Crystal growth, characterization and theoretical studies of 4-aminopyridinium picrate

Crystal growth, characterization and theoretical studies of 4-aminopyridinium picrate

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 805–813 Contents lists available at ScienceDirect Spectrochimica Acta...

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 805–813

Contents lists available at ScienceDirect

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Crystal growth, characterization and theoretical studies of 4-aminopyridinium picrate A. Aditya Prasad, K. Muthu, M. Rajasekar, V. Meenatchi, S.P. Meenakshisundaram ⇑ Department of Chemistry, Annamalai University, Annamalainagar 608 002, India

h i g h l i g h t s

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

 The electron density mapping is

interpreted.  XRD closely resembles the simulated

pattern.  First-order molecular

hyperpolarizability is estimated.  Crystalline cohesion is achieved by

NAH  O and CAH  O hydrogen bonds.

a r t i c l e

i n f o

Article history: Received 3 May 2014 Received in revised form 10 July 2014 Accepted 28 July 2014 Available online 7 August 2014 Keywords: Crystal growth Hyperpolarizability FT-IR Organic picrate HOMO–LUMO

a b s t r a c t Single crystals of 4-aminopyridinium picrate (APP) were grown by slow evaporation of a mixed solvent system methanol–acetone (1:1, v/v) containing equimolar quantities of 4-aminopyridine and picric acid. Structure is elucidated by single crystal XRD analysis and the crystal belongs to monoclinic system with four molecules in the unit cell (space group P21/c) and the cell parameter values are, a = 8.513 Å (±0.015), b = 11.33 Å (±0.02), c = 14.33 Å (±0.03) and b = 104.15° (±0.019), V = 1340 A3 (±6) with refined R factors R1 = 0.0053 and wR2 = 0.0126. The electron density mapping is interpreted to find coordinates for each atom in the crystallized molecules. The various functional groups present in the molecule are confirmed by FT-IR analysis. UV–visible spectral analysis was used to determine the band gap energy of 4-aminopyridinium picrate. Powder X-ray diffraction pattern reveals the crystallinity of the as-grown crystal and it closely resembles the simulated XRD from the single crystal XRD analysis. Scanning electron microscopy reveals the surface morphology of the grown crystal. Optimized geometry is derived by Hartree–Fock theory calculations and the first-order molecular hyperpolarizability (b), theoretically calculated bond length, bond angles and excited state energy from theoretical UV–vis spectrum were estimated. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Despite numerous theoretical and experimental studies, several fundamental questions concerning the geometries of charge transfer complexes remain unanswered. In particular, little is known about the relative importance of classical van der Waals interac⇑ Corresponding author. Tel.: +91 4144221670. E-mail address: [email protected] (S.P. Meenakshisundaram). 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

tions and charge transfer forces in controlling the overall configuration and donor–acceptor contacts in charge-transfer interactions on the internal geometries. Pyridinium cations have been studied especially extensively [1–3] and particular attention has been given to the donor–acceptor properties. Several complexes of picric acid with organic molecules exhibit nonlinear optical applications [4]. During the past decade there has been considerable interest in the physical and structural properties of charge-transfer complexes [5–8].


A. Aditya Prasad et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 805–813 O

O N+


N+ NH 2




O N+












O O-


Scheme 1.

The crystal structure of the 4-aminopyridinium picrate was reported earlier [9]. In the present work, we report the growth, characterization, electron density mapping and theoretical studies using Hartree–Fock and DFT method. Experimental procedure Synthesis and growth

Fig. 1. Photograph of APP crystals.

4-Aminopyridinium picrate was synthesized by mixing stoichiometric amounts of 4-aminopyridine (Sigma–Aldrich) and picric acid (Sigma–Aldrich) in an equimolar ratio 1:1 using methanol as solvent (Scheme 1). The mixture was stirred at room temperature for 3 h and APP was obtained as yellow color precipitate. The product was purified by recrystallization using mixed solvent, methanol and acetone (1:1, v/v). Recrystallized APP was dissolved in mixed solvent system (methanol:acetone, 1:1, v/v) and the solution warmed with con-

Fig. 2. FT-IR spectra of APP (a) experimental and (b) theoretical.

A. Aditya Prasad et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 805–813

stant stirring for an hour to avoid co-precipitation of multiple phases. Transparent crystals were grown by slow evaporation solution growth technique and the crystals were harvested after a period of 8–10 days. Photograph of APP is shown in Fig. 1. Results and discussion FT-IR Fourier transform infrared spectrum (FT-IR) was recorded using AVATAR 330 FT-IR spectrometer by KBr pellet technique in the spectral range of 400–4000 cm1. Theoretical calculations were carried out using DFT/B3LYP/6-31G (d, p) method with Gaussian 09W program [10] package installed in personal computer. In an organic crystal, the crystalline field is usually not strong enough to split the internal modes of the molecule, which may be accidentally degenerate. The comparison of experimental and theoretical FT-IR spectra is shown in Fig. 2. The calculated wavenumbers all are positive and confirm the optimized structure of the title molecule as the most stable conformer. Some bands in theoretical IR spectra were not observed in the experimental spectrum. It is


important to note that computed wavenumbers correspond to gaseous phase of isolated molecular state and this results in variations in wavenumbers. NH2 vibrations The NH2 asymmetric stretching vibrations give rise to a strong band in the region 3365 cm1 and the symmetric NH2 stretching was observed as a weak band in the region 3235 cm1. The theoretically computed wavenumbers for the normal modes fall in the range 3457, 3456, 3432, 3415, 3402, 3401 cm1 by DFT/B3LYP/6-31G (d, p) method. The lowering of NH2 group stretching vibration wavenumbers is due to the intramolecular hydrogen bonding of nitro group of picrate to pyridinium amine as evidenced by the single crystal XRD. CAO vibrations The carbonyl stretching vibrations are observed in the region 1648 cm1 with a strong intense peak and the theoretical values are calculated as 1672 cm1. The CAOAC bending modes of vibrations are predicted weak bands at the frequency of

Fig. 3. UV–vis spectra of APP (a) experimental and (b) theoretical.


A. Aditya Prasad et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 805–813

686 cm1 and theoretically computed vibration mode is observed at 663 cm1. [email protected] and CAC vibrations The ring stretching vibrations are very much important in the spectrum of benzene ring and its derivatives, highly characteristic of aromatic ring itself. The ring [email protected] and CAC stretching vibrations, known as semicircular stretching usually occurs at the region 1400–1625 cm1. The observed bands are at 1648, 1598, 1269, 988 and 822 cm1. Theoretically computed vibrations are present at the range of 1672, 1548, 1254 and 891 cm1 respectively. CACAC bending vibrations are observed at the range of 1189, 1277, 377, 1189 and 724 cm1. CACACAC torsion mode of vibrational peaks is predicted at the range of 965, 609 and 540 cm1. Theoretical modes of vibrations are slightly varied from the experimental frequencies. OAN vibrations A broad peak at 1505 cm1 corresponds to NO2 asymmetric stretching. The sharp peak at 822 cm1 is attributed to NO2 wagging. Theoretically assigned OAN stretching vibrations are observed at the range of 1686, 1669, 1406, 1659, 1371 and 1364 cm1. OANAO bending vibrations are detected at the range of 908, 824, 724, 881 and 727 cm1. OANAC bending modes of vibrations are predicted at the range of 577, 373 and 433 cm1. The peaks are showing less intensity in the theoretical calculations because theoretical computations pertaining to gaseous state and experimental spectrum are recorded in solid mode.

NAH vibrations The very broad peak at 1598 cm1 corresponds to NH asymmetric bending. Theoretically calculated HANAC bending modes of vibrations are observed at the range of 2393 and 1040 cm1. HANAH bending mode of vibrations are predicted at the range of 1426, 1310, 1277, 1274, 2393 and 1364 cm1. UV–vis studies UV–vis spectrum of APP was recorded using CARY5E UV–vis spectrophotometer (Fig. 3a) in the solid phase (barium sulfate). It shows minimum absorption in the visible region. To support experimental observations, the theoretical electronic excitation energies, absorption wavelengths and oscillator strengths were calculated by the CIS–FC, ZINDO and TD–DFT methods using Gaussian 09W [10] program with basis set 6-31G(d,p). Excitation energy from CIS–FC method is 266.92 nm, experimentally is at 250 nm wavelength. The theoretical absorption spectrum of APP obtained from CIS–FC method is shown in Fig. 3b. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of APP are shown in Fig. 4. The frontier orbital gap facilitates in characterizing the chemical reactivity and kinetic stability of the molecule. The red and green colors represent the positive and negative values for the wave function. The HOMO is the orbital that primarily acts as an electron donor and the LUMO is the orbital that mainly acts as an electron acceptor [11–12]. The energy gap between HOMO (0.3114 au) to LUMO (0.0634 au) of the molecule is about 0.3749 eV. The HOMO and LUMO energy gap explains the

Fig. 4. HOMO–LUMO energy gap of APP.

A. Aditya Prasad et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 805–813


Table 1 Mulliken charge population [HF, 6-31G (d, p)] of APP. Atom

Mulliken atomic charge

1O 2O 3O 4O 5O 6O 7O 8C 9C 10 C 11 C 12 C 13 C 14 N 15 N 16 N 17 N 18 C 19 C 20 C 21 C 22 C 23 N 24 H 25 H 26 H 27 H 28 H 29 H 30 H 31 H 32 H

0.60877 0.454915 0.459346 0.420478 0.419105 0.472903 0.519281 0.569788 0.186835 0.065308 0.249984 0.105682 0.226549 0.229054 0.240533 0.223104 0.965744 0.227995 0.304483 0.447508 0.302072 0.252183 0.973721 0.328417 0.326546 0.562478 0.322623 0.273445 0.273813 0.301244 0.414772 0.414936

eventual charge transfer interactions taking place within the molecule. Mulliken [13] has derived the wave functions for the ground state and excited states of the complex and the charge distribution over the atoms thus produces a way of examining the proton transfer process. The proton transfer occurs from picric acid to pyridine. The charge distributions calculated by the Mulliken method [14] for the equilibrium geometry of APP is given in Table 1. The charge

Fig. 6. Tauc plot of APP (a) direct band gap energy and (b) indirect band gap energy.

Fig. 5. Mulliken charge transfer of APP indicated by color.


A. Aditya Prasad et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 805–813

distribution on the molecule has an important influence on the vibrational spectra. Mulliken charge transfer is differentiated by color as shown in Fig. 5. The direct and indirect band gap energies (Fig. 6) of the specimen are calculated as 4.79 eV and 4.96 eV reflections from the Tauc plot by the application of Kubelka–Munk algorithm [15] (theory provides correlation between reflectance and concentration).

FðRÞ ¼ ð1  RÞ2 =2R ¼ a=s ¼ Ac=s where F(R) is the Kubelka–Munk function, R is the reflectance of the crystal and s is scattering coefficient, A is the absorbance and c is concentration of the absorbing species.

Scanning electron microscopy (SEM) analysis The surface morphology was observed using a JEOL JSM 5610 LV scanning electron microscope. SEM images of the APP crystals are shown in Fig. 7. Microscopic structure shows patches with rod like shapes confirming the roughness of the crystal.

Single crystal XRD The structural analysis of APP crystal was carried out for the grown crystal using Bruker AXS (Kappa Apex II) X-ray diffractometer at 293 K temperature. APP crystallizes in the monoclinic system with space group P21/c and the cell parameters are, a = 8.513 Å (±0.015), b = 11.33 Å (±0.02), c = 14.33 Å (±0.03) and b = 104.15° (±0.019), V = 1340 A3 (±6). Earlier the program(s) used to solve & refined structure is SHELXS97 [16]. In the present study the structure was solved using with the Olex2 [17] structure solution program using charge flipping [18] and refined with the Olex2. Refine refinement package using Gauss–Newton minimization. R1 factor is very low with final R1 = 0.0053 (number of reflections 2167). The solved structure is shown in three dimensional graphical images (Fig. 8). Slight variations in the bond length and angles of theoretical and experimental values are observed (Table 2). A three-dimensional view of the electron density in crystal structure is determined from X-ray diffraction experiments. X-rays scatter from the electron clouds of atoms in the crystal lattice. The diffracted waves from scattering planes h, k, l are described by struc-

Fig. 7. SEM photograph of APP.

Fig. 8. Molecular structure and numbering scheme of APP showing OAHAN interaction.


A. Aditya Prasad et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 805–813 Table 2 Experimental and theoretical bond lengths (Å) and angles (°) of APP. Atom

Single crystal XRD

[HF, 6-31G (d, p)]

O(1)AC(1) N(3)AO(7) N(3)AO(6) N(3)AC(6) O(3)AN(1) N(1)AO(2) N(1)AC(2) O(4)AN(2) N(5)AC(10) N(5)AC(9) N(5)AH(5A) N(2)AO(5) N(2)AC(4) C(1)AC(6) C(1)AC(2) C(5)AC(6) C(5)AC(4) C(5)AH(5) C(2)AC(3) C(4)AC(3) C(3)AH(3) C(7)AN(4) C(7)AC(8) C(7)AC(11) N(4)AH(4A) N(4)AH(4B) C(9)AC(8) C(9)AH(9) C(8)AH(8) C(11)AC(10) C(11)AH(11) C(10)AH(10)

1.2360(5) 1.2087(5) 1.2178(5) 1.4442(6) 1.2262(6) 1.2033(6) 1.4531(6) 1.2182(6) 1.3298(7) 1.3387(7) 0.86 1.2226(6) 1.4361(6) 1.4509(6) 1.4525(6) 1.3681(6) 1.3792(6) 0.93 1.3568(7) 1.3792(7) 0.93 1.3217(7) 1.4028(7) 1.4038(7) 0.86 0.86 1.3448(7) 0.93 0.93 1.3531(8) 0.93 0.93

1.2254 1.2047 1.1936 1.4419 1.1963 1.1963 1.4496 1.1972 1.3334 1.3355 1.0206 1.1975 1.4345 1.4483 1.4491 1.3738 1.3824 1.0705 1.3628 1.3934 1.0709 1.3394 1.4084 1.411 0.9921 0.992 1.3624 1.0737 1.0731 1.361 1.0731 1.0723

Bond angles (°) O(7)AN(3)AO(6) O(7)AN(3)AC(6) O(6)AN(3)AC(6) O(2)AN(1)AO(3) O(2)AN(1)AC(2) O(3)AN(1)AC(2) C(10)AN(5)AC(9) C(10)AN(5)AH(5A) C(9)AN(5)AH(5A) O(4)AN(2)AO(5) O(4)AN(2)AC(4) O(5)AN(2)AC(4) O(1)AC(1)AC(6) O(1)AC(1)AC(2) C(6)AC(1)AC(2) C(6)AC(5)AC(4) C(6)AC(5)AH(5) C(4)AC(5)AH(5) C(3)AC(2)AC(1) C(3)AC(2)AN(1) C(1)AC(2)AN(1) C(5)AC(6)AN(3) C(5)AC(6)AC(1) N(3)AC(6)AC(1) C(5)AC(4)AC(3) C(5)AC(4)AN(2) C(3)AC(4)AN(2) C(2)AC(3)AC(4) C(2)AC(3)AH(3) C(4)AC(3)AH(3) N(4)AC(7)AC(8) N(4)AC(7)AC(11) C(8)AC(7)AC(11) C(7)AN(4)AH(4A) C(7)AN(4)AH(4B) H(4A)AN(4)AH(4B) N(5)AC(9)AC(8) N(5)AC(9)AH(9) C(8)AC(9)AH(9) C(9)AC(8)AC(7) C(9)AC(8)AH(8)

120.97(4) 120.46(4) 118.57(4) 122.49(4) 119.83(4) 117.65(4) 120.13(5) 119.9 119.9 122.85(5) 118.58(4) 118.57(4) 125.31(4) 123.54(4) 111.09(4) 119.03(4) 120.5 120.5 124.51(4) 115.80(4) 119.68(4) 115.71(4) 124.66(4) 119.59(4) 121.00(4) 119.73(4) 119.24(4) 119.67(4) 120.2 120.2 120.65(5) 122.46(5) 116.88(5) 120 120 120 121.24(5) 119.4 119.4 120.35(5) 119.8

122.7433 118.9805 118.2418 123.796 118.593 117.5592 121.7112 121.6672 116.6172 124.2596 117.8078 117.9326 124.54 122.8849 112.4846 119.8534 120.0468 120.098 123.8819 116.8865 119.2316 116.5916 123.366 120.0384 120.5242 119.7418 119.7167 119.5633 120.2103 120.2219 121.0206 120.8791 118.1002 121.061 121.1959 117.7296 120.904 115.2296 123.8643 119.1597 119.9221

Table 2 (continued) Atom

Single crystal XRD

[HF, 6-31G (d, p)]

C(7)AC(8)AH(8) C(10)AC(11)AC(7) C(10)AC(11)AH(11) C(7)AC(11)AH(11) N(5)AC(10)AC(11) N(5)AC(10)AH(10) C(11)AC(10)AH(10)

119.8 119.42(5) 120.3 120.3 121.98(5) 119 119

120.9177 119.0766 120.047 120.8761 121.0461 115.8716 123.0818

ture factors. The electron density as a function of position x, y, z is the Fourier transform of the structure factors.

qðxyzÞ ¼ 1=V


FðhklÞ exp½2piðhx þ ky þ lzÞ


The intensity of these reflections is measured for determining the distribution of electrons in the crystal. The result is a map of the crystal that shows the distribution of electrons at each point, which may then be interpreted to find coordinates for each atom in the crystallized molecules. Electron density mapping is generated by using ShelXle [19]. Electron density mapping of F(obs) and Fo–Fc is shown in Fig. 9 and 2132 reflections are used for refinement. An electron density map q(x, y, z), which describes the intensity of the electron density at each point in real space. This reflects the fact that regions in space with a higher electron density will scatter more X-rays, although this is not what you measure directly. Once the mean and the standard deviation (r) of the intensity across the entire map are calculated, the intensity of every point in q(x, y, z) can be described in standard deviation (r) units away from the mean.

P ½FðobsÞ  FðcalcÞ P ½FðobsÞ

Fo–Fc map = 0.04 eÅ3 (r = 0.014). Fo map = 1.3 eÅ3 (r = 1.1). From the electron density mapping we observed that there is no structural disorder in the solved structure so that R factor is very low (0.005) compared to the reported value (9). In the crystal, the cations and anions are linked via NAH  O hydrogen bonds involving the phenolate O atom and the nitrogen atom in the pyridinium ring. The ionic pairs are linked into a ribbon-like structure along by CAH  O hydrogen bonds. This is due to the aggregation of the donor–acceptor molecules manner which contributes to the bulk susceptibility from intermolecular hydrogen bonding. Powder XRD The powder XRD pattern analyzed by Philips X’pert Pro Tripleaxis X-ray diffractometer and refined by Rietveld method with RIETAN-2000. The sharp nature of peaks in XRD pattern indicates the good crystallinity of the grown crystal. All observed reflection peaks were indexed. Most of the peak positions in powder XRD and simulated XRD pattern from single crystal XRD coincide (Fig. 10). However, the relative intensities differ. This could be due to the preferred orientation of the sample used for diffractogram measurement. Also, the mosaic spread of powder and single crystal pattern may differ resulting in intensity variations. First-order molecular hyperpolarizability Theoretical calculations were performed using the Gaussian 09W [10] by Hartree–Fock method, a program package on a


A. Aditya Prasad et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 805–813

Simulated Experimental


bxxx bxxy bxyy byyy bxxz bxyz byyz bxzz byxx bzzz btot (1030)

383 12-4







Counts 10

Table 3 The calculated b components, btot value (esu), dipole moment (l, D) and HOMO– LUMO (eV) characteristic of APP.



131 3-6 1

Fig. 9. Electron density mappings of APP (a) Fobs, (b) Fobs–Fcal and (c) Fobs & Fobs–Fcal.

30 Position [°2 Theta]



Fig. 10. Simulated and experimental powder XRD patterns of APP.


1217.401 270.237 490.608 83.433 156.793 243.907 41.029 94.337 15.795 15.357 7.394 18.88 0.31144 0.06349 0.37493

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personal computer without any constraints on the geometry using 6-31G (d, p) as the basis set [20] with GAUSSVIEW 5.0 molecular visualization [21]. The calculated and first-order molecular hyperpolarizability (b) and dipole moment (l) of the specimen are 7.394  1030 esu (>26 times of urea) and 18.88 D respectively (Table 3). The maximum b is due to the behavior of nonzero l values. High beta value is a required property of an NLO material and it appears that by proper substitution it is possible to sustain nonlinearity at the macro level. Conclusions Pale yellow colored 4-aminopyridinium picrate crystals were grown in methanol and acetone (v/v, 1:1) and characterized. The product formation was confirmed by FT-IR and single crystal XRD analyses. Comparison of bond length and angles of theoretical and experimental values shows slight variations. APP crystallizes in a space group P21/c with four molecules in the unit cell. A good transmission in the visible region is observed with direct band gap energy of 4.79 eV and indirect band gap energy of 4.96 eV. The powder X-ray diffraction study shows the good crystallinity of the material. The crystal cohesion is achieved by intermolecular hydrogen bonds between picrate anions and amino groups of 4aminopyridinium ions. The electron density mapping shows no structural disorder in the solved structure with low R factor. Acknowledgements The authors thank the University Grant Commission (UGC), New Delhi, for financial support through research Grant No. 41270/2012 (SR), and A.A. is grateful to UGC for a project fellowship. The authors thank SAIF, IIT Madras, Chennai for providing single


crystal XRD facility. K.M. is thankful to CSIR [9/3(0009)2K11EMR-I], New Delhi, for the award of a senior research fellowship. References [1] A.S. Dayananda, J.P. Jasinski, J.A. Golen, H.S. Yathirajan, C.R. Raju, Acta Crystallogr., E 67 (2011) o2502. [2] M. Sethuram, G. Bhargavi, M.V. Rajasehakaran, M. Dhandapania, G. Amirthaganesan, Optik 125 (2014) 55. [3] V. Bertolasi, P. Gilli, G. Gilli, Cryst. Growth Des. 11 (2011) 2724. [4] R.P. Sharma, R. Sharma, R. Bala, P. Venugopalan, J. Coord. Chem. 58 (2005) 899. [5] R.S. Mulliken, W.B. Person, Molecular complexes, Wiley-Interscience, New York and London, 1969. p. 498. [6] A. Chandramohan, R. Bharathikannan, M.A. Kandhaswamy, J. Chandrasekaran, V. Kandavelu, Cryst. Res. Technol. 43 (2008) 93. [7] A. Chandramohan, R. Bharathikannan, J. Chandrasekaran, P. Maadeswaran, R. Renganathan, V. Kandavelu, J. Cryst. Growth 310 (2008) 5409. [8] G. Anandha Babu, A. Chandramohan, P. Ramasamy, G. Bhagavannarayana, Babu Varghese, Mater. Res. Bull. 46 (2011) 464. [9] P. Ramesh, R. Akalya, A. Chandramohan, M.N. Ponnuswamy, Acta Crystallogr., E 66 (2010) o1000. [10] M.J. Frisch et al., Gaussian-09, Revision A. 01, Gaussian, Inc., Wallingford (CT), 2009. [11] K.S. Thanthiriwatte, K.M. Nalin de Silva, J. Mol. Struct. (Theochem) 617 (2002) 169. [12] P.S. Liyange, R.M. de Silva, K.M. Nalin de Silva, J. Mol. Struct. (Theochem) 639 (2003) 195. [13] R.S. Mulliken, J. Am. Chem. Soc. 74 (1952) 811. [14] R.S. Mulliken, J. Chem. Phys. 23 (1955) 1833. [15] P. Kubelka, F. Munk, Ein beitrag zur optik der farbanstriche, Z. Tech. Phys. (Leipzig) 12 (1931) 593. [16] G.M. Sheldrick, Acta Crystallogr., A 64 (2008) 112. [17] O.V. Dolomanov, L.J. Bourhis, R.J. Gildea, J.A.K. Howard, H. Puschmann, J. Appl. Crystallogr. 42 (2009) 339. [18] L. Palatinus, Acta Crystallogr., B 69 (2013) 1. [19] C.B. Hubschle, G.M. Sheldrick, B. Dittrich, ShelXle: a Qt graphical user interface for SHELXL, J. Appl. Crystallogr. 44 (2011) 1281. [20] B. Schlegel, J. Comput. Chem. 3 (1982) 214. [21] A. Frisch, A.B. Nielson, A.J. Holder, GAUSSVIEW User Manual, Gaussian Inc., Pittsburgh (PA), 2000.