Rotational Spectrum of Hydrazine in the Submillimeter Range

In the submillimeter range, from 345 to 410 GHz, 472 new rotational transitions in the vibrational ground state of hydrazine have been measured. The majority of recorded lines form eight K=2←1 Q-branches with J≤30, each branch being split due to internal rotation in hydrazine. New rotational transitions together with 443 known earlier have been fitted to an effective group-theoretical Hamiltonian originally developed by Hougen (J. T. Hougen, J. Mol. Spectrosc.89, 296-327 (1981)). A reasonable fit was obtained using several ΔK=0, ±1, ±2, and ±4 matrix elements. For all 915 rotational transitions an overall standard deviation of the fit of 0.59 MHz was obtained using 32 parameters. These were 14 asymmetric rotor constants, 16 NH₂ inversion parameters, and 2 internal rotation parameters.     

A rotational Hamiltonian for the ground vibrational state of hydrazine

A Hamiltonian matrix suitable for fitting rotational energy levels of the hydrazine molecule in its ground vibrational and electronic states was obtained. This matrix is of dimension 16 × 16, where the 16 functions labeling the rows and columns consist of the two members of a near-prolate asymmetric rotor doublet (with given | K_a |) for the eight different, but chemically equivalent, conformers which the molecule can reach by various combinations of -NH₂ inversion at either end of the molecule, and internal rotation about the NN bond. The matrix is derived in a phenomenological fashion, by applying group theoretical arguments to a model in which tunneling among the various frameworks is assumed to be very slow compared with the vibrational frequencies. A comprehensive treatment of the large-amplitude vibrational potential surfaces and associated tunneling pathways has not been carried out, nor have quartic (J⁴) centrifugal distortion effects been considered in a systematic fashion. Preliminary fits indicate that the model developed can be used to fit the hydrazine microwave data in a consistent fashion, and a full treatment of such data has been undertaken.     

Inversion and internal rotation in methyl-d-amine: Theory

A theory has been developed for an analysis of the microwave spectrum of the CH₂DNH₂-type molecule which has an asymmetric internal rotor. First, the Hamiltonian matrix was expressed on the basis of localized wavefunctions, each of which corresponds to a conformer vibrating in the vicinity of a potential minimum. Next, by a symmetrization, the Hamiltonian matrix was factored into four submatrices. By solving these matrices, a general view of the energy-level structure has been given, which should be useful for an interpretation of the observed rotational spectrum. It has been shown that the inversion splitting in each level of the trans form molecule should be sensitive to the amount of trans-gauche coupling through tunneling and therefore the relative height of a trans level with respect to a gauche level can be determined from an observation of the inversion splitting in the trans levels.     

Analysis of the laser magnetic resonance spectrum of HO2

An analysis of the previously detected laser magnetic resonance spectrum of HO₂ is carried out by (i) assigning M_J quantum numbers to each observed Zeeman line, (ii) determining the quantum numbers (N′_Ka′Kc′-N″_Ka″Kc″) and energies of the zero-field asymmetric rotor transitions involved, and (iii) determining the values of the zero-field spin-rotation doublet splittings in the upper and lower states of each asymmetric rotor transition. The rotational transitions obtained lie in the region 50–150 cm^?1, with quantum numbers 4 ≤ N ≤ 19 and 1 ≤ K_a ≤ 4. They are fit to an asymmetric rotor program to obtain the three rotational constants A, B, C and the three symmetric-top centrifugal distortion constants D_K, D_NK, D_N. The spin splittings are fit to an approximate theoretical expression involving two adjustable linear combinations of components of the spin-rotation interaction tensor ?. Because of the lack of spectra from other isotopic species, a unique molecular geometry cannot be derived.     

Far infrared Fourier-transform spectroscopy of mono-deuterated hydrogen peroxide HOOD

We present the gas phase spectrum of singly deuterated hydrogen peroxide, HOOD, in its vibrational ground state, recorded by the high resolution Fourier-transform interferometer located at the AILES synchrotron beamline connected to SOLEIL. More than 1000 transitions in the range from 20 to 143 cm^?1 were assigned, leading to a set of preliminary rotational and centrifugal distortion constants determined by least squares fit analysis. All transitions are split by the tunneling motion of a hindered internal rotation. The splitting has been determined to be 5.786(13) cm^?1 in the torsional ground state and it shows a dependence on the rotational quantum number K_a. Some perturbations were not treated yet, but the present analysis permits to obtain a preliminary set of parameters.     

Microwave spectroscopy of furfural in vibrationally excited states

The results of microwave spectrum investigation of the excited vibrational states of furfural in the frequency range between 49 and 149 GHz are reported. In total 15 excited vibrational states (9 for trans-furfural and 6 for cis-furfural) were assigned and analyzed. Six of the 15 investigated states were assigned for the first time. Accurate values of rigid rotor and quartic centrifugal distortion constants of asymmetric top Hamiltonian have been determined for 13 excited states. Also for some states several sextic and octic level constants were needed in order to fit the data within experimental accuracy. The v_t = 3 and v_s = 1, v_a = 1 states of trans-furfural were found to be strongly perturbed and only rotational transitions with low K_a values can be reliably identified in this study.     

The rotational spectrum of the ground state of methylamine

Recent progress is reported in measuring, assigning, and fitting the rotational spectrum of the ground vibrational state of methylamine, CH₃NH₂, a spectrum complicated both by internal rotation of the methyl top and by inversion of the amino group. New measurements of 513 rotational transitions with J up to 30 and K up to 9 were carried out between 49 and 326 GHz using the millimeter-wave spectrometer in Kharkov. After removing the observed quadrupole hyperfine splittings, these new data along with previously published measurements were fitted to a group-theoretical high-barrier tunneling Hamiltonian from the literature, using 53 parameters to give an overall weighted standard deviation of 0.80 for 850 far-infrared and 673 microwave transitions in the ground state. The root-mean-square deviation of 0.018 MHz obtained for 346 millimeter-wave transitions measured with 0.020 MHz uncertainty represents an approximately 30-fold improvement in fitting accuracy over past attempts.     

The Rotational Spectrum of ClClO2 in Its v4=1 and v6=1 Vibrationally Excited States: An Example of Strong Coriolis Interaction

The two lowest vibrational states of ³⁵Cl³⁵ClO₂, v₄=1 (A′) and v₆=1 (A″), were investigated between 223 and 500 GHz. More than 250 rotational transitions were recorded with J and K_a up to 71 and 34, respectively. The spectra are heavily perturbed by strong c-type and weaker a-type Coriolis interactions. Near degeneracies of rotational levels of the two vibrational states having ΔJ=0, ΔK_a=5 to 1, and ΔK_a+ΔK_c= odd cause moderate to severe perturbations in the rotational structure, preventing the states from being fit as isolated ones. Distortions in the hyperfine structure facilitated the assignment of rotational quantum numbers. Several resonantly interacting levels with ΔK_a=5 to 2 were accessed, and a number of transitions between the states were observed. While resonant Coriolis interaction with ΔK_a=1 occurs only at K_a>40, the effects of this interaction are so severe that nonresonant interaction considerably perturbs the highest K_aQ-branches observed. The observed transitions could be fit to within experimental uncertainties employing the first-order Coriolis coupling constants fixed to those from the harmonic force field, sextic distortion constants fixed to those of the ground state, and some higher order Coriolis terms. The energy difference calculated from the fit agrees well with that obtained from the matrix-isolation infrared spectrum. Quadrupole coupling constants were determined for both Cl nuclei and both vibrational states.     

Vibration-internal rotation-rotation Hamiltonian of an asymmetric top molecule with C3v internal rotor

An expression for the kinetic energy part of the vibration-torsion-rotation Hamiltonian of an asymmetric top molecule containing a C_3v internal rotor has been derived. The terms for various interactions in the molecule, viz. Coriolis interaction between rotation (both overall and internal rotation) and vibration, centrifugal distortion and anharmonicity of molecular vibrations induced by the internal, and overall rotation of the molecule, have been formulated. For a planar molecule with C_s symmetry we have obtained the vibrationally averaged rotation-internal rotation Hamiltonian. Diagonalization of this Hamiltonian for a particular vibrational state will yield the rotation-internal rotation energy levels and hence the transition frequencies. These data will be useful for analysis of high-resolution infrared spectra obtained by laser or Fourier transform spectroscopy of nonrigid molecules with internal rotor. We also present a set of quartic centrifugal distortion coefficients associated with rotation and internal rotation. These data will be helpful for evaluation of vibrational potential constants of the orthorhombic asymmetric top molecules.     

The ground state torsion-rotation spectrum of propargyl alcohol (HCCCH2OH)

The rotation-torsion spectrum of the asymmetric frame-asymmetric top internal rotor propargyl alcohol (HCCCH₂OH) has been extended into the millimeter and submillimeter wave spectral regions. Over 2000 ground torsional state transitions have been measured and analyzed up to rotational quantum numbers J = 80 and K_a = 33 through a frequency of 633 GHz. The newly measured transitions were added to approximately 200 previously reported and now unambiguously assigned microwave transitions to comprise a data set of 2390 transitions which has been fit to 59 kHz using a reduced axis method (RAM) Hamiltonian. The ground state has been confirmed to consist of a symmetric and an antisymmetric gauche conformer with no spectroscopic evidence of stable trans conformer. A complete set of rotation and distortion constants through 6th order and a number of the 8th and one 10th order constants for the normal species are presented along with those determined from a re-analysis of the existing OD species data. The a and b symmetry Coriolis interaction constants and the gauche+ gauche− tunnelling frequency of 652389.4 MHz has been determined for the OH species while the b symmetry Coriolis interaction and the 213 480 MHz tunnelling frequency were determined for the OD species.     

Fourier transform microwave spectroscopy of the reactive intermediate monoiodosilylene,HSiI and DSiI

The pure rotational spectra of three silicon isotopologues of HSiI and two isotopologues of DSiI have been recorded by pulsed-jet Fourier transform microwave (FTMW) spectroscopy. Neon was passed over dry ice cooled H₃SiI or D₃SiI and introduced into the pulsed valve of the FTMW spectrometer. The monoiodosilylenes HSiI and DSiI were produced in situ with a 1000 V DC-discharge nozzle. Only a-type transitions occur in monoiodosilylene from 6 to 26 GHz. We observe K_a = 0 a-type transitions for H²⁸SiI, H²⁹SiI, H³⁰SiI, and D²⁹SiI, and both K_a = 0 and 1 a-type transitions for D²⁸SiI. Rotational constants, centrifugal distortion constants, iodine nuclear quadrupole coupling constants, and nuclear spin–molecular rotation constants were measured.     

The molecular conformation and dipole moment of thiane from the microwave spectrum

The microwave spectrum of thiane, a heterocyclic analog of cyclohexane, has been studied in the region 26.5–40 GHz. The molecule is a highly asymmetric rotor (κ = 0.050154). From the analysis of both the a-type and c-type transitions, the rotational constants determined are (in MHz): A = 3992.719, B = 3005.812, and C = 1914.683. A study of the Stark effect has yielded the dipole moment components (in Debye units) μ_a = 1.684 ± 0.009, μ_c = 0.578 ± 0.002, which give a total dipole moment of μ = 1.781 ± 0.010. Comparison of the spectral data from tetrahydropyran, thiane, and 1,4-thioxane demonstrates the similarity in structure of these three compounds. It is found that a very reasonable set of structural parameters can be found which adequately fits the spectral data of all three molecules.     

Fourier transform microwave spectrum of the CO-dimethyl sulfide complex

The rotational spectrum of the CO-dimethyl sulfide (DMS) complex was measured in the frequency region from 4.8 up to 25 GHz by Fourier transform microwave spectroscopy. For the normal species 27 a-type and 57 c-type transitions were observed, while 16 and 8 c-type transitions were assigned for the species with ³⁴S and ¹³C in the DMS moiety, respectively, in natural abundance. In addition, 7 a-type and 48 c-type transitions were assigned for the complex with the ¹³CO enriched species as a component and 9 a-type and 42 c-type transitions for the complex with enriched C¹⁸O. No splitting was observed, which could be ascribed to the tunneling motion of the CO between two possible potential minima around DMS, while many transitions were split by the internal-rotation of the two methyl tops of the DMS unit. In cases where the K-type splitting was close to the methyl internal-rotation splitting, forbidden transitions were observed which apparently followed b-type selection rules. All of the observed transition frequencies for the normal species were analyzed simultaneously using a two-top internal-rotation and rotation Hamiltonian. The potential barrier height V₃ to internal rotation of the methyl groups of the DMS was determined to be 745.5 (30) cm⁻¹. The transition frequencies observed for all the isotopomers were analyzed using an asymmetric-rotor rotational Hamiltonian, to determine rotational and centrifugal distortion constants. The r_s coordinates calculated from the observed rotational constants led to the conclusion that the CO moiety was located in a plane perpendicular to the skeletal plane of the DMS and bisecting its CSC angle. This structure of the CO-DMS is very much different from that of the CO-DME, in which the CO is located in the DME skeletal plane. The distance between the centers of gravity of the two moieties, R_cm, was calculated to be 3.789 Å for the CO-DMS, which is longer by only 0.11 Å than that in the CO-DME complex: 3.68 Å, in spite of the fact that the van der Waals radius of the S atom is much larger than that of the O atom. The small difference in R_cm is, in part, ascribed to the location of the CO relative to the DMS/DME. The more important reason is that the intermolecular bonding of the CO-DMS is stronger than that of CO-DME; by assuming a Lennard-Jones-type potential, the force constant of the van der Waals stretching mode and the dissociation energy were estimated to be 2.7 Nm⁻¹ and 3.3 kJ mol⁻¹, respectively, which were larger than those of the CO-DME: 1.4 Nm⁻¹ and 1.6 kJ mol⁻¹.     

The microwave spectra of ortho-fluorotoluene with the asymmetric internal rotors CH2D and CD2H

The rotational spectra of αd₁- and αd₂-ortho-fluorotoluene in the ground state of the methyl group torsion have been measured. The evaluation of the spectra has been based on the theory for the internal rotation of an asymmetric internal top formulated earlier by several authors. The barrier potential being threefold symmetric (V₃), each torsional level consists of three nondegenerate substates, designated as sy and ±asy. The sy-state is assigned to the conformation with the unique methyl hydrogen isotope within the molecular heavy-atom plane (sy-rotamer), while the ±asy-states belong to the respective out-of-plane conformation (asy-rotamer). In the torsional ground state the level spacing between the ±asy substates is very small and numerous accidental close degeneracies are present between the rotational level systems based on these torsional substates. The rotational levels involved are strongly perturbed by the coupling between molecular overall rotation and internal rotation. Large deviations from a rigid rotor spectrum and (+) ? (?) intersystem (“tunneling”) transitions are observed. The spectrum of the asy-rotamer can be well reproduced by a “two-dimensional” Hamiltonian containing 11 “rotational constants,” 9 of which are determined by a fit to the spectrum. Several are sufficiently barrier-dependent to derive V₃. We obtain (in cal/mole) 567 ± 48 for αd₁-ortho-fluorotoluene, 711 ± 40 for the αd₂-isotope. The deviations from 649 cal/mole for the normal isotope are appreciable, probably indicating shortcomings of the semirigid model. The sy-rotamer presents a rigid rotor spectrum.     

Fourier transform and CARS spectroscopy of the ν1 and ν3 fundamental bands of 14NH3

The ν₁ and ν₃ fundamental bands of ¹⁴NH₃ have been measured using the techniques of Fourier transform and coherent anti-Stokes Raman spectroscopy. The effective values of the band origins, rotational and centrifugal distortion constants, and parameters of the vibrational-rotational interactions have been obtained by analyzing these bands as essentially regular parallel and perpendicular bands, with the “off-diagonal” local resonance interactions excluded from the fit. The “diagonal” l-type resonance effects have been included into the analysis of the ν₃ band for the +l, K = 1 and ?l, K = 2 levels.     

Study of the torsion-rotation Hamiltonian for symmetric tops using the millimeter wave spectrum of methyl silane

The torsion-rotation Hamiltonian for symmetric tops has been tested in methyl silane by combining recent anticrossing molecular beam measurements in the ground torsional state (v = 0) with pure rotational spectra taken for v as high as 4. The earlier microwave data set which consisted of J = 1 ← 0 and 2 ← 1 has been greatly extended by studying millimeter transitions for J = 4 ← 3, 5 ← 4, and 13 ← 12. An analysis of the 72 rotational frequencies for v ≤ 2 and the 15 anticrossing data for v = 0 yielded an excellent fit using 14 rotational, torsional, and distortion constants including the effective values for the A rotational constant and the barrier height V₃. No satisfactory fit could be obtained when the data set was extended to include measurements for (v = 3) or (v = 4). For each of these higher torsional levels, the difference between the observed frequencies and the predictions based on the best (v ≤ 2) constants can be expressed in terms of a shift δB_v in the B rotational constant, where δB_v is a smooth function of the torsional energy. This disagreement is of particular interest because it may result from the fact that the molecule passes from hindered to free rotation as v is increased from 2 to 4. The possibility of perturbation by a low-lying vibrational level is considered briefly. The information contained in the different types of spectra is discussed; the redundancy relations are treated and a Fourier expansion of the diagonal torsional matrix elements is introduced. For ¹²CH₃²⁹SiH₃, ¹²CH₃³⁰SiH₃, and ¹³CH₃²⁸SiH₃ pure rotational spectra for v = 0 were studied briefly in natural abundance. The results were combined with existing data for two deuterated symmetric rotors to obtain a structure based only on symmetric top rotational constants.     

Analysis and fit of the Fourier-transform microwave spectrum of the two-top molecule N-methylacetamide

The jet-cooled Fourier-transform microwave spectrum of N-methylacetamide (CH₃NHC(O)CH₃), a molecule containing two methyl tops with relatively low barriers to internal rotation, has been recorded and fit to nearly experimental uncertainty. Measurements were carried out between 10 and 26 GHz, with the nitrogen quadrupole splittings resolved for many transitions. The permutation-inversion group for this molecule is G₁₈ (not isomorphic to any point group), with irreducible representations A₁, A₂, E₁, E₂, E₃, and E₄. One of these symmetry species and the usual three asymmetric rotor quantum numbers J_KaKc were assigned to each torsion-rotation level involved in the observed transitions. F values were assigned to hyperfine components, where . Transitions involving levels of A₁ and A₂ species could be fit to an asymmetric rotor Hamiltonian. The other transitions were first fit separately for each symmetry species using a Pickett-like effective rotational Hamiltonian. Constants from these fits show a number of additive properties which can be correlated with sums and differences of effects involving the two tops. A final global fit to 48 molecular parameters for 839 hyperfine components of 216 torsion-rotation transitions involving 152 torsion-rotation levels was carried out using a newly written two-top computer program, giving a root-mean-square deviation of observed-minus-calculated residuals of 4 kHz. This program was written in the principal axis system of the molecule and uses a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. Such an approach requires quite long computation times, but it is much less prone to subtle programming errors (a consideration felt to be important since checking the new program against precise fits of low-barrier two-top molecules in the literature was not possible). The two internal rotation angles in this molecule correspond to the Ramachandran angles ψ and φ often defined to describe polypeptide folding. Barriers to internal rotation about these two angles were found to be 73 and 79 cm⁻¹, respectively. Top-top coupling in both the kinetic and potential energy part of the Hamiltonian is relatively small in this molecule.     

The microwave spectrum of 1-pyrazoline

The microwave spectrum of 1-pyrazoline has been observed from 18 to 40 GHz in the six lowest states of the ring-puckering vibration. It is an a-type spectrum of a near oblate asymmetric top. Each vibrational state has been fitted to a separate effective Hamiltonian, and the vibrational dependence of both the rotational constants and the quartic centrifugal distortion constants has been observed and analyzed. The v = 0 and 1 states have also been analyzed using a coupled Hamiltonian; this gives consistent results, with an improved fit to the high J data. The preferred choice of Durig et al. [J. Chem. Phys.52, 6096 (1970)] for the ring-puckering potential is confirmed as essentially correct, but the A and B inertial axes are shown to be interchanged from those assumed by Durig et al. in their analysis of the mid-infrared spectrum.     

Rotational spectrum of the Ar–dimethyl sulfide complex

The rotational spectra of three isotopomers of the Ar–dimethyl sulfide (DMS) complex – normal, ³⁴S, and ¹³C species – were measured in the frequency region from 3.7 up to 24.1 GHz by Fourier transform microwave spectroscopy. The normal species yielded 43 a-type and 79 c-type transitions. No Ar tunneling splitting was observed, while many transitions were split by the internal rotation of the two methyl tops of the DMS unit. In cases where the K-type splitting was close to that due to methyl internal-rotation, several forbidden transitions were observed that followed b-type selection rules. All of the observed transition frequencies were analyzed simultaneously using a phenomenological Hamiltonian also used in previously published work describing the Ar–dimethyl ether (DME) and Ne–DME complexes. The rotational and centrifugal distortion constants and the potential barrier height to methyl-top internal rotation, V₃, were determined. The rotational constants were consistent with an Ar–DMS center of mass (cm) distance of 3.796 (3) Å and a S–cm–Ar angle of 104.8 (2)°. The V₃ potential barrier obtained, 736.17 (32) cm⁻¹, was 97.8% of the DMS monomer barrier. By assuming a Lennard–Jones-type potential, the dissociation energy was estimated to be 2.4 kJ mol⁻¹, which was close to the value for Ar–DME, 2.5 kJ mol⁻¹.     

The a-type rotational spectrum of isocyanic acid,HNCO, in the excited bending states

Rotational transitions of HNCO in the v₄ = 1, v₅ = 1, and v₆ = 1 vibrational states have been measured. The assignment of the a-type ^qR_K and ^qQ₁ branches has been made with the help of a qualitative discussion of the vibration-rotation interactions. Effective rotational and centrifugal distortion constants have been determined precisely for each vibrational K_a-rotational state, up to K_a = 4 for the lowest excited state and K_a = 3 for the other two excited states. The K_a dependence of the effective rotational constants B and D was observed to be quite anomalous for some of the transitions because of the a-type Coriolis interactions and accidental b-type Coriolis resonances. From a discussion of the selection rules and the effect on B and D of the interactions, the first excited state of the out-of-plane vibration, ν₆, has been assigned definitely to the second lowest excited vibrational state of HNCO.