Formation Of Complex Organic Molecules Research Articles

Radical addition and H abstraction reactions in C2H2, C2H4, and C2H6: A gateway for ethyl- and vinyl-bearing molecules in the interstellar medium

Context. Recent interstellar detections include a significant number of molecules containing vinyl (C2H3) and ethyl (C2H5) groups in their structure. For several of these molecules, there is no clear experimental or theoretical evidence that supports their formation from simpler precursors. Aims. We carried out a systematic search of viable reactions starting from closed-shell hydrocarbons containing two carbon atoms (ethane, C2H6; ethylene, C2H4; and acetylene, C2H2), with the goal of determining viable chemical routes for the formation of vinyl and ethyl molecules on top of interstellar dust grains. Methods. We used density functional theory calculations in combination with semiclassical instantem theory to derive the rate coefficients for the radical-neutral surface reactions. The effect of a surface was modeled through an implicit surface approach, profiting from the weak interaction between the considered hydrocarbons and the dust surfaces. Results. Our results show that both H and OH radicals are key in converting acetylene and ethylene into more complex radicals that are liable to continue reacting and to form interstellar complex organic molecules. The relevant reactions, for example OH additions, present rate constants above 101 s−1 that are likely competitive with OH diffusion on grains. Similarly, H atom addition to acetylene and ethylene is a very fast process, with rate constants above 104 s−1 in all cases, and is greatly enhanced by quantum tunneling. Hydrogen abstraction reactions are less relevant, but may play a role in specific cases involving the OH radical. Reactions with other radicals NH2 and CH3 are likely to have much less impact on the chemistry of ethyl- and vinyl-bearing molecules. Conclusions. The effective formation at low temperatures of four radicals (C2H3, C2H5, C2H2OH, and C2H4OH) through our proposed mechanism opens the gate for the formation of complex organic molecules, and indicates a potential prevalence of OH-bearing molecules on the grain. Following our suggested reaction pathway, we explain the formation of many of the newly detected molecules, and propose new molecules for detection. Our results reinforce the recent view on the importance of the OH radical in interstellar surface chemistry.

Effect of the UV dose on the formation of complex organic molecules in astrophysical ices: irradiation of methanol ices at 20 K and 80 K

ABSTRACT Methanol is a ubiquitous complex organic molecule (COM) in the interstellar medium, thought to be a precursor of larger COMs when it is submitted to different energetic processes, that can trigger chemical reactions in solid and gas phases. Using laboratory experiments, we report the characterization of the evolution of photoproducts generated by the UV irradiation of methanol ice at different UV doses and temperatures (20 and 80 K). We used gas chromatography coupled to mass spectrometry (GC-MS) to analyse the volatile organic compounds (VOCs) recovered during the warming of the photoprocessed methanol ice. We identified 21 molecules (with up to five carbon atoms, including alcohols, aldehydes, ketones, ester, and ethers) and followed their abundance as a function of the UV fluence and ice temperatures. With increasing UV fluence, an increase in the production of heavier COMs is observed, while species with 1 or 2 carbon atoms are depleted or do not increase. Species within a same chemical family show the same pattern of evolution, with heavier molecules present in smaller quantities. Ketones and esters are the chemical families that lead to more complex molecules and start forming at the earliest stages of irradiation. Their formation pathways are driven by radical recombinations with CO as the main building blocks. Aldehydes are formed before their alcohol counterparts, implying they do not form through alcohol dehydrogenation, but via radical recombination around HCO. Ethers seem to be the precursors of a large set of COMs, and alcohols present a steady profile throughout irradiation.

Hydrogen‐atom tunneling reactions in solid para‐hydrogen and their applications to astrochemistry

AbstractH‐atom tunneling reactions play important roles in astrochemistry, but an understanding of these reactions is still in its infancy. The unique properties associated with quantum solid para‐hydrogen provide an effective environment for the generation and reactions in situ of H atoms at low temperature. Several techniques have been employed to generate H atoms to study astrochemically relevant systems that provide significant insight into the formation of complex organic molecules (COM) and help to explain the relations between the abundance of some pairs of stable species. These results introduce new concepts in astrochemistry, including H‐induced H abstraction, H‐induced fragmentation, and H‐induced uphill isomerization in darkness that have been overlooked previously. This mini‐review summarizes the state of the art in this field, discussing fundamental understanding and techniques concerning H‐atom generation, H‐tunneling reactions, and their applications; the perspectives and open questions that await further exploration are discussed.

Exploitation of Synchrotron Radiation Photoionization Mass Spectrometry in the Analysis of Complex Organics in Interstellar Model Ices.

Unravelling the generation of complex organic molecules (COMs) on interstellar nanoparticles (grains) is essential in establishing predictive astrochemical reaction networks and recognizing evolution stages of molecular clouds and star-forming regions. The formation of COMs has been associated with the irradiation of interstellar ices by ultraviolet photons and galactic cosmic rays. Herein, we pioneer the first incorporation of synchrotron vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry (SVUV-PI-ReTOF-MS) in laboratory astrophysics simulation experiments to afford an isomer-selective identification of key COMs (ketene (H2C═CO); acetaldehyde (CH3CHO); vinyl alcohol (H2C═CHOH)) based on photoionization efficiency (PIE) curves of molecules desorbing from exposed carbon monoxide-methane (CO-CH4) ices. Our results demonstrate that the SVUV-PI-ReTOF-MS approach represents a versatile, rapid methodology for a comprehensive identification and explicit understanding of the complex organics produced in space simulation experiments. This methodology is expected to significantly improve the predictive nature of astrochemical models of complex organic molecules formed abiotically in deep space, including biorelated species linked to the origins-of-life topic.

The Radiation Chemistry of NH3···CO Complex in Cryogenic Media as Studied by Matrix Isolation.

The NH3···CO complex can be considered an important building block for cold synthetic astrochemistry leading to the formation of complex organic molecules, including key prebiotic species. In this work, we have studied the radiation-induced transformations of this complex in Ar, Kr, and Xe matrices using FTIR spectroscopy. On the basis of comparison with the quantum chemical calculations at the CCSD(T)/L2a_3 level of theory, it was found that the initial complex had the configuration with hydrogen bonding through the carbon atom of CO. Irradiation of the matrix isolated complex with X-rays at 6 K leads to the formation of a number of synthetic products, namely, HNCO (in all matrices), formamide NH2CHO, NH2CO, and HNCO-H2 (in argon and krypton). The matrix effect on the product distribution was explained by the involvement of different excited states of the complex in their formation. It was suggested that formamide results from the singlet excited states while other species mainly originate from triplet excited states. The latter states are efficiently populated through ion-electron recombination (in all matrices) and through intersystem crossing (particularly, in xenon). High yield of the recombination triplet states is a feature of the processes induced by high-energy radiation (in contrast to direct photolysis). NCO, CN, and NO were found as minor secondary products at high adsorbed doses. The astrochemical implications of the obtained results are discussed.

Radical Recombination during the Phase Transition of Interstellar CO Ice

Complex organic molecules (COMs) can be produced efficiently in ice mixtures that simulate the ice mantle on cosmic dust grains, according to prior experimental studies. However, the mechanism that brings the reactive species together in the ice has been debated. Thermal diffusion, which is widely regarded as the main mechanism to bring reactants together, is inefficient at a typical dense cloud temperature of 10 K. A recent experimental study found that the transition of a thin CO ice film from the amorphous to crystalline phase happens at about 10 K. When a small fraction of CO2 was mixed with CO, the CO2 molecules can separate and form clusters during CO phase transition. It was further proposed that the separation of minor species in the CO ice during phase transition may be an important mechanism to form interstellar COMs without the need for thermal diffusion. In this study, we try to verify this new mechanism through laboratory experiments. An ice mixture of CH3OH and CO, which is an analog of the outer layer of the ice mantle on cosmic dust grains, was exposed to UV irradiation to produce radicals such as HCO and CH2OH, whose concentration was monitored during the subsequent warm-up of the ice. We find clear evidence that during the CO phase transition, most of the radicals recombine to form other molecular species, therefore supporting the recently proposed mechanism of COM formation via CO phase transition.

Chemistry in Acetonitrile–Water Films Induced by Slow (<15 eV) Electrons: Application to the Earth and Space Chemistry

Acetonitrile and water are molecules detected not only in space but also in planetary atmospheres. On the Earth, the nitrile compound is emitted from biomass burning and can be found up to the stratosphere. Trapped in water ice cores, they may be exposed to energetic particles, photons and secondary electrons, and contribute to the formation of complex organic molecules. Here, we show that methanol is the main product from the effective reaction of low-energy (<15 eV) electrons with acetonitrile–water films deposited on a substrate maintained at 85 K. In this process, each of the molecules is decomposed by the colliding electrons, producing respectively methyl and hydroxyl radicals, which further recombine to form methanol, as supported by density functional theory (DFT) calculations. In addition, we also report production of a small amount of glycolonitrile, a key precursor of adenine. This information contributes to a better comprehension and description of chemistry of icy grains.

A numerical approach to model chemistry of complex organic molecules in a protoplanetary disk

Abstract Multiphase astrochemical modeling presents a numerical challenge especially for the simulation of objects with the wide range of physical parameters such as protoplanetary disks. We demonstrate an implementation of the analytical Jacobian for the numerical integration of the system of differential rate equations that govern chemical evolution in star-forming regions. The analytical Jacobian allowed us to greatly improve the stability of the code in protoplanetary disk conditions. We utilize the MONACO code to study the evolution of abundances of chemical species in protoplanetary disks. The chemical model includes 670 species and 6,015 reactions in the gas phase and on interstellar grains. The specific feature of the utilized chemical model is the inclusion of low-temperature chemical processes leading to the formation of complex organic molecules (COMs), included previously in the models of chemistry of COMs in prestellar clouds. To test the impact of analytical Jacobian on the stability of numerical simulations of chemical evolution in protoplanetary disks, we calculated the chemical composition of the disk using a two-phase model and four variants of the chemical reaction network, three values of the surface diffusion rates, and two types of the initial chemical composition. We also show a preliminary implementation of the analytical Jacobian to a three-phase model.

Formation of Complex Organic Molecules in Hot Molecular Cores through Nondiffusive Grain-surface and Ice-mantle Chemistry

Abstract A new, more comprehensive model of gas–grain chemistry in hot molecular cores is presented, in which nondiffusive reaction processes on dust-grain surfaces and in ice mantles are implemented alongside traditional diffusive surface/bulk-ice chemistry. We build on our nondiffusive treatments used for chemistry in cold sources, adopting a standard collapse/warm-up physical model for hot cores. A number of other new chemical model inputs and treatments are also explored in depth, culminating in a final model that demonstrates excellent agreement with gas-phase observational abundances for many molecules, including some (e.g., methoxymethanol) that could not be reproduced by conventional diffusive mechanisms. The observed ratios of structural isomers methyl formate, glycolaldehyde, and acetic acid are well reproduced by the models. The main temperature regimes in which various complex organic molecules (COMs) are formed are identified. Nondiffusive chemistry advances the production of many COMs to much earlier times and lower temperatures than in previous model implementations. Those species may form either as by-products of simple-ice production, or via early photochemistry within the ices while external UV photons can still penetrate. Cosmic ray-induced photochemistry is less important than in past models, although it affects some species strongly over long timescales. Another production regime occurs during the high-temperature desorption of solid water, whereby radicals trapped in the ice are released onto the grain/ice surface, where they rapidly react. Several recently proposed gas-phase COM-production mechanisms are also introduced, but they rarely dominate. New surface/ice reactions involving CH and CH2 are found to contribute substantially to the formation of certain COMs.

Synthetic Approaches to Complex Organic Molecules in the Cold Interstellar Medium

The observation and synthesis of organic molecules in interstellar space is one of the most exciting and rapidly growing topics in astrochemistry. Spectroscopic observations especially with millimeter and submillimeter waves have resulted in the detection of more than 250 molecules in the interstellar clouds from which stars and planets are ultimately formed. In this review, we focus on the diverse suggestions made to explain the formation of Complex Organic Molecules (COMs) in the low-temperature interstellar medium. The dominant mechanisms at such low temperatures are still a matter of dispute, with both gas-phase and granular processes, occurring on and in ice mantles, thought to play a role. Granular mechanisms include both diffusive and nondiffusive processes. A granular explanation is strengthened by experiments at 10 K that indicate that the synthesis of large molecules on granular ice mantles under space-like conditions is exceedingly efficient, with and without external radiation. In addition, the bombardment of carbon-containing ice mantles in the laboratory by cosmic rays, which are mainly high-energy protons, can lead to organic species even at low temperatures. For processes on dust grains to be competitive at low temperatures, however, non-thermal desorption mechanisms must be invoked to explain why the organic molecules are detected in the gas phase. Although much remains to be learned, a better understanding of low-temperature organic syntheses in space will add both to our understanding of unusual chemical processes and the role of molecules in stellar evolution.

Bottom-Up Synthesis of 1,1-Ethenediol (H2CC(OH)2)─The Simplest Unsaturated Geminal Diol─In Interstellar Analogue Ices.

Because of their nucleophilic character and high reactivity, enols─reaction intermediates carrying a hydroxyl group connected to a carbon-carbon double bond─play a key role in the formation of complex organic molecules in astrobiology and biochemistry. Here, we report the first bottom-up preparation of 1,1-ethenediol (H2CC(OH)2)─the simplest unsaturated geminal enol of acetic acid (CH3COOH) and potential precursor for the formation of glycine─in interstellar analogue ices of carbon dioxide and methane processed by proxies of galactic cosmic rays. These enols can easily form via nonequilibrium chemistry in low temperature (10 K) interstellar ices at abundances orders of magnitude higher than thermodynamically predicted. These energetically less favorable tautomers remain stable in ice-coated interstellar nanoparticles in molecular clouds and also upon sublimation into the gas phase in star forming regions thus providing the raw material to a complex and exotic organic chemistry under extreme conditions in deep space.

Systems Astrochemistry: A New Doctrine for Experimental Studies

Laboratory experiments play a key role in deciphering the chemistry of the interstellar medium (ISM) and the formation of complex organic molecules (COMs) relevant to life. To date, however, most studies in experimental astrochemistry have made use of a reductionist approach to experimental design in which chemical responses to variations in a single parameter are investigated while all other parameters are held constant. Although such work does afford insight into the chemistry of the ISM, it is likely that several important points (e.g., the possible influence of experimental parameter interaction) remain ambiguous. In light of this, we propose the adoption of a new “systems astrochemistry” approach for experimental studies and present the basic tenants and advantages of this approach in this perspective article. Such an approach has already been used for some time now and to great effect in the field of prebiotic chemistry, and so we anticipate that its application to experimental astrochemistry will uncover new data hitherto unknown which could aid in better linking laboratory work to observations and models.

Modelling the insertion of O(1D) into methane on the surface of interstellar ice mantles

ABSTRACT The detection of many complex organic molecules (COMs) in interstellar space has sparked the study of their origins. While the formation of COMs detected in hot cores is attributed to photochemistry on warming grain surfaces followed by recombination of radicals and desorption, the formation routes in colder regions are still a debated issue with a number of theories such as cosmic ray bombardment on interstellar ice mantles or non-diffusive surface chemistry. Here, we present another method with reactions involving metastable atomic oxygen in the O(1D) state, which is initially produced by photodissociation of oxygen-containing species in interstellar ices. As a first example, we study the reactions of metastable oxygen atoms and methane in ices to form both formaldehyde and methanol. The reaction is studied incorporating two different surface processes: diffusive and non-diffusive chemistry. The formation of methanol and formaldehyde via metastable oxygen atoms is compared with well-known formation routes of both to understand the O(1D) contributions at different temperatures.

A Photoionization Mass Spectrometry Investigation into Complex Organic Molecules Formed in Interstellar Analog Ices of Carbon Monoxide and Water Exposed to Ionizing Radiation

Abstract The formation of complex organic molecules by energetic electrons mimicking secondary electrons generated within trajectories of galactic cosmic rays was investigated in interstellar ice analog samples of carbon monoxide and water at 5 K. Simulating the transition from cold molecular clouds to star-forming regions, newly formed products sublimed during the temperature-programmed desorption and were detected utilizing isomer-specific photoionization reflectron time-of-flight mass spectrometry. Using isotopically labeled ices, tunable photoionization, and adiabatic ionization energies to discriminate between isomers, isomers up to C 2 H 4 O 2 and C 2 H 6 O 2 were identified, while non-isomer-specific findings confirmed complex organics with molecular formulas up to C 4 H 6 O 4 . The results provide important constraints on reaction pathways from simple inorganic precursors to complex organic molecules that have both astrochemical and astrobiological significance.

Formation of complex organic molecules in molecular clouds: acetaldehyde, vinyl alcohol, ketene, and ethanol via the “energetic” processing of C2H2 ice

Context. The simultaneous detection of organic molecules of the form C2HnO, such as ketene (CH2CO), acetaldehyde (CH3CHO), and ethanol (CH3CH2OH), toward early star-forming regions offers hints of a shared chemical history. Several reaction routes have been proposed and experimentally verified under various interstellar conditions to explain the formation pathways involved. Most noticeably, the non-energetic processing of C2H2 ice with OH-radicals and H-atoms was shown to provide formation routes to ketene, acetaldehyde, ethanol, and vinyl alcohol (CH2CHOH) along the H2O formation sequence on grain surfaces in translucent clouds. Aims. In this work, the non-energetic formation scheme is extended with laboratory measurements focusing on the energetic counterpart, induced by cosmic rays penetrating the H2O-rich ice mantle. The focus here is on the H+ radiolysis of interstellar C2H2:H2O ice analogs at 17 K. Methods. Ultra-high vacuum experiments were performed to investigate the 200 keV H+ radiolysis chemistry of predeposited C2H2:H2O ices, both as mixed and layered geometries. Fourier-transform infrared spectroscopy was used to monitor in situ newly formed species as a function of the accumulated energy dose (or H+ fluence). The infrared spectral assignments are further confirmed in isotope labeling experiments using H218O. Results. The energetic processing of C2H2:H2O ice not only results in the formation of (semi-) saturated hydrocarbons (C2H4 and C2H6) and polyynes as well as cumulenes (C4H2 and C4H4), but it also efficiently forms O-bearing COMs, including vinyl alcohol, ketene, acetaldehyde, and ethanol, for which the reaction cross-section and product composition are derived. A clear composition transition of the product, from H-poor to H-rich species, is observed as a function of the accumulated energy dose. Furthermore, the astronomical relevance of the resulting reaction network is discussed.

Detection of complex organic molecules in young starless core L1521E

ABSTRACT Determining the level of chemical complexity within dense starless and gravitationally bound pre-stellar cores is crucial for constructing chemical models, which subsequently constrain the initial chemical conditions of star formation. We have searched for complex organic molecules (COMs) in the young starless core L1521E, and report the first clear detection of dimethyl ether (CH3OCH3), methyl formate (HCOOCH3), and vinyl cyanide (CH2CHCN). Eight transitions of acetaldehyde (CH3CHO) were also detected, five of which (A states) were used to determine an excitation temperature to then calculate column densities for the other oxygen-bearing COMs. If source size was not taken into account (i.e. if filling fraction was assumed to be one), column density was underestimated, and thus we stress the need for higher resolution mapping data. We calculated L1521E COM abundances and compared them to other stages of low-mass star formation, also finding similarities to other starless/pre-stellar cores, suggesting related chemical evolution. The scenario that assumes formation of COMs in gas-phase reactions between precursors formed on grains and then ejected to the cold gas via reactive desorption was tested and was unable to reproduce observed COM abundances, with the exception of CH3CHO. These results suggest that COMs observed in cold gas are formed not by gas-phase reactions alone, but also through surface reactions on interstellar grains. Our observations present a new, unique challenge for existing theoretical astrochemical models.

Subarcsecond Imaging of the Complex Organic Chemistry in Massive Star-forming Region G10.6-0.4

Abstract Massive star-forming regions exhibit an extremely rich and diverse chemistry, which in principle provides a wealth of molecular probes, as well as laboratories for interstellar prebiotic chemistry. Since the chemical structure of these sources displays substantial spatial variation among species on small scales (≲104 au), high-angular-resolution observations are needed to connect chemical structures to local environments and inform astrochemical models of massive star formation. To address this, we present ALMA 1.3 mm observations toward OB cluster-forming region G10.6-0.4 (hereafter “G10.6”) at a resolution of 014 (700 au). We find highly structured emission from complex organic molecules (COMs) throughout the central 20,000 au, including two hot molecular cores and several shells or filaments. We present spatially resolved maps of rotational temperature and column density for a large sample of COMs and warm gas tracers. These maps reveal a range of gas substructure in both O- and N-bearing species. We identify several spatial correlations that can be explained by existing models for the formation of COMs, including NH2CHO/HNCO and CH3OCHO/CH3OCH3, but also observe unexpected distributions and correlations that suggest that our current understanding of COM formation is far from complete. Importantly, complex chemistry is observed throughout G10.6, rather than being confined to hot cores. The COM composition appears to be different in the cores compared to the more extended structures, which illustrates the importance of high-spatial-resolution observations of molecular gas in elucidating the physical and chemical processes associated with massive star formation.

Chemistry and Photochemistry of Pyruvic Acid at the Air-Water Interface.

Interfacial regions are unique chemical reaction environments that can promote chemistry not found elsewhere. The air-water interface is ubiquitous in the natural environment in the form of ocean surfaces and aqueous atmospheric aerosols. Here we investigate the chemistry and photochemistry of pyruvic acid (PA), a common environmental species, at the air-water interface and compare it to its aqueous bulk chemistry using two different experimental setups: (1) a Langmuir-Blodgett trough, which models natural water surfaces and provides a direct comparison between the two reaction environments, and (2) an atmospheric simulation chamber (CESAM) to monitor the chemical processing of nebulized aqueous PA droplets. The results show that surface chemistry leads to substantial oligomer formation. The sequence begins with the condensation of lactic acid (LA), formed at the surface, with itself and with pyruvic acid, and LA + LA - H2O and LA + PA - H2O are prominent among the products in addition to a series of higher-molecular-weight oligomers of mixed units of PA and LA. In addition, we see zymonic acid at the surface. Actinic radiation enhances the production of the oligomers and produces additional surface-active molecules known from the established aqueous photochemical mechanisms. The presence and formation of complex organic molecules at the air-water interface from a simple precursor like PA in the natural environment is relevant to contemporary atmospheric science and is important in the context of prebiotic chemistry, where abiotic production of complex molecules is necessary for abiogenesis.

A new multi-beam apparatus for the study of surface chemistry routes to formation of complex organic molecules in space.

A multi-beam ultra-high vacuum apparatus is presented. In this article, we describe the design and construction of a new laboratory astrophysics experiment-VErs de NoUvelles Synthèses (VENUS)-that recreates the solid-state non-energetic formation conditions of complex organic molecules in dark clouds and circumstellar environments. The novel implementation of four operational differentially pumped beam lines will be used to determine the feasibility and the rates for the various reactions that contribute to formation of molecules containing more than six atoms. Data are collected by means of Fourier transform infrared spectroscopy and quadrupole mass spectrometry. The gold-coated sample holder reaches temperatures between 7 K and 400 K. The apparatus was carefully calibrated and the acquisition system was developed to ensure that experimental parameters are recorded as accurately as possible. A great effort has been made to have the beam lines converge toward the sample. Experiments have been developed to check the beam alignment using reacting systems of neutral species (NH3 and H2CO). Preliminary original results were obtained for the {NO + H} system, which shows that chemistry occurs only in the very first outer layer of the deposited species, that is, the chemical layer and the physical layer coincide. This article illustrates the characteristics, performance, and future potential of the new apparatus in view of the forthcoming launch of the James Webb Space Telescope. We show that VENUS will have a major impact through its contributions to surface science and astrochemistry.

Observations of the Onset of Complex Organic Molecule Formation in Interstellar Ices

Abstract Isolated dense molecular cores are investigated to study the onset of complex organic molecule formation in interstellar ice. Sampling three cores with ongoing formation of low-mass stars (B59, B335, and L483) and one starless core (L694-2), we sample lines of sight to nine background stars and five young stellar objects (YSOs; A K ∼ 0.5–4.7). Spectra of these stars from 2 to 5 μm with NASA’s Infrared Telescope Facility (IRTF) simultaneously display signatures from the cores of H2O (3.0 μm), CH3OH (C–H stretching mode, 3.53 μm), and CO (4.67 μm) ices. The CO ice is traced by nine stars, in which five show a long-wavelength wing due to a mixture of CO with polar ice (CO r ), presumably CH3OH. Two of these sightlines also show independent detections of CH3OH. For these we find the ratio of the CH3OH:CO r is 0.55 ± 0.06 and 0.73 ± 0.07 from L483 and L694-2, respectively. The detections of both CO and CH3OH for the first time through lines of sight toward background stars observationally constrains the conversion of CO into CH3OH ice. Along the lines of sight, most of the CO exists in the gas phase and ≤15% of the CO is frozen out. However, CH3OH ice is abundant with respect to CO (∼50%) and exists mainly as a CH3OH-rich CO ice layer. Only a small fraction of the lines of sight contains CH3OH ice, presumably those with the highest density. The high conversion of CO to CH3OH can explain the abundances of CH3OH ice found in later stage Class 1 low-mass YSO envelopes (CH3OH:CO r ∼ 0.5–0.6). For high-mass YSOs and one Class 0 YSO, this ratio varies significantly, implying local variations can affect ice formation. The large CH3OH ice abundance indicates that the formation of complex organic molecules is likely during the prestellar phase in cold environments without higher energy particle interactions (e.g., cosmic rays).