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 studies. XXXIII. The MCD spectrum of formaldehyde vapor

The MCD spectrum of formaldehyde in the vapor phase has been measured and analyzed. Triplet bands occur with unusually high ellipticity, allowing identification of several new singlet-triplet bands. Also, an additional singlet progression, based on two quanta of the out-of-plane band and a single quantum of the antisymmetric CH stretch, has been identified.     

Magnetic circular dichroism of cyclic π-electron systems—25: Oxo derivatives of indazole,benzimidazole and purine

The simple perimeter model combined with PMO theory and semimpirical calculations is used to rationalize the observed MCD spectra of a series of oxo derivatives of azaindoles, such as caffeine and xanthine. The MCD spectra indicate the presence of substantial amounts of the 9-H tautomers in aqueous solutions of xanthines.     

Magnetic circular dichroism of azuleno[1,2-b]azulene derivatives

Both 6,12-diphenylazuleno[1,2-b] azulene and 6,12-di-p-bromophenylazuleno [1,2-b] azulene exhibit similar absorption and MCD spectra. PPP calculations reproduce the observed absorption and MCD spectra. The phenyl and p-bromophenyl groups are considered to present no obstacle in investigating the spectroscopic behavior of azuleno[1,2-b] azulene.     

Magnetic circular dichroism studies XXXII

CNDO calculations of the MCD spectra of 7- and 9-methyladenine have been carried out in two approximations. A minor modification of the CNDO/S method of Jaffé and coworkers shows an improvement over Jaffé's parameterization with respect to band separations and the signs of the MCDB terms, but is not applicable ton→ π* transitions.     

Magnetic circular dichroism studies 23. 1. Magnetic circular dichroism spectra of indole alkaloids

The magnetic circular dichroism spectra of a number of indole alkaloids show two B-terms of opposite sign in the 250–330 nm wavelength region associated with the ¹L_b and ¹L_a electronic transitions, the long wavelength, ¹L^b, band being of positive sign, whereas both bands strongly overlap in absorption. Various substituents in different positions of the indole ring cause a red shift of both bands and a broadening of the long wavelength B-term. The sign pattern, howver, remains unchanged in all examples thus far investigated. Dihydroindole and oxindole, on the other hand, exhibit MCD. bands with the opposite sign sequence as compared to the indole chromophore. This observation allows identification of the indole chromophore in alkaloids from the sign pattern of the MCD. bands.     

Magnetic vibrational circular dichroism of the haloforms

The magnetic vibrational circular dichroism of the three haloforms, CHCl₃ CHBr₃ and CHI₃, is presented. In the a¹ symmetry C-H stretch B₀/D₀ increases from <10^t̄6 to ≈ 3 × 10⁻⁵/cm⁻¹ from Cl to I. These values are correlated to ΔE for electronic excitation and to the methyl halide results.     

Matrix isolation magnetic circular dichroism (MCD) spectrum of NH radical produced by a glow discharge technique

MCD and absorption spectra are reported for the X(³Σ^?)→A(³H_i transition of NH produced by a glow discharge and isolated in Ar and Xe matrices. The MCD is strongly temperature dependent and is detectable at concentrations well below those required in a conventional absorption measurement.     

Effect of peripheral substitution on the electronic absorption and magnetic circular dichroism (MCD) spectra of metal-free azo-coupled bisphthalocyanine

A new azo-coupled bisphthalocyanine is synthesized from the corresponding quinoxaline oxime which can be obtained by the reaction of s-trans-chloroethanedial with NN conjugated metal-free phthalocyanine. The phthalocyanine is synthesized by the reaction of 4-nitro-o-phenylenediamine with 2-nitro-9,10,16,17,23,24-hexa(hexylthio)phthalocyanine. Novel compounds are characterized by elemental analysis, UV/vis, IR and ¹H NMR, and MALDI-TOF spectroscopy. The effect of the azo units on the position and intensity of the electronic absorption and magnetic circular dichroism (MCD) spectra of the bisphthalocyanine are examined for the NN conjugated metal-free phthalocyanine.     

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 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 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 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 studies of polystyrene solutions

The magnetic circular dichroism (MCD) spectra of “atactic” and isotactic polystyrene solutions have been found to be sensitive to configurational and conformational differences. The ratio of B terms for “atactic” polystyrene (predominantly syndiotactic) to that of cumene was found to pass through a minimum as the coil expanded in various solvents. The ratio of dipole strengths D also passed through a minimum. For isotactic polystyrene and cumene the ratio of B terms increased with increasing coil size in the range of solvents utilized. The ratio of B terms of isotactic polystyrene over atactic polystyrene was found to go through a maximum as the coil expanded whereas the ratio of dipole strengths was found to continuously decrease. These results have been interpreted on the basis of the change in short range and long range orientation and interaction distances between chromophores during coil expansion. It is concluded that the microscopic conformation of isotactic and atactic polystyrene are very different in very good solvents. The B/D ratio for cumene was found generally to decrease with increasing solvent power.     

Magnetic circular dichroism analysis of crude oil

Optical and magneto-optical properties of solutions of crude oil of different origin (i.e., taken from different fields) are studied in the visible and near-UV region of optical emission. Magnetic circular dichroism (MCD) spectra of oil are obtained in the vicinity of wavelengths of ~410 nm, 533 nm, and 576 nm. It is demonstrated that the intensity of the MCD signal depends on the origin of crude oil, and it is proportional to the oil concentration in the solution. The comparison of the magneto-optical spectroscopy data with the chemical composition of samples allows us to conclude that the observed magneto-optical activity is determined by the presence of VO²⁺ complexes in the oil samples studied. The revealed magneto-optical activity of conventional oil can form a basis of a new method for the analysis of the composition and properties of oil of different origin.     

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.     

Optical circular dichroism of the fullerene amino acid derivatives

The dependence of the circular dichroism spectra of the amino acid derivatives of fullerene C₆₀ on the structure of the initial α-amino acid was studied.     

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.     

Magnetic circular dichroism studies of matrix-isolated metal atoms

Results obtained from work on Ni, Pd, Al and Ga isolated in rare gas matrices are used to illustrate the type of information that can be obtained from MCD experiments.It is possible to identify different species in the same matrix through their temperature dependence. The interaction between guest and host can be seen by the quenching of the orbital angular momentum for Al and Ga. The signed nature of the MCD spectra makes it possible to confirm that the assignment of Ni atom bands is constant when the host gas is changed. The extra sensitivity of MCD allows the detection of a long-lived excited state species of Pd, which cannot be seen in the absorption spectrum.