Polarized infrared and Raman spectra of a CsHSO4 single crystal

The polarized absorption infrared (IR) and polarized Raman spectra of a CsHSO₄ single crystal at room temperature are presented and discussed in relation to the X-ray crystal structure. Breakdown of the selection rules for the X-ray determined C_2h factor group is observed. The vibrational factor group appears to be C₂. This implies C₁ site symmetry for the SO²⁻₄ ions. The polarization features of the HSO⁻₄ ion vibrations are predicted assuming that the longest S---OH bond vibrates independently of the SO₃ group vibrations. The ABC structure of the IR and Raman band arising from the νOH stretching vibration is explained on the basis of Fermi resonance.     

Electronic spectra and transitions of the fullerene C60

Absorption spectra of C₆₀ have been measured in the ranges (a) 190–700 nm in n-hexane solutions at 300 K, (b) 390–700 nm in n-hexane and in 3-methylpentane solutions at 77 K. 40 vibronic bands were observed. They exhibit a large range of bandwidths and intensities, whose significance is discussed. Assignment of electronic transitions has been carried out using the results of theoretical calculations. Vibronic structures have been analyzed within the framework of theories of electronic transitions of polyatomic molecules applied to the I_h symmetry group. Nine allowed ¹T_1u−¹A_g transitions have been assigned in the 190–410 nm region. Observed and calculated allowed transition energies and oscillator strengths are compared. Detailed vibronic analyses of the 1 ¹T_1u−1 ¹A_g and 2 ¹T_1u−1 ¹A_g transitions illustrate the role of Jahn-Teller couplings. Orbitally forbidden singlet-singlet transitions are observed between 410 and 620 nm. Their vibronic structures were analyzed in terms of concurrent Herzberg-Teller and Jahn-Teller vibronic interactions. The 77 K spectra provided useful information on hot bands and on other aspects of the analyses. Vibronic bands belonging to triplet←singlet transitions were detected between 620 and 700 nm.     

Autoionization spectra of Li2 and the XΣg ground state of Li2: Experimental and theoretical investigations

Autoionizing Rydberg levels of Li₂ molecules in a supersonic molecular beam are populated by stepwise excitation with two tunable pulsed dye lasers. The observed autoionization spectra show severe perturbations. Based on calculations of quantum defects and a perturbation treatment of l-uncoupling a tentative assignment of Rydberg series up to n = 32 is proposed. The convergence limits of these series yield a value of IP = 41475 cm⁻¹ for the adiabatic ionization potential and a vibrational constant ω_e = 263 cm⁻¹ for the X²Σ⁺_g ground state of Li⁺₂. The experimental results are compared with ab initio calculations combined with a core polarization potential, which yield the potential curve. the dissociation energy, the quadrupole moment and the vibrational frequency for the X²Σ⁺_g ground state of Li⁺₂, in the excellent agreement with experimental findings.     

Density functional study of molecular properties of hydrazoic acid and methyl azide

Density functional calculations for hydrazoic acid HN₃ and methyl azide CH₃N₃ and for the respective singly ionized structures HN⁺₃ and CH₃N⁺₃ are reported. An analysis of the electrostatic solvent effects, based on the self-consistent reaction field approach, on the molecular properties and conformational equilibrium of CH₃N₃ is also reported. The results are sensitive to the basis set quality and show some dependence on the different representations for the exchange-correlation functions. For HN₃ very good agreement with experiment is observed for several properties, such as the geometry, dipole moment, vibrational frequencies and for the adiabatic first ionization energy. For CH₃N₃ the energy difference between eclipsed (ec) and staggered (st) conformers (δ_ec-st) is 2.5 kJ mol⁻¹, in good agreement with the experimental value (2.9 kJ mol⁻¹). However, for CH₃N⁺₃, δ_ec-st is −3.2 kJ mol⁻, reflecting a significant modification of the methyl group rotational potential after ionization. Solvent effects on the molecular properties of CH₃N₃ are important when it is solvated in a polar medium. The most significant modifications concern the dipole moment and the frequencies related to the CH₃ symmetric stretch and torsion vibrational modes.     

The enthalpy of mixing of liquid NaF and NaMgF3 from drop calorimetry

The enthalpy of mixing of three liquid mixtures of NaF and NaMgF₃ has been measured by drop calorimetry and was found to be negative. This energy release is attributed to a change in the equilibrium

Mg_1/4[MgF²⁻₄]_3/4+f⁻ å MgF²⁻₄ to the formation of complex MgF²⁻₄-ions. A ΔH^M diagram for the system NaF-MgF₂ up to 50 mol % MgF₂ has been constructed.     

Ab initio calculations of stretching vibrational transitions for the linear molecules HCN, HNC, HCCF and HC3N up to high overtones

Accurate electric dipole moment functions have been calculated for the stretching vibrational coordinates of the linear molecules HCN, HNC, HCCF and HC₃N by means of coupled cluster theory with single and double excitation operators plus a quasi-perturbative treatment of connected triples (CCSD(T)). Combining these with anharmonic stretching vibrational wave-functions absolute IR intensities for strecthing vibrational transitions up to high overtones are obtained. For HCN, excellent agreement with experiment is observed up to 7 ν₁ + η₃ at 23047 cm⁻¹. HCCF and DCCF show unusual behavior, with the ν₂ band with origin at 2239.2 cm⁻¹ being strongest in HCCF and the ν₁ band with origin at 2645.1 cm⁻¹ being the most intense in DCCF. The Fermi resonance system 2ν₃/ν₂ of DCCF is analysed in detail. The calculated IR intensities of the stretching fundamentals of HC₃N and DC₃N, which are difficult to obtain with high accuracy, are in very good agreement with the existing experimental data.     

Polarized absorption spectra and polarized Raman spectra of single crystals of trans(Cl2)-[CoCl2(NH3)n(H2O)4−n]Cl complexes (n = 2, 3, 4)

The title cobalt(III) complexes have been investigated by polarized absorption and Raman spectroscopies of the single crystals. The symmetry properties of the d-electron orbitals and of the vibrational modes attributable to the Raman bands of trans(Cl₂)-[CoCl₂(NH₃)_n(H₂O)_4−n]Cl complexes (n = 2, 3, or 4) were examined to elucidated the peculiar observation that ligand substitution causes no splitting of the 15 200-cm⁻¹ absorption band and the 250-cm⁻¹ Raman band. Effects of replacing the NH₃ ligand with H₂O on the electronic structure, atom–atom force constants and vibrational modes of these complex ions are briefly described.     

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.     

Near-IR absorption transition of H4 to F2 for UCl6 complex in M2ZrCl6 host crystals (M = K, Rb, and Cs): an experimental and theoretical study

The near-IR electronic transition from ground-state Г₁ of ³H₄ to the excited-state Г₃ of ³F₂ for the UCl₆²⁻ complex in three M₂ZrCl₆ host crystals (M = K⁺, Rb⁺, and Cs⁺) has been investigated experimentally and theoretically. The vibration frequency of the excited state is found to be sensitive to the alkaline ion substitution and the frequency variation follows the trend predicted by the ‘counterion effect’. In contrast to previous studies, the apparent anomaly in the intensity ratio of the two vibronic peaks from the prediction of a Boltzmann distribution relationship is attributed to the vibration frequency difference between the ground and the excited state. This new interpretation is supported by computer simulation of the experimental optical spectra.     

Polarized infrared and Raman spectra of CsHSeO4 and CsDSeO4 single crystals

The polarized absorption infrared spectra of CsHSeO₄ and CsDSeO₄ single crystals and polarized Raman spectra of the CsHSeO₄ single crystal were measured at room temperature. The polarization features of the internal vibrations of the HSeO⁻₄ ions are predicted on the basis of the X-ray structure assuming strong couping between the vibrations of the two shortest Se---O bonds and an intermediate Se---O bond. The bending methods γOH and δOH of a hydrogen bond appear at 805 cm⁻ and 1258 cm⁻¹, respectively. The νOH absorption has the ABC structure due to Fermi resonance of νOH with the overtones of the δOH and γOH vibrations. A similar shape of the νOH band is observed in the Raman spectra. The νOD absorption has a different shape from that of νOH. Intra-chain coupling was observed for the νOH and νOD vibrations.     

Spectroscopic investigation of ground state pyrrole (C4H5N): the NH stretch

We have recorded the infrared absorption spectrum of pyrrole at 0.005 cm⁻¹ spectral resolution using a Fourier transform interferometer. The rotational analysis of the fundamental N---H stretch (1¹₀) at 3530.811343(82) cm⁻¹ was performed. A set of 13 upper state rovibrational parameters was determined, allowing the 2715 assigned rovibrational lines to be reproduced with a standard deviation of 1.3 10⁻³ cm⁻¹. An attempt to record the fundamental band under slit-jet conditions is reported. The role of hot bands accompanying the series of the N---H stretch excitation is investigated. Effective vibrational parameters — ω⁰₁, X⁰₁₁, Y₁₁₁, X_1,24 — are obtained. The lower level in the hot band series is unambiguously identified as the V₂₄ = 1 level, by retrieving X_1,24 independently, from other spectral data. The observation of the complex band pattern accompanying the N---H series in the higher overtone range is discussed with the help of new data, recorded around the 1⁵₀ band at different temperatures using intracavity laser optoacoustic spectroscopy.     

Jet cooled NO2 intra cavity laser absorption spectroscopy (ICLAS) between 11200 and 16150 cm

We have combined the high sensitivity of the ICLAS technique with the rotational cooling effect of a slit jet expansion in order to observe and to understand the visible and near infrared NO₂ spectrum. By this way, an equivalent absorption pathlength of several kilometers through rotationally cooled molecules has been achieved. Due to the vibronic interaction between the two lowest electronic states, ²A₁ and à ²B₂, this spectrum is vibronically dense and complex. Moreover, the dense room temperature rotational structure is perturbed by additional rovibronic interactions. In contrast, the rotational analysis of our jet cooled spectrum is straightforward. The NO₂ absorption spectrum is vanishing to the IR but, owing to the high sensitivity of the ICLAS technique, we have been able to record the NO₂ spectrum down to 11200 cm⁻¹ with a new Ti:sapphire ICLAS spectrometer. As a result 249 ²B₂ vibronic bands have been observed (175 cold bands and 74 hot bands) in the 11200–16150 cm⁻¹ energy range. Due to the cooling effect of the slit jet we have reduced the rotational temperature down to about 12 K and at this temperature the K = 0 subbands are dominant. Consequently, we have analysed only the K = 0 manifold for N 7 of each vibronic band. The dynamical range of the band intensities is about one thousand. Due to the strong vibronic interaction between the ²A₁ and à ²B₂ electronic states, we observed not only the a₁ vibrational levels of the à ²B₂ state but also the b₂ vibrational levels of the ²A₁ state interacting with the previous ones. By comparison with the calculated density of states, we conclude that we have observed about 65% of the total number of ²B₂ vibronic levels located in the studied range. However, there are more missing levels in the IR because of the weakness of the spectrum in this range. The correlation properties of this set of vibronic levels have been analysed calculating the power spectrum of the absorption stick spectrum which displays periodic motions: the dominant period, at 714 ± 20 cm⁻¹, corresponds to the bending motion of the à ²B₂ state. The other observed periods remain unassigned. In contrast the next neighbor spacing distribution (NNSD) shows a strong level repulsion, i.e. a manifestation of quantum chaos. These two observations, apparently contradictory, can be rationalized as follows: the short time dynamics, for t < 10⁻¹² s, is “regular” while for longer times the dynamics becomes “chaotic”. We suggest that this behavior may be observed directly with a pump and probe fs laser experiment.     

Classical trajectory calculation for ion-pair formation in K+O2 collisions

Three-dimensional trajectory surface hopping calculations were performed on two diabatic energy surfaces. The covalent surface describes the K(²S) + O₂(³Σ⁻_g) state and the ionic surface K⁺(¹S) + O⁻₂(²Π_g). Transitions from one surface to another were computed through the Landau—Zener model. At small deflection angles, the energy loss distribution exhibits two peaks, as observed, due to O⁻₂ in its electronic ground state and to vibrationally excited O⁻₂.     

Superoxide ion formation process on reduced MoO3---TiO2

Two novel Mo⁵⁺ ions, different from either the D_2d symmetry Mo⁵⁺ on anatase or the D_2hMo⁵⁺ on rutile, appeared on partially reduced molybdenum oxide supported on -titanic acid. The oxygen molecule is held on one of these novel Mo⁵⁺ ions but is not reduced to the superoxide ion. Oxygen is reduced to O⁻₂ entirely by the Mo⁴⁺ ion formed from the D_2d symmetry ion and the O⁻₂ is subsequently stabilized after moving to a Ti⁴⁺-site.     

First principles study of the structure, electronic state and stability of SinCm anions

Structure, electronic state and energy of Si_nC⁻ and Si_nC⁻₂ (n=1–7) anions have been investigated using the density functional theory. Structural optimization and frequency analysis are performed at the level B3LYP/6-311G(d). The charged-induced structural changes in these anions have been discussed. The strong C–C bond is also favored over C–Si bonds in the Si_nC⁻_m anions in comparison with corresponding neutral cluster. Among different Si_nC⁻ and Si_nC⁻₂ (n=1–7) anions, Si₃C⁻, Si₅C⁻ and Si₂C⁻₂ are most stable. Their stability has a decreasing tendency with the increase in the size of these clusters.     

The vapour pressure of di-arsenic pentoxide (As2O5)

The arsenic oxide pressure of As₂O₅ has been studied using mass spectrometry and a transportation method. Mass spectrometry revealed the presence of the species As₄O⁺₆, As₄O⁺₇, and As₄O⁺₈ in the vapour. The existence of volatile species up to As₄O₁₀(g) as a result of the reaction As₄O₁₀(g) As₄O_(10−y) (g) +1/2yO₂(g) has been assumed.

The oxygen pressure of this equilibrium builds up very slowly. The equilibrium pressure can be expressed by log(p_O2/atm) (880−952 K) = −(13940±930)/T + (14.53 ± 1.01)

A stationary arsenic oxide pressure has been measured using the transportation method. Since the oxygen pressure in the transportation gas did not influence the arsenic oxide pressure, it is assumed that only the As₄O₁₀(g) pressure has been measured. The results can be expressed by the linear function log(p_As4O10/atm) (865−1009 K) = −(15741 ± 410)/T + (13.87 ± 0.42).     

Ab initio study of the electronic spectrum of dichlorocarbene CCl2

The equilibrium geometries, excitation energies, force constants and vibrational frequencies for seven low-lying electronic states X ¹A₁, ¹B₁, ³B₁, ¹A₂, ³A₂, ¹B₂ and ³B₂ of dichlorocarbene CCl₂ have been calculated at the MRSDCI level with a double-zeta plus polarization basis set. Our calculated equilibrium geometry for the X ¹A₁ state, excitation energy for X ¹A₁ → ¹B₁ and vibrational frequencies for the X ¹A₁ and ¹B₁ states are in good agreement with experimental data. The electronic transition dipole moments, oscillator strengths for the ¹B₁ → X ¹A₁ and ¹B₂ → X ¹A₁ transitions, radiative lifetimes for the ¹B₁ and ¹B₁ states are calculated using MRSDCI wavefunctions, predicting results in reasonable agreement with experiment.     

The infrared spectrum of gaseous Sb4O6

The vibrational spectrum of Sb₄O₆ in the gas phase has been measured at 1000 K by high-temperature infrared spectroscopy. The four infrared-active absorption bands were observed at ν₇ = 785.0 cm¹, ν₈ = 176.2 cm⁻¹, ν₉ = 292.4 cm⁻¹ and ν₁₀ = 415.6 cm⁻¹. By combining these results with data on the molecular geometry and the infrared-inactive modes, as reported in the literature, the thermodynamic functions of Sb₄O₆ have been calculated.     

X-Ray, FTIR and quantum chemical studies of short and asymmetric hydrogen bonds in bis(2,6-dimethylpyridine-N-oxide) sulphate [2,6-(CH3)2C5H3N---OH]2[SO4]

Crystals of bis(2,6-dimethylpyridine-N-oxide) sulphate are monoclinic, space group P2₁/c, a = 14.098(2) Å, b = 7.855(1) Å, c = 15.203(3) Å, β = 104.84(1)°. The crystal structure has been refined to R = 0.0373 (2052 reflections). The disordered SO²⁻₄ anion accepts hydrogen bonds from two protonated 2,6-dimethylpyridine-N-oxides and two alternative conformations of the SO²⁻₄ group are distinguished. The occupancy factor of the predominant orientation is 0.63 and the O...O distances are 2.445(2) and 2.453(4) Å; in the second form (fraction, 0.37), these distances are 2.445(2) and 2.544(9) Å.

The PM3 and AM1 methods predict three minima for the title complex, whereas the SAM1 and BLYP/6-31G methods predict only one. All methods predict that molecular complex 3 is the most stable. The SAM1 geometry is very close to that of BLYP/6-31G.

The Fourier transform IR (FTIR) spectrum shows a very intense and broad (continuum) absorption within the 1600-400 cm⁻¹ region, typical of short hydrogen bonds. There is no absorption in the 3000-2000 cm⁻¹ region expected for the longer hydrogen bond (2.544(9) Å) in the less populated orientation. Isotope and solvent effects are discussed.     

Lifetime measurements of electronically excited C3(Πu) radicals in different vibrational states

The radiative lifetimes of nine vibrational levels of the C₃(¹Π_u) radical were obtained from decay time studies of the C₃(¹Π_u → ¹Σ⁺_g) fluorescence induced by a tunable dye laser. The lifetimes of the different vibronic levels were found to be constant within the experimental error limits, namely, τ_o = (200 ± 10) ns. The collisional deactivation of the C₃(¹Π_u) states by helium gives rate constants between 2.5 and 4 in 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ units.