Molecular structure, spectroscopic (FTIR, FT-Raman, 13C and 1H NMR, UV), polarizability and first-order hyperpolarizability, HOMO–LUMO analysis of 2,4-difluoroacetophenone

Accepted Manuscript Molecular structure, spectroscopic (FTIR, FT-Raman, 13C and 1H NMR, UV), polarizability and first-order hyperpolarizability, HOMO-LUMO analysis of 2,4-difluoroacetophenone S. Jeyavijayan PII: DOI: Reference:

S1386-1425(14)01427-9 http://dx.doi.org/10.1016/j.saa.2014.09.069 SAA 12745

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

31 July 2014 11 September 2014 18 September 2014

Please cite this article as: S. Jeyavijayan, Molecular structure, spectroscopic (FTIR, FT-Raman, 13C and 1H NMR, UV), polarizability and first-order hyperpolarizability, HOMO-LUMO analysis of 2,4-difluoroacetophenone, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa. 2014.09.069

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Molecular structure, spectroscopic (FTIR, FT-Raman, 13C and 1H NMR, UV), polarizability and first-order hyperpolarizability, HOMO-LUMO analysis of 2,4-difluoroacetophenone S. Jeyavijayana,*

a

Department of Physics, J.J.College of Engineering and Technology, Tiruchirappalli 620 009, India

Abstract The FTIR and FT-Raman spectra of 2,4-difluoroacetophenone (DFAP) have been recorded in the regions 4000-400 cm-1 and 3500-50 cm-1, respectively. Utilizing the observed FTIR and FT-Raman data, a complete vibrational assignment and analysis of the fundamental modes of the compound were carried out. The optimum molecular geometry, harmonic vibrational frequencies, infrared intensities and Raman scattering activities, were calculated by density functional theory (DFT/B3LYP) method with 6-31+G(d,p) and 6-311++G(d,p) basis sets. The difference between the observed and scaled wavenumber values of most of the fundamentals is very small. A detailed interpretation of the infrared and Raman spectra of DFAP is also reported based on total energy distribution (TED). Stability of the molecule arising from hyperconjugative interactions, charge delocalization have been analyzed using natural bond orbital (NBO) analysis. The MEP map shows the negative potential sites are on oxygen atom as well as the positive potential sites are around the hydrogen atoms. The UV-Vis spectral analysis of DFAP has also been done which confirms the charge transfer of DFAP. The chemical shifts of H atoms and C atoms were calculated using NMR analysis. Furthermore, the polarizability, the first hyperpolarizability and total dipole moment of the molecule have been calculated. Keywords: FTIR; FT-Raman; DFT calculations; 2,4-difluoroacetophenone, NMR, UV-Vis *

Corresponding author. Tel.: +91 9944702898. E-mail address: [email protected] (S. Jeyavijayan).

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1. Introduction Acetophenones are compounds that exhibit interesting physicochemical and biological properties. They are found in nature [1,2] and they can also be obtained by means of diverse synthesis procedures [3,4]. Antibacterial activity can be mentioned among its biological properties. A recent study has linked the antibacterial activity of 20 acetophenones with their structural characteristics by using electronic and topological indices [5]. It has also been found that diazinium salts with dihydroxyacetophenone skeleton possess antimicrobial activity [6] and that complexes between p-substituted acetophenone and benzoylhydrazones have antifungal activity [7,8]. On the other hand, substituted acetophenones are employed as synthesis reagents of several organic reactions. Acetophenone is one of the most typical aromatic carbonyl, which shows interesting photochemical

properties

[9,10].

Halogen

combined

acetophenone

like

4-chloro-2-

bromoacetophenone possesses NLO properties [11]. NLO (non-linear optics) has wide applications in the field of telecommunication and optical information storage devices. The organic NLO materials play an important role in frequency mixing, electro-optic modulation, optical parametric oscillation and optical bi-stability. Because of these versatile behaviors of acetophenone, Anbarasu et al., have extensively studied the scaled quantum study and vibrational spectra of 5-fluoro-2-hydroxyacetophenone [12]. Seth et al. investigated the spectroscopic and Xray structure of ortho-hydroxy acetophenones [13]. Pei et al. investigated the Franck–Condon region photo dissociation dynamics of p-nitroacetophenone using resonance Raman spectroscopy and density functional theory calculations [14]. Literature survey reveals that to the best of our knowledge no density functional theory (DFT) with 6-31G+(d,p) and 6-311++G(d,p) basis set calculations of 2,4-difluoroacetophenone (DFAP) have been reported so far. Therefore, an

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attempt has been made in the present work to study the detailed theoretical (DFT) and experimental (FTIR and FT-Raman) investigation of the vibrational spectra of DFAP. In particular, for polyatomic molecules the DFT methods lead to the prediction of more accurate molecular structure and vibrational frequencies than the conventional ab initio Hartree–Fock calculations. In DFT methods, Becke’s three parameter hybrid functional combined with the Lee–Yang–Parr correlation functional (B3LYP) predicts best results for molecular geometry and vibrational wave numbers for moderately larger molecule [15–17]. In this study, molecular geometry, optimized parameters and vibrational frequencies are computed and the performances of the computational methods for hybrid density functional method (B3LYP) at 6-31+G(d,p) and 6-311++G(d,p) basis sets are compared. This method predicts relatively accurate molecular structure and vibrational spectra with moderate computational effort. 2. Experimental details The compound under investigation namely DFAP are purchased from Lancaster Chemical Company, UK which is of spectroscopic grade and hence used for recording the spectra as such without any further purification. The FTIR spectrum of the compound were recorded in the range of 4000–400 cm−1 using a BRUKER IFS-66V FTIR spectrometer equipped with an MCT detector, a KBr beam splitter and a globar source. The spectral resolution is ±1cm−1. The FT-Raman spectrum of the title compound have been recorded in the Stokes region (3500–50 cm−1) on a computer interfaced BRUKER IFS model interferometer equipped with FRA-106 FT-Raman accessory using Nd: YAG laser source operating at 1.064nm excitation wavelength, line width with 200mW power. The frequencies of all sharp bands are accurate to ±1cm−1.

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3. Computational details All calculations were performed using the 3-parameter hybrid functional (B3) for exchange part and the Lee–Yang–Parr (LYP) correlation function [18,19], with 6-31+G(d,p) and 6-311++G(d,p) as the basis sets using the Gaussian 09 suite of program [20]. The basis sets 631+G(d,p) and 6-311++G(d,p) are a triple-split valance basis sets that increases the flexibility of the valence electrons. It is useful for most of the studies involving medium-size system [21]. This study included the calculation of thermodynamic properties of DFAP. DFT calculations were reported to provide excellent vibrational frequencies of organic molecules if the calculated frequencies are scaled to compensate for the approximate treatment of electron correlations, for basis set deficiencies, and for anharmonicity. A number of studies have been carried out regarding the calculations of vibrational spectra using B3LYP method with the 6-31+G(d,p) and 6-311++G(d,p) basis sets. As a result, it was found that the experimental vibrational frequencies and IR intensities could be reported very accurately. The scaling factor was applied successfully to the B3LYP method and found to be easily transferable to a number of molecules [22,23]. Therefore, the calculated wavenumbers were scaled by using scaling factor of 0.9613 for B3LYP method [24]. The vibrational frequencies calculated using the B3LYP function with 6-31+G(d,p) and 6-311++G(d,p) as the basis sets can be utilized to eliminate the uncertainties in the fundamental assignment of the IR spectra. Analyses for the vibrational modes of DFAP were presented in some detail in order to describe the basis for the assignments. All parameters were allowed to relax, and all calculations were converged to an optimized geometry that corresponds to a true energy minimum, as revealed by the lack of imaginary values with wavenumber calculations. Cartesian representation of the theoretical force constants has been computed at the fully optimized geometry by assuming the molecule belongs to Cs point group symmetry.

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Transformation force field from Cartesian to internal local symmetry coordinates, scaling of the subsequent normal coordinate analysis (NCA), and calculation of total energy distribution (TED) were done on a PC with the version V7.0–G77 of the MOLVIB program written by Sundius [25]. 4. Results and discussion 4.1. Molecular geometry The molecular structure of DFAP belongs to Cs point group symmetry and the optimized molecular structure is obtained from GAUSSAN 09W and GAUSSVIEW programs as shown in Fig. 1. The title molecule contains fluorine atom and carbonyl group with benzene ring. The experimental and calculated FTIR and FT-Raman spectra of DFAP are given in Figs. 2-3, respectively. The comparative optimized structural parameters such as bond lengths and bond angles of DFAP are presented in Table 1. From the theoretical values, it is found that most of the optimized bond lengths are slightly larger than the experimental values, due to that the theoretical calculations belong to isolated molecule in gaseous phase while the experimental results belong to molecule in solid state [26]. The calculated geometrical parameters represent a good approximation and they are the bases for calculating other parameters, such as vibrational frequencies and thermodynamics properties. The benzene ring appears little distorted and angles slightly out of perfect hexagonal structure. It is due to the substitutions of the fluorine atom and carbonyl groups in the place of H atoms. According to the experimental values [27], order of the optimized length of the six C–C bonds of the ring are as C5–C6 < C2–C3 < C1–C2 < C1–C6 < C4–C5 < C3–C4. According to the calculated values (B3LYP/6-311++G (d,p)), the order of the bond lengths is slightly differed as C3–C4< C5–C6 < C2–C3 < C4–C5 < C1–C2 < C1–C6, since the substitutions are different. The benzene ring appears to be distorted with C1–C2 and C1–C6

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bond lengths exactly at the substitution place 1.398 and 1.405Å, respectively longer than remaining bonds (~1.385Å to ~1.389Å cal.) in the ring. The C–F bond length indicates a considerable increase when substituted in place of C–H. This has been observed even in benzene derivatives [28,29]. Fluorine atoms are in the plane of the benzene ring. The C–F bond length is found to be 1.342Å expt. [30], 0.011Å smaller than the calculated value (1.353Å) by B3LYP/6311++G (d, p). The optimized bond lengths of C–F are in good agreement with experimental results. According to the density functional calculations, the bond angle C2–C1–C7 is lengthened by 6.120 and the bond angle C6–C1–C7 is shortened by 2.530 from 1200 at the C1 position and this asymmetry of the exocyclic angles reveal the repulsion between the COCH3 group and the benzene ring. The asymmetry of the benzene ring is evident by the order of bond angles C6–C1– C2 < C2–C3–C4 < C4–C5–C6 < C5–C6–C1 < C3–C4–C5 < C1–C2–C3. The bond angle C1– C2–C3 and C3–C4–C5 (123.190 and 122.460 cal. by B3LYP/6-311++G(d,p)) is greater than the typical hexagonal angle of 1200, since the substitution of fluorine atoms. 4.2. Vibrational assignments The DFAP consists of 17 atoms, hence undergoes 45 normal modes of vibrations. In agreement with Cs symmetry, the 45 fundamentals are distributed amongst the symmetry species as Γ3N-6 = 30 A΄ (in-plane) + 15 A΄΄ (out-of-plane) All the fundamental vibrations are active in both Raman scattering and IR absorption. In this study, the full set of 59 standard internal coordinates containing 14 redundancies for DFAP were defined as given in Table 2. From these, a non-redundant set of local symmetry coordinates were constructed by suitable linear combinations of internal coordinates following the recommendation of Pulay et al. [31], and they are presented in Table 3. In order to obtain a more

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complete description of the molecular motion involved in the fundamental modes of DFAP, the normal coordinate analysis were carried out. The detailed vibrational assignment of fundamental modes of DFAP along with the calculated IR and Raman frequencies and normal mode descriptions (characterized by TED) are reported in Table 4. 4.2.1. C-H vibrations Aromatic compounds commonly exhibit multiple weak bands in the region 3100-3000 cm-1 due to aromatic C-H stretching vibrations [32]. Hence, the infrared bands appeared at 3115, 3078 and 3055 cm-1 for DFAP has been assigned to C-H stretching vibrations and these modes are confirmed by their TED values. The bands due to C-H in-plane bending vibrations, interact somewhat with C-C stretching vibrations, are observed as a number of sharp bands in the region 1300-1000 cm-1. The C-H out-of-plane bending vibrations are strongly coupled vibrations and occur in the region 900-667 cm-1. The FT-Raman bands observed at 742, 715 cm-1 and infrared bands at 826, 742 cm-1 are assigned to C–H in-plane bending vibrations of DFAP. The out-ofplane bending vibrations of C-H group have also been identified for DFAP and they are presented in Table 4. The theoretically computed values for C-H vibrational modes by B3LYP/6311++G(d,p) method gives excellent agreement with experimental data. 4.2.2. C=O vibrations The carbonyl group vibrational frequencies are the significant characteristic bands in the vibrational spectra of ketones, and for this reason, such bands have been the subject of extensive studies [33]. The intensity of these bands can increase because of conjugation, therefore, leads to the intensification of the Raman lines as well as the increased infrared band intensities. The carbonyl stretching vibrations in ketones are expected in the region 1680–1715 cm−1. In this case, the band observed at 1695 cm−1 in both IR and Raman spectra is assigned as C=O

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stretching vibration. The vibrational bands observed at 582 cm−1 in IR and 294 cm−1 in Raman are assigned to C=O in-plane and out-of-plane bending vibrations for DFAP, respectively. These vibrational assignments are in line with the literature [34]. 4.2.3. C-C vibrations The C-C aromatic stretching vibrations gives rise to characteristic bands in both the observed IR and Raman spectra [35], covering the spectral range from 1600 to 1400 cm-1. Therefore, the C−C stretching vibrations of DFAP are found at 1618, 1498, 1320, 1112, 1082, 1074, 1025, 994 cm-1 in FTIR and 1615, 1500, 1318, 1115, 1085 cm-1 in the FT-Raman spectrum and these modes are confirmed by their TED values. Most of the ring vibrational modes are affected by the substitutions in the aromatic ring of DFAP. In the present study, the bands observed at 690, 626, 600 cm-1 and 692, 625, 602 cm-1 in the FTIR and Raman spectrum, respectively, have been designated to ring in-plane bending modes by careful consideration of their quantitative descriptions. The ring out-of-plane bending modes of DFAP are also listed in the Table 4. The reductions in the frequencies of these modes are due to the change in force constant and the vibrations of the functional groups present in the molecule. 4.2.4. C-F vibrations Assignments of the C–F stretching modes are very difficult as these vibrations are strongly coupled with the other in-plane bending vibrations of several modes. Normally [36], the observed bands of the C–F stretching vibrations have been found to be very strong in the IR spectra and these appear in the range 1000–1300 cm−1 for several fluoro-benzenes. Also, the C–F stretching vibrations strongly coupled with the C–H in-plane bending vibrations in the mono fluorinated benzene and are observed in the region 1100–1000 cm−1 [37]. Sundaraganesan et al. [38] observed two strong bands at 1279 and 1331 cm−1 in IR and at 1280 and 1332 cm−1 in

9

Raman were assigned to C–F stretching mode for 2,3-difluoro phenol. Karabacak et al. [39] have assigned the strong bands at 1254 and 1222 cm−1 in the IR spectrum due to the C–F stretching modes. Their counterpart in the Raman spectrum is at 1274 and 1245 cm−1 [39]. The present molecule has two fluorine atoms which are placed at ortho and para position of the skeletal ring. The C–F stretching vibrations of DFAP are observed at 1265, 1225 cm−1 in FTIR and 1265, 1223 cm−1 in Raman spectrum. The C–F in-plane bending and out-of-plane bending wavenumbers are assigned at 402 cm−1 and 377 cm−1 in Raman, respectively [39]. In the present case, the bands assigned to C–F in-plane bending are at 450, 415 cm−1 in IR and 452 in Raman spectrum. The frequencies of the C–F out-of-plane bending vibration of DFAP are also listed in Table 4. 4.2.5. CH3 group vibrations The DFAP possess only one CH3 group in the ring. The methyl group stretching vibrations are highly localized and generally observed in the range 3000–2900 cm−1 [40,41]. In the present investigation, the bands with sharp peaks are found at 2994, 2942 cm−1 in IR and 3045, 2998, 2940 cm−1 in Raman for the CH3 stretching vibrations of methyl group. These observations agree well with the earlier work [42,43]. The CH3 in-plane bending vibration is observed at 1430 cm−1 in both IR and Raman. The symmetrical methyl deformation mode (CH3sb) is established at 1365 in IR and 1362 cm-1 in the Raman spectrum for DFAP. The CH3 out-of-plane bending vibration is found at 1150 cm−1. These frequencies are in good agreement with those found in the characteristic group frequency table [44,45]. The theoretically computed value by B3LYP6-311++G (d,p) method for CH3 in-plane bending is nearly coincides with the experimental value. The bands obtained at 975, 864 cm-1 and 972 cm-1 in IR and Raman spectra are assigned to CH3 in-plane and out-of-plane rocking modes, respectively, and they show good agreement with the calculated values.

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5. NBO and NLMO analysis The NBO calculations were performed using NBO 3.1 program as implemented in the Gaussian 09 package at the DFT/B3LYP level in order to understand various second-order interactions between the filled orbitals of one subsystem and vacant orbitals of another subsystem, which is the measure of the delocalization or hyperconjugation. By the use of the second-order bond–antibond (donor–acceptor) NBO energetic analysis, insight in the most important delocalization schemes was obtained. The change in electron density (ED) of (σ*, π*) antibonding orbitals and E(2) energies have been calculated by natural bond orbital (NBO) analysis [46] using DFT method to give clear evidence of stabilization originating from various molecular interactions. NBO analysis has been performed on DFAP in order to elucidate intramolecular hydrogen bonding, intramolecular charge transfer (ICT) interactions and delocalization of π-electrons of the benzene ring. The hyperconjugative interaction energy was deduced from the second-order perturbation approach [47]. For each donor (i) and acceptor (j), the stabilization energy E2 associated with the delocalization i → j is estimated as

F (i, j ) 2 E2 = ∆Eij = qi εi − ε j where qi is the donor orbital occupancy, εi and εj are diagonal elements and F(i,j) is the off diagonal NBO Fock matrix element. The most important interactions between Lewis and nonLewis orbitals with oxygen lone pairs are the second order perturbation energy values, E(2), corresponding to these interactions, and the overlap integral of each orbital pair. The second order perturbation theory analysis of Fock matrix in NBO basis of DFAP (Table 5) also indicates intramolecular interactions due to the orbital overlap of π(C1-C2) and π*(C3-C4), resulting in high

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electron density (approx. 0.371e) of anti-bonding π orbitals (C-C). The intra-molecular charge transfer from O atom of the CO-CH3 group to the C−C bond of benzene ring is also apparent from the MEP plot. The orbital overlap between π(C-C) and π*(C-C) results in intramolecular charge transfer causing stabilization of the system. The charge transfer from π(C3 –C4) to π*(C1– C2) amounts to the stabilization of 24.55 kcal/mol. The magnitude of charge transfer from the lone pairs of O8 to anti-bonding C1–C7 and C7–C9, σ orbitals amount to stabilization of 20.31 and 18.68 kcal/mol, respecticely. The energy contribution of n3(F13) → π*(C1 – C2) and n3(F15) → π*(C3 – C4) values are 18.87 and 19.81 kcal/mol, respectively. This interaction is responsible for a pronounced decrease of the lone pair orbital occupancy 1.92179 than the other occupancy, and there is a possibility for hyperconjugation between fluorine atoms and the benzene ring. The natural localized molecular orbital (NLMO) analysis has been carried out since they show how bonding in a molecule is composed from orbital localized on different atoms. The derivation of NLMOs from NBOs gives direct insight into the nature of the localized molecular orbital’s ‘‘delocalization tails’’ [48]. Table 6 shows significant NLMO’s occupancy, percentage from parent NBO and atomic hybrid contributions of DFAP calculated at B3LYP level using 6311++G(d,p) basis set. The most delocalized NLMO shown in Table 6 has only 81.51% contribution from the localized BD(2) C1–C2 parent NBO. The delocalization tail (~18.5%) consists of the hybrids of C1, C2, C3 and C6. Similarly, the NLMO due to BD(2) C3–C4 and BD(2) C5–C6 parent NBOs are having 18% delocalization tail, which indicates that they are strongly delocalized into the regions of vicinal BD*(2) C1–C2, BD*(2) C3–C4, BD*(2) C5–C6 antibonds. This delocalization can also be observed in the perturbation theory energy analysis given in Table 6. Similarly, the NLMOs of LP(2) O8, LP(3) F13 and LP(3) F15 have almost 5% delocalization tail.

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6. Other molecular properties 6.1. Thermodynamic properties The values of some thermodynamic parameters (such as zero point vibrational energy, thermal energy, specific heat capacity, rotational constants, entropy, and dipole moment) of DFAP by DFT/B3LYP with 6-31+G(d,p) and 6-311++G(d,p) basis sets are listed in Table 7. The global minimum energy obtained for structure optimization of DFAP using DFT/B3LYP with 631+G(d,p) and 6-311++G(d,p) basis sets are -583.40101434 a.u. and -583.53538430 a.u, respectively. The difference in the values calculated by both the methods is only marginal. The same trends have been observed in entropy calculations. All the above observations were made without any symmetric constrains. The rotational constants and specific heat are increasing in values from lower to higher basis sets for B3LYP. The thermal energies are also in same trend with global minimum energy. 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. Direction of the dipole moment vector in a molecule depends on the centers of positive and negative charges. Dipole moments are strictly determined for neutral molecules. For charged systems, its value depends on the choice of origin and molecular orientation. In the present study, the total dipole moment of DFAP determined by B3LYP method using 6-31+G(d,p) and 6-311++G(d,p) basis sets is 1.5910 and 1.5274 Debye, respectively. The variation in zero-point vibrational energies (ZPVEs) seems to be significant. The total energy and the change in the total entropy of the title compound at room temperature are also presented. On the basis of vibrational analysis at B3LYP/6-311++G(d,p) level, the standard statistical thermodynamic functions: heat capacity (C), entropy (S), and enthalpy changes (∆H), for the title compound were obtained from the theoretical harmonic frequencies

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and listed in Table 8. From Table 8, 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 [49]. All the thermodynamic data supply helpful information for the further study on the DFAP. They can be used to compute the other thermodynamic energies according to relationships of thermodynamic functions and estimate directions of chemical reactions according to the second law of thermodynamics in thermochemical field [50]. 6.2. Nonlinear optical effects and first hyperpolarizability NLO is at the forefront of current research because it provides the key functions of frequency shifting, optical modulation, optical switching, optical logic, and optical memory for the emerging technologies in areas such as telecommunications, signal processing, and optical interconnections [51,52]. The quantum chemistry based on the prediction of non-linear optical (NLO) properties of a molecule has an important role for the design of materials in modern communication technology [52]. Especially organic molecules are studied because of their larger NLO susceptibilities arising π-electron cloud movement from donor to acceptor, fast NLO response times, high laser damage thresholds and low dielectric constants. In discussing nonlinear optical properties, the polarization of the molecule by an external radiation field is often approximated as the creation of an induced dipole moment by an external electric field. The first hyperpolarizability (β) of this molecular system is calculated using B3LYP/6311++G(d,p) method, based on the finite field approach. The components of dipole moment, polarizability and the first hyperpolarizability of the title compound can be seen in Table 9. The total static dipole moment µ, the average linear polarizability α , the anisotropy of the

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polarizability ∆α, and the first hyperpolarizability β can be calculated by using the following equations[52]: µ =

(µ

α =

1 (α 3

∆α =

2 x

+ µ 2y + µ 2z

xx

+ α

yy

1/2

)

+ α

zz

)

2 2 1  2 α − α + α − α + ( α zz − α xx ) + 6α 2xx   ( ) ( ) xx yy yy zz      2

1

2

2 2 2 β = ( β xxx + β xyy + β xzz ) + ( β yyy + β xxy + β yzz ) + ( β zzz + β xxz + β yyz )   

1

2

The calculated values of total static dipole moment µ, the average linear polarizability α , the anisotropy of the polarizability ∆α, and the first hyperpolarizability β using the DFTB3LYP/6-311++G(d,p) method are 1.5274 Debye, 14.1167 Å3, 34.5292 Å3 and 4.382×10-30 cm5 e.s.u.-1, respectively. Urea is one of the prototypical molecules used in the study of the NLO properties of molecular systems and frequently used as a threshold value for comparative purposes. The values of µ, α and β obtained by Sun et al.[53] with the B3LYP/6-311++G(d,p) method for urea are 1.373 Debye, 3.831 Å3 and 3.729×10-31 cm5 e.s.u.-1, respectively. The first hyperpolarizability of the title compound is 12 times greater than that of urea. According to the magnitude of the first hyperpolarizability, the title compound may be a potential applicant in the development of NLO materials. Thus, this molecule might serve as a prospective building block for nonlinear optical materials.

6.3. UV-Vis and HOMO - LUMO analysis

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The highest occupied molecular orbitals (HOMOs) and the lowest-lying unoccupied molecular orbitals (LUMOs) are named as frontier molecular orbitals (FMOs). The FMOs play an important role in the electric and optical properties, as well as in UV–vis spectra and chemical reactions [54]. In order to evaluate the energetic behavior of the title compound, calculations in DMSO, chloroform, ethanol and gas phase have been carried out by using TD-DFT/B3LYP/6311++G(d,p). The calculated absorption wavelengths (λ), oscillator strengths (f) and excitation energies (E) are given Table 10. The theoretical ultraviolet spectrum of DFAP in gas and solvent phase is shown in Fig. 4. Due to the Frank–Condon principle, the maximum absorption peak (λmax) in an UV–visible spectrum corresponds to vertical excitation. TD-DFT calculations predict three transitions in the UV–vis region for DFAP molecule. The strong transitions at 5.0840 eV (243.87 nm) with an oscillator strength f = 0.2952 in gas phase, at 5.0747 eV (244.32 nm) with an oscillator strength f = 0.3048 and at 5.0940 eV (243.39 nm) with an oscillator strength f = 0.3028 in DMSO and Chloroform and, respectively, are assigned to a π → π* transition. For ethanol solvent, the strong transitions are found at 5.0841 eV (243.87 nm) with an oscillator strength f = 0.2979. The major contributions of the transitions were designated with the aid of SWizard program [55]. In view of calculated absorption spectra, the maximum absorption wavelength corresponds to the electronic transition from the highest occupied molecular orbital HOMO-1 to lowest unoccupied molecular orbital LUMO with 96 % contribution. The other wavelength, excitation energies, oscillator strength and calculated counterparts with major contributions can be seen in Table 10. The HOMO energy characterizes the ability of electron giving; LUMO characterizes the ability of electron accepting [56]. The energy gap between HOMO and LUMO determines the kinetic stability, chemical reactivity and, optical polarizability and chemical hardness–softness of

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a molecule [57]. The hard molecules are not more polarizable than the soft ones because they need big energy to excitation. This is also used by the frontier electron density for prediction of the most reactive position in π-electron systems and also explains several types of reaction in conjugated system [58]. Surfaces for the frontier orbitals were drawn to understand the bonding scheme of present compound. The energies of four important molecular orbitals of DFAP: the second highest and highest occupied MO’s (HOMO and HOMO-1), the lowest and the second lowest unoccupied MO’s (LUMO and LUMO+1) were calculated using TD-B3LYP/6311++G(d,p) and are presented in Table 11. 3D plots of the HOMO-1, HOMO, LUMO and LUMO+1 orbitals computed at the B3LYP/6-311G++(d,p) level for DFAP (in gas phase) are illustrated in Fig. 5. The LUMO: of π nature, (i.e. benzene ring) is delocalized over the whole CC bond. By contrast, the HOMO is located over O=C−CH3 group; consequently the HOMO→LUMO transition implies an electron density transfer to C−C bond of the benzene ring and fluorine atoms from O=C−CH3 group. Moreover, these three orbitals significantly overlap in their position of the benzene ring for DFAP. The energy gap of HOMO–LUMO explains the eventual charge transfer interaction within the molecule, which influences the biological activity of the molecule. Furthermore, in going from the gas phase to the solvent phase, there is an increasing value of the energy gap and molecule becomes more stable. The frontier orbital gap in case of DFAP is found to be 5.3992, 5.4161, 5.4019, 5.3981 eV for DMSO, Chloroform, Ethanol and gas phase, respectively as shown in Table 11. The decrease in energy gap between HOMO and LUMO facilitates intra molecular charge transfer which makes the material to be NLO active. The electronic properties of the molecule are calculated from the total energies and the Koopmans’ theorem. The ionization potential is determined from the energy difference between

17

the energy of the compound derived from electron-transfer (radical cation) and the respective neutral compound; IP = Ecation - En; IP = -EHOMO while the electron affinity is computed from the energy difference between the neutral molecule and the anion molecule: EA = En - Eanion; EA= ELUMO, respectively. The other important quantities such as electronegativity (χ), hardness (η), softness (ξ), and electrophilicity index (ψ) were deduced from ionization potential and electron affinity values [59-61]. Electronegativity

(χ )µ ≈ −χ =

IP + EA 2

Hardness

(η ) ≈

IP − EA 2

Softness

(ξ ) ≈

1 2η

Electrophilicity index

µ2 (ψ ) ≈ 2η

The values of electro negativity, chemical hardness, softness, and electrophilicity index of DFAP are 4.8209, 2.7080, 0.1846 and 4.2912 eV in Chloroform, respectively as shown in Table 11. According to maximum hardness principle (MHP), the most stable structure should have maximum hardness value which being a minimum energy structure at constant chemical potential and hence the principle of maximum hardness has been proved in the present study. 6.4. Electrostatic potential, total electron density and molecular electrostatic potential The electrostatic potential has been used primarily for predicting sites and relative reactivities towards electrophilic attack, and in studies of biological recognition and hydrogen bonding interactions [62,63]. To predict reactive sites for electrophilic and nucleophilic attack

18

for the investigated molecule, the MEP at the B3LYP/6-311 + +G(d,p) optimized geometry was calculated. In the present study, the electrostatic potential (ESP), electron density (ED) and the molecular electrostatic potential (MEP) map figures for DFAP are shown in Fig. 6. The ED plots for title molecule show a uniform distribution. However, it can be seen from the ESP figures, that while the negative ESP is localized more over the oxygen atom and fluorine atoms and is reflected as a yellowish blob, the positive ESP is localized on the rest of the molecule. This result is expected, because ESP correlates with electro negativity and partial charges. The negative (red and yellow) regions of the MEP are related to electrophilic reactivity and the positive (blue) regions to nucleophilic reactivity, as shown in Fig. 6. As can be seen from the figure and the calculated results, the MEP map shows that the negative potential sites are on electronegative O atom of the CO-CH3 group and the positive potential sites are around the hydrogen atoms of the benzene ring. The different values of the electrostatic potential at the surface are represented by different colours. Potential increases in the order red < orange < yellow < green < blue. The color code of these maps is in the range between -0.04663 a.u. (deepest red) to 0.04663 a.u. (deepest blue) in compound, where blue indicates the strongest attraction and red indicates the strongest repulsion. If compared, the maximum negative potential value is -0.0466105 a.u. for oxygen atom and maximum positive region localized on the H14 atom which has the value of +0.0306122 a.u. From these results, one can say that the H atoms indicate the strongest attraction and O atom indicates the strongest repulsion. These sites give information about the region from where the compound can have intermolecular interactions. Thus, it would be predicted that the DFAP molecule will be the most reactive site for both electrophilic and nucleophilic attack and this was also in support with the literature [64].

19

6.5. Mulliken atomic charges Mulliken atomic charge calculation [65] has an important role in the application of quantum chemical calculation to molecular system, because atomic charges affect dipole moment, polarizability, electronic structure, and much more properties of molecular systems. The total atomic charges of DFAP obtained by Mulliken population analysis with DFT (B3LYP) with 6-31++G (d, p) and 6-311++G (d, p) basis sets were listed in Table 12. The negative values on C2, C4 and C6 atom in the aromatic ring lead to a redistribution of electron density. Due to this strong negative charges, C1, C3 and C5 accommodate higher positive charge and the molecule becomes more acidic. In the benzene ring all the hydrogen atoms have a net positive charge; in particular, the hydrogen atom H14 and H16 have charge of 0.224 and 0.212 respectively, owing to bound with more electronegativity atom of oxygen and fluorine. The better represented graphical form of the results has been done in Fig. 7. 6.6. NMR spectra NMR spectroscopy is currently used for structure and functional determination of biological macromolecules. Chemical shifts are recognized as an imperative part of the information contained in NMR spectra. They are valuable for structural interpretation due to their sensitivity to conformational variations. The combined use of NMR and computer simulation methods offers a powerful way to interpret and predict the structure of large biomolecules [66]. The 1H and

13

C chemical shifts were measured in a less polar (CDCl3), DMSO solvents

and gas phase and are compared with experimental values, which are measured in CDCl3. The experimental and calculated 1H and

13

C NMR values for the title compound are represented in

Table 13. The theoretical calculations was performed by Gauge-Including atomic orbital (GIAO)

20

method [67] using B3LYP/6-311+G(d,p) basis set. The experimental 1H and 13C NMR spectrum of DFAP in CDCl3 is shown in Fig. 8. In

13

C NMR spectrum, different signals were observed,

which is consistent with the structure on the basis of molecular symmetry. Aromatic carbons give signals in overlapped areas of the spectrum with chemical shift values from 100 to 150 ppm [68,69]. The fluorine and carbonyl group are highly electronegative and decreases the electron density at the ring carbon. Therefore, the chemical shifts value of C1 bonded to carbonyl group and C2, C4 bonded to fluorine atoms shows very high value. The experimental peak for C1 appeared at 126.79 ppm and the calculated value of C1 is 126.163, 126.495 and126.382 ppm in gas phase, DMSO and chloroform solvents, respectively. In the present investigation, the experimental chemical shift values of aromatic carbons except C2 and C4 are in the range 116.21–126.79 ppm. The experimental chemical shift of C2 and C4 (161.08 and 160.82 ppm) is greater than the other aromatic carbons because of the substitution of fluorine atoms. The calculated chemical shift of C2 and C4 in gas phase, DMSO and chloroform solvents, matches quite well with the experimental shift as shown in Table 13. The atom C7 is de-shielded due to the presence of electronegative oxygen in the CO-CH3 group. The carbonyl group carbon C7 has peak at 197.032, 201.284, 200.077 ppm in gas phase, DMSO and chloroform solvents with respect to TMS, which matches well with the experimental signal at 194.51 ppm. Besides, due to shielding effect which the non-electronegative property of hydrogen atom, the chemical shift value of C9 atom is lower than the others carbon peak. The studied molecule has six hydrogen atoms, three hydrogen atoms attached to the benzene ring and three hydrogens in the part of CO-CH3 group. 1H atom is mostly localized on periphery of the molecules and their chemical shifts would be more susceptible to intermolecular interactions in the aqueous solutions as compared to that for other heavier atoms. Another

21

important aspect is that, hydrogen attached or nearby electron withdrawing atom or group can decrease the shielding and move the resonance of attached proton towards to a higher frequency. By contrast electron donating atom or group increases the shielding and moves the resonance towards to a lower frequency. In this study, the chemical shifts obtained and calculated for the hydrogen atoms of methyl group is quite low. All values are < 3 ppm [70] due to the shielding effect. In the CO-CH3 group hydrogen, the 1H chemical shifts observed at 2.617 ppm and matches well with the calculated shifts in gas phase and solvents. In the 1H NMR spectrum, the experimental chemical shift values for ring hydrogen are 6.871, 6.954, 7.940 ppm. As can be seen from Table 13, there is a good agreement between experimental and theoretical chemical shift results for the title compound. The H14 and H16 chemical shift values are slightly smaller than the H17 value. This is due to the electron donating fluorine atoms cause shielding of the aromatic protons. The hydrogen atoms present in the ortho position to fluorine atoms experience little more shielding than other aromatic hydrogen atoms. The NMR shielding surfaces of C2 is shown in Fig. 9. In the NMR shielding surfaces, the blue region represents shielding and red region represents de-shielding of the title compound.

22

7. Conclusion FTIR, FT-Raman, UV and quantum chemical calculation studies have been performed on 2,4-difluoroacetophenone, in order to identify its structural and spectroscopic properties. A complete vibrational analysis of DFAP was performed using DFT-B3LYP method with 631+G(d,p) and 6-311++G(d,p) basis sets. The influences of carbon–fluorine, carbon–carbonyl group and benzene ring to the vibrational frequencies of the title compound were discussed. NBO results reflect the charge transfer mainly due to the lone pair n2(O8) → σ*(C – C) and π(CC) → π*(C-C). To evaluate the electronic transitions and charge distribution, the UV spectra of title compound were calculated. Theoretical molecular orbital coefficients analysis suggests that electronic transitions are assigned to π→π* type. The MEP map shows the negative potential sites are on oxygen atom as well as the positive potential sites are around the hydrogen atoms. The correlations between the statistical thermodynamics and temperature are also obtained. It is seen that the heat capacities, entropies and enthalpies increase with the increasing temperature owing to the intensities of the molecular vibrations increase with increasing temperature. NMR chemical shifts was calculated and compared with the experimental. Furthermore, the polarizability, the first hyperpolarizability and total dipole moment of title molecule have been calculated and the results are discussed. These results indicate that the DFAP compound is a good candidate of nonlinear optical materials.

23

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Table 1 Optimized geometrical parameters for 2,4-difluoroacetophenone computed at B3LYP/631+G(d,p) and 6-311++G(d,p) basis sets. Parameters Method/Basis set Experimentala B3LYP/6-31+G(d,p) B3LYP/6-311++G(d,p) Bond length (Å) R(1,2) 1.401 1.398 1.390 R(1,6) 1.409 1.405 1.396 R(1,7) 1.507 1.508 1.468 R(2,3) 1.390 1.387 1.370 R(2,13) 1.357 1.353 1.342 R(3,4) 1.387 1.384 1.399 R(3,14) 1.083 1.081 1.090 R(4,5) 1.392 1.389 1.398 R(4,15) 1.352 1.347 1.342 R(5,6) 1.389 1.385 1.369 R(5,16) 1.083 1.082 1.090 R(6,17) 1.084 1.082 1.090 R(7,8) 1.223 1.216 1.215 R(7,9) 1.512 1.511 1.498 R(9,10) 1.094 1.092 1.090 R(9,11) 1.094 1.092 1.090 R(9,12) 1.090 1.088 1.090 0 Bond angle ( ) A(2,1,6) 116.36 116.40 116.7 A(2,1,7) 126.08 126.12 119.7 A(6,1,7) 117.55 117.47 123.6 A(1,2,3) 123.33 123.19 122.3 A(1,2,13) 120.43 120.51 A(3,2,13) 116.22 116.28 A(2,3,4) 117.30 117.49 120.3 A(2,3,14) 121.02 120.92 119.0 A(4,3,14) 121.66 121.58 118.0 A(3,4,5) 122.62 122.46 120.5 A(3,4,15) 118.33 118.40 A(5,4,15) 119.04 119.12 A(4,5,6) 117.97 118.05 120.5 A(4,5,16) 120.06 120.00 A(6,5,16) 121.96 121.94 A(1,6,5) 122.39 122.38 122.1 A(1,6,17) 116.98 117.00 A(5,6,17) 120.62 120.61 A(1,7,8) 118.93 118.99 120.1 A(1,7,9) 120.69 120.40 119.2 A(8,7,9) 120.37 120.59 -

29

A(7,9,10) 110.93 A(7,9,11) 110.93 A(7,9,12) 108.30 A(10,9,11) 106.69 A(10,9,12) 109.98 A(11,9,12) 109.98 a Experimental values are taken from Ref. [27,30].

110.85 110.85 108.38 106.70 110.02 110.02

111.0 111.0 111.0 111.0

30

Table 2 Definition of internal coordinates of 2,4-difluoroacetophenone. No Symbol Type Definition Stretching 1–3 ri C–H C3–H14, C5–H16, C6–H17 4–6 ri C–H C9–H10, C9–H11, C9–H12 (methyl) 7–14 Ri C–C C1–C2, C2–C3, C3–C4, C4–C5, C5–C6, C6–C1, C1–C7, C7–C9 15–16 Qi C–F C2–F13, C4–F15 17 Pi C=O C7–O8 Bending 18–23 βi Ring C1–C2–C3, C2–C3–C4, C3–C4–C5, C4–C5–C6, C5–C6–C1, C6–C1–C2 24–29 αi C-C-H C2–C3–H14, C4–C3–H14, C4–C5–H16, C6–C5–H16, C5–C6–H17, C1–C6–H17 30–32 αi C-C-H C7–C9–H10, C7–C9–H11, C7–C9–H12 (methyl) 33–35 δi H-C-H H12–C9–H10, H12–C9–H11, H11–C9–H10 36–39 θi C-C-F C1–C2–F13, C3–C2–F13, C3–C4–F15, C5–C4–F15 40–42 σi C-C-C C2–C1–C7, C6–C1–C7, C1–C7–C9 43–44 νi C-C-O C1–C7–O8, C9–C7–O8 Out-of –plane bending 45–47 ωi C-H H14–C3–C2–C4, H16–C5–C4–C6, H17–C6–C5–C1 48–49 ψi C-F F13–C2–C1–C3, F15–C4–C3–C5 50–51 πi C-C C7–C1–C2–C6, C9–C7–C1–C2(C6) 52 Σi C-O O8–C7–C1–C2(C6) Torsion 53–58 ti τRing C1–C2–C3–C4, C2–C3–C4–C5, C3–C4–C5–C6, C4–C5–C6–C1, C5–C6–C1–C2, C6–C1–C2–C3 59 ti τC-CH3 (C2,C6)–C1–C7–C9–(H10,H11,H12) For numbering of atom refer Fig.1

31

Table 3 Definition of local symmetry coordinates of 2,4-difluoroacetophenone. No.(i) Symbola Definitionb 1–3 4 5 6 7–14 15–16 17 18

CH CH3 ss CH3 ips CH3 ops CC CF CO Rtrigd

19 20 21–23

Rsymd Rasymd bCH

24 25 26 27 28 29–30 31–32

CH3 sb CH3 ipb CH3 opb CH3 ipr CH3 opr bCF bCC

r1, r2, r3 (r4 + r5 + r6)/ 3 (2 r4 + r5 + r6)/ 6 (r5 – r6)/ 2 R7, R8, R9, R10, R11, R12, R13, R14 Q15, Q16 P17 (β18-β19+β20-β21+β22-β23)/ 6 (-β18-β19+2β20-β21-β22+2β23)/ 12 (β18-β19+β21-β22)/ 2 (α24 – α25)/ 2 , (α26 – α27)/ 2 , (α28 – α29)/ 2 (-α30- α31- α32+δ33+δ34+δ35)/ 6 (-δ33-δ34-2δ35)/ 6 (δ33-δ34)/ 2 (2α30- α31- α32)/ 6 (α31- α32)/ 2

(θ36− θ37)/ 2 , (θ38− θ39)/ 2 (σ40− σ41)/ 2 , σ42 33 bCO (ν43− ν44)/ 2 34–36 ωCH ω45, ω46, ω47 37–38 ψCF ψ48, ψ49 39–40 πCC π50, π51 41 ΣCO Σ52 42 tRtrigd (τ53-τ54+τ55-τ56+τ57-τ58) / 6 43 tRasymd (-τ53+2τ54-τ55-τ56+2τ57-τ58) / 12 44 tRsymd (τ53-τ55+τ56-τ58) / 2 45 tCH3 τ59 a These symbols are used for description of the normal modes by TED in Table 4 b The internal coordinates used here are defined in Table 2

32

Table 4 Vibrational assignments of fundamental modes of 2,4-difluoroacetophenone along with calculated IR, Raman intensities and normal mode descriptions (characterized by TED) based on quantum mechanical calculations using DFT with 6-31+G(d,p) and 6311++G(d,p) basis sets. Calculated frequencies νi (cm-1) TED(%) among types Sl. Species Observed No. Cs fundamentals (cm-1) of internal coordinatesc B3LYP/6-31+G(d,p) B3LYP/6-311++G(d,p) FTIR Raman Unscaled Scaled IR Raman Unscaled Scaled IR Raman νi intensitya activityb νi intensitya activityb 1 A΄ 3115(ms) 3238 3113 5.00 3219 3094 νCH(99) 0.03 4.97 0.04 2 A΄ 3078(w) 3230 3105 1.68 3211 3087 νCH(98) 0.36 1.56 0.35 3 A΄ 3055(w) 3218 3094 0.11 0.17 3199 3075 0.07 0.23 νCH(96) 4 A΄ 3045(s) 3165 3042 3146 3025 CH3SS(92) 6.18 0.02 6.15 0.02 5 A΄ 2994(w) 2998(w) 3121 3000 3103 2983 0.54 CH3ips(94) 0.51 1.96 1.76 6 A΄΄ 2942(w) 2940(w) 3055 2937 0.19 2.02 3043 2925 0.24 1.72 CH3ops(90) 7 A΄ 1695(vs) 1695(s) 1750 1682 1746 1678 νC=O(89) 2.86 0.76 2.81 0.72 8 A΄ 1618(vs) 1615(s) 1654 1590 1646 1582 νCC(87) 2.33 3.51 2.67 3.50 9 A΄ 1498(s) 1500(w) 1631 1568 2.40 1.70 1624 1561 2.30 1.61 νCC(85) 10 A΄ 1430(s) 1430(w) 1523 1465 2.04 1517 1457 CH3ipb(83) 0.45 2.24 0.36 11 A΄ 1365(s) 1362(vw) 1479 1422 1474 1418 CH3sb(84) 5.35 3.73 5.74 3.58 12 A΄ 1320(ms) 1318(w) 1469 1412 32.75 1.81 1466 1409 33.77 1.89 νCC(82) 13 A΄ 1268(vs) 1265(s) 1454 1398 1448 1392 νCF(85) 5.04 0.28 5.64 0.29 14 A΄ 1225(ms) 1223(vw) 1399 1345 1394 1340 νCF(82) 11.81 4.96 11.80 4.87 15 A΄΄ 1150(s) 1150(w) 1362 1309 5.98 0.90 1347 1295 6.80 0.76 CH3opb(83) 16 A΄ 1112(s) 1115(w) 1293 1243 1287 1237 νCC(81) 0.43 4.03 0.36 4.08 17 A΄ 1082(w) 1085(s) 1279 1229 1269 1220 νCC(82) 0.17 0.39 0.00 0.40 18 A΄ 1074(ms) 1240 1192 6.71 19.03 1237 1189 7.34 18.16 νCC(80) 19 A΄ 1025(ms) 1162 1118 1157 1112 νCC(79), bCH(18) 12.72 0.00 11.12 0.04 20 A΄ 994(w) 1128 1084 1123 1079 νCC(78), bCF(17) 55.11 0.11 53.12 0.16 21 A΄ 975(vs) 972(ms) 1079 1037 22.74 7.36 1078 1037 22.62 7.99 CH3ipr(75) 22 A΄΄ 864(s) 1047 1006 1046 1005 CH3opr(76) 61.78 8.00 70.30 6.89 23 A΄ 826(ms) 991 953 990 952 bCH(74), bCO(21) 1.99 0.03 2.77 0.03 24 A΄ 742(s) 742(vs) 985 947 1.48 0.23 982 944 1.40 0.23 bCH(73), Rsymd(19)

33

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

A΄ 715(vw) 974 936 970 933 bCH(75), Rtrigd(18) 6.31 17.04 9.42 17.20 A΄ 690(w) 692(w) 866 833 864 830 Rtrigd(72), bCH(20) 72.90 4.35 80.67 4.72 A΄ 626(s) 625(w) 838 806 28.36 2.09 838 805 20.99 1.82 Rsymd(70), bCC(19) A΄ 600(ms) 602(w) 745 716 745 716 Rasymd(71) bCO(19) 14.94 1.41 17.56 1.20 A΄ 582(ms) 706 679 714 686 55.49 bCO(69), Rsymd(22) 321.58 55.31 329.09 A΄ 569(vw) 685 659 23.81 3.09 684 658 21.02 2.14 bCC(70), bCH(21) A΄ 550(w) 631 607 635 610 bCC(67), Rasymd(20) 2.78 3.70 3.63 3.92 A΄΄ 538(s) 601 578 602 579 ωCH(65), tRtrigd(23) 42.25 3.15 40.46 2.41 A΄΄ 531(ms) 581 558 57.83 2.57 581 559 63.55 2.12 ωCH(63), ωCO(19) A΄΄ 518(ms) 544 523 4.91 546 525 ωCH(61), tRsymd(23) 39.32 33.37 4.65 A΄ 450(vw) 452(w) 520 500 521 501 bCF(68), bCC(19) 12.34 8.83 12.08 8.21 A΄ 415(w) 462 444 43.15 8.62 461 443 46.70 7.56 bCF(69), bCH(20) A΄΄ 398(w) 396 381 397 381 tRtrigd(62), ωCO(21) 56.63 3.82 57.49 3.78 A΄΄ 342(w) 334 321 275.00 334 322 tRsymd(61), ωCH(19) 89.48 272.72 86.18 A΄΄ 310(w) 318 306 194.18 52.03 319 307 200.91 49.04 tRasymd(63), ωCC(21) A΄΄ 294(w) 296 285 295 284 162.20 ωCO(64), tRsymd(22) 1.06 154.52 0.69 A΄΄ 225(w) 226 218 226 217 ωCC(61), ωCH(23) 5.02 47.08 5.15 45.69 A΄΄ 204 204 10.95 86.99 206 206 11.57 83.76 ωCC(62), tRtrigd(20) A΄΄ 187 187 187 187 ωCF(60), ωCO(23) 0.80 35.23 0.86 33.49 A΄΄ 99 99 97 97 3.95 143.92 4.00 139.93 ωCF(59), tRasymd(19) A΄΄ 39 39 1.01 125.66 32 32 1.05 123.39 tCH3(58) Abbreviations used: ν-stretching; ss – symmetric stretching; ass – asymmetric stretching; b-bending; ω-out-of-plane bending; R-ring; t-torsion; s-strong; vs-very strong; ms-medium strong; w-weak; vw-very weak. a Relative absorption intensities in km mol-1 and normalised with the highest peak absorbance. b Relative Raman intensities in Å4amu-1 and normalised to 100. c For the notations see Table 3.

34

Table 5 Second-order perturbation theory analysis of Fock matrix in NBO basis for DFAP. a b E(2) Donor (i) ED (i) (e) Acceptor (j) ED (j) (e) E(j)–E(i) cF (i,j) -1 (kJ mol ) (a.u.) (a.u.) * σ(C1 - C2) 1.97833 σ (C1 – C6) 0.01881 3.25 1.28 0.058 3.87 σ*(C2 – C3) 0.02352 1.28 0.063 π(C1 – C2) 1.64924 π*(C3 – C4) 0.37111 16.76 0.28 0.062 π*(C5 – C6) 0.28934 21.51 0.30 0.073 * π (C7 – O8) 0.13660 16.24 0.30 0.066 σ(C1 – C6) 1.96731 σ*(C1 – C2) 0.03318 3.11 1.25 0.056 σ*(C2 – F13) 0.03291 4.92 0.95 0.061 σ(C2 – C3) 1.97600 σ*(C1 – C2) 0.03318 4.44 1.28 0.068 * σ (C4 – F15) 0.03115 4.02 1.00 0.057 σ(C3 – C4) 1.97660 σ*(C2 – F13) 0.03291 3.76 0.98 0.054 σ*(C4 – C5) 0.02711 3.86 1.29 0.063 * π(C3 – C4) 1.66855 π (C1 – C2) 0.41033 24.55 0.29 0.077 π*(C5 - C6) 0.28934 14.86 0.31 0.061 σ(C3 – H14) 1.97377 σ*(C1 – C2) 0.03318 4.05 1.09 0.059 σ*(C4 – C5) 0.02711 3.73 1.09 0.057 * σ(C4 – C5) 1.98105 σ (C3 – C4) 0.02557 3.80 1.28 0.062 σ(C5 – C6) 1.97427 σ*(C4 – F15) 0.03115 4.30 0.98 0.058 π(C5 – C6) 1.68069 π*(C1 – C2) 0.41033 16.08 0.27 0.061 π*(C3 – C4) 0.37111 26.30 0.27 0.076 σ(C5 – H16) 1.97707 σ*(C1 – C6) 0.01881 3.75 1.08 0.057 σ*(C3 – C4) 0.02557 3.83 1.08 0.058 σ(C6 – H17) 1.97656 σ*(C1 – C2) 0.03318 4.06 1.06 0.059 * π(C7 – O8) 1.97486 π (C1 – C2) 0.41033 4.86 0.38 0.043 σ(C9 – H10) 1.97008 π*(C7 – O8) 0.13660 4.89 0.53 0.047 σ(C9 – H11) 1.97008 π*(C7 – O8) 0.13660 4.89 0.53 0.047 σ(C9 – H12) 1.98845 σ*(C1 – C7) 0.07175 4.15 0.92 0.056 n2(O8) 1.88787 σ*(C1 – C7) 0.07175 20.31 0.68 0.106 σ*(C7 – C9) 0.04854 18.68 0.66 0.100 * n2(F13) 1.97098 σ (C1 – C2) 0.03318 5.77 0.98 0.067 σ*(C2 – C3) 0.02352 5.83 0.98 0.067 n3(F13) 1.92179 π*(C1 – C2) 0.41033 18.87 0.44 0.089 * n2(F15) 1.97178 σ (C3 – C4) 0.02557 6.22 0.98 0.070 σ*(C4 – C5) 0.02711 5.96 0.97 0.068 * n3(F15) 1.91883 π (C3 – C4) 0.37111 19.81 0.43 0.089 π*(C1 – C2) 0.41033 π*(C5 - C6) 0.28934 195.99 0.01 0.081 π*(C7 – O8) 0.13660 105.86 0.02 0.070 π*(C3 - C4) 0.37111 π*(C5 – C6) 0.28934 152.64 0.02 0.082 a E(2) means energy of hyperconjugative interactions. b Energy difference between donor and acceptor i and j NBO orbitals. c F(i,j) is the Fock matrix element between i and j NBO orbitals.

35

Table 6 Significant NLMO’s occupancy, percentage from parent NBO and atomic hybrid contributions of DFAP calculated at B3LYP level using 6-311++G(d,p) basis set. Hybrid contributions Percentage from Bond Occupancy parent NBO Atom Percentage BD(2) C1-C2 2.00000 81.5177 C1 47.484 C2 34.139 C3 2.946 C4 2.825 C5 1.995 C6 6.854 C7 2.425 O8 1.256 BD(2) C3-C4 2.00000 81.9593 C1 2.499 C2 8.887 C3 47.190 C4 34.860 C5 2.927 C6 2.901 BD(2) C5-C6 2.00000 82.7038 C1 2.950 C2 3.473 C3 2.239 C4 8.530 C5 46.501 C6 36.228 LP(2) O8 2.00000 94.3524 C1 1.297 C7 2.737 O8 94.352 C9 1.095 LP(3) F13 2.00000 96.0431 C1 0.744 C2 1.989 C3 0.647 F13 96.043 LP(3) F15 2.00000 95.9323 C3 0.770 C4 2.067 C5 0.638 F15 95.932

36

Table 7 The thermodynamic parameters of 2,4-difluoroacetophenone along with the global minimum energy calculated at the B3LYP/6-31+G(d,p) and B3LYP/6-311++G(d,p) methods. Method/Basis set Parameters B3LYP/6-31+G(d,p) B3LYP/6-311++G(d,p) Optimized global minimum energy,(Hartrees) -583.40101434 -583.53538430 -1 Total energy(thermal), Etotal (kcal mol ) 82.013 81.736 -1 -1 Heat capacity, Cv (kcal mol k ) 0.0355 0.0356 -1 -1 Entropy, S (kcal mol k ) Total 0.0965 0.0968 Translational 0.0410 0.0410 Rotational 0.0302 0.0302 Vibrational 0.0252 0.0255 -1 Vibrational energy, Evib (kcal mol ) 80.236 79.958 -1 Zero point vibrational energy, (kcal mol ) 76.062 75.777 Rotational constants (GHz) A 2.2109 2.2220 B 0.7500 0.7531 C 0.5620 0.5644 Dipole moment (Debye) µx 0.9115 0.8634 µy 1.3040 1.2600 µz 0.0003 0.0000 µtotal 1.5910 1.5274

37

Table 8 Thermodynamic properties at different temperatures at the B3LYP/6-311++G(d,p) level for 2,4difluoroacetophenone.

T (K) 100.00 200.00 298.15 300.00 400.00 500.00 600.00 700.00 800.00 900.00 1000.00

C (J/mol.K) 70.75 117.74 162.28 163.09 203.26 236.23 262.47 283.42 300.39 314.33 325.92

S (J/mol.K) 291.38 354.84 410.26 411.26 463.84 512.88 558.36 600.45 639.44 675.65 709.39

∆H (kJ/mol) 5.00 14.42 28.19 28.49 46.86 68.90 93.88 121.22 150.44 181.20 213.23

38

Table 9 Calculated dipole moment µ (Debye), polarizability (α) and the first hyperpolarizability (β) components (a.u.) for 2,4-difluoroacetophenone.

Components

Values

Components

Values

µx µy µz

0.8634 1.2600 0.0000

αxx αxy αyy αxz αyz αzz

129.2157 1.9714 102.4874 -0.0007 0.0010 54.3777

βxxx βxxy βxyy βyyy βxxz βxyz βyyz βxzz βyzz βzzz

-595.7623 -91.6727 90.5762 -14.8170 0.0045 0.0024 -0.0014 8.9621 1.9268 0.0006

39

Table 10 The calculated absorption wavelength λ (nm), excitation energies E (eV) and oscillator strengths (f) of DFAP calculated by the TDB3LYP/6-311++G(d,p) method in gas and solvent phase.

TD-B3LYP/6-311++G(d,p) DMSO Chloroform Ethanol λ (nm) E (eV) f λ (nm) E (eV) f λ (nm) E (eV) 310.90 3.9879 0.0001 314.39 3.9436 0.0001 311.27 3.9832 261.88 4.7345 0.0578 260.52 4.7590 0.0585 261.61 4.7392 244.32 5.0747 0.3048 243.39 5.0940

0.3028

243.87

5.0841

Major Contribution Gas f λ (nm) E (eV) f 0.0001 310.68 3.9907 0.0001 H-1 → L (96%) 0.0549 261.79 4.7360 0.0536 H-2 → L (94%) H → L (90%) 0.2979 243.87 5.0840 0.2952 H → L (90%)

Assignment π → π* π → π* π → π*

40

Table 11 Calculated .HOMO-LUMO energy values of 2,4-difluoroacetophenone calculated solvent phase Parameters TD-B3LYP/6-311++G(d,p) DMSO Chloroform Ethanol EHOMO energy (eV) -7.5094 -7.5290 -7.5121 ELUMO energy (eV) -2.1102 -2.1129 -2.1102 ∆EHOMO-LUMO energy gap (eV) 5.3992 5.4161 5.4019 EHOMO-1 energy (eV) -7.5741 -7.5350 -7.5687 ELUMO+1 energy (eV) -1.0128 -1.0517 -1.0168 ∆EHOMO-1-LUMO+1 energy gap (eV) 6.5613 6.4833 6.5519 Electronegativity χ (eV) 4.8098 4.8209 4.8112 Chemical hardness η (eV) 2.6996 2.7080 2.7009 Softness ξ (eV)-1 0.1852 0.1846 0.1851 Electrophilicity index ψ (eV) 4.2847 4.2912 4.2851 Dipole moment (Debye) 2.1948 1.9844 2.1708

in gas and Gas -7.5080 -2.1099 5.3981 -7.5766 -1.0108 6.5658 4.8089 2.6990 0.1852 4.2841 2.2061

41

Table 12 The charge distribution of DFAP calculated by the Mulliken (B3LYP/6-31+G(d,p) and 6311++G(d,p) basis sets).

Atoms

C1 C2 C3 C4 C5 C6 C7 O8 C9 H10 H11 H12 F13 H14 F15 H16 H17

Atomic charges (Mulliken) B3LYP/ B3LYP/ 6-31+G(d,p) 6-311++G(d,p) 1.3111 2.0441 -1.5673 -2.1366 0.5426 0.7259 -0.2649 -0.8052 0.3331 0.3760 -0.1681 -0.3102 0.3782 -0.0979 -0.4142 -0.2410 -0.5077 -0.4204 0.1759 0.1669 0.1759 0.1669 0.1775 0.1925 -0.3340 -0.1496 0.1617 0.2249 -0.3265 -0.1572 0.1507 0.2125 0.1758 0.2083

42

Table 13 The experimental (in CDCL3) and theoretical 1H and 13C NMR isotropic chemical shifts (with respect to TMS, all values in ppm) for 2,4-difluoroacetophenone.

Atom

Experimental chemical shift

C1 C2 C3 C4 C5 C6 C7 C9 H10 H11 H12 H14 H16 H17

126.79 161.08 121.82 160.82 118.48 116.21 194.51 31.34 2.617 2.617 2.617 6.871 6.954 7.940

In DMSO Chemical Chemical shielding shift 55.970 126.495 8.282 174.183 73.384 109.081 6.781 175.684 65.865 116.600 43.935 138.530 -18.818 201.284 149.127 33.33 29.027 2.854 29.027 2.854 29.716 2.165 24.826 7.055 24.700 7.181 23.520 8.361

In Chloroform Chemical Chemical shielding shift 56.083 126.382 8.452 174.012 73.744 108.721 6.935 175.529 66.041 116.424 43.601 138.864 -17.612 200.077 149.349 33.115 29.087 2.794 29.087 2.794 29.682 2.199 24.889 6.992 24.750 7.132 23.506 8.376

In gas phase Chemical Chemical shielding shift 56.302 126.163 8.739 173.726 74.564 107.901 7.263 175.202 66.622 115.843 43.020 139.445 -14.566 197.032 149.854 32.611 29.220 2.662 29.220 2.662 29.645 2.237 25.060 6.821 24.898 6.984 23.496 8.385

43

Figure 1

44

Figure 2

45

Figure 3

Molecular Absorbance

46

14000 12000 10000 8000 6000 4000 2000 0 -2000 14000 12000 10000 8000 6000 4000 2000 0 -2000 14000 12000 10000 8000 6000 4000 2000 0 -2000 14000 12000 10000 8000 6000 4000 2000 0 -2000

In DMSO

In Chloroform

In Ethanol

In Gas

50

100

150

200

250

300

350

Wavelength (nm) Figure 4

400

450

500

47

Figure 5

48

Figure 6

49

Figure 7

50

Figure 8

51

Figure 9

52

Figure Captions

Fig.1

Molecular model of 2,4-difluoroacetophenone along with numbering of atoms.

Fig.2

Comparison of observed and calculated IR spectra of 2,4-difluoroacetophenone (a) observed; (b) calculated with B3LYP/6-31+G(d,p); and (c) calculated with B3LYP/6-311++G(d,p).

Fig.3

Comparison of observed and calculated Raman spectra of 2,4difluoroacetophenone (a) observed; (b) calculated with B3LYP/6-31+G(d,p); and (c) calculated with B3LYP/6-311++G(d,p).

Fig.4

Theoretically calculated UV spectra in gas and solvent phase for 2,4difluoroacetophenone.

Fig.5

The atomic orbital compositions of the frontier molecular orbital for 2,4difluoroacetophenone.

Fig.6

(a) Electrostatic potential (ESP); (b) electron density (ED) and (c) the molecular electrostatic potential (MEP) map for 2,4-difluoroacetophenone calculated at B3LYP/6-311++G(d,p) level.

Fig.7

Mulliken population analysis chart of 2,4-difluoroacetophenone.

Fig.8

Experimental (a) 1H difluoroacetophenone.

Fig.9

The NMR shielding surfaces of 2,4-difluoroacetophenone.

NMR

and

(b)

13

C

NMR

spectrum

of

2,4-

53

Graphical abstract

54

•

The optimized geometries and harmonic vibrational wavenumbers of 2,4difluoroacetophenone have been carried out.

•

Stability of the molecule arising from hyperconjugative interactions, charge delocalization has been analyzed.

•

Molecular electrostatic potential, HOMO-LUMO and Mulliken’s charge analysis were performed.

•

The chemical shifts of H atoms and C atoms were calculated using NMR analysis.