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

Weber, Adrian C. J.; Burnell, E. Elliott

doi:10.1016/j.cplett.2011.03.025

Cited by 12 publications

(11 citation statements)

References 29 publications

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“…With the temperature controlled by the Bruker air-flow system, proton NMR spectra were acquired every 5 • from 253.5 to 333.5 K for the sample with butane in 1132 (Ref. 12) and from 263.5 to 298.5 for the sample with butane in 5CB. The spectral parameters defining the anisotropic spectra of all solutes are readily obtained with the use of a CMA-ES (Refs.…”

Section: Methodsmentioning

confidence: 99%

“…This is remarkable agreement -especially considering that the RMS is equal to or better than the best fits found in earlier studies where couplings from a single spectrum were fitted using some model for calculation of order parameters. 12,14,32 The constants can be diagonalized, and the η and θ values obtained pertain to the order matrix. As was the case for the order parameters in the fit pS calculation above, the θ values indicate that PMI and POA axes do not differ greatly, but η values for the gauche conformer are still inconsistent with intuition based on size and shape arguments (see Tables II and III).…”

Section: E Butane Dipolar Couplings Scaled To Ethane Intramethyl Dipmentioning

confidence: 99%

“…In a recent study of n-butane in nematic phases it was shown conclusively that the results obtained for p n were essentially modeldependent. 12 It is clearly important to remedy this unfortunate situation.…”

Section: Introductionmentioning

confidence: 99%

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Nuclear magnetic resonance study of alkane conformational statistics

Burnell

Weber

Lange

et al. 2011

The Journal of Chemical Physics

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NMR spectra of ethane, propane, and n-butane as solutes in the nematic liquid crystals 4-n-pentyl-4(')-cyanobiphenyl (5CB) and Merck ZLI 1132 (1132) are investigated over a wide temperature range. The ratios of dipolar couplings of ethane to propane are constant over the entire temperature range. Assuming that this constancy applies to the butane conformers facilitates the separation of probability from order parameter. This separation allows the investigation of conformational distribution without the need of invoking any model for the anisotropic intermolecular potential. The results give an order matrix that is consistent with that predicted from model potentials that describe the orientational potential in terms of short-range size and shape effects. The isotropic intermolecular potential contribution to the trans-gauche energy difference E(tg) is found to be temperature dependent with the values and variation in agreement with that found when the same results are analyzed using the chord model for anisotropic interactions [A. C. J. Weber and E. E. Burnell, Chem. Phys. Lett. 506, 196 (2011)]. The fit obtained for 9 spectra in 5CB (63 dipolar couplings) has an RMS difference between experimental and calculated dipolar couplings of 2.7 Hz, while that for the 16 spectra in 1132 (112 couplings) is 6.2 Hz; this excellent fit with nine adjustable parameters suggests that the assumption of equal temperature dependencies of the order parameters for ethane, propane, and each conformer of butane is correct. Also the fit parameters (E(tg) and the methyl angle increase) obtained for 1132 and 5CB agree. The results indicate that the chord model, which was designed to treat hydrocarbon chains, is indeed the model of choice for these chains. The temperature variation of E(tg) provides a challenge for theoreticians. Finally, even better fits to the experimental dipolar couplings are obtained when the energy in the Boltzmann factor is used for scaling ethane to butane results. However, in this case the values obtained for E(tg) differ between 1132 and 5CB.

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Section: Methodsmentioning

confidence: 99%

Section: E Butane Dipolar Couplings Scaled To Ethane Intramethyl Dipmentioning

confidence: 99%

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Nuclear magnetic resonance study of alkane conformational statistics

Burnell

Weber

Lange

et al. 2011

The Journal of Chemical Physics

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“…33 Another way of treating the n-pentane conformational problem is by using a continuous intramolecular potential, U iso int, n (φ 1 , φ 2 ); this treatment (using continuous probability distributions) was recently used in NMR studies on n-butane. 25,26,34,35 To do this for n-pentane, the internal energy was calculated in 15 • increments of φ 1 and φ 2 while keeping all other structural parameters fixed to those of the major conformer that exists at the bottom of the local (or global in the case of tt) energy well (see Figure 3 for potential energy plotted as a function of φ 1 and φ 2 ). These energy wells, calculated for the n-pentane molecule isolated in the gas phase, are affected by the environment in the anisotropic liquid phase in which the solute is dissolved.…”

Section: Methodsmentioning

confidence: 99%

A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals

Burnell

Weber

Dong

et al. 2015

The Journal of Chemical Physics

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The proton NMR spectra of n-pentane orientationally ordered in two nematic liquid-crystal solvents are studied over a wide temperature range and analysed using covariance matrix adaptation evolutionary strategy. Since alkanes possess small electrostatic moments, their anisotropic intermolecular interactions are dominated by short-range size-and-shape effects. As we assumed for n-butane, the anisotropic energy parameters of each n-pentane conformer are taken to be proportional to those of ethane and propane, independent of temperature. The observed temperature dependence of the n-pentane dipolar couplings allows a model-free separation between conformer degrees of order and conformer probabilities, which cannot be achieved at a single temperature. In this way for n-pentane 13 anisotropic energy parameters (two for trans trans, tt, five for trans gauche, tg, and three for each of gauche + gauche + , pp, and gauche + gauche − , pm), the isotropic trans-gauche energy difference E tg and its temperature coefficient E ′ tg are obtained. The value obtained for the extra energy associated with the proximity of the two methyl groups in the gauche + gauche − conformers (the pentane effect) is sensitive to minute details of other assumptions and is thus fixed in the calculations. Conformer populations are affected by the environment. In particular, anisotropic interactions increase the trans probability in the ordered phase. C 2015 AIP Publishing LLC. [http://dx

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“…The gauche states are related by symmetry. In treating the butane problem, Weber et al [354] used a size and shape potential, [355] CI(2k), or a modified chord model [356], CCd, to construct the intermolecular nematic potential. However, the derived results did depend on the chosen model.…”

Section: Nmr Of Flexible Alkanesmentioning

confidence: 99%

Recent NMR Studies of Thermotropic Liquid Crystals

Dong¹

2016

Annual Reports on NMR Spectroscopy

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

Cited by 12 publications

References 29 publications

Nuclear magnetic resonance study of alkane conformational statistics

Nuclear magnetic resonance study of alkane conformational statistics

A model-free temperature-dependent conformational study of n-pentane in nematic liquid crystals

Recent NMR Studies of Thermotropic Liquid Crystals

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