Millimeter and submillimeter wave spectroscopic investigations into the rotation-tunneling spectrum of gGg ethylene glycol, HOCH2CH2OH

The rotation-tunneling spectrum of the second most stable gGg^′ conformer of ethylene glycol (1,2-ethanediol) in its ground vibrational state has been studied in selected regions between 77 and 579 GHz. Compared to the study of the more stable aGg^′ conformer, a much larger frequency range was studied, resulting in a much extended frequency list covering similar quantum numbers, J?55 and K_a?19. While the input data were reproduced within experimental uncertainties up to moderately high values of J and K_a larger residuals remain at higher quantum numbers. The severe mixing of the states caused by the Coriolis interaction between the two tunneling substates is suggested to provide a considerable part of the explanation. In addition, a Coriolis interaction of the gGg^′ ground vibrational state with an excited state of the aGg^′ conformer may also contribute. Relative intensities of closely spaced lines have been investigated to determine the signs of the Coriolis constants between the two tunneling substates relative to the dipole moment components and to estimate the magnitudes of the dipole moment components and the energy difference between the gGg^′ and the aGg^′ conformers. Results of ab initio calculations on the total dipole moment and the vibrational spectrum were needed for these estimates. The current analysis is limited to transitions with quantum numbers J?40 and K_a?6 plus those having J?22 and K_a?17 which could be reproduced within experimental uncertainties. The results are aimed at aiding radioastronomers to search for gGg^′ ethylene glycol in comets and in interstellar space.     

The microwave spectrum of piperidine: Equatorial and axial ground states

The microwave spectra of piperidine and N-deuterated piperidine were investigated between 8 and 40 GHz. The ground states of both equatorial and axial conformers have been identified by both type-A and type-C transitions, and the substitution coordinates of the imino hydrogen have been determined for both conformers. Dipole-moment components for the equatorial conformer are μ_a = 0.178 D, μ_c = 0.80 D, μ = 0.82 D, and for the axial conformer are μ_a = 1.07 D, μ_c = 0.521 D, μ = 1.19 D. The quadrupole coupling constants for the axial conformer are: χ_aa = ?3.80 MHz, χ_bb = 2.91 MHz, χ_cc = 0.83 MHz and for the equatorial conformer χ_cc = ?4.83 MHz. The rotational constants indicate a significant flattening of the ring in axial piperidine compared with equatorial piperidine. The equatorial conformer is the more abundant; intensity measurements on several sets of lines indicate the excess energy of the axial conformer to be 3.1 ± 0.3 kJ mole^?1. This represents a significant change from our earlier reported value and is now more in line with measurements obtained by other methods.     

Microwave spectrum and structure of furfural

The microwave spectrum of furfural was investigated in the frequency range 7 GHz-21 GHz and 49 GHz-330 GHz. The ground and first torsional state of trans-furfural and ground state of cis-furfural were assigned and analyzed. A total of 1720 rotational lines with J up to 100 and K_a up to 53 were assigned to the ground state of trans-furfural, 1406 rotational lines with J up to 100 and K_a up to 48 were assigned to the first torsional state of trans-furfural and 2103 rotational lines with J up to 90 and K_a up to 48 to the ground state of cis-furfural. Accurate sets of centrifugal distortion constants for both conformations have been determined for the first time. The spectra of all ¹³C and ¹⁸O singly substituted isotopic species were observed in natural abundance in the 7 GHz-21 GHz range. Molecular structure co-ordinates, bond lengths and angles of the Kraitchman substitution type (r_s) and pseudo-Kraitchman type (r_pKr) are derived from the isotopic studies.     

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.     

The microwave spectrum and centrifugal distortion constants of difluoroborane,HBF2

The microwave spectra of two isotopic species of the unstable molecule difluoroborane have been measured between 8 and 36 GHz. Transitions have been measured up to J = 56 and K_a = 9 for H¹⁰BF₂, and up to J = 66 and K_a = 11 for H¹¹BF₂. Improved values for the rotational constants, boron nuclear quadrupole coupling constants, and quartic and sextic centrifugal distortion constants have been evaluated for both species.     

The rotational spectrum of water between 650 and 50 cm−1: H218O and H217O in natural abundance

In the far infrared spectrum of water, 121 H₂¹⁸O and 48 H₂¹⁷O pure rotational lines have been identified. From the line frequencies and a minimum number of literature values of near infrared rotation-vibration transitions and lines from microwave spectra, the ground-state energy levels have been calculated up to J = 12 and 11, respectively. For low J levels, the agreement with earlier calculations from other microwave lines and near infrared results is within the experimental error, but deviations up to 0.6 cm^?1 are found for high J, high K_a levels.     

Self-broadening coefficients in the ν7 band of ethylene at room and low temperatures

Using a tunable diode-laser spectrometer, we have measured self-broadening coefficients for a few transitions in the ν₇ fundamental band of C₂H₄ at 298 and 174 K. The studied transitions J^′, K_a^′, K_c^′←J, K_a, K_c with 3?J?17, 1?K_a?4, and 1?K_c?14 are located in the spectral range 919-982 cm⁻¹. The collisional widths are measured by fitting each spectral line with Voigt, Rautian, and speed-dependent Rautian profiles. The latter model provides larger broadening coefficients than the Rautian profile and still larger coefficients than the Voigt profile. An approximate semiclassical calculation performed by considering only electrostatic interactions leads to reasonable agreement with the experimental data. By comparing the results obtained at room and low temperatures, the temperature dependence of the self-broadening has been determined both experimentally and theoretically.     

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.     

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.     

1-Butyne microwave spectrum,barrier to internal rotation,and molecular dipole moment

Measurements of rotational transitions of 1-butyne have been made in the range of ～20–130 GHz. Both a-type transitions up to J = 46 and b-type transitions up to J = 42 have been measured and fitted to a rotational Hamiltonian which includes centrifugal distortion terms. In addition to the five quartic centrifugal distortion constants, three sextic coefficients had to be included to reproduce the observed frequencies to within experimental error. The results of the analysis are sufficient for the prediction of all strong transitions throughout the millimeterwave range. A barrier to internal rotation of the methyl group of 3.260 kcal/mole (1 kcal/mole = 4.18 kJ/mole) has been derived from the first excited torsional state. Analysis of the second-order Stark effect has led to an accurate determination of both μ_a and μ_b with μ_a = 0.763(3) D and μ_b = 0.170(4) D.     

The pure rotational spectra of the two lowest energy conformers of the asymmetric ether C4H9OC2H5

An experimental study has been performed shedding light on the conformational energies of the asymmetric ether n-butyl ethyl ether. Rotational spectroscopy between 7.8 GHz and 16.2 GHz has identified two conformers of n-butyl ethyl ether, C₄H₉OC₂H₅. In these experiments spectra were observed as the target compound participated in an argon expansion from high to low pressure causing molecular rotational temperatures to be below 4 K. For one conformer, 95 pure rotational transitions have been recorded, for the second conformer, 20 pure rotational transitions were recorded. Rotational constants and centrifugal distortion constants are presented for both butyl ethyl ether conformers. The structures of both conformers have been identified by exploring the multi-dimensional molecular potential energy surface using ab initio calculations. From the numerous low energy conformers identified using ab initio methods, the three lowest conformers were pursued at increasingly higher levels of theory, i.e. complete basis set extrapolations, coupled cluster methods, and also taking into consideration zero point vibrational energies. The two conformers observed experimentally are only revealed to be the two lowest energy conformers when high levels of quantum chemical methodologies are employed.     

Analysis of the Q-branch transitions of the a2ν2-sν2 band of ammonia

The Q-branch transitions of the a2ν₂-sν₂ band of ¹⁴NH₃ are analyzed on the basis of a polynomial expansion up to eighth order in J and K. A good fit to the data for J ≤ 11 and precise values of the molecular constants are obtained in a 13-parameter fit (ΔG_JJKK = ΔG_KKKK = 0). Results of the statistical analysis are used to calculate wavenumbers and variances for transitions with J ≤ 13 that have not yet been measured.     

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.     

Far infrared spectrum and spectroscopic constants of AsH3 in the ground state

The far ir spectrum of arsine, AsH₃, was recorded in the range 25–100 cm^?1 with a resolution of approximately 0.004 cm^?1. ΔJ = +1, ΔK = 0 rotational transitions were measured and assigned up to J″ = 12. These transitions, together with the presently available microwave and submillimeter-wave data and ground state combination differences, were analyzed on the basis of a rotational Hamiltonian which includes Δk = ±3 and Δk = ±6 interaction terms. The derived ground state molecular parameters reproduced the transition frequencies of both allowed and “perturbation allowed” transitions within the accuracy of the measurements. The equilibrium structure was determined for the AsH₃ molecule.     

Rotational spectra of quinoline and of isoquinoline: spectroscopic constants and electric dipole moments

Rotational spectra of quinoline and of isoquinoline have been observed in the centimeter- and millimeter-wave regions. The spectra were assigned on the basis of bands formed by high-J transitions, which were measured up to J″?128 and ν?234 GHz. Complementary measurements were also made on low-J, centimeter-wave spectra observed in supersonic expansion and with fully resolved nuclear quadrupole hyperfine structure. Accurate rotational, centrifugal distortion and hyperfine splitting constants for the ground states of both molecules are reported. The electric dipole moments for the two molecules were also determined from Stark effect measurements and are μ_a=0.14355(19), μ_b=2.0146(17), μ_tot=2.0197(17) D for quinoline, and μ_a=2.3602(21), μ_b=0.9051(14), μ_tot=2.5278(20) D for isoquinoline. The experimental observables were found to be rather accurately predicted by MP2/6-31G** ab initio calculations, and corresponding molecular geometries are also reported.     

Temperature dependence of pressure broadening of NH3 perturbed by H2 and N2

The temperature dependence of pressure broadening of 134 rovibrational transitions of several branches in the ν₄ and 2ν₂ bands of ammonia perturbed by H₂ and N₂ has been measured using a high-resolution Fourier transform spectrometer. The temperature range covered during the experiments was between 235 and 296 K. The pressure-broadening linewidths were obtained using the method of multipressure fitting to the measured shapes of the lines. These broadenings were also calculated using a semiclassical model leading to a reasonable agreement with the observations and reproduces well the strong systematic experimental J and K quantum number dependencies. The retrieved values of the linewidths, along with those previously determined from the spectra at room temperature, were used to derive the temperature dependence of both H₂ and N₂ broadening of NH₃ lines. The broadening coefficients were shown to fit closely the well-known exponential law. For both experimental and theoretical results, the temperature exponent n has been obtained. Careful inspection of the experimental values shows that, contrary to the linewidths, the coefficient n is nearly K independent within each J multiplet. Also for a given J it does not seem to exhibit any noticeable variation with the type of rotational transition. On the other hand, the calculated n values exhibit a strong J and K systematic dependencies. n increases with K for a given J, decreases with J for a given K and are independent of the type of rotational transition.     

Inversion-rotation spectrum and spectroscopic parameters of 14ND3 in the ground state

The far-infrared spectrum of ¹⁴ND₃ has been recorded in the region between 30 and 220 cm^?1 at a resolution, before deconvolution, of approximately 0.004 cm^?1. ΔJ = +1, ΔK = 0, a ← s and s ← a inversion-rotation transitions have been measured and assigned up to J″ = 19. These transitions, the pure inversion-microwave transitions and ground-state combination differences from the analysis of the ν₂ and ν₄ bands have been fitted simultaneously to an inversion-rotational Hamiltonian which includes Δk = ±3 and Δk = ±6 interaction terms. The ground-state spectroscopic parameters obtained in this way reproduce the transition frequencies within the accuracy of the measurements.     

External cavity tunable diode laser spectra of the ν1 + 2ν4 stretch-bend combination bands of NH3 and NH3

The ammonia ν₁ + 2ν₄ perpendicular stretch-bend combination band has been investigated in spectra of ¹⁴NH₃ and ¹⁵NH₃ recorded in the 6400-6800 cm⁻¹ region with an external cavity tunable diode laser (ECTDL) spectrometer. For ¹⁴NH₃, new assignments were determined initially by extrapolating from published low-J jet-cooled beam results up to transitions of higher J and K. Corresponding ν₁ + 2ν₄ transitions for the ¹⁵NH₃ species were then found by identifying similar patterns of lines with a characteristic downshift of approximately 9.7 cm⁻¹. Assignments were confirmed employing ground-state combination differences. Term values, a-s inversion splittings, l-doubling energies and parameter estimates from simple single-state fits are reported for the two ammonia species.     

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.     

Spin forbidden transitions in the Ka = 0 subband of NO2 in the λ = 592.5 nm region

In a high resolution laser excitation spectrum of NO₂, lines were recorded which do not follow the selection rule ΔN = ΔJ = ΔF of “spin allowed” transitions. Line positions and intensities of these “spin forbidden” lines were investigated for all rotational lines up to N″ = 12 of the K_a = 0 subband around λ = 592.5 nm. While the observed line intensities of “spin allowed” transitions can be well described by the J-coupling scheme, neither the J- nor the G-coupling scheme sufficiently describes the “spin forbidden” transitions. The observations can be fitted satisfactorily by perturbation theory, in which the Fermi interaction in ²A₁ is treated as the perturber. This looks similar to a superposition of J and G scheme in the ²A₁ ground state.