dicyanopyridyl? - Wiley Online Library

Special Issue Article Received: 15 June 2009,

Revised: 3 August 2009,

Accepted: 19 August 2009,

Published online in Wiley InterScience: 11 November 2009

(www.interscience.wiley.com) DOI 10.1002/poc.1622

Matrix isolation and magnetic parameters of septet 3,5-dicyanopyridyl-2,4,6-trinitrene Sergei V. Chapysheva **, Patrik Neuhausb, Dirk Groteb and Wolfram Sanderb * Septet 3,5-dicyanopyridyl-2,4,6-trinitrene was synthesized together with quintet 2-azido-3,5-dicyanopyridyl-4,6-dinitrene, quintet 4-azido-3,5-dicyanopyridyl-2,6-dinitrene, triplet 2,6-diazido-3,5-dicyanopyridyl-4-nitrene, and triplet 2,4diazido-3,5-dicyanopyridyl-6-nitrene by photolysis of 2,4,6-triazido-3,5-dicyanopyridine in solid argon at 4 K. The electronic and magnetic properties of the matrix-isolated nitrenes were studied using electron paramagnetic resonance (EPR) spectroscopy in combination with density functional theory (DFT) calculations. The fine-structure parameters of the nitrenes were determined with high accuracy from spectral simulations. All signals in the EPR spectra of the nitrenes, randomly oriented in the solid phase, were unambiguously assigned based on eigenfield calculations of the Zeeman energy levels and angular dependences of resonance fields. Copyright ß 2009 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this paper. Keywords: EPR spectroscopy; high-spin states; matrix isolation; molecular magnetism; nitrenes

INTRODUCTION Among all organic polyradicals, high-spin nitrenes have the largest zero-field splitting (ZFS) D-parameters and exhibit the strongest magnetic properties.[1–4] Such nitrenes model the molecular magnetic domains and are of considerable interest as model systems for investigation of ferromagnetic exchange interactions in organic molecules. Extensive studies have shown that septet trinitrenes 1a and 1b, formed during the photolysis of the corresponding triazides in 2-methyltetrahydrofuran (2MTHF) glass at 77 K, show jDj values of 0.098–0.100 cm1 and E values close to zero.[1] All attempts to generate the septet trinitrenes 1c and 1d under similar conditions were unsuccessful, presumably due to the high reactivity of these nitrenes toward 2MTHF.[1] However, septet trinitrene 1c could be isolated and spectroscopically characterized in argon at temperatures below 10 K.[3] This finding prompted us to investigate the photolysis of triazide 2 in argon at 4 K using electron paramagnetic resonance (EPR) spectroscopy. Here we report on the photochemical synthesis of septet trinitrene 1d in argon at 4 K and a detailed analysis of its EPR spectrum.

X-band EPR spectra were recorded with a Bruker–Elexsys E500 EPR spectrometer with an ER077R magnet (75 mm gap between pole faces), an ER047 XG-T microwave bridge, and an ER4102ST resonator with a TE102 cavity. The experimental technique for matrix isolation used in this study was similar to that described earlier.[3] Solid argon matrices doped with triazide 2 were prepared by the vacuum co-deposition of two separate molecular beams (Ar and triazide 2 vapor) on the tip of an oxygen-free highconductivity copper rod (75 mm length, 3 mm diameter) cooled at 4 K. The vapor of 2 was produced by an oven heating the polycrystalline 2 to 110 8C. The matrix-isolated samples were irradiated with a high-pressure mercury arc lamp, using a filter passing the light at l > 305 nm, and spectra were recorded at various irradiation times. The computer simulations of EPR spectra were performed by using the EasySpin program package (version 2.7.0).[6] The simulations were performed by using matrix diagonalization methods for S ¼ 1, 2, or 3 using the parameters n ¼ 9.60923 GHz, g ¼ 2.0023 and line widths DH ¼ 5 mT for S ¼ 1, 3 mT for S ¼ 2, and 2.5 mT for S ¼ 3, respectively. The EasySpin program package

* Correspondence to: W. Sander, Lehrstuhl fu¨r Organische Chemie II, RuhrUniversita¨t, D-44780 Bochum, Germany. E-mail: [email protected] ** Correspondence to: S. V. Chapyshev, Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russia. E-mail: [email protected] a S. V. Chapyshev Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russia

EXPERIMENTAL Triazide 2 was synthesized by the reaction of 2,4,6-trichloro-3,5dicyanopyridine with NaN3 according to a literature procedure.[5]

b P. Neuhaus, D. Grote, W. Sander Lehrstuhl fu¨r Organische Chemie II, Ruhr-Universita¨t, D-44780 Bochum, Germany

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SEPTET TRINITRENOPYRIDINE

was also used for the eigenfield calculations of the Zeeman energy levels for canonical orientations of the tenzor D (HjjX, HjjY, HjjZ) and for calculating the angular dependences of resonance fields on rotating the tensor D by angle Q in two planes (f ¼ 0 and f ¼ p/2, where Q and f are the Euler angles). All density functional theory (DFT) calculations were performed with the Gaussian 98 program package.[7] The geometries of the molecules were optimized by using the B3LYP method[8] in combination with the 6-311þG** basis set. The nature of the stationary points was assessed by means of vibrational frequency analysis.

RESULTS AND DISCUSSION Irradiation (2 min, l > 305 nm) of triazide 2, matrix isolated in argon at 4 K, led to the appearance of two strong EPR signals of triplet nitrenes at 7056 and 7346 G, and two weak signals of quintet dinitrenes at 3080 and 3300 G. On further irradiation, a large series of new signals in the 60–7000 G region appeared due to gradual accumulation of quintet dinitrenes 5 and 6, and septet trinitrene 1d. The final EPR spectrum recorded after 77 min irradiation time is shown in Fig. 1.

Figure 1. EPR spectra: (a) simulated spectrum of dinitrene 5 with DQ ¼ 0.209 cm1, EQ ¼ 0.0542 cm1; (b) simulated spectrum of dinitrene 6 with DQ ¼ 0.210 cm1, EQ ¼ 0.039 cm1; (c) simulated spectrum of trinitrene 1d with DS ¼ 0.1011 cm1, ES ¼ 0.0043 cm1; (d) experimental spectrum (n0 ¼ 9.60923 GHz) after 77 min of UV-irradiation of triazide 2 in solid argon at 4 K. T3, T4, Q5, Q6, S, and asterisks show signals of nitrenes 3, 4, 5, 6, 1d, and non-canonical orientations of trinitrene 1d

The EPR spectrum obtained during the photolysis of 2 in argon dramatically differs from that in frozen 2MTHF solutions, in which only signals of triplet nitrenes 3 and 4 were observed.[1] The presence of characteristic[3,4] signals of septet trinitrene 1d at 61, 292, 1795, 1955, 2096, 2435, 2556, 4400, 4700, 5180, 5340, 5860, 5970, and 6775 G as well as that of quintet dinitenes 5 and 6 at 61, 805, 940, 1555, 3080, 3300, 3410, 3600, 4100, 6265, and 6775 G demonstrated that all these highly reactive species are rather stable in argon at 4 K. In analogy to the photolysis of 2,4,6triazido-3,5-difluoropyridine,[3] strong EPR signals at 7056 and 7346 G are tentatively assigned to triplet nitrenes 3 and 4, respectively. DFT calculations predict that nitrene 3 has a lower spin density at the nitrene unit (rN ¼ 1.5650 and 1.5736 for 3 and 4, respectively), and therefore its EPR signal should appear at lower resonance fields.

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S. V. CHAPYSHEV ET AL. According to theory,[9] the effective spin Hamiltonian for molecules with S 1 represents a sum of two terms H ¼ gbHS þ SDS;

(1)

where the first term describes the Zeeman interaction of the total electron spin angular momentum S with the applied magnetic field, and the second term describes magnetic dipole–dipole interactions between unpaired electrons in the molecule. The traceless tensor D is referred to as the zero-field tensor and described by two scalar ZFS parameters, D and E, characterizing the electronic structure and magnetic properties of high-spin molecules. Four basic properties are attributed to high-spin nitrenes: (1) High localization of unpaired electrons at the nitrene units; therefore these units can be considered as spatially separated triplet centers. (2) Strong ferromagnetic exchange interactions (10 kcal/mol) between the triplet centers which results in large energy gaps between the ground high-spin state and excited low-spin states. (3) Very weak spin–orbit interactions; therefore the terms involving higher orders of Sz, Sx, and Sy in the magnetic spin Hamiltonian are negligibly small. (4) High anisotropy of the hyperfine interactions and, as a result, the lack of hyperfine splitting in the EPR spectra. [10]

Based on these features, Itoh has suggested a model that adequately describes the zero-field term of the septet spin state as the product of three interacting triplet centers S1 ¼ 1, S2 ¼ 1 and S3 ¼ 1. Such an approach gives a simple expression for the septet ZFS tensor DS: DS ¼

1 1 ðDt1 þ Dt2 þ Dt3 Þ þ Dij 15 15

(2)

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where Dt1, Dt2, and Dt3 are the zero-field tensors of triplets S1, S2, and S3, and the tensor Dij characterizes dipole–dipole interactions between triplet sites S1, S2, and S3.[10,11] This expression allows one to calculate the tensor DS for a septet molecule with the molecular geometry and Dt values of the triplet centers. In a simple case, when a septet molecule has D3h point symmetry (three nitrene units are magnetically equivalent and all three angles between the C—N bonds are Q ¼ 1208) and the tensor Dij is negligibly small, Eqn (2) results in ZFS parameters DS ¼ Dt/10 and ES ¼ 0. In accordance with this, recent matrix EPR studies have shown that septet 2,4,6-trinitreno-1,3,5-triazine with three magnetically equivalent nitrene units and Dt estimated to 1.461 cm1 showed a DS value of 0.1230 cm1 and an ES value of 0 cm1.[2] The second term in Eqn (2) characterizing the dipole–dipole interactions between the three nitrene units attached to the 2,4,6-triazinetriyl core was about 0.023 cm1 or 19% of the DS-value. When a septet molecule is axially asymmetrical, as the C2v symmetrical trinitrenes 1a, 1c, or 1d, a non-vanishing parameter ES appears in the tensor DS.[4] In this case, the parameters DS and ES depend on Dt and the angle Q between the C—N bonds of two symmetrical triplet sites (Fig. 2).[4] For all septet molecules with Q > 908, the Z-axis of the ZFS tensor DS is directly perpendicular to the molecular plane. When a septet trinitrene has two equivalent symmetrical nitrene units with Dt1 ¼ Dt2 ¼ Dt and the third nitrene unit with Dt3 ¼ Dt (1 þ l) where l ¼ (r3 r1)/r1,


Figure 2. Septet ZFS parameters DS and ES as a function of angle Q for the case of three triplet sites with the parameter Dt. Orientations of the principal axis DS toward the molecular structure are shown

Eqn (2) is transformed into DS ¼ Dt ð1 þ l=3Þ=10 2

ES ¼ Dt ½4 cos ðQ=2Þ 1 þ l=30

(3) (4)

In this case, the ratio Es/Ds is a function of both the angle Q and the parameter l: jES =DS j 31=2 DQ þ l =3 (5) where Du ¼ (2p/3 Q) and jDQj 1.[4] The first term in Eqn (5) appears due to the deviation of Q from 1208, and the second term due to the non-equivalence of the triplet units in C2v symmetrical septet molecules. Thus, by determining DQ and l from quantum-chemical calculations one can estimate from Eqn (5) the ratio ES/DS for septet trinitrenes. Recent matrix EPR studies revealed for septet trinitrene 1a[4] and 1c[3] DS values of 0.1019 and 0.1018 cm1 and ES values of R0.00325 and R0.0037 cm1, respectively, corresponding to Q ¼ 112.8–115.28 and l ¼ 0.049–0.054. The second term in Eqn (2) characterizing dipole–dipole interactions between the three nitrene units at the pyridinetriyl core was estimated to be less than 0.006 cm1. The powder EPR spectra of triplet, quintet, and septet nitrenes were simulated using the EasySpin program package,[6] operating with an exact numerical matrix diagonalization analysis of the spin Hamiltonian (1) for randomly oriented molecules with S ¼ 1, 2, and 3. Triplet pyridyl nitrenes show typical ZFS values DT around 1.0–1.2 cm1 and ET close to zero.[3,4] For quintet pyridyl2,4-dinitrenes ZFS values DQ ¼ 0.19–0.23 cm1 and EQ ¼ 0.039– 0.042 cm1 are frequently observed,[3] whereas for quintet pyridyl-2,6-dinitrenes DQ ¼ 0.19–0.22 cm1 and EQ ¼ 0.054– 0.056 cm1 are typical values.[3,4,12] For septet trinitrene 1c, the ZFS values DS ¼ 0.1018 cm1 and ES ¼ 0.0037 cm1 were determined.[3] Starting with these data, we simulated the EPR spectra (Fig. 1) of all five new nitrenes (3, 4, 5, 6, and 1d) and obtained ZFS values corresponding to the best fits of the experimental spectrum (Table 1). The assignments of signs of D and E were based on the properties of the tensor D for triplet,[13] quintet[12], and septet[4] nitrenes. The ZFS parameters obtained for the nitrenes 3–6 and 1d are in good agreement with the expectation from DFT calculations (Fig. 3). Thus, quintet dinitrene 5 has an about 88 smaller dipolar


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Table 1. Zero-field splitting parameters of nitrenes 3–6 and 1d (g ¼ 2.0023)

units attached to the 3,5-dicyano-substituted pyridine ring. The energy of such interactions DEij is described by DEij ¼ DS þ ðDt þ l=3Þ=10

Nitrene 3 (S ¼ 1) 4 (S ¼ 1) 5 (S ¼ 2) 6 (S ¼ 2) 1d (S ¼ 3)

D/cm

E/cm

þ1.0450 þ1.1590 þ0.2090 þ0.2100 0.1011

0.0000 0.0000 0.0542 0.0390 þ0.0043

angle Q than its isomer 6, and, as a result, a substantially larger ratio jEQ/DQj. The EPR spectra of 5 and 6 are very typical for quintet dinitrenes with Q ¼ 1138–1158 and 1208–1248, respectively.[3,4] From DFT calculations for trinitrene 1d a l value of 0.0492 is obtained. With this parameter l and the ratio jES/DSj from the experimental EPR spectrum, an angle Q1 of 114.28 is determined from Eqn (5), in good agreement with predictions from DFT calculations (Fig. 3). The ZFS parameters of trinitrene 1d also provide information about the dipole–dipole interactions between the three nitrene

(6)

The value DT of triplet nitrene 3 is 1.045 cm1, and the spin density r at the nitrene unit is 1.565. In septet trinitrene 1d the nitrene units in positions 2 and 6 of the pyridine ring show r1 ¼ 1.6144, corresponding to Dt ¼ 1.045 (1.6144/1.5650) ¼ 1.078 cm1. Using l ¼ 0.0492 and Dt ¼ 1.078 cm1, Eqn (6) results in DEij 0.005 cm1 or about 5% of the DS value. Nearly the same value of DEij (0.0052 cm1) has previously been estimated for septet trinitrene 1c. All these studies show that septet 2,4,6-trinitrenopyridines are typical representatives of super high-spin molecular systems with strong one-center spin– spin interactions and p-spin polarization, for which DS results mostly from interactions of electrons localized at the same nitrene units. Substituents in positions 3 and 5 of the pyridine ring of trinitrenes 1a, c, and d mainly affect the spin densities at the nitrene units and the dipolar angles Q1 (Fig. 3). Thus, despite the very small angle Q1 of 112.88, septet trinitrene 1a shows a relatively small value for ES due to the large negative parameter l (0.0542).[4] In contrast, septet trinitrenes 1c[3] and 1d with

Figure 3. Selected data of UB3LYP/6-311G** calculations of nitrenes 3–6 and 1a, c, d

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S. V. CHAPYSHEV ET AL. nearly the same parameters l (0.0489 and 0.0492, respectively) show different ES values, mostly due to different angles Q1. The range of such changes for DS and ES in septet 2,4,6trinitrenopyridines is rather narrow: 0.1011 to 0.1019 cm1 for DS and 0.0032–0.0043 cm1 for ES. To perform a complete and unambiguous assignment of all observable lines in the experimental EPR spectrum, the Zeeman energy levels for the canonical orientations (HjjX, HjjY, HjjZ) and angular dependences of resonance fields for nitrenes 5, 6, and 1d were calculated based on the exact numerical solution of the spin

Hamiltonian (1). These calculations show that only seven transitions (two Xi, three Yi, and two Zi) for the canonical orientations of quintet dinitrene 5 have a high probability. Four transitions at 61 (Y1), 805 (Z1), 3300 (Y2), and 3415 (Z2) G are observed in the experimental spectrum (Fig. 4). One additional transition (marked A in Fig. 4) at 3600 G arises from non-canonical orientations of quintet dinitrene 5 that is rotated in the ZX plane at Q ¼ 408 (the dependences of transitions in EPR spectra of quintet dinitrenes 5, 6 and septet trinitrene 1d from the Euler angles Q and f are given in the Supporting Information). In

Figure 4. Zeeman levels and allowed transitions of quintet dinitrene 5. ‘A’ shows the extra line from non-canonical orientations of dinitrene 5

Figure 5. Zeeman levels and allowed transitions of quintet dinitrene 6. ‘A’ shows the extra lines from non-canonical orientations of dinitrene 6

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SEPTET TRINITRENOPYRIDINE quintet dinitrene 6, nine transitions (three Xi, two Yi, and four Zi) for canonical orientations have a high probability, and the three at 940 (Z1), 1550 (X1), and 3300 (Y2) G are observed in the experimental spectrum (Fig. 5). Dinitrene 6 also displays two extra lines at 4400 and 6265 G from non-canonical orientations with Q ¼ 45 and 658 in the ZX- and ZY planes, respectively. It is interesting to note that the signals of dinitrene 6 are almost two times more intense than that of dinitrene 5, and of the two triplet nitrenes 3 and 4 the latter gives the stronger EPR signals. These

effects show that triplet and quintet nitrenes with higher spin densities at the nitrene units are photochemically more reactive and more efficiently undergo photodissociation of the remaining azido groups. The strong EPR signals of the septet trinitrene 1d in the experimental spectrum suggest that the high-spin nitrenes formed after the complete photolysis of all azido groups are photochemically rather stable in the argon matrix. According to the calculations the powder EPR spectrum of septet trinitrene 1d should contain 23 transitions of high probability (five Xi, seven Yi, and ten Zi) for the canonical orientations (Fig. 6) and 10 extra lines from molecules rotated in the ZY- and ZX planes at different Euler angles Q. Only eight transitions of the canonical orientations at 61 (Z1), 292 (Z2), 1955 (X1 and Y2), 2096 (X2), 2435 (X3), 2556 (Y3), 3250 (X4), and 3550 (Y4) G and ten extra lines of non-canonical orientations in the 1790–6780 G region are observed in the EPR spectrum. In general, the spectrum of the trinitrene 1d much resembles the X-band EPR spectra of the previously studied septet trinitrenes 1a[4] and 1c.[3] Only the line position and spacing between the Xi- and Yi transitions differ due to the slightly different parameters DS and ES in these trinitrenes.

CONCLUSION Matrix isolation provides a convenient way of generating and spectroscopically characterizing highly reactive trinitrenes even in cases where low-temperature organic glasses fail to stabilize these elusive molecules. The highest yields are obtained at very low temperatures below 10 K. The photolysis of 2,4,6-triazido-3,5dicyanopyridine 2 results in the subsequent loss of N2 from all three azido groups. No apparent regioselectivity for the cleavage of the azido groups is observed, and therefore both triplet nitrenes 3 and 4 are formed as initial photoproducts. Further photolysis results in the formation of the two quintet dinitrenes 5 and 6 and finally trinitrene 1d. The well-resolved EPR spectra of the oligonitrenes allowed us to determine the magnetic parameters with high accuracy. From these parameters, valuable information about the molecular structure and the spin densities at the triplet sites of quintet dinitrenes and the septet trinitrene were obtained. Thus, the dipolar angle Q1 of 114.28 derived from the magnetic parameters of the septet trinitrene is close to Q1 predicted by DFT calculations. A comparison of trinitrene 1d with the previously studied septet trinitrenes 1a and 1c reveals nearly the same spin densities at the triplet sites. The X-band EPR spectrum of septet trinitrene 1d much resembles the X-band EPR spectra of septet trinitrenes 1a and 1c. Due to the relatively small angle Q1 and parameter l, 1d shows the smallest DS (0.1011 cm1) and the largest ES (0.0043 cm1) of these three trinitrenes. The synthesis of the septet trinitrene 1d in an argon matrix indicates that the formation of 1d can successfully compete with ring expansions and other rearrangements. 2MTHF glass, on the other hand, is reactive toward the oligonitrenes. Presumably, these reactions take place directly after the photolysis before the intersystem crossing of the nitrenes produces their energetically more favorable, and less reactive, high-spin states.

Figure 6. Zeeman levels and allowed transitions of septet trinitrene 1d. Asterisks show the extra lines from non-canonical orientations of trinitrene 1d

Acknowledgements This work was financially supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie (WS), and the

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S. V. CHAPYSHEV ET AL. Russian Foundation for Basic Research (grant RFBR 09-03-91330DFG).

REFERENCES [1] S. V. Chapyshev, R. Walton, J. A. Sanborn, P. M. Lahti, J. Am. Chem. Soc. 2000, 122, 1580–1588. DOI: 10.1021/ja993131c. [2] T. Sato, A. Narazaki, Y. Kawaguchi, H. Niino, G. Bucher, D. Grote, J. J. Wolff, H. H. Wenk, W. Sander, J. Am. Chem. Soc. 2004, 126, 7846–7852. DOI: 10.1021/ja031794v. [3] S. V. Chapyshev, D. Grote, C. Finke, W. Sander, J. Org. Chem. 2008, 73, 7045–7051. DOI: 10.1021/jo800425k. [4] E. Ya. Misochko, A. V. Akimov, S. V. Chapyshev, J. Chem. Phys. 2008, 129, 174510. DOI: 10.1063/1.3005378. [5] S. V. Chapyshev, U. Bergstrasser, M. Regitz, Khim. Geterotsikl. Soedin. 1996, 67–73. [6] S. Stoll, A. Schweiger, J. Magn. Reson. 2006, 178, 42–55. DOI: 10.1016/ j.jmr.2005.08.013.

[7] M. J. Fisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, ,Jr R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Stain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonazalez, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonazalez, M. Head-Gordon, E. S. Replogle, JA. Pople, Gaussian 98. Revision A.9. Gaussian Inc., Pittsburgh, PA 1998. [8] A. D. Becke, J. Chem. Phys. 1993, 98, 5648–5652. [9] W. Weltner, ,Jr Magnetic Atoms and Molecules, Dover Publications, New York, 1989. [10] K. Itoh, Pure Appl. Chem. 1978, 50, 1251–1259. [11] N. Oda, T. Nakai, K. Sato, D. Shiomi, M. Kozaki, K. Okada, T. Takui, Mol. Cryst. Liq. Cryst. 2002, 376, 501–506. [12] E. Ya. Misochko, A. V. Akimov, S. V. Chapyshev, J. Chem. Phys. 2008, 128, 124504. DOI: 10.1063/1.2840351. [13] E. Wasserman, Prog. Phys. Org. Chem. 1971, 8, 319–337.

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