Di-hydrogen bonds

Comparatively little is, known about H bonding to metal atoms, that is, types X-H—M or M-H—X, and also di-hydrogen bonding of the type M-H—H-X, where X=C,N,0 etc. [46,47]. Examples of P-containing compounds with M-H bonding are probably [48]. 

The tenn di-hydrogen bond was coined to describe an interaction of the type D-H- H-E, where D is a typical hydrogen donor such as N or O. The interesting thing about this type of bond, is that the acceptor atom is also a hydrogen. 

We have also studied a series of nine complexes presenting di-hydrogen bonds (LiH-HCCH, LiH-HCN, LiH-HCF3, NaH-HCCH, NaH-HCN, NaH-HCF3, HBeH-HCCH, HBeH-HCN, HBeH-HCF3) to check the ability of NCI to detect new types of bonds even at the promolecular level. After using the MAPS Platform [89] to set up the initial systems, the complexes were optimized at the MP2/aug-cc-pVDZ level with NWChem [90]. The optimized coordinates were used to perform NCI promolecular analysis. The results are displayed in Fig. 18.13. 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

Finally, none of the ionic liquids were found to be hydrogen bond acids [5], although this may well be a consequence of the salts selected, none of which had a cation that would be expected to act as a hydrogen bond donor. Earlier qualitative measurements on ionic liquid stationary phases of mono-, di-, and trialkylammo-nium salts suggest that hydrogen bond donation can be important where a potentially acidic proton is available [7-9]. More recent work, with [BMIM] salts, also indicates that these ionic liquids should be considered to be hydrogen bond donor solvents [10]. However, this has yet to be quantified. 

Based on the kinetic studies, a mechanism for this oxidation was proposed45 which involves a nucleophilic attack by the sulphide on a cyclic hydrogen-bonded form of the peracid (equation 9). Since oxidation using peracids occurs under very mild conditions, it can be successfully applied to the preparation of base sensitive sulphoxides. Thus, di(a-bromobenzyl) sulphoxide 25, which is very labile in the presence of a base, was obtained by careful oxidation of a-di(a-bromobenzyl) sulphide by means of m-chloroperbenzoic acid (MCPBA)46 (equation 10). 

In the course of this study, the authors determined /Lvalues for dibenzyl, methyl phenyl, methyl p-nitrophenyl, di-p-tolyl, di-isopropyl and tetramethylene sulphoxides and for diethyl, dipropyl and dibutyl sulphites. The /Lscales are applied to the various reactions or the spectral measurements. The /Lscales have been divided into either family-dependent (FD) types, which means two or more compounds can share the same /Lscale, family-independent (FI) types. Consequently, a variety of /Lscales are now available for various families of the bases, including 29 aldehydes and ketones, 17 carboxylic amides and ureas, 14 carboxylic acids esters, 4 acyl halides, 5 nitriles, 10 ethers, 16 phosphine oxides, 12 sulphinyl compounds, 15 pyridines and pyrimidines, 16 sp3 hybridized amines and 10 alcohols. The enthalpies of formation of the hydrogen bond of 4-fluorophenol with both sulphoxides and phosphine oxides and related derivatives fit the empirical equation 18, where the standard deviation is y = 0.983. Several averaged scales are shown in Table 1588. 

Fig. 40. — (a) Stereo view of two turns of the right-handed, 2-fold helix of Rhiz. trifolii capsular polysaccharide (47). The mono- and di-saccharide side chains (filled bonds) are involved in intrachain hydrogen bonds so that the molecule has the appearance of a double helix composed of an inner and outer strands. The vertical line represents the helix axis. 

Diheterolevulosans, 209-211, 240 Dihexulose dianhydrides, 207 -266, see also Caramels Di-D-fructose dianhydrides 13C NMR spectra, 245-246 conformation, electronic control, 224-228 conformational rigidity, energetic outcomes, 228 hexulopyranose rings, 226 historical overview, 210-213 H NMR spectra, 248 -249 intramolecular hydrogen-bonds, 227 isomerization, 231 -232 1,2-linked, ero-anomeric effect, 224-225 listing, 240-241 nomenclature, 208-210 optical rotations and melting points, 242-243 protonic activation... 

The structural investigations have been extended to potassium derivatives of the "super" formamidine HC(NC6H3Pr2-2,6)(NHC6H3Pr2-2,6) ( = HDippForm). Treatment of the free formamidine with KN(SiMe3)2 yielded the formamidinate species [K(DippForm)2K(THF)2l (THF) , which exhibits a macromolecular structure of alternating fj -arene f/ -amidinate bound potassium di-amidinate and potassium di-THF units in a one-dimensional polymeric array (Figure 5). Addition of a further equivalent of HDippForm afforded hydrogen-bonded [K(DippForm)(THF)3](HDippForm). ... 

In some molecules, the twist conformation is actually preferred. In all cis-2,5-di-fert-butyl-l,4-cyclohexanediol, hydrogen bonding stabilizes the otherwise high-energy form and 1,3-dioxane 89 exists largely as the twist conformation shown. Of course, in certain bicyclic compounds, the six-membered ring is forced to maintain a boat or twist conformation, as in norbornane or twistane. 

Another type of steric effect is the result of an entropy effect. The compound 2,6-di-fert-butylpyridine is a weaker base than either pyridine or 2,6-dimethylpyridine. The reason is that the conjugate acid (8) is less stable than the conjugate acids of nonsterically hindered pyridines. In all cases, the conjugate acids are hydrogen bonded to a water molecule, but in the case of 8 the bulky tert-butyl groups restrict rotations in the water molecule, lowering the entropy. 

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