N-ir* Transitions

UV-VIS Aldehydes and ketones have two absorption bands in the ultraviolet region Both involve excitation of an electron to an antibonding tt orbital In one called a TT TT transition the electron is one of the tt electrons of the C=0 group In the other called an n ir transition it is one of the oxygen lone pair electrons Because the tt electrons are more strongly held than the lone parr electrons the transition is of... 

A large body of information is available on the UV spectra of pyrazine derivatives (B-61MI21400, B-66MI21400). Pyrazine in cyclohexane shows two maxima at 260 nm (log e 3.75) and 328 nm (log e 3.02), corresponding to ir->ir and n ir transitions respectively (72AHC(14)99). Auxochromes show similar hypsochromic and bathochromic shifts to those observed with the corresponding benzenoid derivatives. 

The UV spectra of quinoxalines have been examined in several solvents. In cyclohexane, three principal absorptions are observed (Table 2). In hydroxylic solvents the vibrational fine structure disappears and in methanol or water the weak n- ir transitions are obscured by the intense ir->ir transition (79HC(35)l). 

Merrill and Roberts (Z) have examined both PET films and fibers and have attributed the fluorescence (excitation 342 nm, fmision 388 nm) to a 1(n,n ) transition. They have proposed a (n,ir ) transition, since the observed fluorescence is at lower energy than the observed phosphorescence (excitation 313 nm, emission 452 nm, 1.2 sec), which they have proposed from a (tt,tt ) state. 

Subsequently, we reported the same experimental data as Merrill and Roberts but we have attributed the fluorescence to a (ir,ir ) transition rather than a (n,ir ) transition. We also pointed out that.the (n, ir ) state of PET is probably at higher energy than the 1 (tt.tt ) state Figure 2). We attributed the red shift in the fluorescence excitation and emission to aggregates of monomeric units fixed in specific geometry in the polymer matrix. 

As pointed out earlier, RR spectroscopy is a powerful tool for an unambiguous distinction between coordinated and uncoordinated phenoxyl radicals. Upon excitation in resonance with the n —> ir transition of the phenoxyl, the RR bands originating from the modes ula (-1500 cm-1 C-O stretching) and uSa (-1600 cm-1 C=C stretching) are enhanced and clearly detectable. The exact positions of these bands as well as their RR intensity ratio can be used to distinguish between coor-... 

The primary process following a photoexcitation of nltrosamldes XIV Is the dissociation of the N-N bond to form a radical pair XV and the ensuing chemical events are the reactions of amldyl and nitric oxide radicals In the paired state or Individually In the bulk of solutions. Naturally, secondary reactions, thermal or photolytic, have to be taken Into consideration under Irradiation conditions (21). First of all, the relatively straightforward chemistry of selective excitation In the n-ir transition band (>400 nm) will be discussed, followed by the chemistry of Irradiation with a Pyrex filter (>280 nm). As nitric oxide Is known to be rather unreactlve (23), primary chemical processes In the Irradiation with >400 nm light under... 

There are two types of electronic transition commonly responsible for photochemically induced reactions in organic molecules. The first of these is the n—>ir transition in which an electron in a nonbonding atomic orbital is excited to an antibonding ir orbital, the excited state being referred to as n, it. This occurs in nitrogen-, oxygen-, and sulfur-containing molecules, and the nature of the n, 77 state of the carbonyl function has been the subject of considerable study.5,6 Excitation to the n, tt state in aldehydes and ketones occurs at approximately 290 nm. 

Solute-solvent interactions are of two types (1) universal interaction, and (2) specific interaction. Universal interaction is due to the collective influence of the solvent as a dielectric medium. It depends on the dielectric constant D and refractive index n of the solvent and the dipole moment (i of the solute molecule. Such interactions are van der Waals type. Specific interactions are short range interactions and involve H-bonding, charge-transfer or exciplex formation. H-bonding ability may change on excitation specially for n >ir transitions. 

It was noted that I-menthol could induce CD in the n- ir transition of a large number of saturated, achiral, compounds containing a ketone group when these were co-dissolved in a suitable solvent [12,13]. It was concluded in these works that 1 1 association complexes were formed in the solutions, primarily owing to hydrogen-bonding forces between the two... 

Table 6-1 lists the solvent shift on the n- -ir ) transition and its different contributions (see Eq. (6-26)). The first column corresponds to the solvent shift due to... 

Bovine ROS membranes show a CD band at ca. 280 nm attributed to tt-tt transitions of aromatic residues and n-ir transitions of cysteine, as well as two maxima in the visible region at ca. 340 nm and 490 nm (the / and a bands) corresponding to the cis and main peaks of the absorption spectrum. The intensity of the a band in the CD is species-dependent, but is always somewhat blue-shifted relative to the Xmax in all species, and this shift is promoted by detergent solubilization. Strong micellar effects have been observed in the intensity of both a and / CD bands [44] (Fig. 4). 

The fluorescence polarization excitation spectrum has been measured for thymine in aqueous solution. " The depolarization at the red edge is attributed to the hidden n, ir transition. Ionization of the lowest excited singlet and triplet states have been determined by the effect of pH on the absorption, fluorescence, and phosphorescence spectra of purines and pyrimidines. " Spectral, polarization, and quantum yield studies of cytidylyl-(3, 5 )-adenosine have also been published. Intermediates in the room-temperature flash photolysis of adenine and some of its derivatives have been identified hydrated electron, radical cations and anions, and neutral radicals resulting from their reactions have been assigned. Photoionization occurs via the triplet state. FMN encapsulated in surfactant-entrapped water pools interacts with polar head groups, entrapped water molecules, and outer apolar solvent. ... 

A shift toward higher frequency is called a blue shift or a hypsochromic shift. Similarly, a shift toward lower frequency is called a red or bathychromic shift, We will use the blue shift, red shift notation. The designation n —ir refers to the excitation of an electron from a nonbonding orbital (an n orbital) to an excited pi orbital of the chromophoric group (a r orbital). The ir — -ir transition is restricted to pi orbitals of the chromophorc, usually from a bonding to an antibonding orbital. The n —ir transitions are forbidden, whereas the T —T transitions may be allowed. 

Since the spectra show little change in position or intensity as the series is ascended, jt-it transitions are excluded. Figure 6 shows that the spectra are comparatively slightly affected by solution of the chlorides in sulfuric acid absorption cannot, therefore, be due to an n-ir transition from the nitrogen atom. It may be a result of excitation of the unshared electrons on the halogen atoms, consistent with the effect of bromine substitution on the position and intensity of the absorption band. The ultraviolet spectra therefore give no direct information on the structure of the ring. 

The band of longest wavelength absorption in the spectrum of indolizine which shows some relation in shape to that of azulene corresponds to the n ir transition... 

The electronic spectrum of COCIF (Fig. 17.20), at both low and medium resolution, has been obtained [2244a,2245]. The absorption was attributed to a singlet-singlet n—>ir transition. Although no fine structure was detectable in the low resolution spectrum, at... 

---