Dimer spectra

The triplet EPR spectra of the mixed dimers [ZnTPPS/TTAP] and [TPPS/ZnTTAP] also displayed in Figure 5 have the same characteristics as the free base dimer spectra (cf. Figures 3 and 5). A summary of the values of triplet parameters derived from the spectra is given in Table II. Included are literature values on face-to-face covalently-linked diporphyrins (8). 

Dimer spectra. Intermolecular forces are attractive at distant range. Consequently, most pairs of atoms/molecules may exist as a bound van der Waals molecule. Exceptions are a few very light systems which have very shallow wells, for example H2-He, H-He and He-He. 

According to theory, most collision-induced absorption spectra should not only consist of contributions of the free-state to free-state transitions typical of collisional pairs, but also of contributions arising from bound-to-free and bound-to-bound transitions involving van der Waals molecules. In the rotovibrational spectra such dimer bands have been known for some time, but in CIA studies of the rototranslational band, where path lengths have generally been limited to a few meters, dimer features have been seen only recently [268], The dimer spectra are an integral part of the interaction-induced absorption. 

Waals dimers [266, 267, 302, 328], The theoretically predicted dimer Raman spectra [262, 263] have hitherto not been observed in the laboratory, presumably because of the substantial width (%10GHz) of the 5145- and 4880-A argon laser lines commonly employed in such work and because the high gas densities used tend to broaden the rotational features and render them indiscernible [262], Nevertheless, the envelopes of the rotational van der Waals dimer bands have been seen in several gases and mixtures [229, 230, 266, 267, 328]. Under more favorable jet expansion conditions, high-resolution argon dimer spectra have been obtained recently [280], There seems to be little room for doubt that eventually van der Waals dimer spectra will be obtained with ordinary Raman or third-order nonlinear techniques [676]. 

Quantum calculations of spectral profiles based on Eqn. (3) are known isotropic interaction is commonly assumed. Such quantum calculations account for detailed balance in exact ways and consider not only the free-state to free-state transitions of binary complexes but also the bound-free and bound-bound transitions involving bound van der Waals pairs that are an inseparable part of the CILS spectra. These bound-free and bound-bound components can be quite significant, especially at the lower temperatures and for massive systems [266, 302, 328] see also related work on dimer spectra [262, 263], and the review papers on CILS lineshapes [227, 231, 271]. Lineshape calculations are useful for the detailed comparison of the fundamental theory with spectroscopic measurements. For helium pairs, a close agreement between the fundamental theory and the recent measurements is now observed similarly for the neon pair [45]. 

The more recent ASP potentials of MUlot et al. [183] and their counterparts fitted to the experimental dimer spectra, VRT(ASP-W) [52] and VRT(ASP-W)III [53], have been utihzed in diffusion Monte Carlo (DMC) simulation of water clusters of different sizes [192,193]. The three- and higher body effects were described by a polarization model only, similarly as in empirical polarizable potentials. While polarization models are quite efficient in describing the nonadditive induction in the asymptotic regime, they fail to properly model the short-range nonadditivities, which are definitely non-negligible in smaller trimers. 

The potentials discussed above are pairwise or two-body potentials (i.e., potentials describing dimers). Yet, in many cases such potentials are fitted to thermodynamics data for liquids and solids. In such media the pairwise nonadditive effects are usually quite important. Therefore, potentials of this type are called effective two-body potentials since they approximate the many-body effects by an unphysical distortion of the two-body potential relative to the exact two-body potential. As a consequence, the effective two-body potentials perform poorly in predicting pure dimer properties such as dimer spectra or second virial coefficients. In fact, the effective two-body potentials perform poorly also in predicting trimer properties (although the three-body component dominates the nonadditive effects, cf. section III.C). 

Both the Hall- and Shipman-PCM-based studies appear to predict spectral shifts as a function of Ay consistent with general experimental trends observed in Chi dimer spectra blue shifts are observed in dimers with Ay = 3-4A while red shifts occur only when Ay is much larger, i.e., 7-8 A [68, 69]. The Shipman-PCM appears to be a satisfactory as well as economical approximation for use in exciton calculations, and has been employed in further exciton studies of bacteriochlorin and Me-BPheo-a oligomers [66, 70]. 

Ph=9 dimeric spectra 71H2 varied remarkably with Ph values the second order perturliation is involved in the estimation of the EPR parameters... 

The most striking features of the dimer spectra are (1) the complete absence of intensity of peak C, the temperature-variable peak (which is absent also in tRNAp and assigned to Y37 in tRNA ) and (2) an apparent gain in intensity in the main cluster. This region accounts for 50-53 phosphates in each of tRNA " and tRNA and at first inspection 124 resonances in the dimer. However, the dimer main cluster is broader than in... 

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