Minimum Of Potential Energy Surface Research Articles

The intramolecular vibrations of prototypical polythiophenes

Inelastic neutron scattering experiments are combined with infrared and Raman data to obtain a uniquely defined description of the intramolecular vibrations of three oligomers of polythiophene. Through refinement of ab initio force fields, the three vibrational spectra of each oligomer are simulated with remarkable accuracy. Two different basis sets of atomic orbitals are used: the first, is 6-31G* and is used to optimize the geometries and calculate the relevant force fields of α-2T and α-4T, the second is 3-21G* and is used for the same purpose for α-4T and α-6T. To improve agreement with the experiment, the force fields are scaled. In this way, one set of scaling parameters is generated for the 6-31G* basis and another for the 3-21G* basis. The parameters are common to both molecules calculated with either basis sets and are believed to be transferable to higher isomers. The fitting procedure is applied in steps: first, the calculated vibrational frequencies are assigned on the basis of the experimental infrared and Raman activity, then a fitting of the Inelastic Neutron Scattering profile is performed, finally, the infrared and Raman spectra are calculated with the new normal modes and the ab initio derivatives of the dipole moment and the polarizability. The procedure is iterated until the three spectra of each oligomer are satisfactorily reproduced. For α-4T, two scaled force fields are obtained—one for each basis set—and are shown to yield very similar normal modes. It is important to emphasize that not only the vibrational frequencies but also the spectral intensities are well reproduced by the simulations. Implicitly, this means that the dipole moment and the polarization tensor surfaces calculated ab initio at the potential energy surface minimum are of good quality. The procedure is absolutely general and can be applied to any molecular system. In the present case, it leads to well defined force fields that give us a stringent picture of the vibrations of these molecules.

The theory of atoms in molecules as a tool to investigate the reactivity of tetraphosphacubane

Bader's theory of atoms in molecules is used to rationalize the gas-phase reactivity of tetraphosphacubane vs, H+, Li+, Na+, and Be2+. For this purpose we have used MP2 densities obtained at the 6-31G(d,p) level. The characteristics of the C—P bonds of tetraphosphacubane are discussed. The Laplacian of its electron charge density shows that both phosphorus and carbon atoms are active centers for electrophilic substitutions. This is consistent with the fact that both phosphorus and carbon protonated species are minima of the potential energy surface. The strong charge redistribution associated with carbon protonation explains the enhanced stability of the carbon protonated species with respect to the phosphorus protonated one. The Laplacian field also shows the existence of a cavity inside the cage surrounded by high electronic density that can stabilize a cation of the appropriate size. Our results confirm that Li+ and Be2+ fulfil this requirement and the corresponding complexes, where the cation is located inside the cage, are minima of the corresponding potential energy surface. Na+ is far too large and a similar structure is a saddle point of the potential energy surface. Key words: atoms-in-molecules theory, tetraphosphacubane, reactivity, cationization.

High-Pressure Mass Spectrometric Investigations of the Potential Energy Surfaces of Gas-Phase SN2 Reactions

High-Pressure Mass Spectrometric (HPMS) experiments have been carried out to probe the details of the double minimum potential energy surface for gas-phase SN2 reactions. The well depths and entropy changes associated with the formation of entrance and exit channel electrostatic complexes for the chloride and bromide adducts of methyl, ethyl, isopropyl, and tert-butyl chlorides and bromides have been determined from the temperature dependence of the equilibrium constants for adduct formation. In the cases of “symmetric” complexes associated with identity SN2 reactions, there is an increase in well depth as the size and, therefore, polarizability of the alkyl group increases. Concomitant with this is an increase in the magnitude of the negative entropy change for complex formation which is the result of an increase in the frequency of the intermolecular mode(s) of the complex arising from the increased bond strength. The data for the unsymmetrical adducts for the non-identity SN2 reactions show the same pa...

Ion Reaction Dynamics

This article reviews recent progress in the study of ion reaction dynamics in which product energy disposal studies provide detailed information about the potential energy surfaces mediating chemical rea ction. We emphasize product state-resolved studies of the proton tran sfer and hydro­ gen atom tran sfer reactions of 0-. We also discuss several critical experi­ ments that test the validity of the double minimum potential energy surf ace model in describing the bimolecular nucleophilic substitution (SN2) reac­ tion. Photo ionization methods for producing vibrational and vibrational­ rotational state selecti ons of ions are also discussed, and recent examples with Hi and Nt are presented. The role of these state-selection methods in elucidating mode-s elective chemistry in the reactions of NHt and C2Ht are also discussed.

A theoretical study of the conformations of diphenyl ether, its anion, and its dianion

The potential energy surfaces, of diphenyl ether (Ph 2O), its anion and its dianion, were investigated using the AM1 semiempirical description of the electronic wavefunction. Counterions with charges of +1 were used to approximate the solution electrostatic environments for the anion and dianion. The global minimum of the Ph 2O potential energy surface is predicted to be a “twisted” conformation of the “propeller” form while the anion (Ph 2O −) and dianion (Ph 2O 2−) are predicted to be of the “skewed” and “gable” forms, respectively. The relative energies of other critical points on the molecules' potential energy surfaces were calculated. The calculations reported herein indicate that the rings in Ph 2O are nearly free rotors, but that Ph 2O − may be a quasi-rigid structure and Ph 2O 2− is definitely rigid. Charge densities and some vibrational frequencies were calculated to characterize more accurately various conformations.

Ab initioSCF investigation of the intramolecular hydrogen bonding in ?-aminohexanoic acid

Ab initioRHF results (geometry data and potential barriers) are reported for the 14 symmetry unique local minima in the potential energy surface of ϵ-aminohexanoic acid, which contain an intramolecular N HO hydrogen bond. Comparison of characteristic data with those of the homologs with fewer carbon atoms shows that δ-aminopentanoic acid forms the most stable hydrogen bonds. In contrast to these homologs, the COOH group is not limited to anti-periplanar orientation in the H-bonded conformers of ϵ-aminohexanoic acid: In four such conformers, it occurs in syn-clinal orientation. © 1994 John Wiley & Sons, Inc.

Global energy minimum searches using an approximate solution of the imaginary time Schroedinger equation

We present a method for finding the global energy minimum of a multidimensional potential energy surface through an approximate solution of the Schrodinger equation in imaginary time. The wave function of each particle is represented as a single Gaussian wave packet, while that for the n-body system is expressed as a Hartree product of single particle wave functions. Equations of motion are derived for each Gaussian wave packet's center and width. While evolving in time the wave packet tunnels through barriers seeking out the global minimum of the potential energy surface. The classical minimum is then found by setting Planck's constant equal to zero

Remarkable Effect of Fluorine and Chlorine Atoms on the Stability of 1H‐Phosphirene

Ab initio MO calculations at the MP4/6–31G** level show that both fluorine and chlorine atoms exert a strong stabilizing effect on the three‐membered ring 1H‐phosphirene relative to its isomers. While unsubstituted 1H‐phosphirene (12H) is the least stable C2H3P isomer, 1‐fluoro‐1H‐phosphirene (12F) is calculated to be the global minimum of the C2H2FP potential energy surface; 1‐chloro‐1H‐phosphirene (12Cl) is the third most stable of the six C2H2ClP isomers calculated. The remarkable stability of 12F is attributable to the particular strength of the P—F bond.

A combined simulated annealing and quasi-Newton-like conjugate-gradient method for determining the structure of mixed argon-xenon clusters

This paper shows how various limited-memory quasi-Newton large-scale unconstrained minimization methods can be used to speed up the location of global minima of potential energy surfaces related to the structures of mixed Ar-Xe clusters. Both a simulated annealing method and a finite-temperature lattice-based Monte Carlo method are accelerated by the various quasi-Newton limited-memory methods which are then compared for computational efficiency.

Molecular hydrogen complex vs dihydride in ML4 + H2 systems. Influence of the ML4 fragment geometry

The influence of the d 8 ML 4 [Rh(PH 3 ) 4 ] + metallic fragment geometry on the H 2 coordination shape is analyzed with the ab initio MO method. The potential energy surface minima are found to be essentially different when the fragment has a classic C 2v form or a rigid pyramidal C 3v structure. In C 2v symmetry, the stable species turns out to be the dihydride, whereas in C 3v symmetry the η 2 molecular complex is the stable structure, in good agreement with experimental data. Through the analysis of a few selected molecular orbitals, the different hybridization of the Rh d orbitals is shown to be the key factor to explain the experimental behavior

ChemInform Abstract: Pulsed Photoelectron Spectroscopy of Negative Cluster Ions: Isolation of Three Distinguishable Forms of N2O‐ 2.

AbstractThree different types of N2O2‐ ions are selectively prepared by electron impact ionization of different neutral precursors (O2 in N2, N2O, or NO in Ar) in a supersonic expansion and investigated using a pulsed negative ion photoelectron spectrometer.

Pulsed photoelectron spectroscopy of negative cluster ions: Isolation of three distinguishable forms of N2O−2

Three different ionic species with stoichiometry N2O−2 are generated by varying the neutral precursors in an electron beam ionized free jet expansion. In each case, the ion is isolated by mass spectrometry and then probed using pulsed photoelectron spectroscopy (PES) at 532 and 355 nm (2.330 and 3.495 eV, respectively). The neutral starting materials used in the three preparations are (I) O2 seeded 5% in N2, (II) pure N2O, and (III) NO seeded 10% in Ar. Based on their PES and photofragmentation properties, the three species appear to be best described as (I) O−2 ⋅N2, (II) either O−⋅N2O or more likely a chemically bound species, and (III) NO−⋅NO. It is likely that two of these species are trapped intermediates in the O−+N2O→NO−+NO reaction, suggesting a double minimum potential energy surface. The formation mechanisms of these ions in our source are discussed in the context of previous preparation schemes.

Reactive scattering from double minimum potentials: Energetics and mechanism of the gas phase dehydration reaction of lithium ion with t e r t-butyl alcohol

We present a crossed beam study of the reactions of Li+ with tert-butanol, a reactive system hypothesized to proceed on a double minimum potential energy surface, over the collision energy range from 0.85 to 1.80 eV. We observe product energy and angular distributions for the dehydration products as well as for Li+ which is nonreactively scattered from a transient collision complex, thereby probing both wells on the surface. The direct observation of such nonreactive flux provides confirmation of the Brauman model for nonunity reaction efficiency as arising from significant nonreactive decay of the initial encounter complex back to reagents. We also measure the branching ratios for nonreactive to reactive scattering, and the branching ratios for the two dehydration products relative to one another, over the entire kinetic energy range of the experiments. The product angular distributions indicate that the collision complex lifetimes are under 1 ps. The product kinetic energy distributions are in reasonably good agreement with statistical phase space theory predictions, with larger deviations occurring for the nonreactive channel, and with the deviations for the dehydration channels increasing with increasing collision energy. The branching ratio for nonreactive to reactive scattering, in conjunction with statistical calculations, indicates that the intermediate barrier on the double minimum surface is equal to the energy of the asymptotic reagents within ±0.05 eV. The Li(H2O)+/Li(C4H8)+ branching ratios, when compared with statistical calculations including product dissociation, are consistent with products formed in statistical equilibrium. The data suggest that the reaction dynamics in both wells are in substantial agreement with statistical theories, with a reduced number of vibrational modes required to effect the dehydration process at high energies, resulting in significant energy transfer, but a dramatically reduced decay lifetime.

ChemInform Abstract: PREFULVENE AS A STABLE INTERMEDIATE AT THE POTENTIAL ENERGY SURFACE MINIMUM OF THE BENZENE ⇌ BENZVALENE ISOMERIZATION PROCESS

Chemischer InformationsdienstVolume 16, Issue 8 Preparative Organic Chemistry ChemInform Abstract: PREFULVENE AS A STABLE INTERMEDIATE AT THE POTENTIAL ENERGY SURFACE MINIMUM OF THE BENZENE ⇌ BENZVALENE ISOMERIZATION PROCESS S. OIKAWA, S. OIKAWASearch for more papers by this authorM. TSUDA, M. TSUDASearch for more papers by this authorY. OKAMURA, Y. OKAMURASearch for more papers by this authorT. URABE, T. URABESearch for more papers by this author S. OIKAWA, S. OIKAWASearch for more papers by this authorM. TSUDA, M. TSUDASearch for more papers by this authorY. OKAMURA, Y. OKAMURASearch for more papers by this authorT. URABE, T. URABESearch for more papers by this author First published: February 26, 1985 https://doi.org/10.1002/chin.198508109AboutPDF 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. Volume16, Issue8February 26, 1985 RelatedInformation

Prefulvene as a stable intermediate at the potential energy surface minimum of the benzene .dblharw. benzvalene isomerization process

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPrefulvene as a stable intermediate at the potential energy surface minimum of the benzene .dblharw. benzvalene isomerization processSetsuko Oikawa, Minoru Tsuda, Yoriko Okamura, and Tadashi UrabeCite this: J. Am. Chem. Soc. 1984, 106, 22, 6751–6755Publication Date (Print):October 1, 1984Publication History Published online1 May 2002Published inissue 1 October 1984https://doi.org/10.1021/ja00334a047RIGHTS & PERMISSIONSArticle Views204Altmetric-Citations20LEARN 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 (547 KB) Get e-Alerts Get e-Alerts

An extended version of Boyd's force field method applicable to heteroatomic molecules: Part 1. Adenine and uracil

The force field method developed by Boyd is extended to include molecules containing atoms other than C and H (e.g., N, O, P, S, Cl, Br, …). A new set of force field parameters is determined in order to redefine the potential energy functions that govern the dynamics of the internal (valence coordinates) degrees of freedom of a molecule. It is shown that the minimum of the partial potential energy surface is significantly affected by electrostatic intramolecular interactions. In this regard the non-bonded interactions appear to be less important than the dipole—dipole type interactions for a given interatomic distance when heteroatoms are present in the molecular framework. The reliability of the extended method as regards minimized structure, vibrational spectra and thermodynamic properties has been checked for more than 20 polyatomic molecules. From the correlation between calculated and experimental properties it is concluded that the method has good potential for further applications on polyatomic molecules with increasing size and topological compexities such as adenine and uracil.

An extended version of Boyd's force field method applicable to heteroatomic molecules: Part 1. Adenine and uracil

The force field method developed by Boyd is extended to include molecules containing atoms other than C and H (e.g., N, O, P, S, Cl, Br,…). A new set of force field parameters is determined in order to redefine the potential energy functions that govern the dynamics of the internal (valence coordinates) degrees of freedom of a molecule. It is shown that the minimum of the partial potential energy surface is significantly affected by electrostatic intramolecular interactions. In this regard the non-bonded interactions appears to be less important than the dipole-dipole type interactions for a given interatomic distance when heteroatoms are present in the molecular framework. The reliability of the extended method as regards minimized structure, vibrational spectra and thermodynamic properties has been checked for more than 20 polyatomic molecules. From the correlation between calculated and experimental properties it is concluded that the method has good potential for further applications on polyatomic molecules with increasing size and topological compexities such as adenine and uracil.

Calculated frequencies and intensities associated with coupling of the proton motion with the hydrogen bond stretching vibration in a double minimum potential surface

The influence of the coupling of the proton movement and the H bond stretching vibration in a double minimum potential energy surface on the energy levels, transitions, induced dipole moments and polarisabilities is calcualted ab initio as a function of an electric field for the H5O+2 system. The high polarisability of the hydrogen bonds remains to a large extent unchanged due to the coupling. New types of transitions occur, particularly when the tunnelling frequency and the frequency of the bond stretching vibration are comparable in size. Especially in this case numerous Fermi resonances occur due to the shift of the energy levels in the electric field, which leads to a considerable increase in the number of transitions. It is shown that the change of the frequencies of the transitions due to the induced dipole interaction of the bonds with fields from their environment is a decisive cause of the variety of energy level differences observed as a continuous absorption in the i.r. spectrum of such systems.

Possible existence of double minima in potential energy surfaces of AB2-type molecules

Possible existence of double minima in potential energy surfaces of AB2-type molecules