Detection of interstellar ethylene oxide (c-C2H4O)

Rotational Spectroscopy of 1-Cyano-2-methylenecyclopropane (C5H5N)─A Newly Synthesized Pyridine Isomer

The gas-phase rotational spectrum of 1-cyano-2-methylenecyclopropane (C1, C5H5N), an isomer of pyridine, is presented for the first time, covering the range from 235 to 500 GHz. Over 3600 a-, b-, and c-type transitions for the ground vibrational state have been assigned, measured, and least-squares fit to partial-octic A- and S-reduced distorted-rotor Hamiltonians with low statistical uncertainty (σfit = 42 kHz). Transitions for the two lowest-energy fundamental states (ν27 and ν26) and the lowest-energy overtone (2ν27) have been similarly measured, assigned, and least-squares fit to single-state Hamiltonians. Computed vibration-rotation interaction constants (B0-Bv) using the B3LYP and MP2 levels of theory are compared with the corresponding experimental values. Based upon our preliminary analysis, the next few vibrationally excited states form one or more complex polyads of interacting states via Coriolis and anharmonic coupling. The spectroscopic constants and transition frequencies presented here form the foundation for both future laboratory spectroscopy and astronomical searches for 1-cyano-2-methylenecyclopropane.

Comprehensive quantum chemical analysis of the (ro)vibrational spectrum of thiirane and its deuterated isotopologue

The (ro)vibrational spectra of thiirane, c-C2H4S, and its fully deuterated isotopologue, c-C2D4S, have been studied by means of vibrational configuration interaction theory, VCI, its incremental variant, iVCI, and subsequent variational rovibrational calculations, RVCI, which rely on multidimensional potential energy surfaces of coupled-cluster quality including up to four-mode coupling terms. Accurate geometrical parameters, fundamental vibrational transitions and first overtones, rovibrational spectra and rotational spectroscopic constants have been determined from these calculations and were compared with experimental results whenever available. A number of tentative misassignments in the vibrational spectra could be resolved and most results for the deuterated thiirane are high-level predictions, which may guide experiments to come. Besides this, a new implementation of infrared intensities within the iVCI framework has been tested for the transitions of the title compounds and are compared with results obtained from standard VCI calculations.

Theoretical Investigation into a Possibility of Formation of Propylene Oxide Homochirality in Space

The preferential synthesis or destruction of a single enantiomer by ultraviolet circularly polarized light (UV-CPL) has been proposed as a possible triggering mechanism for the extraterrestrial origin of homochirality. Herein, we investigate the photoabsorption property of propylene oxide (c-C₃H₆O) for UV-CPL in the Lyman-α region. Our calculations show that c-C₃H₆O was produced by CH₃⁺ and CH₃CH(OH)CH₃ or C₃H₇^• and O (triplet). The computed electronic circular dichroism spectra show that c-C₃H₆O and the intermediate (CH₃CH(OH)CH₂⁺) could absorb the UV-CPL originating from the Lyman-α emitter spectrum, suggesting that the photolysis of c-C₃H₆O or CH₃CH(OH)CH₂⁺ upon irradiation could induce chiral symmetry breakage.

A Search for Heterocycles in GOTHAM Observations of TMC-1

We have conducted an extensive search for nitrogen-, oxygen-, and sulfur-bearing heterocycles toward Taurus Molecular Cloud 1 (TMC-1) using the deep, broadband centimeter-wavelength spectral line survey of the region from the GOTHAM large project on the Green Bank Telescope. Despite their ubiquity in terrestrial chemistry, and the confirmed presence of a number of cyclic and polycyclic hydrocarbon species in the source, we find no evidence for the presence of any heterocyclic species. Here, we report the derived upper limits on the column densities of these molecules obtained by Markov Chain Monte Carlo (MCMC) analysis and compare this approach to traditional single-line upper limit measurements. We further hypothesize why these molecules are absent in our data, how they might form in interstellar space, and the nature of observations that would be needed to secure their detection.

Synthesis, Purification, and Rotational Spectroscopy of (Cyanomethylene)Cyclopropane-An Isomer of Pyridine

The gas-phase rotational spectrum of (cyanomethylene)cyclopropane, (CH₂)₂C═CHCN, generated by a Wittig reaction between the hemiketal of cyclopropanone and (cyanomethylene)triphenylphosphorane, is presented for the first time. This small, highly polar nitrile is a cyclopropyl-containing structural isomer of pyridine. The rotational spectra of the ground state and two vibrationally excited states were observed, analyzed, and least-squares fit from 130 to 360 GHz. Over 3900 R-, P-, and Q-branch, ground-state rotational transitions were fit to low-error, partial octic, A- and S-reduced Hamiltonians, providing precise determinations of the spectroscopic constants. The two lowest-energy vibrationally excited states, ν₁₇ and ν₂₇, form a Coriolis-coupled dyad displaying small a- and b-type resonances. Transitions for these two states were measured and least-squares fit to a two-state, partial octic, A-reduced Hamiltonian in the I^r representation with nine Coriolis-coupling terms (G_a, G_a^J, G_a^K, G_a^JJ, F_bc, F_bc^J, F_bc^K, G_b, and G_b^J). The observation of many resonant transitions and nine nominal interstate transitions enabled a very accurate and precise energy difference between ν₁₇ and ν₂₇ to be determined: ΔE_17,27 = 29.8975453 (33) cm^-1. The spectroscopic constants presented herein provide the foundation for future astronomical searches for (cyanomethylene)cyclopropane.

Isomerization and Fragmentation Reactions on the [C2SH4] Potential Energy Surface: The Metastable Thione S-Methylide Isomer

Thione S-methylide, parent species of the thiocarbonyl ylide family, is a 1,3-dipolar species on the [C₂SH₄] potential energy surface, not so much studied as its isomers, thiirane, vinyl thiol, and thioacetaldehyde. The conrotatory ring-closure reaction toward thiirane was studied in the 90s, but no complete analysis of the potential energy surface is available. In this paper, we report a computational study of the reaction scheme linking all species. We employed several computational methods (density functional theory, CCSD(T) composite schemes, and CASSCF/CASPT2 multireference procedures) to find the best description of thione S-methylide, its isomers, and transition states. The barrier from thiirane to thione S-methylide amounts to 52.2 kcal mol^-1 (against 17.6 kcal mol^-1 for the direct one), explaining why thiocarbonyl ylides cannot be prepared from thiiranes. Conversion of thiirane to vinyl thiol implies a large barrier, supporting why the reaction has been observed only at high temperatures. Fragmentations of thiirane to S(³P) or S(¹D) and ethylene as well as decomposition to hydrogen sulfide plus acetylene were also explored. Triplet and singlet open-shell species were identified as intermediates in the fragmentations, with energies lower than the transition state between thiirane and vinyl thiol, explaining the preference of the latter at low temperatures.

Laboratory blueprints for interstellar searches of aromatic chiral molecules: rotational signatures of styrene oxide

The tracking of symmetry-breaking events in space is a long-lasting goal of astrochemists, aiming at an understanding of homochiral Earth chemistry. One current effort at this frontier aims at the detection of small chiral molecules in the interstellar medium. For that, high-resolution laboratory spectroscopy data is required, providing blueprints for the search and assignment of these molecules using radioastronomy. Here, we used chirped-pulse Fourier transform microwave and millimeter-wave spectroscopy and frequency modulation absorption spectroscopy to record and assign the rotational spectrum of the chiral aromatic molecule styrene oxide, C6H5C2H3O, a relevant candidate for future radioastronomy searches. Using experimental data from the 2-12, 75-110, 170-220, and 260-330 GHz regions, we performed a global spectral analysis, which was complemented by quantum chemistry calculations. A global fit of the ground state rotational spectrum was obtained, including rotational transitions from all four frequency regions. Primary rotational constants as well as quartic and sextic centrifugal distortion constants were determined. We also investigated vibrationally excited states of styrene oxide, and for the three lowest energy vibrational states, we determined rotational constants including centrifugal distortion corrections up to the sextic order. In addition, spectroscopic parameters for the singly-substituted 13C and 18O isotopologues were retrieved from the spectrum in natural abundance and used to determine the effective ground state structure of styrene oxide in the gas phase. The spectroscopic parameters and line lists of rotational transitions obtained here will assist future astrochemical studies of this class of chiral organic molecules.

Theoretical study on the reaction between silacyclopropenylidene and three-membered heterocyclic compounds (azirane and oxirane): An alternative approach to the formation of heterocyclic silylene

The reaction mechanism between silacyclopropenylidene and three-membered heterocyclic compounds (azirane and oxirane) has been systematically investigated at the B3LYP/6-311+G* level of theory in order to better understand the reactivity of unsaturated cyclic silylene. Geometry optimizations and vibrational analyses have been conducted for the stationary points on the potential energy surface of the system. Calculations show that the Si-spiroheterocyclic intermediate and four-membered heterocyclic silylene compound could be produced through the insertion process and subsequent dissociation process between silacyclopropenylidene and three-membered heterocyclic compounds. For the insertion process, it is easier for silacyclopropenylidene to insert into C-N bond of azirane than into C-O bond of oxirane. This study is helpful to understand the reactivity of silacyclopropenylidene, the evolution of silicon-bearing molecules in space, and to offer an alternative approach to the formation of enlarged heterocyclic silylene compound.

Interaction of hydantoin with solar wind minority ions: O6+ and He2

The laboratory study of prebiotic molecules interacting with solar wind ions is important to understand their role in the emergence of life in the complex context of the astrochemistry of circumstellar environments. In this work, we present the first study of the interaction of hydantoin (C₃N₂O₂H₄, 100 a.m.u.) with solar wind minority multi-charged ions: O⁶⁺ at 30 keV and He²⁺ at 8 keV. The fragmentation mass spectra as well as correlation maps resulting from the interaction are presented and discussed in this paper. Prompt and delayed dissociations from metastable states of the ionized molecule have been observed and the corresponding lifetimes measured. Experimental results are completed by quantum Density Functional Theory (DFT) calculations for energies, structures and dynamics (Internal Reaction Coordinates and Dynamic Reaction Coordinates) of the molecule for its different reachable charge states and the major observed fragmentation pathways. These calculations show that the molecule can only support two charges before spontaneously dissociating in agreement with the experimental observations. Calculations also demonstrate that hydantoin's ring opens after double ionization of the molecule which may enhance its reactivity in the background of biological molecule formation in a cirmcumstellar environment. For the major experimentally observed fragmentations (like 44 a.m.u./56 a.m.u. dissociation), Internal Reaction Coordinate (IRC) calculations were performed pointing out for example the important role of hydrogen transfer in the fragmentation processes.

Testing Densities and Refractive Indices of Extraterrestrial Ice Components Using Molecular Structures - Organic Compounds and Molar Refractions

The use of infrared spectra to determine molecular abundances of icy astronomical objects and to study their chemistry requires laboratory measurements of reference spectra and related quantities, such as the index of refraction (n) and density (ρ) of candidate ices. Here we present new n and ρ measurements on ices involving over thirty C-, H-, and O-containing compounds, both acyclic and cyclic, representing seven chemical families. We examine the results in a way that is rare in the astrochemical literature, namely one in which data from an ice formed from molecules of a particular chemical family are compared to measurements on another member of the same family, such as of a homologous series or a pair of isomers. Apart from the intrinsic usefulness of the n and ρ data, a structure-based comparison can help establish trends and identify possibly spurious results. As liquid-phase data sometimes are used in low-temperature astrochemical work in the absence of solid-phase measurements, we compare our new ice results to those for the corresponding room-temperature liquids. We emphasize the use of our n and ρ data to compute the molar refraction (R _M ) for each of our ices, and how the resulting R _M values compare to those expected from molecular structures. The use of calculated RM values and measured n values to calculate ice densities, in the absence of direct measurements, also is addressed.

On the formation of complex organic molecules in the interstellar medium: untangling the chemical complexity of carbon monoxide-hydrocarbon containing ice analogues exposed to ionizing radiation via a combined infrared and reflectron time-of-flight analysis

Recently, over 200 molecules have been detected in the interstellar medium (ISM), with about one third being complex organic molecules (COMs), molecules containing six or more atoms. Over the last few decades, astrophysical laboratory experiments have shown that several COMs are formed via interaction of ionizing radiation within ices deposited on interstellar dust particles at 10 K (H₂O, CH₃OH, CO, CO₂, CH₄, NH₃). However, there is still a lack of understanding of the chemical complexity that is available through individual ice constituents. The present research investigates experimentally the synthesis of carbon, hydrogen, and oxygen bearing COMs from interstellar ice analogues containing carbon monoxide (CO) and methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), or acetylene (C₂H₂) exposed to ionizing radiation. Utilizing online and in situ techniques, such as infrared spectroscopy and tunable photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS), specific isomers produced could be characterized. A total of 12 chemically different groups were detected corresponding to C₂H_nO (n = 2, 4, 6), C₃H_nO (n = 2, 4, 6, 8), C₄H_nO (n = 4, 6, 8, 10), C₅H_nO (n = 4, 6, 8, 10), C₆H_nO (n = 4, 6, 8, 10, 12, 14), C₂H_nO₂ (n = 2, 4), C₃H_nO₂ (n = 4, 6, 8), C₄H_nO₂ (n = 4, 6, 8, 10), C₅H_nO₂ (n = 6, 8), C₆H_nO₂ (n = 8, 10, 12), C₄H_nO₃ (n = 4, 6, 8), and C₅H_nO₃ (n = 6, 8). More than half of these isomer specifically identified molecules have been identified in the ISM, and the remaining COMs detected here can be utilized to guide future astronomical observations. Of these isomers, three groups - alcohols, aldehydes, and molecules containing two of these functional groups - displayed varying degrees of unsaturation. Also, the detection of 1-propanol, 2-propanol, 1-butanal, and 2-methyl-propanal has significant implications as the propyl and isopropyl moieties (C₃H₇), which have already been detected in the ISM via propyl cyanide and isopropyl cyanide, could be detected in our laboratory studies. General reaction mechanisms for their formation are also proposed, with distinct follow-up studies being imperative to elucidate the complexity of COMs synthesized in these ices.

Solid-State Isomerization and Infrared Band Strengths of Two Conformational Isomers of Cyclopropanecarboxaldehyde, A Candidate Interstellar Molecule

At least a dozen of the known interstellar molecules possess a formyl group (HCO), suggesting that other such species exist and await discovery in the interstellar medium. Here we examine the mid-infrared (mid-IR) spectrum and selected physical properties of one such candidate, cyclopropanecarboxaldehyde, in amorphous ices. Mid-IR transmission spectra of solid cyclopropanecarboxaldehyde are presented for the first time and used to determine the cis-to-trans ratio of conformational isomers present in amorphous samples. The measured ratio is compared to one from an electron-diffraction study of the gas-phase compound. The cis-to-trans isomerization in the amorphous compound is followed and the activation energy is determined. The first IR band strengths for solid cyclopropanecarboxaldehyde are reported. Also presented are refractive indices and densities at 15 K for amorphous forms of two related compounds, cyclopropane and cyclopropanemethanol. Two low-temperature reactions for the interstellar formation of cyclopropanecarboxaldehyde are briefly described.

Far-Infrared Synchrotron Spectroscopy and Quantum Chemical Calculations of the Potentially Important Interstellar Molecule, 2-Chloroethanol

The high brightness of the Australian synchrotron allowed for detailed spectra to be collected at high resolution (0.00096 cm^-1) in the vicinity of the a/ b/ c-type ν₁₉ band of 2-chloroethanol, which involves O-H torsional motion about the C-O bond. A rovibrational analysis was performed for both chlorine isotopologues in the ν₁₉ fundamental (centered at ∼344 cm^-1) which involved the assignment of 7153 lines ( J ≤ 90, K _a ≤ 41). A global fit to these lines in addition to 119 microwave lines ( J ≤ 29, K _a ≤ 11) led to the determination of spectroscopic constants up to the sextic level in both the ground and excited states using Watson's A-reduction Hamiltonian. The constants agree well with those calculated at the anharmonic MP2/cc-pVTZ level and allow for spectroscopically accurate predictions of rotational transitions in the ground vibrational state to be made over a broad range of rotational energies ( T_R < 1000 K). We explored the role that 2-chloroethanol might play in interstellar molecular clouds by performing calculations on the substitution reaction between HCl and ethylene glycol, and the addition reaction between HCl and oxirane, all of which have been observed in Sagittarius B2(N) and are expected to play important roles in the chemistry that occurs on the icy mantles of interstellar dust grains. While both reactions have relatively high activation barriers, the HCl + oxirane reaction was found be much more exothermic; further calculations on it indicate that a water-like environment significantly reduces the barrier while slightly increasing its exothermicity. These results suggest that 2-chloroethanol could be efficiently produced from the cosmic ray bombardment of common interstellar ices.

The Gigahertz and Terahertz spectrum of monodeutero-oxirane (c-C2H3DO)

The rotational spectrum of monodeutero-oxirane was analysed as measured using the Zurich Gigahertz (GHz) spectrometer and our highest resolution Fourier Transform Infrared (FTIR) spectrometer system coupled to synchrotron radiation at the Swiss Light Source (SLS). 112 distinct line frequencies have been newly assigned in the GHz range (extended to 120 GHz, compared to previous work extending to only 59 GHz) including rotational states up to J = 23. We have furthermore assigned 398 lines in the far infrared or Terahertz range (0.75-2.10 THz or 25-70 cm-1) including transitions with rotational quantum numbers up to J = 59. The results are discussed in relation to the possible first astrophysical observation of an isotopically chiral molecule and in relation to molecular parity violation.

Dust evolution, a global view: III. Core/mantle grains, organic nano-globules, comets and surface chemistry

Within the framework of The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS), this work explores the surface processes and chemistry relating to core/mantle interstellar and cometary grain structures and their influence on the nature of these fascinating particles. It appears that a realistic consideration of the nature and chemical reactivity of interstellar grain surfaces could self-consistently and within a coherent framework explain: the anomalous oxygen depletion, the nature of the CO dark gas, the formation of 'polar ice' mantles, the red wing on the 3 μm water ice band, the basis for the O-rich chemistry observed in hot cores, the origin of organic nano-globules and the 3.2 μm 'carbonyl' absorption band observed in comet reflectance spectra. It is proposed that the reaction of gas phase species with carbonaceous a-C(:H) grain surfaces in the interstellar medium, in particular the incorporation of atomic oxygen into grain surfaces in epoxide functional groups, is the key to explaining these observations. Thus, the chemistry of cosmic dust is much more intimately related with that of the interstellar gas than has previously been considered. The current models for interstellar gas and dust chemistry will therefore most likely need to be fundamentally modified to include these new grain surface processes.

Enantiomer excesses of rare and common sugar derivatives in carbonaceous meteorites

Biological polymers such as nucleic acids and proteins are constructed of only one-the d or l-of the two possible nonsuperimposable mirror images (enantiomers) of selected organic compounds. However, before the advent of life, it is generally assumed that chemical reactions produced 50:50 (racemic) mixtures of enantiomers, as evidenced by common abiotic laboratory syntheses. Carbonaceous meteorites contain clues to prebiotic chemistry because they preserve a record of some of the Solar System's earliest (∼4.5 Gy) chemical and physical processes. In multiple carbonaceous meteorites, we show that both rare and common sugar monoacids (aldonic acids) contain significant excesses of the d enantiomer, whereas other (comparable) sugar acids and sugar alcohols are racemic. Although the proposed origins of such excesses are still tentative, the findings imply that meteoritic compounds and/or the processes that operated on meteoritic precursors may have played an ancient role in the enantiomer composition of life's carbohydrate-related biopolymers.

Hydrogen bonding in alcohol–ethylene oxide and alcohol–ethylene sulfide complexes

The O–H⋯O and O–H⋯S hydrogen bonds are of similar strength in the corresponding alcohol–EO and alcohol–ES complexes.

Achiral, acyclic nucleic acids: synthesis and biophysical studies of a possible prebiotic polymer

The search for prebiotic, nucleic acid precursors is, at its best, a speculative undertaking.

ACCURATE SPECTROSCOPIC CHARACTERIZATION OF OXIRANE: A VALUABLE ROUTE TO ITS IDENTIFICATION IN TITAN'S ATMOSPHERE AND THE ASSIGNMENT OF UNIDENTIFIED INFRARED BANDS

In an effort to provide an accurate spectroscopic characterization of oxirane, state-of-the-art computational methods and approaches have been employed to determine highly accurate fundamental vibrational frequencies and rotational parameters. Available experimental data were used to assess the reliability of our computations, and an accuracy on average of 10 cm-1 for fundamental transitions as well as overtones and combination bands has been pointed out. Moving to rotational spectroscopy, relative discrepancies of 0.1%, 2%-3%, and 3%-4% were observed for rotational, quartic, and sextic centrifugal-distortion constants, respectively. We are therefore confident that the highly accurate spectroscopic data provided herein can be useful for identification of oxirane in Titan's atmosphere and the assignment of unidentified infrared bands. Since oxirane was already observed in the interstellar medium and some astronomical objects are characterized by very high D/H ratios, we also considered the accurate determination of the spectroscopic parameters for the mono-deuterated species, oxirane-d1. For the latter, an empirical scaling procedure allowed us to improve our computed data and to provide predictions for rotational transitions with a relative accuracy of about 0.02% (i.e., an uncertainty of about 40 MHz for a transition lying at 200 GHz).

A comparison of two methods for selecting vibrational configuration interaction spaces on a heptatomic system: ethylene oxide

Two recently developed methods for solving the molecular vibrational Schrodinger equation, namely, the parallel vibrational multiple window configuration interaction and the vibrational mean field configuration interaction, are presented and compared on the same potential energy surface of ethylene oxide, c-C(2)H(4)O. It is demonstrated on this heptatomic system with strong resonances that both approaches converge towards the same fundamental frequencies. This confirms their ability to tackle the vibrational problem of large molecules for which full configuration interaction calculations are not tractable.

Spectroscopic diagnostics of organic chemistry in the protostellar environment

A combination of astronomical observations, laboratory studies, and theoretical modelling is necessary to determine the organic chemistry of dense molecular clouds. We present spectroscopic evidence for the composition and evolution of organic molecules in protostellar environments. The principal reaction pathways to complex molecule formation by catalysis on dust grains and by reactions in the interstellar gas are described. Protostellar cores, where warming of dust has induced evaporation of icy grain mantles, are excellent sites in which to study the interaction between gas phase and grain-surface chemistries. We investigate the link between organics that are observed as direct products of grain surface reactions and those which are formed by secondary gas phase reactions of evaporated surface products. Theory predicts observable correlations between specific interstellar molecules, and also which new organics are viable for detection. We discuss recent infrared observations obtained with the Infrared Space Observatory, laboratory studies of organic molecules, theories of molecule formation, and summarise recent radioastronomical searches for various complex molecules such as ethers, azaheterocyclic compounds, and amino acids.

Searches for new interstellar molecules, including a tentative detection of aziridine and a possible detection of propenal

Rotational spectroscopy at millimeter wavelengths is a powerful means of investigating the chemistry of dense interstellar clouds. These regions can exhibit an interesting complement of gas phase molecules, including relatively complex organics. Here we report the tentative first astronomical detection of aziridine (ethylenimine), the possible detection of propenal (acrolein), and upper limits on the abundances of cyclopropenone, furan, hydroxyethanal (glycolaldehyde), thiohydroxylamine (NH2SH), and ethenol (vinyl alcohol) in various interstellar clouds.

The Millimeter- and Submillimeter-Wave Spectrum and the Dipole Moment of Ethylenimine

The rotational spectrum of ethylenimine (aziridine, c-C(2)H(4)NH) has been investigated in selected regions from 118 to 950 GHz using the Cologne terahertz spectrometer. About 320 lines have been measured spanning the quantum numbers 2 </= J </= 59 and 0 </= K(c) </= 50. All lines have been fit together with previously published data to yield refined ground state constants such as A = 22 736.19294(31) MHz, B = 21 192.46114(31) MHz, and C = 13 383.16401(30) MHz and centrifugal distortion constants that permit accurate frequency predictions. FTMW measurements were performed at the ETH Zürich to redetermine the dipole moment, to improve (14)N-quadrupole-coupling constants, as well as to determine (14)N-spin-rotation coupling constants. The most reliable values for the components of the dipole moment are µ(b) = 0.97(12) D and µ(c) = 1.357(25) D determined by the Stark- and additional intensity measurements of appropriate b- and c-type transitions. Moreover, we performed quantum chemical calculations of the dipole moment of ethylenimine that confirm these experimental results. This three-membered ring molecule is of potential astrophysical interest. Copyright 2000 Academic Press.

The composition of interstellar molecular clouds

We consider four-aspects of interstellar chemistry for comparison with comets: molecular abundances in general, relative abundances of isomers (specifically, HCN and HNC), ortho/para ratios for molecules, and isotopic fractionation, particularly for the ratio hydrogen/deuterium. Since the environment in which the solar system formed is not well constrained, we consider both isolated dark clouds where low mass stars may form and the "hot cores" that are the sites of high mass star formation. Attention is concentrated on the gas phase, since the grains are considered elsewhere in this volume.