Relative Barriers Research Articles

A Theory of Melting of Molecular Crystals: the Complete Picture of Transition Temperatures

The theory of melting of molecular crystals with orientational degrees of freedom developed by Pople and Karasz (P and K) [1,2] is extended using a third energy parameter W'', in our previous papers [3-5]. The extention is done as follows: the previous repulsive energy parameter W is devided into two parts. The first part is the interaction between molecules which are on different sites with the same orientations, called again W. The second is the interaction between molecules on different sites and also different orientations, namely W''. W'' is combined with previous W' and the new W energy parameters by the arithmetic mean including an adjustable parameter. The present model is applied to study the thermodynamic properties of melting [3], solid-solid transitions and the complete phase diagrams for transitions under pressure [4], as well as the thermodynamics of solid-nematic and nematic-isotropic liquid transitions [5]. The quantitative predictions of the present model is compared with experimental results for plastic crystals by plotting the entropy of melting as a function of the ratio of melting and the ratio of solid temperatures , the entropy of melting as a function of volume changes and also the melting entropy versus the solid transition entropy. It is found that the agreement between the present theory and experimental data is better than the calculation of P and K theory. In this work, we give the complete pictures of transition temperatures and compare with the P and K theory as well as it volume dependence modification which was done by Chandrasekhar, Shashidar and Tara (CST) [6-7]. In order to study the complete pictures of transition temperatures, first we plot the reduced transition temperatures of solid-solid (S-S), solid-liquid (S-L), solid-nematic (S-N) and nematic-isotropic liquid (N-I) transitions as a function of \nu which is a measure of the relative energy barriers for the rotation of a molecule and for its diffusion to an interstitial site, together with the values given by the P and K and CST theories. Then, entropy and volume changes of S-S, S-L, S-N and N-I transitions as a function of \nu are calculated and plotted along the P and K and CST theories.

Triple Bonds to Silicon. Relative Stabilities and Energy Barriers Separating Silanitriles and Silaisonitriles. An ab Initio Study

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTTriple Bonds to Silicon. Relative Stabilities and Energy Barriers Separating Silanitriles and Silaisonitriles. An ab Initio StudyYitzhak Apeloig and Karsten AlbrechtCite this: J. Am. Chem. Soc. 1995, 117, 27, 7263–7264Publication Date (Print):July 1, 1995Publication History Published online1 May 2002Published inissue 1 July 1995https://doi.org/10.1021/ja00132a034RIGHTS & PERMISSIONSArticle Views170Altmetric-Citations41LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (924 KB) Get e-AlertsSupporting Info (2)»Supporting Information Supporting Information Get e-Alerts

Ab Initio Investigation of the Conformational Energies, Rotational Barriers, Molecular Structures, Vibrational Frequencies, and Dipole Moments of Aldehydes and Ketones

A detailed quantum mechanical study of aldehydes and ketones has been carried out at the HF/6-31G* level. Computed relative conformational energies, rotational barriers, and geometries were calculated for a wide variety of molecules and compared with experiment. For the most part, both the computed relative conformational energies and the barriers are in reasonable agreement with experiment. In several cases there were differences observed between the quantum calculations and experiment which suggested a reinterpretation of the experimental data. For example, in the case of diisopropyl ketone, it was suggested that two conformers rather than the three assumed by experiment were present in equilibrium in the gas phase. For both cyclobutanecarhxaldehyde and methylcyclobutyl ketone the calculations predicted an additional (axiaZ, gauche) low-energy minimum which has not been observed experimentally but should be possible to detect in the microwave spectrum. Relative to experiment, the computed C-0 bond lengths are -0.025 8, smaller and the computed C--O stretches are -280 cm-' higher. For cycloalkanones the calculations qualitatively reproduce the experimentally observed variation in the C-0 bond length and the dramatic decrease in the vibrational frequency with increasing ring size. The dipole moments computed for aldehydes and ketones are -10% higher than experiment with the exception of equatorial, trans-cyclobutanecarboxaldehyde, where it is 59% higher.

Radical Addition to Alkenes: An Assessment of Theoretical Procedures

Ab initio calculations at a variety of levels of theory have been carried out for a number of prototypical radical addition reactions with the aim of identifying levels of theory suitable for general application. The calculated geometries have a relatively small dependence on basis set but a somewhat greater dependence on correlation procedure. The energetic quantities (barriers and reaction enthalpies) are much more sensitive to level of theory, particularly to the correlation procedure used. Absolute barriers require methods such as UCCSD(T) or UQCISD(T) for an accurate description. Relative barriers for a series of related reactions are also best treated with methods such as UQCISD or UQCISD(T), but the simpler RMP2, PMP2, and UB-LYP procedures can fare reasonably well under specific circumstances. UMP2 performs poorly for both absolute barriers and relative barriers, a result that may be attributed to significant and variable spin contamination in the individual systems involved in radical addition reactions.

Influence of Composition and Temperature on Hydrocarbon Migration Through Morrow Fluvial Reservoirs, Las Animas Arch, Colorado: ABSTRACT

Precipitation of wax in pores may impair permeability and prohibit the flow of oil. Crude oil composition and temperature are the most important controlling factors. Oils are chemically complex, may contain up to 45 wax compounds and may vary significantly even in the same pool. High-wax oils are common in the Morrow of eastern Colorado. Narrow fluvial sandstones provide migration paths toward the Las Animas Arch from adjacent basins. Temperatures range from less than 110{degrees}F. on the top of the arch to 180{degrees}F at a structural position only 1,400 feet lower. A range of 30{degrees}F has been observed in individual pools. Wax has precipitated in the 120-140{degrees}F range, creating relative permeability barriers which cut across the sandstones. Wax barriers are impermeable to oil, but may be permeable to gas and water. They account for certain dry holes with high porosity, permeability and oil saturation (and low water saturation) in both core and electrical log analysis. They explain why some oil wells with impaired permeability are adjacent to structurally lower gas wells with good permeability. A network of wax barriers around the Las Animas Arch accounts for approximately 300 feet of variation in the structural position of a line separating oilmore » from gas fields. Since the low temperature bands may be short and discontinuous, wax barriers are more effective in narrow fluvial reservoirs than in blanket reservoirs.« less

Ab Initio Potential Energy Surface for H + OCS Reactions: Extended Basis Sets and Correlation Treatment

Abstract : Ab initio calculations using extended basis sets are presented for the potential energy surface (PES) of H + OCS. There are two major reaction channels on the PES; Reaction (I) is H(2S) + OCS(1(Sigma)) yields OH(2(Pi)) + CS(1(Sigma)), and Reaction (II) is H(2S) + OCS(1(Sigma)) yields SH(2(Pi)) + CO(1(Sigma)). Results of this study substantiate findings from an earlier quantum chemical study using a lower level of theory, including (1) the existence of 12 transition states and 6 stable 4-body intermediates; (2) the qualitative description of the PES (i.e., geometries, relative barriers, and well depths are similar to those in the earlier study); and (3) the entrance channel transition states leading to (II) are tight, as suggested by experiment. The results presented here also support the explanations of observed product energy distributions for (I) and (II) based on the earlier ab initio study. An additional transition state connecting the cis-HOCS and cis-HSCO minima was located, confirming a previous suggestion that Reaction (II) could result from hydrogen migration after HOCS formation. The current results show a substantial improvement in the quantitative agreement with experiment over the previously calculated values. Potential energy surface, Ab initio, MP4, QCISD(T), Electronic structure, Potential energy, Quantum chemistry.

An AM1 study of conformational preference for the herbicide atrazine

The potential energy surface for atrazine, a commercially important herbicide, has been extensively investigated using the quantum mechanical semiempirical molecular orbital method AM1. Full symmetry unconstrained geometry optimisation was performed for each stationary point located on the energy surface. The relative stabilities and rotational barriers for the conformera were also calculated. Four minimum energy conformera were predicted to occur with respect to rotation about the triazine ring carbon atoms and the amino ethyl and amino isopropyl groups. A relatively low energy barrier to bond rotation of 30 kJ mol −1 was predicted. The conformation adopted by triazine molecules on protein binding suggests that this conformation is principally stabilised by intermolecular hydrogen bonding with the protein active site.

Quantitative valence-bond computations of curve crossing diagrams for a gas-phase SN2 reaction: F–+ CH3F → FCH3+ F–

A quantitative curve crossing diagram for the identity SN2 reaction, F–+ CH3F → FCH3+ F–, is computed with the multistructure VB method, at the PS-31 + G* level with a basis set of double-zeta plus polarization quality with pseudo-potentials for the core electrons and additional diffuse functions. The VB value for the central barrier is ca. 11.3 kcal mol–1, in agreement with a recent estimate based on experimental data (reference 9) and with most modern MO computations followed by correlation corrections (reference 7b,d). Because of the low energy of the triple ion structure, F–CH3+ F–, the variational curves can be generated all the way to the crossing point, but beyond it the variational procedure does not terminate in the charge-transfer state despite the natural correlation of the VB structures to this state. The relationship between the natural correlation and the variational VB procedure is discussed. It is shown that this feature of the variational procedure does not affect its ability to provide the key curve crossing quantities at their variational values. These quantities are, in turn, found to follow the trends of the qualitative model. The vertical charge-transfer energy gap is large (241.8 kcal mol–1). The height of the crossing point relative to the ion–dipole cluster is 37.25 kcal mol–1(ca. 15% of the gap). The resonance energy of the transition state is 25.9 kcal mol–1. The relationship between the barrier and the curve crossing quantities is discussed by comparison to the hydride exchange reaction, H–+ CH3H → HCH3+ H–. It is shown that the relative barriers reflect the interplay between the charge-transfer energy gap and the curvature which determines the height of the crossing point relative to the gap. This behaviour is in good agreement with the predictions of the qualitative curve crossing model. The significance of the curvature factor is shown to be related to the characteristic structure–reactivity coefficients of the reaction (e.g., the Bronsted coefficient).

A semiempirical molecular orbital and dynamic NMR study of conformational isomerism in angiotensin-converting enzyme inhibitors

The conformational isomerism in a series of molecules related to inhibitors of the angiotensinconverting enzyme (ACE) is investigated using the semiempirical quantum mechanical molecular orbital model, AM1, and dynamic NMR spectroscopy. The theoretical method is tested by the prediction of relative barriers to rotation about amide C-N and acylhydrazine N-N bonds, and of the trans: cis ratio for a series of model compounds. The conformational energetics of captopril and a series of acylhydrazines are predicted and used to interpret the dynamic NMR spectra of these molecules, presented herein.

An ab initio study of the H2O-mediated 1,3-hydrogen rearrangement in the isoelectric series: X, Y = CH2, NH, or O

Ab initio calculations are performed to investigate the effect of water on the 1,3-intramolecular hydrogen rearrangement in the [Formula: see text] isoelectronic series, for X, Y = CH2, NH or O, by modeling with a single H2O molecule. The geometries for the complexes are optimized at both the 3-21G and 6-31G** basis set levels. MP2 and some MP4 calculations are performed to take into account the correlation effects, and ZPV corrections are calculated at the MNDO level. The results show that the water molecule has a significant effect on the molecules in which hydrogen bonds can be formed or partially formed. The "solvent effects" are also examined using the self-consistent reaction-field (SCRF) approach. A combination of supermolecule and SCRF approach appears to work well at predicting the effect of solvent on relative stabilities and barriers. Key words: tautomerism, ab initio, 1,3-hydrogen rearrangement, solvent effects.

ChemInform Abstract: Oxirenes and Ketocarbenes from α‐Diazoketone Photolysis: Experiments in Rare Gas Matrices. Relative Stabilities and Isomerization Barriers from MNDOC‐BWEN Calculations.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

DREIDING: a generic force field for molecular simulations

We report the parameters for a new generic force field, DREIDING, that we find useful for predicting structures and dynamics of organic, biological, and main-group inorganic molecules. The philosophy in DREIDING is to use general force constants and geometry parameters based on simple hybridization considerations rather than individual force constants and geometric parameters that depend on the particular combination of atoms involved in the bond, angle, or torsion terms. Thus all bond distances are derived from atomic radii, and there is only one force constant each for bonds, angles, and inversions and only six different values for torsional barriers. Parameters are defined for all possible combinations of atoms and new atoms can be added to the force field rather simply. This paper reports the parameters for the nonmetallic main-group elements (B, C, N, 0, F columns for the C, Si, Ge, and Sn rows) plus H and a few metals (Na, Ca, Zn, Fe). The accuracy of the DREIDING force field is tested by comparing with (i) 76 accurately determined crystal structures of organic compounds involving H, C, N, 0, F, P, S, CI, and Br, (ii) rotational barriers of a number of molecules, and (iii) relative conformational energies and barriers of a number of molecules. We find excellent results for these systems.

Transition structures for conjugate and carbonyl additions of cyanide to s-cis- and s-trans-acrolein. An ab initio MO study

Transition structures for the addition of cyanide anion to s-cis- and s-trans-acrolein and to ethylene and formaldehyde have been located with ab initio MO calculations with use of the 3-21 G and 6-31+G* basis sets. Energies of all stationary points, including reactants, ion-dipole complexes, and products, were evaluated at the second-order Moller-Plesset correlation level with the 6-31+G* basis set. The relative reaction barriers for conjugate and carbonyl additions are discussed for gas-phase and solution-phase reactions. Calculations predict that, in the gas phase, the reaction barrier for conjugate addition to s-cis-acrolein is slightly higher than that for carbonyl addition, wheareas the reverse situation is found for addition to s-trans-acrolein. The conjugate addition products are also more stable than the carbonyl addition products. The reaction barrier for carbonyl addition is expected to become smaller than that for conjugate addition in solution, under which conditions the difference in the stabilities of the conjugate and carbonyl products should become relatively small

Reorientational processes in solid (C 6H 6) 2Cr, (C 6H 6)Cr(CO) 3 and (C 6H 5Me)Cr(CO) 3

The reorientational motion of the benzene rings in solid (C 6H 6) 2Cr ( 1) and (C 6H 6)Cr(CO) 3 ( 2) and of the tricarbonyl group in solid (C 6H 5Me)Cr(CO) 3 ( 3), proposed on the basis of solid-state NMR experiments (wide-line 1H and 13C MAS) is studied by means of the pairwise potential energy method. Starting from the known crystal structures of the three species, relative potential energy barriers to motion and root mean square librational amplitudes at the bottom of the potential energy wells are evaluated and compared with the available anisotropic displacement parameters of the atoms involved in the process. The results show that the crystal packing in 2 and 3 forbids rotation of the Cr(CO) 3 group as a whole, while benzene reorientation encounters low potential energy barriers in both 1 and 2, in agreement with the spectroscopic information. An alternative interpretation of the dynamic behaviour shown by 3, based on large amplitude librations of both C 6H 5Me and Cr(CO) 3 groups, is put forward.

Molecular orbital calculations for surface-bound metal species: rhodium(I) dicarbonyls on alumina or silica

The oxide-supported Rh I(CO) 2 species, frequently generated on heterogeneous rhodium catalysts, has been modeled for extended Hückel molecular orbital calculations using, as oxide supports, simple oxyanion structures composed of Si or Al atoms in tetrahedral or octahedral oxygen environments. Calculations for different orientations of Rh I(CO) 2 with respect to the oxyanion structures showed the lowest energy configuration to be that in which the metal car bonyl fragment binds to two oxygen atoms, forming a square planar geometry about the Rh I. Rearrangement to a five-coordinate species with Rh I(CO) 2 bound to three oxygens requires between 0.51 and 1.94 eV of energy, depending upon the oxyanion structure and whether the central atom is Si or Al. The lowest energy for conversion from four- to five-coordination about Rh I occurs upon octahedral anions. The square planar→ five-coordinate conversion is discussed as a surface migration pathway, with relative activation barriers to the migration depending upon the energy of this conversion.

An MNDO molecular orbital study of the reactions of protonated oxirane derivatives (XCHCH2OH+, X = CN, Cl, CH3, Ph) with simple nucleophiles. Implications for regioselectivity in the reactions of electrophiles with nucleic acid bases

AbstractThe gas‐phase analogs of the classical SN1 and SN2 reactions of nucleophiles with a series of protonated oxiranes bearing the substituents CN, Cl, Me, and Ph were studied using MNDO semiempirical molecular orbital theory. The transition states for nucleophilic attack on the intact rings are calculated to become progressively more “SN1‐like,” as the ability of the substituent group to stabilize the corresponding ring opened carbenium ion increases. At the same time the activation barriers become progressively smaller and their relative values for different nucleophiles approach the order expected on the basis of a purely electrostatic attraction between the reacting moieties. Exactly the opposite trends are calculated for the transition states for nucleophilic attack on the intermediate carbenium ions. As the stabilities of the latter increase the extent of bond formation, and transfer of charge to the incoming nucleophile also increase. Here, the relative barriers for attack by different nucleophiles approach the order expected on the basis of a superposition of both covalent and electrostatic interactions in which the former dominate. These results support a previously suggested rationalization for the enhanced reactivities of certain alkylating agents towards the exocyclic oxygen atoms, rather than the ring nitrogens, of the nucleic acid bases. They also suggest a new explanation for the tendencies of aralkylating electrophiles to modify the exocyclic amino groups of the nucleic acid bases: sites which are unreactive towards simple alkylating agents.

Semiempirical mo studies on the proton transfer equilibria of amides 1

Intramolecular proton transfer equilibria of acetamide and methyl carbamate have been studied by AMI MO calculations, and the results are compared with those of the MNDO method. It was found that the two semi- empirical methods predict essentially the same proton transfer mechanism, but MNDO tends to overestimate the activation barriers. Participation of one solvate vater in the proton transfer led to a considerable lovering of the activation barrier, by nearly the same amount, in both methods. One notable conflict between the two methods was that the methoxy-O-protonated form of methyl carbamate, in the gas phase, can be a local energy minimum with MNDO, whereas it leads to dissociation into two species Instead of an optimized structure with AM1. It was concluded that the MNDO method can be useful for this type of process, especially when one is interested in the relative activation barriers only.

ChemInform Abstract: An SCF‐MO Study of the Relative Barriers to Inversion and to Ring Opening in Three‐Membered Ring Carbanions

ChemInformVolume 18, Issue 28 Physical Organic Chemistry ChemInform Abstract: An SCF-MO Study of the Relative Barriers to Inversion and to Ring Opening in Three-Membered Ring Carbanions I. RAJYAGURU, I. RAJYAGURU Dep. Chem., Imp. Coll. Sci. Technol. Med., London SW7 2AY, UKSearch for more papers by this authorH. S. RZEPA, H. S. RZEPA Dep. Chem., Imp. Coll. Sci. Technol. Med., London SW7 2AY, UKSearch for more papers by this author I. RAJYAGURU, I. RAJYAGURU Dep. Chem., Imp. Coll. Sci. Technol. Med., London SW7 2AY, UKSearch for more papers by this authorH. S. RZEPA, H. S. RZEPA Dep. Chem., Imp. Coll. Sci. Technol. Med., London SW7 2AY, UKSearch for more papers by this author First published: July 14, 1987 https://doi.org/10.1002/chin.198728075Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume18, Issue28July 14, 1987 RelatedInformation

An SCF-MO study of the relative barriers to inversion and to ring opening in three-membered ring carbanions

The barriers to inversion of configuration at the centre X and to ring opening in a series of isoelectronic three-membered ring systems (1)–(4) have been investigated at the single determinantal MNDO and ab initio SCF-MO levels of theory. The MNDO-method-predicted barriers to inversion at carbanionic centres are too low compared with experiment and with the ab initio calculations. In the specific case of cyclopropyl carbanion, the MNDO method incorrectly predicts the C– centre to be planar. The two theoretical methods give more comparable results for inversion at nitrogen and oxygen centres. At the highest level of theory employed (RHF-MP4‖6-31 + G), loss of configuration in (1a)–(3a) is predicted to occur via ring opening to give an enolate anion, rather than via direct inversion at C–. The reverse is true in cyclopropyl anion (4), where direct inversion at the carbon centre has the lower barrier. These results lead to the prediction that oxirane and aziridine anions should be configurationally stable species, whereas thiirane carbanions are predicted to ring-open rapidly to give a thioenol.

ChemInform Abstract: Synthesis and Properties of Some Cyclohexadienylrhenium Polyhydride Complexes.

AbstractThe heptahydride (I) reacts with benzene and some of its derivatives to give dihydridocyclohexadienyl complexes (III).