Manifestation of Stereoelectronic Effects on the Calculated Carbon−Hydrogen Bond Lengths and One-Bond¹J_C-HNMR Coupling Constants. Relative Acceptor Ability of the Carbonyl (CO), Thiocarbonyl (CS), and Methylidene (CCH₂) Groups toward C−H Donor Bonds

Abstract

Theoretical examination [B3LYP/6-31G(d,p), PP/IGLO-III//B3LYP/6-31G(d,p), and NBO methods] of six-membered cyclohexane 1 and carbonyl-, thiocarbonyl-, or methylidene-containing derivatives 2-27 afforded precise structural (in particular, C-H bond distances) and spectroscopic (specifically, one-bond (1)J(C)(-)(H) NMR coupling constants) data that show the consequences of stereoelectronic hyperconjugative effects in these systems. Major observations include the following. (1) sigma(C)(-)(H)(ax)() -->(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() (Y = O, S, or CH(2)) hyperconjugation leads to a shortening (strengthening) of the equatorial C-H bonds adjacent to the pi group. This effect is reflected in smaller (1)J(C)(-)(H)(ax)() coupling constants relative to (1)J(C)(-)(H)(eq)(). (2) Comparison of the structural and spectroscopic consequences of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) hyperconjugation in cyclohexanone 2, thiocyclohexanone 3, and methylenecyclohexane 4 suggests a relative order of acceptor orbital ability C=S > C=O > C=CH(2), which is in line with available pK(a) data. (3) Analysis of the structural and spectroscopic data gathered for heterocyclic derivatives 5-12 reveals some additivity of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y), pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)(), n(X) --> sigma(C)(-)(H)(ax)(), n(beta)(O) --> sigma(C)(-)(H)(eq)(), and sigma(S)(-)(C) --> sigma(C)(-)(H)(eq)() stereoelectronic effects that is, nevertheless, attenuated by saturation effects. (4) Modulation of the C=Y acceptor character of the exocyclic pigroup by conjugation with alpha-heteroatoms O, N, and S in lactones, lactams, and methylidenic analogues 13-24 results in decreased sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() hyperconjugation. (5) Additivity of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() hyperconjugative effects is also apparent in 1,3-dicarbonyl derivative 25 (C=Y equal to C=O), 1,3-dithiocarbonyl derivative 26 (C=Y equal to C=S), and 1,3-dimethylidenic analogue 27 (C=Y equal to C=CH(2)).

A theoretical and experimental study on the molecular and electronic structures of artemisinin and related drug molecules

“…Refs. [57][58][59][60][61][62]. According to the DFT/ B3LYP calculations, the dominant contribution to the 1 J(CH)s and 3 J(HH)s of the present molecules is provided by the Fermi-contact term, with the noncontact contributions being comparatively small in all cases.…”

The role of carbonyl and thiocarbonyl groups in the conformational isomerism of haloacetones and halothioacetones

“…The few reports available for simple thiocarbonylic compounds are mostly limited to qualitatively describe the orbital levels of C=S [5] and to their application in synthesis, since thioketones are very reactive, usually evolving to di-or polymerization [5,13]. However, influences of the C=S system on 1 J CH coupling constants have revealed important stereoelectronic features of this functional group [14,15]. Thus, the main goal of this work is to contribute for the understanding of the interactions which rule the conformational isomerism of model compounds, namely halo-propanones and their corresponding sulfur analogues (Fig.…”

Efeito das interações hiperconjugativas na constante de acoplamento ¹J CH da hexametilenotetramina e do adamantano: estudo teórico e experimental

“…A influência da interação hiperconjugativa n→σ* no acoplamento 1 J CH tem sido amplamente estudada por vários autores [12][13][14][15] (Hz) …”

“…A influência da interação hiperconjugativa n→σ* no acoplamento 1 J CH tem sido amplamente estudada por vários autores [12][13][14][15] . Estudos realizados por Juaristi e colaboradores 14 com 1,3-ditianos e 1,3-dioxanos mostraram que as constantes de acoplamento 1 J CH são muito sensíveis às orientações entre os pares de elétrons dos átomos de enxofre e oxigênio e à ligação C-H, pois são observados dois valores para o acoplamento 1 J CH , um para o H na orientação axial e outro para o H na equatorial (Figura 1).…”

“…A interação do tipo n→σ*, conhecida como interação hiperconjugativa do tipo anomérica, causa grandes variações nas constantes de acoplamento a uma e a duas ligações 8 . Interações hiperconjugativas do tipo σ→π* e π→σ* são os principais mecanismos de transmissão de informações para as constantes de acoplamentos de núcleos magnéticos a longa distância em sistemas moleculares que possuem insaturações 9,10 e a interação σ→σ* é o principal mecanismo em compostos saturados 11 .A influência da interação hiperconjugativa n→σ* no acoplamento 1 J CH tem sido amplamente estudada por vários autores [12][13][14][15] (Hz) …”