NMR spectra of symmetric molecules. I. The spin Hamiltonian for twofold symmetry

The 1H and 19F nmr spectra of benzyl fluoride, although very tightly coupled at 300 MHz, are analyzed for dilute CS2 and acetone solutions. 13C, 19F spin–spin coupling constants for benzyl fluoride in a series of solvents are also measured. Geometry-optimized STO 3G MO computations on benzyl fluoride indicate a small twofold barrier to rotation about the exocyclic C—C bond in the gas phase. All the long-range couplings between 19F and ring protons or carbon-13 nuclei and between methylene and ring protons are consistent with the conclusion that the barrier to internal rotation in benzyl fluoride is small and that the conformation in which the C—F bond lies in a plane perpendicular to the benzene plane is stabilized by 2 kJ/mol in going from CS2 to DMSO or acetone solutions. The solvent dependence of the internal barrier may account for the diversity of conformational conclusions in the literature. Furthermore, it is clear that the internal barrier will depend on the presence of substituents at any site in the ring.

Section: Resultsmentioning

confidence: 99%

Solvent-induced conformational changes in benzyl fluoride. Long-range C,F; H,H; H,F spin–spin couplings

Schaefer¹,

Peeling²,

Sebastian³

1985

“…Use of the program NUMARIT (14) in the noniterative mode allowed the extraction of the couplings to the methyl protons, also given in Table 2. The standard deviations quoted in the table suggest an accuracy of 0.02 Hz or better in the spectral parameters.…”

Section: Spectral Analysesmentioning

confidence: 99%

Limits to barriers to internal rotation in benzal fluoride, chloride, and bromide solution by the J method

Schaefer¹,

Danchura²,

Niemczura³

1978

. 56,36(1978). On the basis of the long-range couplings over six bonds between the protons on the side chain and the ring protons in benzal fluoride, chloride, and bromide and some of their derivatives, a hindered rotor model ( J method) yields limits to the barriers to internal rotation. These are 0.0 to 1.8,0.7 to 2.4, and 1.7 to 3.8 kcal/mol for the fluoride, chloride, and bromide, respectively. Refinement of the model suggests barriers of 1.1 f 0.2, 2.2 f 0.3, and 3.5 rt 0.6 kcall mol, respectively. The barrier values in the chloride and bromide are considerably less sensitive to the model parameters than is the barrier in the fluoride. It would be of interest to have gas phase values for these barriers. It is also shown that a o-x mechanism holds for the six-bond proton-proton coupling constant in benzal fluoride but that the analogous coupling between 19F on the side chain and the para ring proton carries a non o-x contribution of +0.3 Hz.TED SCHAEFER, WERNER DANCHURA et WALTER NIEMCZURA. Can. J. Chem. 56.36(1978). [Traduit par le journal]

“…The benzyl portions of the spectra were analyzed as an AA'BBfCM2R system for 2 and as AA1BB'CM2 systems for 3, 4, 5, and 6 using the program NUMARIT (10). Due to many small couplings to the alkyl group protons in compounds 3, 4, 5, and 6, transitions for the M2 part of the spectra were not assigned.…”

Section: J(ch2 H) and 5j(ch2 H)mentioning

confidence: 99%

Conformational preference and internal rotation about the C₁—C_α bond in phenylacetaldehyde and some benzyl alkyl ketones from ¹H nuclear magnetic resonance and abinitio molecular orbital calculations

Penner¹

1987

. Can. J. Chem. 65,538 (1987). Analysis of the 'H nuclear magnetic resonance spectra of the benzyl moieties in phenylacetaldehyde, benzyl methyl ketone, benzyl ethyl ketone, benzyl isopropyl ketone, and 3,5-dichlorobenzyl tert-butyl ketone yields the long-range couplings between ring and a protons. These stereospecific couplings change very little upon replacement of the aldehydic hydrogen by various alkyl groups. The couplings for all the molecules studied fall within the ranges 4~(~~2 , H,) = -0.566 ? 0.008 Hz, 5~( C~2 , H,) = 0.278 ? 0.002 Hz, and 6~(~~2 , Hp) = -0.409 ? O.OlOHz, suggesting that in the ketones the alkyl group prefers to be trans to the phenyl ring and does not interfere with rotation about the CI-C, bond. The long-range couplings are consistent with a potential function V(0) = 8.4 2 1.2 sin2 0 for two-fold rotation about the C,-C, bond; 8 is the angle between the carbonyl and benzene ring plane. Ab initio molecular orbital calculations on phenylacetaldehyde at the STO-3G level with the C=O bond cis to the phenyl group yield a potential of V (0) Chem. 65, 538 (1987). Une analyse des spectres rrnn du 'H de la portion benzylique du phtnylacCtaldChyde, de la benzyl mCthyl cttone, de la benzyl Cthyl cktone, de la benzyl isopropyl cCtone et de dichloro-3,5 benzyl tert-butyl cCtone permet de dktenniner les couplages B longue distance entre les protons du cycle et ceux en a. Ces couplages stCrtospCcifiques ne varient que peu lorsqu'on change l'hydrogkne aldChydique pour divers groupements alkyles. Les couplages pour chacune des molCcules CtudiCes se situent tous dans les intewalles 4~( C~2 , H,) = -0,566 f 0,008 Hz, 5 J ( C~2 , H,) = 0,278 ? 0,002Hz et 6~(~~2 , H,) = -0,409 2 0,010 Hz; ces rksultats suggkrent que, dans les cCtones, le groupement alkyle prtfkre la conformation trans par rapport au noyau phCnyle et qu'il n'interfkre pas avec la rotation autour de la liaison CI-C,. Les couplages B longue distance sont en accord avec une fonction de potentiel qui serait exprim6 par 1'6quation V(0) = 8,4 + 1,2 sin2 0 pour une rotation binaire autour de la liaison C1-C,; 8est l'angle form6 groupement carbonyle et le plane du cycle benztnique. Si l'on effectue des calculs OM a b initio, au niveau STO-3G, sur le phCnylacCtaldChyde en faisant l'hypothkse que la liaison C=O est cis par rapport au groupement phtnyle, le potentiel est alors reprtsentk par 1'Cquation V(0) = (8,65 ? 0,73) sin2 0 + (1,27 k 0,80) sin2 20; cette Cquation est assez proche du potentiel expkrimental, mais elle comporte une petite composante quaternaire. I1 existe une corrklation entre la constante de couplage spin-spin du proton aldkhydique avec les protons en a, et le solvant, qui est en accord avec les donnCes rapportkes anterieurement. Le fait que les valeurs de 4~(CH2, &), 'J(CH2, H,) et 6~(~~2 , Hp) ne varient pas avec la nature du solvant suggkre que (sin2 0) ne dCpend que faiblement sur la rotation du groupement aldkhydique.[Traduit par la revue]