Internal rotation in methyl silane by avoided-crossing molecular-beam spectroscopy

The avoided-crossing molecular-beam electric-resonance technique was applied to methyl silane in the ground torsional state. A new type of anticrossing is introduced which breaks the torsional symmetry and obeys the selection rules ΔJ = 0, K = +1 /a3 ?1. For these “barrier” anticrossings, the values of the crossing fields E_c yield directly the internal rotation splittings; the E_c are independent of the difference (A-B) in the rotational constants. Such anticrossings were observed for J from 1 to 6. Studies were also conducted of several “rotational” anticrossings (J, K) = (1, ±1) /a3 (2, 0) for which E_c does depend on (A-B). The normal rotational transition (J, K) = (1, 0) ← (0, 0) was observed in the ground torsional state using the molecular beam spectrometer. The present data on CH₃²⁸SiH₃ were combined with Hirota's microwave spectra and analyzed with the torsion-rotation Hamiltonian including all quartic centrifugal distortion terms. In addition to evaluating B and several distortion constants, determinations were made of the moment of inertia of the methyl top I_α = 3.165(5) amu-Ų, the effective rotational constant A^eff = 56 189.449(32) MHz, and the effective height of the threefold barrier to internal rotation V₃^eff = 592.3359(73) cm^?1. The correlations leading to these two effective constants are discussed and the true values of A and V₃ are determined within certain approximations. For the isotopic species CH₃³⁰SiH₃, barrier and rotational anticrossings were observed. The isotopic changes in A and V₃ were determined, as well as an upper limit to the corresponding change in I_α.     

Microwave spectrum of silyl fluoride: Coriolis resonance between ν2 and ν5 states

The J = 1 ← 0 and J = 2 ← 1 microwave rotational transitions of SiH₃F and SiD₃F have been measured for the ground and the v₂ = 1, v₃ = 1, v₅ = 1, and v₆ = 1 vibrational states, for which the various rotational and vibration-rotation interaction constants have been obtained. Both molecules show an X-Y Coriolis resonance between the ν₂ and ν₅ vibrational states, whose separation are 29 and 8 cm^?1, respectively. In the case of SiD₃F the resonance is very strong and an exact numerical diagonalization of the energy matrix was employed.     

Microwave spectrum of methyl chloride in the excited vibrational states: Coriolis interaction between the ν2 and ν5 states

The rotational spectra of CD₃³⁵Cl, CD₃³⁷Cl, CH₃³⁶Cl, and CH₃³⁷Cl in the ν₂, ν₃, ν₅, and ν₆ states were observed and analyzed. A few lines of the J = 3 → 2 transition were also detected for ¹²CD₃³⁵Cl in the 2ν₃ state and for ¹³CD₃³⁵Cl in the ν₃ state. For CH₃³⁵Cl in the ν₆ state the present data on the J = 1 ← 0 and J = 2 ← 1 transitions were combined with the millimeterwave spectra reported by Sullivan and Frenkel to determine the molecular constants. Special attention was given to the ν₂ and ν₅ spectra which showed the effect of Coriolis resonance. By transferring some of the constants involved from the laser-Stark spectra we determined B₅^*, B₂^*, and q₅^* for CD₃Cl. The large effective q₅ constant permitted observation of the direct l-doubling transitions of high J. The analysis of the CH₃Cl spectra was much less complete than that on CD₃Cl because of limited data. The B rotational constants obtained were compared with the previous microwave and infrared results when available.By using the infrared data on ν₁ and ν₄ we evaluated the equilibrium B_e constants (α₄^B of CD₃³⁷Cl was estimated), and refined the equilibrium structure of methyl chloride reported by Duncan.     

Microwave spectrum of CH3OD

The microwave spectrum of CH₃OD has been observed in the frequency region between 14 and 92 GHz. All the ground-state transitions with J ≤ 8 and J = 2 ← 1, a-type transitions in the excited torsional states (v = 1 and v = 2) have been observed. The spectrum has been analyzed and rotational constants, torsional constants, torsion-vibration-rotation interaction constants, and centrifugal distortion constants have been evaluated. The Stark effect measurements have been made and the dipole moment components have been determined as μ_a = 0.833 ± 0.008 D and μ_b = 1.488 ± 0.015 D.     

Infrared-microwave double resonance in CH3OH using a Zeeman-tuned 3.5-μm HeXe laser

Infrared microwave double resonance signals have been observed for CH₃OH using the 3.5-μm HeXe laser line. When microwave transitions in the ground vibrational state are pumped, the double resonance signals are obtained on two infrared transitions v = 1 ← 0 of ν^CH(a′); v = 1, J, K, μ = 4, 2, 1 ← v = 0, J, K, μ = 3, 2, 1, and 4, 3, 1 ← 3, 3, 1. Three weak double resonance signals are due to the collision-induced transitions. Their relative intensities have been explained successfully by using the rate constants of collision-induced transitions which are proportional to the dipole matrix elements between the states involved in the transitions.     

Doppler-limited dye laser excitation spectroscopy of HCF

The cw dye laser excitation spectrum of the $\text{A}\text{?}{}^1\text{A″(000) ←}\text{X}\text{?}{}^1\text{A′(000)}$ vibronic band of HCF was observed between 17 188 and 17 391 cm^?1 with the Doppler-limited resolution, 0.04 cm^?1. The HCF molecule was produced by the reaction of discharged CF₄ with CH₃F, and 853 lines were observed, of which 516 transitions were assigned to K′_a ← K″_a = 3 ← 4, 2 ← 3, 1 ← 2, 0 ← 1, 1 ← 0, 2 ← 1, 0 ← 0, 1 ← 1, 2 ← 2, 3 ← 3, 2 ← 0, and 0 ← 2 subbands. A rotational analysis yielded the rotational constants and quartic and sextic centrifugal distortion constants for both the $\text{A}\text{?}$ and $\text{X}\text{?}$ states and the band origin, with good precision. The molecular constants determined reproduce the observed transition frequencies with an average deviation of 0.0038 cm^?1. Small rotational perturbations in the excited state were found at J = 5, 6 and J = 10, 11 of J_1,J and at J = 15, 16 of J_2,J?1 levels.     

Methyl isocyanate: An analysis of the low-J rotational spectrum using the quasi-symmetric top molecule approach

The low-J rotational spectrum of methyl isocyanate (CH₃NCO) has been analyzed in terms of the quasi-symmetric top molecule model, accounting explicitly for the large-amplitude CNC bending motion, and internal and overall rotation. An assignment of 25 J = 1 ← 0 and 2 ← 1 rotational transitions arising from the various CNC bending and torsional states is proposed. The molecule is found to be a nearly freely internally rotating quasi-symmetric top, with a barrier to linearity of the CNCO skeleton of 1049 cm^?1 and an equilibrium CNC valence angle of 140.2°.     

The two-dimensional anharmonic oscillator: The microwave spectrum of silyl isocyanate,SiH3NCO

The J + 1 ← J transitions (J = 2, 3, 4, 5, and 6) in the microwave spectrum of SiH₃NCO have been assigned for the vibrational ground state and for the vibrational states v₁₀ = 1, 2, and 3. The results for v₁₀ = 0 confirm earlier work. The vibration-rotation constants show a remarkable variation with v₁₀ and l₁₀. To a large extent the anomalous behavior of these constants has been explained in terms of a strongly anharmonic potential function for the ν₁₀ vibrational mode.     

Rotational spectrum, structure and internal rotation in CH3CCl3

The rotational spectra of the v₆ = 1 and v₆ = 2 torsional states of CH₃C³⁵Cl₃ have been measured in the millimeterwave range and accurate spectroscopic constants have been determined. The equilibrium structure, the torsional frequency and the barrier to internal rotation have been calculated ab initio. These results are shown to be compatible with the absence of splittings in the rotational spectra.     

Microwave and submillimeter-wave spectra of CH3OH

Frequency measurements and assignments have been made for CH₃OH lines in the 15- to 400-GHz region. The a-type R-branch multiplets are reported up to J = 8 ← 7 for the v_t=0 torsional ground state, and to J = 6 ← 5 for the v_t=1 and v_t=2 excited states. Several new Q branches are listed and many b-type P- and R-branch transitions have been identified over a wide range of J and k values.     

The rotational constants of methyl lodide

The rotational spectra of CH₃l (v = 0; v₃ = 1; v₃ = 2; v₆ = 1), ¹³CH₃l (v = 0), and CD₃l (v = 0) were observed in the submillimeter-wave region. These data have been combined with the microwave and millimeter-wave measurements to determine with high accuracy the rotational and centrifugal distortion constants. An overall fit including rotational data and infrared laser measurements yields a set of molecular parameters for the ν₆ band of CH₃l.     

The microwave spectrum of 1-phosphapropyne,CH3CP: Molecular structure,dipole moment,and vibration-rotation analysis

The J = 4 ← 3 and J = 3 ← 2 rotational transitions of 1-phosphapropyne, CH₃CP, between 26.5 and 40 GHz have been studied by microwave spectroscopy. The spectrum shows the characteristic vibration-rotation satellite patterns associated with a C_3v symmetric rotor. Apart from the most abundant isotope variant, the species ¹²CD₃¹²C³¹P, ¹²CD₂H¹²C³¹P, ¹²CH₂D¹²C³¹P, ¹³CH₃¹²C³¹P, ¹²CH₃¹³C³¹P, ¹³CD₃¹²C³¹P, and ¹²CD₃¹³C³¹P have also been studied. For ¹²CH₃¹²C³¹P the rotational constants B₀ = 4991.339 ± 0.003 MHz, D_J = 0.823 ± 0.092 kHz, D_JK = 66.59 ± 0.18 kHz have been determined. From these data the following structural parameters have been derived: $\text{r}{}_{\mathrm{s}}\text{(}\text{CH}\text{) = 1.107 ± 0.001}\text{A}\text{?}$, ∠_s(HCC) = 110.30 ± 0.09°, $\text{r}{}_{\mathrm{s}}\text{(}\text{CC}\text{) = 1.465 ± 0.003}\text{A}\text{?}$, $\text{r}{}_0\text{(}\text{CP}\text{) = 1.544 ± 0.004}\text{A}\text{?}$. The dipole moment has been determined as 1.499 ± 0.001 D by analysis of the Stark effect of the J = 3 ← 2, |K| = 1 line. The vibrational satellites (v_s = 1, 2, and 3) have been studied and various vibration-rotation parameters derived.     

Microwave measurements and calculations of quadrupole coupling effects in CH3I and CD3I

The transitions J = 1 ← 0, K = 0; J = 2 ← 1, K = 0; and J = 2 ← 1, K = 1 of CH₃I and CD₃I were measured using a Stark-modulated microwave spectrometer. Iodine quadrupole coupling strengths were analyzed to determine variations with deuterium substitution on the methyl group and variations with centrifugal distortion. Quadrupole coupling strengths were described by the expression $\text{eQq}{}_0\text{+ aJ(J + 1) + bK}{}^2\text{+}\text{cK}{}^4\text{J(J + 1)}$. Explicit expressions are given for a, b, and c for a symmetric top in terms of molecular parameters. For CH₃I eQq₀ = ?1934.11 ± 0.02 MHz and for CD₃I eQq₀ = ?1928.95 ± 0.04 MHz. Rotational constants obtained are B(CH₃I) = 7501.274 ± 0.002 MHz and B(CD₃I) = 6040.298 ± 0.007 MHz. The observed fractional change in halogen quadrupole coupling of 0.0027 is related to previous results for methyl chloride and methyl bromide.     

Rotational and Torsional Analysis of the OH-Stretch Third Overtone in CH3OH

We record double resonance spectra of the 4ν₁ band of jet-cooled ¹³C-methanol using single rotational state selection in the ν₁ fundamental and subsequent promotion of the selected molecules to the fourth vibrational level. We then detect transitions to the final excited states by infrared laser assisted photofragment spectroscopy (IRLAPS). The assigned A symmetry transitions reach upper states with K=0 and 1, and J from 0 to 5. For E symmetry, the transitions reach levels with K in the range −3 to 2 and J from 1 to 7. The rotation-torsional analysis determines a value for the torsional tunneling splitting of 2.8±0.4 cm⁻¹ at v₁=4. In a previous paper (J. Chem. Phys.110, 11 359-11 367 (1999)), we reported a trend of monotonically decreasing tunneling splittings in ¹²CH₃OH for v₁=0, 3, and 6 that we explained by a model that incorporates a linear increase in the torsional barrier height with OH stretch excitation. The ¹³CH₃OH tunneling splitting for the 4ν₁ band is in quantitative agreement with the trend found for ¹²CH₃OH.     

Stark spectroscopy of 13CD3OH with the HCN laser

The laser Stark spectrum of ¹³CD₃OH has been studied using the 311 μm line of the HCN laser. An extensive series of absorption lines has been observed and assigned to the J = 3 to 11 members of the K = 2 ← 3E₁, Q-branch transition in the v_t = 1 excited torsional state. Zero-field frequencies for all the assigned transitions are given with improved accuracy over those calculated from available molecular constants. For the Q-branch series, the branch origin and the series expansion coefficients are also presented.     

S1 ← S0 laser excitation spectra of glyoxal in a supersonic jet: High-resolution rotational analysis

The high-resolution rotational spectra of several S₁ ← S₀ vibrational transitions of glyoxal are obtained and analyzed. Undispersed fluorescence excitation spectra are obtained with a CW ring dye laser in a supersonic jet, providing a linewidth down to 100 MHz. Rotational constants and band origins are determined with an asymmetric rotator program. Population distribution in the supersonic jet is studied. The overtones of the torsion mode (ν₇) are examined as an approach to the trans-cis potential barrier. The 8₀¹ and 8₀¹ 7₀² bands of A + B type, which are too congested at room temperature experiments, are well resolved in this jet experiment. As a result, the rotational constants obtained by fitting low-J and -K rotational transitions are in good agreement with the constants obtained by fitting large-J and -K transitions of room temperature spectra. Furthermore, on about 1000 analyzed lines no rotational perturbation has been observed.     

Far-infrared rotational spectra of some freons; chlorotrifluoromethane,dichlorodifluoromethane, and trichlorofluoromethane

The far-infrared rotational spectra of chlorotrifluoromethane, dichlorodifluoromethane, and trichlorofluoromethane have been observed with an interferometric (Fourier transform) spectrometer in the region 10–40 cm^?1 at a resolution of 0.07 cm^?1. CCl₂F₂ exhibits a continuum spectrum at this resolution, but symmetric top rotational fine structure is observed for CClF₃ and CCl₃F. Isotope splitting is also observed in CClF₃, and analysis yields the rotational constants for C³⁵ClF₃ of B₀ = 0.11112 cm^?1, D_J = 1.6 × 10^?8cm^?1; and for C³⁷ClF₃, B₀ = 0.10835 cm^?1, D_J = 1.5 · 10^?8cm^?1. Isotopic shifts can be allowed for in CCl₃F to yield constants for C³⁵Cl₃F of B₀ = 0.0821 cm^?1, D_J = 1 × 10^?8cm^?1. These values are all in agreement with those deduced from microwave studies of the low J transitions apart from B₀ for C³⁵ClF₃, where the difference is outside the expected experimental error.     

Laser magnetic resonance measurement of rotational transitions in the metastable a1Δg state of oxygen

Laser magnetic resonance (LMR) for five rotational transitions, J = 4 ← 3, 5 ← 4, 7 ← 6, 8 ← 7, 9 ← 8, of the oxygen molecule ¹⁶O¹⁶O in its metastable state, a¹Δ_g, v = 0, are observed using six fir laser lines. Taking the known values of the g factors, their zero-field frequencies are obtained as 340.0085(6), 424.9810(9), 594.870(1), 679.780(1), and 764.658(1) GHz, respectively. They are fit by $\text{(}\text{E}\text{h}\text{) = B}{}_0\text{[J(J + 1) ? 4] + D}{}_0\text{[J(J + 1) ? 4]}{}^2\text{+ (?1)}{}^{\mathrm{J}}\text{(}\text{1}\text{2}\text{)qJ (J + 1)[J(J + 1) ? 2]}$, where B₀ = 42.50457(10) GHz, D₀ = 153.14(110) kHz, and q = 0.050(90) kHz.     

The microwave spectrum of sodium tetrahydroborate NaBH4 in excited vibrational states: Coriolis interaction between the Na-BH4 stretching and the BH4 rocking vibrations

Two sets of vibrational satellites have been observed in the rotational spectrum of sodium tetrahydroborate NaBH₄, and have been assigned to the non-degenerate, Na—BH₄ stretching and the degenerate BH₄ rocking (or internal rotation) states. The observation was extended from the J = 11 ← 10 up to J = 20 ← 19 transitions. The vibrational satellites showed anomalous K structure; higher-K lines of the non-degenerate state appeared at higher frequencies, in reverse to those of the ground state, whereas the spectra in the degenerate state exhibited a K pattern similar to but somewhat more widely spread than that of the ground state. These anomalies are ascribed to the Coriolis interaction between the two excited vibrational states. The spectra observed were analysed using a C_3v symmetric-top rotational Hamiltonian, which took into account the Coriolis interaction explicitly. The A rotational constants, the energy difference δE between the two interacting vibrational states, and the first- and second-order Coriolis interaction constants have been derived.     

M-dependence of collisional transfer of rotational energy in OCS mixtures

Collision-induced transitions between rotational levels of OCS in the ground vibrational state have been investigated by steady-state microwave double resonance, with the M sublevels separated by a Stark field. The (2 ← 1)_P-(1 ← 0)_S, (3 ← 2)_P-(1 ← 0)_S, and (4 ← 3)_P-(1 ← 0)_S systems have been studied for pure OCS and for mixtures with excess CH₃OH, He, and H₂. For four-level systems having dipolar connections (ΔJ = 1; ΔM = 0, ± 1; parity ± ? ?) between pump and signal levels, it is found for OCS and the OCS-CH₃OH mixture that the dipole-type ΔJ = 1 transitions always dominate the collisional transfer, but for the OCS-He and OCS-H₂ mixtures that ΔJ = 2 quadrupole-type transitions are dominant. For all four collision partners, significant ΔJ = 2 and ΔJ = 3 collisional transfer is observed in some systems, indicating the presence of high-order terms in the collisional interaction.