McConnell Relation Research Articles

Effects of Restricted Rotations and Dynamic Averaging on the Calculated Isotropic Hyperfine Coupling Constants of the bis-Dimethyl and bis-Di(trifluoromethyl) Nitroxide Radicals

The rotational effects of the CH(3) and CF(3) groups on the electronic structure and nuclear hyperfine coupling constants (HFCCs) in dimethylnitroxide (DMNO*) and ditrifluoro-methynitroxide (TFMNO*) are investigated using the UB1LYP hybrid density functional method. The CH(3) and CF(3) HFCCs of both radicals are found to obey the McConnell relation during rotation. The two CH(3) groups of the DMNO* do not gear with each other, but the rotation of the first CH(3) group induces only a small rocking effect ( approximately 7 degrees ) in the second group. However, in TFMNO*, the fluorine atoms from different CF(3) groups are close enough so that the steric repulsion between them causes them to act as two interlocked gears, where one drives the other. Therefore, both CF(3) groups undergo full rotation. To the best of our knowledge, this is only the second example of "gearing" to be studied. Stabilization due to hyperconjugation is also a major factor that affects the magnitudes of the HFCCs of the CF(3) groups during rotational averaging. Stable configurations at specific CF(3) group orientations have a large overlap with the NO pi-electron cloud because the lobes of the hybridized p(sigma)(F(2)), p(sigma)(F(3)), p(sigma)(F(5)), and p(sigma)(F(6)) orbitals along the F-C bonds have cylindrical symmetry and are of the correct phases for hyperconjugation to occur. The calculated TFMNO* C(1)-N and C(2)-N bond orders range from 0.91 to 0.95 as the CF(3) groups are rotated. Therefore, the C-N bonds are essentially single bonds. This, in conjunction with the low rotational energy barrier of approximately 50 cm(-1), explains why the EPR intensities of the (19)F hyperfine splittings, in the range of 163-297 K, are characteristic of six equivalent rapidly rotating fluorine atoms. The TFMNO* out-of-plane NO vibrations induce additional s character at the (14)N nucleus. This increases the magnitude of the (14)N HFCC and decreases the (19)F HFCCs. As the temperature increases and because of mixing of the first excited out-of-plane vibrational state, the NO vibrational amplitudes also increase. This leads to an increased (14)N HFCC and decreased (19)F HFCCs, which is in agreement with experiment.

Identification, Characterization, and Structure/Function Analysis of a Corrin Reductase Involved in Adenosylcobalamin Biosynthesis

Vitamin B(12), the antipernicious anemia factor, is the cyano derivative of adenosylcobalamin, which is one of nature's most complex coenzymes. Adenosylcobalamin is made along one of two similar yet distinct metabolic pathways, which are referred to as the aerobic and anaerobic routes. The aerobic pathway for cobalamin biosynthesis proceeds via cobalt insertion into a ring-contracted macrocycle, which is closely followed by adenosylation of the cobalt ion. An important prerequisite for adenosylation is the reduction of the centrally chelated metal from Co(II) to a highly nucleophilic Co(I) form. We have cloned a gene, cobR, encoding a biosynthetic enzyme with this co(II)rrin reductase activity from Brucella melitensis. The protein has been overproduced, and the resulting flavoprotein has been purified, characterized, and crystallized and its structure determined to 1.6A resolution. Kinetic and EPR analysis reveals that the enzyme proceeds via a semiquinone form. It is proposed that CobR may interact with the adenosyltransferase to overcome the large thermodynamic barrier required for co(II)rrin reduction.

The McConnell Relation of Newly Modified Indo‐Mo Method

AbstractThe s‐p separation model INDO‐MO method has been introduced and developed for open and closed shell problems1–3 In order to demonstrate the importance of σ–π interaction between 2pπ of carbon and 1s of hydrogen in this newly modified MO1 we apply “McConnell Relation”4–7 to a linear correlation calculation 122 data points of 1H hyperfine coupling constant in the cases of free radicals and ions with π type SOMO (singly occupied molecular orbital). The calculated McConnell constant, Q, is equal to −24.6 gauss. The standard deviation and correlation coefficient of this statistical treatment are 1.011 gauss and 0.980, respectively. Most of the results in this calculation are reasonable.

Unrestricted Hartree—Fock calculations of methoxyl-substituted semiquinone radicals and the proton hyperfine coupling constants

Using the ab initio unrestricted Hartree—Fock method, spin densities are calculated for various members of the family of methoxyl-substituted p-semi, respectively, p-hydrosemiquinones. The empirical McConnell relation has been applied to estimate from these results the proton isotropic hyperfine splittings measured in electron spin resonance (ESR) experiments. The principle of additivity, which in general accurately describes the changes in the hyperfine coupling constants is violated on substituting two neighboring ring protons in p-semiquinone by the bulky methoxyl groups according to the experimental results. The breakdown of this concept, however, is not observed when the theoretically calculated spin densities are used. Therefore, solvent effects cannot be excluded to explain the determined anomalous ESR in the case of two adjacent methoxyl substituents in the ring. Furthermore, the general problem of the interpretation of spin-densities obtained from an unrestricted Hartree—Fock wave function which is not an eigenstate of spin is discussed using a projection operator technique. From the results it is obvious that the UHF-wavefunction obtained for all systems almost corresponds to the pure doublet spin state.

E.p.r. and spin echo study of bituminous coal radicals

Electron spin echo spectrometry was employed to isolate the vitrain maceral free radical spectrum in several bituminous coals by means of the relaxation time differences between the macerals suggested by the coal e.p.r. power and temperature dependence. E.p.r. power saturation corrections are necessary to obtain reliable numbers of spins and the results indicate that the density of spins in the fusain maceral may often be apprecialy less than in the vitrain maceral. The bituminous coal vitrain spin relaxation mechanism at 85–100 K occurs by means of electron-nuclear hyperfine interaction. The coal echo decays with a spin-spin relaxation time T 2. Measured T 2 times are 1.0–1.4 μsec in the temperature range 80–120 K. The echo decay is modulated at frequencies of 2.5 and 5 MHz. One type of radical, with whose CH fragments the observed modulation is consistent, is perinaphthyl. Calculations of hyperfine constants of meta and para protons based on solution spectra and the McConnell relation have been made. The observed echo modulations are predicted and the same model predicts most of the ENDOR lines found in an ENDOR study of a Pittsburgh bituminous coal. The experiments provide very strong evidence that the coal radical spin multiplicity is doublet, S = 1 2 , with weak spin-spin couplings of less than a few hundreds of milliGauss. The optimum pulse duration observed for the coal radicals is 1.4 times longer than for a known triplet S = 1 molecule and relaxation to thermal equilibrium occurs with a single exponential dependence characteristic of a doublet manifold of spin states.

Electron spin resonance study of oxygen-17 enrichedN,N-dimethyl-4-nitroaniline radical anion

N,N-Dimethyl-d6-4-nitroaniline enriched to approximately 11 atom. per cent in oxygen-17 has been prepared and the oxygen-17 isotropic hyperfine interaction in the corresponding radical anion has been examined. In dry dimethylformamide solution we find a O = -8·85 ± 0·02 and a N = 11·39 ± 0·10 gauss, while in moist acetonitrile solution a O = -8·82 ± 0·07 and a N = 12·23 ± 0·11 gauss. These results substantiate the recent suggestion that the isotropic oxygen-17 splittings in π-radicals may be described by a single parameter equation analogous to McConnell's relation for aromatic proton splittings, i.e. a O = Q OρO π, where Q O = -41 ± 3 gauss.

Full configuration interaction for the benzyl radical

AbstractThe electronic structure of the benzyl radical in its ground state has been computed using a model Hamiltonian due to Pariser–Parr with full configuration interaction as well as with different truncated configurational sets built on SCF open‐shell orbitals. The correlation energy corresponding to this model was found to be equal to –0.929722 eV. With the singly excited configurations only 18% of this energy is taken into account. By extending the basis to include the doubly excited configurations one can account for 94% of the correlation energy. An analysis of the accuracy of the proton hyperfine splitting calculation caused by inaccurate computation of the wave function is given. If only singly and even doubly excited configurations are taken into account one cannot hope to obtain splittings with an accuracy of more than 0.5 g. Inclusion of triply excited configurations lowers this error by one order. In addition, the use of the simple McConnell relation may lead to an error in splitting calculations of no less than 1.5 g.

Proton Hyperfine Splittings in the Electron Spin Resonance Spectra of Hydrocarbon Ions: A Comparison of the Colpa-Bolton and Giacometti–Nordio–Pavan Relations

The Giacometti–Nordio–Pavan modification of McConnell's relation between proton hyperfine splittings and spin densities has been shown to reduce to a simpler form than that originally proposed. It has been demonstrated further that the Giacometti–Nordio–Pavan modification should not be regarded as arising from a nearest-neighbor effect. A statistical analysis of the available experimental proton hyperfine splittings from even-alternant hydrocarbon ions indicates, however, that the Colpa–Bolton relation is superior to the Giacometti–Nordio–Pavan relation in explaining the differences between proton splittings in corresponding cation and anionx radicals. These results are valid within the framework of Hückel molecular-orbital theory.

Study of spin density distributions in the radical ions of two non-alternant hydrocarbons by unrestricted Hartree-Fock method: Effect of introducing the variation of β and the inclusion of non-neighbour resonance integrals on the spin density values

UHFAA (unrestricted Hartree-Fock incorporating the annihilation of the quartet spin component) calculations of the spin densities have been made for the cation and anion radicals of two non-alternant hydrocarbons, acepleiadylene (I) and acenaphth [1, 2 (a)] acenaphthylene (II). Effect of varying β (variable β approximation) and the inclusion of non-neighbour resonance integrals on spin density values in these systems have been studied. It has been found that with these modified UHFAA procedures the spin densities obtained are not very different from those obtained by the usual UHFAA procedure. It has been found that in these systems the McConnell relation is as good as Colpa-Bolton relation or Giacometti-Nordio-Pavan relation. If the McConnell relation is to be used a higher value for the proportionality constant (Q) is indicated for the cation radical than for the anion radical in these systems. This result is contrary to the theoretical predictions of Higuchi. For the cation radical of (I) it also appears that with the McConnell relation not only a higher Q value is required, but that this must also vary from a position in a five-membered ring to that in a seven-membered ring. This variation is more marked in the cation radical of (I) than in the anion radical of (I).

Spin densities of polyazine anions

Unrestricted Hartree-Fock wavefunctions have been calculated for a number of conjugated azine anion radicals. Pi-electron spin densities are obtained by projection of the desired doublet state. The results suggest that the McConnell relation which gives good results when applied to the proton hyperfine splittings of conjugated aromatic hydrocarbon radicals, may be less reliable when the proton splittings of heterocyclic molecules are considered. Current values of semi-empirical 14N sigma-pi parameters are discussed from this point of view.

Signs of the Isotropic Hyperfine Splittings in the Electron Spin Resonance Spectrum of the Dihydropyrazine Cation Radical

Linewidth studies of the electron spin resonance spectrum of the dihydropyrazine cation radical have been employed to determine the signs of the isotropic hyperfine splittings. It was found that the nitrogen splitting aN is positive and the proton splitting aNHH of the protons bonded to the nitrogen atoms is negative. The sigma—pi parameter QNHH in McConnell's relation for NH bonds (aNHH=QNHHρNπ) was found to be negative.

Molecular Orbital Theory and Hyperfine Splitting in Electron Spin Resonance Spectra of Semiquinones

Molecular orbital calculations of the unpaired electron density in a number of semiquinone ions have been made as a function of a range of values of the Coulomb integral αO for the oxygen atom and the resonance integral βCO between the oxygen atom and the carbon atom. The unpaired electron densities ρi were compared with the experimental hyperfine splitting constants ai arising from the proton bonded to carbon atom i by the use of McConnell's relation ai=Qρi. Excellent agreement is obtained for the proper choice of αO and βCO. In contrast to most experimental tests of molecular orbital theory, the agreement depends rather critically on the choice of these two parameters. The best values found for the parasemiquinone ions are in the neighborhood of αO≅αC+1.2 βCC and βCO≅1.56 βCC. The values of | Q | are close to 22.5 gauss. A different set of parameters is needed for the ortho-semiquinones. The molecular orbital calculation also fits the experimental splitting constants of the ring protons in the positions meta and para to the methyl group of the tolusemiquinone ion, but there is a large discrepancy at the position ortho to the methyl group. The results of detailed examination of a number of spectra of semiquinone ions are also reported.