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J. Phys. Chem. B 2005, 109, 432-439
Micro-Raman Investigations of the Formaldehyde-Ice System Bertrand Chazallon,*† Nathalie Lebrun,‡ Paul Dhamelincourt,§ Ce´ line Toubin,† and Cristian Focsa† Laboratoire de Physique des Lasers, Atomes et Mole´ cules (UMR 8523), Centre d’Etudes et de Recherches Lasers et Applications (CERLA), UniVersite´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France, Laboratoire de Dynamique et Structure des Mate´ riaux Mole´ culaires (UMR 8024), UniVersite´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France, and Laboratoire de Spectrochimie Infrarouge et Raman, Centre d’Etudes et de Recherches Lasers et Applications (CERLA), UniVersite´ Lille 1, 59655 VilleneuVe d’Ascq, France ReceiVed: March 25, 2004; In Final Form: August 4, 2004
Rapidly frozen aqueous solutions containing variable amounts of dissolved formaldehyde (0.1, 5, 7, 10, 15, and 20 mol %) have been analyzed by micro-Raman spectroscopy at ambient pressure and low temperature. The importance of the formladehyde-ice system has been repeatedly quoted in various contexts, such as atmospheric and snowpack chemistry and interstellar and cometary ices. Understanding and characterizing the effects of freezing and the interactions of formaldehyde with ice are therefore of relevant interest. In this study, the distinct vibrational signatures of the oligomers present in the solution and in the frozen ice mixtures have been identified in the 120-4000 cm-1 spectral range. From the subtle changes of the bands assigned to the CO and CH group frequencies, at least two distinct crystalline phases (pI and pII) are found to coexist with ice at different temperatures. Depending on the cooling-rewarming protocol, pI is found to crystallize in the 163-213 K temperature range. Above ∼213 K, pI gets transformed irreversibly into pII which is stable up to ∼234 K. pII is found to interact more strongly with ice than pI, as revealed, for example, by the drop in frequency of the bands assigned to the O-H stretching as pI transforms into pII. It is suggested that pII consists of a hydrogen-bonded network of oligomers and water molecules. On the other hand, it is suggested that the oligomers mainly present in pI interact through weak forces with the surrounding water molecules.
I. Introduction The formaldehyde-ice system is known to play an important role in atmospheric and snowpack chemistry.1 This is illustrated by its contribution to the production of HOx radicals, which largely determine the lifetime of trace gases (e.g. pollutants) in the atmosphere. Formaldehyde (H2CO) can also be an important sink for free radicals, such as Br atoms that catalyze ozone destruction in the Arctic boundary layer.2 However, the activation of bromine and the contribution of H2CO in the oxidation of halides and molecular halogens are not fully understood.3 Formaldehyde has also proven to have a strong influence on the global mixing rate of ozone and OH radicals, which govern the oxidizing capacity of the troposphere.4,5 As a consequence, H2CO present in the polar ice core potentially provides valuable data on past oxidizing capacity of the atmosphere.6 Importance of the formaldehyde-ice system has also been revealed in the extraterrestrial environment since poly(oxymethylene) (POM), which is a H2CO polymerization product (-(H2CO)n-), was the first polymer identified in space.7,8 Previous investigations involving formaldehyde and ice were mostly focused on reaction products of multicomponent gas condensed-ice mixtures. Furthermore, these studies reported the influence of combined processes that include gas grain chemical * To whom correspondence should be addressed. Phone: +33 (0)320336468. Fax: +33 (0)320336463. E-mail:
[email protected]. † Laboratoire de Physique des Lasers, Atomes et Mole ´ cules. ‡ Laboratoire de Dynamique et Structure des Mate ´ riaux Mole´culaires. § Laboratoire de Spectrochimie Infrarouge et Raman.
reactions and irradiation (UV or cosmic-ray bombardment) in the astrophysical context. Although H2CO is known to be present as trace concentrations in polluted or unpolluted rural areas (∼0.1 to 100 ppbv4,9), its concentration reaches the percent level relative to H2O (the dominant species) in icy mixtures of cometary nuclei10 or in interstellar ice.11 Reactions of gas condensed formaldehyde with ice (e.g., H2O + H2CO + CH3OH + NH3 + CO) have been studied extensively,12-14 under irradiation or by thermal excitation. Schutte et al.12 have demonstrated the ability to produce different complexes of organic materials (e.g., POM), depending on the relative amounts of the different components of the ice mixture and on the excitation process involved. Although polymerization of H2CO may in this case be initiated by unusual processes (irradiation, thermal activation, chemical reactions) and may result from solid reorganization at low temperature (
