Spectra, electronic

Electronic Spectra.—The spectral changes in the u.v. spectra of 31 thiophen-2-sulphonamides were correlated with substituent effects. The u.v. spectra 

Hirai and T. Ishiba, Chem. and Pharm. Bull. (Japan), 1972, 20, 2384. 

Petukhov, V. P. Litvinov, A. N. Sukiasyan, and Ya. L. Gol dfarb, Izvest. Akad. Nauk 

In electronic spectra of metal carbonyls there are bands due to d-d transitions between metal orbitals, transitions within the ligand, charge-transfer transitions M L and L M, and also, in the case of polynuclear carbonyls, transitions between orbitals of metal-metal bonds. Transitions within the CO group probably occur in the vacuum ultraviolet region. 

The lowest spin allowed transitions and n tt for the free CO molecule 

The d-d bands may be divided into two groups (1) spin allowed, caused by transitions between terms of the same spin multiplicity, and (2) spin forbidden. The following transitions serve as respective examples and 

Carbonyl groups, due to their o--donor and Ti-acceptor properties, create a very strong ligand field. Consequently, the values of the splitting parameter for carbonyl complexes are very high, even higher than those of cyanide complexes (Table 2.12). Therefore, high-spin carbonyls are unknown. 

The electronic spectra of lanthanide compounds resemble those of the free ions, in contrast to the norm in transition metal chemistry the crystal-field splittings can be treated as a perturbation on the unsplit levels. Complexes thus have much the same colour as the 

The previous discussion has centred upon the Ln + ions. Most Ln + ions are not stable in solution, but have been prepared artificially in lattices by doping CaF2 with the Ln + 

Electronic absorption bands in the spectrum of PrCls (aq) reproduced with permission from S.A. Cotton, Lanthanides and Actinides, Macmillan (1991) p. 30. 

A unified interpretation of the electronic spectra of 1 1 and 1 2 superoxo and peroxometal complexes has been dveloped by Lever and Gray [177]. The energy levels of the HO radical have been Used to discuss 

MO2M type dinuclear (1 2) superoxo complexes can be discussed on 

Doubly bridged superoxo complexes exhibit similar features [179]. These compounds have an additional OH or NH group connecting the two 

The spectral characteristics of peroxometal complexes are summarized in Table XII. 

Bridging peroxometal complexes are mostly non-planar and have two 

The calculation of UV/vis spectra, or any other form of electronic spectra, requires the robust calculation of electronic excited states. The absorption process is a vertical transition, i.e. the electronic transition happens on a much faster timescale than that of nuclear motion (i.e. Bom-Oppenheimer dynamics, more correctly referred to as the Franck-Condon principle in the context of electronic spectroscopy). The excited state, therefore, maintains the initial ground-state geometry, with a modified electron density corresponding to the excited state. To model the corresponding emission processes, i.e. fluorescence or phosphorescence, it is necessary to re-optimize the excited-state nuclear geometry, as emission in condensed phases generally happens from the lowest vibrational level of the emitting excited state. This is Kasha s Rule. 

HF ground state S-excited state D-excited state 

Orbital configurations used in the definition of Slater determinants, where S, D and T denote any combination of 1,2 and 3 electrons, respectively, promoted fi om occupied orbitals to unoccupied orbitals. For Complete Active Space (CAS) all possible configurations are permitted within the region denoted by the box. 

Evidence of orbital energy levels can be obtained from electronic spectra. The energy of the photons absorbed as electrons are raised to higher levels is the difference in energy between 

A detailed photoelectron-spectroscopic study of the vibrations of thio-phen and other five-membered heterocycles has been undertaken and was found to be of considerable value in the interpretation of Raman and i.r. spectra. 

When we discussed the molecular orbital treatment for the hydrogen molecule, w e obtained two energy levels of the molecule corresponding to the two wave functions  

This result was then generalized to the statement that we could make similar combinations of other wave functions on the atoms to yield symmetric and antisymmetric states of higher energy. We now undertake the systematic description of these electronic states. 

In the case of diatomic molecules, we find that the Schrodinger equation requires that the component of the angular momentum along the molecular axis be quantized. The quantum number. A, describing this component is the basis for the term symbols for diatomic molecules. The quantum number A may have the values, A = 0, 1, 2,. For diatomic molecules, we use a Greek letter code for A. 

The value of A is determined by the resultant value of L for all the electrons in the molecule the possible values of A range from L to 0. 

Similarly, if we calculate the resultant value of the spin quantum number for the electrons outside the closed shells we obtain the total spin quantum number, S. The components of the total spin quantum numbers, S, S — 1,— (S — 1), —S, are the possible values of the quantum number E, which corresponds to Ms in the atom. There are 2S 1 values of E the multiplicity of the state is 25 -h 1. There is a total angular momentum quantum number, Q, which has the values 

The last terms of Eqs. [64] and [65] use the familiar Mulliken two-electron integral notation. Note that the singlet-triplet splitting predicted by this simplest model is given by the exchange integral between orbitals a and i. Since this term is always positive, the simple model presented here predicts that the triplet state is more stable, thereby providing a simple physical justification for Hund s rule. 

Although the model described above may be qualitatively appealing, it 

The discussion of orbital relaxation in the preceding section provides us with a clear way of understanding the shortcomings of the TDA. Recall that any Slater determinant formed from a particular set of basis functions may be obtained from any other Slater determinant that lies in the same basis by means of the exponential transformation 

Let us now assume that the excited state wavefunction is given exactly by some Slater determinant. It should be clear that the TDA does not contain sufficient flexibility to describe this state in the general case because the excited state wavefunctions in this approximation are given by a linear combination of singly excited determinants 

We can derive this method in a nonstandard way by considering the energy associated with I = exp(Ti) I o, that is, 

The introduction of a second heteroatom (other than sulfur) does not change drastically the absorption characteristics of small heterocycles. Oxaziridine and diaziridine are still transparent to light of wavelengths above 220 nm (Section 5.08.2.3.2). 

The first spectroscopic identification of the PH and PD radicals was based on the emission spectrum near 340 nm which was attributed to the 0-0 band of the A rij- X transition see Phosphor C, 1965, pp. 4/5, and [1, pp. 264, 564], [2, p. 306]. More detailed studies of the A-X transition, which could also be observed in absorption, have been carried out repeatedly, and reliable spectroscopic constants for the X and A states were derived. Furthermore, intensity and lifetime measurements enabled insight into the excitation and deexcitation processes of A rij as well as the predissociation effects. 

A second transition which has been studied in some detail is the spin-forbidden b X transition identified with the visible emission near 700 nm. Attempts to observe the other forbidden transition, a A- X expected in the near IR at 1.3 xm, were unsuccessful. 

Absorption measurements in the vacuum UV resulted in the detection of two Rydberg transitions originating from the metastable a state and of one n -X transition which possibly is also of Rydberg type. Another Rydberg transition, s+ b was detected as a two-photon process with near-UV radiation however, this transition was suggested to be one to a state of mainly valence character, d S+ b 

The spin-forbidden b transition was associated with a weak emission system In 

Time-resolved measurements of the whole emission band gave a radiative lifetime of Xrad = 1.25tol3 ms for the b X state [5]. For the collisional quenching of the b X emission, see p. 31. 

Set up a table of microstates for a configuration and show that the term symbol is D, and that the ground term is i 3/2- 

The terms for a configuration are D, F, G, and S. Which is the ground state term Rationalize your answer. 

Explain why a d configuration has the same ground state term as a configuration. 

For annual reviews, see Spectrosc. Prop. Inorg. Organometal. Compounds (A Specialist Periodical Report, Chem. Soc. [London]) 1 [1968] to 19 [1986]. 

Ramsey, B. G., Electronic Transitions in Organometallolds, Academic, New York 1969, 

Watters, K. L., Riesen, W. M., Spectroscopic Studies of Metal-Metal Bonded Compounds, Inorg. Chim. Acta Rev. 3 [1969] 129/54. 

Nakamoto, K., Characterization of Organometallic Compounds by Infrared Spectroscopy, Charact. Organometal. Compounds 1969 73/135. 

Chumaevskli, N. A., Vibrational Spectra of Group IVB and VB Heteroorganic Compounds, Nauka, Moscow 1971. 

To describe the intensity of a band the Beer-Lambert relationship is used  

Lipid autoxidation is accompanied by an increase in the absorption at 233 nm, due to increasing diene conjugation (Klein, 1970 Mead, 1976 Logani and Davis, 1980). Absorption at 233 nm has been used to evaluate the degree of oxidation of human plasma lipids and commercial oils, and in the assay of anti-oxidants (Di Luzio, 1973). 

When the extent of conjugation in a polyenic molecule reaches a certain stage, absorption occurs in the visible range. Among the lipids this is the case 

Visible spectra of carotenoids have been extensively studied (Vetter et al, 1971). In general, the shape of the spectrum is determined by the number of double bonds, the substituents, cis-trans isomerism, and by the solvent. Table 9.22 summarizes these influences. As the length of a chain of conjugated double bonds increases, additional double bonds show diminishing effects on the position of the main absorption band. The apo-carotenal series in Table 9.23 provides a good example. The batho-chromic shift for one additional double bond is 23 nm between the first two compounds, in contrast to only 10 nm between the last two members of this series (Vetter 1971). 

Polymerization is usually monitored by changes in the colour of the compounds. The colour is dependent upon the temperature both during and after irradiation (Leaver et aL, 1983) (Fig. 9.41). 

The spectroscopic constants [1 ] and Morse potential functions for both states were used to calculate the Franck-Condon factors [8,9] and r-centroids [9,10] for the observed and a number of unobserved v v transitions in the b X system. 

In the spectrum emitted from the H-f NF2 reaction at short reaction times ( 5 ms), the predominant 0 0 band and the weaker 1 1,2- 2, 3 3, and 4 4 bands were observed with partially resolved rotational structure (Q heads and clearly outstanding P(N) and R(N) branches). The relative intensities in this Av = 0 sequence indicated a nonthermal vibrational distribution (298 K), which, however, relaxed rapidly in the flow tube, giving only weak 1 1 and 1 0 emission (in addition to 0 0) after about 30 ms [11]. A vibrational distribution of 0.95 0.04 0.01 0.003 for the v = 0 to 3 levels of b 2 was derived from the intensity ratios (peak heights) of the Av = 0, v = 0 to 3 bands recorded after a reaction time of about 0.2 ms [12]. 

The spectrum observed (at low resolution) in the mixing region of NF2 and metastable Ar ( Po 2) atoms exhibited the Av = -1 sequence with v = 0 to 7 at about 570 to 540 nm, the Av = 0 sequence with v = 0 to 6 at about 530 to 510 nm, and a weak band corresponding to the Av=1 sequence around 490 nm. Intensities and calculated Franck-Condon factors indicated a vibrational distribution of approximately 100 18 11 8 5 4 3 2 for the v = 0 to 7 levels. In the flowing afterglow (at ca. 5 cm downstream from the reaction zone flow velocity ca. 16 m/s) only NF(b 2 , v = 0 to 2) with very low v = 2 population could be observed [13]. 

The square of the electronic transition moment Re, which is related to the radiative lifetime and the Franck-Condon factor qvv by xracj = (64jt /3h)- Re 2qvv vJv (variation 

The a X system was also observed in the H + NF2 reaction. To prevent overlap by the vibration-rotation bands of HF(Av = 3) and the B- A system of N2, excess NF2 and high flow velocities were chosen. A short reaction time of about 0.2 ms enabled the detection of the 1 1 

FIGURE 9.13 Contour map of the normalized 5/2 orbital cloud of the O-Mn u-bonds between the Mn atom and two of the O atoms. The values of the contour lines are j = —0.2 for line no. 1 and +0.2 for line no. 9 with increments of 0.05. That means that line no. 5 is a nodal plane where the electron wave function is 0. This is a plot of i, which can he positive or negative, but the electron density p = . The bonds are the buildup of electron probability between Mn and O. (With permission from Johnson, KH. and Smith, F.S., Phys. Rev. B, 5, 831, 1972. Copyright 1972 by the American Physical Society.) 

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Electronic spectra of surfaces can give information about what species are present and their valence states. X-ray photoelectron spectroscopy (XPS) and its variant, ESC A, are commonly used. Figure VIII-11 shows the application to an A1 surface and Fig. XVIII-6, to the more complicated case of Mo supported on TiOi [37] Fig. XVIII-7 shows the detection of photochemically produced Br atoms on Pt(lll) [38]. Other spectroscopies that bear on the chemical state of adsorbed species include (see Table VIII-1) photoelectron spectroscopy (PES) [39-41], angle resolved PES or ARPES [42], and Auger electron spectroscopy (AES) [43-47]. Spectroscopic detection of adsorbed hydrogen is difficult, and... 

Herzberg G 1966 Molecular Spectra and Molecular Structure III Electronic Spectra and Electronic Structure of Polyatomic Molecules (New York Van Nostrand-Reinhold)... 

The above three sources are a classic and comprehensive treatment of rotation, vibration, and electronic spectra of diatomic and polyatomic molecules. 

Electronic spectra are almost always treated within the framework of the Bom-Oppenlieimer approxunation [8] which states that the total wavefiinction of a molecule can be expressed as a product of electronic, vibrational, and rotational wavefiinctions (plus, of course, the translation of the centre of mass which can always be treated separately from the internal coordinates). The physical reason for the separation is that the nuclei are much heavier than the electrons and move much more slowly, so the electron cloud nonnally follows the instantaneous position of the nuclei quite well. The integral of equation (BE 1.1) is over all internal coordinates, both electronic and nuclear. Integration over the rotational wavefiinctions gives rotational selection rules which detemiine the fine structure and band shapes of electronic transitions in gaseous molecules. Rotational selection rules will be discussed below. For molecules in condensed phases the rotational motion is suppressed and replaced by oscillatory and diflfiisional motions. 

High-resolution spectroscopy used to observe hyperfme structure in the spectra of atoms or rotational stnicture in electronic spectra of gaseous molecules connnonly must contend with the widths of the spectral lines and how that compares with the separations between lines. Tln-ee contributions to the linewidth will be mentioned here tlie natural line width due to tlie finite lifetime of the excited state, collisional broadening of lines, and the Doppler effect. 

Duschinsky F 1937 On the interpretation of electronic spectra of polyatomic molecules. I. Concerning the Franck-Condon Principle Acta Physicochimica URSS 7 551... 

Pariser R 1956 Theory of the electronic spectra and structure of the polyacenes and of alternant hydrocarbons J. Chem. Rhys. 24 250-68... 

Marzocchi M P, Mantini A R, Casu M and Smulevich G 1997 Intramolecular hydrogen bonding and excited state proton transfer in hydroxyanthraquinones as studied by electronic spectra, resonance Raman scattering, and transform analysis J. Chem. Phys. 108 1-16... 

Leach S, Vervloet M, Despres A, Brcheret E, Hare P, Dennis T J S, Kroto H W, Taylor R and Walton D R M 1992 Electronic spectra and transitions of the fullerene Cgg Chem. Phys. 160 451-66... 

Rossetti R, Nakahara S and Brus L E 1983 Quantum size effects In the redox potentials, resonance Raman spectra and electronic spectra of CdS crystallites In aqueous solution J. Chem. Phys. 79 1086... 

G. Herzberg, Moleculer Spectra and Molecular Structure III. Electronic Spectra of Polyatomic Molecules, Van Nostrand, New York, 1967. 

You can use Cl to predict electronic spectra. Since the Cl wave function provides groun d state an d excited state energies, you can obtain electron ic absorption frequen cies from the dlfferen ces between the energy of the ground state and the excited states. 

For infrared spectra, both microns and wave numbers (cm. ) are convenient units. For electronic spectra (ultraviolet and visible), the milb-micron is largely used the wave numbers, (cm. i) may range between 13,000 and 50,000 and consequently many authors employ cm.X 10 . ... 

Ultraviolet and visible spectra arise from transitions between the electronic states in molecules. The terms electronic spectra and ultraviolet and visible spectra are synonymous and cover the range 200-800 mp.. The far-ultraviolet region 100-200 mp, only partially transmitted by quartz and appreciably absorbed by air, will not be considered. 

The electronic spectra of benzenoid systems differ in a characteristic manner from their acyclic analogues. Thus benzene, unhke hexatriene. 

Some applications of electronic spectra. These include —... 

Qualitative identification. The spectrum is of help in identifying organic compounds. If two compounds are identical, the electronic spectra must be identical the converse is not necessarily true and in this... 

Quantitative analysis. Spectroscopic analysis is widely used in the analysis of vitamin preparations, mixtures of hydrocarbons (e.y., benzene, toluene, ethylbenzene, xylenes) and other systems exhibiting characteristic electronic spectra. The extinction coefficient at 326 mp, after suitable treatment to remove other materials absorbing in this region, provides the best method for the estimation of the vitamin A content of fish oils. 

Another example of reduced symmetry is provided by the changes that occur as H2O fragments into OH and H. The a bonding orbitals (ai and b2) and in-plane lone pair (ai) and the a antibonding (ai and b2) of H2O become a orbitals (see the Figure below) the out-of-plane bi lone pair orbital becomes a" (in Appendix IV of Electronic Spectra and Electronic Structure of Polyatomic Molecules, G. Herzberg, Van Nostrand Reinhold Co., New York, N.Y. (1966) tables are given which allow one to determine how particular... 

Za,b = integrals are retained. In the INDO approach, the values of these single-atom integrals are determined by requiring the results of the calculation, performed at the Fock-like orbital level, to agree with results of ab initio Fock-level calculations. In the MINDO approach, experimental electronic spectra of the particular atom are used to... 

Semiempirical methods are parameterized to reproduce various results. Most often, geometry and energy (usually the heat of formation) are used. Some researchers have extended this by including dipole moments, heats of reaction, and ionization potentials in the parameterization set. A few methods have been parameterized to reproduce a specific property, such as electronic spectra or NMR chemical shifts. Semiempirical calculations can be used to compute properties other than those in the parameterization set. 

Practically all CNDO calculations are actually performed using the CNDO/ 2 method, which is an improved parameterization over the original CNDO/1 method. There is a CNDO/S method that is parameterized to reproduce electronic spectra. The CNDO/S method does yield improved prediction of excitation energies, but at the expense of the poorer prediction of molecular geometry. There have also been extensions of the CNDO/2 method to include elements with occupied d orbitals. These techniques have not seen widespread use due to the limited accuracy of results. 

The Zerner s INDO method (ZINDO) is also called spectroscopic INDO (INDO/S). This is a reparameterization of the INDO method specihcally for the purpose of reproducing electronic spectra results. This method has been found to be useful for predicting electronic spectra. ZINDO is also used for modeling transition metal systems since it is one of the few methods parameterized for metals. It predicts UV transitions well, with the exception of metals with unpaired electrons. However, its use is generally limited to the type of results for which it was parameterized. ZINDO often gives poor results when used for geometry optimization. 

N. Mataga, T. Kubota, Molecular Interactions and Electronic Spectra Marcel Dekker, New York (1970). 

Use Configuration Interaction to predict the electronic spectra of molecules. The Configuration Interaction wave function computes a ground state plus low lying excited states. You can obtain electronic absorption frequencies from the differences between the energies of the ground state and the excited states. 

Electronic spectroscopy is the study of transitions, in absorption or emission, between electronic states of an atom or molecule. Atoms are unique in this respect as they have only electronic degrees of freedom, apart from translation and nuclear spin, whereas molecules have, in addition, vibrational and rotational degrees of freedom. One result is that electronic spectra of atoms are very much simpler in appearance than those of molecules. 

In electronic spectra there is no restriction on the values that Au can take but, as we shall see in Section 1.2.53, the Franck-Condon principle imposes limitations on the intensities of the transitions. 

As is the case for diatomic molecules, rotational fine structure of electronic spectra of polyatomic molecules is very similar, in principle, to that of their infrared vibrational spectra. For linear, symmetric rotor, spherical rotor and asymmetric rotor molecules the selection mles are the same as those discussed in Sections 6.2.4.1 to 6.2.4.4. The major difference, in practice, is that, as for diatomics, there is likely to be a much larger change of geometry, and therefore of rotational constants, from one electronic state to another than from one vibrational state to another. 

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