An investigation of hydrogen bonding in amides using Raman

Dec 4, 1991 - (49) Doukas, A. G.; Aton, B.; Callender, R. H.; Honig, B. Chem. Phys. Lett. 1978, 56, 248-252. (50) Little, R. G.; Dymock, K. R.; Ibers,...
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(44) Lorenzo, C. F.; Alcantara, R.; Martin, J. J. Romon Spectrosc. 1989, 20, 29 1-296.

(45) Dudik, J. M.; Johnson, C. R.; Asher, S . A. J . Chem. Phys. 1985,82, 1732-1 740. (46) Myers, A. B.; Pranata, K. S . J . Phys. Chem. 1989, 93, 5079-5087. (47) Udagawa, Y.; Iijima, M.; Ito, M. J . Raman Spectrosc. 1974, 2, 313-3 15. (48) Inagaki, F.; Tasumi, M.; Miyazawa, T. J . Mol. Spectrosc. 1974,50, 286-303.

(49) Doukas, A. G.;Aton, B.; Callender, R. H.; Honig, B. Chem. Phys. Lett. 1978, 56, 248-252. ( 5 0 ) Little, R. G.; Dymock, K. R.; Ibers, J. A. J. Am. Chem. SOC.1975, 97, 4532-4539. (51) Tang, S. C.; Koch, S.;Papaefthymiou, G. C.; Foner, S.; Frankel. R. B.; Ibers, J. A.; Holm, R. H. J. Am. Chem. SOC.1976, 98, 2414-2434. (52) Caughey, W. S.; Ibers, J. A. J. Am. Chem. Soc. 1977,99,6639-6645. (53) Stryer, L. Biochemistry, 3rd ed.; W. H. Freeman and Co.: New York, 1988; pp 404, 595.

An Investigation of Hydrogen Bonding In Amides Using Raman Spectroscopy Nancy E. Triggs and James J. Valentini* Department of Chemistry, Columbia University, New York,New York 10027 (Received: December 4, 1991; In Final Form: May 1 1 , 1992)

Raman and preresonant Raman spectra are reported for c-caprolactam and N,N-dimethylacetamide in both the gas phase and the neat liquid and as a function of concentration in aqueous and other solutions. These spectra are analyzed to determine the influence of hydrogen bonding, particularly amide-amide hydrogen bonding, on amide structure and spectroscopy. A shift in intensity from the Am I (carbonyl stretch) band to the Am I1 band (C-N stretch) is observed as the extent of intermolecular amide-amide and amide-water hydrogen bonding increases. For c-caprolactam, which can hydrogen bond to itself, a substantial shift in intensity in these amide bands occurs between the gas and the neat liquid. The formation of hydrogen-bonded complexes, for which there is a clear spectral signature in the Raman spectrum, is indicated. In contrast, for N,N-dimethylacetamidethe Am I to Am I1 intensity shift is seen only upon aqueous solvation and is directly proportional to the mole fraction of water present. Shifts in the Am I vibration to lower frequency are also observed upon solvation for both c-caprolactam and N,N-dimethylacetamide whether or not hydrogen bonding is present. However, the magnitudes of these shifts increase with the extent of hydrogen bonding. The Am I carbonyl band in neat c-caprolactamliquid and in acetonitrile solution consists of two peaks, one of which we assign to the unassociated monomer and the other to the cyclic dimer. In aqueous solution the carbonyl band of N,N-dimethylacetamide also consists of two peaks, which appear to be associated with a free and a hydrogen-bonded form.

Introduction As the repeat unit in both biologically important macromolecules and industrially important polymeric materials, the amide functional group has long been of practical importance and fundamental interest. In an attempt to provide a better understanding of amide macromolecular systems, many studies of small, isolated amides have been reported. Indeed, a variety of techniques such as IR, NMR, Raman, ultrasonic absorption, and UV/vis spectroscopy have been used to characterize both the intermolecular and intramolecular bonding in amide compounds. From a biological standpoint, aqueous solution studies are desirable. In dilute aqueous solution, amide groups are only weakly interacting and amide-water interactions dominate.' However in polyamide materials, amide-amide interactions are substantial, and it is the influence of these forces on the bonding, structure, and dynamics of polyamides that interest us. In the work described here we seek to identify Raman spectral signatures of amide-amide hydrogen bonds and exploit these to investigate hydrogen bonding in cis and trans amide conformers. Our approach is to examine the effects of phase and solvent on the Raman spectra of two prototypical amide compounds, one which can hydrogen bond to itself, c-caprolactam, and one which cannot, N,N-dimethylacetamide. While many of our spectra are recorded upon excitation at optical frequencies far below the first electronic absorption in the amides, we do make use of preresonant enhancement of amide group vibrational modes to identify them among other modes in the spectra. Preresonant enhancement occurs in the Raman spectra that we have obtained at UV wavelengthsnear the very strong II II* transition characteristic of all amides.24 None of the spectra we report here are recorded at Raman excitation wavelengths actually within the II II* absorption band and so are not actually resonant Raman spectra. Nonetheless, our approach is similar to the very successful use

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of resonance Raman spectroscopy as a conformation-sensitive probe of the nature of the ground and excited electronic states of many small peptides and protein^.^-'^ An important consideration in our study is the type of hydrogen bond that the amides form. In N-monosubstituted amides the carbonyl oxygen and the amidic hydrogen predominantly exist in the trans conformation.' As a consequence, if the amidic proton in molecule A hydrogen bonds to the carbonyl of nearest-neighbor molecule B, the carbonyl of A can hydrogen bond only to a different neighbor, C, leading to the formation of long-chain oligomers of varying lengths. The presence of a variety of sizes and geometries of oligomers makes it difficult to associate specific spectral features with particular species and similarly difficult to interpret spectral signatures of hydrogen bonding. In contrast, cyclic aliphatic lactams like c-caprolactam that have ring size less than eight are found only in the cis conformation, due to ring strain, and in nonpolar solvents these lactams readily form hydrogen-bonded cyclic dimers.Ibl8 Thus,they offer the possibility of examining the role of amide-amide hydrogen-bonding interactions under conditions for which the hydrogen-bonded species are unique in chemical makeup and of well-defined geometry. In this paper we report Raman spectra of emprolactam in both the gas and liquid phase. The liquid-phase spectra are reported for neat caprolactam and solutions of caprolactam in various solvents. By monitoring both changes in band position and relative intensity with phase and solvent, we determine how interamide forces are revealed in the Raman spectrum. To separate the effects of interamide hydrogen bonding from the effects of simple solvation, we compare our caprolactam results with those for N,Ndimethylacetamide (DMA). Because it lacks an amidic hydrogen, DMA cannot hydrogen bond to itself, although it can and does hydrogen bond in aqueous solution. To determine how these effects vary with Raman excitation wavelength, we record Raman

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Hydrogen Bonding in Amides

The Journal of Physical Chemistry, Vol. 96, No. 17. 1992 6923

spectra at 532,355, and 266 nm. At 266 nm the spectra should show and indeed do show the beginnings of resonance enhancement due to the amide II XI* transition. Our caprolactam and DMA results show distinct differences as well as some marked similarities, and the comparison of the two clarifies the difference between general solvation effects and hydrogen-bonding effects. Solvent hydrogen bonding has recently become of considerable interest in far-UV, true resonance Raman spectral studies of another prototypical amide, N-methylacetamide (NMA).12,15To connect our investigations with these studies, we have taken a few preresonant and nonresonant Raman spectra of NMA, and we discuss our findings about hydrogen bonding in caprolactam and DMA with particular reference to these pioneering studies of NMA. Our experimentson caprolactam and DMA nicely complement the resonance Raman investigations of NMA. Although our results and interpretation are for the most part consistent with the results and interpretation reported in the resonance Raman NMA studies, the conclusions we draw about the effects of hydrogen bonding and general solvation are not entirely in accord with conclusions drawn from the earlier NMA data.

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Experiment The Raman spectroscopic apparatus used in these studies is similar to that employed in related recent experiments in this lab.19920It consists of an injection-seeded Nd:YAG laser (Continuum NY81C) operating at 10 Hz. The second, third, fourth, and fifth NdYAG harmonics are generated using KD*P and BBO (CSK Co., Ltd.) crystals. Other Raman excitation frequencies are produced by Raman shifting the harmonics of the Nd:YAG. The desired frequency is isolated by dichroic filters and a Pellin Broca prism and brought to a focus near the sampling region. The Raman scattered light is collected at right angles to the incident radiation. The image transfer optics consist of an fl UV-grade fused-silica collection lens and an additional UV-grade lens to image the light into a 1-m single monochromator. The monochromator has a dispersion of 8 A/mm with the grating operated in first order. The collected light is detected with a standard, UV-sensitive S-20 photocathode PMT (EM1 9863B). The PMT output is sampled by a boxcar integrator or a digital oscilloscope, the averaged output of which is then stored by a microcomputer. The Raman spectra reported in this paper were recorded using the second, third, and fourth harmonics of the Nd:YAG laser at 532, 355, and 266 nm, corresponding to nonresonant (532 and 355 nm) and preresonant (266 nm) conditions in the amides studied. Typical pulse energies were 5-15 mJ at 532 nm, 10-15 mJ at 355 nm, and 5-10 mJ at 266 nm. Liquid-phase spectra were recorded in 1-cm X 1-cm quartz cuvettes with an unfocused laser beam. Gas-phase spectra were recorded in a 20-cm-long cylindrical quartz cell fitted with Brewster angle windows and resistively heated by insulated heater wire wound tightly around it. For the gas-phase spectra the laser beam was loosely focused with a 1-m focal length lens to a beam diameter of -3 mm at the sample region. The 266- and 355-nm liquid-phase data were recorded with a slit width of 70 pm, which corresponds to a resolution of 4 cm-I at 355 nm and 8 cm-I at 266 nm. The 532-nm spectra were recorded at a slit width of 100 pm for which the resolution is 3 cm-l. For the gas-phew spectra, the 500-pm slit width gave a resolution of 28 cm- at 355 nm, and a resolution of 35 cm-I at 266 nm was obtained with a slit width of 300 pm. Spectral peak positions were determined by a mercury pen lamp calibration or by comparison with the known solvent peak positions of acetonitrile. IR data, when available, also served to corroborate the peak positions. The wavelength response of the instrument was determined using a calibrated deuterium lamp, and spectra were corrected for this response. Caprolactam, NMA, and DMA were purchased from Aldrich (99.5+%) and used without further purification. Some experiments were also done with anhydrous DMA, which has a stated purity of 99+% and contains less than 0.005% water. DMA is a liquid at room temperature, but caprolactam and NMA are solids with melting points of 342 and 301 K, respectively. In order

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Figure 1. Raman (355 nm) and preresonant Raman (266 nm) spectra of gaseous, neat liquid, and aqueous solution samples of caprolactam. The dotted lines track the Am I and Am I1 band positions in the individual spectra.

to record neat liquid spectra, the NMA and caprolactam samples were heated above their melting points by flowing hot air over the sample cell. The cell was maintained at a temperature sufficient to obtain a sample vapor pressure of a few Torr. Gas-phase spectra of DMA were obtained at room temperature, since the vapor pressure of this compound is greater than that needed to record the spectra. For caprolactam the cell temperature was held at 383 K, and for NMA, at 353 K. The temperatures needed to obtain the desired vapor pressure were estimated from the melting and boiling points of the NMA and caprolactam and from the slope of vapor pressure versus temperature data for other amides for which the vapor pressure data are known.

Results Raman Spectra of Caplactam. Spectral Assignments. Raman spectra of caprolactam at 355 and 266 nm in the gas phase, neat liquid, and aqueous solution are shown in Figure 1. For excitation at 266 nm we are near enough to the very strong II II* transition, peaking at 180-200 nm for the amides?+ that changes in the Raman spectra due to enhancement by this transition are expected and observed, so we infer to the 266-nm spectra as preresonant. The neat liquid and aqueous solution spectra have very high signal-to-noise, typically 20:l or better. The greatly reduced sample number density in the gas phase makes the signal-to-noise ratio in those spectra much lower, but sufficient for our purposes. The important spectral features here are the Am I band (1627-1749 cm-I depending on phase) and the Am I1 (1492 cm-') band, which show marked changes in their relative intensities as a function of both excitation wavelength and phase. As in most Am I is primarily C=O stretch. Am I1 in caprolactam appears to be nearly pure C-N stretch rather than a combination of C-N stretch and C-N-H in-plane bend as in the trans We base this conclusion on the large preresonant enhancement of the 1492-cm-l band in the 266-nm aqueous solution spectra of Figure l and on the absence of a frequency shift

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6924 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992

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Triggs and Valentini

TABLE I: Intensities and Frequencies of Amide Vibrational Bands in the R.r" Swctra of a-Cawohctam frequency (cm-')O integrated intensityb gas liquid aqueous gas liquid aqueousc 355 nm 2.4 (0.8) 3.3 (0.3) 1.8 (0.3) Am I 1744 1674d 1627 1636d Am I1 1496 1492 1494 [2.0] 2.0 (0.2) 2.9 (0.4) band

AmIII

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Am I

1749 1674d 1636d Am I1 1490 1490

266 nm 1633 9.5 (0.7) 4.0 (0.5) 2.6 (0.6) 1495

[4.1]

4.1 (0.5) 7.1 (1.4)

"Uncertainty is k5 cm-'. *Obtained by numerical integration of band areas. The integration includes both Am I peaks for the neat liquid. Uncertainties are given in parentheses and are equivalent to f l standard deviation. Gas-phase intensities are scaled to the Am I1 peak in the neat liquid. The neat liquid and aqueous solution intensities are scaled to the intensity of the methylene rock at 1018 cm-I. '0.06 mole fraction caprolactam. dFor the neat liquid there are two entries because the Am I band consists of two peaks.

Raman 8hR (Cm.3

Figure 2. Raman (355 nm) and preresonant Raman (266 nm) spectra of caprolactam in H20and D20.The dotted lines show the isotope shifts for some bands in the spectra.

upon D-for-H substitution, as indicated by the spectra of caprolactam in H 2 0 and D 2 0 shown in Figure 2. Far-UV resonance Raman spectroscopy on aqueous NMA indicates a large displacement along the C-N coordinate in the 11*excited state, so the C-N stretch mode should be enhanced in our 266-nm preresonant ~ p e c t r a . ~ - l ~ We do not resolve any prominent band in our Raman studies of caprolactam which can be assigned to Am 111, the C-N-H in-plane bend, but IR studies of caprolactam in solution show it to be around 1485 cm-I and also to be very b r ~ a d . ~ ~Figure .*~ 2 shows that the C-N-D in-plane bend is readily observed for caprolactam in D20,but the C-N-H bend appears to be buried beneath the C-N stretch and methylene group motions in H 2 0 . The Am I11 band plays almost no role in the significant results we obtain here, so its assignment and interpretation will not be dealt with any further. Effects of Solvation and Hydrogen Bonding. As shown in the caprolactam spectra of Figure 1, data from which are summarized in Table I, the relative intensities of the Am I and Am I1 bands exhibit a significant phase and wavelength dependence. At both 355 and 266 nm, a loss of intensity in Am I and a gain in intensity in Am I1 are seen as one goes from the gas to the neat liquid to aqueous solution. The ratio of integrated intensities, Int(Am I):Int(Am 11), falls by 40% between the gas and aqueous solution at 355 nm and by 80% at 266 nm. Table I also shows a frequency shift in the Am I band in going from the gas to the condensed phase. In the gas phase, the Am I band is seen at 1744 (1749) cm-l, in the neat liquid at 1677 (1677) cm-l, and in water at 1627 (1633) cm-I for the 355 (266)-nm spectra. In contrast to Am I, the Am I1 band frequency 1490-1496 cm-I is constant within our f5-cm-' accuracy in specifying the Raman frequencies. Clearly, solvation significantly effects not only the band positions but also the observed Raman intensities of caprolactam both near and off resonance. In this regard the behavior of caprolactam generally parallels that of the well-studied prototype amide N-methylacetamide (NMA). NMA shows a shift of the Am I band to lower frequency between the gas phase and the neat liquid and a further shift upon aqueous s o l v a t i ~ n . ~The ~ J ~Am * ~ I~band also loses intensity in

the Raman spectrum as it shifts to lower frequency. This behavior of NMA has been extensively discussed, and both the shift of Am I to lower frequency and the loss of intensity have been attributed to hydrogen bonding in solution.12Js The caprolactam behavior is distinct from that of NMA, however, with regard to the response of the Am I1 band. For NMA, Am I1 shifts up in frequency in going from the gas to the neat liquid to aqueous s01ution.l~~'~ As Table I shows, for caprolactam there is no such shift in the Am I1 vibrational frequency. This difference is readily attributable to the different character of the Am I1 vibration in the two molecules-essentiallypure C-N stretch in caprolactam versus mixed C-N stretch and C-N-H bend in NMA. Table I also reveals the influence of preresonant enhancement on the relative intensities of the Am I and Am I1 bands. In the neat liquid and aqueous solution the preresonant spectrum at 266 nm has a smaller Int(Am I):Int(Am 11) ratio than the corresponding spectra at the off-resonant wavelength 355 nm; that is, the preresonance with the ll 11*absorption band enhances the Am I1 more than the Am I. In the gas phase the enhancement is reversed, the preresonance increases the Int(Am I):Int(Am 11) ratio. This reversal of amide band preresonant enhancement for solvation with hydrogen bond formation has been observed previously for NMA,I2 so it probably reflects a general behavior of the amides. Signatures of the Hydrogen-Bonded Dimer. The change in the spectra with the phase of the sample that most interests us here is the appearance of two peaks in the Am I band for the neat liquid spectra. F w e 1 shows the appearance of two peaks, closely spaced and barely resolved but clearly evident, in the Am I region for the neat liquid spectra at both 355- and 266-nm excitation. The gas-phase and aqueous solution spectra show only one Am I peak. Spectral simulations of the Am I band show that, in the gas phase, the band can be very well fit by a single Lorentzian function with a width (fwhm) of 35 cm-I. In the neat liquid, two Lorentzians are necessary to accurately describe the band shape. These components are separated by 33 cm-' with widths of 35 cm-I and have relative weights of 1.6 for the 1636-cm-I peak and 1.0 for the 1674-cm-l peak, at both 266 and 355 nm. Although we observe only a single Am I peak in the aqueous solution spectra of Figure 1, the spectral simulations give acceptable fits only if two Lorentzians are included, just as with the neat liquid spectra. In the aqueous solution spectrum at 266 nm in Figure 1, the two components, again with widths of 35 cm-I, have relative weights of 1.2 (lower frequency component) and 1.O (higher frequency component) and are separated by 20 cm-l. The presence of two peaks in the Am I region of the neat liquid suggests two different environments for the carbonyl of caprolactam in the neat liquid, presumably differing in the type of amide-amide interaction present. Since caprolactam has a cis orientation of the C - 0 and the N-H, we speculate that the two

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Figure 3. The hydrogen-bonded caprolactam cyclic dimer.

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Figure 5. Plot showing the normalized integrated intensities of the two components comprising the Am I band in acetonitrile (see Figure 4) as a function of caprolactam concentration.

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Raman Shift (cm') Figure 4. Raman (355 nm) and preresonant Raman (266 nm) spectra of the Am I band of caprolactam in acetonitrile showing the effect of concentration. The dot-dash lines show the results of fitting the Am I band to two Lorentzian functions describing the two components of the band. peaks in the neat liquid are due to the monomer of caprolactam and the cyclic dimer (Figure 3) that the cis orientation of the C=O and N-H bonds facilitates. The broadening of the Am I band in the aqueous phase data might be interpreted as the consequence of various types or degrees of hydrogen bonding around the carbonyl. Each of these suggestions seems to be supported by Raman spectra that we have taken. Raman spectra of caprolactam in acetonitrile solution presented in Figure 4 also show two partially resolved peaks in the C-0 region, at some concentrations. The two components are at the same Raman shifts as the two peaks in the Am I band of the neat liquid caprolactam. The dashed lines in Figure 4 show the results of fitting the band to two Lorentzian functions. The intensity of the lower frequency component is strongly concentration dependent and falls sharply as the caprolactam concentration is reduced from 5 to 0.5 M. The results of the two-component Lorentzian fit are shown in Figure 5 . Analysis of the data for low concentration of caprolactam is consistent with an equilibrium between a single hydrogen-bonded species and the monomer, with an equilibrium constant, K,,of 1.5 f 0.3 M-I. Since caprolactam is known to form hydrogen-bonded cyclic dimers in solution,'6J8we interpret this as the equilibrium constant for formation of the cyclic dimer. Other spectroscopic evidence discussed below supports this interpretation. At high caprolactam concentration the data of Figure 5 do not describe a simple two-component equilibrium. This could be. the result of the presence of more than one type of hydrogen-bonded complex at high concentration, or it could merely reflect deviations from ideal solution behavior that are almost certain at high concentration. We have also examined the behavior of the Am I band of caprolactam in carbon tetrachloride solution, the results of which

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Raman Shift (cm-') Figure 6. Raman (355 nm) and preresonant Raman (266 nm) spectra of the Am I band of caprolactam in CCll at various concentrations showing the presence of two components in the Am I band.

are shown in Figure 6. The Am I band is indeed composed of two peaks,the widths and positions of which are very close to those for the Am I in acetonitrile solution and in the neat liquid. However, due to spectral interference from the CC14 solvent, we cannot reliably establish the concentration dependence of either of the two component peaks in the Am I band. The interpretation of the lower frequency peak of the Am I band in the neat liquid and solution samples as due to hydrogen bonding between two caprolactam molecules is supported by other observations. IR spectra of caprolactam in CCl., show evidence in the N-H stretching region for a hydrogen-bonded species, as there is a clear indication of a "bound" N-H stretch.23 However, the lower frequency component of the Am I band that we observe in the Raman spectra is nor seen in IR spectra of caprolactam in CCl,. As we discuss in a later section, this apparent discrepancy is easily understandable and does not contradict the attribution of Raman or IR spectral features to a cyclic dimer; in fact, it lends support to this assignment. Raman Spectra of N,N-Dimethylacetamide. Solvation and Hydrogen-Bonding Effects. Raman investigations on N,N-di-

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Triggs and Valentini

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-L.dL.k 1 600 800 IO0012001400160011

0

Raman Shift (cm-l) Raman spectra at 266 nm of N,N-dimethylacet-

Figure 7. Preresonant amide for gas-phase, neat liquid, and aqueous solutions.

methylacetamide (DMA), which because it lacks an amidic hydrogen cannot hydrogen bond to itself, provide a control experiment that can aid in interpreting the phase and concentration effects we observe in the Raman spectra of caprolactam. Comparison of caprolactam and DMA spectra as a function of phase and concentration also potentially provides a means to separate simple solvation effects from the effects of the specific solvation process hydrogen bonding. In DMA there can be no hydrogen bonding in the neat liquid, but in aqueous solution hydrogen bonding is certainly present. We have recorded Raman spectra of DMA under conditions identical to those used to obtain the caprolactam spectra. Raman spectra of DMA in the gas phase, as the neat liquid, and in aqueous solution at three different concentrations, all with 266-nm excitation, are shown in Figure 7. The aqueous solution spectra in Figure 7 are scaled such that the peak at 1200 cm-I, a methyl rock whose intensity should not be affected by the amount of water present, has the same intensity at all three concentrations. This appears to be a valid scaling procedure as the scale factors are very close to 3:2:1, the ratio of the DMA concentrations in the three samples. As for caprolactam we see that for DMA the amide bands are particularly sensitive to changes in phase and concentration. There is for DMA a substantial shift of the Am I vibrational band to lower frequency in going from the gas to the neat liquid, just as there is for caprolactam. However, for DMA there appears to be little change in the relative intensities of the Am I and Am I1 bands in going from the gas to the neat liquid-the dramatic change in this ratio between the gas phase and the neat liquid which characterizes the caprolactam spectra (see Figure 1 and Table I) is not observed. Hydration, however, does have a large effect on the Am I and Am I1 Raman intensities in DMA-there is a substantial shift in intensity from Am I to Am I1 as we add water. Variation with Excitation Wavelength and Signature of Hydrogen Bonding. This Am I to Am I1 intensity shift is not simply a preresonant effect, associated with the proximity of the 266nm excitation wavelength to the strong II II* absorption of the amides, as the Raman spectra of Figure 8 show clearly. In Figure 8 we compare the Raman spectra of DMA in neat liquid and in

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Figure 8. Raman and preresonant Raman spectra of N,N-dimethylacetamide for the neat liquid and D20solutions.

aqueous solution obtained at 266 nm with those recorded upon 532-nm excitation, a wavelength well removed from any electronic resonances in the molecule. Even at 532 nm we see a loss of Am I intensity and a gain in Am I1 intensity as the mole fraction of amide in the sample is decreased and the water mole fraction is increased. But excitation-wavelength-dependenteffects are observed in the DMA spectra as they are in the caprolactam spectra. For both the neat liquid and the two aqueous solution samples, we observe an enhancement of the Am I1 band relative to Am I for excitation at 266 nm. The most interesting change with excitation wavelength that we observe in the DMA spectra of Figure 8 involves the character of the Am I band. At 532 nm only one peak is observed in the Am I for all three samples, with a fwhm of about 35 cm-’. However, at 266 nm the width of the Am I band depends on the relative amounts of DMA and water, and for the aqueous solution sample with the lowest DMA concentration the Am I splits into two peaks. This is shown even more clearly in Figure 9, where we present on an expanded frequency scale the aqueous solution DMA Raman spectra in the Am I region and include a spectrum for an additional even lower DMA concentration sample. The lower frequency component appears at the same Raman shift as the single peak in the 532-nm excitation aqueous solution spectra, while the higher frequency component occurs at the same Raman shift as the single peak in the neat liquid spectra at both 532- and 266-nm excitation. Thus, we believe that this conentration-dependent splitting of the DMA Am I band into two peaks in aqueous solution spectra at 266 nm reflects distinct hydrogen-bonded and non-hydrogen-bonded DMA carbonyl groups. We should note here that the aqueous DMA spectra presented in Figure 8 were recorded in D20, while H 2 0 is the solvent for the aqueous solution spectra shown in Figure 7. The use of two different solvent isotopomers was designed to eliminate the possibility of interference from the solvent bending vibration Raman band, which for H 2 0 occurs at almost the same frequency as the Am I band in DMA. However, a comparison of the DMA in H20 spectra of Figure 7 with the DMA in D 2 0 spectra of Figure 8 shows no discernible difference and thus no problem with solvent band interference. We also recorded Raman spectra of the pure solvent, to be sure that no interference was likely. For H20solvent

Hydrogen Bonding in Amides

The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 6921

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RMunShift (mi’) Figure 9. Comparison of Raman spectra in the Am I region for nonresonant and preresonant excitation of neat N,N-dimethylacetamideand

aqueous N,N-dimethylacetamide. we find that the bending vibration band that occurs in the region of the DMA Am I band is about 6 times broader than the individual Am I bands and only about as intense as the Am I bands in the least concentrated DMA in D 2 0 spectrum of Figure 8. R a m Spectra of N-Metbylacetamide. Comparison with Caprolactam and DMA. Two recent Raman studies of Nmethy1a~etamide’~J~ have addressed solvation effects on the far-UV resonance Raman spectra. There is a shift in intensity from Am I to Am I1 in going from the gas to the neat liquid and a corresponding decrease in the frequency in the Am I band and an increase in the frequency of the Am I1 band. Shifts in the band frequencies and a transfer of intensity from Am I to Am I1 are also observed for dilute samples of NMA in various solvents. We have taken a few 266-nm preresonant Raman spectra of NMA in the gas and neat liquid phase and as a function of concentration in aqueous solution to make contact with these other experiments and to allow us to compare and contrast the behavior we observe for DMA and caprolactam with that for DMA. O u r preresonant spectra of NMA are shown in Figure 10. The aqueous solution spectra in this figure are scaled such that the peak at 1170 cm-I, a methyl rock whose intensity should not be affected by the amount of water present, has the same intensity at all three concentrations. As in the case of our DMA aqueous solution spectra, this appears to be a valid scaling as the scale factors are very close to 3:21,the ratio of the NMA concentration in the three samples. The shift in intensity from Am I to Am I1 in going from the gas phase to the neat liquid that we observe with caprolactam is also seen in the NMA Raman spectra. The ratio Int(Am I): Int(Am 11) drops from 3.9 f 0.8 in the gas phase to 1.6 f 0.4 in the neat liquid. For caprolactam these ratios are 2.3 f 0.7and 1.O f 0.2. Upon aqueous solvation the intensity shifts further toward Am 11. For 0.07 mole fraction NMA in water the Int(Am I):Int(Am 11) ratio is 0.42 f 0.08. The Am I11 band also gains intensity relative to Am I in going from the gas to the neat liquid and further increases upon aqueous solvation. All this behavior is consistent with what has been reported by others for the far-UV resonance Raman spectra of NMA.s-15

Reman ShM (c”)

Figure 10. Prercsonant Raman spectra at 266 nm of N-methylacetamide for gas-phase, neat liquid, and aqueous solutions. The dotted lines track the positions for the Am I, Am 11, and Am I11 bands.

Also consistent with previous resonance Raman studies of NMA are the frequency shifts we measure upon change of phase and change of concentration in aqueous solution. As we measured for caprolactam (see Figure 1 and Table I), the NMA Am I band position decreases in going from the gas phase to the neat liquid to aqueous solution, from 1736 c m - I in the gas phase to 1673 cm-l in the neat liquid to 1640 cm-I in water. However, in contrast to caprolactam for which the Am I1 band position remains unchanged Over this range of sample environments, the Am I1 band position of NMA increases by about 50 cm-l in going from the gas to neat liquid and shifts up an additional 10 cm-l upon aqueous solvation. The Am I1 frequency shift in NMA might be due to the substantial C-N-H in-plane bend character of this band, which the caprolactam Am I1 lacks. Supporting this conclusion is the fact that the NMA Am 111, which has vibrational character very similar to that of the Am 11, shifts like the Am I1 upon change of phase or solvation. The notable difference between our NMA spectra and those for caprolactam and DMA under identical conditions is that for NMA we never observe two peaks in the Am I band. Regardless of concentration or phase the Am I band of NMA always appears as a single peak. However, simulations of the NMA Am I band in aqueous solution samples require two Lorentzian components, separated by 12-15 cm-I with widths of 35-40 cm-I, for an adequate fit. The widths are identical to those we find necessary to fit the caprolactam Am I band, but the separations are about a factor of 2 or 3 less. Like caprolactam, NMA is most certainly hydrogen bonded in both the neat liquid and aqueous solution. However, as we have already discussed, hydrogen-bonded NMA molecules can form long-chain oligomers in the neat liquid and in aqueous solution, resulting in a range of different possible environments for the carbonyl. The lack of distinct resolvable peaks in the Am I band of NMA probably reflects the absence of well-defined hydrogen-bonded species like the cyclic dimer that is encountered in caprolactam solutions and the single DMA molecule hydrogen bonded to one or two H 2 0 molecules encountered in the DMA aqueous solutions. In far-UV resonance Raman spectra of NMA in very dilute aqueous solution, the Am I intensity is so small as to make this band almost unidentifiable. Mayne et a1.I2 speculated that one

6928 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992

possibility for this was that there are a broad range of frequencies for the C - 0 vibration due to a distribution of hydrogen-bonded carbonyl species. Our studies of NMA in aqueous solution at higher concentration indicate that the Am I peak is not unusually broadened. Rather, the Am I band simply decreases greatly in intensity and shifts toward the Am 11, which is in turn picking up intensity. This suggests that in very dilute aqueous solution Am I is lost underneath Am 11. Wang et al.I5 had previously reached the same conclusion based on their far-UV resonance Raman study of dilute solutions of NMA in solvents of varying polarities.

Discussion Amide-Amide Hydrogen Bonding: The Caprolactam Dimer. The concentration dependence of the relative intensities for the two Am I components in the Raman spectra of caprolactam recorded in acetonitrile (Figures 4 and 5 ) suggests that the lower frequency Am I component is due to hydrogen-bonded complexes and the higher frequency peak to the unassociated carbonyl. At low concentration the data indicate a dimeric complex, and we identify this as the cyclic dimer. The problem with such an assignment is that IR spectra of caprolactam in CCl., at low concentration show only a single peak in the Am I region.*' In the IR studies there was no evidence of a second peak in the Am I band at any concentration studied, from 0.001 to 0.13 M, even though a substantial bound N-H stretch intensity associated with hydrogen-bonded N-H groups was seen. This apparent inconsistency seems to have a simple explanation in terms of the symmetry of the cyclic dimer. The unassociated caprolactam is of low symmetry, and as a consequence the IR and Raman bands are not mutually exclusive. However, in the cyclic dimer a "local" inversion center (see Figure 3) is present, if one considers only the atoms in the hydrogen-bonded ring. Thus, for the cyclic dimer the rule of mutual exclusion should apply, if not rigorously at least approximately. In the complex then, there should be a symmetric and an antisymmetric carbonyl stretch, corresponding to the stretch vibrations of the two almoet identical C g O being in phase or out of phase. The symmetric stretch will be the lower frequency of the two and will be Raman active, while the antisymmetric stretch will be the higher frequency component and active in the IR. Either of these cyclic dimer vibrations will be observable spectroscopicallyonly if the shift relative to the C - 0 stretch of the monomer is larger than the widths of the monomer and dimer bands. We believe that the absence of the Am I vibration of the cyclic dimer in the IR spectra is the consequence of this condition not being satisfied for the antisymmetric stretch vibration. We come to this conclusion by considering a very well studied hydrogen-bonded dimer, that of formic acid. The formic acid is cyclic and possesses an inversion center. The carbonyl frequency for the monomer as well as both the symmetric and antisymmetric stretch frequencies for the carbonyl vibration of the dimer are known. The symmetric stretch of the dimer is 107 cm-I lower in frequency than that of the monomer, while for the antisymmetric stretch the dimer frequency is only 23 cm-I lower than the monomer frequency.*% Our Raman spectra of caprolactam show two peaks for the Am I C=O stretch, with a frequency difference of 30 cm-'. We attribute the two peaks to the monomer and to the symmetric stretch of the dimer. Given this 30-cm-l monomer-cyclic dimer symmetric stretch frequency difference, and assuming the dimer antisymmetric stretch and symmetric C - 0 frequency shifts are in the same ratio in caprolactam as in formic acid, one would expect a shift of 7 cm-l between the C E O stretch of the monomer of caprolactam and the antisymmetric stretch of the cyclic dimer. The fwhm of the carbonyl band observed in the IR studies is approximately 25 cm-', so a shift of 7 cm-' for the dimer relative to the monomer would not be resolved even though the spectra were recorded at a resolution of 1.25 cm-I. Thus, the absence of an observable dimer peak in the C=O region of the IR spectrum does not contradict our assignment of the dimeric species in the Raman spectrum as a cyclic dimer. In fact, the absence

Triggs and Valentini of a dimer IR peak actually provides additional evidence that the dimer is cyclic. If the dimer species were open rather than cyclic, the Am I peak we see in the Raman spectrum should be observed in the IR spectrum, as such complexes would have no local inversion center and a band observed in the Raman spectrum would be observable with the same frequency in the IR spectrum. A frequency shift of the complex of 30 cm-I relative to the monomer would be observable in the 1R spectrum. The Am I two peaks in the Raman spectrum of neat liquid caprolactam have the same widths and occur at the same frequencies as the two Am I peaks in solution samples of caprolactam in acetonitrile and carbon tetrachloride. Therefore, we believe that in the neat liquid we are also seeing the caprolactam monomer and the cyclic dimer. This interpretation has to contend with the fact that we do not observe two Am I peaks in the Raman spectra of aqueous solution caprolactam, aqueous solution NMA, or net liquid NMA, all of which should have hydrogen-bonded C=O groups. However, the absence of two distinguishable Am I peaks in these samples is probably the consequence of there being more than one distinct local hydrogen-bonded carbonyl environment for the aqueous solution samples and for neat liquid NMA and slightly different Am I Raman frequencies for these different carbonyl environments. For neat liquid NMA we have hydrogen-bonded oligomers of various sizes plus non-hydrogen-bonded carbonyl groups. In aqueous solution for both caprolactam and NMA we have both self-hydrogen-bonded amide complexes and water-amide hydrogen-bonded complexes plus some free carbonyl groups. Just slightly smaller frequency shifts between the hydrogen-bonded complexes and the monomer or just slightly broader carbonyl bands would make the two peaks in the Am I band unresolvable. For neat liquid NMA we do indeed see a broader Am I peak, fwhm = 40 cm-I, than in the non-hydrogen-bondedneat liquid DMA, where the single peak has fwhm = 32 cm-',or the monomer plus single hydrogen-bonded complex 0.5 M caprolactam in CH3CN samples where the two resolvable peaks each have fwhm = 25 cm-'.The additional width probably reflects the distribution of oligomer sizes in NMA, each of which contributes to the observed inhomogeneous broadening. The Am I frequency shift of the caprolactam dimer relative to the caprolactam monomer is barely large enough to make the dimer and monomer peaks resolvable. If the carbonyl vibrational shifts of the different hydrogen-bonded forms of NMA in aqueous solution, of caprolactam in aqueous solution, and of NMA in the neat liquid were even just slightly different, it would be impossible to observe distinct peaks in the Am I band. Increased spectral resolution would not make different peaks more clearly observable, as the individual peaks in this band are inhomogeneously broadened with a width that is far greater than the 3-8-cm-' spectral resolution with which we recorded the condensed-phase spectra reported here. Our ability to observe two distinct peaks in the Am I band of caprolactam may also partly be a fortuitous consequence of the symmetry of the cyclic dimer. Because of its high symmetry, the caprolactam cyclic dimer could have a larger Raman cross section than amidewater complexes or other amide-amide complexes, and this would make it more prominent in the Raman spectra. Even when we do not find two discernible peaks in the Am I band of these amides, we do find that when hydrogen bonding is possible the carbonyl band is broader than other peaks in the Raman spectrum. The measured fwhm for the Am I band ranges from 25 to 55 cm-l, while isolated bands due to methylene group motions have widths of about 15 cm-'. Simulations of the Am I band for aqueous samples of NMA and caprolactam always require two Lorentzians, typically 12-15 cm-I apart with widths of 35-40 cm-', to accurately describe the band shape. It seems reasonable to associate the increased width with the presence of more than one C=O environment-free, singly hydrogen bonded, or multiply hydrogen bonded, with hydrogen bonding to both the solvent and other solute molecules. AmideWater Hydrogen Bonding: DMA in Aqueous Solution. Given our interpretation of the additional peak in the Am I region

Hydrogen Bonding in Amides of the neat liquid caprolactam as due to the cyclic dimer, we would not expect to see two Am I peaks in the Raman spectrum of the neat liquid DMA, as DMA lacks the amidic hydrogen necessary to make amideamide hydrogen bonding possible. And indeed in the DMA neat liquid spectrum we see only a single Am I peak. However, we do see two peaks in the Am I band for the 266-nm Raman spectrum of DMA in aqueous solution. Since amideamide hydrogen bonds are not possible in DMA, amide-amide complexes cannot be responsible for the additional structure we see in the Am I band of the aqueous solution DMA. The higher frequency band of these two Am I peaks occurs at the same Raman shift as the single Am I peak in the neat liquid DMA spectrum, so we believe it is associated with DMA C=O groups in aqueous solution that are not hydrogen bonded with the solvent water molecules. In very dilute aqueous solutions we might not expect to find appreciable numbers of DMA molecules that are not hydrogen bonded, but with mole fractions of 0.06-0.37 our DMA aqueous solution samples are not extremely dilute. Even at these concentrations we would not be able to see the Am I band component associated with non-hydrogen-bonded DMA molecules were it not for the preferential preresonant enhancement of this component. We see two peaks in the Am I band only in the preresonant 266-nm excitation spectrum; the nonresonant 532-nm excitation spectrum shows only the Am I band component that we associate with hydrogen-bonded C - 0 groups in DMA. The preresonant enhancement of the higher frequency component in the Am I band in aqueous solution DMA is further support for our assigning it to non-hydrogen-bonded DMA molecules. In neat liqid DMA, gas-phase DMA, gas-phase caprolactam, and gas-phase NMA-all of which are non hydrogen bonded-we observe preferential preresonant enhancement of Am I in the 266-nm Raman spectra. In neat liquid caprolactam, neat liquid NMA, aqueous solution caprolactam,and aqueous solution NMA-all of which are hydrogen bonded-we find preferential preresonant enhancement of Am I1 in the 266-nm Raman spectra. Two Am I peaks are also observed in aqueous solution UV rmnance Raman spectra of formamide.*’ As we see with DMA, one of these formamide carbonyl peaks appears at the same frequency as the formamide carbonyl in the neat liquid. This fact and the concentration dependence of the band lead to it being attributed to large aggregates of hydrogen-bonded formamide molecules in aqueous solution. Since DMA cannot hydrogen bond to itself, the formation of DMA aggregates in aqueous solution seems an unlikely explanation for the appearance of two Am I peaks in our aqueous solution DMA Raman spectra at 266 nm. Nevertheless, the formamide results do seem to support our interpretation of the Am I peak splitting in aqueous solution as due to the existence of two different types of solvation for the DMA molecules in solution. The range of concentrations over which we can observe two distinct Am I peaks with adequate signalto-noise ratio to permit an accurate determination of peak intensities is insufficient to allow us to establish a particular concentration dependence of the peak that we attribute to non-hydrogen-bonded DMA. solvrtion and Hydrogen-Bonding Effects on Amide Vibrational Frequeades. Comparison of gas-phase and liquid (solution)-phase Raman spectra of caprolactam and DMA shows clearly that for both amides solvation and hydrogen bonding alter the relative intensities of the Am I and Am I1 bands and also shift the frequency of the Am I C - 0 vibration. However, comparison of the intensity and frequency shifts for caprolactam with those for DMA shows that the observed frequency and intensity shifts are not so well correlated with each other. This is demonstrated by a plot of the Am I frequency shifts, measured relative to the gas-phase frequencies, for the condensed-phase samples as a function of amide mole fraction, given in Figure 11. As we would expect, the shift of the Am I band to lower frequency is produced by solvation of any type. We observe shifts between gas and neat liquid for both DMA and caprolactam,even though only caprolactam is hydrogen bonded in the neat liquid. However, solvation accompanied with hydrogen bond formation does result in a shift that is about 50% larger than the shift

The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 6929

,

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Figure 11. Plot of the Am I frequency shifts ((vmNe - vW)/vW) observed in Raman spectra of caprolactam and N,N-dimethylacetamideas a function of mole fraction amide in water. For both caprolactam and N,N-dimethylacetamidethere are two entries in the plot for those mole fractions for which two Am I peaks are observed in the Raman spectrum.

produced by non-hydrogen-bonded solvation. For the neat liquid of caprolactam the Am I Raman band shows two peaks with different frequency shifts, one attributable to the cyclic dimer and the other to the monomer. The frequencies of both bands are plotted in Figure 11, which is why there are two squares at x = 1.o. The observed frequency shifts for the caprolactam monomer and dimer in neat liquid caprolactam serve as benchmarks for a hydrogen-bonded and a non-hydrogen-bonded carbonyl. The higher frequency (less negative A v / v in Figure 11) caprolactam monomer Am I peak has a shift identical to that for the single Am I peak in DMA neat liquid where there is no hydrogen bonding upon solvation at the carbonyl. For aqueous solution DMA at small amide mole fraction we also see two Am I peaks in the Raman spectrum. One of these has a a Y / v identical to that for non-hydrogen-bonded neat liquid DMA and caprolactam monomer, while the other has a A v / v identical to that for the hydrogen-bonded dimer in neat liquid caprolactam. For aqueous solution caprolactam the Am I frequency shift is also identical to that for the hydrogen-bonded caprolactam cyclic dimer. These internal consistencies strengthen our interpretation of the two Am I peaks in aqueous DMA as being associated with hydrogenbonded and non-hydrogen-bonded carbonyl groups. It is worth emphasizing here that the presence of both hydrogen-bonded and non-hydrogen-bonded forms of DMA and caprolactam in the same solvent allows us to factor out the effects of solvent polarity and solvent dielectric strength on the amide vibrational frequencies. It is not easy, perhaps not even possible, to separate solvent polarity and solvent dielectric effects from hydrogen-bonding effects by comparing amide Raman spectra in a series of solvents. Solvation and Hydrogen-Bonding Effects on Amide Raman Intensities. While hydrogen bonding at the amide carbonyl does not seem to play a unique role in shifting the frequency of the Am I C 4 vibration in amides, it does play a singular role in altering the relative intensities of the Am I and Am I1 bands in the Raman spectrum. The influence of C - 0 hydrogen bonding on Am I and Am I1 relative intensities is evident in both nonresonant and preresonant Raman spectra. The data plotted in Figure 12, and the data in Table I too, clearly show this behavior for the preresonant Raman spectra excited at 266 nm. For non-hydrogen-bonded neat liquid DMA the ratio of the intensities of the Am I and Am I1 bands is the same as it is in the gas p h a s e t h e Am I being much more intense than the Am 11. However, upon addition of water to the neat liquid the relative intensities of the Am I and Am I1 invert; the Am I1 is now the more intense. For caprolactam, for which the neat liquid is partially hydrogen bonded the Am 1:Am I1 intensity ratio is much different from the gas-phase value. The Am 1:Am I1 ratio falls further upon addition of water to the caprolactam,

J

6930 The Journal of Physical Chemistry, Vol. 96, No. 17, 199'2

2.4

Raman spectra of NMA in dilute ~ o l u t i o n . ' ~It~has ' ~ been ob3 served that as the polarity of the solvent is increased the Am I -

2.2

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Figure 12. Plot of the dependence of the ratio of the Am I and Am I1 intensities as a function of sample phase and solvation in 266-nm preresonant Raman spectra of caprolactam and DMA. The intensities were determined by numerical integration of peak areas. The Am I integrated intensity includes both peaks when two are observed in the spectrum.

reflecting the greater extent of hydrogen bonding present in solution. Figures 11 and 12 show that the Am I frequency shifts are correlated, somewhat but not perfectly, with the Am I to Am I1 intensity transfer. Generally, we see a decrease in the Am I frequency is accompanied by a decrease in the Am 1:Am I1 intensity ratio. However, these two changes do not track each other so well that a knowledge of one allows a reliable prediction of the other. For example, for the three aqueous solution caprolactam samples we see a large change, almost a factor of 3, in the intensity ratio with amide mole fraction, but the Am I frequency shift does not change at all among these three samples. From a far-UV resonance Raman study of NMA in solvents of varying polarity, Wang et al.Is concluded that hydrogen bonding strongly affects the Am I frequency and that the frequency shift was correlated with the intensity shift from Am I to Am 11. Our results show that for fmed solvent polarity and dielectric strength hydrogen bonding does have a larger effect on the Am I frequency than does general solvation. We also concur that hydrogen bonding strongly influences the Am I and Am I1 intensities. We seem to observe a bit weaker correlation between these two effects than Wang et al. imply. However, we hasten to add that Wang et al. do not suggest any particular quantitative correlation between the intensity ratio and the Am I frequency. Molecular Electronic Explanations for the Observed Solvation and Hydrogen-Bonding Effects on Amide Band Intensities and Frequencies. One molecular level rationalization of the effect of hydrogen bonding on the Am I vibrational frequency and the Am 1:Am I1 intensity ratio is the influence of hydrogen bonding on the electron density distribution in the amide g r o ~ p . ~It ~has ~'~ been postulated that hydrogen bonding stabilizes the chargeseparated amide resonance structure B relative to the nominal structure A. This has the effect of reducing the effective C-O H

O

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bond order and increasing the effective C-N bond order, which should reduce the carbonyl stretching (Am I) frequency and increase the C-N stretching (Am 11) frequency. We do indeed observe a decrease in the Am I frequency upon hydrogen bonding for both caprolactam and DMA. However, for neither do we observe any significant frequency shift, either up or down, for the Am 11. This suggests that the solvent interaction, whether hydrogen bonding or not, is based on some interaction that locally influences just the C=O electron distribution. The stabilization of resonance form B of the amide group in hydrogen-bonding solvents has been invoked also to explain variations in the Am 1:Am I1 ratio seen in far-UV resonance

intensity decreases while the Am I1 intensity increases. A shift in the character of the ground electronic state toward resonance form B should make the ground state more like the excited state along the C-O coordinate and less like the excited state along the C-N coordinate, diminishing the displacement along the C-O bond in the resonance Raman excitation and increasing the displacement along the C-N bond. Thus, hydrogen bonding, which stabilizes resonance form B, should result in less activity, that is, less resonance enhancement, in the carbonyl Am I vibration in the resonance Raman spectrum and more activity in the Am I1 vibration. Our nonresonant and preresonant Raman results for DMA and caprolactam are consistent with this interpretation developed for true resonance Raman spectra of NMA. However, as an explanation of Raman intensities it is difficult to test this hypothesis rigorously. Our results do make it possible to differentiate general solvation effects from hydrogen-bonding effects. We observe a shift in the Am 1:Am I1 ratio in going from a gas-phase sample to a neat liquid sample for caprolactam but not for DMA, in other words, only when there is hydrogen bonding in the neat liquid. So, general solvation appears insufficient to alter the relative intensities of the Am 1 and Am I1 modes in the Raman spectrum. Further, the Am 1:Am I1 ratios of caprolactam and DMA in aqueous solution are similar at the same concentration, implying that when either amide is hydrogen bonded at the carbonyl to the same proton donor the Am I to Am I1 intensity shift is very nearly the same. The fact that the intensity shift is the same for DMA and caprolactam further confirms that it is hydrogen bonding at the carbonyl, not at the amidic hydrogen, that affects the intensity shift and by implication alters the ground-state electron distribution. We note that the solvents that were used in these experiments have a large variation in dielectric constant, ranging from about 2 for CC14to 36 for acetonitrile to 78 for water. We do not know the dielectric constant for caprolactam, but DMA has E of 38 and acetamide 50. The wide variation in e could affect the relative contributions of the resonance structures A and B, since the latter has formal charges of +1 on the N and -1 on the 0, while the former has formal charges of 0 on all atoms. However, we note that even for caprolactam in the low dielectric solvent CC14a large shift in intensity from Am I to Am I1 is observed, indicating that the solvent dielectric constant does not govern the Am 1:Am I1 ratio and that hydrogen bonding does play an important role.

Conclusion We have used Raman spectroscopy to study the amideamide and amide-water hydrogen-bonding interactions in caprolactam and DMA. Substantial shifts in the intensity of the amide bands are observed with change of phase and solvation, provided the solvation involves hydrogen bonding. The hydrogen-bond-induced shift is a transfer of intensity from the Am I C I O stretch to the Am I1 C-N stretch, relative to the intensities in the gas phase. In contrast, shifts of the Am I vibrational band to lower frequency are produced by general solvation, although the shifts are larger when hydrogen bonding is present. The intensity shifts from Am I to Am I1 for caprolactam and DMA are consistent with a hydrogen-bond-induced change in the character of the ground electronic state of the amides toward less double-bond character in the C-O and more double-bond character in the C-N. However, the absence of a shift of the Am I1 C-N stretch vibration to higher frequency when we observe a shift of the Am I C - 0 stretch vibration to lower frequency is not consistent with such a perturbation. It seems more likely that solvation and hydrogen-bonding effects on the vibrations of the amide group are more specific to the C=O bond and do much less to alter the C-N. In the cis configuration amide caprolactam, two peaks appear in the Am I band of the Raman spectra for samples of the neat liquid and in non-hydrogen-bonded solvents. Variation of the

J . Phys. Chem. 1992, 96, 6931-6937 intensities of these two peaks with caprolactam concentration show that they are most probably due to the monomer and the cyclic dimer. Acknowledgment. We thank Mr. Raj Punwaney for technical assistance with this project. We are grateful to the Du Pont Company for their support of this work. We also thank Professor T. G. Spiro for useful comments and suggestions in the preparation of this manuscript.

References and Notes (1) Robin, M. B.; Bovey, F. A.; Basch, H. In The Chemistry of Amides; Zabicky, J., Ed.; Interscience Publishers: New York, 1970. (2) Kaya, K.; Nagakura, S. Theor. Chim. Acta 1967, 7, 117. (3) Kaya, K.; Nagakura, S. Theor. Chim. Acta 1967, 7, 124. (4) Nielsen, E. B.; Schellman, J. A. J . Phys. Chem. 1967, 71, 2297. (5) Harada, I.; Sugawara, Y.; Matsuura, H. J . Raman Spectrosc. 1975,

4, 91. (6) Sugawara, Y.; Harada, I.; Matsuura, H.; Shimanouchi, T. Biopolymers 1978, 17, 1405. (7) Mayne, L. C.; Ziegler, L. D.; Hudson, B. J . Phys. Chem. 1985,89, 3395. ( 8 ) Dudik, J. M.; Johnson, C. R.; Asher, S . A. J. Phys. Chem. 1985,89, 3805. (9) Song, S.;Asher, S.A.; Krimm, S.;Bandekar, J. J . Am. Chem. SOC. 1988, 110, 8547.

6931

(10) Krimm, S.;Song, S.;Asher, S.A. J. Am. Chem. Soc. 1989,111,4290. (11) Wang, Y.; Purrello, R.; Spiro, T. G. J . Am. Chem. Soc. 1989, 1 1 1 , 8274. (12) Mayne, L. C.; Hudson, B. J . Phys. Chem. 1991, 95, 2962. (13) Song, S.;Asher, S.A.; Krimm, S.;Shaw, K. D. J . Am. Chem. Soc. 1991, 113, 1155. (14) Wang, Y.; Purrello, R.; Jordan, T.; Spiro, T. J. Am. Chem. Soc. 1991, 113.6359. (15) Wang, Y.; Purrello, R.; Georgiou, S.;Spiro, T. J. Am. Chem. SOC. 1991, 113, 6368. (16) Lord, R. C.; Porro, T. J. Z . Elektrochem. 1960.64, 672. (17) Franzen, J. S.;Stephens, R. E. Biochemistry 1963, 2, 1321 (18) Krikorian, S.E. J . Phys. Chem. 1982, 86, 1875. (19) Phillips, D. L.; Lawrence, B. A.; Valentini, J. J. J . Phys. Chem. 1991, 95, 7570. (20) Phillips, D. L.; Lawrence, B. A.; Valentini, J. J. J. Phys. Chem. 1991, 95, 9085. (21) Miyazawa, T.; Shimanouchi, T.; Mizushima, S.I. J . Chem. Phys. 1956, 24, 408. (22) Warshel, A,; Levitt, M.; Lifson, S.J . Mol. Spectrosc. 1910.33, 84. (23) Chen, C. Y. S.;Swenson, C. A. J . Phys. Chem. 1969, 73, 2999. (24) Bertie, J. E.; Michaelian, K. H. J . Chem. Phys. 1982, 76, 886. (25) Bertie, J. E.; Michaelian, K. H.; Eysel, H. H.; Hager, D. J . Chem. Phvs. 1986. 85. 4779. 126) Chang,'Y. T.; Yamaguchi, Y.; Miller, W. H.; Schaefer, H. F., 111 J. Am. Chem. SOC.1987, 109,1245. (27) Hildebrandt, P.; Tsuboi, M.; Spiro, T. G. J . Phys. Chem. 1990, 94, 2274.

Chemistry and Kinetics of Size-Selected Cobalt Cluster Cations at Thermal Energies. 2. Reactlons with O2 B. C. Guo, K. P. Kerns, and A. W. Castleman, Jr.* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: December 19, 1991; In Final Form: April 10, 1992)

The size-dependent chemistry and kinetics of gas-phase reactions of Con+(n = 2-9) with O2are examined by using a SIDT-LV (selected ion drift tube with laser vaporization source) operated at thermal energies. Con+are observed to display a very high reactivity toward 02,and the clusters tend to undergo successive oxidation reactions. The bimolecular reaction rate constants measured for the primary reactions display a strong correlation between the size of the clusters and their reactivity. As in the case of reactions with other reactant molecules such as CO, clusters containing four and five cobalt atoms exhibit a higher reactivity toward oxygen than clusters of neighboring size. The primary reactions result mainly in a replacement of a Co atom by one 02,which suggests that the oxygen and cobalt atoms in the formed cobalt oxide clusters are bound together by strong chemical bonds. The formed oxide clusters are also very reactive toward oxygen. Except for a few cases, most of the oxide cluster reactions with oxygen proceed via either switching or attachment pathways. The successive oxidation reactions of Co,' virtually terminate when oxide clusters with stoichiometricstructures of (C00),(C0O2),+ (n = &3), c0204,5+, or c0&4,5+ are formed. Compared with the results obtained by other methods, the present work provides another important example demonstrating that SIDT-LV is a very effective technique to examine the reactions of size-selected metal cluster cations at thermal energies.

1. Introduction

Currently, there is intense interest in the chemistry of gas-phase transition metal Understanding the physical and chemical properties of these aggregates has important consequences for both fundamental and applied areas. For instance, metal clusters are viewed as representing a natural bridge between gas-phase molecules or atoms and solids. Hence, the evolution in the onset of metallic behavior from the gas phase to the condensed phase can sometimes be inferred from investigations of the sizedependent properties of the clusters. On the other hand, metal clusters of small finite size are also of direct interest, since they can play a significant role in both homogeneous and heterogeneous catalytic proce~ses.~3'A molecular-level understanding of the catalytic activity of a metal is expected to be of considerable value in the design of new generations of industrial catalysts such as supported catalysts, which are required to be cheap but more efficient and selective. The work described in this paper is part of a continuing effort under way in our laboratory8v9designed to unravel the chemistry

and kinetics of reactions of size-selected metal cluster ions under thermal reaction conditions. In a previous paper, we reported sizedependent reactivity of Co,' toward the CO molecule.8 From the reactions with CO, along with well-developed theories, we were able to gain some insight into the geometric structure of naked cobalt cluster cations. The present work examines the size-dependent chemistry and kinetics of cobalt cluster cation reactions with O2 using a selected ion drift tube with laser vaporization source (SIDT-LV) operated under well-defined energies. The work is motivated by the fact that cobalt is one of the most important catalysts in industry,lWl2especially in the process to convert methane directly into large hydrocarbon molecules. As indicated in a recent review,12 oxygen plays an essential role in the catalytic process, and most of the industrial catalysts use cobalt oxides as promoters to enhance the conversion efficiency. Hence, it is interesting to probe the interaction of cobalt with oxygen from the point of view of systems of varying size. It is also expected that such studies will provide valuable information on the microprocesses occurring on the surface of metal and metal oxide

0022-365419212096-6931 $03.00/0 0 1992 American Chemical Society