Electronic absorption spectra of benzoquinone and its hydroxy substituents and effect of solvents on their spectra

Spectrochimica Acta Part A 56 (2000) 965 – 981 www.elsevier.nl/locate/saa

Electronic absorption spectra of benzoquinone and its hydroxy substituents and effect of solvents on their spectra Musheer Ahmed, Zahid H. Khan * Department of Physics, Laser Spectroscopy Laboratory, Jamia Millia Islamia, New Delhi 110025, India Received 12 March 1999; received in revised form 9 August 1999; accepted 10 August 1999

Abstract The electronic absorption spectra of 1,4-benzoquinone (BQ) and its 2,5-dihydroxy and tetrahydroxy derivatives have been studied in detail. The interpretation of the electronic bands is made on the basis of PPP and CNDO calculations. It is found that the p“ p* transitions are well predicted by the PPP method. The predictions of the CNDO method are however superior both in their accuracy as well as ability to predict the n“ p* transitions. The effect of solvents on the electronic absorption bands have also been investigated in detail. Linear correlations are found between the solvent’s dielectric constant and wavelength of the absorption bands. The solvent shifts are explained on the basis of the polarities of the solute and solvent molecules as well as due to hydrogen bonding. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Electronic spectra; Hydroxy-benzoquinones; Solvent effect; p “ p* transitions; n “ p* transitions

1. Introduction Quinones are an important class of compounds which have various applications in pharmaceutical and dye industries [1]. Para-benzoquinone is the parent of all types of quinones found in living tissues. The derivatives of para-benzoquinone chiefly occur in animal tissues, fungi, bacteria and plants. Such derivatives take part in oxidation-reduction cycles essential to living organisms. Parabenzoquinone is of great interest in carbonyl-olefin chemistry because of the forma* Corresponding author. Tel.: +91-11-6831717; fax: + 9111-6840229. E-mail address: [email protected] (Z.H. Khan)

tion of exciplexes [2], It is the wide usefulness of 1,4-benzoquinone (BQ) that its electronic absorption spectrum has received much attention, both experimentally as well as theoretically [3–12]. However, its hydroxy-substituted systems have not been studied in detail. In particular, no theoretical studies appear to have been made on electronic spectra of hydroxy-substituted benzoquinones. This demands a more comprehensive investigation of the electronic spectra of benzoquinone, including its hydroxy substitutions which would shed light on the effect of such substitution on the electronic absorption bands of the parent benzoquinone. In this paper, we have made a systematic study of electronic absorption spectra of BQ, 2,5-dihydroxy-1,4-benzoquinone

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(DHBQ) and tetrahydroxy-1,4-benzoquinone (THBQ), whose chemical structures are shown in Fig. 1. For the interpretation of the spectra, MO calculations using PPP and CNDO methods are carried out. Another important theme that needs special attention is the proper understanding of the effect of solvents on electronic absorption bands of BQ, including those containing hydroxy groups. With the exception of BQ, for which the solvent shift of electronic bands has been investigated in detail by a number of workers [1,3,13– 16], the effect of solvents on the electronic spectra of hydroxy-substituted quinones has rarely been studied. To examine the effect of solvents on the absorption bands, we have measured the electronic absorption spectra of quinones in solvents of different polarities and have tried to explain the spectral shifts on the basis of the dielectric constants of the solvents.

2. Experimental BQ and THBQ were acquired from Tokyo Kasei (Japan) and DHBQ was obtained from Aldrich Chemical (USA). To study the effect of solvents on the electronic bands, we used solvents of different polarities. Dimethyl sulphoxide (98%), dimethyl formamide (98%), tetrahydrofuran (AR), chloroform (AR), carbon tetrachloride (AR), iso-octane (AR), 1,4-dioxan (AR), methanol (AR), dichloromethane (AR), ethyl ether (AR), 1-propanol (AR), 2-propanol (AR) and 1-butanol (AR) were all procured from Sisco Research Laboratories Pvt. Bombay. Pentane (99%), sec-butylamine (99%) and 2-chlorobutane

(99%) were acquired from Aldrich Chemical Company (USA). Ethyl alcohol (GR) and acetonitrile (GR) were obtained from E. Merck (India), Bombay, and isobutyl alcohol (AR) was acquired from Qualigens Fine Chemicals Glaxo India, Bombay. Electronic absorption spectra of the quinones were measured on a Jasco 570 UV/ VIS/NIR spectrophotometer. The spectrophotometer is computer-controlled, and uses spectroscopy software that enables us to examine more closely a particular spectral region so as to get precise information about spectral peaks.

3. Theoretical details For the interpretation of the electronic absorption spectra of the benzoquinones, we have calculated the electronic transition energies and the oscillator strengths of the transitions for the molecules using the PPP [17,18] and CNDO [19] methods. In the PPP method, the basic formulas for the Fock matrix are: 1 Fmm = − Im + Pmmgmm + % (Pss − Zs )gms 2 s( " n)

(1)

1 Fmn = bmn − Pmngmn, 2

(2)

m" n

where Pmm is the p-electron density, Pmn is the p-bond order, Im is the valence state ionization potential of atom m, bmn, is the resonance integral, Zs is the atomic charge centralized on the atom s, and gmm and gmn are the one- and two-electron repulsion integrals, respectively. Evaluation of the F matrix thus requires computation of Im, bmn, gmm, gmn, Pmm, and Pmn. The ionization potential (Im ) and the electron affinity (Am ) for the carbon sp2 valence state are chosen as: Im = 11.16 eV Am = 0.03 eV This gives the value of the one-electron repulsion integral as: gmm = Im − Am = 11.13 eV

Fig. 1. Molecular structures of 1,4-benzoquinone, 2,5-dihydroxy- 1,4benzoquinone, and tetrahydroxy-1,4-benzoquinone.

(3)

The resonance integral bmn is taken as zero for non-bonded atoms m and n. However, for bonded

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atoms it is chosen empirically, such that it varies from molecule to molecule, and is selected so as to get a better agreement between the theory and the experiment. The two-electron repulsion integrals gmn are calculated from the Mataga-Nishimoto approximation [20]:

the Coulomb and exchange integrals for carbon atom, and hx and kcx are the Coulomb and exchange parameters which are taken as:

gmn =14.3986/{Rmn +14.3986/[0.5(gmm +gnn )]}

For different molecular systems, bcx is chosen empirically so as to achieve better agreement between experiment and theory. Moreover, the molecules are assumed to have planar geometry with equal bond angles of 120°. The bondlengths are taken as follows:

(4)

where Rmn is the bond length of the bond connecting the atoms m and n. The different steps involved in the computation of electronic transition energies and oscillator strengths of the transitions are as follows: 1. Calculation of the starting MOs by the simple LCAO MO (Hu¨ckel) method. 2. Calculation of the density matrix, Pmn. 3. Calculation of the electron repulsion integrals gmm and gmn. 4. Calculation of the F matrix over AO’s, Fmn. 5. Diagonalizing the Fmn matrix to obtain a new set of eigenvalues and eigenvectors. 6. With the new set of eigenvalues and eigenvectors, obtaining a fresh Pmn matrix and an Fmn matrix, diagonalizing the latter and thus obtaining new SCF eigenvalues and eigenvectors. 7. Repeating the process until self-consistency is achieved. 8. Computation of the F matrix over MOs from the F matrix over AOs using the SCF eigenvectors. 9. Calculation of the configuration interaction (C.I.) matrix. 10. Diagonalization of the C.I. matrix to obtain final state eigenvalues and eigenvectors. 11. Calculation of electronic transition energies, wavelengths and oscillator strengths of the electronic bands. The starting MOs are obtained from the Hu¨ckel method, which involves evaluation of the Coulomb integral (a) and resonance integral (b) for heteroatoms. These are calculated from the expressions: ax =ac +hxb cc

(5)

bcx = kcxb

(6)

cc

where the subscripts c and x refer to carbon and hetero atoms, respectively. ac and b cc represent

ho = 2.0 kco = 0.8

R(C–C)=1.40 A, R(CC) =1.40 A, R(C–H)=1.08 A, R(C–O)=1.22 A, The PPP calculations were made using a computer program developed by Khan [21], which was used as such for substituted systems. In the CNDO method, the elements of the Fock matrix are: 1 Fmm = − (Im + Am ) 2



n

1 + (PAA − ZA )− (Pmm − 1) gAB 2 + % (PBB − ZB )gAB

(7)

B( " A)

1 Fmn = b°ABSmn − PmngAB 2

(8)

where − 1/2(Im + Am ) is the ‘core integral’, PAA and Pmm, are the atomic and orbital charge densities, respectively, Pmn is the orbital bond order, ZA is the core charge on atom A, gAB is the electronrepulsion integral, b°AB is the bonding parameter, Smn is the overlap integral between the orbitals m and n, and gAB is the electron repulsion integral. This form for Fmm, shows up the self-consistent character of the theory in a very simple manner. The term 1/2(Im + Am ) is a fundamental electronegativity for the atomic orbital and is closely related to the scale introduced by Mulliken [22]. In Eq. (7), the diagonal element Fmm reduces to −1/2(Im + Am ) if the orbital fm contains one elec-

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tron (Pmm = 1) and if all atoms have zero net charge (PAA =ZA, PBB =ZB ). For the calculation of the bonding parameter b°AB, the following equation is used:

tronic absorption spectra. The spectra measured in iso-octane, a non-polar solvent are taken as reference for explaining solvent shift in solvents of different polarities.

1 b°AB = (b°A +b°B ) 2

4.1. 1,4 -Benzoquinone

(9)

where b°A depends only on the nature of the individual atom A, thus requiring the selection of only a single semi-empirical parameter for each element. The input of density matrix elements Pmn is obtained by estimating the LCAO coefficients by Huckel type theory using matrix elements of the form: 1 F (0) mn = − (Im +Am ) 2

(10)

0 F (0) mn = b ABSmn,

(11)

m" n

The CNDO calculations were carried out using the QCPE Computer Program 333 [23]. For these calculations, we initiate with the standard values of one- and two-electron integrals as described by Pople and Segal [19] which are then modified to obtain the best fit with the experimental data. Also, to achieve a reasonable agreement between the observed and calculated transition energies, some of the parameters of carbon and oxygen atoms are systematically varied in order to get better agreement with the experiments. Similarly, the values of 1/2(Im +Am ) are varied, which result in the change in the core matrix elements Umm for the two p-orbitals of carbon and oxygen atoms. Information about the geometrical parameters of the quinones as required by the CNDO method are adopted from the literature [24 – 26]. The dihedral angles were taken as 0 and 180° for cis and trans structures of the quinones, respectively.

4. Results and discussion In this section we present the results of our experimental and theoretical studies of BQ and its hydroxy-substituted systems. Although detailed results of theoretical calculations are given in separate tables, these are also sketched as stick diagrams in separate figures along with the elec-

The electronic absorption spectrum of BQ has been extensively studied both experimentally as well as theoretically. In particular, the contributions by Morton [1] and Thomson [3] are worth mentioning. Also, Flaig et al. [4] studied the UV spectra of BQ and its methyl derivatives. Trommsdorff [5] used the CNDO method to explain the visible and near ultraviolet spectrum of the molecule. CNDO studies on electronic spectra of BQ have also been made by Stevenson [6] and Bigelow [7]. Merienne-Lafore et al. [8] calculated the transition energies of the excited configurations of the singlet and triplet states of p-benzoquinone using the CNDO method. Kuboyama et al. [9] carried out MO calculations on p-benzoquinone and its methyl derivatives and made assignments of the p“p* transitions. In a later study, Kuboyama et al. [10] performed CNDO/SCI calculations for electronic transitions energies of p-benzoquinone on the basis of which they assigned its p“ p* and n“p* transitions. Recently, Liberko et al. [11] used PPP calculations for the prediction of longer wavelength transitions of quinones. In spite of these extensive studies on BQ, we have reinvestigated its electronic spectrum for the following reasons. (a) The BQ is the parent of all quinones and the experimental and theoretical yardsticks applied to this molecule may then be applied to its hydroxy derivatives in order to get a more coherent picture of electronic spectra of the other quinones of the series. (b) Our another aim is to study the effect of solvent on electronic spectra of benzoquinone and its hydroxy substitutions, for which a more comprehensive information about the electronic spectrum of BQ is required. Our measurement of the absorption spectrum of BQ in isooctane is displayed in Fig. 2 together with those of the PPP and CNDO calculations. The details of the calculated electronic transition

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Fig. 2. Observed and calculated electronic spectra of 1,4-benzoquinone molecule. Vertical solid lines represent the oscillator strength of allowed transitions and the vertical dotted lines indicate the forbidden transitions.

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Table 1 Results of PPP calculations for electronic transition energies of 1,4-benzoquinonea S. No.

E (eV)

n¯ (kK)

l (nm)

f

Mx

My

Main configurations

1 2 3 4

4.49 5.03 6.38 7.21

36.14 40.48 51.35 58.03

276.7 247.0 194.7 172.3

0.0000 0.4754 0.0000 1.6365

0.0000 0.0000 0.0000 0.0000

0.0000 −1.0392 0.0000 −1.6103

0.97 0.83 0.97 0.83

(3 “ 5) (4 “ 5) 0.16 (3“6) (4 “ 6) (3 “ 6) 0.16 (4“ 5)

a

The energies are expressed in eV, wavenumbers in kK, and wavelengths in nm. The Mx and My represent the x and y components of transition moments and ‘f’ stands for the calculated oscillator strength. The last column gives the main configurations that contribute to the final state function, where the quantities in parentheses are one-electron configurations with their coefficients representing the weight factors. The following parameter was used for the calculations, b(CO)=−5.20 eV.

energies and oscillator strengths of the electronic bands using these methods are given in Tables 1 and 2, respectively. A comparison of the observed and calculated transition energies for BQ along with their band assignments is given in Table 3. The present experiment reveals two band systems in the higher wavelength region of BQ; the first one centered at 499.5 nm followed by well structured bands at 478.5 and 457.2 nm, and the other at 448.4 nm with vibrational structure at 433.8, 425.7 and 411.9 nm. These transitions, which could not be predicted by the PPP method, are well predicted by the CNDO calculations and are assigned as n “p* transitions. In the UV-region, we observe a weak band at 285.2 nm and a very intense one at 240.4 nm. These transitions are predicted both by the CNDO and PPP calculations and are assigned as p “ p* transitions. The intensities of these bands are, however, underestimated by the PPP method. The present CNDO calculations give results comparable to that of the CNDO/S method reported by Jaques et al. [13]. The overall agreement between the theory and the experiment is found to be good. In particular, the results of the CNDO calculations show excellent agreement with our observations. To study the effect of solvents on the absorption bands of BQ, we have measured its electronic absorption spectra in a number of solvents some of which are displayed in Fig. 3. The most intense band of BQ in iso-octane lies at 240 nm which gets shifted appreciably in other solvents. Although the spectrum of BQ was measured in twenty one solvents, we have listed the results in Table 4 for sixteen solvents only. This is due to

the reason that some of the solvents, like carbon tetrachloride (CCl4), dimethylformamide (DMF), dimethylsulphoxide (DMSO), ethyl acetate, and 2-butanol absorb up to 265 nm, thus obscuring the 240 nm absorption of BQ. In order to understand the shift of the bands due to the use of solvents of different polarities, we have plotted in Fig. 4 the wavenumbers of the intense higher-energy p“p* transition (240 nm band in iso-octane) as a function of the ‘dielectric constant’ (o) of some selected solvents. In moving from the non-polar solvent (iso-octane) to the polar one (chloroform), the intense band at 240 nm, which is due to the p“p* transition, is red shifted by 6 nm. Moreover, the shift is larger in polar solvents than that in non-polar solvents such as cyclohexane. In BQ, which is almost symmetric and nearly non-polar, the intermolecular interaction between Table 2 Results of CNDO calculations for electronic transition energies of 1,4-benzoquinonea S. No.

E (eV)

n¯ (kK)

l (nm)

f

1 2 3 4 5 6

2.801 2.902 4.166 5.159 6.369 6.580

22.54 23.35 33.53 41.52 51.26 52.96

443.5 428.2 298.2 240.8 195.1 188.8

0.0000 0.0000 0.0006 1.3984 0.0000 0.0000

a The energies are expressed in eV, wavenumbers in kK, and wavelengths in nm. The oscillator strength is denoted by ‘f’. The following parameters were used for the calculations, b(CO)=−37.0 eV, g(C)= 8.00 eV, g(O) =10.00 eV, 1/2(I+ A) (for 2p carbon atom)=−4.000 eV, 1/2(I+A) (for 2p oxygen atom) =−8.761 eV.

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Table 3 Assignment of the observed electronic bands of 1,4-benzoquinone S. No.

Observed

Calculated

Band assignment

CNDO method

1 2 3 4 a b

PPP method

E (eV)

l (nm)

Rel. int.a

E (eV)

l (nm)

fb

2.487 2.770 4.356 5.168

499.5 448.4 285.2 240.4

vw vw m vs

2.801 2.902 4.166 5.159

443.5 428.2 298.2 240.8

0.0000 0.0000 0.0006 1.3984

E (eV)

4.490 5.030

l (nm)

276.7 247.0

fb

0.0000 0.4754

n “ p* n “ p* p “ p* p“ p*

Relative intensity; w: weak; vw: very weak; m: medium; sh: shoulder; s: strong; vs: very strong. Oscillator strength.

Fig. 3. Absorption spectra of 1,4-benzoquinone in iso-octane (1), cyclohexane (2), 1,4-dioxan (3) and chloroform (4).

the solute and the non-polar solvent molecule arises from dispersion forces only and is very weak. However, an electronic excitation in the molecule is accompanied with a displacement of charge in the molecule and, as a result, nodes are introduced into the electronic wavefunction. For this, less work must be required in a dielectric medium than in vacuum. Therefore, the absorption band of BQ corresponding to the p“ p* transitions shifts to longer wavelengths or lower wavenumbers with an increase of dielectric constant of the solvents. The larger the magnitude of the charge displacement, the greater is the effect. In more polar solvents, such as chloroform, the

intermolecular interaction between the solute and the solvent molecules arises mainly from dipoleinduced dipole forces, which are generally stronger than dispersion forces. The polarizability of an excited state is larger than that of the ground state, the excited state is more stabilized than the ground state and, as a result, a larger red shift is expected and is confirmed from the experimental data given in Table 4. In hydroxylic solvents (at S. No. 8–13 and 15–16 of Table 4), formation of a hydrogen bond with the solute is the cause of the red shifting of the p“ p* transition. For example, the high energy band at 240 nm in iso-octane is shifted to 246 nm in water.

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Table 4 Absorption maxima of 1,4-benzoquinone in solvents of different polarities S. No.

Solvent

Dielectric constant (o)

l (nm)

n¯ (cm−1)×104

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Iso-octane Cydohexane 1,4-Dioxan Ethyl ether Chloroform Tetrahydrofuran Dichloromethane 2-Methoxyethanol 1-Butanol Iso-butyl alcohol 1-Propanol Ethanol Methanol Acetonitrile Glycerol Water

1.94 2.02 2.21 4.31 4.81 7.60 9.10 16.9 17.8 17.9 19.9 24.6 32.7 37.5 42.5 78.5

240 241 243 241 246 246 245 243 243 243 243 242 243 242 246 246

4.1666 4.1493 4.1152 4.1493 4.0650 4.0650 4.0816 4.1152 4.1152 4.1152 4.1152 4.1322 4.1152 4.1322 4.0650 4.0650

This may be explained on the basis of the fact that the hydrogen bonding stabilizes the excited state more than the ground state, thus reducing the energy difference between the ground and the excited state. This results in the red shift of the electronic bands in hydroxylic solvents.

4.2. 2,5 -Dihydroxy-1,4 -benzoquinone The study of the electronic spectrum of DHBQ provides the possibility to examine as to how the absorption bands of BQ get modified by the introduction of hydroxyl groups in the parent molecule. Although the electronic absorption spectrum of DHBQ has been studied earlier also [1,3], it has not been investigated in detail. To the best of our knowledge, no theoretical studies have been carried out on the electronic spectrum of DHBQ. In this work, we have studied the electronic spectrum of the molecule both experimentally as well as theoretically, and the results are displayed in Fig. 5. The transition energies and their oscillator strengths calculated from the PPP and CNDO methods are presented in Tables 5 and 6, respectively, and the comparison of the calculated and the observed results is given in Table 7 along with the band assignment. DHBQ shows four absorption bands. The highest wavelength band is located at 395.3 nm

followed by a vibrational structure at 331.5 nm. The second electronic band lies at 273.8 nm which is accompanied by a shoulder at 282.0. It is clearly noticed that incorporation of two hydroxyl groups at the positions 2 and 5 into the benzoquinone nucleus produces a large bathochromic displacement in its three bands. In analogy with the assignment of the corresponding electronic bands of BQ, we assign these electronic bands as p“ p* transitions. Both the transitions are well predicted by CNDO and PPP calculations, but the former gives a better agreement with the observed values.

Fig. 4. Variation of the absorption maximum of the intense higher-energy p “ p* band of 1,4-benzoquinone with dielectric constant of the solvents.

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Fig. 5. Observed and calculated electronic spectra of 2,5-dihydroxy-1,4benzoquinone molecule. For other details see caption to Fig. 2.

The electronic absorption spectra of DHBQ measured in some selected solvents are displayed in Fig. 6, where strong absorption peaks followed by broad absorption in the near UV region are noticed.

The intense higher-energy absorption band is bathochromically shifted by 20 nm in moving from the non-polar solvent, iso-octane, to the polar one, water. The wavelengths and wavenumbers of the absorption maxima of DHBQ in dif-

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Table 5 Results of PPP calculations for electronic transition energies of 2,5-dihydroxy-1,4-benzoquinonea S. No.

E (eV)

n¯ (kK)

l (nm)

f

1 2 3 4 5 6

3.663 4.415 5.846 6.085 6.932 7.222

29.54 35.60 47.15 49.08 55.90 58.25

339.2 281.4 212.5 204.2 179.2 172.0

0.0000 0.5413 0.0000 0.6601 0.0000 1.1121

Main configurations

Mx

My

0.0000 0.2034 0.0000 −0.4539 0.0000 0.2067

0.0000 1.1662 0.0000 −1.0166 0.0000 −1.3103

0.98 0.87 0.93 0.63 0.77 0.63

(6 “ 7) (5 “ 7) (5 “ 8) (6 “ 8) (3 “ 7) (4 “ 7)

0.10 0.04 0.25 0.13 0.25

(6“ 8) (3“7) (4“7) (4“ 8) (6“ 8)

a

The energies are expressed in eV, wavenumbers in kK, and wavelengths in nm. The Mx and My represent the x and y components of transition moments and ‘f’ stands for the calculated oscillator strength. The last column gives the main configurations that contribute to the final state function, where the quantities in parentheses are one-electron configurations with their coefficients representing the weight factors. The following parameters were used for the calculations, b(CO)=−4.6 eV, b(C–OH)= −1.8 eV.

ferent solvents along with the solvent dielectric constants (o) are given in Table 8. A plot of the o-values of the solvents versus the wavenumbers of the higher-energy absorption peak is shown in Fig. 7. The solvent shift in the absorption bands of DHBQ can be explained on the basis of the presence of two hydroxy groups attached to BQ at positions 2 and 5 which makes it non-symmetric and hence more polar in character. In nonpolar solvents, the interaction between the solute and the solvent is of the dipole-induced dipole type. With the increase in the dipole moment of the molecule as a result of the charge displacement due to an electronic transition, the excited electronic state becomes more stabilized by solvation than the ground state. Therefore the corresponding absorption band (p “ p*) is shifted to longer wavelengths with increasing polarizability of the solvent as seen from the data in Table 8. In polar solvents, the interaction between the solute and the solvent is of the dipole–dipole type. The solute dipole moment increases during the electronic transition and the excited state is more stabilized relative to the ground state with the increasing solvent polarity and, as a result, there is bathochromic shifting. It is apparent from the Table 8 that in polar solvents like DMF and DMSO a clear red shifting of the higher-energy absorption maxima is found in moving from isooctane to DMSO. In hydroxylic solvents such as ethanol, methanol and water, the cause of red shifting is the hydrogen bonding between the so-

lute and the solvent. The polar excited state of the p“ p* transition is stabilized by hydrogen bonding in more polar solvents. This lowers the distance between p and p* level, resulting in increase in wavelength of the transition which explains the bathochromic shift in the p“ p* transitions.

4.3. Tetrahydroxy-1,4 -benzoquinone This molecule is formed due to attachment of four hydroxy groups to BQ at positions 2, 3, 5 and 6, which makes the THBQ nearly symmetric. As a result, THBQ is insoluble in many solvents and its solution in pentane added with 1,4-dioxan had to be heated in order to dissolve it properly. Table 6 Results of CNDO calculations for electronic transition energies of 2,5-dihydroxy-1,4-benzoquinonea S. No.

E (eV)

n¯ (kK)

l (nm)

f

1 2 3 4 5 6 7 8

2.085 2.163 3.453 4.525 5.043 6.067 6.373 6.612

16.78 17.41 27.79 36.42 40.59 48.83 51.29 53.22

595.86 574.38 359.79 274.56 246.35 204.77 194.94 187.89

0.0000 0.0000 0.0009 0.9969 0.0000 0.0109 0.0000 1.6305

a

The energies are expressed in eV, wavenumbers in kK, and wavelengths in nm. The oscillator strength is denoted by ‘f’. The following parameters are chosen for the calculations, b(CO)=−25.0 eV, g(C)= 8.33 eV, g(O) =10.00 eV, 1/2(I+ A) (for 2p carbon atom)=−5.572 eV.

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Table 7 Assignment of the observed electronic bands of 2,5-dihydroxy-1,4-benzoquinonea S. No.

Observed

Calculated

Band Assignment

CNDO Method

1 2 3 4 a b

PPP Method

E (eV)

l (nm)

Rel. inta

E (eV)

l (nm)

f

E (eV)

l (nm)

fb

3.145 3.753 4.567 4.537

395.3 331.5 282.0 273.8

vw vw s vs

3.453

359.8

0.0000

3.663

339.2

0.000

p “ p*

4.525

274.6

0.9970

4.415

281.4

1.166

p “ p*

Relative intensity; w: weak; vw: very weak; m: medium; sh: shoulder; s: strong; vs: very strong. Oscillator strength.

Fig. 6. Absorption spectra of 2,5-dihydroxy-1,4-benzoquinone in iso-octane (1), chloroform (2), water (3) and methanol (4).

To the best of our knowledge, the electronic absorption spectrum of THBQ has not been studied before, and the spectrum shown in Fig. 8 is being reported for the first time. The details of the results of the PPP and CNDO calculations are given in Tables 9 and 10, respectively. A comparison of observed and calculated results along with the band assignments is presented in Table 11. The electronic spectrum of THBQ shows a strong absorption peak at 310.8 nm followed by a weak absorption at 499.7 nm. These bands are due to p “p* and n “p* transitions, respectively. It is noticed that the 240.4 nm absorption band of BQ in iso-octane is bathochromically shifted to

310.8 nm when substituted with four hydroxyl groups. It is well predicted both by the CNDO and PPP methods. Furthermore, the well resolved structure of BQ that stretches through the visible region, gets broadened and appears at 499.7 nm. In the visible region, however, the predictions of the two methods differ in that the PPP method reveals only one transition, whereas the CNDO method predicts three transitions all of which are forbidden. In the experiment, however, only one weak band at 499.7 nm is observed which becomes allowed possibly due to vibrational mixing. To examine the effect of solvents on electronic absorption bands of THBQ, we have displayed in

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Table 8 Absorption maxima of 2,5-dihydroxy-1,4-benzoquinone in solvents of different polarities S. No.

Solvent

(o)

l (nm)

6¯ (cm−1)×104

l (nm)

6¯ (cm−1)×104

l (nm)

6¯ (cm−1)×104

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Iso-octane Cyclohexane CCl4 1,4-Dioxan Ethyl ether Chloroform Ethyl acetate THF DCM 2-Methoxy ethanol 1-Butanol Isobutyl alcohol 2-Propanol 1-Propanol Ethanol Methanol DMF Acetonitrile Glycerol DMSO Water

1.94 2.02 2.22 2.24 4.31 4.81 6.60 7.60 8.90 16.9 17.8 17.9 19.9 20.3 24.6 32.7 36.7 37.5 42.5 46.7 78.5

274 275 276 280 278 279 278 253 278 287 289 287 284 290 283 287 287 276 292 286 294

3.6496 3.6363 3.6231 3.5714 3.5971 3.5842 3.5971

327 331

3.0581 3.0211

369

2.7100

388

2.5773

335 332 336

2.9850 3.0120 2.9761

3.5971 3.4843 3.4602 3.4843 3.5211 3.4482 3.5335 3.4843 3.4843 3.6231 3.4246 3.4965 3.4013

331

3.0211

232 335

3.0120 2.9850

369 485 486 488

2.7100 2.0618 2.0576 2.0491

487 406 481 477 407 485 458 490

2.0533 2.4630 2.0790 2.0964 2.4570 2.0618 2.1834 2.0408

Fig. 9 its electronic absorption spectra measured in a number of solvents and the relevant data are listed in Table 12. Since THBQ is symmetric, it is not easily soluble in non-polar solvents, and we have measured its spectrum in the more polar solvents only. It is noticed that the higher-energy intense band, which is due to a p “ p* transition, shifts bathochromically from the less polar solvent (diethyl ether) to the more polar one (dimethyl sulphoxide). On the other hand, the lower energy band due to n “ p* transition is hypsochromically shifted. The linear relationship of the absorption maximum with the dielectric constant of the solvent is shown in Fig. 10. It is also found that the n “ p* bands are blue shifted with an increase in the solvent dielectric constant. In polar solvents the intermolecular interaction between the solute and the solvent molecule is of dipole-induced dipole type. As the THBQ molecule is nearly non-polar, the dipole moment does not change appreciably on absorption of light and thus the intensity of transition is very low. However, in more polar solvents (like dimethyl formamide and dimethyl sulphoxide),

the intensity of the visible transition is comparatively large because of the possibility of the greater transition dipole. The excited state is more stabilized as compared to the ground state and thus the energy difference between the two states decreases. This results in red shift of the p“p* bands. A shifting of 7 nm is found for the p“p* bands in moving from diethyl ether to DMSO. It

Fig. 7. Variation of the absorption maximum of the intense higher-energy p“ p* band of 2,5-dihydroxy-1,4-benzoquinone with dielectric constants of the solvents.

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Fig. 8. Observed and calculated electronic spectra of tetrahydroxy-1,4-benzoquinone molecule. For other details see caption to Fig. 2.

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Table 9 Results of PPP calculations for electronic transition energies of tetrahydroxy-1,4-benzoquinonea S. No.

E (eV)

6¯ (kK)

l (nm)

f

1 2 3 4 5 6

3.030 3.932 5.543 5.770 6.662 6.670

24.441 31.714 44.700 46.542 53.728 53.798

409.9 315.9 224.2 215.3 186.5 186.3

0.0000 0.4823 0.0000 0.9548 0.0000 0.0002

Mx 0.0000 0.0000 0.0000 0.0000 0.0001 −0.0169

My

Main configurations

0.0000 1.1839 0.0000 1.3750 0.0000 0.0000

0.99 0.88 0.97 0.81 0.86 0.54

(8“ 9) (7 “ 9) 0.10 (8“10) (7“ 10) 0.01 (3“ 9) (8“ 10) 0.07 (7“ 9) (6“ 9) 0.09 (5“10) (8 “11) 0.42 (5“9)

a

The energies are expressed in eV, wavenumbers in kK, and wavelengths in nm. The Mx and My represent the x and y components of transition moments and ‘f’ stands for the calculated oscillator strength. The last column gives the main configurations that contribute to the final state function, where the quantities in parentheses are one-electron configurations with their coefficients representing the weight factors. The following parameters were used for the calculations, b(CO)=−4.5 eV, b(C–OH)= −2.0 eV.

is found that the bands are red shifted with an increase in solvent dielectric constant. On the other hand, the n “ p* transitions are found to be blue shifted with an increase in the dielectric constant of the solvent. This is because the ground state is more stabilized than the excited state, and thus the energy difference between the two levels increases. In hydroxylic solvents, the n“p* transitions are found to be blue shifted with an increase in the dielectric constant of the solvent, whereas no distinct shifting of p“ p* bands is found. The effect of hydroxylic solvents is attributed to some extent due to hydrogenbonding interaction between the solute molecule and the solvent molecule. The lone pair of electrons on the oxygen atom of the carbonyl group of the molecule can form a hydrogen bond with the hydrogen atom of the hydroxyl group of the solvent molecule. The formation of this hydrogen bond lowers the energy of the n orbital by an amount approximately equal to the energy of the hydrogen bond. Thus the ground state of the CO group is stabilized by hydrogen bonding with the solvent. In the excited state, the hydrogen bond of a molecule is almost completely broken or, at least, is largely weakened. As a result of the delocalization of the electron, the excited state is less stabilized. Therefore, the energy of the n“ p* transition increases in the presence of hydroxylic solvents. This explains the large hypsochromic shift of n“ p* band in such solvents. The extent of shift increases with increasing hydrogen-bonding ability of the solvent.

4.4. Effects of hydroxy substitution and intra-molecular hydrogen bonding In BQ, DHBQ and THBQ, the lowest-energy p“ p* transition is the most intense one in the entire spectrum. It is noticed that the substitution of the hydroxyl group in BQ results in the red shift of the 240.4 nm band by 33.4 nm in 2,5DHBQ and 70.0 nm in THBQ. This is accompanied by an appreciable change in the band intensities. It might be mentioned that although the functional group OH has no absorption characteristics, it does have a profound effect upon chromophoric systems to which it is attached. The Table 10 Results of CNDO calculations for electronic transition energies of tetrahydroxy-1,4-benzoquinonea S. No.

E (eV)

6¯ (kK)

l (nm)

f

1 2 3 4 5 6 7

2.526 2.794 2.923 3.914 5.880 5.937 6.259

20.33 22.48 23.53 31.50 47.33 47.79 50.38

491.83 444.66 425.03 317.42 211.28 209.26 198.49

0.0001 0.0000 0.0000 1.0150 0.0000 0.3397 0.4998

a

The energies are expressed in eV, wavenumbers in kK, and wavelengths in nm. The oscillator strength is denoted by ‘f’. The following parameters were used for the calculations, b(CO)=−37.0 eV, g(C)= 8.00 eV, g(O) =10.00 eV, 1/2(I+ A) (for 2p carbon atom)=−4.000 eV, 1/2(I+A) (for 2p oxygen atom) =−8.761 eV. Standard parameters as given in [26] are used in this case.

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Table 11 Assignment of the observed electronic bands of tetrahydroxy-1,4-benzoquinone. For other details, refer to Table 3 S. No.

Observed

Calculated

Band assignment

CNDO Method

1 2 3 4 a

PPP Method

E (eV)

l (nm)

Rel. int.a

E (eV)

l (nm)

f

E (eV)

l (nm)

f

2.485

499.7

vw

0.000

n “ p*

vs

0.000 0.000 0.000 1.015

409.9

311.8

491.8 444.7 425.0 317.4

3.030

3.995

2.526 2.794 2.923 3.914

3.932

315.9

0.482

p“ p*

Relative intensity; w: weak; vw: very weak; m: medium; sh: shoulder; s: strong; vs: very strong.

Fig. 9. Absorption spectra of tetrahydroxy-1,4-benzoquinone in ethyl acetate (1), 2-methoxyethanol (2) and in dimethylsulphoxide (3). Inside the figure are shown its absorption spectra in water (1), 2-methoxyethanol (2), methanol (3) and pentane (4).

most conspicuous property of this auxochromic grouping is its ability to provide additional opportunity for charge delocalization, and thus to provide for smaller energy increments for transition to excited states. The auxochrome tends to increase resonance by interacting with the unshared pair of electrons on oxygen atom with the p-electrons of the aromatic ring. This increase in resonance increases the intensity of absorption of light and also shifts the absorption band to longer wavelength. The symmetry also affects the absorption bands; the more the probability and longer the path for a charge to oscillate in a compound, the

longer the wavelength of light that will be absorbed. The intra-molecular hydrogen bonding effects are important both in 2,5-DHBQ and THBQ, which should influence the n“ p* transitions to a larger extent as compared to those of the p“ p* transitions. It contributes significantly to the blue shift of the n “ p* transition in THBQ. However, due to the absence of the n“ p* transition in the present experiment for 2,5DHBQ, no definite conclusion could be drawn about the effect of intra-molecular hydrogen bonding on the electronic spectrum of this molecule.

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Table 12 Absorption maxima of tetrahydroxy-1,4-benzoquinone in solvents of different polarities S. No.

Solvent

Dielectric constant (o)

l (nm)

6¯ (cm−1)×104

l (nm)

6¯ (cm−1)×104

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Pentane 1,4-Dioxan Diethyl ether Chloroform Ethyl acetate Tetrahydrofuran 2-Methoxy ethanol 1-Butanol Isobutyl alcohol 2-Propanol 1-Propanol Ethanol Methanol DMF Acetonitrile Glycerol Dimethyl sulphoxide Water

1.90 2.24 4.31 4.81 6.60 7.60 16.9 17.8 17.9 19.9 20.3 24.6 32.7 36.7 37.5 42.5 46.7 78.5

311 314 311 311 310 314 313 313 313 312 312 312 312 316 308 315 318 308

3.215 3.185 3.215 3.215 3.225 3.184 3.194 3.194 3.194 3.205 3.205 3.205 3.205 3.164 3.246 3.174 3.144 3.246

500 504 510 488 511 502 484 497 489 505 491 487 484 491

2.000 1.984 1.960 2.049 1.956 1.992 2.066 2.012 2.044 1.980 2.036 2.013 2.006 2.036

486 487 483

2.059 2.013 2.070

5. Conclusion This work presents a detailed study of electronic absorption spectra of BQ, DHBQ and THBQ in solvents of different polarities. In particular, the hydroxy-substituted quinones are investigated experimentally as well as theoretically in greater detail for the first time. The assignments of electronic transitions are successfully made on the basis of PPP and CNDO calculations with limited configuration interaction. It is found that the p “p* transitions are well predicted by the PPP method. The predictions of the CNDO method are, however, superior both in their accuracy as well as the ability to predict the n“ p* transitions. The present studies reveal that the increase in the number of hydroxyl groups in the parent benzoquinone molecule results in a red shift of the p “p* and n “ p* bands. Also, an increase in the dielectric constant of the solvents is found to cause a decrease in the wavenumbers or increase in the wavelengths of the transitions. This is well understandable since the increase in the solvent polarity is expected to result in bathochromic shift of the p “ p* absorption bands and hypsochromic shift of the n “ p*

bands. Due to poor intensities of the p“ p* and n“ p* transitions in the visible region of the spectra of DHBQ and THBQ, it could not be possible to study the effects of intra-molecular hydrogen bonding in detail. For this, further spectroscopic studies are to be carried out on these molecular systems, so as to obtain their well-resolved spectra.

Fig. 10. Variation of absorption maximum of the intense higher-energy p“ p* band oftetrahydroxy-1,4-benzoquinone with dielectric constants of the solvents.

M. Ahmed, Z.H. Khan / Spectrochimica Acta Part A 56 (2000) 965–981

Acknowledgements One of the authors (Z.H.K) is grateful to the All India Council for Technical Education (AICTE), New Delhi, for a Research Grant.

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