Unpaired Electron Spin Density Research Articles

Assessment of the Existence of Hyper-Long Axial Co(II)−N Bonds in Cobinamide B12 Models by Using Electron Paramagnetic Resonance Spectroscopy

Protein control of cobalt-axial nitrogen ligand bond length has been proposed to modulate the reactivity of vitamin B(12) coenzyme during the catalytic cycle of B(12)-dependent enzymes. In particular, hyper-long Co-N bonds may favor homolytic cleavage of the trans-cobalt-carbon bond in the coenzyme. X-ray crystallographic studies point to hyper-long bonds in two B(12) holoenzymes; however, mixed redox and ligand states in the crystals thwart clear conclusions. Since EPR theory predicts an increase in Co(II) hyperfine splitting as donation from the axial N-donor ligand decreases, EPR spectroscopy could clarify the X-ray results. However, the theory is apparently undermined by the similar splitting reported for the 2-picoline (2-pic) and pyridine (py) adducts of Co(II) cobinamide (Co(II)Cbi(+)), adducts thought to have long and normal Co-N axial bond lengths, respectively. Cobinamides, with the B(12) 5,6-dimethylbenzimidazole loop removed, are excellent B(12) models. We studied Co(II)Cbi(+) adducts of unhindered 4-substituted pyridines (4-X-py's) in ethylene glycol to separate orbital size effects from Co-N axial distance effects on these splittings. The linear increase in splitting with the decrease in 4-X-py basicity found is consistent with the theoretically predicted increase in unpaired electron spin density as axial N lone pair donation to Co(II) decreases. No adduct (and hence no hyper-long Co(II)-N axial bond) was formed even by 8 M 2-pic, if the 2-pic was purified by a novel Co(III)-affinity distillation procedure designed to remove trace nitrogenous ligand impurities present in 2-pic distilled in the regular manner. Adducts formed by impurities in 2-pic and other hindered pyridines misled previous investigators into attributing results to adducts with long Co-N bonds. We find that many 2-substituted py's known to form adducts with simple synthetic Co models do not bind Co(II)Cbi(+). Thus, the equatorial corrin ring sterically impedes binding, making Co(II)Cbi(+) a highly selective binding agent for unhindered sp(2) N-donor ligands. Our results resolve the apparent conflict between EPR experiment and theory. The reported Co(II) hyperfine splitting of the enzyme-bound cofactor in five B(12) enzymes is similar to that of the relevant free cofactor. The most reasonable interpretation of this similarity is that the Co-N axial bond of the bound cofactor is not hyper-long in any of the five cases.

W- and X-Band Pulsed Electron Nuclear Double-Resonance Study of a Sodium−Nitric Oxide Adsorption Complex in NaA Zeolites

Pulsed electron nuclear double-resonance (ENDOR) spectroscopy at W- and X-band frequencies has been employed to characterize the structure of NO adsorption sites involving sodium cations in zeolite NaA. The principal values of the sodium hyperfine and nuclear quadrupole coupling tensors as well as the orientation of their principal axes system with respect to the g tensor coordinate frame of the Na+−NO adsorption complex could be determined by orientation-selective ENDOR spectroscopy. Such orientation-selective experiments benefit especially from the high spectral resolution at W-band frequencies. Furthermore, the sodium ENDOR spectrum is drastically simplified at high frequencies where the limit of weak hyperfine couplings is fulfilled. The dipolar sodium hyperfine coupling tensor reveals a bent structure of the formed adsorption complexes and gives access to the bond distance between the NO molecule and the cations. The 23Na nuclear quadrupole data indicate that the adsorption complexes are preferentially formed with the sodium ions at the six-membered rings of the NaA zeolite structure. An analysis of the sodium and nitrogen hyperfine coupling data shows that 96% of the unpaired electron spin density in the Na+−NO adsorption complex is localized in the nitrogen and oxygen 2pπ orbitals of the NO ligand molecule.

High-pressure H 2O vapor heating used for passivation of SiO 2/Si interfaces

High-pressure H 2O vapor heating was applied to oxidation of SiO x ( x<2) films formed on silicon substrates at room temperature by thermal evaporation in vacuum, in order to improve properties of SiO x /Si interfaces for passivation of silicon surfaces. The SiO x films were oxidized with an activation energy of 0.035 eV. The spin density of unpaired electron decreased from 2.3×10 17 to 1.4×10 15 cm −3 by the heat treatment at 260°C with 2.1×106 Pa H 2O vapor for 3 h. The surface recombination velocity for excess carriers decreased from 405 (as deposited SiO x film) to 13 cm/s. There was no change in the surface recombination velocity after keeping the samples at an atmospheric pressure and at room temperature for 8000 h. Suitable passivation of silicon surface was achieved by simple heating with high-pressure H 2O vapor at low temperature.

Elucidation of the Paramagnetic R1 Relaxation of Heteronuclei and Protons in Cu(II) Plastocyanin from Anabaena variabilis

The longitudinal paramagnetic dipolar relaxation rates, R1p, of 15N, 13C, and 1H nuclei in plastocyanin from Anabaena variabilis (A.v. PCu) were determined at 11.7 and 17.6 T from the corresponding experimental relaxation rates in reduced (R1d) and partly oxidized (R1o) A.v. PCu. To obtain an accuracy of the relaxation data sufficiently high for the subsequent analysis, the experimental rates were determined by a simultaneous least-squares analysis of all the spectra in a relaxation experiment. Also, a refined solution structure of A.v. PCu was determined from 1459 NOE distance restraints and 87 angle restraints by distance geometry, simulating annealing and restrained energy minimization. The average rms deviation from the mean structure of the 20 structures with the lowest total energy is 0.75 Å for the backbone atoms and 1.21 Å for all heavy atoms. The distance information of the dipolar paramagnetic R1p rates was compared with the corresponding distances in the refined NMR solution structure. The comparison reveals that the point dipolar approximation, which assumes that R1p is caused by a dipolar interaction of the nuclei with the metal-centered unpaired electron of the Cu2+ ion, does not apply to the heteronuclei. In the case of protons it applies only for proton−copper distances shorter than ∼10 Å. In contrast, it is found that the R1p relaxation of the 15N and 13C nuclei is dominated by dipolar interaction with unpaired metal electron spin density delocalized onto the 2pz orbitals of the heteronuclei. From the R1p rates of the heteronuclei and the metal−nuclei distances in the refined NMR solution structure, the delocalized unpaired spin densities ρπ of the individual 15N and 13C nuclei were derived. It is found that ρπ decays approximately exponentially with the metal−nuclei distance and almost isotropically throughout the protein. Possible implications of this decay for the electron-transfer pathways of A.v. PCu are discussed.

Understanding Trends in C-H, N-H, and O-H Bond Dissociation Enthalpies

Theory and experiment are now in good agreement for bond dissociation enthalpies (BDE's) and can be used as checks on each other to identify anomalous data. Both theory and experiment agree that XO-H, XNH-H, and XCH2-H BDE's decrease monotonically by ca. 60 kJ/mol along the series for X = H, but by only ca. 13 kJ/mol for X = CH3. More surprising is the fact that for X = C6H5 the O-H bond is the weakest and the N-H bond is the strongest (by 10-15 kJ/mol). By contrast, BDE's are essentially identical for X = CF3 and X = H. These interesting trends are discussed in terms of a number of concepts useful in advanced organic chemistry courses, including electronegativity, bond length, orbital overlap, unpaired electron delocalization by conjugation and hyperconjugation, and radical stabilization energy. The discussion is supported by EPR data for unpaired electron spin densities and the most recent thermochemical data for BDE's.

Direct Observation of Electron Spin Density on TDAE Cations in the Ferromagnetic State of SolidTDAE−C60

${}^{14}\mathrm{N}$ electron spin echo envelope modulation (ESEEM) and ${}^{1}\mathrm{H}$ nuclear magnetic resonance (NMR) techniques have been employed in order to measure the unpaired electron spin density of amino nitrogens and ${\mathrm{CH}}_{3}$ protons, in the low temperature ferromagnetic state of solid $\mathrm{TDAE}\ensuremath{-}{\mathrm{C}}_{60}$. Well resolved ${}^{14}\mathrm{N}$ ESEEM patterns were obtained for the first time below 9 K, and their analysis reveals an extremely low unpaired electron spin density on specific TDAE nitrogen sites. Also, ${}^{1}\mathrm{H}$ NMR line shape measurements corroborate a stepwise increase of the electron spin density on the ${\mathrm{CH}}_{3}$ protons below 9 K that is accompanied by the onset of an activated law for ${}^{1}\mathrm{H}$ $1/{T}_{1}$ with temperature.

Selenium-containing formate dehydrogenase H from Escherichia coli: a molybdopterin enzyme that catalyzes formate oxidation without oxygen transfer.

Formate dehydrogenase H, FDH(Se), from Escherichia coli contains a molybdopterin guanine dinucleotide cofactor and a selenocysteine residue in the polypeptide. Oxidation of 13C-labeled formate in 18O-enriched water catalyzed by FDH(Se) produces 13CO2 gas that contains no 18O-label, establishing that the enzyme is not a member of the large class of Mo-pterin-containing oxotransferases which incorporate oxygen from water into product. An unusual Mo center of the active site is coordinated in the reduced Mo(IV) state in a square pyramidal geometry to the four equatorial dithiolene sulfur atoms from a pair of pterin cofactors and a Se atom of the selenocysteine-140 residue [Boyington, J. C., Gladyshev, V. N., Khangulov, S. V., Stadtman, T. C., and Sun, P. D. (1997) Science 275, 1305-1308]. EPR spectroscopy of the Mo(V) state indicates a square pyramidal geometry analogous to that of the Mo(IV) center. The strongest ligand field component is likely the single axial Se atom producing a ground orbital configuration Mo(dxy). The Mo-Se bond was estimated to be covalent to the extent of 17-27% of the unpaired electron spin density residing in the valence 4s and 4p selenium orbitals, based on comparison of the scalar and dipolar hyperfine components to atomic 77Se. Two electron oxidation of formate by the Mo(VI) state converts Mo to the reduced Mo(IV) state with the formate proton, Hf+, transferring to a nearby base Y-. Transfer of one electron to the Fe4S4 center converts Mo(IV) to the EPR detectable Mo(V) state. The Y- is located within magnetic contact to the [Mo-Se] center, as shown by its strong dipolar 1Hf hyperfine couplings. Photolysis of the formate-induced Mo(V) state abolishes the 1Hf hyperfine splitting from YHf, suggesting photoisomerizaton of this group or phototransfer of the proton to a more distant proton acceptor group A-. The minor effect of photolysis on the 77Se-hyperfine interaction with [77Se] selenocysteine suggests that the Y- group is not the Se atom, but instead might be the imidazole ring of the His141 residue which is located in the putative substrate-binding pocket close to the [Mo-Se] center. We propose that the transfer of Hf+ from formate to the active site base Y- is thermodynamically coupled to two-electron oxidation of the formate molecule, thereby facilitating formation of CO2. Under normal physiological conditions, when electron flow is not limited by the terminal acceptor of electrons, the energy released upon oxidation of Mo(IV) centers by the Fe4S4 is used for deprotonation of YHf and transfer of Hf+ against the thermodynamic potential.

Investigation of the pure rotational spectrum of magnesium monobromide by Fourier transform microwave spectroscopy

The pure rotational spectrum of the free radical MgBr has been measured in its Σ+2 ground electronic state by Fourier transform microwave spectroscopy. Transitions have been observed for both Mg24Br79 and Mg24Br81 in the v=0 and v=1 vibrational states. Rotational and centrifugal distortion constants have been determined for each isotopomer in each vibrational state. Equilibrium rotational constants have been calculated and an accurate equilibrium bond length has been determined. Spin-rotation constants, for both the unpaired electron and the bromine nuclei, have been calculated along with magnetic and nuclear quadrupole hyperfine constants for the bromine nuclei. From these constants, the electronic structure of MgBr has been investigated and comparisons have been made to similar compounds. The unpaired electron spin density on the bromine nucleus has been found to be very small, suggesting that this is a very ionic compound. However, the Mg–Br bond has been found to have more covalent character than the bond in other alkaline earth monobromides.

Effect of Hydrogen Bonding on the Spin Density Distribution and Hyperfine Couplings of the p-Benzosemiquinone Anion Radical in Alcohol Solvents: A Hybrid Density Functional Study

Hybrid density functional calculations utilizing the B3LYP functional are used to calculate geometries, spin densities, and isotropic and anisotropic hyperfine couplings for the p-benzosemiquinone anion radical. Hydrogen-bonding interactions with methanol, ethanol, 2-propanol, and water are studied for their effects on the above properties. A redistribution of unpaired electron spin density from the oxygen and ring carbon atom positions to the carbonyl carbon atom position is shown to occur on hydrogen bond formation. Outstanding agreement between calculated and experimental hyperfine couplings is observed.

Microwave spectroscopic detection of HCCP in the X 3Σ− electronic state: Phospho-carbene, phospho-allene, or phosphorene?

Microwave spectrum of the HCCP radical was detected for the first time in the X 3Σ− ground electronic state using a source-modulated microwave spectrometer. In total, 24 rotational transitions of HCCP in the 90–360 GHz region, 9 rotational transitions of DCCP in the 260–360 GHz range, and 24 rotational transitions of H13C13CP between 130–360 GHz were measured. Hyperfine structure pertaining to the phosphorus and hydrogen nuclei was observed for HCCP, and in the case of H13C13CP, only for phosphorus. The corresponding hyperfine coupling constants were ascertained in addition to the rotational, centrifugal distortion, and fine structure constants by a least-squares analysis of the measured frequencies. From the hyperfine coupling constants determined, the spin density of unpaired electrons was estimated to be 76% for the phosphorus atom and 42% for the carbon adjacent to the hydrogen. The r0 structure of HCCP was established from the rotational constants of HCCP and its isotopically substituted species: r0(CP)=1.685 Å, r0(CC)=1.241 Å, and r0(CH)=1.057 Å. These structural features are consistent with a linear phospho-allenic form that has been somewhat modified by a phosphorene.

Fourier transform Raman investigation of the electronic structure and charge localization in a bacteriochlorophyll-bacteriopheophytin dimer of reaction centers from Rhodobacter sphaeroides

The Raman spectra of a bacteriochlorophyll (BChl)-bacteriopheophytin (BPhe) heterodimeric primary electron donor from mutant Rhodobacter sphaeroides reaction centers, where histidine M202 has been replaced by leucine, have been obtained in (pre)resonance with its lowest electronic Q y state using 1064 nm excitation. For reaction centers where the heterodimer is in its reduced, neutral state, the dominant Raman scattering is from the BPhe constituent of the ground state heterodimer, and indicates that the 1064 nm-excitation wavelength is interacting with an electronic state or transition which is largely BPhe in character, such as a charge transfer state with a dominant (BChl +BPhe −) configuration. Previous electron paramagnetic studies have established that the unpaired electron spin density on the oxidized, cation radical heterodimer resides almost totally on the BChl constituent. Near infrared electronic absorption spectra of the oxidized heterodimer exhibit a weak band at 900 nm, characteristic of a BChl a ·+ species. The (pre)resonance Raman spectrum of this cation radical, excited at 1064 nm, exhibits a 1723 cm −1 band attributable to the C 9 keto carbonyl group of the BChl constituent whose corresponding band in wild type is observed at 1715–1717 cm −1. This 1723 cm −1 frequency for BChl in the oxidized heterodimer, compared to 1691 cm −1 for the reduced state, represents an oxidation-induced increase in frequency of +32 cm −1, similar to what is observed for the one-electron oxidation of chlorophylls in non-protic solvents. The results presented here indicate that the oxidation-induced change in vibrational frequency of the C 9 keto carbonyl group of the BChl in the reaction center is not significantly perturbed by the protein.

The first ENDOR spectrum of naked metal clusters: 7Li 3, 6Li 7Li 2 and 7Li 3(H 2O) x in an adamantane matrix

The electron double resonance (ENDOR) spectra of naked and hydrated neutral homonuclear metal clusters are reported. 7Li 3, 6Li 7Li 2, and 7Li 3(H 2O) x clusters have been prepared in an adamantane matrix using the rotating cryostat variant of the matrix isolation technique. The centre of the ENDOR doublet for the major cluster, 7Li 3, occurs at 46.15 MHz giving an isotropic hyperfine interaction (hfi) of 92.30 MHz which is close to that of 92.70 MHz determined previously from the EPR spectrum of Li 3. Weaker ENDOR transitions from 6Li 7Li 2 give hfis of 37.1 MHz for 6Li and 51.84 MHz for 7Li. These are consistent with the values for 7Li 3 if account is taken of the statistical factor of two favouring the location of the single 6Li nucleus in 6Li 7Li 2 at the terminal positions of the obtuse angled structures in the minima of the ‘Mexican hat’ hypersurface of the cluster. A weaker doublet, centred at 36.59 MHz is assigned to a hydrated cluster complex 7Li 3(H 2O) x . The reduction in the hfi value compared with that in the neutral cluster 7Li 3 indicates a 20% transfer of unpaired electron spin density to the coordinated water molecules. A complex series of doublets centred at the proton frequency (14.31 MHz) indicate the presence of a number of hydrated Li 3 clusters. These first observations of the ENDOR transitions of neutral trimeric metal clusters suggest that it may be possible to observe the spectra of other, larger and more complex, clusters.

Substrate bias effects on the structural and electronic properties of tetrahedral amorphous carbon.

Tetrahedral amorphous carbon deposited by a filtered cathodic arc has a very high unpaired electron spin density of around ${10}^{20}$--${10}^{21}$ spin/g (c.f. ${10}^{18}$--${10}^{20}$ spin/g for a-Si and a-Ge). Trap states associated with such unpaired spins have a detrimental effect on the electronic properties of both amorphous silicon and germanium and it is therefore desirable to reduce them in tetrahedral amorphous carbon (ta-C). In this paper, we report on the effect of a negative substrate bias, applied during deposition, on the electron spin resonance (ESR) of ta-C films and on their structure, bonding, conductivity, and Raman spectra. The results show a slow decrease in the unpaired electron spin density with increasing bias up to -1350 V followed by an abrupt decrease at -1750 V. Electron energy loss and electrical conductivity measurements indicate a steady increase in the ${\mathit{sp}}^{2}$ hydridized bonding in these films with bias, and it is understood that it is the coupling between electrons at these ${\mathit{sp}}^{2}$ sites that leads to a reduction in the ESR signal. Investigations using electron diffraction and Raman spectroscopy show the formation of ${\mathit{sp}}^{2}$-bonded graphitelike planes in the sample deposited at -1750 V, which provides a mechanism for the absence of unpaired electron spins at this deposition bias. \textcopyright{} 1996 The American Physical Society.

Microwave spectrum and molecular structure of the dihydrophosphoryl radical H2PO in the X̃ 2A′ ground electronic state

The dihydrophosphoryl radical H2PO in the X̃ 2A′ ground electronic state was studied by microwave spectroscopy using a source-modulated spectrometer combined with a free-space cell. The radical was generated by dc-glow discharge in a mixture of PH3 and CO2 and spectral lines of H2PO were observed in the 140–380 GHz region. The spectral pattern, including hyperfine structure, suggests that the radical has a pyramidal structure with Cs symmetry. About 1000 spectral lines of H2P16O and 250 lines of H2P18O were measured for the fine and hyperfine structures of rotational transitions up to N=10−9. Nineteen molecular constants including the hyperfine coupling constants of the phosphorus and hydrogen nuclei were precisely determined by a least-squares fit of the observed lines. Molecular structural parameters were derived from the rotational constants: r(PO)=1.4875(4) Å, r(PH)=1.4287(14) Å, ∠HPO=115.52(10)°, and ∠HPH=102.56(14)° with three standard deviations in parentheses. The PO bond is intermediate between the normal single and double PO bonds. The spin density of unpaired electrons was ascertained from the hyperfine coupling constants for the P and H atoms and is consistent with the molecular structure determined.

Study by 31P NMR spin echo mapping of vanadium phosphorus oxide catalysts

Vanadium phosphorus oxides (VPO) containing vanadium ions in the +3 and +4 oxidation states, namely VPO 4, (VO) 2P 2O 7, VOHPO 4 · 0.5H 2O and VO(H 2PO 4) 2 have been characterized using 31P solid state NMR spectroscopy. Because of couplings between the unpaired electrons of V 4+ + or V 3+ ions and 31P nuclei, 31P NMR lines are drastically broadened and shifted by more than 4500 ppm in the case of the V 3+ phases. It is therefore necessary to use the Spin Echo Mapping technique to obtain the complete NMR information. The technique has proved to be particularly interesting as it permits us to distinguish between various V 4+ phases. The observed frequency shift is proportional to the atomic magnetic susceptibility of the material and, therefore, changes with temperature. For V 4+ phases, we have observed that the inverse shift was proportional to temperature in the working temperature range and that good estimations of the Weiss temperatures could be obtained. Moreover, at low temperature, the 31P NMR spin echo mapping of the pyrophosphate (VO) 2P 2O 7 showed four distinct peaks that could be assigned to various unpaired electron spin densities on the phosphorus atoms. This has been correlated with previous structural determinations, particularly the oxidation state of vanadium ions in these solids. We have also observed that the inverse of the line width increased linearly with temperature in the working range. Special attention was given to VPO 4 where both the evolution with temperature of the observed shift and the line width were very different from those observed on V 4+ phases.

Electron Spin Resonance Super-Hyperfine Splitting Study of Polycrystalline Niobocene and Vanadocene Dichlorides

The electron spin resonance (ESR) spectra of niobocene and vanadocene dichlorides were studied on the title compounds prepared as magnetically diluted species in polycrystalline form. The resolved super-hyperfine splitting of anisotropic ESR spectra of both studied compounds was firstly observed. Computer simulation confirmed that this super-hyperfine splitting is due to interaction of unpaired electron with nuclear spin of two chlorine ligands. Average value of super-hyperfine coupling constant is 13.3 MHz for niobocene dichloride. For vanadocene dichloride, it is estimated to be in the range of 6-10 MHz. It corresponds approximately to 10% delocalization of metal unpaired electron spin density onto chlorine ligands in case of niobocene dichloride and to 4-7% delocalization in case of vanadocene dichloride.

Coordination of the Rieske-type [2Fe-2S] cluster of the terminal iron-sulfur protein of Pseudomonas putida benzene 1,2-dioxygenase, studied by one- and two-dimensional electron spin-echo envelope modulation spectroscopy.

One- and two-dimensional (1D and 2D) electron spin-echo envelope modulation (ESEEM) spectroscopy has been used to investigate the ligand environment of the [2Fe-2S] cluster from the terminal dioxygenase (ISPBED) of the Pseudomonas putida benzene dioxygenase complex. The modulation frequencies observed in the 0.5-8.5 MHz region of the Fourier transforms of 1D and 2D ESEEM spectra measured across the electron paramagnetic resonance (EPR) absorbance envelope (from gz through to gx) are consistent with their assignment to two 14N nuclei. Using hyperfine sublevel correlation spectroscopy (HYSCORE), two sets of correlated double quantum transitions sharing the same hyperfine coupling were observed and were identified as being due to the same two 14N nuclei. On the basis of the isotropic hyperfine and quadrupolar couplings estimated for these 14N nuclei [N(1), Aiso = 3.6 MHz and e2qQ = 2.2-2.8 MHz; N(2), Aiso = 4.8 MHz and e2qQ = 2.2-2.4 MHz], the ESEEM pattern of ISPBED is assigned to two histidine nitrogens which are directly coordinated to the reduced iron-sulfur cluster. Bonding parameters of the two [14N]histidine ligands were calculated from these hyperfine couplings. The primary covalent contributions to the hyperfine interaction arise from 14N-to-Fe2+ sigma bonds. For N(1), our analysis of the percentage of unpaired 2s and 2p electrons gave f2s approximately 1.3% and f2p approximately 0.2%, while values of f2s approximately 1.7% and f2p approximately 1.4% were estimated for N(2). Comparison of these values with those determined from electron nuclear double resonance (ENDOR) data of the Rieske-type [2Fe-2S] center of Pseudomonas cepacia phthalate dioxygenase [Gurbiel, R. J., Batie, C. J., Sivaraja, M., True, A. E., Fee, J. A., Hoffman, B. M., & Ballou, D. P. (1989) Biochemistry 28, 4861-4871] indicates an apparent reduction in unpaired electron spin density residing on the two 14N ligands of ISPBED. Analysis of slices of the HYSCORE spectrum has provided evidence for another 14N nucleus (A approximately 1.1 MHz, e2qQ = 3.3 MHz), which we have attributed to a weakly coupled peptide nitrogen, similar to those observed for ferredoxin-type [2Fe-2S] clusters. This type of weak interaction has not been previously described by the detailed ENDOR and ESEEM studies of Rieske-type centers. The resolution of the spectra demonstrates the effectiveness of 2D ESEEM for the disentanglement of multiple hyperfine interactions to metalloprotein centers.

Carbon-13 hyperfine structure of the CCCCH radical

The fundamental (N = 1 – 0) rotational transitions of the ground 2Σ+ electronic state of the four singly substituted 13C isotopomers of CCCCH have been measured by pulsed-jet Fourier transform microwave spectroscopy. In each isotopomer this transition is split into many well-resolved hyperfine components owing to interaction between the electron spin and the molecular rotation, the proton spin, and the 13C nuclear spin. Here, the hyperfine transition frequencies are analyzed with the higher rotational millimeter-wave frequencies described in the previous paper of McCarthy et al. to produce a precise set of rotational, centrifugal distortion, spin-rotation, and hyperfine coupling constants. In particular, the Fermi-contact interaction of the 13C nucleus has been measured at each substituted position, yielding information on the distribution of the unpaired electron spin density along the carbon chain. The Fermi-contact constants, bF(13C), of 396.8(6), 57.49(5), −9.54(2), and 18.56(4) MHz, for successive 13C substitutions starting furthest from hydrogen indicate that the electronic structure is essentially acetylenic with alternating triple and single bonds.

13C hyperfine coupling constants in MuC 60

13C hyperfine coupling constants of the MuC 60 radical have been measured by muon level-crossing spectroscopy, using a dilute solution of 99% enriched 13C 60 in decalin. The signs as well as the magnitudes of the hyperfine constants were determined. The results range from 52.6 to −25.4 MHz and support those calculations which predict an extended distribution of unpaired electron spin density in radical adducts of fullerenes. The hyperfine constants are consistent with published electron spin resonance results for (CD 3) 3CC 60, but contradict a recent report for HC 60, where a considerably smaller value is reported for the largest splitting.

Hyperfine structure in the electronic spectra of the CdH and CdCH3 radicals

Magnetic hyperfine interactions in the ground and first excited states of CdH and CdCH3 were studied using high resolution electronic spectroscopy. Hyperfine splittings associated with the H nucleus were observed in the X state of CdH; hyperfine splittings associated with 111Cd and 113Cd were observed in both the ground and first excited states of CdH and CdCH3. The hyperfine parameters of CdH were found to be similar to the corresponding parameters of CdCH3. Comparison of the ground state molecular hyperfine constants with values determined in an Ar matrix electron paramagnetic resonance (EPR) study [L. B. Knight, Jr. and W. Weltner, Jr., J. Chem. Phys. 55, 2061 (1971)] indicates clearly a shift of unpaired electron spin density towards Cd, in the matrix. The excited state molecular hyperfine constants indicate that the unpaired electron essentially resides in a Cd 5pπ orbital.