Matrix isolation studies of hydrogen bonding. Vibrational correlation diagram

Abstract

Publication costs assisted by the U S . Office of Naval ResearchThe infrared spectra have been obtained for a number of 1:l hydrogen bonded complexes isolated in a nitrogen matrix at 15'K. The complexes studied and their absorptions (successively, us and Yb, given in cm-l) are as follows: (CH3)2O -HCl, 2280,525; (CH&N -HC1, 1595, 1370; (CH3)sN -HBr, 1870,1600; H3N HBr, 1390, 1211; (CH3)zCO -HBr, 1700; HzO HBr, 2310, 410. For each acid, the value of us in the complex depends systematically, though not monotonically, upon base strength, as measured by proton affinity. We define the normalized proton affinity difference, A = [PA(B) -PA(X-)]/[PA(B) + PA(X-)I, and find that plots of both Au,/uSo and Yb against A correlate all of the systems studied (including H3N a HCl and HzO -HC1). We call such a plot a vibrational correlation diagram and we associate the trends with an increasing extent of proton transfer as A and base strength increase. According to this interpretation, the proton is "completely shared" if A is near -0.23. Partial proton transfer occurs if A is less negative than -0.23, increasing progressively a8 base strength increases.

Interaction of aromatics with Brönsted sites in zeolites: Demarcation line between regions of stable existence of H-complexes and ion pairs for various types of bases

“…One can see from Fig. 3 that AVxa stretching vibrations have a maximum around the transition point of H-complexes into ion pairs, as in the case of solutions [24,25].…”

“…Thus, 5PA=85.+-5 kcal/mol seems to be the demarcation line of the H-complex transition into ion structures of acid-base pairs both in solutions [25] and on surfaces. It is important that this value corresponds to the constant member in the dependence of the base protonation heat on PA'-PA b stated in Ref.…”

“…Ions in ion pairs are known to be connected in many cases via hydrogen bonds. For an Xi..H..Yj fragment, where Xi and Yj are proton-acceptor atoms from acid residues and bases, the change of the stretching vibration frequencies of Xi-H or Yj-H under variation of acid-base properties is described by a common curve with a maximum around the point of the H-complex transition into the ion pair [24,25]. Carbenium ions also form hydrogen bonds in solution [19].…”

Defining the hydrogen bond: An account (IUPAC Technical Report)

“…With the advent of cryogenic matrices [110] and supersonic beams [111] for producing exotic species, there have been several reports of hydrogen-bonded complexes involving HCl [112,113] and H 2 S [114,115]. Moreover, it has been shown recently that on increasing the pressure and decreasing the temperature, H 2 S has a structure similar to that of H 2 O in ice [116].…”

Quantum Structure of the Intermolecular Proton Bond

“…The origin of such large variations in n sp displayed by the analogous neutral complexes (e.g., Me 2 O·HCl) has been subjected to intensive analysis over the years. Pimentel and co-workers (20), for example, correlated the vibrational frequency trends with differences in the proton affinities (DPA) between the two bases, and Barnes (21) later extended this study to a myriad of neutral combinations. Zundel (22) and others (23) considered the infrared spectra of a number of systems in salt crystals, and found that although n sp was correlated with the magnitude of DPA (or the analogous DpK a , where K a is the acid dissociation constant), the n sp bands were also shown to depend on the distance between the atoms connected by the shared proton, which is in turn strongly affected by the overall crystal structure.…”