Hydrogen bonding resonance

Ammonium acetate in acetic acid converts 55a into the imine (59). The hydrogen-bonded, resonance-stabilized form shown is consistent with its high melting point and intense color. The structure is further supported by the ability of the naphthalene analog, which is more soluble, to form a stable complex with cupric perchlorate. ... 

In long and weak hydrogen bonds, resonance between such forms as (1) and (2) cannot make an important contribution to the bonding. We must emphasize that the conditions for resonance include one that the positions of the atomic nuclei must be the same in all contributory forms. The proton, in such bonds, is much nearer to one oxygen atom than to the other, as we have drawn it in (1) and (2). So (2) will have a much higher energy than (1) because it suffers from an uncomfortably long H-0 bond on the... 

H2O solution (6 to 14 ppm) were recorded between 0° and 55°C and the exchangeable protons identified by comparison with the corresponding spectra recorded in 20 solution. The thymidine H-3 imino hydrogen-bonded resonance is observed at 13.0 ppm in the spectrum of poly(dA-dT) at 25.5°C (Figure 1A) and its chemical shift and line width dependence are plotted as a function of temperature in Figures IB and 1C respectively. 

The validity of the structural model in Figure 17d is proven by the XH DQ MAS spectrum in Figure 17b. First, the observation of two hydrogen-bonded resonances is explained A and B correspond to the O—H "N and O—H "0 protons, respectively (this assignment is on the basis of CRAMPS spectra and also results subsequently obtained for a 15N labeled sample). Remembering that a DQ peak is only... 

Alkorta, I., Elguero, J., Mo, O., Yanez, M., and Del Bene, J.E., Are resonance-assisted hydrogen bonds resonance assisted A theoretical NMR study, Chem. Phys. Lett., 411, 411-415 (2005). 

The IR spectrum of [LRu (bqdi)(OH2)], where L= 1,4,7-trimethyl-1,4,7-triazacyclo-nonane bqdi = o-benzoquinone-di-imine, contains vNH of bqdi as a sharp band at 3310 cm showing the absence of hydrogen bonding. " Resonance Raman spectra of [Ru(dmbip)-(Hdchpy)]X, where dmbip = 2,6-bis(l-methylbenzimidazol-2-yl)pyridine H2dchpy = 4,4 -dicarboxy-2,2 -bipyridine X = Cl, NCS or CN, were used to probe the MLCT excited states. 

Marzocchi M P, Mantini A R, Casu M and Smulevich G 1997 Intramolecular hydrogen bonding and excited state proton transfer in hydroxyanthraquinones as studied by electronic spectra, resonance Raman scattering, and transform analysis J. Chem. Phys. 108 1-16... 

Polyamides. The next two compounds are the amide counterparts of the esters listed under item (4). Although the values of AH j are less for the amides than for the esters, the values of T j, are considerably higher. This is a consequence of the very much lower values of AS j for the amides. These, in turn, are attributed to the low entropies of the amide in the liquid state owing to the effects of hydrogen bonding and chain stiffness arising from the contribution of the resonance form... 

The various mechanical properties of polyamides may be traced in many instances to the possibility of intermolecular hydrogen bonding between the polymer molecules and to the relatively stiff chains these substances possess. The latter, in turn, may be understood by considering still another equilibrium, this one among resonance structures along the chain backbone ... 

In dimers composed of equal molecules the dimer components can replace each other through tunneling. This effect has been discovered by Dyke et al. [1972] as interconversion splitting of rotational levels of (HF)2 in molecular beam electric resonance spectra. This dimer has been studied in many papers by microwave and far infrared tunable difference-frequency laser spectroscopy (see review papers by Truhlar [1990] and by Quack and Suhm [1991]). The dimer consists of two inequivalent HE molecules, the H atom of one of them participating in the hydrogen bond between the fluorine atoms (fig. 60). PES is a function of six variables indicated in this figure. 

Only a limited number of coal-denved pitches were examined by H NMR because of their low solubility in solvents commonly used m conventional proton magnetic resonance. Table 12 reports the distribution of hydrogen for three of the pitches. Unlike coal-tar pitches, which typically have over 85% of the hydrogen bonded to aromatic carbon, the matenals listed in Table 12 are characterized by a high content of aliphatic hydrogen. 

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