Indian J. Phys. 64B (6). 447-463 (1990) stimatin f excited state electric diple mment f gme Bubgtitated benznitriles I. Rekha Ra and M A Shashidhar Department f Physics, Karnatak University, Dharwad-580 3, India Received 8 March 1990, accepted 4 September 1990 Abstract: The electric diple mments in the excited state f S, band system 2,3-, 2,4-, 2,6-, 2,6- and 3,4-difliirbenznitriles (DFBN) have been estimated using slvatchrmic shift measurements. The ld prcedure adpted by thers and a new methd prpsed frm this labratry have been applied fr the estimatin f diple mment in the excited state f these mlecules. Tw methds have been discussed and cmpared in the light f the results btained. The grund state electric diple mment fthesemleculeswhichareprerequirement in the determinatin f excited state eiectric diple mment, were als determined experimentally. Keywrds : xcited state diple mments, slvatchrmic shifts, benznitriles. PACS N : 35.20. My Intrductin In cntinuatin f the n-ging wrk in ur labratry (Ayachit et al 1986, 1988, Shashidhar ct l 1976, Deshpande et al 1981, Tnannavar et al 1987) in the area f slvatchrmic shift measurements and estimatin f excited state electric diple mments f mlecules using these measurements, we have measured the slvatchrmic shifts f S i electrnic band system f 2,3-, 2,4-, 2,5-2,6- and 3,4- diflurbenznitriles (DFBN) and estimated the electric diple mments f these mlecules in their excited states f this system. The estimatin f excited state electric diple mments f these mlecules has been carried ut as these give an insight int the mlecular structural parameters in their excited state such as electrnic charge distributin etc. Ledger and Suppan (1967) had earlier suggested a methd fr determining the excited state electric diple mment f a mlecule, using its slvatchrnic shifts frm Me Rae's equatin (Me Rae 1957). Since then the methd has been mdified by several wrkers (Suppan and Tsiamis 1980, Suppan 1975, Prabhumirashl ct al 1983). A new apprach (Ayachit et al 1986) t this prblem with a new methd 447
448 Rekha 8 nd M A Shcshidhar has been prpsed frm this labratry. Th aariiar mathd end the new methd suggested frm this labratry were applied t the Sx band systam f the abve mentfned mlecules and the results are presented and discussed here. The pre-requisite values f grund state electric diple mments in the calculatin f excited state electric diple mments f these mlecules were als determined by the well-knwn mdified Guggenheim's methd (Guggenheim 1951). 2. xperimental The electrnic absrptin spectra (S^ band) f pure samples (Supplied by Aldrich Chemical C., USA) f 2,3-, 2,4-, 2,5-, 2,6- and 3,4-DFBN in different slvents f spectrscpic grade given in Table 1, were recrded using a Hitachi Mdel 150-20 UV/VIS spectrphtmeter with a cell pathlength f 1 cm. The cncentratin f the slute mlecule in different slvents was in the range f 0.01 t 0.02 gm/litre. The data required in the determinatin f grund state electric diple mments, namely, the change in capacitance and refractive indices f the varius' dilute slutins f these plar substances in dixane were measured respectively Using a crystal cntrlled scillatr [1 MHz] and Abbe's refractmeter. 3. Results and discussin In the ld methd. Me Rae's equatin was reduced t tw straight line equatins, namely. heal ( 1) V. - y bca, f ( n» ), (2) where eq. (1) hlds gd fr a set f plar slvents f nearly the same refractive index while eq. (2) hlds nly fr nn-plar slvents. In the new methd prpsed frm this labratry, the Me Rae's equatin has been re-written in the frm f an equatin f a straight line in the intercept frm. Cl C, (3) with X = J [f(d )-f(n *)],.,/
Urn O I Rlnrnln f mcimj m tlm ie dipi, S s; s T- tt r- (0 (0 T* O ^ 0) Or- (D T T s 448 u. Q i CN Tf > t t<s. ^ t ab GO lo 5 S S LO r* t O iir lo t LO LO t t t ft H <30 0 0 T- O r>- ^ t ^ d c' O) m LO IX> 0 3 T -: c0) _> (/ ) >> a a> - O V Q. a> X X <0 X 9 c n Z c Q. D. Otn 5 T3 03 B JC Q
450 Rekha Ra and M A Shashidhar and which hlds fr any set f slvents. In eqs. (1), (2) and (3), I /» I, I / d, O are respectively, psitin f maximum (in cm~^) f a band system in a slvent, the excited state electric diple mment, the grund state electric diple mment and cavity radius f slute mlecule. The ther quantities have the same significance as in ur earlier paper (Ayachit et a l 1986). Here ~ ~ /*») = l /Ij I* - I Pe I I P ( S e 0 is the angle between /tg and jug The values f in different slvents fr all the mlecules studied are given in Table 1, alng with refractive indices and dielectric cnstants f slvents (Aldrich 1985, CRC hand bk 1973-74). Accrding t the ld methd, by chsing tw sets f slvents ne plar f nearly the same refractive index and anther nn-plar, the spectra were reprded. 36^ 363 362 361 360 359 350 Figure I. Plt f k, vs f(n ), f 3,4-diflurbenznitriles. Fr slvents f the first kind, a plt f v, against f(d ), and fr secnd kind, v, against f(n"), were made. By calculating tha slpes (using least-square methd fr bth the plts) f/g and 0 were calculated with a prir knwledge f ffg and a. Figures 1, 2 give the illustrative plts f v, against f(n»), and v, against f(d), in the case f 3,4-DFBN. The methd develped by us is independent f the nature f the slvents used, that is whether ii is nn-plar r plar and this is nt the case with the the(
stimatin f excited state electric diple mment etc 451 mthds mentined earlier. Further it makes use f nly ne equatin, namely, eq. (3). The ther methds in the literature are essentially equivalent t the ld 363 362 361 360 ^ 3S9 02 BZT 06 0^5 if *2 -f(d) Figure 2. Plt f y, vs f(d)^ f 3,4-diflurbenznitrile. methd f Me Rae and they als use tw eqs. (1) and (2). Therefre, it wuld nt b vry usful t use thsse methds and cmpare the results. Using the same data given in Table 1, the values f X and Y were calculated fr each mlecule frm eq. (3) in different slvents. By pltting X against y and j / / / 160 120 / 80 40 20 60 X-lO Figure 3. Plt f X vs Y f 3,4-diflurbenznitriie. calculating and C, (using least square fit methd) /i. { and ff were calculated with a prir knwledge f fj-t and a^. Figure 3 gives an illustratin f a plt f X against Y in the case f 3,4-DFBN. The tw methds mentined abve, d nt take int accunt the ther interactins such as, slvent-slvent interactin (dispersive) hydrgen bnding, effect f
452 Rekha Ra and M A Shashidhar any cmplexes that may be frmed etc. The effect f these factrs can be minimised by a prper chice f the sivents, present study. The estimated values f nn-plar and plar as has been dne in the and e using bth the methds are given in Table 2, alng with U values calculated by the mdified Guggenheim's methd. As fr the mlecular radius Oq, the mlecules were assumed t be spherical in shape and was estimated using the methd f atmic increments and the value f the cavity radius in each mlecule is 3.03 A (dwards 1956). The difference in the values btained by tw methds is essentially because f the Way the methds have been frmulated frm Me Rae's equatin. Table 2. Mlecule fig in D /i. in D in degrees 1 2.3- DFBN 4.35 7.47 (6.97) 60 (56) 2 2.4- DFBN 3.21 4.46 (3.91) 42 (40) 3 2.5- DFBN 4.06 6.64 (6.09) 59 (52) 4 2.6- DFBN 4.04 6.63 (6.27) 61 (56) 5 3.4-DFBN 2.39 3.97 (3.99) 48 (48) The values in the parentheses are estimated values f the and using Me Rae's methd. It can be shwn that the eq. (3) can be reduced t eqs. (1) r (2) under with slvents suitably chsen. q. (3) can be reduced t eq. (1), by taking slvents f nearly same refractive indices, while by taking nn-plar slvents it can j?e reduced t eq. (2). equatin. This means that eq. (3) is a mre general way f representing Me Rae's Further, in eqs. (1) and (2), we neglect the cntributins due t the dispersin term, namely, (A+B)f(n) and ther quantities which are small, while in eq. (3) we neglect quantities which are differences f such small quantities, namely, (A + B) f(n ) i - 2 and etc. In view f these, naturally, the secnd methd may be expected t give a better estimatin f and e. Table 2 shws that the diple mments in the excited state f these mlecules are higher than their grund state values indicating the bserved band system in these mlecules is due t n transitin which is cnsistent with literature (Jaffe and Orchin 1962) mre n the basis f the slvent effect n system. These mlecules exhibit band system In the regin arund A 28 A. The vapur spectra f these mlecules shw a prminent absrptin band system in this regin (Ra et al 1989) and the analysis f this system in each mlecules Is a 77 77 system analgus t ^B^,* transitin f benzene. Hence, the present results are cnsistent with ur earlier bservatins.
Acknwledgments stimatin f excited state electric diple mment etc 463 One f the authrs (RR) thank the Cuncil f Scientific and Industrial Research, New Delhi, fr the award f SRF. References Aldrich Catalg 1985 Handbk f fme Chemicals (Miiwankee, Wiscnsin Aldrich Chemical C.) Ayachlt N H, Deshpande D K, Shashidhar M A and Ra KS 1986 Spectrchim. Acta 42A 585, 1405 Ayachit N H, HuralikppI A M, Ra KS and Shashidhar M A1988 Pramana 30 211 C R C Handbk f Chemistry and Physics 1973-74 ed Weast R C (Cleveland, Ohi; CRC Press) Deshpande D K, Shashidhar M A and Ra KS 1981 Z. Physik, Chem. 282 588 dwards J T 1956 Chem, Ind. 774 Guggenheim A 1949 Trans. Faraday Sc. 45 714 ----1951 Trans. Farady Sc. 47 573 Jaffe N H and Orchin M 1962 Thery and Applicatins f Ultravilet Spectrscpy (N ev^ Y r k : Jhn Wiley) Ledger M B and Suppan P 1967 Spectrchim. Acta 23A 841 Me Rae G 1957 ]. Phys. Chem. 61 562 Prabhumirashi L S, Narayan Kutty D Kand Bhide Z S 1983 Spectrchim. Acta 39A 663 Ra Rekha, Aralakkanavar M K, Ra K S and Shashidhar M A 1989 Spectrchim. Acta 42A 103 Shashidhar M A, Deshpande D Kand Ra KS 1976 Curr. Sci. 45 650 Suppan P 1975J. Chem. Sc. Faraday Trans. 71 539 Suppan P S, Tsiamis C 1980 Spectrchim. Acta 36A 971 Tnannavar J, Jshi A M, Ayachit N H, Shashidhar M A and Ra KS 1987 In d ia n ]. Pure Appt. Phys. 25 425