36 LIGHT AND LIFE 



explanation ot the origin of the characteristic absorption bands as- 

 sociated with carbonyl and thiocarbonyl groups in organic molecules. 

 For an understanding of their interpretation, it is necessary to examine 

 the symmetry properties and spatial distributions of the various 

 molecular orbitals involved. 



Fig. 3 shows schematically the optically important orbitals on the 

 formaldehyde, H^.CO, skeleton (solid contour line designates (+) 

 region, dashed contour line (— ) region of orbital) . The orbitals are 

 arranged in order of decreasing binding energy, left to right. The 

 (T-orbital and the o-*-orbital (with a node perpendicular to the 

 C-O axis) are cylindrically symmetrical about the C-O bond, and 

 are shown as made of hybridized C (approx. sp^) and hybridized 



<r TT n(2p„) TT a- 



Fig. 3. Classification of molecular orbitals (schematic contour 

 diagrams, formaldehyde skeleton). 



O (probably approx sp) orbitals. (The hybridization at the O-atom 

 is unknown. If the o-orbitals are as shown, the second, more tightly- 

 bound lone-pair on the O atom would also be sp; otherwise it would 

 be 25 if non-hybridized. The perturbation produced by the C atom 

 upon bond formation probably mixes the 25 and the 2p orbital of 

 the O atom that lies along the C-O axis.) 



The 77-orbital and 77-*-orbital are both antisymmetric with respect 

 to reflection in the molecular plane; in addition, the 7r*-orbital has a 

 nodal plane perpendicular to the C-O axis. (Since the Tr-orbital has 

 its center of gravity shifted toward the O atom because of its elec- 

 tronegativity relative to the C atom, the 7r*-orbital, in order to be 

 orthogonal to the former, must have its center of gravity shifted 

 toward the C-atom.) The n-orbital is a non-bonding "lone-pair" 2p 

 orbital on the O atom. It is symmetrical with respect to reflection in 

 the molecular plane. In the ground state of formaldehyde, the elec- 

 tronic configuration is . . . a'-TT-n- (neglecting other filled orbitals) . 



Mulliken and McMinray (27, 22) recognized that formaldehyde 

 has a strong absorption band at about 1650A which must correspond 



