Bifurcated dimer

Other hydrogen-bonded dimer configurations have been studied theoretically, although they are rarely observed experimentally. These are the cyclic dimer, 1, and the bifurcated dimer, 2. The latter can be referred to as the bifurcated donor dimer, retaining the description bifurcated acceptor for the three-center bonded trimer [303]. 

In the solid state, 2,6-diphenylphenol has a monomeric structure with an intramolecular O-H—tt interaction to one of the ortho-phenyl substituents.143 Interestingly, the 2,6-diphenyl-4-nitrophenol derivative has a dimeric structure in the solid state with intermolecular, as well as intramolecular, O-H—77 interactions between ortho-phenyl substituents.144 The latter compound represented the first example of bifurcated OH—77 bonding (Fig. 28). 

In contrast with the CH4 molecule, the NC-H + bonds in borane amine (CH3)3NBH3 are slightly polarized. Even such slight polarization is sufficient for the formation of the dimer (CH3)3NBH3- (CH3)3NBH3, which can be formulated as a dihydrogen-bonded complex. Here, two bifurcated CH H-B... 

Three general dimer types were considered for both nitroanilines A, the optimal dimer with two distinct H-bonds, each between one amino-hydrogen and one nitro-oxygen B, a relaxed geometry, with at least one bifurcated H-bond that is the local minimum closest to the crystal structure, and, C, the structure obtained by fixing the H-bonds at their experimental (crystal structure) distances and optimizing the rest of the dimer within the same constraints as A and B. Structure C is closest to the experimental structure. 

The crystal structure can be rationalized by considering the influence of interactions with the neighboring chains. For I, II, and V, the bifurcated structure leaves the second amino hydrogen more available for interactions with a neighboring chain than the more energetically favored head-on dimer. The shortening of the hydrogen bond to the ortho substituent (in C vs. B-type structures) that is often apparent may also serve to bet-... 

Figure 6.20. (a) Projection of a three-dimensional PES K(p,p2,p3) for two-proton transfer in formic acid dimer onto the (p, p,) and (p, p3) planes. In contrast with points A and B, in points C and D the potential along the p3 coordinate is a double well resulting in bifurcation of the reaction path [from Shida et al., 1991b]. (b) The contour lines correspond to equilibrium value of p3 and potential (6.37) when V(Q) = V0(Q4 - 2Q2), V0 = 21 kcal/ mol, C = 5.()9V0, A = 5.351/, Qn = 0.5. When Q > Qc, two-dimensional tunneling trajectories exist in the shaded region between curves 1 and 2. Curve 3 corresponds to synchronous transfer. 

Examples Gas phase dimers with strong acids/bases Acids Minor components of bifurcated bonds... 

The PES of the water dimer system is characterised by three main stationary points the "quasi-linear" structure, representing the global minimum (Fig. 11), and the cyclic and bifurcated configurations (saddle points). 

Table 2 Results of Variational Localized-site Cluster expansions from either a Neel-state based ansatze or a Resonating Valence Bond ansatz. We notice that the lower level NSBA is unable of showing the dimerization of polyacetylene. rc is the critical bifurcation mean bond length, r and r2 are the optimized short and long bond distances (in A). E is the energy per carbon atom (in eV), taking the energy of the Neel state with 1.40 A equal bond lengths as zero of energy. NSB forth order perturbative and Dimer-covering second order perturbative (see Ref. 34), CEPA ab-initio estimate of Kpnig and Stollhoff [52], and the experimental results [46,47] for rx and r2 have been added for comparison.

In protophilic media, amides exist as monomeric H-complexes with a two-centered H-bond and 1 2 H-complexes of the open-chair dimer with a bifurcated (three-centered) hydrogen bond. The formation of a strong bifurcated H-bond weakens the bridging N-H...O=S bond. 

For the bifurcated water dimer, 2, the calculated energy was about half that of the linear dimer. No examples have been observed in hydrate crystal structures, except as components of three-center bonds, see Part IV, Chapter 22. 

Bifurcated hydrogen bonds between dipyridinium cations and coordinated dithiooxalates have been used to assemble tapes [130] with similar geometry to those in which coordinated halides act as acceptors [15]. Replacement of dipyridinium with 4-carboxypiperidinium gave a structure in which the anions are linked by carboxylic acid dimer assembled dications. Similar bifurcated hydrogen bonds were observed in the structures of bipyridinium salts of anilic acids [131]. 

Figure 4.19 Bifurcating PES for the dimerization of cyclopentadiene. Heavy black lines are two trajectories that lead from the Diels-Alder bispericyclic TS 77 to the valley-ridge inflection (VRI) point and then diverge, one into the well of product 76a and one into the well of product 76b.

Quadrelli, P. Romano, S. Toma, L. Caramella, P. Merging and bifurcation of 4-1-2 and 2-1-4 cycloaddition modes in the archetypal dimerization of butadiene. A case of competing bispericyclic, pericyclic and diradical paths, Tetrahedron Lett. 2002, 43, 8785-8789. 

The ubiquitous occurrence of water and its importance as a solvent medium have motivated a great deal of research into the fundamental nature of the interaction between water molecules by theoretical as well as experimental means. Some of the more recent work has been summarized in a review article. Prior to experimental elucidation of the geometry of the water dimer in the gas phase or to the ability of calculations to provide an unambiguous resolution to this question, a number of different candidate structures were considered. In addition to the standard linear arrangment wherein the bridging hydrogen lies near the O O axis in Fig. 2.8, cyclic and bifurcated structures were considered as illustrated in Fig. 2.9. 

While experimental measurements of the water dimer in the gas phase had yielded an unambiguous linear structure, the results for were less clear . Cyclic and bifurcated... 

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