J . Phys. Chem. 1993,97, 4595-4600
4595
Electronic Spectroscopy of Jet-Cooled 1-Phenylcyclohexene: Conformation of the Ground and Excited Electronic States J. P. Finley and J. R. Cable' Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403 Received: December 1 1 , 1992; In Final Form: February 15, 1993
The mass-resolved SI+SO resonant two-photon ionization spectrum of 1-phenylcyclohexenecooledin a supersonic expansion is presented. The origin transition is assigned at 35 066 cm-I, upon which an extended low-frequency progression in a 46-cm-1 mode is observed. This low-frequency mode is assigned to the torsional motion between the phenyl and cyclohexene rings. A hindered rotor analysis is used to calculate the excited-state torsional potential energy surface in terms of V2, V4,and V, cosine terms. The calculated surface displays only two minima in the excited electronic state corresponding to a geometry in which the olefinic bond of the cyclohexene ring is planar with the phenyl ring. A Franck-Condon analysis of the intensities within the torsional progression finds themagnitudeof thechange in the torsional geometry followingexcitation to SIto be 26'. Twoconformations are then expected in the ground electronic state having torsional angles of similar magnitude but opposite sign. Evidence is presented for assigning the origin transition of a second ground-state conformer, establishing the energy difference between the two conformations to be 127 cm-I.
Introduction The structure of 1-phenylcyclohexene, and all substituted styrene derivatives, is governed by an interplay between electronic and steric interactions. Styrene itself adopts a conformation in which both the phenyl ring and the olefinic bond lie in the same plane in both the ground and first electronically excited singlet states.'J Thus, the stabilization energy gained from extending the a electron conjugation outweighs the increase in the steric repulsion between the olefinicand phenyl hydrogens. The addition of a bulky substituent group can greatly increase the steric interactions and force the olefinic bond out of the plane of the benzene ring but only at the expense of the electronic stabilization energy. These electronic interactions are quite sensitive to the nature of the electronic state, and thus molecular conformation can change greatly followingelectronic excitation. In the present study, the conformation and potential energy surface for the torsional motion of 1-phenylcyclohexene in both its ground and first electronically excited singlet states are examined using electronic spectroscopy in a supersonic jet expansion. This torsional coordinate is illustrated graphically below where it is
QtQ represented by the dihedral angle T. For reference, the T = Oo conformation is taken to represent a geometry in which the cyclohexene olefinic bond lies in the same plane as the phenyl ring. In 1-phenylcyclohexenethe dominant steric interactions occur between the ortho phenyl hydrogens and the olefinic and allylic hydrogens on the cyclohexene ring. Similar steric interactions have been investigated by Grassian et al. in a-methylstyrene, also using supersonicjet ~pectroscopy.~There the ground-state conformationwas found to have the olefinic bond displaced 30' out of the plane of the phenyl ring. While having similar steric constraintsto an a-methyl group, a cyclohexenegroup also permits the existence of several distinguishable molecular conformations. Because of the nonplanarity of the cyclohexene ring, a conformation having a torsional angle of T is not equivalent to one having a torsional angle of -T. Thus, if steric repulsions produce a stable conformation at some nonzero value of the torsional
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angle T,a similar but not identical conformation should also exist at a torsional angle near -7. An additional interest in the cyclohexene derivative is that a trimethylcyclohexene group in conjugation with a A electron system (a polyene chain) is found in the retinal chromophore of the rhodopsin proteins. 1-Phenylcyclohexenethus represents one of the simplest models of a conjugated cyclohexene ring and may lend insight into the natureof the torsionalpotential energy surface in the larger retinals. These retinals universally display broad and structureless room temperature absorption spectra. A substantial part of this broadening has been shown to arise from the torsional motion of the trimethylcyclohexene group.4 Not only are the torsional modes vibronically active in the electronic spectrum but a large number of ground-state levels are thermally populated at room temperature due to the very low frequency of this motion. This same type of behavior is found in l-phenylcyclohexene as its room temperature, gas-phase absorption spectrum in Figure 1 shows. As will be demonstrated, the origin transition to the first excited singlet state of 1-phenylcyclohexeneoccurs at 2852.3 A. In this region the room temperature spectrum in Figure 1 shows only a very weak shoulder on the much stronger second absorption band peaking at approximately 2350 A. In analogy with thevery well studied electronic structure of ~ t y r e n e this , ~ second absorption band is composed of transitions to both the second and third excited singlet states. However, theabsorption to the first excited state in 1-phenylcyclohexeneis much more difficult to distinguish than the same transition in styrene and additionally contains none of the resolved vibronic structure tliat is apparent in the spectrum of the sterically unhindered compound. Supersonic jet spectroscopy provides a very powerful tool for uncovering this hidden vibronic structure since molecules can be prepared at very low vibrational temperatures where only the lowest torsional levels are populated> The simultaneousrotational cooling results in very narrow rotational contours that permit closely spaced vibronic features to be resolved. From an analysis of the energy levels for the torsional motion in the excited electronic state, an accurate picture for the torsional potential energy surface. can be derived. Additionally, the intensities of the torsional transitions can be subjected to a Franck-Condon analysis to extract the change in equilibrium torsional geometry between
0022-3654/93/2091-459~~04.00/0 0 1993 American Chemical Society
4596 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 0.14
Finley and Cable
1
0.12 0.10 0.08
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Wavelength (nm)
Figure 1. Room temperature vapor-phase absorption spectrum-of 1-phenylcyclohexencrecorded using a 10cm path length on a diode array
spectrophotometer with 0.5-nm resolution. the ground and excited electronic states, thus locating the minima in the ground-state torsional surface.
Experimental Section The supersonic molecular beam apparatus in which these studies were conducted consists of two differentially pumped chambers. In the first, a free supersonic expansion takes place from a pulsed nozzle (General Valve) having a 1-mm orifice. This chamber is pumped by a 9-in. oil vapor booster pump (Edwards) which maintains a background pressure of approximately 1 mTorr. The vapor pressure of the 1-phenylcyclohexene (Aldrich) was increased by heating the sample reservoir to 90 OC. Optimal cooling of the seeded molecules was obtained using Ar as the carrier gas at a pressure of 2.5 atm. Substantially poorer cooling was observed using He even a t stagnation pressures of up to 6 atm. The free jet is skimmed by a 1-mm skimmer (Beam Dynamics) and directed into a second chamber, maintained at a base pressure of 2 X lo-’ Torr by a liquid nitrogen trapped 6-in. oil diffusion pump. This second chamber contains a linear time-of-flight mass spectrometer (R. M. Jordan) capable of unit mass resolution at a mass of 150. The two chambers are isolated from each other by a sliding gate valve to facilitate access to the nozzle for sample changes and adjustments. The seeded molecular beam is crossed in the ionization region of the linear time-of-flight mass spectrometer with an unfocused laser beam from a tunable, frequency-doubled Nd:YAG pumped dye laser (Continuum). Here the seeded neutral molecules undergo resonant absorption of a first photon followed by nonresonant absorption of a second photon to produce the molecular ion. The ions are accelerated in two uniform electric fields under space-focusing conditions’ into the mass spectrometer, where they drift with a mass-dependent velocity in a field free flight tube before striking a microchannel plate detector. Since the ionization process proceeds through an initial resonant absorption step, an excitation spectrum can be measured by monitoring the intensity of the signal corresponding to the parent ion as a function of the excitation laser wavelength. This is done using a boxcar integrator (Stanford Research Systems) interfaced to a personal computer.
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FREQUENCY (EM-’ 1 Figure 2. Resonant two-photon ionization spectrum of 1-phenylcyclohexene in a supersonic expansionusing 2.5 atm of Ar as the carrier gas. The spectrum is recorded by monitoring the signal from the parent ion (mass 158) and is then corrected by dividing the ion signal by the relative laser power. The SI SOorigin band is assigned to the weak feature at 35 066 cm-I (see Figure 3). The long progression which follows has a spacingof approximately46 cm-I and is assignedto the torsional motion between the two rings.
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Results A. Resonant Two-Photon Ionization Spectrum. The lowest 700 cm-I of the resonant two-photon ionization spectrum of 1-phenylcyclohexene seeded in a supersonic expansion is displayed in Figure 2. Argon was employed as the carrier gas at a stagnation pressure of 2.5 atm. To correct for any wavelength dependence of the laser power, the spectrum in Figure 2 was obtained by dividing the raw ion signal corresponding to the parent ion at mass 158 by the relative power of the excitation laser. Over the length of the scan this correction varies by at most 20%. A linear correction was chosen since the ionization signals were observed to have a nearly linear dependence on the laser power. Imme-
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FREQUENCY ( C M-’ )
Figure 3. Low-energy region of the SI SOspectrum recorded with argon as the carrier gas at a stagnation pressure of 2.5 atm. The sharp peaks at 35 066 and 35 112 cm-I are assigned to the origin transition and the torsional fundamental, respectively. The broader features at lower energy arise from vibrationally excited ground-state molecules.
diately apparent in the spectrum is the long vibrational progression in a low-frequency normal mode of approximately 46 cm-I. This clearly signals that the molecule undergoes a large equilibrium geometry change along this normal coordinate following photoexcitation. The absence of this vibronic activity in spectra of sterically unhindered styrenes strongly supports the assignment of this vibrational mode to the torsional motion between the cyclohexene and benzene rings. Unlike the spectrum of jet-cooled styrene, which shows a dominant origin transition a t 34779 cm-l and only weak vibronic structure,*the electronic origin transition in 1-phenylcyclohexene is extremely weak and therefore difficult to identify. In Figure 3 an expanded view of the origin region of the spectrum is displayed. This spectrum was also obtained using 2.5 atm of Ar as a carrier gas. Sharp vibronic features are seen in Figure 3 a t 35 066 and 35 112 cm-’ and are assigned to the electronic origin and fundamental of the torsional mode, respectively. This assignment is supported by a Franck-Condon analysis of the entire vibrational progression, as will be discussed later. A series of broader peaks is also seen to continue to the red. The width
Electronic Spectroscopy of Jet-Cooled 1-Phenylcyclohexene
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4597
TABLE I: Observed and Calculated Torsional Structure of the SI SOTransition in 1-Phenylcyclohexene torsional torsional spacing energy” calcd torsional re1 calcd re1 u’ (cm-1) (cm-1) energyb (cm-1) intensity intensityc +
0 1 2 3 4 5 6 1 8 9 10
46.1 46.3 46.6 41.4 41.9 48.3 48.4 48.9 49.0 49.5
0.0 46.1 92.4 139.0 186.4 234.3 282.6 331.0 379.9 428.9 418.4
0.0 45.9 92.3 139.2 186.5 234.3 282.5 331.0 379.8 429.0 418.4
0.13d 1.00 2.82 5.52 8.91 12.84 14.25 14.03 11.56 9.16 6.00
0.11 0.15 2.48 5.53 9.30 12.58 14.26 13.94 11.99 9.21 6.41
a Measured relative to the origin transition at 35 066 cm-I. V2 = 3001 cm-I, V4 = - 4 3 4 cm-I, V, = 19 cm-I. ‘ S = 6.61, R = ve/ug = 1.04. The relative intensity of the origin transition was not included in the Franck-Condon analysis.
of these peaks suggests that they are rotationally warm and therefore are probably vibrationally excited as well. When He is used as a carrier gas, these peaks grow considerablyin intensity, indicating less efficient cooling of the vibrational degrees of freedom. These peaks are thus assigned as sequence bands which originatein some unspecified excitedvibrationallevel in the ground electronic state and possess the same torsional mode progression associated with the origin. On the basis of the 35 066-cm-I assignment of the electronic origin, the vibrational energies and relative intensities of the members of the torsional progression are listed in Table I. The relative intensities are derived from the averaged vibronic intensities recorded during several different wavelength scans and are corrected for variations in the laser power. The torsional overtones are seen to appear at fairly constant spacing, ranging from 46.1 cm-I for the fundamental to 49.5 cm-I for the eighth overtone. This small anharmonicity indicates the potential well for the torsional motion rises slightly steeper than the quadratic well of a harmonic oscillator. Besides the torsional progression built off the origin, several other progressions are also visible in the spectrum of l-phenylcyclohexene. Each of these progressionsdisplays similar torsional spacings to that based off the origin. Because of the large change in geometry along the torsional coordinate, the false origins for these progressionsarevery weakand difficulttoidentify. Spectral congestion arising from the overlap of other progressions makes assigning these false origins even more difficult than locating the true electronic origin transition. However, since the envelopes of the different torsional mode progressions appear to be quite similar, the false origin transitions can be located by comparison to the progression based off the origin. This procedure is demonstrated for a second torsional progression whose first visible peakin Figure 2 occurs at 35 239 cm-I, 173cm-I above the origin. The strongest peaks in the progression occur at 35 467 and 35 5 16 cm-I and have roughly 60% of the intensity of the strongest peaks in the origin based progression. Thus, since the first obvious peak at 35 239 cm-’ is about 60% as intense as the torsional fundamental at 35 112 cm-I, this peak can be assigned as terminating in a level with one quantum of excitation in the torsional mode. The false origin for this progression must therefore occur at 35 193 cm-I, 127 cm-I above the origin. This same procedure has been used to identify two additional false origins, 244 and 253 cm-’ above the origin. Again the first peaks of the progressionsvisiblein Figure 2, at 35 356 and 35 365 cm-I, correspond to one quantum of excitation in the torsional mode and so the falseorigin transitions must be located 46.1 cm-1 lower in energy. There are two possible ways in which these false origins can arise. In one case the false origin corresponds to a fundamental
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Figure 4. Resonant two-photon ionization spectrum of l-phenylcyclohexene from 35 000 to 36 200 cm-l recorded under identical conditions as the spectrum in Figure 1 and also corrected for variation of the laser
power. transition of another low-frequency, Franck-Condon active vibrational mode. Each member of the progression would then involve excitation of the torsional mode in combination with one quantum of excitationin another vibration. The Franck-Condon envelope built on the false origin should be the same as the originbased progression in which only the torsional vibration is excited. Alternatively, these false origins can arise from different conformations of the molecule. If, under the low-temperature conditions of a supersonic expansion, 1-phenylcyclohexene cools as a mixture of conformers, each with slightly different electronic energies, separate electronic origin transitions will be present for each conformer. The envelopesof the torsional progressionsfrom different conformers might have small differences depending on the nature of the potential surfaces in the ground and excited states. Displayed in Figure 4 is the higher energy region of the twophoton ionization spectrum from approximately 400 to 1100cm-’ above the origin. This spectrum was taken under conditions identical to those used in Figure 2 and has similarly been normalized for variations in the laser power. The spectrum in this figure is dominated by another torsional progression whose energy level spacings are the same as those in the lower energy region of the spectrum. The previously described procedure for assigningthe peaks in the progression is not as useful here because the true intensities are masked by the steadily rising background and severe spectral overlap. Nevertheless, the false origin for this progression is estimated to occur approximately 580 cm-I above the electronic origin. B. Excited-State Potential Surface. The torsional progression observed in the electronicspectrumof 1-phenylcyclohexenereveals a great deal about the torsional potential energy surface in the excited electronic state. Torsional motion is often treated as separable from the other normal modes of vibration using the following Hamiltonian for a hindered rotor?
+
H = -B d2/d? V(T) (1) Here B represents the reduced rotational constant about the torsional axis and V ( T )represents the one-dimensionaltorsional potential energy surface in the torsional angle, T . The observed torsional energy levels should thus correspond to the eigenvalues of this Hamiltonian. The torsional potential energy surface is typically expanded in a cosine series as ~ ( 7= ) ‘ / 2 C n ~ n (-1cos(n7)) (2) The twofold symmetry of the phenyl rotor requires that V(T)= V(T + T ) , thus restricting the series to only the even n terms. The
Finley and Cable
4598 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993
rotational constant, B, is related to the reduced moment of inertia, I,, through B = h / 8 ~ ~ c ZTodetermine ,. I,, the molecular geometry was optimized using the AM1 Hamiltonian of the MOPAC9 semiempirical molecular orbital package to determine the moments of inertia of the phenyl and cyclohexenyl rotors. Using those values and taking the single bond between the benzene and cyclohexene rings as the torsional axis, a value of 0.357 cm-l was derived for B. No attempt was made to account for any torsional angle dependence in the rotational constant. Although the spacings of the torsional energies are very nearly harmonic, a small negative anharmonicity is present, as evidenced by the gradual increase in the vibrational spacings listed in Table I. To accurately characterize the excited-state potential surface, terms up to V6 were included in the cosine expansion above. Using these parameters, a linear variation calculation of the torsional energy levels was performed, employing the free-rotor sine and cosine eigenfunctions as a basis set. To take advantage of the symmetry of the torsional wavefunctions, the energies corresponding to wavefunctions of odd and even parity were obtained using separate calculations, each with a set of 50 basis functions consisting of either sine or cosine terms. The matrix elements for this Hamiltonian are simple functions of the parameters V, and B."J A nonlinear least-squares fitting procedure was then used to optimize the potential parameters. The best fit to the experimental energies,giving a reduced chi-squared of 0.021 cm-2, was obtained with V2 = 3001, V4 = -434, and V6 = 19 cm-I. These parameters reproduce all of the observed torsional frequencies to within 0.2 cm-I as the calculated values in Table I show. Despite theclose fit, theuncertainty in the three parameters is large, as demonstrated by their standard deviations: u( V2) = 1214,u(V4) = 565,and u(V6) = 122cm-I. Thesensitivityofthe fit to the V6 term was explored by fixing V6 equal to zero while allowing V2 and V4 to vary. Under these conditions a leastsquares fit yielded a reduced chi-squared of 0.024 cm-2, very nearly as good as that from the previous three-parameter fit. The predicted values for V2 and V4 were 2836 and -352 cm-I, respectively. To demonstrate that good fits could also be obtained with a wide range of V, values, fits were performed with VZfixed at 2000 and then at 4000 cm-I while V4 and V6 were allowed to vary. These fits yielded virtually identical potential energy surfaces over the lower energy region (up to 600cm-I), where the energy levels are known but predict dramatically different shapes and barrier heights for the higher energy region of the potential energy surface. Nevertheless, all of these fits predict an excitedstate surface with a double-well potential, suggesting an equilibrium geometry in which the cyclohexene double bond lies in the plane of the benzene ring. C. Ground-State Potential Surface. The hindered rotor analysis above suggests that 1-phenylcyclohexene exists in a conformation with a torsional angle T = 0 in the first excited singlet state. In the ground state T must be significantly different from zero to give the observed extended torsional progression. The measure of this distortion is the shift in the equilibrium torsional coordinate, AT, on going from SOto SI.Since this experiment only assigns transitions from the 0'' = 0 level of the torsional mode, little direct information is available on the groundstate surface. The excited-state surface has been shown to be reasonably approximated as harmonic, with a small negative anharmonicity, and therefore, by assuming that the ground-state surface is also nearly harmonic a t the u" = 0 level, AT can be determined through a conventional Franck-Condon harmonic oscillator analysis. Two parameters are needed to calculate the torsional mode intensities: S , the dimensionless measure of the equilibrium geometry displacement given by AT*/^;^, where is the root mean square torsional displacement in the ground torsional level, and R , the ratio of the excited-state to groundstate torsional frequencies." These are determined through a nonlinear least-squares fit to the torsional progression intensities.
f F B
'I 203
,
0
45
-30
-15
0
15
30
45
torsional angle. r (dcgrss)
Figure 5. Schematicof the torsional potential energy surfaces for SIand SOelectronicstates. The excited-statesurfaceis determined by a hindered rotor analysis and is depicted over the range of energies investigated in this experiment. The two minima on the ground-state surface and the local curvature are determined by a Franck-Condon analysis of the torsional transition intensities. The energy difference between the two conformers, 127 cm-I, is derived from the assignment of the two-photon ionization spectrum.
The 10 data points used in this fit are an average from several different scans. The vibronic intensities corresponding to transitions to u' = 1 through u' = 10 are used, but the origin is not included because its intensity is difficult to accurately measure. These intensities are listed in Table I and are all relative to the intensity of the u ' = 1 transition. The optimum values of S and R calculated from fitting these intensities are 6.61 and 1.04, respectively. The vibronic intensities calculated with these parameters are also listed in Table I for comparison with the observed intensities. The ground-state torsional frequency, vg,is determined from R and the measured excited-state frequency and then used along with S to determine AT according to the equation12
AT^ = 4(B/ug)S
(3)
To provide a check on the assignment of the origin at 35 066 cm-I, three other fits were attempted using different assignments for the torsional quantum numbers. For these fits, the first sharp peak at 35 112 cm-I was assigned in turn to u' = 0, 2, and 3. In each case the overall fit was inferior to that made with the peak at 35 112 cm-I assigned to u'= 1, thus lending additional support to the original assignment of the electronic origin at 35 066 cm-I.
Discussion In Figure 5 the best fit potential energy surface derived from V2,V4,and V6 terms is depicted over the energy region investigated in this experiment. The highest level included in our fitting procedure was the u' = 10 torsional level a t approximately 500 cm-I. This level has classical turning points f32O from the equilibrium geometry, and thus the excited-state potential energy surface can be accurately characterized over a range of about 64O. There are two equivalent minima on this surface (only one is shown) related to each other by a 180° rotation in the torsional angle, and therefore there is only one distinguishable stable conformation in the excited electronic state. In the plot in Figure 5 this minimum is placed a t T = Oo. If the cyclohexene ring were planar, a single minimum would have to be located at exactly T = Oo since, by symmetry, any minimum located at a nonzero value of T would also have to be present at an angle of -T. The cyclohexene ring is not planar, and so this argument does not hold rigorously; nevertheless, the asymmetric steric perturbation from a nonplanar ring is not expected to be large, and therefore the potential minimum must lie very close to T = Oo with the cyclohexene double bond in the plane of the benzene ring. Even though 11 torsional energy levels can be fit to the calculated potential energy surface in Figure 5 with an error of
Electronic Spectroscopy of Jet-Cooled 1-Phenylcyclohexene at most 0.2 cm-I, this still corresponds to only the lowest 500 cm-1 of the torsional potential. While this energy region is very well characterized, the higher energy regions and especiallythe barrier heights for hindered rotation cannot be accurately determined. The best fit V,, Vd, and V, terms predict a barrier of 3020 cm-I between the two equivalent wells. By fixing V2 at 2000 and then 4000 cm-I while letting V, and v 6 vary, reasonable fits to our spectra can also be obtained having reduced chi-squared values fairly similar to the full three-parameter fit. However, the two different cases predict decidedlydifferent barrier heights of 1920 and 4120 cm-I, respectively. Because this experiment involves only transitions from the lowest torsional level of the ground electronic state, the groundstate potential surface cannot be as well characterized as the excited-state surface. The ground-state torsional frequency, vg, and the equilibrium value of T in the ground state can be derived by approximating both the ground- and excited-state wells as harmonic and performing a nonlinear least-squares fit of the observed torsional intensities. This Franck-Condon analysisleads to values for the coupling constant, S,of 6.61 and for the excitedstate toground-statefrequencyratio,R, of 1.04. The fundamental torsional frequency in the excited state is measured as 46.1 cm-I, leading to a value of 44.3 cm-I for v8. From these values of S and vg, eq 3 is used to calculate a value of 26' for (AT[.The sign of AT cannot be determined from the Franck-Condon analysis. Again, if cyclohexene were planar, symmetry would require the two ground-state minima to be at equal but oppositely signed values of T . Since the cyclohexene ring is nonplanar, one of the ortho hydrogens on the benzene ring will be oriented closer to either an essentially axial or equatorial hydrogen on c6 of the cyclohexene ring, depending on the sign of T . Thesteric interaction with the equatorial hydrogen is expected to be larger than with the axial hydrogen, thus accounting for the energy difference between the two conformers. This difference is expected to be small (energy minimization using MOPAC and the AM1 Hamiltonian finds a difference of 77 cm-1) and so if the energy minimum corresponding to one conformation is at an angle T , the other minimum must occur near -T. Each of these two nonequivalent conformations can also be found following a 180' rotation in T leading to a ground-state surface with four potential minima. In Figure 5 the minima for the two nonequivalent conformationsare depicted at T = *26' with an energy difference of 127 cm-1 (discussed later) between the two wells. Because all electronic transitions in this experiment take place from the u" = 0 torsional level, little is known about the higher energy portion of the ground-state surface and, as depicted in Figure 5 , nothing is known about the barrier separating the two wells. It is clear that the origin transition assigned at 35 066 cm-I must originate from the higher of these two wells since there are no spectral features lower in energy, other than those clearly attributable to hot bands. As discussed previously, several less intense torsional progressions, all with the same vibrational spacing, are present in the spectrum of 1-phenylcyclohexene. The false origins for the four progressions seen in Figures 3 and 4 have been identified with frequenciesof 127,244,253, and 580 cm-I. Possible assignments for these lines include other Franck-Condon active vibrations as well as the origin transition for a second ground-state conformer. Distinguishing between these possibilities is complicated by the fact that the vibrational spacings of the excited-state progressions in either conformer must be identical because only a single conformation is present in the excited state. Progressionsarising from different conformations might be distinguished by the different relative intensities within each progression, since the Franck-Condon factors are sensitive to the ground-state geometries. An inspection of the torsional progression envelopes does not reveal significant differences, however, and so it is necessary
The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 4599 to look elsewhere and to make comparisons with other closely related systems. Most helpful for this are styrene and a-methylstyrene. Of primary interest in looking at these other systems is the presence of any low-frequency vibrations which might account for the feature found at 127cm-I in 1-phenylcyclohexene. Besides the torsion, the 127-cm-I line is the lowest energy feature found in the spectrum, and the progression built onto this feature has intensity approaching that of the origin-based progression. Since the energy difference between the two conformations is expected to be small, this line becomes a likely candidate for the origin transition from the lower energy conformer. Comparison with styrene leads to three possible assignments for the 127-cm-1 feature. One possibility is the 28; vibronic transition (using the mode numbering convention for styrene) which corresponds to the benzenelike V6a ring deformationand has fundamental excitedstate frequencies of 395 cm-I in styrene2 and 370 cm-' in a-methyl~tyrene.~ As evidenced by the 25-cm-I red shift found upon methyl substitution, this ring stretching mode is mass sensitive and would be expected to appear at a lower frequency in 1-phenylcyclohexene. However, this stretch is only weakly Franck-Condon active in styrene and a-methylstyrene and would not be expected to become substantially more active in l-phenylcyclohexene. Thus, this seems to be an unlikely assignment. The 29; in-plane bend of the ethylene group and the 41;42; outof-plane bending combination of styrene are found at 238 and 2 8 2 ~ m - respectively.2 I~ Thesemodes should alsobe mass sensitive and therefore red shifted in 1-phenylcyclohexene. They also are only weakly active in styrene and are not assigned at all in a-methyl~tyrene.~The absence of 29; in the spectrum of a-methylstyrene might be attributed to the increased steric restrictions which the methyl group would impose on the bending motion, thereby prohibiting much of a change in the equilibrium ,geometry along this coordinate upon excitation to SI.The same would hold for 1-phenylcyclohexene. These bending modes therefore also seem unlikely to account for the highly FranckCondon active 127 cm-I mode in 1-phenylcyclohexene. Thus, the 127-cm-I false origin appears to be a very likely candidate for the origin transition from the lower energy conformation of 1-phenylcyclohexene. Arguing against this assignment is the appearance of another progression built off of a 253-cm-1 false origin, almost exactly twice the frequency of the 127-cm-1 line. If the 127-cm-' line is assigned as the fundamental of a Franck-Condon active vibration, then the Franck-Condon coupling constant, SIcan be estimated and used to predict the intensity of the overtone. This second progression is approximately 60% as intense as that based off of the origin, leading to a value of S of 0.6 if the ground-state and excited-state vibrational frequencies are the same. Using this value of SIthe progression based on the 253-cm-I overtone would be 18% as intense as the origin-based progression. Experimentally the relative intensity is found to be 35%, roughly twice the intensity expected for the first overtone. This therefore argues against a vibronic assignment for the 127-cm-I peak. An assignment of the 127-cm-1 line to the origin of the lower energy conformation would require that the geometries of the two conformationsbe very similar. The Franck-Condon coupling constant, S, is proportional to the square of AT, and although insensitive to the sign of AT, any appreciable difference in the magnitude of AT between the two conformers should be clearly reflected by a difference in the distribution of intensities within the respectivetorsional progressions. When the second progression beginning at 127 cm-I is analyzed, values of S = 6.41 and R = 1.16 are determined. These values predict a value for IAT~of 27', virtually indistinguishablefrom the 26O obtained from the analysis of the first progression. A further consequenceof this assignment and the similarity of the Franck-Condon factors for the two conformations is that the lower energy conformer appears to be
4600 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993
less populated than the higher energy conformer. It is not readily apparent why this should be the case, although the final conformer distribution is almost certainly determined by the kinetics of the cooling process during the supersonic expansion as opposed to the relative energies of the two conformers. Under all experimental conditions (Ar or He as carrier gas, different backing pressures, different nozzle temperatures) which give vibronically resolved spectra, no appreciable differencein the relative intensities of the two progressions was observed. It is difficult to know just how unusual this situation is because most spectroscopic experiments on jet-cooled molecules probe ground-state conformers which also have different conformations in their excited electronic states.” This makes it impossible to relateground-stateconformer energies since their excited-state energies are also unknown.
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Conclusion
The SI SOresonant two-photon ionization spectrum of jetcooled 1 -phenylcyclohexenehas been presented, and the extended low-frequency torsional progression which is observed has been analyzed and used to examine the torsional potential energy surfaces in both the ground and excited electronic states. A nonlinear least-squares fit of the V,, V4, and V, hindered rotor parameters to the measured torsional energies reveals that the excited electronic state has only a single distinguishable conformation in which the olefinic group of the cyclohexene ring is planar with the phenyl ring. A Franck-Condon analysis of the intensities of the torsional transitions within this progression predicts a change in the equilibrium torsional geometries between the two states, AT^, of 2 6 O . Since the cyclohexene ring is not planar, two distinguishable ground-state conformations are expected having values of T roughly equal in magnitude but of opposite sign. Evidence is presented for assigning the origin transition of a second ground-state conformer leading to an energy separation between the two of 127 cm-I. A Franck-Condon
Finley and Cable analysis of the torsional progression intensities arising from this second conformation leads to a value of 27O for [AT[.Thus, the two ground-state conformers adopt very similar torsional geometries distorted from a planar geometry by torsion in the opposite direction. The small energy separation between the two arises from the asymmetric steric repulsions associated with the nonplanar cyclohexene ring. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for the support of this research. References and Notes (1) Hollas, J. M.; Khalilipour, E.; Thakur, S. N. J . Mol. Spectrosc. 1978, 73, 240. Hollas, J. M.; Ridley, T. Chem. Phys. Lett. 1980, 75, 94. (2) Syage, J. A.; AI Adel, F.; Zewail, A. H. Chem. Phys. Leff.1983,103, 15. (3) Grassian, V. H.; Bernstein, E. R. J. Phys. Chem. 1990, 94, 6691. (4) Myers, A. B.; Trulson, M. 0.; Pardoen, J. A.; Heeremans, C.; Luatenbura, J.; Mathies. R. A. J. Chem. Phvs. 1986. 84. 633. -(5) Himley, R. J.; Leopold, D. G.; Vaiha, V.; Karplus, M. J . Chem. Phys. 1985, 82, 5379. (6) Levy, D. H.; Wharton, L.; Smalley, R. In Chemicaland Biochemical Applicationsof Lasers; Moore, C. B., Ed.; Academic Press: New York, 1977; Vol. 11. (7) Wiley, W. C.; McClarin, I. H. Rev. Sci. Instrum. 1955, 26, 1150. (8) See, for example: Lister, D. G.; MacDonald, J. N.; Owen, N. L. International Rotation and Inversion; Academic Press: New York, 1978; Chapter 2. (9) MOPAC ver. 5.00. Quantum Chemistry Program Exchange No. 455, Department of Chemistry, Indiana University, Bloomington, IN. (10) Lewis, J. D.; Malloy,T. B., Jr.;Chao,T. H.; Laane, J.J. Mol.Strucf. 1972, 12, 427. (11) Manneback, C. Physica 1951, 17, 1001. (12) In the harmonic limit, the force constant for a hindered rotation is given by u2/2B, where Y is the fundamental torsional frequency and B is the rotational constant. This leads to an expression for the mean square torsional displacement of 72 = B / Y . (13) See, for example: Rizzo, T. R.;Park, Y. D.; Levy, D. H. J . Chem. Phys. 1986,85,6945. Breen, P. J.; Warren, J. A.; Bernstein, E. R.; Seeman, J. I. J. Chem. Phys. 1987,87, 1927.
