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.     

Photochemical and nonphotochemical hole burning in dimethyl-s-tetrazine in a polyvinyl carbazole film

Hole burning as well as fluorescence line narrowing experiments have been performed on the system dimethyl-s-tetrazine in polyvinyl carbazole films at low temperatures. The first singlet electronic absorption bands are typical (300 cm^?3 wide) of inhomogeneously broadened bands of guest molecules in amorphous organic hosts. Evidence is presented for both photochemical and nonphotochemical hole burning. The narrowest holes observed were lorentzian, had a width of 0.44 cm^?1 at 1.8 K, and are believed to be of nonphotochemical origin. A model which envisions the guest molecules to occupy different sites in the polymer host with a distribution of energy barriers between sites is used to describe these observations. The fast (20 ps) relaxation time implied by the 0.44 cm^?1 lorentzian linewidth is interpreted as indicative of the rate of site interconversion in the excited state.     

MCD spectroscopy and magnetization studies of matrix-isolated aluminium atoms

The electronic absorption spectra and magnetic circular dichroism (MCD) spectra of matrix-isolated aluminium atoms have been studied with particular reference to the 3p4s and 3p3d transitions. The g values of the isolated atoms also have been measured via the MCD magnetization technique. It is found that, in all matrices, the orbital angular momentum of the atom is heavily quenched giving g values very near 2.0. A consistent analysis of this phenomenon and of the spectra has been developed using a model in which the surrounding noble gas atoms exert an electrostatic field upon the aluminium atom and also enter into molecular orbital formation with it. This interpretation leads to the conclusion that the spin-orbit coupling of the optical electron is negative in both ground and excited states for Al/Kr and Al/Xe, but in the excited state only for Al/Ar. These results confirm and extend the findings of earlier EPR measurements.     

Electronic and vibrational IR MCD in the Creutz-Taube complex

The Creutz-Taube complex was analysed in terms of a simple electronic coupling model and predictions of electronic transitions in the mid-IR (1800–4000 cm⁻¹) were made. Low-temperature MCD measurements in this region show broad features around 2000 and 4000 cm⁻¹ and sharp features near 3200 and 4800 cm⁻¹. Deuteration of the ammine protons indicates that the sharper features are vibrational MCD of anomalous intensity and the broader features are electronic with substantial vibronically induced intensity in the 4000 cm⁻¹ region. Measurements were made by incorporating the complex in a modified PTFE (nafion) film. Thin nafion films are transparent throughout the near- and mid-IR regions and are optically isotropic and transparent. The spectra appear to be the first C-term IR MCD spectra.     

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.     

Perturbed sites and host—guest—host exciton cascade in the biphenyl isotopic mixed crystal phosphorescence

An interesting energy cascade is observed in the phosphorescence spectra of 1% biphenyl-h₁₀ in biphenyl-d₁₀ (2–15 K); strongly perturbed host sites, with energy levels below that of the protonated guest, quench the guest sites at higher temperatures (11–15 K). The identification of the perturbed sites is based on vibrational characteristics (both intensity and frequency), obtained with the help of phosphorescence spectra of biphenyl-h₁₀ and biphenyl-d₁₀ in an argon matrix, indicating an isotope dependent vibronic structure. A partial vibrational analysis is presented, resulting in confirmation of the first triplet state of biphenyl as orbitally ungerade. The dynamics of the triplet excitation are discussed, including several possible mechanisms explaining the non-Boltzmann nature of the low-temperature steady state.     

The magnetic circular dichroism spectra of the linear trinuclear clusters [Fe3S4(SR)4]3− in purple aconitase and in a synthetic model

The low-temperature magnetic circular dichroism (MCD) spectra and MCD magnetization properties of the linear three-iron cluster [Fe(μ₂-S)₂Fe(μ₂-S)₂Fe]⁺ have been compared in a solution of the model compound (Et₄N)₃[Fe₃S₄(SEt)₄] and in the “purple” form of beef heart aconitase. The similarity between the spectra confirms the close equivalence of the structures in the model and the protein. Analysis of the MCD magnetization curves has been carried out in terms of a ground state spin S = 5/2 and E / D values of 0.33 (model) and 0.31 (aconitase) established from the electron paramagnetic resonance (EPR) spectra. This gives estimates of the axial zero-field splitting parameters D = +0.7 ± 0.2 cm⁻¹ and +1.5 ± 0.2 cm⁻¹ for the model and protein, respectively.     

Host–guest complexes of various cucurbit[n]urils with the hydrochloride salt of 2,4-diaminoazobenzene

The interaction products of normal cucurbit[n]urils (n = 7, 8; Q[7] Q[8]) and a sym- tetramethyl-substituted cucurbit[6]uril derivative (TMeQ[6]) with the hydrochloride salts of 2,4-diaminoazobenzene (g·HCl) were investigated in aqueous solution using ¹H NMR spectroscopy, electronic absorption spectroscopy, as well as single crystal X-ray diffraction. The ¹H NMR spectra analysis established a basic interaction model in which inclusion complexes with a host:guest ratio of 1:1 form for the TMeQ[6] and Q[7] cases, while they form with a host:guest ratio of 1:2 for the Q[8] case. Commonly, the hosts selectively bound to the phenyl moieties of the guests. Absorption spectrophotometric analysis in aqueous solution defined the stability of the host–guest inclusion complexes at pH 3.2. Quantitatively, at this pH, complexes with a host:guest ratio of 1:1—those with smaller hosts TMeQ[6] and Q[7]—formed with logK values between 6 and 7. That with host Q[8] and a host:guest ratio of 1:2 formed with a logK value of 10.8. Single crystal X-ray structures of the inclusion complexes TMeQ[6]–g·HCl and Q[8]–g·HCl showed the phenyl moiety of the guest inserted into the host cavity. This result supports the solution-based ¹H NMR spectroscopic study.     

Absorption and magnetic circular dichroism spectra of matrix isolated Ta atoms

The absorption and magnetic circular dichroism (MCD) spectra of Ta atoms isolated in an argon matrix at 14 K have been measured. Over sixty-five MCD bands have been observed and used together with gas phase intensity and energy pattern data to reassign the matrix spectrum. The observed matrix shift (average: 765 cm^?1) to the blue is smaller than determined by previous workers. From this study it is suggested that the assignment of matrix-isolated atomic species can be considerably aided by the determination of their magnetic circular dichroism spectra.     

The magnetic circular dichroism spectrum of matrix-isolated TaO

TaO has been matrix-isolated in an argon matrix at 14 K and 24 K and studied spectroscopically in the visible region (300–850 nm). Both adsorption and magnetic circular dichroism (MCD) spectra have been recorded and analyzed. A determination of the total angular momentum quantum numbers (ω) for fourteen excited electronic states has been made. The g factors for the ground ²Δ_{$\text{3}\text{2}$} and excited ²φ_{$\text{5}\text{2}$} states have been determined from a moment analysis of the MCD and absorption spectra of the 450.3 nm band. The present study indicates the power of the combination of magnetic circular dichroism and matrix isolation for the assignment of excited electronic states of high temperature molecules.     

Far-infrared spectra of some β-hydroquinone clathrates

Far-infrared spectra (400-30 cm^?1) of Nujol mulls of the β-hydroquinone clathrates containing the following guest molecules were investigated: formic acid, formic acid-d₂, methanol, methanol-d₄, acetonitrile, acetonitrile-d₃, sulphur dioxide and also both of methanol and sulphur dioxide. The observed infrared bands of the mulls in the region of 4000-30 cm^?1 were classified into those due to the host lattice and those due to the guest molecules. On the basis of the comparison of the spectra, some bands were assigned to the translational or the rotational vibrations of guest molecules. Appearance of those bands suggested that some guest molecules are considerably bound in the cavities of the host lattice. Effect of temperature change on the bands was also measured.     

Spectroscopic effects of vibronic coupling between nearby 3nπ* and 3ππ* states of dimethylbenzaldehydes in a durene matrix

A vibrational analysis is presented of high resolution ππ* phosphorescence spectra of the 2,4-, 2,5- and 3,4-dimethylbenzaldehyde-1h₁ and ?1d₁, guests in durene host. As previously reported for the fully hydrogenated molecules, the energy gaps between the zero-point levels of the ³ngp*, and ³ππ* states of the guests deuterated on the aldehydic group are determined from the temperature dependence of their phosphorescence spectra and lifetimes. These vibronic energy gaps tend to be larger in the deuterated species.The existence of the vibronic coupling between ³nπ* and ³ππ* states of dimethylbenzaldehydes is suggested by the important activity, in the ππ* phosphorescence spectra, of nontotally symmetric modes involving the aldehydic group and the aromatic ring. Among these vibrations, the most active are the aldehydic CH and CD wagging modes which also show overtone activity. The intensities of the induced bands show a strong deuterium effect which in the 2,4 and 2,5 compounds is anomalous in that the deuterated species shows the higher induced intensity. The observation of phosphorescence spectra from each molecule in two different sites allows an assessment of the crystal field effect which is found to be moderately strong. These observations can be understood qualitatively on the basis of weak pseudo Jahn-Teller-like coupling between the ³ππ* and ³nπ* states which is subject to interference from Herzberg-Teller coupling involving at least one additional state. No convincing evidence is found for nonplanar distortions of the guest molecules.     

MCD studies of biphenyls

The magnetic circular dichroism (MCD) spectra of biphenyl derivatives with electron donating substituents at the C₄ and/or C₄′ positions of biphenyl were measured at room temperature in the wavelength regions of 200 to 350 nm. The biphenyls studied exhibited Faraday B terms caused by a magnetic coupling of the non-degenerate electronic states. A careful inspection of the MCD spectra has enabled us to find hidden bands which were buried in the intense, so-called “conjugation band” of biphenyl, resulting in the preferable assignments of the bands.     

Infrared and Raman studies of halate ions matrix isolated in Sr(ClO3)2 and Ba(ClO3)2·H2O type host lattices

The i.r. and Raman spectra of ClO⁻₃, BrO⁻₃ and IO⁻₃ ions matrix isolated in both Ba(ClO₃)₂·H₂O and Sr(ClO₃)₂ type host crystal lattices are studied in the XO stretching and XO₃ bending mode region. The spectra obtained are an excellent probe of the local potential, as the distortion of the halate ions and the repulsive part of the lattice potential, in the lattice sites under consideration. Thus, both site group splittings of the asymmetric modes ν₃ and ν₄ and separations of ν₁ and ν₃ vary characteristically depending on both the guest ions and the structures of the host lattices. However, the splitting of the bands is still more sensitive to lattice site distortion than are structural data. The mean XO stretching frequencies of the guest ions can be correlated with both the reduced cell volumina [V_u/n(XO⁻₃)] and the ratios of volume increments of the guest and the host ions (V ^g _i/V ^h _i). The intraionic coupling of the stretching and bending vibrations is reduced and changed in the case of strong distortions of the XO⁻₃ ions, which is much larger for Sr(ClO₃)₂ type halates than for Ba(ClO₃)₂·H₂O type compounds.     

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.     

Theory of Monte Carlo simulations of the magnetic circular dichroism spectra of alkali metal/rare gas systems

The history of magnetic circular dichroism (MCD) spectroscopy in the study of alkali metal/rare gas (M/Rg) cryogenic systems is reviewed in the context of developing a better understanding of alkali metal/hydrogen systems of current interest to the U.S. Air Force as enhanced‐performance cryogenic rocket propellants. A new theory for simulating the MCD spectra of M/Rg systems is presented together with a careful discussion of the theory's implicit and explicit approximations and their implications. This theory uses a classical Monte Carlo (MC) simulation scheme to model the perturbing effects of the Rg environment on the ²S → ²P MCD‐active transition of the M atom. The theory sets up the MC–MCD simulation as a 6 × 6 matrix eigenvalue/eigenvector problem in the ²P manifold in which are included the effects of M–Rg interactions, metal atom spin‐orbit coupling in the ²P manifold, magnetic Zeeman perturbations of the ²S and ²P manifolds, Boltzmann temperature factors, and electric dipole transition moment integrals for left circularly polarized (LCP) and right circularly polarized (RCP) light. The theory may be applied to any type of trapping site of the host M in the guest Rg matrix; a single atom substitutional metal atom trapping site (one host Rg atom is replaced by one guest M atom) is modeled in this study for M = Na and Rg = Ar. Two temperature factors are used in these simulations; a lattice temperature to model the mobility of the Rg lattice and a magnetic temperature to model Boltzmann factors in the ²S ground manifold. The 6 × 6 eigenvalue/eigenvector problem is solved for a number of randomly generated and suitably averaged Rg configurations to yield the simulated MC‐MCD spectrum for the single substitutional Na/Ar system. The MC–MCD simulations of Na/Ar give the characteristic triplet MCD spectrum with the correct Boltzmann temperature dependence. The simulated MC–MCD spectrum correctly inverts when the direction of the applied magnetic field is reversed. Addition of the LCP and RCP absorbances gives rise to a characteristic ²S → ²P triplet absorption feature. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

The visible spectrum of fulvene

Vapour phase and mixed crystal of the visible band system of fulvene are reported. The spectra show several interesting features including a main progression of 660 cm^?1 corresponding to the out-of-plane methylene twisting vibration. Strong Herzberg—Teller origins together with the breadth and overall diffuseness indicate two electronic states of symmetry A₁ and B₂ may be responsible for the observed spectrum. The spectra are blue shifted in going from the vapour to mixed crystal and show sequence structure to the blue of the main bands in the vapour phase.     

Laser Spectroscopic Study of Cold Gas‐Phase Host‐Guest Complexes of Crown Ethers

The structure, molecular recognition, and inclusion effect on the photophysics of guest species are investigated for neutral and ionic cold host‐guest complexes of crown ethers (CEs) in the gas phase. Here, the cold neutral host‐guest complexes are produced by a supersonic expansion technique and the cold ionic complexes are generated by the combination of electrospray ionization (ESI) and a cryogenically cooled ion trap. The host species are 3n‐crown‐n (3nCn; n = 4, 5, 6, 8) and (di)benzo‐3n‐crown‐n ((D)B3nCn; n = 4, 5, 6, 8). For neutral guests, we have chosen water and aromatic molecules, such as phenol and benzenediols, and as ionic species we have chosen alkali‐metal ions (M⁺). The electronic spectra and isomer‐specific vibrational spectra for the complexes are observed with various laser spectroscopic methods: laser‐induced fluorescence (LIF); ultraviolet‐ultraviolet hole‐burning (UV‐UV HB); and IR‐UV double resonance (IR‐UV DR) spectroscopy. The obtained spectra are analyzed with the aid of quantum chemical calculations. We will discuss how the host and guest species change their flexible structures for forming best‐fit stable complexes (induced fitting) and what kinds of interactions are operating for the stabilization of the complexes. For the alkali metal ion?CE complexes, we investigate the solvation effect by attaching water molecules. In addition to the ground‐state stabilization problem, we will show that the complexation leads to a drastic effect on the excited‐state electronic structure and dynamics of the guest species, which we call a “cage‐like effect”.     

Different spectra for different sites in mixed crystals

The site splitting in the pure electronic transition and in the 676 cm⁻¹ and 1380 cm⁻¹ vibronic absorption bonds were determined at high spectral resolution at 1.8°K for the system phenanthrene in biphenyl. The site splitting is not the same in different vibronic bands, exposing the fact that the guest molecule force field is different at different sites. Some ideas on site splitting, and phonon effects in mixed crystals spectra are presented.