The Phenomenon\nof Vibrational Circular Dichroism Enhancement:\nA Systematic Survey of Literature Data

While the intensity of vibrational circular dichroism\n(VCD) signals\nis commonly 10<sup>4</sup>–10<sup>5</sup> times smaller than\nthat of corresponding IR signals, several kinds of systems display\nenhanced VCD spectra with <i>g</i>-values (VCD/IR intensity\nratio) above 10<sup>–3</sup> and even reaching 5 × 10<sup>–2</sup> in some exceptional cases. These systems include\ntransition metal and lanthanide complexes, protein and peptide fibrils,\nshort oligopeptide gels, crystalline compounds, gels and solution\naggregates of organic compounds. We review the literature on VCD enhancement,\nfocusing on collecting and analyzing data on enhanced <i>g</i>-values. Special attention is given to the mechanisms proposed to\nproduce these effects.