Harald Møllendal

THE GAS-PHASE FORMATION OF METHYL FORMATE IN HOT MOLECULAR CORES

Methyl formate, HCOOCH3, is a well-known interstellar molecule prominent in the spectra of hot molecular cores. The current view of its formation is that it occurs in the gas phase from precursor methanol, which is synthesized on the surfaces of grain mantles during a previous colder era and evaporates while temperatures increase during the process of high-mass star formation. The specific reaction sequence thought to form methyl formate, the ion-molecule reaction between protonated methanol and formaldehyde followed by dissociative recombination of the protonated ion [HCO(H)OCH3] + , has not been studied in detail in the laboratory. We present here the results of both a quantum chemical study of the ion-molecule reaction between [CH3OH2] + and H2CO as well as new experimental work on the system. In addition, we report theoretical and experimental studies for a variety of other possible gas-phase reactions leading to ion precursors of methyl formate. The studied chemical processes leading to methyl formate are included in a chemical model of hot cores. Our results show that none of these gas-phase processes produces enough methyl formate to explain its observed abundance. Subject headingg ISM: molecules — molecular data — molecular processes — radio lines: ISM

Green Bank Telescope Detection of New Interstellar Aldehydes: Propenal and Propanal

The new interstellar molecules propenal (CH2CHCHO) and propanal (CH3CH2CHO) have been detected largely in absorption toward the star-forming region Sagittarius B2(N) by means of rotational transitions observed with the 100 m Green Bank Telescope (GBT) operating in the range from 18 GHz (λ ~ 1.7 cm) to 26 GHz (λ ~ 1.2 cm). The GBT was also used to observe the previously reported interstellar aldehyde propynal (HC2CHO) in Sagittarius B2(N), which is a known source of large molecules presumably formed on interstellar grains. The presence of these three interstellar aldehydes toward Sagittarius B2(N) strongly suggests that simple hydrogen addition on interstellar grains accounts for successively larger molecular species: from propynal to propenal and from propenal to propanal. Energy sources within Sagittarius B2(N) likely permit the hydrogen addition reactions on grain surfaces to proceed. This work demonstrates that successive hydrogen addition is probably an important chemistry route in the formation of a number of complex interstellar molecules. We also searched for but did not detect the three-carbon sugar glyceraldehyde (CH2OHCHOHCHO).

Microwave spectrum and dipole moment of glycolaldehyde

Abstract The microwave spectrum of glycolaldehyde, CH 2 OH-CHO, has been measured and the rotational and centrifugal distortion constants of the ground and three vibrational excited states have been obtained. Only one isomer, which has the carbonyl and the hydroxyl groups cis to one another, was identified. The dipole moment was determined to be 2.73 ± 0.04 D from Stark-effect measurements.

Rotational spectrum of 13C2-methyl formate (HCOO13CH3) and detection of the two 13C-methyl formate in Orion

Context. Laboratory measurements and analysis of the microwave and millimeter-wave spectra of potential interstellar molecules are a prerequisite for their subsequent identification by radioastronomical techniques. The spectral analysis provides spectroscopic parameters that are used in the assignment procedure of the laboratory spectra, and that also predict the frequencies of transitions not measured in the laboratory with a high degree of precision. Aims. An experimental laboratory study and its theoretical analysis is presented for 13 C2-methyl formate (HCOO 13 CH3) allowing a search for this isotopologue in the Orion molecular cloud. The 13 C1-methyl formate (H 13 COOCH3) molecule was also searched for in this interstellar cloud, using previously published spectroscopic data. Methods. The experimental spectra of 13 C2-methyl formate were recorded in the microwave and sub-mm energy ranges (4–20 GHz, 8–80 GHz, 150–700 GHz). The spectra were analyzed using the Rho-Axis Method (RAM), which takes the CH3 internal rotation and the coupling between internal rotation and global rotation into account. Results. Twenty-seven spectroscopic constants of 13 C2-methyl formate have been obtained from a fit of 936 transitions of the ground torsional state with a standard (unitless) deviation of 1.08. A prediction of line positions and intensities is also produced. This prediction allowed us to identify 230 13 C2-methyl formate lines in the Orion interstellar molecular cloud. We refitted all previously published ground state transitions of the 13 C1-methyl formate molecule in order to provide a prediction of its ground state spectrum. 234 lines of 13 C1-methyl formate were detected in the Orion interstellar cloud using that prediction.

Microwave spectrum, conformational equilibrium, intramolecular hydrogen bonding, inversion tunnelling, dipole moments and centrifugal distortion of ethylenediamine

Abstract The microwave spectrum of ethylenediamine, CH 2 NH 2 CH 2 NH 2 , has been investigated in the 12.4–39.5 GHz spectral region. The two N-C-C-N gauche conformations denoted I and II and shown in Fig. 1, were assigned. The existence of large fractions of further conformations is ruled out. Both rotamers I and II possess an intramolecular hydrogen bond. I is favoured by 0.3 ± 0.2 kcal mol −1 relative to II. The N-C-C-N angles are 63 ± 2° in both conformers. The average CCN angles are 109 ± 1° in I and 111.5 ± 1° in II. The spectra of both rotamers display splittings caused by double minimum potentials. In conformation I the a - and c -dipole moment components were of the “inverting” type, while μ b , is “non-inverting”. The separation between the (+)- and the (-)-energy levels of the double minimum potential of I is 86.356 ± 0.021 MHz. In conformer II the a -axis component of the dipole moment “inverts”, while μ b is “non-inverting”. No c -type lines were observed for this conformation. The energy difference between the (+)- and the (−)-states of the double minimum potential of conformation II is 1.332 ± 0.018 MHz. The first excited state of the C-C torsional motion was assigned for this conformation and the energy difference between the (+)- and (−)-states determined as 1.564 ± 0.066 MHz. The dipole moments were μ b = 1.059 ± 0.007 D, μ b = 0.787 ± 0.032 D, μ c = = 1.179 ± 0.023 D and μ tot = 1.770 ± 0.033 D for conformation I; μ a = 1.952 ± 0.002 D, μ b = 0.867 ± 0.006 D, μ c = 0.538 ± 0.006 D and μ tot = 2.203 ± 0.006 D for II, respectively. All quartic and two sextic centrifugal distortion constants were determined for I, while the quartic distortion coefficients were found for II.

Rotational Spectrum and Tentative Detection of DCOOCH3-Methyl Formate in Orion

New centimeter-wave (7-80 GHz) and submillimeter-wave (580-661 GHz) spectra of a deuterated species of methyl formate (DCOOCH3) have been measured. Transitions with a maximum value of J = 64 and K = 36 have been assigned and fitted together with previous measurements. The internal rotation of this compound was treated using the so-called rho axis method. A total of 1703 transitions were fitted using this method. Only 24 parameters were employed in the final fit, which has an rms deviation of 94.2 kHz. The dipole moment and the nuclear quadrupole coupling constants of the deuterated specie have also been obtained. This new study has permitted a tentative detection of DCOOCH3 in Orion with the IRAM 30 m telescope based on the observation of more than 100 spectral features with low blending effects among the 400 lines expected in the observed frequency domain (for which over 300 are heavily blended with other species). These 100 transitions are above noise and confusion limited without heavy blending and cannot be assigned to any other species. Moreover, none of the strongest unblended transitions is missing. The derived source-averaged total column density for DCOOCH3 is 7.8 × 1014 cm–2 and the DCOOCH3/HCOOCH3 column density ratio varies between 0.02 and 0.06 in the different cloud components of Orion. This value is consistent with the deuteration enhancement found for other species in this cloud.

The molecular structure of benzene derivatives, part 2: 4-chloro-benzaldehyde by joint analysis of gas electron diffraction, microwave spectroscopy and ab initio molecular orbital calculations

The molecular structure of gaseous 4-chlorobenzaldehyde has been determined by a joint analysis of gas electron diffraction data, rotational constants from microwave spectroscopy, and constrained by results from ab initio calculations. The ab initio calculations have been performed at the HF6-311G∗∗ level of theory. The plannar Cs symmetry structure was found to be the only stable conformation. The torsion of the formyl group has been treated as a large amplitude motion. The most important structure parameters (rg) from the joint analysis with estimated total errors (in parentheses) are: (CC)mean = 1.398(1) A, CCl = 1.734(3) A, CC(  O) = 1.482(10) A, C  O = 1.216(5) A, <CCClC = 121.0(5)°, and <CCCHOC = 120.2(8)°. A scaled molecular force field has been determined. The ground state rotational constants have been determined from microwave data.

Microwave and submillimeter spectroscopy and first ISM detection of 18O-methyl formate

Context. Astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict transition frequencies that were not measured in the laboratory very precisely. Aims. We present an experimental study and a theoretical analysis of two 18 O-methyl formate isotopologues, which were subsequently detected for the first time in Orion KL. Methods. The experimental spectra of both methyl formate isotopologues recorded in the microwave and sub-mm range from 1 to 660 GHz. Both spectra were analysed by using the rho-axis method (RAM) which takes into account the CH3 internal rotation. Results. We obtained spectroscopic constants of both 18 O- methyl formate with high accuracy. Thousands of transitions were assigned and others predicted, which allowed us to detect both species in the IRAM 30 m line survey of Orion KL.

Rotational spectrum of butyronitrile: Dipole moment, centrifugal distortion constants and energy difference between conformers

Abstract The molecular rotational spectrum of butyronitrile has been investigated in the vibrational ground state up to 300 GHz. High J transitions have been measured for the two isomers and fitted to a centrifugally distorted Hamiltonian including some sextic coefficients. The results of the analysis are sufficient for the prediction of all strong transitions throughout the millimeter-wave range. The molecular dipole moment components were calculated from measured Stark effect shifts as μ a = 3.597(59) D and μ b = 0.984(15) D for the anti form and μ a = 3.272(37) D and μ b = 2.139(30) D with μ c preset at zero debye for the gauche form. It has been found from intensity measurements that the anti form is slightly more stable than the gauche form with an energy difference of 1.1(3) kJ mol −1 .

THE MILLIMETER- AND SUBMILLIMETER-WAVE SPECTRUM OF GLYCOLALDEHYDE (CH 2 OHCHO)

The simplest monosaccharide, glycolaldehyde (CH2OHCHO), has recently been detected toward the Galactic center in the source Sgr B2(N) at five frequencies from 71-104 GHz. None of the individual lines used in the detection had been measured previously in the laboratory; rather, their frequencies were predicted based on lower frequency measurements. We have now recorded and analyzed many new rotational transitions of glycolaldehyde through 354 GHz using two spectrometers. Lines through 48 GHz in frequency were measured with a spectrometer that uses Stark modulation, while the higher frequency transitions were measured with a FASSST (Fast Scan Submillimeter Spectroscopic Technique) apparatus. Analysis of the data has allowed us to confirm the interstellar identifications and to predict the frequencies of many additional lines not measured in the laboratory.