Potential Energy Hypersurface Research Articles

Directional Noncovalent Interactions: Repulsion and Dispersion.

The interaction energies between an argon atom and the dihalogens Br2, BrCl, and BrF have been investigated using frozen core CCSD(T)(fc)/aug-cc-pVQZ calculations as reference values for other levels of theory. The potential-energy hypersurfaces show two types of minima: (1) collinear with the dihalogen bond and (2) in a bridging position. The former represent the most stable minima for these systems, and their binding energies decrease in the order Br > Cl > F. Isotropic atom-atom potentials cannot reproduce this binding pattern. Of the other levels of theory, CCSD(T)(fc)/aug-cc-pVTZ reproduces the reference data very well, as does MP2(fc)/aug-cc-pVDZ, which performs better than MP2 with the larger basis sets (aug-cc-pVQZ and aug-cc-pvTZ). B3LYP-D3 and M06-2X reproduce the binding patterns moderately well despite the former using an isotropic dispersion potential correction. B3LYP-D3(bj) performs even better. The success of the B3LYP-D3 methods is because polar flattening of the halogens allows the argon atom to approach more closely in the direction collinear with the bond, so that the sum of dispersion potential and repulsion is still negative at shorter distances than normally possible and the minimum is deeper at the van der Waals distance. Core polarization functions in the basis set and including the core orbitals in the CCSD(T)(full) calculations lead to a uniform decrease of approximately 20% in the magnitudes of the calculated interaction energies. The EXXRPA+@EXX (exact exchange random phase approximation) orbital-dependent density functional also gives interaction energies that correlate well with the highest level of theory but are approximately 10% low. The newly developed EXXRPA+@dRPA functional represents a systematic improvement on EXXRPA+@EXX.

Experimental and Theoretical Study of Multi-Quantum Vibrational Excitation: NO(v = 0→1,2,3) in Collisions with Au(111)

We measured absolute probabilities for vibrational excitation of NO(v = 0) molecules in collisions with a Au(111) surface at an incidence energy of translation of 0.4 eV and surface temperatures between 300 and 1100 K. In addition to previously reported excitation to v = 1 and v = 2, we observed excitation to v = 3. The excitation probabilities exhibit an Arrhenius dependence on surface temperature, indicating that the dominant excitation mechanism is nonadiabatic coupling to electron-hole pairs. The experimental data are analyzed in terms of a recently introduced kinetic model, which was extended to include four vibrational states. We describe a subpopulation decomposition of the kinetic model, which allows us to examine vibrational population transfer pathways. The analysis indicates that sequential pathways (v = 0 → 1 → 2 and v = 0 → 1 → 2 → 3) alone cannot adequately describe production of v = 2 or 3. In addition, we performed first-principles molecular dynamics calculations that incorporate electronically nonadiabatic dynamics via an independent electron surface hopping (IESH) algorithm, which requires as input an ab initio potential energy hypersurface (PES) and nonadiabatic coupling matrix elements, both obtained from density functional theory (DFT). While the IESH-based simulations reproduce the v = 1 data well, they slightly underestimate the excitation probabilities for v = 2, and they significantly underestimate those for v = 3. Furthermore, this implementation of IESH appears to overestimate the importance of sequential energy transfer pathways. We make several suggestions concerning ways to improve this IESH-based model.

Temporal disconnectivity of the energy landscape in glassy systems

An alternative graphical representation of the potential energy landscape (PEL) has been developed and applied to a binary Lennard-Jones glassy system, providing insight into the unique topology of the system's potential energy hypersurface. With the help of this representation one is able to monitor the different explored basins of the PEL, as well as how--and mainly when--subsets of basins communicate with each other via transitions in such a way that details of the prior temporal history have been erased, i.e., local equilibration between the basins in each subset has been achieved. In this way, apart from detailed information about the structure of the PEL, the system's temporal evolution on the PEL is described. In order to gather all necessary information about the identities of two or more basins that are connected with each other, we consider two different approaches. The first one is based on consideration of the time needed for two basins to mutually equilibrate their populations according to the transition rate between them, in the absence of any effect induced by the rest of the landscape. The second approach is based on an analytical solution of the master equation that explicitly takes into account the entire explored landscape. It is shown that both approaches lead to the same result concerning the topology of the PEL and dynamical evolution on it. Moreover, a "temporal disconnectivity graph" is introduced to represent a lumped system stemming from the initial one. The lumped system is obtained via a specially designed algorithm [N. Lempesis, D. G. Tsalikis, G. C. Boulougouris, and D. N. Theodorou, J. Chem. Phys. 135, 204507 (2011)]. The temporal disconnectivity graph provides useful information about both the lumped and the initial systems, including the definition of "metabasins" as collections of basins that communicate with each other via transitions that are fast relative to the observation time. Finally, the two examined approaches are compared to an "on the fly" molecular dynamics-based algorithm [D. G. Tsalikis, N. Lempesis, G. C. Boulougouris, and D. N. Theodorou, J. Chem. Theory Comput. 6, 1307 (2010)].

Electron wavepacket dynamics in highly quasi-degenerate coupled electronic states: A theory for chemistry where the notion of adiabatic potential energy surface loses the sense

We develop a theory and the method of its application for chemical dynamics in systems, in which the adiabatic potential energy hyper-surfaces (PES) are densely quasi-degenerate to each other in a wide range of molecular geometry. Such adiabatic electronic states tend to couple each other through strong nonadiabatic interactions. Technically, therefore, it is often extremely hard to accurately single out the individual PES in those systems. Moreover, due to the mutual nonadiabatic couplings that may spread wide in space and due to the energy-time uncertainty relation, the notion of the isolated and well-defined potential energy surface should lose the sense. On the other hand, such dense electronic states should offer a very interesting molecular field in which chemical reactions to proceed in characteristic manners. However, to treat these systems, the standard theoretical framework of chemical reaction dynamics, which starts from the Born-Oppenheimer approximation and ends up with quantum nuclear wavepacket dynamics, is not very useful. We here explore this problem with our developed nonadiabatic electron wavepacket theory, which we call the phase-space averaging and natural branching (PSANB) method [T. Yonehara and K. Takatsuka, J. Chem. Phys. 129, 134109 (2008)], or branching-path representation, in which the packets are propagated in time along the non-Born-Oppenheimer branching paths. In this paper, after outlining the basic theory, we examine using a one-dimensional model how well the PSANB method works with such densely quasi-degenerate nonadiabatic systems. To do so, we compare the performance of PSANB with the full quantum mechanical results and those given by the fewest switches surface hopping (FSSH) method, which is known to be one of the most reliable and flexible methods to date. It turns out that the PSANB electron wavepacket approach actually yields very good results with far fewer initial sampling paths. Then we apply the electron wavepacket dynamics in path-branching representation and the so-called semiclassical Ehrenfest theory to a hydrogen molecule embedded in twelve membered boron cluster (B(12)) in excited states, which are densely quasi-degenerate due to the vacancy in 2p orbitals of boron atom [1s(2)2s(2)2p(1)]. Bond dissociation of the hydrogen molecule quickly takes place in the cluster and the resultant hydrogen atoms are squeezed out to the surface of the cluster. We further study collision dynamics between H(2) and B(12), which also gives interesting phenomena. The present study suggests an interesting functionality of the boron clusters.

Isomers of C46 fullerene with carbyne chains

The dynamics of the processes of isomerization and decomposition of C46 fullerene has been studied by computer simulation in the real-time regime. At an intermediate stage of evolution of the cluster (before it loses the spheroidal shape), isomers in which groups of adjacent pentagons and hexagons of C-C bonds are connected to each other with two-, four-, and six-atomic carbyne chains have been revealed. An analysis of the potential energy hypersurface has demonstrated that the isomers are quite stable and some of them can be observed experimentally.

In search of the dark state of 5-methyl-2-hydroxypyrimidine using a numerical DFT/MRCI gradient

In a recent publication, Lobsiger et al. [Phys. Chem. Chem. Phys. 12, 5032 (2010)] presented infrared and electronic absorption spectra of supersonic jet-cooled 5-methyl-2-hydroxypyrimidine (5M2HP), the enol form of deoxythymine. In addition, they reported on the fast nonradiative decay of the S1 population to a dark state. In the present paper, we have investigated the mechanism and rate constants of this nonradiative decay by means of quantum chemical multi-configuration methods. To this end, minima of the lowest excited singlet and triplet states as well as the minimum-energy crossing point of singlet and triplet potential energy hypersurfaces (PEHs) have been determined employing a numerical DFT/MRCI gradient where DFT/MRCI stands for a combination of density functional theory (DFT) and a semi-empirical multi-reference configuration interaction (MRCI) approach. Rate constants have been calculated in the Condon approximation using a time-dependent approach based on harmonic oscillator functions and electronic spin–orbit coupling matrix elements evaluated at the DFT/MRCI level. It is shown that the first excited triplet state possesses 3(n → π*) character in the gas phase. Fast intersystem crossing is mediated by the low-lying 3(π → π*) state whose PEH crosses both, the S1 1(n → π*) and T1 3(n → π*) PEHs.

Electronic structures, stabilities and spectroscopic properties of Pb m Si n (m + n ≤ 6) clusters

Electronic structures of neutral lead–silicon clusters of up to six atoms are determined by global optimization of the potential energy hyper surface using Metropolis Monte Carlo and genetic algorithm (GA) methods followed by a bond strength propensity model, and by local optimization using density functional theory at the B3LYP/aug-cc-pVTZ-pp level. For each cluster, geometries of those stable isomers whose relative energies are within 1.0 eV are reported. Average binding energy per atom, band gap, ionization potential (IP), electron affinity (EA), and vertical detachment energy of the most stable isomer of each type of cluster are computed to study their dependences on the size of the cluster. Polarizability, chemical potential, hardness and electrophilicity index of the most stable isomers are estimated to study their relative stabilities and chemical reactivities. Relative strengths of metal–metal interactions are determined from a study of all one-step fragmentation processes. IR intensities and Raman activities for all the vibrational modes of the most stable isomers are calculated.

Towards Reference Viscosities of Carbon Monoxide and Nitrogen at Low Density Using Measurements between 290K and 680K as well as Theoretically Calculated Viscosities

An all-quartz oscillating-disk viscometer of very high precision was used to determine the temperature dependence of the viscosities of carbon monoxide and nitrogen at low densities. The measurements were based on a single calibration at room temperature with a value theoretically calculated on the basis of an accurate ab initio pair potential for helium and the kinetic theory of dilute monatomic gases. The uncertainty of the experimental data is conservatively estimated to be ±0.15% at room temperature increasing to ±0.20% at the highest temperature of 682 K. The new data are compared with values recommended by the National Institute of Standards and Technology in the framework of its Standard Reference Data Program REFPROP as well as with experimental data from the literature. Whereas the REFPROP values for nitrogen can be considered as reference values, the new experimental data for carbon monoxide are up to 2% higher than the REFPROP values and should be taken into consideration for a new correlation. The temperature dependence of the viscosities calculated theoretically using ab initio intermolecular potential energy hypersurfaces for carbon monoxide and nitrogen and the kinetic theory of dilute molecular gases should be used for extrapolating the viscosity correlations of both gases to low and high temperatures. In addition, the viscosity ratio of carbon monoxide to nitrogen is investigated with the purpose to establish an improved correlation for carbon monoxide.

Using Density Functional Theory To Interpret the Infrared Spectra of Flexible Cyclic Phosphazenes

The cyclic phosphazene trimer P(3)N(3)(OCH(2)CF(3))(6)and the related cyclic tetramer P(4)N(4)(OCH(2)CF(3))(8) have been synthesized, isolated and their vapor-phase absorption spectra recorded at moderate resolution using an FTIR spectrometer. The interpretation of these spectra is achieved primarily by comparison with the results of high-precision density functional calculations, which enable the principal absorption features to be assigned and conclusions to be drawn regarding the geometries and conformations adopted by both molecules. These in turn allow interesting comparisons to be made with analogous cyclic halo-phosphazenes (such as P(3)N(3)Cl(6)) and other related ring compounds. The highly flexible nature of the two cyclic phosphazenes precludes a complete theoretical study of their potential energy hypersurfaces and a novel alternative approach involving the analysis of a carefully selected subset of the possible molecular conformations has been shown to produce satisfactory results. The two cyclic phosphazene oligomers have been proposed as the major low-to-medium temperature pyrolysis products of the parent polyphosphazene (PN(OCH(2)CF(3))(2))(n), and the identification of vibrational absorption features characteristic of each molecule will enable future studies to test the validity of this proposition.

Effect of Roughness of Confining Surface on Diffusive Motion of Fluid

We propose a model to study self-diffusion of fluid confined between two rough walls. The model is based on microscopic considerations wherein the configuration space of the body systems is divided into cells. Within the cell, it executes harmonic motion unless it finds a saddle point on potential energy hypersurface. Results are obtained for Lennard Jones fluid for different order of rectangular and sinusoidal roughness of confining walls. It is found that the roughness of wall significantly affects the motion of fluid and reduces the average diffusion of particles in agreement with simulation results. The reduction is more for rectangular than for sinusoidal roughness. The study has applicability for fluids flowing in biological systems.

Distinct Photophysics of the Isomers of B18H22 Explained

The photophysics of the two isomers of octadecaborane(22), anti- and syn-B(18)H(22), have been studied by UV-vis spectroscopic techniques and theoretical computational methods. In air-saturated hexane, anti-B(18)H(22) shows fluorescence with a high quantum yield, Φ(F) = 0.97, and singlet oxygen O(2)((1)Δ(g)) production (Φ(Δ) ∼ 0.008). Conversely, isomer syn-B(18)H(22) shows no measurable fluorescence, instead displaying much faster, picosecond nonradiative decay of excited singlet states. Computed potential energy hypersurfaces (PEHs) for both isomers rationalize these data, pointing to a deep S(1) minimum for anti-B(18)H(22) and a conical intersection (CI) between its S(0) and S(1) states that lies 0.51 eV higher in energy. Such an energy barrier to nonradiative relaxation is not present in the PEH of syn-B(18)H(22), and the system therefore has sufficient initial energy on excitation to reach the (S(0)/S(1)) CI and to then decay to the ground state without fluorescence. The computational analysis of the geometries at stationary points along the PEH of both isomers shows that the determining factor for the dissimilar photophysics of anti- and syn-B(18)H(22) may be due to the significant differences in the geometrical rearrangements at their respective conical intersections. Thus, the syn isomer shows one very large, B-B elongation of 1.2 Å from 1.8 Å in the ground state to 3.0 Å at the CI, whereas the anti isomer shows smaller elongations (below 1 Å) in several B-B connectivities at its (S(0)/S(1))(CI). The absorbed energy in S(1) for the anti-B(18)H(22) is therefore redistributed vibrationally into several regions of the molecule rather than almost completely into a single vibrational mode as in the case for the syn isomer. The consequent prolonged S(1) lifetime for the anti isomer allows for relaxation via fluorescence.

Activation of C–Cl by ground-state aluminum atoms: an EPR and DFT investigation

The reaction of ground-state Al atoms with dichloromethane (CH(2)Cl(2)) in an adamantane matrix at 77 K yielded two mononuclear Al species. The magnetic parameters, extracted from the axial EPR spectrum of Species A/A' (g(1) = 2.0037, g(2) = g(3) = 2.0030, a(Al,1) = 1307 MHz, a(Al,2) = a(Al,3) = 1273 MHz, a(35Cl) = 34 MHz and a(37Cl) = 28 MHz) were assigned to the Al-atom insertion product, ClCH(2)AlCl. Density functional theory (DFT) calculations of the values of the Al and Cl hyperfine interaction (hfi) of the Cl(1)-Cl(2)gauche conformer were in close agreement with the experimental values of ClCH(2)AlCl. The second species, B/B', had identical magnetic parameters to those of ClCH(2)AlCl with the exception that the Al hfi was 15% smaller. Coordination of a ligand, possessing a lone pair of electrons, to the Al atom of the insertion product, [ClCH(2)AlCl]:X, could cause the a(Al) to decrease by 15%. Alternatively, it is possible that the Cl(1)-Cl(2) anti conformer of ClCH(2)AlCl is also isolated in the matrix. Support for the spectral assignments is given by calculation of the nuclear hfi of [ClCH(2)AlCl]:H(2)O and the Cl(1)-Cl(2) anti conformer of ClCH(2)AlCl using a DFT method. The potential energy hypersurface for an Al atom approaching CH(2)Cl(2), calculated at the B3LYP level, suggests that Al atom abstraction of Cl forming AlCl and CH(2)Cl is favoured in the gas phase. When produced in a matrix, the close proximity of AlCl and CH(2)Cl could account for the formation of ClCH(2)AlCl. EPR evidence was also found for the formation of the CHCl(2) radical.

Natural molecular fragments, functional groups, and holographic constraints on electron densities

One of the tools of the shape analysis of molecular electron densities, the Density Threshold Progression Approach used in Shape Group studies can also serve as a criterion for the selection of "natural" molecular fragments, relevant to functional group comparisons, reactivity studies, as well as to the study of levels of relative "autonomy" of various molecular regions. The relevance of these approaches to the fragment-based studies of large molecules, such as biopolymers and nanostructures is emphasized, and the constraints represented by the holographic electron density theorem to this and alternative recent fragment approaches are discussed. The analogies with potential energy hypersurface analysis using the Energy Threshold Progression Approach and connections to level set methods are discussed, and the common features of these seemingly distant problems are described.

Searching the “Biologically Relevant”Conformation of Dopamine: A Computational Approach

We report here an exhaustive and complete conformational study on the conformational potential energy hypersurface (PEHS) of dopamine (DA) interacting with the dopamine D2 receptor (D2-DR). A reduced 3D model for the binding pocket of the human D2-DR was constructed on the basis of the theoretical model structure of bacteriorhodopsin. In our reduced model system, only 13 amino acids were included to perform the quantum mechanics calculations. To obtain the different complexes of DA/D2-DR, we combined semiempirical (PM6), DFT (B3LYP/6-31G(d)), and QTAIM calculations. The molecular flexibility of DA interacting with the D2-DR was evaluated from potential energy surfaces and potential energy curves. A comparative study between the molecular flexibility of DA in the gas phase and at D2-DR was carried out. In addition, several molecular dynamics simulations were carried out to evaluate the molecular flexibility of the different complexes obtained. Our results allow us to postulate the complexes of type A as the "biologically relevant conformations" of DA. In addition, the theoretical calculations reported here suggested that a mechanistic stepwise process takes place for DA in which the protonated nitrogen group (in any conformation) acts as the anchoring portion, and this process is followed by a rapid rearrangement of the conformation allowing the interaction of the catecholic OH groups.

Theoretical study of the conformational energy hypersurface of cyclotrisarcosyl

Abstract The multidimensional Conformational Potential Energy Hypersurface (PEHS) of cyclotrisarcosyl was comprehensively investigated at the DFT (B3LYP/6-31G(d), B3LYP/6-31G(d,p) and B3LYP/6-311++G(d,p)), levels of theory. The equilibrium structures, their relative stability, and the Transition State (TS) structures involved in the conformational interconversion pathways were analyzed. Aug-cc-pVTZ//B3LYP/6-311++G(d,p) and MP2/6-31G(d)//B3LYP/6-311++G(d,p) single point calculations predict a symmetric cis-cis-cis crown conformation as the energetically preferred form for this compound, which is in agreement with the experimental data. The conformational interconversion between the global minimum and the twist form requires 20.88 kcal mol-1 at the MP2/6-31G(d)//B3LYP/6-311++G(d,p) level of theory. Our results allow us to form a concise idea about the internal intricacies of the PEHSs of this cyclic tripeptide, describing the conformations as well as the conformational interconversion processes in this hypersurface. In addition, a comparative analysis between the conformational behaviors of cyclotrisarcosyl with that previously reported for cyclotriglycine was carried out

Essential on the Photophysics and Photochemistry of the Indole Chromophore by Using a Totally Unconstrained Theoretical Approach.

Indole is a chromophore present in many different molecules of biological interest, such as the essential amino acid tryptophan and the neurotransmitter serotonin. On the basis of CASPT2//CASSCF quantum chemical calculations, the photophysical properties of the system after UV irradiation have been studied through the exploration of the potential energy hypersurfaces of the singlet and triplet low-lying valence excited states. In contrast to previous studies, the present work has been carried out without imposing any restriction to the geometry of the molecule (C1 symmetry) and by performing minimum energy path calculations, which is the only instrument able to provide the lowest-energy evolution of the system. Relevant findings to the photophysics of bare indole have been obtained, which compete with the currently accepted mechanism for the energy decay in the molecule. The results show the presence of a conical intersection (CI) between the initially populated (1)(La ππ*) and the (1)(Lb ππ*) state, easily accessible through a barrierless pathway from the Franck-Condon region. At this CI region, part of the population is switched from the bright (1)(La ππ*) state to the (1)(Lb ππ*) state, and the system evolves toward a minimum structure from which the expected fluorescence takes place. The reported low values of the fluorescence quantum yield are explained by means of a new nonradiative mechanism specific for the (1)(Lb ππ*) state, in which the presence of an ethene-like CI between the (Lb ππ*) and ground states is the main feature.

Program package for numerical solving time-independent Schrödinger equation for vibrational problems: Inclusion of coordinate dependent reduced masses

In this article an improved program package for variational solving of the time-independent Schrödinger equation (SE) is presented. The program is coded in FORTRAN-95 and is aimed to solve the vibrational SE on a generalized potential energy hypersurface (PES), typically constructed pointwise by quantum chemical calculations. The procedure consists of fitting of the hypersurface to either cubic splines or displaced Gaussians. The central part is the discrete representation of the vibrational Hamiltonian on a uniform grid according to the Fourier Grid Hamiltonian (FGH) scheme. The FGH is tuned for expression of the vibrational problem in terms of generalized internal coordinates (bond distances, angles, torsions, etc.), which is the most convenient and hence widely used representation for chemical systems. In the formalism of generalized internal coordinates the reduced masses assume quite a sophisticated coordinate-dependent form. The FGH matrix is diagonalized and its eigenvalues and eigenfunctions are analyzed giving rise to observables that can be compared with the experimental data (vibrational spectroscopy in the first place). We tested the package for three distinct chemical problems; namely, the umbrella inversion of ammonia, OH stretching motion of sodium hydrogen bissulfate, and hydrogen dynamics in soybean lipoxygenaze-1 (SLO-1), yielding good agreement with the available experimental and previously published computational data. The program package is available from the authors on request.

Multistep conformational interconversion mechanism of cyclododecane. A simple and fast analysis using potential energy curves

AbstractAn ab initio and Density Functional Theory (DFT) study of the conformational properties of cyclododecane was carried out. The energetically preferred equilibrium structures, their relative stability, and some of the transition state (TS) structures involved in the conformational interconversion pathways were analyzed from RHF/6‐31G(d), B3LYP/6‐31G(d,p) and B3LYP/6311++G(d,p) calculations. Aug‐cc‐pVDZ//B3LYP/6311++G(d,p) single point calculations predict that the multistep conformational interconversion mechanism requires 11.07 kcal/mol, which is in agreement with the available experimental data. These results allow us to form a concise idea about the internal intricacies of the preferred forms of cyclododecane, describing the conformations as well as the conformational interconversion processes in the conformational potential energy hypersurface. Our results indicated that performing an exhaustive analysis of the potential energy curves connecting the most representative conformations is a valid alternate tool to determine the principal conformational interconversion paths for cyclododecane. This methodology represents a satisfactory first approximation for the conformational analysis of medium‐ and large‐size flexible cyclic compounds. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Axis-switching in the vibrationless $\widetilde{A}\leftarrow \widetilde{X}$Ã←X̃ transition of the jet-cooled deuterated methyl peroxy radical CD3O2

The jet-cooled high resolution spectrum of the vibrationless Ã←X̃ transition of the deuterated species of the methyl peroxy radical has been recently published in this journal (S. Wu, P. Dupré, P. Rupper, and T. A. Miller, J. Chem. Phys. 127, 224305 (2007)). The spectrum was analyzed using a rigid-rotor model with quadratic spin-rotation coupling. The analysis was based on the fit of ∼350 partially resolved line positions and was quite satisfactory. However, the full simulation of the spectral intensity clearly identifies a lack of ability to reproduce relatively small line clumps ("extra" lines) located between the two main central Q branches. This is indicating of an incomplete initial analysis. In the present paper we reanalyze this electronic transition by considering a reference-frame axis-switching resulting from the nuclear rearrangement associated to the electronic transition (spectra obtained at two different temperatures are considered). The potential energy hypersurfaces of the two electronic states are sufficiently dissimilar to induce changes in the molecule geometry, particularly, the angle COÔ, which induces a rotation (∼1.7°) of the principal axes of inertia located in the molecule symmetry plane. The present analysis is supported by a global fitting of the spectrum intensity and gives rise to a slightly different set of molecular constants. Attention is paid to the wavefunction symmetry assignment of a non-orthorhombic molecule. Couplings due to the torsion of the methyl group are discussed in the following paper (P. Dupre, J. Chem. Phys. 134, 244309 (2011)).

Reference Viscosities of Gaseous Methane and Hydrogen Sulfide at Low Density in the Temperature Range from (292 to 682) K

An all-quartz oscillating-disk viscometer of very high precision was used to measure the temperature dependence of the viscosities of methane and hydrogen sulfide at low densities. The measurements were based on a single calibration at room temperature with a value theoretically calculated using an accurate ab initio pair potential for helium and the kinetic theory of dilute monatomic gases. The uncertainty of the experimental data is conservatively estimated to be ± 0.15 % at room temperature increasing to ± 0.20 % at the highest temperature of 682 K. The new data are compared with experimental data from the literature as well as with values obtained from the Quiñones-Cisneros correlation for methane (2010) and the Schmidt correlation for hydrogen sulfide (2008) both implemented in the REFPROP computer program of the National Institute of Standards and Technology (NIST). The comparison shows that the low-density values of the Quiñones-Cisneros correlation for methane, primarily based on measurements in the range from (211 to 392) K performed at NIST in 2007, can be considered as reference values. On the contrary, the new experimental data for hydrogen sulfide are (2 to 6) % lower than the low-density values of the Schmidt correlation and should be taken into account for an improvement of the viscosity correlation for hydrogen sulfide. Furthermore, the temperature dependence of the viscosities calculated theoretically using ab initio intermolecular potential energy hypersurfaces for methane and hydrogen sulfide and the extended kinetic theory of dilute molecular gases should be used to extrapolate the viscosity correlations to low and high temperatures.