以胍(guanidine)與雙胍(biguanide)作為母體，將其胺基上的氫原子置換成甲基(methyl group)與三氟甲基(trifluoromethyl group)，採用DFT B3LYP/6-31G(d)計算方法，分析胍類與雙胍類分子整體的能量，探討HOMO軌域以及氮原子的負超共軛。胍與雙胍HOMO軌域的主要成分皆含πC-N以及LP(N)，從取代基BO貢獻度的多寡，得知引入甲基會發生超共軛效應，三氟甲基的引入則產生負超共軛效應。除了取代基引起的負超共軛，母體主結構BO之間也有負超共軛效應，LP(N) donor至π*C-N，並藉由二級微擾理論的E(2)值去分析負超共軛穩定LP(N)的能力，發現甲基系列的衍生物其母體自身負超共軛得到的E(2)值高於甲基，因此母體的自身負超共軛具有較強的穩定能力；對於三氟甲基系列的衍生物，LP(N) donor到三氟甲基σ*C-F得到的E(2)值和donor到母體自身π*C-N的E(2)值都相當高，無論是三氟甲基引起的負超共軛或是母體自身的負超共軛，此二者對於LP(N)的穩定性都很重要。
以胍分子為母體：從甲基σC-H與σ*C-H的貢獻度可知，甲基胍衍生物因超共軛效應，其HOMO能量大致上成逐漸上升的趨勢，唯G-4CH3例外；三氟甲基胍衍生物則因負超共軛效應，使得HOMO的能量逐漸降低。
雙胍的部分，從分子結構圖與HOMO軌域電子雲分布圖可知雙胍分子有類似C2的對稱，且分子中心的氮原子銜接左右兩側πC-N的電子雲。陸續引入甲基後，分子中心的氮原子貢獻度逐漸降低甚至出現節面，原本因氮原子銜接一起的πC-N被分成兩個獨立的πC-N系統。能量方面，單一支甲基取代的衍生物，其HOMO能量上升，與胍的經驗相同；多甲基取代的雙胍衍生物其HOMO能量低於母體，此現象與分子中心的氮原子有關，母體中此支BO的能量(-6.62 eV)高於兩側胺基的氮原子能量(-8.24 eV)，隨著甲基的引入高能量的BO貢獻度降低，HOMO能量因此而下降。
此外，四種甲基雙胍衍生物，其中二甲基雙胍是第二型糖尿病患者最常使用的口服藥物，Metformin。我們將利用Weinhold’s NBO method從共軛的觀點解釋為何二甲基雙胍的HOMO能量低於雙胍分子。

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.

1.T.-J. Chang, Y.-D. Jiang, C.-H. Chang, C.-H. Chung, N.-C. Yu and L.-M. Chuang, Journal of the Formosan Medical Association, 2012, 111, 605.
2.S. E. Inzucchi, R. M. Bergenstal, J. B. Buse, M. Diamant, E. Ferrannini, M. Nauck, A. L. Peters, A. Tsapas, R. Wender and D. R. Matthews, Position Statement of the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD), 2012, 35, 1364.
3.A. Garber, M. Abrahamson, J. Barzilay, L. Blonde, Z. Bloomgarden, M. Bush, S. Dagogo-Jack, M. Davidson, D. Einhorn, W. Garvey, G. Grunberger, Y. Handelsman, I. Hirsch, P. Jellinger, J. McGill, J. Mechanick, P. Rosenblit, G. Umpierrez and M. Davidson, Endocrine Practice, 2013, 19, 1.
4.U. K. P. D. S. Group, The Lancet, 1998, 352, 854.
5.A. Decensi, M. Puntoni, P. Goodwin, M. Cazzaniga, A. Gennari, B. Bonanni and S. Gandini, Cancer Prev Res (Phila), 2010, 3, 1451.
6.M.-S. Lee, C.-C. Hsu, M.-L. Wahlqvist, H.-N. Tsai, Y.-H. Chang, Y.-C. Huang, BMC Cancer, 2011, 11, 20.
7.D. Shobac, D. G. Gardner, Greenspan's basic & clinical endocrinology, 9th ed, New York: McGraw-Hill Medical, 2011, Chapter 17
8.Diabetes Fact sheet. WHO. October 2013 (25 March 2014).
9.RSSDI textbook of diabetes mellitus. Rev. 2nd ed, New Delhi: Jaypee Brothers Medical Publishers, 2012.
10.M. Grzybowska , J. Bober, M. Olszewska , Z. C. Medycznej, Postepy Higieny i Medycyny Doswiadczalnej (Online), 2011, 65, 277.
11.M. R. Owen, E. Doran, A. P. Halestrap, Biochem J, 2000, 348, 607.
12.B. Viollet, B. Guigas, N. Sanz Garcia, J. Leclerc, M. Foretz, F. Andreelli, Clinical Science (London, England : 1979), 2012, 122, 253.
13.R. A. Miller, Q. Chu, J. Xie, M. Foretz, B. Viollet and M. J. Birnbaum, Nature, 2013, 494, 256.
14.R. J. Shaw, K.A. Lamia, D. Vasquez, Science, 2005, 310, 1642.
15.M. Foretz, S. Hébrard, J. Leclerc, J Clin Invest, 2010, 120, 2355.
16.C. J. Bailey and C. Day, Practical Diabetes International, 2004, 21, 115.
17.American Diabetes Association, Diabetes Care, 2011, 34, S11.
18.P. v. R. Schleyer, A. J. Kos, Tetrahedron, 1983, 39, 1141.
19.R. S. Mulliken, The Journal of Chemical Physics, 1933, 1, 492.
20.T. W. G. Solomons, Organic Chemistry revised printing, John Wiley＆Sons Inc, New York, 2000.
21.F. A.Carey, R. J. Sundberg, Advanced Organic Chemistry, 4th ed, Kluwer Academic/Plenum Publishers Press:New York, 2000.
22.R. S. Mulliken, The Journal of Chemical Physics, 1933, 1, 492.
23.R. S. Mulliken, The Journal of Chemical Physics, 1935, 3, 517.
24.R. S. Mulliken, The Journal of Chemical Physics, 1939, 7, 339.
25.O. J. Sovers, C. W. Kern, R. M. Pitzer and M. Karplus, The Journal of Chemical Physics, 1968, 49, 2592.
26.L. Goodman, H. Gu, The Journal of Chemical Physics, 1998, 109, 72.
27.L. Goodman, H. Gu, V. Pophristic, The Journal of Chemical Physics, 1999, 110, 4268.
28.L.Goodman, V. Pophristic, F. Weinhold, Acc. Chem. Res, 1999, 32, 983.
29.V. Pophristic and L. Goodman, Nature, 2001, 411, 565.
30.G. W. Wheland, The Journal of Chemical Physics, 1934, 2, 474.
31.A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev., 1988, 88, 899.
32.R. Hoffmann, L. Radom, J. A. Pople, P. v. R. Schleyer, W. J. Hehre, L. Salem, Journal of the American Chemical Society, 1972, 94, 6221.
33.P. v. R. Schleyer, A. J. Kos, Tetrahedron, 1983, 39, 1141.
34.P. Hohenberg, W. Kohn, Physical Review, 1964, 136, B864.
35.W. Kohn, L. J. Sham, Phys. Rev., 1965, 140, A1133.
36.A. D. Becke, The Journal of Chemical Physics, 1993, 98, 5648.
37.A. D. Becke, Phys. Rev. A, 1988, 38, 3098.
38.C. Lee, W. Yang, R. G. Parr, Physical Review B, 1988, 37, 785.
39.L. Goodman, V. Pophristic, F. Weinhold, Acc. Chem. Res., 1999, 32, 983.
40.B. Reimann, K. Buchhold, S. Vaupel, B. Brutschy, Z. Havlas, V. Špirko, P. Hobza, The Journal of Physical Chemistry A, 2001, 105, 5560.
41.S. J. Wilkens, W. M. Westler, F. Weinhold, J. L. Markley, J. Am. Chem. Soc., 2002, 124, 1190.
42.P. Hobza, J. Šponer, E. Cubero, M. Orozco, F. J. Luque, The Journal of Physical Chemistry B, 2000, 104, 6286.
43.N. W. Mitzel, U. Losehand, J. Am. Chem. Soc., 1998, 120, 7320.
44.S. P. Ananthavel, M. Manoharan, Chem. Phys., 2001, 269, 49.
45.A. E. Reed, F. Weinhold, L. A. Curtiss, D. J. Pochatko, The Journal of Chemical Physics, 1986, 84, 5687.
46.H. Anane, A. Boutalib, I. Nebot-Gil, F. Tomás, The Journal of Physical Chemistry A, 1998, 102, 7070.
47.W. Yang, D. G. Drueckhammer, J. Am. Chem. Soc., 2001, 123, 11004.
48.B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett., 1989, 157, 20.
49.G. L. Miessler, D. A. Tarr, Inorganic Chemistry, 2nd ed, Pearson Prentice Hall, 1991.
50.R. H. Petrucci, W. S. Harwood, F. G. Herring, General Chemistry, 8th ed, Pearson Prentice Hall, 2002.
51.G. N. Lewis, Valence and the Structure of Atoms and Molecules, The Chemical Catalog Company, 1923.