- 2H NMR investigations of the hexadecane/urea inclusion compound
-
The molecular behavior of selectively deuterated n-hexadecane in the urea inclusion compound is studied between 100 and 290 K by means of dynamic 2H NMR spectroscopy employing line shape studies and spin-spin and spin-lattice relaxation experiments. Particular emphasis is given to the changes of the guest molecule behavior close to a solid-solid phase transition occurring at lower temperatures which is accompanied by a distortion of the urea lattice. It is demonstrated that spin-lattice relaxation experiments are of particular help for the evaluation of the chain dynamics in such systems. A comprehensive computer analysis of the available experimental data could provide a detailed picture of the hexadecane chains in the low-temperature phase and above the solid-solid phase transition. In the low-temperature phase the alkyl chains are found to undergo fast but restricted rotational motions (rate constant ca. 107 s-1). In addition, intramolecular trans-gauche isomerizations and methyl group rotation contribute to the spin relaxation of the alkane chain ends. In the high-temperature phase the alkyl chains rotate rapidly (rate constant ca. 1010 s-1) and almost unrestricted around the channel long axis, giving rise to a dynamic (rotational) disorder of the embedded alkyl chains. It therefore is concluded that the phase transition can be assigned to a (dynamic) order-disorder transition. The alkane chains furthermore are found to be perfectly aligned with respect to the urea channel long axis. At the same time, they exhibit an internal flexibility gradient toward the chain ends. The conformational order significantly is altered at the phase transition. Thus, the trans population in the low-temperature phase is given by pt, = 0.7 while in the high-temperature phase a value of pt = 0.95 has been derived.
- Schmider, Judith,Müller, Klaus
-
p. 1181 - 1193
(2007/10/03)
-
- Photochemistry of alkyl halides. 12. Bromides vs Iodides
-
Conditions have been developed for optimizing ionic photobehavior material balances from alkyl bromides. Hydroxide ion as an efficient for the byproduct HBr while giving minimal competing photoreduction via electron transfer to the alkyl bomide. The photobehavior of bromides 1, 11, 25, and 40 has examined and with that of the corresponding iodides 2, 12, 26, 41 under conditions. In each case, the bromide higher yields of products derived from out of cage radical intermidiates than the corresponding iodide. However, with the 2-norbornyl bromides 11 and iodides 12 showed that, of products not formed from the out of cage 2-norbornyl radical 13, the bromides 11 gave a higher percentage of products from the ionic intermediates 15 and 16 than did the iodides. Thus, electron transfer within the radical pair 14 is apparently more rapid for bromides than iodides, as expected on the of the relative electronegativities of bromine iodine. It is that the substantially higher yields of out of radical products from alkyl bromides may be due in to formation of the radical pair with greater excess energy, which results in more rapid escape from the cage. The epimeric 2-norbornyl bromides 11x and 11n underwent no detectable interconversion and afforded somewhat different product ratios. The more hindered epimer 11n underwent conversion to products at a slower than 11x. By contrast, 12x and 12n underwent substantial interconversion via out of transfer of an iodine atom from iodide 12 to radical 13. Epimerization was significantly attenuated in the more viscous solvent tert-butyl alcohol.
- Kopp, Paul J.,Adkins, Rick L.
-
p. 2709 - 2717
(2007/10/02)
-
- A Vibrational and Structural Study of the Solid-Solid Phase Transitions in C10H21NH3Cl
-
The joint vibrational and structural study of n-decylammonium chloride results in a fair description of the solid phases and their interconversions during the thermotropic phase transitions.Phase i (stable below 312 K) crystallizes as a well-ordered, interdigitated layer structure, whereas phases α (stable above 318 K), β (stable between 318 and 316 K), δ (stable between 316 and 302 K), and ε (metastable below 302 K) represent intermediate states between a very disordered smectic B and a quite densely packed smectic H.The i -> δ transition consists in a reconstructive separation of the n-decylammonium chains, whereas the other transitions entail a successive disordering of the organic layers and the polar lattice.Disorder in the aliphatic chains takes the form of chain defects (G forms, GTG sequences, and kinks).Disorder in the polar lattice consists of jumps of the ammonium groups between several equivalent potential wells inside an appropriately distorted tetragonal chlorine pyramid.Only short-range order exists in phase α, whereas there are some intralayer correlations in phases β, δ, and ε (phase ε reaches an almost completely ordered state at about 80 K).Comparable populations of G forms, GTG sequences and kinks are observed in phases δ, β, and α; different chain tiltings and packings are realized, however.This difference is also reflected in different molecular motions in phases δ, β, and α.
- Schenk, K. J.,Ogle, C. A.,Chapuis, G.,Cavagnat, R.,Jokic, A.,Rey-Lafon, M.
-
p. 5040 - 5049
(2007/10/02)
-
- Structure and Dynamics of Pyramidic Liquid Crystals by Deuterium MNR and X-Ray Diffraction
-
Pyramidic liquid crystals are mesogens derived from the cone-shaped tribenzocyclononene (TBCN) core, substituted with appropriate side chains.The pyramidic mesophases consist of two-dimensional columnar structures in which the rigid cores of the mesogen molecules are stacked in parallel on top of each other.In the present work X-ray and deuterium NMR measurements on the solid and mesomorphic phases of homologues of the alkyloxy (I-n) and alkanoyloxy series (II-n) are reported.In the PA mesophase of series I-n, the columns for a two-dimensional hexagonal array with a single molecule per unit cell.Deuterium NMR measurements on three specifically deuterated isotopomers of I-8 (including species deuterated at the unsubstituted aromatic sites, crown-ring methylenes and α-methylene of the side chain) indicate that within this mesophase two types of uncorrelated motions with very different correlation times can be distinguished: (i) fast fluctuations of the molecular C3 axis, which results in an effective orientational order of S = 0.85, essentially independent of temperature; (ii) planar reorientation about the C3 axis.The rate of the latter process is within the dynamic range of deuterium NMR, allowing its determination by line-shape analysis.Assuming this motion to correspond to planar diffusion, a diffusion constant at room temperature of DR(300 K) = 6.8E5 rad2/s with an activation energy of ΔE = 63 kJ/mol is derived from the experimental spectra.At least the first three atoms in the alkyloxy side chains do not participate in fast (on the NMR time scale) conformational isomerization in the solid or in the PA mesophase.The high-temperature mesophase of series II-n, PC, is very similar to the PA mesophase of I-n.It has an hexagonal structure with, however, four columns per unit cell.Deuterium line-shape analysis on specifically deuterated II-13 species indicates the same type of averaging motions as in the PA mesophase with an orientational order parameter of S = 0.93 (independent of temperature), and DR(373 K) = 1.4E5 rad2/s with ΔE = 112 kJ/mol.The low-temperature mesophase of this series, PD, is biaxial and appears to be rectangular or very nearly so.It is much less mobile than the PC mesophase, as evidenced by lack of planar reorientation, although there are still fast fluctuations of the molecular C3 axis (giving S = O.93).Unlike in the alkyloxy (I-n) series, in the alkanoyloxy (II-n) series there is fast side-chain isomerization in the PC and PD mesophases and to some extent even in the solid.The line shape of the α-methylene deuterons in the PD mesophase suggest a two-site jump process, while in the high-temperature PC phase, additional motions appear to set in, resulting in further narrowing of the spectrum width.Additional deuterium NMR measurements on deuterated I-8, II-11, and II-13 dissolved in nematic solvents in order to determine molecular structural parameters are reported.
- Poupko, R.,Luz, Z.,Spielberg, N.,Zimmermann, H.
-
p. 6094 - 6105
(2007/10/02)
-
- The Synthesis of Specifically and Selectively Deuteriated 4,4'-Bisalkoxyazoxybenzene Derivatives
-
In connection with deuterium n.m.r. studies of molecular motion in liquid crystals we have developed a number of methods for the synthesis of selectively deuteriated 4,4'-bisalkoxyazoxybenzenes.This paper is concerned with (i) the labelling of specific methylene segments of the alkoxy-chains, (ii) the labelling of the aromatic nucleus, and (iii) the selective enrichment of the deuterium content of one alkoxy-chain relative to the other.Our studies of the n.m.r. spectra of these liquid crystals have shown that there is not (as has sometimes been supposed) a monotonic decrease in C-D quadrupole splitting in passing from the oxygen to the CD3 end of the alkoxy-chain.In -PAA (p-azoxyanisole) we have also shown that the smaller splitting is associated with the CD3O group nearest to the NO end of azoxy-group.Whereas the photorearrangement of azoxy-compounds is normally regiospecific, the photorearrangement of PAA selectively deuteriated in one methoxy-group is unusual in that it leads to isotopically scrambled products.
- Boden, Neville,Bushby, Richard J.,Clark, Leslie D.
-
p. 543 - 551
(2007/10/02)
-