Infrared Spectra of Complexes of 4-Pyridones

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ABSTIWCTThe infrared spectra of complexes of 1-methyl-2-pyridone and l-methyl-2-q~1inolone with Lewis acids are consistent with interaction a t the carbonyl group. The stretching frequency of this g r o~~p , VCO, is lowered on colnplex formation, the change being proportional to the Lewis acidity. Two other bands, a t 1 6 7 0 and 1 3 5 0 cm-' move to lower frequency in much slnaller steps and are associated with ring modes.

Section: Introductionsupporting

confidence: 77%

Section: Vibration Vs and The Carbonyl-stretching Vibrationcontrasting

confidence: 57%

Α-Pyridone COMPLEXES AND THEIR INFRARED SPECTRA

Cook¹

1965

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“…The only other possible donor site in xanthone is the heterocyclic oxygen atom. A growing body of evidence (7) suggests that the Lewis acid complexes formed a t a heterocyclic atom give spectra that are different from the parent heterocycle but are similar amongst themselves. A small trend, about 20 cm-l, in the breathing mode, v l , in pyridine complexes was noted as the Lewis acid was altered (8).…”

Section: Results a N D Discussionmentioning

confidence: 99%

Infrared Spectra of Xanthone: Lewis Acid Complexes

Cook¹

1963

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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

“…protonated carbonyl group stretching mode. I t is not accidental either that this value is close to the carbonyl-stretching frequency in a complex of 1-methyl-2-pyridone with a strong Lewis acid, as both the proton and the strong Lewis acid seem to offer the same degree of perturbation to the carbonyl group (20,21).…”

Section: Vibrations Of the Ring And Carbonyl Groupmentioning

confidence: 76%

PROTONATED CARBONYL GROUPS: V. Α-Pyridone SALTS

Cook¹

1965

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Salts of 1-methyl-2-pyridone and 1-methyl-2-quinolone with strong acids have been prepared and their infrared spectra recorded between 4 000 and 650 cm−1. Protonation of the carbonyl group occurs giving rise to OH stretching, in-plane and out-of-plane bending vibrations which absorb in characteristic regions of the spectrum, namely 1 900 – 3 300 cm−1 (depending on the anion), ~ 1 250 cm−1, and ~ 800 cm−1 respectively, with appropriate shifts on deuteration. Other tentative assignments are made.