Elsevier

Chemical Physics

Volume 288, Issues 2–3, 15 March 2003, Pages 197-210
Chemical Physics

Thermal reactivity of HNCO with water ice: an infrared and theoretical study

https://doi.org/10.1016/S0301-0104(03)00024-7Get rights and content

Abstract

The structure and energy of the 1:1 complexes between isocyanic acid (HNCO) and H2O are investigated using FTIR matrix isolation spectroscopy and quantum calculations at the MP2/6-31G(d,p) level. Calculations yield two stable complexes. The first and most stable one (ΔE=23.3 kJ/mol) corresponds a form which involves a hydrogen bond between the acid hydrogen of HNCO and the oxygen of water. The second form involves a hydrogen bond between the terminal oxygen of HNCO and hydrogen of water. In an argon matrix at 10 K, only the first form is observed. Adsorption on amorphous ice water at 10 K shows the formation of only one adsorption site between HNCO and ice. It is comparable to the complex observed in matrix and involves an interaction with the dangling oxygen site of ice. Modeling using computer code indicates the formation of polymeric structure on ice surface. Warming of HNCO, adsorbed on H2O ice film or co-deposited with H2O samples above 110 K, induces the formation of isocyanate ion (OCN) characterized by its νasNCO infrared absorption band near 2170 cm−1. OCN can be produced by purely solvation-induced HNCO dissociative ionization. The transition state of this process is calculated 42 kJ/mol above the initial state, using the ONIOM model in B3LYP/6-31g(d,p).

Introduction

More than 100 interstellar molecules have been identified in the interstellar medium [1]. Many of these molecules result from an efficient accretion and reaction of atoms and molecules from the gas on dust grains in the dense molecular clouds [2]. Present on the satellites of the outer planets and on the comets, amorphous water ice is also believed to be the major constituent of interstellar dust [3], [4], [5], [6]. To understand the formation and evolution of interstellar molecules, numerous theoretical models with detailed physical and chemical reaction pathways, have been developed [7], [8], [9], [10]. The tiny (micron and submicron) icy dust grains present in cold molecular clouds prove to be important catalysts. Heterogeneous reactions are developed on the ice surface or in the icy mantle of the grains [11] which are assumed to play an important role in interstellar chemistry [12]. For these reasons, we have been interested in small molecules interactions with amorphous ice surface [13], [14], [15], [16], [17], [18], [19], [20].

Since its discovery in 1979 by Soifer et al. [21] in the protostar W33A, the 4.62 μm feature (2167 cm−1) has been extensively hunt up [22] and numerous carriers, such as nitriles [23] and isonitriles [24], have been proposed for this feature called “XCN” band. The position and width of this band led to assigning it to a solid molecular species. In 1987, Grim and Greenberg [25] discussed the spectroscopic validity of the assignment of this XCN band to nitriles and isonitriles and proposed the XCN feature to the intense asymmetric stretch of the isocyanate anion, OCN. Ten years later, Schutte and Greenberg [26] detected three other weak bands of OCN at 1296, 1206 and 630 cm−1 and concluded that OCN is a good candidate for the “XCN” band. The behavior of the XCN feature when the ice matrix was doped by electron donor or electron acceptor molecules gave direct evidence that the carrier is a negative ion [27], [28]. Next, Hudson et al. [29] confirm, from irradiated and photolyzed ices, that the band produced in the laboratory is due to OCN. Novozamsky et al. [30] proposed that the formation of OCN is preceded by photochemical formation of isocyanic acid, HNCO, followed by proton transfer to some base such as NH3.

In this paper, we will be looking at the reactivity of isocyanic acid, which has been detected in the interstellar medium in gas phase but never in the cold (10–100 K) interstellar grains [31], [32], on water ice surface or in bulk. The purpose of this work is threefold: (1) to obtain direct and accurate experiment results of the HNCO…H2O complexes trapped in rare gas cryogenic matrix, (2) to assess the chemical stability of HNCO adsorbed on water ice surfaces, and (3) to determine the favorable conditions for the OCN formation.

Experiments were monitored by FT-IR spectroscopy. Quantum calculations were undertaken to compare the experimental IR spectra with the calculated ones. They are also instrumental in assigning observed absorptions, determining the complex and the structure of absorption sites and furthermore to modeling the reactivity of HNCO on ice surfaces.

Section snippets

Experimental section

Pure HNCO was synthesized using the method described by Herzberg and Reid [33] and modified by Sheludyakov et al. [34]. The isocyanic acid is degassed before each deposition. Moreover, the first fraction of isocyanic acid is evacuated. H2O was degassed by successive freeze–thaw cycles under vacuum before each use.

The apparatus and experimental techniques used to obtain argon matrices have been described elsewhere in the literature [35]. The relative concentrations Ar/H2O (50/1), Ar/HNCO

Study of the HNCOH2O complexes: cryogenic matrix experiments

Argon matrixes containing only HNCO or H2O were prepared, yielding infrared absorptions that were similar to those previously reported. Table 1 summarizes the observed absorptions with spectral assignments based on the works of Ayers [38], [39], Couturier-Tamburelli et al. [40] for H2O, and Teles et al. [41] for HNCO. Prior to the absorption study, matrix isolation experiments were conducted with different compound concentrations. The characteristic absorption bands of free or multimers H2O and

Quantum calculations

Our experimental results point out to the formation of a 1:1 complex in the matrix, adsorption of HNCO on the dangling oxygen sites of the ice surface and thermal formation of OCN. Quantum calculations were carried out to interpret our experimental results. The experimental IR spectra were compared with the calculated ones to assign observed vibrational absorptions and determine the structure of the complex in matrix. The structures of adsorption sites and the thermal ionization of HNCO on ice

Discussion and conclusion

The analysis of our results shows that HNCO trapped in an argon matrix or adsorbed on an amorphous ice surface, acts as an electrophil by means of its acid hydrogen atom. For this reason, only one kind of HOw complex (HOif adsorption site, respectively) is obtained which is stabilized by an hydrogen bond between the acid hydrogen atom of HNCO and the oxygen atom of water (the dangling O on the ice surface, respectively). From theoretical calculations, a second minimum, less stable, is obtained

Acknowledgements

The theoretical part of this work was conducted with the technical means of “Centre Régional de Compétence en Modélisation moléculaire de Marseille”.

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