Typical Hydrogen Bonds

A proton between two heavy atoms (O, N, or F) usually forms one hydrogen bond and one ordinary bond. In rare cases, the hydrogen atom binds equally strongly to two electronegative atoms, with identical bond lengths. 

Some left-right synunetric hydrogen bonds, for example, in [F-H-F] , possess a single eqnilibrinm point The bonding power is then distribnted eqnaUy on the left and right The strength of the bond is in between an ordinary covalent bond and a hydrogen bond. 

FIGURE 9.3 Typical hydrogen bonds (a) methyl alcohol dimer and (b) acetic acid dimer. 

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That way, the Distributed Electrostatic Moments based on the ELF Partition (DE-MEP) allows computing of local moments located at non-atomic centres such as lone pairs, a bonds and n systems. Local dipole contributions have been shown to be useful to rationalize inductive polarization effects and typical hydrogen bond interactions. Moreover, bond quadrupole polarization moments being related to a n character enable to discuss bond multiplicities, and to sort families of molecules according to their bond order. 

FIGURE 2.9 Typical hydrogen bonding (shown as between hydrogen on nitrogen and oxygen for nylon-6,6. 

Hydrogen bonding to substrates such as carbonyl compounds, imines, etc., results in electrophilic activation toward nucleophilic attack (Scheme 3.1). Thus, hydrogen bonding represents a third mode of electrophihc activation, besides substrate coordination to, for example, a metal-based Lewis acid or iminium ion formation (Scheme 3.1). Typical hydrogen bond donors such as (thio)ureas are therefore often referred to as pseudo-Lewis-acids. ... 

In the hatched two-phase region of limited miscibility, the system separates heterogeneously into water-rich and nicotine-rich layers. However, at temperatures below the lower consolute point (about 61°C) or above the upper consolute point (about 210°C), the components become miscible in all proportions, resulting in a uniform homogeneous phase. The molecular-level origins of this extraordinary behavior, as well as more general aspects of consolute behavior in other (typically, hydrogen-bonded) systems, remain deeply obscure. 

A typical hydrogen bonding reaction is equation 11.10. On the left, a hydrogen bond donor,... 

FIGURE 3.14 Left The electrons in water tend to be pulled more towards the oxygen atom, resulting in an electric dipole moment. Right Structure of the water dimer in the gas phase, showing a typical hydrogen bond. The complex rotates freely about the dashed line the atoms do not always lie in a single plane. 

Table 4 provides some estimates of the magnitude of the electrostatic interaction in a typical hydrogen bond, in comparison to other components. The specific system examined in Table 4 is the water dimer the R(OO) distances examined are 3.25 A and 2.75 A. The latter is approximately the interoxygen separation in ice the two values bracket the equilibrium distance of 2.9-3.0 A in the gas-phase dimer, depending upon the level of theory. 

At present some preliminary evidence has been obtained which suggests that sapphyrins can bind anions other than fluoride. In fact, an X-ray diffraction study shows that NJ binds to the monoprotonated form of sapphyrin [157]. Specifically, as shown in Fig. 29, the monoprotonated sapphyrin, H4Sap Nj, does not complex azide anion in an in-plane fashion but in an end-on manner, with the terminal azide nitrogen atom being 1.13 A above the sapphyrin plane [157]. Nonetheless, this atom is still vsdthin typical hydrogen bonding distance (2.8 to 3,0 A) of at least four of the five pyrrolic nitrogens [59, 173-176]. 

Table 1.11 Some typical hydrogen bond donor (HBD) and hydrogen bond acceptor (HBA) solvents.

Hydrogen bonds occur in all phases solid, liquids and gases. Typical hydrogen bond energies are less than 60 kJ mol 1 and bond lengths are approximately 2 A. Compare those values with a typical covalent bond, where the bond energy is 400-500 kJ mol-1 and bond lengths are 1.0-1.3 A. 

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