Karl H. Hausser

Nuclear magnetic resonance of organic free radicals

The conditions under which N.M.R. spectra of organic free radicals can be observed are discussed. The N.M.R. spectra of several radicals are shown and compared with the E.S.R. spectra of the same radicals. The N.M.R. spectra are particularly useful to determine small coupling constants which are below the limit of resolvability of E.S.R. and ENDOR. Futhermore, N.M.R. renders the sign as well as the magnitude of the interaction between the unpaired electron and the nucleus investigated; it can contribute to the understanding of the mechanism of this interaction.

Spin rotation interaction and anisotropic chemical shift in 13CS2

Abstract The 13C nuclear spin-lattice relaxation time T1 was studied in liquid CS2 from −106°C to +35°C at resonance frequencies of 14, 30, and 62 MHz. The relaxation is caused by anisotropic chemical shift and spin-rotation interaction. It is shown that for linear molecules the spin-rotation constant C and the anisotropy of the chemical shift Δσ can be obtained from the relaxation rates without use of adjustable parameters. The analysis yields: C = -13.8 ± 1.4 kHz and Δσ = 438 ± 44 ppm.

Transannular interactions in [2.2] phanes as studied by magnetic resonance and optical spectra

The interaction of π-electrons in [2,2] phanes was studied both experimentally and theoretically. The fluorescence and phosphorescence spectra were measured at liquid helium temperature; in addition, the zero field splitting parameters D and E were determined by ODMR in zero field and by ordinary ESR at X-band. The results for the phanes with two identical aromatic units can be summarized as follows: The rather small reduction of the D and E values of the order of 10% with respect to the monomers indicates, in agreement with the theoretical treatment given in part II, that the two unpaired electrons of the excited triplet state have a high probability to be at a given time in the same half of the molecule. While the fluorescence spectra show the typical behaviour of emission spectra of dimers or excimers, the phosphorescence spectra exhibit some remaining structure. This behaviour which indicates a somewhat weaker coupling among the triplet orbitals as compared to the singlet orbitals can also be understood on the basis of theoretical considerations. For a phane with two different aromatic units the behaviour is found to be more similar to the corresponding aromatic monomer with the lower excited states with some perturbation by the other part of the phase also in agreement with theoretical expectation.

Electronic properties of two isomeric charge transfer [2.2]paracyclophanes

The emission spectra and the zero field splitting parameters of the two diastereomeric 4,7-dicyano-12,15-dimethoxy-[2.2] paracyclophanes 3 and 4 in comparison to the corresponding monomers 1 and 2 were investigated in order to study the orientation dependence of charge transfer (CT) interactions. The general results in glasses (PMMA) are: broad structureless emission bands with large spectral overlap between fluorescence and phosphorescence; strong reduction of the zero field splitting parameters D and D* by a factor of two for the pseudo-ortho isomer 3 and by a factor of four for the pseudo-geminal isomer 4 showing the strong effect of the geometrical orientation. In single crystals of the same phanes the zero field parameters were found to be further reduced to about one fifth of the value of the monomers which indicates, in accordance with the emission spectra, an additional intermolecular interaction between adjacent phanes.

Transanular interaction in [2.2]phanes: [2.2](4,4′)Diphenylophane and [2.2](2,7) fluorenophane

The emission spectra and the zero field splitting parameters D and E of the first excited triplet states of [2.2](4,4′) diphenylophane 3 and of the two stereoisomeric syn- and anti [2.2] (2,7) fluorenophanes 4a and 4b are investigated and compared with those of the corresponding monomers.

PMR and DMR studies of 1,5-diphenylverdazyls

Abstract Using PMR and DMR the hfs H and D coupling constants for a series of substituted 1,5-diphenylverdazyls have been measured and assigned. The C-6 methylene proton resonance has been studied as a function of temperature. The results indicate a ring inversion in verdazyls which is slow on the NMR time scale. Furthermore, the gain in resolution is discussed obtained with PMR in DBNO, and with DMR.

Microwave-Induced Optical Nuclear Polarization (MI-ONP)

Abstract The protons in a single crystal of p -dibromobenzene doped with p -dichlorobenzene placed in an external magnetic field are polarized by simultaneous irradiation with light and microwaves. The selection rules of spin-orbit interaction cause an alignment in the triplet electron spin system of p -DCB. This alignment is transferred to the proton spin system by inducing forbidden transitions Δm s = ± 1, Δm I = ± 1 analogous to the well-known solid-state effect. The sign and the amount of polarization, of about a factor of 10 as compared to the Boltzmann equilibrium, is shown to be in agreement with the expectation for our system. Using proton and chlorine hfs data, the field range with observable proton polarization is calculated and found to be in agreement with the experimental result. Conditions which favor higher nuclear polarizations are discussed.

Optical nuclear polarization as a consequence of the non-crossing rule (Level-Anti-Crossing): III. Experimental results and evidence for guest-host complexes in doped fluorene crystals

Abstract Fluorene crystals doped with the guest molecules acridine, phenazine and anthracene have been studied before due to their strong Optical Nuclear Polarization (ONP) in low magnetic fields around 100 gauss. In this paper, these results together with supporting new data are interpreted in terms of ONP as a consequence of Level-Anti-Crossing (LAC) within the excited triplet state of a guest-host complex. The results permit a first detailed check of the theoretical model of ONP by LAC including the influence of electronic relaxation. Furthermore, they render the spectroscopic parameters needed for the characterization of the triplet-state complexes: • The zero-field splitting tensor and the orientation of its principal axes system with respect to the crystalline axes. • The proton hyperfine tensor, which is essentially due to one-proton spin in the CH2-group of the fluorene host molecule contributing to the complex.

Principles of NMR

The purpose of Chap. 1 of this book is to lay the foundation for a general understanding of NMR, which is then built upon in the following chapters. Nuclear magnetic resonance is just one of the names of the method treated in this book; other terms used are nuclear spin resonance or the abbreviation NMR.

Spin densities in 1,3,5-triphenylverdazyl an NMR study

Abstract NMR spectra of 1,3,5-triarylverdazyls dissolved in DBNO yield all H hfs coupling constants and their assignment for 1,3,5-triphenylverdazyl. The experimental data are compared with McLachlan spin density calculations.