The single vibronic level fluorescence and Raman spectra of styrene-β-D2 vapor and their use in determining the C(1)-C(α) torsional potential function in the state

6-31G levels of ab initio molecular orbital theory suggest that the fourfold component of the barrier to internal rotation about the C,~-C,Ibond in ethylbenzene amounts to about 20% of the twofold component. The 'H nuclear magnetic resonance spectral parameters, extracted by complete analyses of the spectra arising from the ten protons, are reported for ethylbenzene in acetone-d6, CC14, CS2, and perfluoromethylcyclohexane solutions. The long-range proton-proton spin-spin coupling constants demonstrate that the internal barrier is insensitive to the polarity of the solvent, in contrast to polar solute molecules such as benzyl fluoride. The coupling constants do not support a dependence of the internal barrier on the internal pressure of the solvent. TED SCHAEFER, GLENN H. PENNER et RUDY SEBASTIAN. Can. J. Chem. 65, 873 (1987). Des calculs ab initio d'orbitales molCculaires ont Ct C effectuts d'une f a~o n relativement approfondie aux niveaux STO-3G, 4-2lC, 4-3 1G et 6-3 1G et on les a optirnisCs par rapport a la gComCtrie; les rtsultats suggbrent que la barribre la rotation interne autour de la liaison CSp2-CSpl de l'tthylbenzbne comporte une composante quadruple qui correspond 2 20% de la composante binaire. On a dCtermin6 les spectres rmn 'H de 1'Cthylbenzbne en solutions dans I'acCtone-d6, dans le CC14, dans le CS2 et dans le perfluoromtthylcyclohexane et on a dttermint les parambtres spectraux en procCdant a des analyses complbtes des spectres provenant des dix protons. Les constantes de couplage spin-spin, proton-proton a longue distance dCmontrent que la banibre interne n'est pas sensible B la polarit6 du solvant; ce rCsultat est en opposition avec ce qui est obsewt avec des moltcules polaires comme le fluorure de benzyle. Les constantes de couplage ne prtsentent pas de relation entre la barribre interne et la pression interne du solvant.[Traduit par la revue]

Section: Barrier Computations By Molecular Orbital Methodsmentioning

confidence: 66%

“…STO-3G MO computations agree well with the magnitude of V4 but overestimate V2 by 3.5 kJ/mol (18). The 6-31G basis set yields an internal potential (19) effectively equal to that from experiment (17). Such computations demand rather full geometry optimization procedures.…”

Section: Barrier Computations By Molecular Orbital Methodsmentioning

confidence: 75%

¹H nuclear magnetic resonance and molecular orbital studies of the structure and internal rotations in ethylbenzene

Schaefer¹,

Penner²,

Sebastian³

1987

“…For styrene and its trans-P-methyl derivative, these potentials lead to (sin2 8) estimates at 300 K effectively the same as that measured for styrene. For the cis-Pmethyl derivative, it is anticipated that a strongly nonplanar conformation is caused by large repulsive forces between the methyl group and the ortho C-H bond, just as the nonplanarity of styrene itself is plausibly attributable (1) to Pauli overlap of the p and ortho C-H bonds. The minimum in the potential curve in Fig.…”

Section: Molecular Orbital Calculatiotzsmentioning

confidence: 99%

“…TABLE 1. 'H nuclear magnetic resonance spectral parameters for styrene and some of its methyl derivatives in acetone-d6 at 300 K, excluding coupling constants between side chain and ring Parameter Styrenea 4 -c~~~ t r a n s -P -C~~~ c i s -P -C~~' P,P-diCH3' ~-C H~ b2.5 mol% in acetone-d6 containing 0.5 mol% of tetramethylsilane (TMS).…”

Section: Molecular Orbital Calculatiotzsmentioning

confidence: 99%

Molecular orbital calculations and ¹H nuclear magnetic resonance spectra as indicators of internal rotational potentials in some methyl derivatives of styrene

Schaefer¹,

Sebastian²,

Penner³

1988

Can. J. Chem. 66,584 (1988). The 'H nuclear magnetic resonance spectra of the a-methyl, cis and trans-P-methyl, 4-methyl, and P,P-dimethyl styrenes are analyzed to yield long-range proton-proton coupling constants. With the assumptin that the internal rotational potential for styrene in the gas phase is unaltered in solution, a consistent treatment of over 40 of the long-range coupling constants is given in terms of the known coupling mechanisms. Expectation values of sin' 0, where 0 is the angle of twist about the exocyclic carbon-carbon bond, are presented for these molecules. These are compared with theoretical potentials at the 6-3 1G level of molecular orbital theory. The present data indicate rather larger average twist angles than those in the literature. The extrema (at 0 = 0" and 90") in the angle dependent long-range coupling constants appear to be rather smaller in magnitude than are theoretical values obtained from valence bond and molecular orbital approaches.TED SCHAEFER, RUDY SEBAST~AN et GLENN H. PENNER. Can. J. Chem. 66,584 (1988). On a analyse les spectres en resonance magnttique nucltaire du 'H des a-methyl, P-methyl-cis et -trans, mtthyl-4et P,P-dimethyl styrknes afin d'en tirer les constantes de couplage proton-proton A longue distance. En faisant I'hypothkse que le potential A la barrikre interne du styrene est le mZme en phase gazeuse qu'en solution, on peut traiter d'une faqon consistante plus de 40 constantes de couplage A longue distance en fonction des mecanismes connus de couplages. Pour ces moltcules, on presente les valeurs attendues pour sin2 0 , dans lesquelles 0 est I'angle torsion autour de la liaison carbone-carbone exocyclique. On les compare aux potentiels calculCs par la thiorie des orbitales mol&culaires au niveau 6-3 1 G. Les donnCes obtenues dans notre travail suggkrent que les angles moyens de torsion sont plus grands que ceux rapportts dans la litttrature. Les amplitudes des valeurs extremes ( A des valeurs de 0 de 0" et de 90") dans les constantes de couplage A longue distance qui dependent de I'angle semblent stre plus faibles que celles que nous laissent prevoir les approches des liaisons de valence et des orbitales.[Traduit par la revue]

“…Whether, in fact, there exists a (small) barrier to planarity in 2 is a matter of disagreement (1)(2)(3)(4)(5)(6)(7)(8) between experimental and theoretical data. The P-CH bond is absent in 1, while I3c nmr chemical shifts indicate (9) rather similar n conjugational energies in the two molecules.…”

mentioning

confidence: 99%

¹H and¹³C nuclear magnetic resonance and molecular orbital studies of the internal rotational potential and of spin–spin coupling transmission in phenylallene

Schaefer¹,

Kroeker²,

McKinnon³

1995

The 'H nuclear magnetic resonance spectra of phenylallene, diluted in acetone-d6 and benzene-d6, yield long-range coupling constants over as many as eight formal bonds between the ring and side-chain protons. These are discussed in terms of o-and IT-electron spin-spin coupling mechanisms, which are sensitive to the torsion angle between the allenyl and phenyl fragments. The torsion angle is assessed by means of molecular orbital computations of the internal rotational potential, whose height is calculated as 16.0 kJ1mol at the MP216-31G* level of correlation-gradient theory. Comparison with experimental and theoretical internal rotational potentials for styrene suggests that steric repulsions in the planar form of styrene amount to about 4 Idlmol. In a field of 7.0 T, phenylallene is partially aligned, entailing a positive dipolar coupling constant between the methylene protons, from which absolute signs of the spin-spin coupling constants involving these protons can be inferred. Such coupling constants over seven and eight bonds, to the mefa and para protons, are taken as being mediated by the extended a-electron system, providing a measure of IT-electron contributions to coupling constants between mefa protons and those in side chains (spin correlation). Some coupling constants between protons and I3c nuclei in the side chain, as well as between ring protons and these I3c nuclei, are also discussed in terms of spin coupling mechanisms. Solvent perturbations of one-bond proton~arbon coupling constants in the allenyl group do not follow the usual pattern in which an increase in polarity of the solvent is associated with an increase in the magnitude of the coupling constant.