Formaldehyde, orbitals

The electrophilic species must have a low-lying empty orbital. The strongest interaction will be with the ethylene tt orbital and this leads to a strong perturbation which has a stabilizing effect. The same species would lie closer to tt of formaldehyde since the formaldehyde orbitals are shifted to lower energy. The strongest interaction would then be with tt but since both tt and are empty orbitals it does not... 

For example, in formaldehyde, H2CO, one forms sp hybrids on the C atom on the O atom, either sp hybrids (with one p orbital "reserved" for use in forming the n and 7i orbitals and another p orbital to be used as a non-bonding orbital lying in the plane of the molecule) or sp hybrids (with the remaining p orbital reserved for the n and 7i orbitals) can be used. The H atoms use their 1 s orbitals since hybridization is not feasible for them. The C atom clearly uses its sp2 hybrids to form two CH and one CO a bondingantibonding orbital pairs. 

An example will help illustrate these ideas. Consider the formaldehyde molecule H2CO in C2v symmetry. The configuration which dominates the ground-state waveflinction has doubly occupied O and C 1 s orbitals, two CH bonds, a CO a bond, a CO n bond, and two 0-centered lone pairs this configuration is described in terms of symmetry adapted orbitals as follows (Iai22ai23ai2lb2 ... 

The teehniques used earlier for linear moleeules extend easily to non-linear moleeules. One begins with those states that ean be straightforwardly identified as unique entries within the box diagram. For polyatomie moleeules with no degenerate representations, the spatial symmetry of eaeh box entry is identieal and is given as the direet produet of the open-shell orbitals. For the formaldehyde example eonsidered earlier, the spatial symmetries of the nji and nn states were A2 and Ai, respeetively. 

Orbital promotions of this type give rise to states, such as the a and A states of formaldehyde, which are commonly referred to as nn states. In addition, transitions to such states, for example the a — X and A — X transitions of formaldehyde, are referred to colloquially as n — n or n-to-7i, transitions. 

In a molecule with electrons in n orbitals, such as formaldehyde, ethylene, buta-1,3-diene and benzene, if we are concerned only with the ground state, or excited states obtained by electron promotion within 7i-type MOs, an approximate MO method due to Hiickel may be useM. 

Interaction of f. irmeilileliyde frontier orbitals with E and Nu Fig. 1.25. PMO description of interaction of ethylene and formaldehyde with an electrophile (E ) and a nucleophile (Nu ). 

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. 

Here is the output for the relevant orbitals for formaldehyde for reference ... 

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

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

Orbitals 7 and 9 (the latter is the LUMO) of formaldehyde exhibit this same character. Orbital 7 is a bonding 7t orbital, and orbital 9 is a Tt . However, the n orbital formed of the pj orbitals from the carbon and the oxygen (which also lie in the YZ plane) is not the HOMO. Instead, an orbital formed from Pj, orbitals from the carbon and the oxygen and from the s orbitals on the hydrogens is the highest occupied orbital. The contributions from the carbon and oxygen are situated along the double bond while the HOMO in ethylene was perpendicular to this bond. 

This difference is due to the two lone pairs on the oxygen. Of the six valence electrons on the oxygen atom, two are involved in the double bond with the carbon, and the other four exist as two lone pairs. In Chapter 4, we ll examine the IR spectra for these two molecules. The orbitals suggest that we ll find very different frequencies for the two systems. In Chapter 9, we ll look at the transition to the first excited state in formaldehyde. ... 

The reader may be confused to see in ethylene tt orbitals which lie in the molecular plane. In order to avoid any confusion we label the two 7r(Ml2 molecular combinations with a prime 7r cm. Similar orbitals occur in ketene (III.42), formaldehyde (III.20), butadiene (III.Go), etc. 

Sp and sp-carbon atoms such as a carbonyl group and an alkene are reactive cent-ersl in many kinds of reactions. The n orbitals of a simple molecule such as ethylene or formaldehyde are symmetric in magnimde and antisymmetric in sign with respect to reflection in the molecular plane. Various attempts have been made to rationalize stereoselectivities, and several general ideas have emerged, including a steric basis and an orbital basis. 

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. 

When more than two orbitals are involved, the energy change must take into account all important orbital interactions. This will be illustrated for the formaldehyde-ethylene case following the method of Herndon and Giles/131 If we assume the bonds are half-formed in the transition state, then the exchange integral y is just equal to iP- Since p has a value of about 40 kcal/mole, then... 

The Hiickel energy levels and p basis orbital coefficients for formaldehyde and ethylene are shown in Figure 4.9. If we examine the orbital interactions for case 1, we can see that in the orientation such that the lone pair on oxygen is oriented in a way that a bond can begin to form between the oxygen lone pair and the carbon ir orbital, there are two possible orbital interactions. 

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