Question

1.what is it about organic molecules that causes them to absorb specific wavelengths of infrared radiation?

2. why do infrared spectroscopists report their data in reciprocal centimeters . Hint it has something to do with energy and proportionality

Answer 1

1. Ir spectra are based on the vibration of atoms in the molecule. The movement of the atoms and bonds can be demonstrated using balls connected by springs. When the molecule vibrates, the atoms move towards and away from each other at a certain frequency. The stretching and compression of the spring is based on the energy of the system. The vibrational frequency is proportional to the square root of the ratio of the spring force constant to the masses on the spring. If the molecule has lighter the masses on the spring, or bonded tighter the spring, then its vibrational frequency will be high. The vibrational frequencies for stretching and bending of bonds in molecules are depending on the strength of the chemical bonds between the atom and some other interactions like hydrogen bonding, masses of the atoms, and symmetry of the molecule. Only photons with certain energies will excite molecular vibrations. The molecules allows certain energy to vibrate the atom. Each organic molecule, with the exception of enantiomers, has a unique infrared spectrum. Because of these properties organic molecules absorb specific or particular wavelength of infrared radiation.

1. The electromagnetic spectrums have the nature of particle and wave and also it is composed of energy called photons. If we express the energy as a particle, we use the word photon. When we describe this energy as a wave, we use the terms frequency (v) and wavelength (λ). Wavelength is the distance between two adjacent crests. Frequency is the number of wave troughs that pass a given point in a second. According to the equation E = hv = hc / λ , frequency and wavelength are inversely related. So if frequency increases, wavelength decreases. In IR spectroscopy, we are using the unit of measurement called the wavenumber (v). The wavenumber is the number of waves in one centimeter and has the units of reciprocal centimeters (cm-1 ). Since the wavenumber is inversely proportional to wavelength, it is directly proportional to frequency and energy. So we are measuring the frequency and energy of vibration in the IR spectroscopy and this wavenumber system makes more convenient to use.