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The separation of linear from branched hydrocarbons is often required in many situations. There are several methods through which they can be separated but none provides a very high degree of purity or works without considerable expenditure of energy. Recently, a novel method was proposed to separate a mixture of neopentane and n-pentane. The present work demonstrates that the method can be used for separating other mixtures of hydrocarbons as well, by attempting the separation of a mixture of 2,2-dimethyl butane and n-pentane. Intermolecular interaction potentials have been modified to reproduce the experimental heat of adsorption and diffusivity of 2,2-dimethyl butane and n-pentane in zeolite NaY. The method involves choosing the correct host zeolite or other porous solids and introducing hot zones at appropriate positions. This result drives both the components to the opposite ends of the zeolite column, thus leading to separation. The achieved separation factors are much higher than what can be obtained with the help of existing methods. Different properties have been computed to understand the process involved in the separation of the mixture. The approach employed here uses very little energy for separation, making it suitable for green chemistry.

The intramolecular ¹H NMR dipole–dipole relaxation of molecular fluids has traditionally been interpreted within the Bloembergen–Purcell–Pound (BPP) theory of NMR intramolecular relaxation. The BPP theory draws upon Debye’s theory for describing the rotational diffusion of the ¹H–¹H pair and predicts a monoexponential decay of the ¹H–¹H dipole–dipole autocorrelation function between distinct spin pairs. Using molecular dynamics (MD) simulations, we show that for both n-heptane and water this is not the case. In particular, the autocorrelation function of individual ¹H–¹H intramolecular pairs itself evinces a rich stretched-exponential behavior, implying a distribution in rotational correlation times. However, for the high-symmetry molecule neopentane, the individual ¹H–¹H intramolecular pairs do conform to the BPP description, suggesting an important role of molecular symmetry in aiding agreement with the BPP model. The intermolecular autocorrelation functions for n-heptane, water, and neopentane also do not admit a monoexponential behavior of individual ¹H–¹H intermolecular pairs at distinct initial separations. We suggest expanding the autocorrelation function in terms of modes, provisionally termed molecular modes, that do have an exponential relaxation behavior. With care, the resulting Fredholm integral equation of the first kind can be inverted to recover the probability distribution of the molecular modes. The advantages and limitations of this approach are noted.

A series of ReaxFF reactive molecular dynamic simulations are performed to investigate the combustion of n-pentane, isopentane and neopentane. The effects of temperature and the n-pentane-to-O2 ratio on the number of main radical and molecular products for the combustion of n-pentane are analyzed, the combustion characteristics of n-pentane, isopentane and neopentane are compared, and the spontaneous combustion characteristic of n-pentane is discussed in this work. The results indicated that the main reactions involved in the early stage of n-pentane, isopentane and neopentane combustion are the pyrolysis of these working fluids, and then, O2 molecules are involved in the oxidation reactions. The content of O2 in reaction systems has a very obvious effect on the combustion of n-pentane. With the increase in the O2 content, the consumption rate of n-pentane is significantly accelerated, and the rate of increase in the total fragments number is also significantly increased. Neopentane burns most intensely, followed by isopentane, and finally n-pentane. A faster heating rate will be accompanied by more intense spontaneous combustion of n-pentane and more product formation, which will cause greater harm to the surrounding area. Thus, in order to reduce the possibility of damage caused by leakage combustion of flammable working fluid, the heat source temperature and open flame around the organic Rankine cycle system should be strictly controlled, and some flame retardants are recommended to be added to the working fluid to reduce the flammability while ensuring the economy of the organic Rankine cycle. This study is conducive to the safety of n-pentane, isopentane and neopentane used in industrial fields.

The rate constants of ethynyl (C2H) radical reaction with isobutane (kiso) and neopentane (kneo) were calculated for the first time using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) in the temperature range of 150–5000 K. The geometries and frequencies of all the stationary points were calculated at the M06-2X/Aug-cc-pVTZ level of theory. Thermodynamics (ΔH°298 and ΔG°298) results govern that the title reactions are highly exothermic and spontaneous in nature. The rate constants obtained over the temperature range of 150–300 K were used to derive the modified Arrhenius expressions: kiso = 6.84 × 10–19 T2.5 exp[1496/T] and kneo = 1.72 × 10–17 T2.0 exp[938/T] cm3 molecule−1 s−1. Branching ratio calculation for the reaction of C2H radical with isobutane shows that the tertiary-H abstraction channel is dominant (> 95%) over the complete temperature range.

Chromium(VI) dialkyls are remarkably acidic, yielding a new set of anionic chromium alkylidenes upon deprotonation with KN(TMS)2. We describe the characterization of three anionic chromium alkylidenes of the type [(ArN)2Cr(=CHR)(CH2R]- (R = tBu, SiMe3, Ph; Ar = 2,6-diisopropylphenyl). NEt4[(ArN)2Cr(=CHSiMe3)(CH2SiMe3)] has been structurally characterized by X-ray diffraction. We have also studied the C-H activation of cyclohexane effected by the transient neopentylidene (ArN)2Cr(=CHCMe3). This reaction generated a tri-nuclear chromium(IV) complex, namely ‘[Cr(NAr)2]3’, as well as the two organic products cyclohexene and neopentane. A probable intermediate in this reaction could be trapped by PPh3 to form (ArN)2Cr(PPh3)2. The latter undergoes ligand substitution reactions with ethylene and CO to produce p-acid complexes of tetravalent chromium.

The understanding of major zeolite applications is partially based on diffusion of molecules inside or outside microporous networks. However, it is still a challenge to measure such phenomena. The diffusion ordered nuclear magnetic resonance spectroscopy (DOSY) technique has been reported to measure a probe molecule's diffusion inside porous solids. Pulsed-field gradient (PFG)-NMR has been used herein to measure the self-diffusivity of different probe molecules, such as neopentane, benzene, toluene and 1-dodecene with increasing dynamic diameter, respectively, on a series of H-ZSM-5 zeolites. The latter materials exhibit different crystal sizes, Si/Al ratios and the presence (or absence) of crystalline defects. In addition, shaped zeolite bodies representing industrial catalysts were compared with the afore-mentioned samples.

This work aims at the calculation of various inelastic cross sections for three pentane isomers, namely normal pentane, isopentane and neopentane. The direct ionisation, positronium formation, tota...

A highly promising class of three-dimensional polyaromatic hydrocarbons comprises the centropolyindanes. The characteristic feature of these compounds is the mutual fusion of several molecules of indane along the saturated C-C bonds of their cyclopentane rings. Among the polycyclic aromatic hydrocarbons, the centropolyindanes are special because of the saturated core of sp(3)-hybridised carbon atoms embedded in a three-dimensional environment of aromatic building blocks. While the centropolyindanes and their numerous derivatives have been studied in detail by NMR spectroscopy, mass spectrometry and X-ray diffraction, investigation of their vibrational features, and especially those of the neopentane core present in most cases, have not been performed so far. In the present paper, we report the first systematic study of a set of centropolyindanes by vibrational spectroscopy, using inelastic neutron scattering (INS), infrared and Raman spectroscopies.

High resolution infrared absorption spectra of neopentane (2,2-dimethylpropane, C5H12) have been recorded in the mid-infrared region at room temperature and 232 K. Neopentane is a spherical top with Td symmetry. The high symmetry and low temperature allow for detailed comparison of theory and experiment for the analysis of the fundamental vibrations. Four strong bands with characteristic rotational structure of t2 modes were observed at 1257.6, 1369.4, 1472.5, and 1489.0 cm-1, and a fifth very weak band was found near 924.2 cm-1. Quantum chemical calculations (B3LYP/6-311++(d,p)/VPT2 and harmonic CCSD(T)-pVTZ) were carried out in both normal and local mode representations to help with the vibrational assignments and elucidate the couplings. The local mode representation combined with the experimental observations highlights the important potential couplings between neighboring methyl groups and their influence on observed transitions. We have analyzed spectral regions where the models failed to provide agreement and have identified those couplings responsible for the differences.

Single-layer membranes of porous graphene, graphyne derivatives (α/α2/β-graphyne), and porous boron nitride (BN) with similar pore sizes (approximately 8 × 6 Å) have been evaluated for separating pentane isomers using first-principles calculations. In spite of their slightly bigger pore sizes, graphyne derivatives only allow linear pentane molecules to go through their native pores, while in contrast porous graphene and BN membranes only block dibranched isomers, i.e., neopentane molecules. Analyses of the geometric changes reveal that the structural flexibility of the membrane determines the penetration barrier. During the penetration of molecules, rigid membranes like graphyne derivatives may exhibit similar distortion to flexible porous graphene and BN; however the energy increase for the former is twice that for the latter. More importantly, the passing molecules experience about two times geometry distortion and four times energy barrier increase when going through rigid membranes compared with flexible membranes. The more deformed passing molecule and the less deformed rigid membrane can result in a much higher penetration barrier. Our results show that the flexibility of porous BN is comparable to that of porous graphene while graphyne derivatives are much more rigid.

Direct (solute-water) and indirect (water-water) contributions to adsorption at an air-water interface are identified using the Widom potential distribution theorem and quantified using molecular dynamics simulations of a liquid water droplet containing either neopentane or iodide-like solutes with charges of 0 or ±1. The results are used to quantitatively compare direct and indirect energetic and entropic contributions to adsorption, as well as to critically test surface capillary wave, linear response (LR), and mean field (MF) predictions. The negative signs of the total adsorption energies and entropies of both the anionic and cationic solutes are found to result from indirect adsorption induced changes in water-water interactions, rather than from surface capillary wave perturbations, which are found to be asymmetric with respect to solute charge. The LR and MF approximations both accurately describe the adsorption of neutral (hydrophobic) solutes, while for ionic solutes the MF approximation is entirely inappropriate and LR predictions are qualitatively (but not quantitatively) accurate.

Tetrakis(2-pyridylmethyleneimino)neopentane (L) was designed to form a spiro-structure after complex formation, where L plays a role of a double tetradentate bridging ligand. The X-ray diffraction study of [Cu2L(CF3SO3)4] clarified that the molecular structure was topologically spiro but the two coordination basal planes were not orthogonal with the dihedral angle of 49.8(1)°. The χmT value of [Cu2L(CF3SO3)4] showed a small increase on cooling below 10 K, indicating the presence of ferromagnetic coupling. Fitting to a singlet–triplet model gave 2J/kB = +0.293(7) K. The density-functional-theory calculation of the [Cu2L]4+ core reproduced the ferromagnetic coupling. Furthermore, from another calculation after removal of the neopentyl skeleton, through-bond interaction is clarified to be major in the exchange mechanism. The intervening sp3-hybridized carbon atoms may be spin-polarized.

Understanding the fundamental forces such as hydrophobic interactions in a crowded intracellular environment is necessary to comprehensively decipher the mechanisms of protein folding and biomolecular self-assemblies. The widely accepted entropic depletion view of crowding effects primarily attributes biomolecular compaction to the solvent excluded volume effects exerted by the "inert" crowders, neglecting their soft interactions with the biomolecule. In this study, we examine the effects of chemical nature and soft attractive energy of crowders on the water-mediated hydrophobic interaction between two non-polar neopentane solutes using molecular dynamics simulations. The crowded environment is modeled using dipeptides composed of polar and non-polar amino acids of varying sizes. The results show that amongst the non-polar crowders, Leu2 strengthens the hydrophobic interactions significantly, whereas the polar and small-sized non-polar crowders do not show significant strengthening. Distinct underlying thermodynamic driving forces are illustrated where the small-sized crowders drive hydrophobic interaction via a classic entropic depletion effect and the bulky crowders strengthen it by preferential interaction with the solute. A crossover from energy-stabilized solvent-separated pair to entropy-stabilized contact pair state is observed in the case of bulky non-polar (Leu2) and polar (Lys2) crowders. The influence of solute-crowder energy in affecting the dehydration energy penalty is found to be crucial for determining the neopentane association. The findings demonstrate that along with the entropic (size) effects, the energetic effects also play a crucial role in determining hydrophobic association. The results can be extended and have implications in understanding the impact of protein crowding with varying chemistry in modulating the protein free energy landscapes.

A method for the separation of a mixture of n-pentane and neopentane using a nano-crystallite of zeolite Y is reported. This method judiciously combines two well-known, counter-intuitive phenomena, the levitation and the blowtorch effects. The result is that the two components are separated by being driven to the opposite ends of the zeolite column. The calculations are based on the non-equilibrium Monte Carlo method with moves from a region at one temperature to a region at another temperature. The necessary acceptance probability for such moves has been derived here on the basis of stationary solution of an inhomogeneous Fokker–Planck equation. Simulations have been carried out with a realistic and experimentally relevant Gaussian hot zone and also a square hot zone, both of which lead to very good separation. Simulations without the hot zones do not show any separation. The results are reported at a loading of 1 molecule per cage. The temperature of the hot zone is just ∼30 K higher than the ambient temperature. The separation factors of the order of 1017 are achieved using single crystals of zeolite, which are less than 1 μm long. The conditions for including the hot zone may be experimentally realizable in the future considering the rapid advances in nanoscale thermometry. The separation process is likely to be energetically more efficient by several orders of magnitude as compared to the existing methods of separation, making the method very green.

We propose a ‘composite sphere’ model for molecules of tetrahedral symmetry. In particular, we choose as an example the hydrocarbon neopentane. We disassemble the molecule into five moieties: four ...

We demonstrate by molecular dynamics simulation that co-non-solvency manifests itself in the solvent-induced interaction between three hydrophobes, methane, propane and neopentane, in methanol-water mixtures. Decomposition of the potential of mean force, based on the potential distribution theorem, clearly shows that the solute-solvent entropic change is responsible for stabilizing the aggregation of these hydrophobic molecules. Furthermore, we show that the entropic change pertains to the excluded volume effect.

This study explores the autoignition characteristics of three C5 isomers, namely n -pentane, 2-methylbutane ( iso -pentane) and 2,2-dimethylpropane ( neo -pentane). These measurements are intended to enhance understanding of C5 autoignition chemistry, and provide experimental data to guide improvements to a general hydrocarbon oxidation mechanism. To that end, the autoignition behavior of these three C5 isomers was investigated in a modified CFR engine at an intake temperature of 120 °C and a fixed engine speed of 600 rpm to determine the critical compression ratio (CCR) at which hot ignition occurs. To find the critical compression ratio, the engine compression ratio (CR) was gradually increased to the point where CO in the engine exhaust rapidly decreased and significant high temperature heat release was observed, while holding equivalence ratio constant. Fundamental ignition behaviors such as the CCR and the calculated percentage of low temperature heat release (%LTHR) demonstrate the impact of chain length and methyl substitutions on ignition reactivity. The %LTHR shows a stronger two stage heat release for n- pentane than for neo- pentane observed at critical ignition conditions. In contrast, single stage heat release is observed for iso- pentane, leading to the weakest overall oxidation reactivity of the three isomers. Key reaction paths forming conjugate alkenes and C 5 oxygenated species control the autoignition reactivity of n- pentane and iso- pentane within the low temperature and NTC regimes. However, neo- pentane forms no conjugate alkene due to its unique molecular structure, and instead produces iso -butene to retard its oxidation.

The effects of urea and taurine osmolytes on the hydrophobic interactions of methane and neopentane have been studied using molecular dynamics simulations. From the study of potential of mean force, it is observed that both urea and taurine enhance the association of methane and neopentane. The hydrophobic association of methane molecules is stabilized by entropy and destabilized by enthalpy in the mixtures of osmolytes at contact minima while the hydrophobic association of neopentane is stabilized by enthalpy and entropy. In these mixtures, hydrophobic solvation of methane molecules is enthalpy favorable and entropy unfavorable at SSM. The hydrophobic solvation of neopentane is stabilized by entropy and enthalpy. The study of radial distribution functions (RDFs), running coordination numbers, excess coordination numbers of water, urea and taurine molecules around hydrophobic molecules shows that methane shows salting-out (less soluble) and neopentane shows salting-in (more soluble) behavior in aqueous-urea. Methane and neopentane show salting-in behavior in aqueous-taurine solution because of preferential solvation by taurine. Salting-out behavior of methane has been observed in water-urea-taurine mixture. Neopentane shows salting-in behavior in aqueous solution of urea and taurine because of preferential binding with urea. These observations are supported by solvation free energies and spatial density maps.

Previously published experimental viscosity data at low density, originally obtained using all-quartz oscillating-disk viscometers for 12 gases and vapors in the temperature range between 297 K and 691 K, were re-evaluated after an improved re-calibration. The relative combined expanded (k = 2) uncertainty of the re-evaluated data is 0.2% near room temperature and increases to 0.3% at higher temperatures. The re-evaluated data for sulfur hexafluoride, methanol, n-pentane, n-hexane, n-heptane, neopentane, cyclohexane, benzene, toluene, p-xylene, phenol, and triethylamine were arranged in approximately isothermal groups and converted into quasi-isothermal viscosity data using a first-order Taylor series in temperature. Then, they were evaluated by means of a series expansion truncated at first order to obtain the zero-density and initial density viscosity coefficients, η(0) and η(1). When the number of isothermal data or their quality was not adequate, the Rainwater–Friend theory for the initial density dependence of the viscosity was additionally used to derive η(0) and η(1) values. Finally, reliable η(0) and η(1) values, preferably obtained from the isotherms, were recommended as reference values for the 12 gases and vapors in the measured temperature range to be applied when generating any new viscosity formulation.

The paper presents the results of studies on a mesoscopic self-assembling occurring in liquid state of hydrogen-bonded n-, sec-, and tert- butanols. In the first part of the paper it was revealed (basing on the literature data) that an unexpectedly high melting point of tert-butanol does not result from some extraordinary way of intermolecular clustering in that alcohol, but from an exceptional packing of its globular-like molecules. The conclusion arises from a similar behavior of neopentane – the non-polar analogue of tert-butanol. Our results of the static dielectric studies on butanol isomers, in particular the Kirkwood correlation factor and the orientation entropy increment show that the spatial arrangement of the hydrocarbon moiety surrounding –OH group, essentially determines the structure of emerging molecular clusters. The form of clusters are evolving from a chain-like in n-butanol to a great variety of branched entities (including the cyclic ones) in tert-butanol.

An ab initio quantum dynamics study has been performed to explore the distinct structural symmetry of C(CH3)4(+) in the ground electronic state compared to CH4(+). Additionally, the underlying details of the highly diffuse and complex vibronic structure of the first photoelectron band of C(CH3)4 have been investigated. Associated potential energy surfaces over the two-dimensional space of nuclear coordinates, subject to the T2⊗ (e + t2) Jahn-Teller effect, are established from extensive electronic structure calculations and (then) the nuclear dynamics calculations are done on them via wave packet propagation including the nonadiabatic coupling of the three electronic sheets. The theoretical results are in good agreement with experimental observations. The JT stabilization energies due to T2⊗e, T2⊗t2 and T2⊗ (e + t2) distortions in the X[combining tilde](2)T2 electronic manifold of C(CH3)4(+) illustrate that the highest stabilization occurs through the T2⊗t2-JT distortion (in the ground state of C(CH3)4(+)). However, CH4(+) gains such maximum stabilization due to T2⊗ (e + t2)-JT distortion. From this novel result and applying the epikernel principle, we propose that the structural evolution of C(CH3)4(+) from Td to C3v minimum energy configuration occurs via JT active vibrations of t2 symmetry, whereas CH4(+) rearranges to the C2v structure through a combination of JT active e and t2 bending vibrations.

Collisional energy transfer plays an important role in unimolecular reaction kinetics. This Letter presents classical trajectory calculations of the energy transfer processes in collisions between selected alkanes (ethane, propane, isobutane, and neopentane) and krypton at high temperature. The primary aim of this study was to elucidate the validity of the predicted energy transfer parameters by performing master equation analyses of their vibrational relaxation times and comparing the predicted values with the available experimental data. The results demonstrate the ability of classical trajectory calculations to accurately predict the parameters for vibrational energy transfer.

Effects of trimethylamine-N-oxide (TMAO) on hydrophobic and charge–charge interactions are investigated using molecular dynamics simulations. Recently, these interactions in model peptides and in the Trp-Cage miniprotein have been reported to be strongly affected by TMAO. Neopentane dimers and Na+Cl– are used, here, as models for hydrophobic and charge–charge interactions, respectively. Distance-dependent interactions, i.e., potential of mean force, are computed using an umbrella sampling protocol at different temperatures which allows us to determine enthalpy and entropic energies. We find that the large favorable entropic energy and the unfavorable enthalpy, which are characteristic of hydrophobic interactions, become smaller when TMAO is added to water. These changes account for a negligible effect and a stabilizing effect on the strength of hydrophobic interactions for simulations performed with Kast and Netz models of TMAO, respectively. Effects of TMAO on the enthalpy are mainly due to changes in te...

The role of internal motions and molecular geometry on $^1$H NMR relaxation times $T_{1,2}$ in hydrocarbons is investigated using MD (molecular dynamics) simulations of the autocorrelation functions for in{\it tra}molecular $G_R(t)$ and in{\it ter}molecular $G_T(t)$ $^1$H-$^1$H dipole-dipole interactions arising from rotational ($R$) and translational ($T$) diffusion, respectively. We show that molecules with increased molecular symmetry such as neopentane, benzene, and isooctane show better agreement with traditional hard-sphere models than their corresponding straight-chain $n$-alkane, and furthermore that spherically-symmetric neopentane agrees well with the Stokes-Einstein theory. The influence of internal motions on the dynamics and $T_{1,2}$ relaxation of $n$-alkanes are investigated by simulating rigid $n$-alkanes and comparing with flexible (i.e. non-rigid) $n$-alkanes. Internal motions cause the rotational and translational correlation-times $\tau_{R,T}$ to get significantly shorter and the relaxation times $T_{1,2}$ to get significantly longer, especially for longer-chain $n$-alkanes. Site-by-site simulations of $^1$H's along the chains indicate significant variations in $\tau_{R,T}$ and $T_{1,2}$ across the chain, especially for longer-chain $n$-alkanes. The extent of the stretched (i.e. multi-exponential) decay in the autocorrelation functions $G_{R,T}(t)$ are quantified using inverse Laplace transforms, for both rigid and flexible molecules, and on a site-by-site bases. Comparison of $T_{1,2}$ measurements with the site-by-site simulations indicate that cross-relaxation (partially) averages-out the variations in $\tau_{R,T}$ and $T_{1,2}$ across the chain of long-chain $n$-alkanes. This work also has implications on the role of nano-pore confinement on the NMR relaxation of fluids in the organic-matter pores of kerogen and bitumen.

Product formation, in particular ketohydroperoxide formation and decomposition, were investigated in time-resolved, Cl-atom initiated neopentane oxidation experiments in the temperature range 550-675 K using a photoionization time-of-flight mass spectrometer. Ionization light was provided either by Advanced Light Source tunable synchrotron radiation or similar to 10.2 eV fixed energy radiation from a H-2-discharge lamp. Experiments were performed both at 1-2 atm pressure using a high-pressure reactor and also at similar to 9 Torr pressure employing a low-pressure reactor for comparison. Because of the highly symmetric structure of neopentane, ketohydroperoxide signal can be attributed to a 3-hydroperoxy-2,2-dimethylpropanal isomer, i.e. from a gamma-ketohydroperoxide (gamma-KHP). The photoionization spectra of the gamma-KHP measured at low-and high pressures and varying oxygen concentrations agree well with each other, further supporting they originate from the single isomer. Measurements performed in this work also suggest that the "Korcek" mechanism may play an important role in the decomposition of 3-hydroperoxy-2,2-dimethylpropanal, especially at lower temperatures. However, at higher temperatures where gamma-KHP decomposition to hydroxyl radical and oxy-radical dominates, oxidation of the oxy-radical yields a new important channel leading to acetone, carbon monoxide, and OH radical. Starting from the initial neopentyl + O-2 reaction, this channel releases altogether three OH radicals. A strongly temperature-dependent reaction product is observed at m/z = 100, likely attributable to 2,2-dimethylpropanedial.

It is shown that the contact-minimum configuration of two neopentane molecules is favoured on increasing hydrostatic pressure over the range 1–4000 atm, at room temperature, in line with the methane results (Graziano, 2014). This is due to the decrease in water accessible surface area accompanying the association, that leads to a gain in configurational/translational entropy of water molecules, whose magnitude increases with hydrostatic pressure. Of course, the strengthening of pairwise hydrophobic interaction cannot shed light on the pressure-induced denaturation of globular proteins.

We carried out molecular dynamics (MD) simulations with the AMBER force field for four pairs of spherical dimers of different size (neopentane, bicyclooctane, adamantane, and fullerene-C60) using the TIP3P model of water as solvent. For comparison, we performed MD simulations for a linear molecule of n-pentane at the same conditions. To assess the entropy contribution to the Gibbs free energy, MD simulations were run at three different temperatures of 273 K, 323 K, and 348 K, respectively, using umbrella-sampling/the WHAM method. The stability of dimers is described by a potential of mean force (PMF). The shape of PMF curves for hydrophobic interactions is characteristic, and entails a contact minimum, a solvent-separated minimum, and a desolvation maximum. The depth and position of the contact minimum for each pair change with the size of the nonpolar particle, and are consequently shifted to a larger distance for larger molecules. Additionally, the Lennard–Jones contribution to the PMF increases simultaneously with an increase in particle size. For a linear dimer, the contact minimum is shifted to a shorter distance than it is for spherical systems with the same number of carbon atoms. The contact minima on PMF curves increase with temperature, suggesting that the association entropy is positive. Dimensionless PMF curves showed a low dependency (near contact minima regions) on temperature, as the association entropy is low. Copyright © 2014 John Wiley & Sons, Ltd.

Silica-supported Pd and PdZn nanoparticles of a similar size were evaluated for neopentane hydrogenolysis/isomerization and propane hydrogenolysis/dehydrogenation. Monometallic Pd showed high neopentane hydrogenolysis selectivity. Addition of small amounts of Zn to Pd lead Pd–Zn scatters in the EXAFS spectrum and an increase in the linear bonded CO by IR. In addition, the neopentane turnover rate decreased by nearly 10 times with little change in the selectivity. Increasing amounts of Zn lead to greater Pd–Zn interactions, higher linear-to-bridging CO ratios by IR and complete loss of neopentane conversion. Pd NPs also had high selectivity for propane hydrogenolysis and thus were poorly selective for propylene. The PdZn bimetallic catalysts, however, were able to preferentially catalyze dehydrogenation, were not active for propane hydrogenolysis, and thus were highly selective for propylene formation. The decrease in hydrogenolysis selectivity was attributed to the isolation of active Pd atoms by inactive metallic Zn, demonstrating that hydrogenolysis requires a particular reactive ensemble whereas propane dehydrogenation does not.

Using a static analytic method, experimental solubility data were obtained for tetra(5,5-dimethyl-1,3-dioxaphosphorinanyl-2-oxy) neopentane (DOPNP) in acetonitrile, acetone, methanol, ethanol, ethyl acetate, methyl acetate and methylethylketone at temperatures ranging from 293 to 333 K. Several commonly used thermodynamic models, including the ideal, modified Apelblat, Wilson, UNIQUAC and NRTL models, were applied to correlate the experimental solubility data. The binary interaction parameters of the above models were found to have a linear dependency on temperature and the coefficients were regressed. It can be seen that NRTL model is more suitable in describing the solubility data of DOPNP, compared with the other models. By using the van’t Hoff equation, the dissolution enthalpy, dissolution entropy, and Gibbs free energy change of DOPNP are predicted in different pure solvents.

Binding of hydrocarbon guests to supramolecular hosts can lead to unusual geometric changes such as bending or coiling of guests upon encapsulation. Cucurbiturils (CBs) are classic cation binders that were recently used for the selective binding of small-membered hydrocarbons with a very high association constant (Ka ≈ 10(6) M(-1)). In this study, we have systematically investigated the binding of some alkanes to CB-[6] using a series of quantum chemical methods. The calculated binding free energies are very strong and are largely influenced by guest orientations inside the host and reorganization of host and guests. The computed (1)H NMR chemical shifts of the encapsulated alkanes agree with the experimental estimates thus confirming guest encapsulation. Further, we have shown that although binding of both cyclopentane and neopentane have very strong binding affinities (20 kcal mol(-1)), the selectivity of cyclopentane to neopentane at CB-[6] is a kinetically driven process through the computation of approximate transition state structures of both alkanes to CB-[6]. The calculated binding affinities with dispersion corrected density functionals (DFs) are very close to the experimental estimates, whereas DFs that lack dispersion correction predict that alkane binding to CB-[6] is largely unfavorable. Finally, we have investigated the binding of some long chain alkanes to several supramolecular hosts using dispersion corrected semiempirical methods which cannot be routinely studied through density functional theory methods due to the larger size of the system.

Aggregation processes in both the gas phase and aqueous solutions are analyzed by comparing aggregate size distributions obtained from molecular dynamics simulations with analytical predictions pertaining to a nonaggregating random mixture. The latter predictions are obtained by using the binomial distribution to predict the statistical properties of a uniformly mixed solution containing molecules of the same size and concentration as those in the solution of interest. Simulations are performed on systems containing neopentane dissolved in either methane, aqueous methanol, or aqueous NaI solutions. Comparisons of the theoretical and simulation results are used to both classify and quantify the influence of intermolecular interactions on such aggregation processes, including the equilibrium constants and thermodynamic functions pertaining to the partitioning of molecules between the bulk and first coordination shell of neopentane. Although the present results are primarily intended to describe and illustrate the random mixing analysis strategy, they also imply that neopentane has a greater tendency to aggregate with methane in the dense gas phase than with either methanol or iodide ions in aqueous solutions.

The vibrational spectroscopy of C-H stretches in organic molecules is of considerable importance for the characterization of these systems and for exploration of their properties. These stretches are strongly anharmonic, and thus methods including anharmonicity have to be used. The vibrational self-consistent field (VSCF) is applied to the following organic compounds: acetone, dimethylacetylene, neopentane, toluene, ethylene, and cyclopropane. The computed spectra are compared to new experimental data, including Raman measurements of all molecules except cyclopropane and IR of acetone, neopentane, and ethylene. A high level of agreement is found for all of the molecules. The characteristic features of CH3 and CH2 groups are studied and analyzed in detail. A reliable, unambiguous assignment of vibrational modes to spectral peaks is provided. Several characteristic features of CH3 and CH2 vibrations in polyatomic molecules are clarified, providing easier assignments for different types of organic molecules.

Large ZSM-5 zeolite crystals synthesized in fluoride medium show an astonishing activity, stability as well as selectivity towards light olefins in the Methanol-To-Olefins (MTO) reaction. By proper control of the synthesis parameters, ZSM-5 single crystals of unprecedented high quality are produced. The absence of usually uncontrollable variables such as structural defects, external non selective surface acid sites and extra-framework aluminium (EFAl) species was evidenced by SEM, HRTEM, CO-FTIR, 27Al and 19F MAS-NMR, Rietveld structure refinement and N2- and Ar-gas sorption measurements. Interestingly, diffusivity evaluation of different probe molecules (toluene, benzene and neopentane) has been carried out with PFG-NMR, allowing casting light on an interesting structure-diffusion-activity relationship. A “levitation” effect could be experimentally demonstrated and its impact on catalysis is highlighted in a rationalization attempt: Maxwell-Boltzmann based diffusion models properly predict product distributions for this counter-intuitively outstanding Methanol-To-Propylene (MTP) catalyst.

Thermolysis of Cp*W(NO)(Npt)(η3-CH2CHCHSiMe3) [Cp* = η5-C5Me5; Npt = CH2CMe3] at 55 °C leads to the loss of neopentane and the formation of the 16-electron η2-allene intermediate Cp*W(NO)(η2-CH2═C═CHSiMe3), which activates hydrocarbons at their methyl groups. In the case of linear alkanes, only terminal C–H activation occurs. This selectivity persists in the presence of an ether functionality, but not with other oxygen-containing substrates such as aldehydes and alcohols. With these latter substrates, the organometallic complex is oxidized to Cp*W(O)2(Npt). The existence of the allene intermediate has been confirmed by its reaction with PMe3 to form the 18-electron adduct and by its diagnostic reaction with cyclohexene. Carbonylation of Cp*W(NO)(Npt)(η3-CH2CHCHSiMe3) with CO (550 psig) at room temperature results in the clean formation of the corresponding Cp*W(NO)(η1-C(═O)Npt)(η3-CH2CHCHSiMe3) complex, which exists as a mixture of two interconverting isomers differing in their modes of attachment of the ...

The HD (v ′, j ′) product speed distributions from the D + C(CH3)4 → HD(v ′, j ′) + C(CH3)3CH2 reaction at a centre-of-mass collision energy of 1.20 eV (27.7 kcal/mol) were measured using a three-dimensional ion imaging apparatus. Anomalously fast HD (v ′, j ′) molecules are observed that move at speeds beyond the energetically allowed limit for reagents in their ground states plus the average thermal vibrational energy present in neopentane. These products are attributed to a reaction between D atoms and thermally excited C(CH3)4 molecules in which it is speculated that vibrational energy from more than one vibrational mode of neopentane is funneled into product translation in what seems to be an unprecedented coordinated process.

The syntheses of two linear oligogermanes, Ph3GeGePh2GePh2GePh2H and Ge5Ph12, were achieved using a hydrogermolysis reaction starting with HPh2GeGePh2GePh2H. The preparation of the hydride-terminated tetragermane indicates that selectivity is possible using the hydrogermolysis reaction, which had not been observed previously. The structures of both of these compounds were determined, and they were also characterized by UV/visible spectroscopy and electrochemical methods (CV and DPV). The pentagermane Ge5Ph12 exhibits four irreversible oxidation waves in both its CV and DPV, as was observed for other aryl-substituted oligogermanes. The successful synthesis of the neopentane analogue (Ph3Ge)4Ge was also achieved by starting from GeH4 and Ph3GeCH2CN. This material was structurally characterized; the structure of (Ph3Ge)4Ge is highly sterically congested and contains long Ge–Ge single-bond distances that average 2.497(6) A and exhibits an nearly idealized tetrahedral geometry at the central germanium atom wit...

Gentle thermolyses of CpW(NO)(CH2CMe3)(η3-allyl) complexes (Cp = η5-C5Me5 (Cp*), η5-C5Me4H (Cp′)) in neat hydrocarbon solutions result in the loss of neopentane from the metal’s coordination sphere...

Thermolysis of Cp*W(NO)(CH2CMe3)(η3-CH2CHCHPh) (1) at 55 °C leads to the loss of neopentane and the formation of the 16e η2-allene intermediate complex Cp*W(NO)(η2-CH2═C═CHPh) (A), which has been i...

Intermolecular interactions and parameters for use in MD studies of large molecule systems have earlier been determined for hydrocarbons, carbon tetrahalides and sulfur. The paper reports a model representing nonbonding interactions between silane molecules, which were examined in the same way as hydrocarbons in an earlier (neopentane, isopropane, propane, and ethane) study. Intermolecular potentials were determined for 11 combinations of silane compound pairs (silane SiH4, disilane Si2H6, trisilane Si3H8, isotetrasilane Si4H10 and neopentasilane Si5H12) with MP2/aug(df)-6-311G∗ ab initio calculations. The most stable dimer configurations were identified. With use of the modified Morse potential model to represent the interactions, 276 new potential energy surfaces were generated for silane dimers. Separate and generic pair potentials were calculated for the silanes. The pair potentials can be used in MD studies of silanes.

CASPT2 calculations predict the existence of roaming radical pathways for the decomposition of propane, n-butane, isobutane and neopentane. The roaming radical paths lead to the formation of an alkane and an alkene instead of the expected radical products. The predicted barriers for the roaming radical paths lie ∼1 kcal/mol below the corresponding radical asymptotes.

The neopentylidene−neopentyl complex (PNP)Ti═CHtBu(CH2tBu) (1; PNP− = N[2-P(CHMe2)2-4-methylphenyl]2) extrudes neopentane in neat pyridine or picoline (3- or 4-picoline) under mild conditions (25 °C), to generate the transient titanium alkylidyne intermediate (PNP)Ti≡CtBu (A), which subsequently ring-opens the pyridine by ring-opening metathesis of the aromatic N═C bond across the Ti≡C linkage, generating the metallaazabicycles (PNP)Ti(C(tBu)C5H3RNH) (R = H (2), 3-Me (3), 4-Me (4)). Kinetic studies suggest that the C−N activation process obeys a pseudo-first-order process in titanium, with α-hydrogen abstraction being the rate-determining step (the KIE for 1/1-d3 conversion to 2 was 3.8(3) at 25 °C). The activation parameters are ΔH⧧ = 23(3) kcal/mol and ΔS⧧ = −4(3) cal/(mol K). The intermolecular kH/kD ratio is close to unity, 1.07(3) at 25 °C, for the conversion of 1 to 2 in pyridine versus pyridine-d5. Detailed theoretical studies suggest the 1 → 2 transformation proceeds in the following order: (i) fo...

The neopentylidene-neopentyl complex (PNP)Ti═CHtBu(CH2tBu) (1; (PNP− = N[2-P(CHMe2)2-4-methylphenyl]2) extrudes neopentane in neat fluorobenzene under mild conditions (25 °C) to generate the transient titanium alkylidyne (PNP)Ti≡CtBu (A), which subsequently undergoes regioselective 1,2-CH bond addition of a fluorobenzene across the Ti≡C linkage to generate (PNP)Ti═CHtBu(o-FC6H4) (2). Kinetic and mechanistic studies suggest that the C−H activation process is pseudo-first-order in titanium, with the α-hydrogen abstraction being the rate-determining step and the post-rate-determining step being the C−H bond activation of fluorobenzene. At 100 °C complex 2 does not equilibrate back to A and the preference for C−H activation in benzene versus fluorobenzene is 2:3, respectively. Compound 1 also reacts readily, and in most cases cleanly, with a series of hydrofluoroarenes (HArF), to form a family of alkylidene-arylfluoride derivatives of the type (PNP)Ti═CHtBu(ArF). Thermolysis of the latter compounds generates ...

Tetrafluoromethane, CF 4 , is markedly less soluble in water than methane and neopentane around room temperature: it is a superhydrophobic solute. An analysis of the physical origin of this superhydrophobicity is performed, exploiting literature thermodynamic data covering the 5–55 °C temperature range. It results that the CF 4 –water dispersion interactions are markedly weaker in magnitude than those of a ‘hypothetical’ hydrocarbon having the same size of tetrafluoromethane, providing a smaller counterbalancing effect of the work spent to create the cavity in water. The weakness of the CF 4 –water dispersion interactions is due to the very small polarizability of CF 4 , which, in turn, is caused by the strong electronegativity of fluorine atoms.

The grafting of tetraneopentylchromium(IV) onto the surface hydroxyls of silica has been investigated by varying both the method of deposition and the nature of the silica support. Reaction of the volatile organometallic with the silica surface in vacuo is compared with reaction in solution. A faster reaction but a lower ultimate Cr loading was obtained using the solution deposition technique. The reactions of the organometallic compound with Aerosil 200, a fumed silica often used to model catalyst supports, and Sylopol 952, a silica gel used as a carrier for chromium-based ethylene polymerization catalysts, are compared. After thermal pretreatment at 200 °C, grafting on both supports yields bis(neopentyl)chromium(IV) fragments regardless of the grafting method used, suggesting that a paired arrangement of the surface hydroxyl groups exists on both types of silicas. The higher Cr loading achieved on the silica gel is attributed to its higher surface area. Thermally-induced neopentane elimination from the grafted bis(neopentyl)chromium(IV) fragments occurs at the same rate and with the same stoichiometry for both Aerosil- and Sylopol-supported materials. Consequently, interactions of the grafted organometallic fragments with nearby siloxanes appear to be unimportant in the early stages of the transformation of bis(alkyl)chromium(IV) to the alkylidene.

The thermodynamic adsorption characteristics of tetramethylsilane (TMS) molecules on graphitized carbon black (GCB) were for the first time determined experimentally and by molecular statistics methods. The potential function parameters of pair molecular interactions (φ(r)) between Si and C atoms on the basal face of graphite were calculated in the atom-atom approximation of the semiempirical molecular-statistical theory of adsorption. The contributions of Si and C atoms to thermodynamic adsorption characteristics are compared for the example of nonspecific adsorption of TMS and isostructural neopentane molecules on the flat surface of GCB.

The model reactions of neopentane hydrogenolysis and hydroisomerization on transition metal hydrides (≡ Si−O)3MIVH (1), (=eqSi−O)2MIVH2 (2), and (≡Si−O)2MIIIH (3) (M = Zr, Ti) immobilized on the surface of silica were studied by density functional theory. It was shown that the hydrogenolysis of neopentane could occur on all the three types of metal hydrides, the catalytic activity of reaction centers increasing along the series M(IV) monohydrides 1 < M(IV) dihydrides 2 < M(III) hydrides 3. At the same time, the isomerization of the hydrocarbon skeleton of neopentane observed experimentally on titanium-based systems could only be explained by the participation of Ti(III) hydrides.

The solid-state molecular structure of centrohexaindane (1), a unique hydrocarbon comprising six benzene rings clamped to each other in three dimensions around a neopentane core, and the molecular packing in crystals of 1 center dot CHCl3 are reported. The molecular T-d-symmetry and the Cartesian orientation of the six indane wings of 1 in the solid state have been confirmed. The course and limitation of electrophilic aromatic substitution of 1 are demonstrated for the case of nitration. Based on nitration experiments of a lower congener of 1, tribenzotriquinacene 5, the six-fold nitrofunctionalisation of 1 has been achieved in excellent yield, giving four constitutional isomers, two nonsymmetrical (14 and 17) and two C-3-symmetrical ones (15 and 16), all of which contain one single nitro group in each of the six benzene rings. The relative yields of the four isomers (similar to 3 : 1 : 1 : 3) point to a random electrophilic attack of the electrophiles at the twelve formally equivalent outer positions of the aromatic periphery of 1, suggesting electronic independence of its six aromatic pi-electron systems. In turn, the pronounced conformational rigidity of the centrohexacyclic framework of 1 enables the unequivocal structural identification of the isomeric hexanitrocentrohexaindanes 14-17 by H-1 NMR spectroscopy.

The translational and rotational dynamics of neopentane adsorbed in the one-dimensional channels of MIL-47(V) has been studied by quasi-elastic neutron scattering. The rotational motion of neopentane is well-described by the rotational diffusion model, with a correlation time of 41 ps at 300 K. The translational motion of the molecule has been fitted by several models: isotropic diffusion, normal 1D and single-file diffusion. It is found that the observed line shapes can only be reproduced by the single-file diffusion model. The single-file mobility factor, F, is (8 ± 1) × 10(-14) m(2) s(-1/2) at 300 K. This is the first observation of this unusual diffusion behaviour in a MOF.

10 págs.; 8 figs.; 3 tabs., Isomerism is ubiquitous in chemistry, physics, and biology. In atomic and molecular physics, in particular, isomer effects are well known in electron-impact phenomena; however, very little is known for positron collisions. Here we report on a set of experimental and theoretical cross sections for low-energy positron scattering from the three structural isomers of pentane: normal-pentane, isopentane, and neopentane. Total cross sections for positron scattering from normal-pentane and isopentane were measured at the University of Trento at incident energies between 0.1 and 50 eV. Calculations of the total cross sections, integral cross sections for elastic scattering, positronium formation, and electronic excitations plus direct ionization, as well as elastic differential cross sections were computed for all three isomers between 1 and 1000 eV using the independent atom model with screening corrected additivity rule. No definitive evidence of a significant isomer effect in positron scattering from the pentane isomers appears to be present. ©2016 AIP Publishing LLC, G.G. and F.B. would like to acknowledge the Spanish Ministerio de Economía y Productividad (Project No. FIS2012-31230) and the European Science Foundation (COST Action Grants Nos. MP1002–Nano-IBCT and MC1301-CELINA) for financial support. Finally, L.C. thanks the Japan Society for the Promotion of Science for his fellowship.