orbitals & colours

• organic molecules which are coloured and have alternative double and single bonds in long chains is called conjugation
• the molecular orbitals containing the delocalised electrons stretch along the length of the conjugated system
• the more atoms in the conjugated system the smaller the energy gap between HOMO and LUMO

  therefore the excited electrons require less energy
• the more atoms in the conjugated system, the smaller the energy gap between HOMO and LUMO, a lower light frequency and lower energy required

  when the wavelength of light absorbed falls within the visible region, the compound will exhibit the complimentary colour
• most organic molecules are colourless because the energy difference between the HOMO and LUMO is relatively large, which results in absorption of light from the ultraviolet region of the spectrum
• many chemical compounds are coloured because they absorb visible light

  yet most organic compounds are colourless
• electrons can be found in the bonding molecular orbitals,

  the bonding electrons are found in the sigma and pi bonding orbitals
• the non-bonding orbital contains the lone pairs
• the sigma and pi antibonding orbitals usually have no electrons and so are empty
• the energy from photons is used to promote electrons from bonding or non-bonding orbitals into the higher energy antibonding orbitals

  when absorptions occur, electrons are excited and them promoted from the filled orbital to the higher energy antibonding orbital
• filled orbital

  an electron in a sigma or pi bonding orbital or from a lone pair in a non-bonding orbital
• saturated compounds that contain only sigma bonds are colourless,

  and in these compounds, the sigma bonding orbitals is the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) is the sigma antibonding orbital
• the transition between the highest and lowest molecular orbitals is quite large (high energy) and is found to be corresponding to the ultraviolet part of the spectrum

  no visible light is absorbed so the compound is colourless
• with compounds containing simple pi bonds, there's still a large transition from the HOMO to the LUMO

  the compounds also absorb in the ultraviolet part of the spectrum and are colourless
• organic molecules which are coloured have alternating double and single bonds in long chains which is called conjugation
• the alternating sigma and pi bonds of benzene rings allow the electrons to be delocalised among a number of carbon atoms
• the molecular orbitals containing the delocalised electrons stretch along the length of the conjugated system
• the larger the number the number of atoms with delocalised electrons (the larger the conjugated system), the smaller the energy difference between HOMO and LUMO

  the excited delocalised electrons require less energy
• when the energy falls within the visible spectrum, the compounds appear colourless
• a chromophore is a group of atoms within a molecule that is responsible for the absorption of light in the visible range and its colour
• by comparing chromophores, its possible to find out about the energy of light being absorbed
• light can be absorbed when electrons in a chromophore are promoted from the HOMO to the LUMO