Organic Chemistry Across the Universe Extraterrestrial Transformation Spying on things a world away
How do we know which molecules sprinkle our galaxy like needles in a haystack? Very few have actually been physically captured. Mass spectrometers on spacecraft suck gas particles up from space and identify them, but we can not travel very far in our galaxy. The distances that need to be covered in space make this method impossible for revealing the extent of the existence of molecules. What is the fastest thing we know? Light eats up distance and it is electromagnetic radiation, of which visible light is one form, which contains evidence of a molecule in space. Molecules can absorb starlight and other cosmic radiation. Electromagnetic radiation is like Wheatabix in the morning to a molecule. The energy that is transferred to the molecule elates it into an excited state increasing its rotations or vibrations about its bonds like a hyperactive child. Electromagnetic radiation can only interact with matter when there is a change of electric dipole within the molecule. The C-H bond has an electric dipole because the carbon atom is more electronegative than the hydrogen atom. It attracts the two electrons in the bond towards itself. It is as if the carbon atom is winning a tug of war. When the bond stretches, which is equivalent to a vibration, the position of the dipole changes with the length of the bond. The molecule can then absorb energy through vibration. The methane molecule, which has been detected around stars(1), has four molecular vibrations. Only the asymmetric stretch and the asymmetric bend cause a change in electric dipole resulting in the absorption of energy. Only these vibrations will be observed in a vibration spectrum. Spectrums are like graphs. They give a picture of the electromagnetic radiation that is absorbed by the molecule resulting in certain frequencies not entering the telescope. In an absorption spectrum the peaks show which frequency of radiation has been absorbed by the molecule to put it in an excited state. An emission spectrum gives peaks showing the radiation the molecule emits when it relaxes from an excited state back down to its normal energy state. Light shining from distant parts of the galaxy can be examined by telescopes situated on Earth or in Earth's orbit.
How do we know which molecule is getting excited?
To make a bond vibrate or rotate takes a discrete amount of energy. It is like our shoe size. If we are size 8 we can not walk in a pair of size 5 shoes. A bond can not vibrate unless it is given exactly the right amount of energy. The amount of energy a bond needs depends on the type of atoms the bond connects and the strength of the bond. A bond is like a metal spring. It will vibrate at a different frequency depending on the mass connected to the end of the spring. A C-H bond vibrates at a higher frequency then a C-N bond due to the different mass of a hydrogen and nitrogen atom. A carbon triple bond vibrates at a higher frequency to a carbon single bond due to the higher strength of the bond. As if the bond was a stronger spring. Energy is related to frequency by the equation:
E= hv. E= Energy, h=Plank's constant, v= frequency.
The frequencies of the peaks in a vibration spectrum will indicate which bonds occur
in a molecule. Fragments of bonds can then be connected together like a jigsaw puzzle propose a structure for the molecule that is causing the spectrum. In a highly symmetric molecule like CH4 the molecule can be seen to vibrate as a whole unit. In more complex molecules bonds can be looked at separately.
An Illustration of the vibration frequency of different bonds.
