Millimeter-Wave Spectroscopy of Methylfuran Isomers: Local vs. Global Treatments of the Internal Rotation (original) (raw)
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Chemphyschem : a European journal of chemical physics and physical chemistry, 2016
The molecular beam Fourier transform microwave spectra of 2-acetyl-5-methylfuran were recorded in the frequency range 2-26.5 GHz. Quantum chemical calculations calculated two conformers with a trans and a cis configuration of the acetyl group, both of which were assigned in the experimental spectrum. All rotational transitions split into quintets due to the internal rotations of two non-equivalent methyl groups. Using the program XIAM, the experimental spectra can be simulated with standard deviations within the measurement accuracy, yielding well-determined rotational and internal rotation parameters, inter alia the V3 potentials. While the V3 barrier height of the ring methyl rotor does not change for both conformers, that of the acetyl methyl rotor differs by about 100 cm1. The predicted values from quantum chemistry are only in the correct order of magnitude.
The microwave spectrum of 2-methyl-4,5-dihydrofuran
Journal of Molecular Spectroscopy, 1985
The microwave spectrum of 2-methyl-4,5_dihydrofuran has been observed in the frequency range 8-60 GHz. Spectra of the ground and first two states of the ring-bending vibration have been assigned. The variation of the rotational constants with vibrational state confirm the potential function derived by Carreira and Lord from the far-infrared spectrum. The molecule has a bent-ring conformation with a dihedral angle of -20". A-E splittings due to the internal rotation of the methyl group have been observed and there is evidence for an interaction between the methyl torsion and the ring-puckering vibration. The electric dipole moment has been determined as p, = 0.470, pb = 0.760 and Jo = 0.9OD. o 1985 Academic press. IIIC.
Journal of Molecular Spectroscopy, 2011
Four vibrational levels of the five-membered ring molecule furan (C 4 H 4 O) have been rotationally analyzed from far-infrared Fourier transform spectra obtained at the Canadian Light Source synchrotron. We found that the low-lying m 14 and m 11 levels at 602.9 and 599.6 cm À1 interact through a second-order Coriolis resonance. This perturbation was characterized through a coupled analysis of the m 14 and m 18 fundamental spectra and the m 18-m 11 band. The m 19 fundamental spectrum was analyzed as well, and the data for all observed bands were combined with previously reported microwave transitions to produce the final fit. The spectra are an excellent demonstration of the high quality of data that can be obtained when far-infrared synchrotron radiation is used as the radiation source in Fourier transform spectroscopy experiments.
The Journal of Physical Chemistry B, 2006
Rotationally resolved fluorescence excitation spectra of several torsional bands in the S1 <-- S0 electronic spectra of 2-methylanisole (2MA) and 3-methylanisole (3MA) have been recorded in the collision-free environment of a molecular beam. Some of the bands can be fit with rigid rotor Hamiltonians; others exhibit perturbations produced by the coupling between the internal rotation of the methyl group and the overall rotation of the entire molecule. Analyses of these data show that 2MA and 3MA both have planar heavy-atom structures; 2MA has trans-disposed methyl and methoxy groups, whereas 3MA has both cis- and trans-disposed substituents. The preferred orientations (staggered or eclipsed) in two of the conformers and the internal rotation barriers of the methyl groups in all three conformers change when they are excited by light. Additionally, the values of the barriers opposing their motion depend on the relative positions of the substituent groups, in both electronic states. In contrast, no torsional motions of the attached methoxy groups were detected. Possible reasons for these behaviors are discussed.
Microwave and submillimeter-wave rotational spectrum of methyl alcohol in the ground torsional state
Journal of Molecular Spectroscopy, 1992
The frequencies of 440 transitions of the spectrum of the ground torsional-vibrational state of methyl alcohol have been measured in the microwave and submillimeter regions, 130 of these belonging to the main isotopomer CHrOH (J < 42, K < 8), 190 to the isotopomer CDpOH (J =S 37, K 8 IO), and 120 to CDrOD (J < 27, KC 5). For the first time, all the transitions were identified on the basis of directly calculated frequencies. These results were obtained through the use of a new effective torsional-rotational Hamiltonian. Its parameters were calculated based on the frequencies of all earlier known and new transitions of microwave and submillimeter-wave spectra of the ground torsional-vibrational state of methanol The root-mean-square deviation is 710 kHz (800 lines) for the main isotopomer, 144 kHz (370 lines) for CDrOH, and 316 kHz ( 150 lines) for CDSOD.
Far-infrared detection of methylene
Astronomy & Astrophysics, 2005
We present a clear detection of CH 2 in absorption towards the molecular cloud complexes Sagittarius B2 and W49 N using the ISO Long Wavelength Spectrometer. These observations represent the first detection of its low excitation rotational lines in the interstellar medium. Towards Sagittarius B2, we detect both ortho and para transitions allowing a determination of the total CH 2 column density of N(CH 2) = (7.5± 1.1)× 10 14 cm −2. We compare this with the related molecule, CH, to determine [CH/CH 2 ] = 2.7±0.5. Comparison with chemical models shows that the CH abundance along the line of sight is consistent with diffuse cloud conditions and that the high [CH/CH 2 ] ratio can be explained by including the effect of grain-surface reactions.
Journal of Molecular Spectroscopy, 2006
Methylcyanide, CH 3 CN, is an important interstellar species, and therefore the accurate knowledge of precise rest frequencies for rotational transitions as well as ground-state rotational and hyperfine constants is needed. In this work the hyperfine structure of the millimeter-and submillimeter-wave spectra of CH 3 CN has been further investigated. In addition, accurate THz measurements have been carried out for the first time. Consequently, the present investigation allowed us to provide the most accurate ground state rotational and hyperfine parameters known at the moment for CH 3 C 14 N. To resolve the hyperfine structure of the rotational transitions observed, the Lamb-dip technique has been exploited. Both frequency-modulated and video-type detections have been employed.
Astronomy & Astrophysics, 2008
Context. Methylacetylene and methyldiacetylene are the first members of the methylpolyynes series, CH 3 C 2n H (n = 1, 2). Their astrophysical relevance has motivated this spectroscopic study. Aims. The aim of this investigation is to provide very accurate rest frequencies, as well as to improve the spectroscopic parameters available in the literature for these molecules. Methods. The Lamb-dip technique was exploited in order to record the rotational spectra of CH 3 CCH and CH 3 CCCCH at sub-Doppler resolution in the millimeter-and submillimeter-wave frequency ranges. In addition, for CH 3 CCH rotational transitions in the THz region were recorded at Doppler resolution. Results. We report the most accurate ground-state rotational parameters known at the moment for the main isotopic species of methylacetylene, as well as of methyldiacetylene. Conclusions. Since both molecules are known to be of astrophysical relevance, we are confident that the improvement in the accuracy of the ground-state rotational parameters as well as of the rest frequencies of rotational transitions could be useful for future observations purposes.