Complexes hydrogen bonded

Hydrogen fluoride vapors contain monomeric HF molecules, dimers (HF)2 and polymeric species, (HF) , with n varying from 3 to 8. The stmctures of the polymers remain unknown, but the stmcmre of the dimer has been determined by spectroscopic measurements and by high-level quantum chemical calculations. The structure indicated in Fig. 18.8 represents a recommended structure based on information from both experiments and calculations [12]. The complex consists of two monomeric units with essentially the same H-F bond distance as in an isolated monomer. The monomers are joined through a H- F bond which is twice as long as bond distance in the monomer, but nearly 100 pm shorter than the sum of the van der Waals radii of H and F (Table 8.4). Note the similarity to the structure of HFICI (Fig. 18.6). The dissociation energy of the dimer at zero kelvin is Do = 13 kJ mol [13]. 

We now turn our attention of hydrogen-bonded complexes of HCl with ammonia and trimethylamine. Direct combination of gaseous ammonia and hydrogen chloride results in the formation of solid ammonium chloride, (NH4)+C1 . The crystal structure of (ND4)C1 at 5 K has been determined by neutron diffraction [18], The ammonium ions are found to be perfectly tetrahedral with N-D bond distances of 103 pm. Each of them is surrounded by eight chloride ions at the corners of a cube, with N- Cl distances of 382 pm. The cations are oriented in such a manner that the four N-D bonds are pointing towards alternating comers of the cube formed by the anions. This means that the coordination geometry of the H atoms is linear with H- Cl distances of 279 pm. 

Formation of solid ammonium chloride from the gaseous complexes may be divided into three steps. In the first, endothermic, step the complexes are dissociated to form neutral NH3 and HCl molecules in the second, undoubtedly also endothermic, protons are transferred from HCl to NH3 molecules to form the separated NH and Cl ions in the third, exothermic step the ions are combined to form a crystal which is stabilized by sttong Coulomb attfactions between cations and anions. The system NHz HCl, then, provides another demonstration of how intermolecular interactions is the solid phase may lead to more polar or ionic suiicture than in the gas phase. 

We have seen that replacement of H atoms on the donor atom by methyl groups tends to increase the strength of a dative bond. One might therefore expect the dissociation energy of the complex (CHstsN HCl to be significantly higher than that of H3N HC1, and 

The gas-phase structure of the water dimer is presented in Fig. 18.10 [24]. The three atoms of the acceptor molecule and the O atom of the donor are lying in a symmetry plane, and the O- H-0 fragment appears to be linear. Both H-O-H valence angles are close to 104°. The dissociation energy at zero K is 15 kJ moP [25,26,27]. 

Some of the color of lignin is attribnted to complexes between lignin nnits or between a lignin nnit and a metal ion. The strnctures in Fignre 3.8 have been proposed in the literatnre and are discnssed in the following section. 

Imsgard et al. [62] noted that spruce milled-wood lignin contained, in addition to 0.7 quinone groups, 1% catechol groups. Lignin-like catechols form complexes with ferric ions that have a broad absorption from 470-700 nm with around 560-800 nm and s in the range of 1-2 x 10 L mol cm.  

Aqueous ferric ion forms a complex with guaiacol with two discrete maxima and a similar extinction (X = 422 nm, s = 2.4 x 10 Lmol emX = 461 nm, s = 2.3 x 10 LmoHcm ) as the catechol complexes [71]. However, 4-substituted phenols are better models than simple guaiacol for the guaiacyl function in lignin. The extinction coefficients for ferric complexes of such compounds range from 4 to 10 LmoHcm , which is 10-100 times smaller than those of catechol complexes [62,71]. 

Complexes between quinones and phenols have also been proposed as important lignin chromophores in pulping liquors. Furman and Lonsky [75,76] attributed two-thirds of the light absorption of kraft lignin to quinone-phenol charge-transfer complexes. 

Incidental information suggests that similar donor-acceptor complexes contribute to the color of wood or high-yield pulp fibers. For example, solution spectra of 

As numerous NBO studies have shown (see V B, pp. 593-661 and references therein), hydrogen-bonded complexes of generic formula L - H-L may be generally characterized as donor-acceptor complexes of l-0 hl type, driven by intermolecular resonance delocalization from donor lone pair l of one monomer into the acceptor orbital of the other. 

Alternatively (and equivalently), L - H-L may be regarded as a special case of resonance hybrid (8.23) for A=H, namely, 

In terms of resonance hybrid (9.1) or the equivalent NBO donor-acceptor characterization, we can alternatively describe the H-bonded L - H-L complex as  

As a more representative example of weaker neutral H-bonded species, let us consider the (HF)2 dimer, which offers a particularly clear contrast to the dipole-dipole expectations of classical electrostatics. The (HF)2 species is bound by about 5 kcal/mol (in the same range as water dimer and many other common H-bonded species) and exhibits a curiously bent equilibrium geometry, as shown in 1/0-9.1. Although HF has a robust dipole moment (calculated as p = 1.92 Debye) and F HF has the linear geometry expected for an electrostatic ion-dipole complex, the nonlinear geometry of (HF)2 clearly differs from the expected linear geometry of a dipole-dipole model. What s going on here  

Second-order perturbative analysis of (HF)2 shows the leading intermolecular interaction to be the expected delocalization from donor monomer 

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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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