Direct access to polyisocyanide screw sense using vibrational circular dichroism

We show that the screw sense of polyisocyanide helices can be determined in a simple manner from the vibrational circular dichroism (VCD) of their CN-stretching mode. The relation between VCD and molecular structure is obtained using the coupled-oscillator approximation. It is shown that since the C═N groups point approximately radially outward from the helical axis, the CN-stretch region of the VCD spectrum of a polyisocyanide helix consists of a single couplet, the sign of which is directly related to the screw sense of the helix. We use this method to determine the screw sense of poly(R)-2-isocyanooctane and poly(S)-2-isocyanooctane from their VCD spectrum.