Biochemistry - The Chemical Reactions of Living Cells Volume 3 - D. Metzler 1980

Light in Biology
Absorption of Light by Matter
Energy Levels of Molecules

The total energy of a molecule is equal to the sum of its kinetic (translational), rotational, vibrational, and electronic energies. The energy levels corresponding to rotational, vibrational, and electronic energies are always discrete. Light quanta with a wavelength of 0.2–20 mm (50–0.5 cm^-1, with frequencies from 1.5∙10¹² to 1.5∙10¹⁰ s^-1) carrying an energy of 0.6–0.006 kJ∙einstein^-1 excite molecules, promoting them from an initial rotational energy level to a higher one. In the far-infrared or microwave regions corresponding to these energies, the absorption spectra often consist of series of closely spaced lines. For example, the rotational spectrum of gaseous HCl is a series of lines spaced 20.7 cm^-1 apart, starting from the specified wavenumber and ending at 186 cm^-1 (54 μm). It should be noted that the energy of these quanta is much lower than the activation energy of conventional Chemical Reactions and lower than the average translational energy of molecules in solution at room Temperature or 3.7 kJ∙mol^-1 at 25°C).

Nevertheless, these energy values are much higher than the energy of nuclear transitions in NMR spectra (Ch. 2, Sec. 3.7). Compare, for example, the frequency of 100 MHz (10⁸ s^-1) used in NMR spectroscopy with the frequencies of 10¹⁰–10¹² s^-1 of microwave spectra.

The range of vibrational energies spans from 6 to 100 kJ∙mol^-1, which corresponds to frequencies of ~500–8000 cm^-1. The corresponding absorption bands lie in the infrared region. The excitation energy of electronic levels ranges from 120 to 1200 kJ∙mol^-1, with characteristic frequencies of 10,000–100,000 cm^-1 (wavelengths of 1000–100 nm), and the spectra are located in the visible and ultraviolet regions.