Double-cross-polarization detection of labeled chemical bonds in hydrogen cyanide polymerization

Abstract

nearly linear. The different spatial averages are all about the same. Only a few special orientations may not fit this averaging pattern, thereby resulting in a slight curvature. That is, nonlinearity in Tlp(C) can arise from contributions from those methylene carbons having a carbon-proton internuclear vector lying along the sample rotation axis. Carbons approximately satisfying this condition will have relatively long Tlp(C)'s which will remain longer than the average for the whole sample even with magic-angle spinning.Regardless of the details of the orientational averaging, the observation remains that relative methylene-carbon (T^ADRFD's can be calculated if the corresponding Hh's are known. This means estimates of spin-spin contributions to methylene-carbon <Tlp(C))'s for any polymer can be made without recourse to elaborate nine-step ADRF procedures. A proton T2 or a JBSLF determination of Hh, in combination with the use of eq 8, may be sufficient. The more elaborate accurate determination of the ratio of (Tlp(C)) to (TIS(ADRF)) can then be reserved for particularly difficult cases.Estimates of (T^ADRF)) for other types of carbons are also possible. Thus, the 7-kHz HL for the protonated aromatic carbon of poly(a-methylstyrene)23 suggests a (TIS(ADRF)) of about 70 ms (eq 9). A more accurate estimate would probably result if parameters from an aromatic reference compound were used in eq 9 rather than those from glycine. Nevertheless, it is clear that the observed < Tlp(C)) at 37 kHz of 19 ms (Table V) must be dominated by spin-lattice contributions.Registry No. Glycine,; polyethylene (homopolymer), 9002-88-4; dipalmitoylphosphatidylcholine, 2644-64-6; poly(amethylstyrene) (homopolymer), 25014-31-7; polystyrene (homopolymer), 9003-53-6; hexadecane-urea clathrate, 3311-88-4; poly(o-bromostyrene) (homopolymer), 27290-16-0; isotactic polystyrene (homopolymer), 25086-18-4.

HCN polymers characterized by solid state NMR: Chains and sheets formed in the neat liquid

“…The latter detects only carbons that have formed at least one new bond with nitrogen because the contact time for the CP from 15 N to 13 C was optimized for single bond transfer. Previously, 14,15 such experiments have demonstrated the formation of new saturated and unsaturated carbon-nitrogen bonds in the dark polymer. The present results for the pale polymer indicate that the dominant signals at ϳ183 and ϳ102 ppm are due to carbons that have formed new bonds with nitrogen.…”

“…27 The sharp signals illustrate the improved resolution attainable at higher field strengths and MAS frequencies than were previously available. [13][14][15] Notable features of the 13 C spectra ͑Fig. 4͒ are the unusually large chemical shifts in the polymer signals ͑saturated at 70-100 ppm and unsaturated at 140-185 ppm͒ and the varying amount of residual nitrile in the polymer ͑isotropic signal at ϳ124 ppm flanked by characteristically large spinning sidebands marked with asterisks͒.…”

“…However, the structures of these polymers remain unknown despite extensive effort. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Figures 1 and 2 show the proposed polymer structures. The "ladder" structures in Fig.…”

“…Chemical shifts help to identify functional groups and dipolar interactions allow mapping of chemical connectivity. Previous solid state NMR studies of base-catalyzed HCN polymerization [13][14][15] determined that existing C-N bonds were not broken, new C-N bonds were created, and some secondary amides were formed. However, no further conclusions were drawn as to the structures of the polymers.…”

Dark matter in the solar system: Hydrogen cyanide polymers

“…In particular, the unambiguous presence of secondary amide groups, as in peptides, has been established by double-cross-polarization studies on polymers synthesized from equimolar amounts of H13CN and HC15N (Schaefer et al, 1982;Matthews et aL, 1984;McKay et al, 1984;Garbow et al, 1987).…”