Two-Dimensional Correlation Spectroscopy Study of Temperature

8674

J. Phys. Chem. 1996, 100, 8674-8680

Two-Dimensional Correlation Spectroscopy Study of Temperature-Dependent Spectral Variations of N-Methylacetamide in the Pure Liquid State. 2. Two-Dimensional Raman and Infrared-Raman Heterospectral Analysis Isao Noda* Miami Valley Laboratories, The Procter and Gamble Company, P.O. Box 538707, Cincinnati, Ohio 45253-8707

Yongliang Liu and Yukihiro Ozaki*,† Department of Chemistry, School of Science, Kwansei-Gakuin UniVersity, Uegahara, Nishinomiya 662, Japan ReceiVed: NoVember 20, 1995; In Final Form: March 6, 1996X

This paper demonstrates the potential of two-dimensional infrared-Raman (2D IR-Raman) heterospectral correlation spectroscopy in analyses of IR and Raman spectra of complex systems and of correlations between IR and Raman bands. Temperature-dependent spectral variations of N-methylacetamide (NMA) in the pure liquid state have been investigated as the first example of 2D IR-Raman correlation spectroscopy study. IR and Raman spectra of NMA in the pure liquid state have been measured over a temperature range of 30-65 °C. Generalized 2D Raman correlation spectroscopy has been also performed to assist the 2D IR-Raman correlation analysis. The 2D Raman correlation spectroscopic analysis reveals that the amide I Raman band of NMA consists of at least three distinct bands at 1665, 1650, and 1635 cm-1, and the 2D IR-Raman study shows that they are correlated with the amide I IR bands at the same frequencies, respectively. In the preceding paper, these IR bands at 1635, 1650, and 1665 cm-1 have been ascribed to large and medium-size oligomers and dimer of NMA, respectively. Therefore, the above Raman bands may also be assignable to them. Comparison of the IR spectra in the 3500-2700 cm-1 region and the Raman spectra in the 1750-1050 cm-1 region gives the following correlation: 3275 cm-1 (IR, amide A)-1650 and 1635 cm-1 (Raman, amide I)1305 cm-1 (Raman, amide III). Combining this correlation with the correlation found for the IR amide bands in the preceding paper, the following correlation can be proposed: 3275 cm-1 (IR)-1635 cm-1 (IR)1570 cm-1 (IR)-1300 cm-1 (IR)-1650 and 1635 cm-1 (Raman)-1305 cm-1 (Raman). All the bands involved in this correlation decrease in the low-temperature range and thus may be due to the large or medium-size oligomers of NMA. Similar correlation has been obtained for the dimer.

Introduction In the preceding paper,1 we reported about generalized twodimensional (2D) infrared (IR) correlation spectroscopy study of temperature-dependent spectral variations of NMA in the pure liquid state. This paper describes the results of the first indepth attempt to conduct the generalized 2D heterospectral correlation analysis by comparing the similarity or dissimilarity of the spectral features of Raman and IR spectra.2 Two separate sets of temperature-dependent spectra were obtained for Nmethylacetamide (NMA) by mid-IR absorption and Raman scattering measurements carried out at the same temperature conditions. The basic idea of heterospectral correlation analysis, where two different types of spectroscopic data are correlated against each other, had been described as early as in the first paper which introduced the field of 2D IR spectroscopy.3 In that paper, the possible construction of 2D correlation spectra based on the pairwise comparison of spectroscopic measurements, such as IR vs Raman, X-ray vs fluorescence, etc., was discussed as an obvious extension of 2D correlation approach used for IR. The concept was later reduced to practice in the dynamic rheo-optical study of a microphase separated block copolymer system.4 The first set of 2D heterocorrelation spectra were * Authors to whom correspondence should be addressed. † Fax: +81-798-51-0914. X Abstract published in AdVance ACS Abstracts, May 1, 1996.

S0022-3654(95)03414-9 CCC: $12.00

obtained from the angle X-ray scattering and IR dichroism measurements of a dynamically strained block copolymer film. The molecular level orientational dynamics of polymer chain segments probed by IR spectroscopy was directly correlated to the structural reorganization of supermolecular structure of the microphase-separated specimen monitored by the change in the scattering pattern of X-ray. The correlation between IR and Raman spectroscopy is a very attractive one. Because of the complementary nature of the two spectroscopic techniques, the analysis is expected to provide rich insight and clarification into the vibrational spectra of complex systems. In the present series of studies we have performed separately both generalized 2D IR and Raman correlation spectroscopy investigation of temperature-dependent spectral changes of NMA first and then tried 2D IR-Raman heterospectral correlation analysis for the same IR and Raman data. This paper reports the results of the 2D Raman analysis first and then those of the 2D IR-Raman investigation. Background In the conventional 2D correlation analysis proposed by Noda,2 such as 2D IR or 2D Raman spectroscopy, the observed variations of spectral intensities at two different spectral variables (i.e., wavenumbers) are compared pairwise over a fixed observation interval of time, temperature, or any other physical variable affecting the spectra. The similarity or dissimilarity under the influence of such physical variable is then expressed © 1996 American Chemical Society

Spectral Variations of NMA in the Pure Liquid State

J. Phys. Chem., Vol. 100, No. 21, 1996 8675

quantitatively in the form of a complex correlation function. For 2D IR spectroscopy, the correlation function comparing IR absorption intensities A(ν1) and A(ν2) is obtained as

〈A(ν1),A(ν2)〉 ) Φ(ν1,ν2) + iΨ(ν1,ν2) and for 2D Raman comparing the scattering intensities I(∆ν1) and I(∆ν2) at two Raman shifts becomes

〈I(∆ν1),I(∆ν2)〉 ) Φ(∆ν1,∆ν2) + iΨ(∆ν1,∆ν2) In two-dimensional heterospectral correlations, the correlation function takes a similar but slightly more complex form consisting of two different spectra data. For example, in 2D IR-Raman spectroscopy, one obtains a correlation function in the form

〈A(ν1),I(∆ν2)〉 ) Φ(ν1,∆ν2) + iΨ(ν1,∆ν2) Experimental Section The sample of NMA of high purity (purity greater than 99.0%) was the same as that used in the previous and preceding papers.1,5,6 The IR data treated in this paper are the same as those used in the preceding paper.1 The FT-Raman spectra were measured at a resolution of 8 cm-1 with a JEOL JRS FT-Raman spectrometer equipped with an InGaAs detector. An excitation wavelength at 1064 nm was provided by a CW Nd:YAG laser (Spectron SL 301 1355), and the laser power at the sample position was about 300 mW. Fifty scans were accumulated to ensure an acceptable signal-to-noise ratio. A quartz cell of 1 cm thickness was employed for the Raman measurements, and the scattered light was collected in a backscattering configuration. The equipment used for the temperature control with the precision of (0.1 °C was the same as that described in the preceding paper.1 Each set of spectra were measured sequentially between 30 and 65 °C in the ascending order at a fixed 5 °C interval. The detailed features of the temperature-dependent behavior of two spectral data sets were than compared by using the generalized two-dimensional correlation formalism. Results and Discussion 2D Raman Correlation Spectroscopy. Figure 1, a, b, and c, shows FT-Raman spectra in the 1750-1000 cm-1 region of NMA in the pure liquid measured at 30, 45, and 65 °C, respectively. The corresponding IR spectra were presented in the preceding paper.1 Table 1 summarizes the Raman assignments previously proposed based upon the isotopic shift and theoretical calculations.7,8 As in the case of the IR spectra of NMA,1 the intensities of all the Raman bands decrease with temperature. Thus, the observed bands are assignable to polymeric forms of NMA. Figure 2, A and B, shows a fishnet plot and the corresponding counter map of the synchronous 2D Raman correlation spectra of NMA. The spectrum represents the temperature-dependent spectral intensity variations of NMA between 30 and 65 °C in the 1750-1075 cm-1 region. A number of autopeaks and cross peaks are clearly observed in the plot and counter map. The autopeaks at 1650, 1438, 1415, 1373, 1303, and 1161 cm-1 are attributable to the Raman bands due to the amide I, asymmetrical methyl bending, symmetrical methyl (-N) bending, symmetrical methyl (-C) bending, amide III, and methyl (-N) rocking modes, respectively. One of the notable features of the plot and map is the appearance of numerous ridges connecting various correlation peaks; the development of ridges is one of the classical symptoms of the contribution of baseline intensity

Figure 1. FT-Raman spectra of NMA in the pure liquid measured at (a) 30, (b) 45, and (c) 65 °C.

TABLE 1: Band Assignments of Raman Frequencies of N-Methylacetamide7,8 Raman band (cm-1)

assignments

1655 1438 1415 1373 1300 1161

amide I CH3 (-C), CH3 (-N) asym bending CH3 (-N) sym bending CH3 (-C) sym bending amide III CH3 (-N) rocking

fluctuations. The unfortunate result of large baseline fluctuations of Raman spectra is the parasitic or coincidental matching of variational pattern of essentially unrelated spectral signals. There is no physical basis for the apparent correlation. It is purely the limitation of experimental precision. Such baseline effects may obscure more interesting but subtle spectral features representing the individual responses of vibrational modes toward the temperature change. In Figure 2C an asynchronous 2D Raman spectrum of NMA in the same spectral region is plotted. The dominating effect of baseline fluctuation is again apparent in the asynchronous spectrum. It is already clear from the sign of cross peaks in the asynchronous spectrum of NMA that the changes in the baseline occurs earlier, i.e., at temperatures lower than the temperature where most of the Raman scattering peak intensity of major vibrational modes in this spectral region changes. Figure 3, A and B, presents the close-up view of the synchronous correlation spectra in the region of the amide I and III modes, respectively. It is apparent from Figure 3A that an autopeak at 1650 cm-1 is located at somewhat lower frequency than the peak maximum of the average Raman scattering peak of the amide I mode. This observation suggests that the amide I mode consists of at least two bands near 1665

8676 J. Phys. Chem., Vol. 100, No. 21, 1996

Noda et al.

Figure 3. (A) Synchronous 2D Raman correlation spectrum of NMA in the 1700-1600 cm-1 region. (B) Synchronous 2D Raman correlation spectrum of NMA in the 1350-1250 cm-1 region.

Figure 2. (A) Full view of the pseudo-three-dimensional stacked-trace representation of the 2D Raman synchronous correlation spectrum of NMA in the pure liquid. (B) Corresponding counter map representation. (C) Asynchronous 2D Raman correlation spectroscopy in the same spectral region.

and 1650 cm-1. It is also noted that the autopeak is significantly extended into the lower frequency region. This may be an evidence for the existence of the third amide I band near 1635 cm-1. These three amide I modes probably correspond to IR amide I modes at 1665, 1650, and 1635 cm-1 observed in the preceding paper.1

The position of an autopeak at 1305 cm-1 is slightly higher than the peak maximum of the amide III Raman band near 1300 cm-1. It seems therefore that there is one more band near 1290 cm-1 assignable to the amide III mode. Figure 4 directly compares the amide I and III band regions. It is clear from this figure that the amide I bands at 1650 and 1635 cm-1 are correlated with the amide III band at 1305 cm-1. 2D IR-Raman Correlation Spectroscopy. Figure 5, A and B, shows a fishnet plot and the corresponding counter map of the synchronous 2D IR-Raman heterospectral correlation spectra of NMA in the pure liquid, respectively. The appearance of synchronous correlation peaks observed in these spectra indicates that the thermally induced dynamic variations of spectral intensities of IR absorption and Raman scattering of NMA occur at somewhat similar temperature ranges. The influence of CO2 contaminant detected during the IR measurements is present in the 2D correlation spectra. The formation of spectral ridges in the 2D correlation spectrum due to the baseline fluctuation of Raman spectra is also observed. The spectral region of IR spectrum of NMA could be roughly separated into three distinct areas: NH and CH stretching-mode region above 2700 cm-1, amide I, II, III, and methyl bending-


J. Phys. Chem., Vol. 100, No. 21, 1996 8677

Figure 4. Localized view of the off-diagonal position of a synchronous 2D Raman spectrum of NMA between the 1700-1500 cm-1 and 13501250 cm-1 region.

mode region below 1750 cm-1, and the in-between region where only the contribution from CO2 contamination is observable. Figure 6 is the corresponding counter map representation of the asynchronous correlation spectrum in the same spectral region. The spectrum clearly indicates the existence of asynchronicity, representing the difference in the temperaturedependent behavior of the spectral components of IR and Raman spectra. More detailed features become apparent as we examine the close-up view of these 2D correlation spectra. Figure 7, A and B, presents the synchronous and asynchronous 2D IR-Raman heterospectral correlation spectrum of NMA in the spectral region between 1750 and 1075 cm-1, respectively. These 2D correlation spectra are significant in that they represent the first set of 2D correlation spectra obtained by the generalized 2D correlation analysis between two fundamentally different spectroscopic techniques, such as IR and Raman, in the same spectral wavenumber regions. Figure 7A shows that the synchronous 2D correlation spectrum is no longer symmetric with respect to the diagonal line, as two sets of spectral data were measured by completely different techniques. It is true that the synchronous spectrum has some correlation peaks located at the spectral coordinate where ν1 ) ∆ν2. These peaks, however, arise from the synchronous correlation of simultaneious variations of temperature-dependent intensities of IR and Raman bands of NMA. Since these peaks do not arise from the comparison of a dynamic spectral signal to itself (i.e., autocorrelation), they should not be considered as autopeaks. In short, a synchronous 2D heterocorrelation spectrum consists exclusively of cross peaks. The ubiquitous presence of synchronous cross peaks in Figure 7A indicates the similarity between the temperature-dependent variations of IR and Raman spectra of NMA. For example, the amide I band of the IR spectrum around 1650 cm-1 is correlated with the amide I band found at the same wavenumber in the Raman spectrum. Similarly, a strong correlation is observed between the amide III Raman band around 1300 cm-1 with IR amide bands. All the other bands in the IR and Raman spectra, such as methyl bending modes, also develop synchronous cross peaks. The asynchronous 2D correlation spectrum in Figure 7B reveals an interesting feature. None of the asynchronous cross peaks indicate the correlation between major bands of IR absorption and Raman scattering by NMA in this spectral region.

Figure 5. (A) Full view of the pseudo-three-dimensional stacked-trace representation of the 2D IR-Raman synchronous correlation spectrum of NMA in the pure liquid between the 3500-1100 (IR) and 17501050 cm-1 (Raman) regions. (B) Corresponding counter map representation.

Figure 6. Asynchronous 2D IR-Raman correlation spectrum of NMA in the same spectral region as Figure 5.

The cross peaks appear at the spectral coordinates where the absorption peaks of IR spectrum are located. The coordinates

8678 J. Phys. Chem., Vol. 100, No. 21, 1996

Noda et al.

Figure 7. (A) Synchronous 2D IR-Raman correlation spectrum of NMA in the 1750-1075 cm-1 region. (B) Corresponding asynchronous 2D IR-Raman correlation spectrum of NMA in the same spectral region.


for Raman scattering, on the other hand, are all located around the areas where no major Raman peaks are observed. Instead, the dominant contribution observed in the region arises from the baseline fluctuations. Thus, the presence of peaks in the asynchronous 2D heterospectral correlation spectrum of NMA points to the fact that the spectral variations of IR absorption bands are synchronous to major scattering bands of Raman spectrum but not to the baseline fluctuations. The signs of asynchronous peaks in Figure 7B indicate that the baseline fluctuations of Raman spectrum must be occurring at temperatures lower than those causing the band intensity variations of IR spectrum. The observation is consistent with the previous 2D correlation analysis of the same Raman data. The baseline fluctuations of Raman spectra occurred at temperatures below those for the major Raman scattering bands. Figure 8A characterizes the similarity of temperature-dependent behavior of amide vibrational modes of IR and Raman spectra of NMA. The similarity of the temperature dependence of the amide I and amide II bands of IR spectrum at 1650 and 1635 cm-1 and 1565 cm-1 compared to the amide I variation of Raman spectrum at 1650 cm-1 is quite obvious. Amide II mode is not active in Raman spectrum so that the correlation peak is absent in this spectral coordinate. It is interesting to

note that position of the synchronous peak at the IR wavenumber coordinate of 1565 cm-1 is slightly higher than the peak position of the average IR peak at 1560 cm-1. This differentiation of 1560 and 1565 cm-1 bands is observed more clearly in the 2D IR correlation spectrum presented in the preceding paper.1 It is also noted that position of the synchronous peak at the Raman coordinate of 1650 cm-1 is slightly lower than the peak position of the average Raman band at 1660 cm-1. This result again suggests that there is one more Raman band near 1665 cm-1. As discussed above (Figue 3A), there may also be the Raman band near 1635 cm-1. The apparent asynchronicity observed between the IR and Raman bands of amide vibrations, as shown in Figure 8B, must be arising from the effect of temperature-dependent baseline changes of Raman spectrum which are absent in IR spectrum. The asynchronous peaks are observed predominantly where there is little contribution from actual Raman scattering peaks. Instead, they are located between the low-intensity region of Raman spectrum and peak positions of IR absorption spectrum. The conspicuous absence of asynchronous peaks at the spectral coordinates corresponding to the peak positions of IR absorption and Raman scattering indicates that the thermally induced variations of IR and Raman peaks are more or less synchronized,



i.e., occurring at similar temperatures. Of course, such apparent synchronicity must be evaluated with respect to the vast difference in the temperature dependent behavior of the Raman baseline and IR peaks. The comparison between Figure 8, A and B, reveals that the positions of the synchronous and asynchronous correlation peaks in the IR wavenumber coordinates are substantially different. The synchronous peaks are located around 1650, 1635, and 1565 cm-1, while the asynchronous peaks are found around 1665 and 1555 cm-1. By combining the above observation with the findings from the preceding 2D IR correlation study,1 the order of temperature-dependent changes of intensities are given by Raman baseline < 1635 cm-1 (IR, Raman) < 1650 cm-1 (IR, Raman), 1565 cm-1 (IR) < 1665 cm-1 (IR, Raman), 1555 cm-1 (IR). Amide I band is largely due to a CdO stretching mode of the amide groupe,7,8 and thus its frequency reflects the strength of the hydrogen bonding of the CdO group. In the preceding paper,1 we assigned the IR bands at 1635, 1650, and 1665 cm-1 to the amide I modes of large, and medium-size oligomers, and dimers of NMA, respectively, based upon the correlation between the frequency and the strength of the hydrogen bonding. The correlations between the Raman and IR bands lead us to

J. Phys. Chem., Vol. 100, No. 21, 1996 8679

Figure 10. (A) Synchronous 2D IR-Raman correlation spectrum of NMA between the 3500-2700 cm-1 (IR) and the 1750-1050 cm-1 (Raman regions. (B) Corresponding asynchronous 2D IR-Raman correlation spectrum of NMA for the same spectral region.

propose the Raman assignments. The Raman bands at 1635 and 1650 cm-1, which are correlated with the IR bands at 1635 and 1650 cm-1, respectively, are due to the amide I modes of the large and medium-size oligomers, respectively, while that at 1665 cm-1 is assignable to the amide I of the dimer. Very similar conclusions could be drawn from the comparison of amide III bands of IR and Raman spectra of NMA as shown in Figure 9, A and B. The synchronous spectrum is located at 1305 cm-1 of IR and Raman spectral coordinates while the IR coordinate for asynchronous peak is around 1285 cm-1. In the 2D IR,1 the 1305 cm-1 band was synchronous with the 1650 and 1565 cm-1 bands. The sequence of intensity changes as the temperature of the system increases becomes Raman baseline < 1305 cm-1 (IR, Raman) < 1285 cm-1 (IR, Raman). Figure 10, A and B, shows the comparison of the IR absorption in the NH and CH stretching-vibration region and Raman scattering in the amide I, II, III, and methyl bending vibrations. It is interesting to note that the temperaturedependent intensity variations of Raman peaks at 1650 and 1305 cm-1 are synchronously correlated with the lower side of IR peak around 3275 cm-1 of the NH stretching mode of associated NMA oligomers. The asynchronous peaks representing the

8680 J. Phys. Chem., Vol. 100, No. 21, 1996 difference between the baseline fluctuations of Raman spectrum and peak intensity changes of IR spectrum, on the other hand, are located at the higher wavenumber side around 3335 cm-1.

Noda et al. All the bands in this correlation can be assigned to the largesize oligomers of NMA.1 Similarly, the following correlation can be obtained: This correlation is concerned with the dimer of NMA.1

Conclusion 2D IR-Raman heterospectral correlation spectroscopy offers rich information about temperature-dependent spectral variations of NMA which is not readily obtainable from conventional onedimensional spectroscopy. A number of correlations between the IR and Raman bands can be obtained from 2D IR-Raman analysis. In the preceding paper,1 we found the following correlation for IR bands:

3275 cm-1 (amide A)-1635 cm-1 (amide I)1570 cm-1 (amide II)-1300 cm-1 (amide III)

3335 cm–1 (IR)–1665 cm–1 (IR)-1545 cm–1 (IR)-1285 cm–1 (IR)

1665 cm–1 (Raman)

1285 cm–1 (Raman)

In conclusion, we have demonstrated the potential of the 2D IR-Raman heterospectral correlation spectroscopy in the study of temperature-dependent variations of NMA. This technique should have considerable promise for researches in widespread field. References and Notes

Here, we obtain the new correlation for the IR and Raman bands:

3275 cm-1 (IR)-1650 cm-1 (Raman)-1305 cm-1 (Raman) Combining two results, correlations between two series can be obtained: 3275 cm–1 (IR)–1635 cm–1 (IR)-1570 cm–1 (IR)-1300 cm–1 (IR)

1635 cm–1 (Raman)

1305 cm–1 (Raman)

(1) Noda, I.; Liu, Y.; Ozaki, Y. J. Phys. Chem. 1996, 100, 8665. (2) Noda, I. Appl. Spectrosc. 1993, 47, 1329. (3) Noda, I. Appl. Spectrosc. 1990, 44, 550. (4) Noda, I. Chemtracts-Macromol. Chem. 1990, 1, 89. (5) Liu, Y.; Czarnecki, M. A.; Ozaki, Y. Appl. Spectrosc. 1994, 48, 1095. (6) Liu, Y.; Ozaki, Y.; Noda, I. J. Phys. Chem. 1996, 100, 7326. (7) Miyazawa, T.; Shimanouchi, T.; Mizushima, S. J. Chem. Phys. 1958, 29, 611. (8) Sugawara, Y.; Hirakawa, A. Y.; Tsuboi, M. J. Mol. Spectrosc. 1984, 108, 206.

JP9534141

Two-Dimensional Correlation Spectroscopy Study of Temperature

Recommend Documents