Peter D. Godfrey

Models of class II methanol masers based on improved molecular data

The class II masers of methanol are associated with the early stages of formation of high-mass stars. Modelling of these dense, dusty environments has demonstrated that pumping by infrared radiation can account for the observed masers. Collisions with other molecules in the ambient gas also play a significant role, but have not been well modelled in the past. Here we examine the effects on the maser models of newly available collision rate coefficients for methanol. The new collision data do not alter which transitions become masers in the models, but do influence their brightness and the conditions under which they switch on and off. At gas temperatures above 100 K the effects are broadly consistent with a reduction in the overall collision cross-section. This means, for example, that a slightly higher gas density than identified previously can account for most of the observed masers in W3(OH). We have also examined the effects of including more excited-state energy levels in the models, and find that these play a significant role only at dust temperatures above 300 K. An updated list of class II methanol maser candidates is presented.

The missing conformers of glycine and alanine: relaxation in seeded supersonic jets

Abstract Certain conformers of glycine and alanine that are predicted by ab initio calculations to be among those of the lowest energy have not been detected in jet spectroscopy studies of these amino acids, although others of similar or higher predicted energy are observed. Selective conformational relaxation of the missing conformers to lower energy species during the free jet expansion, broadly consistent with the criteria established by Ruoff et al. and by Felder and Gunthard, has been found to provide a satisfactory explanation. Relevant regions of the molecular potential energy hypersurface have been modelled via ab initio calculations [ MP 2 6–31 G(d,p) ] and these show that glycine 4 (see Fig. 3) and glycine 7 can relax to glycine 1 , leading to the conclusion, in accordance with observation, that only the latter and glycine 2 should be detected in the jet. In the case of alanine, the ab initio study of the molecular potential energy hypersurface has yielded revised data on predicted spectroscopic constants for some conformers. Because of the combined effects of relaxation and conformational coalescence through large-amplitude vibrational motion, the revised data do not change the interpretation of the previous spectroscopic study of alanine. The barrier separating conformers 2 and 3 (see Fig. 8) is predicted to be no more than 129 cm −1 , so that in addition to alanine 1 only a single additional coalesced conformer { 2 , 3 }, undergoing a large-amplitude motion in a double-minimum potential, is expected to appear in the jet spectrum rather than two separate species. The ab initio calculations on alanine indicate that conformers 4 and 5 are separated by a low energy barrier and a similarly low barrier separates 4 from the much lower energy conformer 1 . The resulting coalesced { 4 , 5 } is effectively a “ledge” on one wall of the potential well containing 1 , and is occupied via an excited vibrational state of 1 . In this light, the conformer { 4 , 5 } is not expected to be observed in the jet, as the vibrational relaxation of low-frequency vibrational modes is known to be very efficient in free jet expansions. Overall, no conflicts are found to remain between the experimental observations of jet spectra and the ab initio calculations for glycine and alanine, provided that the scope of such calculations is broadened from a narrower focus on equilibrium structures to consideration of the potential hypersurface. This approach should be important in the correct interpretation of the spectra of other multi-conformational species in free jet expansions.

The microwave spectrum of HNC: identification of U90.7

THE detection and characterisation of hydrogen isocyanide, HNC, was for a long time a challenge to science, the first real progress coming from matrix-isolation studies of its infrared spectrum in 19631 and it has more recently been detected by its infrared chemiluminescence in certain flames involving atomic nitrogen2. In 1971 an interstellar line (U90.7) was attributed by radioastronomers3,4 to this molecule, the best estimate of the rest frequency of the line being6 90663.59 ±0.15 MHz. The possibility that this line represents the J = 2 →1 transition of Si C has also been advanced5. We have now succeeded in generating HNC in the gas phase and observing its microwave spectrum. The agreement between our observations of the frequency of the J = 1←0 transition and the radioastronomical data render the identification of U90.7 as HNC secure.

A study of the major gas-phase tautomer of adenine by microwave spectroscopy

Abstract The microwave spectrum of the nucleic acid base adenine in a cw seeded supersonic beam has been analyzed indicating that the amino, N (9) H tautomer is the most stable in an isolated environment. A comparison of the experimental rotational constants is made with those obtained using ab initio 3–21G basis SCF calculations for three of the lower- energy tautomers.

The microwave spectrum of urea

Abstract The microwave spectrum of urea has been recorded between 5 and 50 GHz. The rotational constants obtained from a least-squares fit of 47 b-type transitions are A = 11233.333, B = 10369.369, C = 5416.668 MHz. Centrifugal distortion constants were evaluated subject to the planarity constraints, giving satisfactory agreement up to at least J = 20. They are: τaaaa = −0.04423, τbbbb = −0.04075, τaabb = 0.02268, τabab = −0.02806. The diagonal elements of the nuclear quadrupole coupling tensor are χaa = 2.16, χbb = 1.88, χcc = −4.04. The dipole moment is μ = μb = 3.83 D (12.8 × 10−30 Cm).

Detection of a new carbon-chain molecule, CCO

We have detected a new carbon-chain molecule, CCO(3 sigma-), in the cold, dark molecular cloud TMC-1. The excitation temperature and the column density of CCO are, respectively, approximately 6 K and approximately 6 x 10(11) cm-2. This column density corresponds to a fractional abundance relative to H2 of approximately 6 x 10(-11). This value is two orders of magnitude less than the abundance of the related carbon-chain molecule CCS, and about half that of C3O. The formation mechanism for CCO is discussed.

The shape of urea

Abstract The shape of the urea molecule has been studied by analysing the microwave spectra of several isotopic species, yielding r s coordinates of all atoms except that of C, the latter being derived from first-moment equations, and by ab initio molecular-orbital calculations at the MP 2 6-311 ++ G(d,p) level. The derived bond lengths and angles are: r (CO), 1.22 1 A; r ( CN ), 1.37 8 A; r (NH (5) ), 0.99 8 A; r (NH (6) ), 1.02 1 A; ∠NCN, 114.7°; sum of pyramidal angles around N, 350.6°. The conformer of lowest energy is predicted to be nearly planar with C 2 symmetry, a second minimum for a shape of C 8 symmetry being higher in energy by 421 cm −1 . However, these are separated by a barrier estimated to be no higher than about 130 cm −1 . Thus the two shapes are likely to be parts of the potential energy surface domain that is associated with the most stable shape of urea, i.e. one in which the zero-point vibration covers both C 2 and C s geometries, the most probable ( r p ) geometry being C 2 . Computed ab initio frequencies and their eigenvectors (harmonic approximation) imply that the lowest frequency vibration is a CN torsion, in conflict with Kings assignment of NH 2 wag. However, the substantial anharmonicity of the large-amplitude motion (LAM) associated with the interconversion between the C s and C 2 forms seems likely to perturb the v = 1 level of this LAM so that its 1 ← 0 transition becomes lowest in frequency, so removing the apparent conflict in assignment.

The conformation of formamide

Abstract A reanalysis of vibration-rotation spectral data of formamide using the semi-rigid-bender technique adopted for unsymmetrical 1 large-amplitude motions (LAM) reveals that formamide has a very shallow single-minimum inversion potential. During inversion the amino group rotates around the CN bond with the syn -hydrogen staying closer to the NCO plane than the anti -hydrogen. The formyl hydrogen moves in the opposite direction to the amino hydrogens while the CN bond lengthens as the amino hydrogen move out-of-plane. The findings are similar to those recently reported for cyanamide and vinylamine. The present analysis overcomes the deficiencies of the previous analyses of formamide that failed to account for the position of the strong vibrational satellites relative to the ground state lines in the microwave spectrum. Our analysis now accounts for all of the experimental data relating to the planarity of formamide and its out-of-plane “NH 2 ” vibration. Published data on thioformamide suggest that the inversion motion for this molecule is similar to that of formamide.

A search for propylene oxide and glycine in Sagittarius B2 (LMH) and Orion

We have used the Mopra Telescope to search for glycine and the simple chiral molecule propylene oxide in the Sgr B2 (LMH) and Orion KL, in the 3-mm band. We have not detected either species, but have been able to put sensitive upper limits on the abundances of both molecules. The 3 sigma upper limits derived for glycine conformer I are 3.7 x 10(14) cm(-2) in both Orion-KL and Sgr B2 ( LMH), comparable to the reported detections of conformer I by Kuan et al. However, as our values are 3s upper limits rather than detections we conclude that this weighs against confirming the detection of Kuan et al. We find upper limits for the glycine II column density of 7.7 x 10(12) cm(-2) in both Orion-KL and Sgr B2 ( LMH), in agreement with the results of Combes et al. The results presented here show that glycine conformer II is not present in the extended gas at the levels detected by Kuan et al. for conformer I. Our ATCA results have ruled out the detection of glycine ( both conformers I and II) in the compact hot core of the LMH at the levels reported, so we conclude that it is unlikely that Kuan et al. have detected glycine in either Sgr B2 or Orion-KL. We find upper limits for propylene oxide abundance of 3.0 x 10(14) cm(-2) in Orion-KL and 6.7 x 10(14) cm(-2) in Sgr B2 (LMH). We have detected fourteen features in Sgr B2 and four features in Orion-KL which have not previously been reported in the interstellar medium, but have not been able to plausibly assign these transitions to any carrier.

NEW CLASS II METHANOL MASERS IN W3(OH)

We report interferometric observations of nine class II methanol maser candidate lines toward W3(OH). Narrow maser emission spikes at vLSR = -43.1 km s-1 are present in three of the lines: 31-40 A+, 72-63 A+, and 72-63 A-. For all three lines the maser position is near the northern edge of the W3(OH) ultracompact H II region (maser emission is also seen near the southern edge in the 31-40 A+ line). For the remaining six lines there is no obvious counterpart to the narrow maser spike at -43.1 km s-1. Additional spatially extended emission is present in all nine lines over the range from -41 to -48 km s-1. By comparing our observed flux densities with an extensive set of model calculations, we infer physical characteristics of the maser region. In these calculations the methanol is excited by infrared radiation from warm dust, and this excited gas amplifies the free-free background emission from the ultracompact H II region. The gas forming the narrow maser spikes appears to have both high kinetic temperature, Tkin ≥ 110 K, and high density, n ≈ 107 cm-3. Low-temperature solutions are ruled out by the observed line ratios and low-density solutions by the unphysically large path length that would be required. The gas is rich in methanol (2NM = NA + NE 10-6N), and the methanol column density in the tangential direction for each symmetry species (divided by line width) is NM/ΔV ≈ 1012 cm-3 s. Somewhat lower values of n and NM/ΔV are also acceptable. The size of the region emitting the maser spike is of order 100 × 1000 AU. In most of the lines the broad emission from -41 to -48 km s-1 can also be attributed to weak maser action, produced in gas with similar physical conditions (high density and temperature). It differs from the narrow spike emission mainly through a beaming factor that can be interpreted as an elongation factor for clumps of maser gas. The combination of narrow and broad emission can arise naturally from an ensemble of clumps of different elongations and orientations. In this unified picture the best fit to the data is provided by n ≈ 2 × 106 cm-3 and NM/ΔV ≈ 4 × 1011 cm-3 s, somewhat lower than the values obtained for just the spike component. The methanol maser clumps may be present in an expanding shell surrounding the H II region, similar to the material producing OH maser emission in this source.