Rotational isomerism. XI. Raman spectra of <i>n</i>-butane, 2-methylbutane, and 2, 3-dimethylbutane

Verma, A. L.; Murphy, William F.; Bernstein, H. J.

doi:10.1063/1.1681228

Cited by 135 publications

(45 citation statements)

References 8 publications

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“…For example, J2., = 7.5 Hz (6s-PGII/DiO) (3) gives p,(2-3) = 0.73, in excellent agreement with the value of 0.72 obtained from the gaucheltrans butane energy difference of 0.966 kcallmol (28).…”

Section: Resultssupporting

confidence: 73%

The conformations of prostacyclin and related prostaglandins

Beierbeck¹,

Kotovych²,

Sugiura³

1985

Can. J. Chem.

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HELMUT BEIERBECK, GEORGE KOTOVYCH, and MAKIKO SUGIURA. Can. J. Chem. 63, 1 143 (1985). The conformations of prostacyclin, PG12, and three of its analogues, 6R-and 6s-PGII and carbacyclin, were studied by high field 'H nmr spectroscopy. The cis-bicyclo-and cis-oxabicyclo[3.3.O]octane ring conformations were completely assigned. The minima for the pseudorotational conformations are observed at 7~/ l z~ for PGI?, :,T/:~T for 6R-PGI,, and 6 E / 1 '~ for 6s-PGI, and carbacyclin. The data indicate that each molecule adopts a narrow pseudolibrational range, if not a single conformation. The a-and w-side chain conformations were characterized, but not unambiguously. Vicinal coupling constants --and nuclear Overhauser enhancements proved to be the most useful spectroscopic parameters in this study.HELMUT BEIERBECK, GEORGE KOTOVYCH et MAKIKO SUGIURA. Can. J. Chem. 63, 1143Chem. 63, (1985. In a recent series of articles (1-4) we reported ' H nmr chemical shifts and coupling constants for prostacyclin and three of its derivatives, compounds of considerable pharmacological interest. Prostacyclin or PG12, first isolated in 1976, is a potent vasodilator and inhibitor of platelet aggregation (5, 6). It is rather unstable, however, and a number of biologically active analogues were developed ( 3 , among them the isomeric 5,6-dihydroprostacyclins, 6R-PGII and 6s-PGII, and carbacyclin, in which the ether oxygen is replaced by a methylene group. In the present study we have undertaken a detailed conformational analysis, based on ' H nmr coupling constants and nuclear Overhauser effects, of this important group of prostaglandins. ExperimentalThe samples of PGI?, 6s-PGI,, 6R-PGI,, and carbacyclin used in this study were gifts from Drs. R. A. Johnson, J. E. Pike, and D. R. IMorton of Upjohn, Kalamazoo. For the measurements of coupling 'Paper No. 10 in a series of high-field nmr studies on the prostaglandins and leukotrienes.2Author to whom correspondence should be addressed.constants in chloroform solution, CD,OD was distilled into nmr tubes containing PGI? methyl ester or 6s-PGII, respectively, to exchange labile protons; the methanol was then evaporated and CDCI, added. The prostaglandin concentrations were -0.005 M. The samples were sealed after four freeze-pump-thaw cycles. PGIz samples were prepared in glycine/D20 buffer solutions at pH 2 10. Solutions were 0.005 M PGIz in a 0.20 M and 0.015 M buffer and 0.017 M in 0.02 M buffer to check for the effect of concentration on the T , data. The high pH was necessary to prevent PGI? hydrolysis. For 6s-and 6R-PGI,, a 0.10 M phosphate/D20 buffer of pH = 7.0 was used. The samples were weighed into nmr tubes, buffer solution added, the solvent evaporated, fresh 100% D 2 0 added, and oxygen removed in four freeze-pump-thaw cycles. 6s-PGI, concentrations were 0.005 M, 0.008 M, and 0.013 M while the 6R-PGI, solutions were 0.005 M. All glassware was cleaned extensively, including a nitric acid wash, to eliminate paramagnetic impurities.All nmr measurements were carried out on a Bruker WH-40...

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Section: Resultssupporting

confidence: 73%

The conformations of prostacyclin and related prostaglandins

Beierbeck¹,

Kotovych²,

Sugiura³

1985

Can. J. Chem.

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“…The three rotational states of bonds involving quaternary carbons are taken to be equally probable, with zero rotational energies and statistical weights of 1.0. The remaining three bond types correspond to butane, 2-methylbutane, and 2,3-dimethylbutane, whose rotational energy differences are available from Raman spectroscopic studies (6). The rotation potential for 2,3-dimethylbutane has been confirmed by low temperature 'T nmr spectroscopy (7).…”

Section: Resultsmentioning

confidence: 84%

“…The chemical shifts calculated from these conformer resonances and probabilities agreed reasonably well with the experimental values. However, magnetic nonequivalence in asymmetric molecules was greatly underestimated, and the rotamer populations derived by optimization did not agree too well with experimental values (6,7).…”

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Analysis of ¹³C nuclear magnetic resonance chemical shifts of acyclic hydrocarbons

Beierbeck¹,

Saunders²

1980

Can. J. Chem.

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HELMUT BEIERBLLK and JOHN K SAUNDERS. Can. J . Chem. 58, 1258Chem. 58, (1980. The IZC nmr chemical shifts of aliphatic hydrocarbons were derived from chemical shifts and probabilities of individual chain conformations. Conformer resonances were calculated with parameters developed for conformationally homogeneous cyclic molecules. Conformer populations were derived from bond rotation potentials and steric energy contributions, available from independent sources. This analysis contains non adjustable parameters. It represents a unified treatment of cyclic and acyclic hydrocarbon resonances, which is capable of dealing with molecularasymmetry and thus permits the determination of configuration in aliphatic hydrocarbons. The early work by Grant and Paul (1) on aliphatic hydrocarbons provided one of the first demonstrations of the additivity of substituent effects on I3C nmr chemical shifts. Unfortunately, their analysis was based on a topological, rather than geometric, description of molecular structure, and conformational interconversions were not explicitly taken into consideration. Consequently their substituent parameters could neither deal with molecular asymmetry, nor could they be applied to conformationally homogeneous cyclic substrates. Lindeman and Adams (2) later proposed an expanded and modified parameter set, but it too was topological and therefore incapable of dealing with molecular chirality. HELMUT BEIERBECKStructure elucidation obviously includes the determination ofconfiguration at asymmetric centers, and chemical shift parameters must be able to reproduce magnetic nonequivalence. Since chemical shift nonequivalence in asymmetric chains is the result of geometric nonequivalence in individual conformations, it cannot be understood without a knowledge of the physical properties of these conformations. In other words, the observed chemical shifts are conformational averages, and therefore have to be compared to conformational averages, calculated from conformer resonances and populations.We have used such an approach to analyse the carbon chemical shifts of a series of saturated aliphatic hydrocarbons (3). The conformer resonances were calculated with a parametrization scheme developed for rigid cyclic substrates (4). The conformer populations were taken to be products of rotamer populations, and the rotational states of adjacent bonds were assumed to be independent. Steric contributions to conformer energies, although considered important (3, were ignored in order to simplify the analysis. Numerical values for the chemical shift and rotamer population parameters were derived by a least-squares fit of calculated to observed conformationally averaged resonances. The chemical shifts calculated from these conformer resonances and probabilities agreed reasonably well with the experimental values. However, magnetic nonequivalence in asymmetric molecules was greatly underestimated, and the rotamer populations derived by optimization did not agree too well with experimental values (6, 7).The insensitivi...

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“…Flory suggested the effect was likely negligible [2]; however, this was not borne out in experiment. In particular, for the simplest flexible alkane n-butane, experiments report a trans-gauche energy difference, E tg , range of 788-884 cal/mol in the gas phase [3][4][5][6][7] and 502-681 cal/mol [8][9][10][11] in the condensed phase.…”

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Conformational statistics of n-butane in the condensed phase and the effects of temperature

Weber

Burnell

2011

Chemical Physics Letters

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Rotational isomerism. XI. Raman spectra of n-butane, 2-methylbutane, and 2, 3-dimethylbutane

Cited by 135 publications

References 8 publications

The conformations of prostacyclin and related prostaglandins

The conformations of prostacyclin and related prostaglandins

Analysis of ¹³C nuclear magnetic resonance chemical shifts of acyclic hydrocarbons

Conformational statistics of n-butane in the condensed phase and the effects of temperature

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Rotational isomerism. XI. Raman spectra of n-butane, 2-methylbutane, and 2, 3-dimethylbutane

Cited by 135 publications

References 8 publications

The conformations of prostacyclin and related prostaglandins

The conformations of prostacyclin and related prostaglandins

Analysis of 13C nuclear magnetic resonance chemical shifts of acyclic hydrocarbons

Conformational statistics of n-butane in the condensed phase and the effects of temperature

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Analysis of ¹³C nuclear magnetic resonance chemical shifts of acyclic hydrocarbons