J. Phys. Chem. 1986,90, 6793-6800 of approximately 0.3 eV. (For example, the binding energy of (CH3),NH is 0.32 eV higher than that of (C2H5)2NH.6) Second is the effect of replacing the two hydrogens on the boron atom with fluorine atoms. We assume that this causes an increase of 0.9 eV, equal to the increase in the CH3binding energy on going from CH3CH3to CH3CHF2. Summation of these effects yields a net estimated shift of 1.2 eV, corresponding to a nitrogen 1s binding energy of 405.9 eV for (CH3)2NBF2. Similar treatment of the data for the other aminoboranes, together with the SN(CH3) value of -0.32 eV from Table I, leads to the estimated binding energies for NH2BH2,NH(CH3)BF2,and NH2BF2given in Table VI. The nitrogen 1s binding energy and nitrogen p r ionization potential of the planar ammonia molecule are 405.3 and 9.8 eV, re~pective1y.l~The nitrogen p r orbital of planar ammonia is strictly nonbonding. By applying the approximation that shifts in nonbonding valence orbital ionization potentials are eight-tenths of the corresponding core binding energy shifts, we calculate the "localized orbital ionization potentials" (LOIPs) for the nitrogen p-xorbitals of the various aminoboranes given in Table VI. These are the ionization potentials that the nitrogen p?r orbitals would have if they were strictly nonbonding. The last column of Table VI gives the difference between the actual ionization potentials and the LOIP values. Positive differences correspond to net stabilization of the nitrogen p r orbitals and constitute evidence for N-B ?r bonding; negative differences correspond to net destabilization off the nitrogen p r orbitals and constitute evidence for repulsive interactions of those orbitals. It can be seen that the nitrogen p r orbital of NH2BH2is stabilized by 1.3 eV, corresponding to a significant degree of N-B ?r bonding. In NH2BF2 this stabilization is considerably reduced, yet still large enough to constitute evidence for N-B ?r bonding. The reduction in N-B T bonding from NH2BH2 is probably due to a reduction in the ?r acceptor character of the boron atom by the donation of electron
6793
density to the boron atom from the p?r orbitals of the fluorine atoms. When one of the NH, hydrogen atoms is replaced by a methyl group, repulsion between the CH3bonding orbital and the nitrogen p?r orbital further reduces the net stabilization of the nitrogen p r orbital, an effect which leads to an overall destabilization of 0.8 eV with two methyl groups. The fact that the nitrogen p r orbital undergoes overall destabilization in NH(CH3)BF2and N(CH3)2BF2should not be interpreted as evidence for the lack of N-B ?r bonding. The A values in Table VI are the net effects of the repulsive interactions of the methyl groups and the stabilizing, *-acceptor interactions of the BF2 group. Indeed the N-B ?r bonding in NH(CH3)BF2 and N(CH3)2BF2is probably greater than in NH2BF2because the repulsive effect of the methyl group makes the NH(CH3) and N(CH3)2groups better *-donors than the N H 2 group. Hence, contrary to the conclusions of Kroto and McNaughton, who were B(p?r) unwilling to interpret their data in terms of N(p?r) bonding, we conclude that there is significant N-B ?r bonding in NH2BF2and even more in NH(CH3)BF2 and N(CH3)2BF2. Although there are uncertainties of *0.2 or f0.3 eV in the estimated nitrogen 1s binding energies of Table VI, and although uncertainty in the factor of 0.8 used in the LOIP method introduces a further uncertainty of f O . l eV, we believe that the combination of these uncertainties is still small enough that the qualitative conclusions we have drawn from the A values of Table VI are correct.
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Acknowledgment. This work was supported by the Director, Office of Basic Energy Sciences, Chemical Sciences Division, of the US.Department of Energy under Contract No. DE-AC0376SF00098. Registry No. F, 14762-94-8; C1, 22537-15-1; Br, 10097-32-2; I, 14362-44-8; Mn(CO)5, 15651-51-1; NH2BHz, 14720-35-5; NH2BF2, 50673-31-9; NH(CH,)BF,, 99646-58-9; N(CHj)IBFz, 359-1 8-2.
Two-Photon, Thermal Lensing Spectroscopy of Monosubstituted Benzenes in the
Jane K. Rice* Chemistry Department, University of Southern California, Los Angeles, California 90089-0484
and Roger W. Anderson Chemistry Board of Studies, University of California, Santa Cruz, Santa Cruz, California 95064 (Received: September 17, 1984; In Final Form: June 12, 1986)
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The 'B2,('Lb) 'AIg('A) and 'BIu('La) 'AIg('A) electronic transitions in neat benzene, fluorobenzene, toluene, phenol, and aniline are examined by two-photon thermal lensing spectroscopy. The energies, integrateajntensitie, and polarization ratios (circular/linear) are given for both transitions. The 'B2,,(lLb) 'A18('A) transition shows intensity mainly through vibronic coupling, in particular through the vI4 vibration. In addition, pure electronically allowed intensity is seen in toluene and aniline. The integrated intensities vary by a factor of 2 for the molecules benzene, fluorobenzene, toluene, and phenol. Aniline has an integrated intensity of 9 times that for benzene. The 0 transition energies for phenol and aniline are lowered from those for the other molecules indicating the presence of charge-transfer contributions. However, only in aniline is the integrated intensity enhanced. The lBI,,('La) 'A18('A) intensity strongly increases with the strength of inductive substituents in the order F C OH C NH2 where the latter has an intensity 160 times that of benzene. Again the 0-0 energies for phenol and aniline indicate the presence of charge-transfer contributions. The 'BlU('La) 'A18('A) intensity increases moderately for the hyperconjugative substituent -CH3. These results display the importance of the pseudoparity designation of altemant hydrocarbons in determining two-photon transition strengths. This work is consistent with current theoretical approaches which consider explicitly or implicitly pseudoparity designations. The results also point out the necessity of considering the effect of hyperconjugativesubtituents like methyl in theoretical treatments. The results indicate that when integrated intensities are used to give two-photon intensities charge-transfer states behave like inductive perturbations.
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Introduction The research in the nature of benzene electronic states has &n prolific, and Ziegler and Hudson' have provided a thorough review (1) Ziegler,
L. D.;Hudson, B. S. Excired Stares 1982, 5, 41.
0022-3654/86/2090-6793$01 .50/0
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of experimental results. Recent experimental advances in twophoton, multiphoton, and resonance Raman spectroscopies have provoked new questions about the mechanism of intensities in unsubstituted and substituted benzene electronic bands. Experimental and theoretical workf3 have demonstrated that one-photon 0 1986 American Chemical Society
6794 The Journal of Physical Chemistry, Vol. 90, No. 26, 1986
excitation of the first excited singlet state in benzenes is enhanced by inductive substituents while two-photon excitation of this state is enhanced by vibronic coupling. The theoretical approacheszs3 also predict that the type of coupling is reversed for excitation of the second excited singlet state. For the second excited state, vibronic coupling promotes one-photon excitations. The primary goal of this work is to provide experimental evidence for the predictions of the theories for two-photon excitation of the first and second excited states in benzenes. We provide two-photon intensities for excitation of both states for benzene and four substituted benzenes. Since we include toluene in our molecules, our data also allows evaluation of the effects of hyperconjugative as well as inductive perturbations. For the Dshsymmetry group of benzene, states with 'Eluand ,A2, overall symmetry can be excited by one-photon dipole-allowed transitions from the ,Al, ground state. A one laser experiment allows the observation of two-photon transitions from the ground state to states of ,Alg, ,E,,, or ,Ez, symmetries. In addition states of ,Az, symmetry can be excited if two photons with different frequencies are used. Hence of the valence states IB2,, lBlu, and ,Eluthat are produced by one electron H O M O to LUMO excitation, none can be produced by an allowed electronic transition which uses two photons and only the latter can be produced by using one photon. One-photon excitation of the first two states and two-photon excitation of all three states are made possible only through the involvement of vibronic coupling that produces states of total symmetry of ,Alg, ,Elg,or ,Ets. However, recent workZhas shown that the group theory selection rules can explain only part of the intensities of one- and two-photon absorptions. The pseudoparity properties of the electronic states must also be considered. For benzene, an alternant hydrocarbon, the x electronic states can be classified as having + or - pseudoparity. With the inclusion of pseudoparity the ground and the first two excited states become ,Al;, lBz,,-, and IB,,'. Since the electronic transition dipole operator alters the pseudoparity, one-photon excitation of IBz; and two-photon excitation of lB1,+ from the ground state are also forbidden by the pseudoparity properties of the states. However, Callis, Scott, and AlbrechtZ have shown that, to first order in perturbation theory, inductive perturbations alter pseudoparity so that such perturbations can promote one-photon excitation of IBZ; and two-photon excitation of IB1,+. The perturbation theory of Goodman and Rava3 gives the same result without explicitly using pseudoparity. Upon substitution, the D6,,symmetry group of benzene becomes Cz, symmetry and the benzene states of lBzu and lBlu symmetry demote to states of 'B2 and 'A, symmetry. Consequently, oneand two-photon transitions are formally allowed by group theory arguments. But since the substituent only weakly perturbs the benzene x electronic states in low order, the allowed group theory designation is not manifested. Similarly the pseudoparity selection rules of benzene are weakened very little by substitution and therefore remain relevant. Since inductive perturbations alter pseudoparity and since states of opposite pseudoparity are effectively coupled by an odd number of photons, we expect that inductive substituents will determine the strength of the onephoton excitation of IBT ,A,-. Likewise, since even photon transitions combine states with the same pseudoparity, an inductive perturber ,Al-. will also enhance the two-photon excitation of ,AI+ In the commonly used Platt notation4 the lBZuand lBlustates in D6h symmetry or the lBZand 'Al states in C, symmetry are classified as 'Lb and 'La states, respectively. For the remainder of this paper the singlet specification will be omitted since we are only considering singlet states, and the two lowest excited states of either &h or C, symmetry will be labeled as Lband La. The Platt notation allows consistent designation of the two lowest states of benzene when substituents are added. The symmetry designations corresponding to these states should be kept in mind since they facilitate the determination of vibronic activity and the in-
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(2) Callis, P. R.; Scott, T. W.; Albrecht, A. C. J . Chem. Phys. 1983, 78, 16. (3) Goodman, L.; Rava, R. P. Adu. Chem. Phys. 1983, 54, 177-230.
Rice and Anderson terpretation of polarization ratios. Also the pseudoparity designations of Lb and La are - and +, respectively, for both symmetry groups. We will not discuss the excitation of the E,, state,5 nor the elusive Ez,:6 However, we note that the only two-photon allowed transition in the energy region below 52 000 cm-l is the El, state at 51 085 cm-'. This state has been seen in the gas phase by Johnson' and in a 3% benzene solution by Scott and AlbrechL8 The state has been assigned to the Rydberg transition of a 2px electron to a 3s orbital. The state could not be observed with the two-photon fluorescence technique in neat benzene. In this work we present the results of a single laser two-photon thermal lensing spectroscopy (TPTL) experiment on the excitation of Lb and La states in benzene and four monosubstituted benzenes in the condensed phase. We have measured the relative intensities of the transitions to these states and the polarization ratios (circular/linear). The intensity information allows us to evaluate the theoretical ~ o r kon~ the , ~complementary nature of one- and two-photon absorptions as well as the complementary nature of the intensities for Lb A and La A transitions under inductive or vibronic coupling substituent perturbations. The polarization ratios help determine the symmetry of the vibrational modes that promote vibronic activity. For a single laser two-photon experiment on benzene, vibrations with symmetries bz,, ezu, and e,, allow excitation of Bzu(Lb) electronic states because they result in states with total symmetries of A,,, E,,, and Ezq Similarly, excitation of B,,(La) electronic states becomes possible with vibronic coupling of vibrations with b,,, ezu,and e,, symmetries which also produce resultant total symmetries of A,,, E,,, and E2,, respectively. A polarization ratio of 3/2 is expected for the antisymmetric resultant symmetries (E,,, EZg). Hence vibronic assistance by vibrations of symmetry ezu or e,, can be identified for excitation of either Lb or La states. Two-photon excitation of states in benzene with total symmetry of A,, can result in polarization ratios between 0 and 213 depending on the ratio of in-plane to out-of-plane absorption strength? Pure in-plane polarization results in a polarization ratio of 1/4, and pure out-of-plane polarization results in a ratio of 213. Polarization ratios can also help determine the vibronic borrowing electronic state for two-photon transitions. In this way Friedrich and McClaing have determined that the ground A,, state is efA two-photon excitation in benzene. fective for Lb This work is the first systematic study of the relative intensities of the Lb A and La A two-photon excitations. Similar work has been done by Scott, Callis, and Albrecht'O who have examined neat benzene and fluorobenzene using the two-photon fluorescence (TPF) technique. The Lb A and La A two-photon transitions of neat, liquid benzene and toluene were examined by Razumova and Starobogatov using fluorescence excitation detection. Twarowski and Kligerl2 examined neat benzene with TPTL spectroscopy and they were the first to show that the technique was effective in measuring both Lb A and La A absorptions. The other studies have concentrated on the Lb state in benzene and substituted benzenes. Tam and PatelI3 used two-photon opto-acoustic spectroscopy to document the stronger v18 progression in the condensed phase compared to the gas phase. Friedrich et al.14 examined the 0-0 band in condensed-phase benzene, fluo-
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(4) Platt, J. R. J . Chem. Phys. 1949, 17, 484. (5) Ziegler, L. D.; Hudson, B. S . Chem. Phys. Lett. 1980, 71, 1 1 3 . (6) Nakashima, N.; Sumitani, M.; Ohmine, I.; Yoshihara, K. J . Chem. Phys. 1980, 72, 266. Nakashima, N.; hove, H.; Sumitani, M.; Yoshihara, K. J . Chem. Phys. 1980, 73, 5976. (7) Johnson, P. M. J. Chem. Phys. 1975,62,4562. J . Chem. Phys. 1976, 64, 4143. (8) Scott, T. W.; Albrecht, A. C. J . Chem. Phys. 1981, 74, 3807. (9) Friedrich, D. M.; McClain, W. M. Chem. Phys. Lert. 1975, 32, 541. (10) Scott, T. W.; Callis, P. R.; Albrecht, A. C. Chem. Phys. Lett. 1982, 93, 1 1 1 . (1 1) Razumova, T. K.; Starobogatov, I. 0. Opt. Spectrosc. (USSR) 1981, 49, 654. (12) Twarowski, A. J.; Kliger, D. S. Chem. Phys. 1977, 20, 259. (13) Tam, A. C.; Patel, C. K. N. Nature (London) 1979, 280, 304. (14) Friedrich, D. M.; Van Alsten, J.; Walters, M. A Chem. Phys. Lett. 1980, 76, 504.
TPTL Spectroscopy of Monosubstituted Benzenes
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6195
robenzene, toluene, and pdifluorobenzene using the TPF excitation technique. Dick et al.15 examined the two-photon excitation of the Lb A transition in toluene and several larger aromatics. This study included polarization measurements. Gas-phase experimental work includes the MPI and TPF excitation results of Goodman and c o - w o r k e r ~ .They ~ ~ ~have ~ ~ ~measured the relative peak heights for vibrational bands in the Lb A excitation to determine the importance of vibronic and inductive effects for a large class of monosubstituted benzenes. Squire et a1.25have used MPI with mass spectrometric detection to study the intensities of the 0-0 bands of benzene, fluorobenzene, and toluene. The two perturbation theoriesz3 for the intensities of two-photon transitions in benzenes have been mentioned earlier. Both theories predict that one-photon Lb A and two-photon La A transitions are enhanced by inductive perturbations while vibronic perturbations will enhance the one-photon La A and two-photon Lb A. Both theories are qualitative and both use the Pariser pseudoparity designations either explicitlyZ or i m p l i ~ i t l y . ~ Quantitative calculations of two-photon intensities are lacking except for the Callisz6calculation which applies CNDO/s calculations to over 30 monosubstituted and polysubstituted benzenes. His calculated results will be compared to our experimental results. The Goodman and Rava perturbation theory has been developed in a number of papers.3~2J3~z7 It is derived from work of Moffitt28 and Murrell and L o n g u e t - H i g g i n ~ . ~In~ this theory, vibronicand substituent-induced electrostatic perturbations on the ring are included. In addition, if the perturbing substituent has a low-lying K orbital, charge-transfer states between the substituent and the ring are included in the perturbation. With the inclusion of these charge-transfer states comes the ability to predict the effect of a substituent which forms hyperconjugation with the benzene K system. To date, there have been only qualitative discussions from the Goodman theory. Their qualitative arguments have explained the general behavior of their experimental results on the Lb A transition. We will discuss their hyperconjugation arguments in reference to our toluene results. The Goodman and Rava theory indicates that the presence of charge-transfer states should enhance both of the two-photon Lb and La excitations. Goodman and Rava3 have also explored mass perturbations in A transitions. They are able to explain the two-photon Lb observed intensities for isotopically substituted benzenes. In the substituted benzenes used in this work, the masses of the substituents are approximately equal. In their extensive experimental work on monosubstituted benzenes, Goodman and c o - w ~ r k e r s ~ zhave J ~ - made ~ ~ a distinction between transition intensity due to a pure electronically allowed component and a vibronically coupled component. They refer to the former as Franck-Condon intensity and one determination of its strength is the 0-0 band intensity of the transition. We use the term electronically allowed (EA) for this component. Goodman and co-workers have used the v I 4vibrational band in the Lb A transition as an indication of vibronic coupling in-
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(15) Dick, B.; Gonska, H.; Hohlneicher, G. Ber. Bunsenges. Phys. Chem. 1981, 85, 746.
(16) Krogh-Jespersen, K.; Rava, R. P.; Goodman, L. Chem. Phys. Lett. 1979, 64, 413.
(17) Rava, R. P.; Goodman, L.; Krogh-Jespersen, K. Chem. Phys. 1979, 68, 337. (18) Krogh-Jespersen, K.; Rava, R. P.; Goodman, L. Chem. Phys. 1979, 44, 295. (19) Krogh-Jespersen, K.; Rava, R. P.; Goodman, L. Chem. Phys. 1980, 47, 321. (20) Rava, R. P.; Goodman, L. Chem. Phys. Lett. 1980, 76, 234. (21) Rava, R. P.; Goodman, L.; Krogh-Jespersen, K. J. Chem. Phys. 1981, 74, 273. (22) Goodman, L.; Rava, R. P. J . Chem. Phys. 1981, 74, 4826. (23) Rava, R. P.; Goodman, L. J . Am. Chem. SOC. 1982, 104, 3814. (24) Rava, R. P.; Goodman, L.; Philis, J. G. J. Chem. Phys. 1982, 77, 4912. (25) Squire, D. W.; Barbalas, M. P.; Bernstein, R. B. J. Phys. Chem. 1983, 87, 1701. (26) Callis, P. R. Chem. Phys. Lett. 1984, 107, 125. (27) Goodman, L.; Rava, R. P. Acc. Chem. Res. 1984, 17, 250. (28) Moffitt, W. J. J. Chem. Phys. 1954, 22, 230. (29) Murrell, J. N.; Longuet-Higgins, H. C. Proc. Phys. SOC.(London), Sec. A 1955, A68, 329, 601, 969.
VIT Figure 1. Experimental apparatus for two-photon thermal lensing spectroscopy: PL, pulsed laser; ML, monitoring laser; PLD, pulsed laser wave Fresnel detector; MLD, monitoring laser detector; I, iris; FR, Rhomb; S P scattering plate; BS, beam splitter (1/4 in. flat); S, sample cell ( 1 cm length); M I and M2, mirrors; L,, pulsed laser focusing lens; L2, monitoring laser focusing lens; L3, divergent lens to fill M L detector; F,, 10-8, bandwidth interference filter centered at 6328 8,; F2, 100-8, bandwidth filter centered at 6328 8,; P, pinhole; VIT, vibration isolation table; BF, N2laser beam flag; F3,variable neutral density filter. Details about the equipment are given in the text.
tensity. We continue to use the ~ 1 intensity 4 as an indicator of vibronic coupling (VC); however, we note that the v I 4intensity is not constant in the monosubstituted benzenes studied. For molecules with small fluorescence quantum yields like the benzenes in this study, the TPTL technique yields accurate absorption spectra even if the fluorescence quantum yield changes with excitation energy. However, TPF measurements are distorted by changes in the fluorescence quantum yields. From the onephoton work of Braun et al.30 and Birks et al.” on the variation of fluorescence yields on excitation wavelength for benzene and similar molecules in solution, there is a decrease in fluorescence yield of 45 to 65% for benzene over the energy range used in these experiments. This suggests that TPF excitation results which have not been corrected for wavelength-dependent fluorescence quantum yields will display two-photon cross sections 2 to 3 times smaller than TPTL values in the La energy range and undistorted results in the Lb energy range. The fluorescence yield dependence on wavelength is similar for toluene, and aniline.33
Experimental Section Spectral purity benzene was used without further purification. Fluorobenzene was distilled and passed through neutral alumina. Toluene was stored under argon overnight followed by distillation over CaH2. Phenol was dried in a vacuum desiccator, transferred in a dry bag, and zone refined. Aniline was purified by storing over KOH for 1 h, fractional distillating over SnClZ,and cycling three times with freezepumpthaw before sealing under vacuum in the 1-cm glass cuvette that was used to obtain the spectrum. Braun, C. L.; Kato, S.; Lipsky, S. J . Chem. Phys. 1963, 39, 1645. Birks, J. B.; Conte, J. C. Walker, G. J . Chem. Phys. 1968, SIB, 934. Kohler, G.; Getoff, N. Trans. Faraday SOC.1976, 72, 2101. Kaler, C.; Getoff, N. Sitzungrber. dsterr. Akad. Wiss., Math. Naturwiss. KI., Abt. 2 1974, 183, 158. (30) (31) (32) (33)
6796 The Journal of Physical Chemistry, Vol. 90, No. 26, 19616
One-photon spectra were taken of the neat solutions in an absorption cell constructed of two Suprasil plates squeezed together in a threaded hollow tube. Although exact O D comparisons could not be made with this cell due to the undetermined short path length, the Lb A and La A absorptions for most cases were very similar to more dilute one-photon spectra. The exception was neat aniline which was quite broad with little similarity to its dilute solution spectrum. The experimental apparatus used to collect TPTL spectra is shown in Figure 1. The two-photon excitation source was a Molectron DL200 dye laser with an intracavity linear polarizer that was pumped by a UVlOOO nitrogen laser. After passing through an iris, the beam was reflected from two mirrors and passed through a five-sided 1 / 4 wave Fresnel rhomb which produced either circular or linear polarization. The laser light was then focused into the center of the sample with a 380-mm focal length lens. The thermal lens was monitored with a Spectra Physics 2-mW H e N e laser that was focused with a 100-mm lens. The monitor beam was coaxial to the pump laser but entered the sample from the opposite direction. Its focus was placed about one confocal parameter from the pump laser focus so that the thermal lens was probed with diverging light. After exiting the sample cell the monitor beam was reflected off a flat glass plate to a 300-pm-diameter pinhole located 50 cm away from the plate. The monitor light then passed through an interference filter with a band pass of 10 8, centered a t 6328 8, before its intensity was measured with a Hamamatsu R446 photomultiplier. The dye laser intensity measurement was made by reflecting a portion of the dye laser onto a MgO scattering plate. A Hamamatsu S1137-1010BQ photodiode detected the scattered light. Due to the enormous range of the thermal lensing (TL) signal intensity and the variations of laser intensity for different dyes, a variable neutral density filter was placed between the dye laser and the T L optical equipment. This was adjusted with dye changes but not during the individual dye ranges. The dye laser was operated with either horizontal or vertical linear polarization depending on the wavelength region. No systematic errors greater than 7% were evident when cross sections were computed with both horizontal and vertical polarization. The polarization ratio of a one-photon benzene vibrational overtone was found to be 1 f 0.1 indicating that any one-photon components in the cross-section values did not introduce systematic errors in the polarization ratios. The data collection for TPTL was computer controlled by a Z80 based m i ~ r o c o m p u t e r . ~With ~ the computer, two-photon cross sections could be calculated in real time for individual flashes and then averaged to obtain a cross-section value at each wavelength. The excitation laser intensity was varied over a large range at each wavelength to accurately determine the photon dependence of the absorption. The T L signals were €it to the quadratic equation y = bx cx2 where y is the T L signal and x is the measured dye laser intensity. Hence bx is the one-photon component and cx2 is the two-photon component for the TL signal. The accuracy of this fitting routine was carefully tested with a one-photon overtone in neat benzene. The two-photon fraction of the T L signal was determined by dividing the 200 flash average of cx2 by the average T L signal. This fraction varied from a value of 1.O which repA resents a pure two-photon event to about 0.6 in the Lb excitation regions between vibrational peaks. Figure 2 displays a sample plot of 200 two-photon T L data points collected at one wavelength. This data was collected in a region where no vibrational overtones or any other one-photon signals were present. A one-parameter fit representing a strict two-photon excitation is shown with the experimental data. In regions of the spectra where onephoton overtone signals have been confirmed by one-photon spectroscopy or published work by others, the bx term in the y = bx + cx2 equation was seen to increase. In this work the c coefficient is used to represent the two-photon cross sections. This eliminates the unwanted one-
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(34) Rice, J. K. Ph.D. Thesis, University of California Santa Cruz, 1983.
Rice and Anderson TOLUENE
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Figure 2. A typical plot of TPTL signal vs. laser intensity. The 200 data points are collected at one wavelength and the smooth curve is the best quadratic fit 0, = cx2) to the data points where y is the thermal lensing signal, c is a parameter, and x is the pulsed laser intensity.
" 32
34
36
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Energy ( k K )
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Figure 3. The neat TPTL spectrum (solid line) and the one-photon Cary-14 spectrum (dotted line) for the Lb A transition in benzene. The asterisk indicates the 0-0 band position in the references cited in the text. (1 kK = 1000 cm-'.)
photon portion of the T L signals. Solvent absorption and onephoton overtones are usually a drawback in the use of T L as a twephoton spectroscopic tool, but the fitting routine circumvented these problems. The fitting routine allowed determination of two-photon cross sections in the 360-400-nm region where significant one-photon contributions were found for aniline and phenol and to a lesser degree in toluene and fluorobenzene. Further discussion of the fitting procedure has been given by Rice.j4 The relative TPTL cross sections were obtained by correcting the thermal lensing signals by a X4 factor3' and for the photodiode wavelength response. No normalization factors were used at the edges of the dye ranges. The spectra for all monosubstituted benzenes were set on the same relative intensity scale by a comparison experiment. In this experiment, two-photon cross sections for the five molecules were measured consecutively through a dye range keeping all alignment and laser adjustments constant. The results of this experiment were used to Oormalize the individual spectra. These spectra are shown in Figures 3-12. Table I1 summarizes the TPTL cross-section results and includes a correction for differences in the thermal properties, neat concentrations, and fluorescence quantum yields among the molecules. The values used for the correction appear in Table I and indicate that there is a factor of 2 difference in the cross sections when this normalization is made. Although the (35) Twarowski, A. J.; Kligcr, D. S . Chem. Phys. 1977, 20, 253.
The Journal of Physical Chemistry, Vol. 90, No. 26, I986 6791
TPTL Spectroscopy of Monosubstituted Benzenes ln
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Figure 5. Spectrum displayed as in Figure 3 for toluene. I
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Figure 8. The neat TPTL spectrum showing the Lb A and L, A transitions in benzene is shown in the lower panel and are obtained with linear laser polarization. The upper panel shows the circular/linear polarization ratios. The polarization ratios given in solid lines are ratios of the c coefficients in the fit y = cx2 for the spectra taken with circular and linear light polarizations. The ratios given in dotted lines are the ratios of the c coefficients in the fit y = bx + cx2. The asterisk indicates the 0-0 band position for the Lb A transition.
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Energy ( k K )
Figure 6. Spectrum displayed as in Figure 3 for phenol at 45 O C .
fluorescence quantum yield (4) measurements were made in dilute solutions, the yield in neat solutions is expected to be lower due to self-quenching. Therefore the (1 - 4) factor which is used in the TL data correction is thought to be less than 15% in error. All other data in Table I1 match the conditions of the T L experiments and we estimate that each two-photon and cross-section value is accurate within a factor of two with the possible exception of the value for aniline in the La A transition which may error as much as a factor of 3. This uncertainty is a result of the comparison test which was used to scale the spectra of the monosubstituted benzenes to one another. As can be seen by the consistency of our spectra with previous data published on benzene, fluorobenzene, and toluene, the accuracy of the L, and Lb tran-
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Figure 9. Spectrum as in Figure 8 for fluorobenzene.
sition strengths for each molecule is quite good. This is a difficult task when using thermal lensing detection. However, thermal lensing spectroscopy provides the best opportunity to make state-to-state and molecule-to-molecule comparisons in systems with low fluorescence quantum yields. TPTL Spectra and Polarization Ratios. The two-photon T L spectra of the Lb state is compared with the corresponding one-
Rice and Anderson
6798 The Journal of Physical Chemistry, Vol. 90, No. 26, 1986
is mainly due to the larger fraction of one-photon T L signals in this spectral region (0.1 to 0.6). The reason that there is more scatter in the ratios for the two-parameter fits and that some ratios exceed 3/2 is that the b coefficients for the circular and linear polarization data are not constrained to be equal.
Results Benzene. The TPTL spectrum of benzene is shown in Figures 3 and 8, and this spectrum is in basic agreement with previous work. We observe a weak feature at 37 550 f 50 cm-' which we assign to the 0-0 band that Friedrich et al.I4 have previously assigned at 37 595 cm-' in neat benzene. We see the 14A1: vibrational progression in the Lb A excitation as has been noted in gas phaseg and liquid phase.13 The v18 vibration is also present. The polarization ratios are similar to those measured by Friedrich et aI.l4 and Tam and Patel.13 The La state is difficult to observe in benzene because twophoton transitions to it are weak and are overshadowed by the El, state in the condensed phase and the E,, Rydberg state in the gas phase. Our spectrum of this transition shows no clear vibrational structure and its peak height is about 0.3 the peak height of the Lb state. Vibrational structure for this state has only been observed for crystalline samples.36 Scott et a1.I0 observed the peak A transition compared the to Lb A height for the La transition to be 0.14. This discrepancy can be explained by the decrease in fluorescence efficiency of benzene for higher energy excitations noted earlier. Razumova and Starobogatov" observe a ratio of about 0.3. With the fluorescence quantum yield correction, this differs from our results by a factor of 2. Fluorobenzene. The TPTL spectrum for the Lb A transition in fluorobenzene is shown in Figure 4. Although we do not see the 0-0 band for the Lb A transition, we use the 0-0 energy (37600 cm-') determined by Friedrich et al.14 to make peak assignments to vI, ~14,and possibly u6a fundamentals. A 1; progression occurs through vibronic coupling of vI4just as in benzene. There is no sign of any v18 vibration in fluorobenzene as was present in benzene. The general shape and band positions are in good agreement with the TPF excitation spectrum of solvated fluorobenzene observed by Scott et al.1° The average polarization A transition is 0.50. value of the Lb A comparison of the L, A one- and two-photon absorptions is also given in Figure 4. The peak spacings in both spectra reflect the v1 energy. The 0-0 band is the strongest feature in the one-photon spectrum. The strongest features of the two-photon spectrum are the vibronically induced 1: progressions promoted 4 This is an indication of the difference in the by the ~ 1 mode. source of intensity for the two spectra. The one-photon spectrum has pure electronically allowed (EA) intensity while the two-photon spectrum derives its intensity from vibronic coupling (VC). This data matches the results of Krogh-Jespersen et al.I9 and Squire et aLzs in this respect. The TPTL La A transition in fluorobenzene, shown in Figure 9, has also been observed in a TPF excitation spectrum by Scott et al.IO The similarity between the results of TPTL and TPF is very good. As in benzene, the TPTL technique gives a signal 2 times larger than the TPF technique. The ratio of La A to Lb A intensity in the TPTL spectrum in 3.5:l from the peak height values. The polarization ratio at the red end of the La state is 1.2 and about 0.8 on the three other peaks that have also been observed by Scott et al.1° This is consistent with 1lo vibronically coupled by a bl vibration such as vl2. If this is correct, the 0-0 band would be positioned around 46000 cm-'. However, because of the trends seen in this transition for the other monosubstituted benzenes, it is more reasonable to assign the strength of this band to EA intensity. We also note that the half-width of this band is taken to be about 3000 cm-I from the TPTL spectrum which agrees with the TPF spectrum. Since the La A transition in fluorobenzene shows the least masking from the E,, transition, we will use this
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Figure 10. Spectrum as in Figure 8 for toluene. 1.4
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Figure 11. Spectrum as in Figure 8 for phenol at 45 OC.
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TABLE I: Thermal Properties, Concentration, and Fluorescence Data for the Correction of Two-Photon Cross Sections of Monosubstituted Benzenes
thermal properties"
CP molecule cal g-l K-l 0.416 benzene fluorobenzene 0.365 toluene 0.406 phenol (0.499)d aniline 0.497
concn, dq/dt g mol-' 0.00064 78.12 96.1 1 0.00049 O.OOO55 92.15 0.00050 94.11 0.00052 93.13
normalization factor: bb cal mol-' K-I 0.07 1.o 0.13 1.5 0.17 1.5 0.10 1.9 0.08 1.8
"Reference 37. The Cp values from top to bottom are for the temperatures 25 O C , 26.8 O C , 25 O C , extrapolated to 25 OC,and 26.4 O C . Reference 38. Fluorescence quantum yield (@)concentrations in cyclohexane (from top to bottom): 3 mL/L, 5 mL/L, 3 mL/L, 0.1 g/L, and 3 mL/L. 'Reference 35. The normalization factor is defined as pCp (dq/dt)-' (mwtlp) (1 - @)-l and is multiplied by the uncorrected TPTL cross sections. The benzene value is set to one. dReference 39. The C, of phenol was extrapolated to 25 "C from higher temperature liquid values.
photon spectra in Figures 3-7 for the five molecules studied in this work. The two-photon T L spectra of the La state and polarization ratios are shown in Figures 8-1 2. Two polarization ratio values are shown on the upper part of these spectra. The dotted lines are the polarization ratios when the two-photon cross sections are computed by assuming no one-photon contribution. This is accurate if there is not a large one-photon component in the T L signal. The solid lines are the polarization ratios using the c parameters determined from the two-parameter fits described previously. In the Lb A region of the spectra, the polarization ratios from the two fits are similar. The La region shows much more discrepancy between the two sets of polarization ratios. This
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(36) Bree, A. C.; Talilani, C.; Thirunacmachandran,T. Chem. Phys. 1981, 56, 288.
TPTL Spectroscopy of Monosubstituted Benzenes
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6799
TABLE II: Energies, Two-Photon Integrated Intensities, 0-0 Intensities, Peak Heights, Polarization Ratios, R, and Theoretical Intensities for L, A and L. A Transitions in Monosubstituted Benzenes' Lb A La A molecule energyb aC o-od PH' R calcdf energy d R calcdf 0.03 1.0 0.8-1.2 10 0.3-0.9 1.0 46000 0.3 37550 1.0 1.0 benzene 0.4-0.6 1.2 46000 5.5 1.7 0.8-1.2 0.06 1.5 14 37 6008 fluorobenzene 0.8-1.0 0.40 3.3 2.4 26 0.6-1.2 1.2 454OOh 37050 2.1 8.2 to1u en e 8.6 8.7 0.8-1.0 0.48 0.6 1.2 44000 4.2 35450' 0.8 phenol 0.7 3.1 40200 49 65 0.8-1.2 7.1 8.8 25 99 aniline 33100 +
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'The integrated intensities, 0-0 intensities, and peak height values were corrected for thermal properties, concentration, and fluorescence quantum yields given in Table I. bO-O energy in cm-I. 'Integrated linear polarization, two-photon intensities normalized for benzene Lb A = 1.0. dO-O peak height normalized within Lb + A and La+ A transitions with benzene = 1. ePeak height for the 14; band normalized for benzene 0-0 = 1.0. /Calculated two-photon intensity for linearly polarized light taken from ref 26 (see Appendix A for details). #From ref 14. *Tentative assignments as 0-0 band. 'Deduced from 14; position and the 0-0 band in one-photon spectroscopy.
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half-width to calculate integrated absorption intensities for the La A transitions in the other molecules in this work. Toluene. The toluene m L Lb A spectrum shown in Figure 5 is similar to the benzene and fluorobenzene spectra. However, the 0-0 band found at 37050 f 50 cm-' is much stronger. The second feature has been assigned as a superposition of v18, a symmetric vibration, and v i , a pure electronically allowed fundamental." The rest of the Lb A transition mimics the benzene 4 Lb A transition in the 1; progressions coupled by the ~ 1 vibration. The toluene and fluorobenzene Lb A transitions show increased intensity between the 14;l; bands relative to benzene. Hence, integrated absorptions will be relatively greater than would be expected on the basis of peak height comparisons. The average polarization ratio is approximately 0.9 for the Lb A transition. This is higher than is found for benzene and fluorobenzene. The 0-0 band shows a polarization ratio of 0.7 which disagrees with the ratio of 1.2 found by Friedrich et al.I4 and 1.3 found by Dick et al.15 However, the polarization ratios of the remaining vibrations agree. This includes the polarization ratio of the second band which is 1.2and approaches the expected value of 1.5 for a v18 or a vI assignment. The strong 0-0 band in the two-photon Lb A spectrum indicates the presence of EA intensity. There is also substantial VC intensity evidenced by the 14;l; progression. These two sources of the two-photon intensity have been previously discussed in MPI work.'8,2z*25 In the TPTL La A transition, shown in Figure 10,the toluene spectrum shows intensity and vibronic structure similar to fluorobenzene. The ratio of the La A to the Lb A intensity using peak height is 1.75:1. An examination of this transition by Razumova and Starobogatov using fluorescence excitation detection" lead to a ratio of 1:l. This is in agreement with the work presented here when the drop in fluorescence quantum yield in this wavelength region is considered. Phenol. The TPTL Lb A spectrum for neat, liquid phenol at 45 OC is shown in Figure 6. It is weak and shows little structure compared to the other substituted benzenes. We expect the 0-0 band to appear at about 35 450 cm-l based on an expected 900cm-' red shift in the liquid-phase value compared to the gas-phase ~ ~do not see the 0-0 transition, although value of 36 344 ~ m - ' .We the weak lasing efficiency of the R6G and C495 dyes in the two-photon energy region 34 800 to 36 800 cm-' may have precluded its observation. A tentative assignment of the lowest energy band at 37050 f 50 cm-l in the TPTL spectrum is 14;. A polarization ratio of about 0.6 is consistent with other 14; viA transitions in the other brational bands seen for the Lb monosubstituted benzenes. The rest of the Lb A intensity is assumed to be the Y , progression coupled by v14. The VC intensity of this two-photon transition is evident when the two-photon spectrum is compared with the one-photon spectrum in Figure 6. Although the one-photon spectrum is for 90% phenol and 10% methanol (to depress the freezing point), no changes in the one-photon spectrum were observed for more dilute phenol solutions. The TPTL La A spectrum of phenol is shown in Figure 1 1. The first band at 44000 cm-l is probably the 0-0 band. The ratio of La A to Lb A intensity based on peak height is 8:l. The
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strong intensity for the La A transition indicates that it has primarily EA intensity. Aniline. The TPTL spectrum of the Lb A transition in aniline is shown in Figure 7. The shoulder at 33 100 cm-' has been assigned as the C-0 band which corresponds to a 930-cm-' ~ bands red shift from the gas-phase value of 34032 ~ m - l .The at 33600 and 34700 cm-' are assigned to 6a; and 14;. The 34 000-cm-I feature is comprised of I: and 12;. All of the transitions are all red shifted from their corresponding gas-phase energies. Because there is no variation in the polarization ratio (0.7)for this transition, there is no evidence of V I 8 that is seen in benzene and toluene. This transition exhibits a strong integrated intensity which is 9 times larger than found in benzene. The TPTL spectrum of the aniline La A transition is shown in Figure 12. This transition is the strongest of the monosubstituted benzenes studied. The ratio of the intensities for the La A to Lb A transition determined by peak heights is 4:l.The band at 40200 cm-' is probably the 0-0 band and it shows a polarization ratio of about 0.8. The three bands that follow have energy spacings of v1 progressions, but there is a large change in the polarization ratio across these three bands. This may be due to EA intensity from an adjacent transition. The large strength of the La A transition is evidence of EA or allowed intensity.
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Comparison of the Spectra The energies, relative integrated intensities, relative 0-0 intensities, relative 14; peak heights, and polarization ratios are summarized in Table I1 for the La A and Lb A transitions found in this work. The table also includes relative two-photon linear polarization cross sections from a CNDO/s calculation by calli^.^^ The integrated intensities are determined by integration of the spectrum for the Lb A transition and by using 3000 cm-l as the half-width for the La A transition. The 0-0 intensities indicate the allowed or EA contribution to the transition. In the Lb A transition, the 14; peak height is used as a measure of the VC contribution to the transition. The energies of the Lb A and La A transitions are similar for benzene, toluene, and fluorobenzene. However, phenol and aniline are shifted, indicating charge transfer is present, since pure inductive perturbations will cause little change in the state energies. A intensities for phenol and fluorobenzene are The 0-0 Lb missing from the table because of experimental limitations, but Rava and Goodmanz1have previously found that the 0-0 band intensity increases in the series fluorobenzene, phenol, and aniline. This increase resembles the one-photon absorption spectra. However, since most of the two-photon Lb A strength is derived from VC (with the exception of aniline), the integrated intensities do not parallel the 0-0 intensities. For the La A transition the 0-0 and integrated intensities are roughly proportional to each other, and they increase with the inductive and/or charge-transfer strength of the substituent group.
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Discussion We have found that the Lb A transition in monosubstituted benzenes show predominantly VC intensity with the appearance of the v14vibration. In addition to VC intensity, toluene and aniline show moderate EA intensity. In phenol the Lb A transition +-
Rice and Anderson
6800 The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 z 0
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Figure 12. Spectrum as in Figure 8 for aniline.
is surprisingly weak and in aniline the transition is 4 times stronger than the other inductive perturben. With the exception of aniline, we find the Lb A transition is only weakly affected by strong inductive or conjugative substituents. On the other hand, the La A transition is markedly enhanced by strong inductive substituents. This is seen particularly in the TFTL spectra of aniline, which is enhanced two orders of magnitude over benzene. The transition intensity also increases moderately with the conjugative substituent, CH3. Both the Callis, Scott, and Albrecht2 and the Goodman and Rava3 theoretical approaches predict the basic observation that the Lb + A transition gains intensity from substituents through vibronic coupling in two-photon spectra and inductive perturbers in one-photon spectroscopy. Also, both predict, in general, that the La A transition behaves in a complementary manner in that it gains two-photon intensity from inductive perturbers and onephoton intensity from vibronic coupling. This is not surprising since both theories include pseudoparity either explicitly or implicitly. Experimental data supported these predictions for the Lb state, and this paper provides substantial support for the theories for the La state. The Callis, Scott, and Albrecht theory is qualitative, but their pseudoparity selection rules appear to work well in generally explaining our results. However, the theory is formulated only to consider vibronic coupling and inductive perturbations. Our systems also involve charge-transfer and/or hyperconjugative perturbations. Since we find that these perturbations enhance the La A transition, such perturbations tend to act primarily as coulomb operators and alter pseudoparity. It is likely that an extension of the pseudoparity approach to include charge transfer will result in this correspondence. The Goodman and Rava theory is also qualitative, but their perturbation theory equations are transparent enough to make clear predictions about the effects of vibronic coupling and inductive substituents on one- and two-photon transitions to the Lb and La states. These predictions which are the same as those of the Callis theory agree with our results. Goodman and Rava also have extended their theory to treat charge-transfer states. Their perturbation expressions are not very transparent, but they state that the presence of charge transfer will enhance two-photon transitions to both Lb and La. Our results support this in the case of aniline but not phenol. One explanation for the anomalous behavior of phenol is that extensive hydrogen bonding present in neat phenol may inhibit the resonance effect of the -OH substituent in the liquid-phase experiment and thereby lower the two-photon cross section. Since hydrogen bonding is weak in neat aniline,40 the resonance effect of -NH2 is retained in the liquid
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(37) Timmermans, J. Physico-Chemical Constants of Pure Organic Compounds; Elsevier: New York, 1950. (38) Berlman, I. Handbook of Fluorescence Spectra of Aromatic Molecules; Academic: New York, 1965. (39) Andon, R. J. L.; Counsell, J. F.; Herington, E. F. G.; Martin, J. E. Trans. Faraday SOC.1963, 59, 830.
phase resulting in an enhancement of the two-photon cross section. Although the inclusion of charge-transfer states can explain enhanced two-photon intensity in toluene and aniline, we think the charge-transfer predictions of Goodman and Rava must be qualified to address only the EA portion of the transition strength. It is also unclear if the limited set of intermediate states used in the perturbation expansion is sufficient to correctly predict TP intensities. Precise prediction of two-photon cross sections must be made with electronic structure calculations, and the CNDO/s calculations of CallisZ4are a good start. Callis has calculated o n e and two-photon cross sections for 30 molecules using 60 singly excited configurations. The predictions of the pseudoparity theory should be valid for a calculation using singly excited configurations, and these calculations a r e consistent with the qualitative theory. The calculated results for fluorobenzene are not correct, however, because of problems with parameterization for this system. We have calculated the relative two-photon cross sections for linearly polarized light in Table I1 from the calculated two-photon tensor invariants (see Appendix A). Omitting the calculated fluorobenzene results, we find that the calculated La two-photon cross sections are qualitatively correct in predicting substituent effects. However, the calculated values are about one-tenth the values found experimentally. Some of the differences present in the calculation may be corrected with a more careful treatment of vibronic coupling and molecular geometry. Since we have determined experimentally that most of the Lb cross section is vc induced, VC must be more adequately considered in calculations. The order of magnitude discrepancy in the relative cross sections for the L, transition will also be corrected if the Lb cross sections have been underestimated. It would be interesting to know how the inclusion of doubly excited configurations would affect the two-photon cross sections calculated with the C N D O method. In summary, we find that our two-photon intensity results are in agreement with the predictions of the Callis, Scott, and Albrecht2 and the Goodman and Rava3 theories.
Acknowledgment. We gratefully acknowledge Professor D. S. Kliger for the use of his laboratory and equipment. We also thank Professors L. Goodman and P. Callis for valuable discussions. Appendix A The calculated values given in Table I1 are taken from Callis.26 The following equation is used to convert the theoretical values, 6G aH and Q,to values proportional to our experimental results using linearly polarized light, 6tt. 6G + &H 6tt = constl+Q In the case of benzene and aniline, we have quoted calculation 4 geometries were used. We have set values in which ~ 1 distorted the theoretical cross section for benzene ( ~ 1 4 ) to the value 1 and normalized the other theoretical cross sections accordingly. To allow the best possible comparison of experiment to theory we have compensated for the VC component exclusion in the fluorobenzene, toluene, and phenol molecules. In the Lb A transition calculations, the value 1 representing the VC component in benzene has been added to fluorobenzene toluene and phenol values. This assumes that the VC component in all the substituted benzenes has the same strength. In the La A transition, we assume the benzene experimental integrated intensity result of 0.3 is representative of the VC contribution. However, since the calculated values in this transition are about 10 times too low for toluene, ,phenol, and aniline, we cannot simply add 0.3 to the theoretical cross sections. Instead, we will add 0.03 or l/lOth of 0.3 to the Callis calculations to represent the VC contribution. Although this is not perfect, it allows a more accurate comparison of experiment to theory.
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Registry No. Benzene, 7 1-43-2; fluorobenzene, 462-06-6; toluene, 108-88-3; phenol, 108-95-2; aniline, 62-53-3. (40) Nageswara Rao, B. D.; Venkateswarlu, P.; Murthy, A. S. N.; Rao, C. N . R. Can. J . Chem. 1962,40,963.
