Electron spin resonance investigation of carbon-13 hyperfine interactions in nitrobenzene anion radical

Synthesis of carbon-13 enriched nitrobenzene which contains 22·5 mole per cent of nitrobenzene-1-¹³C and an equal amount of nitrobenzene-4-¹³C has been accomplished. We report the carbon-13 hyperfine interactions in the corresponding anion radicals in a variety of solvent media. In hexamethylphosphoramide a _C1 = -7·05 gauss and a _C4 = 6·14 gauss. These couplings change to -9·03 gauss and 5·31 gauss, respectively, when the solvent medium is dimethylformamide containing 0·598 mole fraction of water. The solvent dependence of a _C1 is consistent with the radical remaining planar as opposed to adopting a pyramidal conformation at the nitrogen when hydrogen bonds form between the radical anion and protic solvents. Data reported here provide an experimental means to estimate the spin density distribution in nitrobenzene anion.     

Electromigration and permeation of hydrogen and deuterium in silver

A steady state flow technique was used to measure the effective charge number (Z*) and permeability (N) of hydrogen and deuterium in silver. Over the range of temperature (485–720°C) and pressure (220–750 torr) the effective charge number is a constant. The interstitial impurity migrates in the direction of the electron wind with Z_H* = ?6·8 and Z_D* = ? 18. The values of Z* are of the same order as self-electromigration but the size of the isotope effect is surprising. The quantum theories used to explain the isotope effect for hydrogen electromigration in Fe and Ni appear to fail here. In order to determine the effective charge number is was necessary to measure the permeability. For both H₂ and D₂, the permeability in silver follows the equation N = N_O exp(? Q/RT) where N_0D = 2·39 ± 0·40, Q_D = 14400 ± 300, N_OH = 2·86 ± 0·70 and Q_H = 14200 ± 500. Here Q is in units of cal/mol and N is in units of cc(ntp)/(sec - atm² - cm) The isotope effect ratio N_H/N_D = 1·25 was smaller than the classically expected value of (2)^1/2, but could be explained by the theory of Ebisuzaki, Kass and O'Keeffe.     

Optical and Zeeman studies of the first excited singlet state of zinc porphin in a single crystal of n-octane: Evidence for Jahn-Teller instability

The absorption and fluorescence spectra of Zn porphin in an n-octane single crystal at 4·2 K are reported in the region between 17 400 and 18 500 cm^-1. A strong peak appears in both spectra at 17 961 cm^-1 and is assigned to the origin of one component (|x, 0>) of the nearly degenerate Q-band. In absorption a second strong line occurs at a frequency δ = 109 cm^-1 above the first; a corresponding line is almost totally absent in the emission spectrum at 4·2 K, but it appears as a hot band of appreciable intensity when the temperature is raised to 80 K. This feature is assigned to the origin of the other component (|y, 0>) of the Q-band. The lifting of the degeneracy of the Q-band is interpreted as a crystal field splitting of the Jahn-Teller unstable ¹ E_u state.

The Zeeman effect is investigated for the 0–0 transition of the phosphorescence spectrum and the |x, 0> and |y, 0> components of the Q-band absorption spectrum. From the phosphorescence experiment it is concluded that the great majority of the ZnP guests are oriented in the host with an angle of about 25° between the out-of-plane molecular axes and the crystal a-axis. The analysis of the Zeeman effect in absorption (H//crystal a-axis) is complicated by the Jahn-Teller instability which causes two additional unknowns to appear in the problem: the frequency ν and the nuclear displacement parameter α of the active mode. When not making an assumption about these parameters one can only derive a lower limit for the matrix element of the orbital angular momentum between the two electronic components: Λ > 4·6. If is identified with the low-frequency mode of 180 cm^-1 appearing in the absorption spectrum, then it follows that Λ = 6·1 ± 0·6 with α = 1·4 ± 0·1.     

E.S.R. of Cl17O2

The E.S.R. spectrum of ¹⁷O-labelled ClO₂ was studied in an X-irradiated single crystal of KClO₃ and in toluene solution. The isotropic hyperfine constant measured in solution is 12·0 gauss and allows the correct choice of signs to be made in analysing the dipolar tensor in the solid. The values of the anisotropic tensor indicate a spin density of 0·27 on each oxygen.     

Negative Ions,Ozone, and Metastable Components in dc Oxygen Glow Discharge

In a dc glow discharge in oxygen, the concentrations of minor components of O₂(a¹Δ_g), O₂(b¹ Σ_g), O₃, O(¹D), as well as nagative ions and electrons have been measured. Balance equations have been derived which describe satisfactorily the stationary concentrations of these components as functions of gas pressure and discharge current. For the first time, the rate constants of important aeronomical reactions (a) O^? + O₂(a¹Δ_g) → O₃ + e, (b) O₂^? + O₂(a¹Δ_g) → 2O₂ + e and (c) e + O₃ → O₂^? +O have been measured as functions of gas temperature T and mean energies of ions E_i and electron E₆: K_a = (2.5 ± 0.5) · 10^?9 · (T/300)^{4 ± 0.4}· (E_i/0.04)^{?2.6 ± 0.4} cm³/s for T = 385?605 K and E_i = 0.10 ? 0.66 eV; K_b = (1.0 ± 0.3) · 10^?10 · (T/300)^{?2 ± 0.5} · (E_i/0.04)^{0.23 ± 0.05} cm³/s for T = 330?605 K and E_i = 0.09 + 1.5 eV; K_c for E_e = 0.8÷5 eV.     

Estimation of hyperfine coupling constants of 29Si nuclei in the radical anions of trimethylsilyl-substituted π-electron systems

An attempt has been made to relate the coupling constant a _Si of a ²⁹Si nucleus in the radical anions of nine trimethylsilyl derivatives of π-electron systems to the spin populations ρ _Si and of the silicon and the substituted carbon centre , respectively. For the parameter Q _CSi of the relation an absolute value of circa 20 gauss is found, whereas Q _Si appears not to be significantly different from zero. The coupling constants a _Si calculated by means of this relation agree moderately well with the experimental values (standard error circa 0·6 gauss). An HMO model in which the trimethylsilyl substituents are treated as pseudo-π-centres, with the parameters h _Si = -1·5 and k _CSi = 0·55, reproduces satisfactorily the spin distribution in the corresponding radical anions.     

Measurement of vibrational energy transfer of OH (A2Σ+,v′=1→0) in low-pressure flames

2 Σ⁺) was measured in a low-pressure H₂/O₂ flame for three rotational levels of OH (v^′=1). Rate coefficients for collisions with H₂O and N₂ were determined. At 1600 K, k_VET (N₂) is (in 10^-11 cm³s^-1) 10.1±2, 6.1±1.8, and 3.8±1.3 for N^′=0, 5, and 13, respectively. The k_VET (H₂O) is <1.1±1.8. The k_Q (N₂) is <2.4±8 for both vibrational levels. The k_Q (H₂O) in v^′=1 is 59.1±6.5, 54.7±6.4, and 54.9±6.6 for N^′=0, 5, and 13, respectively, and, determined indirectly, 74.6±10.4, 70.6±10.3, and 63.4±7.3 for N^′=0, 5, and 13 in v^′=0. A multi-level model of OH population dynamics, which is being developed for the quantitative simulation of experimental LIF spectra, was extended to include VET. It was attempted to simulate state-to-state-specific VET coefficients for N₂ collisions. From these simulations it appears that angular momentum conservation does not determine the N dependence of the vibrational relaxation step. Received: 9 September 1996/Revised version: 6 January 1997     

E.S.R. and ENDOR of an α-chloro radical in X-irradiated 5-chlorodeoxyuridine single crystals

The most prominent radical formed from X-irradiation of the nucleic acid constituent analogue 5-chlorodeoxyuridine at room temperature is shown to be an α-chloro radical formed by hydrogen addition to C₆. The E.S.R. analysis of the ³⁵Cl hyperfine interaction combined with theoretical simulation of the spin hamiltonian yields tensor components a_xx =46·98 MHz, a_yy =-10·98 MHz and a_zz =-17·01 MHz with a quadrupole coupling constant of eqQ=72 MHz. The principal g-tensor values are g_xx =2·0012, g_yy =2·00862 and g_zz =2·00687. Three additional hyperfine interactions in the radical are observed combining E.S.R. and ENDOR spectroscopy. Besides the two nearly equivalent β-protons on C₆ with principal values of 103·39 MHz and 110·12 MHz, there is hyperfine interaction with the ¹⁴N nucleus of nitrogen N₃ (a_xx =9·81 MHz, a_yy =a_zz ? 0 MHz) and with the proton of the hydrogen bonded to N₃. The latter interaction has tensor components of 2·65 MHz, -10·80 MHz and -8·09 MHz as obtained from ENDOR data. The chlorine hyperfine coupling parameters are related to those observed in other α-chloro radicals. The mechanism of the formation of the radical in 5-chlorodeoxyuridine is discussed briefly.     

Anharmonic force constants of GaSb from the measured third order elastic constants at 300°K

The third-order elastic constants of single crystal GaSb are determined using ultrasonic pulse interferometer at 10 MHz. The constants at 300°K, in units of 10¹¹ N.m.⁻², are C_l11 = ™ 4 ·75 ± 0·06 C₁₄₄ = + 0·50 ± 0·25 C₁₁₃ = ™ 3 ·08 ± 0·02 C₁₆₆ = ™ 2·16 ± 0·13 C₁₂₃ = ™ 0 ·44 ± 0·29 C₄₅₆ = ™ 0·25 ± 0·15 These constants are used to evaluate the three anharmonic first and second neighbour force constants based on modified Keating’s model. The constants are (in units of 10¹¹ N.m⁻²)γ=− 2·406;δ=0·407;ε=−0·222.     

Optically detected E.P.R. and low-field ENDOR of triplet benzophenone

The ENDOR spectrum of ¹³C substituted (carbonyl carbon) triplet excited benzophenone was studied at ca. 100 G applied magnetic field by means of optical detection. The benzophenone was substitutionally dissolved in dibromodiphenylether (DDE). The ENDOR transitions and the E.P.R. transitions were studied as a function of applied field direction in the ab and bc crystal planes. The angle ? between the line joining the phenyl group centres and a principal axis of the fine structure tensor was found to be 23·6° ± 0·02° (standard deviation limited to one set of measurements). The principal axes of the ¹³C hyperfine tensor were within experimental error (±10°) coincident with the fine structure axes. The principal values of the hyperfine tensor were found to be : |A_xx /h| = 43·16 ± 1·84, |A_yy /th| = 22·66 ± 2·50, |A_zz /h| = 10·25 ± 1·30 MHz. The low-field ENDOR does not provide an indication of the signs of these tensor elements, so the value of the isotropic coupling constant cannot be measured. Two values of the anisotropic (dipole) hyperfine tensor elements were deduced on the assumption that the transitions frequencies are mainly determined by interactions between the nuclear spin and electron density on the carbonyl carbon atom. These values were (i) A _xx / ^d /h = 17·82, A_yy /h = -22·68, A_zz /h = -15·14 MHz and (ii) A_xx /h = 39·73, A_yy /h = -26·09, A_zz /h = -13·63 MHz. Set (ii) is consistent with recent calculations of the spin density distribution in triplet benzophenone in which the orbital spin densities are 0·64 for the carbonyl carbon π-orbital, 0·17 for the oxygen π-orbital, and 0·88 for the non-bonding orbital on oxygen (all expressed as fractions of one electron).

Isotope effects on the fine structure constants of triplet benzophenone were measured and found to be consistent with the changes occurring in the spin-orbit coupling with the ground state.

The kinetic parameters for the excitation and de-excitation of the triplet substates were deduced from transient ODMR studies of benzophenone in DDE. The T_z spin state is mainly populated (85–88 per cent) and this is also the most strongly radiative state (k_z ^r = 0·88). The steady-state populations of the three triplet levels are similar.     

Mößbauereffekt in Eisenstilben und FeCl2·H2O

Using Mößbauer effect measurements in the temperature range between 3 °K and 310 °K the magnetic fields at the nucleus in iron-stilbene, FeCl₂·H₂O and FeCl₃ are determined to beH _T=0=(250±10) kOe, (252±18) kOe and (468±10) kOe; a Néel-temperature ofT _N=(23±1) °K is measured for iron-stilbene. The electric quadrupole splittings atT=0 °K for iron-stilbene and FeCl₂ ·H ₂ O, ΔE=(+2.52±0.02) mm/sec and (+2.50±0.05) mm/sec, yield electric field gradients at the iron nucleus ofq _z=+9.7·10¹⁷ V/cm² and +9.6·10¹⁷ V/cm², whereq _z⊥H; Debyetemperatures of θ=162 °K and 188 °K are obtained. The energy of the excited 3d-electron levels in iron-stilbene is estimated to Δ₁=309 cm^?1 and Δ₂=618cm^?1 as deduced from the temperature dependence ofΔE. In contrast to the suggestion ofEuler andWillstaedt bivalence of the iron in ironstilbene is found; its composition is shown to be 4(FeCl₂ ·H ₂O)·stilbene.     

The 4 A 2→2 E transitions of 2[Cr(en)3Cl3]KCl. 6H2O in strong magnetic fields

The Zeeman effect of the R absorption lines for the pure compound 2[Cr(en)₃Cl₃]KCl.6H₂O has been measured photographically, using a strong pulsed magnet with fields of up to 200 000 gauss at 77 K. The results indicate g _∥ (⁴ A ₂) = 1·82 , g _⊥(⁴ A ₂) = 1·90 with g _∥(ē) = -1·82 and g _∥ (2ā) = 1·82. The uncertainties are of the order of ± 0·05.     

Zeeman spectroscopy of the 4 Eu → 2 B 1g phosphorescence of copper porphin in an n-alkane single crystal

The phosphorescence spectrum of the metastable ⁴ E_u state of copper porphin in single crystals of n-octane (C₈) and n-decane (C₁₀) has been studied between 2·3 and 35 K, with and without a magnetic field B. The crystal field splitting between the orbital components observed at 35 K is δ = 30·3 ± 0·3 (C₈), 13·8 ± 0·2 cm^-1 (C₁₀). From the Zeeman shifts we derive the effective orbital angular momentum Λ′ = 0·8 ± 0·2 (C₁₀), the spin-orbit coupling parameter |Z′| = 1·5 ± 1·0 cm^-1 (C₁₀), the spin-spin dipolar interaction parameters D = -0·1 ± 0·2 cm^-1 (C₈, C₁₀) and |E| = 0·31 ± 0·03 cm^-1 (C₈, C₁₀), and the g-factors g _∥ = 2·14 ± 0·04 (C₈, C₁₀) and g _⊥ = 2·00 ± 0·03 (C₈, C₁₀).     

Hyperfine coupling constants and their dependence on charge densities

In paramagnetic aromatic systems the relation between the hyperfine coupling constants a _H for the protons and the spin density ρ on the adjacent carbon atom is usually given by the relation a _H = -Qρ (McConnell) in which Q is a constant characteristic for the C-H bond. In the paramagnetic ions of aromatic molecules the carbon atoms have in general besides a non-zero spin density also a non-zero excess charge density.

If one takes into account both the effect of the spin density ρ and the excess charge density ε on the C-H bond, the following formula is obtained for the proton hyperfine coupling constant:

in which Q and K are predicted to be positive constants of the same order of magnitude.

Comparison with experimental data gives Q = 31·2 gauss; K = 17 gauss.     

Kinetics of formation of O2(1Σ) molecules in reactions involving excited O2(1Δ) oxygen molecules and I(2P1/2) iodine atoms

The results of our experimental study of the kinetics of formation of O₂(¹Σ) molecules in energy-exchange reactions O₂(¹Δ) + I(⁵ p,² P _1/2) and O₂(a,¹Δ) + O₂(a,¹Δ) are presented. The ratio of rate constants was obtained for these reactions (4800 ± 300). Setting the rate constant of the deactivation of O₂(¹Σ) molecules on CO₂ molecules at 4.1 · 10^–13 cm³/s, we evaluated the rate constants for these reactions at a temperature of approximately 330 K: (1.7 ± 0.2) · 10⁻¹³ and (3.6 ± 0.5) · 10⁻¹⁷ cm³/s, respectively.     

The temperature dependence of the spectral width of singlet oxygen dimole emission

In the temperature range of T = 150–400 K, the dependence of spectral widths (cm⁻¹) on temperature, 182 + 0.38(±0.01)T and 217 + 0.48(±0.01)T, respectively, has been obtained for dimole emission of O₂(a, 0) + O₂(a, 0) → O₂(X, 1) + O₂(X, 0) + hν (λ = 703 nm) and O₂(a, 0) + O₂(a, 0) → O₂(X, 0) + O₂(X, 0) + hν (λ = 634 nm). It was shown that the ratio of dimole emission rate constants does not depend on temperature in the range of 150–400 K and is 1.06 ± 0.01.     

The far infrared spectrum of 14N16O2

High resolution Fourier transform spectra in the 8–200 cm^-1 spectral region have been used to analyse the pure rotation spectrum of nitrogen dioxide. In this way, the spin rotation levels of the (000) state were accurately measured for K_a up to 14 and N up to 54. Using a hamiltonian which takes the spin-rotation and the hyperfine operators explicitly into account, it has been possible to derive a complete set of molecular parameters (rotational, spin-rotation and hyperfine constants) for the (000) state of ¹⁴N¹⁶O₂ from these experimental data and from the available microwave measurements. Numerous perturbations due to the hyperfine Fermi contact operator were analysed as well as a local resonance [42 0 42, J = 41·5] ? [41 2 40, J = 41·5] due to the electron spin-rotation interaction. Finally, a synthetic spectrum of the (000) ← (000) band of ¹⁴N¹⁶O₂ including all hyperfine transitions has been computed, covering the 0–235 cm^-1 spectral region.     

The v 1 and v 3 bands of ND3

High resolution (0.004cm^-1 instrumental bandwidth) interferometric Fourier transform infrared spectra of ¹⁴ND₃ were obtained on a BOMEM DA002 spectrometer under essentially Doppler limited conditions. An analysis is reported of the ND₃ stretching fundamentals with band centres at [EQUATION]₁ ⁰ (s ← a) = (2420.056 ± 0.001)cm^?1, [EQUATION]₁ ⁰(a ← s) = (2420.650 ± 0.001)cm^?1, [EQUATION]₃ ⁰(a ← a) = (2563.8840 ± 0:0005)cm^?1 and [EQUATION]₃ ⁰ (s ← s) = (2563.9161 ± 0.0005)cm^?1, with inversion tunnelling splittings Δ[EQUATION]₁ = 0.5412cm^?1 and Δ[EQUATION]₃ = 0.0209cm^?1 in the vibrationally excited levels. About 50 parameters of the effective Hamiltonian for this band system could be determined accurately. Assignments were established with certainty by means of ground state combination differences. The results are important for and are discussed in relation to the mode selective inhibition and promotion of inversion at the nitrogen atom by exciting ND stretching vibrations, and treatments of isotope e? ects on inversion of ammonia by means of effective Hamiltonians and true molecular Hamiltonians on high dimensional potential hypersurfaces.     

The rate constants of singlet oxygen collision-induced emission at 634 and 703 nm wavelengths

Absolute spectral luminosity from an O₂–O₂(a)-H₂O gas flow formed by a chemical singlet oxygen generator was measured at 600–800 and 1230–1310 nm wavelengths. The results were used to determine the rate constants for O₂(a, 0) + O₂(a, 0) → O₂(X, 0) + O₂(X, 0) + hν (λ = 634 nm) and O₂(a, 0) + O₂(a, 0) → O₂(X, 1) + O₂(X, 0) + hν (λ = 703 nm) collision-induced emission ((6.72 ± 0.8) × 10⁻²³ and (7.17 ± 0.8) × 10⁻²³ cm³/s, respectively).