Electron spin resonance study of oxygen-17 enriched N,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.     

The dipole moment of water: Stark effects in D2O and HDO

The dipole moment of D₂O has been determined from Stark effect measurements for the 3₁₃–2₂₀ and 4₄₁–5₃₂ microwave transitions as 1·857 ± 0·006 and 1·869 ± 0·005D respectively. A rotational dependence of dipole moment has also been established for HDO through μ_a in the 2₂₀–2₂₁ and 5₃₂–5₃₃ transitions; μ_a was determined as 0·662 ± 0·001 and 0·644 ± 0·001D respectively. The total dipole moment for HDO has been determined from the 3₂₁–4₁₄ transition to be 1·85 ± 0·01D and to lie within 0·1° of the bisector of the HOD angle. High resolution Stark spectroscopy has been performed on the 6₂₄–6₁₅ transition of D₂O with improved precision using the 337 μm emission line of the HCN laser. This experiment has confirmed the dipole results from the microwave work and the frequency of the 6₂₄–6₁₅ transition in D₂O has been determined as 890 395 ± 3 MHz.

The slight increase of dipole moment with deuteration is consistent with the dipole moment for H₂O determined from the dielectric constant. This increase is discussed for the vibrational ground state (as for ammonia) in terms of anharmonicity in the bending vibration. The change of μ with rotational transition is interpreted in terms of large changes in molecular geometry for certain rotational states due to centrifugal distortion.     

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).     

Isotope effects on the degree of order and the deuterium quadrupole coupling constants,as measured by N.M.R. of oriented benzene-d 1, 1,4-benzene-d 2 and 1,3,5-benzene-d 3

Proton and deuteron magnetic resonance spectra of benzene-d ₁, 1,4-benzene-d ₂ and 1,3,5-benzene-d ₃, oriented in various liquid crystal solvents have been analysed. Isotope effects on the degree of order have been measured. The ratios of S-values in the benzene plane are found to be S_xx/S_zz =1·0201 ± 0·0009 in benzene-d ₁ and 1·0389 ± 0·0004 in 1,4-benzene-d ₂ with the deuterons substituted on the z-axis. Deuteron quadrupole coupling constants of 190·4 ± 1·2 kHz and 192·4 ± 1·2 kHz have been obtained for 1,4-benzene-d ₂ and 1,3,5-benzene-d ₃.     

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.     

Formulae which relate pressure,activity and potentials of mean force to sums of integrals over Ursell-Mayer correlation functions of the distribution functions

The pure rotational spectrum in the far-infrared between 30 and 170 cm^-1 and its absolute intensity has been measured for CH₂D₂ in the vibrational ground state by high-resolution interferometric Fourier transform techniques. The analysis of the integrated cross-sections in the essentially water-free spectrum results in an accurate value for the permanent, vibrationally induced ground state electric dipole moment of CH₂D₂|μ₀| = (6·40±0·33) x 10^-3D.The influence of centrifugal effects on intensities and on the determination of the permanent dipole moment was investigated. Although centrifugal effects are important for the explanation of single band profiles, they appear to be of little relevance for the resulting permanent dipole moment. A new, more general 9- dimensional dipole moment function for methane is derived from ab initio calculations and experimental band strength information of CHD₃. Quantum Monte Carlo calculations using this function and a new, more general 9- dimensional analytical, anharmonic potential function for methane yield a semi-theoretical estimate μ₀ ^z = – (7·8±0·5)x10^-3D for CH₂D₂.     

Radioactive decay of Os191 and Os191m

Beta and gamma spectra of Os¹⁹¹ were studied using a magnetic double-focusing beta-spectrometer and a scintillation spectrometer. The isomeric state Os^191m decays through the 74·4 ± 0·1 keV (E3/M4=50) transition with a half-lifeT _1/2=13·0 ± 0·5 hours. A continuous beta spectrum withE _max=147 ± 3 keV and the gamma transitions 41·83 ± 0·05 keV (E3), 82·5 ± 0·3 keV and 129·4 ± 0·1 keV (70%M1 + 30%E2) were observed in the decay of the ground state of Os¹⁹¹. The conversion coefficient of the last transition was determined as 1·94 ±± — 0·10. Gamma transitions with energies of 47 keV and 185·8 keV were not observed.     

The infra-red spectra of silyl fluoride and silyl fluoride-d 3

Infra-red spectra have been recorded for silyl fluoride and silyl fluoride-d ₃ at a resolution of circa 0·3 cm^-1. Rotational structure has been observed for parallel fundamentals in both molecules, and for all perpendicular fundamentals. In both SiH₃F and SiD₃F the A ₁ and E species deformation modes interact strongly via a Coriolis perturbation; this has been analysed, and the band origin of v ₅ for SiH₃F is reassigned. A hybrid-orbital force field based on the experimental data is also reported.     

Measurement of intensities of multiplets in the 2ν3-band of methane at low temperatures

The integrated intensities of the multiplets P(1)–P(10), R(0)–R(9), and of the Q-branch in the 2ν₃-band of ¹²CH₄ have been measured at 102°K, 152°K, 202°K, 251°K, and 300°K. Comparison of our data with theoretical line strengths confirms, at all of the temperatures mentioned, the intensity anomalies observed by Margolis⁽⁵⁾ for lines in this band. The integrated intensity of the 2ν₃-band is found to be S_v = (1·76±-0·04)(300/T (°K)) cm^?2 atm^?1.     

Hydrogen bond studies

The deuteron magnetic resonance spectra from partially deuterated single crystals of NaHC₂O₄. H₂O have been studied. The quadrupole coupling tensors for stationary deuterons are determined at about 25°C. The quadrupole coupling constants and the assymmetry parameters η of the tensors for the two deuterons in the water molecule are, 235·2 ± 1·9 and 228·5 ± 1·6 kHz, and 0·09 ± 0·01 and 0·13 ± 0·01, respectively. The principal axes corresponding to the largest components both deviate by 0·9 ± 0·5° from their respective O-H directions. For one of the tensors this angular displacement is in the plane of the H₂O molecule and towards the other O-H direction. The displacement for the other tensor is out of the plane of the H₂O molecule.     

Microwave and Raman spectra,dipole moment,barrier to internal rotation,and vibrational assignment of silylphosphine-d2

The microwave spectra of SiH₃PD₂ have been recorded in the range 26.5–40.0 GHz. Both a- and c-type transitions were observed and assigned. The rigid rotor rotational constants were determined to be A = 37589.06 ± 0.11, B = 5315.70 ± 0.02, and C = 5258.70 ± 0.02 MHz. The barrier to internal rotation has been calculated from the A-E splittings to be 1512 ± 26 cal/mole. The dipole moment components of |μ_a| = 0.22 ± 0.01, |μ_c| = 0.56 ± 0.01, and |μ_t| = 0.60 ± 0.01 D were determined from the Stark effect. By using previously determined microwave data for SiH₃PH₂, several structural parameters have been calculated and their values are compared to similar ones in other compounds. The Raman (0–2500 cm^?1) spectra of gaseous, liquid, and solid SiH₃PH₂ and gaseous SiH₃PD₂ have been recorded and interpreted in detail on the basis of C_s molecular symmetry.     

Amorphous germanium III. Optical properties

The optical properties of thick sputtered films (～30μ) of amorphous Ge, grown with different substrate temperatures (0ˇ-T _sˇ-350°C), were obtained between 0·05 and 4·5 eV by a combination of reflectance, transmittance and ellipsometric measurements. The refractive index at 0·15 eV decreases monotonically with increasing T _s, or equivalently, with increasing density, and is 4·13±0·05 eV in the highest density films. The absorption edge is approximately exponential (10²?α?10⁴ cm^?1) but shifts monotonically to higher energy and increases in slope with increasing T _s. Similarly, the peak in ε₂ grows by about 10% and shifts by about 0·15 eV to higher energies, reaching a maximum of about 23 at 2·90±0·05 eV in the high density films. The peak in the transition strength ω²ε₂ occurs at 4·2±0·2 eV in all films, but increases in magnitude with increasing T _s. The sum rules for n _eff(ω) and ε_0,eff(ω) are evaluated for ▄ω?5 eV and vary monotonically with T _s. These trends are neither compatible with Galeener's void resonance theory nor with changes in the oxygen content of the films, determined by the examination of absorption peaks at 0·053 eV and 0·09 eV. An explanation, suggested here and expanded in I, is based on the observed changes in the structure of the network and voids.     

Mössbauer studies of europium and ytterbium hydrides

Mössbauer absorption spectra were obtained for the 21·6 ke V transition of ¹⁵¹Eu in EuH₂ at various temperatures and for the 84·3 keV transition of ¹⁷⁰Yb in YbH₂ at 4·1°K. The isomer shift of EuH₂ relative to Eu³⁺: Sm₂O₃ is ? 12·1 ± 0·3 mm. sec^?1, and the magnetic hyperfine field equals ? 305 ± 5 kOe at saturation. The Curie temperature is found to be 16·2 ± 0·05°K, and the critical parameters of the transition are D = 1·17 ± 0·02 and β = 0·35 ± 0·01. The magnetic field is perpendicular to the principal axis of the electric field gradient and the values of the quadrupole hyperfine interaction e²qQ₀(3 cos² θ ? 1)/8 is ? 28 ± 4 Mc . sec^?1. A large increase of the resonance area (21%) occurs at the transitio to the ferromagnetic state. The isomer shift of YbH₂ relative to Yb: TmAl₂ is ?0·11 ± 0·01 mm . sec^?1. The value of the quadrupole coupling constant e²qQ_c/4is ? 91·5 ± 2 Mc . sec^?1 and the asymmetry parameter of the electric field gradient equals 0·89 ± 0·05. The data for EuH₂ and YbH₂ is shown to be consistent with the hydridic model for the rare earth hydrides.     

Proton N.M.R. study of the dynamics of the ammonium ion in ferroelectric langbeinite, (NH4)2Cd2(SO4)3

The proton N.M.R. lineshape of polycrystalline Langbeinite, (NH₄)₂Cd₂(SO₄)₃, has been studied in the temperature range 300 K to 1·8 K. The resonance line is motionally narrowed over the entire temperature range, and the low temperature proton line shows clear evidence for tunnelling motion of the ammonium ion between spin-symmetry states. From a computer simulation of the lineshape, we obtain an estimate for the tunnelling splitting parameter, J, of the torsional ground state of the ammonium ion, as 375 ± 125 gauss. For an undistorted tetrahedral crystal field this corresponds to a tunnelling splitting Δ = 4J = 6·3 ± 2·1 MHz.

Pulsed proton N.M.R. studies have also been carried out on the above compound at 30·8 MHz and 48·2 MHz and the spin-lattice relaxation time (T ₁) has been measured by the π - t - π/2 pulse sequence as a function of temperature down to 77 K. At 30·8 MHz, a T ₁ minimum of 13 ms occurs at 105·8 K, and is ascribed to random reorientations of the NH₄ ⁺ ion. An activational energy barrier of 0·74 ± 0·1 kcal/mole and an associated pre-exponential factor of 8·0 × 10^-13 s are calculated for the above motional process, and the value of the activation energy is correlated with the tunnelling splitting of the torsional ground state.

An anomaly in T ₁ has been observed at the ferroelectric Curie point (95 K), indicating the order-disorder nature of the transition. This is the first experimental evidence relating to the nature of the transition in Langbeinite.     

Symmetry enforced gauge invariance of nuclear magnetic shielding constants

E.S.R. experiments performed at 1·3 K by optical detection are reported for the photo-excited triplet state of palladiumporphin in a single crystal of n-octane, and the observation of a level anticrossing signal is described.

In the crystal the orbital degeneracy of the ³ E _u state of the free molecule is lifted by the crystal field and in n-octane the energy difference between the two orbital components |x> and |y> is found to be 58 ± 2 cm^-1. The spinorbit coupling (SOC) and the orbital Zeeman interaction couple the triplet manifolds of |x> and |y>, and for a proper understanding of the magnetic properties of these states it is necessary to work in the basis of the six spin-orbit functions deriving from the ³ E _u state of the free molecule. It is shown that either of the two triplet states can be described by an effective spin hamiltonian of the common form and expressions for the zero-field parameters D and E and the principal values of the g tensor are given. The experimental values of the parameters in the lowest triplet state are D = -24·38 ± 0·03 GHz, |E| = 320 ± 60 MHz, g _‖ = 1·677 ± 0·001 and g _⊥ = 1·989 ± 0·002. The matrix element of the SOC connecting the |x> and |y> triplet manifolds amounts to qZ = 15 ± 3 cm^-1 and the vibronic orbital angular momentum (in units of ?) in the ³ E _u state of the free molecule to qΛ = 1·5 ± 0·3. A tentative value of 0·63 for the orbital reduction factor q is obtained by comparison with a theoretical estimate of Λ. The value of q is indicative of weak Jahn-Teller coupling.     

N.M.R. investigation of critical orientational order fluctuations below the nematic-isotropic transition in liquid crystals

The high-resolution infra-red spectrum of SiH₄ in the region 2101 cm^-1 to 2265 cm^-1 has been analysed. Most of the lines observed have been assigned to transitions of the v ₃ and v ₁ bands of the abundant isotopic species ²⁸SiH₄. The v ₁ band is formally forbidden in the infra-red, but a vibration-rotation interaction between v ₁ and v ₃ lends intensity to the v ₁ transitions. The spectrum has been fitted by diagonalizing the v ₃ = 1 and v ₁ = 1 hamiltonians coupled by the vibration-rotation interaction term. 500 transitions have been fitted with an overall standard deviation of 0·007 cm^-1, using only 15 adjustable parameters (ten in the v ₃ = 1 hamiltonian, four in the v ₁ = 1 hamiltonian, and one interaction coefficient). The calculated intensities are also in good agreement with experiment. Transitions of the other isotopic species ²⁹SiH₄ and ³⁰SiH₄ have also been observed, but these spectra have not been analysed in detail.     

Oscillator strengths from line absorption in a high-temperature furnace—II. The (0,0) band of the B2σ+—X2σ+ transition in CN*

Quantitative high-resolution absorption spectroscopy was applied to the (0,0) violet band of CN. The CN radical was prepared in a furnace at 1421°K containing pure cyanogen gas. Since the calculated CN concentration is dependent on the controversial CN heat of formation, only the relationship, f_υ = 6·84 X 10^-3exp (0·354δ), where f_υ is the excess over the initially assumed ΔH⁰_f(CN) = 100·8 kcal/mole, could be directly determined in this study with an estimated error in f_υ of ±20%. For δ = 0, our f_υ is a factor of 4·8 smaller than an average value of 0·033±0· derived from other measurements. If this latter value of f_υ is assumed, our relationship yields ΔH⁰_f(CN) = 105·3±1· kcal/mole or D₀(CN) = 7·66±0·05 eV. The rotational temperature and line widths for this band were also measured.     

Influence of the surface on the quantum P.E.M. effect in InP

The photoelectromagnetic effect of InP is studied in quantizing magnetic fields at 4·2 K in an energy range 1·4–1·5 eV for linearly polarized light. Depending on the sample surface condition two types of spectral oscillations may appear, those associated with interband transitions between Landau levels or the LO phonon type usually seen in photoconductivity. An analysis of the spectral oscillations gives: E₀ = 1·423±0·001 eV; Δ₀ = 0·102±0·006 eV; hω_L = 0·036 eV.     

Magnetic circular dichroism of the first spin-forbidden transition in 2[Cr(en)3Cl3] . KCl . 6H2O

The longitudinal Zeeman effect of the ² E ←⁴ A ₂ transition of the Cr⁺³ ion in single crystals of 2[Cr(en)₃Cl₃] . KCl . 6H₂O has been measured using circularly polarized light as a function of magnetic field strength between 0 and 160 kgauss at ～80 K. The g factor for the ground state was determined to be g _‖(⁴ A ₂) = 2·01±0·05, in agreement with the E.S.R.-determined value. The ratio between the excited state g values and that of the ground state was determined; using the value g _‖(⁴ A ₂) = 1·99, the g factors g _‖(2) = 2·61±0·04 and g _‖() = 1·47±0·04 were obtained.