170,280 research outputs found
FAR INFRARED SPECTRA, RING-PUCKERING VIBRATION AND MOLECULAR CONFORMATION OF SOME ANALOGUES OF BICYCLO [3.1.0] HEXANE
No full textAuthor Institution: Spectroscopy Laboratory, Massachusetts Institute of TechnologyThe far-infrared spectra of molecules of the general structure where X, Y = O or have been investigated. The lowest frequency mode, the ring-puckering vibration, gives rise to a series of Q branches which may be interpreted as single quantum jumps within a single-minimum asymmetric potential function of the form v(x) = + + , where x is the ring-puckering coordinate and a, b and c are all positive
ANALYSIS OF THE RING-PUCKERING SPECTRA OF 1,3-DISILACYCLOBUTANE AND ITS DERIVATIVE.
No full textAuthor Institution: Department of Chemistry, Texas A\&M UniversityThe low-frequency infrared and Raman spectra of 1,3-disilacyclobutane, , and its derivative have been analyzed using the usual quartic-qudratic potential energy function for the ring-puckering vibration. A puckering model which allows rocking of the groups was utilized in order to obtain the kinetic energy expansions. The incorporation of the rocking motion permits analysis of the data for both isotopic molecules with the same potential energy function, which has a barrier to inversion of
Dependence of pyranose ring puckering on anomeric configuration: methyl idopyranosides
No full textIn the aldohexopyranose idose, the unique presence of flexibility, rendering it challenging to study experimentally and an three axial ring hydroxyl groups causes considerable conformational excellent model for rationalizing the relationship between puckering and anomeric configuration. Puckering in methyl alpha- and beta-L-idopyranosides was predicted from kinetically rigorous 10 mu s simulations using GLYCAM11 and three explicit water models (TIP3P, TIP4P, and TIP4P-EW). In each case, computed pyranose ring three-bond (vicinal) H-1-H-1 spin couplings ((3)J(H,H)) trended with NMR measurements. These values, calculated puckering exchange rates and free energies, were independent of the water model. The alpha- and beta-anomers were C-1(4) chairs for 85 and >99% of their respective trajectories and underwent C-1(4)-> C-4(1) exchange at rates of 20 mu(-1) and 1 mu s(-1). Computed alpha-anomer C-1(4)<-> C-4(1) puckering rates depended on the exocyclic C6 substituent, comparing hydroxymethyl with carboxyl from previous work. The slower kinetics and restricted pseudorotational profile of the beta-anomer were caused by water occupying a cavity bounded by the anomeric 1-O-methyl and the C6 hydroxymethyl groups. This finding rationalizes the different methyl alpha- and beta-L-idopyranoside (3)J(H,H) values. Identifying a relationship between idopyranose anomeric configuration, microsecond puckering, and water structure facilitates engineering of biologically and commercially important derivatives and underpins deciphering presently elusive structure-function relationships in the glycome
THE FAR-INFRARED SPECTRA OF SELECTED RING COMPOUNDS WITH ASYMMETRIC PUCKERING POTENTIAL FUNCTIONS
No full text C. S. Blackwell, L. A. Carreira, J. R. Durig, J. M. Karriker, R, C. Lord, J. Chem. Phys. 56, 1706 (1972). L. A. Carreira and R. C. Lord, J. Chem. Phys, 51, 2735 (1959). J. N. Bevan, A. C. Legon, and D. J. Millen, Proc, Roy. Soc. London A354, 491 (1977).""Author Institution: Department of Chemistry, University of CalgaryThe far-infrared spectra of several ring compounds with asymmetric puckering potential functions are either re investigated or reported for the first time, Among these are cyclopentene trimethylene sulfoxide (TMSO), 2-methyl oxetan and 3-methyl oxetan. In some cases the potential function can be determined uniquely only with the help of very weak transitions in the vicinity of the inversion barrier. For TMSO the microwave results of Bevan are used to verify the assignments and to estimate a reduced mass for the puckering motion
Seam Puckering as a Mechanical Instability Phenomenon
No full textResults are presented from a study directed toward establishing quantitative relationships among the important systems parameters involved in the puckering of fabric seams treating the phenomena as a problem in mechanical instability. Using idealized models to simulate the mechanical behavior of a simple seam, the elastic stability problem encountered is that of a compressed member supported on a nonlinear foundation which is discontinuous and depends on the final shape of buckling. An energy method is used to obtain approximate theoretical solutions for the buckling behavior of such a model for the case of a straight edge seam subjected to excessive sewing thread tension. These solutions both identify the important system variables and predict what combinations will produce puckering. An experimental study conducted on simulated seam models to verify the theory shows acceptable agreement with predicted results.</jats:p
VAPOUR PHASE RAMAN SPECTRA AND THE RING PUCKERING VIBRATION OF TRIMETHYLENE OXIDE AND SEVERAL DEUTERATED ANALOGUES
No full textAuthor Institution: Division of Chemistry, National Research Council of Canada; Department of Chemistry, University of CalgaryThe vapour phase Raman spectra of trimethylene oxide and several of its deuterated analogues (, , and ) are reported. At low frequency () the transitions of the pure ring puckering vibration are observed. Various other spectral regions show evidence of combination progressions with the pure puckering energy levels. Several examples will be given of ``sum’’, ``difference’’ and ``hot band’’ sequences found particularly in the C-H and C-D stretching and bending regions. The discussion will emphasize how these results may be used as an aid in assigning some fundamental vibrations
Investigating proline puckering states in diproline segments in proteins
No full textIn the current study, the puckering states of the Proline ring occurring in diproline segments (LPro-LPro) in proteins has been investigated with a segregation made on the basis of cis and trans states for the Pro-Pro peptide bond and the conformational states for the diproline segment to investigate the effects of conformation of the diproline segment on the corresponding puckering state of the Proline ring in the segment if any. The value of the endocyclic ring torsional angles of the pyrrolidine ring has been used for calculating and visualizing various puckering states using a proposed new sign convention (+/-) nomenclature. The results have been compared to that obtained in a previous study on peptides from this group. In this study, quite interestingly, the Planar (G) conformation that was present in 14.3% of the cases in peptides, appears to be nearly a rare conformation in the case of proteins (1.9%). The present study indicates that the (C-exo/C-exo), (C-exo/Twisted C-exo-C-endo) and (Twisted C-endo-C-exo/Twisted C-endo-C-exo) categories are the most preferred combinations. For Proline rings in proteins, the states C-exo, Twisted C-exo-C-endo and Twisted C-endo-C-exo are the most preferred states. Within diproline segments, the pyrrolidine ring conformations do not show a strong co-relation to the backbone conformation in which they are observed. It is likely that five-membered rings have a considerable plasticity of structure and are readily deformed to accommodate a variety of energetically preferred backbone conformations
FOURIER TRANSFORM MICROWAVE SPECTROSCOPY OF 1, 1-DIMETHYLSILACYCLOBUTANE. INTERPLAY OF TWO TYPES OF LARGE-AMPLITUDE MOTIONS: TWO-TOP INTERNAL ROTATION AND RING PUCKERING
No full textAuthor Institution: The Graduate University for Advanced Studies; Department of Physics, Faculty of Science Kanazawa University; Department of Applied Chemistry, Kanagawa Institute of TechnologyThe rotational spectra of 1, 1-dimethylsilacyclobutane (CH3)2SiCH2CH2CH2 were observed to consist of eight components due to the four internal- rotation states AA, AE, EA and EE, each further split into the symmetric and antisymmetric states of puckering, and were analyzed by two formulations: one being a combination of a standard two-top internal rotation and ring puckering theory and the other a theory of large- amplitude motions developed by Hougen and coworkers, to yield two sets of molecular parameters, which were in good agreement with each other. Using spectra of Si and C isotopic species, molecular structure parameters were derived, including the puckering angle: 28.64 or 30.26 (two possible sets, I and II). The splitting between the two lowest puckering states was determined to be 11.90(22) MHz, which, combined with the equilibrium puckering angle, led to the puckering potential barrier of 395.3 and 347.0 cm-1 for the two sets I and II, respectively. The first-order (or linear) internal rotation terms were analyzed to give the potential barrier to CH3 internal rotation to be 567.8 and 505. 1 cm-1, respectively, for the AE (two CH3 groups rotating in the same direction, as viewed from Si) and EA (two CH3 rotating in the opposite direction) states
Competition between ring-puckering and ring-opening excited state reactions exemplified on 5H-furan-2-one and derivatives
No full textInternational audienceThe influence of ring-puckering on the light-induced ring-opening dynamics of heterocyclic compounds was studied on the sample 5-membered ring molecules γ-valerolactone and 5H-furan-2-one using time-resolved photoelectron spectroscopy and ab initio molecular dynamics simulations. In γ-valerolactone, ring-puckering is not a viable relaxation channel and the only available reaction pathway is ring-opening, which occurs within one vibrational period along the C-O bond. In 5H-furan-2-one, the C=C double bond in the ring allows for ring-puckering which slows down the ring-opening process by about 150 fs while only marginally reducing its quantum yield. This demonstrates that ring-puckering is an ultrafast process, which is directly accessible upon excitation and which spreads the excited state wave packet quickly enough to influence even the outcome of an otherwise expectedly direct ring-opening reaction
Ritz assignment and Watson fits of the high-resolution ring-puckering spectrum of oxetane
No full textOxetane is a four-membered ring molecule exhibiting a large-amplitude ring-puckering motion. In order to analyze this vibration we recorded a rotationally resolved far-infrared spectrum between 50 and 145 cm(-1). The analysis of the ring-puckering fundamental band with the assignment of 1108 lines, has been presented in a previous paper. In the present work we present a list of further 6531 assigned transitions between the five lowest excited ring-puckering states. The 4983 term values involved in the transitions assigned in this and in the preceding work have been evaluated by the "Ritz" program, and are now available. An A-reduced Watson Hamiltonian in any of the three representations I-r, IIr, and IIIr was used to perform a fit of the assigned transitions. Precise rotational constants and quartic as well as a full set of sextic centrifugal distortion constants were obtained for the investigated ring-puckering states. For the first time, high-resolution values for the vibrational G(v) parameters have been obtained, and we have added terms in x(6) and x(8) to the double minimum-potential well describing the ring-puckering motion, in order to reproduce their values within the experimental accuracy. The same potential still reproduces the lower resolution values of the Q-branch origins involving higher ring-puckering states up to nu(rp) = 14 found in the previous literature. (C) 2003 Elsevier Science (USA). All rights reserved
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