Indian Journal of ChemistryVol. 28A, August 1989, pp. 635-638

Determination of inductive effect parameter of methyl group fromgas-phase charge transfer bands of molecular complexes of

N-phenyl-tetrachlorophthalimide with methyl substituted benzenes

M Roy, A K Mukherjee & B K Seal*

Department of Chemistry, Burdwan University, Burdwan 713 104, India

Received 2 August 1988; revised and accepted 4 November 1988

The charge transfer absorption bands of electron donor-acceptor (EDA) complexes of Nephenylte-trachlorophthalimide as acceptor with a series of methyl substituted benzenes as donors have beenstudied in dioxan, chloroform, dichloromethane and dimethylformamide as solvents. The absorptionmaxima (v CT ) of the complexes in these solvents deviate greatly from Mulliken's equation when plottedagainst donor ionisation potentials but the gas-phase values of v CT, determined from the CT transitionenergies in the first three weakly polar solvents, give a reasonable fit to Mulliken type plot with thedonor ionisation potentials. The energies of the HOMOs of methyl substituted benzenes, obtained asfunctions of the inductive effect PMO parameter (hMe) of the methyl group by Coulson-Longuet-Higgins perturbation technique, give a linear correlation with the gas-phase h v CT values of the com-plexes. From the slope of this plot, hMe is found to be - 0.1843, a value in fair agreement with that( - 0.21) obtained by Lepley from studies of CT complexes of TCNE with a series of methyl substitut-ed benzenes and naphthalenes. .

Phthalimide and some N-alkylphthalimides havebeen reported 1-3 to act as acceptors in electrondonor-acceptor (EDA) complexes involving organ-ic compounds. There is, however, no report on si-milar behaviour of Nearylphthalimides (also called'phthalanils'). Pratt and Perkin", who first de-scribed the synthesis of such compounds, reportedthat they developed colour when mixed withmethylaniline and N,N-dimethylaniline, but nosystematic study of the new absorption band wasdone. Expectedly, such colour development mightbe due to a charge-transfer (CT) type of trans-ition. Such colours may be claimed to have a CTorigin if, working with a fixed acceptor and a seri-es of structurally similar donors, the transition en-ergies are found to follow the relation (1)5.

... (1)

I

where ID is the vertical ionisation potential of thedonor, EA the electron affinity of the acceptor andC is a term summing the contributions from non-bonding species, polarisation and solvation. Themain ~art of C comes from the electrostatic at-traction between I the negatively charged acceptorand positively charged donor in the excited staten.A series of such CT transition energies may be ut-ilised to obtain important structural informationabout the donor and perturbational parameters of

some heteroatoms and alkyl groups as used in theextended Hiickel theory"!", In the present investi-gation it has been shown that an acceptor likeN-phenyltetrachlorophthalimide gives new absorp-tion bands when mixed with a series of donorslike methyl substituted benzenes; the frequenciesof the band maxima vary systematically with sol-vent dielectric constant and the gas-phase valuesof transition energies follow the relation (1). Fol-Iowing the method described in literature 7 an in-ductive effect Hiickel parameter for the methylgroup has been obtained utilising the CT trans-ition energies of the above mentioned complexesin the gas-phase.

Materials and MethodsN-PhenyltetracWorophthalimide was prepared

by adding freshly distilled aniline to a boiling solu-tion of tetrachlorophthalic anhydride in acetic ac-id. The resulting precipitate was filtered and rec-rystallised from xylene with activated charcoal.The product was kept immersed in acetone for24hr, filtered and "dried, m.p. 268° (lit" m.p. 268-69°).

Benzene (thiophene-free), toluene, three isomer-ic xylenes, mesitylene, durene, pentamethylben-zene (PMB) and hexamethylbenzene (HMB) wereused as donors and purified by repeated distilla-tion, crystallisation [PMB from absolute ethanol

635

INDIAN J CHEM, SEe. A, AUGUST 1989

(m.p. 53°) and HMB from benzene-ethanol (1:5)(m.p.) 166°]. Anhydrous chloroform, dichlorome-thane, dimethylformamide (DMF) and dioxanwere freshly distilled before use and their puritieswere ascertained.

The UV spectra of the donor-acceptor mixturesin the above mentioned solvents were recorded ona Beckmann model 26 spectrophotometer using1em matched quartz cells.

Results and DiscussionThe CT-band maxima of the systems studied in

different solvents are recorded in Table 1. Onlythe CT spectra of HMB with the acceptor in thethree solvents are shown in Fig. 1. The plot of thetransition energies in dioxan against the ionisationpotentials (10) of the methyl substituted benzenesis shown in Fig. 2(a). A linear correlation is ob-served but the slope is 0.1286, whereas it is ex-pected to be unity from Eq. (1). Such a deviationfrom Mulliken's equation (1) is attributable to sol-vent dielectric effects 12.

In CT phenomenon, there is a large change indipole moment on electronic excitation. Conse-quently the dipolar contribution to solute-solventinteraction is likely to be much more pronouncedthan dispersive effect. Under such circumstancesthe shift in the spectra of a C'l-complex in goingfrom solution to gas phase is givenl3•14 by Eq. (2)

(g)_ 50In._1..- (2n~+ 1)2! ( _ )hvcr hVcT - a ' n~+2 2fLgfLg fLe

(£-1 n~-I) (n~-I) 2 2)

x £+2 - n~+2 + n~+2 (fLg-fLe)

Equation (2) is applicable to solutions in weaklypolar solvents. In Eq. (2), 'a' is Onsager cavity ra-dius to be calculated from molar volume; fLgand

... (2)

fLe respectively are the dipole moments of thecomplex in ground and excited states; and no and£ are the refractive index and dielectric constantof the solvent respectively. Making measurementsin three different weakely polar solvents and usingEq. (1), one can find the gas-phase transition ener-gy,hv~.

For a given acceptor and a structurally similardonors one may take EA and C of Eq. (1) to beconstant. The donor ionisation potential (10) maybe taken to be the negative of the energy (ED) ofthe HOMO of the donor. Thus Eq. (1) takes theform (3)

h VCT= - ED + constant ... (3)

0.3

A

0.2

B

c:io C

0.1

340 360 380

",nm

Fig. 1- Plot of absorbance versus wavelength for HMB + N-phenyltetrachlorophthalirnide in different solvents [A-Dioxan;

B-chloroform, and Cvdimethylformamide].

Table l-Charge-transfer absorption maxima of complexes of N-phenyltetrachlorophthalimide with methylbenzenes in fourdifferent solvents at 25°C, the calculated gas-phase transition energies and perturbational coefficients

10 eV CT absorption maxima (ern - I) in hviSJ.,eV1 2: I 2Donor c.,

(ref. 18) n

CHCi3 CH2Ciz Dioxan DMF

Benzele 9.24 29175 29425 29841 29585 4.5740 0Toluene 8.82 28410 28571 28901 28985 4.1998 0.333o-Xylene 8.52 . 28328 28490 28735 28818 4.1098 0.500m-Xylene 8.48 28328 28248 28735 28525 4.2600 0.500p-Xylene 8.60 28248· 28409 28818 28653 4.2560 0.667MesityIene 8.14 28328 28490 28653 28653 4.0306 0.667Durene 8.052 28089 28248 28570 28311 4.1478 1.000PMB 7.92 28011 28089 28328 28200 3.8790 1.133

HMB 7.90 27777 27827 28169 28328 3.660 1.333

636

ROY et al.: INDUCTIVE EFFECT PARAMETER FROM GAS-PHASE CHARGE TRANSFER BANDS

•...u

:)

93.6 •••.0;;...· --I ~

7~ 8~ "lD' LV

Fig. 2- Plot of CT-transition energies of complexes ofN-phenyltetrachlorophthalimide with various methyl substi-tuted benzenes against ionisation potentials of the donors in

(a) dioxan and (b) in gas-phase.

ED of a methylbenzene can be obtained from thatof benzene (EoO) by applying Coulson-Longuet-Higgins perturbation technique 15. After taking intoconsideration the equivalence of many locations inbenzene molecule, expression (4) relates ED withEOo

... (4)

where hMe is the perturbational parameter of themethyl group required to express the change incoulomb integral of the carbon atom in benzenedue to the inductive effect of the methyl group att-ached to the carbon atom 10: B is the standard re-sonance integral for C - C bonds in benzene; n isthe number of equivalent structures of the methyl-benzene under consideration; C; is the atomicorbital coefficient of the r th carbon atom in thej th MO of the mehylbenzene and j is a suffix usedto denote the HOMO. The derivation of Eq. (4)and the values of the calculated perturbational co-

efficients !2:2:c;j are collected from reference (7)n II r

and have been tabulated in Table 1 together withthe values of gas phase transition energies of thecomplexes studied in the present investigation.

Fig. 3 - Plot of gas-phase CT transition energies versus versus

The gas-phase values of transition energies(h v /:!f) correlated well with the Io's of methylben-zenes in accordance with Eq. (1) as shown in Fig.2(b). The slope, 0.5714 is significantly greater thanthe slope (0.1286) in dioxane. The plot of hv~

against 1. LLc ~j is also linear (Fig. 3). The slopen n r

of this plot (- 0.5714eV), when used in Eq. (4),gives hMe = - 0.1846 with B = - 3.1eV as obtainedfrom the first four singlet-singlet transition ener-gies of benzene. This value of hMe is in closeagreement with .that ( - 0.21) obtained by Lepley'?from the charge-transfer spectra of the rt-complexes formed by a number of methyl substi-tuted benzenes and naphthalenes with tetracyano-ethylene. This value is in good agreement withother recent treatments (h = - 0.21 from reductionpotentials) 17.

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INDIAN J CHEM, SEe. A, AUGUST 1989

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