Conformational analysis of guaiazulene in a supersonic jet - The

4226

J . Phys. Chem. 1985,89, 4226-4230

Conformational Analysis of Guaiazulene In a Supersonic Jet Michael M. Carrabba, Timothy M. Woudenberg, and Jonathan E. Kenny* Department of Chemistry, Tufts University, Medford, Massachusetts 021 55 (Received: April 24, 1985)

-

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 5, 2015 | http://pubs.acs.org Publication Date: September 1, 1985 | doi: 10.1021/j100266a016

The low energy region of the S2 So transition of guaiazulene (1,4-dimethyl-7-isopropylazulene) has been studied by using free-jet fluorescence excitation spectroscopy. Two spectroscopic origins have been observed, at 27 483 and 27 589 cm-', corresponding to two stable conformations of the isopropyl internal rotor. Two different temperature variation techniques, variation of nozzle temperature at a fvred distance downstream,and variation of distance downstream at fixed nozzle temperature, have consistently yielded a ground-state energy difference between the two conformers of 0.9 f 0.2 cm-l. In the excited state, an energy difference of 105 cm-I, in the opposite sense, has been inferred from the ground-state value and the spectral shifts. In addition, differences in transition dipoles and cross sections for collisional relaxation of the two conformers have been estimated. Finally, the ground-state barrier to internal rotation of the isopropyl group has been inferred to be very small, in agreement with recent gas-phase work on related molecules and in disagreement with solution-phase work and calculations.

Introduction Barrier heights for internal rotation about the C-C single bond joining a substituent to an aromatic ring vary widely. Complicated rotational-torsional structure in the electronic spectra of methyltetrazine and toluene have recently been analyzed'**in terms of the free-rotor model; the barrier in the latter molecule is k n o w d 4 to be about 5 cm-I. At the other extreme, phenylcyclohexanesS can have barriers as high as 10000 cm-' if the six-membered rings are substituted at the 2,6,2', and 6' positions. One problem in comparing and understanding barrier heights is that, in many cases, only calculated values exist; furthermore, when experimental data are available, they often disagree with calculations. A recent article6 by True and co-workers reviewed the situation for ethyl- and isopropylbenzenes and presented low resolution microwave data consistent with a 20-18O-cm-' twofold barrier for internal rotation of the isopropyl group in gas-phase pisopropylbenzaldehyde. This is in contrast to the 700 f 70 cm-l twofold barrier in 3,5-dibromoisopropylbenzene,determined by solution-phase NMR,' or the 1370-cm-' barrier calculated for the analogous equatorial phenylcyclohexane.8 A recent improved calculation* on the last-named compound yields a barrier height of 853 cm-I. We have employed the unique properties of free-jet expansions to study the potential surface for internal rotation of the isopropyl substituent with respect to the aromatic ring system of guaiazulene (1,4-dimethy1-7-isopropylazulene; Figure l ) , using electronic spectroscopy as the probe. The dramatic cooling of internal degrees of freedom in these expansions provides a convenient way to prepare molecules in their lowest available states of internal motion; whether these are local or absolute minima depends on the details of the potential surface. Recently, several groups have studied conformational isomers using this technique, including trans and gauche isomers of alkylbenzenesg and alkylanilines,1° different internally hydrogen-bonded forms of methyl salicylate," and cis and trans isomers of meta-substituted phenols and P-naphthol.I2 In addition, similar techniques were used to elucidate Seliskar, C. J.; Leugers, M . A. Chem. Phys. Lett. 1984, 1 1 1 , 141. (2) Leugers, M. A.; Seliskar, C. J. J . Mol. Spectrosc. 1982, 92, 150. (3) Rudolph, H. D.; Dreizler, H.; Jaeschke, A,; Wendling, P. 2. Naturforsch. A 1967, 22A, 940. (4) Kreiner, W. A,; Rudolph, H. D.; Tan, B. T. J. Mol. Spectrosc. 1973, 48, 86. ( 5 ) Jaime, C.; Osawa, E. J . Mol. Srrucr. 1985, 126, 363. (6) True, N. S.; Farag, M. S.; Bohn, R.K.; MacGregor, M. A.; Radhakrishnan, J. J. Phys. Chem. 1983, 87, 4622. (7) Schaefer, T.; Parr, W. J. E.; Danchura, W. J. Magn. Reson. 1977, 25, (1)

the details of ground-state torsional motion in dipheny1a~etylene.l~ All the non-hydrogen-bonded isomers observed in these studies showed spectral shifts of 50-300 cm-I at the 0; band of the SI So transition. In addition, roughly equal intensities for both isomers were observed; however, ground-state energy differences were not measured, so the ground- and excited-state contributions to the spectral shifts could not be separated. Our study of the S2 So fluorescence excitation spectrum of guaiazulene has yielded similar results; in addition, a detailed investigation of relaxation dynamics in the jet has allowed precise determination of the energy differences of the two stable conformers in both electronic states and provided strong evidence regarding the ground-state barrier height which corroborates the True result.6 In fact, our results suggest that the lower limit given in ref 6 may be the most accurate value, indicating an even larger discrepancy among the gas-phase, solution, and theoretical results. We have also estimated the relative transition dipole strengths of the conformers, and the cross section for conformer relaxation in collisions with the helium carrier gas.

-

+-

Experimental Section Freshly sublimed guaiazulene (Aldrich) was transferred to a temperature-controlled reservoir in a stainless steel gas line. Helium carrier gas at 1.O atm flowed over the guaiazulene sample, and the mixture was expanded into vacuum (50.010 torr) through a 2 5 0 - ~ mpinhole glued to the end of a modified Bosch fuel injector, whose temperature was separately regulated at a value slightly higher than that of the sample. The free jet was probed at variable distances downstream of the nozzle by an NRG nitrogen-pumped dye laser. Either the second harmonic of the dyes LDS-698 and Rh-640 or the fundamental of the dye PBD was used; in either case, the excitation bandwidth was about 2 cm-I fwhm. Fluorescence was collected at 90' to the laser and jet axes by a 5-cm-focal-length spherical mirror used at f11.2 and imaged through a Corion LG-450 cutoff filter and an optical mask onto an Amperex 56 TVP photomultiplier tube. The PMT output was processed by either a homemade boxcar based on an Evans 41 30 gated integrator board or an EG&G Model 162 boxcar with 166 plug-in unit. The boxcar was operated in summation mode, and the integrated signal from 100 laser shots was recorded at 1-cm-l intervals and stored on an HP-87 microcomputer. The apparatus is described in more detail e1~ewhere.l~The spectrum was scanned from 27 000 to 29 500 cm-I at various laser energies, and showed significant saturation effects;15 however, only the low-frequency region shown in Figure 2 is of interest here. The full spectrum

167. ( 8 ) Allinger, N. L.; Tribble, M. T. Tetrahedron Lett. 1971, 35, 3259. (9) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J . Chem. Phys. 1980, 72,

5039. (10) Powers, D. E.; Hopkins, J. B.; Smalley, R. E. J . Chem. Phys. 1980, 72, 5721. (11) Heimbrook, L. A,; Kenny, J. E.; Kohler, B. E.; Scott, G. W. J . Phys. Chem. 1983, 87, 280.

0022-3654/85/2089-4226!$01 SO10

(12) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1984, 88, 5 180. (13) Okuyama, K.; Hasegawa, T.; Ito, M.; Mikami, N. J . Phys. Chem. 1984. 88. 1711. (14) Carrabba, M. M.; Kenny, J. E.; Moomaw, W. R.; Cordes, J.; Denton, M. J . Phys. Chem. 1985,89, 674. ( 1 5 ) Carrabba, M. M.; Woudenberg, T. M.; Kenny, J. E., in preparation.

0 1985 American Chemical Society

Conformational Analysis of Guaiazulene

The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4221

O”O1

I

E

0

i -

0

.

1

0

1

,

0.7 0.19 021 023 0.25 0.27 1 / T trans

I I

27483

022

I


0

90 180 270 DEGREES

Figure 1. Stable conformers (rotational isomers) of guaiazulene: “front” and “edge-on” views, and schematic representation of the ground (So) and excited-state (S,) potential surfaces for internal rotation. 0 is the angle of rotation about the bond joining the isopropyl group to the sevenmembered ring.

I

025

I

I

I

0.28 0.31 l/Tno&

0.34

Figure 3. Plot of In A / B , the conformer intensity ratio, vs. l/temperature from an experiment in which nozzle temperature was varied. The solid line represents the best weighted least-squared fit to a Boltzmann equation (eq 2 in the text); error bars equal to one standard deviation u are shown. The two l / T scale correspond to the two limits of no cooling (TNouls)and complete cooling (T,,,,,), and provide bounds on the ground-state energy difference. The lables on the 1/ Tnorrlc scale have been multiplied by 100.

07

I

0.4

0.0 0.1

02

0.3 0.4

,

,

0.5

l/Ttmls I

RELATIVE ENERGYCVACUUM WAVENUMBERS) Figure 2. Fluorescence excitation spectra of jet-cooled guaiazulene, showing the low vibrational energy region of the S2 So electronic transition. The peaks at 0 and 106 cm-’ are assigned as spectroscopic origins for the two conformers depicted in Figure 1, the lower energy peak appearing at 27 483 cm-I. The lower trace was taken with a laser pulse energy of 30 pJ, and shows the effect of saturation; the upper trace was taken with a pulse energy of 2 pJ.

-

will be discussed in a future publication.I6 The relative intensities of the two intense peaks at 27 483 and 27 589 cm-’ were monitored in two separate temperature variation experiments. These were done by performing short scans (1-cm-’ steps, 100 laser shots per step) which sufficed to determine baseline, peak height, and peak width accurately. During these experiments, the reservoir and nozzle temperatures were controlled to within fO.l to f0.5 OC with a modified RFL Model 70A temperature controller with thermistor probes. The first temperature variation experiment was performed at a fixed distance between the nozzle and laser, x = 2.5 mm, or x / D = 10 nozzle diameters. The nozzle temperature was varied from 301 to 414 K. The average of five separate intensity ratio measurements at each temperature constituted a data point, with an associated standard deviation, u. Peak heights (average of 100 laser shots) were used to calculate these intensity ratios; ratios computed from peak areas showed no significant difference. These data points were used with l / a z weighting in subsequent fits; in Figure 3, points taken within a 4-deg range were averaged before plotting to avoid clutter. The second temperature variation experiment was a true “relaxation” experiment; the nozzle temperature was held constant (16) Carrabba, M. M.; Kenny, J. E., in preparation.

0.0

I

5.6

I

I

I

1

9.4 12.8 16.0 18.9

X/D Figure 4. Plot of In A / B vs. l/Ttrancand x / D in the relaxation experiments, in which the nozzle temperature was held constant and the distance from the nozzle was varied. The two least-squares fits are discussed in the text. Error bars (LJ) are shown.

at 358.3 K, and the jet was probed at distances downstream varying from x / D = 3.15 to x / D = 17.5. At each x / D value, an intensity ratio was calculated from the areas obtained in 100 shots/point scans through the two peaks; three such ratios were averaged to give the values shown in Figure 4.

Results and Discussion General Features of the Spectrum. The fluorescence excitation spectrum of guaiazulene contains about 50 vibronic bands in the first 400 em-’ above the lowest energy intense peak at 27 483 em-’ (corrected to vacuum). By contrast, the free-jet fluorescence excitation spectrum of azulene” shows only four vibronic bands in the same interval above the S2origin. The intensities of these low-frequency features were comparable to that of the lowest energy band when scanned at high laser pulse energies (Figure 2, lower trace). However, the line widths of the two intense low-energy bands were significantly greater than those of the higher energy bands. As laser pulse energy was decreased, these two bands became relatively taller and sharper until at the lowest pulse energies their bandwidths approached that of the laser. A t pulse energies low enough to avoid saturation15 of these two peaks, most of the other features in Figure 2 were unobservable. The experiments of Smalley and co-workers on alkylbenzenes6 established the use of free-jet expansions as useful media for (17) Fujii, M.; Ebata, T.; Mikami, N.; Ito, M. Chem. P h p . 1983, 77, 191.


4228

The Journal of Physical Chemistry, Vol. 89, No. 20, 1985

conformational analysis. For n-alkylbenzenes with alkyl chains of three or more carbons, spectra of trans and gauche isomers were observed with roughly equal intensity. However, only one isomer of isopropylbenzene could be observed, presumably the one in which the terminal carbons of the alkyl group lie symmetrically above and below the plane of the benzene ring. The corresponding geometry was calculated to be the most stable one in equatorial phenylcy~lohexane,~*~ again giving rise to only one isomer. Breaking the symmetry of the aromatic ring with appropriate substituents gives rise to two stable conformers. True and coworkers6 accomplished this in p-isopropylbenzaldehyde: the carbonyl oxygen is fixed in the plane of the benzene ring by conjugation effects, and the rotation about the C-C bond between the formyl and phenyl groups may be ignored. Their low resolution microwave spectrum was analyzed by assuming that the observed triplet structure was due to transitions of two conformers with different rotational constants, as well as free-rotor transitions of those molecules in states lying above the potential barrier. In guaiazulene, the aromatic framework is intrinsically asymmetric about the isopropyl-bearing ring carbon. The two corresponding isomers are shown in Figure 1. A calculation using a modified version of Allinger's MM2 program18 predicts that these two minima in isopropylazulene are nearly equal in energy ( A E = 9 cm-I), with a barrier of about 1390 cm-l located at 90°, and that no other minima exist on this potential surface. Thus, we can assign the two strongest features in Figure 2, at 27 483 and 27 589 cm-I, to two conformations which we refer to as rotamer B and rotamer A, respectively. Most of the other major features in the spectrumI6 occur in pairs that are separated by about 100 cm-I. In many ways, the spectrum of guaiazulene is similar to the SI So spectra of meta-substituted phenols reported in ref 12. In those molecules, the meta substituent provides the asymmetry to allow cis and trans isomerism about the phenyl-hydroxyl single bond, and two lone pairs on the oxygen play the role of the terminal methyls of the isopropyl group. In those spectra also, the splitting of the 0; bands appears often, but not always, in the other peaks of the spectrum, making a complete vibrational analysis difficult. Determination of the Energy Difference in So and S 2 . The observation that the A/B intensity ratio changes as the nozzle temperature is varied indicates that the two isomers are not isoenergetic in the ground electronic state. The observation that the ratio may be either greater or less than unity indicates that the transition dipole moments pAand kBfor these two 0; transitions are unequal. (This is a slight oversimplification to which we shall return.) Furthermore, the observed trend in intensity ratios indicates that rotamer A is more stable in the ground state and has the smaller transition dipole. The fact that its 0; transition lies to the blue of the 0; transition of rotamer B indicates that the relative stability reverses in the excited electronic state (see Figure 1). The relationship may be stated:

-

106 cm-l = t(OX,A) - Z(O8.B) = (EB - E,),,

+ ( E A- EB)es (1)

The temperature variation experiments can, in principle, provide numerical information on the ground-state energy differences. Whether they do or not depends on the kinetics of relaxation of the internal rotor degree of freedom, most importantly on the ground-state barrier height. It is instructive to consider two limiting cases. First, suppose the barrier is extremely high. The yardstick to be used, of course, is the relative energy of a guaiazulene-helium collision, which is equal to 3 / 2 k T ,on average. Even at the highest nozzle temperatures of 414 K, this corresponds to 430 cm-I, much lower than the barrier height calculated in ref 5 and 8. Thus, an extreme limit to cooling of the conformer equilibrium is that molecules initially in well A or well B will relax toward the bottom of the well they are in at the start of the expansion, but not cross over into the other well."J2 The energy difference in this case could be measured by using the 0; intensity ratios in a Boltzmann (18) Allinger,N. L. J . Am. Chem. SOC.1977, 99, 8127

350 Y

-.

Carrabba et al.

$a-

!"I 10

5-

0

10 X/D

20

Figure 5. Plot of temperature vs. distance from nozzle, showing different relaxation behavior for different degrees of freedom. The lowest curve shows the translation temperature, which undergoes essentially complete cooling, while the uppermost curve shows the limit of zero cooling that would obtain for an internal rotation with an extremely high barrier. The intermediate curve is a smooth fit to the observed conformer temperatures (data points) in the relaxation experiment; similar curves for rotation and vibration would be expected to be below and above the conformer curve, respectively.

expression employing the nozzle temperature, Le., T, = Tnozzler where T, is the effective temperature of the internal rotor degree of freedom. The other limiting case corresponds to a barrier height that approaches zero. In this case, we could measure the energy difference by using 0; intensity ratios and local carrier gas temperatures in the Boltzmann equation, at least as long as rarefaction has not proceeded too far toward free molecular flow. This local temperature will be referred to as T,,,, since it characterizes the width of the velocity di~tributionl~ and the kinetic energy of collisions. This limit corresponds, then, to assuming T, = Ttrans. Thus, the nozzle temperature variation data of Figure 3 are to provide bounds plotted as In ( A / B ) vs. 1/ Tnovlcor vs. 1/ Ttrans on the ground-state energy difference, where the notation A intensity of 0; band of rotamer A at 27 589 cm-' and B intensity of 0; band of B at 27 483 cm-I, is used:

Since the ratio of Ttransto Tnozzlcis fixed at fixed x / D , one fit suffices to give the two limiting values of AE, which are related by this temperature ratio. A weighted least-squares fit of eq 2 yields a lower limit of ( E B- EA)&= 0.85 f 0.16 cm-I, while the upper limit thus determined is 66 f 12 cm-l. In either case, p A 2 / p B 2 = 0.76 f 0.04. The upper limit energy difference calculated above can be immediately dismissed upon inspection of the results of the x / D variation experiment. Figure 4 clearly shows that the conformers are able to interconvert even in the extremely low-energy collisions which occur in the later stages of the expansion. For example, 3 / 2 k T 5 cm-I at x / D = 10, indicating that the barrier separating the conformers must be very low, indeed. A more detailed analysis, presented below, confirms that the lower-bound energy difference calculated above, under the assumption of a small barrier, is in agreement with all the experimental data. There is an extensive literature on relaxation of various degrees of freedom of molecules in purez0 or mixed-gas2' expansions. A typical22relaxation plot for internal degrees of freedom in free-jet expansions is shown in Figure 5. The limit of complete cooling corresponds to maintenance of equilibrium between the internal degree of freedom and the translational bath. Departure from

-

(19) Smalley, R. E.; Wharton, L.; Levy, D. H. Acc. Chem. Res. 1977, 10, 139. (20) Miller, D. R.; Andres, R. P. J . Chem. Phys. 1967, 46, 3418. (21) Tusa, J.; Sulkes, M.; Rice, S. A. Proc. Natl. Acad. Sci. U.S.A.1980,

77, 2367. (22) Gallagher, R. J.; Fenn, J. B. J . Chem. Phys. 1974, 60, 3487


Conformational Analysis of Guaiazulene


equilibrium with the bath occurs when the necessary collision frequency (in general, different for each degree of freedom) can no longer be maintained, due to rarefaction. Thus, the temperature of each degree of freedom gradually diverges from the translational temperature, and is often said to "freeze out" as it approaches a horizontal asymptote. The top curve in the figure, T = TnoEzle = constant, corresponds to the conformer temperature in the limit of a very high barrier, discussed above. In fact, the arguments advanced in the previous paragraph suggest that the conformer temperature diverges very little from the translational temperature. If such is the case, then a plot of In (AIL?)vs. l/T,,,,, should be linear for measurements performed at small x/D, and should provide an accurate value of the ground-state energy difference. Figure 4 bears out this expectation well; a straight line can be drawn through the first three data points, with deviations in the expected direction beyond. The slope of this line yields AE = 0.95 f 0.09 cm-I and ~ ~ = 0.89 ~ f 0.01. / This p energy ~ difference ~ is within experimental error of the lower bound obtained in the nozzle temperature experiment; the agreement in dipole strength ratios is reasonable (within about 15%), although outside the estimated error bars of one standard deviation. We note for completeness that a least-squares fit of all seven points in Figure 4 yields a poorer fit, with AE = 0.46 f 0.05 cm-I and / , L A ~ / / . L B=~ 0.94 f 0.01. However, given the well-known character of relaxation curves discussed above, and the size of the experimental error, we feel that the seven-point fit actually represents a more biased interpretation of the observations than the three-point fit, and thus prefer the latter. In any case, the value of A E can be confidently reported to lie between 0.5 and 1.1 wavenumbers; we suggest 0.9 f 0.2 cm-' as the best value. By eq 1, then, EA - E , in the excited state is 105 cm-I. Relaxation Kinetics and Estimated Barrier Height. The limited data allow only an approximate analysis of the relaxation kinetics. If we define the conformer temperature as discussed above (3) where AE = (EB- E A ) g s = 0.95 cm-' and pA2/KB2 = 0.89, then we can calculate T, for the seven points of Figure 4 and produce the relaxation curve shown in Figure 5. We can use the divergence between the translational and conformational temperatures to estimate a cross section, au2, for relaxation of guaiazulene conformers in collisions with the helium carrier gas. The approach is that of ref 20 and 2 1, where the rate of conformational cooling is proportional to the difference T, - Ttrans (4)

where n is the carrier gas number density and ( u , ~ is~ the ) average relative velocity between guaiazulene and helium.23 The equations for isentropic expansion can be used to rewrite24eq 4 in terms of the nozzle diameter D and temperature To,the mass m,specific heat ratio y, and initial density no of the helium carrier gas, the reduced mass p for relative motion of guaiazulene and helium, and the Mach number, M d(Tc/To)

-

d(x/D) - ( 8 m / y a ~ ) ' ~ ~ a a ~ n ~ ( T+ , /(yT ~ )1)@/2] [l - 1) M[l

+ (y - 1)@/2]7/(7-l)

(5)

where the Mach number M i s givenI9by 3 . 2 6 ( ~ / 0 ) ~ Following /~. the lead of ref 21, we fit our seven T,'s to a simple function of (x/D). This was explicitly differentiated to provide an analytical expression for the left-hand side of eq 5. The equation was then rearranged to give an expression for m2,and evaluated at the four points ( x / D = 10, 12.5, 15, and 17.5) not used in the AE,pA2/pB2 (23) Frost, A. A,; Pearson, R. G . "Kinetics and Mechanism", 2nd ed.; Wiley: New York, 1961; p 60. (24) A minor error in eq 3 of ref 21 has been corrected here; in addition, we note that the definition of the quantity Cobelow their eq 2c also contains a minor error: C,, is, in fact, the speed of sound in the carrier gas before expansion, and is given by ( y k T o / m ) ' / * .

4229

determination to yield four values of the cross section, whose mean and standard deviation are 39.3 and 7.5 ,A2, respectively. The actual values showed no trend with decreasing T,/To, so the prospect of obtaining an activation energy corresponding to the internal rotational barrier from the temperature dependence of the cross section23was abandoned. Instead, we note that the mean collision energy at x / D = 12.5, where slight cooling was still observed, was 3.6 cm-].

Discussion Our free-jet temperature/relaxation study of guaiazulene has allowed us to infer a great deal of information on the potential surface for internal rotation of the isopropyl group with respect to the planar aromatic ring system. This information should be relevant to the more general problem of internal rotation of dialkylmethyl substitution on an aromatic ring, at least if the alkyl substitutents are not too bulky. Support for this statement is provided by two sources. The first is the result of an MM2X calculationzs on the molecule 5-isopropylazulene, which predicts that conformers A and B represent the only minima on the potential surface, with nearly equal energies (AE = 9 cm-l), and a barrier at 90° equal to 1390 cm-I, essentially identical with that calculated8 for equatorial phenylcyclohexane. The second is the semiquantitative agreement of our gas-phase experiments with the gas-phase results of True and co-workers6 on isopropylbenzaldehyde. Their AE (estimated by us from their Figure 4) is 1.1 cm-I, which is in excellent agreement with our value of 0.9 cm-I. Their data were consistent with a barrier height anywhere between 20 and 180 cm-I, and they suggest a "best" value of 90 f 50 cm-I. The continued slow cooling observed at single-digit translational temperatures in our relaxation experiment suggests that their lower limit of 20 cm-I may indeed be reasonable (this would correspond to a slightly lower AE also). The ground-state potential information may be summarized as follows: the only available gasphase data indicate very small barriers and energy differences. These results are at odds with solution-phase work and calculations, but provide supporting evidence for the recent suggestion26that solvent internal pressure effects are responsible for the difference. While we have no information on the excited-state barrier, the energy difference between the two conformers in the Sz state is seen to be 105 cm-I, which is small on an absolute scale, but large relative to the ground-state difference. Since the stability of the two isomers reverses upon electronic excitation, we have attempted to correlate this observation with other information on the So and S2states. The only detailed studies17.z7-30 on electronic states of azulenes have been carried out on the parent, and, to a limited extent, on perdeuterated azulene. Structural data obtained from M O calculation^^^ or inferred from rotational constants2gindicate little geometry change in the S2 So transition; this is borne out by the lack of long progression^.'^^^^ Thus, steric factors should not be very different in the two states. One obvious characteristic of nonalternant aromatic hydrocarbons is the charge alternation on adjacent carbons, so we considered this as a possible cause for the observed energy differences. In fact, Pariser's calculations on ground-state azulene (Hiickel MO's, no CI) yield *-electron densities at C6 and C8 (C, and C6 in the unsubstituted parent) of 0.85 and 0.87 in the ground state, and 1.01 and 0.97 in the Sz state. The calculation was repeated with CI in the ground state only, with little change in the values. Thus, we see a very small difference in the ground state, and a larger difference, in the opposite direction, in the excited state, in the a-electron densities at the two ring carbons with which the terminal methyls of the +-

(25) This calculation was performed for us by Jay Ponder of Harvard University, using a slightly modified version of Allinger's MM2 program, ref 18. (26) Ross, B. D.; True, N. S.; Decker, D. L. J. Phys. Chem. 1983,87,89. (27) Pariser, R. J . Chem. Phys. 1956, 25, 1 1 12. (28) Sidman, J. W.; McClure, D. S. J . Chem. Phys. 1956, 24, 757. (29) McHugh, A. J.; Ross, I. G.Aust. J . Chem. 1968, 21, 3055. (30) Lawrance, W.D.Ph.D. Thesis, Griffith University, Queensland, Australia, 1983. The recent literature on azulene is extensive and a fairly complete bibliography may be found in this reference.


4230

Carrabba et al.


isopropyl group interact most strongly. Since the difference in energies is so subtle, even in the excited state, we are unable to definitively assign one of the geometries sketched in Figure 1 to rotamer A as opposed to B. The labels used throughout the paper are unambiguous only in specifying the spectral lines. However, we note that the cis and trans isomers of meta-substituted phenols have S& excitation energies that differ by 100-300 cm-’; since ground-state differences were not determined, it is not possible to partition these differences into individual AE’s in each state. However, the isomerization itself is due to the meta substitution which induces inequivalence in *-electron densities at the two ring positions ortho to the internal rotor. Thus, the observed splittings in these compounds are consistent with the explanation offered above. Furthermore, the trans and gauche isomers of the n-alkylbenzenes, while not geometrically analogous to our rotamers A and B, have different energies due to differences in the amount of interaction between the alkyl group and the *-electrons of the ring. The isomer with the greater alkyl-* interaction (gauche) exhibits a greater “solvent” red shift, or greater stabilization in the excited state of the A T* transition. Reasoning analogously, we might suppose that rotamer B, lying to the red of rotamer A in the a P* transition studied, has a greater interaction with the *-electrons, Le., is closer to the ortho carbon that has the greater *-electron density in the excited state, carbon 6 . Thus, a rather tenuous assignment of A and B to the geometries depicted in Figure 1 may be made. We now briefly discuss the relaxation dynamics of the conformers. The cross section obtained for this process in heliumguaiazulene collisions was about 40 AZ.This is similar to the value found for rotational relaxation21 of I, in collisions with helium (20-70 A2, depending on collision ener y) and rotationaltranslational relaxationm of pure N, (10-50 2). Thus, relaxation of conformers mimics relaxation of rotations, indicating again the likelihood of a very small barrier to interconversion. Finally, we discuss the observation of a small difference in the 0; transition dipole moments of the two rotamers. This result was obtained under the implicit assumption of equal fluorescence quantum yields, a, which, while it may seem reasonable, is no more or less so than equal transition dipoles. Thus, we should say pA2aA/fiB2aB = 0.89 f 0.04, which is more correct, but still subject to the assumption of equal detection efficiencies for the two emission spectra. Again, such an assumption seems reason-

-

-

w

able, but the vast literature on the excited-state dynamics of azulenes often belies such reasonable assumption^.^' The subtleties involved in making any more accurate statement regarding the two 0; bands of the rotamers are currently under investigation.l5*I6 We note briefly that, because of extensive vibronic coupling to S4 (and probably S3 as well), small changes in intensities in the two conformers due to differences in S2-S4(and S+3) gaps could easily account for the observed ratio. However, the fluorescence quantum yield depends (through the internal conversion rate) on the S2-Sl gap (and probably S2-Soas well), and this dependence could also account for our observations. Currently, only the So-S2 gaps are known (from this work) in the gas phase. We conclude by emphasizing the dramatic impact we expect rotational conformers to have on studies of vibronic coupling and excited-state dynamics. These isomers reflect the subtlest kind of difference possibleg0that can give rise to appreciable changes in energy gaps between electronic states, and should play an important role in unraveling the mysteries of vibronic coupling, intramolecular vibrational relaxation, internal conversion, and intersystem crossing in azulenes and other aromatic molecules. Acknowledgment. This work was supported by a grant from Research Corporation. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We thank Prof. R. R. Dewald of Tufts University for assistance in sample purification, Prof. Robert Stolow of Tufts University for helpful discussions on conformational analysis, Jay Ponder of Harvard University for performing the MM2X calculations, and Dr. Lewis Rubin of EG&G/PAR for use of equipment. M.M.C. and T. M.W. thank the Mallinckrodt Foundation for support during part of this work. Registry No. Guaiazulene, 489-84-9. (31) Of course,molecules that differ only in rotational state represent an even more subtle difference, which can, indeed, produce dramatic effects in excited-state dynamics; see, for example: Lorincz, A,; Smith, D. D.; Novak, F.; Kosloff, R.;Tannor, D. J.; Rice, S . A. J . Chem. Phys. 1985, 82, 1067. Maburnoto, Y.; Spangler, L. H.; Pratt, D. W. Chem. Phys. Left. 1983, 98, 333. However, the change in electronic energy gaps in this case is too small to be of practical value in determining the sensitivity of couplings to this parameter.

Conformational analysis of guaiazulene in a supersonic jet - The

Recommend Documents