–667 cm1 [26]. The CH sym.

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Table 8 Observed and theoretical vibrational assignments of mannitol. Experimental FT-IR

Theoretical FT-Raman

3392 3286

3665 3050

2971 2902

2984 2970 2950

1459

1419

1402 1386

1321 1279

1231

1133 1118

1081

1090

1036 1022 990 930 871 625

897 875 647

567 517 492 416 345

300 260 219 140

82 75

B3LYP

IR

HF

IR

Vib. No.

Assignmentsa

3804 3802 3801 3733 3696 3618 3062 3055 3023 3008 3004 2987 2972 2948 1489 1485 1470 1447 1437 1423 1411 1394 1358 1350 1344 1326 1315 1283 1263 1249 1234 1224 1210 1190 1139 1112 1105 1097 1093 1076 1067 1057 1014 997 942 897 883 719 669 616 561 549 518 487 433 370 344 338 323 300 282 266 231 222 166 145 114 99 82 71 36

26 55 27 54 60 471 42 38 75 3 3 81 43 32 4 31 21 18 18 15 22 1 27 16 0 68 12 21 6 36 20 26 37 24 35 64 19 18 135 27 100 24 69 33 10 9 9 146 1 61 4 113 95 3 9 8 5 16 8 80 36 84 4 2 5 1 30 8 37 25 9

4165 4164 4122 4118 4106 4082 3276 3242 3219 3191 3186 3175 3163 3151 1627 1621 1618 1606 1574 1562 1539 1525 1501 1485 1480 1434 1413 1399 1386 1376 1371 1338 1319 1295 1251 1239 1225 1210 1199 1191 1166 1147 1124 1074 1054 968 944 736 666 643 616 600 543 518 507 472 399 383 362 352 307 289 257 244 220 180 150 133 110 72 51

61 101 38 239 63 404 38 48 64 89 68 11 24 10 0 8 8 5 3 0 51 7 8 71 157 46 2 35 33 36 9 90 92 61 29 25 48 281 28 12 4 35 19 18 54 26 4 13 123 74 33 107 75 138 9 3 6 5 6 5 6 116 70 53 5 7 3 5 1 0 4

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 v55 v56 v57 v58 v59 v60 v61 v62 v63 v64 v65 v66 v67 v68 v69 v70 v71 v72

tOH(100) tOH(96) tOH(96) tOH(100) tOH(96) tOH(97) tOH(87) tasymCH2(98) tCH3(94) tCH2(88) tCH2(97) tsymCH2(96) tsymCH(90) tCH(88) dHCH(63) + s(HCCC)2(25)

t, stretching; d, bending; s, torsion; c, out of plane bending; asym, asymmetric; sym, symmetric. a

Total Energy Distribution (TED).

dHOC(41) dHCH(35) + s(HCCC)2(37) + dHOC(10) dHOC(20)+ dHCC(17) + tCC(11) + sHCCC(11) tCC(11) + d(HOC)2(25) + dHCH(30) dHCH(17) + d(HOC)2(29) + cCCCH(14) d(HOC)2(40) + cCCCH(24) d (HOC)2(40) + cCCCH(24) dHCC(19) + c(CCCH)2(23) dHOC(15) + cCCCH(20) + dHCO(25) dHCC(14) + c(CCCH)2(45) dHCO(20) + cCCCH(22) dHCO(10) + dHOC(10) + dHCC(12) + cCCCH(10) dHCO(49) + sHCCC(13) dHCO(14) + sHCO(10) + c(CCCH)2(24) dHCC(13) + dHOC(11) + cCCCH(14) dHOC(21) + dHCO(37) + sHCCC(10) dHOC(29) + sHCCC(13) dHCC(14) + sHCCC(18) + d(HOC)2(32) dHOC(12) + dHCC(14) tOC(29) + dHOC(10) tCC(24) + tOC(27) tOC(16) + sHCCC(12) t(OC)2(21) tCC(19) + dHOC(12) + t(OC)2(36) t(OC)2(38) tCC(38) tOC(16) + tCC(11) + cOCCC(11) tOC(33) tCC(12) + tOC(13) tCC(15) + tOC(11) + sOCCC(12) + sHCCC(12) + dHCO(10) tCC(23) + tOC(22) tCC(30) sHOCC(82) sHOCC(13) + dOCC(16) dOCC(13) + cOCCC(10) tOC(12) + dOCC(14) + cOCCC(19) sHOCC(78) sHOCC(77) dOCC(15) + cOCCC(18) dOCC(27) + cOCCC(16) d(OCC)3(41) d(OCC)2(54) d(OCC)2(24) + dCCC(11) + cCCNC(14) d(OCC)3(34) + dCC(11) + cOCCC(10) sHOCC(56) dOCC(10) + sHOCC(20) + cOCCC(11) sHOCC(87) dCCC(29) + dOCC(15) + sCCCC(14) dCCC(17) + dOCC(28) d(CCC)2(32) + sOCCC(18) sCCCC(21) sHOCC(17) + sOCCC(48) dCCC(20) + sCCC(11) sHOCC(53) + s(CCCC)2(25) sHOCC(18) + sCCCC(68) s(CCCC)2(18)

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Fig. 8. Experimental and theoretical FT-IR spectra of mannitol.

and asym. vibration frequency has computed in the range 2972 cm1 (symmetric), 3276 cm1 (asymmetric) by B3LYP method and 3163 cm1 (asymmetric) by HF method which is good agreement with FT-IR (2971 cm1) FT-Raman (2970 cm1) (sym), and FT-IR (3286 cm1) FT-Raman 3050 cm1 (asym.) frequencies the theoretically calculated scaled down vibrations corresponding to CH stretching show good agreement with the experimentally observed vibrations at 2902 cm1 in FT-IR spectrum and 2950 cm1 in FT-Raman spectrum of mannitol. The strong band observed at 1036 cm1 in the FT-Raman spectrum is assigned to CH-in-plane bending vibration [27]. The theoretically computed frequencies for CH in-plane bending vibration by B3LYP level with ccpVDZ basis set show excellent agreement with recorded FT-Raman spectrum. The medium band observed at 1321 cm1 in FT-Raman spectrum is assigned to CH-out-of-plane bending vibration. The theoretically computed frequencies for CH out of plane bending vibration by B3LYP level with cc-pVDZ basis set show excellent agreement with recorded FT-Raman spectrum. O–H vibrations

421

Fig. 9. Experimental and theoretical FT-Raman spectra of mannitol.

to be depolarized [28]. The asymmetric CH2 stretching vibrations are generally observed below 3000 cm1, which the symmetric stretch will appear between 3000 and 2700 cm1 [29–31]. In this molecule mannitol, the asymmetric and symmetric stretching vibrations are observed at 3050 cm1and 2989 cm1 FT-Raman spectrum only for mannitol. They are very pure modes since their TED contributions are above 97%. The computed CH2 scissoring is in the range 1489 and 1470 cm1 (B3LYP) and 1627 and 1618 cm1 (HF) using cc-pVDZ basis set. CH2 wagging observed theoretically at 1411 and 1539 cm1 by B3LYP and HF respectively, which is good agreement with FT-IR (1419 cm1), FT-Raman (1402 cm1) frequencies. CH2 Rocking observed theoretical at 883 cm1and 968 cm1 by B3LYP and HF respectively, which is good agreement with FT-IR (871 cm1) FT-Raman (875 cm1) frequencies. The computed CH2 twisting modes are assigned in the range of 1358–1283 cm1 and 1501–1399 cm1 by B3LYP and HF respectively. This is in agreement with experimental value of 1279 cm1 (FT-IR) and 1363 cm1 (FT-Raman) CH2 twisting. CH3 vibrations

The OH group gives rise to three vibrations stretching, in-plane bending and out-of-plane bending. The hydroxyl group exhibit very strong hydrogen bonds due to intermolecular hydrogen bonding. The theoretical wavenumbers of OH stretching were shown in Table 8. The bands observed at 3392 cm1 in FT-IR and 3665 cm1 in FT-Raman spectra are associated with OH stretching. The strong band observed at 517 cm1 and 345 cm1 in the FT-Raman 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-Raman spectrum. The theoretically calculated values at 300 and 282 cm1 are assigned to the O–H out-of-plane bending vibration.

The title molecule under consideration possesses a CH3 group in the side substituted chain. For the assignments of CH3 group frequencies one can expect that nine fundamentals can be associated to each CH3 group. These vibrations are CH3 ss (symmetric stretching), CH3 ips (in-plane stretching), CH3 ipb (in-plane bending), CH3 sb (symmetric bending), CH3 ipr (in-plane rocking), CH3 opr (out-of plane rocking), tCH3 (twist), CH3 ops (out-of-plane), CH3 opb (outof-plane bending) vibrations, respectively. The C–H methyl group stretching vibrations are highly localized and generally observed in the range 3000–2800 cm1 [32,33]. In the present investigation, the bands found at 3219 cm1 in B3LYP/cc-pVDZ and 3023 cm1 in HF/cc-pVDZ are assigned CH3 stretching vibrations.

CH2 vibrations

Conclusion

For the assignment of CH2 group frequencies basically six fundamental can be associated to each CH2 group namely CH2 sym. stretching, CH2 asym. stretching, CH2 scissoring and CH2 rocking which belongs to in-plane vibration and two out-of-plane vibrations viz, CH2 wagging and CH2 twisting modes and are expected

In this present investigation molecular structure, vibrational frequencies, HOMO, LUMO, NBO and polarizability analysis of mannitol have been studied using ab initio HF and DFT (B3LYP/ cc-pVDZ) calculation. The FT-IR, FT-Raman and NMR (1H and 13C) spectral studies were carried out at first time. Any discrepancy

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noted between the observed and calculated frequencies may be due to the fact that the calculations have been actually done on a single molecule in the gaseous state contrary to the experimental values recorded in the presence of inter-molecular interactions. On the basis of the agreement between the calculated and observed results, assignments of fundamental vibrational modes of mannitol were examined and assignments are proposed. This study demonstrates that DFT/B3LYP calculations are powerful approach for understanding the vibrational spectra of medium sized organic compound. The MEP map shows that the negative potential sites are on halogen atoms as well as the positive potential sites are around the hydrogen atoms. To sum up, this study not only shows the way to the identification of the molecules but also helps to researchers for the future studies in both the fundamental researches and applications in technology and industry. The UV spectrum was measured in water solution. Acknowledgements The first author of this manuscript would like to thank Dr. G.R. Ramkumaar and Mr. S. Swaminathan for their technical support. We are also thankful to the Department of Physics, C. Kandaswami College for Men, Anna Nagar East, Chennai 600102 for its support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.08.066. References [1] A. Wakai, I. Roberts, G. Schierhout, Mannitol for acute traumatic brain injury, Cochrane Database Syst. Rev. 24 (1) (2007). [2] E.C. Botez, W.P. Stephens, Crystal structure of anhydrous D-mannitol, Powder Diffr. 18 (3) (2003). [3] P. Lawson, Mannitol, Blackwell Publishing Ltd., 2007. pp. 219–225.

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