Molecular-Weight Dependence of the Optical Rotation of Poly((R)-2-deuterio-n-hexyl isocyanate)

Abstract

The extraordinarily large optical rotations [a] of poly((R)-2-deuterio-n-hexyl isocyanate) were investigated with a particular emphasis on the chain length dependence of [a]. Data for [a] in hexane, 1-chlorobutane, and dichloromethane were obtained as functions of temperature and degree of polymerization N and analyzed by a statistical mechanical theory developed recently (Lifson, S.; Andreola, C.; Peterson, N. C.; Green, M. M. J . Am. Chem. SOC. 1989, 111, 8850). The theory is based on a model whereby a polymer chain consists of an alternate sequence of M helix (left-handed) and P helix (righthanded), interrupted by helix reversal points, and la1 originates from the excess presence of the P helix over the M helix due to the chiral substitution of a deuterium atom on the side chain. Detailed analysis of [a] vs N data substantiated the validity of the theory and allowed a separate estimate of enthalpic and entropic contributions to the isotope effect, revealing that the P helix dwells in a slightly narrow but deep energy well in the conformational space compared with the M helix. In hexane at 25 "C, this isotope effect favors the P helix over the M helix by 0.74 cal mol-' on a monomer unit basis, implying that the P helix would exist in excess of the M helix only by 0.12%. However, for a long chain with N = 2000, this minute excess is amplified by the cooperative mechanism to 67:33 in [a], whereas the helix reversal costs 3900 cal mol-l and appears only once in 762 units on the average. As reported in previous c~mmunications,~-~ solutions of poly((R)-1-deuterio-n-hexyl isocyanate) (1, aPdHIC) show extraordinarily large optical rotations (OR, negative) compared with its monomer in organic solvents. Recently, Green et al.5 have found that poly((Rb2-deuterio-n-hexyl isocyanate) (2, PPdHIC), which has a chiral center on the ,&carbon on the side chain, also exhibits substantially the same OR behavior, although to a less extent. This is a remarkable phenomenon on three accounts. First, it is an enormous amplification of a structural deuterium isotope effect, since the deuterated parent monomer shows only a very small optical rotation. Second, the optical rotation is highly temperature dependent, approaching its maximum value at a very low temperature. Third, the OR is remarkably molecular-weight dependent, being close to zero for oligomers and approaching its maximum value at very high degrees of polymerization for any given temperature. The molecular-weight dependence of OR of hexane, 1-chlorobutane, and dichloromethane (DCM) solutions of pPdHIC is the subject of the present study. These solvents were chosen because they allowed optical rotation measurements in the ultraviolet region and the stiffness of the polymer chain differs significantly in them. Taken in part from the doctoral thesis of C. Andreola, @ Abstract published in Advance ACS Abstracts, January 15, Polytechnic University, 1991. 1995. 0024-929719512228-1016$09.0010 0 0 1 2Polyisocyanates, -(CO-N(R)),-, are known to be stiff po1ymers6-l2 because of their...

Molecular Mechanisms for the Optical Activities of Polyisocyanates Induced by Intramolecular Chiral Perturbations

“…33 • 34 Thus we take the results shown in Figure 6 to be consistent with the assumed molecular mechanism. This v value implies that there exists one chain with a helix * 1 If it is assumed that optical rotation arises only from the helical conformation of the main chain, it is regitimate to use mean residue rotation ([IX] based on the mass concentration times M,.,) as a measure for 2/M-l.…”

The Macromolecular Route to Chiral Amplification

“…Fitting this ªchiral solvationº in the identical manner to the theory discussed above for the deuterated polyisocyanates [23] for one of the chiral solvents, 2-chlorobutane, yields a remarkably minute energy. The chiral bias for the units is 0.04 cal mol…”

“…The qualitative picture drawn for the polyisocyanates and the understanding of the source of the chiral amplification are supported quantitatively. [23] The key energy parameters defining the excess energy of the helical reversal and the energetic bias favoring one helical sense arising from the deuterium substitution were revealed. [23] These results took the work in two unrelated directions based on these different energy terms.…”

“…Only a tiny amount of energy, on the order of a single calorie per mole per unit [23] (approximately 1 / 600 of the ambient energy), distinguishes the left-and right-handed helices in the deuterated polyisocyanates, where the R configuration at the a-and b-carbon atoms favors opposing helical senses (Figure 6). In general terms, substitution of deuterium for hydrogen leads to a lower zero-point energy, and the stronger the carbon ± hydrogen bond is, the greater is the decrease in the zero-point energy.…”

Molecular weight dependence of helical conformation of amino acid‐based polyphenylacetylenes

“…Lifson, Green, Teramoto, Sato and coworkers proposed the statistical mechanical theory for helix inversion of polyisocyanates, in which a polymer chain of optically active polyisocyanates is composed of biased right-and left-handed long helical sequences separated by the reversal points. 1,[9][10][11][12][13] This theory expects that the chiroptical properties of the polymers considerably depend on their molecular weights, because the molecular weight and the average length of the one-handed helical sequence determine the number of helix reversal per molecule. It was actually proven by the experiments using poly[(R)-2-deuterio-1-hexyl isocyanate]; for example, the specific rotation [a] at a wavelength of 300 nm rapidly increases with the degree of polymerization (N) below about 1000 but tends to level off to be limited to an apparent constant value in a higher molecular weight region.…”