The Donor Characteristics of the Carbonyl: Group:ii. The Effect of Angular Variations at the Carbonyl Group

Abstract

The effect of variations in the angle X-C-X' (in a typical carbonyl compound XXIC=O) on the carbonyl stretching frequency (PC-0) is examined in the light of recently published empirical relations between vc ,o and the ionization potential. INTRODUCTIONIn a recent paper dealing with the donor characteristics of the carbonyl group (1) empirical relationships were demonstrated between the carbonyl stretching frequency (vc=,) and the ionization potential (IP). Two classes of compounds were noted: those having no group conjugated with the carbonyl group (A), and those with a group capable of such conjugation (B). The parameters of those molecules were expressed, respectively, by the equations The variation of these parameters within each class was interpreted in terms of inductive and resonance effects due t o the different groups attached t o the carbonyl group.T h e variation of I P with the carbony1 force constant kc=o was considered many years ago by Walsh (2). He pointed out the relationship between IP, kc=o, and C=O bond length and a parameter defining the percentage of polarity of the C=O bond. Since that time Inany more IP's have been accurately measured though the number of determinations of force constants of C=O bonds is still relatively small.Previously (1) no allowance was made for the effect of variations in angle XCX' (a) (in the compound C=O) on v c =~. This will now be considered. x / VARIATION OF vc-0 WITH orT h e fact that v c =~ increases on decreasing the size of a cycloalkanone ring has been known for some time (3). The phenomenon is not restricted t o simple cyclic ketones, for similar compounds with heteroatoms in the ring show the same effect. Figure 1 shows a graph of vc=, and a for a wide variety of cyclic carbonyl compounds, the data and source being listed in Table I. Points lying on the same curve in Fig. 1 are for compounds containing the same functional group, with different ring sizes depending on the number of -CH2-groups contained therein. These lines do not have quite the same slope but get steeper from the bottom line t o the top in a fairly smooth manner.

Infrared Spectra of Complexes of 4-Pyridones

“…(Susz has recently summarized (8) the results of carbonyl shifts in various complexes of ketones with Lewis acids, and found that the shift, Av, for the same Lewis acid increased in the series acetone < 4-heptanone < acetophenone < benzophenone. This is in the order of decreasing carbonyl frequency, ionization potential, and therefore increasing donor ability (7,9).) There are a few irregularities in the shifts that one particular Lewis acid generates with different donors.…”

PROTONATED Γ-Pyrones

“…The circumstances leading to enhanced basicity in carbonyl groups have been discussed in detail (2,3). T h e y-pyrones satisfy most of the required conditions.…”

Distortion of the amide bond in amides and lactams. Photoelectron-spectrum and electronic structure of 3,5,7-trimethyl-1-azaadamantan-2-one, the most twisted amide

“…Based on the results of various theoretical methods like AM1, 12 PM3 13 and B3LYP, 14-16 the first ionization potential is assigned to the removal of an electron from the nitrogen lone-pair MO, n(N), and the second IP is assigned to the analogous process related to the n(O) orbital. Compared with other tertiary lactams, the first IP of 20 appears at an unusually low energy, about 0.6 eV lower than in 1-methylpiperidin-2-one (9). The n(N) ionization of 20 is, however, 0.36 eV higher than that of 1-azaadamantane (21).…”

“…In this way a value of 9.00 eV is obtained for IP[n(O)] which compares well to that found experimentally (Table 1). for various secondary and tertiary lactams (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). Azaadamantanone 20 has a carbonyl frequency of 1732 cm Ϫ1 11 from which ν 120 (C᎐ ᎐ O) = 1716.4 cm Ϫ1 is obtained by eqn.…”