Dimers T-shaped

Figure 9.46 Rotational structure of the Ojj bands in the fluorescence excitation spectra of s-tetrazine dimers at about 552 run. Bottom Ojj band of planar dimer. Middle Ojj band of T-shaped dimer with transition in monomer unit in stem of T. Top Ojj band of T-shaped dimer with transition in monomer unit in top of T. (Reproduced, with permission, from Haynam, C. A., Brumbaugh, D. V and Levy, D. H., J. Chem. Phys., 79, f58f, f983)...

There are only a few gas-phase experiments of the benzene dimer. Some experiments point to a T-shape dimer while others ° implicate a sandwich or more likely a displaced stracture. Hole-burning experiments find evidence for multiple different structures. The experimental estimates of the binding energy of 77 are 1.6 0.2 and 2.4 0.4 kcal mol ... 

Fig. 12 Chemical structure of T-shaped dimeric liquid crystal molecules that broadened the temperature range of blue phases to 13 K [24]...

Waals complexes. One is that the vibrational frequencies of each monomer change little upon dimer formation. A similar statement holds for the bond distances and angles. Second, there is often a considerable gap between the frequencies of the monomer units and the van der Waals frequencies. Finally, it is interesting to compare the C—H frequencies and their shifts in the linear and T-shaped dimers. In the linear isomer, the C—H frequency of the HCN unit is free, while the acetylene C—H unit is bound as it forms part of the van der Waals bond. This difference in the C—H environment is reflected in the frequency shifts. That is, the HCN frequency is shifted by only 1 cm, while the bound acetylene frequency is shifted by 27 cm. In the T-shaped isomer, it is the other way around in that the acetylene C—H stretch is free while the HCN stretch is bound. (Apparently, the acetylene C—H stretch is not totally free in the T-shaped dimer as its shift is still 10 cm . )... 

First, one must consider the accepted model of the geometric configuration of the dimer, shown in Fig. 43 and referred to as the T-shaped dimer. The geometric picture is of crucial importance for a correct arrangement of the algebraic model and for comparison with the (few... 

T-shaped dimer containing a terminal and a laterally linked mesogenic unit. 

Table 7 MP2 and CCSD(T) Interaction Energies of the Stacked and T-shaped Dimers of Acetylene and Diacetylene ...

Electrostatics stabilizes the T-shaped dimer, but destabiUzes the sandwich and slipped-parallel dimers, as shown in Table 4. The electrostatic interaction in the slipped-parallel dimer becomes attractive when the molecules have... 

CCSD(T) calculations of the naphthalene dimer using a large basis set are extremely difficult therefore, the MP2 method was used even in recently reported calculations [61-63]. The MP2 interaction energy depends significantly on the basis set (- 3.74 to - 10.69 kcal/mol) nevertheless, all the MP2 calculations concluded that the slipped-parallel dimer is stabler than the T-shaped dimer. Local MP2 calculations also show the same preference [64]. Very recently reported CCSD(T) interaction energies of the naphthalene dimers at the basis set limit show that the slipped-parallel and rrn j dimers are the stablest (- 5.28 and - 5.73 kcal/mol, respectively) and the T-shaped and sandwich dimers are substantially less stable (- 4.34 and - 3 78 kcal/mol respectively) [30]. /. ... 

Table 4 shows that electrostatic interactions stabilize the T shaped dimer... 

Although the benzene dimer has been extensively studied as a prototype for the tt/tt interaction, its relevance to general properties of aromatic residues in proteins was questioned. The toluene dimer was instead proposed as a better prototype for the tt/tt interaction in proteins [65]. MP2 calculations show that the stacked dimers are substantially stabler than the T-shaped one [65,66] and that the antiparallel dimer is 0.9 kcal/mol stabler than the parallel dimer [66]. Recently reported CCSD(T) calculations of the toluene dimer show that the cross dimer (- 4.08 kcal/mol) is slightly stabler than the antiparallel and parallel dimers (- 3.77 and - 3.41 kcal/mol, respectively), as shown in Table 4 [10]. These stacked dimers are substantially stabler than the T-shape dimer (- 2.62 kcal/mol). Spectroscopic measurements show that the toluene dimer consists of at least two isomers [67,68). The interaction energy of the toluene dimer ( e) obtained from a CCSD(T) calculation (- 4.1 kcal/mol) is not very different from the experimental Eg value (- 3.6 kcal/mol) [69]. 

CCSD(T) interaction energies of the thiophene dimer show that the T-shaped dimer is stabler than the parallel dimer (Fig. 9), as shown in Table 4 [31]. Although dispersion is the major source of the attraction in the thiophene dimer, electrostatics plays an important role for stabilizing the T-shaped... 

The benzene dimer has two nearly isoenergetic structures (sHpped-parallel and T-shaped) [9,11]. Dispersion stabiUzes the slipped-parallel benzene dimer strongly, while the electrostatic interaction is repulsive in the slipped-parallel dimer. However, the electrostatic interaction stabihzes the T-shaped benzene dimer, while the stabilization due to dispersion is small in the T-shaped dimer [9]. On the other hand, both dispersion and electrostatic interactions stabihze the stacked hexafluorobenzene-benzene complex, producing a large attractive force between hexafluorobenzene and benzene. 

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