Spectrochimica Acta Part A 62 (2005) 1131–1139 Gas–solid reactions of single crystals: A study of reactions of NH 3 and NO 2 with single crystalline organic substrates by infrared microspectroscopy Samantha L. Jenkins, Matthew J. Almond ∗ , Samantha D.M. Atkinson, Peter Hollins  , John P. Knowles School of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK Received 26 January 2005; accepted 18 March 2005 Abstract Reaction of single crystals of benzoic and trans-cinnamic acids with 200Torr pressure of ammonia gas in a sealed glass bulb at 20 ◦ C generates the corresponding ammonium salts; there is no sign of any 1:2 adduct as has been reported previously for related systems. Isotopic substitution using ND 3 has been used to aid identification of the products. Adipic acid likewise reacts with NH 3 gas to form a product in which ammonium salts are formed at both carboxylic acid groups. Reaction of 0.5 Torr pressure of NO 2 gas with single crystals of 9-methylanthracene and 9-anthracenemethanol in a flow system generates nitrated products where the nitro group appears to be attached at the 10-position, i.e. the position trans to the methyl or methoxy substituent on the central ring. Isotopic substitution using 15 NO 2 has been used to confirm the identity of the bands arising from the coordinated NO 2 group. The products formed when single crystals of hydantoin are reacted with NO 2 gas under similar conditions depend on the temperature of the reaction. At 20 ◦ C, a nitrated product is formed, but at 65 ◦ C this gives way to a product containing no nitro groups. The findings show the general applicability of infrared microspectroscopy to a study of gas–solid reactions of organic single crystals. © 2005 Elsevier B.V. All rights reserved. Keywords: Gas–solid reactions; Carboxylic acid; Hydantoin; Solvent-free nitration; Infrared microspectroscopy; Raman microspectroscopy 1. Introduction In recent years there has been a strong renewal of interest in the reactions of solid crystalline organic substrates [1–5]. Reactions of this type are desirable from an environmental point of view in that they obviate the need for large quan- tities of solvent as the reaction medium [4,6]. At the same time it has become increasingly recognised that such reac- tions give considerable stereochemical control, as the form of the reaction product may be directed by the arrangement of reactant molecules within the crystal [2,3,7–11]. Performing reactions in the solid-state has been found in many examples to increase yields and improve specificity. The explosion of ∗ Corresponding author. Fax: +44 1 1893 16332. E-mail address: m.j.almond@reading.ac.uk (M.J. Almond).  Deceased. interest in combinatorial chemistry where a wide range of reagents are tested on solid beads of substrates provides a further impetus to research in this area. We have utilised the techniques of infrared and Raman microspectroscopy to monitor reactions of this type and to characterise reaction products [1–3,12]. The principal advan- tages of vibrational microspectroscopy in this context are that single crystals may be monitored in situ and that the spec- tra give considerable structural information about reactants and products. Such structural information is much less read- ily forthcoming from an alternative approach, which has been used elsewhere [4,6,13,14] in which atomic force microscopy is employed. Here morphological changes to the crystals are seen upon reaction, but it is difficult to relate these changes to the chemical processes ongoing within the crystal. Diffrac- tion methods have, of course, also been used [8,9,15] but are still undergoing development for use in time-resolved 1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2005.03.030