Argon trimer

Figure 6. Effective potential energy (left) and local K-entropy (right) for the argon trimer, as functions of the bending angle (left to right axis) and the (equal) lengths of the two Ar-Ar bonds (back to front axes, 3.05 A at back to 4.05 A at front. [Reprinted with permission from D. J. Wales and R. S. Berry, J. Phys. B 24, L351 (1991). Copyright 1991, Institute of Physics.]...

In low-temperature, solid inert matrixes (argon, xenon 7 = 9 K) pyrrole forms hydrogen-bonded aggregates that are predicted to be mainly cyclic trimers and tetramers, based on DFT analysis, with a significant cooperativity effect <2004PCA6953>. 

In previous electron spin resonance (ESR) studies of matrix isolated Naj (X, 1) and K3 3, 4), alkali trimers have been shown to be chemically bound and well described both by simple bonding ideas (1, 3) and by the more sophisticated calculations recently employed for Li3 (5), Na3 6) and Kj 7). For the potassium trimer in argon, two distinct ESR spectra are observed 3). An obtuse angled isomer corresponds to one of three static Jahn-Teller distortions from 03 symmetry, and is surprisingly similar to the... 

In Reference 3, two distinct ESR spectra were identified for potassium clusters in argon matrices. Both spectra have doublet ground states (S = h) and a well resolved hyperfine (hf) structure arising from the Fermi contact interaction of the unpaired electron spin with three I = h nuclei. Seven groups of four transitions each were assigned to a potassium trimer of 62 symmetry whose apical and two equivalent terminal atoms have hf splitting... 

Figure 3(b) shows the ESR spectrum of the trimer Lis in an argon matrix at 28.5 K. Despite complications due to the presence of the heptamer Lij and an unknown impurity LiX, seven equally spaced signals can be seen. These have the correct intensity ratio to belong to a molecule with three equivalent Li nuclei (I = 1). It appears that the Lis trimer is rapidly pseudorotating in an argon matrix at this temperature thus the system behaves as an isotropic one and only one set of g values are seen. 

Pyrolysis of 32 was carried out at 850 °C. The trimer was sublimed at ca 105 °C with argon as a carrier gas. Under these conditions, pure quinone methide 31 was matrix isolated on a 7.6 K KBr target as evidenced by the IR spectrum. In a similar experiment, the pyrolysis of trimer 32 was carried out without argon. When the neat quinone methide 31 was warmed above —92°C, new IR bands appeared, and the absorptions due to the quinone methide decreased. These newly formed bands became much stronger when the target was warmed further to —65 °C, and they are attributed to the formation of dimer 34 and trimer 32 by comparison with the IR spectrum of the trimer obtained in the preparative FVP work. Moreover, a low temperature NMR experiment revealed the existence of dimer 34. After the target was warmed to room temperature, additional bands due to the tetramer appeared. It was readily concluded that dimerization would be the first step of reaction of quinone methide 31. 

Fig. 12. (A) Time-resolved fluorescence spectra of A. nidulans phycobilisomes measured at 77 K. Excitation by 6-ps, 580-nm argon laser pulse. Three small ticks in the topmost spectrum (at 932 ps) indicate locations of maximum fluorescence at 0 ps. (B) Rise and decay of various fluorescent components derived from deconvolution of the fluorescence spectra. Assignment of individual fluorescent components are shown in the right margin. (C) Energy flow among individual chromophores in the phycobilisomes. The asterisk in (B) and (C) indicates a linker polypeptide is attached to the trimer. See text for discussion. Figure source Mimuro (1989) Studies on excitation energy How in the photosynthetic pigment system structure and energy transfer mechanisms. Bot Mag Tokyo 103 244.

The interaction energies of clusters of molecules can be decomposed into pair contributions and pairwise-nonadditive contributions. The emphasis of this chapter is on the latter components. Both the historical and current investigations are reviewed. The physical mechanisms responsible for the existence of the many-body forces are described using symmetry-adapted perturbation theory of intermolecular interactions. The role of nonadditive effects in several specific trimers, including some open-shell trimers, is discussed. These effects are also discussed for the condensed phases of argon and water. 

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