Carbon-13 NMR Chemical Shifts of Dimeric Model Compounds of Poly(propylene Oxide):  A Proof of Existence of the (C−H)···O Attraction

Abstract

Conformational energies of poly(propylene oxide) (PPO) have been determined from 13 C NMR chemical shifts of its six dimeric model compounds. The model compounds were prepared and fractionated by supercritical fluid chromatography into three components: CH 3 OCH 2 CH(CH 3 )OCH 2 CH(CH 3 )OCH 3 (headto-tail); CH 3 OCH 2 CH(CH 3 )OCH(CH 3 )CH 2 OCH 3 (head-to-head); CH 3 OCH(CH 3 )CH 2 OCH 2 CH(CH 3 )OCH 3 (tailto-tail). Carbon-13 NMR measurements using 1 H broad-band decoupling and DEPT techniques were carried out for the benzene solutions at 25 °C, and all observed NMR peaks were assigned to the methine, methylene, pendant methyl, and terminal methoxy carbons. By the simulation based on the rotational isomeric state (RIS) scheme and γ-and δ-substituent effects for the 13 C NMR chemical shifts, the γ-and δ-effect parameters and the conformational energies were optimized; the root-mean-square error between the calculated and observed chemical shifts was minimized to 0.12 ppm. Values of the γ-anti effect (-2.6 ppm) of oxygen, the γ-gauche effects of carbon (-4.9 ppm) and oxygen (-7.9 ppm), and the δ-effect (2.1 ppm) of oxygen for the g + gconformations were obtained to fall within the allowable ranges, as shown in the parentheses. The conformational energies evaluated here are comparable to those determined for isotactic PPO in our previous studies. These results confirm our interpretation of the gauche oxygen effect: The gauche stability of the C-C bond in the main chain of PPO is due to the intramolecular (C-H)‚‚‚O hydrogen bonding.

Structure–Property Relationships of Poly(glycolic acid) and Poly(2-hydroxybutyrate)

“…The crystallographic software PLATON did not detect any classical hydrogen bond in the optimized structure but reported a close contact: C–H···O (C–H = 1.09 Å, H···O = 2.41 Å, C···O = 3.4959 Å, and ∠C–H···O = 173°). Our previous studies on polyethers have estimated the interaction energies of such weak C–H···O attractions to be about −1 kcal mol –1 . − Therefore, some more attractions would be formed in the all-trans crystal. For example, a short distance of 3.038 Å can also be found between interchain carbonyl C δ+ and O δ− atoms.…”

“…The α form of nylons 4 and 6 includes all-trans chains that are bridged by the classical CO···H–N hydrogen bond, , which causes comparatively large negative Δ E CP ’s and Δ H u ’s and, furthermore, high T m 0 ’s. ,, Of the polymers in Table , PGA exhibits the largest negative Δ E CP (in cal g –1 ) although it forms no classical hydrogen bond. The all-trans crystallization is probably due mainly to the electrostatic and dipole–dipole attractions and partly to the weak C–H···O hydrogen bond, − , and these attractions are more effective in the all-trans structure than in the tgt conformation. The stabilization due to such dipole–dipole interactions was also found in the crystal of poly­(ethylene sulfide), ,, whose T m 0 is as high as 216 °C (cf.…”

“…12,61,62 Of the polymers in Table 12, PGA exhibits the largest negative ΔE CP (in cal g −1 ) although it forms no classical hydrogen bond. The all-trans crystallization is probably due mainly to the electrostatic and dipole−dipole attractions and partly to the weak C−H•••O hydrogen bond, [41][42][43][44][45][46][47]63 and these attractions are more effective in the all-trans structure than in the tgt conformation. The stabilization due to such dipole−dipole interactions was also found in the crystal of poly(ethylene sulfide), 45,64,65 whose T m 0 is as high as 216 °C66 (cf.…”

Predictive elucidation of conformational characteristics and configurational properties of poly(1-methylphosphirane) and poly(1-phenylphosphirane) as a molecular design

“…No other intramolecular attractions were found in the two polyphosphiranes. The lone pair of BDMePE and BMePhPE adopts an sp hybrid orbital, whereas that of tetra-MEDA yields sp 6 hybridization. The high s character (50%) of the phosphines is the reason for their soft basicity, low proton (hydrogen) affinity, and lack of intramolecular PÁ Á ÁH-C attractions.…”

“…1 shows the principal elements included in skeletal bonds of polymers. So far, we have carried out conformational analyses of polymers whose backbones contain oxygen, sulfur, selenium, nitrogen, or silicon; we have elucidated conformational characteristics, configurational properties, crystal structures, electric and optical properties, and crystallization and melting of polyethers (including O), [1][2][3][4][5][6][7][8][9][10] polysulfides (S), 1,2,7,10,11 polyselenoethers (Se), 12 polyimines (N), 13,14 and polysilanes (Si) 15,16 especially in terms of weak intramolecular interactions generated by the heteroatoms. In a series of studies, we have also aimed to relate the structures and properties of the above polymers to the periodic table, i.e., electronic structures of the heteroatoms and, furthermore, to develop a methodology for molecular design of polymers.…”