Hydrogen-bond complexes

Another example of current interest is the vibrational predissociation of hydrogen bonded complexes such as (HF) ... 

Ewing G E 1980 Vibrational predissociation in hydrogen-bonded complexes J. Cham. Phys. 72 2096-107... 

Miller R E 1990 Vibrationally induced dynamics in hydrogen-bonded complexes Accou/rfs Chem. Res. 23 10-16... 

Dyke T R 1984 Microwave and radiofrequency spectra of hydrogen-bonded complexes in the vapor phase Topics in Current Chemistry 120 85-113... 

Hydrogen-bonded complexes are common throughout chemistry. They generally involve a hydrogen attached to a heteroatom (usually nitrogen or oxygen) interacting with another heteroatom. 

The process of the direct methylation is interpreted in the following way f Eq. (9) ] initially the hydrogen-bonded complex 1 is... 

Beyer, A., Karpfen, A., and Schuster, P. Energy Surfaces of Hydrogen-Bonded Complexes in the Vapor Phase. 120, 1-40 (1984). 

Excess Volume Comparison Figure 7.5 compares V for the three systems for which we have compared H, G, and 5, plus the (cyclohexane + decane) system.5 The comparatively large negative for the (ethanol + water) system curve (4) can be attributed to the decrease in volume resulting from the formation of hydrogen-bonded complexes in those mixtures. The negative for the (hexane + decane) system curve (3) reflects an increased packing... 

In the above formulation the proton is transferred in the step in which the intermediate is formed. Such proton transfer is not essential for base catalysis. An alternate mode of catalysis is one in which the transition state for intermediate formation is a hydrogen-bonded complex, e.g. L, but in which this complex collapses to VI and the catalyst rather than to VIII. For such a formulation the only significant intermediate determining the rates would be VI, which would now be formed by the additional steps... 

TABLE 14. Thermodynamic parameters of hydrogen-bonded complexes of p-FC6H4OH with sulphoxides and some other bases... 

Fig. 1. The structure of gas hydrates containing a hydrogen-bonded framework of 46 water molecules. Twenty molecules, arranged at the comers of a pentagonal dodecahedron, form a hydrogen-bonded complex about the comers of the unit cube, and another 20 form a similar complex, differently oriented, about the centre of the cube. In addition there are six hydrogen-bonded water molecules, one of which is shown in the bottom face of the cube. In the proposed structure for water additional water molecules, not forming hydrogen bonds, occupy the centres of the dodecahedra, and...

An alternative strategy for promoting Diels-Alder reaction by proton involves the activation of dienophile by hydrogen bonding [93]. Biphenylene diol 143 (Scheme 4.26) forms doubly hydrogen-bonded complexes with a,j]-unsaturated carbonyl compounds, which strongly accelerate the Diels-Alder... 

Complexes. The structure of an n a charge-transfer complex between quinoxaline and two iodine atoms has been obtained by X-ray analysis and its thermal stability compared with those of related complexes. The hydrogen bond complex between quinoxaline and phenol has been studied by infrared spectroscopy and compared with many similar complexes. Adducts of quinoxaline with uranium salts and with a variety of copper(II) alkano-ates have been prepared, characterized, and studied with respect to IR spectra or magnetic properties, respectively. 

The actual proton transfer takes place in the second step— the first step is formation of a hydrogen-bonded complex. The product of the second step is another hydrogen-bonded complex, which dissociates in the third step. 

Tertiary amines can be converted to amine oxides by oxidation. Hydrogen peroxide is often used, but peroxyacids are also important reagents for this purpose. Pyridine and its derivatives are oxidized only by peroxyacids. In the attack by hydrogen peroxide there is first formed a trialkylammonium peroxide, a hydrogen-bonded complex represented as R3N-H202, which can be isolated. The decomposition of this complex probably involves an attack by the OH moiety of the H2O2. Oxidation with Caro s acid has been shown to proceed in this manner ... 

Trialkyl phosphates form volatile 1 1 adducts with acids such as nitric and chloroacetic, from which the esters are recovered by base treatment. I.r. and n.m.r. spectral data suggest that these are hydrogen-bonded complexes. At low temperatures, in FSOaH-SbFj, trialkyl phosphates were shown (by n.m.r.) to give protonated species in which there appears to be considerable pir-d-rr back-donation from oxygen to phosphorus. These species are not stable the tri-n-butyl ester decomposing over the course of two days to MeaC+ and (HOiP. ... 

Figure 3.10. Comparison of the experimental TR spectra of p-methoxyacetophenone (MAP) obtained in MeCN (b) and 50% H2O/50% MeCN (v v) (c) with the DFT calculated spectra for the free triplet state (a) and triplet of the carbonyl hydrogen-bonded complex (d). (Reprinted with permission from reference [42]. Copyright (2005) American Chemical Society.)...

The intense absorption band at v (CO) 2154 cm" that appears at high CO pressure can be attributed to the hydrogen-bonded complex of CO with silanol OH-groups. Its appearance corresponds to a slight shift to lower frequencies of the absorption band at v (OH) 3742 cm corresponding to the stretching of Si-OH groups. The absorption band at v (CO) 2136 cm can be ascribed to physical-adsorbed CO over sepiolite. From these results we can conclude that on this particular kind of silica no acid sites are present, whereas sepiolite shows both weak and medium Lewis acid sites. 

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