The IR spectra of DCOOH and HCOOD crystals at low temperatures

We show the IR spectra of HCOOD and DCOOH crystals in the region 1000–3000 cm⁻¹ at 7, 77 and 125 K. This article is a complement to previous articles which described spectra of HCOOH and DCOOD crystals recorded in the same conditions. In this article we also compare the results obtained on the four isotopic species and specially focus our attention on a comparison of the moments or order 0, 1 and 2 of the v_O---H…O (O---H…O) and v_O---D…O bands. Furthermore, we precise some experimental aspects, particularly those concerning polarization effects. In the following paper, we shall deduce from the first moments the relative importance of the different anharmonicities governing the stretching motion of the H or D atoms in the H- or D---bonds.     

Vibrational dependence of the NH3 (v2)+NO and NO(v)+NH3 charge transfer cross sections

A tandem quadrupole mass spectrometer is used to study the charge transfer reactions NH₃⁺ + NO and NO⁺ + NH₃ over a collision energy range 1.5–13 eV. The vibrational state of the reagent ions is selected by resonance-enhanced multiphoton ionization. For the 0.9 eV exothermic process NH₃⁺ + NO → NH₃ + NO⁺ excitation of the v₂ umbrella bending mode (v₂ = 0–12) causes no marked change in the charge transfer cross section, while in the reverse process NO⁺ + NH₃ → NO + NH₃⁺ excitation of the NO⁺ vibration (v = 0–6) strongly enhanced the charge transfer cross section.     

The electronic quenching rates of NO(AΣ, v=0–2)

The electronic quenching rates of NO(A²Σ⁺, v^′=0–2) are measured for the gases He, Ar, Xe, N₂, O₂, CO₂, N₂O, and SF₆. The variations of the fluorescence intensity were measured for the (0,0), (1,0), and (2,0) bands of the γ band system when the quencher gases were added. The quenching rates were determined by using the Stern–Volmer plots with the known radiative lifetimes of the excited states. The electronic quenching rate constants are fast for the group of gases of O₂, CO₂, N₂O, and SF₆, whose quenching rate constants are in the order of 10⁻¹⁰ cm³/s. The quenching rate constants are slow for the group of gases including He, Ar, Xe, and N₂ whose rate constants are in the order of 10⁻¹⁴ cm³/s. For the slow group, the quenching rate constants increase rapidly for v^′=2 compared with those of v^′=0 and 1.

The charge transfer model and collision complex model are used to understand the quenching mechanism. For the fast group which mainly consists of gases with positive electron affinities, the charge transfer model adequately describes the mechanism. For the slow quenching group, a theoretical background is provided by consider the coupling of initial and final states in the complex potential surfaces.     

Étude infrarouge de complexes aldéhyde-trifluorure de bore Rôle des doublets libres de l'oxygène sur la vibration v(CH)ald

The infrared spectra of some aldehyde-BF₃ complexes were measured from 2.5 μ to 19 μ. The intensity and force constant of the v(CH)_ald range is dependent on the lone-pair electrons of the neighbouring oxygen atom. In the BF₃ complexes the intensity of v(CH)_ald is decreased and shifted 150 cm⁻¹ towards higher frequencies, while the band becomes simple. The addition of BF₃ is verified by checking the v(C=O) band, which is shifted 70 cm⁻¹ towards lower frequencies.     

Laser magnetic resonance spectrum of the OD radical (Π3/2, v=0) at 118.6 μm

Far infrared laser magnetic resonance (FIR LMR) spectrum of OD (²Π_3/2, v=0) has been observed. Data are presented for the Zeeman components of the rotational transition J=3.5→4.5 observed at 118.6 μm using a discharge water vapour laser spectrometer. Theoretical values of the transition magnetic field strengths have been calculated using the best available molecular constants. The agreement between theory and experiment confirms the spectroscopic assignments.     

Conformational isomerism of chlorophenylthioureas and intramolecular N---H … Cl hydrogen bonds: IR spectra of N---H stretching vibrations

The vN---H regions of the IR spectra of thioureas with chlorophenyl (ClPh) groups and those with halophenyl groups were measured in dilute CCl₄ solution. The observed vN---H bands were classified into eight groups according to the wavenumbers and the two substituent groups. The suggested conformational states and the formation of intramolecular N---H … Cl hydrogen bonds in these compounds were discussed in comparison with those of the urea analogs. It was found that these thiourea derivatives are more stable in the cis form than the urea analogs and that thioureas with o-ClPh groups form fewer intramolecular N---H … Cl hydrogen bonds than do the urea analogs.     

Geometry and nature of the binding of the pre-reactive complex C2H4…ClF from its rotational spectrum

The encounter complex C₂H₄…ClF was isolated by using a fast-mixing nozzle before chemical reaction could occur between the components and was characterised by Fourier-transform microwave spectroscopy. Rotational constants, centrifugal distortion constants, Cl nuclear quadrupole constants and Cl spin-rotation constants were determined for the isotopomers C₂H₄…³⁵ClF and C₂H₄…³⁷ClF. The complex has C_2v symmetry with the ClF subunit perpendicular to the plane of C₂H₄ and oriented so that Cl is closer to C₂H₄. Both the centrifugal distortion constant Δ_J and the Cl nuclear quadrupole coupling constants indicate that the complex is relatively weakly bound and it is concluded that the interaction between the subunits is largely electrostatic in origin.     

Chain-length and mode-delocalization dependent amide-I anharmonicity in peptide oligomers

The diagonal anharmonicities of the amide-I mode in the alanine oligomers are examined in the normal-mode basis by ab initio calculations. The selected oligomers range from dimer to heptamer, in either the α-helical or β-sheet conformations. It is found that the anharmonicity varies from mode to mode within the same oligomer. For a given amide-I mode, the anharmonicity is closely related to the delocalization extent of the mode: the less it delocalizes, the larger the anharmonicity it has. Thus, the single-mode potential energy distribution (PED(max)) can be used as an indicator of the magnitude of the anharmonicity. It is found that as the peptide chain length increases, the averaged diagonal anharmonicity generally decreases; however, the sum of the averaged diagonal and off-diagonal anharmonicities within a peptide roughly remains a constant for all the oligomers examined, indicating the excitonic characteristics of the amide-I modes. Excitonic coupling tends to decrease the diagonal anharmonicities in a coupled system with multiple chromophores, which explains the observed behavior of the anharmonicities. The excitonic nature of the amide-I band in peptide oligomers is thus verified by the anharmonic computations. Isotopic substitution effect on the anharmonicities and mode localizations of the amide-I modes in peptides is also discussed.     

Hydrogen-bonding effect on C NMR chemical shifts of amino acid residue carbonyl carbons of some peptides in the crystalline state

¹³C cross polarization-magic angle spinning NMR spectra were measured for a series of peptides containing -valine, -leucine and -aspartic acid residues, for which the crystal structures were already determined by X-ray diffraction, in order to investigate the relationship between hydrogen-bond lengths (R_N…O) and ¹³C chemical shifts of amide carbonyl carbons in the peptides. From these experimental results, it was found that the isotropic ¹³C chemical shifts (δ_iso) of the amino acid residues move linearly downfield with a decrease in R_N…O within the hydrogen-bonded length range considered here and also shown in our previous work on glycine and -alanine residues as expressed by δ_iso(ppm) = a − bR_N…O(Å) where a and b are 215.4 (ppm) and 14.2 (ppm Å⁻¹) for the -valine residue, 202.2 (ppm) and 10.0 (ppm Å⁻¹) for the -leucine residue, and 199.0 (ppm) and 9.6 (ppm Å⁻¹) for the -aspartic acid residue, respectively. Using these relations, the R_N…O values of some polypeptides in the crystalline state were determined through the observation of the amide carbonyl carbon chemical shifts. These values were compared with those determined by the X-ray diffraction method. Furthermore, quantum-chemical calculation of the ¹³C shielding constant for a model compound was carried out by the finite perturbation theory INDO method in order to ascertain the ¹³C shielding behavior in the formation of hydrogen bonds.     

Ab initio molecular orbital study of Fe2Cl6 and FeAlCl6

Ab initio molecular orbital theory was used to determine the equilibrium structure and vibrational frequencies of Fe₂Cl₆ and FeAlCl₆. The equilibrium structure the Fe₂Cl₆ dimer has D_2h symmetry with a planar arrangement of the four membered {FeCl_brFeCl_br} ring, similar to the Al₂Cl₆ dimer. The calculated bond distances and vibrational frequencies are in good agreement with experiment. The potential energy surface for the puckering of the {FeCl_brFeCl_br} ring is extremely flat. This prevents an unambiguous assignment of either D_2h or C_2v symmetry to the Fe₂Cl₆ structure in electron diffraction measurements. The FeAlCl₆ molecule is found to have a C_2v structure similar to Fe₂Cl₆ with vibrational frequencies in good agreement with experiment.     

Electric dipole moments of thietane in excited states of the ring puckering vibration: an experimental and ab initio study

The electric dipole moment in the ground state (v_p = 0) and the first five excited states (v_p = 1 … 5) of the ring puckering vibration of thietane have been determined from Stark shifts of rotational transitions. The results are: 0|μ_a|0 = 1.87583(16) D, 1|μ_a|1 = 1.87341(18) D, 2|μ_a|2 = 1.89759(28) D, 3|μ_a|3 = 1.88688(29) D, 4|μ_a|4 = 1.90036(18) D, 5|μ_a|5 = 1.88596(59) D. The dependence of these values on v_p shows the zig-zagging behaviour typical of modes with double minimum potentials. A combined analysis of the ground and first excited states yielded also a precise value for the transition moment, 0|μ_c|1 = 0.24023(49) D.

A potential and electric dipole moment function has been derived from ab initio calculations, using MP2 and the 6–31G** basis set. Expectation values of the dipole and transition moments were determined from these data. Absolute values are about 5% in error, but the variation with vibrational state is reproduced excellently by the theoretical values.     

The vibrational state distributions in both products of the reaction: O(P) + NO2 → O2 + NO

A laser pulse-and-probe method has been used to determine the nascent vibrational populations in NO(v=0–4) and O₂(v=6–11) formed in the thermal reaction: O(³P) + NO₂ → O₂(v) + NO(v). A frequency-tripled Nd: YAG laser is used to photolyse NO₂, diluted tenfold in Ar, and laser-induced fluorescence spectroscopy in the NO A ²Σ⁺-X ²Π and O₂ B ³Σ⁻_u -X ³Σ⁻_g electronic band system is used both to follow the kinetics of individual vibrational states and to determine the nascent vibrational distributions. The majority of the NO product is formed in v = 0 and the average vibrational yield is ≈ 4.6%. The O₂ populations fall monotonically from v = 6 to 11 in a distribution close to what is expected on prior grounds. Based on a surprisal analysis, the average vibrational energy yield in O₂ is ≈ 26%. The nature of the reaction dynamics is discussed.     

The molecular structure of beryllocene, (C5H5)2Be. A reinvestigation by gas phase electron diffraction

The electron scattering pattern of gaseous dicyclopentadienylberyllium, Cp₂Be, has been recorded from s = 2.00 to 39.00 Å⁻¹ with a nozzle temperature of about 120°C. Molecular models of D_5d symmetry or models containing one π-bonded and one σ-bonded Cp ring are not compatible with the data. The possibility the gaseous Cp₂Be consists fo a mixture D_5d and π-Cp, σ-Cp conformers is considered and rejected. A model of C_5v symmetry can be brought into satisfactory agreement with the data. It is also found that a slip sandwich model obtained from the C_5v model by moving sideways the ring which is at the greatest distance from Be, while keeping the two rings essentially parallel is compatible with the electron diffraction data. The best fit between experimental and calculated intensity curves is obtained with a model with a sideways slip of 0.8(1) Å. This model is similar to that indicated by the X-ray diffraction investigations by Wong and coworkers [4,5]. It is suggested that the potential energy of the molecule does not change much as the magnitude of the slip changes and that the molecule thus undergoes large amplitude vibration.     

Energy decay mechanisms and anharmonic lattice dynamics: The case of solid nitrogen

The anharmonic frequencies and linewidths of the lattice phonons in -N₂ are calculated on the basis of three different intermolecular potentials which include atom-atom and electrostatic interactions. The distinction between stationary anharmonicity and decay anharmonicity is stressed and the mechanism of energy transfer between the optical lattice phonons and the two-phonon manifold of the crystal is discussed in detail. The temperature dependence of the phonon self-energy is also considered. The results thus obtained for -N₂ are compared with predictions from previous lattice dynamics. SCP and molecular dynamics calculations. The calculated anharmonic effects are substantially independent of the adopted potential: the agreement with experimental data is reasonably good as far as the linewidths are concerned, while the anharmonic deformation of the potential wells (and thus the frequency shifts) is overestimated. We suggest that, while higher orders in the diagram expansion are necessary for a proper account of the stationary anharmonicity, the decay anharmonicity limits its effectiveness to two-phonon processes, thus allowing proper predictions of the phonon lifetimes by using the lowest-order diagrams. Finally, -N₂ is compared to -CO, and the role played by the translation-rotation coupling is discussed.     

Determinations cinetiques par analyse microcalorimetrique differentielle. XVII. Decomposition spontanee et induites du percarbonate de O,O-t-butyle et O-vinyle en solution

The mechanism of the free-radical decomposition of O,O-t-butyl and O-vinyl peroxycarbonate looks like that of the decomposition of the O,O-t-butyl and O-isopropenyl homolog. In such solvents as triisopropylbenzene, di-n-butyl phthalate or diphenyl oxide oxo-2-ethyl [HC(O)CH₂] and acetyl [CH₃C(O)^·] radicals add themselves to the peroxycarbonate double bond. This addition gives rise to an induced decomposition which is associated with the “spontaneous” thermolysis. In octadecane, the induced process originates in the addition of radicals produced from the solvent. As for its intrinsic thermal behaviour, O,O-t-butyl and O-vinyl peroxycarbonate ranks, before O,O-t-butyl and O-isopropenyl peroxycarbonate or t-butyl peroxybenzoate, among the most stable peroxyesters.     

Time-resolved vibrational cemiluminescence: Rate constants for the reaction F + HC1 → HF + Cl and for the relaxtion of HF(v = 3) by HCl, CO2, N2O, CO, N2 and O2

The reaction: F + HCl→ HF (v 3) + Cl (1), has been initiated by photolysing F₂ using the fourth-harmonic output at 266 nm from a repetitively pulsed Nd: YAG laser By analysing the time-dependence of the HF(3,0) vibrational chemiluminescence, rate constants have been determined at (296 ± 5) K for reaction (1), k₁ = (7.0 ± 0.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, and for the relaxation of HF(v = 3) by HCl, CO₂, N₂O, CO, N₂ and O₂: k_HCl = (1.18 ±0.14) × 10⁻¹¹ k_CO2 = (1.04 ± 0. 13) × 10⁻¹², k_N2O = (1.41 ± 0.13) × 10⁻¹¹ k_CO = (2.9 ± 0.3) × (10⁻¹², k_N2 = (7.1 ± 0.6) × 10⁻¹⁴ and k_O2 = (1.9 ± 0.6) × 10⁻¹⁴ cm³molecule⁻¹s⁻¹.     

The orientation—inversion motions of ammonia trapped in matrix an inertial model

Good doorway stationary states for the inversion and orientation of ammonia trapped in rare gas and nitrogen matrices are determined in order to interpret the narrowing of the v₂ inversion doublet and the hindering of the molecular rotation. A pseudomotional crystal model (MC) is developed to describe the coupling between the optical (inversion + orientation) modes and the lattice vibrations. With respect to the rigid crystal model already studied, the MC model introduces additional statical, dynamical and kinetical contributions in the optical mode hamiltonian. Among these additional terms, the cubic and quartic anharmonicities of the motion of the molecule center of mass, the effective mass corrections and the damping due to the matrix surrounding are shown to be very different in the three matrices Ar, Xe and N₂. Numerical results on the inversion doublet splitting and on the rotational level spacing of ammonia agree with the observed data.     

Two-colour bound—free—bound spectroscopy of the [E1/2]c6S Rydberg states of CH3I and CD3I

In this resonantly enhanced multiphoton ionization (REMPI) experiment, an extended vibrational progression in the CI stretching mode (v₃) of methyl iodide (-h₃ and -d₃) is observed in the 1 + 1′ excitation of the [1/2] 6s; 0 Rydberg state when the pump photon wavelength lies in the bound → free absorption continuum. This is in contrast with one-colour coherent (non-resonant) two-photon excitation, where the v₃ mode is not excited. By working at several different fixed probe wavelengths and scanning the pump frequency, the relative contributions from the three intermediate repulsive states can be explored through changes in the relative strengths of the Ω = 0 and 1 components of the final Rydberg states. Extensive predissociation in the Rydberg states curtails the vibrational progression.     

Multidimensional anharmonic calculation of the vibrational frequencies and intensities for the trans and cis isomers of HONO with the use of normal coordinates

The equilibrium geometry and the potential energy and dipole moment surfaces have been determined for the cis and trans isomers of the HONO molecule by an ab initio Moller–Plesset (MP2) calculation with a wide set of atomic orbitals. The multidimensional anharmonic vibrational Schrodinger equations are solved using the variational method with the Hamiltonian and wave functions written in the normal coordinates of cis and trans isomers. All one- and two-dimensional and a number of three-dimensional vibrational problems are solved to obtain the energy levels and vibrational eigenfunctions. The frequencies and intensities for the fundamental, overtone and some combination bands are determined in good agreement with the available experimental results. The calculation shows the strength of coupling between different vibrational modes and reveals the presence of strong resonances between the (v₁, v₃, v₆) and (v₁, v₃−1, v₆+2) states of cis-HONO. This fact may be important for understanding the energy redistribution between the intermolecular degrees of freedom. The magnitude and direction of vibrationally averaged ground-state dipole moment of both isomers, as well as the direction of transition dipole moments, are in good agreement with the experimental findings. The changes in the values of dipole moment and some geometrical parameters of cis- and trans-HONO on vibrational excitation are also computed.     

Spectral density analysis of nuclear spin-spin coupling : I. A Hulthén potential LCAO model for JX-H in hydrides XH4

A spectral density function has been calculated for the indirect nuclear spin-spin coupling in hydrides XH₄, using a simple LCAO model for the X-H fragment. The basis consists of bound and continuum solutions is a Hulthén screened Coulomb potential, and a hydrogen atomic orbital. The reduced coupling constant, K, is an integral over all momenta, k, of this spectral density function. The calculated values of K are about half from the experimental ones, but the 23-fold increase of K from C-H to Pb-H is successfully interpreted by the model. The “high-energy tail” contribution to K from k > 10² au is found to be of the relative order of 10⁻⁴ for all systems considered.