Lowest Torsional States Research Articles

Rotational spectroscopy of CH3OD with a reanalysis of CH3OD toward IRAS 16293–2422

We have started a measurement campaign of numerous methanol isotopologs in low-lying torsional states in order to provide extensive line lists for radio astronomical observations from an adequate spectroscopic model and to investigate how the intricate vibrationtorsion-rotation interactions manifest themselves in the spectra of different isotopic species. After CD3OH and CD3OD, we turn our focus to CH3OD, which is an important species for studying deuteration in prestellar cores and envelopes that enshroud protostars. Notably, deuteration is frequently viewed as a diagnostic tool for star formation. The measurements used in this study were obtained in two spectroscopic laboratories and cover large fractions of the 34 GHz-1.35 THz range. As done in previous studies, we employed a torsion-rotation Hamiltonian model for our analysis that is based on the rho-axis method. The resulting model describes the ground and first excited torsional states of CH3OD well up to quantum numbers J ⩽ 51 and Ka ⩽ 18. We derived a line list for radio astronomical observations from this model that is accurate up to at least 1.35 THz and should be sufficient for all types of radio astronomical searches for this methanol isotopolog in these two lowest torsional states. This line list was applied to a reinvestigation of CH3OD in data from the Protostellar Interferometric Line Survey of IRAS 16293–2422 obtained with the Atacama Large Millimeter/submillimeter Array. The new accurately determined value for the column density of CH3OD implies that the deuteration in methanol differs in its two functional groups by a factor of ~7.5.

Internal methyl rotation and molecular structure of trifluorotoluenes: microwave rotational spectra of 2,3,4- and 2,4,5-trifluorotoluene

The microwave rotational spectra of 2,3,4- and 2,4,5-trifluorotoluenes, along with all13C isotopic species in natural abundance, have been recorded in the frequency range 8–27 GHz employing pulsed-jet Fourier transform microwave spectroscopy. The analysis of the spectra in the lowest torsional state has yielded the rotational constants, centrifugal distortion constants, three-fold barrier to methyl rotation, and the direction of the internal rotation axis in the moment of inertia principal axes systems of these trifluorotoluenes. For both molecules, the molecular constants of their eight isotopologues have been used to obtain the substitution rsstructures of the ring and the methyl-carbon. The potential barriers hindering the internal rotation of the methyl top in 2,3,4- and 2,4,5-trifluorotluene are 2.5878(80) and 2.2809(23) kJ/mol, respectively.

Microwave and FIR spectroscopy of dimethylsulfide in the ground, first and second excited torsional states

We present a new global study of the millimeter, submillimeter and far-infrared (FIR) spectra of dimethylsulfide ((CH3)2S). New microwave measurements have been carried out between 49 and 660 GHz using spectrometers in IRA NASU (Ukraine), and PhLAM Lille (France). Pure rotational transitions belonging to the three lowest torsional states (ν11,ν15) = (0,0), (1,0), (0,1) of the molecule as well as to intertorsional transitions (ν11,ν15) = (1,0) ↔ (0,1) observable in the millimeter and submillimeter wave ranges due to intensity borrowing via avoided-crossing interactions were combined with new assignments in the fundamental torsional band (AILES beamline of the synchrotron SOLEIL) and analyzed using a model developed for molecules with two equivalent methyl rotors and C2v symmetry at equilibrium (PAM_C2v_2tops program). The upper limit of the frequency range coverage for the microwave data was extended from 300 GHz to 660 GHz and all symmetry species in the torsional excited states and fundamental torsion band of the molecule were analyzed. The final fit included 87 parameters to give an overall weighted root-mean-square deviation of 0.71 for the dataset with J up to 60 consisting of 22277 microwave line frequencies belonging to the three lowest torsional states (ν11,ν15) = (0,0), (1,0), (0,1) as well as to intertorsional transitions (ν11,ν15) = (1,0) ↔ (0,1) and 5720 FIR line frequencies belonging to the observed fundamental band associated with the methyl-top torsion mode (ν11,ν15) = (0,1) ← (0,0). This investigation presents a twofold expansion in the J quantum number for the ground torsional state (ν11,ν15) = (0,0) and the fundamental torsional band (ν11,ν15) = (0,1) ← (0,0); and significantly extends the analysis for the excited torsional states with one torsional quantum excited (ν11,ν15) = (1,0) and (0,1) for which only 18 transitions with J ≤ 5 were analyzed in previous work.

Far infrared spectrum of Methanol-D2 in the lowest torsional state (e0)

The infrared (IR) and far infrared (FIR) absorption spectra have been measured for the Methanol-D1 and D2 species in the wave number range of 20–1200cm−1 at the Justus Liebig Universität in Giessen, Germany using a Brüker Fourier Transform spectrometer at a resolution of 0.002cm−1. The spectra looked very complicated but analyses were possible for the lower lying states. At higher wave number regions spectra of few vibrational fundamental modes were visible. In this communication, the details of the assignments and analyses of the lowest lying internal rotational state (e0) for Methanol-D2 are reported. A catalog of about 900 assigned spectral lines has been prepared and is made available from the author to conserve space. As application of this work, it was possible to assign some optically pumped FIR laser lines to quantum states. The results should be useful for “radio astronomers” and in the field of optically pumped FIR and Sub-millimeter Wave (SMMW) lasers which are used in Tokamaks for plasma diagnostics and as local oscillators in radio detection from space.

High-resolution Fourier transform infrared synchrotron spectroscopy of the NO2 in-plane rock band of nitromethane

The high-resolution rotationally resolved Fourier-transform infrared spectrum of the NO2 in-plane rock band (440–510cm−1) of nitromethane (CH3NO2) has been recorded using the Far-Infrared Beamline at the Canadian Light Source, with a resolution of 0.00096cm−1. About 1773 transitions reaching the upper state levels m′=0;Ka′⩽7;J′⩽50 have been assigned using an automated ground-state combination difference program together with the traditional Loomis–Wood approach. These data from the lowest torsional state, m′=0, were fit using the six-fold torsion-rotation program developed by Ilyushin et al. (2010). The analysis reveals that the rotational energy level structure in the upper vibrational state is similar to that of the ground vibrational state, but the sign and magnitude of high-order constants are significantly changed suggesting the presence of multiple perturbations.

Torsion-vibration coupling in methanol: The adiabatic approximation and intramolecular vibrational redistribution scaling

The four-dimensional model Hamiltonian of Wang and Perry [J. Chem. Phys. 109, 10795 (1998)] is used to compare the approximate adiabatic separation of the torsion and CH stretches in methanol to an exact solution of the same Hamiltonian. The adiabatic approximation accounts for the pattern of the energy levels in the lowest torsional states, including the inverted tunneling splittings, but does not account for the pattern of systematic two- and four-fold near degeneracies at high torsional excitation. In the adiabatic basis, the nonadiabatic couplings mix the torsional and vibrational degrees of freedom and hence are a source for intramolecular vibrational redistribution (IVR). These IVR matrix elements are found to decrease by only a factor of 2 or 3 with each higher coupling order, in agreement with the results of Pearman and Gruebele [Z. Phys. Chem. Munich 214, 1439 (2000)]. This gentle scaling behavior, which contrasts with a steeper falloff with coupling order in more rigid molecules, points to a more important role for direct high-order couplings in torsional molecules. In this model, the scaling behavior derives from a single coupling term that is low order in the torsional angular momentum in combination with one-dimensional torsional functions that include contributions from many torsional angular momenta.

Torsional splittings in the v 12=1 vibrational state of Si2H6: analysis of the ν6+ν12 and ν9+ν12(E) combination bands in the high resolution infrared spectrum

The ν6 + ν12 and ν9 + ν12(E) vibrational combination bands of disilane (Si2H6) have been measured at high resolution and analysed. A few K-branches of the hot band associated with ν9 + ν12(E), originated from the torsional state v4 = 1, have also been observed and analysed. These observations give information on the effects of the v12 = 1 vibrational state, not directly accessible in the infrared, on the molecular internal rotation. From the vibrational frequencies of ν9 + ν12(E) and its related hot band, determined in this work, and those of ν9 and its hot band determined previously, we can determine that the torsional barrier height changes by −19.25 cm−1 if the ν12 mode is excited by one quantum. This causes the torsional splitting to increase by about 0.0038 cm−1. The observed torsional splitting in the ν6 + ν12 vibrational state is larger than in the ground vibrational state by 0.0024 cm−1, with a 0.0033 cm−1 change caused by the lowering of the barrier height and a −0.0009 cm−1 change caused by the head–tail and torsional Coriolis mechanisms. The torsional splitting observed in the ν9 + ν12(E) state, −0.0043 cm−1, is inverted with respect to the ground vibrational state (0.0150 cm−1 in the lowest torsional state), showing that this state is closer to the E2s than to the E1d vibrational symmetry description. The analysis of perturbations observed in the rotation–torsion structure of ν9 + ν12(E) yields information on the vibrational states ν2 + ν4 and ν4 + ν11, and on the E3d torsional component of 4ν4 + ν12.

The forbidden rotational Q branch of CH 3SiF 3: A study in internal rotation

The pure rotational spectrum driven by the small dipole moment produced perpendicular to the symmetry axis by centrifugal distortion has been investigated for CH 3SiF 3 in the ground vibrational state using a Fourier transform waveguide spectrometer. Between 10.9 and 17.0 GHz, four ( k + 3 ← k) series in the Q branch have been measured in the lowest torsional state v 6 = 0 for k = 4, 5, 6, and 7 with 54 ⩽ J ⩽ 65. In each transition, the quantum number σ = 0, +1, −1 labelling the different torsional sub-levels is conserved. For given ( J, k), splittings from ∼10 to ∼45 MHz have been observed between lines with different values of σ. The global data set includes the anticrossing molecular beam energy differences of [W.L. Meerts, I. Ozier, Chem. Phys. 71 (1982) 401–415] as well as the mm-wave R branch frequencies and ( A 1 − A 2) splittings of [P. Dréan, J.-M. Colmont, J. Demaison, L. Dore, C. Degli Esposti, J. Mol. Spectrosc. 176 (1996) 23–27]). A good fit was obtained by varying 15 molecular parameters characterizing the torsion–rotation Hamiltonian H TR for the vibrational ground state. Because of the strong correlation between two of the quartic torsion–distortion parameters ( F 0,3 K and D 0, Km ) and a redundancy connecting the centrifugal distortion constants, four models were obtained yielding comparable fits. In each case, effective values were determined for the A-rotational constant and the height of the potential hindering the internal rotation. A high precision determination of the structural parameter ρ was made that is the same in all four models. For the off-diagonal quartic centrifugal distortion constant ε 0 and the sextic constants H 0, J , H 0, JK , H 0, KJ , and h 0,3, the differences in the values obtained in the two different reductions used have been explained in terms of the redundancy connecting these parameters. For σ = 0, +1, −1, the energy level pattern for (| k| = 3) is discussed for the case where the pure torsional energy splitting and the matrix elements off-diagonal in k are of comparable magnitude. A method is described of using an R branch study of the resulting σ-splittings for (| k| = 3) to probe the zeroth-order torsional Hamiltonian.

Vibration–Torsion–Rotation Study of the ν 5 State of CH 3CF 3

An investigation of the torsion–rotation–vibration energies in the ν 5 vibrational state in CH 3CF 3 has been carried out using infrared and mm-wave spectroscopy. The lowest frequency parallel fundamental band ν 5 near 600 cm −1 has been measured at a resolution of 0.00125 cm −1 with Fourier transform spectroscopy for the two lowest torsional states v 6=0 and 1. The cold band ( v 5=1, v 6=0)←( v 5=0, v 6=0) showed no torsional splittings and looked much like a parallel band in a C 3 v molecule. The hot band ( v 5=1, v 6=1)←( v 5=0, v 6=1) consisted of three distinct subbands, one for each torsional sublevel σ=0, +1, and −1. For the state ( v 5=1, v 6=1), the torsional splitting was increased from ∼0.001 cm −1 to ∼0.022 cm −1 by torsion-mediated Fermi-type interaction primarily with the dark state ( v 5=0, v 6=5). The effects of this coupling on the spectrum are striking in spite of the fact that the two interacting states are ∼100 cm −1 apart and differ by four units in v 6. The large amplitude character of the state ( v 5=0, v 6=5) is seen to be largely responsible for the unusual ( k, σ) dependence of the energies in the state ( v 5=1, v 6=1). The pure rotational spectrum in the state ( v 5=1, v 6=0) has been measured between ∼50 and 370 GHz with Doppler-limited resolution; no σ-splitting was detected. The 3590 infrared and mm-wave frequencies measured here have been analyzed together with the 1494 measurements reported earlier by Wang et al. in an analysis of the vibrational ground state (2001, J. Mol. Spectrosc. 205, 146–163). A good fit was obtained here by varying 36 parameters in a Hamiltonian which takes into account the interaction between the torsional stacks of levels for v 5=0 and 1, as well as the ( A 1− A 2) splittings measured earlier for v 5=0. The explicit treatment of the interstack interactions is shown to lead to significant changes in the parameters ( V 0,3, V 0,6) that characterize the torsional potential for v 5=0. These changes have been explained quantitatively by examining the contact transformation that is implicitly applied when the interstack coupling is neglected.

Stark and Frequency Measurements in the FIR Spectrum of H2O2

The submillimeter-wave spectrum of H2O2has been recorded by means of a tunable FIR spectrometer. Stark measurements have been performed on three selected transitions in then= 0 state, namely, the 220–110(τ = 4 ← 2) at 1272297 MHz, the 826–716(τ = 1 ← 3) at 882451 MHz, and the 928–818(τ = 1 ← 3) at 962933 MHz. Accurate values of the transition dipole moments of the molecule have been derived by considering the interaction between the levels involved in the transition and the close near-resonant levels. About 40 new lines, belonging to therQ4andrQ5subbranches of the rotational transitions between the lowest torsional states (τ = 1, 2, 3, 4n= 0), have been measured in the 2.8 and 3.4 THz spectral regions and analyzed together with the previously measured millimeter- and submillimeter-wave, as well as IR, transitions.

ChemInform Abstract: Millimeter‐Wave Spectrum of CH3CD3 in the Three Lowest Torsional States.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Millimeter-wave spectrum of CH3CD3 in the three lowest torsional states

The pure rotational spectra of CH3CD3 in the three lowest torsional states has been observed using a mm-wave spectrometer. A total of 87 rotational frequencies were measured between 230–363 GHz for J=7←6 to 11←10 in v6=0, 1, and 2, where v6 is the torsional quantum number. For the lowest two torsional states, the spectra have the classic form expected for a symmetric top (with no internal rotation) in the ground vibrational state. For v6=2 and for a given (J+1)←J, a markedly different splitting pattern is observed as a result of the (K,σ) dependence of the effective rotational constant B̂, where σ labels the torsional sublevels. In order to identify the individual features in the (v6=2) spectrum, an assignment procedure was developed which is based on the fact that the ratio of moment of inertia of the top about the molecular symmetry axis to that of the whole molecule about the same axis is to a very good approximation 1/3. The torsion–rotation Hamiltonian discussed earlier in connection with CH3SiH3 [N. Moazzen-Ahmadi et al. J. Mol. Spectrosc. 119, 299 (1986)] was used to analyze the rotational frequencies along with the molecular beam anticrossing data and the origin of the torsional fundamental. Several constants which characterize the J-dependence of the energy levels were determined. Effective values for the barrier height Ṽ3 and the shape parameter Ṽ6 associated with the first-order correction in the Fourier expansion of the potential function were obtained. The effect of redundancies on the interpretation of the measurements is discussed.

Internal rotation of molecules with two equivalent tops: The microwave spectrum of 2-fluoropropane

The microwave spectrum of 2-fluoropropane in the three lowest torsional states has been investigated to determine parameters V 3 and V′ 12 of the torsional potential function and the angles between the principal axes of inertia and the internal rotation axes. This required the development of a computer program taking into account the top-top coupling. The energy levels and linestrength are calculated using a stepwise procedure of diagonalization of appropriately truncated matrices.

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 CH 3OH and CH 3SH. The root-mean-square deviation of the fit is only 685 kHz. An analogous least-squares fit to 335 v t = 0 lines yields a root-mean-square deviation of 269 kHz.

A new analysis and additional measurements of the millimeter and submillimeter spectrum of methanol

Over 100 new spectral lines of gas-phase methanol in the millimeter and submillimeter region (viz. 150–1000 GHz) corresponding to rotational transitions in the two lowest torsional states have been measured and assigned. Some of these lines have been combined with spectral lines measured by previous investigators below 1000 GHz into a global data set consisting of 470 lines involving transitions with J ≤ 8 and v t (torsional quantum number) ≤ 2, and then analyzed via a nonlinear least-squares procedure. The analysis involves supplementing the standard IAM Hamiltonian with a number of new distortion and “interaction” constants. Direct diagonalization has been employed in preference to an earlier perturbation scheme. The quality of the least-squares fit compares favorably with the previous quantum mechanical approach.

Rotational tunnelling in methylpyridines as studied by NMR relaxation and inelastic neutron scattering

Nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS) techniques have been applied to study the phenomenon of methyl group tunnelling of various picolines and lutidines in the crystalline state over a wide range of temperatures. In most of these materials, two maxima—one is field independent—and two different apparent activation energies in the proton spin lattice relaxation rate have been observed. The NMR-T1 results are discussed in terms of Haupt’s model, where nuclear relaxation occurs via successive coupling of the spins with the tunnelling CH3 rotator and the phonon system. The meaning of correlation times and associated activation energies, and the connection between classical and quantum mechanical theory are considered. By INS, the tunnel splittings and the energy differences between the lowest torsional states have directly been determined. For 3,5-dimethylpyridine the tgemperature dependence of the tunnel splitting has been measured; there is agreement between this data and the NMR average tunnel frequency. Potential curves have been derived using tabulated eigenvalues. The INS results confirm Haup’s model.

Microwave spectrum and internal rotation of isopropyl alcohol

Two rotational isomers, trans and gauche, were detected for the isopropyl alcohol molecule from the analysis of the microwave spectrum. The rotational constants of the trans form were obtained A = 8488.99, B = 8041.75, and C = 4765.19 MHz for the H-species, (CH 3) 2CHOH, and A = 8098.72, B = 7917.76, and C = 4680.75 MHz for the D-species, (CH 3) 2CHOD. Rotational constants averaged for the two lowest torsional states were estimated for the normal species of the gauche form to be A = 8566.03, B = 7992.73, and C = 4714.59 MHz. Dipole moment of the trans form of the H-species obtained was μ b = 1.40 ± 0.03, μ c = 0.73 ± 0.02, and μ t = 1.58 ± 0.03 D. The a-type lines, characteristic of the gauche form, were observed for the H-species. From a preliminary analysis of these a-type lines, the energy difference between the ground and the first excited torsional state was estimated to be Δ = 46 GHz. The barrier height was obtained to be V 3 = 1.68 kcal/mole by assuming a threefold symmetry for the internal rotation of the OH group.