Vibronic structure of the near-infrared and visible electronic transitions

J. Phys. Chem. 1983, 87,3657-3664

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into incompletely surrounded bridge CO:

Thus, the reaction of one surrounded bridge CO is accompanied with the disappearance of the other surrounded bridge CO. If that is the case, the difference in the rate constant among three types of adsorbed CO is not essential, and the rates of the hydrogenation of three types of adsorbed CO species are essentially equal to each other and to the rate of CHI formation. The relative reactivities of adsorbed CO species seem to depend on the catalyst. Heal et al.l9 concluded from the comparison of IR spectra with catalytic reactions that linear CO is involved in CHI formation on Ni/Si02. Fujimoto et alZoconcluded from the IR and reactive thermal studies that bridge CO is more active than linear CO on Ru/Al2O3and Rh/A1203. Rate-Determining Step of Hydrogenation of Adsorbed CO. The formation of CHI from adsorbed CO requires the dissociation of the C-0 bond and the hydrogenation of carbon species. As mentioned above, the rate of disappearance of adsorbed CO measured by EDR agreed well with the rate of CH4 formation measured by PSRA. This indicates that the rate-determining step of CHI formation (19) M. J. Heal,E. C. Leisegang, and R. G. Torrington, J. Catal.,42, 10 (1976). (20) K. Fujimoto, M. Kameyama, and T. Kunugi, J. Catal., 61, 7 (1980).

3657

is the C-0 bond dissociation of the adsorbed CO species and the hydrogenation of surface carbon species proceeds rapidly. If the C-O bond dissociation proceeds rapidly and the hydrogenation of the surface carbon species is the rate-determining step, the rate constant of the C-0 bond dissociation measured by EDR should be much larger than that of CHI formation measured by PSRA. This does not agree with the results shown in Table I. Further, the absence of a C-H stretching band in the IR spectra supports the conclusion that CH, species are rapidly hydrogenated to CH,. We have concluded in the previow paper5 that the rate-determining step of CHI formation on supported Ni catalysts is the dissociation of the C-0 bond of partially hydrogenated CO species. A similar conclusion has been obtained on supported Pd catalysts.6 In the present study, appreciable adsorption bands could not be observed in the 0-H and C-H regions where the partially hydrogenated species would have the adsorption bands. However, this does not necessarily mean the absence of the partially hydrogenated CO species, because adsorbed CO and the partially hydrogenated CO species are considered to be in equilibrium and the concentration of the latter may be lower than the f~rmer.~f' After all, it can be concluded that the rate-determining step is the dissociation of the C-0 bond of the adsorbed CO species or the partially hydrogenated CO species. The detailed reaction mechanism will be discussed in a separate paper on the basis of the results of PSRA and the kinetic isotope effect6 Registry No. CO, 630-08-0; Pd, 7440-05-3; Ni, 7440-02-0;

methane, 74-82-8.

Vibronic Structure of the Near-Infrared and Visible Electronic Transitions of 7,7,8,8-Tetracyanoquinodimethane Radical Anion Ines Zanon' and Cesare Peclle Institute of Physical Chemistry, The Unlversiiy, 35100 Padova, Italy (Received:July 27, 1982; In Final Form: March 10, 1983)

The near-IR and visible absorption spectra of 7,7,8,8-tetracyanoquinodimethane(TCNQ), TCNQ-d,, and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (TCNQ-F,) anions in a 2-methyltetrahydrofuran (MTHF) glassy solution at 77 K are reported. The vibronic structure of the near-IR 2B3,-2BZg single electronic transition of the TCNQ anion is interpreted by taking advantage of deuterium and fluorine substitution and by a careful consideration of the intensity distribution. The dominant role in the vibronic coupling of the excited-state totally symmetric vibrational modes ut3= 1597,w', = 1257,and = 330 cm-' is definitely established, thereby improving substantially the previously reported assignment. The vibronic linear coupling constants have been calculated by a CNDO/S method, utilizing the Cartesian eigenvectors of the vibrational modes obtained from a reported normal-coordinateanalpis. The calculated values compare well with the experimental Franck-Condon (FC) factors. The proposed interpretation agrees with the available preresonance Raman excitation profile data if the latter are appropriately reinterpreted. A qualitative vibrational analysis has been attempted also for the visible absorption system for which the data are poorer and less favorable. The vibronic structure of this system, attributed to the overlapping of a strong 2B3u-2B2gand a weak 2A,-2Bzg.transition, is reasonably assigned to a coupling with the former transition of the = 1630 and o', = 885 cm-' mbrational modes, whereas no vibronic structure is identified for the second transition.

Introduction Research interest in the field of charge-transfer salts is rapidly increasing because these nonmetallic materials display physical properties which can reach metallic behavior.' They are made up of parallel chains of molecules arranged in stacks of donor and/or acceptor molecules, thereby acquiring the characteristics of quasi-one-dimen(1) L. Alcacer, Ed., "The Physics and Chemistry of Low Dimensional Solids", Reidel, Dordrecht, 1980, and references therein.

sional systems. Consideration of the molecular nature of these solids led to the idea that the interaction of conduction electrons with intramolecular optical phonons may contribute significantly to their physical properties. The relevance of this interaction has been well recognized2and, (2) M. J. Rice, C. B. Duke, and N. 0. Lipari, Solid State Commun. 17, 1089,1975; C. B. Duke in "Synthesis and Properties of Low-Dimensional Material", G. S.Miller and A. J. Epstein, Eds. Ann. N . Y. Acad. Sci., 313, 166 (1978);E. M. Conwell, Phys. Reu. Lett., 39,777 (1977); R. Bozio and C. Pecile, J. Chem. Phys.: Solid St. Phys., 13, 6205 (1980).

OO22-3654/83/2087-3657$OI.5O/O 0 1983 American Chemical Society


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naturally, suggests study of vibronic coupling at the level of a single molecular or ionic unit. We have undertaken a study of vibronic coupling in the lower electronic transitions of the radical anion of 7,7,8,8-tetracyanoquinodimethane(TCNQ), which is a leading structure in the field of organic charge-transfer systems. Previously reported assignments from independent analyses of electronic absorption3r4and Raman ~ p e c t r a led ~ - ~to conclusions that are not satisfactory and often contradictory. Thus, we briefly recapitulate the methods used for the assignment of vibronic coupling and the data so far available on the TCNQ anion. The assignment can be based on the analysis of the electronic absorption spectra and of the resonance Raman (RR) spectra and on semiempirical calculations. In the F i t case one has to face the problem of identifying the progressions in the absorption spectra, taking into account the intensity distribution or the Franck-Condon (FC) factors. The vibrational frequencies of the electronic excited states usually can be recognized by correlation with those of the ground state. The analysis of the RR spectrum refers to the fact that “a typical RR spectrum consists of two separate contributions, namely, a fluorescence type (RF) and a preresonance type (PR) contrib~tion”.~ Furthermore, it is possible, in principle, to get a good resolution of the spectra by studying the excitation profiles (EP), a methodwhich allows one to reach a high resolution also from solution ~ p e c t r a . ~ The electron-vibration interaction can be derived also from the fact that the FC factors are functions of some linear coupling constants gTi.l0 They are the derivatives of the difference between the potential curves of the initial and final states, AET, with respect to the normal coordinates Qi,gTi = [1/21/2hwi](ahEy/aQi)0, where wi is the frequency of the i-th normal mode. The TCNQ anion spectrum shows two strong absorption systems, one in the near-IR (A = 900-600 nm) and the other in the visible (A = 470-350 nm) spectral The first displays a rich vibrational structure, whereas the second exhibits, even at liquid-nitrogen temperature, only a few vibronic peaks superimposed on an electronic background. The vibrational envelope of the LiTCNQ near-IR absorption system, analyzed in 2-methyltetrahydrofuran (MTHF) glassy solution at 77 K,3*4has been intepreted as mainly due to vibronic coupling with the w5 and wg totally symmetric (aJ vibrational modes ( w ’ ~= 1264 and wrg = 335 cm-l).*l In the room-temperature RR solution spectrum5v6the w3, w,, and w g vibrational modes = 1615,a”, = 1390, w ” ~= 337 cm-’) have been shown to display a strong intensity enhancement, whereas a less marked enhancement has been observed for the w5 and w2 modes (d5 = 1195 and w f r 2 = 2206 cm-l). Moreover, from the E P analysis,’ the (3)I. Haller and F. B. Kaufman, J . Am. Chem. Soc., 98,1464 (1976). (4)F. B. Kaufman and I. Haller, Chem. Phys. Lett., 33, 30 (1975). (5)W. T.Wozniak, G. Depasquali, and M. W. Klein, Chem. Phys. Lett., 40,93 (1976). 98,4029 (6)D. L.Jeanmaire and R. P. Van Duyne, J. Am. Chem. SOC., (1976). (7)D.L. Jeanmaire and R. P. Van Duyne, J. Am. Chem. Soc., 98,4034 (1976). (8)R. Bozio, A. Girlando, and C. Pecile, J . Chem. SOC.,Faraday Trans. 2,71, 1237 (1975). (9)M. Mingardi and W. Siebrand, J. Chem. Phys., 62,1074(1975);W. Siebrand and M. 2.Zgierski, ibid., 71,3561 (1979). (10)L.S.Cederbaum and W. Domcke, J . Chem. Phys., 64,603(1976); N.0.Lipari, C. B. Duke, and L. Pietronero, ibid., 65,1165 (i976). (11) Primed and doubled primed values refer to the excited and ground-state frequencies, respectively, according to Mulliken’s spectroscopic convention, in J . Chem. Phys., 23,1977 (1955).

following excited-state frequencies have been proposed: 1613,~ ‘ = 4 1335,u‘? = 330,~ ’ = 5 1198,and ~ ’ = 2 2150 cm-’. Except for the assignment of the wg mode and of its excited-state frequency, the agreement in the assignment is very poor both for what concerns the vibrational modes involved and for their excited-state frequencies. The visible absorption system of MTHF glassy solution at 77 K of LiTCNQ has also been reported, but no attempt has been made to assign the few peaks of clear vibronic rigi in.^,^ Moreover, the vibrational modes involved in the vibronic coupling, even if not their excited-state frequencies, are suggested by the available PR scattering data,5p8which have shown that the w7, w3, w,, and w5 modes display strong intensity enhancement when one analyses a LiTCNQ solution in CH3CN at room temperature. As to the electronic states involved in the two TCNQ anion electronic absorption systems, some ab initio12and semiempirical13-15 MO calculations, including various amounts of configuration interaction (CI), have been reported. The results agree in the indication of a ground 2B 12-15,16 and a first excited 2B3ustate, but the absolute an$ relative energies of the next 2B3uand 2Auexcited states seem strongly dependent on the degree of CI considered. Consequently, the near-IR system appears attributable to a 2B3u-2B2ptransition. For the visible system the assignment remms uncertain between a 2B3u-%29 and a 2Au-2B29 transition, with the first apparently favored by the higher value of the calculated oscillator strength. The polarized spectra of KTCNQ single crystal13 and of LiTCNQ in liquid crystals3 confirm the assignment of the former system to a single transition long-axis polarized (2B3u-2Bzg).The same experimental measurements give for the visible system a weak component short-axis polarized (2Au-2B2g) and a strong one long-axis polarized (2B3u-2Bzg). The aim of the present paper is to clear up the vibronic assignment of the near-IR electronic absorption of the TCNQ anion and to attempt a vibronic assignment of the visible absorption. For this purpose we will take advantage of the substituent effect on the vibrational structure, considering the analogous spectra of LiTCNQ-d4 and LiTCNQ-F,. Furthermore, we reexamine the intensity distribution of the LiTCNQ absorption spectrum and reconsider the reported RR5s6and EP’ data. Additional support for the assignment is also sought through a calculation of the TCNQ anion linear coupling constants gyi,whose values can be used for comparison with the experimental FC factors. ~ ‘ = 3

Experimental Section TCNQ, TCNQ-d4, and TCNQ-F4 were prepared and purified as previously reported.”J8 The corresponding 1:l Li salts were obtained by using the method of Melby et al.19 MTHF (Uvasol Merck) was purified by distillation under vacuum from sodium, stirred with sodium and an(12)H. Johansen, Int. J. Quantum Chem., 9,459 (1975). (13)S.Hiroma, M. Kuroda, and H. Akamatu, Bull. Chem. Soc. Jpn., 44,9 (1971). (14)(a) D. A. Lowitz, J . Chem. Phys., 46, 4698 (1967);(b) H. T. Jonkman and J. Kommandeur, Chem. Phys. Lett., 15,496(1972);(c) K. Krogh-Jespersen and M. A. Ratner, Theor. Chim. Acta, 47,283(1978). (15)A. Bieber and J. J. Andre, Chem. Phys., 5,166 (1974). (16)z in plane axis along C=€ external bonds and x axis normal to the molecular plane. (17)B. Lunelli and C. Pecile, J . Chem. Phys., 52,2375 (1970);Gazz. Chim. Ital., 99,496 (1969). (18)R.C.Wheland and E. L. Martin, J . Org.Chem., 40,3101 (1975). (19)L. R. Melby, R. J. Harder, W. R. Hertler, W. Mahler, R. E. Benson, and W. E. Mochel, J . Am. Chem. SOC.,84,3374 (1962).

Vibronic Coupling in TCNQ Anion

The Journal of Physical Chemistty, Vol. 87, No. 19, 1983

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TABLE I: Vibronic Frequencies (F/cm-' ) and Assignments of the Near-IR Absorption System of TCNQ and TCNQd, Anions in MTHF at 77 Ka TCNQ-

IF1

b


loo 47 14.6 4.5 31 44.2 19.2 7 7.7 11.6 9.5 4 2.6 2.5 2.4 1.5 0.9 1 0.9 0.7 0.7

exptl 11685 1 2 015 1 2 337 1 2 640 sh 1 2 947 1 3 282 13 611 1 3 908 br 14 190 1 4 535 14 881 1 5 207 15 446 1 5 785 1 6 130 1 6 466 16 722 17036 17382 1 7 720 1 8 050 1 8 280 1 8 690

TCNQ-d,A5

330 652 955 1262 1597 1926 2223 2505 2850 3196 3522 3761 4100 4445 4781 5037 5351 5697 6035 6365 6595 7005

exptlC 11685 1 2 015 1 2 346 1 2 575 sh 1 2 947 1 3 263 1 3 596 1 3 888 1 4 190 1 4 518 1 4 847 1 5 165 15 442 15 770 1 6 098 1 6 420 1 6 717 1 7 018 1 7 364 17 671 1 8 008 1 8 280 1 8 650

isot shift A5

330 661 890 1262 1578 1911 2203 2505 2833 3162 3480 3757 4085 4413 4735 5032 5333 5679 5986 6323 6595 6965

exptl 0 0 +9 -65 0 -19 -15 - 20 0 -17 - 34 -42 -4 -15 -32 -46 -5 -18 -18 -49 -42 0 -40

calcd

-17 -17 -17 - 34 - 34 -17 - 34 -51

assignment UU

ub

UC

0 0 0

0 0 0

0

0 1 1

1 0 0

0 0 1

0

2 1

0 0 0 1 0 0 0 0 0 0 0 0 0 0 0

1 2 2 0 1 2 3

0 -17 - 34 -51 -68 -17 - 34

1 2 3 4 1 2

0 0 3 2 1 0 4 3 2

1 0 4 3

1 2

a Italicized frequency values correspond to those of the excitation profile peaks (ref 7 ) bearing in mind that the latter are Intensities relative to that of the 11685-cm-' band, assumed as 100. The rablue shifted when CH,CN solvent is used. tios 1:10:30 have been taken into account for the absorbance obtained from the three different concentrations used ( 2 X lo-$, 2 x lo-,, and 6 x mol dm-3,respectively, at room temperature. sh: shoulder; br: broad. Frequency values of bands with relative intensity less than 1 are affected by higher inaccuracy.

thracene, and distilled just before its use. Spectroscopic-grade ethanol (EtOH) was dried on molecular sieves. CH3CN (Uvasol Merck) was carefully dried through P205 and Na2C03. Solutions for low-temperature spectra were prepared in a glovebag under nitrogen atmosphere and transferred to l-cm quartz cells which were sealed and cooled to liquidnitrogen temperature in an optical Pyrex Dewar. Spectra at 77 K in MTHF were obtained for the three salts at concentrations ranging from 2 X to 6 X lo-, mol dm-3 at room temperature. The glassy solution spectra in EtOH were measured for LiTCNQ only in order to examine the solvent effect on the near-IR absorption system. This analysis of the low-temperature visible system is prevented by the presence of the dimeric species20 at analytical concentrations as low as 7 X mol dm-3. Room-temperature spectra of LiTCNQ were also obtained in CH3CN in order to correlate the vibronic absorption and EP' data. Absorption spectra were taken on a Cary 14 spectrophotometer equipped with a pen period attachment which allows one to decrease the noise level. The reading precision was about 1nm, that is, from 15 to 30 cm-' in the first absorption system and from 50 to 70 cm-' in the second one. Raman spectra of LiTCNQ-F4solutions in CH3CNwere measured in a rotating cell on a Jarrell Ash 25-300 spectrophotometer with the 457.9-nm exciting line of an Ar ion laser.

Results Near-Infrared Absorption System. Table I gives the vibronic frequencies of LiTCNQ and LiTCNQ-d4 in a glassy solution of MTHF at 77 K. The relative intensities and the frequency shifts from the 0-0 band (11685 cm-l) are also reported. Both frequencies and relative intensities of LiTCNQ are in good agreement with those previously The spectra of LiTCNQ in EtOH glassy so(20)

R.H.Boyd and w.D. Phillips, J. Chem. Phys., 43,2927 (1965).

lution display no appreciable modification except for a blue shift of about 80 cm-' over the whole absorption range. Relative to the MTHF spectra at 77 K, constant average blue shifts (cm-') were observed at room temperature: MTHF, 70; EtOH, 200; and CH3CN, 190. The spectra of light and deuterated salts exhibit a very similar intensity distribution and the isotopic shifts (sixth column, Table I) become clearly appreciable in going into the higher frequency range of the spectrum. Inspection of the isotopic effect shows that the sharp band at 13282 cm-' in the LiTCNQ spectrum is 19 cm-' red shifted in LiTCNQ-d,. Approximately the same displacement or its multiple is found for the sharp maxima (isotopic shift in parentheses) at 14535 (15), 14881 (34), 15785 (15), 16130 (32), and 16466 (46) cm-'. These results suggest vibronic coupling of a normal mode with an isotopic shift of about 17 cm-'. In the light salt, its frequency, as given by the distance from the origin of the first shifted band, is 1597 cm-'. Thus, this mode gives rise to a progression as well as to combinations with other vibrations. At distances from the origin less than 1597 cm-l, strong isotopically unshifted bands are found at 1262 and 330 cm-l. For the former one observes harmonics with decreasing intensity at 2505,3761, and 5037 cm-', which gives a 1257-cm-' average value (1256 cm-' in LiTCNQ-d4). This four-term progression corresponds to one previously asFor the 330-cm-' mode only the first harmonic is observed. On the basis of the three fundamental vibrations sorted out above, all but two vibronic bands of the spectra (Table I) can be represented by 3 = 300

+ u,w'a + ubw'b + u,w',

where vo0 = 11685 cm-', u, = 0, 1, 2, 3,4; w', = 1597 cm-' for LiTCNQ and 1580 cm-' for LiTCNQ-d,; u b = 0, 1, 2, = 1257 cm-l; u, = 0, 1, 2; w', = 330 cm-' for both 3,4; salts. The above proposed assignment is reported in Table I together with the isotopic shifts evaluated taking into

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The Journal of Physical Chemistry, Vol. 87, No. 19, 1983

Zanon and Pecile

TABLE 11: Vibronic Frequencies (Tlcm-')and Assignment of the Near-IR Absorption System of TCNQF, Anion in MTHF at 7 7 K

Ire1

exptl

AT

100 43 14 22 27 41 21 9.8 9.8

11494 11810 12117 12763 12943sh 13117 13417 14382 14668 15 058

316 623 1269 1449 1623 1923 2888 3174 3564

assignment

rine shift"

Vu

Vb

Vc

191 205 220 184

0 0 0 0

0 0

0 1 2 0

165 194 153 213 149

1 1 1 2 2

0 0 1

0 1

0 1 0 0 1

0 0


a The frequency shifts are evaluated with respect to TCNQ anion frequencies, taking into account the corresponding relative intensities.

account how the v, mode contributes. The agreement between the expected and observed isotopic shifts is satisfactory with some worsening at higher energy where the wavenumber determination is less accurate. Small anharmonicity effects, Fermi perturbations, or overlapping by other weak transitions is not appreciable; thus, their possible presence connot change the present assignment. The tentative assignment of the two weak bands at 955 and 2220 cm-' from the origin (isotopic shifts 65 and 20 cm-', respectively) is discussed below. In the light of their relative intensity, an attribution to fundamental modes must be preferred to an assignment to combination tones (3v,and lu, + 3 4 ,respectively, in our notation) previously prop~sed.~~~ Table I1 gives the vibronic frequencies, the relative intensities, and the frequency shifts relative to the 0band (11491 cm-') of LiTCNQ-F4in a glassy solution of MTHF at 77 K. Figure 1 compares the absorption system of LiTCNQ with that of LiTCNQ-F4 making clear the correspondence in the intensity distribution and band positions. The red shifts due to fluorination (Table 11, fourth column) are between 150 and 220 cm-'. This fact strongly suggests that the vibrational modes involved in the transition have a similar frequency in all three compounds studied, leading to the assignment for LiTCNQ-F4 proposed in Table I1 where only the shoulder at 1450 cm-' from the origin is left unassigned. Visible Absorption System. Table I11 reports the vibronic frequencies of the visible absorption spectrum of LiTCNQ, LiTCNQ-d4,and LiTCNQ-F4 in MTHF glassy solution. In the last two columns the frequency displacements of the deuterated and fluorinated compounds relative to the parent compound are included. Figure 2 shows the absorption envelopes for the three compounds. The LiTCNQ spectrum agrees with that reported in the literature3s4for which, however, no interpretation of the vibronic structure has been attempted. From Table 111, penultimate column, it appears that the whole absorption system is shifted upon isotopic substitution. This behavior

Flgure 1. Near-IR absorption spectra of MTHF glassy solutions at 77 K in the 650-900-nm spectral range: LiTCNQ (upper part) and LiTCNQ-F, (lower part). Dashed lines refer to optical density from 1 to 2. The whole absorption system of LiTCNQ is reported in ref 3 and 4. 2

15

T

>

k

m

E0 ' 1

a 2 + a

0

05

Flgure 2. Visible absorption spectra of MTHF glassy solutions at 77 K: LiTCNQ (full line), LiTCNQ-d, (dashed line), and LiTCNQ-F, (dotted line).

indicates the presence of a shift of the electronic transition whereas the vibronic coupling involves one or more vibrational modes of a type insensitive to isotopic substitution. Coupling with only one vibrational mode must be rejected because it is impossible to identify a single progression. In the light and deuterated salt some regular

TABLE 111: Vibronic Frequencies (Tlcm-') in the Visible Absorption System of TCNQ, TCNQd,, and TCNQ-F, Anions in MTHF at 7 7 K TCNQ-d -

TCNQa b C

d e f

22 925 23 810 24 555 25 440 26 115

885 1630 2515 3190

23 055 23 945 24 670 25 565 26 315

890 1615 2510 3260

TCNQ-F,. 23 845 24 320 24 780 25 590 26 060 26 525

475 935 1745 2215 2680

130 135 115 125 200

920 510 225 150 - 55



frequency differences are evident from Table 111. Namely, using the ordering letters of the first column (in parentheses the values for LiTCNQ-d4), one finds the following: c-a, 1630 (1620); d-b, 1630 (1615); and e-c, 1560 (1645) cm-'. Tentatively a frequency value of 1630 cm-' can be associated with an excited-state normal mode. Analogously the difference (b-a and d-c) of 890 cm-' may correspond to a second normal mode. According to the above hypothesis, the vibronic frequencies of LiTCNQ and LiTCNQ-d, are given by the equation


fi

=

200

+ Uaw', + ubw'b

where ,P = 22925 cm-l (deuterated: 22 035 cm-'), ub = 0, 1, 2; w'b = 1630 cm-' (deuterated: 1615 cm-'); u, = 0, 1; ora= 890 cm-' for both compounds. Extending the same type of analysis to the LiTCNQ-F4 spectrum (Table I11 and Figure 21, the vibronic frequencies turn out to be given by the same equation with P~ = 23845 cm-' and u, = 0 , 1 , 2; w', = 470 cm-l; u b = 0, 1; w'b = 1745 cm-'. Calculation of TCNQ Anion Coupling Constants. The calculation of the TCNQ anion linear coupling constants g,i was performed according to a reported methodlo through (i) calculation of the electronic states involved in the transitions, (ii) calculation of the Cartesian eigenvectors associated with each normal coordinate, and (iii) evaluation of the dependence of the electronic transition energies on the variation of the Cartesian eigenvectors. The TCNQ anion orbitals and the electronic transition calculations were carried out on a CDC-CYBER 76 computer by a CNDO/S method. A Pariser-Parr parameterization,21 suitable for open-shell molecular systems, was adopted together with interaction over 60 single excited configurations. The energies (eV) and oscillator strengths (f) obtained for the first three transitions are as follows: 2B3u-2B2, AE = 1.65, f = 0.105; 2B3u-2B2g,AE = 2.97, f = 0.44; and 2AU-2B2g,AE = 2.99, f = 0.025. Examination of the CI coefficients shows the following: the 2B2gstate is essentially (97%) the ground configuration; the lowest excited 3," state is mostly a mixture of two configurations, 77% of the configuration 36 37, arising from the promotion of an electron from the doubly occupied 7(b3J orbital to the singly occupied 7*(bzg)orbital, and 19% of the configuration 37 38 involving the promotion of the unpaired electron into the empty 7r*(b3J orbital; the 2B3u state at 2.97 eV is mainly the 37 38 and the 36 37 configurations (73% and 15%, respectively); the 2Austate is essentially (95%)the configuration 37 39, arising from the promotion of the unpaired electron into the empty a*(a,) orbital. These results compare satisfactorily with those previously obtained by the PPP13J5and IND014Cmethods. The Cartesian eigenvectors were calculated by using a program written by Schachtschneider22and a normal-coordinate analysis of the ground-state totally symmetric modes available in the The gYi coupling constants were extracted from the slopes of the electronic transition energies vs. the magnitude of the normal-mode displacements and their values are collected in Table IV.

-

-

-

-

-

Discussion Near-Infrared Absorption System. The presented results, both the vibrational analysis and the solvent effect, (21) H. M. Chang, H. H. Jaff6, and C. A. Masmanidis, J. Phys. Chem., 79, 1118 (1975). (22) J. H. Schachtachneider, Technical Report No. 57-65, Shell Development Co., Emeryville, CA, 1962.

3661

TABLE IV : Calculated Dimensionless Linear Coupling Constants g Y ifor the First Three Electronic Transitions of TCNQ Anion w calcj'

cm

1

w ew{/

2B3U-2B2g2B,U-2B2g 2AU-ZB2g

cm

3052 2186 1589

2206

1403 1206 954

1391 1196 978

1615

-0.01 0.11 -0.61

-0.01 0.06 -0.40

-0.06

-0.13 0.03

-0.40

0.01 0.75 0.14 -0.11 -0.48 -0.50 742 725 0.61 0.42 607 613 0.10 -0.09 337 337 -0.45 0.45 0.76 126 148 -0.45 0.21 0.09 a Vibrational ground-state frequencies (from ref 8 ) of the 10 ag modes running from w , to w i(,. -0.18 0.11 -0.16 0.09

"a

Figure 3. Three-dimensional plot of relative Franck-Condon coefficients (R, /R0)vs. the vibrational quantum numbers v,, v,, and v,. The dashed lines in the plane v,-v, refer to the assignment of ref 3.

agree with the assignment of the vibrational absorption system of LiTCNQ to one electronic transition (2B3u-2Bzp) as from the previous studies reviewed in the Introduction. 3,4,13-15 Besides the already elucidated isotopic effect, the vibrational analysis in terms of intensity distribution is also relevant. The reduced intensity, R, and the relative FC factors R,/Ro = (I,ijo/Io~,)1/2 have been obtained from the optical densities and the latter are plotted vs. the u,, u b , and u, assignment in Figure 3. Their regular decrement along the principal progression axes as well as in the planes of u, + u b and u, u, combinations is clearly evident. Thus, the intensity distribution supports the assignment to three coupled vibrational modes as indicated by the isotopic shift. The regular decrement of the intensity, together with the regularity in the observed frequency values pointed out already, allows one to rule out the presence of any appreciable perturbing effect. The assignment of ub and u, progressions coincides also in terms of FC factors with that previously However, the attribution of a third progression to a u b + u, combin a t i ~ ninstead ~ . ~ of to a u, fundamental is not acceptable. The assignment to the combination is not supported by the isotopic effect and would imply (Figure 3, dashed lines) an intensty distribution going through a maximum, behavior which could not be justified. For the vibrational assignment we assume the following: (i) in the excited molecular state the D2h molecular symmetry is retained; (ii) the description of the normal vibrations is not significatively changed in respect to the

-+

3002

\* ",

"6

P

B Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 9, 2015 | http://pubs.acs.org Publication Date: September 1, 1983 | doi: 10.1021/j100242a018

Zanon and Pecile


\

-

-

"2

"3

>iK "9

"4

strongly involved in the vibronic coupling. As far as the weak band at 2220 cm-l and the shoulder a t 995 cm-' (Table I) are concerned, the first may be associated with ut2mode (CN stretch). The second is left unassigned since its attribution to the wr5 mode (CH in plane bending, w " ~= 1196 cm-') would imply an isotopic shift of about 300 cm-l, whereas one observes only 55 cm-'. The near-IR absorption system of LiTCNQ-F4permits a vibrational attribution in terms of the same criteria adopted above for the hydrogenated analogue. A vibrational assignment of the ground state together with a zero-order normal-coordinate analysis has been carried out in our laboratory and the results24are directly utilized in the present discussion. If one chooses the frequencies of polarized Raman lines of a solution of TCNQ-F4anion, the assignments are as follows (cm-'): u, w t Z = 1620, = 1652; u, w'8 = 315, w"8 = 301. The vibronic bands a t 1449 and 1269 cm-' (Table 11)can be safely associated with w " ~and o " ~ (1456 and 1269 cm-', respectively). The description of the ag modes sorted out above is as follows: all2,C=C ring stretching; external C=C stretching; d4, CF and ring C-C stretching; d ' 8 , CCC external ring bending. Thus, the description of the w2, w3, and modes of TCNQ-F4 anion corresponds to that of the w3, 04, and wg vibrations of TCNQ anion, respectively (see Figure 4). However, the most important point is the strong activity in the coupling of the wt2 vibration of TCNQ-F4 anion which parallels that of the of TCNQ anion, a fact which further supports our assignment of LiTCNQ. Useful information can be drawn from the a priori calculated coupling constants of the TCNQ anion in the first electronic transition. Table IV values predict that the vibrational modes w3, 04,wg, and wl0 dominate the others in vibronic coupling since the values of the corresponding constants are the highest. Moreover, Table IV data allow one to calculate the relative intensity of the vibronic spectrum through the Poisson distribution equation

Ino = exp(-CaJll,

a

n,. where Inois the intensity of the n-th transition relative to the origin, ai = gi2, and ni are the vibrational quantum "5 numbers. Since all the givalues are smaller than unity, Figure 4. Totally symmetric normal modes of TCNQ anion. for the first 'B3,-'B2, transition the above equation predicts a maximum of intensity a t the 0-0 band, in qualiground state as indicated by the intensity maximum of the tative agreement with what is observed (Figures 1 and 3). transition which coincides with the 0band. Accordingly, From the Poisson equation and by using the experimental only the 10 agvibrations may be involved in the vibronic relative intensities, one can extract the experimental values coupling with frequencies and isotopic shifts close to the of g,, g,, and gg. They turn out to be 0.79,0.73, and 0.33, corresponding ground-state values. The and modes respectively, in good agreement with the calculated ones (essentially CH stretching and bending, r e s p e c t i ~ e l y ) ~ ~ (Table ~~ IV). The equation predicts a significant activity are rejected since they would imply a too large isotopic also for the wl0 vibrational mode, but no vibronic band effect. The and modes (2206 and 148 cm-') are related with this low-frequency fundamental = 148 excluded since their association with u, and u, would imply cm-l) is observed. A remarkable decrease of the dl0with an unacceptable frequency variation. For LiTCNQ the respect to the o"lo frequency could justify an apparent best assignment is (cm-l, isotopic shifts in parentheses) as absence of this fundamental band, caused by an insuffifollow^: U, ~ ' = 3 1597, ~ " 3= 1615 ( A d 3 = 17, Aw"3 = cient resolution. However, this fact cannot justify the lack 34); Ub ~ ' = 4 1257, 0"4 = 1391 ( A d 4 = 0, A w " ~= 0); U, of the expected overtone bands. The calculated value of w'g = 330, d ' g = 337 ( A d g = 0, A d ' g = 0). the gloconstant may be relatively overestimated, because The description of the nature of the assigned wr3, wr4, in the case of the lowest frequency fundamentals the apand utgvibrational modes, assumed unchanged in the proximate description of the vibrational mode is less reelectronic transition, is shown in Figure 4, where the totally liable. symmetric mode amplitudes of TCNQ anion in the elecIt is interesting now to reconsider the reported PR536and tronic ground state are reported. These Cartesian disEP7 spectra since they are related to the coupling of the placements were obtained with the force field and the structural data previously used23 for the TCNQ anion. (23) R. Bozio, I. Zanon, A. Girlando, and C. Pecile, J. Chem. SOC., Figure 4 shows that benzenoid ring (ut3and d4)and Faraday Trans. 2,74, 235 (1978). internal coordinates of TCNQ anion are (24) R. Bozio and C. Pecile, unpublished results. skeletal (d9) i

+

--f

+

i

The Journal of Physical Chemktry, Vol. 87, No. 19, 1983 3663


TABLE V : Vibronic Frequencies of TCNQ Anion Near-IR Absorption and Excitation Spectrum and Assignments assignments u ( absorption, excitation, ref 7' this work ref 3, 4 ref 7 this work)"/ T/cm-' monitor. modeC w ' ~ w g 4 w f 9 w ' ~ wig w ' ~ wf3 w ' ~ w ' ~ cm- ' 1 5 207 1 5 446 15 785 16 722 17 036 17 832

15 400 15 630 15 955 16 900 1 7 230 1 7 560

W'I4

W"4

w'ls

1

W"* W t 1 2

Wtt2

a MTHF glassy solution at 77 K. excitation spectra.


2

W'1 9

1 2

1 3 2 4 3 2

CH,CN solution at room temperature.

vibrational modes. The PR spectra5" show that among the potentially active vibrational modes a dominant role is modes, which are certainly played by the w " ~ ,w",, and dfg those assigned in the present paper. The high quality of the detailed E P spectra reported7 permitted a reproduction of part of the absorption spectrum from which the excited-state vibrational frequencies were attributed as from the sixth column of Table V. The frequencies in the first column have been chosen for the sake of comparison as those corresponding to the sharpest and strongest maxima in the excitation spectrum. The monitoring vibrational modes used for obtaining the excitation spectra are recalled in the third column. The frequencies in the first and second columns are practically the same if one bears in mind the blue shift of about 190 cm-l, mentioned in the Results section, between the absorption spectrum of a MTHF glassy solution and that of a room-temperature CH&N solution. The reported assignment of the band at 15 400 cm-' of the E P spectrum,7namely, 2 ~ 'l d~g , corresponds to our proposal. All the other bands from the E P spectrum included in Table V find a completely satisfactory explanation in terms of the assignment of the absortion spectrum presented in this paper, but this interpretation is rather different from that previously proposed? The latter is questionable since it is based on the following criteria: (i) thew'3 (1613 cm-') and w ' ~(1196 cm-') were assumed unchanged from the ground to the excited state; (ii) the w', (1335 cm-l) value was deduced from a CNDO/S calculation of the bond order variation in going from the ground to the excited state and determined from the experimental differences between intensity maxima in the E P spectra. This procedure7 was directed to fit the observed frequencies in the spectral range examined, disregarding the intensities. In fact the PR spectrum does not support the assignment of the w f 2 and or5modes, which, despite their poor resonance e n h a n ~ e m e n t ,have ~ , ~ been even invoked as components of ternary combinations. The reinterpretation of the E P spectra given here parallels the present assignment of the absorption near-IR system but cannot, in our opinion, be advantageously extended to all the reported E P data. The limiting factors are intrinsic to the nature of the room-temperature measurements available, whose complicated structure will be disentangled only if one can base the interpretation of the E P spectra on an excitation moved toward the transition origin, preferably at low temperature. This would in fact allow one to examine a spectrum free from the complications caused by the presence of thermally occupied levels, namely, the Aui = 0, symmetry-independent transitions, in addition to the w9 hot transition^.^ Visible Absorption System. As previously reported13-15 and confirmed by our CNDO/S calculation, the visible absorption system of TCNQ anion (Figure 2 and Table 111) is attributable to the overlappingof a very strong 2B3u-2Bzp

+

2 3 3 4 4 4

3 1 1 2

1 1

2 1 1

W ' ~

1 1 2 2 2

2 2 2

1 2

Vibrational modes used for monitoring RR

and to a weak 2Au-2B2gtransition, in agreement with the calculated oscillator strengths given above. The absorption system (emax N 3 X lo4cm-l mol-' dm3,f N 0.45) is known to display a prevailing (287%) long-axis p~larization.~ In the analysis of the electronic band envelope, an intensity borrowing, due to a Herzberg-Teller vibronic coupling through the b3gvibrational modes, cannot be neglected. This effect is expected on account of the closeness of the two electronic states and of the presence in the RR spectraE of LiTCNQ solution of two weak depolarized lines which have been assigned to b3gmodes = 1499 and wffU = 1320 cm-l). However, at the present experimental resolution, the regularity and the values of the distances between the vibrational peaks together with the intensity distribution do not evidence any overlapping of the vibronic structure of the allowed transition with that of a possible transition induced by the Herzberg-Teller interaction. Moreover, the reported PR Raman spectra, obtained with excitation on the long-wavelength tail of the absorption, evidence that the ag vibrational modes play a dominant role in the vibronic coupling and their depolarization ratios, close to 0.33, suggest a resonance with a single electronic excited state. Therefore, on the basis of the above considerations, we have undertaken an analysis of the diagonal electron-vibrational coupling for the visible absorption system according to the scheme adopted in the foregoing section. The vibrational envelope of the light and deuterated radical anion spectra (Figure 2) finds an explanation in terms of vibrational modes insensitive to deuteration. In LiTCNQ the ub vibration (w'b = 1630 cm-') can be directly identified as the w3 mode ( w " ~= 1615 cm-l) and the u, ( w ' ~ = 885 cm-') as the w6 or w7 mode (w"6 = 978, w " ~= 725 cm-I). The comparison with LiTCNQ-F4 absorption (Figure 2) is useful in deciding the u, assignment. Table I11 shows that the blue frequency shifts following fluorination are decreasing with an irregular trend moving toward higher frequencies. Thus, more than one vibrational mode seems to be involved in the transition and the frequency of at least one of them should be strongly influenced by fluorination. Bearing in mind the internal coordinate description, the u, vibration in LiTCNQ-F4 can only be attributed to the w7 C-C ring stretching = 492 cm-') strongly sensitive to fluorination ( w " ~= 725 cm-I in LiTCNQ). The association of the w3 mode with the u b vibration in TCNQ anion is confirmed by the association of the w2 mode ( w " ~= 1652 cm-') in TCNQ-F, anion, since these modes have essentially the same description (C=C ring stretching, see Figure 4). Consequently, the vibrational modes involved in the vibronic coupling and attributable from Figure 2 are w7 and w3, the latter corresponding to w2 in the case of TCNQ-F4 anion. The assignment of w7 agrees well with the indication given by the highest value of the coupling constant g7, calculated for the 2B3u-%zptransition (Table IV). Substantially the same

J. Phys. Chem. 1983, 87,3664-3670


3664

can be said for the w3 assignment even if the g 3 value is comparable with that of g6 and g,. However, an assignment of w6 has been already ruled out and that of the ogmode cannot be compared, given its low ( w ” ~ = 337 cm-’) frequency value, with the experimental vibronic frequencies. The LiTCNQ PR5,6Raman spectra support our assignment of the w3 and o7vibrations since they are reported as the most strongly intensity enhanced. The calculated coupling constants allow one to reject the attribution of the vibronic structure of the visible absorption system to the 2AU-2B2g transition. In fact, the values reported in the fifth column of Table V suggest that the w g and w4 vibrational modes are the most heavily involved in a possible vibronic pattern of this transition. The first has been already excluded and the association of the w4 mode with the u, experimental frequency is improbable. In fact, this mode being practically insensitive to deuterium or fluorine substitution, this association would require a too large variation in the normal coordinates in going to the excited state. In conclusion, the transition which displays the vibronic structure in the visible spectral range corresponds to the

2B3u-2Bzg, coupled to the w3 and w7 vibrational modes. The fit between experimental assignment and calculated coupling constants is reasonably successful if one considers that the closeness of the two electronic transitions certainly reduces the reliability of the calculated value^.'^^^^ One can get more precise information through an absorption profile calculation, taking into account both the diagonal and the off-diagonal coupling constants, as well as the i~ bond order variations. Such calculation is planned for future work in this laboratory.

Acknowledgment. Financial support by the National Research Council and by the Ministry of the Education of Italy is acknowledged. We thank Drs. R. Bozio and A. Girlando for useful discussions. The technical assistance of Dr. C. Ricotta in the chemical syntheses has been much appreciated. Registry No. TCNQ radical anion, 34507-61-4;LiTCNQ-d4, 86392-67-8; LiTCNQ-F4,34473-33-1. (25) L. S. Cederbaum and W. Domcke, Chem. Phys. Lett., 25, 357 (1974).

Raman Spectroscopic Study of Aqueous LiX and CaX, Solutions (X = CI, Br, and I ) in the Glassy State H. Kanno” Department of Chemistty, Meisei Universiw, Hino, Tokyo 19 1, Japan

and J. Hlralshl National Chemical Laboratory for Industry, 1- 1 Yatabe, Tsukubagun, Ibarakl305, Japan (Received: October 27, 1982; In Final Form: March 16, 1983)

Raman spectra of aqueous LiX and CaX2 solutions (X = C1, Br, and I) in the glassy state were measured with special attention given to the low-frequencyRaman bands and to the Raman band intensity changes with anion or salt concentration. The importance of the charge-transfer states of hydrogen bonds OH...X- (X- = halide ion) is confirmed for the intensity changes of all the Raman bands in these halide solutions. It is shown that detection and characterization of low-frequency Raman bands for aqueous solution are easily achieved in a Raman spectrum for the solution in the glassy state. Spectral changes associated with going from the liquid state to the glassy state were also studied for the OH stretching Raman spectra of the LiCl solutions.

Introduction Raman spectroscopy is in the important position of being able to clarify the structural characteristics of aqueous electrolyte ~olutions.~-~ This is because the OH stretching, H-0-H bending, and low-frequency Raman bands of water are all sensitive to the structural changes involved for the addition of an electrolyte to water. Numerous infrared and Raman data, from which the structural information about hydrogen bonding in the solutions (1) Lilley, T. H. “Water, A Comprehensive Treatise”;Franks, F. Ed.; Plenum Press: New York, 1973; Vol. 3, Chapter 6. (2) Irish, D. E.; Brooker, M. H. In “Advances in Infrared and Raman Spectroscopy”; Clark, R. J., Hester, R. E., Eds.; Heyden: London, 1976; Vol. 2, Chapter 6. (3) Scherer, J. R. In “Advancesin Infrared and Raman Spectroscopy”; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1978; Vol. 5, Chapter 3.

and vibrational characteristics between water and dissolved ionic species are obtained, have been rep~rted.l-~ In recent pape1-9,~ we have shown that Raman spectroscopy of aqueous solutions in the glassy state can provide an important clue for getting valuable information which is difficult to obtain from Raman spectra of the solutions in the liquid state. The important aspect of Raman spectroscopy for a glassy aqueous solution is the ability to observe low-frequency Raman bands which are obscured and/or hidden by the intense Rayleigh scattering wing in a Raman spectrum for the solution in the liquid state. In addition, the spectral changes associated with going from the liquid state to the glassy state can yield information about the overall effect of a temperature (4) Kanno, H.; Hiraishi, J. Chem. Phys. Lett. 1979, 62, 82; 1980, 72, 541.

0022-3654/83/2087-3664$01.50/00 1983 American Chemical Society

Vibronic structure of the near-infrared and visible electronic transitions

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