gerade, g, or if the atomic orbital does not maintain its original character, ungerade, u. The final symmetry-labeled atomic orbital is now known as an irreducible representation. Carbon dioxide’s molecular orbitals are made by the linear combination of atomic orbitals of the same irreducible representation that are also similar in atomic orbital energy. Significant atomic orbital overlap explains why sp bonding may occur. Strong mixing of the oxygen 2s atomic orbital is not to be expected and are non-bonding degenerate molecular orbitals. The combination of similar atomic orbital/wave functions and the combinations of atomic orbital/wave function inverses create particular energies associated with the nonbonding (no change), bonding (lower than either parent orbital energy) and antibonding (higher energy than either parent atomic orbital energy) molecular orbitals. Water For nonlinear molecules, the orbital symmetries are not σ or π but depend on the symmetry of each molecule. Water () is a bent molecule (105°) with C2v molecular symmetry. The possible orbital symmetries are listed in the table below. For example, an orbital of B1 symmetry (called a b1 orbital with a small b since it is a one-electron function) is multiplied by -1 under the symmetry operations C2 (rotation about the 2-fold rotation axis) and σv'(yz) (reflection in the molecular plane). It is multiplied by +1(unchanged) by the identity operation E and by σv(xz) (reflection in the plane bisecting the H-O-H angle). The oxygen atomic orbitals are labeled according to their symmetry as a1 for the 2s orbital and b1 (2px), b2 (2py) and a1 (2pz) for the three 2p orbitals. The two hydrogen 1s orbitals are premixed to form a1 (σ) and b2 (σ*) MO. Mixing takes place between same-symmetry orbitals of comparable energy resulting a new set of MO's for water: 2a1 MO from mixing of the oxygen 2s AO and the hydrogen σ MO. 1b2 MO from mixing of the oxygen 2py AO and the hydrogen σ* MO. 3a1 MO from mixing of the a1 AOs. 1b1 nonbonding MO from the oxygen 2px AO (the p-orbital perpendicular to the molecular plane). In agreement with this description the photoelectron spectrum for water shows a sharp peak for the nonbonding 1b1 MO (12.6 eV) and three broad peaks for the 3a1 MO (14.7 eV), 1b2 MO (18.5 eV) and the 2a1 MO (32.2 eV). The 1b1 MO is a lone pair, while the 3a1, 1b2 and 2a1 MO's can be localized