NEER principle

The photoisomerization of all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K illustrates the need for a new mechanism to explain the observed behavior [150]. Upon irradiation, all-s-trans-all-trans 1,3,5,7-octatetraene at 4.3 K undergoes conformational change from all-s-trans to 2-s-cis. Based on NEER principle (NonEquilibrium of Excited state Rotamers), that holds good in solution, the above transformation is not expected. NEER postulate and one bond flip mechanism allow only trans to cis conversion rotations of single bonds are prevented as the bond order between the original C C bonds increases in the excited state. However, the above simple photochemical reaction is explainable based on a hula-twist process. The free volume available for the all-s-trans-all-trans 1,3,5,7-octatetraene in the //-octane matrix at 4.3 K is very small and under such conditions, the only volume conserving process that this molecule can undergo is hula-twist at carbon-2. 

From G. Tchobanouglous, H. Theissen, and R. Eliassen, Solid Wastes Engi-neering Principles and Management Issues, McGraw-Hill, New York, 1977. 

Figure 7.43. Illustration of the NEER principle interconversion of the excited-state conformers of precalciferol is prohibited by the high probability of decay to the ground state through the conical intersection CI., (by permission from Bemardi et al., 1992b).

Wastes En neering Principles and Management Issues, McGraw-Hill, New York, 1977. 

The short lifetimes of excited diaryl polyene singlet states precludes the equilibration of conformers following excitation and, as a result, the conformational distribution in the ground state controls the product distribution. Because the absorption spectra of different conformers usually differ, this results in an excitation-wavelength dependence of the product distribution (NEER principle 501 cf. Section 6.1.1 and Case Study 6.3). 

Explain the following concepts and keywords photostationary state Rydberg state NEER principle bicycle-pedal mechanism phototransduction phototherapy photochemical deracemization enantiodifferentiating photosensitization photochromism di-Tt-methane rearrangement asymmetric photochemical synthesis single-crystal-to-single-crystal photochemistry photochemotherapy rule of five. 

Usually rotational isomerization cannot take place in the excited state as proposed by Havinga (NEER principle) [105,106]. The excited singlet state of 2c follows this principle, t. -ever, in 2-vinylanthracene and its alkyl derivatives. 

The NEER principle implies that each type of conformer yields its own specific assortment of photoproducts. The primary product composition originating from irradiation thus should form a qualitative and quantitative reflection of the composition of the ground-state conformational equilibrium. Hammond and Liu (64) even followed a comparable line of thought in the interpretation of the product formation resulting from triplet sensitized excitation of butadienes. Results of some further studies, in particular those of Baldwin (65) and of Dauben (66), brought additional support to the validity of the NEER hypothesis. Baldwin found a linear relationship between the optical rotatory power of a-phellandrene In solution at various temperatures and... 

Unsubstituted Z-hexa-1,3,5-trlene, In contradistinction to prevltamln D, was found not to show photocycllzatlon to cyclo-hexadlene to any notable extent (Section IIB). The rationalization oh the basis of the NEER principle Is Indicated In Figure 6. The value of the molar extinction coefficient (e 40,000) Is characteristic for a Z-trlene In the stretched tZt conformation. The predominance of that conformation makes no difficulty In thermal cycllzatlon. However, for photochemical cycllzatlon the consequence of the NEER postulate Is that by far the major part of the excited species will stay In the stretched form which cannot undergo cycllzatlon. Its pathways of relaxation lead back to the original Z-trlene, or to the E-lsomer via rotation about the central bond that In the excited state has more single-bond character (70). 

FIGURE 6. Z-1,3,5-hexatrlene e = 41,000 (254 nm) the tZt conformation predominates. The NEER principle rationalizes the nonoccurrence of photoreactions other than Z - E Isomerization. 

The results certainly are Induclve to accept the validity of the NEER principle and demonstrate Its predictive value. However, one cannot exclude the possibility of other models— for example, equilibration between conformers In the excited state resulting In a mixture of these excited species that Is similar to that of the ground state. Such a hypothesis Is not a fortiori unlikely, comparable sterlc factors being operative In the ground and the excited state of a given molecule. With the... 

To conclude this section It can be said that the photochemistry of vitamin D now offers a rounded off picture. The six overlrradlatlon products (Figure 11) are exactly those that could be predicted on the basis of the NEER principle for a trlene compound having the cZt conformation as established for vitamin D. 

The previous sections have already shown the richness and variability of the photoreactIons emerging from Z-l,3,5-hexa-trlenes, and later sections will present further examples of the abundance of pathways of relaxation of excited conjugated Z-trlenes. Until recently the photochemistry of E-hexatrlenes appeared to be much more simple, being essentially restricted to E/Z-lsomerlzatlon of the central double bond and. If suitably substituted, also of the terminal double bonds. However, on the basis of the NEER principle. In conjunction with reasonable assumptions on the conformational features of E-trlenes, a more diverse photochemistry should be expected. From the mixture of conformers present In E-hexatrlenes (Figure 12), the forms with... 

In conclusion, this section Illustrates that In addition to E/Z-lsomerlzatlon, various other pathways of chemical relaxation are open to electronically excited E-trlene molecules. Moreover, the usefulness of the NEER principle In predicting some of these pathways Is confirmed. 

If a sufficient amount of cEc or cEt form (Figure 17) were present In the mixture of conformers of tachysterol, the NEER principle would also predict [1.5] hydrogen shift products to occur In the Irradiation mixture (cf. Section VI). In fact one such compound, toxlsterol Dl, has been Isolated (51). However,... 

From the experimental data presented earlier in this survey the NEER principle appears to have good predictive value. It yields consistent pictures also in the case of the phenomena reported in the elegant studies of Hammond (64) (for triplet species ), of W. G. Dauben (66), of John E. Baldwin (65), and of Courtot (82, 97). However, alternative descriptions are not difficult to conceive. 

FIGURE 26. Formation of photoproducts from conformers of Z-hexatriene according to the NEER principle. 

Furthermore, there Is Indication for an extension of the NEER principle. The variation of product composition with changing wavelength of Irradiation may not be solely due to the different ratios of the extinction coefficients of the various rotamers at the wavelengths used. Probably one and the same... 

The NEER principle, which Implies product composition to reflect the percentages of the various conformers present In the ground state and their extinction coefficients at the wavelength of Irradiation, and... 

In Section XI a modest elaboration was presented on the influence of Irradiation wavelength on product composition and on the validity of the NEER principle. In this chapter the authors stick out their necks in dealing with another provoking aspect. It concerns the difference In photochemical behavior between the Z-triene system of vitamin D and, at first sight, the not too different one of previtamin D. 

The NEER principle now extended by the evidence that photoproducts of one and the same conformer may vary with change of wavelength of Irradiation ... 

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