Li-Hong Xu

A Survey of Organic Volatile Species in Comet C/1999 H1 (Lee) Using NIRSPEC at the Keck Observatory

The organic volatile composition of the long-period comet C/1999 H1 (Lee) was investigated using the —rst of a new generation of cross-dispersed cryogenic infrared spectrometers (NIRSPEC, at the Keck Observatory atop Mauna Kea, HI). On 1999 August 19¨21 the organics spectral region (2.9¨3.7 km) was completely sampled at both moderate and high dispersion, along with the CO fundamental region (near 4.67 km), revealing emission from water, carbon monoxide, methanol, methane, ethane, acetylene, and hydrogen cyanide. Many new multiplets from OH in the 1¨0 band were seen in prompt emission, and numerous new spectral lines were detected. Several spectral extracts are shown, and global production rates are presented for seven parent volatiles. Carbon monoxide is strongly depleted in comet Lee relative to comets Hyakutake and Hale-Bopp, demonstrating that chemical diversity occurred in the giant

Methanol as a diagnostic tool of interstellar clouds - I. Model calculations and application to molecular clouds

We present a detailed analysis of the diagnostic properties of methanol, (CH3OH), in dense molecular clouds, made possible by the availability of new (CH3OH-He) collisional rate coefficients. Using a spherical Large Velocity Gradient (LVG) model, the dependence on kinetic temperature and spatial density of various millimeter and submillimeter line bands is inves- tigated over a range of physical parameters typical of high- and low-mass star-forming regions. We find CH3OH to be a good tracer of high-density environments and sensitive to the kinetic temperature. Using our LVG model, we have also developed an innovative technique to handle the problem of deriving physical parameters from observed multi-line spectra of a molecule, based on the simultaneous fit of all the lines with a synthetic spectrum, finding the best physical parameters using numerical methods.

On the physical interpretation of torsion-rotation parameters in methanol and acetaldehyde: Comparison of global fit and ab initio results

Equilibrium structural constants and certain torsion–rotation interaction parameters have been determined for methanol and acetaldehyde from ab initio calculations using GAUSSIAN 94. The substantial molecular flexing which occurs in going from the bottom to the top of the torsional potential barrier can be quantitatively related to coefficients of torsion–rotation terms having a (1−cos 3γ) dependence on torsional angle γ. The barrier height, six equilibrium structural constants characterizing the bottom of the potential well, and six torsion–rotation constants are all compared to experimental parameters obtained from global fits to large microwave and far-infrared data sets for methanol and acetaldehyde. The rather encouraging agreement between the Gaussian and global fit results for methanol seems both to validate the accuracy of ab initio calculations of these parameters, and to demonstrate that the physical origin of these torsion–rotation interaction terms in methanol lies primarily in structural rela...

Methanol and the optically pumped far-infrared laser

New results on the generation and spectroscopic analysis of optically pumped far-infrared (FIR) laser emission from CH/sub 3/OH have been obtained as part of a systematic study of methanol isotopomers as FIR laser sources utilizing the extended line coverage available from a recently developed high-resolution CO/sub 2/ laser of high efficiency. For normal CH/sub 3/OH, six new short-wavelength lines have been found using a 2 m long Fabry-Perot FIR laser cavity. Accurate heterodyne frequency measurements are reported for 14 CH/sub 3/OH FIR laser lines, nearly all above 100 cm/sup -1/, as well as accurate frequency offsets for most of the corresponding CO/sub 2/ pump lines. Spectroscopic assignments are presented for nine high-frequency FIR laser lines in four pump systems.

Exploring molecular complexity with ALMA (EMoCA): Alkanethiols and alkanols in Sagittarius B2(N2)

Context. Over the past five decades, radio astronomy has shown that molecular complexity is a natural outcome of interstellar chemistry, in particular in star forming regions. However, the pathways that lead to the formation of complex molecules are not completely understood and the depth of chemical complexity has not been entirely revealed. In addition, the sulfur chemistry in the dense interstellar medium is not well understood. Aims. We want to know the relative abundances of alkanethiols and alkanols in the Galactic center source Sagittarius B2(N2), the northern hot molecular core in Sgr B2(N), whose relatively small line widths are favorable for studying the molecular complexity in space. Methods. We investigated spectroscopic parameter sets that were able to reproduce published laboratory rotational spectra of ethanethiol and studied effects that modify intensities in the predicted rotational spectrum of ethanol. We used the Atacama Large Millimeter Array (ALMA) in its Cycles 0 and 1 for a spectral line survey of Sagittarius B2(N) between 84 and 114.4 GHz. These data were analyzed by assuming local thermodynamic equilibrium (LTE) for each molecule. Our observations are supplemented by astrochemical modeling; a new network is used that includes reaction pathways for alkanethiols for the first time. Results. We detected methanol and ethanol in their parent 12 C species and their isotopologs with one 12 C atom substituted by 13 C; the latter were detected for the first time unambiguously in the case of ethanol. The 12 C/ 13 C ratio is ~25 for both molecules. In addition, we identified CH 3 18 OH with a 16 O/ 18 O ratio of ~180 and a 13 CH 3 OH/CH 3 18 OH ratio of ~7.3. Upper limits were derived for the next larger alkanols normal - and iso -propanol. We observed methanethiol, CH 3 SH, also known as methyl mercaptan, including torsionally excited transitions for the first time. We also identified transitions of ethanethiol (or ethyl mercaptan), though not enough to claim a secure detection in this source. The ratios CH 3 SH to C 2 H 5 SH and C 2 H 5 OH to C 2 H 5 SH are ≳21 and ≳125, respectively. In the process of our study, we noted severe discrepancies in the intensities of observed and predicted ethanol transitions and propose a change in the relative signs of the dipole moment components. In addition, we determined alternative sets of spectroscopic parameters for ethanethiol. The astrochemical models indicate that substantial quantities of both CH 3 SH and C 2 H 5 SH may be produced on the surfaces of dust grains, to be later released into the gas phase. The modeled ratio CH 3 SH/C 2 H 5 SH = 3.1 is lower than the observed value of ≳21; the model value appears to be affected most by the underprediction of CH 3 SH relative to CH 3 OH and C 2 H 5 OH, as judged by a very high CH 3 OH/CH 3 SH ratio. Conclusions. The column density ratios involving methanol, ethanol, and methanethiol in Sgr B2(N2) are similar to values reported for Orion KL, but those involving ethanethiol are significantly different and suggest that the detection of ethanethiol reported toward Orion KL is uncertain. Our chemical model presently does not permit the prediction of sufficiently accurate column densities of alkanethiols or their ratios among alkanethiols and alkanols. Therefore, additional observational results are required to establish the level of C 2 H 5 SH in the dense and warm interstellar medium with certainty.

Can a local mode picture account for vibration–torsion coupling? Ab initio test based on torsional variation of methyl stretching and bending frequencies in methanol

The torsional dependence of the CH3 stretching and bending modes of methanol has been explored in terms of a local mode internal coordinate picture [X. Wang and D. S. Perry, J. Chem. Phys. 109, 10795 (1998)]. First, the torsional variations of the small-amplitude vibrational frequencies along the mass weighted intrinsic reaction coordinate from the top to the bottom of the torsional potential barrier were calculated by means of ab initio frequency projection utilizing GAUSSIAN 98. The resulting curves for the three C–H stretch ab initio frequencies as functions of the torsional angle cannot be reproduced by the original 3×3 local mode model incorporating stretch–torsion and stretch–stretch couplings at lowest-order only, but are well-fitted if the model is extended to include higher-order coupling terms. For the CH-bending modes, with internal coordinates chosen to give a high degree of localization, bend–torsion and bend–bend coupling parameters were determined from the ab initio projected frequencies, a...

A rotation–torsion–vibration treatment with three-dimensional internal coordinate approach and additional FTIR spectral assignments for the CH3-bending fundamentals of methanol

A theoretical model has been developed to account for certain features of both newly observed and previously reported CH3bending subbands between 1450 and 1570 cm � 1 in the high-resolution Fourier transform infrared spectrum of CH3OH [Can. J. Phys. 79 (2001) 435]. The features include (i) an apparent inversion of the rotationless E–A torsional splitting with respect to the ground state,i.e.,the A state located above the E state,(ii) a pronounced upward slope in the K-reduced torsion–vibration energy pattern for the subband origins,and (iii) unexpected A1=A2 inversion of the K¼ 2A and K¼ 3A J-rotational levels that led to ambiguity in identifying the vibrational mode as m4 ðA1Þ or m10 ðA2Þ. The model is an effective internal coordinate Hamiltonian constructed in G6 molecular symmetry with the CH3-bends coupled to each other and to torsion and including a- and c-type Coriolis coupling. With this model,33 out of 36 experimental upper-state K-term values for newly assigned m4; m5,and m10 subbands plus previous m4 subbands have together been fitted successfully,employing 9 adjustable parameters and 17 fixed parameters to give a standard deviation of 0.14 cm � 1 . The Pc Coriolis term appears to be the leading cause of the upward shift in the K-reduced energies. When J-dependence is introduced via a rotational Hamiltonian including b- and c-type Coriolis terms in addition to molecular asymmetry,the observed A1=A2 inversion of the K¼ 2A and 3A rotational levels can also be reproduced. Predictions using the fitted K-rotation–torsion–vibration Hamiltonian show an interesting Coriolis-induced crossover and mixing of the m5 and m10 torsion– vibration energy patterns. These predictions played a role in identifying two of the new m5 subbands in the crossing region,thereby helping to validate the model. 2003 Elsevier Science (USA). All rights reserved.

The microwave spectrum and OH internal rotation dynamics of gauche‐2,2,2‐trifluoroethanol

The microwave spectra of CF3CH2OH and CF3CH2OD have been investigated from 5 to 26 GHz with a pulsed‐nozzle Fourier‐transform microwave spectrometer and from 26 to 42 GHz with an electric resonance optothermal spectrometer. Tunneling of the OH proton between the two isoenergetic gauche conformations splits the observed transitions into two tunneling components. An effective rotation‐tunneling Hamiltonian is used to fit the aan both isotopomers to better than 5 and 13 kHz for the OH and OD forms, respectively. The tunneling splittings determined from the fits for the OH and OD isotopomers are 5868.6952(16) and 208.5037(42) MHz, respectively. A structural analysis using the moments of inertia of the OH and OD isotopomers determines that the hydroxyl hydrogen is directed toward the fluorine with a F...H separation of 2.561(1) A and a dihedral angle of φ(CCOH)=68.97(6)°. The observed tunneling splittings are fit to a double‐minimum potential, giving gauche–gauche tunneling barriers of 763 and 720 cm−1 and OH ...

Rotational spectra of four of the five conformers of 1-pentene

The rotational spectra of four of the five expected conformers of 1-pentene, together with their monosubstituted 13C isotopic forms, have been measured in a molecular beam using a pulsed-nozzle Fourier-transform microwave spectrometer. One of the conformers has Cs point-group symmetry while the other three conformers have C1 point-group symmetry. The measurements are compared to results from molecular modeling calculations using the MM3 molecular-mechanics force field of Allinger et al. and to ab initio electronic structure calculations (MP2/6-31G*, MP2/6-311G*, MP4/6-31G*, MP4/6-311G*). Both types of calculations suggest the existence of five distinct conformers of 1-pentene, four of C1 symmetry and one of Cs symmetry. Both the MM3 and ab initio rotational constants deviate from the measured values by ⩽5%. The relatively high barriers between the four conformers limit the conformational cooling in the expansion, allowing all four conformers to be observed at the <2 K rotational temperature of the molecul...

Fourier transform spectroscopy of 13CD3OH: Assignment of far-infrared laser lines

High-resolution infrared (IR) and far-infrared (FIR) Fourier transform spectroscopic studies of r3CD30H have been applied to the identification of the IR-pump/FIR-laser transition systems associated with optically pumped FIR Iaser emission. Assignments are considered for 13 reported or proposed pump systems and are supported for most of these by rigorous closed-loop frequency ~mbi~tion relations. Three of the systems originate from Fermi resonances between C-O stretch levels and high-lying excited torsional levels of the vibrational ground state. Spectral positions have been obtained for the majority of the FIR laser lines to an estimated accuracy of ~0.001 cm-’ from spectroscopic combination differences. Q 1992 Academic PESS, IW.