Isotope Effect in Molecular Tunneling

The intramolecular binding of carbon monoxide (${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$/${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$/${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ mixtures) to myoglobin at 20 and 60 K is observed with time-resolved Fourier-transform infrared spectroscopy. The binding rates (at ≈ 1 ks) for the molecules ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$, and ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ are in the ratio 1:0.84:0.65 at 20 K, 1:0.87:0.84 at 60 K. The large isotope effect confirms earlier evidence for molecular tunneling. The fact that ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ binds faster than ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ implies that structure effects are significant in molecular tunneling.