The results show that substituents on the phenyl ring alter the bond length C-NdC-N, charge on atom N of amino group QN, dipole momentµ and the pKa of compounds.. There is a good correla
Trang 1§¹i häc Vinh T¹p chÝ khoa häc, tËp XXXVI, sè 1A-2007
Studying substituent effects on the properties of aniline with approximate quantum chemistry method AM1
Truong Van Nam (a), Nguyen Xuan Dung (a)
Abstract Substituents effects on the properties of aniline were studied by approximate quantum chemical methods using HyperChem software 7.52 (latest version) AM1 method has been used to optimize geometry and calculate some properties of molecules The results show that substituents on the phenyl ring alter the bond length C-N(d(C-N)), charge on atom N of amino group (QN), dipole moment(µ) and the pKa of compounds
There is a good correlation between these properties and Hammett σ constants, which traditionally have been used to characterize quantitatively the substituent electronic effect, except for d(C-N), QN of meta substituted compounds
I Introduction
The influence of substituents on physical and chemical properties of compounds has attracted much interest in chemistry Generally, substituents change reactivities, conformations, equilibrium relationships, spectra, and thermal properties of the substituted compounds
Aniline is an interesting object to study the influence of substituents Aniline molecule was studied well [1-5] On the basis of resonance theory, electron - withdrawing constituents, especially at the para position, should favor quinoid resonance structures, which are more planar than aniline itself, decreasing both negative charge on N atom and the C-N bond length Conversely, electron-donating substituents should favor KekulÐ-type structures with more nearly tetrahedral conformations at the nitrogen atom and increase charge on N atom and d(C-N) bond length [6]
H
X
H
X
N H H
X
N
N
HyperChem is a sophisticated molecular modeling environment that is known for its quality, flexibility, and ease of use It unites 3D visualization and animation with quantum chemical calculations, molecular mechanics, and dynamics [7]
‘
NhËn bµi ngµy 07/8/2006 Söa ch÷a xong 05/10/2006
Trang 2§¹i häc Vinh T¹p chÝ khoa häc, tËp XXXVI, sè 1A-2007
HyperChem Release 7.52 is a full 32-bit application, developed for the Windows 95, 98, NT, ME, 2000 and XP operating systems HyperChem Release 7.5 incorporates even more powerful computational chemistry tools than ever before, as well as newly incorporated modules, additional basis sets, new drawing capabilities and more
In this article, we used HyperChem software 7.52 to study the influence of substituents on some properties of compounds
II Methods
Hammett constants for the substituents were taken from [8] The pKa values for the aniline derivatives were taken from [9]
Quantum chemical calculations were performed using HyperChem 7.52 To optimize geometry of molecules, we used AM1 method Statistical analyses were done by EXCEL program
III Results and discussion
To simplify the analysis of substituent effects, we restricted our attention to four representative properties: the C-N bond length (d(C-N)), charge on atom N (QN), dipole moment(µ) and the acid dissociation constant (pKa)
The results of geometry optimization of aniline by AM1 method are shown in the Table 1 The obtained values are reasonable close to the experimentally observed ones [10] but for <HNH bond angle Therefore, we used this method to optimize aniline derivatives
Table 1 Some properties of aniline optimized by AM1 method
AM1 experimental value AM1 experimental value AM1 experimental value AM1 experimental value
Substituent effects on properties of aniline derivatives were examined by AM1 method Calculated results for d(C-N), QN and µ of its meta - and para-substituted derivatives are shown in table 2
Table 2 Calculated properties, Hammett constants and experimental pKa value for aniline derivatives
Order substituents d(C-N) (A0) QN µ σ pKa
2 m-amino 1.4000 -0.407 1.528 -0.16 4.88
4 m-chloro 1.3939 -0.398 2.603 0.37 3.34
6 m-fluoro 1.3941 -0.399 2.825 0.34 3.59
7 m-hydroxy 1.3950 -0.400 1.347 0.12 4.17
Trang 3§¹i häc Vinh T¹p chÝ khoa häc, tËp XXXVI, sè 1A-2007
8 m-methoxy 1.3971 -0.398 2.342 0.12 4.20
9 m-methyl 1.3990 -0.402 1.416 -0.07 4.69
10 m-nitro 1.3932 -0.398 6.303 0.71 2.50
11 p-amino 1.4102 -0.422 0.000 -0.66 6.08
12 p-bromo 1.3947 -0.404 3.304 0.23 3.91
13 p-chloro 1.3956 -0.407 3.086 0.23 3.98
14 p-cyano 1.3832 -0.398 5.446 0.66 1.74
15 p-fluoro 1.402 -0.409 3.285 0.06 4.65
16 p-hydroxy 1.4083 -0.415 2.354 -0.37 5.50
17 p-methoxy 1.4075 -0.410 1.879 -0.27 5.29
18 p-methyl 1.4017 -0.412 1.432 -0.17 5.12
19 p-nitro 1.3783 -0.386 7.640 0.78 1.02
The variations observed for the bond length d(C-N) upon substitution are seen to be relatively small Electron-donating substituents tend to slightly increase d(C-N), whereas electron-withdrawing substituents slightly decrease this bond length As expected, meta substituents exert a smaller influence than do para substituents, although, with the exception of p-cyano, p-nitro groups A fair correlation between d(C-N) and the Hammett σ values of the substituents is found : d(C-N) = -0.0071σ + 1.3972 ; R2 = 0.6225(meta substituents);
d(C-N) = -0.0229σ + 1.3992; R2 = 0.9454 (para substituents);
R=the correlation coefficient This is illustrated in Figure 1
R2 = 0.6225
R2 = 0.9454 1.3750
1.3800 1.3850 1.3900 1.3950 1.4000 1.4050 1.4100 1.4150 1.4200
σ
m-
p-Fugure 1 Correlation of the bond length d(C-N) with the Hammett σ constants of the substituents
The para substituents show a much better correlation with d(C-N) (R2 = 0.9454) than do the meta substituents (R2 =0.6225) The para substituents have a greater effect on the C-N bond length (angle factor =-0.0229) than do the meta substituents( angle factor =-0.0071)
The correlation between QN and Hammett constants was also examined :
QN = 0.0109σ - 0.4034 ; R2 = 0.5007 (meta substituents);
QN = 0.0208σ - 0.4081 ; R2 = 0.9057 ( para substituents)
Trang 4§¹i häc Vinh T¹p chÝ khoa häc, tËp XXXVI, sè 1A-2007
The results show that electron-donating substituents increase nagative charge on N meanwhile electron-withdrawing substituents have opposite effect However, para-subtituted compounds are much better fit(R2 = 0.9057) than meta-substituted compounds (R2= 0.5007)
p-: Q N = 0.0208σ - 0.4081
R2 = 0.9057
m-: Q N = 0.0109σ- 0.4034
R2 = 0.5007
-0.430 -0.420 -0.410 -0.400 -0.390 -0.380
σ
Q N
m-
p-Fugure 2 Correlation of QN with the Hammett σ constants of the substituents
According to the table 2, electron-donating substituents decrease the dipole moment of compounds and electron-withdrawing substituents have opposite results The para substituents show better correlation with µ (R2=0.8552) than do the meta substituents (R2=0.8108) The meta substituents is more effective than the para ones because angle factor of m - line is greater than angle factor of p-line
µ = 5.0230 σ + 1.5148 ; R2 = 0.8108 (meta substituents);
µ = 4.5064 σ + 2.7802 ; R2 = 0.8552 (para substituents)
m-:µ = 5.023σ + 1.5148
R 2 = 0.8108
p-: µ = 4.5064σ + 2.7802
R 2 = 0.8552
-1 0 1 2 3 4 5 6 7 8
σ
p-
m-Figure 3 Correlation of the dipole moment µ with the Hammett σ constants
of the substituents
It is evident from the table II that electron-donating substituents on the phenyl ring increase the pKa of aniline’s amino group, and electron-withdrawing substituents decrease the pKa Hammett σ constants were initially developed to
Trang 5§¹i häc Vinh T¹p chÝ khoa häc, tËp XXXVI, sè 1A-2007
describe the electronic influences of substituents on chemical reactions and equilibria Accordingly, it is reasonable to expect that they might also provide a reliable measure of the substituent’s effects on the pKa of the amino moiety, and this is indeed the case:
pKa = -2.8437σ + 4.4898 ; R2 = 0.9892 (meta substituents);
pKa = -3.5197σ + 4.335 ; R2 = 0.9403 (para substituents)
In this case, the meta derivatives are better fit (R2 = 0.9892) than the para derivatives (R2=0.9403)
m- :pKa = -2.8437σ+ 4.4898
R2 = 0.9892
p-: pKa = -3.5197σ + 4.335
R 2 = 0.9403 -1
1 3 5 7 9
σ
m-
p-Figure 4 Correlation of the pKa value with the Hammett σ constants of the substituents
Table 3 lists correlation coefficients between Hammett σ constants and bond length C-N, negative charge on atom N, dipole moment and the pKa of compounds
Table 3 Correlation coefficients among the properties studied
meta substituents para substituents
It is apparent from the information in the table 3 that for d(C-N), QN and µ, para substituents provide better correlation than meta substituents, and for pKa, it shows contrary results
IV Conclusion
The semi-empirical AM1 level calculation performs well in terms of its geometrical description of aniline as indicated by the close agreement with the experimental bond lengths
As one would anticipate from general principles, electron-withdrawing substituents enhance planarity of the aniline framework, thereby shortening d(C-N) The accompanying loss of electron density from the amino nitrogen atom decreases the pKa of the amino group Conversely, electron-donating substituents favor pyramidal sp3 hybridization at the amino nitrogen, leading to a longer C-N
Trang 6Đại học Vinh Tạp chí khoa học, tập XXXVI, số 1A-2007
bond length Also, the pKa of the amino nitrogen is increased by electron-donating substituents
The variations of the three properties are closely tied together Hammett
σ constants which used as regression indicator to investigate the change of properties yield rather good correlations for these properties except for d(C-N), QN of meta – derivatives The correlation of pKa with σ is best fit
References
[1] Lister, D G.; Tyler, J K Chem Commun, (1966), 152–153
[2] Lister, D G.; Tyler, J K.;Hỉg, J H.;Larsen, N W J Molec Struct, 23 (1974), 253–264
[3] Brand, J C D.; Williams, D R.; Cook, T J J Molec Spectrosc, 20(1966), 193–
195
[4] Brand, J C D.; Jones, V T.; Forrest, B J.; Pirkle, R J J Molec Spectrosc, 39(1971), 352–356
[5] Quack, M.; Stockburger, M J Molec Spectrosc, 43(1972) , 87-116
[6] Kenvin C Gross, Paul G Seybold, International Journal of Quantum Chemistry, 80((2000),1107-1115
[7] http://www.hyperchem.com
[8] Hansch, C.; Leo, A.; Taft, R W Chem Rev, 91(1991), 165–195
[9] Albert, A.; Serjeant, E P Ionization Constants of Acids and Bases; Methuen: London, 1962
[10] David R.Lide, CRC Handbook of Chemistry and Physics, 75 ed, CRC press, London (1994), 9-50
Tóm tắt
Nghiên cứu ảnh hưởng của nhóm thế lên tính chất
của anilin bằng phương pháp lượng tử gần đúng AM1
ảnh hưởng của nhóm thế lên tính chất của aniline được nghiên cứu bằng phương pháp tính hoá lượng tử gần đúng sử dụng phần mềm HyperChem phiên bản 7.52 Phương pháp AM1 được dùng để tối ưu phân tử và tính các tham số cấu trúc Kết quả nghiên cứu cho thấy nhóm thế làm thay đổi độ dài liên kết C-N, điện tích trên nguyên tử N, mômen lưỡng cực và giá trị pKa của các dẫn xuất thế Hằng số hammett đặc trưng định lượng cho ảnh hưởng của nhóm thế có sự tương quan khá tốt đối với các thuộc tính này ngoại trừ độ dài liên kết C-N và điện tích trên nguyên
tử N của dẫn xuất thế meta