Spectra UV-visible

In addition to the absorption corresponding to the anionic part (tetraphenylborate, iodide, etc.), the electronic absorption spectra of phosphonium salts are typical of those of the independent chromophoric group present in the cationic part108. However, as the phosphorus atom is positively charged, its 3d orbitals become able to overlap with any nearby n orbital this d-n interactions leads to bathochromic and hyperchromic 

Field-desorption mass spectrometry (FDMS), where no evaporation prior to ionization is required, has been successfully used in the analysis of in volatile phosphonium salts113, although a direct thermal process gave similar spectra114. In the case where the FD spectra are complex, a chemical ionization technique may give wider applicability115. The cation is the base peak for monophosphonium salts when the [2M + anion]+ cationic species is the one for bisphosphonium compounds. 

Considering the anionic counterpart, their geometries, contact distances (including the possibility of hydrogen bonding with the cations), stacking and crystal packing have 

FIGURE 11. Comparative view of the two equivalent parts of the molecule along the aromatic plane of the phosphinolium ring66 

The choice of the NDO method depends on several factors including your previous experience and preferences. If you want to compare the results to other studies, you must use the same semi-empirical method. Since some methods can converge much more quickly than others, you might want to use a fast method to obtain an approximation of the final answer, and then a more accurate method for the final result. 

Eor transition metals the splitting of the d orbitals in a ligand field is most readily done using EHT. In all other semi-empirical methods, the orbital energies depend on the electron occupation. HyperChem s molecular orbital calculations give orbital energy spacings that differ from simple crystal field theory predictions. The total molecular wavefunction is an antisymmetrized product of the occupied molecular orbitals. The virtual set of orbitals are the residue of SCE calculations, in that they are deemed least suitable to describe the molecular wavefunction. 

Normally, you would expects all 2p orbitals in a given first row atom to be identical, regardless of their occupancy. This is only true when you perform calculations using Extended Hiickel. The orbitals derived from SCE calculations depend sensitively on their occupation. Eor example, the 2px, 2py, and 2pz orbitals are not degenerate for a CNDO calculation of atomic oxygen. This is especially important when you look at d orbital splittings in transition metals. To see a clear delineation between t2u and eg levels you must use EHT, rather than other semiempirical methods. 

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When computing UV visible spectra, you should do a Cl singles calculation, RHFor L H F ealcu lation s arc sufficient to reproduce the proper order of molecular orbitals in most complexes. 

The Microstate Cl Method lowers the energy of the uncorrelated ground state as well as excited states. The Sngly Btcited Cl Method is particularly appropriate for calculating UV visible spectra, and does not affect the energy of the ground state (Brillouin s Theorem). 

The similarity in the electronic structures of the eight r-electron systems R2PN3S2 (3.25) and [S3N3O2] (3.26) is also reflected in their UV-visible spectra. Both these heterocycles have an intense purple colour due to a visible band at ca. 560 nm attributed to the HOMO (tt )... 

Some acyclic sulfur-nitrogen compounds also exhibit intense colours. For example, S-nitrosothiols RSNO are either green, red or pink (Section 9.7). Their UV-visible spectra show an intense band in the 330-350 nm region (no it ) and a weaker band in the visible region at 550-600 nm... 

Comparison of the photoelectron spectra and electronic structures of M-NS and M-NO complexes, e.g., [CpCr(CO)2(NX)] (X = S, O), indicates that NS is a better a-donor and a stronger r-acceptor ligand than NO. This conclusion is supported by " N and Mo NMR data, and by the UV-visible spectra of molybdenum complexes. 

In solution the cis and trans isomers may co-exist, as demonstrated by N NMR and UV-visible spectra. The N NMR chemical shift of the trans isomer is shifted ca. 60 ppm downfield relative to the cis isomer." The visible absorption band of S-nitrosothiols corresponds to a weak n K transition in the 520-590 nm region. The absorption maxima of trans conformers are red-shifted by ca. 30 nm relative to those of the cis isomer. Two absorptions are observed in the 520-590 nm region in the experimental spectra of RSNO derivatives." ... 

These iridium(IV) complexes have UV-visible spectra dominated by intense absorptions around 500 nm (X = Cl) and 700 nm (X = Br) assignable to 7tx —> Ir(t2g) ligand-to-metal charge-transfer bonds. 

Despite the unpromising UV-visible spectra and flash photolysis studies, the carbene complexes presented in this chapter have a rich photochemistry at wavelengths exceeding 300 nm. A wide range of synthetically useful transformations has been developed, and continued studies are likely to reveal more. 

A somewhat more detailed study of vanadium atoms and dimers has also appeared 108). Figure 1 shows the UV-visible spectra of V and V2 as a function of vanadium concentration. Figure 2 shows a tjqiical, metal-concentration plot illustrating the aforementioned kinetic anal-... 

Fig. 5. Correlation of the UV-visible spectra of Co atoms and Coj molecules isolated in Ar, Kr, and Xe matrices under identical conditions of temperature and gas and metal deposition rates (49, 154).

The UV-visible spectra of Ni and Nia have also been identified in argon matrices (93) Ni absorbed at 377, 529, and 4l0 nm, with vi-bronic structure on the first two bands, and with spacing of—330 cm , and Nis absorbed at 420 and 480 nm, the latter band showing vibrational spacing of -200 cm" . Higher-nuclearity clusters were observed, but not characterized. After prolonged warm-up of these matrices, nickel colloid was formed (93). 

Fig. 16. The UV-visible spectra of Ag,jj /Kr mixtures (Ag/Kr = l/10 )at 10-12K (A) After a 30-min irradiation centered at the atomic resonance absorption lines. (B ) The outcome of a 10-min, 423-nm Agj irradiation, showing major decay of the bands associated with Ag, (indicated by arrows) and the appearance of two new bands near 450 nm. (C) The result of a 5-min, 25K bulk thermal annealing period, showing regeneration of the original Ag3 spectrum eind loss of the new band near 445 nm USD.

Fig. 18. UV-visible spectra of Agi,j.s/Ar mixtures (Ag Ar = 1 1(F) at 10-12K. Note the growth of Agj and Ag, clusters and loss of Ag atoms as a result of 305 nm, Ag atom excitation. Spectra A, B, and C represent irradiation times of 0,1, and 4 min, respectively (149).

The cocondensation of nickel atoms and CS2 at 12 K resulted in the formation of three binary, mononuclear, nickel/CS complexes, NKCSjln, n = 1-3 (145). Mixed CS2/ CS2 isotopes were used to identify the lowest stoichiometry species. An interpretation of the IR and UV-visible spectra, as well as normal-coordinate analyses (144), suggested that these species are best considered as normal 7r-complexes, with the nickel atom coordinated to the C=S bond in a manner analogous to C=C bond coordination (123). 

The UV-visible spectra of the H- and nifro-azobenzene dendrimers in chloroform solution showed strong absorption bands within the visible region due to the transitions of azobenzene chromophores (Table 2). Because of the stronger delocalization of n-electrons in nitro-azobenzene, the maximum absorption band is at a longer wavelength compared with that for H-azoben-zene. There was little spectral shift of the absorption maximum for dendrimers with different numbers of azobenzene units, indicating that dendrimers did not form any special intermolecular aggregates. 

A cationic molybdenum sulfide cluster [Mo3S4(H20)9] " with incomplete cubane-type structure and a cationic nickel-molybdenum mixed sulfide cluster [Mo3NiS4Cl(H20)9p " with complete cubane-type structure were introduced into zeolites NaY, HUSY and KL by ion exchange. Stoichiometry of the ion exchange was well established by elemental analyses. The UV-visible spectra and EXAFS analysis data exhibited that the structure of the molybdenum cluster remained virtually intact after ion exchange. MoNi/NaY catalyst prepared using the molybdenum-nickel sulfide cluster was found to be active and selective for benzothiophene hydrodesulfurization. 

Figure 1. UV-visible spectra of (a) physical mixture of NaY and the chloride salt of 1, (b) Mo/NaY, (c) Mo/HUSY and (d) Mo/KL.

UV-visible spectra of the cluster 1 and molybdenum/zeolite catalysts are shown in Figure 1. The cluster 1 showed bands at 300, 390 and ca. 650 nm. Similar bands were observed for the spectrum of each molybdenum/zeolite catalyst, suggesting that the structure of cluster 1 was practically unchanged after ion exchange. 

In order to obtain more structural information about the molybdenum species in Mo/NaY, EXAFS measurements of the cluster 1 and Mo/NaY were carried out. The Fourier transforms of the EXAFS data are shown in Figure 2. Structural parameters (Table 3) showed no change of the Mo-0, Mo-S and Mo-Mo distances, suggesting that there is no significant structural difference between the cluster 1 and the molybdenum compound in the Mo/NaY. From these EXAFS parameters and the UV-visible spectra, it is considered the structure of cluster 1 remained vinually intact after ion exchange. 

The optical absorption spectra of sulfonyl radicals have been measured by using modulation spectroscopy s, flash photolysis and pulse radiolysis s techniques. These spectra show broad absorption bands in the 280-600 nm region, with well-defined maxima at ca. 340 nm. All the available data are summarized in Table 3. Multiple Scattering X, calculations s successfully reproduce the experimental UV-visible spectra of MeSO 2 and PhSO 2 radicals, indicating that the most important transition observed in this region is due to transfer of electrons from the lone pair orbitals of the oxygen atoms to... 

As already briefly mentioned in the introduction, some metals exhibit so-called plasmon resonances in the UV-visible spectra, attributed to the interaction of electromagnetic waves (visible light) and the confined electron gas, if a critical size on the nanoscale is reached. The process is sketched in a simplified manner in Figure 8. 

Fig. 34.2, UV-visible spectra of mixtures of fluoranthene and chrysene (see Fig. 34.3 for the pure spectra).

Spectra at p (=20) wavelengths. Because of the Lambert-Beer law, all measured spectra are linear combinations of the two pure spectra. Together they form a 15x20 data matrix. For example the UV-visible spectra of mixtures of two polycyclic aromatic hydrocarbons (PAH) given in Fig. 34.2 are linear combinations of the pure spectra shown in Fig. 34.3. These mixture spectra define a data matrix X, which can be written as the product of a 15x2 concentration matrix C with the 2x20 matrix of the pure spectra ... 

Solvatochromic pareuaeters, so called because they were Initially derived from solvent effects on UV/visible spectra, have been applied subsequently with success to a wide variety of solvent-dependent phenomena and have demonstrated good predictive ability. The B jo) scale of solvent polarity is based on the position of the intermolecular charge transfer absorption band of Reichardt s betaine dye [506]. Et(io> values are available for over 200 common solvents and have been used by Dorsey and co-%rarkers to study solvent interactions in reversed-phase liquid chromatography (section 4.5.4) [305,306]. For hydrogen-bonding solvents the... 

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