Infrared induced conformational change in the two strong hydrogen bonded complexes 2-1 between dimethyl ether and hydrogen iodide in nitrogen matrix

Abstract

Ahatraet-The IR spectra of the complexes between dimethylether (DME) and hydrogen iodide in nitrogen matrices were reinvestigated with careful attention to the temperature and IR radiation effects upon the higher aggregates. Irradiation using a broad band IR source was found weakly active leading to a nearly stationary population of the two (DME)r HT species. Irradiation with CO* laser lines induced a conformational interconversion of both aggregates to non-hydrogen-bonded forms. The breaking of the strong hydrogen bond was principally governed by irradiation in the range 2600-15OOcn-' corresponding to the dCHp region whereas the reverse process was observed by increasing temperature above 27 K under the annealing limit or irradiation below llOOcm-'. No accurate evidence was found for the occurrence of photochemical proton transfer between the centrosymmetrical and asymmetrical cations characterizing the two investigated complexes.THE PROTON transfer process between two interacting molecules is one of the most interesting problems in the hydrogen bonding phenomenon. It is governed both by the proton affinities of the opposing groups (Ref.[l] and others cited therein) and by environmental effects [2]. Recently it has been shown that the reaction proton transfer takes place in the electronically excited state of the neutral clusters formed in free jetcooled supersonic expansion [3] or isolated in low temperature rare gas matrices [4]. Besides, recent studies in matrices showed that a large class of hydrogen bonded systems involving HI as proton donor were IR photosensitive (Refs [5,6] and others cited therein). In this view, it could be expected that a single quantum of excitation of a (B),(HI), cluster vibrational mode could induce proton motion by a structural change in a part of the cluster as was recognized by the end of the 1970s [7,8]. In this paper, we explore this idea showing the effects of monochromatic CO* laser or broad band (globar) irradiation on the two strong hydrogen bonded 2-l complexes involving dimethylether (DME) and hydrogen iodide, previously identified in nitrogen matrices as ion pairs (DME)*H+I- [9]. The first one, IR photosensitive, was characterized by a centrosymmetrical potential function for the proton, whereas the second one showed a proton potential function probably of the symmetrical double minimum type. The one-to-one complex was not accurately identified but it was not surprising as since that time it has been observed that complexes of HI with weak to medium strength bases quickly dissociate in the beam of the spectrometer at low temperature [6]. Also we briefly reinvestigated the DME-HI system trapped in nitrogen, and then we give results on the irradiation of the two cations (DME)*H+I-.