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The electronic spectroscopy of 4-ethylphenol, 4-propylphenol, and 4-propylanisole in a supersonic jet
JOURNAL
OF MOLECULAR
SPECTROSCOPY
137,420-426 (1989)
The Electronic Spectroscopy of 4-Ethylphenol, 4-Propylphenol, and 4-Propylanisole in a Supersonic Jet SELSO J. MARTINEZ
III, JOSEPH
C. ALFANO, AND DONALD H. LEVY
The James Franck Instituteand The Departmentof Chemistry, The Universityof Chicago, Chicago, Illinois60637 We have obtained 5uorescence excitation spectra of the origin regions of 4-ethylphenol, 4propylphenol, and 4-propylanisole in a supersonic jet. Power saturation techniques reveal the existence of one, three, and three conformers, respectively. Two of the three origins of 4-propylphenol and 4-propylanisole were interpreted as resulting from a splitting of gauche structural conformers; the third results from an anti structural conformer. The 4-propylanisole spectrum indicates that steric and/or electronic interactions between the propyl group and the hydroxyl group are responsible for the observed splitting in the spectrum of 4-propylphenol. Q 1989Academic Press, Inc.
In a recent investigation, Song and Hayes observed supersonic jet fluorescence excitation and dispersed fluorescence spectra for a series of seven 4-alkyl-substituted phenols ( 1). The spectroscopy of these molecules is of some interest since a 4-alkylsubstituted phenol is the chromophore of the amino acid tyrosine. Thus these molecules are important models which can be used in interpreting the spectrum of tyrosine. In their study, Song and Hayes assigned many vibrational bands and discussed the effect of multiple structural conformers on the spectra of these molecules (I ). Their work was very important in providing an understanding of the spectroscopy of these molecules, particularly in demonstrating the spectral complexity that exists even in a supersonic jet spectrum due to combination of vibrational structure and the existence of several conformers. In their work they were not able to differentiate unambiguously between vibrational structure and structure due to conformers, and in some cases only an upper limit was placed on the possible number of existing conformers. In previous work on the amino acid tryptophan, we used the power saturation behavior of various spectral features to assign these features as either vibronic bands or conformer origins (2). We have recently studied the laser power dependence of the spectra of 4alkylphenols and have definitively identified the origin transitions of the several molecular conformers. This letter is a report of that work. Fluorescence excitation spectra were taken of 4-ethylphenol, 4-propylphenol, and 4-propylanisole at power densities that varied by approximately two orders of magnitude. The power dependence behavior of these spectra revealed that 4-ethylphenol has one conformer, in agreement with the work of Song and Hayes. We identified three conformers for both 4-propylphenol and 4-propylanisole whereas Song and Hayes could only set an upper limit of five on the number of conformers of 4-propylphenol ( I ) . 0022-2852189 $3.00 Copyright 0 kll r&s
1989 by Academic Press, Inc.
of reproduction in any form reserved.
420
SPECTROSCOPY OF 4-ALKYLPHENOLS
421
The importance of identifying the conformers of these molecules becomes apparent when one tries to interpret the electronic spectrum of tyrosine, an amino acid which contributes to the near-ultraviolet absorption and fluorescence of proteins (3-5) and which plays an important role in protein photophysical processes (6, 7). Only by identifying the number of conformers of the 4-substituted phenols and discussing their most reasonable structures can one attempt to understand the more complicated electronic spectrum of tyrosine (8). The compounds 4-ethylphenol and 4-propylphenol were purchased from Aldrich Chemical Company and used without further purification, and the compound 4-propylanisole was synthesized according to the procedure outlined by Zoran, Sasson, and Blum (9). The purity of the sample was confirmed by ‘H NMR measurements. A standard oven source heated to 30-40°C was used to volatilize the samples, and a free jet was produced by passing 3.4 atm of helium over the heated sample reservoir and expanding the mixture through a 0. I mm diameter nozzle. The output of a Nd:YAG laser excited dye laser operating on Rhodamine 590 laser dye was frequently doubled in a KDP crystal, producing about one mJ of ultraviolet light. This was used to excite the samples. Fluorescence was detected at right angles to the jet propagation direction and the laser light path and collected with a fluorescence collection optics system similar to that described in Ref. ( 10). In our power dependence studies a variable attenuator (Newport Corporation) was used to reduce the laser power before the laser beam was directed into the vacuum chamber. When saturation of the spectral transitions was desired, the attenuator was replaced by a 50 cm focal length lens which focused the excitation beam into the vacuum chamber. The fluorescence excitation spectra of 4-ethylphenol in the region 35 45 1 to 35 96 1 cm -’ taken at low and high power densities are shown in Fig. 1. As in previous studies, the power saturation behavior of these spectra is used to identify different molecular conformers (2). Different transitions having a common initial level and different transition strength (e.g., vibronic bands with the zero-point vibrational level of the ground electronic state as the initial level and different vibrational levels of the excited electronic state as the final level) will power saturate at different rates, and their relative intensity will change as a function of laser power. Transitions having different initial levels and the same transition strength (e.g., different conformer origins) will power saturate at the same rate and will have a constant relative intensity that does not depend on laser power. In Fig. 1, all features grow in intensity relative to the feature at 35 503.0 cm-‘, and this feature has been assigned as the 0: band of the single conformer of 4-ethylphenol. The other features in the spectrum are vibrations built on this single conformer and have been previously assigned ( 1) . One should note that the high power density spectrum (Fig. 1b) more closely resembles the 4-ethylphenol spectrum reported by Song and Hayes (1). These spectra further support other investigations which propose that the ethyl group is oriented in a plane perpendicular to the plane of the benzene ring, passing through the 1 and 4 positions of the ring (I, 11-14). The fluorescence excitation spectra of 4-propylphenol in the region 35 405 to 36 155 cm-i taken at low and high power densities is shown in Fig. 2. Once again, the high power density spectrum (Fig. 2b) more closely resembles the 4-propylphenol spectrum
422
MARTINEZ, ALFANO, AND LEVY
(a)
JJ 35500
35700
35600
3S600
35900
CM-’ (b) 1
I
1
35500
I
1
I
35600
35700
35600
35900
CM-’ FIG. 1. Fluorescence excitation spectra of 4-ethylphenol in the region 35 451 to 35 961 cm-r taken at different laser power densities. The spectrum in (b) is taken at - 100 times the power density of that in (a). The oven temperature is 40°C the carrier gas is helium at a backing pressure of 3.4 atm, and the nozzle diameter is 0.1 mm.
* (a)
1,
I
I
a.
,
35700
35500
*a.
I
35900
36100
35900
36100
.
CM-’
* (b)
1..
35500
.
1.
*
35700
I
CM-’ FIG. 2. Fluorescence excitation spectra of 4-propylphenol in the region 35 405 to 35 600 cm-’ taken at different laser power densities. The spectrum in (b) is taken at - 100 times the power density of that in (a). The oven temperature is 42”C, the carrier gas is helium at a backing pressure of 3.4 atm, and the nozzle diameter is 0.05 mm.
SPECTROSCOPY
423
OF 4-ALKYLPHENOLS
reported by Song and Hayes ( 1) . In their investigation, Song and Hayes stated that a definitive assignment of conformers could not be made without additional information but reduced the number of conformers to five, based on the argument that only the bands lower in energy than the most intense band in the region 35 400 to 35 600 cm-’ are considered origins (1). Here we provide definitive evidence for the existence of three conformers. The spectral features marked with asterisks have a constant intensity relative to each other that does not change as the power is increased. At higher power, all other features grow relative to these three; these three have therefore been assigned as the 08 bands of three distinct conformers of the molecule. Their positions are listed in Table I. In an investigation of profyltoluenes, Breen et al. reported that the multiphoton ionization spectrum of the O0 region of n-propylbenzene exhibits two conformers. After examining the multiphoton ionization spectrum of the 0: region of m-n-propyltoluene, Breen et al. assigned the low- and high-frequency features in the spectrum of n-propyltoluene, as resulting from gauche and anti conformations, respectively ( 15). Their empirical force field calculations supported their assignments but did not specifically take into account the CH-a attractive interactions postulated by Hirota et al. ( 16, I 7). However, a set-butylphenol study indicates that conformers in which the alkyl group interacts with the benzene ring are less stable than conformers in which this interaction is minimized ( 1). Based on these studies and microwave studies of phenol which locate the hydroxyl group in the plane of the benzene ring (18), we propose the conformer structures shown in Fig. 3 for 4-propylphenol. The
(a)
CH3
(b)
oNH
H\ 0
$ 0
H
H FIG. 3. Proposed conformer structures for 4-propylphenol:
CH3
H
H
(a) anti conformer and (b)
gauche ‘conformers.
424
MARTINEZ, ALFANO, AND LEVY
anti conformer, corresponding to the anti conformer of n-propylbenzene (15), has its origin at 35 502.5 cm-‘. As this conformer is symmetric with respect to the plane perpendicular to the benzene ring passing through carbon atoms 1 and 4, the two different orientations of the hydroxyl group give rise to indistinguishable geometries and therefore yield only a single origin transition. On the other hand, the gauche conformer, corresponding to the gauche conformer of n-propylbenzene (15), is not symmetric with respect to this plane. Thus, the two different orientations of the hydroxyl group give rise to two distinct conformers. These conformers occur at 35 444.1 cm-’ and 35 455.6 cm-’ and exhibit a splitting of 11.5 cm-‘. Two possible mechanisms were considered to explain the observed splitting in 4propylphenol. The first attributes the splitting to steric and/or electronic interactions between the propyl group and the hydroxyl group; the second attributes the splitting to a dipole-dipole interaction of the hydroxyl group with the propyl group. The molecule 4-propylanisole was chosen for study because it offers a way to determine which of the two mechanisms is likely to be correct. It differs from 4-propylphenol in that it contains a methoxy group rather than a hydroxyl group attached to the benzene ring. The methoxy group is bulkier than the hydroxyl group; one may therefore expect the gauche splitting in 4-propylanisole to be larger than that of 4-propylphenol if the steric and/or electronic interaction mechanism is correct. On the other hand, the methoxy group has a smaller dipole moment than the hydroxyl group. Although only the difference in dipole interactions between the ground and excited states can be measured, one may expect the gauche splitting in 4-propylanisole to be smaller than
3ssoo
35700
35900
36100
(b)
FIG. 4. Fluorescence excitation spectra of 4-propylanisole in the region 35 440 to 36 165 cm-’ taken at different laser power densities. The spectrum in (b) is taken at - 100 times the power density of that in (a). The oven temperature is 33”C, the carrier gas is helium at a hacking pressure of 2.7 atm, and the nozzle diameter is 0.1 mm.
425
SPECTROSCOPY OF 4-ALKYLPHENOLS TABLE I Origin Transitions (in cm-‘)” in the Electronic Spectra of 4-Ethylphenol, 4Propylpheno1, and 4Propylanisole 4-ethylphenol
4.propylphenol
cpropylmisole
35503.0
35444.1
35496.1
35455.6
35537.1
35502.5
35574.7
LITypical uncertainty is +O.S cm-‘.
that of 4-propylphenol if the dipole-dipole mechanism is correct. Figure 4 shows the fluorescence excitation spectrum of 4-propylanisole in the region 35 440 to 36 165 cm-’ taken at low and high power densities. Three distinct conformers of the molecule (marked with asterisks in Fig. 4) were identified and their positions are listed in Table I. The gauche splitting is 41 .O cm-’ which indicates that steric and/ or electronic interactions between the hydroxyl group and the propyl group are the mechanism most probably responsible for the observed splitting in 4-propylphenol. For comparison, the electronic spectra of me&-substituted phenols (m-fluorophenol, m-chlorophenol, and m-cresol) in supersonic free jets exhibited splittings in the range from 100 to 300 cm-’ (19). To summarize, powerdependent fluorescence excitation spectra of 4-ethylphenol revealed one conformer, in agreement with previously reported spectra. Power-dependent fluorescence excitation spectra of Cpropylphenol and 4-propylanisole revealed three conformers in both cases. In these spectra the two lower frequency features are assigned to gauche structural conformers while the third higher frequency feature is assigned to an anti structural conformer. The mechanism responsible for the gauche splitting observed in the 4-propylphenol spectrum is attributed to steric and/or electronic interactions between the propyl group and the hydroxyl group. ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Grant CHE-8311971. S.J.M. acknowledges support from the IMGIP Fellowship Program. J.C.A. acknowledges support from the National Science Foundation Graduate Fellowship Program. RECEIVED:
May 30, 1989 REFERENCES
2. K. SONG AND J. M. HAYES, J. Mol. Spectrosc. 134, 82-97 ( 1989). 2. T. R. RIZZO,Y. D. PARK, L. A. PETEANU,ANDD. H. LEVY,J. Chem. Phys. 84(S), 2534-2541( 1986). 3. 1. WEINRYBAND R. F. STEINER,in “Excited States of Proteins and Nucleic Acids” (R. F. Steiner and I. Weinryb, Eds.), Plenum, New York, 1971. 4. D. CREED,Photochem. Photobiol. 39,563-575 (1984). 5. C. TANFORD, J. D. I-LUJENSTEIN, ANDD. G. RAND& J. Amer. Chem. Sot. 77,6409-6413 ( 1955). 6. H. EDELHOCH, L. BRAND,AND M. WILCHECK, Iv. J. Chem. 1,216-217 ( 1963).
MARTINEZ, ALFANO, AND LEVY
426 7. 8. 9. IO. II.
R. W. COWGILL,B&him. Biophys. Acta 75,272-273 ( 1963). LIANGLI AND DAVID M. LUBMAN,Appl. Spectrosc. 42(3), 418-424 ( 1988). A. ZORAN,Y. SASSON,ANDJ. BLUM,J. Mol. Catal. 26,321-326 ( 1984). W. SHARRN,K. E. JOHNSON,L. WHARTON,AND D. H. LEW, J. Chem. Hp. 71,1292-1299 ( 1979). P. J. BREEN,J. A. WARREN,E. R. BERNSTEIN, AND J. I. SEEMAN,J. Chem. Phys. 87(6), 3269-3275
22. 13. 14. IS.
U. BERG,T. LILJEFORS, C. ROUSSEL,ANDJ. SANDSTROM, Act. Chem. Rex 18,80-86 ( 1985). W. J. E. PARRANDT. SCHAEFER, Act. Chem. Res. 13,400-406 ( 1980). J. KAO, D. LEISTER,AND M. BTO, Tetrahedron Lett. 26, 2403-2406 ( 1985). P. J. BREEN,J. A. WARREN,E. R. BERNSTEIN, AND J. I. SEEMAN,J. Chem. Phys. 87(4), 1927-1935
(1987).
(1987). 16. M. HIROTA,T. SEKIYA,K. ABE, H. TASHIRO,M. KARATSU,M. NISHIO,ANDE. O~AWA, Tetrahedron 39,3091-3099 (1983). 17. M. HIROTA,K. ABE, H. SUEZAWA,AND M. NISHIO,J. Mol. Struct. 126,455-460 ( 1985). 18. T. KOJIMA,J. Phys. Sot. Japan 15,284-287 (1960). 19. A. OIKAWA, H. ABE, N. MIKAMI,AND M. ITO, J. Phys. Chem. 88,5180-5186 (1984).
OF MOLECULAR
SPECTROSCOPY
137,420-426 (1989)
The Electronic Spectroscopy of 4-Ethylphenol, 4-Propylphenol, and 4-Propylanisole in a Supersonic Jet SELSO J. MARTINEZ
III, JOSEPH
C. ALFANO, AND DONALD H. LEVY
The James Franck Instituteand The Departmentof Chemistry, The Universityof Chicago, Chicago, Illinois60637 We have obtained 5uorescence excitation spectra of the origin regions of 4-ethylphenol, 4propylphenol, and 4-propylanisole in a supersonic jet. Power saturation techniques reveal the existence of one, three, and three conformers, respectively. Two of the three origins of 4-propylphenol and 4-propylanisole were interpreted as resulting from a splitting of gauche structural conformers; the third results from an anti structural conformer. The 4-propylanisole spectrum indicates that steric and/or electronic interactions between the propyl group and the hydroxyl group are responsible for the observed splitting in the spectrum of 4-propylphenol. Q 1989Academic Press, Inc.
In a recent investigation, Song and Hayes observed supersonic jet fluorescence excitation and dispersed fluorescence spectra for a series of seven 4-alkyl-substituted phenols ( 1). The spectroscopy of these molecules is of some interest since a 4-alkylsubstituted phenol is the chromophore of the amino acid tyrosine. Thus these molecules are important models which can be used in interpreting the spectrum of tyrosine. In their study, Song and Hayes assigned many vibrational bands and discussed the effect of multiple structural conformers on the spectra of these molecules (I ). Their work was very important in providing an understanding of the spectroscopy of these molecules, particularly in demonstrating the spectral complexity that exists even in a supersonic jet spectrum due to combination of vibrational structure and the existence of several conformers. In their work they were not able to differentiate unambiguously between vibrational structure and structure due to conformers, and in some cases only an upper limit was placed on the possible number of existing conformers. In previous work on the amino acid tryptophan, we used the power saturation behavior of various spectral features to assign these features as either vibronic bands or conformer origins (2). We have recently studied the laser power dependence of the spectra of 4alkylphenols and have definitively identified the origin transitions of the several molecular conformers. This letter is a report of that work. Fluorescence excitation spectra were taken of 4-ethylphenol, 4-propylphenol, and 4-propylanisole at power densities that varied by approximately two orders of magnitude. The power dependence behavior of these spectra revealed that 4-ethylphenol has one conformer, in agreement with the work of Song and Hayes. We identified three conformers for both 4-propylphenol and 4-propylanisole whereas Song and Hayes could only set an upper limit of five on the number of conformers of 4-propylphenol ( I ) . 0022-2852189 $3.00 Copyright 0 kll r&s
1989 by Academic Press, Inc.
of reproduction in any form reserved.
420
SPECTROSCOPY OF 4-ALKYLPHENOLS
421
The importance of identifying the conformers of these molecules becomes apparent when one tries to interpret the electronic spectrum of tyrosine, an amino acid which contributes to the near-ultraviolet absorption and fluorescence of proteins (3-5) and which plays an important role in protein photophysical processes (6, 7). Only by identifying the number of conformers of the 4-substituted phenols and discussing their most reasonable structures can one attempt to understand the more complicated electronic spectrum of tyrosine (8). The compounds 4-ethylphenol and 4-propylphenol were purchased from Aldrich Chemical Company and used without further purification, and the compound 4-propylanisole was synthesized according to the procedure outlined by Zoran, Sasson, and Blum (9). The purity of the sample was confirmed by ‘H NMR measurements. A standard oven source heated to 30-40°C was used to volatilize the samples, and a free jet was produced by passing 3.4 atm of helium over the heated sample reservoir and expanding the mixture through a 0. I mm diameter nozzle. The output of a Nd:YAG laser excited dye laser operating on Rhodamine 590 laser dye was frequently doubled in a KDP crystal, producing about one mJ of ultraviolet light. This was used to excite the samples. Fluorescence was detected at right angles to the jet propagation direction and the laser light path and collected with a fluorescence collection optics system similar to that described in Ref. ( 10). In our power dependence studies a variable attenuator (Newport Corporation) was used to reduce the laser power before the laser beam was directed into the vacuum chamber. When saturation of the spectral transitions was desired, the attenuator was replaced by a 50 cm focal length lens which focused the excitation beam into the vacuum chamber. The fluorescence excitation spectra of 4-ethylphenol in the region 35 45 1 to 35 96 1 cm -’ taken at low and high power densities are shown in Fig. 1. As in previous studies, the power saturation behavior of these spectra is used to identify different molecular conformers (2). Different transitions having a common initial level and different transition strength (e.g., vibronic bands with the zero-point vibrational level of the ground electronic state as the initial level and different vibrational levels of the excited electronic state as the final level) will power saturate at different rates, and their relative intensity will change as a function of laser power. Transitions having different initial levels and the same transition strength (e.g., different conformer origins) will power saturate at the same rate and will have a constant relative intensity that does not depend on laser power. In Fig. 1, all features grow in intensity relative to the feature at 35 503.0 cm-‘, and this feature has been assigned as the 0: band of the single conformer of 4-ethylphenol. The other features in the spectrum are vibrations built on this single conformer and have been previously assigned ( 1) . One should note that the high power density spectrum (Fig. 1b) more closely resembles the 4-ethylphenol spectrum reported by Song and Hayes (1). These spectra further support other investigations which propose that the ethyl group is oriented in a plane perpendicular to the plane of the benzene ring, passing through the 1 and 4 positions of the ring (I, 11-14). The fluorescence excitation spectra of 4-propylphenol in the region 35 405 to 36 155 cm-i taken at low and high power densities is shown in Fig. 2. Once again, the high power density spectrum (Fig. 2b) more closely resembles the 4-propylphenol spectrum
422
MARTINEZ, ALFANO, AND LEVY
(a)
JJ 35500
35700
35600
3S600
35900
CM-’ (b) 1
I
1
35500
I
1
I
35600
35700
35600
35900
CM-’ FIG. 1. Fluorescence excitation spectra of 4-ethylphenol in the region 35 451 to 35 961 cm-r taken at different laser power densities. The spectrum in (b) is taken at - 100 times the power density of that in (a). The oven temperature is 40°C the carrier gas is helium at a backing pressure of 3.4 atm, and the nozzle diameter is 0.1 mm.
* (a)
1,
I
I
a.
,
35700
35500
*a.
I
35900
36100
35900
36100
.
CM-’
* (b)
1..
35500
.
1.
*
35700
I
CM-’ FIG. 2. Fluorescence excitation spectra of 4-propylphenol in the region 35 405 to 35 600 cm-’ taken at different laser power densities. The spectrum in (b) is taken at - 100 times the power density of that in (a). The oven temperature is 42”C, the carrier gas is helium at a backing pressure of 3.4 atm, and the nozzle diameter is 0.05 mm.
SPECTROSCOPY
423
OF 4-ALKYLPHENOLS
reported by Song and Hayes ( 1) . In their investigation, Song and Hayes stated that a definitive assignment of conformers could not be made without additional information but reduced the number of conformers to five, based on the argument that only the bands lower in energy than the most intense band in the region 35 400 to 35 600 cm-’ are considered origins (1). Here we provide definitive evidence for the existence of three conformers. The spectral features marked with asterisks have a constant intensity relative to each other that does not change as the power is increased. At higher power, all other features grow relative to these three; these three have therefore been assigned as the 08 bands of three distinct conformers of the molecule. Their positions are listed in Table I. In an investigation of profyltoluenes, Breen et al. reported that the multiphoton ionization spectrum of the O0 region of n-propylbenzene exhibits two conformers. After examining the multiphoton ionization spectrum of the 0: region of m-n-propyltoluene, Breen et al. assigned the low- and high-frequency features in the spectrum of n-propyltoluene, as resulting from gauche and anti conformations, respectively ( 15). Their empirical force field calculations supported their assignments but did not specifically take into account the CH-a attractive interactions postulated by Hirota et al. ( 16, I 7). However, a set-butylphenol study indicates that conformers in which the alkyl group interacts with the benzene ring are less stable than conformers in which this interaction is minimized ( 1). Based on these studies and microwave studies of phenol which locate the hydroxyl group in the plane of the benzene ring (18), we propose the conformer structures shown in Fig. 3 for 4-propylphenol. The
(a)
CH3
(b)
oNH
H\ 0
$ 0
H
H FIG. 3. Proposed conformer structures for 4-propylphenol:
CH3
H
H
(a) anti conformer and (b)
gauche ‘conformers.
424
MARTINEZ, ALFANO, AND LEVY
anti conformer, corresponding to the anti conformer of n-propylbenzene (15), has its origin at 35 502.5 cm-‘. As this conformer is symmetric with respect to the plane perpendicular to the benzene ring passing through carbon atoms 1 and 4, the two different orientations of the hydroxyl group give rise to indistinguishable geometries and therefore yield only a single origin transition. On the other hand, the gauche conformer, corresponding to the gauche conformer of n-propylbenzene (15), is not symmetric with respect to this plane. Thus, the two different orientations of the hydroxyl group give rise to two distinct conformers. These conformers occur at 35 444.1 cm-’ and 35 455.6 cm-’ and exhibit a splitting of 11.5 cm-‘. Two possible mechanisms were considered to explain the observed splitting in 4propylphenol. The first attributes the splitting to steric and/or electronic interactions between the propyl group and the hydroxyl group; the second attributes the splitting to a dipole-dipole interaction of the hydroxyl group with the propyl group. The molecule 4-propylanisole was chosen for study because it offers a way to determine which of the two mechanisms is likely to be correct. It differs from 4-propylphenol in that it contains a methoxy group rather than a hydroxyl group attached to the benzene ring. The methoxy group is bulkier than the hydroxyl group; one may therefore expect the gauche splitting in 4-propylanisole to be larger than that of 4-propylphenol if the steric and/or electronic interaction mechanism is correct. On the other hand, the methoxy group has a smaller dipole moment than the hydroxyl group. Although only the difference in dipole interactions between the ground and excited states can be measured, one may expect the gauche splitting in 4-propylanisole to be smaller than
3ssoo
35700
35900
36100
(b)
FIG. 4. Fluorescence excitation spectra of 4-propylanisole in the region 35 440 to 36 165 cm-’ taken at different laser power densities. The spectrum in (b) is taken at - 100 times the power density of that in (a). The oven temperature is 33”C, the carrier gas is helium at a hacking pressure of 2.7 atm, and the nozzle diameter is 0.1 mm.
425
SPECTROSCOPY OF 4-ALKYLPHENOLS TABLE I Origin Transitions (in cm-‘)” in the Electronic Spectra of 4-Ethylphenol, 4Propylpheno1, and 4Propylanisole 4-ethylphenol
4.propylphenol
cpropylmisole
35503.0
35444.1
35496.1
35455.6
35537.1
35502.5
35574.7
LITypical uncertainty is +O.S cm-‘.
that of 4-propylphenol if the dipole-dipole mechanism is correct. Figure 4 shows the fluorescence excitation spectrum of 4-propylanisole in the region 35 440 to 36 165 cm-’ taken at low and high power densities. Three distinct conformers of the molecule (marked with asterisks in Fig. 4) were identified and their positions are listed in Table I. The gauche splitting is 41 .O cm-’ which indicates that steric and/ or electronic interactions between the hydroxyl group and the propyl group are the mechanism most probably responsible for the observed splitting in 4-propylphenol. For comparison, the electronic spectra of me&-substituted phenols (m-fluorophenol, m-chlorophenol, and m-cresol) in supersonic free jets exhibited splittings in the range from 100 to 300 cm-’ (19). To summarize, powerdependent fluorescence excitation spectra of 4-ethylphenol revealed one conformer, in agreement with previously reported spectra. Power-dependent fluorescence excitation spectra of Cpropylphenol and 4-propylanisole revealed three conformers in both cases. In these spectra the two lower frequency features are assigned to gauche structural conformers while the third higher frequency feature is assigned to an anti structural conformer. The mechanism responsible for the gauche splitting observed in the 4-propylphenol spectrum is attributed to steric and/or electronic interactions between the propyl group and the hydroxyl group. ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Grant CHE-8311971. S.J.M. acknowledges support from the IMGIP Fellowship Program. J.C.A. acknowledges support from the National Science Foundation Graduate Fellowship Program. RECEIVED:
May 30, 1989 REFERENCES
2. K. SONG AND J. M. HAYES, J. Mol. Spectrosc. 134, 82-97 ( 1989). 2. T. R. RIZZO,Y. D. PARK, L. A. PETEANU,ANDD. H. LEVY,J. Chem. Phys. 84(S), 2534-2541( 1986). 3. 1. WEINRYBAND R. F. STEINER,in “Excited States of Proteins and Nucleic Acids” (R. F. Steiner and I. Weinryb, Eds.), Plenum, New York, 1971. 4. D. CREED,Photochem. Photobiol. 39,563-575 (1984). 5. C. TANFORD, J. D. I-LUJENSTEIN, ANDD. G. RAND& J. Amer. Chem. Sot. 77,6409-6413 ( 1955). 6. H. EDELHOCH, L. BRAND,AND M. WILCHECK, Iv. J. Chem. 1,216-217 ( 1963).
MARTINEZ, ALFANO, AND LEVY
426 7. 8. 9. IO. II.
R. W. COWGILL,B&him. Biophys. Acta 75,272-273 ( 1963). LIANGLI AND DAVID M. LUBMAN,Appl. Spectrosc. 42(3), 418-424 ( 1988). A. ZORAN,Y. SASSON,ANDJ. BLUM,J. Mol. Catal. 26,321-326 ( 1984). W. SHARRN,K. E. JOHNSON,L. WHARTON,AND D. H. LEW, J. Chem. Hp. 71,1292-1299 ( 1979). P. J. BREEN,J. A. WARREN,E. R. BERNSTEIN, AND J. I. SEEMAN,J. Chem. Phys. 87(6), 3269-3275
22. 13. 14. IS.
U. BERG,T. LILJEFORS, C. ROUSSEL,ANDJ. SANDSTROM, Act. Chem. Rex 18,80-86 ( 1985). W. J. E. PARRANDT. SCHAEFER, Act. Chem. Res. 13,400-406 ( 1980). J. KAO, D. LEISTER,AND M. BTO, Tetrahedron Lett. 26, 2403-2406 ( 1985). P. J. BREEN,J. A. WARREN,E. R. BERNSTEIN, AND J. I. SEEMAN,J. Chem. Phys. 87(4), 1927-1935
(1987).
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