Anal. Chem. 1990, 62, 1623-1627 (3) Sleszynskl, N.; Osteryoung, J.; Carter, M. Anal. Chem. 1984, 56, 130. (4) Bard, A. J.; Crayston, J. A.; Kittlesen, G. P.; Shea, T. V.; Wrighton, M. S. Anal. Chem. 1986, 5 8 , 2321. (5) Chdsey, C. E.; Feldman, B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1986, 58, 601. (6)Thormann, W.: van den Bosch, P.; Bond, A. M. Anal. Chem. 1985,
57,2764. (7)Weber. S.G. Anal. Chem. 1989, 67,295. (6) Caudlll, W. L.; Howell, J. 0.; Wightman, R. M. Anal. Chem. 1982, 5 4 , 2532. (9) Fosdick. L. E.; Anderson, J. L. Anal. Chem. 1988, 58, 2481. (lo) Peterson, S.L.: Weisshaar. D. E.; Tallman, D. E.; Schulze, R. K.; Evans. J. F.; DesJarlais, S. E.; Engstrom, R. C. Anal. Chem. 1988, 60,
2385. (11) Stroehben, W. E.; Smith, D. K.; Evans, D. H. Anal. Chem., submitted for publication.
(12) Magee, L. J.; Osteryoung, J. Anal. Chem. 1989, 67,2124. (13) DeAbreu. M.: Purdy, W. C. Anal. Chem. 1987, 59, 204. (14) Singleton, S.T.; O'Dea, J. J.; Osteryoung, J. Anal. Chem. 1989, 61, 1211. (15) Kovach, P. M.; Caudill, W. L.; Peters, D. G.; Wightman, R. M. J . Electroanal. Chem. 1985, 185, 265. (16) Wehmeyer, K. R.; Deakin, M. R.; Wightman, R. M. Anal. Chem. 1985. 57, 1913. (17) Hill, H. A.; Klein, N. A.; Psalti. I . S. M.; Walton, N. J. Anal. Chem. 1989, 67,2200. (18)Amatore, C.;Saveant, J. M.; Tessier, D. J . Nectroanal. Chem. 1983, 746, 37.
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(19) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J . Electroanal. Chem. 1982, 738, 65. (20) Gueshi, T.; Tokuda, K.; Mats&, H. J. Ek%3manal. Chem. 1979, 707, 29. (21) Rhodes, R. K.; Kadish, K. M. Anal. &em. 1981, 5 3 , 1539. (22)Fitch, A,, Personal communication. (23) Engstrom, R. C.; Johnson, K. W.; DesJarlais, S. Anal. Chem. 1987. 59. 670.
(24) Wehmeyer, K. R.; Wightman, R. M. J. Electroanal. Chem. 1985, 796, 417. (25) Thormann, W.; Bond, A. M. J. Ekctroanal. Chem. 1987. 278, 167. (26)Andrieux, C. P.; Garreau, D.; Hapiot, P.; Pinson, J.; Saveant, J. M. J. Electroanal. Chem. 1988, 243, 321. Krlstensen, E. W.; Deakin, M. R.; Wightman, R. M. Anal. (27)Wipf, D. 0.; Chem. 1988, 60, 306. (28) Bard, A. J.; Faulkner, L. R. Electrochembal Mefhods fundamentals and Appllcations; Wiley: New York, 1960;p 218. (29) Bowyer, W. J.; Engelman, E. E.; Evans, D. H. J. flectroanal. Chem. 1989, 262, 67. (30) Shea, T. V.; Bard, A. J. Anal. Chem. 1987, 59,2101. (31) Fitch, A.; Evans, D. H. J. Electroanal. Chem. 1986, 202, 83. (32) Reller, H.; Kirowa-Eisner, E.; Gileadi, E. J. Electroanal. Chem. 1984, 767. 247.
RECEIVED for review February 20,1990. Accepted April 19, 1990. This research was supported by a grant from Research Corporation and by Hobart and William Smith Colleges.
Differentiation of Stereoisomers Using High-Resolution Electronic Spectroscopy Applied to Methyl-Substituted Tetrahydropyrans Timothy J. Cornish a n d Tomas Baer* Chemistry Department, University of North Carolina, Chapel Hill, North Carolina 27599-3290
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The n 3s Rydberg spectra of jet-cooled methyl-substltuted tetrahydropyrans (THPs) have been collected by the technique of 2 1 resonance-enhanced multiphoton ionization (REMPI) in the wavelength region 420-375 nm. As with the prevlously Investigated methyl-substituted cyclohexanones, the transltion energies of the THPs are hlghly sensltlve to both the positlon of substitution around the ring and its orientation (axial vs equatorial). Furthermore, these shlfts are addltlve. It Is thus readily posslble to distlngulsh on the basis of the spectra cls- and trans-dimethyl-THPs.
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INTRODUCTION Ultraviolet spectroscopy has enjoyed a rebirth as a spectroscopic and analytical tool for intermediate size molecules because of efficient cooling of gaseous samples to a few degrees kelvin provided by pulsed molecular beam technology and by the use of pulsed lasers as excitation sources (1-14). The effect of cooling the rotational and vibrational energy has resulted in sharp absorption peaks and the simplification of the spectra due to hot band suppression. Of particular interest is that these methods are no longer limited to intermediate size molecules because techniques have been developed that allow large or nonvolatile molecules to be vaporized and entrained in a beam of rare-gas atoms (15-17). A particularly interesting electronic transition that we have recently investigated by 2 + 1 resonance-enhanced multiphoton ionization (REMPI) for cyclic and linear ketones is that of the n 3s excitation (12-14). In many ketones, the 3s Rydberg state lies at an energy that is greater than two-
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thirds of the ionization energy. The n 3s transition is thus very convenient for REMPI studies because the laser wavelength lies in the easily accessible range of 420-375 cm-' and because the absorption can be efficiently monitored by collecting the ions resulting from the absorption of the third photon. A characteristic feature of the spectra are very sharp peaks, often exhibiting little vibrational excitation. We have found during the course of our investigations of many methyl-substituted cyclic ketones that the 3s Rydberg spectra are surprisingly sensitive to molecular stereochemistry. The n 3s transition origin for cyclohexanone was found to be 50717 cm-' (394.3 nm in a two-photon excitation scheme). The most stable conformation of the six-membered ring is a chair form, in which substituents are oriented in either the more stable equatorial or the less stable axial direction. When methyl groups are substituted in equatorial orientations at the 2, 3, or 4 positions, the n 3s transition origin shifts by -546, +109, and -7 cm-', respectively. These shifts depend strongly on whether the methyl groups are attached in axial or equatorial orientations. Thus, cis-3,5-dimethylcyclohexanone (diequatorial) and trans-3,5-dimethylcyclohexanone (equatorial-axial) have origins at 50 955 and 50 464 cm-I, respectively. Not only are these shifts sensitive to the location and orientation of the methyl groups, they are largely additive. This means that stereoisomers can be identified on the basis of their transition origin. The additivity applies as well to multiring compounds such as norcamphor derivatives (18). The standard spectroscopic method for structure determination of such stereoisomers is NMR (19). We recently compared the optical n 3s transition energy shifts with the carbonyl carbon 13C NMR shifts and found a remarkable
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0003-2700/90/0362-1623$02.50/0 0 1990 American Chemical Society
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correspondence (20). Linear relationships with correlation coefficients in excess of 0.95 were discovered between these shifts for similar groups of molecules. The four groups considered were six-membered rings, five-membered rings, straight-chain ketones, and bicyclic ketones. The origin of this linear relationship is not entirely understood but seems related to the influence of the HOMO energy on both the optical and NMR spectroscopies. A property that distinguishes one-photon and two-photon excitation is the dependence of the absorption cross section on the laser polarization. The ratio of the peak intensity, R, under circular and linear polarization is sensitive to the molecular symmetry and thus provides useful information about the molecule's conformation (21,22). The previously studied cyclic ketones had R values between 1.5 and 0.6 (12,13). The higher value is expected for an electronic transition in which the molecular orbital symmetries of the upper and lower states are different. Deviations from this maximum value for different methyl isomers appear to reflect distortions of the backbone geometry. We recently determined that the MO symmetries of the upper and lower states of tetrahydropyran (THP) and dioxane are the same (14). The reported R values for these two molecules is 0.08. Distortions from C, symmetry caused by substituents are expected to increase the Q value for the THPs. In this paper we present the n 3s 2 + 1 REMPI spectra for methyl-substituted tetrahydropyran (THP), a ring system that is quite important in biochemistry since it constitutes the backbone structure for hexose sugars. This study includes Rydberg spectra for a number of related methyl isomers and comparisons of their spectral shifts relative to the unsubstituted parent compound THP. The immediate goal of this research is the extension of the n 3s Rydberg spectroscopic technique to a new class of molecules. The ultimate goal is to develop a new spectroscopic method for conformational analysis of biologically interesting and important molecules.
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EXPERIMENTAL APPROACH The REMPI apparatus has been described in detail earlier (12, 13). Briefly, the output from a Lumonics dye laser pumped by a Lumonics excimer laser was focused with a 25-cm focal length lens at the center of the photoionization region where it intersected a seeded and skimmed molecular sample beam at right angles. The laser power was held at a constant intensity over the entire scan with a feedback loop coupled to a Scientech calorimeter. The pump laser intensity was adjusted with a motor-driven, mechanical flag inserted into the path of the excimer beam. In this manner, the dye laser output was held constant at 400 pJ/pulse. The molecular beam was generated with a Laser Technics pulsed valve by diluting a few Torr of sample gas in about 700 Torr of Ar. Typical pressures in the nozzle and experimental chambers during the experiment were and lo4 Torr, respectively. The electrons and ions were extracted with a strong electric field of 1500 V/cm. The electrons then passed through a 10-cm drift region and were detected with a set of microchannel plates. The microchannel plate output was sent t o a gated integrator followed by digital conversion and processing using a PC. The spectra were produced by scanning the laser while collecting the total electron signal. The ions were accelerated in the opposite direction before entering a 1-m drift tube and collected with a set of microchannel plates. Ion time-of-flightmass spectra were obtained at selected laser wavelengths in order to verify the purity of the samples. The spectra required about 10 min of collection time and were scanned with a single laser dye mixture. However, different dyes were used for different spectra. A good spectrum could be obtained with 1 mg of sample. The accuracy of the wavelength scale was on the order of f 3 cm-'. The THP samples were prepared as discussed in detail by Eliel et al. (23) or were purchased from Aldrich. Configurational isomers were separated by preparative GC on an SF-96 column.
I
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1 374
316
L 318
382
380
384
386
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388
I
@
I
390
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392
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Wavelength (nm)
Figure 1. Two-photon n 3s REMPI spectra of tetrahydropyran (THP) and methyl-substituted isomers of THP. -+
Stereoisomer authenticity was confirmed by 13C NMR spectroscopy. RESULTS The spectra for the lowest lying electronic transition in tetrahydropyran (THP) and 21 of its methyl-substituted isomers are plotted in Figures 1-4. All spectra shown in these figures were taken with the laser under linear polarization. The spectra are arranged according to the location of the methyl groups on the ring in order to show the effect of substitution at various ring positions on the transition energy. Thus, all molecules with methyl groups in the 4-position are shown in Figure 1, while Figure 2 contains the spectra of molecules with methyl groups at the 3- and 5-positions. The spectra of the remaining compounds having at least one methyl group in the 2-position are shown in Figure 3. The unsubstituted T H P spectrum is plotted in these three figures as a point of reference for the transition energy shifts. Finally, the spectra of three configurational isomers of 3,4,5-trimethyl-THP are displayed in Figure 4. Table I lists the transition energies, the relative shifts, and the polarization ratios for these spectra. Spectroscopic arguments (24) as well as results from ab initio molecular orbital calculations ( 1 4 ) have established that this lowest lying optical excitation in the THPs is attributable to the n 3s Rydberg transition. In all of the T H P isomers, the intensity of the 3s origin band is quite strong, indicative of an allowed electronic transition. Because of the very sharp and strong Q branches, the peaks are much more intense than those of the corresponding cyclic ketones when the spectra are collected by using linearly polarized laser light. Optical Shifts. The kydberg spectrum of unsubstituted T H P (1) has been discussed in detail in an earlier paper (14). A striking aspect of the n 3s Rydberg spectra of the methyl substituted THPs in Figures 1-4 are the substantial transition origin shifts exhibited by these compounds. As in the case of the cyclic ketones, the two important properties of each methyl group are its position relative to the oxygen atom and
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
380
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1825
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W a w l w y t h (nm)
Figure 4. Two-photon n
3s REMPI spectra of three configurational isomers of 3,4,5-trimethyl-THP.
Table I. Transition Origin Energies, Relative Shifts, and Polarization Ratios for the Methyl Isomers of Tetrahydropyran 376
380
384
388
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392
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Wavelength (nm)
Figwe 2. Two-photon n
400 &I
3s REMPI spectra of tetrahydropyran (THP)
and methyl-substituted isomers of THP.
origin,
shift,
compd
ern-'
cm-'
(1) THP (2) 4-Me (3) trans-3,4-Mez (4) cis-3,4-Mez (5) cis-2,4-Me2 (6) 4,4-Mez (7) 3-Me (8) cis-3,5-Me2 (9) trans-3,5-Mez (10) 3,3,5-Me3 (11) 3,3-Me2 (12) 2-Me (13) cis-2,6-Mez (14) trans-2,6-Mez (15) 2,2,6-Me3 (16) trans-2,5-Mez (17) cis-2,5-Mez (18) 2,2-Mez (19) 3,4,5-Me3(eq, eq, eq) (20) 3,4,5-Me3(eq, eq, ax) (21) 3,4,5-Me9(ea, ax, ea)
51907 52 384 51920 51319 51722 51995 51579 51387 50 955 50 626 50 521 51393 50 906 50 671 50 389 51294 50 631 50 689 51 511 51 167 50316
+477 +13 -588 -185 +88 -328 -521 -952 -1281 -1386 -514 -1001 -1236 -1518 -614 -1277 -1218 -396 -740 -1591
0
polarization ratio (n) 0.09 0.09 0.14 0.08 0.08 0.06 0.18 0.68 0.39 0.88 0.40 0.24 0.48 0.68 1.05 0.41 0.26 0.57 0.10
0.08 1.0
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Table 11. n 3s Transition Origin Shifts (cm-') for Methyl Substitution in THP
eq ax
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400
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Wavehgth (nm)
Flgure 3. T w w h o t o n n 3s REMPI spectra of tetrahydropyran (THP) and methyl-substituted isomers of THP.
its orientation (equatorial vs axial). For instance, a number of multisubstituted structures are displayed in Figures 1-4. For the case of dimethyl substitution, each pair of configurational isomers is clearly resolved into two unique spectra separated by at least several hundred wavenumbers. Furthermore, all spectra of the TUP isomers having an axial
CZ -514 -722
c3
c4
-328 -925
+477 -630
methyl group are red shifted relative to the corresponding equatorially substituted isomers. The effect of the more stable equatorial methyl groups in the 4-, 3-, and 2-positions can be determined quantitatively by comparing the spectrum of T H P with those of molecules 2 , 7 , and 12, respectively. The observed shifts for 4-, 3-, and 2-methyl-THP of +477, -328, and -514 cm-' are shown in Table 11. The spectral shifts resulting from an axial methyl group can also be determined from the methyl-THP spectra. Because of the interconverting chair conformations, a small fraction of the methyl-THPs will have their methyl group oriented in the axial position. In the case of the 4- and 3methyl-THPs, these higher energy conformers have sufficient
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with the laser circularly and linearly polarized. The intensity of the origin band was used for this purpose. The 9 values, which are listed in Table I, depend very strongly on the symmetry of the molecule and the nature of the transition. In general, the THPs have low il values, while the cyclic ketones had values that ranged between 0.6 and 1.5. However, we found several notable exceptions to the low Q values in the T H P molecules studied here. These are cis-3,5-dimethylTHP, cis-2,6-dimethyl-THP, and 3,4,5-trimethyl-THP (eq, ax, eq). These three molecules were expected to be symmetric and to have the same low Q value as does THP. The reason for this discrepancy is not clear. However, we note that many of the spectra with higher 9 values have substantial vibrational structure excited. Thus it appears that the change in ring geometry upon excitation is related to the high 9 values.
DISCUSSION t
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Figure 5. Two-photon n
3s REMPI spectra of 4-methyl-THP and 3-methyl-THP on an expanded scale showing the presence of less stable axial conformers at higher laser wavelengths.
concentration at room temperature that the spectra due to these structures can be detected as small peaks to the red of the main spectrum. These are shown in Figure 5. They have been reported and discussed previously in connection with room-temperature equilibrium constant determinations for the equatorial-axial conformational equilibria (25). The 4and 3-methyl axial substituents shift the origin to the red by 630 and 925 cm-', respectively. The shift resulting from an axial methyl group in the 2-position could not be determined from the 2-methyl-THP spectrum because this axial conformer has a very low concentration and the peak suspected to be due to the axial conformer could not be unequivocally distinguished from very weak hot bands. However this shift can be determined indirectly from disubstituted THPs. The truns-2,6-dimethyl-THP differs from the 2-methyl-THP by the addition of a single axial methyl group. The difference in the transition energies between these two molecules is 722 cm-l, which we assign as the shift of an axial methyl group in the 2-position. These axial shifts are also listed in Table 11. Vibrational Structure. The spectra of Figures 1-4 exhibit a considerable range of vibrational excitation. For instance, the spectrum of gem-4,4-dimethyl-THP (6) has almost no vibrations excited, while the other two gem-dimethyl-THPs (11 and 18) exhibit a great deal of vibrational excitation. Overall these spectra are considerably more complex than are the spectra of the methylcyclohexanones (13). This is not unexpected since the electron in T H P is removed from an atom within the ring whereas in cyclohexanone the 0 atom is not a part of the ring. Although we have performed preliminary analyses of the THP and dioxane spectra (14), we did not attempt to analyze the vibrational structure of the substituted THPs. Without substantial calculations of the excited-state surface as well as isotopic labeling studies, such an attempt would be rather futile. Nevertheless, we can conclude that these complex spectra are indicative of considerable rearrangement in the backbone of the molecules as they are excited to the 3s Rydberg state. This is especially true of the two gem-dimethyl-THPs. The excitation of the lowest energy ring vibrations (between 250 and 500 cm-') is good evidence of this structural change. Polarization Effects. All spectra were collected with both linear and circular polarization. The results can be cast into a single parameter, 9, which is the ratio of the signal intensity
The overall trends of the Rydberg shifting parameters (sign and approximate magnitude) for both equatorial and axial methyl groups are consistent over the entire set of THP isomers represented here. In addition, these shifts are somewhat larger than those reported previously for the cyclic ketones. In addition, the pattern of the shifts appears to be different. That is, the blue shift in the THPs is associated with the equatorial methyl group in the 4-position, whereas it is associated with the 3-position in the cyclic ketones. However, we note that if the substituent position is labeled according to the distance from the oxygen chromophore, the patterns are identical. That is, in both the THPs and in the cyclic ketones, a blue shift is observed if the methyl group is positioned three atoms away from the oxygen atom. This pattern extends also to the five-membered cyclic ketones in which the 3-methylcyclopentanone origin is shifted to the blue by 240 cm-' (13). These sizable shifts, especially in the case of the THPs, indicate that there exists a considerable interaction between the oxygen chromophore and the substituent methyl group. Many of the shifts are additive. This is most clearly the case in the 2-methyl-THPs. While a single equatorial methyl group shifts the origin by -514 cm-', a second equatorial methyl group shifts the onset [spectrum for cis-2,6 dimethyl-THP (13)] an additional -487 cm-'. This additivity carries over even to the gem-2-2-dimethyl-THP (18), which has a total shift of -1218 cm-' compared with the predicted shift of -514 - 722 = -1236 cm-'. The cis- and truns-2,5-dimethyl-THPs are less well predicted. The observed shifts of -1277 and -614 cm-' are less than the shifts predicted on the basis of those in Table I1 (-1439 and -842 cm-', respectively). Finally, we predict for the 2,2,6-trimethyl-THP (15) a shift of 2(-514) - 722 = -1750 cm-'. The observed shift for this compound is -1518 cm-', which is lower than predicted. The shifts are less additive for the C3 position. While one equatorial methyl group shifts the onset by -328 cm-', the second shifts it only an additional -193 cm-'. The trans3,5-dimethyl-THP (9) has an onset shifted by -952 cm-' compared to the predicted shift of -1253 cm-'. As in the case of the 2-methyl-THPs, the gem-3,3-dimethyl-THP (11) should have the same shift as does 9, but this is clearly not the case. Nor is the shift for the spectrum of 11 very consistent with the shift for 3,3,5-trimethyl-THP (10). Three configurational isomers of 3,4,5-trimethyl-THP (19, 20, and 21) are plotted in Figure 4. On the basis of the shifts from the single and double methyl substitution, we can immediately associate the spectra with the structure of the corresponding molecules. Thus, the molecule with all three methyl groups in the equatorial position (19) has its origin farthest to blue. Its predicted shift, based on three equatorial methyl groups, is -179 cm-l, compared to the experimental shift of -396 cm-l. Similarly, molecule 20, with two equatorial
ANALYTICAL CHEMISTRY, VOL. 02, NO. 15, AUGUST 1, 1990
groups and one axial group in the 3-position, has a predicted shift of -328 477 - 925 = -776 cm-', compared to the observed shift of -740 cm-l. Finally, the 3,4,5-trimethyl-THP (eq, ax, eq) is shifted -1591 cm-' from the T H P origin. We predict a shift of -1286 cm-', which is within 80% of the observed shift. However, if we base the prediction on molecule 19 with its -396-cm-' shift and simply convert the equatorial methyl group in the 4-position into an axial one, we predict a total shift of -1503 cm-', which is in excellent agreement with the observed shift. In general, the absolute shifts in the methyl-substituted T H P isomers are not as additive as they are among the cyclic ketone spectra. For example, the calculated spectral shift of trans-3,4-dimethyl-THP is +149 cm-', while the observed shift is +13 cm-', giving an error of 136 cm-'. A similar comparison for trans-3,4-dimethylcyclohexanone reveals an error of only 9 cm-'. Other transition energies in the dimethyl-THP isomers calculated from the monomethyl shifts have comparable discrepancies from the predicted value. The likely explanation for the reduced additivity in the T H P ring system can be inferred from the high level of vibrational excitation that is observed in the excited-state spectrum. That is, the backbone conformation of the ring in the 3s Rydberg state is significantly perturbed from the ground-state geometry. In contrast, the cyclohexanone Rydberg spectrum has a much lower degree of vibrational excitation, indicating little change in geometry between the ground and excited states. The shifts of the transition origins upon substitution of methyl groups in the equatorial positions are intriguing, in particular the occurrence of the blue shift at specific positions in the T H P or cyclohexanone rings. This effect would be much less interesting if substitutions at all ring positions resulted in shifts to the red. If this were the case, it would be straightforward to suggest that methyl groups at all positions serve to stabilize the chromophore in the excited state, thereby decreasing the energy required for electronic transition. This is the explanation for the optical shifts in UV spectra of conjugated or aromatic systems, which generally exhibit red shifts as substituents are added. Red shifts upon methyl substitution are also observed in the ionization energies of most molecules. It is thus rather odd that the n 3s transition shows such blue shifts because, as the first member of a Rydberg series converging to the ionization energy of the molecule, one would expect that these shifts would reflect the change in the ionization energies. Accurate IPS of the methyl-substituted THPs have not yet been measured. However, if they follow the normal trend, the IPS would decrease with increasing size or methylation (12,26). Similar diverging patterns of Rydberg energies and IPSwere noted in a series of cyclic ketones of various ring sizes (12). It has already been pointed out that the n 3s transitions in the methyl-substituted cyclic ketones show shifts similar to the cyclic ether methyl isomers, although the blue shift in those molecules occurs when an equatorial group is added to the 3-position. The model discussed previously for the cyclic ketone system attributes the Rydberg shifts to long range van der Waals interactions between the carbonyl group and the methyl substituents in the ground electronic state (13). This same model was used to describe the chemical shifts in 13C NMR spectra of these molecules by Li and Chesnut (27). The degree of local van der Waals interactions, calculated by using molecular mechanics methods (MM2), was correlated to shifting parameters for a number of cyclic hydrocarbons.
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Although the definitive explanation for the similar trends in the n 3s optical spectra and the 13C NMR shifts has not been offered, the fact that the rather unusual shift patterns are correlated is strong evidence that they both originate from a common orbital energy property of the molecule. CONCLUSIONS We have demonstrated that the Rydberg absorptions of T H P and its methyl isomers are quite sensitive to the stereochemistry. The shifts in the transition origins follow predictable patterns as a function of substituent position and orientation. Thus, stereoisomers can be readily identified by their spectra. Significant changes in ring conformation resulting from electronic excitation tend to reduce the precision of the additive parameters compared to the cyclic ketone ring system, although the overall trends are consistent throughout the entire set of methyl substituted isomers. ACKNOWLEDGMENT We are grateful to Professor Ernest L. Eliel and Dr. K. Michal Pietrusiewicz for providing us with most of the compounds used in this investigation as well as for a number of useful discussions concerning the stereochemistry of these molecules. We also thank Mary Neumann, Johnathan Tschritter, and Dr. Norman Boggs for synthesizing several of the compounds. LITERATURE CITED Trembreuli, R.; Lubman, D. M. Anal. Chem. 1984, 56, 1962. Sin, C. H.; Trembreull, R.; Lubman, D. M. Anal. Chem. 1984, 56, 2776. Lubman, D. M. Anal. Chem. 1988, 59. 31A. Donaldson, D. J.; Richard, E. C.; Strickler, S. J.; Vaida, V. J. Phys. Chem. 1988, 92, 5514. Gaines, G. A.; Donaidson, D. J.; Strickler, S. J.; VaMa, V. J. Phys. Chem. 1988, 92, 2762. Breen, P. J.; Bernstein, E. R.; Seeman, J. I.; Secor, H. V. J. Phys. Chem. 1989, 93, 6731. Breen, P. J.; Bernsteln, E. R.; Secor, H. V.; Seeman, J. I . J. Am. Chem. SOC. 1989, 111, 1958. Seeman, J. I.; Secor, H. V.; Breen, P. J.; Grassian. V. H.; Bernstein, E.R. J. Am. Chem. SOC. 1989, 171,3140. Brady, B. B.; Peteanu, L. A.; Levy, D. H. Chem. Phys. Lett. 1988, 147, 538. Martlnez, S. J.; Aifano, J. C.; Levy, D. H. J. Mol. Spectrosc. 1989, 137, 420. Lipert, R. J.; Colson, S. D. J. Phys. Chem. 1889, 93, 3894. Cornish, T. J.; Baer, T. J. Am. Chem. SOC. 1987, 109, 6915. Cornish, T. J.; Baer, T. J. Am. Chem. SOC. 1988, 110, 3099. Cornish, T. J.; Baer, T. J.; Pedersen, L. 0. J. Phys. Chem. 1989, 93, 6064. Li, L.; Lubman, D. M. Anal. Chem. 1987, 59. 2538. Grotemeyer, J.; Schlag, E. W. Org. Mass Spectrom. 1987, 22, 758. Cable, J. R.; Tubergen, M. J.; Levy, D. H. J. Am. Chem. SOC. 1987, 109, 6198. Cornish, T. J.; Baer, T. Unpublished work. Stothers, J. B.; Tan, C. T. Can. J. Chem. 1974, 52, 308. Cornish, T. J.; Baer, T. J. Am. Chem. SOC.1988, 110, 6287. Monson, P. R.; McClain, W. M. J. Chem. Wys. 1970, 53. 29. Lin, S. H.; Fujimura, Y.; Neusser, H. J.; Schlag, E. W. I n MuMphoton Spectroscopy of Molecules; Academic Press: Orlando, FL, 1984; p 116. Eiiel, E. L.; Manoharan, M.; Pietrusciewicz, K. M.; liargrave, K. D. Org. Magn. Reson. 1983, 21, 94. Robin, M. B. Higher Excited States of Polyatomlc Molecules; Academic Press: New York. 1975; Vol. I . Cornish, T. J.; Baer, T. J. Phys. Chem. 1990. 94, 2852. Lias, S. G.;Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref. Data 1988, 17, Gas Phase Ion and Neutral Thermochemistry. LI, S.: Chesnut, D. B. Magn. Reson. Chem. 1985, 23, 625; 1986, 24, 93.
RECEIVED for review March 3, 1990. Accepted April 9,1990. We thank the National Science Foundation and Glaxo, Inc., for financial support of this work.
