When the frequency of light is equal to the natural frequency of a molecule, molecule absorbs light.
My question is whether the molecule can have only one natural frequency? If thats the case why the natural frequency should match the energy gap between the ground and excited state.
To the first part of your question on the number of frequencies a molecule has:
A molecule will absorb light at any number of frequencies depending on its size, properties, and what aspect of the molecule is being photo-excited.
Infrared light generally matches the frequency of molecular vibrational transitions. The number of frequencies at which the molecule will absorb is therefore related to the number of atoms the molecule has, and how high you can excite it before the bonds break and the molecule dissociates.
Ultraviolet light generally matches the frequency of electronic transitions, so the number of frequencies of absorption will depend on the number of electrons and how high you can excite them before they become ionized.
Different frequencies of light can also excite other aspects of a molecule, for instance radio waves can match nuclear spin state transitions to give NMR spectra, microwaves can match rotational transitions to give rotational spectra, etc.
A molecule has many different excited states, and therefore has many different frequencies of light that match the energy gaps between them. How many, and what those gaps are will depend on the particular molecule.
I hope this answers your question,
Magnus.
Thnks dear Magnus...I am familiar with UV, IR and electronic spec..
My question is about the natural frequency of a molecule..
How do you define natural frequency of a molecule? how the natural frequency can be sometimes equal to vibrational or electronic energy levels spacing?
I am unable to understand the physical meaning of a natural frequency..
I wil be very happy if you can explain me about this..
It seems that any frequency of rob-vibrational mode of a molecule can be called the natural frequency. This term refers to the absence of the external oscillating field (that would modify the frequency of molecular vibrations is certain cases) and any interactions with neighbouring molecules.
So if a molecule is isolated, it oscillates with its natural frequency. Note that (in the simplest, harmonic picture) all the normal modes are active, albeit being in their ground states.
Hope it helps.
Marcin
I'm curious where you came across the concept of the natural frequency; that would help with interpretation. However, usually when I come across the concept, it is in the context of a Lorentz model for optical response. In this model we apply more-or-less classical mechanics to an electron bound by a harmonic force to derive things like absorption line shapes, index of refraction vs frequency, etc. The natural frequency is then the harmonic frequency of the electron, root(k/m). The spring constant is basically arbitrary here.
When applying this model to reality, you'll use any resonant frequency of the molecule as the "natural frequency", usually the closest resonance to the spectral range you're considering. So in the visible, the frequency of the HOMO-LUMO transition will take the place of the natural frequency. In the IR or microwave, you could use vibrational or rotational frequencies. However, the Lorentz model is less useful in these spectral ranges, as it is mainly valuable when you can pretend the response is dominated by a single frequency.
The natural frequencies of a molecule are related to its possibilitis to vibrate.
For a non'linear molecule of n atoms thera are 3n-6 degrees of freedom (each atom can move along x or y or th z axis => 3n possibilities. Then one substract the rigid motions of the whole molecule i.e., 3 translations (along x, y, z axis) and 3 rotations (arond the x,y,zaxis) leading to 3n--6 motions of vibrations WITH 3N-6 NATURAL FREQUENCIES; ex wter molecule 3 atoms => 3x3-6= 3 motions of vibrations (2 strechting modes along the OHbonds and one bending vibration OHO )
A linear molecule as only 3n-5 vibrations: ex for the X-Y molecule, one has 3x2- 1 stretch motion at one single frequency.
The whole theory of molecular vibrations is a little more complicated, specially for large molecule, depending essentially on the type of symetry of the molecule. It is also possible to define NATURAL frequency of PARTS of the molecule (CH2, CH3 groups, ABN bases, sugars and phosphated groups etc).
- Badges
- Science topic