Weak H-bonds

Weak H-bonds are for instance tt hydrogen bonds that have acceptors that are not atoms with nonbonding orbitals, but a set of atoms with polarizable orbitals such as Tr-orbitals 

The Hydrogen Bond Formation, Thermodynamic Properties, Classification 

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Weak H-bond acceptor (or weak electron donor)... 

First let us restrict the comparison to very weak H bonds because in such a case we can expect that the adiabatic approximation is fulfilled. We made some sample calculations shown in Figs. 8(c) and 8(d), with a small dimensionless anharmonic coupling parameter, ac = 0.6. The figures displays /sf ex [dashed line (d)] and 7Sf [dashed line (c)] at 300 K. Each one is compared with the adiabatic spectra 7f (superimposed full lines). Note that the adiabatic spectra (c)... 

In the full quantum mechanical approach [8], one uses Eq. (22) and considers both the slow and fast mode obeying quantum mechanics. Then, one obtains within the adiabatic approximation the starting equations involving effective Hamiltonians characterizing the slow mode that are at the basis of all principal quantum approaches of the spectral density of weak H bonds [7,24,25,32,33,58, 61,87,91]. It has been shown recently [57] that, for weak H bonds and within direct damping, the theoretical lineshape avoiding the adiabatic approximation, obtained directly from Hamiltonian (22), is the same as that obtained from the RR spectral density (involving adiabatic approximation). 

Owing to the above remarks, we shall study in the following the features of the lineshapes of weak H bonds in which there is a dephasing of the fast mode and simultaneously a damping of the slow mode to which the fast one is anharmo-nically coupled. The quantum spectral density is then [96]... 

The anharmonic coupling parameter is able to reproduce a relatively large half-width of a weak H bond. 

According to the above conclusions, some remarks may be made regarding the nature of the damping mechanism that may be inferred to be efficient in any actual lineshape of weak H bond, according to its experimental features ... 

The pure quantum approach of the strong anharmonic coupling theory performed by Marechal and Witkowski [7] gives the most satisfactorily zeroth-order physical description of weak H-bond IR lineshapes. 

The ability of quaternary ammonium halides to form weakly H-bonded complex ion-pairs with acids is well established, as illustrated by the stability of quaternary ammonium hydrogen difluoride and dihydrogen trifluorides [e.g. 60] and the extractability of halogen acids [61]. It has also been shown that weaker acids, such as hypochlorous acid, carboxylic acids, phenols, alcohols and hydrogen peroxide [61-64] also form complex ion-pairs. Such ion-pairs can often be beneficial in phase-transfer reactions, but the lipophilic nature of H-bonded complex ion-pairs with oxy acids, e.g. [Q+X HOAr] or [Q+X HO.CO.R], inhibits O-alkylation reactions necessitating the maintenance of the aqueous phase at pH > 7.0 with sodium or potassium carbonate to ensure effective formation of ethers or esterification [49,64]. 

Treatment of compound 222, containing a 1,2,4-trioxane ring fused to a cyclopentene ring, with O2 leads to formation of a hydroperoxide (223) with ene displacement, as shown in equation 76. The structure of 223 was determined by single-crystal XRD analysis. A contact of the hydroperoxy group with the endocyclic ether O atom of a neighboring molecule (287.4 pm) points to weak H-bonding. ... 

A) For a system with a weak H-bond showing separate potential minima for the... 

The first feature of interest is the variation of the peak height of the 3630 cm "3 (monomer) band with temperature and with concentration. Fig. 3 shows the apparent absorption coefficient, em, of the monomer band in ethanol as a function of concentration at several temperatures. The general decrease of em with increasing concentration is, of course, the result of depletion of monomer by formation of H-bonded species. Tt should be noted that the intercepts at zero concentration vary with temperature, with the highest value coming at the lowrest temperature. We reported this effect previously [10] and attributed it to interaction between alcohol monomers and the 0014 solvent, probably similar to weak H-bonding. Others have also noted this effect [11]. 

The origin of this absorption may be water molecules with one OH group bonded to the membrane and the second OH group non-H-bonded or water molecules with H-bonds to weak acceptors of the membranes. In both cases the water transport through these membranes may be related to weak H-bonds water-membrane. In addition the spectroscopic observations show in celluloseacetate — or polyimide — membranes are less water molecules of the type of liquid water. In agreement with this observation we have found in model glass membranes with low salt rejection (about 70%) at high relative humidity water spectra not far from the spectra of liquid water. As result of these first experiments we may discuss two possible mechanism of membranes for desalination processes ... 

The small destabilization of the relaxed crystal, B, and crystal structures, C, relative to the optimal dimer are likely overcome by other interactions in the crystal such as attractions between planes and weak H-bonding between adjacent chains. For example, the amino hydrogen not involved in an H-bond in IC can form a weak interaction with a nitro-group on the adjacent chain. 

More recently, Engel and Olesik have also concluded that formic acid is a good modifier for carbon dioxide in SFC. The results they obtained on porous glassy carbon stationary phases with 1.5% (w/w) formic acid in CO2 (16) showed that formic acid was effective because of its strong H-bond donor and very weak H-bond acceptor characteristics higher concentrations of formic acid (3%), however, were found to polymerize on the porous glassy carbon surface (17). 

Jablonski M, Palusiak M (2010) Atoms-in-molecules Dependence of results on basis set was not strong in a study of weakly H-bonded systems. J Phys Chem A 114 2240... 

The position of the intercalated DMSO molecule in the interlayer space of kaolinite is such that the first S-C group is almost parallel with the surface of kaolinite and the second S-C group is directed into the tetrahedral cavity of kaolinite [130, 131]. The S=0 group has 40.3° inclination to the basal surface of kaolinite. The methyl group of DMSO is influenced by both the opposing mineral surfaces. In addition, the intercalation of DMSO molecule results in the expansion of kaolinite from 7.2 to 11.19 A phase [132]. The formation of H-bonds between DMSO and surface OH groups of the octahedral side and weak H-bonds with the tetrahedral side of kaolinite was suggested [133-139]. 

The Adiabatic Approximation [51] Dealing With the Strong Anharmonic Coupling Theory of Weak H-Bonds Partition of the Full Hamiltonian into Diabatic and Adiabatic Parts Weakness of the Diabatic Hamiltonian [122]... 

Appendix K shows that, in the absence of damping, the IR line shape of the vx-h °f weak H-bonded species calculated with the aid of Eqs. (l)-(3) is equivalent to that in which the line shape is computed with the aid of the following equations ... 

The basic quantum theories [47-50], dealing with the IR line shape of the vX-h °f weak H-bonded species working within the linear response theory have been performed with the aid of Eq. (4) in place of (1) used by Bratos [45] and Robertson and Yarwood [46]. 

Owing to the equivalence between the two approaches in the absence of damping, we study here the IR line shape of the vx-11 of weak H-bonded species, within the linear response theory, with the aid of the simple method using Eqs. (4—6), even in the presence of damping. 

Besides, p(0), the IR absorption transition operator at time t = 0 and p(t) is this same operator at time t. Then, beyond the harmonic approximation and in view of Eq. (9) giving the expression of the transition moment operator, the ACF of the bare weak H-bond, may be written... 

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