Theory of pulsed reaction yield detected magnetic resonance

We propose pulse sequences for Reaction Yield Detected Magnetic Resonance (RYDMR), which are based on refocusing the zero-quantum coherences in radical pairs by non-selective microwave pulses and using the population of a radical pair singlet spin state as an observable. The new experiments are analogues of existing EPR experiments such as the primary echo, Carr-Purcell, ESEEM, stimulated echo and Mims ENDOR. All pulse sequences are supported by analytical results and numerical calculations. The pulse sequences can be used for more efficient and highly detailed characterization of intermediates of chemical reactions and charge carriers in organic semiconductors.     

Laser magnetic resonance spectra of the PH radical

The PH radical has been detected by laser magnetic resonance spectroscopy at 118.6 μm in the reaction products when hydrogen atoms were passed over red phosphorus. The spectra have been identified as the N = 4 → 5 rotational transition in the ground ³∑^? state and J = 4 → 5 transition in the a¹ Δ state. The hyperfine constants for the ¹Δ state are a_p = 775 MHz and a_H = 28 MHz.     

The rotational and fine-structure spectrum of FeH, studied by far-infrared laser magnetic resonance

Transitions between the spin-rotational levels of the FeH radical in the upsilon=0 level of the X (4)Delta ground state have been detected by the technique of laser magnetic resonance at far-infrared wavelengths. Both pure rotational and fine-structure transitions have been observed; lambda-type doubling is resolved on all the observed transitions. The energy levels of FeH are strongly affected by the breakdown of the Born-Oppenheimer approximation and cannot be modeled accurately by an effective Hamiltonian. The data are therefore fitted to an empirical formula to yield term values and g factors for the various spin-rotational levels involved. Many of the resonances show a doubling that arises from the proton hyperfine structure. These splittings are analyzed in a similar manner.     

Far-infrared laser magnetic resonance spectrum of NHD

Far-infrared laser magnetic resonance spectra of NHD (X ²A′') (0,0,0) have been observed. Data are presented for the rotational transitions 3₁₃ ← 2₀₂ and 4₁₃ ← 3₂₂ observed at wavelengths of 211.3 and 374.1 μm respectively. Theoretical values of the transition magnetic field strengths have been calculated using the best available molecular constants. The agreement between theory and experiment confirms our spectroscopic assignments, but indicates that with further experimental data, considerable refinement of the molecular constants will be obtained.     

Zero-field optically detected magnetic resonance of model compounds for pheophytins

ZFS parameters and kinetic constants of the lowest triplet state of chlorin and tetraphenylchlorin free base in n-octane have been determined by fluorescence-detected ODMR at 4.2 K. These compounds can be considered as model compounds for pheophytin, a compound of biological interest. For both compounds the middle spin-level is the most active one in the populating and depopulating pathway. In the lowest triplet state the NHNH axis in both chlorins is probably fixed to one orientation not involving the reduced ring, and no evidence was found for the occurrence of two tautomeric forms as in the corresponding porphyrins.     

The spin mixing process of a radical pair in low magnetic field observed by transient absorption detected nanosecond pulsed magnetic field effect

The spin mixing process of the radical pair in the sodium dodecyl sulfate (SDS) micelle is studied by using a novel technique nanosecond pulsed magnetic field effect on transient absorption. We have developed the equipment for a nanosecond pulsed magnetic field and observed its effect on the radical pair reaction. A decrease of the free radical yield by a reversely directed pulsed magnetic field that cancels static field is observed, and the dependence on its magnitude, which is called pulsed MARY (magnetic field effect on reaction yield) spectra, is studied. The observed spectra reflect the spin mixing in 50-200 ns and show clear time evolution. Theoretical simulation of pulsed MARY spectra based on a single site modified Liouville equation indicates that the fast spin dephasing processes induced by the modulation of electron-electron spin interaction by molecular reencounter affect to the coherent spin mixing by a hyperfine interaction in a low magnetic field.     

Optically detected zero-field triplet state magnetic resonance in photosynthetic bacteria

Triplet state transitions of the photosynthetic bacteria Rhodospirillum Rubrum, Rhodopseudomonas Spheroides and Chromatium Vinosum in chemically reduced preparations have been observed by zero-field optical detection of magnetic resonance at 2 K. For each bacterial preparation two sharp, structureless, zero-field EPR transitions were observed as microwave-induced decreases in the fluorescence intensity of the frozen cellular preparations. The depopulating rate constants for the spin sublevels of the triplet states observed in R Rubrum and R Spheroides were also measured. The similarities of the triplet state frequencies, spectral features and intersystem crossing rates suggest a common structure for the reaction centers in the photosynthetic bacteria.     

Solvent effects on optically detected magnetic resonance in triplet spin labels

We have calculated solvent effects on the zero-field splitting (ZFS) constants induced by electron spin–spin coupling (SSC) in the low-lying triplet states of azaaromatic molecules in solutions using multiconfiguration self-consistent-field wave functions and the polarizable continuum model. The second-order spin–orbit coupling (SOC) contribution to the splitting of the ^3* states is found to be almost negligible, and the calculations therefore provide a good estimate of the ZFS parameters and their solvent dependence based only on the electron spin–spin coupling expectation values. The correlation between the shift in the ZFS and the phosphorescence frequency that has been observed in optically detected magnetic resonance experiments in low-temperature glasses is supported by our direct SSC calculations without taking SOC into account. This makes it possible to distinguish between the two theories that earlier were proposed to explain the inhomogeneous broadening of triplet state spectra, and discard the one that is exclusively based on the SOC-induced mixing of the singlet and triplet states.Contribution to the Jacopo Tomasi Honorary IssueAcknowledgments. This work was supported (B. M.) by the Swedish Royal Academy of Science (KVA). This work was also supported by the Norwegian Research Council through a grant of computer time from the Program for Supercomputing. We are grateful to B. Schimmelpfenning for his valuable assistance in the computations.     

Optically detected magnetic resonance study of the phosphorescent states of thiouracils

The triplet states of 1-methyl-2-thiouracil (1-Me-s²U), 1-methyl-4-thiouracil (1-Me-s²s⁴U) and 1-methyl-2,4-dithiouracil (1-Me-s² s⁴) have been investigated by optically detected magnetic resonance in zero magnetic field. The zero field splittings (ZFS) and the individual sublevel kinetic parameters are reported. The ZFS (|D|, |E|) values (in cm^?1) are found to increase in the order: 1-Me-s² U (0.2895, 0.0728) < 1-Me-s⁴U, (0.605, 0.0500) < 1-Me-s²s⁴ U (0.870, 0.0458). The triplet state lifetimes decrease in the same order, and both effects are attributed to an internal heavy atom effect of sulfur substitution. The vibronic structure of the phosphorescence emission indicates that the thiouracil phosphorescent states are ³(π, π^*). The low phosphorescence quantum yields of 1-Me-s⁴ U and of 1-Me-s²s⁴U result from radiationless decay of the triplet state rather than from inefficient intersystem crossing from the excited singlet state. The efficient radiationless decay of the triplet state appears to be a feature of the S-substitution at the 4-position of uracil. Phosphorescence polarization measurements of the individuals triplet sublevel emissions at ca. 1.2 K are consistent with 1-Me-s²U and 1-Me-s⁴U being non-planar in the phosphorescent state; the thiouracil phosphorescence from each triplet sublevel is polarized in the average plane of the distorted molecule. In the absence of σπ separability, spin—orbit mixing of ¹(π, π^*) and ³(π, π^*) states is enhanced and the radiative properties of the triplet state may be dominated by this mechanism rather than by the mixing of ¹(n, π^*), ¹(σ, π^*), or ¹(π, σ^*) with ³(π, π^* states which generally is the dominant mechanism for planar aromatic molecules.     

Calculations of magnetic resonance parameters for the protonated nitroxide radical

Electron and magnetic resonance parameters of the protonated H₂NO radical have been calculated by the INDO and CNDO/SP methods for different models. Calculated changes of magnetic resonance parameters on protonation are consistent with experiment. The most appropriate structure has been found to be one in which the proton is in the plane of the radical with r(O?.H⁺) = 1.05 A.Calculated signns of the proton spin density for the models concerned are opposite to those of the spin density on the proton of a ligand involved in the hydrogen bonding for analogous models of hydrogen bond systems formed by the nitroxide radical.In the case of the protonated radical, taking into account the interaction with a solvent molecule leads to more reasonable results for large O?.H⁺ distances.     

Tetrazoloazines. 15N nuclear magnetic resonance and infrared absorption spectroscopy

The reaction of 4-chloro-2-phenylquinazoline with K¹⁵NN₂ has been studied by ¹⁵N-NMR. spectroscopy. ¹⁵N-chemical shifts in 5-phenyl-1 (3)-[¹⁵N]-tetrazolo[1,5-c]quinazoline and -Nα(Nγ)-[¹⁵N]-4-azido-2-phenylquinazoline are reported. The characteristic IR. absorption frequencies of the tetrazole group have been determined in a series of annelated ¹⁵N-labelled compounds. From these studies and the chemistry of the labelled tetrazoles, it is concluded that all haloazines examined react with KN₃ by the direct nucleophilic substitution mechanism. An addition of nucleophile-ring opening-ring closure (ANRORC) mechanism was not observed. The synthesis of several ¹⁵N-labelled tetrazoloazines is described.     

Theoretical study of spin-one nuclear quadrupole effects in triplet-state optically detected magnetic resonance

Nuclear quadrupole effects on the triplet-state optically detected magnetic resonance (ODMR) of molecules having spin I = 1 nuclei have been investigated by theoretical methods. Beginning with an exact diagonalization of the triplet-state spin Hamiltonian, these methods have involved a systematic variation in quadrupolar parameters, which allows us to distinguish between “allowed” and “forbidden” transitions and also enables us to assign the observed lines in any 2 E ODMR transition.