N-rr* transitions

The spectral properties of the A -phosphorin ring system can be identified more easily in molecules with alkyl substituents such as 2.4.6-tri-tert-butyl-A -phosphorin 24 (Fig. 7) or 2.4.6-trimethyl-A -phosphorin. Markl attributes the long-wave absorption (shoulder at 312 nm) to an n -> rr transition (see, however, p. 38), the absorption in the center to an L(a)-transition, and the short-wave absorption to an L(p)-transition. The values are listed in Table 5 together with the absorption bands of the unsubstituted X -phosphorin. 

In most simple aldehydes and ketnoes, including benzophenone, the longest wavelength absorption is a low intensity n - rr transition. The promotion of a n-electron, localized on O-atomto a re-orbital, leaves behind a positive hole on this atom. The charge density on C-atom is increased creating a bipolar state. The dipole moment of >C = O bond is reduced. Three primary processes are commonly encountered for this electrophilic centre ... 

There is a correlation between reactivity of the pyrrolinones, 3-ethoxyisoindolenone and the oxazolinones and their UV spectra5 2 The pyrrolinones all have distinct n-7r bands as the longest wavelength transitions in their UV spectra (Table 1), they all react by a-cleavage, and they do not cycloadd to olefins. The molecules studied which cycloadd to olefins and do not a-cleave are 3-ethoxyisoindolenone (50) and the methoxyphenyloxazolinones (65 b and 65c). These show no resolved n-rr transitions (Table 4). Of the two molecules, 65a and 65d which undergo both modes of photochemical reactivity, 65d has a slightly resolved n-7r band as a shoulder on the intense tt-tt transition (Table 4). 

Good test cases would be the solvent effects on the UV-vis absorption spectra of formaldehyde and acetone that have been the subject of innumerous theoretical studies. Innovative theoretical methods have been applied to formaldehyde (see also the compilation of results in [20,32,113,114,115,116]). Unfortunately the experimental result for formaldehyde in water is not clear because of chemical problems mostly associated to the aggregation and formation of oligomers. Therefore a better test case is the UV-vis spectra of acetone, because reliable experimental solvent shifts and several theoretical results are available (see the compilation of results in [117]). The Stokes shift of the n-rr transition of acetone has been critically discussed by Ohrn and Karlstrom [118], Grozema and van Duijnen [17] studied the solvatochromic shift of the absorption band of acetone in as much as eight different solvents. Acetone is known to shift the maximum of the n-rr band by 1500-1700 cm 1 when immersed in water [119,120,121], Using the conventional HF/6-31 G(d) point charges, Coutinho and Canuto [54] simulated acetone in water and performed INDO/CIS... 

The spectra of pyrazine, pyrimidine, and pyridazine, all in cyclohexane, are compared by Albert (1462). They show two bands with associated fine structure, the peaks for pyrazine center around 260 and 328 nm. The near-ultraviolet spectrum of pyrazine has been measured in several solvents [and at various pH values (1463)] and the transitions assigned (1474, 1482). The diffuse system at 260 nm has been attributed to rr->-7r transitions whereas the sharp system at 328 nm has been ascribed to n rr transitions (1467, 1474). Semiempirical calculations have been made on the electronic structure of pyrazine with reference to its -> rr transition (1483-1486), calculations have been made of transition energies in N-... 

From comparisons with model compound spectra, we attribute the 330-350 nm absorption to the n- rr transition of the benzophenone moiety and the 255 nm absorption to the transition of the same group. 

Carbonyl group. Saturated carbonyl compounds like acetone exhibit three bands a weak band around 280 m/x, a more intense band around 190 mp and a still more intense band around 150 mp, which are assigned to the n -> 7T, n rr transitions respectively5. The n rr transitions of a large number of carbonyl derivatives have been reported in the literature5. 

The other nitrogen-oxygen linkages which exhibit weak bands due to n -> rr transitions are alkyl nitrites12 14, alkyl nitrates12, nitroso compounds14-18 and azoxy compounds19. The absorption maxima and extinction coefficients of a few typical compounds have been summarized in Table 3.6. 

Kasha8 and McConnell84 have proposed solvent effects as a criterion for distinguishing the n rr and the v v transitions (see Chapter 2). They defined bands as red-shift or blue-shift5 bands according to their displacement in the solvent order, paraffin, alcohol, water, without reference to the gas phase frequency. McConnell2 and a number of other authors have found that all the known n->rr transitions give blue-shift bands (see Chapters Written in collaboration with A. Baiasubramanian. 

The n — rr transition for methyl vinyl ketone is at 324 nm, and the rr rr transition is at 219 nm. Both A ax values are at longer wavelengths than the corresponding Amax values of acetone because methyl vinyl ketone has two conjugated double bonds. 

Atoms as well as molecules have electronic transitions that are not of the Rydberg type. For atoms the famous D-lines of sodium (3s,3p) are an example. For molecules all the familiar (vr, n ) and (n, rr ) transitions of olefins and aromatic molecules are examples of non-Rydberg, valence-shell (or intravalency) type transitions. For typical valence-shell transitions the orbital of the excited electron is not much larger than the molecular core. Bands due to such transitions cannot be ordered into series. The orbital of the excited electron is usually antibonding in one or more bonds wliile Rydberg orbitals because of their large size are, in most cases, essentially non-bonding. 

Figure 5.32 Expected absorption spectrum for the molecule in Fig. 5.31 dissolved in a polar solvent such as ethanol. There is a bathochromic (red) shift in the rr —> tt transition and a hypso-chromic (blue) shift in the n —> rr transition.

Synthesis.—Mannich-type Reactions. The double Michael addition of amines to cyclohepta-2,6-dienone has been extended by the use of a series of optically active primary amines to the synthesis of chiral 8-azabicyclo-[3,2,l]octanes. Yields are typically ca. 70% circular dichroism studies show interesting effects. Perturbation of the n - rr transition must originate in the asymmetry of the nitrogen substituents which lie in the symmetry plane of the ring. The analysis of the tertiary amines is complicated by... 

For pyridine, the picolines and the 3-halogenopyridines, the intensity of absorption in the region 34-37,500 cm i decreases when the solvent is changed from iso-octane to ethanol, and that at about 38,500 cm i increases. In the 2-halogenopyridines, the n->rr transition cannot be clearly identified because it is shifted to shorter wavelengths and obscured by the 77 77 band. According to Sklar s vectorial treatment, the intensities of... 

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