Ab initio determination of the torsional spectra of acetic acid

The torsional potential energy surface and the favourite geometries of acetic acid are determined with MP4/cc-p VTZ ab initio calculations. The molecule shows two planar trans and cis conformers whose energy difference is 1882.7 cm^?1. Both minimum energy geometries are separated by a barrier of 4432.1 cm^?1. The most stable trans-conformer shows a quite low methyl torsion barrier of 169.8 cm^?1. The roto-torsional energy levels have been calculated up to J = 10. The two torsional fundamental frequencies of the trans-conformer, the methyl and the OH torsion are 82.857 (A²) and 77.050cm^?1 (E) and 568.532 (A²) and 568.418cm^?1 (E). The V₃ barrier causes a splitting of 0.315cm^?1 in the ground vibrational state where the quartic centrifugal distortion constants have been predicted to be D_J = 90.4kHz, D_JK = ?301.5kHz and D_K = 165.4kHz. Finally, the far-infrared spectra of two isotopomers have been simulated from ab initio calculations.     

Study of the NMR spectra of hydrogen isotopic analogs and estimation of the magnetic moment of the triton

Nuclear magnetic resonance spectra of the hydrogen isotopic analogs TH and TD on the triton resonance and HT, HD, and H₂ on the proton resonance in the gas phase have been studied. The triton and proton spectra from a single sample have been recorded simultaneously by using the inductivity of a common receiver LC circuit. The energies of the interaction between the magnetic moments of the nuclei of the hydrogen isotopic analogs have been determined: J _tp = 299.3(1) Hz, J _td = 45.5(1) Hz, and J _pd = 43.3(1) Hz. The ratio of the resonance frequencies of the HT molecule nuclei: F _t (TH)/F _p (HT) = 1.066693898(2), which is equal to the ratio of the magnetic moments of the nuclei in the bound state, has been obtained. If the value Δσ(TH) = 2.04 × 10⁻⁸ calculated previously is used for the difference in the screening of nuclei in the HT molecule, then the ratio of the magnetic moment of the triton to the magnetic moment of the proton is μ _t /μ _p = 1.066693920(2), where the statistical standard deviation of the data is given in the parentheses in the units of the last digit.     

Temperature dependence of nitrogen and oxygen broadening of the 16O3 ν 3 band

High-resolution Fourier transform absorption spectra of ozone broadened by N₂ and O₂ have been recorded at room temperature and at 225 K. Nitrogen- and oxygen-broadened half-widths and their temperature dependence for respectively 160 ro-vibrational transitions on the ¹⁶O₃ ν ₃ band with a wide range of rotational quantum numbers J (2–48) and K_a (0–8) have been determined.     

Raman study of CF4 in liquid Ar

The rotational behaviour of CF₄ dissolved in liquid argon is derived from the band contour analysis of Raman spectra recorded over the concentration range from 10^-2 mole fraction to 1 mole fraction at 85 K. The angular momentum correlation times have been determined with the help of the J-diffusion model and used to apply the rough hard sphere model. A study of the transition dipole-transition dipole (TD-TD) interaction is made using the v ₃ mode at 85 K for different concentrations.     

Optical spectra of nearest neighbour nickel pairs in KMgF3 and KZnF3 crystals

The absorption, MCD, and emission spectra associated with the nearest neighbour Ni²⁺ pairs in KMgF₃ and KZnF₃ have been studied and analysed on the basis of the theory which includes spin dependent excitation transfer as well as spin independent transfer. It is shown that excitation transfers are important in determining the pair energy levels for the case where the singleion states involve orbital degeneracy. The antiferromagnetic J value of the ground state is determined to be 68 cm^-1.     

General Features of the X Part of an ABX Spin System in Isotropic and Liquid Crystalline Phases as Illustrated by theC–{H} Spectra of 2,2′-Difluorobiphenyl

The¹³C–{¹H} NMR spectra of 2,2′-difluorobiphenyl dissolved in isotropic and liquid crystalline solvents have been obtained and analyzed. They are examples of the X part of an ABX spectrum. It is shown that the spectrum of the isotropic solution yieldsJ_AX,J_BX,J_AB, δ_AB, and δ_X, but only if all the transitions are detected, and that intensities as well as frequencies of the transitions are used in the analysis. It is demonstrated that for 2,2′-difluorobiphenyl this requires that for some of the carbons it is necessary to detect very weak transitions. For the spectra of liquid crystalline solutions of ABX systems it is shown that the dipolar couplingsD_AX,D_BX, andD_ABare obtained only if these couplings are in a certain sensitive range of relative values. The sensitive range can be adjusted by using variable angle sample spinning (VASS). It is demonstrated that VASS spectra taken near the magic angle can be used to obtain the absolute signs of the scalar couplings.     

Hyperfine interactions in the gas-phase E.P.R. spectra of 35ClO (2Π3/2) in the J = 5/2 and J = 3/2 levels

Gas-phase E.P.R. spectra of ³⁵ClO have been measured accurately both in the J=5/2 and J=3/2 rotational levels, using C-band (4·4 GHz) and X-band (9·4 GHz) microwaves, respectively. Special attention has been paid to separate the spectral lines of ³⁵ClO from those of ³⁷ClO. Analysis of the spectra has resulted in the simultaneous determination of the hyperfine parameters h and b and, when combined with a-(b+c)/2 obtained by microwave spectroscopy, the hyperfine parameters a, b and c are all determined. Molecular constants which describe the distribution of the unpaired electron around the chlorine nucleus are derived from them. The molecular g values of the rotational levels indicate the mixing of the ²Π state both with the ²Σ+ and with the ²Δ electronic excited states.     

High-resolution infrared spectrum and analysis of the ν2 band of CF3Cl

The rotational structure of the ν₂ band of CF₃Cl, with natural isotopic abundance, has been investigated using a tunable diode laser spectrometer. The spectra have been obtained for a low-temperature (200 K) sample, to reduce the interfering contributions of hot-band transitions. Due to the very small value of the (ΔA − ΔB) constant for both isotopic species, the K structure of most P(J) and R(J) multiplets is generally not resolvable. Only for CF₃³⁵Cl, the K structure could be resolved for P(J) multiplets with J≥55. Molecular constants for the ν₂ fundamental of both isotopic species have been obtained using least-squares fitting routines in combination with band contour simulations for unresolved K structure.     

Determination of Self-, Air-, and Oxygen-Broadening Coefficients of Pure Rotational Absorption Lines of Ozone and of Their Temperature Dependencies

Room temperature measurements of self-, air-, and oxygen-broadening coefficients are reported for 101 pure rotational absorption lines for ¹⁶O₃ covering a range of 7≤J″≤34 and 3≤K₋₁″≤11 in the spectral region 50 to 90 cm⁻. In addition, the temperature dependence of the coefficients has been determined for most of these lines. A total of 14 high-resolution Fourier transform far-IR spectra (0.004 cm⁻ resolution) of ozone/air/dioxygen mixtures were recorded at various temperatures (212, 252, and 296 K). The broadening coefficients and corresponding temperature exponents were deduced by analyzing all of the 14 spectra simultaneously for each absorption line using a nonlinear least-squares fitting technique. Several sources of systematic errors were taken into account: the values of the partial pressures of the species involved, the value of the total optical path length, the sample temperature, thermal emission from sources other than the spectrometer source (e.g., sample emission), the effect of convolution with the instrument function, and intensity contributions from weak and neighbouring absorption lines. The variation of the determined broadening coefficients and temperature exponents with the rotational quantum numbers J″ and K₋₁″ has been examined. Furthermore the determined pressure-broadening parameters have been compared to the data reported in the 2000 edition of the HITRAN compilation.     

A High-Resolution Fourier Transform Infrared Study of the ν3, ν4, and ν5 Bands of Deuterated Formyl Chloride (DCOCl)

High-resolution infrared spectra of the low-lying ν₃, ν₄, and ν₅ fundamentals of the transient molecule DCOCl are reported. These type-A/B hybrid bands have been analyzed in detail, providing extensive rotational assignments for the DCO³⁵Cl and DCO³⁷Cl isotopomers. The ground state constants have been refined by a simultaneous fit of the available microwave data and FTIR combination differences from the three bands. The excited state constants have been determined by fitting assignments over a wide range of J and K_a values. A small perturbation was found at high K_a values in the ν₄ band and determined to be due to a ΔK_a = −2 interaction with the rotational levels of the 6¹ vibrational state.     

Iterative Lineshape Fitting of MAS NMR Spectra: A Tool to Investigate HomonuclearJCoupling in Isolated Spin Pairs

Possibilities and limitations of iterative lineshape fitting procedures of MAS NMR spectra of isolated homonuclear spin pairs, aiming at determination of magnitudes and orientations of the various interaction tensors, are explored. Requirements regarding experimental MAS NMR spectra as well as simulation and fitting procedures are discussed. Our examples chosen are the isolated³¹P spin pairs in solid Na₄P₂O₇· 10H₂O, (1), and Cd(NO₃)₂· 2PPh₃, (2). In both cases the two³¹P chemical shielding tensors in the molecular unit are related byC₂symmetry, and determination of the orientations of these two tensors in the molecular frame is possible. In addition, aspects of homonuclearJcoupling will be addressed. For 1, both magnitude and sign of²J_iso(³¹P,³¹P) (J_iso= −19.5 ± 2.5 Hz) are obtained; for 2, (J_iso= +139 ± 3 Hz) anisotropy ofJwith an orientation of theJ-coupling tensor collinear, or nearly collinear, with the dipolar coupling tensor can be excluded, while absence or presence of anisotropy ofJwith any other relative orientation of theJ-coupling tensor cannot be determined.     

The relative signs of fluorine N.M.R. spin coupling constants in aromatic compounds

In two disubstituted 1,2,4-trifluorobenzenes, it has been shown by double irradiation that J ₀ ^FF has an opposite sign to J_m ^FF and J_p ^FF.     

High-resolution FTIR spectrum of vinyl chloride: rovibrational analysis of the v 10 and v 11 fundamental bands

The Fourier transform gas-phase infrared spectra of the v ₁₀ and v ₁₁ bands of natural CH₂=CHCl have been measured with a resolution of 0.005 cm^?1 in the frequency range 820–1010 cm^?1. These vibrations of symmetry species A″ give rise to c-type bands and the transitions observed are characterized by δK _a = ±1 and δK _c = 0, ±2. Both J and K structures have been resolved in different subbranches and about 1800 (J ≤ 64, K _a ≤ 13) and 2800 (J ≤ 72, K _a ≤ 14) transitions for the v ₁₀ and v ₁₁ fundamentals, respectively, have been identified for the ³⁵Cl isotopomer. Combined analysis of the assigned data with the available ground state constants allowed the determination of the band origins, rotational and centrifugal distortion parameters for the v ₁₀ = 1 and v ₁₁ = 1 excited states of CH₂=CH³⁵Cl isotopic species. The molecular constants obtained account for slight perturbations in the v ₁₀ vibrational level.     

High resolution spectroscopy of and

Extensions in frequency coverage coupled with sensitive spectroscopic techniques have enabled high resolution measurements of pure rotational spectra of deuteromethane and its 13-C substituted counterpart up to J^′=7. The current work reveals a small inconsistency in previously reported frequency measurements of ¹²CH₃D at J^′=5.     

Electronic structure and indirect spin-spin interactions in bournonite (CuPbSbS3) according to antimony nuclear quadrupole resonance

A complex sulfide CuPbSbS₃ (bournonite) has been studied by the nuclear quadrupole resonance on ^121,123Sb. The temperature dependences of the spectroscopic and relaxation parameters in the temperature range of 10–295 K have been obtained. The crystallochemical features of the environment of the two non-equivalent Sb positions in the unit cell have been revealed from the nuclear quadrupole resonance spectra. The existence of the lattice vibrations with the frequency ω = 110 cm^?1 has been demonstrated on the basis of the temperature dependence of the nuclear quadrupole resonance frequencies. Slow beats have been observed on the decay curve of the spin echo signal. Experimental data have been analyzed in order to reveal the existence of the indirect spin-spin interactions involving Sb atoms. The indirect spin-spin coupling constant has been estimated as J = 2.5 ± 0.5 kHz.     

Thermally stimulated recombination processes and luminescence in Li<Subscript>6</Subscript>(Y,Gd,Eu)(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

The thermally stimulated recombination processes and luminescence in crystals of the lithium borate family Li₆(Y,Gd,Eu)(BO₃)₃ have been investigated. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence spectra), the temperature dependences of the X-ray luminescence intensity, and the glow curves for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃: Eu, and Li₆Gd(BO₃)₃: Eu compounds have been measured in the temperature range 90–500 K. In the X-ray luminescence spectra, the band at 312 nm corresponding to the ⁶ P _J → ⁸ S _7/2 transitions in the Gd³⁺ ion and the group of lines at 580–700 nm due to the ⁵ D ₀ → ⁷ F _J transitions (J = 0–4) in the Eu³⁺ ion are dominant. For undoped crystals, the X-ray luminescence intensity of these bands increases by a factor of 15 with a change in the temperature from 100 to 400 K. The possible mechanisms providing the observed temperature dependence of the intensity and their relation to the specific features of energy transfer of electronic excitations in these crystals have been discussed. It has been revealed that the glow curves for all the crystals under investigation exhibit the main complex peak with the maximum at a temperature of 110–160 K and a number of weaker peaks with the composition and structure dependent on the crystal type. The nature of shallow trapping centers responsible for the thermally stimulated luminescence in the range below room temperature and their relation to defects in the lithium cation sublattice have been analyzed.     

Low temperature laser absorption spectra of methane in the near-infrared at 1.65 μm for lower state energy determination

Direct absorption spectra of the 2v₃ band of methane (CH₄) from 6038 to 6050 cm^-1 were studied at different low temperatures using a newly developed cryogenic cell in combination with a distributed feedback (DFB) diode laser. The cryogenic cell can operate at any stabilized temperature ranging from room temperature down to 100 K with temperature fluctuation less than ±1 K within 1 hour. In the present work, the CH₄ spectra in the range of 6038-6050 cm^-1 were recorded at 296, 266, 248, 223, 198, and 176 K. The lower state energy E″ and the rotational assignment of the angular momentum J were determined by a “2-low-temperature spectra method” using the spectra recorded at 198 and 176 K. The results were compared with the data from the GOSAT and the recently reported results from Campargue and co-workers using two spectra measured at room temperature and 81 K. We demonstrated that the use of a 2-low-temperature spectra method permits one to complete the E″ and J values missed in the previous studies.     

Spin state and exchange in the quasi-one-dimensional antiferromagnet KFeS2

We report the optical spectra and single crystal magnetic susceptibility of the one-dimensional antiferromagnet KFeS₂. Measurements have been carried out to ascertain the spin state of Fe³⁺ and the nature of the magnetic interactions in this compound. The optical spectra and magnetic susceptibility could be consistently interpreted using aS=1/2 spin ground state for the Fe³⁺ ion. The features in the optical spectra have been assigned to transitions within thed-electron manifold of the Fe³⁺ ion, and analysed in the strong field limit of the ligand field theory. The high temperature isotropic magnetic susceptibility is typical of a low-dimensional system and exhibits a broad maximum at ∼565K. The susceptibility shows a well defined transition to a three dimensionally ordered antiferromagnetic state atT _N=250 K. The intra and interchain exchange constants,J andJ′, have been evaluated from the experimental susceptibilities using the relationship between these quantities, andχ _max,T _max, andT _N for a spin 1/2 one-dimensional chain. The values areJ=−440.71 K, andJ′=53.94 K. Using these values ofJ andJ′, the susceptibility of a spin 1/2 Heisenberg chain was calculated. A non-interacting spin wave model was used belowT _N. The susceptibility in the paramagnetic region was calculated from the theoretical curves for an infiniteS=1/2 chain. The calculated susceptibility compares well with the experimental data of KFeS₂. Further support for a one-dimensional spin 1/2 model comes from the fact that the calculated perpendicular susceptibility at 0K (2.75×10⁻⁴ emu/mol) evaluated considering the zero point reduction in magnetization from spin wave theory is close to the projected value (2.7×10⁻⁴ emu/mol) obtained from the experimental data.     

Measurement ofJand Dipolar Couplings from Simplified Two-Dimensional NMR Spectra

Simple procedures are described for recording complementary in-phase and antiphaseJ-coupled NMR spectra. The sum and difference of these spectra contain only the upfield and the downfield components of a doublet, making it possible to measure theJsplitting directly from these combinations without an increase in resonance overlap relative to the decoupled spectrum. The approach is demonstrated for measurement of¹J_NHsplittings and²J_HNC′splittings in oriented and isotropic ubiquitin. Dipolar couplings obtained from differences in the splittings measured in the oriented and isotropic phases are in excellent agreement with dipolar couplings obtained from direct measurement of the splitting or from a conventional E.COSY-type measurement.     

Molecular orbital calculations of hydrogen-hydrogen coupling constants in substituted cyclopropanes

The SCF finite perturbation theory of indirect spin-spin coupling constants was applied to ² J _HH and ³ J _HH in a series of mono-substituted cyclopropanes, C₃H₅X. where X=Li, H, CN, CO₂H, COCH₃, NH₂, OH and F. The calculated results are qualitatively consistent with the experimental patterns exhibited by ² J _HH and ³ J _HH. Problems were encountered for substituents for which through-space interactions may be important, particularly if X is a -I^- substitutent.