Raman spectroscopy of water

Leng, W., Woo, H. Y, Vak, D., Bazan, G. C. and Kelly, A. M. (2006) Surface-enhanced resonance Raman and hyper-Raman spectroscopy of water-soluble substituted stilbene and distyrylbenzene chromophores. J. Raman Spectrosc., 37, 132-141. 

Fourier Transform Infrared and Raman Spectroscopy of Water-Soluble Polymers... 

Dangling OH bonds exit, which are not included in hydrogen bonds. These entities persists far longer than water molecule vibrational periods, and hence may hold the key to the structurally sensitive band shapes that arise in infrared and Raman spectroscopy of water and its solutions. 

Lilley, T. H. Raman Spectroscopy of Aqueous Electrolyte Solutions, in Water — a Comprehensive Treatise (ed. Franks, F.), Vol. 3, chapter 6, New York, Plenum Press 1973... 

Vibrational spectroscopy can help us escape from this predicament due to the exquisite sensitivity of vibrational frequencies, particularly of the OH stretch, to local molecular environments. Thus, very roughly, one can think of the infrared or Raman spectrum of liquid water as reflecting the distribution of vibrational frequencies sampled by the ensemble of molecules, which reflects the distribution of local molecular environments. This picture is oversimplified, in part as a result of the phenomenon of motional narrowing The vibrational frequencies fluctuate in time (as local molecular environments rearrange), which causes the line shape to be narrower than the distribution of frequencies [3]. Thus in principle, in addition to information about liquid structure, one can obtain information about molecular dynamics from vibrational line shapes. In practice, however, it is often hard to extract this information. Recent and important advances in ultrafast vibrational spectroscopy provide much more useful methods for probing dynamic frequency fluctuations, a process often referred to as spectral diffusion. Ultrafast vibrational spectroscopy of water has also been used to probe molecular rotation and vibrational energy relaxation. The latter process, while fundamental and important, will not be discussed in this chapter, but instead will be covered in a separate review [4],... 

The differences in selection rules between Raman and infrared spectroscopy define the ideal situations for each. Raman spectroscopy performs well on compounds with double or triple bonds, different isomers, sulfur-containing and symmetric species. The Raman spectrum of water is extremely weak so direct measurements of aqueous systems are easy to do. Polar solvents also typically have weak Raman spectra, enabling direct measurement of samples in these solvents. Some rough rules to predict the relative strength of Raman intensity from certain vibrations are [7] ... 

Dubessy, J., Lhomme, T., Boiron, M.-C., Rull, F. 2002. Determination of chlorinity in aqueous fluids using Raman spectroscopy of the stretching band of water at room temperature application to fluid inclusions. Applied Spectroscopy, 56, 99-106. 

Waters DN. Raman spectroscopy of powders effects of light absorption and scattering. Spectrochimica Acta Part A—Molecular and Biomolecular Spectroscopy 1994, 50, 1833-1840. 

Aqueous solutions can also be studied, since the Raman spectrum of water is weak and quite broad. Possible samples would be 1 Mto 3 Msolutions of Na2C03, NaN03, Na2S03, or Na2S04. Such samples are difficult to study using infrared spectroscopy because of solvent absorption and the solubility of most infrared cell windows. 

Water is only moderately suitable as a solvent for IR spectroscopy, since even thin layers of water give rise to a broad IR spectrum with a strong background (Fig. 4.1-21 A). Although the Raman spectrum of water exhibits the same bands, these are usually much weaker than those of the solute. In D2O (Fig. 4.1-2IB), all vibrational frequencies are shifted, so that overlapping of the solvent bands with those of the solute can be avoided. 

Chumanov, G.D., Efremov, R.G., and Nabiev, LR. (1990) Surface-enhanced Raman spectroscopy of biomolecules 1. Water-soluble proteins, dipeptides and amino acids. Journal of Raman Spectroscopy, 21, 43-48. 

Time-resolved resonance Raman spectroscopy of 25 in 50% aqueous CH3CN proved that the final product 26 appears with a rate constant of 2.1 x 109 s 1 following pulsed excitation of 25.207 The appearance of 26 was slightly delayed with respect to the decay of (25), A = 3.0 x 109s, that was determined independently by optical pump probe spectroscopy in the same solvent. The intermediate that is responsible for the delayed appearance of 26, t 0.5 ns, is attributed to the triplet biradical 327.462 It shows weak, but characteristic, absorption bands at 445 and 420 nm, similar to those of the phenoxy radical. ISC is presumably rate limiting for the decay of 327, which cyclizes to the spiro-dienone 28. The intermediate 28 is not detectable its decay must be faster than its rate of formation under the reaction conditions. Decarbonylation of 28 to form p-quinone methide (29) competes with hydrolysis to 26 at low water concentrations. Hydrolysis of 29 then yields p-hydroxybenzyl alcohol (30) as the final product. 

One more advantage of Raman spectroscopy is due to the fact that the Raman spectrum of water exhibits only a few signals of low intensity. Thus, careful dehydration of zeolites, which is crucial in many IR experiments, does not play the... 

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