Continuous-wave terahertz spectroscopy of biotin: vibrational anharmonicity in the far-infrared

A high-resolution continuous-wave terahertz spectrometer has been used to investigate the absorption spectrum of biotin in a polyethylene matrix from 6 to 115 cm−1. Gaussian linewidths from least squares fits at 4.2 K are found to increase from 1 to 5 cm−1 over this region establishing conservative lower limits on the vibrational lifetimes from 5 to 1 ps. The calculated partition function of 1.0 at 4.2 K is shown to increase to >108 at 298 K indicating enormous changes in the sequence level populations with increasing temperature. Lineshape models that include mechanical anharmonicity were therefore necessary to obtain satisfactory fits at 298 K. Anharmonicity factors (χeν̄e/ν̄e) range from 0.1% to 1% and establish their importance for refining model predictions.