Types of electronic transitions in polyatomic molecules

An electronic transition consists of the promotion of an electron from an orbital of a molecule in the ground state to an unoccupied orbital by absorption of a photon. The molecule is then said to be in an excited state. Let us recall first the various types of molecular orbitals. 

A cr orbital can be formed either from two s atomic orbitals, or from one s and one p atomic orbital, or from two p atomic orbitals having a collinear axis of symmetry. The bond formed in this way is called a a bond. A n orbital is formed from two p atomic orbitals overlapping laterally. The resulting bond is called a n bond. For example in ethylene (CH2=CH2), the two carbon atoms are linked by one a and one n bond. Absorption of a photon of appropriate energy can promote one of the n electrons to an antibonding orbital denoted by n. The transition is then called Ti — 7i. The promotion of a a electron requires a much higher energy (absorption in the far UV) and will not be considered here. 

A molecule may also possess non-bonding electrons located on heteroatoms such as oxygen or nitrogen. The corresponding molecular orbitals are called n or- 

The energy of these electronic transitions is generally in the following order  

When one of the two electrons of opposite spins (belonging to a molecular orbital of a molecule in the ground state) is promoted to a molecular orbital of higher energy, its spin is in principle unchanged (Section 2.3) so that the total spin quantum number (S = Es , with s — I y or 1) remains equal to zero. Because the multi- 

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This brief allusion to types of transition will serve merely to give the reader an idea that the detailed description of electronic transitions in polyatomic molecules is complicated and for the moment not a realm of universal agreement. Zimmerman46 has extended the mechanistic ideas currently used by physico-organic chemists to organic photochemistry. It is possible in this way to give systematic descriptions of many organic photochemical phenomena. A systematization of this type will ultimately be necessary merely because of the multitude of facts which must be made useful. 

The +, —, e, and/labels attached to the levels in Figure 7.25 have the same meaning as those in Figure 6.24 showing rotational levels associated with and Ig vibrational levels of a linear polyatomic molecule. Flowever, just as in that case, they can be ignored for a Z — I, type of electronic transition. 

In attempting to understand the properties of, and/or processes involving, excited electronic states in polyatomic molecules, it is often necessary to consider radiationless transitions. Radiationless processes can be classified into a number of types ... 

M. Kasha has evolved still another system of notation in which electronic states are expressed in terms of the initial and final orbitals involved in a transition. This form of description is less precise than the symmetry notation but is very convenient for photochemical purposes, specially for designating energy levels of polyatomic organic compounds. In general, four types of molecules can be identified. 

Condition 4. For most polyatomic molecules monochromatic light is impossible to obtain. Monochromatic light for experiments of this type would be defined as one for which the absorption coefficient is constant. To say the least, the incident radiation will cover much unresolved rotational structure within a single band. The more common case found with high-pressure arcs and color filters will be that two or more vibration transitions within a given electronic transition will be involved. Since the absorption coefficient will never be a true constant, the strongly absorbing parts of the spectrum will be important at low pressures and will decrease in relative importance as the pressure increases. 

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