Forces from absorption spectra

Azulene. The absorption spectrum of azulene, a nonbenzenoid aromatic hydrocarbon with odd-membered rings, can be considered as two distinct spectra, the visible absorption due to the 1Lb band (0-0 band near 700 nm) and the ultraviolet absorption of the 1L0 band.29 This latter band is very similar to the long wavelength bands of benzene and naphthalene CLb) and shows the same 130 cm-1 blue shift when adsorbed on silica gel from cyclohexane.7 As in the case of benzene and naphthalene, this blue shift is due to the fact that the red shift, relative to the vapor spectra, is smaller (305 cm"1) for the adsorbed molecule than in cyclohexane solution (435 cm"1). Thus it would appear that the red shifts of the 1La band are solely due to dispersive forces interacting with the aromatic molecule, in agreement with Weigang s prediction,29 and dipole-dipole interaction is negligible. 

The presence of a photoconductivity peak at 610 nm at the threshold of the absorption spectrum (curve 4) is a common phenomenon in inorganic semiconductors and is explained by competition between surface and volume recombination processes of the charge carriers. The optical activation energy determined from the spectral photoconductivity threshold is equal to 1.82 + 0.02 eV. The thresholds of the photoelectromotive force and the absorption spectra are likewise in agreement with this value. It is remarkable that the same value has been found for the activation energy of the dark conductivity in this polymer... 

The observation of a bent Cr-H-Cr bond in the tetraethylammonium salt without an accompanying substantial deformation of the linear architecture of the nonhydrogen atoms in the [Cr2(CO)io(M2-H)] monoanion reflects the inherent flexibility of the bond. The deformability of the[M2(CO)io(M2-H)] monoanion species to adopt an appreciably bent, staggered carbonyl structure was first reported by Bau and co-workers (23) from neutron diffraction studies of two crystalline modifications of the electronically equivalent, neutral W2(CO)9(NO)(m2-H) molecule. Subsequent x-ray diffraction studies (15) of the analogous [W2(CO)io(m2-H)] monoanion found that the nonhydrogen backbone can have either an appreciably bent structure for the bis(triphenylphosphine)-iminium salt or a linear structure for the tetraethylammonium salt, with the W-W separation 0.11 A less in the bent form. Crystal packing forces probably were responsible (15) for the different molecular configurations of the monoanion in the two lattices. In solution, however, all known salts of the [W2(CO)io(m2-H)] monoanion exhibit the same three-band carbonyl ir absorption spectrum char-... 

In contrast to the results obtained with EB, weak excitonic interactions in ACR intercalated at the BAZrP galleries result in broadened absorption spectrum only at high loadings (Fig. 23), suggesting that the intermolecular forces are relatively weak for this chromophore [55]. The fluorescence spectra of ACR intercalated into BAZrP show a large blue shift from 480 nm (free form) to 450 nm (bound form) (Fig. 24), and the fluorescence intensity also decreases monotonously with the concentration of ACR, ascribed to self-quenching. This blue shift of ACR fluorescence is also in contrast to the 20-nm red shift observed for EB bound to BAZrP. 

The forces responsible for a second or third layer to be adsorbed cannot be very strong. We may, therefore, not expect multimolecular adsorption to occur at moderately low relative pressures on flat and homogeneous surfaces. By means of absorption spectra a bimolecular adsorption of p-nitrophenol could be established on CaF2 or BaF2 layers (250). The absorption spectrum of the first layer was quite different from that of p-nitrophenol itself, the absorption spectrum of the second layer being practically the same as that of pure p-nitrophenol. The first layer is apparently sufficiently polarized by the salt surface, to enable a second layer to adsorb the second layer, however, cannot accumulate a third layer. It may not be entirely excluded, however, that there is no more place in the capillary space between the salt layers to accommodate a third layer of p-nitrophenol. 

Within the HA, the prediction of a vibrational absorption spectrum amounts to the calculation of the harmonic normal mode frequencies, vi7 and dipole strengths, Dt. The frequencies are obtained from the harmonic force field (HFF). With respect to Cartesian displacement coordinates, this is the Hessian (d2WG/dXXadXx,a,)0. Diagonalization (after mass-weighting) yields the force constants ky the frequencies, vy and the normal coordinates, <2 , i.e. the transformation matrices, SXa4. The dipole strengths depend in addition on the APTs these require calculation of (dtpG/dXXa)0. 

As described in earlier sections, any two material bodies will interact across an intermediate substance or space. This interaction is rooted in the electromagnetic fluctuations— spontaneous, transient electric and magnetic fields—that occur in material bodies as well as in vacuum cavities. The frequency spectrum of these fluctuations is uniquely related to the electromagnetic absorption spectrum, the natural resonance frequencies of the particular material. In principle, electrodynamic forces can be calculated from absorption spectra. 

Within the separable harmonic approximation, the < f i(t) > and < i i(t) > overlaps are dependent on the semi-classical force the molecule experiences along this vibrational normal mode coordinate in the excited electronic state, i.e. the slope of the excited electronic state potential energy surface along this vibrational normal mode coordinate. Thus, the resonance Raman and absorption cross-sections depend directly on the excited-state structural dynamics, but in different ways mathematically. It is this complementarity that allows us to extract the structural dynamics from a quantitative measure of the absorption spectrum and resonance Raman cross-sections. 

The antisymmetric stretching frequency (Vg 490 cm" ) has been observed in the infrared absorption spectrum of gaseous MgBr by Randall et al. (7). The symmetric stretching (v.) and bending (u ) frequencies are calculated from force constants by " 5 —1... 

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