N >7T* transitions

Kamlet, JACS 88, 4761 (1966) Trichloronitromethane. This article supplements the material in Vol 3, C256-R under Chloroform and Derivatives CA Registry No 76-06-2. The IR spectrum and assignment of peaks are given in Ref 2 and in the UV there is a n+7T transition at 35907cm-1 (Ref 3). 

The intensity of an n-> 7T transition (transition of one p-electron of a lone electron pair from a nonbonding (n) to an antibonding (tt ) orbital) is about two magnitudes less than the dominant transitions and thus has no effect on the... 

Popovych O, Rogers LB (1959) Fluorescence in the 8-quinolinol family and the n-7t transition. Spectrochim Acta 15 584—592... 

In contrast, the n ==> Jt transition has a ground-excited state direct product of B2 x Bj = A2 symmetry. The C2V s point group character table clearly shows that the electric dipole operator (i.e., its x, y, and z components in the molecule-fixed frame) has no component of A2 symmetry thus, light of no electric field orientation can induce this n ==> Jt transition. We thus say that the n ==> 7t transition is El forbidden (although it is Ml allowed). 

An example of an El forbidden but "vibronically allowed" transition is provided by the singlet n ==> 7t transition of H2CO that was discussed earlier in this section. As detailed there, the ground electronic state has A] symmetry, and the n ==> Jt state is of 1A2 symmetry, so the El transition integral... 

Conversely, in the case of the n >7T transition of a ketone, the interaction of the lone pair of the ground state carbonyl with a polar solvent lowers the energy of the lone pair n orbital, and thereby increases the energy required to promote an electron to the ti energy level. Consequently, there is often a blue (hypsochromic) shift - to lower wavelength - observed when the UV spectrum of a ketone is recorded in ethanol rather than cyclohexane. 

Similar curves are obtained with other synthetic polypeptides, and in most cases they are reasonably independent of the nature of the amino acid side chains. In synthetic polypeptides and proteins the observed Cotton effects do not arise from isolated chromophores but are composite curves resulting from several transitions assigned to the amide bonds in the 200-m/x region. The a-helical curve, for example, results from three optically active absorption bands. One around 222 m/ arises from an n — 7T transition of nonbonding electrons, and the other two at 208 and 191 m/ji are attributed to w — tt transitions parallel and perpendicular to the axis of the helix. These transitions of the a-helix and the resulting Cotton effects characteristic of the a-helix are at present of great interest in interpreting ORD curves of membranes. 

Let us consider first lipid-lipid interaction. Urry et al, showed the existence of a positive CD band at 218 m/x and a negative CD band at about 192 m/z in phosphatidyl choline and phosphatidyl ethanolamine dissolved in trifluoroethanol (86). The 192-m/z band was not characterized in detail, but the 218-m/z band is of such position and shape that the addition of lipid and protein CD bands could produce a composite CD band, and hence an ORD Cotton effect, which is red shifted. As noted by Urry, the 218-m/z CD extremum of lecithin must arise from n — 7T transitions in the fatty acid ester groups. Although the optical activities of solutions of deproteinized membrane phospholipids determined at the same concentration as in the intact membrane are negligibly small, in membranes an ordered array of lipids could greatly enhance rotation. Such an effect could yield information on the nature of lipid-lipid association. This can be tested experimentally. Halobacterium cutirubrum offers a unique system since Kates has shown that the lipids in this extreme halophile contain ether bonds rather than ester bonds (43, 44), Hence, the n — tt transition essential to the CD band at 218 m/z in phospholipids does not exist. Nevertheless, we found that the ORD... 

Acetone is a moderately polar molecule that can hydrogen-bond with water. Which electronic state of acetone would be stabilized more by moving the molecule from hexane to water Will this shift the n—>7t transition to longer or shorter UV wavelengths Explain. 

The conformational properties of peptides linked to these capping templates were investigated by CD spectroscopy, NMR-measurements, and X-ray diffraction analysis.204 Figure 32 shows the CD spectra of the two template-linked model peptides 102,103, and an uncapped 12-mer reference peptide 104 with comparable amino acid sequence (Boc-Ala Aib-Alag-Aib-Alaj-pIa). The CD spectra exhibit the negative maxima at about 207 and 222 nm, characteristic for a-helical peptides. Based on the ellipticity values at 222 nm (amide n — 7t transition) the N-capped peptide was calculated to be approximately 90% a-helical, whereas the helicity of the reference peptide is approximately 50% in TFE/H20 (1 1). 

Table 13-2. Vertical electronic n —> 7T transition energy of acetone in gas phase, Egas, and aqueous solution, Ewat, in units of eV. Excitation energy in aqueous solution was obtained from the combined QM/MM calculations treating acetone molecule at the quantum mechanical level of theory as indicated in the first column and using the polarizable potential for water molecules as a statistical average over 1200 molecular configurations extracted from classical MD simulation. The solvent shift in excitation energy, AE (in cm-1), is evaluated as a difference between excitation energies in water and in vacuum...

This procedure of including several AOs on an atom in the linear combination of atomic orbitals (LCAO) scheme and obtaining MOs which are essentially pure AOs may prove useful in future work for studying intramolecular electron-transfer processes and n 7t transitions. 

The n ------> 7t Transition exhibit a weak bond in their absorption spectrum is found in... 

Figure 8 also shows dichroism in the weak band in the 2200-2300 A range, which may correspond to the weak amide absorption assigned as an n —> 7T transition by Hunt and Simpson (1953). This may quite possibly be the transition involved in an apparent Cotton effect seen in the optical rotatory dispersion of a-helical structures (Simmons et al., 1961). If this is true, and it appears possible, it raises a serious question about the... 

Amino-5-deoxy-D-xylopyranose (34 = 17) is like 2-piperidinol (2-hydroxypiperidine) in existing in a pH-dependent equilibrium with its dehydration product (33 = 16). The ultraviolet peak of the n-7T transition of the C=N chromophore of 33 is not suitable for structural elucidation. However, the asymmetric nature of this chromophore gives rise to a Cotton effect. A solution of free 5-amino-5-deoxy-D-xylose shows a negative Cotton effect at 300 nm that is well demonstrable by measurement of circular dichroism for this purpose, the optical rotatory dispersion is much less sensitive. The Cotton effect is ascribable to 33, as 34 and 22 would exhibit no Cotton effect in this region. Thin-layer chromatograms of 34 always show a second spot which, like 34, gives a red coloration with o-aminobenz-aldehyde this is, presumably, caused by 33. 

Weakly absorbing chomophores (i.e., simple ketones, sulphides etc., or n a or n -> 7t transitions) are indicated by weak bands in the absence of strong chromophores. 

In aromatic azides the main absorption region (2100-2900 A) has been assigned to n, 7t transitions as the main vibronic features of the parent hydrocarbon are retained a less intense absorption (3000-3500 A) appears to originate from an n,n transitions. Because of the requirements of spin conservation, nitrenes... 

The uv absorption spectrum of simple aliphatic nitrocompounds consists of an extended band centered at 2700-2800 A and is most probably associated with an n 7t transition involving oxygen-atom lone-pair electrons . Both the photochemical and thermal decompositions of nitrocompounds are extremely complex, in part owing to secondary reactions between products and substrate although the overall mechanisms are still unclear in many cases, the primary modes of decomposition of some molecules have been established. 

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