Why carbon dioxide and methane do not have dipole moment in absence of electric field?
Answers
Molecular rotations require little energy to excite them. Pure rotation spectra occur in the microwave region of the spectrum (~1 - 200 cm-1). It is important to note that a molecule cannot rotate about some arbitrary axis - the principle of conservation of angular momentum dictates that only a few rotations are possible. In general, rotation must be about the centre of mass of a molecule, and the axis must allow for conservation of angular momentum. In simple cases, this can often be recognised intuitively through symmetry - such as with the water molecule.
Molecular rotations require little energy to excite them. Pure rotation spectra occur in the microwave region of the spectrum (~1 - 200 cm-1). It is important to note that a molecule cannot rotate about some arbitrary axis - the principle of conservation of angular momentum dictates that only a few rotations are possible. In general, rotation must be about the centre of mass of a molecule, and the axis must allow for conservation of angular momentum. In simple cases, this can often be recognised intuitively through symmetry - such as with the water molecule.A pure rotation spectrum can only arise when the molecule possesses a permanent electric dipole moment. Like with vibrational spectroscopy, the physical effect that couples to photons is a changing dipole moment. Since molecular bond lengths remain constant in pure rotation, the magnitude of a molecule's dipole cannot change. However, since electric dipole is a vector quantity (it has both size and direction) rotation can cause a permanent dipole to change direction, and hence we observe its spectra. Since homonuclear molecules such as dinitrogen (N2) have no dipole moment they have no rotation spectrum. Highly symmetric polyatomic molecules, such as carbon dioxide, also have no net dipole moment - the dipoles along the C-O bonds are always equal and opposite and cancel each other out. It is important to recognise also that if a molecule has a permanent dipole, but this dipole lies along the main rotation axis, then the molecule will not have a rotational spectrum - such as for a water molecule.
Answer:
thanks carbon dioxide has a linear geometry it has carbon at its centre and oxygen on its both sides since oxygen is more electronegative than carbon electron cloud is more pulled towards oxygen and both oxygen pull the electron cloud with the same tedency from both side therefore the net resultant becomes zero hence the ...