Computational evaluation concerning the deviation of the atoms in 1 ...

Front. Chem. China 2011, 6(2): 91–97 DOI 10.1007/s11458-011-0223-z

RESEARCH ARTICLE

Computational evaluation concerning the deviation of the atoms in 1- and 4postions on the six-member ring and the effects on 1 HNMR chemical shift Asadollah FARHADI (✉)1, Mohammad Ali TAKASSI1, Hamid Reza MEMARIAN2 and Mousa SOLEYMANI Density functional theory (DFT) calculations were carried out on some cyclohexane derivatives to investigate the deviation atoms on the 1- and 4-positions of chair plane. The deviations of chair plane of two position in the cyclohexane derivatives were calculated at the B3LYP/6–31 ++ G(d,p) level. Furthermore, we investigated the correlation between deviations of two positions from chair plane on the chemical shift hydrogen atoms on the 4-position. Keywords cyclohexane, heterocyclohexane, deviation, B3LYP/6–31 ++ G(d, p), chemical shift

1 Introduction Cyclohexane, an alicyclic hydrocarbon that forms a major component of petroleum products, is more reactant than even n-alkanes or monoaromatic hydrocarbons [1,2]. It is often used as an industrial solvent [3] and as food preservatives, and may be found as an intermediate of ethylene medium-pressure solution polymerization [4] because it is colorless, inflammable and highly volatile. In addition, cyclohexane has recently been used as an alternative solvent for replacing benzene, which is a carcinogenic compound [5]. However, many papers published in chemical and pharmaceutical industries demon-

strate the characterization of cyclohexane and its derivatives [6]. For the first time Sachse has proposed a clear model of the cyclohexane chair and later, the boat [7]. (Note: Sachse called these isomers “symmetrical” and “unsymmetrical,” respectively. The silly but convenient names “chair” and “boat” came later.) Moreover, from this type of compounds valuable information on the structural point of view has been reported [8–12]. Furthermore, the conformational analysis of these compounds is particularly well developed [13–18]. Cyclohexane and its derivatives lend themselves to thorough analysis because they are characterized by a small number of energy minima. The most stable conformations are separated by barriers that are somewhat higher and more easily measured than rotational barriers in acyclic compounds or other ring systems. The most stable conformation of cyclohexane is the chair [12]. However, two nonchair conformations of cyclohexane that have normal bond angles and bond lengths are the twist and the boat both of which are less stable than the chair [19]; the chair conformation was determined to be 5.5 kcal/mol lower in energy than the twist, and the twist and the boat conformations are more flexible than the chair, but are destabilized by torsional strain, as the bonds along the “sides” of the boat are eclipsed [19,20]. In addition, the boat conformation is further destabilized by van der Waals repulsion between the “flagpole” hydrogens. Both van der Waals repulsion and the torsional strain are somewhat reduced in the twist conformation [19]. However, the data of IR spectroscopy confirmed that the chair conformation is more stable than the twist conformation [20]. The NMR spectroscopy data obtained from iodocyclohexane at low temperature shows two distinct peaks in the area of the CHI signal, which are referred to as σ-bond field effect of cyclohexane [21] and the deviation of this position from chair plane can also cause changes of this parameter. In this study, at first the deviation atoms on the 1- and 4position of cyclohexane and its derivatives from chair plane in chair conformation were investigated, because the deviation of these atoms in two positions may affect the rate of reaction of cyclohexane and its derivatives and may impact the region of NMR signal of 1- and 4-posion of cyclohexane [21–24]. General structures of the cyclohexane, heterocyclohexane and the conformation of their derivatives studied in this work are shown in Fig. 1.

2 Computational methods Received January 20, 2011; accepted March 24, 2011 1. Faculty of Science, University of Petroleum Technology, Ahwaz 61981-44471, Iran 2. Department of Chemistry, Faculty of Science, University of Isfahan, Isfahan 81746-73441, Iran E-mail: [email protected]

Geometries of cyclohexane and its derivatives were optimized using density functional theory (DFT) B3LYP method with 6– 31 ++ G(d,p) basis set. All computations are carried out using G98W program package [25].

©Higher Education Press and Springer-Verlag Berlin Heidelberg 2011

92

Asadollah FARHADI, Mohammad Ali TAKASSI, Hamid Reza MEMARIAN and Mousa SOLEYMANI

Figure 1 General structure of the cyclohexane and heterocyclohexane and those derivatives conformation and the numbering scheme used in this work. Table 1 Selected optimized geometrical parameter obtained for cylohexanes and heterocyclohexnes at B3LYP/6–31 ++ G(d,p) level of theory (bond lengths are given in Å). See Fig. 1 for numbering scheme.

3 Results and discussion General structure of cyclohexanes and heterocyclohexanes shown in Fig. 1, respectively, in which the numbering scheme used to describe these structures is also introduced. In Table 1 the optimized lengths of the C1-C2, C2-C3 and C3-C4 are listed and these data show that the bond lengths in cyclohexane and its derivatives are not equal. Analysis of the data reported in Tables 1 and 2 show that the bond lengths in cyclohexane and its derivatives depend on the type and position of the substituent on the ring. For example, in compound c with the methyl group on the axial position of the cyclohexane, all bond lengths are increased as compared to compound a. (Just as C3-C4 length for a and b is 1.5367 Å vs. 1.5376 Å, respectively.) However, we observed this trend in the heterocyclohexane derivatives. For example, the bond lengths in heterocyclohexane such as k and l are 1.5372 Å and 1.5381 Å, respectively).

Comp.

C1-C2

C2-C3

C3-C4

a

1.5375

1.5373

1.5374

b

1.5461

1.5386

1.5376

c

1.5405

1.5375

1.5367

d

1.5508

1.5383

1.5364

e

1.5494

1.5494

1.5454

f

1.5357

1.5370

1.5379

g

1.5392

1.5369

1.5376

h

1.5409

1.5371

1.5374

i

1.5330

1.5378

1.5371

j

1.5357

1.5356

1.5381

3.1 Deviations of the C1 and C4 atoms from the chair plane (C2-C3-C5-C6) Thus, the deviations of C1, C4 and X1 atoms of the cyclohexane, heterocyclohexane and their derivatives

Frontiers of Chemistry in China Vol. 6, No. 2, 2011

Computational evaluation concerning the deviation of the atoms in 1- and 4-postions on the six-member ring Table 2


2 ¼ 180 –
1

(3)

C6 A ¼ C6 C1 cos y1

(4)

C2 B ¼ C2 C1 cos y2

(5)

C1 A ¼ C6 C1 sin y1

(6)

C1 B ¼ C2 C1 sin y2

(7)

The same as Table 1, but for heterocyclohexanes

Comp.

X1-C2

C2-C3

C3-C4

k

1.4270

1.5306

1.5372

l

1.4276

1.5232

1.5381

m

1.4207

1.5302

1.5379

n

1.4676

1.5396

1.5377

o

1.4646

1.5382

1.5383

p

1.4632

1.5444

1.5407

q

1.8374

1.5320

1.5379

r

1.8452

1.5328

1.5393

s

1.8344

1.5342

1.5404

93

In triangle C6C2A, C2 A ¼ C6 A2 þC2 C26 – 2C6 A
C2 C6
cos
2

(referenced to the plane containing C6, C5, C3 and C2 atoms) from the chair plane may affect the spectroscopy data of these compounds. In this section, calculations and analysis of these deviations are worked out. These calculations are based on the geometries and trigonometrics given in Figs. 2 and 3. y1 ¼ 180 – α1

(1)

y2 ¼ 180 – α2

(2)

cos φ2 ¼


1=2

C2 C26 þ C2 A2 – C6 A2 2  C6 C2

φ3 ¼ 180 – ðφ1 þ φ2 Þ

(8) (9) (10)

In triangle BC2A, AB ¼ðC2 A2 þC2 B2 – 2C2 A
C2 B
cos φ3 Þ1=2

Figure 2 General geometry defining deviations of the C1, C4 and X1 atoms from the chair plane (C2-C3-C5-C6) in the cyclohexane, heterocyclohexane and their derivatives.

Figure 3 a) The trigonometric frame used for the calculation of the deviation of C1 atom (the height hC1) from the C2-C3-C5-C6 plane, Fig. 2; b) A closer view of the triangle defining the height of C1 atom (hC1) from the C2-C3-C5-C6 plane as described in part (a).

Frontiers of Chemistry in China Vol. 6, No. 2, 2011

(11)

94

Asadollah FARHADI, Mohammad Ali TAKASSI, Hamid Reza MEMARIAN and Mousa SOLEYMANI

In triangle C1AB, cosαB ¼

C1 B2 þ AB2 – C1 A2 2  C1 B
AB

(12)

In triangle C1HB, C1 H ¼ C1 B
sin αB

(13)

For these calculations, we obtained the θ1, f1, α1 and α2 angles and the C2-C6 distance from the optimized structures of cyclohexane and heterocyclohexane using the trigonometric relations (1)–(13) given above. The same algorithm is used for the calculation of the deviation of the C4 and X1 atoms from the same chair plane (hC4 and hX1); the only difference is that the geometry is based on the C4C5C3 and C2X1C6 triangle (Fig. 2). The calculated values of the deviation of the C1 (hC1), C4 (hC4) and X1 (hX1) atoms of the cyclohexane and heterocyclohexane series are reported in Tables 3 and 4, respectively. Table 3 Calculation based on the geometries given in Figs. 2 and 3 by using relations (1)–(13) for the hC1 (C1) and hC4 (C4) (Å) from chair plane (C2-C3-C5-C6) for cyclohexanes. Comp.

hC1(C1)/Å

hC4(C4)/Å

a

0.6498

0.6531

b

0.6524

0.6497

c

0.6616

0.6574

d

0.6512

0.6575

e

0.6157

0.6476

f

0.6529

0.6660

g

0.6503

0.6626

h

0.6422

0.6626

i

0.6477

0.6620

j

0.6037

0.6492

Table 4 Comp.

The same as Table 3, but for heterocyclohexanes. hX(X1)/Å

hC(C4)/Å

k

0.6406

0.6497

l

0.6461

0.6351

m

0.6261

0.6498

n

0.5898

0.6497

o

0.5958

0.6299

p

0.6122

0.6517

q

0.9013

0.6767

r

0.9254

0.6569

s

0.9098

0.6569

The data reported in these Tables show that the deviations of C4 atom from the chair plane in cyclohexane and its derivatives are also more or less larger than that of C1 atom for each compound; compounds b and c are exceptions. Furthermore, we show this trend in heterocyclohexane, although in series of compounds is an exception (compound l is an exception).

Analysis of the data reported in the Tables 3 and 4 shows that deviations of the C1, C4, and X1 atoms from the chair plane also depend on the type and position of the substituent on cyclohexane and heterocyclohexane. For example, in the b and c isomers in cyclohexane with the same substituent (methyl) on the axial and equatorial positions on the cyclohexane, this deviation is smaller for the former isomer (0.6524 Å vs. 0.6616 Å, respectively) and in the l and m isomers with the same substituent (hydroxyl) on the 2- and 3positions on the heterocyclohexane we observed the change of the deviations for X1 and C4 atoms (0.6461 Å vs. 0.6261 Å and 0.6351 Å vs. 0.6498 Å, respectively). The increase of the deviation of C1 atom in the a and b depends on the 1,3-diaxial interaction of methyl group with hydrogen atom. For example, the deviations of C1 (hC1) for a and b compounds are 0.6498 Å and 0.6524 Å, respectively. While the deviations of C1 (hC1) for b isomer are smaller than the c isomer (0.6524 Å vs. 0.6616 Å, respectively), although the 1,3diaxial interaction in the b isomer is stronger than c isomer. This difference referred to the other parameters in the geometry of cyclohexane. Such as, the bond lengths of C3C4 for b and c isomers are 1.5376 Å and 1.5367 Å, respectively. A review of the data reported in Table 3 shows that hC1 for f compound is slightly bigger than that for g compound (0.6529 vs. 0.6503 Å, respectively) because the bond length of C1-OH is slightly shorter than the bond length of C1-SH (1.4473 Å vs. 1.8770 Å, respectively). Because of the decrease of the 1,3-diaxial interaction of Methoxy group with hydrogen atoms on the 3- and 5-positions the height of C1 for i compound is shorter than f compound (0.6477 Å vs. 0.6529 Å, respectively). The distances of OCH3 and OH groups from hydrogen atoms on the 3- and 5-positions are 2.7840 Å and 2.7061 Å, respectively. This explains that the height of C1 for h compound is shorter than f compound (0.6422 Å vs. 0.6529 Å, respectively). The data from a comparative analysis reported in the Table 4 show that hydrogen bound is very effective on the deviation of X1 from chair plane in the heterocyclohexane compounds. This parameter can be effective in increasing the height of X1 atom from chair plane. For example, the height of X1 (hX1) in l compound with 2-OH substituent is 0.6461 Å as compared to k compound (0.6406 Å). However, the data obtained for (n, o) and (q, r) pairs compounds show this parameter effect (0.5898 Å, 0.5958 Å) and (0.9013 Å, 0.9254 Å), respectively. However, the same substituent on the 3-position of these compounds causes hX1 (Å) from chair plane to decrease. For example, the height of X1 atom for (k, m) and (q, s) pairs compounds are (0.6406 Å, 0.6261 Å) and (0.9013 Å, 0.9098 Å), respectively. This difference referred to the repulsive interaction between X1 atom and 3-substiutent group, although n and p compounds are exceptions to this

Frontiers of Chemistry in China Vol. 6, No. 2, 2011

Computational evaluation concerning the deviation of the atoms in 1- and 4-postions on the six-member ring

explanation (0.5898 Å, and 0.6122 Å, respectively). Analysis of the data reported in the Tables 3 and 4 shows that the deviation of C4 from chair plane in the cyclohexanes and heterocyclohexanes are also dependent on the type and position of the substituent on these compounds because different substituent on the cyclohexane and derivatives can affect the bond lengths of cyclohexanes. According to this data no pure trend could be extracted for individual contributions. Fig. 4 shows the correlation between the deviations of the C4 and C1 atoms from the chair plane and the type and position of the substituent on the cyclohexane and derivatives at different positions of ring. 3.2 The correlation between deviation atoms from chair plane with chemical shift δ of 1HNMR Varying parameters such as temperature, aromatic ring effect and σ-bound effect can affect the chemical shift (δ) of hydrogen atom on the compounds [26]. Therefore, in this paper, we investigated the effect of the height atom from chair plane that affect the chemical shift δ of hydrogen atoms.

95

According to the data reported in Tables 5 and 6 we observed that the δ of hydrogen atom also depends on the height of C4 atom from chair plane because this parameter has an effect on the σ-bound effect in the cyclohexane and heterocyclohexane compounds. The data reported in these tables show that changes in the deviations of C4 atom from the chair plane cause changes in the chemical shifts δ (ppm) of hydrogen atom although on the 4-position there is no substituent. For example, in a and b, the δ (ppm) for H4ax is 1.12 Å and 1.06 Å respectively, corresponding to the heights of the C4 atom, 0.6531 Å and 0.6497 Å. Furthermore, we observed this relation between δ (ppm) for H4eq and the height on this atom from chair plane. For example, in a and e, the δ (ppm) for H4eq is 1.60 ppm and 1.38 ppm respectively, corresponding to the heights of the C4 atom, 0.6531 Å and 0.6476 Å. No pure trend could be extracted for individual contributions and it is incorrect to investigate the effect of deviation of C1 atom with δ (ppm) of H1-position because the effect of the substituent on this position is stronger than that for the height of C1 atom. Analysis of the data reported in the Tables 5 and 6 shows

Figure 4 Correlation between deviations of the C1, C4 and X1 atoms from the chair ring plane and the type and position of the substituent on the ring for (a) cyclohexanes and (b) heterocyclohexanes

Frontiers of Chemistry in China Vol. 6, No. 2, 2011

96

Asadollah FARHADI, Mohammad Ali TAKASSI, Hamid Reza MEMARIAN and Mousa SOLEYMANI

Table 5 Chemical shift δ of 1HNMR analysis of the axial and equatorial hydrogen (denoted with Hax and Heq respectively) atoms on the 1- and 4positions that are calculated for cyclohexane and heterocyclohexane using Chem 3D. Ultra 8 method. See Fig. 1 for sign scheme. Comp. δ (C1 Hax)/ppm δ (C1 Heq)/ppm δ (C4 Hax)/ppm δ (C4 Heq)/ppm 1.12

1.60

1.12

1.60

b

–

1.93

1.06

1.58

c

1.27

–

1.06

1.58

d

–

–

1.00

1.56

e

–

1.67

1.31

1.38

f

–

3.89

1.14

1.52

g

–

3.43

1.12

1.60

h

–

3.15

1.12

1.60

i

–

3.89

1.14

1.52

j

–

3.62

1.37

1.50

a

Table 6

The same as Table 5, but for heterocyclohexanes. δ (C4 Hax)/ppm

δ (C4 Heq)/ppm

k

1.60

1.60

l

1.55

1.65

m

1.63

1.88

n

1.50

1.50

o

1.45

1.55

p

1.60

1.85

q

1.70

1.70

r

1.65

1.75

s

1.84

2.09

Comp.

4 Conclusion

that the effect of the deviation C4 atom from chair plane is stronger than the inductive effect of heteroatoms on the 1position on the value of the δ (ppm) of Hax and Heq. For example, in k and q compounds the δ (ppm) for Hax and Heq are 1.60 and 1.70 ppm, corresponding respectively to the heights of the C4 atom, 0.6497 Å and 0.6767 Å although the electronegativity of oxygen atom is higher than that of sulfur atom. Furthermore, in Table 7 the results of 1H-NMR data of Table 7 Observed proton chemical shifts of some of cyclohexanes and analogs a) [27,28] Comp. δ (C1-Hax)/ppm δ (C1-Heq)/ppm δ (C4-Hax)/ppm δ (C4-Heq)/ppm a

1.1

1.6

1.1

1.6

b

–

1.93

1.04

1.58

c

1.26

–

1.04

1.58

d

1.36

1.48

–

–

f

–

3.89

1.12

1.52

g

–

3.43

1.1

1.6

h

–

3.15

–

–

k

–

–

1.6

1.6

n

–

–

1.5

1.5

q

–

–

1.7 (average)

1.7 (average)

a) Data observed at t = – 100°C and – 80°C

some cyclohexanes and analogs are reported [27,28]. A comparison analysis of data depicted on Tables 3, 4 and 7 shows that the deviation of the 1- and 4-positions of the atoms (hC1, hX1 and hC4) from the C2-C3-C5-C6 plane can have effect on the chemical shift δ of axial and equatorial hydrogen on the C1 and C4 atoms. For example, in compounds a and b the δ (ppm) for C1-Heq are 1.6 ppm and 1.93 ppm respectively, corresponding to the heights of C1 atom, 0.6498 Å and 0.6524 Å from chair plane although the Methyl group is a better electron donor than hydrogen atom. However, these data show that the effect of the deviation C4 atom from chair plane is more than the inductive effect of heteroatom on the 1position on the value of the δ (ppm). For example, in q compound, the δ (ppm) for C4-Hax and C4-Heq is slightly bigger than the k compounds; however, the electronegativity of sulfur atom is less than oxygen atom (1.7 ppm vs. 1.6 ppm, respectively). The deviation of 1- and 4-position for the two compounds are hX1 = 0.9013 Å, hC4 = 0.6767 Å and hC1 = 0.6406 Å, hC4 = 6497 Å, respectively.

A comparison data have been obtained for deviations of 1and 4-postions in the cyclohexane and derivatives, which shows a dependence of this parameter with type and position of substituent on the ring. Furthermore, the deviation of C4 can have an effect on the chemical shift of hydrogen atoms on this position and it seems that this effect is stronger than the inductive effect that heteroatoms on the 1-position have on the chemical shift of hydrogen atom on the 4-position. However, the chemical shifts of the protons in the axial and equatorial C4-position of thiacyclohexane (q compound) are slightly stronger than those for oxacyclohexane (k compound) as the electronegativity of oxygen atom is higher than that of sulfur atom (Table 7).

References 1. Walker, J. D.; Colwell, R. R., Appl. Environ. Microbiol. 1976, 31, 189–197 2. Solano-Serena, F.; Marchal, R.; Lebeault, J. M.; Vandecasteele, J. P., Lett. Appl. Microbiol. 2000, 30, 19–22 3. Brzostowicz, P. C.; Walters, D. M.; Jackson, R. E.; Halsey, K. H.; Ni, H.; Rouvière, P. E., Environ. Microbiol. 2005, 7, 179– 190 4. Mannsville, C. P. S., Cyclohexane; Mannsville Chemical Products Corporation, New York, 1993 5. Uribe, S.; Rangel, P.; Espínola, G.; Aguirre, G., Appl. Environ. Microbiol. 1990, 56, 2114–2119 6. Available from < http://chemicalland21.com/ > in 2006/07/20. 7. Sachse, H., Berichte 1890, 23, 1363–1370

Frontiers of Chemistry in China Vol. 6, No. 2, 2011

Computational evaluation concerning the deviation of the atoms in 1- and 4-postions on the six-member ring 8. Tomé, L. I. N.; Rosado, M. T. S.; Eusébio, M. E. S.; Redinha, J. S., J. Mol. Struct. THEOCHEM 2007, 804, 65–67 9. Kleinpeter, E.; Bölke, U.; Frank, A., Tetrahedron 2008, 64, 10014–10017 10. Kassaee, M. Z.; Arshadi, S.; Beigi, M., J. Mol. Struct. THEOCHEM 2002, 589–590, 153–159 11. Kośnik, W.; Bocian, W.; Chmielewski, M.; Tvaroška, I., Carbohydr. Res. 2008, 343, 1463–1472 12. Dega-Szafran, Z.; Dutkiewicz, G.; Fojud, Z.; Kosturkiewicz, Z.; Szafran, M., J. Mol. Struct. THEOCHEM 1971, 9, 447–454 13. Geise, H. J.; Buys, H. R.; Mijlhoff, F. C., J. Mol. Struct. THEOCHEM 1981, 46, 1959–1962 14. Allinger, N. L.; Hirsch, J. A.; Miller, M. A.; Tyminski, I. J.; VanCatledge, F. A., J. Am. Chem. Soc. 1968, 90, 1199–1210 15. Van de Graaf, B.; Baas, J. M. A.; Wepster, B. M., Recl. Trav. Chim. Pays Bas 1978, 97, 268–273 16. Baas, J. M. A.; Van de Graaf, B.; Van Veen, A.; Wepster, B. M., Recl. Trav. Chim. Pays Bas 1980, 99, 228–233 17. Antúnez, S.; Juaristi, E., J. Org. Chem. 1996, 61, 6465–6469 18. Allinger, N. L.; Miller, M. A., J. Am. Chem. Soc. 1961, 83, 2145–2148 19. Kellie, G. M.; Riddell, F. G., Top. Stereochem. 1974, 8, 225–269 20. Squillacote, M. R.; Sheridan, S.; Chapman, O. L.; Anet, F. A. L., J. Am. Chem. Soc. 1975, 97, 3244–3246 21. Jensen, F. R.; Bushweller, C. H.; Beck, B. H., J. Am. Chem. Soc. 1969, 91, 344–351 22. Li, G.; Xu, M.; Larsen, S. C.; Grassian, V. H., J. Mol. Catal.

97

Chem. 2003, 194, 169–180 23. Hao, J.; Wang, J.; Wang, Q.; Yu, Y.; Cai, S.; Zhao, F., Appl. Catal. A Gen. 2009, 368, 29–34 24. Anisia, K. S.; Kumar, A., J. Mol. Catal. Chem. 2007, 271, 164– 179 25. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A. Jr; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ciolowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; HeadGordon, M.; Replogle, E. S.; Pople, J. A., Gaussian 98; Gaussian Inc., Pittsburg PA, 1998 26. Silverstein, R. M.; Clayton Bassler, G.; Morrill, T., Spectrometric Identification of Organic Compound; 5th Edition, New York, 1991 27. Pretsch, E.; Clerc, T.; Seibl, J.; Simon, W., Tabellen zur Structuraufklärang organischerVerbindurgen, springer-verlag, Berlin Heidelberg, New York, 1989 28. Abraham, R. J.; Warne, M. A.; Griffiths, L., J. Chem. Soc. Perki Trans. 2 1997, 2, 31–39

Frontiers of Chemistry in China Vol. 6, No. 2, 2011