Ir -» transition

FIGURE 13 38 The ir ir transition in as trans 1 3 cyclooctadiene involves exci tation of an electron from the highest occupied molec ular orbital (HOMO) to the lowest unoccupied molecu lar orbital (LUMO)... 

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). 

In which of the following molecules could there be an M-to-ir transition Explain your choices, (a) Formic acid, HCOOH (b) ethyne, C2H2 (c) methanol, CH,OH ... 

FIGURE 2 In a TT-to-ir transition, an electron in a bonding 7i--orliit.il is excited into an empty antibonding 7r -orbital. 

FIGURE 3 In an n-to-ir transition of a carbonyl group, an electron in a nonbonding orbital (one localized wholly on the oxygen atom) is excited into an antibonding TT -orbital spread over both atoms. 

Ir/transition metals Description of a new model (Atomic cell model) for the interpretation of isomer shift values, with electronegativity and cell boundary electron density as parameters... 

A further interesting feature of the gallium phosphides and arsenides is that the former compounds are colorless whereas the latter range from yellow to orange. Color can arise from ir- ir transitions in main group compounds for example, in the disilylenes and digermenes R2E = ER2 (E = Si, Ge) in which the it- tt transitions occur at lower energy than... 

In a detailed study of the IR transitions of matrix-isolated 1-methylsilene and its isomer dimethylsilylene the structures of these species154 have been clearly defined. 

The stable disilenes are pale yellow to orange-red in the solid state and have electronic absorption maxima in solution between 390 and 480 nm (Table I). The longest wavelength absorptions have been assigned to the ir-ir transition.28... 

Ir(ppy)2hat]+ exhibits dual emission at 77 K. The bimetallic complex [(ppy)2Ir(/i-hat)-Ru(bpy)2]3+ has three reversible reduction waves the first two involve the bridging hat ligand, and the third process is attributed to a bpy-based process.330 The complex also has an irreversible oxidation. Low-energy ( 19,000 cm-1) bands are assigned as Run hat ir transitions. 

Since upon excitation an electron is promoted from a bonding orbital to an antibonding orbital, the bond length is expected to increase. Hence, for ir-+ir transitions, in diagrams such as Figure 1.2, the excited state is drawn with a larger internuclear distance than the ground state. 

IR differences key on frequencies of band maxima and band widths, whereas VCD varies in terms of band shapes associated with those IR transitions, thus being secondarily influenced by frequency and width as well. Two strong bands, the amide I and II, dominate the mid-IR, whereas the amide III is weak and mixed with local CaH deformation modes, spreading its small intensity over a broad region (Diem et al, 1992 Baello et al., 1997 Asher et al., 2001). 

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. 

A recent success in the detection of H species has been that of the molecular ion H3+. All of the models of ion-molecule chemistry in hydrogen-dominated regions are controlled by reactions of H3+ but until recently the H2+ molecular ion had not been detected. However, the modes of vibration of H3"1" provide for an allowed IR transition at 3.668 pin used for its detection. These ro-vibrational transitions have now been observed in a number of places, including the interstellar medium and in the aurorae of Jupiter. Not all astronomical detection and identification problems have been solved, however, and the most annoying and compelling of these is the problem of diffuse interstellar bands. 

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-... 

However, the rapid decay of Fe(II) protoporphyrin IX DME in benzene where no axial ligands are present indicates that energy levels between the ir > ir transitions are available even without axial ligands. Based on the temperature independent quadrapole splitting in the Mossbauer spectra Dolphin et (21 ) have concluded that there are no low-lying exciteJ states in the square planar Fe(II) octaethylporphyrins. Calculations by Zerner and Gouterman (2 ) and Zerner (23) show that the d orbitals are... 

In order to evaluate the vibrational frequencies defined within the model described in Section 2.1, the second derivative of the electronic energy with respect to the nuclear coordinates (usually the normal coordinates) must be evaluated. There are three different methods of evaluation of the second derivative namely, it is possible to perform numerical second differentiation, numerical first differentiation of analytical derivatives, or direct analytical second differentiation. These derivatives provide the matrix of force constants which when diagonalized gives frequencies of the IR transitions as well as their normal modes (the degree and direction of the motion of each atom for a particular vibration). ... 

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... 

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