Frequency OH stretch

The temperature-dependent Raman spectra are depicted in Fig. 4-27a, b. Figure 4-27a shows the spectra of H2O-I (the water molecules in the inner coordination sphere) from 133-223 K. Figure 4-27b shows the spectra of H2O-II (the water molecules in the outer sphere). The spectra above 223 K are not shown because of the overlap with fluorescence that is observed with the 514.5 nm excitation. Plots of the variations of band frequency with temperature are illustrated in Fig. 4-28a, b for H2O-I and H2O-II. Two discontinuities are observed at 195 5K and 140 5K, indicative of three distinct phases occurring in the temperature range studied, as indicated in Fig. 4-28a. The higher-frequency OH stretch region, as shown in Fig. 4-28b does not show any discontinuities for H2O-I. A plot of full width at half maximum intensity (FWHM) vs. T for H2O-I shows a discontinuity at 140 K (Fig. 4-28c, d). Additional support for these phase transitions was found from the temperature dependences of the UO vibrational mode, lattice vibrations and the NO3 ion vibrations (translations and rotations). 

Y, H-Y, H-MOR, H-ZSM-5) and correlated especially the shift, Av (OH), of the (high-frequency) OH stretching bands to the acid strength (Sanderson s charge on the H atom). The shifted OH bands appeared as broad signals between 3025 (AN H-X) and 2540 cm (AN - H-ZSM-5) for the neutral complexes. Also, the in-plane, 6(OH) and out-of-plane, y(OH), bending vibrations were measured and compared with the results of Fermi resonance calculations. Interaction of di-acetonitrile with NH4-, H, Na-, H-, and Co-Beta led to the detection of Brqnsted and Lewis sites through the appearance of bands at 2297 and 2325 cm", respectively. Na+ and Co + were identified by bands at 2284 and 2308 cm, respectively. 

Figure Bl.5.15 SFG spectrum for the water/air interface at 40 °C using the ssp polarization combination (s-, s- and p-polarized sum-frequency signal, visible input and infrared input beams, respectively). The peaks correspond to OH stretching modes. (After [ ].)...

Infrared absorption studies have shown that correlates with an absorption at 3 p.m associated with an OH-stretching frequency (20). Indeed, infrared absorption provides a useful tool for evaluation in rapid production quaUty control. Infrared and other studies show that degradation is caused by proton inclusion in the grown quartz. 

Would you expect the OH stretching frequencies in 2,3-dimethyl-2,3-butanediol to be shifted from the value in tert-butyl alcohol, even in dilute solution. Identify the OH stretching frequencies in the diol and compare them to tert-butyl alcohol. Rationalize your observations by comparing the geometry of the diol with those of tert-butyl alcohol and tert-butyl alcohol dimer. 

The evidence presented for the formation of hydrogen bonds with sulphoxides and sulphones was first reported by Barnard. Fabian and Koch, who measured the characteristic infra-red stretching frequency shifts of the S—O bond in the presence of MeOH in Simultaneously the OH stretching band of MeOH at... 

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],... 

In addition to the effects of motional narrowing, vibrational line shapes for the OH stretch region of water are complicated by intramolecular and intermolecular vibrational coupling. This is because (in a zeroth-order local-mode picture) all OH stretch transition frequencies in the liquid are degenerate, and so the effects of any... 

As described above, it is probably adequately clear that the vibrational spectroscopy of water is complicated indeed One can simplify the situation considerably by considering dilute isotopic mixtures. Thus one common system is dilute HOD in D2O. The large frequency mismatch between OH and OD stretches now effectively decouples the OH stretch from all other vibrations in the problem, meaning that the OH stretch functions as an isolated chromophore. Of course the liquid is now primarily D2O instead of H2O, which has slightly different structural and dynamical properties, but that is a small price to pay for the substantial simplification this modification brings to the problem. 

Now suppose that the system of interest has N vibrational chromophores whose frequencies are in the same region, all of which need to be treated quantum mechanically. Such, for example, is the situation for the OH stretch region of... 

H2O, where N/2 molecules have N OH stretch chromophores. In this case we need to label the transition dipoles and frequencies by an index i that runs from 1 to N. In addition, in general these chromophores interact, with couplings (in frequency units) coy. In this case the above mixed quantum/classical formula can be generalized to [95 98]... 

Within the mixed quantum/classical approach, at each time step in a classical molecular dynamics simulation (that is, for each configuration of the bath coordinates), for each chromophore one needs the transition frequency and the transition dipole or polarizability, and if there are multiple chromophores, one needs the coupling frequencies between each pair. For water a number of different possible approaches have been used to obtain these quantities in this section we begin with brief discussions of each approach to determine transition frequencies. For definiteness we consider the case of a single OH stretch chromophore on an HOD molecule in liquid D2O. 

Each of the approaches is based on the premise that it makes sense to focus on the Born Oppenheimer potential for the OH stretch for fixed bath variables. Such a potential has vibrational eigenvalues, and for example h times the transition frequency of the fundamental is simply the difference between the first excited and ground state eigenvalues. Thus in essence this is an adiabatic approximation the assumption is that the vibrational chromophore is sufficiently fast compared to the bath coordinates. To the extent that the h times frequency of the chromophore is large compared to kT, and those of the bath are small compared to kT, this separation of time scales exists and so this should be a reasonable approximation. For water, as discussed earlier, some of the bath variables (librations) have frequencies somewhat larger than kT/h, and... 

We wanted to extend this approach to include dynamical effects on line shapes. As discussed earlier, for this approach one needs a trajectory co t) for the transition frequency for a single chromophore. One could extract a water cluster around the HOD molecule at every time step in an MD simulation and then perform an ab initio calculation, but this would entail millions of such calculations, which is not feasible. Within the Born Oppenheimer approximation the OH stretch potential is a functional of the nuclear coordinates of all the bath atoms, as is the OH transition frequency. Of course we do not know the functional. Suppose that the transition frequency is (approximately) a function of a one or more collective coordinates of these nuclear positions. A priori we do not know which collective coordinates to choose, or what the function is. We explored several such possibilities, and one collective coordinate that worked reasonably well was simply the electric field from all the bath atoms (assuming the point charges as assigned in the simulation potential) on the H atom of the HOD molecule, in the direction of the OH bond. 

Figure 1. Bottom panel OH stretch frequencies, to , for water clusters and the surrounding point charges, versus electric field ) (in atomic units). The solid line is the best quadratic fit. Top panel Dipole derivative, / ], (relative to the gas phase value) for water clusters and the surrounding point charges, versus electric field ). The solid line is the best linear fit.

Figure 2. The histogram is the distribution of OH stretch frequencies for the water clusters and surrounding point charges, and the solid line is the distribution of frequencies from the quadratic electric field map.

---