Spectroscopic study of intermolecular interaction in solutions involving phosphonium salts

Abstract

The present paper describes an infrared spectroscopic study of the mechanism of intermolecular interactions involving molecules of phosphonium salts. For the studies we synthesized the previously unknown high symmetrical tetraalkylphosphonium iodides R4P+I (R = CsHIi, C6H13, CsHIT), obtained by the reaction of the higher trialkyl phosphines with the alkyl iodides in boiling toluene. The corresponding tetraalkylphosphonium bromides were prepared from the iodides by replacing the iodine by bromine. The constants and chemical properties of these compounds were given in [1]. Since studies of intermolecular interaction involving phosphonium salts have not been car r ied out unti l recent ly , the resu l t s were compared with those p r e viously obtained for ammonium salts [2-11]. A mmonium salts in solvents with low die lect r ic constant (CCI4, C~H6, etc.) take an active part both in the fo r mation of hydrogenbonds with molecules which are p ro ton donors and in dipole-dipole in te rmotecular i n t e r action with polar molecules . It has been shown [2-4, 7-9] that the formation of a hydrogen bond between a proton donor (alcohols, haloforms) and an ammonium salt leads to a low-frequency shift of the v(XH) absorpt ion band and a high-frequency shift of the v(NH) band of the t e r t i a ry sal ts [6,11], in the same sense as the change in the proton-accept ing proper t ies in H-bonding in molecules of ammonium sal ts , in which the acceptor center is the anion. For the ammonium halides, the proton-accept ing proper t ies decrease in the order C1> B r > I . On the other hand, the high-frequency shift of the v(NH) band of t e r t i a ry ammonium hatides, which inc reases with increase in the dipole moment of the pa r tne r -molecu le [5], and the low-frequency shift of the P(C~--~N) band of acetoni t r i le (P(CH3CH) ~ 4D) on in teract ion with the molecules of ammonium salts [10] showed that the polar molecules of the ammonium salts (# = 7-15D [12]) in solution also form molecular complexes of the dipole-dipole type. A remarkab le feature is that the v(C~--N) band is shifted toward lower f r e quencies. It is known that interact ions involving the free pai r of e lect rons of the ni t rogen atom of the ni t r i t e group (H-bonding [13], coordination to metal cations [14]) lead to an increase in the frequency of the stretching vibration of the C~-N group [15]. The increase in the low-frequency shift of the P(C~-~N) band with increase in the dipole moment of the molecule of the salt confirms the proposal that the molecular associates involve dipole-dipole interaction. On the basis of the analogy between the molecules of ammonium and phosphonium salts, we assumed that phosphonium salts in inert solvents are also able to form complexes with hydrogen bonding and complexes of the dipole-dipole type. We studied the changes in the v(OH) absorption bands of methanol and the p(CN) absorption bands of acetonitrile in solution CCt 4 when phosphonium salts were added. The spectra were recorded on a UR-20 spectrometer in the range of the LiF prism at room temperature. The solutions were placed in sectional ceils with CaF 2 windows. It was assumed that methanol would be an effective proton donor in H-bonding with molecules of phosphonium salts, so that the changes in the spectral characteristics of the v(OH) absorption band would be sufficient for the identification of the observed interaction with hydrogen bonding. Relative to the molecules of the salts, the CH3CN molecules are chiefly carriers of dipole moment. Considering that the dipole moment is localized chiefly in the C~N group, and taking into account the structure of the CH3CN molecule, it was assumed that the spectral changes in the v(C~N) band are the chief data from which one can deduce the formation of dipole-dipole molecular associates. Figure 1 (curve I) shows the v(OI-I) = 3640-cm -I band for methanol in CCI 4 at a concentration of 0.025 M. The addition of tetraoctytphosphonium iodide solution leads to the appearance on the low-frequency side of a new broad band at 3400 cm -I whose intensity increases