Hydrogen bond, isotropic proton

Isotropic H chemical shifts for weakly hydrogen bonded hydrates have recently been compared [81] with previous data on carboxylic acids with O-H 0 hydrogen bonds of strong and medium strengths. The values of SH for the hydrogen bonded protons in this work varied from 4.8 ppm in NaCl04-H20 to 20.5 ppm in potassium hydrogen malonate. 

A clear correlation between isotropic H chemical shift and the frequency of the O-H stretching vibration has been reported [61] for surface hydroxyl groups in zeolites and related materials, as well as for water molecules in solid hydrates and strongly hydrogen bonded protons in inorganic solids. 

Similar to the situation for 13C, isotropic 15N chemical shifts and the principal components of 15N chemical shift tensors have been used to study N-H- -0=C hydrogen bonds in peptides. It has been shown that isotropic 15N chemical shifts of proton donors (such as N-H) are displaced downfield by ca. 15 ppm, whereas those of proton acceptors are shifted upfield by ca. 20 ppm [110-112]. Amongst the CSA components, S33 (parallel to the C-N bond) has been shown to be most sensitive to the hydrogen bond strength, as reflected by the N- -O distance [113]. Detailed studies of the principal components and orientations of 15N chemical shift tensors for amide nitrogens in simple peptides have been reported recently [114]. This work confirmed that S33 and Siso are the 15N chemical shift parameters that are the most sensitive to details of the hydrogen bonding. It was also found that N-H... 

These strong and weak hydrogen bonds are randomly and isotropically distributed in the ice structure and their populations should correspond to the ratio of the integrated peak areas. The observed value, low-energy mode to high-energy mode, is about 1 2 which agrees well with the assumption of the protons in the structure is complete disordered. The difference between the force... 

For the 1SN isotropic chemical shift the protonation-induced shifts are of the order of 100 ppm towards lower frequencies for aromatic amines, and about 25 ppm towards higher frequencies for aliphatic amines [62]. For example, in the case of glycine residues in solid oligopeptides, a relationship between the N-O distance and the 15N chemical shift tensor in C=0 H-N hydrogen bonds has been proposed [63]. 

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