Anti-H-bonding

Figure 5.40 The optimized structure and leading n->- a donor-acceptor interaction of the anti-H-bonded H2Pt PtH2 complex.

Finally, Figure 5.40 displays the surprising structure of the (PtH2)2 dimer, an anti-H-bond n—a donor-acceptor complex. As expected, the leading NBO donor-acceptor interaction is of npt crptH type, but the attack by npt is now on the backside of the cjrh NBO, leading to unusual Pt - -Pt—H coordination to the hydride antibond as depicted in Fig. 5.40b. 

The Pt - -Pt—H anti-H-bond linkage can also be viewed as a hypervalent 3c, 4e (co-bond) resonance hybrid of the form... 

Figure 3.54 Energy levels of a dipolar, hydrogen bonding molecule M (excited state M ) in a polar, non-protic solvent (left) and a polar, protic solvent (right). H is the H-bond energy in the ground state, H in the excited state. (1), (2), (4) and (7) correspond to solvation relaxation, (3) and (6) to anti-H-bond dissociation, and (5) and (8) to H-bond formation...

In the table, and also elsewhere in the present chapter, we use the notation H-bonded to specify a structure bound by a hydrogen bond, and anti-H-bonded to specify a structure where VdW binding is opposite to that of a hydrogen bond. 

Table 5.5 Dipole polarizabilities a and relative stabilities A of H-bonded with respect to anti-H bonded structures in Rg-HF interactions (atomic units)...

From the data reported in this work, we obtain Table 5.5, which gives the relative stability A of H-bonded with respect to anti-H-bonded structures in the whole series of heterodimers.The distances for the heavier rare gases (Ar, Kr, Xe) are those given by Hutson and Howard (1982) for the well depths of the potentials at the absolute minima of the H-bonded structures. The experimental values of the dipole polarizabilities a of the Rg gases taken from Soldan etal. (2001) are given in the second column of the table. It is seen that the stability of the H-bonded structures increases with the dipole polarizability of the heavier rare gases. 

Brandi, M., Meyer, M., and Suhnel, J. (2001) Quantum-chemical analysis of C-H- - -O and C H- - -N interaidions in RNA base pairs—H-bond versus anti-H-bond pattern, J. Biomol Struct. Dyn. 18, 545-555. 

The chapter hrst presents a history of the problem, how the CH- -O H-bond went from a nonissue to one of recognized importance. This history also discusses the contributions made by computational chemistry along the way, at each stage. We then turn our attention to a unique and surprising property of the CH- -O bond (or at least certain of them) that led some to initially deny its characterization as a H-bond at all, and others to go so far as to dub it an anti-H-bond . This property is then analyzed and placed into proper perspective through the power of modern quantum chemistry. The chapter closes with a perspective on the paths that future work on this problem is apt to take. 

Continued work in this direction led to expansion of the so-called anti-H-bond concept to nonaromatic systems, as for example the pairing of CHF3 with ethylene oxide... 

Blue-shifted. - Cubero et used AIM to examine the nature of what they termed the anti-hydrogen bond interaction . Upon formation of the anti H-bond the X-H bond shortens and its vibrational frequency blue-shifts contrary to a H-bond. Amongst others the topological properties of p have been determined for a series of C-H... n complexes, which exhibit either anti H-bond or normal H-bond character. According to a set of topological criteria utilized to characterize conventional H-bonds no relevant difference was found in the two classes of C-H... 71 complexes. Since the anti H-bonds are just a special type of H-bond the qualifier anti is misleading and was abandoned in later work. 

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