在H3-n(Me)nC-C≡N 這一系列分子中，C(3)-C(2) 鍵隨著甲基取代的數目增加而變弱，此一現象可由超共軛觀點來解釋，此超共軛可視為σC(3)H(4) 與 σ*C(2)C(3).之“電子提供－電子接受”軌域作用。然而，由天然鍵結軌域分析本系列分子中軌域作用能量，E(2)，發現 σC(3)H(4) 與 σ*C(2)C(3) 作用小於 σC(3)C(Me) 與σ*C(2)C(3) 之作用，而與超共軛的預測相反。包含 σC(2)C(3) 為電子提供軌域之作用及以σ*C(2)C(3) 為電子接受軌域之作用，扮演解釋C(2)C(3) 鍵結強度變化的主要因素。前者的E(2)值 隨著甲基取代的數目增加分別為：13.06，17.08，21.34，25.22 kcal/mol；後者E(2)值也呈現相同的傾向：23.88，29.12，34.08，40.44 kcal/mol，此傾向主要來自於 σC(3)C(Me) 與 σ*C(2)C(3) “電子提供－電子接受” 軌域之作用，隨甲基取代數目之增加而變大。因而在本研究中，此兩類軌域作用變化的傾向，可以取代超共軛，作為 “ C(3)-C(2) 鍵隨著甲基取代的數目增加而變弱” 此現象之另一種解釋。

在單取代苯環和吡啶的對位碳和對位氮的電場梯度可以分別的計算出來。我們可以發現碳或氮電場梯度僅受取代基π效應的影響，而且與電荷呈線性關係，有如Townes-Dailey 理論所預測。比例常數非常接近qo值，即表示一個電子填入2p軌域所產生的電場梯度。本研究結果可用來解釋文獻上所提出計算 “ 一氧化碳過渡金屬錯合物回饋電子計算所用的半經驗關係式 ”。換言之，在僅考慮π效應的系統中，本研究可以作為支持Townes-Dailey 理論的有效性。

Weakening of the C(3)-C(2) bonds in the series: H3-n(Me)nC-C≡N has been explained by the well-known hyperconjugation concept which may be classified as the donor-acceptor interaction between σC(3)H(4) and σ*C(2)C(3). Natural Bond Orbital (NBO) analysis of the orbital interaction energy, E(2), in this series of molecules reveals that the σC(3)H(4) and σ*C(2)C(3) interaction is in general smaller than σC(3)C(Me) and σ*C(2)C(3). Orbital interactions involving σC(2)C(3) as donating bond orbital and σ*C(2)C(3) as accepting bond orbital play critical roles concerning the strength of C(2)C(3) bonds. The values of E(2) for the former interactions increase in the trend: 13.06, 17.08, 21.34, 25.22 kcal/mol and E(2) for the latter indicates the same trend as well: 23.88, 29.12, 34.08, 40.44 kcal/mol, which primarily arises from the increasing σC(3)C(Me) and σ*C(2)C(3) donor-acceptor interactions. Both trends can be employed to account for the weakened C(2)-C(3) bonds since the increasing number of methyl substitution leads to more or higher orbital interactions involving the C(2)-C(3) bonding and antibonding orbitals.

The electric field gradients (EFG) of para-carbon and para-nitrogen atoms in mono-substituted benzene and pyridine were calculated, respectively. The linear relationship between EFG and charge for either carbon or nitrogen atom, which is only influenced by the π-effect of the substituent, R, has been found. Furthermore, the relative proportionality constants can be related to the value of qo, representing the EFG resulting from one electron filled in the 2p-orbital. Our results can be employed to rationalize the semi-empirical relationship proposed in the literature for evaluation of backbonding electrons in transition-metal carbonyls. In other words, the work presented here provides evidence in support of the Townes-Dailey theory applied to systems in which only π-effects are relevant.

1. R. S. Mulliken; J. Phys. Chem., 1, 492(1933); 3, 520(1935); 7, 339(1937).

2. (a) E. Schemp and P. J. Bray; ed. In “Physical Chemistry An Advance Treatise,”
6th Handerson Ed., McGraw-Hill, New York,(1955). Chapter 11.
(b) H. Negita and P. Bray; J. Chem. Phys., 33, 1876(1960).
(c) E. A. C. Luken; “Nuclear Quadrupole Resonance Spectroscopy,” Academic
Press, New York (1958).

3. (a) P.v. R. Schleyer and A. J. Kos; Tetrahedron, 39, 1141(1983).
(b) A. E. Reed and P.v. R. Schleyer; J. Am. Chem. Soc.,109, 7362(1987).
(c) A. E. Reed and P.v. R. Schleyer; J. Am. Chem. Soc., 112, 1434(1990).
(d) U. Salzner and P.v. R. Schleyer; Chem. Phys. Lett., 190, 401(1992).
(e) K. B. Wiberg and P. R. Rablen; J. Am. Chem. Soc., 115, 614(1993).

4. (a)L. Goodman, V. Pophristic and F. Weinhold; Acc. Chem. Res., 32, 983(1999).
(b)S. J. Wilkens, W. M. Weatler, F. Weinhold and J. L Markley; J. Am. Chem. Soc.,
124, 1190(2002).

5. (a) K. S. Wang, D. Wang, K. Yang, M. G. Richmond and M. Schwartz; Inorg.
Chem., 34, 3241(1995).
(b) S. P. Wang, M. G. Richmond and M. Schwartz; J. Am. Chem. Soc., 114,
7595(1992).
(c) S. P. Wang, P. Yuan and M. Schwartz; Inorg. Chem., 29, 484(1990).

6. C. H. Townes and B. P. Dailey; J. Chem. Phys. 17, 782(1949).

7. T. M. Ho and C. C. Chang; Int. J. Quant. Chem., 57, 229(1996).

8. (a) R. M. Sternheimer; Phys. Rev., 146, 140(1966).
(b) R. M. Sternheimer; Phys. Rev., 130, 1423(1963).
(c) R. M. Sternheimer; Phys. Rev., 95, 736(1954).
(d) R. M. Sternheimer; Phys. Rev., 86, 316(1952).
(e) R. M. Sternheimer; Phys. Rev., 84, 244(1951).

9. T. P. Das E. L. Hahn; “Nuclear Quadrupole Resonance Spectroscopy,”
Academic Press: New York.(1969).

10. W. Gordy and R. L. Cook; “Microwave Molecular spectroscopy,” Wiley,
New York, (1980).

11. R. G. Barnes and W. V. Smith; Phys. Rev., 93(1954).

12. Y. Kato, U Furukane and H. Takeyama; Bull. Chem. Soc. Japan, 32, 527(1959).

13. (a)M. L. Martin, J. J. Delpueh and J. G. Martin; “Practical NMR Spectroscopy,”
Heyden and Sons, Ltd., London, (1980).
(b)T. C. Farrar and E. D. Becker; “Pulse and Fourier Transform NMR.
Introduction to Theory and Methods,” Academic Press: New York, (1971).

14. E. A. C. Lucken; “Nuclear Quadrupole Coupling Constants,” Academic Press:
New York, (1969).

15. R. S. Drago; “Physical Methods for Chemists,” 2nd Ed., Mexico, (1992). Chap 14.

16. F. W. Wehrli, A. P. Marchand and S. Wehrli; “Interpretation of Carbon-13 NMR
Spectra,” 2nd Ed., Wiley, New York, (1989).

17. W. E. Lamb; Phys. Rev., 60, 817(1941).

18. H. M. McConnell; J. Chem., Phys., 26, 226(1957).

19. J. A. Pople; Proc. Roy. Soc., A1, 1038(1971).

20. M. Karplus and J. A. Pople; J. Chem. Phys., 38, 2803(1963).

21. W. J. Hehre, L. Radom and P. v. R. Schleyer; “ Ab Initio Molecular Orbital
Theory,” Canada, (1986).

22. P. A. Cassabella and P. J. Bray; J. Chem. Phys., 28, 1182(1958).

23. R. Verma and K. S. Buckton; J. Chem. Phys., 46, 1565(1967).

24. S. Huzinaga; “Gaussian Basis Sets for Molecular Calculations,” Elsevier,
New York, (1984).

25. A. E. Reed, L. A. Curtiss and F. Weinhold; Chem. Rev., 88, 899(1989).

26. J. P. Foster and F. Weinhold; J. Am. Chem. Soc., 102, 7211(1980).

27. R. McWeeney; “Coulson’s Valence,” 3rd Ed., Oxford University Press:
New York, (1979).

28. A. E. Reed, F. Weinstock and F. Weinhold; J. Chem. Phys., 83, 1736(1985).

29. R. S. Mulliken; J. Chem. Phys., 23, 1833, 1841, 2338, 2343(1955).

30. R. S. Mulliken, C. A. Rieke and W. G. Brown; Presented at the 5th annual
symposium of the division of Physical and Inorganic Chemistry of the
American Chemical Society, Columbia University, New York, Jan. 1st,
(1941).

31. R. S. Mulliken; J. Chem. Phys., 23, 1833(1955).

32. L. O. Brockway; J. Phys. Chem., 41, 185(1937).

33. J. D. Roberts, R. L. webb and E. A. McElhill; J. Am. Chem. Soc., 72, 408(1950).

34. R. Radom, L. Hofmann, J. A. Pople P. v. R. Schleyer, W. J. Hehre and L. Salem;
J. Am. Chem. Soc., 94, 6221(1972).

35. D. S. Friedman M. M. Francl and L. C. Allen; Tetrahedron, 41, 499(1985).

36. Gaussian 98 (Revision A.1), M. J. Frisch, G. W. Trucks, H. B. Schlegel,
G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski,
J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam,
A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone,
M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz,

B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts,
R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen,
M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle and J. A. Pople,
Gaussian, Inc., Pittsburgh PA, (1998).

37. NBO Version 3.1, E. D. Glendening, A. E. Reed, J. E. Carpenter and F. Weinhold.
Cited in reference 36.