Ab-Initio Computational Study : The Activation Energy Changes and Steric Effects In Peptide Synthesis Of Ac-AA-NH2 and Ac-AP-NH2

Ab-Initio computational method can be used for simulating reaction mechanisms, such as concerted reaction mechanism on peptide synthesis. The concerted reaction is one of many possible pathways on how peptide can be synthesized. The purpose of this study are probing the concerted reaction mechanism and comparing the steric effect to the reaction, given by different side-chain of alanine (A) and proline (P). Two dipeptides formed from alanine and proline were computed at HF-SCF/6-31G** theory level: Ac-AA-NH2 and Ac-AP-NH2. The res.lts show the activation energy of Ac-AA-NH2 and Ac-AP-NH2 forming via concerted pathway are 167.541 kJ/mol and 161.044 kJ/mol, respectively. The steric difference in side-chain affects the dihedral angle of the structure, and also gives difference to the entropy value of reaction.

Hydrogen atom transfer in the photochemical isomerization of hydrazones of 1,2,4-oxadiazole derivatives

DFT calculations on the photoisomerization of hydrazones of 1,2,4-oxadiazole derivatives to 1,2,5-triazoles have been performed showing that the reaction occurred through the first excited singlet state. The Z isomer gave the reaction through a hydrogen atom transfer of the hydrazonic nitrogen atom to the nitrogen atom in four position on the oxadiazole ring. In this case, the isomerization was a concerted reaction. The E isomer could undergo the same reaction. However, it could not be a concerted reaction but required the presence of a ring opening intermediate.

Bypassing the Multi-reference Character of Singlet Molecular Oxygen. Part 2: Ene-reaction

Theoretical calculations involving singlet molecular oxygen (O2(1g)) are challeng- ing due to their inherent multi-reference character. We have tested the quality of re- stricted and unrestricted DFT geometries obtained for the reaction between singlet oxy- gen and a series of alkenes (propene, 2-methylpropene, trans-butene, 2-methylbutene and 2,3-dimethylbutene) which are able to follow the ene-reaction. The electronic en- ergy of the obtained geometries are rened using 3 dierent methods which account for the multi-reference character of singlet oxygen. The results show that the mechanism for the ene-reaction is qualitatively dierent when either one or two allylic-hydrogen groups are available for the reaction. When one allylic-hydrogen group is available the UDFT calculations predict a stepwise addition forming a biradical intermediate, while, the RDFT calculations predict a concerted reaction where both hydrogen abstrac- tion and oxygen addition occur simultaneously. When two allylic-hydrogen groups are available for the reaction then UDFT and RDFT predict the same reaction mechanism, namely that the reaction occurs as a stepwise addition without a stable intermediate between the two transition states. The calculated rate constants are in reasonable agreement with experimental data, except for trans-butene where the calculated rate constant is three orders of magnitude lower than the experimental one. In conclusion we nd that the simple bypassing scheme tested in this paper is a robust approach for calculations of reaction involving singlet oxygen in the limit that the transition state processes low multi-reference character. 2

Ethynamine - Ketenimine - Acetonitrile - Rearrangements: A computational Study of Flash Vacuum Pyrolysis Processes

<p>The rearrangements of ethynamine <b>3</b> (H-CºC-NH₂) to ketenimine <b>4</b> (CH₂=C=NH) and acetonitrile <b>5</b> (CH₃CN) were investigated computationally up to the MP4(SDTQ)/6-31G*//MP2(FU)/6-31G* level. The calculated barrier for a concerted reaction <b>3</b> -> <b>4</b> is very high, 74 kcal/mol, the structure of the transition state very unusual, and this path is discredited. A lower barrier of about 60 kcal/mol via aminovinylidene <b>2</b> and imidoylcarbene <b>15</b> has been found. The calculated barrier for a concerted second step <b>4 </b>-><b> 5</b> is 61 kcal/mol, and the transition state structure is again very unusual with a virtually linear CCN backbone, but this does not appear to correspond to physical reality. Instead, CASPT2 calculations predict reaction via vinylnitrene <b>9</b> and/or homolysis of <b>4 </b>to the radical pair ·CH₂CN + H· (<b>11</b>) with a barrier of 67-70 kcal/mol in agreement with experimental shock-tube data. Recombination (maybe via roaming) affords acetonitrile <b>5</b>. There is strong experimental evidence for homolytic paths in pas-phase pyrolyses of ketenimines.</p>

Ethynamine - Ketenimine - Acetonitrile - Rearrangements: A computational Study of Flash Vacuum Pyrolysis Processes

<p>The rearrangements of ethynamine <b>3</b> (H-CºC-NH₂) to ketenimine <b>4</b> (CH₂=C=NH) and acetonitrile <b>5</b> (CH₃CN) were investigated computationally up to the MP4(SDTQ)/6-31G*//MP2(FU)/6-31G* level. The calculated barrier for a concerted reaction <b>3</b> -> <b>4</b> is very high, 74 kcal/mol, the structure of the transition state very unusual, and this path is discredited. A lower barrier of about 60 kcal/mol via aminovinylidene <b>2</b> and imidoylcarbene <b>15</b> has been found. The calculated barrier for a concerted second step <b>4 </b>-><b> 5</b> is 61 kcal/mol, and the transition state structure is again very unusual with a virtually linear CCN backbone, but this does not appear to correspond to physical reality. Instead, CASPT2 calculations predict reaction via vinylnitrene <b>9</b> and/or homolysis of <b>4 </b>to the radical pair ·CH₂CN + H· (<b>11</b>) with a barrier of 67-70 kcal/mol in agreement with experimental shock-tube data. Recombination (maybe via roaming) affords acetonitrile <b>5</b>. There is strong experimental evidence for homolytic paths in pas-phase pyrolyses of ketenimines.</p>