An ab initio calculation of the potential surface and rotation—vibration energies of the silyl radical

We have calculated 64 points on the ground electronic state potential energy surface of the silyl radical (SiH₃) using the MRD CI technique. This potential surface gives an inversion barrier of 1951 cm^?1 and an equilibrium geometry of r_e = 1.480 Å and α_e(HSiH) = 111.2°. Using the non-rigid invertor Hamiltonian with this potential we determine for SiH₃ that ν₁ = 2424 cm^?1, ν₂ = 778 cm^?1, ν₃ = 2106 cm^?1, and ν₄ = 976 cm^?1; the inversion splitting is calculated to be 0.11 cm^?1. Rotational constants and centrifugal distortion constants have also been calculated.     

On spectroscopic properties and isotope effects of vibrationally stabilized molecules

Results of quantum and semiclassical calculations obtained for two different potential-energy surfaces are used to discuss spectroscopic properties and isotope effects of the linear IHI and IDI molecules. The potentials are a purely repulsive LEPS surface and a DIM-3C potential with two van der Waals type minima for equivalent IH ··· I and I ··· HI configurations. Both systems are dominated by the effect of vibrational bonding giving rise to some very unusual spectroscopic phenomena, which are discussed in detail. The different vibrational frequencies and rotational constants are roughly estimated as ν₁ = 120 (100) cm^?1, ν₂ = 280 (210) cm^?1, ν₃ = 360 (160) cm^?1 and B = 0.0194 (0.0196) cm^?1 for IHI (IDI). A detailed discussion of the dependence of ν₁, ν₂ and B on ν₃, their sensitivity to variations of the potential-energy surface, and a comparison with the vibrational frequencies of I₂ and HI (ID) is given. It is predicted that there exists only one excited level of the antisymmetric stretching mode. The numbers of symmetrical stretching and bending levels are fairly constant or may even decrease upon deuteration. Simultaneously deuteration destabilizes the molecule. These unusual phenomena are rationalized by our calculations. A set of criteria for observing infrared and Raman bound-to-bound and bound-to-resonance state transitions are presented for the IHI and IDI molecule.     

The microwave and infrared spectra and structure of hydrothiophosphoryl difluoride

The microwave spectra of ³²SPHF₂, ³⁴SPHF₂ and ³²SPDF₂ have been analyzed. The structural parameters obtained from this analysis are: d(S-P) = 1.867±0.005Å, d(P-F) = 1.551 ±0.005Å, (P-H) = 1.392±0.005Å, ∠SPF = 117.4 ± 0.2 °, ∠SPH = 119.2±0.2 °, ∠FPF = 98.6±0.2 °. Centrifugal distortion coefficients were obtained for ³²SPHF₂. The spectra of two vibrational excited states of ³²SPHF₂ were observed. The two sets of rotational constants (A) 8336.72, 3726.70, 2807.56 MHz and (B) 8344.88, 3727.73, 2798.75 MHz were associated with the vibrational states with measured infrared frequencies 419 cm^?1 and 344 cm^?1 respectively. An analysis of the infrared spectrum is included. Dipole moment measurements yielded μ = 1.87±0.03 D for ³²SPHF₂ and μ = 1.86±0.03 D for ³²SPDF₂     

CEPA calculations on open-shell molecules

Ab initio calculations at SCF and CEPA levels using large Gaussian basis sets have been performed for the two lowest electronic states,X ² Σ⁺ andA ² Π, of HeAr⁺. Spin-orbit coupling (SOC) effects have been added using a semiempirical treatment. The resulting potential curves for the three statesX,A ₁, andA ₂ have been used to evaluate molecular constants such as vibrational intervals ΔG(v + 1/2) and rotational constantsB _v as well as — by means of a Dunham expansion — equilibrium constants such asR _e , ω _e ,B _e etc. Comparison with the experimental data from UV emission spectroscopy shows that the calculated potential curves are slightly too shallow and have too large equilibrium distances:D _e = 242 cm^?1 andR _e = 2.66 Å compared to the experimental values of 262 cm^?1 and 2.585 Å, respectively, for theX ²Σ⁺ ground state. However, the ab initio calculations yield more bound vibrational levels than observed experimentally and allow for a more complete Dunham analysis, in particular for theA ₂ state. The experimental value of 154 cm^?1 for the dissociation energyD _e of this state is certainly too low; our best estimate is 180±5 cm^?1. For theA ₁ state our calculations are predictions since this state has not yet been observed experimentally.     

Microwave spectra of isotopic glycolaldehydes,substitution structure,intramolecular hydrogen bond and dipole moment

The microwave spectra of ¹³CH₂OH-CHO, CH₂OH-¹³CHO, and CH₂OH-CH¹⁸O are reported and have been used in combination with previously published data on other monosubstituted glycolaldehydes to determine the substitution structure of the molecule as r(CO) = 1.209 Å, r(C-O) = 1.437 Å, r(C-C) = 1.499 Å, r(O-H) = 1.051 Å, r(C-H_ald) = 1.102 Å, r(C-H_alc) = 1.093 Å, r(O β H) = 2.007 Å, r(O β O) = 2.697 Å, ∠(C-CO) = 122°44', ∠(C-C-H_ald) = 115°16', ∠(C-C-O) = 111°28', ∠(C-O-H) = 101°34', ∠(C-C-H_alc) = 109°13', ∠(H-C-H) = 107°34', ∠(O-H β O) = 120°33', ∠(H β OC) = 83°41', and ∠(O-H, C0) = 24°14'. The intramolecular hydrogen bond and the other structural parameters are discussed and compared to related molecules. The dipole moment is redetermined to be μ_a = 0.262 ±0.002 D, μ_b = 2.33 ± 0.01 D, and μ_tot = 2.34 ± 0.01 D. Relative intensity measurements yielded 195 ± 30 cm^?1 for the C-C torsional fundamental and 260±40 cm^?1 for the lowest in-plane skeletal bending mode. Computations performed by the CNDO/2 method correctly predict the observed cis hydrogen-bonded conformer to be the energetically favoured one and in addition yield some indication of the existence of at least two other non-hydrogen-bonded forms of higher energy.     

Analysis of the Vibrational Structure of the <Emphasis Type="Italic">n</Emphasis>–π* Transition in the High-Resolution UV Absorption Spectrum of Methacryloyl Fluoride in the Gas Phase

The UV absorption spectrum of methacryloyl fluoride molecule in the gas phase is obtained in the wavenumber range of 32300–35900 cm^?1. The resolved vibrational structure of this spectrum consists of 153 absorption bands. The assignment of all bands has been made for the first time. Values ν_00trans = 35670.0 сm^?1 and ν_00cis = 35371.1 cm^?1 are determined. The fundamental frequencies for isomers in the S₀ and S₁ states are found. Several Deslandres Tables (DTs) are constructed for the torsional vibration of the s-trans- and s-cis-isomers of the investigated molecule using the NONIUS program. The origins in these DTs correspond to bands attributed to ν₀₀, and to the fundamental frequencies of each isomer in states S₀ and S₁. These DTs are used to determine harmonic frequencies ω_e, anharmonicity coefficients х₁₁, and the frequencies of torsional vibration 0–v transitions up to high values of vibrational quantum number v for s-trans- and s-cis-isomers in both electronic states. The frequencies of torsional vibrations for the s-trans-isomer and the s-cis-isomer in the S₀ state are ν″₁ = 80.9 сm^?1 and ν″₁ = 59.8 сm^?1, respectively. The frequencies for the s-trans- isomer and the s-cis-isomer in the S₁ state are ν′₁ = 134.1 сm^?1 and ν′₁ = 103.6 cm^?1, respectively.     

An ab initio theoretical study of the optical spectrum of CF2

The ab initio calculation methods have been used to calculate the spectral and electronic characteristics of difluorocarbene in the ground electronic state (¹A₁), the lowest-lying singlet (¹B₁) and triplet (³B₁) states. The optimized equilibrium geometries, rotational constants, harmonic vibrational frequencies and energy gaps, electronic charges, dipole moments of these states have been computed with different basis sets. The calculated vibrational frequency of ³B₁ state (v₂=522 cm^?1) and the energy separation (2.26 eV) between ³B₁ and ¹A₁ states are in good agreement with the experimental results (519 cm^?1, 2.46 eV respectively). According to the calculations the previous assignment of vibrational symmetries of ¹B₁ state was corrected, and some experimentally undetermined vibrational frequencies were predicted.     

A new approach to electron diffraction analysis of symmetric triatomic molecules with large-amplitude bending motion: The equilibrium geometry,force field and vibrational frequencies of BaI2 as determined by electron diffraction

Diffraction data on BaI₂, analyzed by a new approach, indicate an anharmonic potential with a barrier of 71(12) cm^?1 at a linear geometry. The structural and vibrational parameters were found to be r_e^h(Ba-I_o) = 3.150(7)Å, ∠_eIBaI = 148.0(9) °, f_q = 0.69(8) mdyn/Å,f_qq= 0.14(6) mdyn/Å, k₂ = ?0.0075(15) mdyn/Å, k₄ = 0.0025(9) mdyn/ų, v₁ = 106(12) cm^?1 and v₃ = 145(21) cm^?1. The bending frequency v₂ is predicted to be near 16 cm^?1.     

The B̃2B1 ← X̃2A1 absorption system of NO2 in Xe matrices: evidence for Renner—Teller interaction

A tentative vibrational assignment of the B?²B₁ ← X?²A₁ absorption system of NO₂ in solid Xe is reported. About 65 bands were analysed, yielding normal vibration energies of ν₁ = 1230, ν₂ = 450 and ν₃ = 2040 cm^?1. The electronic transition energy can be estimated to be T₀₁₀ = 14160 cm^?1 (14220 cm^?1 for the gaseous phase). These observations are in good agreement with predictions made using ab initio calculations. Evidence for Renner—Teller interaction is documented by a systematic staggering of frequency intervals between successive bands in the ν₂ progression of the B? state.     

Vibronic coupling of pyrene in the first ππ* excited state

The two-photon fluorescence excitation spectrum of pyrene in n-hexane and n-heptane matrices has been measured at 10 K in the region of the first electronic transition (26800–30200 cm^?1). The spectrum consists of a rich number of sharp bands, being in general better resolved in n-hexane than in n-heptane matrix. Shpol'skii multiplets have been observed for the most intense bands. A strong two-photon band dominates the spectrum = 1495 cm^?1 from the 0—0 line and was assigned to B_1u × b_1u = A_g symmetry. Other weaker vibronic origins occur in the spectrum which were correlated to vibrational modes of b_1u, b_2u, b_3u and a_u symmetry. Intense vibronic bands are observed close to the origin of the second electronic transition and were interpreted as combination bands of B_1u × b_1u × b_3g symmetry. A two-photon vibronic theory to account for their intensity is proposed where the electronic moment is linearly expanded in powers of the nuclear displacements.     

Vibrational-translational/rotational and vibrational-vibrational processes in methane: Optoacoustic measurements

Both V-T,R and V-V processes in methane have been studied optoacoustically following excitation of the ν₃ level with a He-Ne laser at 2947.9 cm^?1. The lifetime of the V-T,R process is 1.55 ± 0.05 μs atm. The rate constants for the fast equilibration between the bending modes is k(ν₂ → ν₄) = 60 μs^?1 atm^?1 and k(ν₄ → ν₂) = 13 μs^?1 atm^?1. The decay of the ν₃ and ν₂ stretching modes, which are in very rapid equilibrium, shows a rate constant of 0.23 ns^?1 atm^?1 and, within experimental error, produces exclusively the ν₄ stretching mode. Part of this decay, 4.6%, is by a single-quantum process producing a large amount of translational/rotational energy; the dominant process, 95.4%, is double-quantum through the 2ν₄ overtone. Both the yield of the single-quantum process and the exclusive production of the ν₄ bending mode from the (ν₃, ν₂) level are in dispute with current theoretical models.     

2-Hydroxy-4-methylbenzenesulfonic acid dihydrate: Crystal structure, vibrational spectra, proton conductivity, and thermal stability

The crystal and molecular structure of 2-hydroxy4-methylbenzenesulfonic acid dihydrate C₆H₃(CH₃)(OHSO^? ₃ H₅O₂ ⁺ (I) was studied by X-ray diffraction and vibrational spectroscopy. The compound crystallized in the monoclinic crystal system; crystal data: a=10.853(2) Å, b=7.937(2) Å, c=12.732(3) Å, β=112.13(3)°, V=1015.9(4)ų,Z=4,d_calc=1.466g/cm³,spacegroupP2₁/c,R_f=0.0486,GOOF=1.161.The S-O distances in the sulfonate group differed substantially (S1-O2 1.439(2) Å, S1-O3 1.455(2) Å, and S1-O4 1.464(2) Å. The symmetry of the H₅O₂ cation decreased due to proton displacement toward one of the two water molecules. XRD data on the asymmetry of H5O2 were confirmed by IR and Raman spectral data. The strong triplet at 2900, 3166, 3377 cm^?1 in the IR spectrum of I corresponds to different types of H-bond and shifted to 2185, 2363, 2553 cm^?1 after deuteration. The proton conductivity of the compound was measured by impedance spectroscopy: 6 × 10^?7 S/cm at 298 K (32 rel %), E _act=0.4±0.01 eV. The conductivity increased to 10^-3 S/cm, E_act=0.1 eV when ambient humidity increased to 60 rel %.     

Absorption,fluorescence and phosphorescence spectra of the singlet and triplet states of s-tetrazine in the crystal and in mixed crystals at low temperatures

The electronic absorption, fluorescence and phosphorescence spectra of s-tetrazine at low temperatures (4.2-1.5 K) are reported and analyzed in the neat crystal and in several mixed crystals. The ³B_3u-¹A_g (nπ^*) origin is at 18414 ± 5 cm^?1 for neat tetrazine. In the mixed crystal several sites identified. The lowest energy origin is at 17453 cm^?1 for tetrazine in pyrazine; 17 701 cm^?1 in pyrimidine; and 17 676 cm^?1 in pyridazine. The ^eB_3u-¹A_g (nπ^*) origin is at 14 096 ± 2 cm^?1 for the neat crystal. The phosphorescence lifetime of neat tetrazine is measured to be 96.8 ± 2.1 μs at 4.2 and 1.8 K. All the spectra are predominately composed of members of progressions in a single totally symmetric mode (ν_6a) built upon site origins and vibrational fundamentals. The ν_6a interval is: 743 (¹A_g), 715 (³B_3u), and 709 cm^?1 (¹B_3u) in the neat tetrazine crystal; 732 (¹A_g) and 705 cm^?1 (¹B_3u in pyrazine host, 737 (¹A_g) and 701 cm^?1 (¹B_3u) in pyrimidine host, and 732 (¹A_g) and 703 cm^?1 (¹B_3u) in pyridazine host mixed crystals. All emission spectra may be analyzed by Oi → (ν″_6a)^o_n (i), i indicating the observed s     

Electronic absorption spectrum of Fe3+ doped in ammonium chloride single crystal

The electronic absorption spectrum of the Fe²⁺ ion doped in ammonium chloride has been studied at room and liquid air temperatures. The observed bands have been assigned transitions from the ground ⁶A_1g(S) state to the excited ⁴A_1g(⁴E_g), ⁴T_1g(G) and ⁴T_2g(G) states. The cubic field approximation with D_q = 675 cm^?1, B = 645 cm^?1 and C = 4.4 B is found to give a good fit to the observed band positions.It is further concluded that the site symmetry of the Fe³⁺ ion in the crystal is lowered from O_h to C_4v symmetry at liquid air temperature.     

Reexamination of the phosphorescence of Ba2Pt2(H2P2O5)4 via thermally modulated emission spectroscopy

Reexamination of the phosphorescence of Ba₂Pt₂(H₂P₂O₅)₄ reveals that the ≈10 K spectrum is a superposition of two electronic transitions [³A₂u(E_u,A_1u → A_1g] separated by ≈40 cm^?1. Each band displays a prominent 110 cm^?1 vibrational progression. Franck-Condon analysis yields a ≈0.25 Å distortion of the PtPt bond in the excited states, interpreted as a contraction.     

Raman analysis of SF6 molecular beams excited by a cw CO2 laser

In a molecular beam the effects of vibrational pumping of SF₆ (ν₃ = 948 cm^?1) are studied, using a line-tunable cw CO₂ laser. Intracavity spontaneous Raman scattering is used for analysis. For excitation in the collision regime (x_E/D ≤ 1), a thermal redistribution of the ν₃ excitation over all vibrational modes is found, together with an average absorption up to six photons per molecule. The infrared absorption profile shows a red-shift of 6 cm^?1. For excitation in the relatively rare collision regime (x_E/D ? 4), a structured non-thermal ν₁ Raman spectrum is observed, especially in the case of seeded molecular beams (10% in He). The observed hot-band peaks can be explained in terms of single-photon absorptions and collision-induced near-resonant V-V energy transfer, leading to single, double and triple excitations of the ν₃ mode. The value of T_rot in the beam is found to influence sensitively the non-resonant energy-transfer rate [e.g. hν₃(948 cm^?1)+ΔE_rot → h(ν₄ + ν₆)(962 cm^?1) relative to the near-resonant transfer rate (hν₃ + hν₃ → 2hν₃ + 3.5 cm^?1)].     

The 3400 Å band system of sulphur dioxide

The contour of the band of SO₂ at 29937 cm^?1 has been shown to be of type-c, and an approximate excited state structure derived as r_SO = 1.50 Å, valence angle = 112°. For a number of reasons it is proposed that the principal bands between 3150 and 3400 Å correspond to vibronically induced B₁-A₁ transitions of a ¹A₂-¹A₁ forbidden electronic transition rather than to an allowed ¹B₁-¹A₁ transition.     

Darstellung und spektroskopische Charakterisierung der reinen Bindungsisomeren [OsCl5(NCS)]2− und [OsCl5(SCN)]2−

Preparation and Spectroscopic Characterization of the Pure Bondisomers [OsCl₅(NCS)]^2? and [OsCl₅(SCN)]^2? The oxidation of [OsCl₅I]^2? with (SCN)₂ in CH₂Cl₂ yields the bondisomers [OsCl₅(NCS)]^2? and [OsCl₅(SCN)]^2?, which are isolated as pure compounds by ion exchange chromatography on DEAE-Cellulose. Only the salts of the N-isomer show significant shifts in the vibrational and electronic spectra caused by polarization of the terminal S depending on the size of the cations and the polarity of the solvents. In the IR and Raman spectra ν_CN(S), ν_CS(N) and δ_NCS are found at higher wave numbers than ν_CN(N), ν_CS(S) and δ_SCN. In the optical spectrum of the red [OsCl₅(SCN)]^2? the charge-transfer S→Os is nearly constant at 538 nm, but the N→Os transition of the yellow to violet coloured N-isomer shifts from 480 nm in organic solvents or in presence of large alkylammonium cations to 516 nm in aqueous solution and to 544 nm in the solid Cs-salt. The optical electronegativities are calculated to χ_opt(–SCN) = 2.6 and χ_opt(–NCS) = 2.6–2.8. According to spinorbit coupling and to lowered symmetry (C_4v) the splitted intraconfigurational transitions are observed at 10 K as weak peaks in the regions 600, 1000 and 2000 nm. The O? O transitions are calculated from the vibrational fine structure. The lowest level of both isomers is confirmed by peaks in the electronic raman spectra. With the parameters ζ(Os^IV) = 3200 cm^?1 and B(? SCN) = 316 cm^?1 or B(? NCS) = 288 cm^?1 there is a good fit of calculated and experimental data, resulting in the nephelauxetic series: F^? > CI^? > SCN^? > Br^? > NCS^? > I^?.     

Tunable biode laser high resolution spectroscopy of the ν3 vibration of matrix isolated SO2

True lineshape of the ν₃(b₁) vibrational transition of ³²SO₂ isolated in an Ar matrix was measured with a high resolution (< 10^?3 cm^?1) tunable diode laser spectrometer and temperature effects on line frequencies and linewidths are reported.     

The importance of ground-state vibronic mixing in molecular Raman scattering

We consider Albrecht's theory for Raman scattering of fundamentals in the far and pre-resonance regions. Destructive interferences inherent to the A and B terms augment the conventionally suppressed C term dramatically. Raman excitation profiles for the ν₁(a_1g) and ν₆(e_2g fundamentals in benzene can be well fitted with theoretical C-term profiles involving the ¹E_1u(π-π^*) state at 1800 A.