Methane ground states

TABLE 6.5. Characteristic Al-0, Si-0,0-H and C-H Distances (A) in the cluster of Figure 6.17 in the Presence of Physisorbed Methane (Ground State) and in the Transition State of H-D Exchange... 

The energy of atomization of a ground state molecule at 0 K, for example, methane, is the energy of the reaction... 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

A similar investigation of methane (21) has shown that between 15.5 and about 20 e.v. the fragment ion CH2 + is formed in the ground state, but above 20 e.v. in an excited state that can cause ion-molecule reactions of different kind. 

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

Frequently we are dealing with the special but common situation that the system has an even number of electrons which are all paired to give an overall singlet, so-called closed-shell systems. The vast majority of all normal compounds, such as water, methane or most other ground state species in organic or inorganic chemistry, belongs to this class. In these... 

While this scheme is useful in helping to predict products from di-rr-methane rearrangements, all evidence indicates that the structures drawn are not intermediates in the reaction. That is, they do not represent energy minima on the potential energy surface leading from the excited state of the reactant to the ground state of the product. 

More subtle change can occur to a molecule s structure following photo-excitation. For example, the bond angle in methanal (formaldehyde) increases, so the molecule is flat in the ground state but bent by 30° in the first excited state see Figure 9.14. 

Figure 9.14 The structure of methanal changes following photo-excitation, from a flat ground-state molecule to a bent structure in the first photo-excited state...

In terms of the spin multiplicity of the ground state and excited states of methanal, the ground state is a singlet state (S0), with the excited states being either singlets (Si, S2, etc.) or triplets (T2, T2, etc.) (Figure 1.10). 

Figure 1.10 Configurations of the ground state and excited electronic states of methanal...

Figure 7.7 Molecular geometry of ground-state and (n,Jt ) excited-state methanal molecules...

Di-TT-methane rearrangements are typical examples of reactions that occur in the excited state exclusively. These rearrangements have never been observed in the ground-state chemistry of 1,4-unsaturated compounds. 

We believe that the rearrangements of the di-ir-methane type observed in the DCA-sensitized irradiations of 1-aza- and 2-aza-1,4-dienes are important because the di-ir-methane process has been considered until now a paradigm of reactions that take place in the excited-state manifold only. Our results show that rearrangements of this type can also occur in the ground states of radical-cation intermediates. This opens the possibility of promoting di-ir-methane-type rearrangements by alternative thermal means. 

Another interesting observation in this study is the boron trifluoride etherate-catalyzed rearrangement of tri-ir-methane systems 141 that afford the corresponding cyclopentenes 142. These reactions can be considered as the first examples of tri-ir-methane rearrangements in the ground state. Interestingly, compounds 141 only undergo conventional di-TT-methane reactions on irradiation. The mechanism shown in Scheme 25 is proposed to account for this novel reaction [79]. 

Thus, the UV spectrum of 22, was interpreted as incompatible with a a /a configuration. At the same time, the IR data were interpreted in favor of the a-ir/a-ir configuration. In addition, the reactivity pattern of 22 differs significantly from that of 6, which has a ground-state configuration. For example, 22 does not appear to ring expand upon irradiation (as 6 is known to do) and reacts with hydrocarbons (like methane) via hydrogen-atom abstraction (rather than by C—H insertion, as is the case with 6) [86,87]. Thus, the conclusion that 22 is a aTr/a-ir biradical was reached. 

Beside charge transfer excitation, the independently excited molecule could conduct an electron transfer reaction. Photooxidation of Leuco Ethyl Crystal Violet (tris (p-N, N-diethylaminophenyl) methane) in the presence of carbon tetrachloride is thought to proceed by electron transfer from excited LECV to CC14 (74). Complex formation in the ground state is hardly observed. 

---