Formaldehyde molecular orbitals

Here are the energies and symmetry designations for the next set of molecular orbitals for formaldehyde ... 

For formaldehyde, molecular orbital number 8 is the HOMO, and molecular orbital number 9 is the LUMO. In this case, the energy also changes sign at the point separating the occupied from the unoccupied orbitals. 

Lowest unoccupied molecular orbital (or formaldehyde (tapi and ethylene... 

Highest occupied molecular orbital for formaldehyde (tapI and ethylene... 

Ab initio molecular orbital calculations are being used to study the reactions of anionic nucleophiles with carbonyl compounds in the gas phase. A rich variety of energy surfaces is found as shown here for reactions of hydroxide ion with methyl formate and formaldehyde, chloride ion with formyl and acetyl chloride, and fluoride ion with formyl fluoride. Extension of these investigations to determine the influence of solvation on the energy profiles is also underway the statistical mechanics approach is outlined and illustrated by results from Monte Carlo simulations for the addition of hydroxide ion to formaldehyde in water. 

The 7i bonding molecular orbital of formaldehyde (HCHO). The electron pair of the n bond occupies both lobes. 

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

The case of formaldehyde is interesting because it contains all the three types of electrons and the energies for the various types of molecular orbitals differ significantly. 

Fig. 2.1. Energy levels of molecular orbitals in formaldehyde (HOMO Highest Occupied Molecular Orbitals LUMO Lowest Unoccupied Molecular Orbitals) and possible electronic transitions.

In absorption and fluorescence spectroscopy, two important types of orbitals are considered the Highest Occupied Molecular Orbitals (HOMO) and the Lowest Unoccupied Molecular Orbitals (LUMO). Both of these refer to the ground state of the molecule. For instance, in formaldehyde, the HOMO is the n orbital and the LUMO is the n orbital (see Figure 2.1). 

Figure 2.16 Molecular orbitals, their approximate energy levels and types of transitions in formaldehyde molecule.

Figure 3.11 Symmetry and charge distribution m molecular orbitals of formaldehyde n (b2), n (bj) and w (bj) and o (r ).

Figure 18-10 Molecular orbital diagram of formaldehyde, showing energy levels and orbital shapes. The coordinate system of the molecule was shown in Figure lfF9. [From w. l Jorgensen and L Salem, The Organic Chemist s Book ot Orbitals (New York Academic Press, 1973).]...

In an electronic transition, an electron from one molecular orbital moves to another orbital, with a concomitant increase or decrease in the energy of the molecule. The lowest-energy electronic transition of formaldehyde promotes a nonbonding (ri) electron to the antibonding pi orbital (it ).1 1 There are two possible transitions, depending on the spin quantum numbers in the excited state (Figure 18-11). The state in which the spins are opposed is called a singlet state. If the spins are parallel, we have a triplet state. 

The highest occupied molecular orbital (HOMO) in formaldehyde and heteroaldehydes, H2C=E, is the lone pair at E (nE), and the second highest MO (SOMO) is the C=E 77-bonding orbital. The LUMO is the 77 CE orbital composed of the antibonding combination of pz(C) and pz(E). The ionization energy of the HOMO in formaldehyde is 10.88 eV and of the SOMO 14.5 eV, as determined by photoelectron spectroscopy.33 The ionization energy of the HOMO and the SOMO both decrease considerably when the oxygen atom in formaldehyde is replaced by sulfur or selenium (see Fig. 1, data are compiled from Refs. 33-37). 

These deviations from linearity indicate the existence of an oligomeric distribution of chiral ligands. Noyori proposed a rationale as follows Due to the different dissociability (stability) of homochiral and heterochiral dimer, the enantiopurity of the remaining reactive catalyst (monomer) is improved as compared with that of the submitted chiral ligand 6 (Scheme 9.5) [11]. Heterochiral dimer is thermodynamically more stable than homochiral dimer, which is consistent with Noyori s rationale mentioned above [12a]. An ab initio molecular orbital study was also reported in a simplified model reaction between formaldehyde and dimethylzinc catalyzed by achiral 2-aminoethanol [12b]. 

The influence of substituents on the energetics of the uncatalyzed Mukaiyama aldol reaction was studied using ab initio molecular orbital calculations at the G3(MP2) level <2005JOC124>. For the reaction between formaldehyde and trihydrosilyl enol ether, a concerted pathway via a six-membered transition state was favored over a stepwise pathway and an oxetane intermediate. 

For hydrogen bonds weaker than OH 0=C or NH 0=C, theory predicts smaller differences in the X-H bond lengths and negligible changes in the nonhydrogen bonds. TWo models have been studied theoretically with the ab-initio molecular orbital method at the HF/3-21G level of approximation. These are formaldehyde oxime cyclic dimer versus the monomer [378] and formaldehyde hydrazone cyclic dimer versus the monomer [379]. Except for the O-H and N-H bonds, these differences are too small to be detected by single crystal X-ray or neutron diffraction methods, as shown in Fig. 5.2. 

Figure 7 Proposed molecular orbital scheme for uncomplexed and complexed formaldehyde...

The geometries found for the complexes of formaldehyde with Brst and second row cations in theoretical studies were analyzed in terms of molecular orbitals. Based on the results of photoelectron spectroscopy, it was argued that the carbonyl group contains two nonequivalent lone pairs an sp-hybridized orbital contains one pair of electrons along the C—O axis and a second, higher energy lone pair in a p-like orbital lies perpendicular to the C=0 axis (Figure 7). ... 

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