Absorption spectrum

Phase Absorption bands wavelength, mm Ref. Year Remarks 

FIGURE 7.13 The absorption spectrum, visible region, of a copper sulfate solution. 

Notice also that this absorption spectrum is a continuous spectrum, meaning that the spectrum is an unbroken pattern, left to right. It does not display any breaks or sharp peaks of absorption at particular wavelengths, but rather shows that a smooth band of wavelengths in a given region, such as the red region, is absorbed. 

FIGURE 7.14 The absorption spectrum, in a narrow portion of the ultraviolet region, of gaseous copper atoms. 

FIGURE 7.15 The energy transitions possible between two electronic states, each with five vibrational levels superimposed. 

The characteristic spectral properties of tryptophanase in the visible and near UV region are due to pyridoxal-P-lysine aldimine (Schiff base).32 In the presence of K+ or NH4+ ions the enzyme displays pH-dependent spectra with maxima at 420 and 337 nm (Fig. 9.6). The 420-nm band predominates at acid pH and gradually diminishes as pH is raised from 6 to 9, whereas another band at 337 nm increases in intensity. From a plot of absorbance versus pH at these wavelengths the pA a value of the coenzyme chromophore was found to be 7.232) or 7.5.33) The species with 2max 337 nm is the active enzyme form because it prevails at the pH-optimum for tryptophanase activity. 

Morino and Snell32 ascribed the 420-nm species to a protonated internal pyridoxal-P-lysine aldimine and the 337-nm species to either a dipolar ionic form of the aldimine or to a substituted aldamine (an adduct at C-4 ). However, the dipolar ionic form absorbs at about 360 nm in aspartate transaminase and in the model systems containing Schiff bases of A-methylated PLP.34 It was suggested that the 337-nm species may be the enolimine tautomer.35,36  

A striking feature of tryptophanase spectra is their dependence on monovalent cations. In the absence of cations, e.g. in (CH3)4N/ Hepps buffer or in the presence of Na+ ions, the enzyme exhibits an increased pH-independent absorption peak at 420 nm and little or no absorption at 337 nm even at pH 8.O.32 Addition of activating cations, e.g. K+, to the enzyme solution in Hepps buffer at pH 8 results in a decrease of absorbance at 420 nm and an increase at 337 nm, i.e. produces an effect similar to that of raising pH.37  

Addition of substrates (L-tryptophan, S-methyl-L-cysteine and others) or competitive amino acid inhibitors to tryptophanase solutions induces an intense narrow absorption peak near 500 nm with a shoulder at 470-475 nm the peak rapidly disappears as the substrates are decomposed, but it is relatively stable in the presence of amino acid 

A quinonoid intermediate is formed only in the presence of activating cations, which seem to be required for labilization of the a-hydrogen of the enzyme-substrate complex. Suelter and Snell37 found that Vmax observed with most cations except NH4+ is directly proportional to the extent to which they elicit formation of the 500-nm absorbing species. 

The absorption spectra of 0.025 mol% 07283 and 0.1 mol% Tm283 single-doped and codoped ChH glass samples in the range between 400 nm and 1,800 nm are shown in Fig. 6.8, where the corresponding energy levels from Tm and Dy ions are observed. 

Absorption spectra of the GGCl, GGC5, and GGC8 glasses at room temperature. 

UV VIS absorption spectra are called electronic spectra, because the observed absorption bands indicate the frequencies of the electromagnetic radiation at which the energy of a photon, hv, matches the energy difference between the electronic ground state and an electronically excited state of the absorbing molecules, AE = hv. The absorption and emission spectra (Section 3.4) of a substrate should be determined at the outset of any 

The non-SI unit cm 1 (1 im 1 = 104cm J) is mostly used in IR, Raman and microwave spectroscopy. In this text, absorption spectra are shown on a wavenumber scale A(v), v = vjc = 1/2, [v] = im, mnning from right to left, so that the wavelengths, shown on top of the diagrams on a nonlinear scale, still increase to the right as is more customary to the chemist. To convert to a molecular energy scale we use the relation E = N. hc v to obtain the practical Equation 3.1. 

For the determination of molar absorption coefficients, s, care must be taken to use clean glassware and solvents and the instrumental baseline must be recorded or calibrated prior to measurement. Absorbance measurements are most accurate in the range 0.1 A 1.5 lower absorbances are prone to baseline errors and those exceeding 1.5 should be avoided, because the amount of light passing the sample cell becomes very small and the readings are 

As previously noted, there are a number of factors that can affect the emitted spectra before it reaches the sample. One of the most important of these items is the container, specifically its color, thickness, and geometry. 

Colors and impurities obviously filter out certain radiation bands. Increasing the thickness of a container will obviously increase the absorption of any given absorbed band. One of the most difficult items to measure is the effect of the container geometry on the exposure. Depending on the physical shape of the container and the angle(s) of incidence of the radiation (cosine law effect), radiation can be reflected or refracted by flat surfaces and possibly focused internally by rounded corners. 

All of these items make it difficult to determine the amount of radiation actually absorbed by a sample. For liquids, the best technique is to fill a duplicate 

It is frequently assumed that the absorption spectrum of a test substance will accurately represent its activation or action spectrum because only absorbed radiation can bring about a photochemical change. Frequently neglected are solvent (bathochromic or hypsochromic) or matrix effects such as polarity, pH, complexation, dimerization, binding, self-filtering, and quantum efficiency differences between various absorption bands, etc., which may occur. 

one encounters comparisons of spectra taken in different media in the pharmaceutical photostability literature. An often encountered example is the failure to recognize the effect of oxygen capacity of the various mediums used on the results obtained, e.g., comparing the results obtained in aqueous to those obtained in methanolic media. 

One feature common to all hemoglobin absorption spectra (Fig. 12,13) is the occurrence of an intense band in the region 400—420 mp. The milh-molar extinction coefficient smM hes in the range of 120 to 140. This band is known as the Soret band and is due to an electronic transition in the prophyrin structure which is common to all hemoglobins. To discuss other features of the spectra, it is necessary to distinguish between the ferric and ferrous compounds and these classes must further be subdivided into compounds in which the iron is either in a state of high spin or low spin. 

Ferric (Fe +) Hemoglobins (Fig. 12) In high spin compounds like ferrihemoglobin fluoride the Soret band is located at 405—410 mp.. In the 

Ferrous (Fe +) Hemoglobins (Fig. 13) In addition to the Soret band it is common for ferrous derivatives to have two absorption bands in the range 525—580 m(jt. with approximately the same intensity (smM = 13 —15). 

It may be mentioned that metal porphyrins have spectra which consist of a Soret band located in the vicinity of 400 m(i and two other bands, generally designated as the a- and (3-bands situated at about 650 and 550 mjx respectively. The precise locations of the bands will vary according to the particular compound (7, 25). Metal-free porphyrins have a Soret band and four other rather weak absorption bands. 

Absorption of sunlight induces photochemistry and generates a variety of free radicals that drive the chemistry of the troposphere as well as the stratosphere. This chapter focuses on the absorption spectra and photochemistry of important atmospheric species. These data can be used in conjunction with the actinic fluxes described in the preceding chapter to estimate rates of photolysis of various molecules as well as the rate of generation of photolysis products, including free radicals, from these photochemical processes. 

There are several highly useful sources of data on the absorption spectra and photochemistry of atmospheric species. NASA publishes on a regular basis a summary of kinetics and photochemical data directed to stratospheric chemistry (DeMore et al., 1997). However, much of the data is also relevant to the troposphere. This document can be obtained from the Jet Propulsion Laboratory in Pasadena, California. Alternatively, the data are available through the Internet (see Appendix IV). IUPAC also publishes regularly in The Journal of Physical Chemical Reference Data a summary directed more toward tropospheric chemistry (Atkinson et al., 1997a, 1997b). Finally, Nolle et al. (1999) have made available a CD-ROM containing the UV-visible spectra of species of atmospheric interest. 

We do not attempt a comprehensive treatment of the literature on each of the compounds discussed herein. With apologies to our colleagues whose work may not be explicitly cited, we shall rely on these exhaustive evaluations carried out by NASA (DeMore et al., 1997) and IUPAC (Atkinson et al., 1997a, 1997b) whenever possible. The reader should consult these evaluations, in addition to the original literature after 1998, for details and more recent studies. 

The absorption of light by both molecular oxygen and ozone is a strong determinant of the intensity and 

The absorption cross sections between 205 and 240 nm recommended by the NASA evaluation (DeMore et al., 1997) are shown in Table 4.1. 

When the lamp is turned on, the white image of each slit is projected on the center of the screen. A visible spectrum appears on 

Color Plate 15a shows the spectrum of white light and the spectra of three different colored solutions. You can see that potassium dichromate, which appears orange or yellow, absorbs blue wavelengths. Bromophenol blue absorbs orange wavelengths and appears blue to our eyes. Phenolphthalein absorbs the center of the visible spectrum. For comparison, spectra of these three solutions recorded with a spectrophotometer are shown in Color Plate 15b. 

This same setup can be used to demonstrate fluorescence and the properties of colors.7 

Approximate low-energy cutoff for common infrared windows  

For infrared measurements, cells are commonly constructed of NaCI or KBr. For the 400 to 50 cm 1 far-infrared region, polyethylene is a transparent window. Solid samples are commonly ground to a fine powder, which can be added to mineral oil (a viscous hydrocarbon also called Nujol) to give a dispersion that is called a mull and is pressed between two KBr plates. The analyte spectrum is obscured in a few regions in which the mineral oil absorbs infrared radiation. Alternatively, a 1 wt% mixture of solid sample with KBr can be ground to a fine powder and pressed into a translucent pellet at a pressure of —60 MPa (600 bar). Solids and powders can also be examined by diffuse reflectance, in which reflected infrared radiation, instead of transmitted infrared radiation, is observed. Wavelengths absorbed by the sample are not reflected as well as other wavelengths. This technique is sensitive only to the surface of the sample. 

If the cation has been unchanged, its ability to act as a hydrogen-bond donor has been unchanged, so why is an effect seen at all I propose that there is competition between the anion and the Reichardt s dye solute for the proton. Thus, the values of the ionic liquids are controlled by the ability of the liquid to act as a hydrogen bond donor (cation effect) moderated by its hydrogen bond acceptor ability (anion effect). This may be described in terms of two competing equilibria. The cation can hydrogen bond to the anion [Equation (3.5-2)]  

The cation can hydrogen bond to the solute (Reichardt s dye in this case) [Equation (3.5-3)]  

It can easily be shown that the value of K is inversely proportional to the value of K and that K is dependent on both the cation and the anion of the ionic liquid. Eience, it is entirely consistent with this model that the difference made by changing the anion should depend on the hydrogen bond acidity of the cation. 

Attempts have also been made to separate non-specific effects of the local electrical field from hydrogen-bonding effects for a small group of ionic liquids through the use of the k scale of dipolarity/polarizability, the a scale of hydrogen bond donor acidity, and the (i scale of hydrogen bond basicity (see Table 3.5-1) [13, 16]. 

The n values were high for all of the ionic liquids investigated (0.97-1.28) when compared to molecular solvents. The n values result from measuring the ability of the solvent to induce a dipole in a variety of solute species, and they will incorporate the Coulombic interactions from the ions as well as dipole-dipole and polarizability effects. This explains the consistently high values for all of the salts in the studies. The values for quaternary ammonium salts are lower than those for the monoalkylammonium salts. This probably arises from the ability of the charge center on the cation to approach the solute more closely for the monoalkylammonium salts. The values for the imidazolium salts are lower still, probably reflecting the delocalization of the charge in the cation. 

The effect of heteroatoms on the LUMO energy of mesomeric betaines can be treated similary, and this leads to an estimate of the HOMO-LUMO splitting. If E is the splitting in an AH anion, the change (A ) upon substitution of a heteroatom at position r is given by Eq. (2). 

The frequency of the first Tt - Tt absorption band (v) is related to frontier orbital separation by the expression 

Calculated change in HOMO-LUMO splitting of heterocycles isoconjugate with the benzyl anion vs frequency of their first n- n absorption in ethanol solution. 

The correlations shown on Figs. 1 and 2 are particularly remarkable when other factors which influence the spectra are considered. A primary complication is the effect of solvent polarity. Ideally, UV spectra should be recorded in nonpolar hydrocarbon solvents to minimize the effect of 

The formal rt-electron density at each atom in an odd AH radical (e.g., 408) is unity, and to a first approximation it will be the same in isoelectronic radical cations (e.g., 409). Approximately (because inductive effects are neglected), a unit positive charge is localized on the heteroatom. Introduction of aa additional electron into these cations (e.g., 409) gives mesomeric betaines (e.g., 410). Because the electron enters a NBMO, it is restricted to 

It should also be remarked that uranyl complexes tend to emit a bright green fluorescence under UV irradiation, from the first excited state. This is used by geologists both to identify and to assay uranium-bearing minerals in deposits of uranium ores. 

The absorption spectrum of (1) [U02(0Ac)4] in liquid Et4N0Ac.H20, showing the lack of vibronic structure, due to hydrogen bonding (2) [U02(0Ac)3] in MeCN solution, showing the progression due to the 0=U=0 stretching vibration (from J.L. Ryan and W.E. Keder, Adv. Chem. Ser., 1967, 71, 335 and reproduced by permission of the American Chemical Society). 

Absorption spectra of THF solutions of 1 [U02Cl -CH(Ph2PNSiMe3)2 (thf)] and and 2 [U02Cl jj -N(Ph2PNSiMe3)2 (thf)] (reproduced with permission of the Royal Society of Chemistry from MJ. Sarsfield, H. Steele, M. Helliwell, and S.J. Teat, Dalton Trans., 2004, 3443). 

Using Eq. (2) we can estimate the HOMO-LUMO splitting of the nitrogen heterocycles 396-398 (Fig. 1). Assuming that perturbations due to exocyclic oxygen are constant, the separation of the frontier orbitals (Ehet) given by 

A plot of absorption frequency against the factor — C, r) should 

The role of the anion is less clear-cut. It can be seen that ionic liquids with the same cation but different anions have different EXN values. However, the difference in the values for [Et4N]Cl and [Et4N][N03] is only 0.006, whereas the difference between [EtNH3]Cl and [EtNH3][N03] is 0.318. Less dramatically, the difference in the values for [OMMIM][BF4] and [OMMIM][Tf2N] is only 0.018, whereas the difference between [BMIM][BF4] and [BMIM][Tf2N] is 0.031. Hence, itis clear that the effect of changing the anion depends on the nature of the cation. 

The first solvatochromic dye to be used with a number of ionic liquids was Nile Red (Fig. 3.5-1) [13]. The ionic liquids used were composed of [RMIM]+ with [N02] , [NOs] , [BF4] , [PFis] , bis(trifluorosulfonyl)imide ([N(Tf)2] ) ions. The values of the energy of the electronic transition, Fnr. did not vary greatly and fell in the same region as short chain alcohols. 

A recent study of the effects of molecular solvents on the nr value has shown it to be correlated with the Kamlett-Taft tz and a scales (see below), and so it is thought to arise from a combination of hydrogen bond donation and dipolarity and polarizability effects [14]. 

The effect of hydrogen-bond donation to Nile Red can be seen when comparing the Tnr values of imidazolium salts protonated on one of the ring nitrogen atoms 

The longest wavelength absorption band of Reichardf s dye (2,4,6-triphenylpyridinium-N-4-(2,6-diphenylphenoxide) betaine (Pig. 3.5-2) shows one of the largest solvatochromic shifts known (375 nm between diphenyl ether and water) 

Results of theoretical calculation at the B3LYP-DFT level using the 6-31 + G(d,p) basis set in ground and triplet states of stUbenes (c-1, t-1) and of 2,3-diphenylnor-bornene (2) were described [6]. Pronounced pyramidalization at the olefinic C atoms giving a PhCCPh dihedral angle of 51.0° in 32 was shown. 

There are three types of electronic transitions involving lanthanide ions sharp intraconfigurational 4f—4f transitimis, broader 4f-5d transitions, and broad 

Ln ions doped in a low-symmetry crystal, LaFa. Redrawn 

In addition to the parity selection rule, other rules are operative, for instance, on AS (spin selection rule, requiring no change of spin for all three mechanisms, AS = 0), AL, and AJ they will be detailed below. The selection rules are derived under several hypotheses which are not always completely fulfilled in reality (in particular 4f wavefunctions are not completely pure), so that the terms forbidden and allowed transitions are not accurate. Let s say that a forbidden transition has a low probability and an allowed transition a high probability of occurring. 

The spectrum of visible light can be projected on a screen in a darkened room in the following manner Four layers of plastic diffraction grating are mounted on a cardboard frame that has a square hole large enough to cover the lens of an overhead projector. This assembly is taped over the projector lens facing the screen. An opaque cardboard surface with two 1 X 3 cm slits is placed on the working surface of the projector. 

When the lamp is turned on, the white image of each slit is projected on the center of the screen. A visible spectrum appears on either side of each image. When a beaker of colored solution is placed over one slit, you can see color projected on the screen where the white image previously appeared. The spectrum beside the colored image loses its intensity in regions where the colored solution absorbs light. 

The distinction between Rydberg and valence states is not that clear in larger molecules. A good indicator for the state character can be obtained from the (r ) expectation value (a measure of spatial extension) difference for state n with respect to the ground state. This is given by 

The problems of most density functionals with Rydberg states are also related to the electronic potential far away from the nuclei. It is well known that the local density approximation (EDA) exchange potential oc p(r)  

This behavior of the DF is also observed in molecular applications. Vertical singlet excitation energies from CASPT2 % TDDFT-B3LYP, and 

The experimental absorption spectrum of coumarin 102 obtained in ethanol solution exhibits two intense absorption bands at 3.2 and 5.9 eV and one band with a lower intensity located at 4.8 As for thioindigo, 

Due to its crucial role in combustion chemistry and biology, the phenoxyl radical has been the subject of intense experimental and theoretical studies for many years (for recent work, see Ref. 127 and references cited therein). For these reasons we present it here as an example of UV spectroscopy of (neutral) radicals (for the application of DFT to the spectra of aromatic radical cations, see, e.g.. Ref. 128 and for results of MRCI computations, see Ref. 129). The phenoxyl radical is well known as a difficult case for electronic structure methods because it requires a sophisticated treatment of electron correlation similar to closed-shell aromatic hydrocarbons. 

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Fig. XVn-16. Infrared absorption spectra of H2 physisorbed on NaCl(lOO) at 30 K. See text for explanations. (From Ref. 96. Reprinted with permission from American Institute of Physics, copyright 1993.)...

Vibrational Spectroscopy. Infrared absorption spectra may be obtained using convention IR or FTIR instrumentation the catalyst may be present as a compressed disk, allowing transmission spectroscopy. If the surface area is high, there can be enough chemisorbed species for their spectra to be recorded. This approach is widely used to follow actual catalyzed reactions see, for example. Refs. 26 (metal oxide catalysts) and 27 (zeolitic catalysts). Diffuse reflectance infrared reflection spectroscopy (DRIFT S) may be used on films [e.g.. Ref. 28—Si02 films on Mo(llO)]. Laser Raman spectroscopy (e.g.. Refs. 29, 30) and infrared emission spectroscopy may give greater detail [31]. 

Pollard W T, Lee S-Y and Mathies R A 1990 Wavepacket theory of dynamic absorption spectra in femtosecond pump-probe experiments J. Chem. Phys. 92 4012... 

The traditional instruments for measuring emission and absorption spectra described above set the standard for the types of infonnation which can be obtained and used by spectroscopists. In the more recent past, several new... 

The interpretation of emission spectra is somewhat different but similar to that of absorption spectra. The intensity observed m a typical emission spectrum is a complicated fiinction of the excitation conditions which detennine the number of excited states produced, quenching processes which compete with emission, and the efficiency of the detection system. The quantities of theoretical interest which replace the integrated intensity of absorption spectroscopy are the rate constant for spontaneous emission and the related excited-state lifetime. 

Most stable polyatomic molecules whose absorption intensities are easily studied have filled-shell, totally synuuetric, singlet ground states. For absorption spectra starting from the ground state the electronic selection rules become simple transitions are allowed to excited singlet states having synuuetries the same as one of the coordinate axes, v, y or z. Other transitions should be relatively weak. 

Mantini A R, Marzocchi M P and Smulevich G 1989 Raman excitation profiles and second-derivative absorption spectra of beta-carotene J. Chem. Phys. 91 85-91... 

In 1960, Harrick demonstrated that, for transparent substrates, absorption spectra of adsorbed layers could be obtained using internal reflection [42]. By cutting the sample in a specific trapezoidal shape, the IR beam can be made to enter tlirough one end, bounce internally a number of times from the flat parallel edges, and exit the other end without any losses, leading to high adsorption coeflScients for the species adsorbed on the external surfaces of the plate (Irigher than in the case of external reflection) [24]. This is the basis for the ATR teclmique. 

Figure Bl.22.4. Differential IR absorption spectra from a metal-oxide silicon field-effect transistor (MOSFET) as a fiinction of gate voltage (or inversion layer density, n, which is the parameter reported in the figure). Clear peaks are seen in these spectra for the 0-1, 0-2 and 0-3 inter-electric-field subband transitions that develop for charge carriers when confined to a narrow (<100 A) region near the oxide-semiconductor interface. The inset shows a schematic representation of the attenuated total reflection (ATR) arrangement used in these experiments. These data provide an example of the use of ATR IR spectroscopy for the probing of electronic states in semiconductor surfaces [44]-...

Yaroslavskii N G and Terenin A N 1949 Infrared absorption spectra of adsorbed molecules Dokl. Akad. Nauk 66 885-8... 

The development of ultrafast spectroscopy has paralleled progress in the teclmical aspects of pulse fomiation [Uj. Because mode-locked laser sources are tunable only with diflSculty, until recently the most heavily studied physical and chemical systems were those that had strong electronic absorption spectra in the neighbourhood of conveniently produced wavelengths. 

Porter G and Topp M R 1968 Nanosecond flash photolysis and the absorption spectra of excited singlet states Nature 220 1228-9... 

Knickelbein M B and Menezes W J C 1992 Optical response of small niobium clusters Rhys. Rev. Lett. 69 1046 Ceilings B A, Athanassenas K, Lacombe D, Rayner D M and Hackett P A 1994 Optical absorption spectra of AUy,... 

Alvarez M M efa/1997 Optical absorption spectra of nanocrystal gold molecules J. Phys. Chem. B 101 3706... 

Haug, K., Metiu, H. A test of the possibility of calculating absorption spectra by mixed quantum-classical methods. J. Chem. Phys. 97 (1992) 4781-4791... 

To verify effectiveness of NDCPA we carried out the calculations of absorption spectra for a system of excitons locally and linearly coupled to Einstein phonons at zero temperature in cubic crystal with one molecule per unit cell (probably the simplest model of exciton-phonon system of organic crystals). Absorption spectrum is defined as an imaginary part of one-exciton Green s function taken at zero value of exciton momentum vector... 

In Fig. 1 the absorption spectra for a number of values of excitonic bandwidth B are depicted. The phonon energy Uq is chosen as energy unit there. The presented pictures correspond to three cases of relation between values of phonon and excitonic bandwidths - B < ujq, B = u)o, B > ujq- The first picture [B = 0.3) corresponds to the antiadiabatic limit B -C ljq), which can be handled with the small polaron theories [3]. The last picture(B = 10) represents the adiabatic limit (B wo), that fitted for the use of variation approaches [2]. The intermediate cases B=0.8 and B=1 can t be treated with these techniques. The overall behavior of spectra seems to be reasonable and... 

Figure 1 Absorption spectra for system of excitons linear and locally coupled with strength S to nondispersive phonons with energy u>o calculated in NDCPA. B is excitonic bandwidth, and 7 is excitonic decay rate.

Determination of the dissociation constants of acids and bases from the change of absorption spectra with pH. The spectrochemical method is particularly valuable for very weak bases, such as aromatic hydrocarbons and carbonyl compounds which require high concentrations of strong mineral acid in order to be converted into the conjugate acid to a measurable extent. 

Section A,7, Applications of infrared and ultraviolet absorption spectra to organic chemistry, should provide a brief introduction to the subject. 

Figure 3.4. TJV-vis absorption spectra of 3.10c in water and in water containing 3.0 mM of Cu (glycine) complex, 3.0 mM of Cu(N-methyl-Ftyrosine) and 3.0 mM of Cu(L-abrine).

Computed optical properties tend not to be extremely accurate for polymers. The optical absorption spectra (UV/VIS) must be computed from semiempiri-cal or ah initio calculations. Vibrational spectra (IR) can be computed with some molecular mechanics or orbital-based methods. The refractive index is most often calculated from a group additivity technique, with a correction for density. 

An intramolecular charge transfer toward C-5 has been proposed (77) to rationalize the ultraviolet spectra observed for 2-amino-5-R-thiazoles where R is a strong electron attractor. Ultraviolet spectra of a series of 2-amino-4-p-R-phenylthiazoles (12) and 2-amino-5-p-R-phenylthiazoles (13) were recorded in alcoholic solution (73), but, reported in an article on pK studies, remained undiscussed. Solvent effects on absorption spectra of 2-acetamido and 2-aminothiazoles have been studied (92). 

The thiazolium ring, as most heterocycloammoniums, is a Lewis acid conferring to the carbon atom in the 2-position the carbocationic property of adding the free pair of a base either organic or mineral that may be the molecule of solvent as ROH (Scheme 11). For many nuclei of suitable acidity, these equilibria can be observed in dilute solution by means of absorption spectra when species A and C possess different characteristics (24). For example, benzothiazolium and benzoxazolium in methanol and ethanol give at 10 mole liter 8 and 54% of the alkoxy derivatives for the former and 29 and 90% for the latter respectively. 

Since the very beginning of chemistry, many efforts have been devoted to find out basic relationships between the characteristics of absorption spectra and the molecular structure of dyes. 

The dyes prepared in this way show a positive solvatochromism as the dielectric constant of the solvent increases, indicating that they possess a predominantly nonpolar structure. Substituents on the phenyl group in the 4-position of the selenazole ring have little influence on the absorption spectra. 

The ultraviolet absorption spectra of most new thiazoles currently synthesized have been described and occasionally used for structural... 

TABLE 1-16 ULTRAVIOLET ABSORPTION SPECTRA OF THIAZOLE AND ITS MONOMETHYL DERIVATIVES... 

TABLE 1-17. ULTRAVIOLET ABSORPTION SPECTRA OF SOME TYPICAL derivatives OFTHIAZOLE IN EtOH SOLUTION... 

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