C—H … O=C and C—H … H—C interactions in the side chains of 2-isopropylbenzaldehyde. A negative ⁵J(CHO,CH(CH₃)₂)

Abstract

. Can. J. Chem. 69, 919 (1991).The 'H nuclear magnetic resonance spectra of 2-isopropylbenzaldehyde in CS2/C6DI2 and acetone-d6 solutions provide the chemical shifts and coupling constants of all the protons. The long-range coupling constants involving the side-chain protons yield certain sums of the populations of the four putatively planar conformations. The o-anti conformers have a fractional population of 0.55(3) in the polar and of 0.49(3) in the nonpolar solvent. The conformers in which the methine C-H bond lies cis to the aldehyde group have a fractional population of 0.83(3) in both solutions. ' The close approach of the methine and aldehydic hydrogen atoms in one conformer is indicated by a negative proximate coupling constant between their protons of -0.39(1) Hz. The chemical shifts of the ring and of the side-chain protons are consistent with the conformer populations deduced from the long-range coupling constants and also with the indications that the side chains do not, on average, deviate from "coplanarity" with the ring by much more or less than in the parent compounds. The C-H . . .H-C and C-H.. .O==C interactions in the o-syn and o-anti conformers are most likely repulsive and of very similar magnitude and lead to a significant deshielding of the protons in these moieties. Molecular orbital computations are also reported and are an aid in estimating the populations of the individual conformers. The STO-3G MO structures have H . . . H and H . . .O distances well below the sums of the van der Waals radii of hydrogen and oxygen atoms in the conformers with the methine C-H bond placed cis to the aldehyde group, yet these are computed to be by far the most abundant by the STO-3G as well as by AM1 algorithms.Key words: 2-isopropylbenzaldehyde, conformations of; 2-isopropylbenzaldehyde, proximate spin-spin coupling constants in; MO calculations, STO-3G, and AM1 on 2-isopropylbenzaldehyde, 'H NMR and long-range spin-spin coupling constants in 2-isopropylbenzaldehyde.

1H nuclear magnetic resonance and molecular orbital studies of 2-formylstyrene. Three significant nonplanar conformers at 300 K

“…The same can be said for the coupling constants between ring protons in these two compounds and also for 2-formylisopropylbenzene (42). Such small changes, also measured for monosubstituted benzenes in this laboratory, follow patterns to a large extent independent of the nature of the substituent and are certainly not artifacts of the spectral analytical procedures.…”

“…Cfl Coupling constants between olefinicprotons and the formyl proton 5~7 , , o is -0.163(1) Hz and is most likely a proximate coupling constant, similar in nature to the -0.38(1) Hz determined for the coupling constant between the formyl and methine protons in 2-formylisopropylbenzene (42). Such coupling constants are very sensitive to interatomic distance (43,44).…”

“…A speculative mechanism invokes the through-space, or proximate, interaction between the P-CH and the ortho C-H bonds, postulated for styrene (2). In all known cases (2,(35)(36)(37)(38)(39)(40)(41)(42) such an interaction leads to a negative coupling constant between the protons, as indicated in C by the parallel crossed arrows for H-6 and , CHO C H-8. Schematically, the repulsive overlap of the 1s orbitals centered on these protons is consistent with a parallel-spin preference of the electrons in these orbitals.…”

“…Such coupling constants are very sensitive to interatomic distance (43,44). In the isopropylbenzene derivative, the experimental and theoretical evidence (42) is that the methine C-H bond is rather closely confined to the aromatic plane. In consequence, its hydrogen atom approaches that of the formyl C-H bond rather more closely on average than for the analogous interaction in 2-formylstyrene.…”

NMR spectra of the porphyrins. Part 39. Paramagnetic shifts in cobalt(II) porphyrins

“…Consequently, we used the value 115", which is the average of this range but which has also been optimized by previous ab initio studies (STO-3G). 29 The proton paramagnetic shifts used in the optimization were those of the ortho-, meta-and parahydrogens on the second phenyl ring. Values for the first phenyl ring, which is the phenyl group directly attached to the porphyrin macrocycle, were not considered as this ring had already been optimized at 65" to the porphyrin plane.…”