Microwave spectrum, structure, dipole moment, and internal rotation of fluoromethyl methyl ether

Microwave spectra of fluoromethyl methyl ether and its 10 isotopically substituted species were measured. The r_s structure of this molecule was determined from the observed moments of inertia. Structural parameters obtained for this molecule, which was in the gauche form, were compared with those of the analogous molecules. Dipole moments of the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment is 1.744 ± 0.029 D making an angle of 100°54′ with the O---CH₂ bond toward the C---F direction and lies in the plane whose dihedral angles with the FCO and COC planes are 114°9′ and 44°56′, respectively. The barrier to internal rotation of the methyl group was calculated taking into account the coupling effect with the skeletal torsion using the observed splitting data of the spectra in the ground, first excited methyl torsional, and skeletal torsional states. The barrier, skeletal torsional frequency, and coupling term were determined to be V₃ = 1538 ± 40 cal/mole, ω_t = 158 ± 4 cm⁻¹, and V_s = 490 ± 500 cal/mole, respectively.     

Microwave and low-frequency vibrational spectra of bullvalene

The microwave spectrum of bullvalene has been investigated in the region 18–40 GHz. In addition to transitions in the ground vibrational state, transitions arising from five excited vibrational states below 600 cm⁻¹ have also been observed. A combination of microwave intensity measurements and infrared and Raman data has been utilized to assign these vibrations. Three of the vibrations are E-type modes at 241, 355, and 588 cm⁻¹. One is an A₁-type mode at 445 cm⁻¹, and another is an A₂-type at 266 cm⁻¹. The microwave spectrum indicates the presence of a first-order Coriolis interaction for the E modes at 241 and 588 cm⁻¹. The first-order Coriolis coupling constant q = 0.557 MHz for the 241 cm⁻¹ vibration. The spectral results are consistent with C_3v symmetry for bullvalene.     

Methyl barrier to internal rotation and evidence of torsion-torsion interaction in methyl thiolcyanoformate

The rotational spectrum of methyl thiolcyanoformate has been measured in the ground state and in four vibrationally excited states. The methyl group has the conformation syn with respect to the carbonyl group. The E component lines were assigned only in the ground state and the methyl group barrier to internal rotation has the value V₃ = 705 ± 20 cal/mole. The A species rotational transitions of the first excited state of the methyl and skeletal torsions do not follow a semirigid-rotor pattern probably because they strongly interact.     

Internal rotation in acetaldehyde

The microwave spectrum of acetaldehyde has been investigated in the frequency range from 7 to 40 GHz. A rather complete assignment of rotational transitions in the ground and torsionally excited states has been found with the help of microwave-microwave double resonance techniques. The spectral data have been analyzed using three different models for the overall and internal rotation problem including a nonrigid model. The threefold component of the internal rotation barrier was determined to be V₃ = 400 ± 2 cm⁻¹. The sixfold contribution V₆ = −10.9 ± 0.3 cm⁻¹ could only be adjusted reliably from data for both ground and torsionally excited states using the nonrigid model. The methods of barrier determinations have been critically reviewed. In an appendix, the Hamiltonian for a nonrigid model is derived based on structure relaxation of the methyl top during internal rotation.     

Microwave spectrum and barrier to internal rotation of 1-chloro-2-butyne

Rotational transitions of the μ_a and μ_b type have been identified with microwave-microwave double resonance measurements for 1-chloro-2-butyne in the ground vibrational state. In the first excited state of the methyl torsion only μ_a-type transitions have been identified. The A-type transitions of the ground vibrational state can be described perfectly by the rigid rotor approximation with centrifugal corrections. Using the internal axis method the barrier to internal rotation was determined from the A,E splittings: V₃ = 10.05 ± 0.09 cm⁻¹. A model which allowed for geometry relaxation upon internal rotation was used to fit one set of parameters to the transition frequencies of both ground state and first excited torsional state. The sixfold contribution to the barrier was found to be negligible: V₆ = −0.4 ± 0.3 cm⁻¹.     

Microwave spectrum of chloromethyl methyl ether

The microwave spectrum of chloromethyl methyl ether has been studied in the region 12.4–40 GHz. For ³⁵Cl species, a- and c-type transitions have been assigned for the ground state, the first excited state of the chloromethyl torsional mode, and the first excited state of the methyl torsional mode. Assignments were also made for the ground state of ³⁷Cl species. The assigned transitions are due to the gauche conformer. The nuclear quadrupole coupling constants were determined for the ground state of ³⁵Cl and ³⁷Cl species. The observed A-E splittings of the rotational transitions arising from the three vibrational states indicate a strong coupling between the two torsional vibrations. A model calculation based on the Hamiltonian previously used by Butcher and Wilson (J. Chem. Phys.40, 1671 (1964)), was carried out to account for the splittings and the vibrational frequencies of the two torsional modes. The barrier to internal rotation of the methyl group is estimated to be V₃ = 647 ± 17 cm^?1 (1.84 ± 0.05 kcal/mole).     

Infrared and microwave spectra, molecular conformation and electric dipole moment of methyl thiolformate

The microwave spectrum (41-10 GHz) and the infrared spectrum (4000-50 cm⁻¹) of methyl thiolformate have been obtained and analyzed. The spectra are consistent with a single molecular conformation having a planar array of heavy atoms and with the alkyl group cis to the carbonyl group. The measured rotational constants are: A, 11042.22 MHz; B, 5118.27 MHz; C, 3562.03 MHz (κ = −0.5839). No internal rotation doublets were observed in the microwave spectrum for the ground vibrational state, which implies that the barrier hindering internal rotation of the methyl group is either much larger or much smaller than the corresponding value for methyl formate. If the former is true then a lower limit of 10.5 kJ mol⁻¹ may be placed on the barrier height.The dipole moment of methyl thiolformate was measured using the Stark effect to be 1.58 ± 0.05 Debyes (μ_A = 1.52 D; μ_B = 0.43 D) for the vapor, and for dilute solutions in benzene at 295 K the value of 1.6 ± 0.1 D was found from capacitance measurements.SCF computations using minimal basis sets of STO/3G atomic orbitals and extended basis sets of STO/4.31G atomic orbitals have been carried out for methyl thiolformate and methyl formate. Energy differences between rotational isomers and estimates of barrier heights are given together with the calculated dipole moments.     

Millimeter-Wave and High-Resolution Infrared Spectra of Monoisotopic SCSe: Equilibrium, Ground State, and ν1, mν2, and nν3 Rovibrational Parameters

The Fourier transform infrared spectrum of monoisotopic SC⁸⁰Se has been investigated in the ν₂, ν₃, 2ν₂, 2ν₃, and ν₁ regions with a resolution between 3 and 4 × 10⁻³ cm⁻¹. In addition, the millimeter-wave spectrum has been studied in the region 150 to 320 GHz, and ground and ν₂ = 1 excited state transitions have been measured. Ground state constants, B₀ = 2043.285 4(4) MHz and D₀ = 146.53(5) Hz, have been determined from a merge of millimeter-wave data and ground state combination differences spanning J values up to 77 and 143, respectively. The band centers ν₂ = 352.341 075(9) cm⁻¹ and ν₃ = 505.480 06(5)cm⁻¹ have been determined. The rovibrational parameters of numerous overtone and combination levels (ν₁ν^l2₂ν₃) = 02⁰0, 02²0, 03¹0, 03³0, 04⁰0, 04²0, 00⁰2, and 00⁰3 have been obtained from polynomial analyses whose standard deviations ranged from 0.7 to 3.5 × 10⁻⁴ cm⁻¹. The 10⁰0 level, ν_eff 1435.840 cm⁻¹, is anharmonically perturbed by the 04⁰0 level, with an avoided crossing at J = 55, and W₁₂₂₂₂ = 0.963 09(1) cm⁻¹. Transitions to both the upper (E⁺) and lower (E⁻) sublevels of the dyad were observed for 1 ≤ J′ ≤ 117 and 4 ≤ J′ ≤ 171, respectively, and the deperturbed wavenumbers ν₁ = 1435.542 76(2) and 4ν⁰₂ = 1432.725 00(3) cm⁻¹ were derived. Furthermore, a local crossing of the E⁻ and 04²0 levels involving l-type resonance was observed at J = 91.     

A new electronic transition of antimony chloride in the visible region

The emission spectrum of SbCl has been photographed at high resolution in the region 400 to 640 nm. In addition to bands of two previously reported transitions in this region, A₁-X and A₂-X, 36 bands of a new system have been identified. A vibrational analysis has been made with ν₀₀ ≈ 20 679 cm⁻¹, and 7 of the bands have been rotationally analyzed. The electronic transition has ΔΩ = 0 with lower state constants which match published data for the ground state X³Σ⁻(0⁺). The upper state is characterized by the following ¹²¹Sb³⁵Cl molecular parameters: B′₀ = 0.0922 cm⁻¹, D′₀ = 3.1 × 10⁻⁸ cm⁻¹.     

Analysis of the 2ν3 band of CD3F at 5.06 μm recorded under high resolution

The 2ν₃(A₁) band of ¹²CD₃F near 5.06 μm has been recorded with a resolution of 20–24 × 10⁻³ cm⁻¹. The value of the parameter (α^B − α^A) for this band was found to be very small and, therefore, the K structure of the R(J) and P(J) manifolds was unresolved for J < 15 and only partially resolved for larger J values. The band was analyzed using standard techniques and values for the following constants determined: ν₀ = 1977.178(3) cm⁻¹, B″ = 0.68216(9) cm⁻¹, D″_J = 1.10(30) × 10⁻⁶ cm⁻¹, α^B = (B″ − B′) = 3.086(7) × 10⁻³ cm⁻¹, and β^J = (D″_J − D′_J) = −3.24(11) × 10⁻⁷ cm⁻¹. A value of α^A = (A″ − A′) = 2.90(5) × 10⁻³ cm⁻¹ has been obtained through band contour simulations of the R(J) and P(J) multiplets.     

Internal rotation in the spectrum of acetaldehyde

The geometrical structure of acetaldehyde from the microwave spectrum and torsional transitions from the far infrared spectrum have been fitted with a semirigid model in order to obtain the torsional parameters V″₃ = 415.0 and V″₆ = 22.3 cm^?1 and the torsional energy levels of the isotopic species CH₃CHO, CH₃CDO, CD₃CHO, and CD₃CDO. These have been used in fitting torsional sequences in the 182-nm system of the electronic spectra of these species to obtain the excited state parameters V′₃ = 880 and V′₆ = 77 cm^?1. Both the ground and excited state parameters are in good agreement with ab initio predictions.     

Internal rotation and inversion doubling in the microwave spectrum of CH3NHCl

The microwave “a” and “c” type spectra of four isotopic species of CH₃NHCl in the ground state and of CH₃NHCl³⁵ and CH₃NDCl³⁵ in the first excited torsional state have been analyzed. From the A-E torsional splittings of the excited state the torsional barrier height has been determined to be V₃ = 3710 ± 46 cal/mole. The “c” type transitions show an inversion doubling of 4.60 ± 0.10 MHz in the ground state and of 5.25 ± 0.10 MHz in the first excited torsional state. Such doublings are independent on the rotational quantum numbers within the experimental errors. The height of the inversion barrier has been roughly evaluated by using the Dennison-Uhlenbeck potential.     

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.     

The Microwave Spectrum of m-Tolunitrile: Methyl Internal Rotation and N Nuclear Quadrupole Coupling

The microwave spectrum of m-tolunitrile (3-methylbenzonitrile, m-C₆H₄CH₃CN) has been investigated in the frequency range from 1 to 4 and 8 to 26.5 GHz. The spectra in the two lowest states of internal methyl rotation (m = 0, ±1) were recorded by means of pulsed molecular beam Fourier transform microwave (MB-FTMW) spectrometers. The interpretation of the spectra was based on an asymmetric frame–symmetric top Hamiltonian with inclusion of centrifugal distortion terms and first-order contributions from ¹⁴N nuclear quadrupole coupling. A least-squares analysis yielded the rotational constants A = 3295.9103(10) MHz, B = 1199.1188(2) MHz, C = 883.9223(1) MHz, all elements of the nuclear quadrupole coupling tensor χ_aa = −3.626(1) MHz, χ_bb = 1.684(1) MHz, χ_cc = 1.943(1) MHz, and χ_ab = −1.870(3) MHz, as well as the threefold barrier to internal rotation, V₃ = 14.2 cm⁻¹, and the angle between the internal rotor axis and the principal moment of inertia a axis, θ = 42.66°, using fixed values for the sixfold barrier term V₆ (−11 cm⁻¹) and the moment of inertia of the methyl top I_α (3.16 u Ų).     

The far-infrared spectrum and spectroscopic parameters of PH3 in the ground state

The far-infrared spectrum of phosphine, PH₃, was recorded in the region between 30 and 200 cm⁻¹ at a resolution of 0.002 cm⁻¹. ΔJ = +1, ΔK = 0 rotational transitions in the ground state were measured and assigned up to J″ = 22 and K = 19. These transitions were analyzed together with the presently available microwave and submillimeter-wave data on the basis of different formulations of the rotational Hamiltonian, which included Δk = ±3 and/or Δk = ±6 interaction terms. An upper limit for the constant of the inversion splitting was obtained by fitting the same transitions to an appropriate inversion-rotational Hamiltonian. Rotational transitions in the v₂ = 1 and v₄ = 1 vibrational states were also observed.     

The π ← n transition of formic acid

New sharp bands of formic acid have been observed in the near ultraviolet at the long wave-length end of the previously observed diffuse band system (2250–2500 Å) by considerably extending the absorption path length. Both the diffuse and sharp bands belong to the same vibrational system which is assigned to the π^* ← n electronic transition in the carbonyl group. Extensive progressions are observed in the carbonyl stretching frequency which is greatly reduced in the excited state (fundamental ν₃′ ≈ 1080 cm⁻¹) and many intervals of about 400 cm⁻¹ are assigned to the OCO bending frequency ν₇′.A band contour analysis of the 2593 Å band shows that the molecule is nonplanar in the excited state because of the magnitude and sign of the inertial defect. From this analysis, the rotational constants for the excited state are S^‘=1.8755, B^‘0.4042, C^‘=0.3378cm⁻¹ By the plausible assumption that the important changes in the molecule are in the C=0 bond length, the OCO angle, and the nonplanarity due to the formyl hydrogen, the following excited state parameters are derived.rC=0 = 1.407A.The changes in formic acid are closely analogous to the changes in formyl fluoride as a result of the π^* ← n transition.     

Ab initio determination of the torsional spectra of acetic acid

The torsional potential energy surface and the favourite geometries of acetic acid are determined with MP4/cc-p VTZ ab initio calculations. The molecule shows two planar trans and cis conformers whose energy difference is 1882.7 cm^?1. Both minimum energy geometries are separated by a barrier of 4432.1 cm^?1. The most stable trans-conformer shows a quite low methyl torsion barrier of 169.8 cm^?1. The roto-torsional energy levels have been calculated up to J = 10. The two torsional fundamental frequencies of the trans-conformer, the methyl and the OH torsion are 82.857 (A²) and 77.050cm^?1 (E) and 568.532 (A²) and 568.418cm^?1 (E). The V₃ barrier causes a splitting of 0.315cm^?1 in the ground vibrational state where the quartic centrifugal distortion constants have been predicted to be D_J = 90.4kHz, D_JK = ?301.5kHz and D_K = 165.4kHz. Finally, the far-infrared spectra of two isotopomers have been simulated from ab initio calculations.     

Analysis of the fundamental asymmetric stretching mode of T2O

The ν₃ mode of T₂O, observed at 0.04 cm⁻¹, has been analyzed. By an iterative process of fitting and assignment 210 lines were assigned; 167 of these were used in the final fitting. The standard deviation of the fit is 0.023 cm⁻¹. The rotational constants, A, B, and C, as well as the quartic distortion constants, have been evaluated for the excited state.     

High-Resolution Laser Spectroscopy of YbCl: The AΠ–XΣ Transition

The A²Π–X²Σ⁺ transition of ¹⁷⁴Yb³⁵Cl and ¹⁷²Yb³⁵Cl has been rotationally analyzed for the first time. Doppler-limited laser excitation spectroscopy with selective detection of fluorescence was used to obtain spectra of the 0–0 and 1–0 bands with a measurement accuracy of approximately 0.0035 cm⁻¹. Resolved fluorescence was used to record the 0–1, 0–2, and 0–3 bands and to unequivocally assign the rotational numbering, N, to the laser excitation spectra. In total, over 1300 line positions have been measured and assigned for each of the two isotopomers and employed in least-squares fits of molecular parameters. The principal results for the A²Π state are A_e = 1491.494(2) cm⁻¹ and R_e = 2.4433(1) Å, and for the X²Σ⁺ state, R_e = 2.4883(2) Å and γ_e = 4.59(2) × 10⁻³ cm⁻¹. The interaction between the X²Σ⁺ and A²Π states has been investigated and is shown to be the main contributor to the spin–rotation splitting in the ground state.     

The spectra and structure of indium iodide: The rotational and vibrational constants of the A0 state

The rotational constants of the A0⁺ state of InI are reported for the first time as B_e = 0.038077 cm⁻¹ and α_e = 0.0002373 cm⁻¹, while T_e = 24402.91 cm⁻¹ for the A0⁺-X0⁺ transition. Accurate vibrational constants for both the A0⁺ and X0⁺ states are computed from the derived band origins.