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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.

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.

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.

The kinetic relevance and rates of elementary steps involved in C–C bond hydrogenolysis for isobutane, neopentane, and 2,3-dimethylbutane reactants were systematically probed using activation enthalpies and free energies derived from density functional theory. Previous studies showed that C–C cleavage in alkanes occurs via unsaturated species formed in fast quasi-equilibrated C–H activation steps, leading to rates that decrease with increasing H2 pressure, because of a concomitant decrease in the concentration of the relevant transition states. This study, together with previous findings for n-alkanes, provides a general mechanistic construct for the analysis and prediction of C–C hydrogenolysis rates on metals. C–C cleavage in alkanes is preceded by the loss of two H atoms and the formation of two C-metal (C–M) bonds for each 1C and 2C atom involved in the C–C bond. Metal atoms transfer electrons into the 1C and 2C atoms as C–C bonds cleave and additional C–M bonds form. 3C and 4C atoms of isobutane, neo...

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.

Silica-supported Pt and Pd nanoparticles from 1 to 10 nm in diameter were evaluated for neopentane conversion (hydrogenolysis and isomerization). Characterization of the catalysts was conducted uti...

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 "alkane branching effect" denotes the fact that simple alkanes with more highly branched carbon skeletons, for example, isobutane and neopentane, are more stable than their normal isomers, for example, n-butane and n-pentane. Although n-alkanes have no branches, the "kinks" (or "protobranches") in their chains (defined as the composite of 1,3-alkyl-alkyl interactions-including methine, methylene, and methyl groups as alkyl entities-present in most linear, cyclic, and branched alkanes, but not methane or ethane) also are associated with lower energies. Branching and protobranching stabilization energies are evaluated by isodesmic comparisons of protobranched alkanes with ethane. Accurate ab initio characterization of branching and protobranching stability requires post-self-consistent field (SCF) treatments, which account for medium range (∼1.5-3.0 Å) electron correlation. Localized molecular orbital second-order Møller-Plesset (LMO-MP2) partitioning of the correlation energies of simple alkanes into localized contributions indicates that correlation effects between electrons in 1,3-alkyl groups are largely responsible for the enhanced correlation energies and general stabilities of branched and protobranched alkanes.

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.

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 ...

Bimetallic PtPd on silica nanoparticle catalysts have been synthesized, and their average structure has been determined by Pt L3 and Pd K edge extended X-ray absorption fine-structure spectroscopy. The bimetallic structure is confirmed from elemental line scans by scanning transmission electron microscopy of the individual 2-nm-sized particles. A general method is described to determine the surface composition of bimetallic nanoparticles even when both metals adsorb; for example, CO, by combining the quantitative characterization by X-ray absorption near-edge structure spectra at L edges with CO adsorption with the adsorption stoichiometry determined by Fourier transform infrared spectroscopy. Determination of the surface composition leads to a better understanding of the changes in catalytic chemistry that accompany alloy formation. Although monometallic Pt and Pd have similar turnover rates for neopentane hydrogenolysis and isomerization, on the basis of the surface composition, it appears that in the b...

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...

The first examples of a new class of gallium hydride clusters with direct Ga-Ga bonds and common hydrocarbon structures are reported. Neutron powder diffraction was used to find a Ga[GaH(3)](4)(5-) ...

The isosteric enthalpy of adsorption for neopentane at relative pressures down to 3 × 10(-8) in MCM-41 was predicted for the temperature range from -15 to 0 °C. At such low pressures and temperatures, experimental measurements become problematic for this system. We used an atomistic model for MCM-41 obtained by means of a kinetic Monte Carlo method mimicking the synthesis of the material. The model was parametrized to represent experimental nitrogen adsorption isotherms at 77 K using grand canonical Monte Carlo simulations. The simulated isosteric enthalpy of adsorption shows very good agreement with available experimental data, demonstrating that GCMC simulations can predict heats of adsorption for conditions that are challenging for experimental measurements. Additional insights into the adsorption mechanisms, derived from energetic analysis at the molecular level, are also presented.

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 ...

The isotropic (n-6) Lennard-Jones temperature-dependent potential (LJTDP) together with the Hohm−Zarkova−Damyanova mixing rules is used to predict second interaction pVT virial coefficients BAB(T),...

A methodology recently introduced to describe orientational order in liquid carbon tetrachloride is extended to the plastic crystal phase of XY4 molecules. The notion that liquid and plastic crystal phases are germane regarding orientational order is confirmed for short intermolecular distances but is seen to fail beyond, as long range orientational correlations are found for the simulated solid phase. It is argued that, if real, such a phenomenon may not to be accessible with direct (diffraction) methods due to the high molecular symmetry. This behavior is linked to the existence of preferential orientation with respect to the fcc crystalline network defined by the centers of mass. It is found that the dominant class accounts, at most, for one-third of all configurations, with a feeble dependence on temperature. Finally, the issue of rotational relaxation is also addressed, with an excellent agreement with experimental measures. It is shown that relaxation is nonhomogeneous in the picosecond range, with a slight dispersion of decay times depending on the initial orientational class. The results reported mainly correspond to neopentane over a wide temperature range, although results for carbon tetrachloride are included, as well.

Several different synthetic approaches to a total of 13 novel B, Al, and Sm complexes derived from the tridentate tripodal triamine ligand [N3]H3 with a neopentane, trisilylmethane, or trisilylsilane backbone and different N-substituents, as well as applications of the selected complexes to polymerization catalysis, are reported. Salt metathesis between HC[SiMe2N(CH2Ph)]3Li3(THF)2 (THF = tetrahydrofuran) and AlCl3 in Et2O/hexanes leads to complete elimination of LiCl and formation of the corresponding tripodal triamido alane HC[SiMe2N(CH2Ph)]3Al·(THF) (1). On the other hand, the reaction of {MeC[CHN(SiMe3)]3Li3}2 and AlCl3 in Et2O/hexanes yields a LiCl-containing compound MeC[CH2N(SiMe3)]3AlCl[Li(Et2O)] (2). Alkane elimination involving [N3]H3 and 1 AlMe3 produces diamido−amino aluminum methyl HC[SiMe2NHAr][SiMe2NAr]2AlMe [Ar = 4-MeC6H4 (3), CH2Ph (4)], while the reaction using ≥2 AlMe3 gives amido−amino aluminum dimethyl [ArHNMe2Si](H)C[SiMe2NAr]2(AlMe2)2 (Ar = 4-MeC6H4, 5) and [(Me3Si)HNCH2](Me)C[CH2N(S...

The neopentylidene-neopentyl complex (PNP)Ti=CH(t)Bu(CH2(t)Bu) (2; PNP(-) = N[2-P(CHMe2)(2-)4-methylphenyl]2), prepared from the precursor (PNP)Ti[triple bond]CH(t)Bu(OTf) (1) and LiCH2(t)Bu, extrudes neopentane in neat benzene under mild conditions (25 degrees C) to generate the transient titanium alkylidyne, (PNP)Ti[triple bond]C(t)Bu (A), which subsequently undergoes 1,2-CH bond addition of benzene across the Ti[triple bond]C linkage to generate (PNP)Ti=CH(t)Bu(C6H5) (3). Kinetic, mechanistic, and theoretical studies suggest the C-H activation process to obey pseudo-first-order in titanium, the alpha-hydrogen abstraction to be the rate-determining step (KIE for 2/2-d(3) conversion to 3/3-d(3) = 3.9(5) at 40 degrees C) with activation parameters DeltaH = 24(7) kcal/mol and DeltaS = -2(3) cal/mol.K, and the post-rate-determining step to be C-H bond activation of benzene (primary KIE = 1.03(7) at 25 degrees C for the intermolecular C-H activation reaction in C6H6 vs C6D6). A KIE of 1.33(3) at 25 degrees C arose when the intramolecular C-H activation reaction was monitored with 1,3,5-C6H3D3. For the activation of aromatic C-H bonds, however, the formation of the sigma-complex becomes rate-determining via a hypothetical intermediate (PNP)Ti[triple bond]C(t)Bu(C6H5), and C-H bond rupture is promoted in a heterolytic fashion by applying standard Lewis acid/base chemistry. Thermolysis of 3 in C6D6 at 95 degrees C over 48 h generates 3-d(6), thereby implying that 3 can slowly equilibrate with A under elevated temperatures with k = 1.2(2) x 10-5 s(-1), and with activation parameters DeltaH = 31(16) kcal/mol and DeltaS = 3(9) cal/mol x K. At 95 degrees C for one week, the EIE for the 2 --3 reaction in 1,3,5-C6H3D3 was found to be 1.36(7). When 1 is alkylated with LiCH2SiMe3 and KCH2Ph, the complexes (PNP)Ti=CHtBu(CH2SiMe3) (4) and (PNP)Ti=CHtBu(CH2Ph) (6) are formed, respectively, along with their corresponding tautomers (PNP)Ti=CHSiMe3(CH2tBu) (5) and (PNP)Ti=CHPh(CH2tBu) (7). By means of similar alkylations of (PNP)Ti=CHSiMe3(OTf) (8), the degenerate complex (PNP)Ti=CHSiMe3(CH2SiMe3) (9) or the non-degenerate alkylidene-alkyl complex (PNP)Ti=CHPh(CH2SiMe3) (11) can also be obtained, the latter of which results from a tautomerization process. Compounds 4/5 and 9, or 6/7 and 11, also activate benzene to afford (PNP)Ti=CHR(C6H5) (R = SiMe3 (10), Ph (12)). Substrates such as FC6H5, 1,2-F2C6H4, and 1,4-F2C6H4 react at the aryl C-H bond with intermediate A, in some cases regioselectively, to form the neopentylidene-aryl derivatives (PNP)Ti=CHtBu(aryl). Intermediate A can also perform stepwise alkylidene-alkyl metatheses with 1,3,5-Me3C6H3, SiMe4, 1,2-bis(trimethylsilyl)alkyne, and bis(trimethylsilyl)ether to afford the titanium alkylidene-alkyls (PNP)Ti=CHR(R') (R = 3,5-Me2C6H2, R' = CH2-3,5-Me2C6H2; R = SiMe3, R' = CH2SiMe3; R = SiMe2CCSiMe3, R' = CH2SiMe2CCSiMe3; R = SiMe2OSiMe3, R' = CH2SiMe2OSiMe3).

The hydrogen/deuterium exchange reaction of 2,2-dimethylpropane (neopentane) over D(2)O-exchanged zeolites (MOR, FAU, BEA, MFI) using a batch recirculation reactor was studied by means of gas chromatography coupled with mass spectrometer. In the temperature range 473-573 K, H/D exchange proceeds without side reaction such as cracking at short contact times. Indeed the C-H bond has appeared favorably involved in the activation of neopentane compared to the less accessible C-C bond. The transition state allowing hydron exchange is most likely a carbonium species (pentacoordinated carbon) as in the case of the H/D exchange between methane and solid acid. The activation energies of the H/D exchange between neopentane and zeolites are the same for all zeolites indicating a common carbonium ion type transition state. On the basis of previous results in the case of the exchange between methane and liquid superacids, the deuterium exchange rates in neopentane were tentatively related to the acidity of the solids. However the order of activity MORMFIBEAFAU seems to be related to the size of the pores, which may suggest the involvement of a confinement effect in the zeolites cavities. Moreover we found that H/D exchange takes also place between neopentane and deuterated sulfated zirconia (SZ) emphasizing its strong acidity.

Gentle thermolysis of the bis(neopentyl) complex Cp*W(NO)(CH 2 CMe 3 ) 2 (1) at 70 °C in various substituted benzenes results in the loss of neopentane and the generation of the transient alkylidene complex Cp*W(NO)(=CHCMe 3 ) (A), which subsequently effects single C-H bond activations of the benzenes. These activations exhibit a pronounced selectivity for the C-H linkages ortho to the benzene substituents. Thus, thermal reactions of 1 with C 6 H 5 X lead to the preferential formation of the corresponding Cp*W-(NO)(CH2CMe3)(o-C6H4X) complexes, the ortho-selectivity, i.e., X = F > OMe > Cl > Br > C≡CPh, diminishing as the steric demands of the substituents increase. Consistently, thermolyses of 1 in o-C 6 H 4 X 2 afford Cp·W(NO)(CH2CMe3)(2,3-C6H3X2) complexes with 98% selectivity when X = F and 82% selectivity when X = Cl. Similarly, the principal organometallic products resulting from the thermolyses of 1 in m-C 6 H 4 X 2 are the ortho-para-activated isomers Cp*W(NO)(CH 2 CMe 3 )(2,4-C 6 H 3 X 2 ) with 84% selectivity when X = F and 89% selectivity when X = Cl. Finally, thermolyses of 1 in para-disubstituted benzenes p-C 6 H 4 XY (X = F or OMe, Y = Cl or OMe) again reveal that the ortho-directing abilities of the substituents diminish in the order F > OMe > Cl. Some mechanistic insights into these activation processes have been obtained by monitoring of the early stages of the thermolysis of 1 in chlorobenzene by 1 H NMR spectroscopy. This monitoring reveals that the selectivity of the alkylidene intermediate A for forming the ortho-activated isomer is thermodynamic rather than kinetic in nature. Thus, the meta-and para-activated isomers are formed initially, but then convert to the ortho-activated product. The principal organometallic complexes resulting from ortho-C-H activations are formally 16-electron species in which there are no interactions between the tungsten centers and the Lewis-basic benzene substituents. Instead, the Lewis-acidic tungsten centers engage in intramolecular agostic interactions with the methylene C-H bonds of their neopentyl ligands to acquire some additional electron density. Being coordinatively and electronically unsaturated, these tungsten complexes react readily with added Lewis bases. Thus, treatment of Cp*W(NO)(CH 2 CMe 3 )(o-C 6 H 4 F) with Et 4 NCN and CO produces [Et 4 N] + [Cp*W(NO)(CH 2 -CMe 3 )(CN)(o-C 6 H 4 F)] - and Cp*W(NO)(η 2 -C(=O)CH 2 CMe 3 )(o-C 6 H 4 F), respectively, and the solid-state molecular structure of the latter complex has also been established by a single-crystal X-ray crystallographic analysis.

The reaction of Hf(CH2tBu)4 with a silica surface treated at 800 °C affords a unique surface organometallic species in only one surface environment, (⋮SiO)Hf(CH2tBu)3 (1-SiO2-(800)). In contrast, with SiO2-(500) two surface species, (⋮SiO)Hf(CH2tBu)3 (1-SiO2-(500)) and (⋮SiO)2Hf(CH2tBu)2 (2−SiO2-(500)), in a molar ratio of 70:30 are obtained. Thermal treatment of 1-SiO2-(800) at increasing temperatures leads to the successive evolution of neopentane, isobutene, and isobutane as well as several alkanes varying from C1 to C5. Polyisobutenes are also formed on the surface. The mechanism by which such decomposition occurs suggests a succession of γ-H eliminations with formation of neopentane followed by β-methyl transfer and formation of isobutene and [Hf]−Me. This isobutene is reinserted into [Hf]−Me with formation of isopentene and [Hf]−H. A comparison of the analytical data of 1-SiO2-(800) and (⋮SiO)Zr(CH2tBu)3 indicated the hafnium complex exhibits in EXAFS shorter Hf−C and Hf−O bonds and a larger Hf−Cα−C...

In a preceding project, functional forms for “short” Helmholtz energy equations of state for typical nonpolar and weakly polar fluids and for typical polar fluids were developed using simultaneous optimization. In this work, the coefficients of these short forms for the equations of state have been fitted for the fluids acetone, carbon monoxide, carbonyl sulfide, decane, hydrogen sulfide, 2-methylbutane (isopentane), 2,2-dimethylpropane (neopentane), 2-methylpentane (isohexane), krypton, nitrous oxide, nonane, sulfur dioxide, toluene, xenon, hexafluoroethane (R-116), 1,1-dichloro-1-fluoroethane (R-141b), 1-chloro-1,1-difluoroethane (R-142b), octafluoropropane (R-218), 1,1,1,3,3-pentafluoropropane (R-245fa), and fluoromethane (R-41). The 12 coefficients of the equations of state were fitted to substance specific data sets. The results show that simultaneously optimized functional forms can be applied to other fluids out of the same class of fluids for which they were optimized without significant loss of a...

Neopentane and TMS are used as model M(CH3)4 systems to investigate intramolecular interactions. The nonbonded site−site potential between two proximal hydrogen atoms on different methyl groups, Vnb(dHH), is not Lennard−Jones- or Morse-like but is found to be pseudolinear in hydrogen−hydrogen internuclear separation, dHH, for both neopentane and TMS. The Morse potential is found to be a poor basis in which to expand Vnb(dHH). The nonbonded site−site potential is conformation-dependent and not transferable between molecules. The individual contributions to Vnb(dHH) are presented. The local mode parameters for neopentane and TMS are calculated ab initio for a variety of molecular conformations. The ab initio values of the local mode frequency and local mode anharmonicity are increasingly blue-shifted with increasing steric hindrance. Electron correlation is found to be increasingly important with decreasing internuclear separations, dHH.

Mechanisms for H-transfer from 1,1-dimethylallyl (DMA) to neopentyl (CH 2 t-Bu) in the complex Cp*W(NO)(CH 2 t-Bu)(η 3 -DMA) are examined by B3LYP-DFT calculations. Either the mid-H or the methyl H on the DMA can transfer tothe W-bound neopentyl to generate neopentane (2,2-dimethylpropane) and Cp*W(NO)(η 2 -1,1-dimethylallene) or Cp*W(NO)(η 2 -2-methylbuta-1,3-diene) intermediates, respectively. The open coordination site created by the neopentane loss is the site of the subsequent C-H bond activation of alkanes and arenes. The similarities in the overall enthalpic barriers for these mechanisms, 23-24 kcal/mol, explain the observed deuterium distribution (65% on the mid-carbon and 35% on the methyls of the DMA) when benzene-d 6 reacts as substrate.

Kinetic acidities are reported for methane, ethane, propane, cyclopropane, isobutane, neopentane, tetramethylbutane, norbornane, nortricyclene, and adamantane by tritiodeprotonation or deuteriodeprotonation in cyclohexylamine catalyzed by cesium cyclohexylamide.

We have acquired high-resolution carbon 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of methane, ethane, propane, isobutane, and neopentane. These experimental measurements are complemented by high-quality ab initio calculations, performed with the improved virtual orbital approximation. The degree and character of Rydberg-valence mixing in the preedge of the NEXAFS spectra of these species is explored. Significant Rydberg-valence mixing only occurs when there are excited states of valence sigma(C-H) character that have the appropriate symmetry to interact with excited states of Rydberg character. Our results show that this mixing is only present when there are C-H bonds to the core excited carbon atom.

Rate coefficients for the reactions of ethane, propane, n-butane, isobutane, and neopentane with Ta atoms and Ta clusters consisting of up to 30 Ta atoms have been measured in a fast-flow reactor. A near-monotonic dependency of the rate coefficients on cluster size is observed for each of these hydrocarbons, with the exception of Ta 1 1 , which exhibits rate coefficients that are several times that observed for the analogous reaction with Ta 1 0 and Ta 1 2 , for propane, n-butane, and isobutane. Modeling the general trend in the rate coefficients using transition state theory is found to require a relatively loose transition state with free rotation of the hydrocarbon about the bonding point on the cluster, consistent with the rate-determining step being cluster removal by an association reaction process. It is demonstrated that, in general, metal cluster reactions with reagents that can form strongly bound association complexes may exhibit little evidence of cluster-size-based selectivity in the rate of cluster removal, despite exhibiting significant selectivity in terms of the extent of the dehydrogenation reaction involved.

This is the first of two papers aimed at understanding, from an atomistic perspective, hydrophobicity at different length scales and how properties such as local densities and angular profiles change for hydrophobic solutes of different sizes. In a subsequent publication we will describe the hydrophobic hydration and hydrophobic interaction of platelike molecules of nanoscale size. Molecular dynamics is used to compute radial and orientational distribution functions of water around three different molecules: argon, methane, and neopentane. In addition, the potential of mean force between two neopentane molecules is computed. The results for the full OPLS/AA1 force field are compared with the solute−solvent WCA truncated OPLS/AA force field for these systems. This work addresses the question of whether a molecule of the size of neopentane is large enough to induce a hydrophobic response similar to that of large hydrophobic molecules or paraffin walls. We answer this question in the affirmative. The orient...

Applying the G3 and G3(MP2) models and using both the isodesmic and atomization schemes, the heats of formation (ΔHf) at 0 and 298 K are calculated for mono-tert-butylmethane (2,2-dimethylpropane or neopentane, abbreviated as mono-TBM), di-tert-butylmethane (di-TBM), tri-tert-butylmethane (tri-TBM), and tetra-tert-butylmethane (tetra-TBM). Upon examining the results, it is found that all of the calculated ΔHf298 values are well within ±10 kJ mol-1 of the available experimental data for the first three compounds. Hence, for tetra-TBM, a compound that has not yet been synthesized, the G3(MP2) results reported in this work should be reliable estimates. Moreover, we have found that the atomization scheme is slightly more suitable for the study of the smaller molecules, while the isodesmic scheme is more suitable for the larger molecules. Structurally, it is found that the equilibrium structures of mono-TBM, di-TBM, tri-TBM, and tetra-TBM have Td, C2, C1, and T symmetry, respectively. The energy-minimized stru...

The reductive elimination of neopentane from (dtbpm)Pt(Np)H (dtbpm = bis(di-tert-butylphosphino)methane, Np = neopentyl) results in the formation of the highly reactive intermediate [(dtbpm)Pt0]. This species reacts with a variety of electron-deficient benzenes (1,4-bis(trifluoromethyl)benzene and 1,3,5-tris(trifluoromethyl)benzene) and biphenyls (3,5,4‘-tris(trifluoromethyl)biphenyl and 3,5,3‘,5‘-tetrakis(trifluoromethyl)biphenyl) to form η2-arene complexes. X-ray crystallography confirms the η2-bonding motif. The arene complexes all display broad 1H NMR spectroscopic features at room temperature that sharpen considerably at lower temperatures. Thermolysis or photolysis of the arene complexes results in C−H activation and the formation of platinum(II) aryl hydride complexes. The products observed depend on whether the reaction is thermal or photolytic. Reaction with 4,4‘-bis(trifluoromethyl)biphenyl yields two C−H activated products directly. Oxidative addition occurs at the 2 and 3 positions of the biph...

A comparison of the CH vibrational overtone spectra of vapor phase neopentane-d0 (C(CH3)4), -d6 (C(CH3)2(CD3)2), and -d9 (C(CH3)(CD3)3) and tetramethylsilane (TMS) in the frequency range ΔvCH = 4−8 (10 800−18 200 cm-1) has revealed pronounced differences between the spectra of TMS and the neopentanes, and subtle differences among the spectra of the neopentanes. These spectral differences are interpreted as a manifestation of geometry and vibrational frequency dependent differences in coupling efficiencies that facilitate the de-excitation of local modes of vibration via IVR. Fermi resonance plays a key role in this coupling. Some of the states are perturbed by through space coupling (a collision-like van der Waals interaction) that can facilitate IVR. The normal modes of vibration, which are implicated in Fermi resonance of the neopentanes and TMS, have been calculated ab initio using density functional theory and are shown to be affected by through space interactions.

The molecular adsorption dynamics of methane, propane, and neopentane on Pd(111) was studied using supersonic molecular beam techniques and stochastic trajectory simulations. The sticking probability of the alkanes was measured as a function of incident energy, incident angle, and alkane coverage. In each case, the trapping probability decreased with increasing incident translational energy, which is expected for nonactivated molecular trapping. Nonnormal energy scaling was observed for the trapping probability of all of these alkanes, indicating corrugation of the gas−surface interaction potentials. The modified Kisliuk model was employed to describe the self-coverage dependence of the alkane on Pd(111). The trapping probability of each of these alkanes on Pd(111) is higher than that on Pt(111), consistent with the lower mass of palladium. Using the Morse potential for the methyl (methylene)−platinum two body potential obtained from the stochastic trajectory analyses of alkanes trapping on Pt(111), the t...

The trimorphism of (CH3)3CCl, a member of the (CH3)4-nCCln series, is reexamined as a function of the temperature and pressure. Its topological p−T diagram is constructed resorting to the available thermodynamic and crystallographic data for the four observed crystalline solids and the data extrapolated from binary phase diagrams sharing compounds of this series. The p,T coordinates of the five stable triple points are calculated together with the slopes of the two-phase equilibrium curves. Through these results, the uncharacterized phase IV (a high-pressure orientationally disordered stable phase) is shown to coincide with the rhombohedral phase (R) which is the orientationally disordered stable phase for the members with n = 2, 3, and 4 at ordinary pressure. In addition, the existence of a isomorphous high-pressure R phase is postulated for the first member [neopentane, (CH3)4C] of the series.

A 1:1 mixture of In(CH2CMe3)3 and HP(i-Pr)2 at room temperature undergoes a very slow hydrocarbon elimination reaction to form [(Me3CCH2)2InP(i-Pr)2]2 and neopentane. The indium phosphide [(Me3CCH2)2InP(i-Pr)2]2 has also been prepared by reacting KIn(CH2CMe3)3H with ClP(i-Pr)2 in pentane. When pentane solutions of these reagents were combined at −78 °C and then maintained at ∼20 °C for 2 days, [(Me3CCH2)2InP(i-Pr)2]2, (Me3CCH2)3In·P(H)(i-Pr)2, (Me3CCH2)3In·P2(i-Pr)4, (Me3CCH2)3In·P(CH2CMe3)(i-Pr)2, H2, indium metal, and KCl were observed. The formation of all products is explained by a set of experimentally verified reactions. An X-ray structural study of [(Me3CCH2)2InP(i-Pr)2]2 shows each molecule to have an unusual puckered four-membered In2P2 core.

The (19)F NMR shielding for the alkyl fluorides from methyl fluoride to tert-butyl fluoride has been calculated using IGAIM and has been separated into the contribution from each of the molecular orbitals. The relatively large change in fluorine shielding, in contrast to the adjacent carbon, was found to be due to the tensor components normal to the C-F bond axis. As the number of adjacent p-orbitals increases, the lone-pair p orbitals at fluorine become involved with MOs using these orbitals. The increase in the number of occupied orbitals associated with the fluorine leads to increased opportunities for mixing with virtual orbitals and to the increase in paramagnetic deshielding. The same pattern is seen on going from acetylene to 2-tert-butylacetylene and is also seen in the methyl (13)C shielding in the series ethane, propane, isobutane, and neopentane. The fluorine shielding in the series of fluoromethanes also decreases with increasing fluorine substitution. With carbon tetrafluoride, the decreased shielding arises from the highest occupied MO, which is a nonbonding linear combination of pure p functions at the fluorines.

The effect of six newly discovered heavy hydrate-forming compounds on the hydrate phase boundary of a gas mixture, a natural gas, and a model oil has been measured. The study, which is a continuation of a previous work,1 investigates the effect of cyclopentane, cyclohexane, neopentane, isopentane, methylcyclopentane, and methylcyclohexane at various concentrations. The first three compounds form structure-II hydrates, while the last three compounds are known to promote structure-H hydrates. The experimental hydrate dissociation data have been successfully predicted by the use of a thermodynamic model. The results show that structure-II is the stable hydrate structure for the systems investigated at various heavy hydrate former concentrations (from 0.27 mol % to 59.66 mol %). Also, the structure-II heavy hydrate formers promoted structure-II hydrate formation whereas the structure-H heavy hydrate formers (at the concentrations used) did not change the stable hydrate structure to structure-H and hence inhib...

Local structures and motions of radical reactants in highly selective tunneling abstraction reaction in neopentane-ethane mixture are investigated with CW-ESR, ENDOR, and ESEEM spectroscopy at diff ...

Solubility measurements proved that, at 25 °C, methane and ethane are more soluble in water than in 7 M aqueous urea, whereas propane, i-butane, n-butane, and neopentane are more soluble in 7 M aqueous urea than in water. No convincing explanation of these experimental data has been provided up to now. An extension of an emerging theory of hydrophobic hydration is devised to account for the solubility of aliphatic hydrocarbons in 7 M aqueous urea. The conclusions reached are: (a) the work of cavity creation is always greater in 7 M aqueous urea than in water, contrasting the transfer; (b) the solute−solvent van der Waals interaction energy is always greater in magnitude in 7 M aqueous urea than in water, favoring the transfer. The latter contribution increases in magnitude with hydrocarbon size more rapidly than the difference in the work of cavity creation, explaining the existence of a threshold size for the solubility enhancement. The reorganization of H-bonds in both the solvent systems is a compensa...

Reaction of the dimeric Ni(I) chloride complex [(dtbpe)NiCl](2) (1) with dimesitylsilyl potassium affords the three-coordinate Ni(I) silyl complex (dtbpe)Ni(SiHMes(2)) (2). Alternatively, 2 can be prepared by an oxidative-addition reaction of Mes(2)Si(H)OTf (Tf = CF(3)SO(3)) with the nickel(0) complex [(dtbpe)Ni](2)(mu-C(6)H(6)) (3), with (dtbpe)Ni(OTf) (4) formed as an easily separable byproduct. The one-electron oxidation of 2 by ferrocenium affords diamagnetic [(dtbpe)Ni(mu-H)SiMes(2)][BAr(F)(4)] (5), a Ni(II) complex formed by partial 1,2-H migration from silicon to nickel and featuring an unusual 3-center, 2-electron bonding motif between Ni, Si, and the bridging H. Complex 5 was also obtained from Mes(2)SiH(2) activation by the neopentyl complex salt [(dtbpe)Ni(CH(2)CMe(3))][BAr(F)(4)] (6) with elimination of neopentane.

The experimental systems considered in this paper are isobaric binary plastic-crystalline mixed crystals of neopentane derivatives, and their properties studied are (i) solid−liquid phase behavior and (ii) the thermodynamic mixing properties, actually the excess enthalpy, entropy, and volume. More in particular, 10 systems are considered, nine being face-centered cubic and one body-centered cubic, and their components are derived from neopentane, C(CH3)4, by substituting one or more −CH3 groups by one or more −CH2OH groups and/or one −NO2 or −NH2 groups. The (mean) number of −CH2OH groups per molecule appears to have a dominating influence with regard to melting temperature and degree of compactness. The values of the excess properties vary from system to system, whereas their quotients are system-independent. For the group of fcc systems, the quotient of excess enthalpy and excess entropy (compensation temperature) has the uniform value of 630 K, and the quotient of excess enthalpy and excess volume the ...

The decomposition of neopentyl thiol on clean and oxygen-modified Fe(100) surfaces has been investigated under ultrahigh vacuum conditions using temperature-programmed reaction spectroscopy, Auger electron spectroscopy, and high-resolution electron energy loss spectroscopy. On the clean Fe(100) surface upon adsorption at 100 K, the S−H and C−S bonds of the neopentyl thiol cleave, resulting in adsorbed S, H, and neopentyl alkyl fragments, in contrast to the formation of the alkyl thiolate observed on the adsorption of n-alkane thiols on this surface. Upon heating, some neopentyl alkyl fragments react with adsorbed surface hydrogen to evolve neopentane at 260 K. On the oxygen-modified surface, only S−H bond cleavage is observed, and a neopentyl thiolate surface species is formed. No initial neopentane evolution from the oxygen-modified surface is observed upon heating. However, on both surfaces, isobutene and trace amounts of neopentane are seen to desorb at 380 K as a result of an elimination reaction path...

Our methodology for producing a spectroscopically determined force field (SDFF) (i.e., a molecular mechanics energy function) that, in addition to structures and relative energies, reproduces vibrational frequencies to spectroscopic standards, has been extended from its previous implementation on the linear to include branched saturated hydrocarbon chains. To the ab initio force fields of the 14 stable conformers of n-pentane and n-hexane, we have now added those of the 7 stable conformers of isopentane, 3-methylpentane, and neopentane, plus specific force constants from a secondary set of branched molecules, to optimize the parameters of the SDFF. This SDFF reproduces 791 ab initio non-CH stretch frequencies with a root-mean-square deviation of 6.2 cm-1. When applied to other molecules not in the optimization set, viz., cyclobutane, cyclohexane, isobutane, tri-tert-butylmethane, and tetra-tert-butylmethane, not only are ab initio as well as experimental geometries and frequencies well reproduced, but the...

When H atoms are produced by the photolysis of HI or the radiolysis of neo-C5H12 in the neo-C5H12−i-C4H10(2 mol %) mixtures above 40 K, the H atoms react selectively with the solute i-C4H10. The selective reaction is suppressed below 30 K. The reactions have been studied by electron spin resonance and electron spin echo measurements at 4−100 K. The drastic temperature effect on the reaction by H atoms suggests that the H atoms concerned are not hot atoms, but thermal atoms. The thermal H atoms react with the solute alkane by quantum tunneling at low temperature. Temperature effects on the ESR spectra and the spin−lattice relaxation times of the solvent neo-C5H11 radicals and the solute t-C4H9 radicals indicate that local motions of the solute i-C4H10 molecules take place above 40 K, while motions of the solvent neo-C5H12 molecules are severely restricted even at 100 K. It is concluded that the local motions of the solute i-C4H10 assist the tunneling abstraction reaction by H atoms, resulting in the select...

We have studied by volumetric adsorption isotherm measurements the sorption properties of different AlPO4-5 or SAPO-5 samples, for sorbate species characterized by a large range of molecular size: D2, Kr, Ar, C2D2, CO2, CH4, CF4, and C(CH3)4. The highest sorption capacities, Qmax, greatly depend on the sorbate molecular size. A difference of 1 order of magnitude is observed between hydrogen and neopentane. In addition, we have found that the sorption capacities of the four analyzed AlPO4-5 or SAPO-5 samples vary by almost a factor of 2. This result clearly shows that, as a consequence of the one-dimensional character of the AFI micropore network, Qmax is very sensitive to small amounts of crystalline defects or impurities. For methane, Qmax is equal to 6 mol/uc, a quantity that is not compatible with the commonly accepted value of the internal diameter of the AlPO4-5 micropore diameter, ⌀M = 7.3 A. Moreover, an interesting phase transition phenomenon has been observed during the sorption of krypton and me...

The most successful empirical method for predicting hydrate-phase equilibrium in water−hydrocarbon system is based upon vapor−solid equilibrium ratio (Kvs values) correlations. Equilibrium ratio charts for known hydrate formers, with n-butane as the heaviest compound, are available in the literature. With the discovery of heavier hydrate-forming compounds, their equilibrium ratio charts are required for reliable prediction of the hydrate-free zone of hydrocarbon mixtures, particularly reservoir fluids. In this work, an in-house thermodynamic model, validated against experimental hydrate equilibrium data of four structure-II heavy hydrate formers (HHF), benzene, cyclopentane, cyclohexane, and neopentane, has been used for generating their Kvs value charts. The resulting Kvs values have been used in predicting a hydrate-free zone of binary and ternary mixtures of HHF with methane and/or nitrogen, demonstrating success when compared with experimental data.