Interaction of water-ice and acetonitrile has been studied at low temperatures in their codeposited mixtures, in ultrahigh vacuum conditions. They interact strongly at low temperatures (in the temperature range of 40-110 K), which was confirmed from the new features manifested in the reflection absorption infrared spectra of the mixtures. This interaction was attributed to strong hydrogen bonding which weakens upon warming as the acetonitrile molecules phase segregate from water-ice. Complete phase separation was observed at 130 K prior to desorption of acetonitrile from the water-ice matrix. Such a hydrogen-bonded structure is not observed when both the molecular solids are deposited as water on acetonitrile or acetonitrile on water overlayers. A quantitative analysis shows that in a 1:1 codeposited mixture, more than 50% acetonitrile molecules are hydrogen bonded with water-ice at low temperatures (40-110 K). © 2015 American Chemical Society.

Pradeep Thalappil

Department Of Chemistry

Radha Gobinda Bhuin

acetonitrile

Hydrogen bonds

Mixtures

Molecules

Phase separation

Spectroscopic analysis

ACETONITRILE MOLECULES

HYDROGEN-BONDED STRUCTURES

INFRARED SPECTROSCOPIC STUDIES

Infrared spectrum

Low temperatures

REFLECTION ABSORPTION

Temperature range

ULTRAHIGH VACUUM CONDITIONS

Fulltext

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IIT Madras

Journal of Physical Chemistry C

Clathrate hydrates (CHs) typically nucleate under high-pressure conditions, but their existence in ultrahigh vacuum (UHV) is an open question. Here, we report the formation of tetrahydrofuran (THF) hydrate in UHV, using reflection absorption infrared spectroscopy (RAIRS). Annealing both sequentially and co-deposited mixtures of THF and H2O to 130 K for adequate time, originally prepared at 10 K, led to the formation of THF hydrate, at 10-10 mbar. Nucleation of THF hydrate was associated with the crystallization of amorphous solid water. Crystallization kinetics was examined through isothermal kinetic measurements using RAIRS in the temperature range of 120-130 K. The kinetic measurements revealed that the THF hydrate formation was a diffusion-controlled process and the overall activation energy for the process was found to be ∼23.12 kJ mol-1. This considerably lower activation energy as compared to that for the crystallization of pure ice established the spontaneity of the process. The results provide valuable insights into the low-pressure characteristics of CHs and associated thermodynamics. © 2019 American Chemical Society.

Spontaneous Formation of Tetrahydrofuran Hydrate in Ultrahigh Vacuum

Clathrate hydrates (CHs) are ubiquitous in earth under high-pressure conditions, but their existence in the interstellar medium (ISM) remains unknown. Here, we report experimental observations of the formation of methane and carbon dioxide hydrates in an environment analogous to ISM. Thermal treatment of solid methane and carbon dioxide–water mixture in ultrahigh vacuum of the order of 10 −10 mbar for extended periods led to the formation of CHs at 30 and 10 K, respectively. High molecular mobility and H bonding play important roles in the entrapment of gases in the in situ formed 5 12 CH cages. This finding implies that CHs can exist in extreme low-pressure environments present in the ISM. These hydrates in ISM, subjected to various chemical processes, may act as sources for relevant prebiotic molecules. © 2019 National Academy of Sciences. All Rights Reserved.

Proceedings of the National Academy of Sciences of the United States of America

Clathrate hydrates in interstellar environment

Temperature-dependent interaction of acetonitrile with methanol and ethanol, as codeposited and sequentially deposited films, was studied in the 10-130 K window, in ultra high vacuum. Films in the range of 50-100 monolayers were investigated using temperature-dependent reflection-absorption infrared spectroscopy (RAIRS), Cs+ ion scattering mass spectrometry, and temperature-programmed desorption (TPD). Acetonitrile interacts with methanol and ethanol through intermolecular hydrogen bonding. When a codeposited system was annealed, acetonitrile underwent a phase segregation at 110 K, and large changes in the infrared spectrum were observed. The OH stretching of methanol gave two peaks characteristic of the change to the α-phase of methanol, while ethanol gave three peaks at the same temperature. The surface composition of the systems probed by 40 eV Cs+ scattering showed that both the alcohols and the acetonitrile were of equal intensity below 110 K, while above 110 K the intensity of the latter went down substantially. We find that the presence of acetonitrile does not allow ethanol to undergo complete phase transition prior to desorption, while methanol could do so. This behavior is explained on the basis of the size, extent of hydrogen bonding, and phase transition temperature, of the two alcohols. Additional peaks in the hydroxyl region observed in alcohols in the 110-130 K window may be used as a signature of the presence of acetonitrile mixed with alcohol, especially ethanol, and hence this may be used in observational studies of such molecular environments. © 2017 American Chemical Society.

Interaction of Acetonitrile with Alcohols at Cryogenic Temperatures

Here we report the first evidence for a reversible phase change in an ethanethiol ice prepared under astrochemical conditions. InfraRed (IR) spectroscopy was used to monitor the morphology of the ice using the S[sbnd]H stretching vibration, a characteristic vibration of thiol molecules. The deposited sample was able to switch between amorphous and crystalline phases repeatedly under temperature cycles between 10 K and 130 K with subsequent loss of molecules in every phase change. Such an effect is dependent upon the original thickness of the ice. Further work on quantitative analysis is to be carried out in due course whereas here we are reporting the first results obtained. © 2017 Elsevier B.V.

Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy

Qualitative observation of reversible phase change in astrochemical ethanethiol ices using infrared spectroscopy

Extremely surface specific information, limited to the first atomic layer of molecular surfaces, is essential to understand the chemistry and physics in upper atmospheric and interstellar environments. Ultra low energy ion scattering in the 1-10 eV window with mass selected ions can reveal extremely surface specific information which when coupled with reflection absorption infrared (RAIR) and temperature programmed desorption (TPD) spectroscopies, diverse chemical and physical properties of molecular species at surfaces could be derived. These experiments have to be performed at cryogenic temperatures and at ultra high vacuum conditions without the possibility of collisions of neutrals and background deposition in view of the poor ion intensities and consequent need for longer exposure times. Here we combine a highly optimized low energy ion optical system designed for such studies coupled with RAIR and TPD and its initial characterization. Despite the ultralow collision energies and long ion path lengths employed, the ion intensities at 1 eV have been significant to collect a scattered ion spectrum of 1000 counts/s for mass selected CH 2+. © 2014 AIP Publishing LLC.

Review of Scientific Instruments

Development of ultralow energy (1-10 eV) ion scattering spectrometry coupled with reflection absorption infrared spectroscopy and temperature programmed desorption for the investigation of molecular solids

Ion scattering using ultralow energy projectiles is considered to be a unique method to probe the nature of molecular surfaces because of its capacity to probe the very top, atomically thin layers. Here, we examine one of the most studied molecular solids, water-ice, using this technique. When ice surface undergoes the amorphous to crystalline transition, an ultralow energy reactive projectile identifies the change through the reaction product formed. It is shown that ultralow energy (2, 3, 4, 5, 6, and 7 eV) CH2+ (or CD2+) collision on amorphous D2O (or H 2O) ice makes CHD+, while crystalline ice does not. The projectile undergoes H/D exchange with the dangling -OD (-OH) bond present on amorphous ice surfaces. It is also shown that H/D exchange product disappears when amorphous ice is annealed to the crystalline phase. The H/D exchange reaction is shown to be sensitive only to the surface layers of ice as it disappears when the surface is covered with long chain alcohols like 1-pentanol as the ice surfaces become inaccessible for the incoming projectile. This article shows that ultralow energy reactive ion collision is a novel method to distinguish phase transitions in molecular solids. © 2013 American Chemical Society.

Distinguishing amorphous and crystalline ice by ultralow energy collisions of reactive ions

A study was conducted to demonstrate low-energy ionic collisions at molecular solids. The investigations were limited to molecular surfaces and associated chemical processes occurring during impact of a hyperthermal energy ion. The term 'molecular solid' comprised the wide range of molecular materials that were used in ion/surface collision experiments. The energy regime considered in the investigations lied in an intermediate range. Primary ion energies of several kiloelectronvolts used in a secondary ion mass spectrometry (SIMS) experiment was considered in the case of condensed molecular solids to allow correlation with the results of chemical sputtering (CS) or other low-energy sputtering (LES) experiments.

Chemical Reviews

Low-energy ionic collisions at molecular solids

Topmost layers of amorphous ice undergo a structural transformation before the onset of crystallization and well before the premelting transitions. Ultralow-energy (∼1 eV) mass-selected Ar+ scattering has been used to detect this structural change. The transformation is manifested in the form of drastic changes in the scattered ion intensity in the ≤2 eV collision energy range. The changes are limited to the first few monolayers as larger thicknesses produce no additional effects. The technique becomes chemically sensitive but insensitive to the morphology and structure as the energy is increased above 3 eV. A similar behavior is observed with He+, Kr+, and a polyatomic ion, CH3+. Experiments revealed that the structural changes occurred only on amorphous ice and no such change occurred on crystalline ice. H2O and D2O ices behave similarly. © 2008 American Chemical Society.

Structural reorganization on amorphous ice films below 120 K revealed by near-thermal (∼1 eV) ion scattering

In this work, we have examined the diffusive mixing of chloromethanes in different molecular solids H2O, D2O, and CH3OH by monitoring their chemical sputtering spectra due to the impact of Ar + ions in the collision energy range of 3-60 eV, focusing on amorphous solid water. The chemical sputtering spectra have been monitored over the temperature window accessible by liquid nitrogen, and the coverages of the molecules of interest and ice have been varied from one to several hundred monolayers. Instrumentation and sensitivity of the technique have been discussed. It is found that while the diffusion of CCl4 in the molecular solids investigated is hindered, other choloromethanes such as CHCl3 and CH2Cl2 undergo diffusive mixing over the same temperature range. Quantitatively, while ∼4 monolayers (ML) of ice are found to block CCl4 diffusion, the numbers are ∼250 and ∼600 ML for CHCl3 and CH2Cl2, respectively. Crystallinity of ice does not have any effect on the diffusivity of water molecules when it is deposited below the chloromethanes. The effect of substrate was insignificant, and the rise in temperature increased diffusive mixing wherever the process was observed at a lower temperature. © 2007 American Chemical Society.

Journal	Data powered by TypesetJournal of Physical Chemistry C
Publisher	Data powered by TypesetAmerican Chemical Society
ISSN	19327447
Open Access	No