Indian Journal of Chemistry Vol. 23A, July 1984, pp. 546-550 Electronic Spectra of Fluorobenzoic Acids DHARAM V S JAIN*, FATEH S NANDEL & (Mrs) PREM SING LA Department of Chemistry, Panjab University, Chandigarh 160014 Received 25 November 1983; revised and accepted 30 January 1984 The absorption and emission spectra offluorobenzoic acids have been studied in different solvents, viz. water, ethanol and cyclohexane. The data are supplemented by theoretical calculation of electronic spectra by CNDO-SjCI method. The first excited singlet and triplet states of the acids are found to be of Imr* and 3nn*-types respectively. The presence of these transitions accounts for the non-fluorescent and strongly phosphorescent nature of these compounds via coupling of 3 nn * and 1nn*. It is found that at low temperature (17K) all the isomers show weak fluorescence in water and ethanol. This is explained by hydrogen bonding of carbonyl oxygen with the solvent molecules. The electronic spectra of fluorobenzoic acids do not seem to have been studied in detail and there is only one reference in the literature 1. As a part of our programme+:' of systematic study of the electronic spectra of disubstituted benzenes, the electronic spectra of fluorobenzoic acids in different solvents have now been studied and the results supplemented by theoretical calculations of the spectra. The concentrations of benzoic acids are so chosen as to avoid the formation of dimeric species which may lead to complication in the interpretation of the spectra. Materials and Methods Ortho-, meta- and para-fluorobenzoic acids were prepared from corresponding fluorobenzaldehydes by oxidation with Jones' agent", and the melting points of the fluorobenzoic acids (or tho- 394K, meta-396K and para-455K) agreed with the literature values. Absolute ethanol was twice distilled over sodium metal and stored over molecular sieves. Water was distilled over potassium dichromate and potassium permanganate solutions. Cyclohexane ' was stirred with sulphuric acid, washed with water, dried over fused calcium chloride and distilled over sodium, b.p. 352.6K. The UV spectra were obtained at room temperature with a Carl-Zeiss UV-VIS spectrophotometer. The fluorescence and phosphorescence spectra were taken on a Perkin-Elmer MPF-44B spectrophotometer. Method of Calculation The calculations were performed using a CNDO/S technique" described previously and found suitable 7,8 in describing the excited states of a variety of molecules. After the electron density on each atomic center had converged to within ±0.005 electrons in SCF calculation, the excited configurations were generated and interacted (CI) to form the excited state. 546 For CI 30, lowest excited states were considered. The molecular axis and numbering of atoms are designated in Fig. I. The values of the bond distances and bond angles were taken from literature't-!", The electron densities on various atoms in the excited state j were given by Eq. (I) CI A jPA=OPA + LLqm(C~f-C~J ... (1) m 11 In which 0 PAis the ground state electron density on atom A and C jm being the coefficient of the mth configuration contributing to the jth state. C llf and C lli are respectively the orbital coefficients of the final and the initial molecular orbitals involved in the mth configuration contributing to the jth state. The characterization of the different states was done on the basis of configurational analysis. All the calculations were done on DEC-2050 machine. Results and Discussion The absorption and emission spectra of fluoroben- zoic acids were recorded in different solvents, viz. water, ethanol and cyclohexane. The absorption spectra of all the isomers exhibit three moderate to very strong bands. The V max (in kK) together with the oscillator strength (j) for all the isomers in different solvents are summarised in Table 1. The presence of an additional band near 33 kK could be detected only by taking their excitation spectra and hence its oscillator strength could not be calculated. ct ·"', /? , ' C ', ..' '0 , FH o-FBA Fb-'" ,9 : ', C v.. 'o-H m-FBA Fig. 1--- Molecular axis designation [Y-axis being perpendicular to the molecular plane]
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Indian Journal of ChemistryVol. 23A, July 1984,pp. 546-550
Electronic Spectra of Fluorobenzoic Acids
DHARAM V S JAIN*, FATEH S NANDEL & (Mrs) PREM SING LADepartment of Chemistry, Panjab University, Chandigarh 160014
Received 25 November 1983; revised and accepted 30 January 1984
The absorption and emission spectra offluorobenzoic acids have been studied in different solvents, viz. water, ethanol andcyclohexane. The data are supplemented by theoretical calculation of electronic spectra by CNDO-SjCI method. The firstexcited singlet and triplet states of the acids are found to be of Imr* and 3nn*-types respectively. The presence of thesetransitions accounts for the non-fluorescent and strongly phosphorescent nature of these compounds via coupling of 3nn* and1nn*. It is found that at low temperature (17K) all the isomers show weak fluorescence in water and ethanol. This is explainedby hydrogen bonding of carbonyl oxygen with the solvent molecules.
The electronic spectra of fluorobenzoic acids do notseem to have been studied in detail and there is onlyone reference in the literature 1. As a part of ourprogramme+:' of systematic study of the electronicspectra of disubstituted benzenes, the electronicspectra of fluorobenzoic acids in different solventshave now been studied and the results supplementedby theoretical calculations of the spectra. Theconcentrations of benzoic acids are so chosen as toavoid the formation of dimeric species which may leadto complication in the interpretation of the spectra.
Materials and MethodsOrtho-, meta- and para-fluorobenzoic acids were
prepared from corresponding fluorobenzaldehydes byoxidation with Jones' agent", and the melting points ofthe fluorobenzoic acids (or tho- 394K, meta-396K andpara-455K) agreed with the literature values.
Absolute ethanol was twice distilled over sodiummetal and stored over molecular sieves. Water wasdistilled over potassium dichromate and potassiumpermanganate solutions. Cyclohexane ' was stirredwith sulphuric acid, washed with water, dried overfused calcium chloride and distilled over sodium, b.p.352.6K.
The UV spectra were obtained at room temperaturewith a Carl-Zeiss UV-VIS spectrophotometer. Thefluorescence and phosphorescence spectra were takenon a Perkin-Elmer MPF-44B spectrophotometer.
Method of CalculationThe calculations were performed using a CNDO/S
technique" described previously and found suitable 7,8
in describing the excited states of a variety ofmolecules. After the electron density on each atomiccenter had converged to within ±0.005 electrons inSCF calculation, the excited configurations weregenerated and interacted (CI) to form the excited state.
546
For CI 30, lowest excited states were considered. Themolecular axis and numbering of atoms are designatedin Fig. I. The values of the bond distances and bondangles were taken from literature't-!",
The electron densities on various atoms in theexcited state j were given by Eq. (I)
CI A
jPA=OPA + LLqm(C~f-C~J ...(1)m 11
In which 0PAis the ground state electron density onatom A and Cjm being the coefficient of the mthconfiguration contributing to the jth state. Cllf and Clli
are respectively the orbital coefficients of the final andthe initial molecular orbitals involved in the mthconfiguration contributing to the jth state. Thecharacterization of the different states was done on thebasis of configurational analysis. All the calculationswere done on DEC-2050 machine.
Results and DiscussionThe absorption and emission spectra of fluoroben-
zoic acids were recorded in different solvents, viz.water, ethanol and cyclohexane. The absorptionspectra of all the isomers exhibit three moderate tovery strong bands. The Vmax (in kK) together with theoscillator strength (j) for all the isomers in differentsolvents are summarised in Table 1. The presence of anadditional band near 33 kK could be detected only bytaking their excitation spectra and hence its oscillatorstrength could not be calculated.
·The oscillator strength (J) values in cyclohexane could not be calculated due to poor solubility of fluorobenzoic acids.
None of the isomer is fluorescent at roomtemperature but becomes fluorescent at lowertemperature. All the three acids are stronglyphosphorescent at 17K. The Vf and vp are given inTable 1. The Vf and vp values for water and ethanolrefer to the acidic conditions to ensure that these valuesare for the undissociated acids. It is interesting to notethat in Ethanol Propanol Acetone solvent at 17K, allthe three isomers display two fluorescence maxima.This indicates the presence of two types of fluorescingspecies.
The calculated transition energies together with theoscillator strengths for all the isomers are given inTable 2. The molecular orbitals involved in the lowenergy transitions together with the orbital descriptionfor all the molecules are given in Table 3. The nature ofthe molecular orbitals are described by giving thesymbols n, n ..... etc. The different monoexcitedconfigurations which contribute to some of the lowfrequency transitions are given in Table 2. Thecalculated results predict the lowest excited state to beof nn" type for all the isomers near 28kK. As explainedearlier this band is missing in the absorption spectradue to its low intensity and further it overlaps thestrong nn· band (- 36.5 kK). However, thetheoretically predicted ntt" band has been observed inthe excitation spectra and the experimental values thusobtained agree with the theoretical values for all thethree isomeric fluorobenzoic acids.
The next higher energy transition SO-+S2 1t7t* (11)polarized along the short axis (X-direction) is assigned
to the observed band near 36.10 kK for the ortho andmeta-fluorobenzoic acids while this band appears at ahigher frequency (38.02 kK) forming a tail of thestrongly allowed high energy band in p-fluorobenzoicacid. The next theoretically predicted transition So-+S 3 (44.50 kK for ortho, 45.12 kK for meta and 44.28kK for para) is attributed to observed bands at 44.64,44.54 and 43.17 kK in ortho-, meta- and para-fluorobenzoic acids, respectively. This transition is nn*in character and is polarised along the long axis (Z-axis), hence is interpreted as ILa band. SO-+S4transition merges into the higher frequency side of thisband. The intense band which is not clearly detecteddue to its occurrence beyond 50.00 kK comprised thetheoretically predicted transitions So -+ Ss to Ss in paraand So-+Ss to S7 in meta- and ortho-fluorobenzoicacids. The agreement between the experimental andtheoretical values of transition energies and otherquantities seem to confirm that our assignments arecorrect.
Charge transfer nature of IL, and ILa bandsIt can be seen from Table 1 that the frequencies of
IL; and ILa bands increase with the polarity of thesolvent. Normally the frequencies decrease for nn*transitions. Therefore, it was decided to calculatecharges in the ground as wel1 as both these excitedstates. The electron density calculated according to Eq.(I) for the benzene ring, fluoro group and carboxylicgroup are given in Table 4 for the IL, and IL" bandsalongwith the electron density in the ground state. It is
Subscripts: r signifies that the orbital is localised on the benzene ring only; del means that the orbital is delocalised over the entire molecule.
obvious from Table 4 that for the 1L,band the chargeis transferred from ring to carboxylic group (in orthoisomer, there is some charge transfer from fluorogroup also, probably due to hydrogen bonding). This isin conformity with the observation that the acidicnature of the carboxylic group decreases."? in theexcited state. Theoretical results indicate that pK for1La state should be less than that for the ground statefor ortho-fluorobenzoic acid. The charge transfer forthe meta- and para isomers in the 1La state iscomparatively less than that in 1L, state. In the meta-isomer the charge is transferred from the fluoro groupto the carboxylic group while for the para-isomer thecharge is transferred both from ring and fluoro groupto the carboxylic group.
EmissionFor the monomer the lowest excited singlet is of nit"
and as mentioned earlier at room temperature all the
Table 4-Ground and Excited State Charge Densities inFluorobenzoic Acids
State Ring COOH F group Remarksgroup
ortho-Fluorobenzoic acid0..286 -0..0.83 -0..204 CT from ring to the0..354 -0..159 -0..195 -COOH group
-0..0.40 0..230. -0..193 CT from -COOHgroup to the ring
meta-Fluorobenzoic acid0..312 -0..098 -0..215 CT from ring tp the0..350. -0..149 ~0..204 -COOH group0..320. -0..119 -0..197
para-Fluorobenzoic acid0..310. -0..097 -0..214 CT from ring to the0..383 -0..175 -0..219 COOH group0..325 -0..132 -0..195
549
INDIAN J. CHEM., VOL. 23A, JULY 1984
isomeric fluorobenzoic acids are non-fluorescent. Infact very few compounds with 1rm" are known 12,13 tobe fluorescent. However, at low temperature (17K) allthe three acids are weakly fluorescent. Thefluorescence in the non-polar solvent is probably dueto dimer formation. However, in polar solvent (wateror ethanol) the dimer formation is very small.Therefore, we suggest that the fluorescent species is thefluorobenzoic acid hydrogen-bonded to the solventthrough the oxygen of its carbonyl group. This leads toa decrease in energy of nn* transition. The 1ntt" stateshould increase in energy because the stabilization ofground state by the hydrogen bond is greater than thatof the excited state where the oxygen has lost non-bonding electron density. This is supported by the twofluorescence maxima around 27.3 and 25.6 kK inEthanol Propanol Acetone solvent at 77 K. Theposition of the first maximum is exactly the same whenethanol is used as a solvent. Thus it is not thevibrational structure of the second fluorescence bandat 25.6 kK. The position of the second maximum tallieswith the Vr for fluorobenzoic acid in non-polar solventsand is attributed to the formation of dimer. Thus inEPA both the monomer and the dimer of fluro-benzoic acids. exist.
The observation of Baba and Kitamura 14 thatbenzoic acid is non-fluorescent in isopentane +methylcyclohexane at low temperature in the presence ofether is also consistent with our arguments andobservations. Here, the hydrogen bond is formedbetween the hydroxyl group of acid and the oxygen of
550
the ether, leaving the non-bonding electron of thecarbonyl group unutilized. The lowest excited singletstate here is still mr* type.
For ortho-, meta- and para-fluorobenzoic acids thelowest triplet states are calculated to be of nn* type andoccur at 22.56, 22.68 and 22.51 kK, respectively. This isin good agreement with the experimental values inethanol (23.42 kK for ortho, 23.15 kK for meta- and23.15 kK for para-isomers).
AcknowledgementOne of us (PS) is highly thankful to the Department
of Atomic Energy (India) for financial support.
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