Spectroscopic investigation of some electron withdrawing groups substituted TTF donor

SAA-117849; No of Pages 12 Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx

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Spectroscopic investigation of some electron withdrawing groups substituted TTF donor V. Mukherjee a,⁎, D.P. Ojha b a b

SUIIT, Sambalpur University, Sambalpur, Odisha, India School of Physics, Sambalpur University, Sambalpur, Odisha, India

a r t i c l e

i n f o

Article history: Received 11 September 2019 Received in revised form 19 November 2019 Accepted 23 November 2019 Available online xxxx Keywords: IR Raman TTF DFT NBO NCA

a b s t r a c t The structure optimization and spectroscopic properties for some derivatives of tetrathiafulvalene in both the neutral and ionized forms have been studied. The electron withdrawing groups like -CN, -CF 3 and -CO 2 Me were considered to study the effect on structure, vibrational and electronic properties of tetrathiafulvalene. All the calculations were carried out at density functional theory incorporated with B3LYP exchange functional. The CAM-B3LYP exchange–correlation energy functional was also assessed for the determination of molecular structures. The normal coordinate analysis was performed to compute potential energy distributions of the normal modes which were used for the subsequent normal modes assignment. The temperature dependent Raman spectra have been presented showing the relative reduction in Raman intensity. The ionization of TTF-CN leads a very interesting effect. The IR spectrum of neutral TTF-CN contains a very strong CN stretching band at 2251 cm−1 while it is completely diminished in IR spectrum of TTF-CN cation. NBO analyses were also performed to study the electronic structure, second order perturbation and HOMO-LUMO and their energies. Some important thermodynamical parameters have been presented in which entropy calculation reveals below 50% contribution of vibrational motion in all the three molecules. © 2019 Elsevier B.V. All rights reserved.

1. Introduction One of the applications of vibrational spectroscopy is to detect the valence of molecules in organic charge-transfer salts. Some of the vibrational modes, called charge-sensitive modes, change their frequencies depending on the valence of the molecule. For example, the C_C stretching mode in tetrathiafulvalene (TTF) derivatives is utilized as a probe for detecting the valence. As the chemical bond is usually much stronger than intermolecular interactions, the vibrational frequency and symmetry of a free molecule is approximately preserved in the solid state except in the case where a strong vibronic interaction occurs between a vibrational mode and a low energy electronic state (electron-molecular vibration coupling). A vibrational mode that strongly interacts with the electronic state is called structure-sensitive mode. Knowledge of the normal mode of a free molecule is essential, because a deviation

⁎ Corresponding author. E-mail address: [email protected] (V. Mukherjee).

from this mode in the solid state tells us which interaction is occurring [1]. Investigations of radical cation salts of TTF functionalized by electron-withdrawing groups (EWG) are less documented, essentially because the presence of such substituent as halogen, acyl, ester, amide or nitrile on the TTF core dramatically increases its oxidation potential and destabilizes the radical cation form [2]. This strong anodic shift is particularly noticeable in tetra substituted TTF such as TTF-(CO2Me)4, TTF-(CF3)4, or TTF(CN)4 [3–5]. The associated instability of these radical species in moist air hindered in most cases their isolation in cation radical salts. This is all the more unfortunate since the electronegative atoms (O, N, Hal) within such EWG are expected to be able to engage, in the solid state at the organic– inorganic interface, in a variety of secondary nonbonding interactions such as hydrogen or halogen bonding [6,7]. However, it was also recognized that the introduction of only one or two of such EWG on the TTF core could limit this anodic shift, and accordingly, several tetrathiafulvalenes bearing only one or two ester [8], nitrile [9], amide [10], thioamide [11], or halogen [12] substituents were successfully engaged in radical cation salts by electro-crystallization with intermolecular hydrogen or halogen bond interactions.

https://doi.org/10.1016/j.saa.2019.117849 1386-1425/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: V. Mukherjee and D.P. Ojha, Spectroscopic investigation of some electron withdrawing groups substituted TTF donor, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117849

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The charge-sensitive mode was applied for the first time to the distribution of charges in the unit cell of Cs2TCNQ3, where TCNQ represents 7,7,8,8-tetracyanoquinodimethane, wherein the infrared-active external ring C_C stretching modes of TCNQ0 and TCNQ− were separately observed [13]. A detailed normal mode analysis of TCNQ0 and TCNQ− was reported by Bozio et al. [14]. Matsuzaki et al. extended the application to the TCNQ salt with fractional charge [15]. Subsequently, vibrational spectroscopy was applied to mixed-valence compounds of TCNQ [16], where one of the CN stretching modes of TCNQ was utilized as the probe. The IR and Raman spectra of TTF and TTF-d4 were reported earlier in which an assignment of the fundamental vibrational modes were presented only for planar modes [17]. However, vibrational spectroscopic studies of TTF and its several derivatives have been reported so far [18–25]. The relative effects of the EWG such as CF3, CO2Me and CN on the TTF core have been investigated from a combination of structural, electrochemical, spectrochemical and theoretical investigations [2]. We observed that the vibrational and electronic spectra of singly substituted TTF (TTF-X, X = CF3, CN, CO2Me) are not available so far. Therefore, we have planned to perform structure optimization and to calculate vibrational and electronic structures of neutral TTF-X donors and their cation. Besides, the temperature dependent Raman spectra and some thermodynamical parameters viz. thermal energy, specific heat, entropy and zero point vibrational energy have also been studied. 2. Calculation details The first principle ab initio calculations have been performed to optimize molecular structures and to calculate vibrational and electronic structures of TTF-X (X = CN, CF3 , CO 2 Me). The density functional theory (DFT) in addition with exchange functional B3LYP [26,27] and CAM-B3LYP [28] and standard basis set 6-311+ +g(d,p) were used for the computational purpose at Gaussian 09

software [29]. Yanai et al. introduced the hybrid exchange– correlation energy functional for DFT based on the Coulombattenuating method (CAM) [28]. The approximation, denoted CAM-B3LYP, follows original works of Hirao and coworkers [30,31] and Savin and coworkers [32,33]. Preliminary investigations have demonstrated that CAM-B3LYP provides significantly improved Rydberg and charge transfer (CT) electronic excitation energies due to an improved description of the long-range exchange interaction over B3LYP [34]. The optimized geometries corresponding to the minimum on the potential energy surface have been obtained by solving self consistent field (SCF) equation iteratively. Harmonic vibrational frequencies have been calculated using analytic second order derivatives to confirm the convergence to minima on the potential surface and to evaluate the zero-point vibrational energies without imposing any molecular symmetry constraints. It is a well known fact that ab initio calculations tend to overestimate the vibrational frequencies with respect to the experimental ones. This is due to several reasons, for instance, the use of finite basis set, the incomplete implementation of the electronic correlation and the neglect of anharmonicity effects in the theoretical treatment. However, the calculated ab initio force field can be improved by using the scaled quantum mechanical force field (SQMFF) methodology. For subsequent normal coordinate analysis (NCA), the force field obtained in Cartesian coordinates and dipole derivatives with respect to atomic displacements were extracted from the archive section of the Gaussian 09 output and transformed to a suitably defined set of internal coordinates (given in supplementary material, Table S1) by means of a modified version of the MOLVIB program [35,36]. The scale factors were taken from the earlier work of TTF for the present force field scaling [37]. To reproduce Raman spectra, Gaussian calculated Raman activities were converted in corresponding Raman intensities using the empirical relation of Raman scattering theory [38,39]. The excitation wavelength

Table 1 Optimized structure (TTF only) of TTF-X (neutral) (X = CN, CF3 and CO2Me). Parametersa

C1=C3 C2=C4 C5=C6 C2-S7 C2-S8 C4-S9 C4-S10 C3-S7 C1-S8 C6-S9 C5-S10 C1-H12 C3-H11 C5-H13 C6-C14 S7-C2-S8 S9-C4-S10 C2-S7-C3 C2-S8-C1 C4-S9-C6 C4-S10-C5 C1-C3-S7 C5-C6-S9 C4-C2-S7-C3 C2-C4-S9-C6 Energy (E) ∇E a

TTF-CN

TTF-CF3

TTFa

TTF-CO2Me

B3LYP

CAM-B3LYP

B3LYP

CAM- B3LYP

B3LYP

CAM- B3LYP

1.3338 1.3476 1.3465 1.7818 1.7825 1.7825 1.7873 1.7620 1.7622 1.7855 1.7427 1.0811 1.0812 1.0815 1.4154 113.63 113.61 94.66 94.66 93.67 94.52 117.92 117.05 169.55 163.42 −1916.16 −92.4

1.3266 1.3387 1.3369 1.7716 1.7722 1.7736 1.7772 1.7539 1.7541 1.7716 1.7365 1.0804 1.0805 1.0809 1.4190 113.84 113.82 94.69 94.69 93.72 94.55 117.94 117.46 171.09 165.65 −1915.96 −92.2

1.3339 1.3474 1.3374 1.7826 1.7830 1.7836 1.7869 1.7621 1.7623 1.7757 1.7495 1.0812 1.0812 1.0810 1.4960 113.62 113.55 94.63 94.63 93.61 94.33 117.94 117.72 169.31 164.70 −2161.05 −337.29

1.3266 1.3385 1.3291 1.7723 1.7727 1.7743 1.7768 1.7540 1.7541 1.7651 1.7422 1.0804 1.0805 1.0805 1.4921 113.82 113.82 94.68 94.68 93.61 94.37 117. 95 117.97 170.90 166.20 −2160.81 −337.08

1.3341 1.3473 1.3438 1.7843 1.7849 1.7839 1.7886 1.7617 1.7622 1.7757 1.7408 1.0812 1.0813 1.0811 1.4742 113.58 113.84 94.67 94.69 94.07 94.69 118.03 117.43 169.91 168.45 −2051.84 −228.08

1.3268 1.3384 1.3345 1.7740 1.7747 1.7746 1.7782 1.7536 1.7540 1.7646 1.7350 1.0805 1.0806 1.0805 1.4731 113.82 114.13 94.72 94.74 94.05 94.71 118.05 117.76 171.99 170.75 −2051.60 −227.84

1.339 1.352 1.339 1.787 1.787 1.787 1.787 1.763 1.763 1.763 1.763 1.083 1.083 1.083 – 113.74 113.74 96.64 96.64 96.64 96.64 117.96 117.96 – – −1823.76 0

Unit of bond length is Å, bond angle and dihedrals are degree and energy is a.u.

Please cite this article as: V. Mukherjee and D.P. Ojha, Spectroscopic investigation of some electron withdrawing groups substituted TTF donor, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117849

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Fig. 1. Optimized structures of TTF-CN (a-transverse view, b-longitudinal view), TTF-CF3 (c-transverse view, d-longitudinal view) and TTF-CO2Me (e-transverse view, f-longitudinal view).

785 nm and room temperature at 300 K were used for the calculation of Raman intensity. Natural bond orbital (NBO) analysis has also performed which gives the accurate possible natural Lewis structure picture of orbital because all orbitals are mathematically chosen to include the highest possible percentage of the electron density. Interaction between both filled and virtual orbital spaces information correctly explained by the NBO analysis, it could enhance the analysis of intra and inter–molecular interactions. The interaction between bonding and antibonding orbitals represent the deviation of the molecule from the Lewis structure and can be used as the measure of delocalization. This non-covalent bonding–antibonding interaction can be quantitatively described in terms of the second order perturbation interaction energy E (2) [40–43]. This energy represents the estimate of the off-diagonal NBO Fock Matrix elements. It can be deduced from the second-order perturbation approach [44].

3. Results and discussions 3.1. Geometry optimization The optimized geometrical parameters viz. bond lengths, bond angles and dihedrals of TTF-X at B3LYP and CAM-B3LYP are collected in Table 1. The optimized structure of TTF at B3LYP [37] are also taken for comparison purpose. The optimized structures with atomic labelling of all the TTF-X are shown in Fig. 1. Present optimization of all the TTF-X exhibits boat shape structure with a fall of symmetry as compared to the parent TTF which is obviously due to the mono substitution of EWGs. The bending is larger in TTF-CN and smaller in TTF-CO2Me. The symmetry point group of TTF is C2h [37] while C1 in all the three TTF-X. The atomization energy in all the TTF-X significantly increases making them more stable as compared to TTF. The relative energies of TTF-X with respect to that for TTF are also given in Table 1 which reveals that the global minimum energy was

Table 2 Optimized structure (TTF only) of TTF-X (cation) (X = CN, CF3 and CO2Me) at B3LYP and comparison with neutral TTF-X donor. Parametersa

TTF-CN+

TTF-CN0

TTF-CF+ 3

TTF-CF03

TTF-CO2Me+

TTF-CO2Me0

C1=C3 C2=C4 C5=C6 C2-S7 C2-S8 C4-S9 C4-S10 C3-S7 C1-S8 C6-S9 C5-S10 C1-H12 C3-H11 C5-H13 C6-C14 S7-C2-S8 S9-C4-S10 C2-S7-C3 C2-S8-C1 C4-S9-C6 C4-S10-C5 C1-C3-S7 C5-C6-S9 C4-C2-S7-C3 C2-C4-S9-C6 Energy (E) ∇E

1.3437 1.3959 1.3521 1.7442 1.7447 1.7440 1.7501 1.7378 1.7378 1.7643 1.7283 1.0820 1.0820 1.0824 1.4164 114.40 114.80 95.60 95.59 95.21 95.77 117.20 116.50 180.00 179.99 −1915.91 0.25

1.3338 1.3476 1.3465 1.7818 1.7825 1.7825 1.7873 1.7620 1.7622 1.7855 1.7427 1.0811 1.0812 1.0815 1.4154 113.63 113.61 94.66 94.66 93.67 94.52 117.92 117.05 169.55 163.42 −1916.16 0

1.3436 1.3963 1.3426 1.7443 1.7445 1.7442 1.7485 1.7381 1.7380 1.7530 1.7346 1.0819 1.0820 1.0823 1.5148 114.41 114.81 95.58 95.58 95.09 95.49 117.21 117.15 180.00 179.98 −2160.80 0.25

1.3339 1.3474 1.3374 1.7826 1.7830 1.7836 1.7869 1.7621 1.7623 1.7757 1.7495 1.0812 1.0812 1.0810 1.4960 113.62 113.55 94.63 94.63 93.61 94.33 117.94 117.72 169.31 164.70 −2161.05 0

1.3429 1.3959 1.3471 1.7455 1.7455 1.7423 1.7506 1.7391 1.7393 1.7533 1.7315 1.0818 1.0818 1.0820 1.4941 114.40 114.90 95.56 95.59 95.12 95.42 117.26 116.98 179.99 180.00 −2051.61 0.23

1.3341 1.3473 1.3438 1.7843 1.7849 1.7839 1.7886 1.7617 1.7622 1.7757 1.7408 1.0812 1.0813 1.0811 1.4742 113.58 113.84 94.67 94.69 94.07 94.69 118.03 117.43 169.91 168.45 −2051.84 0

a

The energy difference (∇E) was calculated as the difference of energy of neutral and cation donors.

Please cite this article as: V. Mukherjee and D.P. Ojha, Spectroscopic investigation of some electron withdrawing groups substituted TTF donor, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117849

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and 1.3374 Å in TTF-CN, TTF-CO2 Me and TTF-CF 3 respectively. Therefore, it is slightly elongated in TTF-CN and TTF-CO2 Me and contracted in TTF-CF3. The mono substitution of EWGs also affected the C\\S bonds in all the three TTF-X. The four C\\S bonds coupled with C2-C4 bond were identical and optimized at 1.787 Å in TTF, however, in present study, one C\\S bond (C4-S10) was found slightly affected and rest three bonds were considerably contracted in all the three TTF-X as compared to TTF [37]. The corresponding C\\S bond lengths in all the three TTF-X exhibit the pattern, (C\\S)TTF-CN b (C\\S) TTF-CF3 b (C\\S) TTF-CO2Me . Another two C\\S bonds namely, C1-S8 and C3-S7 were optimized very close to that in TTF with slight contraction in all the three TTF-X whereas rest two C\\S bonds of the ring having EWGs were greatly affected. The bond C6-S9 was elongated while C5-S10 was contracted by ~0.02 Å as compared to TTF [37]. The contraction

observed in TTF-CF3 as compared to other two TTF-X. A perusal of Table 1 concludes that, in most cases, CAM-B3LYP results are slightly less than the values at B3LYP and the standard deviation is 0.003. The atomization energy is also slightly less in CAM-B3LYP as compared to B3LYP. Some B3LYP results are discussed here. The bonds C1-C3 and C2C4 were optimized at ~1.334 Å and ~1.347 Å respectively in all the TTF-X which means that these parameters are less sensitive towards the nature of substitution. However, these two bond lengths have been reported at 1.339 Å and 1.352 Å respectively in TTF which indicates that both the bonds in all the three TTF-X are contracted as compared to TTF [37]. The optimization of the bond C5-C6 is different in different TTF-X which is due to the different electronegativity of EWGs. The two identical bonds C1-C3 and C5-C6 were optimized at 1.339 Å in TTF while C5-C6 differ s C1-C3 in all the three TTF-X. The bond C5-C6 was optimized at 1.3465 Å, 1.3438 Å

Table 3 Normal mode assignment of TTF-CN0 and TTF-CN+ at DFT/B3LYP/6-311++g(d,p). TTF-CN0 ν

a

3230 3220 3210 2321 1613 1589 1571 1283 1233 1120 1106 970 865 835 826 799 797 771 726 670 645 619 549 515 511 477 459 417 415 388 308 274 227 158 152 90 77 73 33

TTF-CN+ b

c

d

IR

R

ν

0.003 0.082 0.051 1 0.06 0.753 0.265 0.001 0.032 0.02 0.183 0.004 0.001 0.377 0.004 0.697 0.398 0.458 0.072 0.023 0.89 0.01 0.137 0.039 0.074 0.004 0.03 0.147 0.001 0.005 0.001 0.016 0.002 0.072 0.074 0.03 0.018 0.042 0

0.013 0.006 0.005 0.104 0.015 0.014 0.201 0.002 0.006 0.008 0.026 0.003 0.001 0.036 0.005 0.01 0.011 0.002 0.016 0.035 0.007 0 0.009 0.028 0.015 0.064 0.024 0.008 0.01 0.042 0.139 0.027 0.392 0.161 0.042 0.278 0.159 0.095 1

3068 3059 3049 2251 1581 1557 1540 1257 1208 1098 1084 955 852 822 814 787 785 759 715 663 638 612 543 510 506 472 455 413 411 384 304 270 225 156 150 88 76 72 33

ν

e

3231 3220 3215 2342 1561 1535 1430 1294 1236 1128 1108 1031 886 880 851 831 812 812 743 704 680 635 548 519 515 496 480 444 421 376 321 300 248 168 158 117 86 54 37

i f

g

PEDs

h

IR

R

ν

0.395 0.355 0.182 0.008 0.093 1 0.004 0.036 0.018 0.001 0.01 0 0 0.078 0.101 0.111 0.269 0.377 0.049 0.652 0.008 0.004 0.06 0.007 0.002 0.007 0.141 0.05 0 0.001 0.018 0 0.002 0.02 0.029 0.043 0.017 0 0.003

0.023 0.01 0.007 0.216 0.579 0.017 0.315 0.005 0.013 0.012 0.023 0.002 0 0.007 0.127 0.015 0.003 0.004 0.016 0.001 0.013 0.033 0.014 0.206 0.006 0.528 0.055 0.046 0.009 0.018 0.002 0.109 0.85 0.025 0.557 0.084 1 0.159 0.059

3070 3059 3054 2272 1530 1504 1401 1268 1212 1106 1086 1016 872 867 838 819 800 800 731 697 674 628 543 514 510 491 475 440 417 372 318 297 245 167 156 116 86 53 37

99rCH 99rCH 100rCH 87rCN + 13rCC2 46rCC + 35rCC1 + 6rCS + 5bCCH 73rCC1 + 9bCCH + 6rCC2 49rCC + 34rCC1 + 6rCS 90bCCH + 8rCS 77bCCH + 8rCS 95bCCH 57rCC2 + 14rCS + 9bCCS + 7bCCH + 6rCN 52rCS + 43βCC 53γCH + 47τring 61rCS + 11βCN + 11bCCS + 9βCC2 87rCS + 5bCSC 52rCS + 31bCCS + 14βCH 56γCH + 37τring 66rCS + 18bSCS + 7βCC + 7bCSC 92rCS 36βCN + 36rCS + 10βCC2 81γCH + 17τring 60rCS + 16bCCS + 15bCSC + 5βCH 45γCC2 + 35γCN + 11βCN 53γCC + 17βCN + 9rCS + 8τring 56βCN + 23γCN + 8rCS + 6γCC 36rCS + 23βCN + 10bCSC + 9γCN + 7bCCS 41rCS + 15bCCS + 13rCC2 + 10bCSC + 5γCC 20rCS + 19bCSC + 9bCCS + 8bSCS + 8τring + 7γCC 43τring + 34γCH + 6rCS 46γCN + 24τring + 15γCC2 + 7βCN + 5γCH 38βCC + 37rCS + 15βCN 49γCC + 23τring + 11rCS + 8bCSC 19bSCS + 18γCC + 15τring + 11γCN + 8rCS + 8βCC 31τring + 26βCC2 + 21γCN + 6γCH + 5βCN 56γCN + 31βCN + 7βCC2 31γCN + 23βCC + 16βCN + 14τCC + 8τring + 5βCC2 63τring + 12τCC + 6βCN + 6βCC + 5γCN 37τCC + 28τring + 24γCN 56τring + 38γCN

The bold numbers represent the contribution of the corresponding mode. Less than 5% contribution is not considered. The abbreviations are: b = angle bending, r = stretching, β = inplane bending, γ = out of plane bending, τ = twisting, ω = wagging, μ = rocking, ρ = scissoring. a DFT calculated frequency in cm−1for neutral TTF-CN donor. b IR intensity (relative, in scale 1.0). c Raman intensity (relative, in scale 1.0). d Scaled frequency in unit cm−1. e DFT calculated frequency in cm−1 for TTF-CN cation. f IR intensity (relative, in scale 1.0). g Raman intensity (relative, in scale 1.0). h Scaled frequency in unit cm−1. i Potential energy distributions.

Please cite this article as: V. Mukherjee and D.P. Ojha, Spectroscopic investigation of some electron withdrawing groups substituted TTF donor, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117849

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of C5-S10 was observed large in TTF-CO 2Me as compared to the two TTF-X. The geometry optimization of TTF-X+ was also performed and the optimized data along with that for neutral TTF-X0 are collected in Table 2. The optimization of cations reveals planar structure which is similar to our earlier work [37]. The boat shaped structures of neutral molecules were converted into planar structure upon ionization. Interestingly, the energies of all the three cations were increased almost by same amount (~0.25 a.u.). The intermediate bond of the two rings C2C4 was elongated by ~0.048 Å while all the eight C\\S bonds were shortened by ~ 0.038 Å in all the cations as compared to their neutral part.

5

3.2. Vibrational spectra The vibrational frequencies of both the TTF-X0 and TTF-X+ along with the IR and Raman intensities and potential energy distributions of the modes were collected in Tables 3–5. The DFT frequencies were overestimated due to approximations which were further scaled using the scale factors given for TTF. The corresponding scaled IR and Raman spectra of all the three TTF-X neutral were shown in Figs. 2–3 respectively. The DFT calculations provide Raman spectrum in Raman shift vs. Raman activity. However, Fig. 3 is labeled with Raman intensity which was calculated using

Table 4 Normal mode assignment of TTF-CF03 and TTF-CF+ 3 at DFT/B3LYP/6-311++g(d,p). TTF-CF03

TTF-CF+ 3

i

PEDs

νa

IRb

Rc

νd

νe

IRf

Rg

νh

3229 3226 3210 1636 1605 1576 1283 1280 1230 1142 1120 1108 972 962 864 827 820 806 797 771 726 712 664 645 618 605 543 515 479 473 432 415 376 336 327 305 264 208 164 136 88 73 72 29 22

0 0.016 0.008 0.136 0.115 0.003 0.032 1 0.157 0.49 0.003 0.615 0.008 0.18 0 0.001 0.081 0.075 0.067 0.071 0.009 0.049 0.033 0.144 0.001 0.011 0.005 0.009 0.001 0 0.032 0 0.001 0.003 0.003 0 0.002 0 0.004 0.003 0.002 0.005 0.002 0.001 0

0.004 0.002 0.001 0.002 0.005 0.034 0 0.004 0.001 0.001 0.002 0 0.001 0 0 0.001 0.004 0.002 0.004 0 0.005 0.005 0.002 0.002 0 0 0.002 0.001 0.008 0.015 0 0.003 0.003 0.002 0.005 0.02 0.002 0.104 0.018 0.014 0.02 0.015 0.028 1 0.745

3068 3064 3049 1603 1573 1544 1258 1254 1205 1119 1098 1086 957 948 851 815 808 793 785 759 715 702 657 638 612 599 538 510 474 468 428 410 372 333 324 302 260 206 163 135 87 72 71 29 22

3232 3222 3215 1599 1538 1431 1294 1260 1232 1181 1151 1129 1033 977 885 882 835 831 823 813 742 723 703 673 634 613 550 517 506 481 472 420 378 336 331 312 299 217 163 153 112 79 48 37 17

0.074 0.082 0.037 0.025 0.111 0.008 0.006 1 0.392 0.426 0.567 0 0.001 0.083 0 0.018 0.036 0.022 0.064 0.062 0.012 0.018 0.133 0.041 0.001 0.012 0.003 0 0.002 0 0.04 0 0.003 0.004 0 0.005 0 0.002 0.003 0 0.005 0.002 0 0 0

0.002 0.001 0.001 0.016 0.009 0.017 0 0.002 0 0.002 0 0.001 0 0.001 0 0 0.004 0.001 0 0 0.002 0.001 0 0 0.003 0 0.001 0 0.057 0.001 0.001 0.001 0.001 0.011 0 0 0.008 0.07 0.015 0.005 0.001 0.04 0.214 0.296 1

3070 3062 3055 1567 1507 1403 1268 1235 1207 1157 1128 1106 1018 963 872 868 822 818 811 800 731 712 696 667 628 607 545 512 501 476 467 416 374 332 328 309 296 215 161 152 111 78 47 37 17

99rCH 99rCH 100rCH 61rCC1 + 13rCC + 9rCC2 + 5βCH 50rCC1 + 32rCC + 7βCH + 6rCS 54rCC + 31rCC1 + 7rCS 89βCH + 8rCS 42rCC2 + 21rCF3ss + 20bCF3sb 73βCH + 8rCS 70rCF3ips + 10μCF3opr + 9bCF3ipb 95βCH 74rCF3ops + 11μCF3ipr + 10bCF3opb 50rCS + 40βCC 39rCF3ss + 18rCS + 10bCCS + 8βCH + 8rCC2 + 7bCF3sb 53γCH + 47τring 87rCS + 5bCSC 55rCS + 10rCF3ss + 9γCH + 8bCCS + 6τring 52γCH + 28τring + 9rCS 53rCS + 31bCCS + 15βCH 65rCS + 18bSCS + 7βCC + 7bCSC 91rCS 39bCF3sb + 32rCS + 20rCF3ss 24bCF3sb + 24rCS + 13bCCS + 9bCF3ipb + 8bCSC + 7βCC2 82γCH + 18τring 61rCS + 16bCCS + 16bCSC + 5βCH 38bCF3opb + 21μCF3ipr + 16γCC2 + 11rCF3ops + 6τring 59μCF3opr + 16rCS + 11rCF3ips + 10μCF3ipr 78γCH + 9τring 31μCF3ipr + 23rCS + 9γCC2 + 9bCF3opb + 9μCF3opr + 6bCSC 32rCS + 26μCF3ipr + 9bCF3opb + 8γCC2 + 7bCSC + 5τring 28rCS + 24bCSC + 13bSCS + 13γCC + 7bCCS + 5βCC 54τring + 42γCH 43bCF3ipb + 24μCF3opr + 12rCS + 6μCF3ipr + 6βCC2 19rCC2 + 17rCS + 11bCF3sb + 7CF3opb + 6bCSC + 6βCC 30bCF3opb + 23τring + 10μCF3ipr + 9γCH + 6γCC2 38rCS + 35bβCC + 10bCF3ipb 55γCC + 27τring 19bSCS + 13rCS + 10γCC + 10bCSC + 9τring + 8βCC 41βCC2 + 23bCF3ipb + 20βCC + 6rCS 37τring + 30γCC2 + 21bCF3opb + 6γCH 42τCC + 32βCC + 15τring 82τring + 6βCC 53τCC + 24τring + 13βCC 86τring 71τCF3 + 12τring + 10γCC2

The bold numbers represent the contribution of the corresponding mode. Less than 5% contribution is not considered. The abbreviations are: b = angle bending, r = stretching, β = inplane bending, γ = out of plane bending, τ = twisting, ω = wagging, μ = rocking, ρ = scissoring. a DFT calculated frequency in cm−1 for neutral TTF-CF3 donor. b IR intensity (relative, in scale 1.0). c Raman intensity (relative, in scale 1.0). d Scaled frequency in unit cm−1. e DFT calculated frequency in cm−1 for TTF-CF3 cation. f IR intensity (relative, in scale 1.0). g Raman intensity (relative, in scale 1.0). h Scaled frequency in unit cm−1. i Potential energy distributions.

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3.2.1. CH vibrations The CH stretching vibrations seem less sensitive towards the substitutions as well as ionization. Three CH stretching frequencies were calculated at ~3049, ~3064 and ~3068 cm−1 in all the three TTF-X0

empirical equation keeping excitation wavelength at 785 nm and temperature 293.15 K. Also, the IR and Raman spectra of all the three TTF-X cations along with that for neutral molecules were depicted in Figs. 4–9.

Table 5 Normal mode assignment of TTF-CO2Me0 and TTF-CO2Me+ at DFT/B3LYP/6-311++g(d,p). TTF-CO2Me0 ν

a

3228 3226 3208 3159 3126 3052 1758 1617 1594 1574 1498 1482 1471 1287 1283 1220 1209 1171 1120 1079 974 928 864 846 829 827 796 774 765 742 727 677 643 618 517 474 441 435 429 413 351 321 291 258 205 201 143 135 108 90 70 68 58 22

TTF-CO2Me+ b

c

d

IR

R

ν

0 0.01 0.005 0.021 0.027 0.071 0.574 0.032 0.249 0.036 0.022 0.016 0.042 1.000 0.004 0.205 0.096 0.001 0.002 0.274 0.002 0.042 0 0.053 0.008 0.002 0.05 0.014 0.072 0.06 0.01 0.005 0.107 0.001 0.004 0.001 0.009 0.007 0.001 0 0.003 0.028 0.018 0.001 0.003 0.007 0.002 0 0.004 0.002 0.002 0.004 0.001 0.004

0.004 0.002 0.001 0.001 0.001 0.003 0.018 0.005 0.007 0.065 0.001 0.002 0 0.019 0.001 0.001 0.003 0.001 0.003 0.001 0.003 0.005 0 0.02 0.001 0.008 0.004 0.001 0.003 0 0.005 0.003 0.002 0 0.001 0.035 0.001 0.005 0.003 0.003 0.009 0.018 0.031 0.009 0.115 0.022 0.004 0.013 0.008 0.006 0.052 0.016 0.018 1

3067 3064 3048 3001 2970 2899 1722 1584 1561 1542 1468 1452 1441 1261 1257 1195 1184 1147 1097 1057 959 913 851 832 816 814 784 762 753 730 716 670 637 611 512 469 437 430 424 409 347 317 287 255 202 199 141 133 106 89 69 67 57 22

ν

e

3232 3226 3216 3186 3153 3068 1780 1574 1540 1491 1485 1474 1432 1297 1294 1222 1210 1169 1128 1068 1032 915 884 880 852 833 829 815 772 748 742 700 687 634 522 505 472 438 433 417 350 324 310 282 218 212 142 142 119 101 72 72 40 36

i f

g

PEDs

h

IR

R

ν

0.039 0.038 0.02 0.005 0.007 0.018 0.27 0.013 0.056 0.014 0.017 0.037 0.001 1 0.004 0.09 0.005 0.001 0 0.061 0 0.06 0 0.009 0.012 0.012 0.014 0.04 0.012 0.041 0.005 0.076 0.011 0 0 0 0.017 0 0.001 0 0 0.002 0.022 0.016 0.006 0.002 0.003 0.001 0.001 0 0.002 0 0.003 0

0.012 0.004 0.003 0.003 0.002 0.009 0.017 0.21 0.029 0.004 0.004 0.001 0.129 0.037 0.003 0.006 0.001 0.001 0.006 0.008 0.001 0.029 0 0.002 0.045 0.001 0.009 0.001 0.011 0.001 0.005 0 0.006 0.014 0.001 0.352 0.006 0.004 0.006 0.002 0.09 0 0.008 0.077 0.009 0.405 0.003 0.004 0.029 0.049 0.199 0.073 0.468 1

3071 3065 3055 3026 2995 2915 1744 1542 1509 1462 1455 1444 1404 1271 1268 1198 1185 1146 1106 1047 1017 902 871 867 840 821 817 803 761 736 730 693 680 628 516 500 467 434 429 413 347 321 307 279 216 209 141 140 118 100 71 71 39 36

99rCH 99rCH 100rCH 97rCH3ips 100rCH3ops 97rCH3ss 80rCO2 + 6rCC2 44rCC + 36rCC1 + 6rCS 76rCC1 + 9βCH 53rCC + 32rCC1 + 7rCS 58μCH3opr + 21μCH3ipr + 11bCH3ipb + 9bCH3sb 69μCH3ipr + 23μCH3opr + 7bCH3opb 83bCH3sb + 7μCH3opr 30rCC2 + 27rCO1 + 14ρCO2 + 7μCO2 89βCH + 8Rcs 68βCH + 6rCS + 6rCO1 74bCH3ipb + 8rCO1 + 7μCH3opr 93bCH3opb 95βCH 53rOC + 12rCO1 + 11rCC2 + 5βCH 51rCS + 42βCC 41rOC + 26rCO1 + 7rCS + 5bCCS 53γCH + 47τring 55rCS + 12bCCS + 8βCC2 + 6βCH + 5ρCO2 41γCH + 25ωCO2 + 24τring 84rCS + 5bCSC 54rCS + 30bCCS + 14βCH 47rCS + 18ρCO2 + 9bSCS + 7βOC 44rCS + 24ρCO2 + 9bSCS + 9βOC 71ωCO2 + 16γCH + 7τring 92rCS 39rCS + 14μCO2 + 11bCCS + 7ρCO2 + 7bCSC + 7βCC2 82γCH + 17τring 60rCS + 17bCCS + 16bCSC + 5βCH 83γCC + 10τring 68rCS + 15bCSC + 6bCCS 18γCC2 + 17rCS + 17μCO2 + 11τring + 8bCSC 24γCC2 + 18rCS + 14bCSC + 14τring + 7Bscs 36μCO2 + 20rCS + 7bCSC + 6γCC2 53τring + 43γCH 25rCC2 + 22rCS + 10bCCS + 8βCC + 7bCSC + 6βOC 42βOC + 16βCC + 15rCS + 12ρCO2 + 11βCC2 28βOC + 22rCS + 18βCC + 12ρCO2 57γCC + 27τring 14bSCS + 13rCS + 12τring + 10τOC 36τOC + 23τring + 9γCC2 + 6τCH3 + 6γCH 29βCC + 28βCC2 + 28μCO2 + 7βOC 79τCH3 + 7τring 52τOC + 22τring + 10γCC2 + 6ωCO2 47τCC + 23τCO2 + 17τring 46βCC + 16τring + 15βCC2 + 8μCO2 72τring + 16τCC 55τCO2 + 19τring + 19τCC 88τring

The bold numbers represent the contribution of the corresponding mode. Less than 5% contribution is not considered. The abbreviations are: b = angle bending, r = stretching, β = inplane bending, γ = out of plane bending, τ = twisting, ω = wagging, μ = rocking, ρ = scissoring. a DFT calculated frequency in cm−1 for neutral TTF-CO2Me donor. b IR intensity (relative, in scale 1.0). c Raman intensity (relative, in scale 1.0). d scaled frequency in unit cm−1. e DFT calculated frequency in cm−1for TTF-CO2Me cation. f IR intensity (relative, in scale 1.0). g Raman intensity (relative, in scale 1.0). h scaled frequency in unit cm−1. i Potential energy distributions.

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Fig. 2. Theoretical IR spectra of neutral TTF-CN, TTF-CF3 and TTF-CO2Me. Fig. 3. Theoretical Raman spectra of neutral TTF-CN, TTF-CF3 and TTF-CO2Me. +

and TTF-X . However, comparison between DFT calculated frequencies of TTF and TTF-X reveals that the CH frequencies in TTF-X were red shifted by ~20 cm−1 as compared to TTF [37]. Also, all the three CH bands in all the three TTF-X0 and TTF-X + were very weak in both the IR and Raman spectra except for TTF-CN+, for which IR spectrum contains a very strong CH band. Similar to stretching modes, CH in plane bending modes also seem less sensitive. Two frequencies out of three CH in plane bending modes were calculated exactly at 1257 cm−1 and 1098 cm−1 for all the three TTF-X0. The third frequency was slightly differ in different TTF-X and calculated at 1208 cm −1, 1205 cm−1 and 1195 cm−1 for TTF-CN, TTF-CF3 and TTF-CO2Me respectively. The former two frequencies with doublet at each have also been assigned in TTF [37] while the third frequency reveals that one of the CH in plane bending modes frequency lowered in each TTF-X by ~50 cm−1 that is attributed to the EWGs. In case of TTF-X +, all the three CH in-plane bending frequencies were slightly increased. However, their intensity pattern remains same. A similar trend was also observed in CH out of plane bending modes. Two frequencies out of three CH out of plane bending modes were calculated exactly at 851 cm−1 and 638 cm−1 . The third frequency was slightly differ in different TTF-X0 and calculated at 785 cm−1, 793 cm−1 and 816 cm−1 for TTF-CN, TTF-CF3 and TTFCO2Me respectively. In case of TTF-X+, all the three CH out of plane bending frequencies were up shifted by 20–50 cm−1 with variable intensities. 3.2.2. Ring vibrations The C2_C4 bond (the intermediate bond between two rings) is the important bond in TTF as the mobile electrons are available

at this site. The stretching mode of this bond was assigned at 1540 cm−1 in TTF-CN 0, 1544 cm −1 in TTF-CF 03 and 1542 cm−1 in TTF-CO 2Me0 . This mode was found strongly coupled with ring CC stretching vibrations. This CC stretching mode has been assigned at 1496 in TTF [37]. Therefore, the substitution of mono EWGs produces blue shift of ~45 cm−1 in CC stretching vibration in all the three TTFX as compared to TTF. The concerning band was appeared strongly in Raman spectra. In case of TTF-X+, the result is quite different in TTFCO2Me+ than other two cations. The CC stretching frequency was diminished by ~140 cm−1 in both the TTF-CN+ and TTF-CF+ 3 while it was diminished by ~80 cm−1 in TTF-CO2Me+. In addition, the IR intensity was decreased and Raman intensity was increased in both the TTF-CN+ and TTF-CF+ 3 while Raman intensity was also decreased in TTF-CO2Me+. There is one C_C bond in each ring elaborating two CC stretching modes. The frequency pair is (1557 cm −1 , 1581 cm −1 ) in TTF-CN 0, (1573 cm −1 , 1603 cm −1) in TTF-CF03 and (1561 cm−1, 1584 cm −1 ) in TTF-CO 2 Me 0 . And, also it has been assigned at 1521 cm −1 and 1541 cm −1 in TTF which indicates that both the ring CC stretching frequencies were increased in all the three TTFX as compared to TTF. The corresponding bands were very weak in Raman spectra and having moderate IR intensity. The IR intensity of the band at 1557 cm −1 of TTF-CN is very strong while it was diminished and appeared with medium of the band at 1603 cm −1 of TTF-CF 3 and medium strong of the band at 1561 cm −1 of TTF-CO 2 Me. These two bond stretching frequencies were lowered by 50–70 cm −1 in corresponding cations thereby

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Fig. 4. Theoretical IR spectra of TTF-CN0 and TTF-CN+.

Fig. 5. Theoretical IR spectra of TTF-CF03 and TTF- CF+ 3 .

confirming that these C_C normal modes are charge sensitive modes. The IR intensities are slightly changed in TTF-CN + and TTF-CF + 3 whereas a medium strong band was converted into a very weak band in TTF-CO2Me+. There are eight C\\S bonds in each TTF-X molecule that elaborate eight C\\S stretching normal modes and the present calculation exhibits appreciable contribution of C\\S stretching vibration in many frequencies. Anyhow, six common frequencies in all the three TTF-X are 970 cm −1 , 826 cm −1 , 771 cm −1 , 726 cm −1 , 619 cm −1 and 477 cm −1 which possess enough contribution of C\\S stretching and so these frequencies were assigned to the six C\\S stretching modes. These six C\\S stretching mode frequencies were unaffected upon type of substitutions. A doublet at 732 cm−1 has been assigned to the C\\S stretching modes in TTF [37] which, in case of TTF-X, split into two components (771 cm −1 and 726 cm−1).

effects the C-CN stretching frequency should be higher. Anyhow, the latter two frequencies were appeared with most strong IR intensity. The bands at 156 cm−1 in TTF-CN, 163 cm−1 in TTF-CF3 and 141 cm−1 in TTF-CO2Me were assigned to the C-X in plane bending mode which indicates that this mode is slightly affected by the substitutions. However, the C-X out of plane bending mode assignment exhibits that this mode is very sensitive. The corresponding frequencies are 543 cm−1 in TTF-CN, 135 cm−1 in TTF-CF3 and 430 cm−1 in TTF-CO2Me.

3.2.3. C-X (X = CN, CF3, CO2Me) vibrations The C-X stretching mode was assigned at 1084 cm−1 for TTF-CN, 1254 cm−1 for TTF-CF3 and 1261 cm−1 for TTF-CO2Me. The bond length C-CN is the smallest among the three C-X and its stretching frequency is comparatively lower than that in TTF-CF3 and TTF-CO2Me which seems contradictory to force constant and mass effects. According to these two

3.2.4. Temperature dependent Raman spectra The temperature dependent Raman spectra of TTF-CN are depicted in Fig. 10 and given in supplementary data (Figs. S1–S2) for other two molecules. The spectra were plotted at 100 K, 200 K and 300 K and a reduction in Raman intensity in all the three molecules was occurred at low temperature. Also, the lower modes intensities were diminished more as compared to higher modes intensities which indicate that the low frequency vibrations are less populated while lowering the temperature. Therefore, the stretching vibrations are more dominating at low temperature. 3.3. Thermodynamics Some thermodynamical parameters viz. thermal energy, specific heat, entropy and zero point vibrational energy (ZPVE) of all

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Fig. 6. Theoretical IR spectra of TTF-CO2Me0 and TTF- CO2Me +. Fig. 7. Theoretical Raman spectra of TTF-CN0 and TTF-CN+.

the three TTF-X at both the B3LYP and CAM-B3LYP levels are given in Table 6. All these parameters were computed at 300 K. The total value of all the parameters and vibrational contribution only are given. In case of thermal energy and specific heat, both are mainly contributed by vibrational motion whereas in entropy, the vibrational contribution is 30–40% only for all the three TTF-X. Therefore, entropy is also sufficiently contributed by rotational, translational and electronic motions. Also, entropy is lower in TTF-CN as compared to other two molecules at same temperature therefore, thermally TTF-CN is more stable. The zero point vibrational energy (ZPVE) is almost equal in TTF-CN and TTF-CF3 while it is high in TTF-CO2Me by ~23 kcal/mol. 3.4. NBO analysis Natural bond orbital analysis was performed for all the three TTF-X and the orbital occupancy and energy of some important NBOs were given in Table 7. A perusal of Table 7 indicates that the ring with no EWG having relatively less population as compared to the ring with EWG and it is quite less at π(C5-C6). The population at this bonding orbit is 1.914e in TTF-CN, 1.949e in TTF-CF 3 and 1.905e in TTF-CO2Me which indicates that the electron withdrawing strength is in the order, -CO2Me N -CN N -CF3. Also, the population of all the C\\S bonding orbitals are nearly equal (1.970e)

except the population of C6-S9 in TTF-CN which was found at 1.961e and the population of C5-S10 in all the TTF-X which was found at ~1.976e. The second order perturbation analysis provides E (2) energy given in Table 8 which is associated in the transition between bonding donor orbitals to anti-bonding acceptor orbitals. The transition energy for C2_C4 σelectron to all the four neighbouring C\\S indicates that the EWG CO2Me perturbs the TTF system more as compared to the other two groups. The E(2) energy for σ(C5C6) → σ*(C6-C14) is different in different molecule. It is 4.59 kcal/ mol in TTF-CN, 1.93 kcal/mol in TTF-CF3 and 2.50 kcal/mol in TTFCO2Me. The HOMO-LUMO of all the three TTF-X are shown in Fig. 11 which is entirely different for different molecules. The calculated orbitals are quite different than the work of Jeannin et al. [2]. In case of TTF-CN, HOMO is contributed mainly by S atom of the ring containing no CN and LUMO is mainly contributed by C5_C6 and C6-C14 orbitals while in earlier work [2], they reported that HOMO is equally contributed by all the bonding orbitals of TTF ring. In case of TTF-CO 2 Me, the HOMO is mainly contributed by C\\S orbits of the ring containing CO2Me with partial contributions of core electrons of C2 and C5 while LUMO is equally shared by core electrons of TTF ring. The HOMO-LUMO energies and their gap are

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Fig. 8. Theoretical Raman spectra of TTF-CF03 and TTF- CF+ 3 . Fig. 9. Theoretical Raman spectra of TTF-CO2Me0 and TTF- CO2Me +.

given in Table 9 which reveals that the gap is more in TTF-CF 3 (6.671 eV) as compared to the other two molecules. Also, the HOMO-LUMO and all their possible transitions will lead UV absorption. 4. Conclusions The structure optimization, vibrational, electronic and some thermodynamical properties of three singly substituted TTF by EWGs namely cyano (-CN), easter (-CO2Me) and fluoromethyl (-CF3) in both the neutral and ionized forms were studied successfully. Newly proposed CAMB3LYP exchange functional was tested along with the conventional B3LYP for density functional calculations. The magnitude of global minimum energy was slightly less in CAM-B3LYP. Among the three TTF-X, TTF-CF3 was found more stable as compared to the parent TTF. It is a well known fact that TTF resembles boat shape structure and in present study, the mono substitution of EWGs in TTF did not affect the shape of structure however, the curvature is relatively large in TTF-CN. The IR spectrum reveals different intensity pattern for different molecule which means that absorption intensities were moderated upon EWGs due to their different strength of electron withdrawing tendency. Conversely, Raman spectra of all the three TTF-X exhibit overlapping in

some extent except the spectral lines of the concerning group specially for stretching modes. The strongest Raman line appeared in the vicinity of 210 cm−1 in all the three TTF-X. The ionization of TTF-CN leads a very interesting effect. The IR spectrum of neutral TTF-CN contains a very strong CN stretching band at 2251 cm−1 while it is completely diminished in IR spectrum of TTF-CN cation. The C_C normal modes were found charge sensitive and the concerning frequencies were changed greatly in cations as compared to neutral molecules. The temperature dependent Raman spectra show a reduction in most of the Raman lines in all the three TTF-X and also the lower modes get more diminish as compare to the higher modes. NBO analysis reveals that the order of electron withdrawing strength is in the order, -CO2Me N -CN N -CF3. Supplementary data to this article can be found online at https://doi. org/10.1016/j.saa.2019.117849. Authors contributions V. Mukherjee: Conceptualization; Data Curation; Investigation; Methodology; Resources; Software; original draft.

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Table 8 Second order perturbation analysis. Donora

Acceptorb

E(2) (kcal/mol) TTF-CN

TTF-CF3

TTF-CO2Me

σ(C2=C4) σ(C2=C4) σ(C2=C4) σ(C2=C4) σ(C6-C14) σ(C6-C14) σ(C5-C6)

σ*(C2-S7) σ*(C2-S8) σ*(C4-S9) σ*(C4-S10) σ*(C5-C6) σ*(C5-S10) σ*(C6-C14)

0.65 0.65 0.69 0.63 4.87 3.37 4.59

0.65 0.65 0.67 0.64 3.29 3.51 1.93

0.63 0.65 0.70 0.59 4.24 3.45 2.50

a b

σ represents bonding orbitals. σ* represents anti-bonding orbitals.

D.P. Ojha: Formal Analysis; Funding Acquisition; Project Administration; Supervision; Validation. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement We are grateful to DST-SERB India to provide financial assistance in form of a project (SB/FTP/PS-096/2014 dated 27/03/2015). Prof. T. Sundius is also gratefully acknowledged to provide us his own program MOLVIB-7.0. Fig. 10. Temperature dependent Raman spectra of TTF-CN. Table 6 Some thermodynamical parametersa. Parameters

TTF-CN

TTF-CO2Me

TTF-CF3

B3LYP

CAM-B3LYP

B3LYP

CAM-B3LYP

B3LYP

CAM-B3LYP

Thermal energy (kcal/mol)

57.63 (55.85)

62.45 (60.67)

Specific heat (Cv) (Cal/mol·K) Entropy (S) (Cal/mol·K) ZPVE (kcal/mol)

42.21 (36.25)

58.68 (56.90) 41.36 (35.40) 110.25 (35.91) 51.41

63.62 (61.84) 47.89 (41.93) 121.39 (45.58) 55.20

86.97 (85.20) 51.58 (45.62) 125.92 (50.20) 77.77

88.34 (86.56) 50.59 (44.63) 125.27 (49.59) 79.28

a

110.80 (36.43) 50.23

48.90 (42.94) 122.32 (46.32) 53.88

The total value of the concerning parameter is given outside the parenthesis and the vibrational contribution is given within the parenthesis.

Table 7 Orbital occupancy and energy of some important NBOs. NBO

Occupancy (in e unit)

Energy (kcal/mol)

TTF-CN

TTF-CF3

TTF-CO2Me

TTF-CN

TTF-CF3

TTF-CO2Me

Ring with no EWG σ(C1-C3) π(C1-C3) σ(C1-S8) σ(C2-C4) π(C2-C4) σ(C2-S7) σ(C2-S8) σ(C3-S7)

1.990 1.983 1.976 1.990 1.974 1.970 1.970 1.976

1.990 1.983 1.976 1.990 1.975 1.970 1.970 1.976

1.990 1.984 1.976 1.990 1.978 1.970 1.971 1.976

−0.812 −0.330 −0.675 −0.831 −0.352 −0.678 −0.678 −0.675

−0.809 −0.327 −0.671 −0.827 −0.346 −0.674 −0.674 −0.671

−0.803 −0.320 −0.665 −0.820 −0.336 −0.664 −0.665 −0.664

Ring with EWG σ(C4-S9) σ(C4-S10) σ(C5-C6) π(C5-C6) σ(C5-S10) σ(C6-S9) σ(C6-C14)

1.969 1.970 1.981 1.914 1.976 1.961 1.976

1.969 1.970 1.986 1.949 1.976 1.969 1.983

1.969 1.970 1.987 1.905 1.978 1.970 1.972

−0.678 −0.678 −0.820 −0.336 −0.698 −0.666 −0.780

−0.674 −0.673 −0.825 −0.336 −0.687 −0.675 −0.745

−0.660 −0.665 −0.807 −0.320 −0.688 −0.657 0.715

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V. Mukherjee, D.P. Ojha / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx

Fig. 11. Frontier orbitals of TTF-CN, TTF-CF3 and TTF-CO2Me. Table 9 HOMO-LUMO energies of TTF-X. Orbitals

HOMO LUMO HOMO-LUMO

Energy (eV) TTF-CN

TTF-CF3

TTF-CO2Me

−7.365 −0.826 6.539

−7.266 −0.595 6.671

−6.985 −0.390 6.595

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Please cite this article as: V. Mukherjee and D.P. Ojha, Spectroscopic investigation of some electron withdrawing groups substituted TTF donor, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117849