A Quantum Chemical Study Of The Effect Of Impurities On The Intramolecular Proton Transfer Of Hydroxyl Carbene

Since 2010,a series of low-temperature experimental studies had proposed that the quantum mechanical tunneling（QMT）of proton existed in the intramolecular proton transfer process in hydroxycarbenes（R-C-OH）to generate corresponding aldehydes（R-CHO）.One of the most important evidences for drawing such a conclusion was the prohibitively high energy barriers（EBs）（28.0–33.3 kcal/mol）predicted by combined theoretical studies using classical transition state theory（TST）.For instance,the energy barrier of 28.0 kcal/mol was predicted for direct proton transfer from trans-methylhydroxycarbene（CH₃-C-OH）to generate acetaldehyde（CH₃-CHO）.According to TST theory,such a reaction cannot proceed because it was recorded at the extremely low temperature（around 10 K）,so such a TST result was taken as an evidence to exclude the possibility of classical chemical reaction and to verify the existence of QMT process.Similarly,QMT of proton was thought to play the dominant role in the proton transfer reactions,including CF₃-C-OH to CF₃-CHO,HC≡C-C-OH to HC≡C-CHO,CN-C-OH to CN-CHO,and Ph-C-OH to Ph-CHO.However,corresponding TST studies did not consider the pathways involving the impurities in the experimental system.Given that the impurities like H₂O can significantly lower the EBs of proton transfer reactions,we devote ourselves in this thesis to study the influence of impurities on the proton transfer reactions,with the aim to find the more reasonable explanation.The details our work are summarized below:（1）Influence of Common Impurity H₂O on Proton Transfer ReactionsWe studied the effect of the common impurity H₂O as an auxiliary bridging molecule on proton transfer in various reaction systems at the CCSD（T）/aug-cc-p VTZ level of theory.The extremely expensive full geometry optimization and harmonic frequency analysis were performed for reactants,transition states（TSs）,and products at this level.The results reveal that,the EBs decrease dramatically for all reaction systems when H₂O was taken as the bridging molecule:the EBs are lowered from 28.3,31.4,31.9,and 33.2kcal/mol to 6.0,8.4,8.3,and 9.4 kcal/mol for R-C-OH（R=CH₃,CF₃,HC≡C,C≡N）,respectively.We also studied the larger Ph-C-OH system at the CCSD（T）//M06-2X level（CCSD（T）/aug-cc-p VTZ single point energy+Gibbs free energy corrections at M06-2X/aug-cc-p VTZ level）and the results reveal that H₂O can lower the EB from 28.6kcal/mol to 6.2 kcal/mol.To verify the results from classical TST studies,we had performed the Born-Oppenheimer molecular dynamics（BOMD）simulations at the PBE/DZVP level and the results revealed that though H₂O can allow the proton transfer at the relatively low temperatures（75–180 K）,the desired proton transfer cannot be witnessed at the experimental temperature（around 10 K）.Therefore,we can draw a conclusion:though TST and BOMD studies cannot exclude the possibility of QMT of proton as the dominant manner of proton transfer,we can confirm that even if the proton transfer needs to proceed through QMT,such QMT should proceed through the H₂O-bridged indirect pathway rather than the previously reported direct pathway due to the much lower EBs.（2）Influence of Specified Impurities on Proton Transfer in each SystemAccording to description in the literatures,besides the common impurity H₂O,several specified impurities also exist in each hydroxycarbene system.Therefore,we had studies the influence of these impurities on the proton transfer in each hydroxylcarbene system.Our results suggest that,with impurity of hydrogen fluoride（HF）as the bridging group,the EB for proton transfer in CF₃-C-OH system can be lowered to 3.7 kcal/mol,which allows the reaction to proceed through classical chemical reaction even in the extremely low experimental temperature around 10 K,and the reasuts of BOMD shows that proton transfer can still be observed at very low temperatures of 10 K,that is,QMT process is even not necessary to explain the reaction mechanism in CF₃-C-OH system.In addition,ethylene（C₂H₄）and acetylene（C₂H₂）are the specified impurities in HC≡C-C-OH system and they can lower the EBs for proton transfer to 25.6 and 15.6kcal/mol,respectively.Similarly,C₂H₄and hydrogen cyanide（HCN）are the specified impurities in C≡N-C-OH system and they can lower the EBs for proton transfer to 26.2and 16.3 kcal/mol,respectively.In comparison with the direct proton transfer,these organic molecules can also effectively lower the EBs,while in comparison with the inorganic molecule HF and H₂O as the bridging molecule,the latter can lower the EBs more effectively（to less than 9.4 kcal/mol）.（3）Explaining the Difference of Different Impurities in Lowering the EBsTo give a reasonable explanation on the differences of different bridging molecule in lowering the EBs,the energy decomposition analysis（EDA）had been performed.Focusing on the difference（ΔΔΔE）between the fragment interaction energies of reactants and TS,we found the following crucial factor for lowing the EBs:orbital interaction(ΔΔΔE_orb)favors HF in lowering the EBs more effectively than H₂O and favors C₂H₂as well as HCN in lowering the EBs more effectively than C₂H₄.Pauli repulsion(ΔΔΔE_Pauli)favors H₂O over C₂H₂and HCN,while the synergistic effects from electrostatic interaction(ΔΔΔE_elstat)andΔΔΔE_orbfavor the H₂O over C₂H₄.