Thermal Synthesis of Carbamic Acid and Its Dimer in Interstellar Ices: A Reservoir of Interstellar Amino Acids

Reactions in interstellar ices are shown to be capable of producing key prebiotic molecules without energetic radiation that are necessary for the origins of life. When present in interstellar ices, carbamic acid (H2NCOOH) can serve as a condensed-phase source of the molecular building blocks for more complex proteinogenic amino acids. Here, Fourier transform infrared spectroscopy during heating of analogue interstellar ices composed of carbon dioxide and ammonia identifies the lower limit for thermal synthesis to be 62 ± 3 K for carbamic acid and 39 ± 4 K for its salt ammonium carbamate ([H2NCOO–][NH4+]). While solvation increases the rates of formation and decomposition of carbamic acid in ice, the absence of solvent effects after sublimation results in a significant barrier to dissociation and a stable gas-phase molecule. Photoionization reflectron time-of-flight mass spectrometry permits an unprecedented degree of sensitivity toward gaseous carbamic acid and demonstrates sublimation of carbamic acid from decomposition of ammonium carbamate and again at higher temperatures from carbamic acid dimers. Since the dimer is observed at temperatures up to 290 K, similar to the environment of a protoplanetary disk, this dimer is a promising reservoir of amino acids during the formation of stars and planets.


Experimental Methods
2][3] The apparatus is housed in a hydrocarbon-free stainless steel ultra-high vacuum (UHV) chamber which is maintained at a few 10 −11 Torr by magnetically levitated turbomolecular pumps. 4A closed cycle helium refrigerator (Sumitomo Heavy Industries, RDK-415E) cools a mirror-polished silver wafer (15.1×12.6 or 32.8×32.8mm for PI-ReToF-MS or FTIR-TPD experiments, respectively) to 5-12 K.The refrigerator-wafer assembly is rotatable about the vertical axis because it is mounted on a doubly differentially pumped rotatable flange (Thermionics Vacuum Products, RNN-600/FA/MCO), and can be translated along its rotation axis via an adjustable bellows (McAllister, BLT106).Ices were prepared by passing ammonia (NH3, Matheson, 99.99%; 15 NH3, Aldrich, 98% atom 15 N; ND3, Aldrich, 99% atom D) and carbon dioxide (CO2, Airgas, research grade; 13 CO2, Cambridge Isotope Laboratories, 99% 13 C, < 1% 18 O; C 18 O2, Aldrich, 95% atom 18 O) through separate glass capillary arrays (10 or 25.4 mm diameter for PI-ReToF-MS or FTIR-TPD experiments, respectively) directed at the cooled wafer.Partial pressures of each ice component were adjusted by the use of leak valves to achieve approximately a 1:1 ratio of [NH3]: [CO2].Ice thickness was determined by monitoring the ice deposition with a helium-neon laser (CVI Melles-Griot, 25-LHP-230, 632.8 nm) at a 4° angle of incidence and measuring variations in reflected power due to thin film interference by the ice. 5The index of refraction of the mixed ice is unknown though this parameter is necessary to determine ice thickness from interferometric measurements.This parameter was approximated by the average of the indexes of refraction of the two components, 1.33 for ammonia at 18 K and 1.27 ± 0.02 at 15 K for acetaldehyde, with an average of 1.32 ± 0.03. 6,7Details of the composition and thickness of ices studied are reported in Table S1.
The photoionization reflectron time-of-flight mass spectrometry (PI-ReToF-MS) technique has been discussed in detail elsewhere. 1Ices were heated to 320 K with temperature programmed desorption (TPD) at a rate of 1 K min −1 .During TPD, pulsed 30 Hz coherent vacuum ultraviolet (VUV) light was passed 1-2 mm above the surface of the ice to photoionize subliming molecules.
VUV light was produced via resonant difference four-wave mixing (ωVUV = 2ω1 ± ω2) schemes (Table S7).After generation of the selected ω1 and ω2, the lasers were made collinear and directed through a lens (Thorlabs, LA5479, f = 300 mm) and focused into a jet of rare gas in the VUV generation vacuum chamber.Coherent VUV light exiting this chamber was separated from ω1 and ω2 by passing the collinear beams through an off-axis lithium fluoride (LiF) biconvex lens (Korth Kristalle, R1 = R2 = 131.22mm) which imparts an angular separation between the three frequencies and directs only the VUV light through an aperture to the ionization region.Ions formed are massanalyzed in a reflectron time-of-flight mass spectrometer (ReToF-MS; Jordan TOF Products) and detected with a dual microchannel plate (MCP) detector in the chevron configuration (Jordan TOF Products).MCP signal was amplified (Ortec, 9305) before discrimination and amplification to 4 V (Advanced Research Instruments Corp., F100-TD) and recorded by a multichannel scaler (FAST ComTec, MCS6A).Ion arrival times were recorded to 3.2 ns accuracy.New mass spectra were accumulated every two minutes during TPD.
The adiabatic ionization energies are computed by taking the energy difference between the neutral and cationic species that correspond to similar conformation.The adiabatic ionization energies at this level of calculations (CCSD(T)/CBS//B3LYP/cc-pVTZ) are expected to be accurate within  0.05 eV. 18,19The GAUSSIAN16 program 20 was utilized in the electronic structure calculations.

Table S3 .
Carbamic acid dimer zero-point corrected electronic energy (E at 0 K) and ionization energy (IE) at the CCSD(T)/CBS level with zero-point vibrational energy, Cartesian coordinates (Angstrom), vibrational frequencies, and vibrational intensities calculated at the B3LYP/cc-pVTZ level.

Table S4 .
Carbamic acid dimer cation zero-point corrected electronic energy (E at 0 K) at the CCSD(T)/CBS level with electronic energy, zero-point vibrational energy, Cartesian coordinates (Angstrom), vibrational frequencies, and vibrational intensities calculated at the B3LYP/cc-pVTZ level.

Table S7 .
Four-wave mixing schemes were employed to generate vacuum ultraviolet (VUV) photons for photoionization in experiments 1-13 (TableS1).All experiments use at least one dye laser (Sirah Lasertechnik, Cobra-Stretch) pumped by a neodymium yttrium-aluminum garnet (Nd:YAG, Spectra-Physics, Quanta Ray PRO 270-30 or PRO 250-30) laser harmonic (355 or 532 nm) appropriate for the dye in use.Because of the orientation of the FTIR and mass spectrometric probes in the experimental chamber, only one may be measured during TPD, these experiments are those for which the PI-ReToF-MS technique was used.
a Nd:YAG harmonic

Table S8 .
Parameters used in irradiation dose calculation and resulting doses.