Charge Transfer Complexes Formed by Heterocyclic Thioamides and

Aug 29, 2017 - Tetracyanoethylene (TCNE) as one of the most versatile organic compounds is involved in various chemical reactions with electron transf...
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Charge Transfer Complexes Formed by Heterocyclic Thioamides and Tetracyanoethylene: Experimental and Theoretical Study Tatiana S. Kolesnikova,† Margarita S. Chernov’yants,*,† Mikhail E. Kletskii,† Oleg N. Burov,† Gennady I. Bondarenko,† and Pavel A. Knyazev‡ †

Chemical Department, Southern Federal University, Zorge st. 7, Rostov-on-Don 344090, Russia Institute of Physical and Organic Chemistry, Southern Federal University, Stachki av,194/2, Rostov-on-Don 344092, Russia

‡

S Supporting Information *

ABSTRACT: Tetracyanoethylene (TCNE) as one of the most versatile organic compounds is involved in various chemical reactions with electron transfer. Charge transfer complexes (CTCs) of a few antioxidants, nitrogen containing thioamides [pyrrolidine-2-thione (I), 1,3-H-imidazolidine-2-thione (II), 1,3-H-Imidazoline-2-thione (III), pyridine-2-thione (IV), 5-trifluoromethylpyridine-2-thione (V), 4-trifluoromethylpyrimidine-2-thione (VI), quinoline-2-thione (VII), 3,4,5,6-tetrahydropyrimidine-2-thione (VIII)] as π-donors and TCNE as π-acceptor were studied. The DFT PCM/UB3LYP/6-31+ +G(d,p) and SA-CASSCF quantum chemical calculations were used to study the structures and relative stabilities of these complexes in the ground and lowest excited electronic states. The formation of a weak molecular associates in the chloroform and acetonitrile solutions was confirmed by UV/vis and IR absorption spectroscopy. The stability constants and molar extinction coefficients were estimated by UV/vis spectroscopy. The highest stability in acetonitrile is found for associates formed by quinoline-2-thione and pyridine-2-thione with TCNE, the lowest one is found for CTC formed by imidazolidine-2-thione. Molecular associate formed by pyridine-2-thione and TCNE has the greatest stability in the chloroform solution. 5-Trifluoromethylpyridine-2-thione and 4-trifluoromethylpyrimidine-2-thione do not form CTC in CH3CN due to the presence of an electron acceptor group in the molecules. The molar extinction values of CTC vary within the range of 0.4 × 103 to 1.0 × 104 M−1 cm−1. An analytical strategy of thioamides identification based on wavelength and intensity of CTCs absorption band has been suggested.

1. INTRODUCTION

Independent and reliable methods for evaluation the antioxidant activity of heterocyclic thioamides are based on kinetic parameters of thioamide interaction with 2,2′-diphenyl1-picrylhydrazyl (chromogenic radical).9 However, the formation of CTC (or radical ion pair) between the thioamide molecule (π-donor) and π-deficient (TCNE) molecule (πacceptor) as a result of electron transfer theoretically is also possible (Scheme 2). Today there are a limited number of published studies on the interaction of thioamides with π-acceptor agents.10,11 A set of theoretical approaches have been used to study the ionization potentials and electronic charge transfer (CT) transitions in the complexes formed by TCNE and mono- and bicyclic thioamides.12

In the last few decades, charge transfer complexes (CTCs) have been applied to various domains, such as organic conductive materials,1,2 organic luminescent materials,3 and photovoltaic devices.4 Heteroaromatic thioamides are vital heterocyclic systems which influence the regulating function of the thyroid gland. The mechanism through which drugs containing thiocarbamide group exert antithyroidal activity currently attracts a great interest in developing of new drugs with fewer adverse side effects. Heteroaromatic thioamides may also be involved in the interception of free radicals that is caused by the presence of a thioamide group−active reaction center. The kinetic and thermodynamic characteristics of hydrogen atom transfer from a donor (D) to an acceptor (radical R•) are widely represented in scientific periodicals5−8(Scheme 1).

Scheme 2. Transfer of Electron from Donor (D) to Acceptor (A) with Formation of Radical Ion Associate

Scheme 1. Transfer of Hydrogen Atom from Donor (D) to Acceptor (Radical R•)

© 2017 American Chemical Society

Received: January 18, 2017 Revised: August 24, 2017 Published: August 29, 2017 7000

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Figure 1. Structures of the thioamides; (I) pyrrolidine-2-thione; (II) 1,3-H-imidazolidine-2-thione; (III) 1,3-H-imidazoline-2-thione; (IV) pyridine2-thione; (V) 5-trifluoromethylpyridine-2-thione; (VI) 4-trifluoromethylpyrimidine-2-thione; (VII) quinoline-2-thione; (VIII) 3,4,5,6tetrahydropyrimidine-2-thione.

The authors of a recent article13 have successfully constructed a series of novel CT crystals from pyren derivatives and 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) with tunable molar ratio, which could be controlled by solvent mediated method. X-ray diffraction data, vibrational spectroscopy, and thermal behaviors were exploited to investigate their structures. This implies that solvents play a significant role in CT and molar ratio between donor and acceptor. TCNE undergoes numerous reactions and is reported to present in many structural motifs. The review14 discusses the assignment of structure and formal charge for TCNEcontaining compounds. Numerous IR and Raman data of νCN absorptions and their frequencies provide insight with respect to the specific forms and charge on the TCNE fragment. The research is directed to encourage the synthesis of new TCNEbased materials by providing an enhanced ability to analyze and assign the structure of new materials. Dixon and Miller have prepared the dianion containing {[Co(C5Me5)2]+}2[TCNE]2− and studied its spectroscopic characteristics by IR, Raman, and UV/vis spectroscopic techniques.15 The crystal and molecular structure of the substance have been determined by single-crystal XRD analysis. Organic dianions (2,3-dichloro-5,6-dicyanobenzoquinone, (DDQ)2−) have been stabilized by (M(C5(CH3)5)+2) counterion, where M is iron or cobalt atom, which allowed to determine the structural and spectroscopic characterization of these dianions. The molecular and crystal structure of (M(C5(CH3)5)+2)(DDQ)2− has been determined by X-ray crystallography.16 Absorption spectra of the π−π complexes formed by several aromatic amines and nitrogen heterocycles (acting as donors) with acceptors, TCNE and chloranil, were measured in acetonitrile. DFT quantum chemical calculations in solvent were carried out to determine the probable geometric structures of the complexes that are responsible for the absorption bands. On the basis of the calculated results, which are in a good agreement with experiment, the nature and origins of the absorption spectra of the various molecular complexes were clarified.17 The CTCs formed by of thioamides containing heterocyclic nitrogen, oxygen, and sulfur atoms simultaneously and TCNE in CHCl3 and CH2Cl2 were studied by UV/vis and EPR spectroscopy.11 Spectral characteristics and formation constants are discussed in the terms of electron donor and electron acceptor (TCNE) affinity and the nature of organic solvent. Stability of CTCs with TCNE increased in the following order: benzoxazoline-2-thione < benzothiazoline-2-thione < 5-methylbenzimidazoline-2-thione. As previously reported18 for the first time a reliable method for evaluating of the antioxidant activity of thioamides derivatives of pyridine, quinoline, imidazole, pyrimidine, pyrrolidinebased on kinetic parameters of the thioamide

interaction with chromogenic radical was offered. Furthermore, CTCs formed by thioamide (electron donor) and molecular iodine as σ-acceptor, were studied.11,19−22 In light of these important results10 and with the lack of sufficient data about the interaction of thioamides with acceptors, it was of interest to study the interaction of thioamides as electron donors with TCNE as π-electron acceptor. The present paper reports the study of CTCs based on fiveand six-membered nitrogen containing heterocycles with TCNE using UV/vis, IR, and EPR spectroscopy. The experimental UV/vis spectra of different mixtures of donor and acceptor were used to evaluate the formation constants of complexes. With quantum chemical UDFT calculations, the structures of CTCs were studied, and the energies of electron transitions and ionization potentials were obtained and discussed. An analytical strategy of thioamides identification based on wavelength and intensity of CTC absorption band has been suggested.

2. EXPERIMENTAL SECTION Thioamides I, III, and IV were obtained from Alfa Aesar, II was obtained from Fluka, and V, VI, VII, VIII, and TCNE were supplied by Aldrich. The structures of these compounds are shown in Figure 1. Solvents used were of spectral grade. Chloroform was purified according to known procedure.23 Stock solutions of donors and acceptors were freshly prepared. The UV/vis absorption spectra were scanned on a Cary 50 spectrophotometer in quartz cells with the optical length of 1.0 cm at 370−720 nm. For the purpose of UV/vis spectral determination of the formation constants (β) and molar extinction coefficients (ε) stock solutions of the donors and acceptor in a proper solvent were freshly prepared prior to use. Spectra were recorded after the maximum formation of CTCs (immediately after mixing the components (II, VII, VIII), 1 day (I, III, IV) or 2 days (V, VI) in chloroform). The concentration of donors in the reaction mixtures was kept fixed at 1.0 × 10−3 M for I, III−VIII and 3.0 × 10−3 M for II while the concentration of the acceptor (TCNE) was varied from 1.0 × 10−3 to 9.0 × 10−3 M. First derivative EPR spectra of chloroform solutions of each CTC were recorded at X-band on a Radiopan SE/X 2543 spectrometer. Stock solutions of samples were prepared in deoxygenated chloroform prior to use. To extract the oxygen from chloroform approximately 500 mL of chloroform was refluxed for several hours under argon, and then it was distilled in the same way under argon. Spectra were referenced to an external standard of DPPH (2,2′-diphenyl-1-picrylhydrazyl). The isotropic spectra were simulated by taking into account the signals from radical cations of thioamides and radical anion of TCNE. The experimental spectra were best approximated by the theoretical data by minimization of the error functional: 7001

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∑ (Ii obs − Ii t)2 /N

Formation constants (β) and molar extinction coefficients (ε) of the investigated thioamides I−VIII with TCNE in CH3CN, and CHCl3 were determined by UV/vis spectroscopy. The Benesi−Hildebrand24 eqs 1 and 2 were used to evaluate the formation constants (β) and molar extinction coefficients (ε).

i

Iiobs

where are experimental intensities of the EPR signal processed as a file of points with a constant step along the magnetic field B. Iit (theoretical intensities at the same B values) were calculated as a sum of the Lorentz and Gaussian functions derivatives centered at the resonance values of B; N is the number of points. The line width was specified by the equation

l × CD 1 1 1 = + × εD·TCNE βD·TCNE εD·TCNE C TCNE Ai

ΔB = α + βmI + γmI 2

(1)

l × CD 1 1 = + × εD·TCNE βD·TCNE εD·TCNE Ai 1

where mI is the projection of the nitrogen nuclear spin onto the direction of the external magnetic field and α, β, and γ are parameters. The theoretical parameters (g, aN, aH, α, β, γ) were varied until the minimum of the error functional was obtained. The minimization was terminated when R reached the value at which the theoretical spectrum agreed well with the experimental data and further iterations did not lead to changes in the values of R. Generally, this was achieved at R < 0.02.

CTCNE −

(

Ai (l × εD·TCNE

)

(2)

where CD, CTCNE = concentrations of donor and acceptor, respectively. The absorption (Ai) of the CTC solutions was measured in each case. The ionization potentials (IPs) of thioamides were determinate out of the CT bands frequencies for the complexes with TCNE.25 IPs have been evaluated from empirical equation reported by Aloisi and Pignataro:26

3. RESULTS AND DISCUSSION 3.1. Spectral Characteristics and Formation Constants of the CTCs. Possible CT interaction within a molecular complex formed by an electron donor D and TCNE is shown in Scheme 3:

IP (eV) = 5.21 + 1.65 × 10−4νmax(CTC)

(3)

CT energies in CTCs were calculated on transformed Planck’s equation, eq 4:

Scheme 3. Possible Stages of Donor−TCNE Interaction

ECT =

The absorption spectra of the mixed donor−acceptor solutions are characterized by appearance of a new absorption bands in the visible region (370−720 nm). Neither donor nor acceptor absorbs in this region. Therefore, these new absorption bands are attributed to the formation of CTC. The UV/vis absorption spectra of solutions containing thioamide (I) at a constant concentration and TCNE at concentrations varied from 2.0 × 10−3 up to 9.0 × 10−3 M are represented in Figure 2. The band of pyrrolidine-2-thione· TCNE CTC appears at 484 nm.

hc λmax × 10 × 1.60 × 10−19 −7

(4)

−1

where νmax (cm ) or λmax (nm) is the wavenumber corresponding to the CT band. IP values of thioamides were calculated in acetonitrile from the absorption maxima data for CTCs. In this solvent the IP values for donor molecules decrease in the sequence I > II > IV > VIII > VII. In chloroform, the picture changes slightly, and IPs decrease in the sequence I, VII > VI > II > V > III > VIII > IV. Change of the quinoline-2-thione (VII) and pyridine-2thione (IV) positions in these series may be explained by the different polarities of solvents. Donors III, V, and VI do not form π-complexes in acetonitrile solutions. It should be noted that the plot of the CT energy (ECT) values of π-complexes in CH3CN vs ionization potentials of the thioamides as electron donors is approximately linear (Figure 3). This confirms that the observed new absorption bands are from CT. The obtained results are summarized in Table 1. For the complexes of thioamide with TCNE stability constants β were calculated according to the eqs 1 and 2. Then from the equation ΔG = −RT ln β, we have calculated Gibbs free energies of complex formation ΔG295. Their values did not exceed 3.5 kcal/mol, indicating a low stability of associates. The experimental data obtained correlate with previously published data for stabilization of complexes of TCNE with donors of different nature.27 In the series of CTCs based on azoles the most stable is pyrrolidine-2-thione in both solvents. The molar extinction values at the maximum absorption bands of complexes in chloroform vary in order of II > III > I and in acetonitrile I > II (III does not form CTC in CH3CN). It should be noted that the polarity of the solvent considerably influences the CTC band position and stability of I·TCNE complex. Stability constants for CTC formed by TCNE and thioazines (IV, VII,

Figure 2. Electronic absorption spectra of chloroform solutions for pyrrolidine-2-thione (CD = 1.0 × 10−3 M (1)), TCNE (CTCNE= 1.0 × 10−3 M (2)) and mixtures of donor (CD = 1.0 × 10−3 M) and acceptor (CTCNE ranged from 2.0 × 10−3 to 9.0 × 10−3 M). 7002

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Figure 3. Plot of CT energies for thioamides·TCNE CTCs vs IPs of thioamides. Figure 4. EPR spectrum of the supposed complex III·TCNE in the CHCl3 solution recorded at room temperature, C = 1.0 × 10−6 M. The experimental curve is shown by a solid line, and the theoretical curve is shown by a dotted line.

VIII) measured in acetonitrile are much higher than for the azole-based compounds I and II. At the same time V and VI do not form CTCs in CH3CN. The complexes of thioamides VI, VII, VIII show minor stability in CHCl3. The strongest light absorption has a CTC formed by 5-trifluoromethylpyridine-2thione (V) in CHCl3 (ε = 1.0 × 104 M−1 cm−1). 3.2. EPR and IR Spectroscopy. Study of the complexes formed in solutions at room temperature by TCNE and III, IV, and VII (C = 1.0 × 10−6 M) recorded the spectra, which were interpreted as the spectra of cation radical D+•. Hyperfine structure of these spectra is determined by the interaction of the unpaired electron with nitrogen nuclei (IN = 1) and hydrogen (IH = 1/2) (Figure 4). The values of isotropic gfactors (from 2.0028 to 2.0036) of these CTCs are close to the g-factor of a free electron (ge = 2.00232) which is typical for organic radicals. For the remaining complexes formed by TCNE (I, II, V, VI, and VIII at C = 8.0 × 10−4 M), EPR signals were not detected. These systems gave a clear but weak five-line hyperfine structure indicative of electron coupling to an I = 1 14N species superimposed upon a broad signal under identical experimental conditions. TCNE undergoes numerous interactions and is reported to present in many structural motifs.14,16,28 Identification of such forms and motifs can be challenging. For a wide range of systems, a successful attempt to reveal the nature of the molecular complexes of TCNE and a number of donors has been made by Miller and coauthors. Using some spectral data (IR, Raman, UV) and X-ray data they have managed to divide

the considered systems in accordance with the forms of TCNE existence. In the IR spectra the absorption frequencies corresponding to the oscillations of a CN triple bond were observed, and the registration of spectra was carried out in the absence of oxygen: νCN frequencies values provide insight with respect to the specific form and charge on the TCNE fragment. The transfer of an electron density from the thioamide moiety to TCNE was confirmed by the appearance of absorption bands in the region 2210−2140 cm−1, while formation of the dianion shows intense absorption band in the 2104−2070 cm−1 region.14 In the IR spectra of the investigated CTCs a set of bands at 2300−2100 cm−1 corresponds to formation of various forms of TCNE. Part of these bands may be attributed to the absorption of nitrile groups in the radical ion form resulting from single electron transfer. Also the spectra present absorption band of the nonassociated TCNE and absorption bands of molecular complexes. For example, the IR spectra obtained for VIII· TCNE exhibit three absorptions at 2100−2350 cm−1, 2169 cm−1, and 2202 and 2349 cm−1. The calculations of the absorption spectra for the complexes formed by I−VIII attribute the bands at 2350−2300 cm−1 exactly to the vibrations of TCNE nitrile groups in the molecular associates.

Table 1. Spectral Characteristics (λ, ε), Charge Transfer Energy (ECT), Gibbs Free Energies (ΔG295) of CTCs Formation, and Constants of Formation (β) for the Associates of Thioamides I−VIII with TCNE in CH3CN and CHCl3 donor·TCNE ε, M‑1 cm‑1

λ, nm donor

CHCl3

CH3CN

I II III IV V VI VII VIII

484 496 510 572 489 501 484 527

395 501 − 505 − − 518 517

CHCl3 2.18 4.99 2.84 2.70 1.00 3.25 3.46 1.82

× × × × × × × ×

103 103 103 102 104 103 103 103

β, M‑1

ECT, eV CH3CN

2.89 1.22 − 4.29 − − 1.61 4.53

× 103 × 103 × 102

× 103 × 102

ΔG295, kcal/mol

CHCl3

CH3CN

CHCl3

CH3CN

CHCl3

CH3CN

2.57 2.50 2.44 2.17 2.54 2.48 2.57 2.36

3.15 2.48 − 2.46 − − 2.40 2.40

79 2.0 22 185 111 47 24 13

37 13 − 338 − − 388 257

−2.56 −0.41 −1.81 −3.05 −2.76 −2.25 −1.86 −1.50

−2.11 −1.48 − −3.41 − − −3.49 −3.25

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Figure 5. Probable coordination, S···C and N···H distances in π-complexes, shifted π-complexes and σ-complexes: (A) I·TCNE; (B) III·TCNE; (C) IV·TCNE; (D) VII·TCNE. Systems (V, VI)·TCNE are stable in geometries close to IV·TCNE; systems (II, VIII)·TCNE are close to I·TCNE.

Thus, the results of EPR and IR spectroscopy measurements have not led to a definite, clear conclusion about the nature of the weak complexes formed by thioamides and TCNE. 3.3. Structural and Electronic Information. We have performed a DFT PCM/UB3LYP/6-31++G(d,p) quantum chemical calculations of CTCs formed by TCNE and systems I−VIII. In this connection, molecular associates of different structure (π-complex, shifted π-complex, and σ-complex; Figure 5A−D) were calculated to obtain structural and electronic information.29−31 The stationary points of potential energy surface (PES) were characterized by Hessian (frequencies) calculations. Solvent effects were considered for all PES stationary points on the basis of the polarizable continuum model PCM,32−35 chosen due to it well recommended effectiveness.33,35 All computations were carried out with the Gaussian 03 suite of programs.36

Three types of thermodynamically stable structures (for each structure, the number of negative eigenvalues of Hessian matrixes is equal to zero) were found on the PESs: σ-complex, π-complex, and shifted π-complex. For all associates, our calculations gave almost the same picture: π-complexes form more or less plane parallel stacked structures with the interplanar distance at the range 4.0−5.5 Å for II·TCNE and VII·TCNE, respectively. In the shifted π-complexes, one of the CCN groups lies above the CS bond (S···C distance ∼3 Å). These structures represent the most stable geometries almost for all complexes. It agrees well with those previously obtained for 1-methylimidazoline-2-thione and 3-methyl-1-ethoxycarbonilimidazoline-2-thione.12 This conclusion is totally consistent with the scheme of frontier MOs interaction: in all systems in-phase (but not very strong) interaction between sulfuric AOs in HOMOs and carbon AOs in LUMOs is present (Figure 6). Coulomb two7004

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When approaching the CN group to the thioamide by a distance less than the sum of the van der Waals radii (the distance CN··· HN is ∼2.2 Å), a sufficiently stable σ-complexes are formed. Only σ-complexation for V·TCNE energetically is more favorable, but the adducts of this type have lower values of dipole moment and CT, as compared to shifted π-complexes. The results of quantum chemical calculations of the stabilization energies, charge transfer values and the dipole moments of all types of CTCs in acetonitrile are shown in Table 2. The ability of different quantum mechanical schemes (in particular, DFT) to reproduce Gibbs free energy for intermolecular complexes were analyzed in refs 37 and 38. More detailed information on the results of quantum chemical calculations is given in Tables S1 and S2. CTCs formed by compounds IV, VII, and VIII, with sufficiently large values of the calculated dipole moments (13.4, 12.7, and 11.3 D) have the highest stability in polar acetonitrile solutions. The decrease of the dipole moments of the CTCs (down to 9.8 D) correlates with a decrease in the stability of complexes I and II in a polar medium. Also we have calculated the ionization potentials (IPs) of all complexes in solution as an important parameter for the complexes formation, and compared them to the experimental data. In all cases, we see a good correspondence between theoretical and experimental results (Table S1). Another important parameter, characterizing the stability of complexes, is the energy of CT from thioamide fragment to the TCNE. These values were estimated as the difference between frontier orbitals energies (ELUMO − EHOMO) of thioamide and TCNE (see Figure 6). Overall, they were ∼0.5 kcal/mol lower than that determined from experiment (Table 1), as a consequence of coexistence of D·TCNE σ- and π-complexes in reality (the energy difference of isomers is rather small, Table 2). In the investigated complexes we have calculated the CT values (Table 2), representing a total Mulliken charge on

Figure 6. Frontier MOs interaction in the D·TCNE shifted πcomplexes.

center attraction between positively charged H atom of heterocyclic moiety and negatively charged terminal N atom of TCNE is an additional factor of stabilization: for example appropriate Mulliken charges are approximately +0.16e̅ on H atom and −0.23e̅ on N atom for the complexes I·TCNE and III·TCNE. The HOMO−LUMO difference lies in the narrow interval 0.97−1.72 eV, and due to a weak interaction between C and S atoms the resulting HOMO′−LUMO′ difference is also rather insignificant (0.86−2.19 eV), Figure 6 and Tables S1 and S2.

Table 2. Relative Gibbs Free Energies ΔG*, Charge Transfer Values and the Dipole Moments of the Three Types of CTCsa compound pyrrolidine-2-thione (I) imidazolidine-2-thione (II) imidazoline-2-thione (III)

pyridine-2-thione (IV)

5-trifluoromethylpyridine-2-thione (V) 4-trifluoromethylpyrimidine-2-thione (VI) quinoline-2-thione (VII)

3,4,5,6-tetrahydropyrimidine-2-thione (VIII)

molecular complex geometry

ΔG, kcal/mol

CT, e̅

ECT, eV

μ, D

shifted π-complex σ-complex shifted π-complex σ-complex π-complex shifted π-complex σ-complex π-complex shifted π-complex σ-complex shifted π-complex σ-complex shifted π-complex σ-complex π-complex shifted π-complex σ-complex shifted π-complex σ-complex

8.5 7.6 8.2 9.4 9.3 7.0 8.5 6.2 7.2 7.1 9.0 8.1 8.8 9.3 7.1 8.4 8.3 7.7 8.1

0.168 0.040 0.162 0.031 0.088 0.285 0.035 0.003 0.239 0.045 0.127 0.023 0.061 0.006 0.002 0.170 0.043 0.243 0.040

2.02 1.28 2.05 1.38 1.18 1.91 0.86 1.20 2.00 1.09 1.97 1.39 1.96 1.57 1.30 1.88 1.21 2.19 1.29

9.852 7.795 10.446 8.441 7.533 12.915 8.233 8.983 13.391 8.982 7.400 4.779 11.798 11.0 9.010 12.678 8.893 11.300 9.203

ΔG values were calculated as a difference between the associate total Gibbs free energies and the sum of total Gibbs free energies of initial isolated thioamides and TCNE. DFT PCM/UB3LYP/6-31++G(d,p) calculations in acetonitrile. a

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crystals failed, preferable geometry and electronic structure of corresponding molecular associates was studied quantum chemically for different electronic states. The CT interaction leads to the formation of weak and close in energy associates: πcomplexes, shifted π-complexes, and σ-complexes. The most important general structural conclusion states that almost all associates are stable in π-shifted geometry. Stability constants of CTCs in chloroform and acetonitrile have been evaluated by UV/vis spectroscopy. The effect of solvent polarity on the stability of CTCs was found. An analytical strategy of the identification of thioamides by UV/vis spectroscopy has been suggested. The values of CT and the dipole moments of the molecular associates have also been determined. The formation of the molecular associates of thioamides with TCNE in the chloroform solution was confirmed by IR spectroscopy. From theoretical IR spectra, it follows that systems I−VIII should detect CN group vibrations at ∼2300 cm−1, and this is the second important general feature linking associates under investigation. The isotropic spectra of all complexes in the refined EPR experiments do not represent the hyperfine structures and hereby do not exhibit single-electron transfers in the considered weak D·TCNE complexes. The latest conclusion was supported by the quantum chemical calculations.

TCNE fragment or equal in absolute value total charge on thioamide fragment: the CT from one molecule to another lies in the interval 0.12−0.24 e.̅ In the molecular complexes the dipole moments directions and their absolute values correspond to the dipole moments in the isolated thioamides. In the associated molecules, the dipole moment values slightly increase. Also we believe that small CT values indicate the preferable existence of complexes in the ground singlet (S0) electronic state, which was chosen as a most favorable. To confirm the absence of the complexes biradical nature, the calculations by the EPR III method39−41 were carried out for the ground S0 and lowest excited T1 states of molecular associates. In the S0 state, the stability of a wave function was tested, and the calculations showed the absence of spin-electron decoupling. In the T1 electronic state, single occupied MOs are localized mainly on the sulfur atom and on the double CC bond of TCNE. In all associates the energy of the vertical excitation is rather significant (for example in the case of IV· TCNE is equal to 40.3 kcal/mol). This clearly indicates the existence of complexes in the ground S0 state without any hyperfine splitting. Complexes in the excited T1 state were also studied, and as it turned out, they were less stable. The average rising of energy in the case of optimized T1 states (adiabatic excitation) was approximately about 8−9 kcal/mol for σ-complexes and 12−13 kcal/mol for π-complexes. As for vertical excitation such complexes were less stable by 12−41 kcal/mol (Table S3). This important conclusion was checked by means of more sophisticated calculations using Firefly 8.1.0 QC package,42 which is partially based on the GAMESS (US) source code.43 Optimized geometries of complexes have been obtained in the framework of the state-averaged (ground state and four low-lying excited states) complete active space self-consistent field (SA-CASSCF) level of theory with 6e−,4o active space. Single point calculations were carried out for optimized geometry by XMCQDPT2 method (extended multiconfiguration quasi-degenerate second order perturbation theory).44 The cc-pVDZ valence-split basis of Dunning45 was used for all calculations. The results for two shifted π-complexesI·TCNE and III·TCNEare represented in Figure S1 and S2 and Tables S4 and S5. The same data were obtained for all other complexes, and it is evident that, for complexes under investigation, excited electronic states really are higher in energy than the ground singlet and not less than ∼40 kcal/mol. In connection with the received calculations data we have made the second attempt of the EPR studies: all measurements were recorded at X-band on Bruker EMX Plus spectrometer. To remove oxygen from solutions samples a standard freeze− pump−thaw technique was applied. The EPR study was carried out at temperatures of 298 and 77 K. In these measurements, complexes (I−VIII)·TCNE (C = 1 × 10−3 M and C = 8.0 × 10−4 M) did not show characteristic spectra for triplet biradicals. These results were obtained at room temperature, as well as at conditions of matrix stabilization (toluene/ chloroform, 77 K) of complexes IV·TCNE and VII·TCNE for which the hyperfine splitting was found earlier (see above). Therefore, both from experimental and theoretical data it is evident that for shifted π-complexes D·TCNE closed shell singlet (S0) electronic state is really the ground one.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b00564. Electronic and geometric characteristics of thioamides and molecular associates of thioamides with TCNE (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(M.S.C.) E-mail: [email protected]. ORCID

Margarita S. Chernov’yants: 0000-0001-6784-4521 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Quantum chemical calculations were financially supported by the Ministry of Education and Science of the Russian Federation in the frame of Governmental Contract No. 4.129.2014/K. Also we wish to acknowledge our indebtedness to Nina A. Kirdaneva and Alexander I. Turbin and especially to Anton V. Lisovin for SA-CASSCF calculations.

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REFERENCES

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