NMR gives precise cyclobutane structure
3
NMR gives precise cyclobutane structure Near-planar molecule bends at 18°; methylenes tilt at 6° TRACERS. Dr. Β. Β. Buchanan, Dr. D. I. Arnon, and Dr. M. C. W. Evans (left to right) at Berkeley traced the new photosynthetic path by feeding radio- active COo to bacteria in these jugs oxalacetate, and α-ketoglutarate to iso- citrate—pose no thermodynamic prob- lems and don't depend directly on fer- redoxin. To learn whether this pathway is followed in a photosynthetic bacte- rium, the Berkeley workers first con- firmed that C. thiosulfatophilum con- tains the 17 enzymes required by the cycle. They incubated extracts of the bacterium with carbon-14 labeled substrates and identified the products by paper chromatography and radio- autography. The activities of these enzymes, measured in the direction of the cycle, seem adequate to drive it in the proper direction. The lowest rate of activity found for any enzyme in the extracts was about one tenth of the rate of carbon dioxide fixation by whole cells. Other workers have re- ported that the lowest rate of activity in extracts of enzymes in the Calvin cycle is about one thirtieth of the rate of fixation in whole cells. The California workers traced the new pathway by exposing suspensions of the bacterium to labeled carbon di- oxide and identifying the products of the labeled carbon at various time in- tervals. Glutamate was the first stable product they identified. It accounted for three quarters of the labeled car- bon fixed in the first 30 seconds. (Glutamate is one of the main prod- ucts of photosynthetic bacteria. It can be converted to other amino acids in the cell.) Small amounts of suc- cinate and aspartate also appear quickly and, like glutamate, gradually decline. No more than 10% of the labeled carbon fixed went into phos- phate esters. All these results are consistent with the new cycle. A precise determination of the molec- ular structure of cyclobutane has been made from nuclear magnetic reso- nance spectroscopy in a nematic sol- vent by two scientists at Bell Tele- phone Laboratories, Murray Hill, N.J. Dr. Saul Meiboom and Dr. Lawrence C. Snyder have found that by measur- ing the NMR spectrum of cyclobutane dissolved in p,p'-di-n-hexyloxyazoxy- benzene, they can determine cyclobu- tane's dihedral angle and the angle of tilt of the methylene group. Some organic substances exist in one or more liquid crystalline phases at temperatures between those where the solid phase and the normal isotropic liquid phase occur. The three main types of liquid crystals are nematic, smectic, and cholesteric. In the ne- matic phase, molecules tend to orient themselves under the influence of a magnetic field, as in an NMR spec- trometer. In such cases, the axis of highest susceptibility of the molecular aggregates is parallel to the field. Dr. Alfred Saupe of Physikalisches Institut der Universitàt, Freiburg, West Germany, and Dr. Gerhard Eng- lert of Hoffmann-La Roche & Co., Ltd., Basel, Switzerland, earlier found that molecules in a nematic solvent experience an anisotropic environment and give highly resolved NMR spectra (C&EN, March 16, 1964, page 42). The NMR lines are relatively sharp because the dissolved molecules are sufficiently mobile to average dipole- dipole interactions with the solvent. The anisotropy of the molecular mo- tion leads to large intramolecular mag- netic dipole-dipole interactions be- tween nuclei, and to a corresponding fine structure in the NMR spectrum. The most important terms in the spin Hamiltonian that Dr. Meiboom and Dr. Snyder used in their molecu- lar structure determinations in nematic solvents are the direct spin-spin cou- plings. To determine the structure of a molecule dissolved in a nematic sol- vent, the Bell scientists first assume a certain geometry and motion for the molecule. Using the spin Hamilto- nian, they compute a theoretical NMR spectrum and compare it with the ex- perimental NMR spectrum of the mol- ecule in the nematic medium. If the experimental and calculated spectra do not agree, the assumed ge- ometry and motion for the molecule are incorrect. A new spectrum is then calculated for a different geometry and motion. This procedure is re- peated until close agreement between calculated and experimental NMR spectra are obtained. This method can be used to determine molecular structure with all distances determined relative to one other distance. Dr. Meiboom and Dr. Snyder tested their method on cyclopropane, a mole- cule whose structure has been deter- mined by electron diffraction. They used ρ,ρ'-di-n-hexyloxyazoxybenzene as the nematic solvent. With the as- sumption that the distance between a pair of protons on the same side of the cyclopropane ring has the electron dif- fraction value of 2.521 Α., they ob- tained values of 1.485 A. for the C—C distances, 1.106 A. for the C-H dis- OTHERS. Using NMR spectra obtained in nematic solvents, Dr. Saul Mei- boom (seated) and Dr. L. C. Snyder are now studying the structure of simple organic molecules in addition to cyclopropane and cyclobutane APRIL 25, 1966 C&EN 51
Transcript of NMR gives precise cyclobutane structure
NMR gives precise cyclobutane structure Near-planar molecule bends at 18°; methylenes tilt at 6°
TRACERS. Dr. Β. Β. Buchanan, Dr. D. I. Arnon, and Dr. M. C. W. Evans (left to right) at Berkeley traced the new photosynthetic path by feeding radioactive COo to bacteria in these jugs
oxalacetate, and α-ketoglutarate to iso-citrate—pose no thermodynamic problems and don't depend directly on fer-redoxin.
To learn whether this pathway is followed in a photosynthetic bacterium, the Berkeley workers first confirmed that C. thiosulfatophilum contains the 17 enzymes required by the cycle. They incubated extracts of the bacterium with carbon-14 labeled substrates and identified the products by paper chromatography and radio-autography. The activities of these enzymes, measured in the direction of the cycle, seem adequate to drive it in the proper direction. The lowest rate of activity found for any enzyme in the extracts was about one tenth of the rate of carbon dioxide fixation by whole cells. Other workers have reported that the lowest rate of activity in extracts of enzymes in the Calvin cycle is about one thirtieth of the rate of fixation in whole cells.
The California workers traced the new pathway by exposing suspensions of the bacterium to labeled carbon dioxide and identifying the products of the labeled carbon at various time intervals. Glutamate was the first stable product they identified. It accounted for three quarters of the labeled carbon fixed in the first 30 seconds. (Glutamate is one of the main products of photosynthetic bacteria. It can be converted to other amino acids in the cell.) Small amounts of succinate and aspartate also appear quickly and, like glutamate, gradually decline. No more than 10% of the labeled carbon fixed went into phosphate esters. All these results are consistent with the new cycle.
A precise determination of the molecular structure of cyclobutane has been made from nuclear magnetic resonance spectroscopy in a nematic solvent by two scientists at Bell Telephone Laboratories, Murray Hill, N.J. Dr. Saul Meiboom and Dr. Lawrence C. Snyder have found that by measuring the NMR spectrum of cyclobutane dissolved in p,p'-di-n-hexyloxyazoxy-benzene, they can determine cyclobu-tane's dihedral angle and the angle of tilt of the methylene group.
Some organic substances exist in one or more liquid crystalline phases at temperatures between those where the solid phase and the normal isotropic liquid phase occur. The three main types of liquid crystals are nematic, smectic, and cholesteric. In the nematic phase, molecules tend to orient themselves under the influence of a magnetic field, as in an NMR spectrometer. In such cases, the axis of highest susceptibility of the molecular aggregates is parallel to the field.
Dr. Alfred Saupe of Physikalisches Institut der Universitàt, Freiburg, West Germany, and Dr. Gerhard Eng-lert of Hoffmann-La Roche & Co., Ltd., Basel, Switzerland, earlier found that molecules in a nematic solvent experience an anisotropic environment and give highly resolved NMR spectra (C&EN, March 16, 1964, page 42). The NMR lines are relatively sharp because the dissolved molecules are sufficiently mobile to average dipole-dipole interactions with the solvent. The anisotropy of the molecular motion leads to large intramolecular mag
netic dipole-dipole interactions between nuclei, and to a corresponding fine structure in the NMR spectrum.
The most important terms in the spin Hamiltonian that Dr. Meiboom and Dr. Snyder used in their molecular structure determinations in nematic solvents are the direct spin-spin couplings. To determine the structure of a molecule dissolved in a nematic solvent, the Bell scientists first assume a certain geometry and motion for the molecule. Using the spin Hamiltonian, they compute a theoretical NMR spectrum and compare it with the experimental NMR spectrum of the molecule in the nematic medium.
If the experimental and calculated spectra do not agree, the assumed geometry and motion for the molecule are incorrect. A new spectrum is then calculated for a different geometry and motion. This procedure is repeated until close agreement between calculated and experimental NMR spectra are obtained. This method can be used to determine molecular structure with all distances determined relative to one other distance.
Dr. Meiboom and Dr. Snyder tested their method on cyclopropane, a molecule whose structure has been determined by electron diffraction. They used ρ,ρ'-di-n-hexyloxyazoxybenzene as the nematic solvent. With the assumption that the distance between a pair of protons on the same side of the cyclopropane ring has the electron diffraction value of 2.521 Α., they obtained values of 1.485 A. for the C—C distances, 1.106 A. for the C-H dis-
OTHERS. Using NMR spectra obtained in nematic solvents, Dr. Saul Meiboom (seated) and Dr. L. C. Snyder are now studying the structure of simple organic molecules in addition to cyclopropane and cyclobutane
APRIL 25, 1966 C&EN 51
φ A D M CHEMICALS
Secret agent with a license to thrill
She doesn't know it, but an Adol oleyl alcohol has her covered. From head to toe, this fine ingredient makes her cosmetics dangerously exciting to use.
Imparting sheen and body to her hair shampoos. Acting as an emollient in her mascara and lipstick, making it smooth and even. Penetrating deeply into her skin from creams and hand lotions... leaving her feeling luscious and alluring.
ADM's unsaturated oleyl alcohols kindle the creative instincts of product development people. Formulators like the light color, improved stability, resistance to hydrolysis, and compatibility with emulsifying agents.
A dossier on the entire Adol family of fatty alcohols will give you a good idea how ADM can tailor an oleyl alcohol for you.
There's no secret to getting your hands on it.
ADM qmiCALS/KESINS · PLASTICS · PLASTICIZERS · FATTY ACIDS AND TRIGLYCERIDES · FATTY ESTERS · FATTY AMINES, AMIDES, QUATERNARIES
52 C & E N
MORE NEWS ABOUT
ADM CHEMICALS
LOTIONS GLOW LIKE LIQUID PEARL
Hydrofol Acid 336 in solid form exhibits a glistening needle-like structure. This high-purity eutec-tic mixture of stearic and palmitic acid adds pearlescent brilliance to lotions and creams. Other advantages: Practically odorless and resists color change for long shelf life, outstanding for pharmaceuticals as well as cosmetics.
TAILOR-MADE BUTYL STEARATES
Starfol butyl stéarate 100 serves as a solubilizer, lubricant, and emollient in high quality cosmetic creams. It adds "s l ip" to lipsticks and gloss to nail polish. Prevents drying in nail polish removers. And if this one doesn't meet your needs, ADM will tailor a Starfol that will.
REPORT '66 You'll f ind interest ing reading and possibly, solutions to problems currently confronting you in ADM Chemicals Report '66. Let us send you this 12-page illustrated fact-filled booklet. Write us at the address below.
ADM CHEMICALS ARCHER DANIELS MIDLAND COMPANY
7 3 3 MARQUETTE AVENUE · DEPT. 9 0
MINNEAPOLIS. MINNESOTA 5 5 4 4 0
CP-7A
AGREEMENT. The experimental NMR spectrum (top) of cyclobutane dissolved in pfp'-di-n-hexyloxazoxybenzene, obtained by Dr. Saul Meiboom, agrees with the theoretical spectrum (bottom) computed by Dr. L. C. Snyder. Refinements of the analysis give almost perfect agreement
tances, and 114.6° for the H-C-H angle. The corresponding values of 1.510 Α., 1.089 Α., and 115.1° were obtained from electron diffraction data by Dr. O. Bastiansen, Dr. F. N. Fritsch, and Dr. Kenneth Hedberg at the University of Oslo (Norway).
The Bell workers then went on to study the structure of cyclobutane, a molecule that scientists believe is slightly bent from a planar configuration. (Dr. Meiboom disclosed the NMR work at the 151st ACS National Meeting. )
After testing various assumptions for the geometry and motion of the cyclobutane molecule, they have found one in which the experimental NMR spectrum agrees quite closely with the calculated spectrum. For cyclobu-tane, they arrive at a value of 18° ± 1° for the molecule's dihedral angle (between the two planes defined by the carbon atoms) and 6° ± 1° for the angle of tilt of the methylene groups. There are two such equivalent geometries. Dr. Meiboom and Dr. Snyder thus conclude that cyclobutane is rapidly flipping between the two.
The Bell team now is studying the position of the carbon relative to the protons in cyclobutane using proton NMR spectra of molecules containing carbon-13 in natural abundance. They are also studying the structures of other simple organic molecules.
New methylides are stable Sulfonium ylides in which the negative charge on carbon is stabilized only by sulfonium groups have been isolated for the first time by two chemists at the University of Massachusetts, Amherst [/. Am. Chem. Soc, 88, 1560 ( 1966) ]. An ylide is an internal salt of a carbanion and a nonmetal.
The university's Dr. C. Peter Lillya and Phillip Miller have prepared the three "strikingly" stable bis(dialkylsul-fonium) methylides by treating the corresponding methylene bis(dialkyl-sulfonium ) difluoroborates with one equivalent of potassium hydroxide in methanol. The alkyl groups are methyl or ethyl groups.
bis(dialkyl8ulfonium) methylide
Dr. Lillya and Mr. Miller launched their attempts to synthesize the bis-(dialkylsulfonium)methylides because they anticipated that the compounds would be unusually stable. They also expected that these ylides would react with carbonyl compounds. Thus, they reasoned, the ylides would be useful reagents for organic syntheses.
To prepare the bis(dialkylsulfo-nium ) difluoroborates, the Massachusetts team reacts bis (methylthio) methane or bis (ethylthio) methane with tri-ethyl- or trimethyloxonium fluorobo-rate [/. Am. Chem. Soc, 88, 1559 (1966)]. These bissulfonium salts precipitate from aqueous solution as glistening white crystals when ethanol is added slowly.
The bissulfonium salts behave on monobasic acids in aqueous solution and can be titrated potentiometrically with alkali in dilute aqueous solution. These neutralization reactions can be followed by observing the ultraviolet spectra of the bissulfonium salts.
A 10-3M. solution of bis(diethyl-sulfonium) methane difluoroborate in methanol gives an absorption maximum at 214 m/i, Dr. Lillya and Mr. Miller find. When they add small amounts of hydrogen chloride to this solution, the absorption at 214 πΐμ, is
APRIL 25, 1966 C&EN 53
