The millimeter-wave spectrum of acetaldehyde in its two lowest torsional states

A large number of new millimeter-wave spectral lines of gaseous acetaldehyde have been measured at frequencies up to 250 GHz. These lines arise from rotational transitions of acetaldehyde in its two lowest (v_t = 0,1) torsional states and involve angular momentum quantum numbers J ≤ 12 and K ≤ 10. A global data set consisting of 562 lines has been obtained by combining the millimeter-wave lines with previously measured lower frequency data involving the two lowest torsional states. This data set has been analyzed via an internal axis method previously used to study the spectra of CH₃OH and CH₃SH. The root-mean-square deviation of the fit is only 685 kHz. An analogous least-squares fit to 335v_t = 0 lines yields a root-mean-square deviation of 269 kHz.     

The rotational spectrum of methoxyethyne: Centrifugal distortion and internal rotation analysis

In the rotational spectrum of methoxyethyne 173 new transitions (J ≤ 30) have been measured between 150 and 240 GHz. In the centimeter range 25 new transitions (J ≤ 11) of the first excited torsional state have also been assigned. An overall fit of the measurements using a structure relaxation model has allowed us to accurately determine the internal rotation parameters. For the A substate effective rotational parameters are given which allow the calculation of transition frequencies of possible astrophysical interest.     

The microwave spectrum of methyl isothiocyanate

The microwave spectrum of methyl isothiocyanate, CH₃NCS, has been measured in the range from 5 to 26 GHz. Almost 300 lines have been assigned to the a-type J + 1 ← J rotational transitions (J = 0 to 4) arising from molecules in the ground state and excited torsional and CNC bending states. The observed spectrum has been analyzed and successfully interpreted in terms of the five-dimensional quasi-symmetric top molecule model accounting explicitly for the large-amplitude CNC bending motion, and internal and overall rotation. In the least-squares fitting to the observed transition energies the values of the parameters of the CNC bending-torsional potential function have been determined.     

The microwave spectrum of methyl isocyanate

The microwave spectrum of methyl isocyanate, CH₃NCO, has been measured in the range from 8 to 40 GHz. More than 200 lines have been assigned to the a-type J + 1 ← J rotational transitions (J = 0 to 3) arising from molecules in the ground state and excited torsional and CNC bending states. The observed spectrum has been analyzed and successfully interpreted in terms of the five-dimensional quasi-symmetric top molecule model accounting explicitly for the large-amplitude CNC bending motion, and internal and overall rotation. In the least-squares fitting to the observed transition energies the values of the parameters of the CNC bending-torsional potential function have been determined.     

A study of the rotational-torsional spectrum of hydrogen peroxide between 80 and 700 GHz

One hundred and thirty-three new rotational transitions that occur between pairs of torsional sublevels in the ground vibrational state of hydrogen peroxide have been measured in the 80 to 700 GHz region of the spectrum. These data, in combination with the 50 previously measured lines, have been theoretically analyzed to within the expected experimental uncertainty (?0.1 MHz). The rotational constants for the τ = 1, 2 state are (in MHz): A = 301 878.857 ± 0.015, B = 26 211.9019 ± 0.0059, C = 25 099.1400 ± 0.0059, and for the τ = 3, 4 state: A = 301 583.825 ± 0.075, B = 26 156.337 ± 0.13, C = 25 185.771 ± 0.13. The splitting between the torsional levels is 342 885.03 ± 0.05. These measurements and analysis accurately characterize the nine branches that make substantial contributions in the spectral region below 700 GHz.     

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.     

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.     

An investigation of centrifugal distortion in the microwave spectrum of formamide

Measurements of the microwave spectrum of formamide have been extended in order to account accurately for the effects of centrifugal distortion. A total of 22 new transitions involving J ≤ 29 have been measured for ¹⁴NH₂¹²CH¹⁶O in the ground vibrational state. Combined with previous observations, these transitions have been fit to a model containing five quartic distortion terms and seven sextic terms with a rms deviation of 64 kHz. A large number of resolved quadrupole shifts were fit with an rms deviation of 42 kHz. The remainder of the spectrum for J ≤ 30 has been calculated with standard deviations less then 3 MHz. Correct weighting of the observed transitions has been found to be important.     

Microwave spectrum of trans 3-fluorophenol in excited torsional states

The microwave rotational spectra of the trans conformer of 3-fluorophenol have been observed in excited torsional states and analyzed in the frequency range 12.0-43.0 GHz using conventional microwave and Radio-Frequency Microwave Double Resonance (RFMWDR) techniques. Analysis of the ground torsional state spectrum has been extended to higher rotational states. Least-squares analysis of three sets of rotational transitions yield rotational and centrifugal distortion constants for the ground and first two excited torsional states. A nonlinear behavior of the variation of inertial defect with the torsional quantum number was observed.     

THz Spectrum of Methanol-D4 and Microwave Spectrum of Methanol-D1

The sub-THz spectra of CD₃OD have been observed in the frequency ranges of 461-486 GHz and 596-610 GHz using the Backward Wave Oscillator based Technique. The 218 transitions of CD₃OD are newly assigned to J=31 and K=12 in the first three torsional states (n=2). The assigned lines include several new series of both a-type and b-type transitions. These THz transitions combined with the previous published millimeter-wave (MMW) and microwave (MW) data and recently observed high resolution Fourier transform far infrared (FIR) spectra have been used in a global fit. The data set contains 1320 MW, MMW, SMMW and FIR transitions with n=2, J=31 and K=12. Using the reduced torsion-rotational Hamiltonian with 67 parameters the fit converges with an RMS deviation of 277 kHz for the MW transitions and of 0.00024 cm^-1 for the FIR transitions. Thus molecular parameters are well determined. The MW spectrum of CH₂DOH (in the range 97.7 -118.0 and 128.5-146.5 GHz) is also presented which will help astronomers for astrophysical detection and theoretical spectroscopists to get further information on torsion-rotation-vibration interaction in an internal rotor with an asymmetric top.     

Global fit of rotational transitions of methyl formate (HCOOCH3) in the ground and first excited torsional states

The microwave spectrum of methyl formate (HCOOCH₃) has been observed in the frequency range from 7 to 200 GHz. New 348 lines were assigned to E-species in the first excited torsional state. By combining these lines with previously reported data, a global analysis of A- and E-species lines of the ground and first excited torsional states were carried out on the basis of the internal axis method. A total of 3862 transitions were fitted to a Hamiltonian model involving 69 molecular parameters to a weighted unitless standard deviation of 1.96.     

The millimeter wave rotational spectrum of pyruvic acid

The rotational spectrum of pyruvic acid has been investigated for the first time in the millimeter-wave region, at 160-314 GHz, and also in supersonic expansion, at 10-17.4 GHz. The analysis of the broadband spectra recorded in this work was carried out with the newly developed AABS software package for Assignment and Analysis of Broadband Spectra, and precise spectroscopic constants are reported for the ground state, the first excited state of the low-frequency skeletal torsional mode ν₂₄, and the first excited state of the methyl torsional mode ν₂₃. Limited results have also been obtained for several higher excited states. The dataset for the ground state currently exceeds 1500 lines and for both the A and E internal rotor sublevels spans the complete range of values of K_a at the mid values of J for the measured transitions. The results were analysed with three freely available computer programs employing different strategies for dealing with internal rotation and a comparative discussion of their merits is made.     

The v10=1 Level of Propyne, H3CCCH, and Its Interactions with v9=1 and v10=2

Almost 300 new rotational transitions within the fundamental vibrational level v₁₀=1 of propyne have been measured in selected regions between 495 and 925 GHz spanning the quantum numbers 28≤J≤54 and 0≤K≤16. The accuracies are mostly between 10 and 20 kHz. In addition, the J″=4 and 5 transitions near 85 and 103 GHz have been remeasured. Simultaneous analyses with refined rovibrational data have been performed, showing that even this lowest and seemingly isolated vibrational level needs a global treatment when high K transitions are involved. The global model with the v₁₀=1 level coupled to the next higher cluster of levels, v₁₀=2/v₉=1, by Fermi and Coriolis resonances is necessary for a quantitative reproduction of both the rovibrational and rotational data within their experimental uncertainties. Included are also improved ground state spectroscopic parameters from a fit of previous pure rotational data and Δk=3 ground state combination loops as well as additional data obtained in course of the present study.     

Fourier Transform Infrared spectrum of the OCD bending mode in methanol-D1

The infrared (IR) spectra corresponding to OCD bending vibration of asymmetrically deuterated methanol species CH₂DOH have been recorded with a Fourier Transform Spectrometer. The spectrum shows a typical structure of a parallel a-type band. This is expected because the bending vibration mainly executed parallel to the symmetry axis The Q-branch lines are grouped closely around 896 cm⁻¹ and the P- and R-Branches show complex structure. Nonetheless it was possible to assign a-type P- and R-branch lines up to K value of 8 and J value up to about 20 in most cases. The Q-branch lines for higher K values can be followed to about J = 15, the presence of which confirmed the assignments. The observations suggest that in the OCD bend some energy levels are highly interacted by highly excited torsional state from the ground torsional state. A full catalogue is presented along with the effective molecular parameters. An intensity anomaly was also observed in the transitions. So far it has been possible to assign only transitions between e₀ ← e₀ states. Plausible explanations of intensity anomaly are presented. Lastly, a number of optically pumped far infrared (FIR) laser lines have been assigned either to exact or tentative quantum states. These assignments should prove valuable for production of new FIR laser lines.     

Microwave spectra and centrifugal distortion constants of oxetane

The microwave spectra of oxetane (trimethylene oxide) and its three symmetrically deuterated isotopic species have been observed on a Hewlett-Packard microwave spectrometer from 26.5 to 40 GHz. For the parent species, the β-d₂ and the αα′-d₄ species, about 300 lines have been assigned for each molecule, and for the d₆ species more than 600 lines have been assigned. The assignments range from v = 0 to v = 5 in the puckering vibration; although they are mostly Q transitions, either 3 or 4 R transitions have been observed for each vibrational state.The spectra have been interpreted using an effective rotational hamiltonian for each vibrational state, including five quartic distortion constants according to Watson's formulation, and a variable number of sextic distortion constants; in general, the lines are fitted to about ± 10 kHz. The distortion constants show an anomalous zig-zag dependence on the puckering vibrational quantum number, similar to that first observed for the rotational constants by Gwinn and coworkers. This is interpreted according to a simple modification of the standard theory of centrifugal distortion, involving the double minimum potential function in the puckering coordinate.     

Line shifts in the ν2, 2ν2, and ν4 bands of NH3 perturbed by O2

Pressure-induced line shift coefficients have been measured for more than 200 rovibrational lines of NH₃ perturbed by O₂ at room temperature (T = 295 K) in some branches of the ν₂, 2ν₂, and ν₄ bands. These lines with J values ranging from 1 to 13 are located in the spectral range 800-1800 cm⁻¹. Experiments were made with a high-resolution Fourier transform spectrometer. The treatment of vibration-rotation lines includes interference effects caused by the overlapping of lines. The O₂ pressure-induced shift coefficients have been derived from the non-linear least-squares multi-pressure fitting technique. The results illustrate a vibrational dependence of line shifts with vibrational quantum number. Most of the measured shifts are negative in the ν₄ band. They are positive for the ν₂ and 2ν₂ bands. The measured shift coefficients are compared with previous measurements and with those calculated from a semiclassical theory based upon the Robert-Bonamy formalism extended to the case of symmetric top molecule with inversion motion. The predictions are generally in satisfactory agreement with the experimental data. Analyses of measured and predicted results illustrate that these shifts mainly originate from the isotropic part of the intermolecular potential.     

A new joint analysis of the ground and first excited torsional states of methylformate

A global fit within experimental accuracy of microwave rotational transitions in the ground and first excited torsional states (v_t = 0 and 1) of methylformate (HCOOCH₃) is reported, which combines older measurements from the literature with new measurements from Kharkov. In this study the so-called ‘‘rho axis method’’ that treats simultaneously both A and E species of the ground and first excited torsional states is used. The final fit requires 55 parameters to achieve an overall unitless weighted standard deviation of 0.71 for a total of 10533 transitions (corresponding to 9298 measured lines) with rotational quantum numbers up to J ? 62 and K_a ? 26 in the ground state and J ? 35 and K_a ? 23 in the first excited torsional state. These results represent a significant improvement over past fitting attempts, providing for the first time a fit within experimental accuracy of both ground and first excited torsional states.     

Ground and first excited torsional states of acetamide

We present a first global study involving rotational levels in the lowest three torsional states of acetamide (CH₃CONH₂). New measurements of this spectrum, consisting of approximately 1600 lines and involving torsion-rotation transitions with J up to 20 and K_a up to 11, have been carried out between 49 and 149 GHz using the millimeter-wave spectrometer in Kharkov. After removing the observed quadrupole hyperfine splittings, the new data were combined with previously published measurements and fitted using a rho-axis-method (RAM) torsion-rotation Hamiltonian in conjunction with a new computer-automated v_t and K labeling algorithm. The final fit used 48 parameters to give an overall weighted standard deviation of 0.72 for 759, 587, and 265 lines belonging, respectively, to the ground, first, and second excited torsional states and 95 lines corresponding to Δv_t=1 transitions falling in the millimeter wave range. Separate root-mean-square (rms) deviations for the A (25 kHz) and E (27 kHz) species, as well as for the v_t=0 state (26 kHz), v_t=1 (25 kHz), v_t=2 (22 kHz), and Δv_t=1 transitions (35 kHz) indicate a similar quality of the fit for the two symmetry species and for the three torsional states. The RAM Hamiltonian was found to have rather good predictive ability when fits of only v_t=0 (or only v_t=0 and 1) transitions were used to calculate v_t=1 (or v_t=2) lines. In addition, the combination of this Hamiltonian and the computer-automated v_t and K labeling algorithm seems to provide a rather powerful tool for the precise assignment and fitting of torsion-rotation transitions in C_3v internal-rotor molecules with low torsional potential barriers.     

New measurements and analysis of the far-infrared spectrum of CH2DOH in the lowest torsional vibrational state (e0)

In this work the far infrared (FIR) absorption spectrum has been measured for the asymmetrically mono deuterated Methanol (CH₂DOH) species in the wavenumber range of 15–1200 cm⁻¹ better accuracy and signal/noise ratio than known before. Assignments have been made for b-type transitions in the lowest lying torsional vibrational state trans-(e₀) for a wide range of rotational angular momentum. The assignments have been rigorously confirmed by the residual loop defect methods. The ^rR–branch wavenumbers are analyzed by the usual state dependent expansion parameters and the Q-Branch origins. These origins have been used to calculate the torsional and torsional-rotation interaction contributions. These findings are in good agreement with predicted from the Hamiltonian model described in recent publications. A large number of assignments have also been made in the millimeter wave spectrum recorded earlier and thereby evaluated the asymmetry splitting parameters for 4 different axial rotational angular momentum quantum numbers. The analysis and interpretation of the spectra are reported. New assignments for about 260 transitions are included the text and a catalog of about 1500 transitions belonging to the e₀ species is prepared (Appendix 1) and is made available through the open server in “Research Gate” and will be freely available to others.