Fluorine hyperfine coupling constants: an INDO study of the fluorobenzyl radicals

The underlying assumptions of Roothaan's symmetry-restricted SCF method for open-shell systems are considered. A ‘restricted Brillouin's theorem’ is formulated and applied in a discussion of total energy discontinuities arising in restricted SCF calculations of systems which exhibit Jahn-Teller instability, such as the tetrahedral ² T ₂ state of CH₄ ⁺.     

An INDO study of the hyperfine coupling constants and equilibrium geometries of some tetra-atomic radicals

Equilibrium geometries and isotropic hyperfine coupling constants have been estimated for the series of radicals CH _n F_3-n , CH _n Cl_3-n , CCl _n F_3-n and S₁H _n F_3-n using the INDO method. The theoretical hyperfine splittings agree well with experimental values including the unusually large proton splittings of 34·6 G and 89·9 G recently reported for SiH₂F and SiHF₂.     

Semiempirical molecular orbital calculations of anisotropic 1H, 13C and 19F hyperfine coupling constants in hydrocarbon and fluorocarbon radicals

The anisotropic hyperfine coupling constants (AHCC) from the electron spin resonance (E.S.R.) spectra of a variety of atoms in organic radicals have been calculated by means of semiempirical molecular orbital wavefunctions in the INDO approximation. Hyperfine tensors involving ¹H, ¹³C and ¹⁹F nuclei are obtained for the ?H, ?H₃, CH₃?H₂, (CH₃)₃? hydrocarbon radicals, malonic acid radical, ?H₂F, ?F₂H, ?F₃ and CF₃?H₂ radicals. The calculated values are compared with available experimental, non-empirical and semiempirical values for these radicals. All integrals of the operator entering the electronic contributions have been evaluated over Slater type orbitals. The introduction of deorthogonalized wavefunctions gives generally better calculated results. In particular, the tensor components of the ¹⁹F AHCC are in good agreement with the experimental results without the necessity of readjusting the effective nuclear charges.     

Degenerate four-wave mixing diagnostics on OH and NH radicals in flames

Degenerate four wave mixing (DFWM) is applied as a diagnostic to study OH and NH radicals in flames. DFWM is a coherent technique which offers the advantages of a highly collimated signal beam permitting efficient rejection of interfering radiation and requiring minimal optical access. Rotational temperatures have been determined from the DFWM spectra and are in close agreement with the temperatures measured using coherent anti-Stokes Raman scattering of nitrogen.Work performed at the Combustion Research Facility, Sandia National Laboratories, supported by the U.S. Department of Energy, Energy Conversion and Utilization Technologies Program and Office of Basic Energy Sciences, Division of Chemical Sciences     

The inversional dependence of hyperfine quadrupole coupling in 14NH3

A simple theoretical model explaining the inversional dependence of hyperfine quadrupole coupling in ¹⁴NH₃ is presented. The model, when related to available experimental hyperfine splittings, can be used to predict the effective hyperfine quadrupole coupling constants for all (low-lying) ro-inversional levels. The actual predictions reveal that the inversional dependence of the coupling constants is not only rather strong but also non-monotonic.     

The hyperfine coupling tensors of muonated radicals in single‐crystal benzophenone

The hyperfine coupling tensors of muonated radicals in a large single crystal of benzophenone have been determined by measuring the hyperfine couplings of the radicals as the crystal was rotated about three orthogonal axes. Signals from four radicals were observed, corresponding to the four molecules per unit cell in the crystal. Due to an ambiguity in the data we are unable to discriminate between two similar hyperfine coupling tensors. Both lead to an isotropic hyperfine coupling of \sim32 MHz, contrasting with the solution value of 42 MHz obtained earlier in diethyl ether. The anisotropic components of the tensor are relatively large, at approximately -15 MHz, -6 MHz and 21 MHz, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Absolute signs of hyperfine coupling constants as determined by pulse ENDOR of polarized radical pairs

A novel method that allows the determination of absolute signs of hyperfine coupling constants in polarized radical pair (RP) pulse electron-nuclear double resonance (ENDOR) spectra is presented, The variable mixing time (VMT) ENDOR method used here leads to a separation of ENDOR transitions originating from different electron spin manifolds by employing their dependence on the time-dependent parameters of the pulse sequence. The simple kinetic model of the RP VMT ENDOR experiment shows very good agreement with the experiments performed on the P ₇₀₀ ^.+ A ₁ ^.- RP in photosystem I. This method relies on the selective excitation of absorptive or emissive lines of one radical in the RP EPR spectrum and therefore requires high spectral resolution. This condition was fulfilled for the system studied at the low-field edge of the RP EPR spectrum obtained at Q-band. The method presented here has a very high sensitivity and does not require any equipment additional to the one used for RP pulse ENDOR. The VMT ENDOR method offers the possibility for selective suppression of signals from different electron spin manifolds.     

Microwave spectrum of the32S17O radical. Magnetic hyperfine structure and quadrupole coupling constants

Summary The millimetre wave spectrum of the³²S¹⁷O free radical has been observed and analysed. The fine- and hyperfine-structure parameters have been determined. Ther ^s andr ^m structures have been also determined.

---

Riassunto è stato osservato ed analizzato lo spettro millimetrico del radicale libero³²S¹⁷O. Sono stati determinati i parametri di struttura fine ed iperfine e le strutturer ^s er ^m.

---

Резюме Наблюдался д проведен анализ спектра миллиметровых волн свободного радикала³²З¹⁷О. Определены параметры тонкой и сверхтонкой структуры. Также определены структурыr ^s иr ^m.

---

---

To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.

---

The supporting organizations are the CNR and the M.P.I.     

Localizability and coupling constants

Let us consider for free elementary systems the postulates: (i) localizability of systems would not favor in a physical sense some inertial frames and that (ii) standard quantum mechanics (or, at least, a skeleton of it) applies for localizability. It is known that, at least for the lower values of spin, (i) and (ii) imply a unique solution of the localization problem for the no-interaction case. Extrapolating the analysis to the case when interactions are present, we offer arguments in favor of theconjecture that if elementary systems under interaction are localizable, then (i) and (ii) imply restrictions on the coupling constants, and probably their single-valuedness.     

Microwave spectrum,centrifugal distortion constants,and quadrupole coupling constants of 3-chloropyridine

The microwave spectrum of 3-chloropyridine has been measured in the frequency region of 8.2 to 18 GHz. The rotational constants, centrifugal distortion constants, and the quadrupole coupling constants for the ³⁵Cl species are A = 5839.448 ± 0.027 MHz, B = 1604.152 ± 0.005 MHz, C = 1258.327 ± 0.004 MHz, Δ_J = 0.10 ± 0.01 KHz, Δ_JK = 0.36 ± 0.09 KHz, Δ_K = 1.18 ± 0.07 KHz, δ_J = ?0.008 ± 0.005 KHz, δ_K = 0.88 ± 0.20 KHz, χ_aa = ?70.04 ± 0.38 MHz, χ_bb = 36.68 ± 0.19 MHz. The values of rotational constants and quadrupole coupling constants for the ³⁷Cl species are A = 5840.052 ± 0.034 MHz, B = 1559.354 ± 0.01 MHz, C = 1230.739 ± 0.016 MHz, χ_aa = ?54.20 ± 1.26 MHz, χ_bb = 29.49 ± 0.48 MHz. The double bond character in the CCl bond is found to be 2%. The smaller than expected value of rotational constant A points to a “fattening” of the pyridine ring about the a-axis in contrast to 2-chloropyridine, where no such substitution effect was observed.     

Perturbation theory for large coupling constants

The perturbation technique for large coupling constantsg ² is used for obtaining the solutions of Schrödinger equation for a double exponential potential. In particular the solution valid forg ² e ^-r/2 g²e^-r/2 ? 1, is obtained int erms of confluent hypergeometric functions. A noteworthy aspect of this application is that the procedure developed can also be used for solving certain singular potentials.     

Singlet-triplet transitions in substituted benzenes containing OH and NH2 groups

The electronic absorption spectra of substituted benzenes containing OH and NH₂ groups are investigated in the 2500–5000 Å region. It is shown that the absorption band in the long-wave region is a singlet triplet absorption band.     

利用电子自旋共振测量自由基未成对电子的朗德g因子和超精细结构

应用了电子自旋共振技术来研究有机半导体1,1-二苯基-2-三硝基苯肼(DPPH)中自由基未成对电子的超精细结构和塞曼劈裂.通过测量共振磁场大小和微波频率,我们得到了自由基未成对电子的朗德g因子.在实验中,一种混合接头(magic-T)波导管被用来提高信噪比.朗德g因子的测量结果为2.033±0.0003,与其他实验中测得的期望值2.0036相吻合.这个方法可以用以研究各种材料、生物系统中自由基未成对电子,并可进一步应用于研究电子芯片微电感上的自由基.     

Microwave spectrum,quadrupole coupling constants,and barrier to internal rotation of 2-iodopropene

The microwave spectrum of 2-iodopropene has been investigated between 7.7 and 18 GHz. The measured transition frequencies of the ground and two vibrationally excited states have been analyzed using direct diagonalization of the rotational and quadrupole Hamiltonian. The following rotational and quadrupole coupling constants have been determined in a leastsquares fit for the ground state: A = 9285.153(20) MHz; B = 2337.2198(14) MHz; C = 1887.5871(14) MHz; and χ_cc = ?1820.783(33) MHz; χ_ab = 147.5(10) MHz; χ_bb = 957.018(41) MHz; and χ_cc = 863.765(40) MHz. The quadrupole coupling constants have been transformed to their principal axis system. From the splittings of some transitions of the first torsionally excited state a value of V₃ = 905(3) cm^?1 has been found for the threefold barrier hindering the internal rotation of the methyl group.