H-shaped dimers

H-shaped dimer in which the mesogenic units are laterally attached... 

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)...

Uracil dimers were studied at the MP2 level using the 6-3IG basis with modified polarization functions. Eleven low-energy minima were located Seven of them are H-bonded, one is T-shaped, and three correspond to various stacked arrangements. The most stable structure was found to be the H-bonded dimer with two Ni-H O2-H bonds (Scheme 85) [98JPC(A)6921]. 

In this chapter, we shall examine two different approaches to explain the nature of the hydrogen bond and the structure of H-bonded dimers. First, a qualitative MO model where H-bonding is assumed to stem from electron transfer from an electron-rich donor MO1 on one partner to an electron acceptor MO2 centred at the H atom of the partner molecule. Second, a quantitative electrostatic approach, where the H-bond and the shape resulting therefrom for the dimers, can be understood in terms of the long-range interactions between the first few permanent multipole moments of the interacting molecules, yielding the electrostatic model. 

The shapes of some H-bonded dimers resulting from electrostatic calculations involving permanent multipole moments5 up to R 6 are shown in Table 5.3, where angles refer to the coordinate system of Figure 5.3. Apart from the (NH3)2 dimer, a substantial agreement is found between theoretical predictions and experiment. 

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 . )... 

Figure 5.2 shows molecular aggregate models examined in this study. We assume a A-type three-state monomer with the excitation energies of 10,000 and 30,000 cm and the transition moments of 5 D. The H- and L-shaped dimer models are also investigated in order to clarify. the molecular orientation effect on EIT. The excited states of the molecular aggregate models are calculated by solving Eq. 5.6 under the assumption of the dipole-dipole coupling between monomers. The excitation... 

Fig. 5.6 Absorption spectra (Im[a]) of the probe field (with

Apparently, the reduced absorption by BIT shown as MINJm[a] is independent of the intermonomer distance both for the H- and the L-shaped dimer models, whereas MAX Jm[ot] gradually increases with the increase in R (the decrease in the intermonomer interaction) in case of the L-shaped dimer model. This result shows that BIT can be realized even when several states contribute to the optical response for the probe and coupling fields. This robustness of BIT is considered to originate in the frequency matching between the coupling field and the energy difference between the intermediate and metastable states. 

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