Acetone, LUMO

Finally, select acetone from the molecules on screen. Here, both the LUMO and the LUMO map are available under the Surfaces menu. First, select LUMO and display it as a Solid. It describes a 7U-type antibonding ( i ) orbital concentrated primarily on the earbonyl carbon and oxygen. Next, turn off this surface (select None under the LUMO sub-menu), and then seleet LUMO Map under the Surfaces menu. Display the map as a transpareni solid. Note the blue spot (maximum value of the LUMO) directly over the carbonyl carbon. This reveah the most likely site for nucleophilic attack. 

LUMO of acetone reveals likely site of nucleophilic attack. 

HOMO (top) and LUMO (bottom) of acetone change occupancy upon absorption of light. 

Examine and describe both the highest-occupied and lowest-occupied molecular orbitals (HOMO and LUMO, respectively) oiground state acetone. On which atom(s) is the HOMO primarily concentrated Is it in the o system or in the 7t system Repeat your analysis for the LUMO. 

Figure 1.3 Calculated charge density (HF-31G ) and LUMO energy of acetone and acetaldehydes. ...

LUMO (acetone) = a-0.878p LUMO (acetic acid)= a— 0.917ft LUMO(methyle acetate) = a— 0.925/ ... 

The LUMO energies in benzophenone (a — 0.294ft) and acetophenone (a — 0.458/1) are lower than in acetone (a - 0.847/ ). So why does NaBH4 reduce them seven times more slowly than acetone 33... 

The FO approximation was introduced in 1952 when all calculations were done using the Hiickel method. In these conditions, the n frontier orbitals were also the chemically reactive MOs. Semi-empirical and ah initio calculations, which are frequently used now, take into account both o and n orbitals. With these methods, the orbitals to be considered in FO theory are not necessarily the formal frontier orbitals, i.e. the highest occupied and lowest unoccupied MOs Consider, for example, the reduction of acetone by LiAlH4. The chemically important MO is of course the Jt co of the ate complex, which is in fact the LUMO + 1. The formal LUMO is the empty s orbital of the lithium cation. 

A similar rationale shows that the barrier to rotation of a methyl group should lower along the series hydroxyacetone > acetone > fluoroacetone.85 Here, the dominant interaction involves the carbonyl LUMO and the methyl group HOMO. As the LUMO energy rises, the barrier to rotation falls. 

Hence, a quick analysis might use LUMO energies from semi-empirical calculations for the analysis of the reduction of 3 (Scheme 3). Because the LUMO energy of acetone as model for site (a) in 3 is the lowest within the three electrophores, one must expect that an electron will be accepted at this site. This is indeed demonstrated in the PET reduction of 3 to a ketyl radical anionic intermediate using tri-ethylamine as sensitizer [8]. 

It is noteworthy that the LUMO of a Fischer type carbene resembles the LUMO (determined using semi-empirical MO theory) of a typical ester such as methyl acetate (Figure 10-9a) or a ketone such as acetone (Figure 10-9b). In fact, the analogy between Fischer carbenes and carbonyl compounds should be useful to readers already familiar with organic chemistry. Indeed, the chemistry of these two types of compounds, seemingly so different in structure, is similar in several ways. 

In the sulfur-capped trimetallic clusters, the antibonding LUMO is centred on the metallic triangle (in particular, the FeCo2 core, it is mainly localized on the Co-Co edge). It has been briefly reported that the present chromium adduct undergoes a quasireversible one electron reduction just at the same potential value (E° = — 0.28 V, in Acetone solution) as that exhibited by FeCo2(S)(CO)9 [59],... 

Fig. 11.5 Positions of the HOMO and LUMO levels for various Ir-dyes, compared to some charge-transporting materials frequently used in OLEDs [27], Here, niBr stands for N-2,6-dibro-mophenyl-1,8-naphthali-mide, Irppy for lr(ppy)3, Btlr for bis(2-phenyl-benzothiozolato-n,C2)iri-d i u m (acetyl-acetonate), Btp(lr) for btp2lr(acac), Alq3 for Aluminium(lll)-tris(8-hydroxyquinoline), NPD for 4,4 -bis(N-(l-napthyl)-N-phenylamino)-biphenyl and CBP for 4,4 -N,N -dicarbazolebi-phenyl.

Make a model of acetone and submit it to an AMI calculation with geometry optimization. Also select two surfaces to display, the LUMO and a mapping of the LUMO on a density surface. 

PROBLEM 16.73 The Imine formation reaction shows the methanamine attacking acetone in the first step. The alkoxide intermediate is then protonated by an acidic ammonium species in a proton transfer process. It is known that imine formation occurs faster under slightly acidic conditions (pH 5). Why not show the acetone being protonated in the first step as we did in Carbonyl hydration and Acetal formation Select the LUMO track for this animation and examine this MO on acetone by pausing the reaction at the beginning. There are several important facts that are confirmed by this calculated image. Which atom of the molecule has the most LUMO density ... 

What is the role of water under these conditions It has been suggested that water suppresses the formation of proHne-oxazoUdinone, which has been considered to be a parasitic species [11]. Then, the role of water is to prevent deactivation rather than to promote activity. Studies, carried out on the proUne-catalyzed reaction between acetone and 2-chlorobenzaldehyde allow one to hypothesize a conflicting role of water. Water increases the total catalyst concentration due to suppression of unproductive species and decreases the relative concentration of productive intermediates by shifting the iminium ion back to proline [12]. Addition of water suppresses formation of both on- and ofF-cyde iminium ions 1 and 2 by Le Chatelier s principle (Scheme 24.2a). The net effect of added water on the globally observed rate will depend on the relative concentrations of iminium ions 1 and 2, which may be different for different aldehydes and can be a function of substrate concentrations and rate and equilibrium constants. Seebach and Eschen-moser have raised doubts about the fact that oxazoUdinones are unproductive and parasitic species in proline-catalyzed aldol reactions [13]. The small excess of water will potentially facilitate proton-transfer in the transition state (Scheme 24.2b), which both lowers the LUMO of the incoming electrophile as well as directs the enantioselectivity of the newly formed stereocenters. 

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