An X-ray and neutron diffraction refinement of the structure of p-terphenyl

Rietveld, H. M.; Maslen, E. N.; Clews, C. J. B.

doi:10.1107/s0567740870003023

Cited by 236 publications

(118 citation statements)

References 3 publications

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“…For a long time, polyphenyl molecules have been considered, from diffraction results, as being planar in the crystal at room temperature, this planarity being associated with large librational amplitudes (Pickett, 1936;Trotter, 1961;Robertson, 1961;Hargreaves & Rizvi, 1962;Dejace, 1969;Rietveld, Maslen & Clews, 1970). The space group for the monoclinic room-temperature structures is P2~/a, which is very common for planar organic molecules.…”

Section: Starting Point: the Characteristics Of Disorder In The High-mentioning

confidence: 99%

Structural phase transition in polyphenyls. X. Potential barrier heights in crystalline polyphenyls and in gaseous biphenyl determined uniquely from diffraction data

Baudour¹

1991

Acta Crystallogr Sect B

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The analysis from X-ray or neutron diffraction data, of the librational motion around the long molecular axis of the p-terphenyl central ring permits resolution of its disorder: the phenyl rotation angle on either side of the average molecular plane is ¢ = _+ 13.3 c. This disorder is associated with a double-well potential between two twisted conformations, and the overall libration on each site, at the bottoms of the double well, has the mean-square amplitude (02) = 52.5 deg 2 at room temperature. The analysis also permits separation in (02) of the mean-square amplitude of the torsional g mode (0~) = 35 deg 2 from that of the external mode (0~) = 17.5deg 2. Thus it becomes possible to scale the parameters of a simple model describing inter-and intramolecular interactions in the whole family of polyphenyls. It is shown that the intramolecular potential between two adjacent phenyl rings cannot be described by a simple sinusoidal function but exhibits a steeper gradient near the planar conformation. This double potential well model accounts for disorder and libration in crystalline p-terphenyl and p-quaterphenyl. an estimation of potential barrier heights: V = 4.57 kJ mol-1 in p-terphenyl and 7.91 kJ mol-' in p-quaterphenyl in good agreement with NMR results. It agrees with the displacive nature of the biphenyl transition by giving in this case a barrier height, V = 1.25 kJ mol-~, significantly less than the thermal energy kT and a singly peaked probability density function associated with a large libration amplitude as observed experimentally. Combined with the estimation of librational amplitude from diffraction data it predicts a large increase of intermolecular interactions in biphenyl at low temperature and gives an estimation of the interactions involved in the low-temperature incommensurate phase of biphenyl. Indirectly it is one of the very few experimental determinations of the potential-energy curve for gaseous biphenyl, giving V0 = 10"82kJmol ~ for the planar conformation in agreement with the results of gas-phase electron diffraction for deuterated biphenyl.

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Section: Starting Point: the Characteristics Of Disorder In The High-mentioning

confidence: 99%

Structural phase transition in polyphenyls. X. Potential barrier heights in crystalline polyphenyls and in gaseous biphenyl determined uniquely from diffraction data

Baudour¹

1991

Acta Crystallogr Sect B

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“…The Fourier transform of the Gaussian probability distribution of harmonically vibrating atoms is the temperature factor component of the time-averaged atomic form factor first introduced by Cruickshank (1956). It has also been realized, however, that there exist not uncommon phenomena in which anharmonic motion might be a significant effect, for example pseudorotation (Cremer & Pople, 1975), ring oscillations in biphenyl-type compounds (Rietveld, Maslen & Clews, 1970), anharmonic vibrations in alloys (Kontio & Stevens, 1982) and semiconductors (McIntyre, Moss & Barnea, 1980) and many others. Internal vibrations of bonds in molecules have a less dominant effect in crystallographic work, but evidence for their anharmonicity is abundantly present in spectroscopic data.…”

Section: Introductionmentioning

confidence: 99%

The Gram–Charlier and multipole expansions in accurate X-ray diffraction studies: can they be distinguished?

Mallinson¹,

Koritsánszky²,

Elkaïm³

et al. 1988

Acta Cryst Sect A

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The Gram-Charlier temperature factor formalism has been applied to a set of accurate low-temperature data on bis(pyridine)(meso-tetraphenylporphinato)iron(II), and to a theoretical set of static structure factors on the hexaaquairon(lI) ion. The refinements are compared with the multipole treatment for atomic asphericity due to chemical bonding. In a treatment of the experimental data in which only the iron atom asphericity is considered, the 'thermal motion' formalism is as efficient as the multipole formalism in accounting for the observations. It is slightly less efficient when applied to the static theoretical data, though model maps based on the two treatments are remarkably similar. A high-order Gram-Charlier refinement of the porphyrin data, followed by a multipole refinement of all data with the Gram-Charlier parameters initially fixed, and later varied, shows that simultaneous refinement of anharmonic and aspherical effects is possible, though the resulting separation may not be accurate. A combined Gram-Charlier multipole refinement on the static data, however, leads to non-significant thermal parameters. It is concluded that the statistical GramCharlier formalism is remarkably successful in representing bonding effects in the valence charge density if these are not specifically accounted for in the scattering formalism. Statistical anharmonic thermal motion formalisms should only be used for X-ray data analysis in combination with a formalism accounting for the effect of bonding on the atomic charge density. * Permanent address: Department of Chemistry, The University, Glasgow G12 8QQ, Scotland.t Permanent address: Central Research Institute for Chemistry, Hungarian Academy of Sciences, Budapest 11, Pusztaszeri UT 59-67, H-1525 Budapest, POB 17, Hungary. ~t To whom all correspondence should be addressed.0108-7673/88/030336-07503.00 IntroductionIt is commonly assumed in crystallographic studies that thermal motion can be adequately described by a formalism based on a harmonic force field. The Fourier transform of the Gaussian probability distribution of harmonically vibrating atoms is the temperature factor component of the time-averaged atomic form factor first introduced by Cruickshank (1956). It has also been realized, however, that there exist not uncommon phenomena in which anharmonic motion might be a significant effect, for example pseudorotation (Cremer & Pople, 1975), ring oscillations in biphenyl-type compounds (Rietveld, Maslen & Clews, 1970), anharmonic vibrations in alloys (Kontio & Stevens, 1982) and semiconductors (McIntyre, Moss & Barnea, 1980) and many others. Internal vibrations of bonds in molecules have a less dominant effect in crystallographic work, but evidence for their anharmonicity is abundantly present in spectroscopic data. Indeed, inclusion of anharmonic covariant tensor coefficients has been reported to be essential in some structure determinations (e.g. Marsh & Abrahams, 1987;Zucker & Schulz, 1982;Johnson, 1969; Willis, 1969). Such anharmonicity leads to devia...

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“…With good data, deviations from a Gaussian distribution can be detected and related to a non-parabolic potential function by Boltzmann's formula. For p-terphenyl an evaluation of this potential, based on van der Waals interactions and orbital alignment energy, has been given (Rietveld, Maslen & Clews, 1970). To compare with these calculations we have tried to specify the potential curve for the central-ring librations, directly from diffraction data.…”

Section: Introductionmentioning

confidence: 99%

Structural phase transition in polyphenyls. IV. Double-well potential in the disordered phase of p-terphenyl from neutron (200 K) and X-ray (room-temperature) diffraction data

Baudour¹,

Cailleau²,

Yelon³

1977

Acta Crystallogr Sect B

209

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New neutron-diffraction data collected at 200 K with deuterated p-terphenyl, and published X-ray data collected at room temperature with protonated p-terphenyl, have been used to determine the characteristics of the double-well potential for the librations of the central ring about the long molecular axis, in the disordered phase. A picture of the double well is obtained simply by halving the atoms outside the molecular axis and imposing constraints on their geometries and thermal parameters. The R values compared with that obtained with the simple-well model, are significantly improved. The deuterated benzene rings, compared to the protonated ones, show a systematic and hard-to-explain distortion: all the internuclear C-C bond lengths parallel to the long axis are shortened and the others lengthened, the mean bond lengths being the same in the two compounds. The double-well potential-barrier height is found to be about 0.6 kcal mo1-1 at 200 K and at room temperature. This value is in good agreement with that given by potential-energy calculations. The rotation angle between the two wells is about 26 °. Only for the internal g mode is the thermal energy kT sufficient to bring the rings near the top of the barrier. For the central-ring librations the doubly peaked distribution function is an argument for an order-disorder regime. However, the smallness of the double-well barrier height suggests that the p-terphenyl structural phase transition is near the boundary between the order-disorder and displacive regimes.

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An X-ray and neutron diffraction refinement of the structure of p-terphenyl

Cited by 236 publications

References 3 publications

Structural phase transition in polyphenyls. X. Potential barrier heights in crystalline polyphenyls and in gaseous biphenyl determined uniquely from diffraction data

Structural phase transition in polyphenyls. X. Potential barrier heights in crystalline polyphenyls and in gaseous biphenyl determined uniquely from diffraction data

The Gram–Charlier and multipole expansions in accurate X-ray diffraction studies: can they be distinguished?

Structural phase transition in polyphenyls. IV. Double-well potential in the disordered phase of p-terphenyl from neutron (200 K) and X-ray (room-temperature) diffraction data

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