Conformation internal hydrogen bonding

Aformamide-water complex109 was studied by Sim et al. by means of the DFT(SVWN), DFT(PW86/P86), and DFT(B88/P86) calculations. The LDA led to qualitatively wrong results for conformational energies of this hydrogen-bonded complex. Furthermore, the poor performance of the LDA calculations was observed in studies of conformational equilibria in malonaldehyde, a molecule with the internal hydrogen bond. 

Caminati, W., A. C. Fantoni, B. Velino, K. Siam, L. Schafer, J. D. Ewbank, and C. Van Alsenoy. 1987a. Conformational Equilibrium and Internal Hydrogen Bonding in 2-Methylallyl Alcohol Detection of a Second Conformer by Microwave Spectroscopy on the Basis of Ab Initio Structure Calculations. J. Mol. Spectrosc. 124, 72-81. 

Figure 18. Conformations of diol epoxides of BP. (a) Diagrams of the anti- and syn-isomers of a diol epoxide showing the possibility for internal hydrogen-bonding in the syn-diol epoxide.

C. J. Cramer and D. G. Truhlar, Quantum chemical conformational analysis of 1,2-ethanediol Correlation and solvation effects on the tendency to form internal hydrogen bonds in the gas phase and aqueous solution, J. Am. Chem. Soc. 116 3892 (1994). 

Figure 8. IR and Raman OH stretching spectra of n propanol monomers and dimers reflecting conformational diversity. The Raman spectrum reveals the dominance of the internally hydrogen bonded Gt monomer most clearly, whereas the IR spectrum indicates more than five different dimer conformations in the red shifted dimer spectrum [69].

Torsional parameters and VdW parameters for internal hydrogen bonds in the N—C—N moiety were obtained by fitting the ab initio rotational profiles of methylenediamine (MDA, 15) and /V-methylmelliylenediamine (NMMDA, 16). A comparison of relative conformational energies between ab initio and MM2 results for 15 and 16 is provided in Table 6. Bond length correction terms for inner and outer C—N bonds (K, K2 and... 

The symbols were defined in Section IV.B.) A full discussion of the ortfio-effect as revealed in this work would be inappropriate here. We must restrict ourselves to the more limited task of indicating the role of o-N02. We discuss first the work involving alcohols as solvents. To apply the extended Hammett equation, i.e. to determine the regression coefficients a, fi and

intercept term h, it is first necessary to select a set of substituents which can be expected to be well-behaved . Particular problems for or and v may be caused by conformational effects, and internal hydrogen-bonding may occur... 

Concluding, we may state that in aqueous solutions of APTS the amine is involved in acid-base interactions. The conformation of the molecule changes with changing pH of the medium. At natural pH (10.8) the hydrolyzed aminosilane forms a 6- or 7-membered ring structure, stabilized by internal hydrogen bonding or protonation. 

Fig. 6.13 Internally hydrogen-bonded conformation of 1-naphthyl-taddol.

There has been considerable interest in the factors that control the stereoselectivity of cyclobutanol formation. Three main factors were identified quite early pre-existing conformational preferences due to steric effects or to internal hydrogen bonding solvation of the OH group and variable rotational barriers for cyclization. More recently Griesbeck has proposed that orbital orientation favoring soc produces another form of conformational preference in triplet biradicals [55], These factors have different importance depending upon the molecule. 

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