Application to Hydrogen-Bonded Complexes

These results can be compared with those recently obtained using a much simpler chemical electrostatic theory [228, 229, 296]. In this approach, the elec- 

The results of some theoretical calculations on other hydrogen-bonded dimers with functional groups of biological relevance are given in Thble 4.4. For these dimers, comparison with experiment is not generally possible, since they are insufficiently stable in the gas phase. 

Early theoretical calculations on hydrogen-bonded water polymers predicted cooperativity. Even when it was possible to carry out calculations only at relatively low approximation levels, comparison of the relative values for the water dimers, trimers, tetramers and higher polymers were very informative. They predicted the nonadditivity of hydrogen-bond energies and the phenomenon of cooperativity, discussed in Chapter 1 and elsewhere throughout this text [103-105]. One such calculation involved successive polymers, the results of which are shown in Thble 4.5. 

The energetic advantage of homodromic cyclic bonding was predicted. The 

Because of the crystal field distortion, the hydrogen-bond lengths tend to be shorter in crystals than observed experimentally or predicted theoretically in hydrogen-bonded dimers. The information from gas-phase diffraction and micro-wave spectroscopy on hydrogen-bonded dimers is necessarily limited to the relatively few dimers which are stable in the gas phase. X-ray and neutron diffraction crystal structure analysis, in contrast, provides a plethora of structural data relating to hydrogen-bond formation, as is demonstrated in Part IB, Chapter 7 and subsequent chapters. 

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Baker, J., Buckingham, A. D., Fowler, P. W., Steiner, E., Lazzeretti, R, Zanasi, R. (1989). The electrostatic model of field gradients at nuclei -an application to hydrogen-bonded complexes of... 

Esterification. Carboxylic acids can be esterified by reaction with alkyl halides and DBU in benzene at 25 or 80° (equation I). The reaction is widely applicable to hindered or unstable acids and to BOC- or CBZ-protected amino atids. Yields are invariably high. Triethylamine is of less value as the base. Presumably the hydrogen-bonded complex of DBU and the acid plays a significant role in the reaction. The DBU can be recovered by treatment of the hydrohalide with sodium hydroxide. ... 

Bouteiller, Y., Mijoule, C., Karpfen, A., Lischka, H., and Schuster, P., Theoretical vibrational investigation of hydrogen-bonded complexes Application to CIH-NH, ... 

The linear response function [3], R(r, r ) = (hp(r)/hv(r ))N, is used to study the effect of varying v(r) at constant N. If the system is acted upon by a weak electric field, polarizability (a) may be used as a measure of the corresponding response. A minimum polarizability principle [17] may be stated as, the natural direction of evolution of any system is towards a state of minimum polarizability. Another important principle is that of maximum entropy [18] which states that, the most probable distribution is associated with the maximum value of the Shannon entropy of the information theory. Attempts have been made to provide formal proofs of these principles [19-21], The application of these concepts and related principles vis-a-vis their validity has been studied in the contexts of molecular vibrations and internal rotations [22], chemical reactions [23], hydrogen bonded complexes [24], electronic excitations [25], ion-atom collision [26], atom-field interaction [27], chaotic ionization [28], conservation of orbital symmetry [29], atomic shell structure [30], solvent effects [31], confined systems [32], electric field effects [33], and toxicity [34], In the present chapter, will restrict ourselves to mostly the work done by us. For an elegant review which showcases the contributions from active researchers in the field, see [4], Atomic units are used throughout this chapter unless otherwise specified. 

Taft et al. (1969) have defined a base parameter, pA hb, which measures the relative strength of the acceptor when a hydrogen-bonded complex is formed using any suitable hydroxyl reference acid. Values of pA hb are not applicable to reference acids involved in intramolecular hydrogen bonding. The reader is referred to the original article for details. 

The quantitative evaluation of the corresponding interaction energies had to await the development of computers and ab initio systems in the 1960s. By the early 1970s it was apparent that self-consistent field (SCF) theory provides a reasonably accurate description of hydrogen-bonded complexes like (H O) while theories that explicitly account for electron correlation " must be used for systems which are predominantly bound by dispersion forces, such as He-H2 and Hcj. Rapid developments in both hardware and software have since taken place and ab initio calculations on weakly bound systems are now routinely being carried out. Useful information is gathered in this way and the potential surfaces obtained find application in simulation studies of liquids, solids and various solvation problems. ... 

Theoretical Vibrational Investigation of Hydrogen-Bonded Complexes Application to ClH-NHj, CIH-NH2CH3 and BrH-NH.,. 

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