Intramolecular hydrogen bond distances

The diketo tautomer adopts the U (Z,Z) conformation with the oxygen atoms 277 pm apart and the two carbonyl groups at a dihedral angle of 48.6° to each other. The enol tautomer was planar and the intramolecular hydrogen bond distance, R(0"0) was only 238 pm. The hydrogen bond was linear and symmetric as would be expected for such a short bond71. A bond this short was unheard of for a neutral molecule, and still is but we have reason to believe that this information about AA is unsound. 

Conjugation is more important 1 3 Cyclohexanedione exists mainly in its enol form in spite of the fact that intramolecular hydrogen bonding is impossible due to the distance between the carbonyl group and the enohc —OH group... 

The alkaloid Nigellicine proved to be the pyridazino[l,2-u]indazolium-l 1-carboxylate (234) and forms yellow crystals (Scheme 77). It was isolated from the widely distributed herbaceous plant Nigella saliva L., which is used as a spice and for the treatment of various diseases (85TL2759). The structure was determined by an X-ray crystal structure analysis. The carboxylate bond distances are essentially equal (123.3 and 125.6 pm). An intramolecular hydrogen bond was found between the carboxylate oxygen atom and the hydroxy group. In mass spectrometry, the molecular peak was found at mjz —246 (20) and the base peak at mjz —202 which corresponds... 

P2j Z = 2 D = 1.57 R = 0.048 for 931 intensities. The base exists in the thioxo form, with C-8=S and N-7 protonated. The 8-thio substituent causes the base to assume the syn (—102.6°) orientation. The o-ribosyl group is 2T3 (174.8 °, 44.1 °). The exocyclic, C-4 -C-5 bond orientation is trans (—173.2°). This does not favor intramolecular hydrogen-bonding of 0-5 to N-3 of the syn base. The C=S distance is 166.8 pm. The S atom is involved in a weak, acceptor hydrogen-bond to a water molecule, S H-O(w) = 361 pm. The bases are stacked head-to-tail, with overlap of the C=S bonds and the purine ring, in contrast to the known, related structure l-/ -D-ribofuranosyl-2-thioxo-3ff-benzimidazole,197 where similar head-to-tail stacking of the bases involves overlap of the base rings only. 

The structure of 11 2IBr is (TBP, TBP), which represents the first example of a T-shaped adduct of a selenium compound with IBr.39a The anomalous bond distances in ll-2IBr (Se-Br (A)/Se-I (A) 2.897(2)/2.606(2) and 2.766(2)/ 2.629(2) for the two Br-Se-I units, respectively) have been explained on the basis of different polarities of the two bonds and on the basis of the involvement of Br in intramolecular hydrogen bondings. Reactions and/or structures are similarly investigated for A-methylthiazolidine-2(3H)-selone (12), N-methylbenzothiazole-2(3H)-selone (13), tV,7V -dimethylbenzoimidazole-2(3H)-thione (14), and -selone (15) with bromine,39b,c and more recently, for 14 and 15 with IBr.39d In this last case, the prediction of a 10-1-2 adduct for 14 IBr and a 10-Se-3 adduct for 15 IBr from the DFT calculations has been experimentally confirmed.39d... 

These configurational changes will affect the total pair correlation in the model compounds. In addition, in the real molecules we have also a large effect due to changes in proton-proton distances (as well as differences in solvent effects and intramolecular hydrogen bonding). 

Malonaldehyde, CH2(CHO)2, exists as an intramolecularly hydrogen-bonded enol (86) in the vapour phase. Molecular dynamics calculations suggest that while a short 0—0 distance favours proton transfer to an (identical) tautomer, such proximity is neither a sufficient nor a necessary condition. 

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