Fragmentation Mechanism of Two Kinds of Violated Even-electron Rule Compounds with Doubly Charged Ions in Mass Spectrometry†

doi:10.7503/cjcu20170507

Abstract

Abstract:

The formation of odd electron ion(OE^+·) by cleavage of even electron ion(EE⁺) is an anomalous reaction violates the “even-electron rule”. Two kinds of liquid crystal compounds, which have the structures of biphenyl with C≡≡C or CF₂O moiety as a linkage group, were analyzed by gas chromatography-mass spectrometer(GC-MS). The results show that the spectra of the compounds are obtained by remarkable odd electron radical ion peaks and doubly charged fragment ion peaks, so the cleavage pathways were similar for the two kinds of compounds. We noted a sequential loss of alkyl, alkyl, or phenoxyl radicals under electron impact, which seems to be frequent in the case of liquid crystal compounds. The achievement of a higher degree of aromaticity may be the driving force for these processes. For the alkyl biphenyl compounds with C≡≡C central-bridge-bond, the cleavage of the alkyl carbon-carbon bond is initiated by the loss of alkyl radical via the α-cleavage, followed by the the losing of the other alkyl radical by σ bond cleavage, i.e., α-σ cleavage. For the alkyl biphenyl compounds with CF₂O central-bridge-bond, the cleavage of the carbon-oxygen bond is initiated by the loss of phenoxyl radical via the i-cleavage, followed by the losing of the other alkyl radical by σ bond cleavage, i.e., i-σ cleavage. For the biphenyl compounds with double CF₂O central-bridge-bond, the cleavage of the carbon-oxygen bond is initiated by the loss of phenoxyl radical via the i-cleavage, followed by the losing of the other phenoxyl radical by σ-bond cleavage, i.e., i-σ cleavage. The obtained results suggest the reason of σ-bond breakage, which should be caused by an internal conversion(IC). Generally, doubly charged ions are found in the electron ionization(EI) mass spectra of hydrocarbons, but the intensities of them are weak and cannot be used in chemical analyses. In contrast, in this paper we showed two kinds of liquid crystal compounds, as interesting examples, which can eliminate two internal alkyl or phenoxyl groups as radicals and yield the most abundant doubly charged ions in their EI mass spectrum by αα/iα or ii cleavage reactions. As an example, the DFT method is adopted to simulating the cleavage pathway of compound 2d. A conclusion can be draw that the proposed fragmentation mechanism is agreed by both experimental data and theoretical research.

Key words: Gas chromatography-mass spectrometer, Even-electron rule, Doubly charged ions, Liquid crystal compound, Fragmentation mechanism

CLC Number:

O657.63

TrendMD:

WANG Yi, ZHANG Ping, WU Shengxiu, SUN Yuanyuan, ZHAO Tong, LIU Shixi. Fragmentation Mechanism of Two Kinds of Violated Even-electron Rule Compounds with Doubly Charged Ions in Mass Spectrometry^†[J]. Chem. J. Chinese Universities, 2018, 39(3): 435.

Figures/Tables 13

Table 1 Structures and stereochemistry of alkyl biphenyl compounds with C≡≡C central-bridge-bond*

Compound	R¹	X¹	X²	X³	X⁴	R²
1	C₃H₇	F	H	H	H	C₂H₅
2	C₃H₇	H	H	H	H	C₄H₉
3	C₃H₇	F	H	H	H	C₃H₇
4	C₃H₇	F	H	H	H	C₄H₉
5	C₃H₇	F	H	H	H	C₅H₁₁
6	C₃H₇	H	H	F	F	C₂H₅
7	C₂H₅	F	F	H	H	C₃H₇
8	C₃H₇	H	H	F	F	C₄H₉
9	C₄H₉	F	F	H	H	C₃H₇

Fig.1 EI mass spectra of alkyl biphenyl compounds with C≡≡C central-bridge-bond for compounds 1—9(A—I) ■ Doubly charged ion; ▲ even electron ion; ● odd electron ion; * molecular ion. Insets of (D) arethe expanded EI mass spectra, showing the presence of 13C isotope for doubly charged ion.

Table 2 Results of Q-TOF-HRMS of compound 4

Type of ion	Theoretical value	Measured value	10³ Error	Elemental composition
[odd]^+·	370.2091	370.2060	3.1	C₂₇H₂₇F^+·
[even]⁺	341.1700	341.1736	3.6	C₂₅H₂₂F⁺
[even]⁺	327.1544	327.1513	3.1	C₂₄H₂₀F⁺
[even]⁺	311.1231	311.1198	3.3	C₂₃H₁₆F⁺
[odd]^+·	298.1152	298.1147	0.5	C₂₂H₁₅F^+·
[odd]^+·	276.0934	276.0896	3.8	C₂₂ $H 12 + ·$
[even]²⁺	185.1043	185.1030	1.3	C₂₇H₂₇F²⁺
[even]²⁺	149.0573	149.0568	0.5	C₂₂H₁₅F²⁺

Table 2 Results of Q-TOF-HRMS of compound 4

Type of ion	Theoretical value	Measured value	10³ Error	Elemental composition
[odd]^+·	370.2091	370.2060	3.1	C₂₇H₂₇F^+·
[even]⁺	341.1700	341.1736	3.6	C₂₅H₂₂F⁺
[even]⁺	327.1544	327.1513	3.1	C₂₄H₂₀F⁺
[even]⁺	311.1231	311.1198	3.3	C₂₃H₁₆F⁺
[odd]^+·	298.1152	298.1147	0.5	C₂₂H₁₅F^+·
[odd]^+·	276.0934	276.0896	3.8	C₂₂ $H 12 + ·$
[even]²⁺	185.1043	185.1030	1.3	C₂₇H₂₇F²⁺
[even]²⁺	149.0573	149.0568	0.5	C₂₂H₁₅F²⁺

Scheme 1 Possible cleaving pathways of alkyl biphenyl compounds with C≡≡C central-bridge-bond * The reaction violates the “even-electron rule”.

Table 3 Structures and stereochemistry of alkyl biphenyl compounds with CF2O central-bridge-bond*

Compd.	R¹	X¹	X²	X³	X⁴	X⁵	Compd.	R¹	X¹	X²	X³	X⁴	X⁵
1a	C₂H₅	H	F	OCF₂CFCF₂	F	H	1b	CH₃	H	F	F	F	H
2a	C₃H₅	H	F	F	F	H	2b	C₂H₅	H	F	F	F	H
3a	CH₃	H	H	OCF₃	H	H	1c	H	H	F	F	F	H
4a	C₂H₅	H	H	OCF₃	H	H	2c	C₃H₇	H	F	F	F	H
5a	C₄H₉	H	F	F	F	H	3c	C₄H₉	H	F	F	F	H
6a	C₃H₇	H	F	F	F	H	4c	C₅H₁₁	H	F	F	F	H
7a	C₂H₅	H	F	F	F	H	5c	C₆H₁₃	H	F	F	F	H
8a	C₂H₅	CH₃	F	F	F	H	6c	C₅H₁₁	H	F	F	F	H
9a	No Replace	H	F	F	F	H

Fig.2 EI mass spectra of alkyl biphenyl compounds [a] with CF2O central-bridge-bond for compounds 1a—9a(A—I) ■ Doubly charged ion; ▲ even electron ion; ● odd electron ion; * molecular ion.

Fig.3 EI mass spectra of alkyl biphenyl compounds [b] with CF2O central-bridge-bond for compounds 1b(A) and 2b(B) ■ Doubly charged ion; ▲ even electron ion; ● odd electron ion; * molecular ion.

Fig.4 EI mass spectra of alkyl biphenyl compounds [c] with CF2O central-bridge-bond for compounds 1c—6c(A—F) ■ Doubly charged ion; ▲ even electron ion; ● odd electron ion; * molecular ion. Insets of (B) is the expanded EI mass spectrum, showing the presence of 13C isotope for doubly charged ion.

Table 4 Results of Q-TOF-HRMS of compound 2c

Type of ion	Theoretical value	Measured value	10³ Error	Elemental composition
[odd]^+·	562.1537	562.1592	5.5	C₃₁H₂₂F₈O^+·
[even]⁺	519.0990	519.0972	1.8	C₂₈H₁₅F₈O⁺
[even]⁺	415.1480	415.1496	1.6	C₂₅H₂₀
[odd]^+·	372.0932	372.0901	3.1	C₂₂H₁₃ $F 5 + ·$
[even]⁺	371.0854	371.0894	4.0	C₂₂H₁₂
[even]²⁺	186.0463	186.0449	1.4	C₂₂H₁₃ $F 5 2 +$

Table 4 Results of Q-TOF-HRMS of compound 2c

Type of ion	Theoretical value	Measured value	10³ Error	Elemental composition
[odd]^+·	562.1537	562.1592	5.5	C₃₁H₂₂F₈O^+·
[even]⁺	519.0990	519.0972	1.8	C₂₈H₁₅F₈O⁺
[even]⁺	415.1480	415.1496	1.6	C₂₅H₂₀
[odd]^+·	372.0932	372.0901	3.1	C₂₂H₁₃ $F 5 + ·$
[even]⁺	371.0854	371.0894	4.0	C₂₂H₁₂
[even]²⁺	186.0463	186.0449	1.4	C₂₂H₁₃ $F 5 2 +$

Scheme 2 Possible cleaving pathways of alkyl biphenyl compounds with CF2O central-bridge-bond* The reaction violates the “even-electron rule”.

Scheme 3 Structures and stereochemistry of biphenyl compounds [d] with double CF2O central-bridge-bond 1d: Biphenyl; 2d: quaterphenyl.

Fig.5 EI mass spectra of biphenyl compounds with double CF2O central-bridge-bond for compounds 1d(A) and 2d(B) ■ Doubly charged ion; ▲ even electron ion; ● odd electron ion; * molecular ion.

Scheme 4 Possible cleaving pathways of alkyl biphenyl compounds with double CF2O central-bridge-bond* The reaction violates the “even-electron rule”.

References 29

[1]	Mathur B. P., Abbey L. E., Burgess E. M., Moran T. F., Org. Mass Spectrom., 1980, 15(6), 312—316
[2]	Mathur B. P., Burgess E. M., Bostwick D. E., Moran T. F., Org. Mass Spectrom., 1981, 16(1), 92—96
[3]	Jones B. E., Abbey L. E., Chatham H. L., Hanner A. W., Teleshfsky L. A., Burgess E. M., Moran T. F., Org. Mass Spectrom., 1982, 17(1), 10—18
[4]	Hanner A. W., Abbey L. E., Bostwick D. E., Burgess E. M., Moran T. F., Org. Mass Spectrom., 1982, 17(1), 19—28
[5]	Teleshefsky L. A., Jones B. E., Abbey L. E., Bostwick D. E., Burgess E. M., Moran T. F., Org. Mass Spectrom., 1982, 17(106), 481—492
[6]	Appling J. R., Jones B. E., Abbey L. E., Bostwick D. E., Moran T. F., Org. Mass Spectrom., 1983, 18(7), 282—294
[7]	Appling J. R., Musier K. M., Moran T. F., Org. Mass Spectrom., 1984, 19(9), 412—422
[8]	Appling J. R., Burdick G. W., Moran T. F., Org. Mass Spectrom., 1985, 20(5), 343—350
[9]	Shamma M., Dudock B. S., Cava M. P., Rao K. V., Dalton D. R., Dejongh D. C., Shrader S. R., Chem. Commun., 1966, 1(1), 7—8
[10]	McLafferty F., Tureek F., Interpretation of Mass Spectra, University Science Books, California, 1993, 115—116
[11]	Gross J.H., Mass Spectrometry, Science Press, Beijing, 2012, 257—259
[12]	Karni M., Mandelbaum A., Org. Mass Spectrom., 1980, 15(2), 53—64
[13]	Bowen R. D., Harrison A. G., Org. Mass Spectrom., 1981, 16(16), 180—182
[14]	Ceraulo L., Agozzino P., Ferrugia M., Lamartina L., Natoli M. C., Org. Mass Spectrom., 1991, 26(4), 279—286
[15]	Attygalle A. B., Bialecki J. B., Nishshanka U., Weisbecker C. S., Ruzicka J., J. Mass Spectrom., 2008, 43(9), 1224—1234
[16]	Vessecchi R., Carollo C. A., Lopes J. N. C., Crotti A. E. M., Lopes N. P., Galembeck S. E., J. Mass Spectrom., 2009, 44(8), 1224—1233
[17]	Cai Y., Mo Z., Rannulu N. S., Guan B., Kannupal S., Gibb B. C., Cole R. B., J. Mass Spectrom., 2010, 45(3), 235—240
[18]	Chai Y. F., Gan S. F., Pan Y. J., Acta Chim. Sinica, 2012, 70(17), 1805—1811
	(柴云峰, 甘世凤, 潘远江.化学学报,2012, 70(17), 1805—1811)
[19]	Nizigiyimana L., Rajan P. K., Haemers A., Claeys M., Derrick P. J., Rapid Commun. Mass Spectrom., 1997, 11(16), 1808—1812
[20]	Ji H.Y., Synthesis and Properties of Novel Polyaryls, East China Normal University, Shanghai, 2009
	(冀海英. 新型联芳共轭有机化合物的合成及其性能研究, 上海: 华东师范大学, 2009)
[21]	Gao A. A., Zheng Y. Y., Du W. S., Chinese Journal of Liquid Crystals and Displays, 2014, 29(2), 159—171
	(高嫒嫒, 郑远洋, 杜渭松.液晶与显示,2014, 29(2), 159—171)
[22]	Meng F. B., Lian J., Gao Y. M., Progress in Chemistry, 2008, 20(4), 499—507
	(孟凡宝, 廉娇, 高永梅.化学进展,2008, 20(4), 499—507)
[23]	Gao H.J., Liquid Crystal Chemistry, Tsinghua University Press, Beijing, 2011
	(高鸿锦. 液晶化学, 北京:清华大学出版社, 2011)
[24]	Liu Y., Zhang Z. Y., Ren Z. D., Chinese Journal of Liquid Crystals and Displays, 2010, 25(4), 490—493
	(刘运, 张智勇, 任占冬.液晶与显示,2010, 25(4), 490—493)
[25]	Zhang F. M., Han Y. H., Shang H. Y., Chinese Journal of Liquid Crystals and Displays, 2010, 25(4), 510—514
	(张芳苗, 韩耀华, 尚洪勇.液晶与显示,2010, 25(4), 510—514)
[26]	Wang Y., Li M., Journal of Chinese Mass Spectrometry Society, 2015, 36(3), 255—260
	(王毅, 李敏.质谱学报,2015, 36(3), 255—260)
[27]	Wang Y., Zhang P., Wu S. X., Chinese Journal of Liquid Crystals and Displays, 2016, 31(11), 1046—1054
	(王毅, 张苹, 吴生秀.液晶与显示,2016, 31(11), 1046—1054)
[28]	Wang C.H., Techniques and Methods in Organic Mass Spectrometry, China Light Industry Press, Beijing, 2011
	(王聪慧. 有机质谱技术与方法, 北京: 中国轻工业出版社, 2011)
[29]	Alex A., Harvey S., Parsons T., Pullen F. S., Wright P., Riley J. A., Rapid Commun. Mass Spectrom., 2009, 23(17), 2619—2627