Isotopic Frequency Shifts Research Articles

Vibrational Spectra of HNIW and its Isotopologues: A Combined Experimental and Computational Study

AbstractIncorporating isotopically labelled materials in degradation experiments could help unravel the mechanism(s) of decomposition through use of the kinetic isotope effect. Characterizing synthesized isotopologues however requires an understanding of what observable signals are affected by the isotopic substitution. As vibrational spectroscopy can distinguish between isotopologues, it is an ideal characterization technique to evaluate isotopic variants. To this end, the vibrational spectra of HNIW and its deuterated (2H), 13C, 15N (all), 15N (nitro), and 18O isotopologues have been computationally predicted in the gas phase using density functional theory. These results are compared to experimentally measured FTIR/ATR and Raman spectra of both unsubstituted HNIW and 15N‐labeled HNIW in which the six nitro groups were synthetically tagged with 15N atoms (15Nnitro−HNIW). The experimental isotopic frequency shift for the −NO2 asymmetric stretching frequencies agrees with that theoretically calculated (∼35.7 cm−1 vs. 36.5 cm−1, respectively). Furthermore, analysis of the theoretically predicted frequency shifts for all isotopologues suggest the −NO2 bending modes are lower in frequency than previously reported. This assignment is supported by the experimentally measured isotopic shift of ∼10.1 cm−1 for these features (consistent with the predicted shift of ∼13.1 cm−1). This work expands our current understanding of the vibrational modes in HNIW as well as provides a method for future work on similar systems.

Fingerprints of carbon defects in vibrational spectra of GaN considering the isotope effect

In this paper, we examine the carbon defects associated with peaks of infrared (IR) absorption and Raman scattering appearing in GaN crystals at carbon ($^{12}\mathrm{C}$) doping in the range of concentrations from $3.2\ifmmode\times\else\texttimes\fi{}{10}^{17}$ to $3.5\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. Here, 14 unique vibrational modes of defects are observed in GaN samples grown by HVPE (hydride vapor phase epitaxy) and then compared with defect properties predicted from first-principles calculations. The vibrational frequency shift in two $^{13}\mathrm{C}$-enriched samples related to the effect of the isotope mass indicates six distinct configurations of the carbon-containing point defects. The effect of the isotope replacement is well reproduced by the density functional theory (DFT) calculations. Specific attention is paid to the most pronounced defects, namely, tricarbon complexes (${\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$) and carbon substituting for nitrogen (${\mathrm{C}}_{\mathrm{N}}$). The position of the transition level $(+/0)$ in the bandgap found for ${\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$ defects by DFT at 1.1 eV above the valence band maximum suggests that ${({\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}})}^{+}$ provides compensation of ${\mathrm{C}}_{\mathrm{N}}^{\ensuremath{-}}$. Here, ${\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$ defects are observed to be prominent yet have high formation energies in DFT calculations. Regarding ${\mathrm{C}}_{\mathrm{N}}$ defects, it is shown that the host Ga and N atoms are involved in the delocalized vibrations of the defect and significantly affect the isotopic frequency shift. Much more faint vibrational modes are found from di-atomic carbon-carbon and carbon-hydrogen (C--H) complexes. Also, we note changes of vibrational mode intensities of ${\mathrm{C}}_{\mathrm{N}}, {\mathrm{C}}_{\mathrm{N}}=\mathrm{C}={\mathrm{C}}_{\mathrm{N}}$, C--H, and ${\mathrm{C}}_{\mathrm{N}}--{\mathrm{C}}_{i}$ defects in the IR absorption spectra upon irradiation in the defect-related ultraviolet/visible absorption range. Finally, it is demonstrated that the resonant enhancement of the Raman process in the range of defect absorption $>2.5\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ enables the detection of defects at carbon doping concentrations as low as $3.2\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$.

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The IR transmittance spectrum of an isotopic mixture of chlorodifluoromethane (CHF2Cl, Freon-22) with a 33% fraction of 13C and a natural ratio of chlorine isotopes was measured in the frequency range 1400-740 cm–1 with a resolution of 0.001 cm–1 at a temperature of 20C. An ab initio calculation of the structure and sextic potential energy surface and surfaces of the components of the dipole moment has been carried out by the the electronic quantum-mechanical method of Möller-Plesset, MP2/cc-pVTZ. Then the potential was optimized by replacing the harmonic frequencies with the frequencies calculated by the electronic method of coupled clusters, CCSD(T)/aug-cc-pVQZ. The fundamental and combination frequencies were calculated using the operator perturbation theory of Van Vleck (CVPTn) of the second and fourth order (n=2,4). Resonance effects were modeled using an additional variational calculation in the basis up to fourfold VCI excitation (4). The average prediction error for the fundamental frequencies of the 12C isotopologues was ~1.5 cm–1. The achieved accuracy made it possible to reliably predict the isotopic frequency shifts of the 13C isotopologues. It is shown that the strong Fermi resonance ν4/2ν6 dominates in the 12C isotopologues and is practically absent in 13C. The literature assumption [Spectrochim. Acta A, 44: 553] about the splitting of ν1 (CH) due to the resonance ν1/ν2+ν7+ν9 is confirmed. The coefficients of the polyadic quantum number are determined. The analysis made it possible to carry out a preliminary identification of the centers of the vibrational-rotational bands of isotopologues 13CHF235Cl и 13CHF237Cl in the spectrum of the mixture in preparation for individual analyzes of the vibrational-rotational structures of individual vibrational transitions.

Неэмпирический анализ изотопических сдвигов и резонансных эффектов в инфракрасном спектре высокого разрешения фреона-22 (CHF-=SUB=-2-=/SUB=-Cl), обогащенного -=SUP=-13-=/SUP=-C

The IR transmittance spectrum of an isotopic mixture of chlorodifluoromethane (CHF2Cl, Freon-22) with a 33% fraction of 13C and a natural ratio of chlorine isotopes was measured in the frequency range 1400-740 cm–1 with a resolution of 0.001 cm–1 at a temperature of 20C. An ab initio calculation of the structure and sextic potential energy surface and surfaces of the components of the dipole moment has been carried out by the the electronic quantum-mechanical method of Möller-Plesset, MP2/cc-pVTZ. Then the potential was optimized by replacing the harmonic frequencies with the frequencies calculated by the electronic method of coupled clusters, CCSD(T)/aug-cc-pVQZ. The fundamental and combination frequencies were calculated using the operator perturbation theory of Van Vleck (CVPTn) of the second and fourth order (n=2,4). Resonance effects were modeled using an additional variational calculation in the basis up to fourfold VCI excitation (4). The average prediction error for the fundamental frequencies of the 12C isotopologues was ~1.5 cm–1. The achieved accuracy made it possible to reliably predict the isotopic frequency shifts of the 13C isotopologues. It is shown that the strong Fermi resonance ν4/2ν6 dominates in the 12C isotopologues and is practically absent in 13C. The literature assumption [Spectrochim. Acta A, 44: 553] about the splitting of ν1 (CH) due to the resonance ν1/ν2+ν7+ν9 is confirmed. The coefficients of the polyadic quantum number are determined. The analysis made it possible to carry out a preliminary identification of the centers of the vibrational-rotational bands of isotopologues 13CHF235Cl и 13CHF237Cl in the spectrum of the mixture in preparation for individual analyzes of the vibrational-rotational structures of individual vibrational transitions.

Analysis of the hyperfine splitting spectral structure of rubidium atomic D1 and D2 lines with pressure broadening

A physical model is established to describe the broadening mechanism of rubidium atomic D1 and D2 lines, hence to improve rubidium vapor laser efficiency. Hyperfine energy level structures of rubidium D1 and D2 transitions are analyzed and plotted. Considering the effects of the isotopic frequency shift, the pressure broadening and the hyperfine energy levels splitting, contributions of every hyperfine component to the cross sections are calculated and discussed. The relationship between cross sections and the ratio of helium pressure to ethane is achieved, and the composition of the buffer gas is optimized. The broadening effects of Rb D1 line and D2 line under different buffer gas pressure are also presented. The calculation results have a good agreement with that of experiments, which shows the validity of this model. The simulations data provides a reference for the experiment to further improve the bandwidth matching between alkali atomic D2 absorption line and the diode laser pump source.

Structural and vibrational characterization of 1-(diaminomethylene)-thiouron-1-ium 4-aminobenzoate

The single crystals of 1-(diaminomethylene)thiouron-1-ium 4-aminobenzoate, (C2H7N4S)-(NH2C6H4COO), (1) suitable for the X-ray analysis were grown using a solution growth technique room temperature. Recrystallization of 1 in heavy water (three-times) yield respective deuterated analogue, (C2D7N4S)(ND2C6H4COO), (1d). The X-ray single crystals analyses show that both, protiated and deuterated, compounds crystallize in the same crystal system with quite similar lattice parameters. The conformation of the 1-(diaminomethylene)thiouron-1-ium cation is not strictly planar, but slightly twisted. The 4-aminobenzoate(-) anion is also non-planar. The planar COO− group is inclined to the plane of aromatic ring by 5.2(2)o. In the crystal the oppositely charged units, i.e. 1-(diaminomethylene)thiouron-1-ium cation and 4-aminobenzoate anion interact via two of almost linear NH…O hydrogen bonds with a graph of R22(8), that are further interact each other forming three dimensional hydrogen-bonded network. Hirshfeld surface and the analysis of 2D fingerprint plots are illustrating both quantitatively and qualitatively interactions governing the supramolecular assemblies. The compound was also characterised by the FT-IR and Raman spectroscopy. The vibrational assignments have been supported by the isotopic frequency shift as well as by the DFT calculations.

Isotope Shift of Nuclear Magnetic Resonances in 129Xe and 131Xe Caused by Spin-Exchange Pumping by Alkali Metal Atoms

The effect of isotope shifts of nuclear magnetic resonance (NMR) frequency in xenon isotopes 129Xe and 131Xe polarized by optically-oriented alkali metal atoms is not only of fundamental interest, but also of practical significance, since it is the main factor limiting the accuracy of a whole class of prospective navigation and metrological devices. Our study of the parametric dependences of the isotope shift has shown that this effect is largely due to incomplete averaging of an inhomogeneous local magnetic field by two xenon isotopes, and therefore, its value can be influenced by applying an external magnetic field gradient. A numerical model is derived that qualitatively describes the effect of the isotopic frequency shift, as well as its dependence on the cell temperature and the external magnetic field gradient. The model provides a good quantitative agreement with the experiment – e.g., at a temperature of 85°C, the isotope shift values predicted by the model coincide with experimental data with an accuracy of up to the experimental error, which, in turn, does not exceed 8% of the total change in the isotopic shift in the entire range of applied magnetic field gradients.

Supramolecular diversity in solid-state architecture formed between the 1-(diaminomethylene)thiourea and citric acid

Three different single crystals formed between the 1-(diaminomethylene)thiourea and citric acid from water solution depending on the concentration and the molar composition of 1-(diaminomethylene)thiourea to citric acid, i.e. 1-(diaminomethylene)thiouron-1-ium citrate monohydrate (1), tris(1-(diaminomethylene)thiouron-1-ium) citrate (2) and tris(1-(diaminomethylene)thiouron-1-ium) citrate monohydrate (3), were obtained. X-ray single crystal diffraction technique was used for the analysis of the diversity in supramolecular arrangement between the units constituting the crystals. Compound 1 and 3 crystallize in the centrosymmetric space groups (1 in P-1) and 3 in P21/c) whereas compound 2 crystallises in non-centrosymmetric space group P212121. As show the X-ray single crystal analysis the conformation of the 1-(diaminomethylene)-thiouron-1-ium cation in each of studied crystals is not strictly planar, but twisted with the angle of 3.5 (1)o in 1, 4.8 (1) to 13.4 (1)o in 2 and 2.1 (1) to 8.7 (1)o in 3. The flexible carbon chain of the citrate (3-) anion exhibits conformational disorder in 3, whereas the citrate (3-) anion in 2 as well as the citrate (−) mono anion in 1 are ordered. The interaction between the units constituting the crystals including the NH⋯O and OH⋯O hydrogen bonds determine the supramolecular diversity in solid. Hirshfeld surface and the analysis of 2D fingerprint plots are used for illustrate both qualitatively and quantitatively interactions between the units governing the supramolecular assemblies. The compounds were also characterised by vibrational spectroscopy. The vibrational assignment have been supported by the isotopic frequency shift. Within the 1–3 compounds only compound 2 crystallises in non-centrosymmetric space group therefore for it the SHG was measured.

Structural and spectroscopic characterization of bis[1-(diaminomethylene)thiouron-1-ium] naphthalene-1,5-disulfonate

Single crystals of bis[1-(diaminomethylene)thiouron-1-ium] naphthalene-1,5-disulfonate (1) are grown using growth solution technique. The compound crystallizes in the centrosymmetric C2/c space group of the monoclinic system with four molecules per unit cell. The arrangement of oppositely charged units, i.e. 1-(diaminomethylene)thiouron-1-ium cations and naphthalene-1,5-disulfonate anions in the crystal is mainly determined by the ionic and the NH⋯O hydrogen bonding interactions. Hirshfeld surface and the analysis of 2D fingerprint plots are illustrating both qualitatively and quantitatively interactions governing the supramolecular arrangement. The compound was also characterized by FT-IR and Raman spectroscopies. The vibrational assignment of the characteristic bands of the functional groups have been supported by the isotopic frequency shift.

Solid-state supramolecular structure formed between the 1-(diaminomethylene)thiourea and mellitic acid

The single crystals of hexakis(1-(diaminomethylene)thiouron-1-ium) benzene-1,2,3,4,5,6-hexacarboxylate (mellitate(6-)) tetrahydrate suitable for the X-ray analysis were grown using a solution growth technique at room temperature. The compound crystallises in centrosymmetric space group of the triclinic system. The conformation of six crystallographically independent 1-(diaminomethylene)thiouron-1-ium cations is not strictly planar, but twisted. Both planar arms of the cations are oppositely rotated by 6.1(1) ÷ 20.9(1)o around the CN bonds involving the central N atom. All of the carboxylate groups of mellitate(6-) anion are inclined to the plane of aromatic six-membered carbon ring by 43.0(1) ÷ 72.0(1)o. Oppositely charged components i.e. 1-(diaminomethylene)thiouron-1-ium cations and mellitate(6-) anion interact each other via NH⋯O hydrogen bonds forming hexakis(1-(diaminomethylene)thiouron-1-ium) mellitate units that further interact with the water molecules forming hexakis(1-(diaminomethylene)thiouron-1-ium) mellitate tetrahydrate supramolecular architecture. Hirshfeld surface and the analysis of 2D fingerprint plots are used for illustrate both qualitatively and quantitatively interactions between the units governing the supramolecular assemblies. The compound was also characterised by vibrational spectroscopy. The vibrational assignment have been supported by the isotopic frequency shift.

Matrix-Isolation and Quantum-Chemical Analysis of the C3v Conformer of XeF6, XeOF4, and Their Acetonitrile Adducts.

A joint experimental-computational study of the molecular structure and vibrational spectra of the XeF6 molecule is reported. The vibrational frequencies, intensities, and in particular the isotopic frequency shifts of the vibrational spectra for 129XeF6 and 136XeF6 isotopologues recorded in the neon matrix agree very well with those obtained from relativistic coupled-cluster calculations for XeF6 in the C3v structure, thereby strongly supporting the observation of the C3v conformer of the XeF6 molecule in the neon matrix. A C3v transition state connecting the C3v and Oh local minima is located computationally. The calculated barrier of 220 cm-1 between the C3v minima and the transition state corroborates the experimental observation of the C3v conformer and the absence of the Oh conformer in solid noble gas matrices. For comparison matrix-isolation spectra have also been recorded and analyzed for the 129XeOF4 and the 136XeOF4 isotopologues. The matrix-isolation complexation shifts obtained for the XeF6·NCCH3 relative to those of free matrix isolated XeF6 and CH3CN are in good agreement with those reported for crystalline XeF6·NCCH3.

Factors of 18O/16O fractionation in garnets: Evidence from calculations of isotope frequency shifts

Based on the density functional theory (DFT), frequency shifts of garnet end-members upon isotope substitutions (18O/16O, as well as “non-traditional” stable isotopes) were calculated. According to the calculations, the temperature dependencies of fractionation factors (β-factors) suggested for garnets: 1000lnβprp = 9.68511x–0.19204x 2 + 0.00567x 3; 1000lnβgrs = 9.14697x–0.15682x 2 + 0.00412x 3; 1000lnβadr = 8.72470x–0.15092x 2 + 0.00402x 3; 1000lnβuvr = 8.71526x–0.14749x 2 + 0.00385x 3; 1000lnβsps = 9.33600x–0.17598x 2 + 0.00499x 3; 1000lnβalm = 9.45295x–0.18465x 2 + 0.00539x 3, x = 106/T (K)2. The values obtained combined with with the known values of β-factors may be applied in geothermometry of garnet-bearing rocks.

Solid-state structure of 1-(diaminomethylene)thiouron-1-ium propionate

The single crystals of 1-(diaminomethylene)thiouron-1-ium propionate suitable for the X-ray analysis were grown using a solution growth technique room temperature. The compound crystallises in the centrosymmetric C2/c space group of the monoclinic system. The conformation of the 1-(diaminomethylene)thiouron-1-ium cation is not strictly planar, but slightly twisted. Both planar arms of the cation are oppositely rotated by 2.1(1)° around the CN bonds involving the central N atom. The propionate(−) anion is also non-planar, the carboxylate group is turned by 4.3(1)° in relation to the planar carbon chain. The arrangement of oppositely charged components, i.e. 1-(diaminomethylene)thiouron-1-ium cations and propionate(−) anions in the crystal is mainly determined by ionic and NH⋯O hydrogen bonding interactions forming two-dimensional network aligned to (100) plane. The neighbouring 2D layers interact via much weaker NH⋯S hydrogen bonds forming three-dimensional hydrogen bonded network. Hirshfeld surface and the analysis of 2D fingerprint plots are illustrating both quantitatively and qualitatively interactions governing the supramolecular assemblies. The compound was also characterised by the FT-IR and Raman spectroscopy. The vibrational assignments have been supported by the isotopic frequency shift.

Accuracy of finite-difference harmonic frequencies in density functional theory.

Analytic Hessians are often viewed as essential for the calculation of accurate harmonic frequencies, but the implementation of analytic second derivatives is nontrivial and solution of the requisite coupled-perturbed equations engenders a sizable memory footprint for large systems, given that these equations are not required for energy and gradient calculations in density functional theory. Here, we benchmark the alternative approach to harmonic frequencies based on finite differences of analytic first derivatives, a procedure that is amenable to large-scale parallelization. Not only for absolute frequencies but also for isotopic and conformer-dependent frequency shifts in flexible molecules, we find that the finite-difference approach exhibits mean errors < 0.1 cm-1 as compared to results based on an analytic Hessian. For very small frequencies corresponding to nonbonded vibrations in noncovalent complexes (for which the harmonic approximation is questionable anyway), the finite-difference error can be larger, but even in these cases the errors can be reduced below 0.1 cm-1 by judicious choice of the displacement step size and a higher-order finite-difference approach. The surprising accuracy and robustness of the finite-difference results suggests that availability of the analytic Hessian is not so important in today's era of commodity processors that are readily available in large numbers. © 2017 Wiley Periodicals, Inc.

Distinguishing the Protonation State of the Histidine Ligand to the Oxidized Iron-Sulfur Cluster from the MitoNEET Family of Proteins.

The iron-sulfur cluster located in the recently discovered human mitoNEET protein (and related proteins) is structurally similar to the more well-known ferredoxin and Rieske clusters. Although its biological function is uncertain, the iron-sulfur cluster in mitoNEET has been proposed to undergo proton-coupled electron transfer involving the histidine ligand to the cluster. The cluster is also released from the protein at low pH. This contribution reports density functional calculations to model the structures, vibrations, and Heisenberg coupling constants (J) for high-spin (HS), broken symmetry (BS) singlet, and extended broken symmetry (EBS) singlet states of the oxidized iron-sulfur cluster from mitoNEET. This work suggests that J values or 15 N isotopic frequency shifts may provide methods for determining experimentally whether the histidine ligand to the oxidized iron-sulfur cluster in human mitoNEET and mitoNEET-related proteins is protonated or deprotonated.

An inverted crossover resonance aiding laser cooling of ^171Yb

We observe an inverted crossover resonance in $\pi$-driven four-level systems, where $F'-F=0,+1$. The signal is observed through saturated absorption spectroscopy of the $(6s^{2})$ $^{1}S_{0}$ $-$ $(6s6p)$ $^{3}P_{1}$ transition in $^{171}$Yb, where the nuclear spin $I=1/2$. The enhanced absorption signal is used to generate a dispersive curve for 556 nm laser frequency stabilisation and the stabilised light cools $^{171}$Yb atoms in a two-stage magneto-optical trap, achieving temperatures of 20 $\mu$K. The Doppler-free spectroscopy scheme is further used to measure isotopic frequency shifts and hyperfine separations for the intercombination line in ytterbium.

Vibrational spectra of tris(acetylacetonato)chromium(III)

The molecular structure and vibrational spectra of tris(acetylacetonato)chromium(III), Cr (AA)3, its mono-deuterated, Cr (DAA)3, and per-deuterated analogous, Cr(D7AA)3, were investigated using density functional theory (DFT) calculations. The geometry of the titled compound was fully optimized at the B3LYP level using 6-311 + G* and 6-31G** basis sets. The harmonic vibrational frequencies were obtained at the B3LYP/6-311 + G** level. Raman scattering activities were calculated at the B3LYP/6-31G** level. The calculated frequencies are compared with the experimental IR and Raman spectra. All of the measured IR and Raman bands were interpreted in terms of the calculated vibrational modes and isotopic frequency shifts. The scaled theoretical frequencies and the structural parameters are in excellent agreement with the experimental data. Analysis of the vibrational spectra indicates a strong coupling between the chelated ring modes. Several bands at 933, 680, 611, 594, 495, 250, 224, and 179 cm−1 are found to be engaged in Cr–O vibrational movements. The IR bands at 611 and 594 cm−1 are attributed to the asymmetric Cr–O stretching and the very strong Raman band at 460 cm−1 is assigned to the totally symmetric Cr–O stretching mode (a1 species).

13C═18O/15N Isotope Dependence of the Amide-I/II 2D IR Cross Peaks for the Fully Extended Peptides

We have used a combination of 2D IR spectroscopy with 13C═18O labeled amide-I and 15N-labeled amide-II modes to reveal how vibrational coupling between labeled peptide units depends on secondary structure. Linear and 2D IR measurements and simulations of Cα,α-diethylglycine homotetrapeptide show that this compound adopts the fully extended (2.05-helical) conformation in CDCl3, consistent with previous work on the Ac-capped peptide. The amide-I/II cross peaks of isotopomers exhibit only a marginal isotope frequency shift between labeled modes that are separated by two peptide units, indicating a very weak coupling. This result is in sharp contrast with a large cross-peak shift observed in 310-helical peptides, in which the labeled amide-I and -II modes are connected through an inter-residue C═O···H–N hydrogen bond. The discovered 3D-structural dependence indicates that the 13C═18O/15N labeled amide-I/II cross peaks can distinguish the formation of a single 310-helical turn from the fully extended polypepti...

Vibrational spectra and normal coordinate analysis of lithium pyruvate monohydrate and its isotopic compounds

IR and Raman spectra of lithium pyruvate monohydrate and its O- and C-deuterated and 13 C- and 18 O-substituted compounds have been recorded in the solid state, and the observed bands have been assigned by using the isotope effects and the normal coordinate calculations based on the gem -diol structure (lithium 2,2-dihydroxypropionate). The refined force constants have well reproduced the observed frequencies and the 13 C- and 18 O-shifts. These results support the structures of these compounds discussed by many authors. The potential energy distributions show that many vibrational modes are very complicated except for the well-known group vibrations. The additive property of the isotopic frequency shifts has also been discussed.

Infrared and reflectron time-of-flight mass spectroscopic study on the synthesis of glycolaldehyde in methanol (CH3OH) and methanol-carbon monoxide (CH3OH-CO) ices exposed to ionization radiation.

We present conclusive evidence on the formation of glycolaldehyde (HOCH2CHO) synthesized within astrophysically relevant ices of methanol (CH3OH) and methanol-carbon monoxide (CH3OH-CO) upon exposure to ionizing radiation at 5.5 K. The radiation induced chemical processes of the ices were monitored on line and in situ via infrared spectroscopy which was complimented by temperature programmed desorption studies post irradiation, utilizing highly sensitive reflectron time-of-flight mass spectrometry coupled with single photon fragment free photoionization (ReTOF-PI) at 10.49 eV. Specifically, glycolaldehyde was observed via the v14 band and further enhanced with the associated frequency shifts of the carbonyl stretching mode observed in irradiated isotopologue ice mixtures. Furthermore, experiments conducted with mixed isotopic ices of methanol-carbon monoxide (13CH3OH-CO, CH3(18)OH-CO, CD3OD-13CO and CH3OH-C18O) provide solid evidence of at least three competing reaction pathways involved in the formation of glycolaldehyde via non-equilibrium chemistry, which were identified as follows: (i) radical-radical recombination of HCO and CH2OH formed via decomposition of methanol--the "two methanol pathway"; (ii) via the reaction of one methanol unit (CH2OH from the decomposition of CH3OH) with one carbon monoxide unit (HCO from the hydrogenation of CO)--the "one methanol, one carbon monoxide pathway"; and (iii) formation via hydrogenation of carbon monoxide resulting in radicals of HCO and CH2OH--the "two carbon monoxide pathway". In addition, temperature programmed desorption studies revealed an increase in the amount of glycolaldehyde formed, suggesting further thermal chemistry of trapped radicals within the ice matrix. Sublimation of glycolaldehyde during the warm up was also monitored via ReTOF-PI and validated via the mutual agreement of the associated isotopic frequency shifts within the infrared band positions and the identical sublimation profiles obtained from the ReTOF spectra and infrared spectroscopy of the corresponding isotopes. In addition, an isomer of glycolaldehyde (ethene-1,2-diol) was tentatively assigned. Confirmation of the identified pathways based on infrared spectroscopy was also obtained from the observed ion signals corresponding to isotopomers of glycolaldehyde. These coupled techniques provide clear, concise evidence of the formation of a complex and astrobiologically important organic, glycolaldehyde, relevant to the icy mantles observed in the interstellar medium.