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Headspace Raman Spectroscopy ELECTRONICALLY REPRINTED FROM SEPTEMBER 2014 ® Molecular Spectroscopy Workbench Headspace Raman Spectroscopy We examine vapor-phase Raman spectroscopy through the acquisition of spectra from gas mol- ecules confined to the headspace of sealed containers. Studying the Raman spectra of the liquid and vapor phases of compounds with different functional groups, degrees of hydrogen bonding, and polarity provides insight into the energetics of molecular interactions. David Tuschel ou were probably first introduced to rotational in energy and bandwidth (primarily from collisional and vibrational-rotational spectroscopy as an broadening) between the vapor and condensed phases Yundergraduate. Perhaps you even applied what provides insight into the variation of molecular interac- you were learning in the lecture hall and from text tions of compounds of different molecular structure and books in a physical chemistry laboratory involving functional groups. We also explore the phenomenon of infrared absorption spectroscopy of gases. If so, that Fermi resonance and discuss the effect of spin statistics may well have been your last encounter with vibra- on high spectral resolution vibrational-rotational di- tional spectroscopy of compounds in the vapor phase. atomic molecules. Those of us who perform Raman spectroscopy do so almost entirely with materials in the condensed Carbonated Beverages phase; that is, liquids or solids. However, many of the Carbonated beverages such as soda, beer, or sparkling compounds with which we work have some measure water have a substantial amount of CO2 dissolved of volatility, particularly liquids that evaporate quite in them, which provides the fizz when first opening rapidly, such as methanol, isopropyl alcohol, and ac- the bottle or can and the tingle when drinking the etone. Consequently, sealed containers of compounds, contents. We’ve all experienced drinking a soft drink solutions, or mixtures will have some number of that has lost most of its CO2; we say the drink has volatilized molecules in the headspace between the gone flat. Our own experience with beverages that condensed phase and the container seal. A transpar- have gone flat teaches us that CO2 is not particularly ent container offers Raman spectroscopists the ability soluble in water at atmospheric pressure and, for that to probe both the condensed and vapor phase in the reason, carbonated beverages must be securely sealed. headspace above the sample. Consequently, carbonated beverages have a high par- In this installment, we examine vapor-phase Raman tial pressure of CO2 in the headspace. spectroscopy through the acquisition of spectra from One such carbonated beverage is beer. Fluorescence gas molecules confined to the headspace. Compari- overwhelms the Raman scattering from the liquid sons of the vapor-phase and condensed-phase spectra portion of the sealed beer sample when using 532-nm reveal the significant effects of molecular interactions, excitation (the excitation wavelength for all spectra particularly hydrogen bonding, on the vibrational spec- shown in this article). However, guiding the laser trum. The degree to which certain Raman bands differ beam above the liquid through the headspace yields the spectrum of CO2 shown in Figure 1. One of the most striking features of Raman spectra from 1387.5 Headspace the vapor phase are how narrow 1500 the bands are relative to those ob- 1284.8 FWHM: 0.9 cm–1 tained in the liquid phase. The full width at half maximum (FWHM) 1000 -1 of the CO2 bands is 0.9 cm ! The spectrum consists of six bands: one 1369.3 attributed to O2 and the remaining Intensity (counts) 500 five bands attributed to CO . This O 2 1264.4 1408.8 2 may seem a little strange given that group theory predicts only one 1200 1250 1300 1350 1400 1450 1500 1550 1600 Raman active mode for CO2, the ν1 Raman shift (cm–1) symmetric stretching mode. A phenomenon called Fermi resonance accounts for the ap- Figure 1: Raman spectrum of CO2 and O2 from the headspace of beer. pearance of multiple CO2 Raman bands. The symmetric stretching + mode of CO2 is of ν g symmetry and is expected at approximately 1387.7 -1 1330 cm (1). The ν2 doubly de- 1285.0 FWHM: 0.9 cm–1 generate bending mode is Raman forbidden, but does appear in the Headspace infrared absorption spectrum at -1 O2 667 cm (2). However, the over- + tone (2ν2) has ν g symmetry and is 1633.6 expected at 1334 cm-1. So, we have 1379.6 a fundamental vibrational mode 1272.7 Liquid and an overtone of approximately equal energies and the same sym- Intensity (arbitrary units) metry. Therefore, these two energy 1200 1300 1400 1500 1600 1700 1800 1900 states can interact, and this inter- action is called Fermi resonance. Raman shift (cm–1) This type of resonance can pro- duce quite striking effects with Figure 2: Raman spectra of CO2 and O2 from the headspace of sparkling water and the liquid. respect to both Raman scattering strength and perturbation of the vibrational states — that is, Raman band positions. The assignments of the beer 3340 headspace CO2 Raman bands are N-H shown in Table I in accordance Headspace 3659 with the assignments of Hanf and O-H coworkers (3). The very strong bands at 1284.8 and 1387.5 have been assigned to ν1 and 2ν2, re- Liquid spectively. Fermi resonance has caused the ν1 and 2ν2 bands of ap- proximately equal energy to split 3314 3424 such that the lower energy state Intensity (arbitrary units) N-H O-H shifts lower and the higher energy 2800 3000 3200 3400 3600 3800 state shifts higher. The greater the interaction of the two states (that Raman shift (cm–1) is, the stronger the Fermi reso- nance), the greater the splitting of Figure 3: Raman spectra of liquid- and vapor-phase household ammonia. the energy states and the observed ν 1 ν 1 Table I: Assignment of Raman bands in cited state or so-called hot ( 1 and 2 2 ) bands appear in the Raman spectrum. beer headspace spectrum of Figure 1 The Raman spectra obtained from sparkling water Molecular Raman Band offer us an opportunity to compare the spectrum of Symbol CO in the vapor phase with that dissolved in water. Formula (cm–1) 2 Raman spectra of CO2 and O2 from the headspace of ν 1/ν 12C16O /13C16O 1264.4 sparkling water and the liquid itself are shown in Fig- 1 1 2 2 ure 2. The headspace Raman spectrum of sparkling ν 12C16O 1284.8 water is nearly identical to that from beer. Now, with- 1 2 out the overwhelming fluorescence generated by the 13 16 2ν2 C O2 1369.3 liquid beer, the Raman spectrum of the CO2 in liquid sparkling water can be obtained. The liquid spectrum ν 12C16O 1387.5 -1 2 2 2 consists of bands at 1272.7 and 1379.6 cm (CO2) and 1633.6 cm-1 (the bending mode of H O). The effect of 2ν 1 12C16O 1408.8 2 2 2 the solvent (H2O) on the solute (CO2) can be seen in the positions and widths of the CO2 ν1 and 2ν2 bands. The shifts between the vapor phase and the water sol- -1 ubilized CO2 ν1 and 2ν2 bands are -12.3 and -8.1 cm , respectively. Given that the shift in the ν1 band is 50% 2846 greater than that observed for the 2ν2 band, it is not 2956 surprising that the intensity of the solubilized CO2 3684, 3697 ν1 band is significantly more diminished. Also, the Headspace O-H FWHM of the 2ν2 bands in the vapor phase and water solubilized CO are 0.9 and 9.9 cm-1, respectively. The 2832 2941 2 band width of the solvated CO2 is 10 times greater Liquid than that in the vapor phase, thereby indicating the 3340 Intensity (arbitrary units) O-H strength of the interactions of CO2 with the water solvent. The Raman spectra of the headspace and sol- 2600 2800 3000 3200 3400 3600 3800 vated CO demonstrate that Raman spectroscopy is Raman shift (cm–1) 2 well suited for the study of molecular interactions of Figure 4: Raman spectra of liquid- and vapor-phase methanol. solute and solvent. Effect of Hydrogen Bonding on the Vibrational Spectrum In the previous section, we compared the spectra of a N2 2952.8 gas in the vapor phase and solubilized by water. Here, we compare the spectra of compounds in the liquid and 2262.9 Headspace FWHM: 3.5 cm–1 vapor phases and see the effect of hydrogen bonding manifest in the vibrational spectrum of the liquid. Our 2940.2 first example consists of household ammonia purchased 2249.6 at the grocery store. In this case, our headspace Raman Liquid FWHM: 7.6 cm–1 spectrum consists of both the solute (NH3) and solvent Intensity (arbitrary units) (H2O). The Raman spectra of the liquid and headspace of household ammonia are shown in Figure 3. The spec- 2000 2200 2400 2600 2800 3000 3200 trum of the liquid consists of a very broad band ranging Raman shift (cm–1) from approximately 3000 cm-1 to -1 Figure 5: Raman spectra of liquid- and vapor-phase acetonitrile. 3700 cm and consisting of two partially resolved broad peaks. This is the Raman scattering due to water and the enormous bandwidth is attributed to hydrogen Raman bands. Here, we see a difference of 103 cm-1. bonding. The much narrower peak protruding at 3314 -1 In addition to the band splitting, one also observes cm is the ν1 symmetric stretch of NH3. The Raman a so-called borrowing of intensity with Fermi reso- bandwidths clearly indicate that the strength of hydro- nance.
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