Asymmetric hydrogen bonds

Perhaps more remarkable than the accuracy of equation (2) is the gross oversimplification that it contains. The formulation of this equation rests upon the presumption that during the first half of its transfer, while the proton is closer to the lefthand subunit, it feels no effect whatsoever from the other subunit. Upon crossing the midpoint, the equation prescribes that the lefthand subunit suddenly vanishes and the proton feels only the influence of the right subunit. It is likely that the equation fares as well as it does due to a cancellation of errors along the transfer potential. 

Simplifying assumptions notwithstanding, the accuracy of the Marcus equation (2) provides an immeasurably useful tool to estimate proton transfer 

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Chapter 9, on entropy and molecular rotation in crystals and liquids, is concerned mostly with statistical mechanics rather than quantum mechanics, but the two appear together in SP 74. Chapter 9 contains one of Pauling s most celebrated papers, SP 73, in which he explains the experimentally measured zero-point entropy of ice as due to water-molecule orientation disorder in the tetrahedrally H-bonded ice structure with asymmetric hydrogen bonds (in which the bonding proton is not at the center of the bond). This concept has proven fully valid, and the disorder phenomenon is now known to affect greatly the physical properties of ice via the... 

An asymmetric hydrogen bond is common even where a proton coordinates two equivalent anions. The rc-bond repulsive forces between two coordinated anions tend to prohibit a close X-H-X separation, so competition between the two equivalent anions for the shorter X-H bond may set up a double-well potential for the equilibrium proton position between the two coordinated anions. With oxide anions, an O-H-O separation greater than 2.4 A sets up a double-well potential and creates an asymmetric hydrogen bond, which we represent as O-H O. Although displacement toward one anion may be energetically equivalent to a displacement toward the other, one well is made deeper than the other by an amount AH, as a result of the motion of the proton from the centre of the bond. 

The dielectric relaxation of water (Hasted, 1973) can be characterized by a relaxation time m = 9-3 X 10-12 s at 293 K with activation energy 20 kj mol-1. The spread of relaxation times is remarkably small for such a complicated liquid. The data are interpreted in terms of rotation by water molecules having two hydrogen bonds, the spread of relaxation times showing that symmetrically hydrogen bonded and asymmetrically hydrogen bonded water molecules have slightly different relaxation times. 

O atoms come into contact when the CM3 distance is larger that twice the O-H distance expected for a symmetrically two-coordinated H atom. To compensate, the H atom moves off-center until its bond valence sum reaches 1.0 e (electrons), giving rise to the characteristically asymmetric hydrogen bond see Hydrogen Bonding) ... 

Further insight into the nature of proton-coupled electron transfer can be obtained from donor-acceptor pairs featuring an asymmetric hydrogen bonding in-... 

Hayashida, O., Ito, J.-L, Matsumoto, S., and Hamachi, I., Preparation and unique circular dichroism phenomena of urea-functionalized self-folding resorcinarenes bearing chiral termini through asymmetric hydrogen-bonding belts, Org. Biomol. Chem., 3, 654-660 (2005). 

R.E. Lechner (2001). Solid State Ionics, 145, 167-177. Proton conduction mechanism in MjH(X04)2 crystals the trigonal asymmetric hydrogen bond model. 

A related problem of instantaneous pictures is the asymmetric hydrogen bonding between R3NH+ and R3N having a vibrational dispersion of the position of the proton with the consequence that two somewhat broadened N1 s signals are separated by a distance somewhat smaller than between clear-cut cases of protonated and un-proton-ated (75) amines. 

Fig. 10.1. Bond-valence - bond-length correlation for H-0 bonds. The dotted lines indicate the valences and lengths found in normal asymmetric hydrogen bonds the thin solid line indicates the valence and length found in symmetric hydrogen bonds...

Enediolate C2v valence tautomer of croconate. To create an asymmetric hydrogen-bond donor environment that stabilizes the C2v valence tautomer of CsOs , we... 

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