Magnetic circular dichroism of matrix-isolated group IVb diatomic oxides

The absorption and magnetic circular dichroism spectra of the diatomic group IVb metal oxides have been measured in argon matrices. Temperature independent MCD spectra for the three oxides TiO, ZrO, and HfO confirm the assignments of their ground states to the magnetically nondegenerate ³Δ₁, ¹ Σ⁺, and ¹Σ⁺ states, respectively. The observed MCD spectra for the three oxides consist of A terms, B terms, or no MCD signal. From simple selection rule considerations, the appearance (or nonappearance) of particular MCD terms has been used to assign a number of heretofore unknown states and to confirm known assignments. Transitions due to the atomic Ti and Zr species codeposited with the corresponding oxide have also been observed. Temperature dependent C term behavior enabled a straightforward differentiation of the atomic bands from the temperature independent molecular bands. Assignments of the atomic transitions consistent with the observed C term signs are reported.     

Solvent interactions with the indole chromophore

Comparison of solvent behaviour upon indole, N-methylindole, 5-methoxyindole and benzimidazole, both at the first S₁(¹L_a, ¹L_b) and second S₂(¹B_b) excited states, provides new information upon solvent interactions for the biologically important indole chromophore.     

Magnetic circular dichroism and molecular orbital studies on bicyclo[6,2,0] decapentaene

The magnetic circular dichroism (MCD) spectrum of bicyclo[6,2,0] decapentaene has revealed four skeletal π → π^* electronic transitions in the visible and ultraviolet region. The four MCD bands are assigned to the B₂ ← A₁, A₁ ← A₁. B₂ ← A₁ and A₁ ← A₁ electronic transitions in increasing order of energy.     

Magnetic circular dichroism of heterocycles: thiophene

The existence of two transitions with opposite MCD in the 38000–45000 cm^?1 absorption of thiophene is demonstrated. The B-term for the lowest transition is negative, while it is positive for the higher one. Quantum mechanical calculations in the π-electron approximation resulted in energies, oscillator strengths and MCD terms in good agreement with the observed spectra.     

The magnetic circular dichroism spectrum of acetylene

The magnetic circular dichroism (MCD) spectrum of gas phase acetylene has been studied in the spectral region 320-140 nm. No MCD signal was detected in the region of the first transition. The observation of irregular MCD bands beginning at 185 nm confirm assignment of this latter band as the 0-0 band of the second transition. The discrete and relatively narrow bands in the region 155-140 nm exhibit pure A-term character, lending unambiguous confirmation to the assignment of ¹Π_u for the state. The observed absorption spectrum in this region is interpreted as consisting of two non-interacting systems, one involving the ¹Π_u Rydberg state and the other, a broad continuous background, cannot be assigned. It is however suggested that it may be due to the ¹Σ_u⁺ state of the π → π^* excited configuration.     

The absorption and circular dichroism spectra of chiral olefins

Absorption and CD spectra over the quartz and vacuum UV region down to 165 nm are reported for a range of chiral alkenes in the vapour phase and in solution from +70° to ?185°C. A major couplet of oppositely-signed CD bands with comparable band areas, near 48 and 55 kK, is observed in a number of dissymmetric olefins and in some cases a weaker Rydberg CD absorption is found at lower frequency. The Rydberg CD band is characterised by its sharp vibronic structure in the vapour phase and by large blue-shifts produced on passing to the condensed phase and by a reduction in temperature. The olefin couplet of major CD bands with opposite sign is assigned to a near-complete mixing of the electric-dipole π_x→π_x^* and the magnetic-dipole π_x→π_y^* excitations, producing a pair of isotiopic absorption bands with the same polarisation and comparable dipole strengths associated with the CD couplet. Three mixing mechanisms are discussed; sterically-induced π-bond torsion, a first-order static field model, and a second-order dynamic-coupling model dependent, respectively, upon the effective charge and upon the mean polarisability of dissymmetrically-located substituent. The latter two models give the octant rule previously proposed empirically connecting the sign of the rotational strength of the lower- and higher-frequency member of the olefin CD couplet with the position of the substituent in the chromophore coordinate frame.     

Magnetic circular dichroism spectra of 1,5-naphthyridine

The absorption and MCD spectra of 1,5-naphthyridine (1) are reported. The transition energies, oscillator strengths and Faraday B terms computed within the PPP SCF π MO Cl method are in reasonable agreement with experiment. The Faraday B terms of two bands in the middle wavenumber region [(30.0–45.0) × 10³ cm^?1] of 1 are larger than those in quinoline.     

Magnetic circular dichroism studies of aromatic polymers

The magnetic circular dichroism (MCD) spectra of syndiotactic and isotactic polymers which contain aromatic chromophores have been found to be sensitive to configurational and conformational differences. For isotactic polymers it was determined that as the aromatic ring moved farther from the main chain the ration of B terms of the polymers to those of their model compounds reached a minimum but increased significantly when the aromatic ring was separated from the main chain by four atoms. This enhancement of MCD is believed to be caused by the alignment of the more flexible side chains which would allow the interaction of the aromatic rings with neighboring groups and could result in a favorable mixing of the ester electronic transition with the aromatic ¹A_1g?¹B_2u transition. This effect was not felt to any great extent by the syndiotactic polymers because the necessary nearest-neighbor interaction was sterically unfavorable. The ratio of the B terms of isotactic poly(phenyl methacrylate) to its model compound decreased as the polymer coil expanded, whereas it increased for the syndiotactic polymer. This effect reflects the different changes that the side chain interaction and orientations undergo in these polymers during coil expansion. The MCD ratios for iso- and syndiotactic poly(phenylethyl methacrylate) were not so sensitive during coil expansion. The ratio of the dipole strengths of the polymers and model compounds paralleled the MCD results, but the ultraviolet (UV) technique was less sensitive than MCD to subtle conformational differences. Poly(benzyl methacrylate) and benzyl pivalate were unsuitable systems for studying the MCD effect because the B terms of these materials approached zero.     

Magnetic circular dichroism of manganese pentacarbonyl bromide

The absorption and magnetic circular dichroism (MCD) spectra of manganese pentacarbonyl bromide Mn(CO)₅Br in a solution of 2-methyl tetrahydrofuran were measured at various temperatures over the 3500–4500 Å region. Within experimental error the MCD spectra are not temperature dependent and in addition to a negative B term exhibit a positive A term, indicative for a degenerate excited state. Following the irreducible tensor method we conclude that the transition involved will be mainly e[π, Br] → a₁[σ^*, 3d₂²] and for the excited state we calculate a magnetic moment which agrees quite closely with the experimental value.     

Magnetic circular dichroism spectrum of the triphenylene-iodine complex

The MCD CT bands of the triphenylene (9,10-benzphenanthrene) -I₂, triphenylene-TCNE and I₂-I₂ complexes are indicative of a structure of C_3v symmetry where the I₂ is located centrally and axially over the triphenylene ring.     

Magnetic circular dichroism and computational study of mononuclear and dinuclear iron(iv) complexes

High-valent iron(iv)-oxo species are key intermediates in the catalytic cycles of a range of O₂-activating iron enzymes. This work presents a detailed study of the electronic structures of mononuclear ([Fe^IV(O)(L)(NCMe)]²⁺, 1, L = tris(3,5-dimethyl-4-methoxylpyridyl-2-methyl)amine) and dinuclear ([(L)Fe^IV(O)(μ-O)Fe^IV(OH)(L)]³⁺, 2) iron(iv) complexes using absorption (ABS), magnetic circular dichroism (MCD) spectroscopy and wave-function-based quantum chemical calculations. For complex 1, the experimental MCD spectra at 2–10 K are dominated by a broad positive band between 12 000 and 18 000 cm^–1. As the temperature increases up to ∼20 K, this feature is gradually replaced by a derivative-shaped signal. The computed MCD spectra are in excellent agreement with experiment, which reproduce not only the excitation energies and the MCD signs of key transitions but also their temperature-dependent intensity variations. To further corroborate the assignments suggested by the calculations, the individual MCD sign for each transition is independently determined from the corresponding electron donating and accepting orbitals. Thus, unambiguous assignments can be made for the observed transitions in 1. The ABS/MCD data of complex 2 exhibit ten features that are assigned as ligand-field transitions or oxo- or hydroxo-to-metal charge transfer bands, based on MCD/ABS intensity ratios, calculated excitation energies, polarizations, and MCD signs. In comparison with complex 1, the electronic structure of the Fe^IV O site is not significantly perturbed by the binding to another iron(iv) center. This may explain the experimental finding that complexes 1 and 2 have similar reactivities toward C–H bond activation and O-atom transfer.     

Comparative sensibility of S1 ←S0 and S2 ←S0 indole electronic transitions to environment perturbations. The positions of the 0—0 bands in polar media

The modifications under environmental perturbations of the well separated two first electronic systems of carbazole are used to demonstrate the possibility of including solute-ether complexes in polyethylene films. The application of this technique to indole allows to differentiate the overlapping ¹L_a and ¹L_b transitions in the spectrum of hydrogen bonded indole. The ground state interactions of indole, 5-methoxyindole and 3-methylindole in several well defined environment conditions (hydrogen bond in a polar cage, hydrogen bond in a nonpolar cage) are investigated through UV absorption spectroscopy between 293 and 88 K. A strong red shift of the ¹L_a bands under the hydrogen bonding occurs with all the indoles investigated. The comparative analysis of the spectra in three types of environment, allows to give the accurate location of the first bands for the two overlapping vibronic systems of these molecules in polar media. Particularly, it is unambiguously shown that the first ¹L_a band and the first ¹L_b band of indole in alcohols are superposed at 288 nm. These results offer reliable new bases for the study of the emission properties of indole and its derivatives.     

Magnetic circular dichroism spectra of triphenylene and coronene

The absorption spectra of triphenylene (I) and coronene (II) are assigned by using the MCD spectra and the results of the SCF screened potential π-MO CI calculations. The calculated Faraday A/D values for the ¹E and ¹E_1u excited states of I and II are in excellent agreement with the experimental values, in contrast to the calculated Faraday B value for the ¹E_1u state of II.     

Synthesis and circular dichroism studies of steroidal schiff bases of biogenic amines

A number of imines (Schiff bases) have been prepared from ketosteroids (estrone, estrone-3-methyl ether, androsterone, epiandrosterone, 5α-cholestan-3-one, 6-ketocholestanol and testosterone) and primary amines (phenethylamine, tyramine and hexanamine). The CD spectra of these compounds have been measured in methanol. Except for the testosterone imines, the azomethine n-π^* band (~ 235 nm) exhibited the same sign (but not as high an ellipticity) as the parent ketone's n - π^* band (~295 nm). The imines of the estrogens (estrone and estrone 3-methyl ether) gave CD spectra characterized by a negative ¹L_b Cotton effect ([θ] ? -1000) near 280 nm and a band near 230 nm ([θ] ? +19,000) which is considered the sum of a positive azomethine n - π^* Cotton effect and positive aromatic ¹L_a Cotton effect(s). The ¹L_a band originating in the amine moiety was prominent in the CD of 5α-cholestan-3-one and 6-ketocholestanol phenethylamine imines. When aromatic Cotton effects originating in the amine moiety were detected, they were of the same sign as the n - π^* Cotton effect.     

Conformation and chiroptical properties of mandelic acids

Using CD data (solution, solid state, various temperatures), X-ray data and MO calculations for a number of substituted mandelic acids (phenylglycolic acids) an attempt was made to explain the different results for the relation between sign of the ¹L_b CD band and substitution pattern as described in the literature for various types of aromatic compounds. Moreover the reported sector rule has been reconsidered taking into account the sign and magnitude of the spectroscopic moments. It has been found that in any explanation of the signs of the ¹L_b CD bands of substituted aromatic compounds both the conformational behaviour and the spectroscopic moments should be taken into account.     

Magnetic circular dichroism of some benzene derivatives

The MCD spectra of phloroglucin, 1,3,5-trimethoxybenzene, 1,3,5-tricyanobenzene, 1,3,5-trinitrobenzene, 1,3,5-benzenetricarboxylic acid and 1,3,5-benzenetricarbonyl chloride were measured. For the ¹E′ ← ¹A′₁ transition of phloroglucin or 1,3,5-tricyanobenzene and the ¹E′ ← ¹A′ transition of 1,3,5-trimethoxybenzene, the Faraday A term was observed and the A/D value was extracted. However, 1,3,5-trinitrobenzene, 1,3,5-benzenetricarboxylic acid and 1,3,5-benzenetricarbonyl chloride showed no magnetic circular dichroism in this spectral region. The magnetic moments in the ¹E′ states of these molecules seem to be quenched by the effects of substituents. The magnetic moments in the ¹E′ states of benzene derivatives are sensitive to substitution in the benzene ring.     

Magnetic circular dichroism of Eu(C2H5SO4)3·9H2O due to magnetic dipole transitions

The magnetic circular dichroism (MCD) for the three low-energy absorption bands of the Eu(C₂H₅SO₄)₃·9H₂O crystal has been measured at room temperature. It may be well understood from the MCD as well as from other experimental results that two of these bands, ⁷F₁ → ⁵D₀ and ⁷F₀ → ⁵D₁, are of magnetic dipole origin. However, the MCD of another band at about 18650 cm^?1, ⁷F₁ → ⁵D₁, cannot be interpreted as being only one electric dipole in origin, which has been pointed out by several investigations. The present MCD analysis indicates that although this broad band consists of one electric- and two magnetic-dipole transitions, each of which has a sizeable absorption strength, the MCD spectrum originates exclusively from the two magnetic dipole transitions     

Polarized two-photon fluorescence excitation spectra of indole and benzimidazole

Polarized two-photon fluorescence excitation spectra of indole in hexane, benzimidazole in isopropanol, and benzimidazole cation in methanol-H₂SO₄, all at 0.2 M and 25°C are reported for the excitation range 470–600 nm, the region of their L_b, and L_a bands. Relative two-photon absorptivities are deduced by correcting for different fluorescence response and are compared to toluene's L_b band. The indole integrated absorptivity is about 10 times greater than that of toluene. The L_a band of indole appears less dominant than in one-photon but still outweighs the L_b band by a factor of 4. The two-photon polarization spectrum for indole indicates that the L_a origin lies ≈500–1000 cm⁻¹ above the L_b origin in hexane. The benzimidazoles absorb only about twice as strongly as toluene and show strong vibronic peaks; the L_a, bands are only faintly seen. Two-photon properties calculated from INDO/S CI wavefunctions with doubly excited configurations are in good agreement with those of indole, but predict the benzimidazole TPA to be several times stronger than observed. For the cation, the predicted results are nearly two orders of magnitude too high.     

Magnetic circular dichroism of the 3A2g → 3T2g band of MgO: Ni2+

The absorption and magnetic circular dichroism (MCD) spectra of the ³A_2g → ³T_2g transition in Ni²⁺:MgO have been studied. MCD data confirm the magnetic dipole character of the zero-phonon lines. The phonon side bands can be explained in terms of an A_1g lattice mode.     

Magnetic circular dichroism spectra of the methyl halides. Resolution of the n → σ* continuum

The MCD (magnetic circular dichroism) of CH₃l, CH₃Br and CH₃Cl has been studied in the spectral region of the n → σ^* continuum (3500-1700 Å). The MCD spectrum of CH₃I makes it possible to resolve the three excited states contributing to the continuum. The peak energies and the oscillator strengths of the components are obtained.