Symmetric hydrogen bonds

A description of the double well O-H-O bond in so-called chromous acid (in reality a Cr(III) compound, CrO(OH)) has been derived in agreement with some early INS work [7], see Fig 9.3. In symmetric H-bonds the vibrations are best described as, the symmetric stretch, VsCO-H-0), and the antisymmetric stretch, VasCO-H-O) wherev (O-H-O) 

In strong symmetric H-bonds Vs(O-H-O) can rise to about 500 cm and v as(O-H-O) can fall to around 800 cm. Thus, the main spectral features, at least of moderate and strong H-bonds, fall in the range from 200 to 1500 cm , which is optimal for neutron spectroscopy. 

All of the important chemical and biological properties of water, and most of the physical properties, are related to its unique ability to H-bond, both to other molecules and to itself. It is not only a hydrogen acceptor but also a hydrogen donor, but weakly so in the gas phase [8]. 

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The timescale of a microwave observation is ca 10 12s so that an average of the properties of the species in equilibrium (35) is obtained if the equilibrium occurs in a time shorter than this. The X-ray photoelectron spectra of intramolecularly hydrogen-bonded species in the gas phase have been studied in an attempt to obtain an instantaneous picture of the structure of these molecules. In this technique the ionisation of core electrons which occurs within 10 16s is observed. For malondialdehyde, 6-hydroxy-2-formyl-fulvene, 2-hydroxy-1,1,1,5,5,5-hexafluoropent-2-ene-4-one, 9-hydroxyphen-alenone [19], and tropolone [20], two peaks are observed in the Ou region of the photoelectron spectrum (Brown et al., 1979). If these molecules existed in the C2v form with a symmetrical hydrogen bond and equivalent oxygen... 

Figure 10 O-H radial distribution function as a function of density at 2000 K. At 34 GPa, we find a fluid state. At 75 GPa, we show a covalent solid phase. At 115 GPa, we find a network phase with symmetric hydrogen bonding. Graphs are offset by 0.5 for clarity.

The 0-0 and H-H RDFs (not shown) indicate that no 0-0 or H-H covalent bonds are formed during the simulations at all densities. The g(Roti) shows a lattice-like structure at 115 GPa, which is consistent with proton diffusion via a hopping mechanism between lattice sites.65 At 34 GPa, the coordination number for the first peak in g(RQH) is 2, which indicates molecular H20. Between 95 GPa and 115 GPa, however, the coordination number for the first peak in g(RQH) becomes four, which indicates that water has formed symmetric hydrogen bonds where each oxygen has four nearest-neighbor hydrogens. 

The second isotope effect, 87 , requires the proton and deuteron to be accurately located. The distance between the equilibrium positions of the potential energy well of double minima, symmetrical hydrogen bonds, which Ichikawa calls 7 h/h defined as q — 2i o . This distance can... 

The first step of the catalytic process is the hydrogen bond directed assembly and orientation of the reactants. In this example, the azlactone and methanol form a ternary starting complex with the organocatalyst (Fig. 1) [39]. The pseudo-Lewis acidic thiourea forms two bifurcated, nearly symmetric hydrogen bonds (2.147 A, < (0,H,N) = 155.5° and 2.146A, <(0,H,N) = 155.8°) to the carbonyl oxygen atom of the azlactone. 

Scheme 4.2 Bond energy as a function of hydrogen position (black solid line), assuming identical pff, values for the donor and acceptor, relative to the lowest vibrational energy level of the hydrogen atom (highlighted by a dotted line), (a) A standard, symmetric hydrogen bond (b) the corresponding low-barrier hydrogen bond (LBHB). The red line represents the probability density function [27, 28].

However, under appropriately constrained conditions, symmetric hydrogen bonds are known (Section 7.4), but not with the predicted 0-0 distance of 220 pm (point E). Since the shortest 0-0 distance allowed by the anion-anion repulsion is 244 pm (point C), the O-H bonds are stretched from 110 to 122 pm and, because they are stretched, the valence sums at H+ are less than 1.0 vu and the bonds are constrained to be linear. 

The structure of naphthazarin presents an interesting problem regarding the position and the bonding of the hydroxylic hydrogens. Structure I certainly does not describe adequately eitherr the chemical or the physical properties. A structure involving symmetrical hydrogen bonds (II) has been proposed in several instances. Some infrared [1, 2] and also... 

Taking the spectroscopic value of /3 equal to 32 000 cm-1, the long wavelength n-n transition of 17 400 cm-1 was obtained for the model with the non-symmetric hydrogen bonds. With the two parameters used for the symmetric model, the values for the transition... 

The position of the rOH and vOD bands in naphthazarin seem to be shifted by about 600 cm-1 with respect to the free phenolic OH groups. This is a shift which corresponds to strong hydrogen bonding but it is not as much as might be expected for a symmetric hydrogen bond [11]. 

The same conclusion about the non-symmetric hydrogen bond is reached from the electronic spectrum. The observed position (510 m/j, = 19 600 cm "1) of the ti-ji transition may be compared to that in 5,8-diacetoxy-1,4-naphthoquinone (420 m.fi) [13] and the difference of 4200 cm-1 is not unusually large for a hydrogen bond. Also, the agreement with the value of 17 400 cm-1 calculated by the MO method for the non-symmetric bonding is very satisfactory, whereas the values calculated for the models with symmetric hydrogen bonds are much too low. 

Thus, the spectroscopic results eliminate the model with symmetric hydrogen bonds, but they do not tell us much about the tunnelling of the protons. Actually, this should cause a splitting of the vibrational levels, but a rough estimation [14] of the splitting of the levels for a linear (which is probably not the case) O H 9 O system with an assumed 0—O distance of 2 7 A, yields a value of 15 cm- 1, This is too small a splitting to be observable with such broad bands, having moreover the contour complicated by the overlap with other bands. 

Symmetrical Hydrogen Bonds between Oxygen Atoms.—It was... 

The difference 0.24 A between the O—H distance in this compound and in water corresponds to bond number 0.40 (Equation 7-7). Accordingly the formation of a symmetrical hydrogen bond permits the satisfaction of the valence of the oxygen atom. 

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