The pure rotational spectrum of AlCCH in its ground electronic state (X∼1Σ+) has been measured using Fourier transform microwave (FTMW) and mm/sub-mm direct absorption spectroscopy. AlCCH was created in a DC discharge from HCCH and aluminum vapor, either produced by a Broida-type oven, or generated from Al(CH3)3 in a supersonic jet source. Rotational transitions were measured for five isotopologues of AlCCH, with 13C and deuterium substitutions. From these data, rotational and Al and D quadrupole parameters were determined, as well as an accurate structure. AlCCH appears to exhibit an acetylenic arrangement with significant covalent character in the Al–C single bond.Graphical abstractHighlights► Pure rotational spectrum of AlCCH measured using FTMW and millimeter spectroscopy. ► Accurate structure for AlCCH was determined from measurements of five isotopologues. ► AlCCH has an acetylenic structure with little modification of C–C triple bond. ► Quadrupole constant measured for aluminum indicates a chiefly covalent Al–C bond.

The pure rotational spectrum of TiS in its X3Δr ground state has been measured using millimeter–wave direct-absorption techniques in the frequency range of 313–425 GHz. This free radical was created by the reaction of titanium vapor, produced in a high-temperature Broida-type oven, with H2S. Eight to ten rotational transitions were recorded for the main titanium isotopologue, 48TiS, in the v = 0 and v = 1 levels, as well as for the v = 0 state of 46TiS, observed in natural abundance (48Ti:46Ti = 74:8). All three Ω components were observed in almost every recorded transition, with no evidence for lambda-doubling. The data were fit with a Hund’s case(a) Hamiltonian, and rotational, spin–orbit, and spin–spin constants were determined, as well as equilibrium parameters for 48TiS. Relatively few fine structure parameters were needed for the analysis of TiS (A, AD, and λ), unlike other 3d metal species. The rotational pattern of the three fine structure components suggests the presence of a nearby excited 1Δ state, lying ∼3000 cm−1 higher in energy. From the equilibrium parameters, the dissociation energy for TiS was estimated to be ∼5.1 eV, in reasonable agreement with past thermochemical data.

The pure rotational spectrum of ZnS (X1Σ+) has been measured using direct-absorption millimeter/sub-millimeter techniques in the frequency range 372–471 GHz. This study is the first spectroscopic investigation of this molecule. Spectra originating in four zinc isotopologues (64ZnS, 66ZnS, 68ZnS, and 67ZnS) were recorded in natural abundance in the ground vibrational state, and data from the v = 1 state were also measured for the two most abundant zinc species. Spectroscopic constants have been subsequently determined, and equilibrium parameters have been estimated. The equilibrium bond length was calculated to be re ∼ 2.0464 Å, which agrees well with theoretical predictions. In contrast, the dissociation energy of DE ∼ 3.12 eV calculated for ZnS, assuming a Morse potential, was significantly higher than past experimental and theoretical estimates, suggesting diabatic interaction with other potentials that lower the effective dissociation energy. Although ZnS is isovalent with ZnO, there appear to be subtle differences in bonding between the two species, as suggested by their respective force constants and bond length trends in the 3d series.

The pure rotational spectrum of ZnO has been measured in its ground X1Σ+ and excited a3Πi states using direct-absorption methods in the frequency range 239–514 GHz. This molecule was synthesized by reacting zinc vapor, generated in a Broida-type oven, with N2O under DC discharge conditions. In the X1Σ+ state, five to eight rotational transitions were recorded for each of the five isotopologues of this species (64ZnO, 66ZnO, 67ZnO, 68ZnO, and 70ZnO) in the ground and several vibrational states (v = 1–4). Transitions for three isotopologues (64ZnO, 66ZnO, and 68ZnO) were measured in the a3Πi state for the v = 0 level, as well as from the v = 1 state of the main isotopologue. All three spin–orbit components were observed in the a3Πi state, each exhibiting splittings due to lambda-doubling. Rotational constants were determined for the X1Σ+ state of zinc oxide. The a3Πi state data were fit with a Hund’s case (a) Hamiltonian, and rotational, spin–orbit, spin–spin, and lambda-doubling constants were established. Equilibrium parameters were also determined for both states. The equilibrium bond length determined for ZnO in the X1Σ+ state is 1.7047 Å, and it increases to 1.8436 Å for the a excited state, consistent with a change from a π4 to a π3σ1 configuration. The estimated vibrational constants of ωe ∼ 738 and 562 cm−1 for the ground and a state agreed well with prior theoretical and experimental investigations; however, the estimated dissociation energy of 2.02 eV for the a3Πi state is significantly higher than previous predictions. The lambda-doubling constants suggest a low-lying 3Σ state.

The pure rotational spectrum of HZnCl (X 1Σ+) has been recorded using sub-millimeter direct-absorption methods in the range of 439–540 GHz and Fourier transform microwave (FTMW) techniques from 9 to 39 GHz. This species was produced by the reaction of zinc vapor and chlorine gas with H2 or D2 in a d.c. glow discharge for the sub-millimeter studies. In the FTMW measurements, HZnCl was created in a discharge nozzle from Cl2 and (CH3)2Zn. Between 5 and 10 rotational transitions were measured in the sub-millimeter regime for four zinc and two chlorine isotopologues; four transitions were recorded with the FTMW machine for the main isotopologue, each consisting of several chlorine hyperfine components. The data are consistent with a linear molecule and a 1Σ+ ground electronic state. Rotational and chlorine quadrupole constants were established from the spectra, as well as an rm(2) structure. The Zn–Cl and Zn–H bond lengths were determined to be 2.0829 and 1.5050 Å, respectively; in contrast, the Zn–Cl bond distance in ZnCl is 2.1300 Å, longer by ∼0.050 Å. The zinc–chlorine bond distance therefore shortens with the addition of the H atom. The 35Cl electric quadrupole coupling constant of eQq = −27.429 MHz found for HZnCl suggests that this molecule is primarily an ionic species with some covalent character for the Zn–Cl bond.

The interstellar medium is enriched primarily by matter ejected from old, evolved stars1, 2. The outflows from these stars create spherical envelopes, which foster gas-phase chemistry3, 4, 5. The chemical complexity in circumstellar shells was originally thought to be dominated by the elemental carbon to oxygen ratio6. Observations have suggested that envelopes with more carbon than oxygen have a significantly greater abundance of molecules than their oxygen-rich analogues7. Here we report observations of molecules in the oxygen-rich shell of the red supergiant star VY Canis Majoris (VY CMa). A variety of unexpected chemical compounds have been identified, including NaCl, PN, HNC and HCO+. From the spectral line profiles, the molecules can be distinguished as arising from three distinct kinematic regions: a spherical outflow, a tightly collimated, blue-shifted expansion, and a directed, red-shifted flow. Certain species (SiO, PN and NaCl) exclusively trace the spherical flow, whereas HNC and sulphur-bearing molecules (amongst others) are selectively created in the two expansions, perhaps arising from shock waves. CO, HCN, CS and HCO+ exist in all three components. Despite the oxygen-rich environment, HCN seems to be as abundant as CO. These results suggest that oxygen-rich shells may be as chemically diverse as their carbon counterparts.

Mass loss from evolved stars results in the formation of unusual chemical laboratories: circumstellar envelopes. Such envelopes are found around carbon- and oxygen-rich asymptotic giant branch stars and red supergiants. As the gaseous material of the envelope flows from the star, the resulting temperature and density gradients create a complex chemical environment involving hot, thermodynamically controlled synthesis, molecule “freeze-out,” shock-initiated reactions, and photochemistry governed by radical mechanisms. In the circumstellar envelope of the carbon-rich star IRC+10216, >50 different chemical compounds have been identified, including such exotic species as C8H, C3S, SiC3, and AlNC. The chemistry here is dominated by molecules containing long carbon chains, silicon, and metals such as magnesium, sodium, and aluminum, which makes it quite distinct from that found in molecular clouds. The molecular composition of the oxygen-rich counterparts is not nearly as well explored, although recent studies of VY Canis Majoris have resulted in the identification of HCO+, SO2, and even NaCl in this object, suggesting chemical complexity here as well. As these envelopes evolve into planetary nebulae with a hot, exposed central star, synthesis of molecular ions becomes important, as indicated by studies of NGC 7027. Numerous species such as HCO+, HCN, and CCH are found in old planetary nebulae such as the Helix. This “survivor” molecular material may be linked to the variety of compounds found recently in diffuse clouds. Organic molecules in dense interstellar clouds may ultimately be traced back to carbon-rich fragments originally formed in circumstellar shells.