Dipole moments, charge-transfer parameters, and ionization potentials of the methyl-substituted benzene-tetracyanoethylene complexes

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1 Dipole moments, chargetransfer parameters, and ionization potentials of the methylsubstituted benzenetetracyanoethylene complexes R. K. CHAN AND S. C. LIAO Department of Chemistry, University of Western Ontario, London, Ontario Received September 22, 1969 Can. J. Chem. Downloaded from by on 02/05/18 The dipole moments of a series of chargetransfer complexes of methylbenzenes with tetracyanoethylene in carbon tetrachloride solutions at 25 "C and the various parameters derived from Mulliken's theory have been evaluated. The energies of various states of the complexes were calculated via their relationships with the parameters, chargetransfer transition energies, and heats of formation of the complexes by means of the variation principle. Vertical ionization potentials of the donors were obtained from the calculated energies of the dative structures of the complexes. The dipole moments contributed from the chargetransfer interaction can also be reasonably interpreted as chargetransfer energies in terms of vertical ionization potentials of the donors. Canadian Journal of Chemistry, 48, 299 (1970) Introduction It is worthwhile to investigate the characteristic relationships among various parameters of a molecular complex in accordance with the existing Mulliken theory (I), primarily among the dipole moment, heat of formation, and chargetransfer transition energy. The dipole moments and energy states of the iodine complexes with several aliphatic amines and with some oxygen, sulfurcontaining donors have been determined by Kobinata and Nagakura (2) and by Bhat and Rao (3), respectively. The former assume that the dipole moment of the nobond structure, po, is negligibly small compared with that of the dative structure, p,. In both papers the authors apply the results of the variation method with the assumption that the energy of the ground state is small compared to that of the dative state, W, << W,, and is approximately equal to the energy of the nobond state, W, z Wb; energy of the excited state is also assumed approximately equal to that of the dative state, WE z W, (4). In the present study tetracyanoethylene complexes with a series of methylbenzenes have been investigated. Dipole moments of the complexes have been determined in carbon tetrachloride solutions at 25 "C. Calculations for various chargetransfer parameters and energies of the different states are carried out according to Mulliken's theory and the variation method without assuming WG << W,, W, z W,, and WE z W,. Vertical ionization potentials of the donors obtained in this work are in satisfactory agreement with the literature values. Experimental Materials Tetracyanoethylene (TCNE) supplied by Eastman was recrystallized from chlorobenzene and purified twice by sublimation; m.p "C in a sealed tube. The liquid methylbenzenes and carbon tetrachloride were purified and dried over calcium hydride by fractional distillation and their purity was checked by gas chromatography. The solid methylbenzenes (I,2,4,5tetramethylbenzene, pentamethylbenzene, and hexamethylbenzene) were recrystallized in alcohol and dried in a vacuum desiccator; their purity was checked by gas chromatography. Procedure The solutions for the dielectricconstant determinations were prepared by adding increments of TCNE to carbon tetrachloride solvent containing the methylbenzene in a dried atmosphere. The concentration ratio of methylbenzene to TCNE in the solution was of the order of 100:l. The dielectric constants of the solutions were measured by the WTW DM01 Dipole Meter, based upon the superposition principle, using a cylindrical goldplated condenser cell, Type DFLI. Densities of the mixed solvents were measured by the use of a bicapillary pycnometer with scales on both stems, and some densities were calculated directly from the weights and densities of pure components. The refractive indices were determined by the use of a Bausch and Lomb Abbe3L refractometer. All measurements were made at a constant temperature of "C. The dipole moments of the complexes were evaluated by the Guggenheim method (5, 6) where k is Boltzmann's constant; No, Avogadro's number; M,, the molecular weight of the complex; dl and E, represent the density and dielectric constant of the mixed solvent at temperature T, respectively; a, and a, represent the slopes of a plot of dielectric constant and

2 300 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 square of refractive index, respectively, against weight fraction of the complex in the solution. Values of the dipole moments of the donors were selected from McClellan's book (7). Chargetransfer spectra of the complexes in CCIL solutions were measured by a Cary 14 recording spectrophotometer from 1 to 50 "C at 5to10degree intervals. The temperature was maintained to i0.5" by circulation of mater from a thermostat and was measured by means of a copperconstantan thermocouple with a Hewlett Packard 7100 B recorder. Since there was no absorption except by the complex itself in the visible region, the BenesiHildebrand equation (8) was used for calculating the equilibrium constant K,, in terms of mole fraction, where [TCNE] is the initial molar concentration of TCNE and ND refers to the donor concentration in mole fraction, 1 represents the thickness of the cell in cm, A is the absorbance, and e,, is the extinction coefficient of the complex. The heat of complex formation, AH, calculated from the temperature dependence of the equilibrium constant, is constant within experimental errors. Results According to the Mulliken theory, the ground state and excited state wave functions of a chargetransfer complex with 1 : 1 stoichiometric ratio may be described by the resonance wave functions where D and A represent the donor and acceptor molecules, respectively; a, b, a*, and b* are coefficients defining the participation of corresponding nobond structure (D... A) described by wave function +, and dative structure (Df... A) described by $,. The dipole moment of the complex at the ground state, p,, which is experimentally determined, can be expressed by where vi is the vector distance of the ith electron from any convenient origin and S is the overlap integral between $, and +,, which is taken as 0.1 for most weak complexes (3, 9); p, and y, refer to the dipole moments of the nobond and dative structures, respectively. On the basis of data from the Xray structure investigation of anbn molecular complexes formed by TCNE with naphthalene (10) and perylene (1 1) in which the 2 planar components are parallel to one another with an intermolecular distance of 3.3 A, the dipole moment of the dative structure is thus perpendicular to that of the nobond structure. Therefore, in terms of magnitude, eq. [5] can be written in the following form By substituting the experimental value of pg and known values of p,, p,, and S into eq. [6] with the aid of the normalization condition a2 + b2 + 2abS = I, we may evaluate a and b. The percentage of chargetransfer, b2 + abs, from the donor to the acceptor of the complex and the dipole moment contributed from the chargetransfer interaction in the complex, pct = p1 (b2 + abs), were also evaluated. It is possible to calculate the energies of the various states of the complex through the relationships of the ground state energy, WG, excited state energy, WE, chargetransfer transition energy at maximum wavelength, hv, and the heat of formation, AH, obtained from spectroscopic data. Starting with the variation principle for the energies of the ground and excited states, the following equations are derived (4) where W, and W, are the energies of the nobond and dative states, respectively; H,, is the resonance integral between these states. Equations [9] and [12] are obtained by elimination of a,b and a*, b* from eqs. [7], [8] and [lo], [ll], respectively. For the purpose of convenience, let PG and p, represent the two quantities (H,, WGS) and (H,, WES), respectively. Therefore, the energies of various states can be calculated without approximation from the chargetransfer spectra and dipole moment determination using the following equations [13] PG = [(a*s b*)ahv]/(aa* + bb*)

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4 302 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 [15] RG = +{ (S2hv 2PGS hv) + [(S2hv 2PGS hv)' 4PG2]'] [16] WG = AH [17] Wo = AH R, [18] WE=AH+hv [I91 RE = PE2/(W~ W ~) 210 [20] W1 = WE RE \ E where RG and RE refer to the resonance energies between the dative and nobond structure for the ground and excited states, respectively. The 200 results for the complete series of TCNEmethylbenzene complexes are summarized in Table 1. The energy diagrams of benzene and hexamethylbenzene complexes are also shown in Fig According to Fig. 1 [21] hv= wl wo + R E RG? 220 '8 0 o 8/ I Wt ( kcai/rnole I 60 I [22] W, = ID EA GI FIG. 2. Plot of vertical ionization potentials of donors vs. dative state energles of TCNEmethylbenzene complexes. Numbers refer to the donors as shown in Table 1. where ID refers to the vertical ionization potential of the donor, EA is the electron affinity of the acceptor, G1 represents the interaction energy between donor and acceptor at the dative state. GI is the sum of the Coulombic interaction at a separation of 3.3 and the energy, X, due to the polarization of the D'. A bond at the dative state [23 I GI = (e2/r) + X The value of X, which is expected to be a few kcal/mole (9, 12), was evaluated by plotting the literature values of the vertical ionization potentials against the energies of the dative states as shown in Fig. 2. It was found to be 2 kcal/mole for the TCNEmethylbenzene complexes. The electron affinity of TCNE is taken as 2.04 ev or 47.1 kcal/mole from Trotter (13). Therefore, it is possible to evaluate the vertical ionization potentials of donors directly from eq. [22] with the aid of the energy of the dative state. The results are listed in Table 2. Discussion C6(CH3)6 The dipole moment of the nobond structure FIG. 1. Energy diagrams of TCNE colnplexes with the can be taken as that of the benzene and hexamethylbenzene. due to the nonpolar nature of the acceptor,

5 CHAN AND LIAO: ON BENZENETETRACYANOETHYLENE COMPLEXES 303 TABLE 2 Vertical ionization potentials of methylbenzenes in kcal/mole Ionization potential Literature No. Compound Calcd. (14) (15) (16) (17) (18) (19) (20) (22) Can. J. Chem. Downloaded from by on 02/05/18 *Also ref. 21. TCNE; for the nonpolar donor it can be considered as zero. Thus eq. [6] is reduced to This means that the dipole moment of the complex is due to the partial chargetransfer interaction from the donor to the acceptor. The value of p,, caused by the charge transfer of one electron to the acceptor from the donor, with the intermolecular distance 3.3 A, is taken as 15.8 D for all complexes in this series. A change of 0.1 A in distance causes a change of only in the parameter a, in b, 0.2 in 100(b2 + abs), and 0.2 kcal/mole in most energy terms, which are within the experimental errors. For some of the donors, such as 1,2,3,4tetramethylbenzene, which do not have appropriate literature values of dipole moment, the values of p, derived from vectorsummation of bond moments are chosen. A variation of 0.07 D in p, due to the different literature data corresponds to a change of in a, in b, 0.1 in 100(b2 + abs), and 0.1 kcal/mole in energy terms. The values of a and b found are relatively insensitive to the dipole moment of the complex, as are the quantities derived from a and b. For instance, there is only a change of in a, 0.01 in b, 0.6 in 100(b2 f abs), and 0.4 kcal/mole in energy when the dipole moment of the complex is varied by 0.1 D, which is approximately equal to the exuerimental scatter of the data. In the calculation of the dipole moment of the complex, we assume that all of the acceptor in the solution is in the form of the complex, since the concentration ratio of donor to acceptor is of the order of 100 : 1 ; the complete complexation (23) is shown by the linear relationship of the increment of the dielectric constant of solution with the weight fraction of acceptor for a given amount of donor. For those complexes having two maxima in the absorption spectra, such as 1,4dimethylbenzene, 1,2,4trimethylbenzene, and 1,2,4,5tetramethylbenzene complexes, the wavelength of the maximum absorption without splitting is calculated according to Briegleb's empirical halfwidths relationship (24, 25) where v, and v, refer to the frequency at half the maximum intensity on the high and lowenergy sides of the peak located at v,,,. Several values of v,,, at different intensities are used to obtain an average value of v,,,. For instance, transition energies of 1,2dimethylbenzene and 1,3dimethylbenzene complexes are 67.3 and 66.7 kcal/mole, respectively. The lowerenergy band of the 1,4dimethylbenzene complex, 62.3 kcall mole, is close to the energies of the unsplit charge'transfer bands of the trimethylbenzene complexes, whereas the higherenergy band, 72.0 kcal/mole, is higher than the unsplit chargetransfer band of the methylbenzene complex. Using eq. [25], the average value of hv,,, for the

6 304 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, ,4dimethylbenzene complex was calculated to be 66.4 kcal/mole which is comparable and consistent with the unsplit spectra of 1,2 and 1,3dimethylbenzene complexes (see Table 1). The energy of the noband state, Wo, including the sum of the electrostatic energy and van der Waals energy, is expected to be small, ranging from 0.5 kcal/mole for the benzene complex to 2.3 kcal/mole for hexamethylbenzene. Thus, such complexes are formed under weak interaction. The resonance energy of interaction between the nobond and dative structures in the ground state, RG, is larger in magnitude than Wo, indicating that the resonance with the dative structure in the ground state is a major force for stabilizing the complex. It is shown in Table 1 that the contribution of resonance energy to the formation of the complex, AH or W, is by no means equal or nearly equal to Wo, and the forces leading to complex formation are not primarily electrostatic. On the contrary, the main contribution of the excited state energy, WE, is largely due to the dative structure energy, W,: 92% for benzene and 84% for hexamethylbenzene; the rest is contribution from the resonance energy, RE. The values of the estimated ionization potentials are in agreement with the literature data. The vertical ionization potential of 1,2,3,4tetramethylbenzene, which is not available in the literature, is given in Table 2. According to Matsen and coworkers (26), the relationship between chargetransfer energy and vertical ionization potential can be written as where Obviously, the coefficients C, and C,, which vary with the donor, are really not constants. However, they may be calculated and the values obtained are approximately 151 kcal/mole for C, and 552 (k~al/mole)~ for C,. The values of C, and C, obtained from Fig. 3 are 150 kcal/mole and 557 (k~al/mole)~, respectively, in good agreement FIG. 3. Plot of chargetransfer energies of TCNEmethylbenzene con~plexes vs. vertical ionization potentials of donors. Numbers refer to the donors as shown in Table 1. with the calculated values. The approximate linear relationship of these quantities can be expressed in terms of kcal/mole as The values of the constants in the linear equation have no immediate theoretical significance; the fact that the slope is less than unity is a result of the nonzero resonance interaction (27). A similar equation was obtained by Voigt and Reid (25). With increasing number of methyl substituents on the benzene ring, the dipole moment of the complex is increased. For the same number of methyl substituents, the more polar donor gives rise to higher dipole moment of the complex. The following relationship can be established between the dipole moments caused by the chargetransfer interaction, p, and vertical ionization potentials of the methylbenzenes as shown in Fig. 4. where pct is in Debye units and 6, in kcaljmole. It can also be shown that [31] (b2 + abs) = the lower the ionization potential of the donor for a given acceptor, the higher the degree of chargetransfer, i.e., the higher the dipole moment contribution from the chargetransfer interaction.

7 CHAN AND LIAO: ON BENZENETETRACYANOETHYLENE COMPLEXES 305 Can. J. Chem. Downloaded from by on 02/05/18 FIG. 4. Plot of dipole moments caused by chargetransfer interaction vs. ionization potentials of donors for TCNEmethylbenzcnc complexes. htumbers refer to the donors as shown in Table I. The authors are indebted to the National Research Council of Canada for financial support of this work. 1. R. S. MULLIKEK. J. Amer. Chem. Soc. 74, 811 (1952). 2. S. KOBINATA and S. NAGAKURA. J. Amer. Chem. Soc. 88, 3905 (1966). 3. S. N. BHAT and C. N. R. RAO. J. Amer. Chem. Soc. 90, 6008 (1968). 4. G. BRIEGLEB. ElektronenDonatorAcceptorKomp!exe. SpringerVerlag, Berlin p E. A. GUGGENHEIM. Trans. Faraday Soc. 45, 714 (1949). 6. J. W. SMITH. Trans. Faraday Soc. 46, 394 (1950). 7. A. L. MCCLELLAN. Table of experimental dipole moments. W. H. Freeman and Co., San Francisco H. A. BENESI and J. H. HILDEBRAND. J. Amer. Chem. Soc (1949). 9. W. B. PERSOK.' J. hem. ~hys. 38, 109 (1963). 10. R. M. WILLIAMS and S. C. WALLWORK. Acta Cryst. 22, 899 (1967). 11. I. IKEMOTO and H. KURODA. Bull. Chen~. Soc. Jap. 40, 2009 (1967). 12. T. M. CROMWELL and R. L. SCOTT. J. Amer. Chem. SOC. 72, 3825 (1950). 13. P. J. TROTTER. J. Cheni. Phys. 48, 2736 (1968). 14. F. MEYER and A. G. HARRISON. Can. J. Chem. 42, 2256 (1964). 15. F. H. FIELD and J. L. FRANKLIN. J. Chem. Phys. 22, 1895 (1954). 16. G. F. CRABLE and G. L. KEARNS. J. Phys. Chem (1962). H. BABA, I. OMURA, and K. HIGASI Bull. Chem. Soc. Jap. 29, 521 (1956). A. D. BAKER, D. P. MAY, and D. W. TURNER. J. Chem. Soc. B, 22 (1968). R. E. HO~IG. J. Chem. Phys. 16, 105 (1949). B. CAUTOIYE. F. CRASSO. and S. PIG~ATRARO. Mo1. Phvs (1966). V:K. POTAPO;, D. N. SHIGORIPI, A. D. FILYUGIVA, and V. V. SOROKIN. RLISS. J. Phys. Chen~. Engl. Transl. 40, 1256 (1966). G. BRIEGLEB, J. CZEKALLA, and G. REUSS. 2. Phys. Chem. Frankfurt, 30, 333 (1961). M. DAVIS. J. Chem. Educ. 46, 17 (1969). G. BRIEGLEB and J. CZEKALLA. 2. Phys. Chem. Flankfurt, 24, 37 (1960). E. M. VOIGT and C. REID. J. Amer. Chem. Soc. 86, 3930 (1964). S. H. HASTIRGS, J. L. FRAYKLIY, J. C. SCHILLER, and F. A. ~~ATSEV. J. Amer. Chem. Soc. 75,2900 (1953). R. S. MULLIKEN and W B. PERSON. Ann. Rev. Phys. Chern. 13, 107 (1962).