Avoided-crossing molecular-beam spectroscopy of CH3SiF3

The avoided-crossing molecular-beam method has been applied to CH₃SiF₃ in the ground torsional state. Stark and hyperfine rotational anticrossings have been studied, along with barrier anticrossings in which the zero-field energy differences depend only on the torsionial splittings. For υ = 0, pure rotational spectra were measured forJ = 13 ← 12 and 14 ← 13 with a mm-wave spectrometer and for J = 1 ← 0 with the molecular-beam spectrometer. Stark and Zeeman studies have been carried out with conventional beam techniques. From a simultaneous analysis of existing microwave data for υ ? 2 and the current measurements, it was found that the moment of inertia of the methyl top I_α = 3.170(2) amu A², the effective rotational constants A^eff = 4059.522(22) Mhz, and the effective height of the barrier V₃^eff = 413.979(14) cm^?1. The true values of A and V₃ have been obtained within certain approximations. The rotational constant B and several distortion constants including D_K were evaluated. In addition to determining the electric dipole moment μ 2.33938(14) D, the data yielded values for the distortion dipole constants μ_D and μ_J, and the molecular g-factors g_| and g_⊥.     

Microwave spectrum,dipole moment and barrier to internal rotation of trans-methyl glyoxal

The microwave spectrum of the trans conformer of methyl glyoxal has been investigated in the frequency range from 8 to 40 GHz. The rotational constants have been determined for the A state: A = 9102.4332(31), B = 4439.8832(27) and C = 3038.9404(22) MHz. Quantitative measurements of the Stark effect have yielded the components of the electric dipole moment: μ_a = 0.1597(11), μ_b = 0.9620(7) and μ_total = 0.9751(7) D. From the splittings of rotational transitions the three-fold barrier to internal rotation of the methyl top has been found to be V₃ = 269.1 (3) cm^?1.     

Allycyanide gauche revisited

The microwave spectrum of the ground state of the gauche rotamer of allylcyanide (CH₂=CHCH₂ CN) has been remeasured. The obtained rotational constants A = 19 707.9 ± 0.1, B = 2 619.74 ± 0.05 and C = 2 497.43 ± 0.05 (in MHz) were in good agreement with a structural model. The dipole moment components were also fitted as |μ_a| = 3.50 ± 0.05, |μ_b| = 1.70 ± 0.02 and |μ_c| = 0.19 ± 0.04 (in Debye). The results are in both cases in good agreement with a CCCC dihedral angle near the expected 120°.     

Electric dipole moment of BrCl

The electric dipole moment of the BrCl molecule has been determined by a study of the Stark effect on nine hyperfine components of the J = 1 ← 0 rotational transition lying at 9 GHz. A direct diagonalisation procedure of the energy matrix of the total hamiltonian has been used and the electric dipole moment derived for the ground vibrational state of BrCl is |μ_o| = 0.519 ± 0.004 D.     

Photophysics and photochemical dynamics of methylanisole molecules in a supersonic jet

The fluorescence excitation, dispersed fluorescence and hole burning spectra, and fluorescence lifetimes of jet-cooled o-, m-, and p-methylanisoles (MA) were measured. The low-frequency ring methyl internal rotational bands observed for their S₀ and S₁ states were assigned. In the case of m-MA, the rotational isomers of cis and trans conformers, which arise from the orientation of the OCH₃ group with respect to the CH₃ group, were assigned by hole-burning spectroscopy. The observed level energies and relative intensities of the methyl internal rotation were reproduced by a calculation using a free rotor basis set. Furthermore, their potentials in the S₀ and the S₁ states were determined. The potential barrier heights for the S₀ states of m- and p-MA were quite low, suggesting that the methyl groups are freely rotating, while changing from S₀ to S₁ states, the potential barrier height increases. The potential barrier heights of o-MA drastically decreased in going from S₀ to S₁ states. The decrease would be due to the hydrogen bonding between O atom and one H atom of the methyl group. The torsional bands of the methoxy group (–OCH₃) were also observed for p- and o-MA. The –OCH₃ modes are found to couple with the level of the e species for the methyl internal rotation.Fluorescence lifetimes (τ_f) of the methyl internal rotational bands in the S₁ states of o-, m-, and p-MA were measured in order to investigate the photochemical dynamics. The values of the nonradiative rate constant (k_nr) were estimated from the τ_f values and Franck–Condon factors. The k_nr values drastically increased with the excitation of methyl internal rotation. Accordingly, the methyl internal rotation should enhance the nonradiative process, presumably intersystem crossing (ISC). The enhancement should be caused by the increase of the state density (ρ) effectively coupled with triplet manifolds. The drastic increase in the ρ value should be caused by level mixing. In addition, the methyl internal rotational motion may enhance the increase of the coupling matrix elements through the vibronic coupling between the excited singlet states. The remarkable rotational quantum species dependence on the ISC rate constant (k_ISC) value clearly appeared in m-MA. The dependence should result from the difference of the ρ value between a₁ and e species, since the e species are doubly degenerate. The species dependence was apparently related to the potential barrier height, suggesting that the large barrier height should have an influence on the ρ value of the triplet states.     

Methyl group tunneling rotation in the lowest nπ* state of toluquinone. An optically detected ENDOR,LAC and CR study

In this paper we report and discuss the effects of methyl group tunneling rotation on the methyl proton ENDOR, LAC and CR spectra in the lowest triplet state of toluquinone at 1.8 K. From a detailed analysis of the ENDOR spectra in the lowest rotational state (A) we obtain for the methyl protons the following isotropic hyperfine interaction constants: A₁ = A₂ = 34.4 MHz and A₃ = ?53.7 MHz. The tunneling frequency (3J) is calculated to be 2.9 GHz. The most likely equilibrium configuration of the torsional oscillator is found to be the one whereby the molecular plane is a mirror plane and the out-of-plane methyl protons point towards the closest oxygen atom. We also show that cross-relaxation between |A> and |E> states of translational equivalent toluquinone molecules is responsible for the observed sidebands in the level-anti-crossing spectra of toluquinone.     

Investigation on the potential barrier to internal rotation in molecules——Ab initio calculation and energy partition of the internal rotation barrier in complex molecule H_3N-BH_3

Based on the ab initio/6-31G~* calculation, the potential barrier to internal rotation in mol-ecule H_3N--BH_3 has been studed by means of PD/LSF atomic charge model and Buckingham(exp--6-1)energy partition method. The results indicate that the order of the contributions of the componentsto the total energy barrier △E is |△V_(es)|(electrostatic) >|△V_(et)| (charge transfer)>|△V_(ex)| (exchangerepulsion)>|△V_(di)| (dispersion). For △V_(es) there are maxima at θ= 30°and 90°, and a saddle atθ= 60°. There are good linear relationships for the total barrier △E, △V_(ex) and △V_(di) with cos3θrespectively, and the same for the dipole moment from PD/LSF model (μ_(PD)) and that from abinitio calculation (μ_(QM)) vs. cos3θ respectively.     

Rotational energy transfer in NO (A2Σ+, v = 0 and 1) studied by two-color double-resonance spectroscopy

Collision-induced rotational relaxation in the A²Σ⁺, v = 0 and 1 states of NO has been measured by using step-wise double-resonant ionization spectroscopy. Multiple quantum rotational energy transfer is occurring to at least |ΔJ| = 6 and the observed cross sections ranging from tens to hundreds of A² are larger than the gas-kinetic collisional cross section. The energy-transfer efficiency is slightly enhanced by the vibrational excitation. Energy-based scaling laws are successfully applied to reproduce the observed rotational distribution.     

Untersuchung der π-σ-delokalisation an den radikalionen des rubrens mit endor in lösung

From the negative and positive radical ions of rubrene (tetraphenyltetracene) ENDOR- in-solution spectra have been recorded. Proton hfs-constants could be assigned unambiguously to the various positions by also recording ENDOR spectra of partially deuterated species of this compound. The positive sign of one of the phenyl coupling constants was established by NMR measurements. The observed order |a_H^meta|? |a_H^para| = |a_H^ortho| of the phenyl proton coupling constants is in contrast to the predictions of pure π-MO-theories. These results have been interpreted by direct delocalization of the unpaired π-electron into the σ-systems of the phenyl rings. The synthesis is given for the various deuterated compounds used.     

The direct l-type resonance spectrum of CF3CCH in the vibrational state v10=3: Extension of the theory of reduction to H6n terms

The direct l-type resonance spectrum of CF₃CCH has been measured in the v₁₀=3 vibrational state by means of waveguide microwave Fourier transform spectroscopy in the range 8–26 GHz. Three types of direct l-type resonance transitions induced by the (Δk=Δl=±2) interaction could be observed: 140 transitions following the ΔJ=0, Δk=Δl=±2, Δ|l|=0, selection rule covering values of J=17–61 and G=|k−l| from 1 to 7, 94 transitions following the ΔJ=0, Δk=Δl=±2, Δ|l|=±2, selection rule covering values of J=26–57 and G from 15 to 24, 90 transitions following the ΔJ=0, Δk=Δl=±4 selection rule covering values of J=34–80 and G from 3 to 12 and 13 transitions following the ΔJ=0, Δk=Δl=±6, k=∓1, l=∓1⇔k=±5, l=±3 selection rule for J=71-83. 14 A₁–A₂ splitting transitions with kl=+1 were measured from J=33 to 49. Strong perturbations due to Δ(k−l)=±3 interactions made possible the observation of perturbation-allowed transitions with selection rule k=±4, l=±3⇔k=±3, l=∓1. Furthermore, the rotational transitions J=2–1, 3–2 and 4–3 have been measured. To make use of the multiple fitting concept, the theory of reduction developed for the v_t=2 vibrational state by Sarka and Harder [K. Sarka, H. Harder, J. Mol. Spectrosc. 197 (1999) 254] has been extended to the v_t=3 vibrational state including terms up to H₆₂. Owing to problems in the reproduction of data with low K and A₁–A₂ symmetry, two types of fits have been performed using five reduced forms of the effective Hamiltonian. The first type of fit used a reduced data set for which part of the transitions has been omitted. The unitary equivalence of the determined parameter sets has been demonstrated including newly derived relations for parameters in v₁₀=3. In the second type of fit, the complete data set has been included, yielding satisfactory results only in one reduction. In both type of fits, parameters up to sixth order have been determined including the axial rotational constant A and the Coriolis constant Aζ for both values of l. General features of the direct l-type resonance spectrum in a v_t=3 level are discussed.     

Ionization Constants of o-Phthalic Acid and Standard pH Values of Potassium Hydrogen Phthalate Buffer Solutions in Glycerol + Water Solvent Mixtures at Normal and Subzero Temperatures

Determination of primary standards for pH measurements in glycerol + water solventmixtures has been carried out based on reversible emf measurements of the cellPt|H₂|KHPh (m _PS) + KCl (m _Cl)|AgCl|Ag|Ptwhere KHPh denotes the potassium hydrogen phthalate buffer solution of molalitym _PS = 0.05 mol-kg^–1, at glycerol mass fractions w _G = 0.2 and 0.4, within thetemperature range –10 to 40°C. A multilinear regression procedure as a functionof electrolyte molality, glycerol mass fraction w _G, and temperature T has beenapplied for the data processing leading to the values of primary standards pH_PS.These can be represented by the following regression equationpH_PS = (4.007037±0.001113) + (3.55844±0.01776)x _G+(0.39622±0.01410)z + (4.3084±0.3377)z ²– (50.66±10.53)x _G z ² + (457.10±78.48)x _G ² z ²where z = (T – 298.15)/298.15. Parallel values of the first ionization constantof o-phthalic acid (H₂Ph; benzene-1,2-dicarboxylic acid, the parent acid of KHPh),which are essential for the above calculations, have been determined fromreversible emf measurements of the cellPt|H₂|H₂Ph (m ₁) + KHPh (m ₂) + KCl (m ₃)|AgCl|Ag|Ptover the range of solvent composition and temperatures mentioned above.     

Electric dipole moment of the diatomic AgBr molecule

Stark effect measurements on hyperfine components of the J = 3 – 2 rotational transition of ¹⁰⁷ Ag ⁷⁹ Br and ¹⁰⁹ Ag⁷⁹ Br at 11.5 GHz were carried out. The electric dipole moment derived for AgBr in its ground electronic and vibrational state is |μ_o| = 5.620 ± 0.030 D.     

N2 rotational energy distributions in cold,supersonic beams from electron excited fluorescence measurements of N+2

Ground-state rotational energy distributions of N₂ molecules produced in pure and He-seeded supersonic expansions have been determined by measurements of the N⁺₂ first negative band rotational line intensities produced by 800 eV electron impact on cooled pure and He-seeded N₂ supersonic beams. Sufficient spectral resolution was employed to resolve completely both P and R branches of the first negative bands. Rotational state distributions were obtained to much higher values of J than in previous investigations. The data show that at 800 eV, the electric dipole selection rule, |ΔJ| = 1, is consistent with the observed N⁺₂ emission bands and that the rotational energy distributions produced in the cooled, supersonic beam are non-Boltzmann with a large population in the first few rotational states followed by a long, high-energy fail to quite high J values.     

ΔF = 2–6 transitions in the hyperfine spectrum of λ = 601.8 nm line of 151Eu

The magnetic field effect of the a ⁸ S _1/2-z ⁸ P _9/2, F = 7 transition lines of ¹⁵¹Eu in magnetic fields up to 400 Gauss has been studied by using laser-atomic-beam-spectroscopy. δF = 2–6 transitions were observed. Results show that the transitions originate from the mixing of the Zeeman states induced by the magnetic field. The condition favourable to observation of the |δF| ≥ 2 transitions especially the transitions with high |δF| values discussed     

Microwave spectrum and molecular structure of arsenic tribromide1

The microwave spectrum of arsenic tribromide has been recorded in the frequency range 26.5–40.0 GHz. From the rotational constants for the symmetric top species ⁷⁹AsBr₃ and ⁸¹AsBr₃ the following structural parameters have been deduced: r_z(AsBr) = 2.324 ± 0.003 Å, θ_z(BrAsBr) = 99.8 ± 0.2°. These results are in excellent agreement with the parameters obtained by Samdal et al., in a concurrent electron diffraction study.     

Ionization constants ofo-phthalic acid in (propylene carbonate + water) and (ethylene carbonate + water), and thermodynamics of the cellPt|H2|HCl(m)| Hg2Cl2|Hg

The potential differences E of the cells Pt|H₂|H₂Ph(m₁)  +  KHPh(m₂)  +  KCl(m₃) in Z|AgCl|Ag and Pt|H₂|H₂Ph(m₁)  +  KHPh(m₂)  +  KCl(m₃) in Z|Hg₂Cl₂|Hg have been measured at T =  298.15 K in mixtures Z =  (W + S) of water (W) with cosolvents S =  propylene carbonate (PC) and S =  ethylene carbonate (EC), to determine the first ionization constants K of the o -phthalic acid H₂Ph(benzene-1,2-dicarboxylic acid), which are indispensable for the determination of primary pH-metric standards based on the potassium hydrogen phthalate buffer (KHPh) in such solvent mixtures. The value of K is seen to decrease progressively with increasing mass fraction w_sof the organic cosolvent, as with all of the other cosolvents studied earlier, but no simple relationship with the cosolvent permittivity is discernible. Since the required values of the standard potential difference E^oof the second cell were hitherto missing, they have now been obtained based on potential difference measurements of the cell Pt|H₂|HCl(m) in Z|Hg₂Cl₂|Hg. The primary medium effect (E_W^o − E_Z^o, by Owen’s definition) upon HCl in water-rich mixtures Z is seen to increase linearly with increasing w_s, as in earlier investigations. In this comparative context, the slope of the primary medium effect against w_splots for the aprotic cosolvents increases regularly with decreasing permittivity, whereas for the protic (alcoholic) cosolvents the slope is ill-defined.     

Thermodynamic properties of the intermediate phases of the Ag-Sb-Se system

The emf (?) dependence of C|Ag|AgI|glass Ag₂GeS₃|D|C galvanic elements on temperature is studied in the range of 470 to 575 K (C represents current electrodes, D denotes equilibrium three-phase alloys of individual Ag-Sb-Se systems, and AgI|glass Ag₂GeS₃ is a bilayer membrane with purely ionic (Ag⁺) electroconductivity). Analytical equations ? = ?(T) are used to calculate the thermodynamic functions of saturated solid solutions of the AgSbSe₂, Sb₂Se₃, and Ag₂Se phases of the Ag-Sb-Se system in the standard state.     

Microwave spectrum of propionyl chloride

The microwave spectrum of propionyl chloride has been investigated in the region 18.0–40.0 GHz, and transitions due to a cis conformer have been assigned. This form has a heavy atom planar configuration and the methyl group and the carbonyl oxygen atom are cis to each other. Using the substitution structures of propionic acid and acetyl chloride as molecular models for the propionyl chloride molecule, good agreement is found between observed and calculateò effective rotational constants. For the ³⁵Cl species satellite spectra assigned to the first four excited states of the C-C torsional mode have been observed together with the first excited state of the methyl torsional mode. The ground state spectrum has also been assigned for the ³⁷Cl species. Relative intensity measurements yielded the lowest C-C torsional vibration frequency of 86 ± 10 cm^?1. The CH₃ internal rotation frequency was found to be 197 cm^?1. Nuclear quadrupole coupling constants were determined for the ground state of the ³⁵Cl and ³⁷Cl species. From observed A-E splittings of ^bQ-branch transitions of the first excited state of the methyl torsional mode a barrier to internal rotation was estimated to be V₃ = 2480 ± 40 cal mol^?1 (867 ± 14 cm^?1).     

The rotational barrier in dimethyl acetylene and related molecules

The rotational barriers in dimethyl acetylene (CH₃CCCH₃), diamino acetylene (H₂NCCNH₂), dihydroxy acetylene (HOCCOH), methyl trifluoro methyl acetylene (CF₃CCCH₃), silyl methyl acetylene (SiH₃CCH₃), propene, cis and trans 2-butene and ethane were studied by ab initio molecular orbital methods using various basis sets. The eclipsed structure for dimethyl acetylene and its CF₃ and SiH₃ analogs was found to be the most stable, as has been inferred from experimental work, and the barrier height for these compounds has been predicted. The barriers in the OH and NH₂ substituted acetylenes, propene, butene and ethane were studied in order to more clearly understand the important influences in determining the barrier mechanism; specifically, the delocalized molecular orbital and Pauling VB model have been compared.     

Electric dipole moment of the diatomic if molecule

The electric dipole moment of the IF molecule has been determined by a study of the Stark effect on the hyperfine components of the J = 1 ← 0 rotational transition lying in the 17 GHz region. The production of molecules was by microwave discharge of I₂ and C₆F₁₀(CF₃)₂. A direct diagonalisation of the energy matrix of the total hamiltonian has been used and the electric dipole moment derived for the ground vibrational state of IF is |μ₀ |= 1.948(20)D.