Solvents basic, localizing

Figure 5-4. Procedure for selectivity optimization in NPC based on mixtures with hexane of nonlocalizing solvent (CH2CI2), a basic-localizing solvent (MTBE), and a nonbasic localizing solvent (ACN or ethyl acetate). All mobile phases are of equal strength. (Reprinted from reference 1, with permission.)...

A typical basic localizing B solvent is tert, butylmethyl ether. 

In order to obtain selectivity changes it is necessary to choose solvents which differ in their localization and basicity. In many cases the mobile phase consists of two solvents, A and B. The usual A solvent is hexane which has no strength, localization or basicity. For B it is best to use either a non-localizing, a non-basic localizing, or a basic localizing solvent. For systematic selectivity tests and maximum changes in elution pattern, the separation should be tried with all these types of B solvents. 

Typical non-basic localizing B solvents are acetonitrile and ethyl acetate. Acetonitrile is only slightly miscible with hexane ethyl acetate has a high UV cutoff of 260 nm. 

The spectroscopic behavior of the probe used to construct this scale (1) is typical of a structure exhibiting highly localized charge on its carbonyl group in the electronic ground state, see scheme III, and hence strong stabilization by effect of increased solvent polarity and acidity. The electronic transition delocalizes the charge and results in an excited state that is much less markedly stabilized by increased polarity or acidity in the solvent. The decreased dipole moment associated to the electronic transition in this probe also contributes to the hypsochromie shift. The small contribution of solvent basicity to the transition of the probe (1) is not so clear, however. 

In summary, all the experiments expressly selected to check the theoretical description provided fairly clear evidence in favour of both the basic electronic model proposed for the BMPC photoisomerization (involving a TICT-like state) and the essential characteristics of the intramolecular S and S, potential surfaces as derived from CS INDO Cl calculations. Now, combining the results of the present investigation with those of previous studies [24,25] we are in a position to fix the following points about the mechanism and dynamics of BMPC excited-state relaxation l)photoexcitation (So-Si)of the stable (trans) form results in the formation of the 3-4 cis planar isomer, as well as recovery of the trans one, through a perpendicular CT-like S] minimum of intramolecular origin, 2) a small intramolecular barrier (1.-1.2 kcal mol ) is interposed between the secondary trans and the absolute perp minima, 3) the thermal back 3-4 cis trans isomerization requires travelling over a substantial intramolecular barrier (=18 kcal moM) at the perp conformation, 4) solvent polarity effects come into play primarily around the perp conformation, due to localization of the... 

Local density functional theory may be introduced within the RF model of solvent effects thorugh the induced electron density. The basic quantity for such a development is the linear density response function [39] ... 

In the case of naphthalene, transitions to the two lowest excited states (again, often indicated with Lb and La) are two-photon forbidden, as in benzene. However, due to vibronic coupling, the Lb band is visible in the 2PA spectrum of naphthalene in the 575-650 nm region (see Fig. 5), while La gains intensity in the IPA spectrum and peaks around 275 nm [44-46], but is basically absent from the 2PA spectrum this is again in line with predictions based on the pseudoparity of the states. Polarization ratio data were used to aid the band assignment. A weak 0-0 peak of the Lb band can actually be seen in the 2PA spectrum (at 630.5 nm for naphthalene in cyclohexane [45] and at 631.8 nm in carbon tetrachloride [47]), probably because of local perturbation of the symmetry due to the solvent environment or other effects [44,45]. The 2PA... 

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