P. N. Schatz

Magneto‐Optical Rotatory Dispersion of Porphyrins and Phthalocyanines

Recent MORD data in the region of the visible and near‐uv absorption bands of some porphyrins and phthalocyanines are analysed. New computer programs are used to fit general formulas to the experimental data and to extract molecular parameters. Theoretical expressions for the latter are derived, adopting the current interpretation of the observed transitions. The over‐all agreement with experiment is good, supporting the assignments. The theory also shows that the magnetic moments of the excited states can be obtained from the data. Values obtained agree with a priori calculations in order of magnitude. Further experimental and theoretical work is suggested.

Faraday Effect of Charge‐Transfer Transitions in Fe(CN)63−, MnO4−, and CrO42−

Measurements of the magneto‐optical rotatory dispersion (MORD) and magnetic circular dichroism (MCD) through the accessible charge‐transfer bands of Fe(CN)63−, MnO4−, and CrO42− in the visible and ultraviolet are described and interpreted. The apparatus employed combines a superconducting solenoid furnishing maximum fields of 45 000 G with a commercial ORD—CD instrument which covers the range 185–700 mμ. The MCD measurements are found to be more reliable and are much easier to make; they are also more useful for theoretical interpretation. However, Kramers—Kronig transforms of the MORD results agree well with the experimental MCD measurements. Comparison of the theoretically derived Faraday parameters for Fe(CN)63− with those extracted from experiment shows that the 24 000‐, 33 000‐, and 38 500‐cm−1 bands are due, respectively, to 2T2g→2T1u, 2T2u and 2T1u, ligand‐to‐metal(t2g) charge‐transfer transitions. A similar analysis confirms the assignment of the lowest band in MnO4− and CrO42− to a t1(π)→e(π) tra...

The magnetic circular dichroism and absorption spectra of C60 isolated in Ar matrices

Abstract The absorption and magnetic circular dichroism (MCD) spectra of C60 isolated in Ar matrices have been measured in the visible and near-ultraviolet. The first two allowed bands (at ≈ 326 and ≈253 nm) show substantial negative A terms. Sharp, weak vibronic structure showing clear evidence of Jahn—Teller activity is observed in both the 400 and 600 nm regions. Detailed assignments are proposed for the resolved spectral features.

On the Robinson and Price (Kramers—Kronig) Method of Interpreting Reflection Data Taken through a Transparent Window

It is shown that the usual integral formula used to obtain the phase shift from the reflectivity (at all frequencies) must be modified when the reflectivity is measured off a surface covered by a transparent window. The discussion is first restricted to reflection at normal incidence but is later generalized to include all possible angles of incidence. There are found to be three distinct cases defined by the conditions (1)sin2α e0/ew, where e0 is the static dielectric constant of the sample, ew is the dielectric constant of the window, and α is the angle of incidence. Case (1) involves the addition of an extra factor to the usual expression for the phase shift. This extra factor involves either zero, one, or two unknown parameters depending upon the particular values of α, ew and e0. In case (2) the expression for the phase shift is quite complicated and of doubtful practical value. In case (3), it was found possible to write a simple expression for the phase shift in t...

Magnetic Circular Dichroism of Benzene, Triphenylene, and Coronene

The magnetic circular dichroism of benzene, triphenylene, and coronene in regions of π→π absorption (20 000‐45 000 cm−1) are reported. The data support the conventional molecular‐orbital assignments of the absorption spectra. The lowest 1E′ and 1E1u states of triphenylene and coronene, respectively, are shown to be little affected by Jahn‐Teller perturbations. Magnetic moments are obtained for these states which are in good agreement with values calculated from the free‐electron network model and from molecular‐orbital wavefunctions if Lowdin orthogonalized atomic basis functions are employed.

Magnetic Circular Dichroism of Charge‐Transfer Transitions: Low‐Spin d5 Hexahalide Complexes

The magnetic circular dichroism (MCD) of IrCl62−, IrBr62−, and RuBr63− have been measured through the visible and near ultraviolet. Analysis leads to new assignments of the ligand‐to‐metal (t2g) charge‐transfer bands. In particular, the conventional order of the first two allowed charge‐transfer states is reversed so that the 2T1u state lies at lower energy than 2T2u.

Moment Analysis of Magnetic Circular Dichroism: Diamagnetic Molecular Solutions

The method of moments enables more rigorous analysis of magnetic circular dichroism data than rigid shift models. Moment analysis is here applied to magnetic circular dichroism measurements on room temperature solutions of MnO4−, CrO42−, PdCl42−, SbCl6−, Ni(CN)42−, Fe(CN)64−, triphenylene, and coronene. Qualitative conclusions previously reached on the basis of rigid shift models are shown to be very little affected. More accurate values for the magnetic moments of degenerate excited state manifolds are obtained. The new data for Fe(CN)64− support the assignment of the 47 000 cm−1 absorption band to an intramolecular, t2g → t1u metal‐to‐ligand transition.

Argon matrix photolysis and photoionization studies of benzene. Absorption spectrum of benzene cation and benzene dimer cation

Argon/benzene samples subjected to argon resonance photoionization during condensation at 21±1 K revealed a sharp new 552.5 nm product absorption, sharp C−2 bands and a broad 900 nm absorption. The 552.5 and 900 nm absorptions were destroyed and the C−2 bands decreased by mercury arc photolysis, which suggests that the 552.5 and 900 nm absorptions are due to molecular cations. The broad 900 nm absorption is in agreement with an earlier assignment to the intervalence transition in (C6H6)2+. Near agreement between the 552.5 nm absorption energy and the difference between the sharp origins for the first two benzene photoelectron bands invites consideration of the 552.5 nm absorption for C6H6+. A tentative assignment of the sharp, weak 552.5 nm band to the vibronically allowed 2E2g+e2u←2E1g transition of C6H6+ is in accord with deuterium isotopic shifts and predictions from the gas phase fluorescence spectra of fluorine substituted benzene cations. A strong 310 nm band system is attributed to the C6H7 radical...

Magnetic circular dichroism spectra of alkaline earth metal atoms, dimers, and aggregates in noble gas matrices

The magnetic circular dichroism (MCD) of Mg, Ca, and Sr atoms, dimers, and higher aggregates have been studied after codeposition with noble gases at low temperatures. The1Σ+g→1Πu transition in each of the M2 species (M = Mg, Ca, Sr) occurring to the blue of the 1S→1P resonance transition, is unambiguously identified by measurement of the excited state magnetic moment. The distinct band at ∼30,000 cm−1 in magnesium samples is identified as Mg3, and the measured excited state magnetic moment favors the assignment, 1A1′→1E′, corresponding to an equilateral triangular (D3h) geometry. A broader band at ∼33 000 cm−1 in Ar is assigned to Mg4, and the very small excited state magnetic moment is consistent with the assignment, 1A1→1T2, of a tetrahedral species. In Ca/Ar, Ca/Kr, and Sr/Ar matrices, the 1S→1D atomic transition is seen with about 5% of the intensity of the corresponding 1S→1P transition. The accompanying MCD patterns unambiguously demonstrate the presence of two distinct sites for Ca atoms and three...

Magnetic circular dichroism of the 1S→1P transition of Mg atoms in noble gas matrices

A magnetic circular dichroism (MCD) and absorption study has been made of the 1S (3s2) →1P (3s3p) transition of Mg atoms in Ne, Ar, Kr, and Xe matrices. Triplet structure is observed in the latter three, and a detailed analysis of the MCD shows that this structure results from a splitting of the 1P excited state at a single site. An analysis of the zeroth and first MCD and absorption moments shows that the orbital angular momentum of this 1P state is partially quenched, the quenching factors being approximately 0.65, 0.75, 0.50, and 0.43 in Ne, Ar, Kr, and Xe matrices, respectively. This quenching is necessarily a consequence of out‐of‐state mixing (i.e., orbital mixing outside the atomic 1P excited manifold) and almost certainly reflects strong mixing of the 3p Mg orbitals with noble gas orbitals on neighboring host atoms. An analysis of higher absorption and MCD moments assuming octahedral site symmetry shows a dominant noncubic (Jahn–Teller active) mode contribution to the bandwidth in the three hosts ...