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Vibrational Spectroscopy of Perfluorocarboxylic Acids from the Infrared to the Visible Regions† Nabilah Rontu and Veronica Vaida* Department of Chemistry and Biochemistry and CIRES, Campus Box 215, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: June 26, 2007; In Final Form: August 29, 2007
We report here the gas-phase mid-IR, near-IR, and visible vibrational spectra of perfluoropropionic, perfluorooctanoic, and perfluorononanoic acids using Fourier transform and cavity ring-down spectroscopy. The impetus for this work was to provide insight into the behavior of environmentally persistent perfluorocarboxylic acids. Although the most intense transition for all three perfluorocarboxylic acids in the fundamental spectra is the CF3 stretch, the O-H stretch carries the most intensity in the overtone region. We record the O-H stretching transition frequencies and absorption cross-sections in the ∆VOH ) 1-5 regions. Our work suggests that perfluorocarboxylic acids have more harmonic O-H bonds, smaller transition frequencies, and smaller intensities as compared to shorter-chain hydrocarbon acids, alcohols, and peroxides.
Introduction Vibrational overtone spectra are sensitive to bond properties and have been used to study molecular conformation and intramolecular energy flow. Interest in nonstatistical energy flow1-4 led to experimental and theoretical studies using activation through vibrational overtone pumping.5-12 Hynes and his collaborators investigated intramolecular vibrational energy transfer from overtone excited CH (benzene)13 and OH (H2O and H2O2),14,15 contributing to understanding the role of vibrational overtone transitions in spectroscopy and chemical dynamics. The double harmonic approximation has been used successfully to treat vibrational spectra in the fundamental region.16 In this approximation, the potential energy surface is expanded to the second-order term leading to the normal mode picture, and the dipole moment function is considered to be linear. Consequently, the observations of vibrational overtones and combination bands cannot be understood using this approximation. For polyatomic molecules containing relatively high-frequency X-H (X ) O,C,N) vibrations, local mode descriptions have been used effectively to assign spectra of high-energy transitions.17-20 In the local mode view, vibrations become less harmonic as the vibrational energy increases and the vibrational pattern is more localized. As the internal energy of a molecule increases, the anharmonicity of the potential energy surface becomes important and the normal mode description of molecular vibrations becomes invalid. Furthermore, in the transition of high-energy X-H stretching states, the nonlinear terms of the dipole moment function contribute to the transition moment. Higher vibrational levels can be excited directly resulting in weak vibrational overtone spectra. Spectroscopic investigations of vibrational overtones are a prelude to the study of energy flow and reaction dynamics. Recently, renewed interest in studies of vibrational overtone spectra and photochemistry followed the realization that such processes are important in atmospheric radiative transfer21-24 and atmospheric chemistry.25-32 †
Part of the “James T. (Casey) Hynes Festschrift”. * Corresponding author. Tel.: (303) 492-8605. Fax: (303) 492-5894. E-mail:
[email protected].
The O-H component of several long-chained perfluorocarboxylic acids is the topic of investigation in this report. Vibrational spectra of perfluorocarboxylic acids of varying carbon chain lengths are examined to obtain properties of fundamental and overtone transitions. Intensities, frequencies, and anharmonicities of vibrational overtones of the OH and CF transitions are evaluated in this series of compounds and compared to their hydrocarbon analogues. Perfluorocarboxylates and acids, especially perfluorooctanoic acid (CF3(CF2)6COOH, denoted PFOA), have been under scrutiny from the EPA since the late 1990s for concerns of their widespread distribution, persistence, and bioaccumulation.33 They have a variety of uses; for example, perfluorocarboxylic acids or their salts are used in the synthesis of fluoropolymers,34 small chain perfluoroalkyl sulfonates act as surface protectors in carpets, leather, paper, packaging, and upholstery,35 and fluorotelomer alcohols and olefins are intermediates in products such as surfactants and polymers.36 Some perfluorocarboxylic acids have been recently detected in ecosystems.37 For example, rainwater samples from Sweden and Finland measured between 11 and 17 ng/L of PFOA.38 Other perfluorinated compounds are even observed in the Arctic food chain,39,40 detected in human blood,41-43 and the tissues of animals.44,45 In the present work, perfluoropropionic, perfluorooctanoic, and perfluorononanoic acids are used to understand the O-H vibrational overtone spectroscopy to provide insight into the properties of environmentally persistent perfluorocarboxylic acids. We present here the preliminary data needed to determine if these compounds have the potential to undergo sunlight-initiated photochemistry via vibrational overtone-pumping. Experimental Methods All acids were used as received without further purification. Perfluoropropionic acid [denoted PFPA] (pentafluoropropionic acid, 97%), perfluorooctanoic acid [denoted PFOA] (pentadecafluorooctanoic acid, 96%), and perfluorononanoic acid [denoted PFNA] (heptadecafluorononanoic acid, 97%) were purchased from Aldrich Chemical Co., Inc. (Milwaukee, WI).
10.1021/jp0749773 CCC: $40.75 © 2008 American Chemical Society Published on Web 10/10/2007
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Figure 1. IR spectra of PFOA and PFNA between 1000 and 7100 cm-1 at 318 K.
Fourier Transform Spectroscopy. The absorption spectra of the acids were measured in the mid-IR and near-IR from 1000 to 11 000 cm-1 using a Bruker IFS 66v/s Fourier transform spectrometer. Measurements made in the mid-IR used a silicon carbide light source, KBr beamsplitter, and a MCT liquid nitrogen cooled detector. The near-IR measurements consisted of using a tungsten light source, CaF2 beamsplitter, and an InGaAs detector. For both sets of experiments, the instrument was used in the external mode with the light source directed onto a detector with a parabolic mirror. The experiments coadded 1000 scans at 0.5 cm-1 resolution. The sample chamber, previously described,46,47 had a path length of 75 cm and was wrapped with heating tape and insulated for temperature control. A bubbler connected to the cell through Teflon tubing housed the perfluorinated acids. Samples were placed in a temperature-controlled water bath, and the vapor above was flowed into the cell using nitrogen as a carrier gas at 1 standard liters per minute (SLPM). Spectra were collected when the temperature of the sample was equal to the cell temperature. Unless noted otherwise, all experiments were conducted at 318 K and at atmospheric pressure. Cavity Ring-Down Spectroscopy. The experimental setup and methods for the cavity ring-down spectroscopy used in these studies have been previously described.48-50 Briefly, acid samples were placed inside a 77 cm heated glass cell connected to two stainless steel mirror mounts. The cell was wrapped with heating tape for temperature control. Condensation of sample on the mirrors was prevented by introducing a small purge volume of helium in front of the mirrors at a constant flow rate of 1 SLPM through 1/4 in. diameter ports. This flow was also used as the main flush gas through the sample volume. All experiments were performed at constant flow rates at atmospheric pressure. Two sets of high-reflectivity mirrors were connected to the cell depending on the wavelength region. The mirrors used for
observation of the third O-H stretching overtone [∆V ) 4] (R > 99.997%) had a 1 m radius of curvature and gave a maximum observable ring-down time constant of 176 µs at 756 nm. For ∆V ) 4, spectra were collected at 318 ( 1 K. Observations of the fourth O-H stretching overtone [∆V ) 5] were comprised of mirrors (R > 99.993%), which had a 6 m radius of curvature and gave a maximum observable ring-down time constant of 100 µs at 620 nm. For best signal-to-noise in this region, spectra of ∆V ) 5 were collected at 393 ( 1 K. A Northern Lights tunable dye laser (NL-5-2-MF6) pumped by a Big Sky frequency doubled Nd:YAG laser was used as the light source for the absorption experiments. LDS 751 was used to observe the ∆V ) 4 transition region (20.3 mW at 750 nm, 3 × 10-4 M).51 For the ∆V ) 5 transition region, Sulforhodamine 640 was used (23.2 mW at 615 nm, 2 × 10-4 M).51 The dye laser output was directed through an isolator composed of a polarizer (Newport, 10GLO8AR.14) and waveplate (Newport, 05RP). To collimate the beam to a spot size similar to the low-order transverse electric modes of the optical resonator, the beam passed through a 50 cm focal length lens. The beam was then redirected toward two turning mirrors before entering the cell. Upon exiting the cell, the residual beam was directed onto a turning mirror and through a negative lens of focal length of 50 cm, where it was expanded onto a commercial PMT (Hamamatsu, R943-02). The PMT was interfaced to a commercially available data acquisition card (Gage Compuscope 1250). The card sampled at 10 MHz for all experiments. In both regions of interest, 10 ring-downs were averaged per wavelength interval at a resolution of 0.01 nm. Results IR Spectra: FTS Experiments. Gas-phase spectra of PFOA and PFNA between 1000 and 7100 cm-1 at 318 K are shown in Figure 1. As illustrated in the figure, the spectrum of PFOA
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Figure 2. The fundamental (a) and first (b) O-H stretching overtones of PFOA and PFNA at 318 K.
has a higher measurable absorbance than that of PFNA at this temperature as a result of its higher vapor pressure (approximately by a factor of 2).52 The stretching modes of these molecules dominate the fundamental spectra. Both spectra show that the CF3 stretch is the most intense mode, centered at approximately 1251 cm-1. This is very similar to PFPA, which has its most intense transition centered at approximately 1232 cm-1 attributed to the CF3 stretch.47 Like PFPA, the C-F, C-C, and C-O skeletal modes of PFOA and PFNA appear between 1000 and 1400 cm-1 as shown in the inset; however, because of limited available data on the infrared vibrational spectra of PFOA and PFNA, we were unable to assign vibrational modes to every observed transition in this region. The CdO stretch is the next most intense mode centered approximately at 1818 cm-1 for all three perfluorocarboxylic acids. In the fundamental spectrum, the highest energy transition for all acids studied here is the O-H stretch, which occurs centered at around 3575 cm-1 and is less intense than the CdO stretch. The high-energy inset in Figure 1 shows the first O-H stretching overtones of PFOA and PFNA. It is well known that intensities of X-H stretching overtones decrease by approximately an order of magnitude with increasing quantum numbers.5 Figure 2 shows the intensity trends of the fundamental and first overtones of the O-H stretch for PFOA and PFNA. While Figure 2a is an enlargement of Figure 1 in the regions between 3510 and 3670 cm-1, Figure 2b was collected under different conditions using the near-IR setup as described in the Experimental Methods, hence, the signal-to-noise ratio of the first O-H stretching overtones in Figure 2b higher than that in Figure 1. The intensity drops by about a factor of 20 from the fundamental O-H stretch to the first O-H stretching overtone for PFOA. Similarly, for PFNA, the intensity drops by about a factor of 25 from the fundamental O-H stretch to the first O-H stretching overtone. In a previous study,47 PFPA at 363 K showed a drop in intensity by a factor of 20 from the fundamental to the first O-H stretching overtone. The second O-H stretching overtone was not observed for either PFOA or PFNA, but dropped by a factor of 50 from the first O-H stretching overtone to the second O-H stretching overtone for PFPA at 363 K.47 It is interesting to note that, although the CF3 stretch carries the most intensity for all three of the perfluorocarboxylic acids reported here, its overtone was not observed. Rontu et al.47 provided an upper limit for the drop in intensity from the fundamental to the first C-F stretching overtone for PFPA and obtained a factor of 1.5 × 104. This
phenomenon, as explained later, could be caused by bond anharmonicity and the dipole moment function. Vapor pressures of PFOA and PFNA and the constants of their Antoine equation have been previously reported between 332-463 and 372-476 K, respectively.52 Because our experiments were carried out under flow conditions and at lower temperatures, calibration of vapor pressure data was necessary. This was done by first collecting spectra of both PFOA and PFNA under static conditions at 353 and 373 K, respectively. From these data, the integrated cross-sections of the fundamental O-H stretches were determined using Beer’s law, and the number densities at the lower temperatures were back-calculated. The number densities were then used to determine the integrated cross-sections of the first O-H stretching overtones of PFOA and PFNA. At 318 K, number density of PFOA was determined to be 1.98((0.4) × 1017 molec/cm3 and 3.19((0.5) × 1017 molec/cm3 for PFNA. A similar method was used to determine the number densities and O-H cross-sections for PFPA.47 Table 1 summarizes the experimental frequencies, bandwidths (full width at half-maximum, fwhm), and the relative intensities of the various O-H stretches of PFPA, PFOA, and PFNA. For all three acids, the fundamental and first O-H stretching overtone frequencies are almost identical. The fundamental integrated O-H stretching absorption cross-sections determined from our experiments for PFPA, PFOA, and PFNA are (1.09 ( 1.2) × 10-21 cm2 molecule-1 cm-1,47 (3.84 ( 0.03) × 10-20 cm2 molecule-1 cm-1, and (3.19 ( 0.15) × 10-21 cm2 molecule-1 cm-1, respectively. Values for the integrated first O-H stretching overtone absorption cross-sections for PFPA, PFOA, and PFNA were determined to be (5.17 ( 2.1) × 10-23 cm2 molecule-1 cm-1, (2.06 ( 0.05) × 10-21 cm2 molecule-1 cm-1, and (1.31 ( 0.11) × 10-22 cm2 molecule-1 cm-1, respectively. PFPA has much smaller O-H cross-sections than both PFOA and PFNA. We believe the discrepancy with PFPA is primarily a result of inaccurate literature vapor pressure. The vapor pressure data for PFOA and PFNA come from much more reliable methods and source52 than PFPA.53 In comparison, PFOA has larger cross-sections than PFNA, and our data suggest that lengthening the carbon chain length decreases the absorption cross-section, but further theoretical calculations are needed to determine the accuracy of this trend. However, it should be noted that the largest source of error in the calculations for all acids studied is due to uncertainties in the number density of molecules under investigation.
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Figure 3. The third O-H stretching overtone of PFOA and PFNA at 318 K.
TABLE 1: Experimental Transition Frequencies, Bandwidths, and Relative Intensities for the O-H Stretches of PFPA, PFOA, and PFNA experimental frequency (cm-1)
experimental fwhm (cm-1)
relative intensity
fundamental OH stretch first OH-stretch overtone second OH-stretch overtone third OH-stretch overtone fourth OH-stretch overtone
Perfluoropropionic Acid (PFPA) 3574 ( 4a 8 ( 4b 6986 ( 1a 19 ( 1b 10 233 ( 1a 23 ( 1b 13 225.0 ( 0.7 16 273.2 ( 0.6
fundamental OH stretch first OH-stretch overtone third OH-stretch overtone fourth OH-stretch overtone
Perflourooctanoic Acid (PFOA) 3574.7 ( 0.5 8.1 ( 0.5 6984.8 ( 0.5 10.1 ( 0.5 13 320.9 ( 0.3 17.1 ( 0.2 16 258.9 ( 0.5
1 0.05 ( 0.03 0.00037 ( 0.00001
fundamental OH stretch first OH-stretch overtone third OH-stretch overtone fourth OH-stretch overtone
Perfluorononanoic Acid (PFNA) 3574.9 ( 0.5 7.8 ( 0.6 6985.2 ( 0.6 9.1 ( 0.8 13 320.7 ( 0.4 17.2 ( 0.1 16 259.5 ( 0.4
1 0.04 ( 0.03 0.00029 ( 0.00004
1b 0.05 ( 0.03b 0.0009 ( 0.0005b
a Values reported from: Rontu, N.; Vaida, V. J. Mol. Spectrosc. 2006, 237, 19-26. b Values calculated from: Rontu, N.; Vaida, V. J. Mol. Spectrosc. 2006, 237, 19-26.
Near-IR and Visible Spectra: CRD Experiments. Examination of the third and fourth O-H stretching overtones for PFPA, PFOA, and PFNA utilized the cavity ring-down spectrometer. Table 1 summarizes the frequencies of PFPA and the frequencies and relative intensities for the third O-H stretching overtones of PFOA and PFNA. Unlike the frequencies of the fundamental and first O-H stretching overtones of the acids, the third O-H stretching overtone of PFPA is slightly higher in energy centered at approximately 13 325 cm-1 as compared to PFOA and PFNA occurring at approximately 13 320 cm-1. The third O-H stretching overtones of PFOA and PFNA at 318 K are shown in Figure 3. PFOA with twice the vapor pressure of PFNA gives rise to a higher measurable absorbance, by approximately a factor of 15. We were able to calculate the absorption cross-sections for the third O-H stretching overtones for PFOA and PFNA, but not for PFPA. The reasons for this are 2-fold. First, as previously mentioned, the literature vapor pressure data for PFOA and PFNA are available in different temperature ranges and are more accurate than for PFPA. The
number density is an essential component in determining the absorption cross-section, and, for PFPA, the available literature data are not sufficient. Second, the concentrations of PFOA and the PFNA remained fairly constant over the course of these experiments. Minor loss in signal was compensated by adding more sample between collection of every five spectra. The glass cells for the FTS and CRD experiments are comparable in length, and because spectra of PFOA and PFNA were collected at the same temperature and flow rates for both sets of experiments, we assumed that the number densities remained constant and were able to determine the absorption crosssections for PFOA and PFNA. PFPA, on the other hand, has a much higher vapor pressure than both PFPA and PFNA, and, even at lower flow rates, its concentration fluctuated significantly, precluding a determination of the absorption crosssection. For PFOA, the integrated third O-H stretching overtone absorption cross-section was found to be (1.43 ( 0.08) × 10-23 cm2 molecule-1 cm-1 and (9.33 ( 0.22) × 10-25 cm2 molecule-1 cm-1 for PFNA.
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Figure 4. Birge-Sponer plot for the O-H stretches in PFPA, PFOA, and PFNA.
TABLE 2: Calculated Anharmonicities and Harmonic Frequencies of PFPA, PFOA, and PFNA
perfluoropropionic acid (PFPA) perfluorooctanoic acid (PFOA) perfluorononanoic acid (PFNA)
anharmonic constant (ωeχe, in cm-1)
harmonic frequency (ωe, in cm-1)
-80.21 -80.81 -80.84
3733.8 3735.6 3735.8
The fourth O-H stretching overtones for PFPA, PFOA, and PFNA were observed at 393 K for best signal-to-noise considerations. Spectra collected below this temperature did not have sufficient intensity for either PFOA or PFNA to be observed in this study. Because of the elevated temperature, the concentrations of all three perfluorocarboxylic acids considerably fluctuated, hindering the determination of an integrated cross-section and transition bandwidth; thus, we do not show the fourth O-H stretching overtones in a figure. The last row for each of the acids in Table 1 shows the experimental transition frequencies. Similar to the third O-H stretching overtone transition, PFPA is again slightly higher in energy than PFOA and PFNA for the fourth O-H stretching overtone. We were unable to determine the absorption cross-sections for any of the perfluorocarboxylic acids, and thus a relative intensity, as a result of the uncertainty in the number density. Although the vapor pressure data of PFOA and PFNA at 393 K are available, the inconsistency in the sample concentrations during the experiment prevented the determination of accurate number densities. More importantly, both PFOA and PFNA are solids at room temperature with melting points at approximately 328 and 332 K, respectively. Upon placing the acids in the cell, the sample immediately melted and caused a loss in signal due to aerosol formation, which scattered the laser beam. Even though low flow rates were used, regaining signal took approximately 5 min during the course of which a significant amount of sample was lost, resulting in unstable sample concentrations. Discussion Overtone intensities are governed by vibrational wavefunctions and the dipole moment function.54,55 In the ground electronic state, the mechanical and electrical anharmonicity gives rise to vibrational overtone transitions. Absorption intensities are proportional to the square of the transition dipole moment matrix, expressed by
Iabs ∝ |〈Ψ′′ν′′|µ(r-re) |Ψ′ν′〉|2
where Ψ′′ν′′ and Ψ′ν′ are the ground- and excited-state vibrational wave functions, respectively, and µ(r-re) is the dipole moment as a function of the internuclear distance. Because of the electrical anharmonicity, the variation of the dipole moment with the internuclear distance is not strictly linear.55 Consequentially, the electrical anharmonicity will cause the appearance of overtones even if the mechanical oscillators can be approximated to be harmonic. In these cases, the second-order term of the dipole moment function is responsible for the transition to the first overtone. Therefore, for better approximations, the cubic and higher terms in r-re in the expansion of the dipole moment function must be included because the dipole moment function is not perfectly linear. Thus, the vibrational anharmonicity and the dipole moment function determine the intensity of an overtone transition. Generally, X-H oscillator (X ) O, C, N) overtones dominate the overtone spectra due to their high anharmonicity.56 In the fundamental spectra of PFPA, PFOA, and PFNA, the C-F stretching transitions are more intense than the O-H stretch. However, only overtones for the O-H stretch were observed in this study. For the C-F bond, the fundamental and first overtone stretch occur at a low-energy region of the potential well where the harmonic oscillator approximation is valid, indicating that their intensities follow the rules of a harmonic vibration. Hence, the great decrease in the C-F intensity in going from the fundamental to the first overtone stretch is due to the large difference in the absolute ratio between the linear term coefficient and the second-order coefficient in the dipole moment for this bond. The low overtone intensity probably signifies the small second-order term of the dipole moment function; however, theoretical calculations need to be performed to fully clarify this. Hence, in comparison to the O-H bond, the C-F bond of the three acids studied here behaves more harmonically. For PFPA, PFOA, and PFNA, the O-H bond is the most anharmonic bond in the molecule. Previous work by Rontu et al.47 reported an anharmonic constant of ωeχe ≈ -82 cm-1 for PFPA, but these values were only based on three (ν1, 2ν1, and 3ν1) transitions. A Birge-Sponer plot, shown in Figure 4, was constructed from the experimental frequencies of PFPA, PFOA, and PFNA from this study. Table 2 summarizes the anharmonic constants (ωeχe) and harmonic frequencies (ωe) for all three perfluorocarboxylic acids. Morse potentials of O-H oscillators are anharmonic with typical values of ωeχe between -80 and -85 cm-1. As shown in the figure and the table, the three compounds have comparable values of anharmonic constants (∼ -80 cm-1) and harmonic frequencies (∼3735 cm-1).
Vibrational Spectroscopy of Perfluorocarboxylic Acids As compared to their hydrocarbon analogues, the fundamental O-H stretches of the perfluorocarboxylic acids have a slightly lower vibrational frequency by approximately 5 cm-1. The fundamental O-H transition is centered at 3581.2 cm-1 for propionic acid,57 3578.5 cm-1 for octanoic acid,46 and 3578.2 cm-1 for nonanoic acid.46 Havey et al.57 reported an anharmonic constant for propionic acid as approximately -85 cm-1, which is slightly higher than its fluorinated analogue acid. A recent study suggests that this characteristic is due to the strong electronegative nature of the fluorine atom.50 Similarly, our results show that the O-H stretch of perfluorocarboxylic acids studied here are more harmonic than their hydrocarbon analogs. The perfluorocarboxylic acids studied here were found to have smaller O-H absorption cross-sections than other small carbonchain acids, alcohols, and peroxides. A plot of log(intensity) versus upper vibrational quantum number showed that the perfluorocarboxylic acids have smaller intensities as compared to shorter-chain hydrocarbon acids, alcohols, and peroxides by approximately 2 orders of magnitude.58 However, the intensities of the perfluorocarboxylic acids investigated follow the same trend as their hydrocarbon analogues in losing about an order of magnitude in successive overtones. Furthermore, because lengthening the C-F chain did not change the peak positions of O-H stretching overtones, the reported trend suggests that the O-H stretching transitions of perfluorocarboxylic acids like their hydrocarbon analogues can be treated using local mode models.58 Conclusion We report the gas-phase spectra and the O-H absorption cross-sections of PFPA, PFOA, and PFNA in the mid-IR and near-IR using both Fourier transform and cavity ring-down spectroscopy. The emphasis of the work was on the O-H stretching overtones of the molecules. For PFPA, ∆VOH ) 1-5 transitions were observed, whereas for PFPA and PFNA, we only detected the ∆VOH ) 1, 2, 4, and 5 transitions. Although the CF3 stretch is the most intense stretching mode in the fundamental IR spectra for all of the perfluorocarboxylic acids studied, we were unable to observe overtones of this vibrational mode. For the O-H stretches, the three acids have comparable transition frequencies and anharmonicities, which are slightly smaller in value as compared to their analogue hydrocarbon acids. Vibrational overtone transitions were investigated in this study because they can be excited in the atmosphere by solar radiation.26 Interest in studying perfluorocarboxylic acids is motivated by their uses in various products, widespread atmospheric distribution, and environmental persistence. The spectroscopic data obtained in this study serve as the basis in understanding the proper description of overtones, energy, and energy flow in these molecules. Future work includes studying the overtone-induced photochemistry and reaction dynamics for this class of compounds. Acknowledgment. We thank K. J. Feierabend and K. Takahashi for helpful discussions. N.R. thanks DuPont for funding. V.V. thanks NSF for partial funding of this work. References and Notes (1) Hase, W. L. In Dynamics of Molecular Collisions; Miller, W. H., Ed.; Plenum Press: New York, 1976; Part B. (2) Oref, I.; Rabinovitch, B. S. Acc. Chem. Res. 1979, 12, 166-175. (3) Rice, S. A. In Excited States; Lim, E. C., Ed.; Academic Press: New York, 1975; p 11, Vol. 2.
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