| Summary: | 碩士 === 國立成功大學 === 化學系 === 104 === Methylation performed on guanidine would destabilize the energy of the highest occupied molecular orbital (HOMO), as well as found earlier for benzene. The N,N-dimethylated biguanide is known as Metformin, which has currently replaced guanidine and biguanide for biomedical use in treating type-2 diabetes. Di-methylation, however, result in a node-like Nc in the HOMO plots, namely the HOMO of biguanide is separated into two fragmental π-systems on both ends in Metformin. Moreover, the HOMO-energy in Metformin is unexpected lower than that of the parent biguanide. This observation can be understood based on theoretical studies employing the natural bond orbital method: exclusion of the high-lying lone-pair bond orbital on Nc leads to a stabilized HOMO on going from biguanide to metformin. This might underline the biomedical use of metformin for reducing side-effects found for biguanide.
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