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 does not alter which transitions become masers in the models, but does 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.Comment: 14 pages, 4 figures, Accepted for publication in MNRA

Class II methanol masers are found in close association with OH main‐line masers in many star‐forming regions, where both are believed to flag the early stages in the evolution of a massive star. We have studied the formation of masers in methanol and OH under identical model conditions for the first time. Infrared pumping by radiation from warm dust at temperatures >100 K can account for the known maser lines in both molecules, many of which develop simultaneously under a range of conditions. The masers form most readily in cooler gas (<100 K) of moderately high density (105–108 cm‐3), although higher gas temperatures and/or lower densities are also compatible with maser action. The agreement between the current model (developed for methanol) and the established OH maser trends is very encouraging, and we anticipate that further tuning of the model will further improve such agreement. We find the gas‐phase molecular abundance to be the key determinant of observable maser activity for both molecules. Sources exhibiting both 6668‐MHz methanol and 1665‐MHz OH masers have a typical flux density ratio of 16; our model suggests that this may be a consequence of maser saturation. We find that the 1665‐MHz maser approaches the saturated limit for OH abundances >10−7.3, while the 6668‐MHz maser requires a greater methanol abundance >10−6. OH‐favoured sources are likely to be less abundant in methanol, while methanol‐favoured sources may be less abundant in OH or experiencing warm (>125 K), dense (∼107 cm−3) conditions. These abundance requirements offer the possibility of tying the appearance of masers to the age of the new‐born star via models of gas‐phase chemical evolution following the evaporation of icy grain mantles.

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The tautomerism of 2-hydroxypyridinef2-pyridinone has been investigated by microwave spectroscopy using both a conventional spectrometer and also a jet-cooled millimeter-wave spectrometer. We have observed spectra attributable to both the (Z)-hydroxy tautomer and the pyridinone tautomer and also their monodeutero isotopomers, the relative abundances in both spectrometers being about 3:l in favor of the hydroxy form. From relative intensity and dipole moment measurements, we estimate the energy difference between the vibrational ground states of the two tautomers to be = 270 (30) cm-I. The rotational constants for 2-pyridinone yield an inertial defect that demonstrates the planarity of this tautomer, in contrast to recent reports of the nonplanarity of pyridinone. We can detect no lines that could be assigned to such a conformer. There are no lines correspondingto the E isomer of 2-hydroxypyridine, indicating that its relative abundance in the jet is less than -5% of the 2 isomer. The rotational spectra of several vibrational satellites have also been assigned. Identification of species is based on comparison of rotational constants determined experimentally with the values predicted from molecular orbital calculations and independently confirmed by comparison of observed and theoretically predicted hyperfine multiplets. The observed rotational constants for both tautomers are within 2% of thevalues predicted by ab initio molecular orbital calculations at the basis set level of 6-31G*, a degree of agreement similar to that found in some previous studies of nitrogen heterocycles. Nitrogen quadrupole coupling constants have been determined for both species: (Z)-2-hydroxypyridine, xOO = 0.4 (3) MHz, Xbb = -2.3 (3) MHz; 2-pyridinone, xcc = -2.8 (2) MHz. These values provide an independent confirmation of the identification of the two species of the molecule.

It has been found possible to understand the observed relative abundances of the several different tautomers and conformers observed in the millimeter-wave jet spectroscopy of histamine by using ab initio energy calculations at the MP2/6-311++G(d,p) level and including an estimate of thermal free energies. Conformational relaxation, previously found to be important for several molecules of comparable complexity, has been found to have a significant role in determining the relative abundances of histamine species detectable in the cooled jet, the present example being indicative that this phenomenon is expected to influence generally the interpretation of multiconformational jet spectra. The need to include free-energy corrections is also noteworthy. The thermal equilibration of histamine tautomers in the gas phase within the millimeter-wave spectrometer prior to jet expansion has been confirmed experimentally by comparing abundances measured with high surface area and low surface area inlet systems. Thus, it is valid to use relative abundances to indicate relative free energies. The presented ab initio calculations yield spectroscopic constants (e.g., planar moments) that tighten the identification of the four species detected in jet spectroscopy. In particular, the identity of the least abundant species (3 G-Ic) is confirmed. Nitrogen quadrupole coupling constants computed at the MP2/6-311++G(d,p) level are in noticeably better agreement with experiment than previously reported calculations, it being noteworthy that they require no scaling for the 14N nuclear quadrupole moment.

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