¹H and ¹⁹F NMR conformational studies of the monofluorostyrenes in solution. Comparison with theory and vapor phase behavior

Abstract

. Can. J. Chem. 68, 1383 (1990). The parameters for the high resolution 'H and "F NMR spectra of 2-, 3-, and 4-fluorostyrene are reported for solutions in CS, and acetone-d6 at 300 K. The populations of the planar cis and trans conformers of 2-and 3-fluorostyrene are deduced from the long-range coupling constants involving the meta and a protons. These populations are insensitive to solvent and appear to be in reasonable agreement with previous 6-3 1G MO computations for the free molecule; they are also compared with populations deduced from recent rotational and vibronic spectra. The long-range coupling constants for the protons of 4-fluorostyrene imply an internal barrier to rotation about the exocyclic carbon-carbon bond very similar to that in styrene, in agreement with the 6-3 1 G results. The signs of the coupling constants involving "F and the protons in the side chain are reported and discussed in terms of coupling mechanisms for the three molecules. An earlier surmise, of a positive a electron contribution to the coupling constant over six bonds in an all-trans arrangement, is confirmed for the meta and trans-p protons in 3-fluorostyrene.Key words: monofluorostyrenes, 'H and "F NMR, conformations, long-range coupling mechanisms, MO calculations of internal barriers.TED SCHAEFER et RUDY SEBASTIAN. Can. J . Chem. 68, 1383 (1990).On a dttermink les paramktres pour les spectres RMN a haute rksolution du 'H et du "F des 2-, 3-et 4-fluorostyrknes, en solutions dans le CS2 et dans I'acktone-d6, a 300 K. On a dCduit les populations des conformkres plans cis et trans prtsents dans les 2-et 3-fluorostyrknes en se basant sur les constantes de couplage a longue distance impliquant les protons me'ta et a . Ces populations sont insensibles la nature des solvants et elles semblent en bon accord avec des prkvisions faites antkrieurement en se basant sur des calculs d'OM effectuCs au niveau 6-31G, sur la molCcule libre; on a aussi compark ces rCsultats avec les populations qui ont rkcernment Ct C dCduites de spectres de rotation et de vibration. Les constantes de couplage a longue distance qui sont observkes dans le 4-fluorostyrkne impliquent que la barrikre interne la rotation autour de la liaison carbone-carbone exocyclique est trks semblable a celle qui est observke dans le styrkne; cette conclusion est en accord avec les prkdictions faites a l'aide de calculs au niveau 6-3 I G. On rapporte aussi les signes des constantes de couplage entre le "F et les protons de la chaine latCrale et on en discute en fonction de mkcanismes de couplage opCrant dans ces trois moltcules. Pour les protons me'ta et trans-p du 3-fluorostyrkne, on confirme la prkdiction antCrieure a l'effet que, dans un arrangement complktement trans, il existe une contribution a klectronique positive a la constante de couplage travers six liaisons.

Experimental and theoretical assessments of the substituent and medium dependence of the internal rotational potentials in benzyl fluoride. 3,5-Difluorobenzyl fluoride and 4-fluorobenzyl fluoride

“…'Except where otherwise noted, the standard deviations in all parameters were <lo-' Hz. /The marked solvent dependence of "J(H, F) within the phenyl group for a variety of fluorobenzene derivatives is well documented (32)(33)(34)44); more polar solvents increase 'J(H, F) algebraically, that is, a negative 'J(H, F), for example, becomes smaller in magnitude (35,36). Tables 1 and 2 for details) were filtered into 5 mm 0.d.…”

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

“…In 3-fluorostyrene, however, 5~5 9 is +0.06 HZ (4). All methods agree that the cis conformer, which favors a positive 6~5 9 , is most abundant (4,(31)(32)(33)(34). Yet (cos20) is not smaller than in styrene (4), so that the expression above cannot reproduce 6~5 9 :…”

“…Interatomic distances in the planar conformers, even for 0 = 0, imply substantial steric strain and, therefore, that conformers characterized by 0 > 0 are likely, the torsional barrier of the vinyl being much smaller than that of the formyl group. Many of the long-range coupling constants between vinyl or formyl protons and the aromatic protons depend on conformation and on torsion angles in known ways (2)(3)(4)(5). Hence their measurement and interpretation in terms of 0-cis and 0-trans orientations, together with nonzero values of 0 (see A), is informative.…”

Proton NMR studies of dihalogenated phenyl benzamides: two‐dimensional higher quantum methodologies

“…It is interesting to note that in 3-phenylpentane which is a 16 spin system, 6 J HH and 7 J HH have been determined to the precision of 0.001 Hz by carrying out the analyses treating it as an A 3 A 3 / BB / CC / D spin system [16] . Significantly large value of 0.53 Hz has been observed for 7 J HH in ring substituted styrenes [17,18] . There are also examples of detection of through space couplings, including 8 J HH [19,20] .…”