UV—vis spectroscopy

As seen in the previous two chapters, spectroscopy allows us to probe molecular structure by studying the interaction between matter and electromagnetic radiation. Recall that the frequency of light determines the energy of a photon, and the range of all possible frequencies is known 

The ground state of butadiene can absorb a photon of UV light to produce the excited state, in which an electron has been promoted to a higher energy MO. 

An energy diagram showing the relative energy levels for MOs for various conjugated tt systems. 

TABLE 17.3 THE WAVELENGTH OF MAXIMUM ABSORPTION FOR A SIMPLE DIENE, TRIENE, AND TETRAENE 

In the previous section, we noted that a compound containing a conjugated it system will absorb UV-Vis light. The region of the molecule responsible for the absorption (the conjugated tt system) is called the chromophore, while the groups attached to the chromophore are called auxochromes. 

All kinds of CNTs are active in the UV-Vis region and exhibit characteristic absorption bands corresponding to additional absorption due to 1D van Hove singularities. The spectra obtained are 

Consequently, it allows a quick and simple determination of the optimal exposure time to ultrasound. Obtaining the optimum debundling of the CNTs, while minimizing as much as possible damage caused by sonication, by reducing the exposure time, are crucial issues — the sine qua non conditions to produce technically interesting C NT/polymer nanocomposites, among others. I n the same order of ideas, UV-Vis spectroscopy can also be used to quantitatively characterize the colloidal stability in time of CNT dispersions.  

The same Carbolex and HiPCO SWCNT dispersions as those previously studied in Section 3.1.2 were examined with UV-Vis spectroscopy. Samples were taken regularly during the sonicating process, diluted and UV-Vis spectra were recorded. Since desorption processes are typically quite slow, it was assumed that the amount of SDS molecules adsorbed on the CNT walls was not significantly influenced by dilution, and UV-Vis spectra were typically immediately recorded after dilution. Please note that the dilution factor — in other words, the CNT concentration after dilution [i.e., 6.7 x 10 wt% for the standard SWCNT dispersions) — was chosen in such a way that all the UV-Vis absorbance values remained below 1 so that the error inherently present in the measurement itself is reduced. At this dilution, the contribution of scattering can be ignored.  

Quasi one-dimensional confinement ofthe electronic and phonon states of the SWCNT 7i-electron system causes theCNTdensityof state to show very sharp and characteristic van Hove maxima at energies depending on the CNT diameters and chiralities. As a result, the UV-Vis spectrum of the SDS-CNT dispersions is a superposition of distinct electronic transitions, generated by a variety of SWCNTs with different diameters and chiralities. This is reflected in the separated absorption features (like little humps ) observed in the UV-Vis spectra of the SDS-SWCNT dispersions. These observed features become narrower and narrower as the debundling proceeds. This constitutes an additional indication that CNT individualization from the CNT ropes occurs. It has, indeed, been observed and proven that CNT bundling leads to broadening of spectral features of CNT dispersions.  

Nevertheless, these features could not be observed in the UV-Vis spectra of Carbolex CNTs, not even at the end of the debundling, whereas these van Hove singularities could be observed for the debundling of the same type of CNTs in sulfuric acid. Vaccarini et al. showed that the absorption peak width does not necessarily exclusively correlate with the level of debundling, and the interaction between the CNT and the dispersion medium can have a noticeable effect on the peak width. It can thus be conjectured that Carbolex and H iPCO CNTs might not have the same interaction with SDS molecules adsorbed on their walls. 

Despite the widespread use of UV-Visible spectroscopy in analytical chemistry, it has seen much more limited application to the study of electrochemical phenomena relevant to EEIs. However, the studies available are excellent representations of the kind of unique insight that could be gained from these tools. Absorption spectroscopy has been applied to the study of the electrochemical degradation of aromatic molecules on the surface of electrodes under working conditions [166], 

The shape of an electronic absorption band will be determined to a large extent by the spacing of the vibrational levels and the distribution of band intensity over the vibrational sublevels. In most cases these effects lead to broad absorption bands in the UV-vis region. 

The wavelength maximum at which an absorption band occurs in the UV-vis region is generally referred to as the Xmax of the sample (where wavelength is determined by the band maximum). 

The quantitative relationship of absorbance (the intensity of a band) to concentration is expressed by the Beer-Lambert equation  

A = absorbance, expressed as Iq/I Iq = the intensity of the incident light I = the intensity of the light transmitted through the sample 8 = molar absorbtivity, or the extinction coefficient (a constant characteristic of the specific molecule being observed) values for conjugated dienes typically range from 10,000 to 25,000 c = concentration (mol/L) 

The calculated extinction coefficient and solvent are usually listed with the wavelength at the band maximum. For example, data for methyl vinyl ketone (3-buten-2-one) would be reported as follows  

Last but not least, ageing and destruction processes can be monitored in polymers under application, and structural and quantitative analysis of unknown additives (stabilizers etc.) is possible in commercial polymers using UV-vis spectroscopy. Advantage can be taken here of the fact that the position of an electronic absorption in unsaturated systems depends only weakly on the surroimd-ing medium. Even though UV-vis spectroscopy is not very specific in the absorption band, it is highly sensitive and therefore much better than NMR or IR spectroscopy to detect small amounts of chromophors. 

Last but not least, ageing and destructiOTi processes can be monitored in polymers under application, and structural and quantitative analysis of unknown 

Reprinted with permission from the American Chemical Society (1 993, 1 998 and 2005), Wiley-VCH Verlag GmbH (2001) and the Chemical Society of Japan (1 995). For individual citations, please see the references listed at the end of this chapter. 

Survey of highly HT-regioregular alternating copoly(3-substituted thiophenejs 

Spectra of [Co2(C02EtL2)(CH3COO)2] in acetonitrile (blue) and ethanol (black). A broad transition in the NIR region around 1,100 nm is apparent in addition to 2-3 bands between 400 and 600 nm which are typical for octahedral high-spin Co(ll) complexes [21]. Table 6.3 lists the observed transitions and extinction coefficients for all complexes. 

While the extinction coefficients for the first transition around 470 nm range 

The UV-Vis-NIR diffuse reflectance spectra (Fig. 6.13) of the Co(II) complexes showed a characteristic broad NIR-band around 1,100 nm corresponding to the Tig T2g d-d transition of hexacoordinate Co(II) that was observed in some of the spectra measured in solution [14]. 

The visible absorption bands between 400 and 600 nm arise from the Tig Tig(P) transition, which is split into three bands due to distortion from Oj, symmetry [41]. These bands are better resolved than in the respective solution spectra. Two distinct phenolate O Co(II) bands present below 400 nm were assigned to tt 2g and 71 eg LMCT transitions [14]. Interestingly, the diffuse reflectance spectra exhibit close similarities to the spectra collected in solution. 

Hence the structures found in the solid state (crystal structure, IR) are close to those in solution, which is in agreement with the findings from mass spectrometry supporting the formation of [Co2(H iLX)(CH3COO)2] species in solution. 

The identification of the prewave observed on voltammograms for ethyl xanthate on chalcocite and copper with chemisorbed xanthate was disputed by Mielczarski et aL on the basis of Fourier transform infrared (ITIR) and XPS studies. These authors concluded that the surface species in the undeipotential region could not be xanthate itself and suggested that it was probably a decomposition product of this compound. A similar conclusion with respect to silver was reached in later work.  

It was found that, if oxygen was not rigorously removed from solution, a small peak developed in the spectrum at 350 nm. The peak was assigned to perxanthate this species is known to be formed by the reaction of xanthate and peroxide  

It was considered that hydrogen peroxide is formed at silver and chalcocite surfaces when oxygen is present as this compound is an intermediate in the cathodic reduction of oxygen. Clearly, then, there is interaction between the two reactions that make up the mixed potential system, namely, xanthate oxidation and oxygen reduction. Thus, care must be taken in considering the individual processes in isolation. 

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Fortunately, azachalcone derivatives (2.4a-g, Scheme 2.4) turned out to be extremely suitable dienophiles for Lewis-add catalysed Diels-Alder reactions with cyclopentadiene (2.5). This reaction is outlined in Scheme 2.4 and a large part of this thesis will be devoted to the mechanistic details of this process. The presence of a chromophore in 2.4 allows kinetic studies as well as complexation studies by means of UV-vis spectroscopy. Furthermore, the reactivity of 2.4 is such that also the... 

In the kinetic runs always a large excess of catalyst was used. Under these conditions IQ does not influence the apparent rate of the Diels-Alder reaction. Kinetic studies by UV-vis spectroscopy require a low concentration of the dienophile( 10" M). The use of only a catalytic amount of Lewis-acid will seriously hamper complexation of the dienophile because of the very low concentrations of both reaction partners under these conditions. The contributions of and to the observed apparent rate constant have been determined by measuring k pp and Ka separately. ... 

After in situ neutralisation, the complexation behaviour of 4.44 was studied using UV-vis spectroscopy. The absorption maximum of this compound shifted from 294 nm in pure water to 310 nm in a 10 mM solution of copper(II)nitrate in water. Apparently, 4.44, in contrast to 4.42, does coordinate to copper(II)nitrate in water. 

Kinetic measurements were performed employii UV-vis spectroscopy (Perkin Elmer "K2, X5 or 12 spectrophotometer) using quartz cuvettes of 1 cm pathlength at 25 0.1 C. Second-order rate constants of the reaction of methyl vinyl ketone (4.8) with cyclopentadiene (4.6) were determined from the pseudo-first-order rate constants obtained by followirg the absorption of 4.6 at 253-260 nm in the presence of an excess of 4.8. Typical concentrations were [4.8] = 18 mM and [4.6] = 0.1 mM. In order to ensure rapid dissolution of 4.6, this compound was added from a stock solution of 5.0 )j1 in 2.00 g of 1-propanol. In order to prevent evaporation of the extremely volatile 4.6, the cuvettes were filled almost completely and sealed carefully. The water used for the experiments with MeReOj was degassed by purging with argon for 0.5 hours prior to the measurements. All rate constants were reproducible to within 3%. 

All kinetic measurements were performed using UV-vis spectroscopy (Perkin Elmer 2, 5 or 12 photo spectrometers) as described in Chapter 2. 

Ultraviolet visible (UV VIS) spectroscopy, which probes the electron distribution especially m molecules that have conjugated n electron systems Mass spectrometry (MS), which gives the molecular weight and formula both of the molecule itself and various structural units within it... 

As diverse as these techniques are all of them are based on the absorption of energy by a molecule and all measure how a molecule responds to that absorption In describing these techniques our emphasis will be on then application to structure determination We 11 start with a brief discussion of electromagnetic radiation which is the source of the energy that a molecule absorbs m NMR IR and UV VIS spectroscopy... 

With this as background we will now discuss spectioscopic techniques mdividu ally NMR IR and UV VIS spectroscopy provide complementaiy mfoimation and all are useful Among them NMR provides the mfoimation that is most duectly related to moleculai stiuctuie and is the one we 11 examine hist... 

The main application of UV VIS spectroscopy which depends on transitions between electronic energy levels is in identifying conjugated tt electron systems... 

The structural unit associated with an electronic transition m UV VIS spectroscopy IS called a chromophore Chemists often refer to model compounds to help interpret UV VIS spectra An appropriate model is a simple compound of known structure that mcor porates the chromophore suspected of being present m the sample Because remote sub stituents do not affect Xmax of the chromophore a strong similarity between the spectrum of the model compound and that of the unknown can serve to identify the kind of rr electron system present m the sample There is a substantial body of data concerning the UV VIS spectra of a great many chromophores as well as empirical correlations of sub stituent effects on k Such data are helpful when using UV VIS spectroscopy as a tool for structure determination... 

Molar absorptivity (Section 13 21) Ameasure of the intensity of a peak usually in UV VIS spectroscopy Molecular dipole moment (Section 1 11) The overall mea sured dipole moment of a molecule It can be calculated as the resultant (or vector sum) of all the individual bond di pole moments... 

Ultraviolet visible (UV VIS) spectroscopy (Section 13 21) An alytical method based on transitions between electronic en ergy states in molecules Useful in studying conjugated systems such as polyenes... 

Typical cells used in UV/Vis spectroscopy. Courtesy of Fisher Scientific. 

The evaluation of instrumentation for molecular UV/Vis spectroscopy is reviewed in the following pair of papers. Altermose, 1. R. Evolution of Instrumentation for UV-Visible Spectrophotometry Parti, /. Chem. Educ. 1986, 63, A216-A223. 

Listed below is a two-part series on the application of photodiode arrays in UV/Vis spectroscopy. 

Stability of the chromophore was observed usiag uv-vis spectroscopy, the authors conclude that this sol—gel method of chromophore encapsulation does not provide any real thermal or oxidative protection in either the covalendy or noncovalently bonded state. 

Instrumentation. The k region was developed usiag dispersive techniques adapted as appropriate from uv—vis spectroscopy. Unfortunately, k sources and detectors tend to be kiefficient compared to those for other spectral regions. 

Sohd-state multi-element detector arrays in the focal planes of simple grating monochromators can simultaneously monitor several absorption features. These devices were first used for uv—vis spectroscopy. Infrared coverage is limited (see Table 3), but research continues to extend the response to longer wavelengths. Less expensive nir array detectors have been appHed to on-line process instmmentation (125) (see Photodetectors). 

Ketenes absorb near 2100-2130cm . When the photolysis was carried out and the IR spectrum of the solution monitored, it was found that a band appeared at 2118 cm , grew, and then decreased as photolysis proceeded. The observation of this characteristic absorption constitutes good evidence for a ketene intermediate. As with UV-VIS spectroscopy, the amount of intermediate that can be detected depends both on the intensity of the absorption band and the presence of interfering bands. In general, IR spectroscopy requires somewhat higher concentration for detection than does UV-VIS spectroscopy. 

With this as background, we will now discuss spectroscopic techniques individually. NMR, IR, and UV-VIS spectroscopy provide complementary information, and all are useful. Among them, NMR provides the information that is most directly related to molecular- structure and is the one we ll examine first. 

Section 13.21 Transitions between electronic energy levels involving electromagnetic radiation in the 200-800-nm range form the basis of UV-VIS spectroscopy. The absorption peaks tend to be broad but are often useful in indicating the presence of particular- tt electron systems within a molecule. 

Molar absorptivity (Section 13.21) Ameasure of the intensity of a peak, usually in UV-VIS spectroscopy. 

In the field of soluble conducting polymers new data have been published on poly(3-alkylthiophenes " l They show that the solubility of undoped polymers increases with increasing chain length of the substituent in the order n-butyl > ethyl methyl. But, on the other hand, it has turned out that in the doped state the electro-chemically synthesized polymers cannot be dissolved in reasonable concentrations In a very recent paper Feldhues et al. have reported that some poly(3-alkoxythio-phenes) electropolymerized under special experimental conditions are completely soluble in dipolar aprotic solvents in both the undoped and doped states. The molecular weights were determined in the undoped state by a combination of gel-permeation chromatography (GPC), mass spectroscopy and UV/VIS spectroscopy. It was established that the usual chain length of soluble poly(3-methoxthythiophene) consists of six monomer units. 

Further structural information is available from physical methods of surface analysis such as scanning electron microscopy (SEM), X-ray photoelectron or Auger electron spectroscopy (XPS), or secondary-ion mass spectrometry (SIMS), and transmission or reflectance IR and UV/VIS spectroscopy. The application of both electroanalytical and surface spectroscopic methods has been thoroughly reviewed and appropriate methods are given in most of the references of this chapter. 

UV-Vis spectroscopy may also provide valuable information if small molecules are studied. However, the photochemical sensitivity of many sulfur-containing molecules may trigger changes in the composition of the sample during irradiation. For instance, this phenomenon has been observed in Raman spectroscopy using the blue or green hnes of an argon ion laser which sometimes decompose sensitive sulfur samples with formation of Sg [2, 3]. Reliable spectra are obtained with the red hnes of a krypton ion or a He-Ne laser as well as with the infrared photons of a Nd YAG laser. 

The red tetrasulfide radical anion 84 has been proposed as a constituent of sulfur-doped alkali hahdes, of alkah polysulfide solutions in DMF [84, 86], HMPA [89] and acetone [136] and as a product of the electrochemical reduction of 8s in DM80 or DMF [12]. However, in all these cases no convincing proof for the molecular composition of the species observed by either E8R, Raman, infrared or UV-Vis spectroscopy has been provided. The problem is that the red species is formed only in sulfur-rich solutions where long-chain polysulfide dianions are present also and these are of orange to red color, too (for a description of this dilemma, see [89]). Furthermore, the presence of the orange radical anion 8e (see below) cannot be excluded in such systems. 

Using CO-saturated hydrocarbon matrices, Pearsall and West" photolyzed sily-lene precursors at 77 K and monitored CO coordination to the silylenes by UV-vis spectroscopy (Scheme 13). Bis(trimethylsilyl)silanes 44a-c or SifiMcji were irradiated at 254 nm to create silylenes 45a-d, which reacted with CO, causing new peaks to ca. 290 and 350 nm, which were attributed to complex 46a-d, a resonance structure of silaketene 47a-d. Silylene adducts form fairly weak bonds, as seen by warming of the matrices. In the case of silylene adducts where one R = Mes, the CO dissociates and the corresponding disilene 48a-c peaks in the UV-vis spectra observed upon warming (R2 = Me most likely produced silane rings Si, Me6. etc.). 

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