'The vibrational spectrum of the pyridiniuln ion, CjHjI\TH+, and the N-deuterated species has been studied in several pyridinium salts. By comparison with benzene and deuterobenzene (with which the p y r i d i n i~~~n ion is isoelectronic) a fairly complete assignment has been made. The N-H bonds, which are hydrogen bonded to the anions, have stretching vibrations which show systematic variations depending on the nature of the anion. The N-H in-plane deformation vibrations show little variation while the out-of-plane deformation vibrations, some of which do vary, are probably affected, in addition, by other factors. INTRODUCTIONPyridine forms salts with many strong mineral and organic acids. In these salts it is assumed that complete proton transfer taltes place between the acid and the pyridine molecule, giving the pyridinium ion I and an anion.Little spectroscopic data on such salts is available in the published literature." Greenwood and Wade (I) assigned a few bands from their published spectra of C5H6NH+Cl-and C5HSNH+BC14-, while Kynaston and co-workers (2) listed all the bands in the latter salt. Spinner published a list of frequencies in the infrared and Raman spectrum (aqueous solution) of C6H6T\TI-I+C1-(3) and made a few assignments.In these papers emphasis was placed on sinlilarities between these salts and C5HsN. I t is believed that a more pertinent and helpful co~npariso~l is between the pyridiniurn ion and C61-16. These n~olecules contain the same number of atoms, are isoelectronic, and, apart from the mass and electronegativity of the nitrogen atom, would be identical. This physical similarity brealts down, however, when the synlrnetry of the two ~nolecules is compared. C6Ho has D6h, and C6H6NH+ is expected t o have Cz, symmetry. Thus C6H6 has only four (strictly) infrared-active fundamentals and seven Ranlan fundamentals. C6H6NH+ will have 30 Raman-active fundamentals and 27 infrared-active fundamentals. (The selection rules are shown in Table I.) The conlparison between C6HBD and C6HSND+, however, reveals a much closer similarity, since both molecules now have Cz, symmetry, with identical selection rules.

Xanthone, a member of the 7-pyrone species, which have basic carbonyl groups, forms an excellent series of solid complexes with Lewis acids. In these complexes the carbonyl oxygen atom is the donor site, and the carbonyl stretching vibration moves to progressively lower frequency as the Lewis acid strength increases. The carbonyl frequency in the BIS complex, 1400 cm-I, is one of the lowest encountered in complexes of this type.The wide range of Lewis acids used to form these complexes has enabled a quantitative estimate of the Lewis acid strength to be made, which compares reasonably well with previous estimates. IKTRODUCTIOSThe y-pyrones have basic carbonyl groups and forin salts with mineral and organic acids (1). They also form stable complexes with Lewis acids (2). The perturbation of the carbonyl stretching frequency in Lewis acid complexes of 2,6-dimethyl-4-pyrone was used recently to identify the carbonyl vibration in such pyrones, and to prove the site of protonation as the carbonyl oxygen atom. Later, all the vibrations due to motions of the proton were identified by the study of several salts and their deuterated analogues (3).In the 2,6-dimethyl-4-pyrone complexes only a small range of Lewis acids was used. This was due in part to the ease with which the Lewis acid, MX,, could be hydrolyzed -(unless scrupulous precautions against adventitous moisture were taken) to produce halogen acids H X , which either protonated the pyrone directly, or in coinbination with the Lewis acid as HfMX,+l. Fortunately, xanthone is free from this limitation, as it forms s d i d protonated salts only with difficulty (4, 5 ) . Thus a wide range of Lewis acids, including those susceptible t o hydrolysis could be used without fear of protonation of the base. The solid complexes so formed have been-examined from the point of view of the Lewis acid's ability t o coordinate the free electron pair of the carbonyl oxygen atom, for which the term Lewis acidity, or Lewis acid strength is used. A satisfactory criterion and comparison of Lewis acid strength is the free energy of formation of a complex having a coordinate bond between Lewis acids and a constant donor, in the absence of constraints due to steric factors. T h e technique of nleasuring such properties irivolving precise measurements of equilibrium constants and heats of mixing is painstaking and time consuming. In principle, the relative perturbation of the stretching frequency of a carbonyl group with different Lewis acids should give similar information (6). This criterion will be examined for the complexes discussed in this paper. RESULTS A N D DISCUSSIONConsideration of the spectra of the 2,6-dimethyl-4-pyrone complexes (2) led to the conclusion that the carbonyl stretching vibration and the double-bond stretching vibration (ring mode of A l species) were reversed in position, the CO vibration being a t lower frequency but with a slight interaction between them. The unperturbed value

On the whole the present results for the ionic entropies seem to be more consistent with those of Latimer and Jolly than with those of Coulter. The differences are mainly in the choice of free energy values rather than a result of the temperature differences. Although the molar entropy of Ca++ in liquid ammonia is expected to be more negative than that of Sr++ the difference can hardly be expected to be so great as that (124 e.u.) obtained if Coulter's result for Ca++ and the present result for Sr++ are both correct. The former result is based upon some e.m.f. measurements by Pleskov (cf . Table 11) and this comparison supports the view that these e.m.f. measurements are inconsistent with our amalgam partition measurements involving Sr .For the most part, a detailed consideration of these results is deferred pending the resolution of several uncertainties in the auxiliary data, particularly those relating to the energetics of formation of rubidium amalgam from the elements. How-ever one observation may be made independently of these data. By comparing the equilibrium constant of reaction 1 a t 0" with that for the same reaction a t 0", but with HzO in place of NH3 as the solvent,' we can calculate AFO for the reaction 1 -Mi'i+ (H20) + Na+(SHs) + Zi 1 Mi')+ ( "3)In this way we obtain Mi K Rb cs Sr AF+J 1.55 1.91 2.35 -1.06 kcal./mole These results are clearly the opposite of what one would expect on the basis of the simple Born charging equation, which predicts that the smaller, more highly charged ion should show the greater preference for the solvent of higher dielectric constant. The donor characteristics of molecules containing the carbonyl group are examined in terms of the ionization potential (IP), the carbonyl stretching frequency ( P C ,~) , the value of the fundamental stretching frequency of HCl (YHCl), dissolved in such solvents, and the value of the acetylenic C-H stretching frequency of CcHbCCH ( YCH) in solution in such solvents. Empirical relationships have been found between these quantities for a large number of carbonyl compounds. Donor strengths are discussed in quantitative terms. C=O in terms of the properties of X are examined. The experimental evidence presented suggests that such carbonyl compounds can be divided into two clearly distinguishable classes, ( A ) where no conjugation between X and C=O exists, and ( B ) where there is such conjugation.Modifications of the carbonyl group of "i X There have been several studies of the relationship between vc = 0 and carbon-oxygen distance, and other fundamental properties of the carbonyl b0nd.l The important result from such studies is that short bonds have high stretching frequencies and force constants and vice versa. The emphasis in such studies has been placed on the electrons in the C=O bond, and little or no attention has been paid to the lone pair electrons.The carbonyl group consists of a C atom hybridized in its sp2 state with three planar u-bonding orbitals, the interbond angle being 120". The fourth orbital is a p orbital a t right an...

The infrared absorption bands due to S-H stretching rriodes in protonated bases are diagnosticaily of little value in determining the protonation site. However, the deformation mode, 6xH, shows promise of becoming a useful group frequency whose value is characteristic of the type of atom protonated. The following values, which have been confirmed by deuterium substitution of the sensitive hydrogen atom, are typical:Occasionally splittings occur so that two components of the bending mode are observed. These have been attributed in lnany cases to correlation field coupling in the solid state.

An infrarecl s t~l d y of the liquid complex of aluminum chloricle with acetyl chloride has been undertaken. 'The results indicate that the pure liquid complex is not a simple mixture, but con-C1sists of (1) (1) is present. The carbon-chlorine boncl in (1) is modified. The mechanism of Friedel-Crafts ketone synthesis is briefly exan~ined in the light of these results.A variety of metallic halides have been used as catalysts in the Friedel-Crafts ketone synthesis. The most widely used of these is A1C13, but others, such as BF3, AlBr3, SbC13, SbC15, FeC13, TiCl,, SnC14, and ZnClz etc., have all been used. All these halides have in common the function of Lewis acids, due either to electron deficiency, or to hybridization changes to give complex ions.Numerous examples exist in the literature of complexes formed from such Lewis acids and various donors. The characteristics of the dollor molecule are usually those of welldirected lone-pair electro~ls. Thus compoullds like R 2 0 , R2C=0, RCN, R3N, RNOz etc.can all form more or less stable compleses with Lewis acids, the electron deficiency of the latter being satisfied by sharing the lone-pair electrons of the donor group. Some of the Lewis acids can function as acceptors in a slightly different way. Thus NaCl and A1C13 form the compound NaAlC14, which has been shown t o consist of Na+ and [AIC14]-ions (1, 2).The presence of two functional groups in an acyl halide, namely a carbony1 donor group ancl an ionizable chlorine atom, suggests that two types of intermediates may be possible in the Friedel-Crafts ketone synthesis: ( a ) the ions [CH3CO]+ and [AlCl.I]-, and (b) the donor-acceptor complex formed between the carbonyl-oxygen lone-pair electrons and the vacant orbital of the AlCl3, as suggested by Pfeiffer (3). An extension of (b) proposed by Dilthey (4) involved loss of a C1-from the donor-acceptor complex to give a complex ion.I t is the purpose of this paper, from the spectroscopic evidence presented, to sllow that the donor-acceptor complex as in (b) is present in solution in solvents of low dielectric constant, but that in the pure complex or in its solution in solvents of high dielectric constant ions such as in (a) are present as well as the donor-acceptor conlplex.After the experimental work described here was completed, two papers by Susz and Wuhrmann (5, 6) appeared on the infrared spectra of the conlplexes CH3COF:BF3 and CH3COC1:A1C13. These gave the first clear indication of the presence of the [CH,CO]+ ion. The present work corroborates their finclings but important differences exist which are detailed below.