A positive ⁶J(H,CHO) in some meta derivatives of benzaldehyde. A simple model

Abstract

6J(H,CHO), the long-range coupling constant between the aldehydic and para protons in benzaldehyde, has not been detected, possibly because the σ–π interaction giving rise to a negative coupling is intrinsically rather small and because the internal barrier to rotation about the [Formula: see text] bond is large. However, 6J(H,CHO) in some meta substituted derivatives is actually positive and as large as 0.09 Hz in 3,5-difluorobenzaldehyde; the barrier to internal rotation in this molecule is some 4 kJ/mol lower than in the parent molecule. The magnitude of 6J(H,CHO) in these derivatives correlates well with σR.ST values, a recent set of substituent constants derived from 13C nuclear magnetic resonance chemical shifts of the β carbon in styrene derivatives. It is hypothesized that the substituents with negative σR.ST values stabilize an ionic valence bond structure that has a positive 6J(H,CHO). A brief discussion of 4J(H,CHO) in some of these molecules is also presented. Keywords: NMR, spin coupling NMR, benzaldehyde.

The conformational distribution of 2-cyanobenzaldehyde and 3-cyanobenzaldehyde in solution and in the vapor

“…In benzaldehyde, 5~ is 0.43 l(2) and 0.430(1) Hz in CS2/C6D12 and acetone-d6, respectively (2). As discussed in detail previously (3), all the available evidence implies that 5~ vanishes for a conformation such as o-cis in 3CNB and that it has its maximum value for the o-trans orientation.…”

“…Km follows as 1.27(3) and, at 300K, the free energy of the o-trans is 0.60(5) kJ/mol lower than that of the o-cis conformer. In CS2 solution, however, the two 'J values sum to 0.894(2) Hz, substantially higher than the 0.862(2) Hz for benzaldehyde (2). If the extrema in 'J3 and 'JS are both taken as 0.894(2) Hz, then KO follows as 8.2(2), the fractional population of the cis conformer being 0.11 and the free energy difference between the conformers, 5.2(l)kJ/mol at 300K.…”

“…Thus, for 2CNB in acetone-d6 solution, the sum of the 5~ values is 0.858(2) Hz, compared to 0.860(2) for benzaldehyde (2). Km follows as 1.27(3) and, at 300K, the free energy of the o-trans is 0.60(5) kJ/mol lower than that of the o-cis conformer.…”

“…specific (I), is apparently a pure a electron mediated interaction 7 for effectively planar molecules (2), and, when measured with precision, is perhaps the most sensitive indicator available for For personal use only. "A saturated solution in CS,, being considerably less than 2 mol% in 2CNB, containing also 10 mol% of C6D,,, and 0.5 mol% of TMS.…”

Computations and measurements of the structures and conformations of 2- and 3-fluorobenzaldehyde in the gas and in solution

“…A recent analysis of the 'H nmr spectra of benzaldehyde in the same solvent mixtures (apart from a tiny amount of C6F6) as in Table 1 gave 5J(H, CHO) as 0.43 l(2) and 0.430(1) Hz in CS2/C6D12 and acetone-d6, respectively (15). The methyl groupreplacement procedure (16) showed (15) that 5~( H , CHO) is a pure a electron coupling (for these essentially planar benzaldehyde derivatives) and hence 5J(H-5, CHO) is expected to vanish in the 0-cis forms above, as it does in 2-hydroxybenzaldehyde (17) and, from recent unpublished work in this laboratory, in 3-fluoro-2-hydroxybenzaldehyde.…”