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Mass spectra of liquid crystals. III.Phenylpyrimidine derivatives

Citation for published version (APA):

Leclercq, P. A., & Bogaert, van den, H. M. (1991). Mass spectra of liquid crystals. III.Phenylpyrimidine derivatives. Organic Mass Spectrometry, 26(3), 135-138. https://doi.org/10.1002/oms.1210260304

DOI:

10.1002/oms.1210260304 Document status and date: Published: 01/01/1991

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ORGANIC MASS SPECTROMETRY, VOL. 26, 135-138 (1991)

Mass Spectra of Liquid Crystals

IIIt-Phenylpyrimidine

Derivatives

~ ~ _ _ _

P. A. LeclercqS

Laboratory of Instrumental Analysis, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands

H. M. van den Bogaert

Philips Research Laboratories, PO Box 80000,5600 JA Eindhoven, The Netherlands

The 70 eV electron impact mass spectra of 34 l-phenyl-2,5pyrimidine derivatives are presented. Based on the observed mass shifts by the various substituents, the nature of the main fragment ions is rationalized.

INTRODUCTION

Liquid crystals are of considerable industrial interest. Worldwide, 900 million twisted nematic liquid crystal displays (TN-LCDs) were manufactured in 1987.' The development of LCD technology is closely related to the progress made in the production of low relative molecular mass thermotropic liquid crystal materials since the early 1970s. Modern liquid crystal mixtures contain up to 20 components from different liquid crystal classes. These mixtures are blended for the simultaneous optimization of more than ten parameters which determine the performance of LCDs for specific applications.

Liquid crystal components used in TN-LCDs are polar or non-polar molecules, with mostly two, three or four six-membered ring structures. Introduced in 1977,3 phenylpyrimidine derivatives were among the first het- erocyclic nematic liquid crystals with electro-optical properties that allow improved digital addressability of TN-LCDs. These compounds are used extensively today, justifying the publication of their mass spectra for analytical purposes.

1-3. None of these spectra have been reported in the literature (the fragmentation of cyanophenyl- pyrimidines, however, was mentioned briefly4).

In many instances, the various substituents R2 in this class of compounds cause peak shifts in the spectra of over 3 u (X - OH+', Table 2) and 5 u (X - F', Table 3) with respect to Table 1 (X - CH2+). This affords the tentative structure postulated for many of the ions given. It should be stressed, however, that the structural formulae presented here are not to be taken literally. They merely indicate the different moieties of the molec- ular ions incorporated into the fragment ions, to account for the observed mass shifts.

In Tables 1-3, molecular ions M + ' are present in all instances. Tropylium- or benzyl-like ions are responsible for the base peaks in all spectra but two (compounds 10 and 11).

The spectra of the bisalkyl derivatives 1-9 (Table 1)

contain the base peaks at m/z (168

+

R'), corresponding to the ions [R'C4N,H2C6H4CH2]

+.

The alternative ions [H,CC4N2H2C6H4R2]+ at m/z (168

+

R') and the odd-electron ions [H2CC4N2H2C6H4CH,]+' at m/z 182 occur to a much lower extent. The only other frag- ment ion of importance appears at m/z 116, and corre- sponds to [NCC6H4CH,]+. Minor peaks are found at m/z 169 ([C4N2H3C6H4CH2]+), 168 and 155 (of equal intensity within each spectrum) and 142, 103,89 and 53. EXPERIMENTAL

P' Gas chromatographic/mass spectrometric analyses were

carried out under the conditions reported in Part I.'

RESULTS AND DISCUSSION

The electron impact (EI) mass spectra of 23 phenylpyri- midine derivatives, 1-23 (class I), are presented in Tables

t

For Parts I and 11, see Ref. 1.

1 Author to whom correspondence should be addressed. 0030-493X/91/03013544 $05.00

0

1991 by John Wiley & Sons, Ltd.

Class I

Class 11

Class 111

Received 30 August 1990 Revised manuscript received 6 November I990

(3)

136 P. A. LECLERCQ AND H. M. van den BOGAERT

Table 1. Relative abundances (YO) of characteristic ions in the EI mass spectra of bisalkylphenylpyrimidines (class I, R3 I H)’

168 168 141 No. R ’ R’ RMM M I ’ [ M - 1 1 ’ + R ‘ + A 2 + R ’ 182 Et Et Et Pr Pr Bu Bu Hx Hx Et Pr Bu Bu Pe Bu Pe Bu Pe 212 84 49 226 87 32 240 65 18 254 26 - 268 31 - 268 27 - 282 27 - 296 40 - 310 40 - 1 00 100 100 1 00 100 1 00 100 100 100 c 59 38 15 7 18 25 15 c 8 30 25 13 19 14 21 24 19 23 30 31

a Et = ethyl; Pr =n-propyl; Bu =n-butyl; Pe =n-pentyl; Hx =n-hexyl. brnfz of base peak. “Seernil (168+R’). 169 168 155 142 3 3 5 12 4 10 2 2 3 2 2 2 2 2 4 4 4 3 mlz 132 6 6 9 116 45 58 42 14 15 12 13 18 15 1 03 10 1 1 8 2

-

- - 2 2 89 77 53 14 9 1 1 14 10 6 13 1 1 9 4 - -4 - 2 3 - 2 3 - 2 4 - 3 3 - 3 41

-

12 20 5 9 5 6 1 1 9 Base peakh 197 197 197 21 1 21 1 225 225 253 253

In the spectra of compounds 1-3, the intensities of the last four peaks are slightly higher. These ethyl- pyrimidine derivatives also yield ions [M - 13’ at m / z

(141

+

R’),

and ions at m/z 132 and 77.

The peaks at m / z 155 could correspond to [C4N,H2

- C,H,]

’,

but more likely originate from the loss of

HCN from the aforementioned ion at m / z 182 (cf. m / z 158 in Table 2 and m/z (154

+

R 3

+

Rz) in Table 3).

The spectra of the alkoxyphenyl alkylpyrimidines 12-19 are summarized in Table 2. As an example, the spectrum of nonyloxyphenyl nonylpyrimidine (19) is

given in Fig. 1. The spectra of these compounds are dominated by the base peak at m / z 185, corresponding

to [H,CC4N,H2C6H40H]+, and the peaks at m/z

(171

+

R’) from the odd-electron ions [R’C4N2H,C6H40H]+’. The spectra of the esters 10 and 11 contain the same peaks, but with reversed inten- sities.

The spectra of all compounds 10-19 show a peak at

m / z 158. This peak can be assigned to ions formed by

the loss of HCN from m/z 185. Peaks at m / z 119 corre- spond to the odd-electron ion [NCC,H,OH] +*. Ion

Table 2. Relative abundances (%) of characteristic ions in the EI mass spectra of phenylpyrimidine ethers and esters (class I, R 3 = H)’ miz 171 Base No. R ’ R’ R M M M + ‘ + R ’ 297 283 269 255 241 227 213 199 186 185 158 156 155 120 119 57 55 43 41 peakb 10 11 12 13 14 15 16 17 18 19 Hx Yc Hp Zd Hp OHx HP OHP Hp ONo Oc OHx Oc ONo No OHx No OOc No ONo 346 388 354 368 396 368 41 0 382 41 0 424 10 4 33 31 29 23 23 51 1 1 42 100 100 68 70 86 16 27 12 19 25 - 3 1 1 1 14 9

_ -

_ _

2 4 3 5 7 7 14 14 1 1 1 1 - 2 4 4 4 7 9 20 22 30 - 3 12 80 3 1 1 19 74 4 17 26 100 4 16 25 100 4 17 28 100 8 39 50 100 9 43 53 100 12 58 62 100 16 68 62 100 17 83 83 100 ” M e = methyl; Hx=n-hexyl; Hp=n-heptyl; Oc =n-octyl; No=n-nonyl.

bm/z of base peak. “ Y = OCOCHCIMe. d Z = OCOCHCICH(Me),. 1 2 7 6 3 5 -1 -1 6 5 3 5 -1 7 4 3 5 7 -1 5 4 3 4 5 1 1 1 5 4 3 4 5 6 2 5 6 5 7 1 1 7 21 5 4 5 7 1 0 2 4 5 4 7 8 7 2 5 3 2 5 6 1 9 2 5 5 4 7 7 1 3 3 26 5 7 9 9 1 1 8 3 16 6 8 27 19 28 48 37 44 27 53 8 12 17 22 20 34 28 29 36 37 256 270 185 185 185 185 185 185 185 185

Table 3. Relative abundances (YO) of characteristic ions in the EI mass spectra of fluorophenylpyrimidine derivatives (class I)’ mi2 1 6 7 + 154+ 140+ 102+ 1 0 1 + 7 5 + 74 + Base No R‘ R 2 R3 R M M M + ‘ R 3 + R 2 R ” + R ’ R 3 + R 2 R ’ + R 2 R ’ + R 2 R 3 + R 2 R 3 + R 2 75 53 41 peakb 187 20 Et F H 202 82 100 20 32 39 97 15 21 Pe F H 244 24 100 3 34 12 22 4 6 3 6 16 187 22 Pr F F 234 34 100 3 42 13 39 7 8 3 8 1 1 205 - 5 10 217 23 Pr OMe F 246 42 100 -

a Me = methyl; Et = ethyl; Pr = n-propyl; Pe = n-pentyl. ’m/. of base peak.

22 10 19 -

- -

(4)

MASS SPECTRA OF LIQUID CRYSTALS. PART 111 137

1

1298

100 200 3 0 0 4 0 0

m / z

Figure 1. 70 eV E l mass spectrum of 5-nonyl-2- [4-(nonyloxy)phenyl]pyrirnidine (19).

series at m/z (185

+

n x 14), with n = 1-5, become apparent in the spectra of the higher homologues, and the abundance of the ions at m/z 186 increases accord- ingly.

The spectra of compounds 10 and 11 show chloro- containing ions [H3CCHClCO]+ at m/z 91 (3%) and [H,CCHCl]+ at m/z 63 (17%), and [(H,C),CHCHCl]+ at m/z 91 (8%), respectively (not included in Table 2).

The fluorinated phenylpyrimidines 2&23 (Table 3) yield spectra with base peaks at m/z (167

+

R3

+

R2), corresponding to the ions [H2CC,N2H,C6H3R3R2]+. Loss of HCN from these ions results in peaks at m/z (140

+

R3

+

R2). Peaks at m/z (101

+

R 3

+

R2) orig- inate from [NCC6H3R3R2] + ' ions, and are accompa-

nied by peaks 1 u higher. Ions [C,N,H3C6H3R3R2]+ at m/z (154

+

R 3

+

R2) and (bis)fluorophenyl ions at

m/z (75

+

R 3

+

R2) and (74

+

R3

+

R2) are found in these spectra as well.

The EI spectra of seven biphenylpyrimidine deriv- atives 24-30 (class 11) are listed in Table 4. No reference to mass spectrometry of this class of compounds was found in the literature.

The molecular ions are abundant. The fragmentation of these compounds is similar to that of the mono- phenylpyrimidines, so that the m/z values in Table 4 are 76 u higher than those in Tables 1 and 3. The addi- tional peaks at m/z (151

+

A) in Table 4 correspond to the ions [C12H7CH2]+ (24-28) and [C,,H,F]+' (29,

30), respectively. All compounds yield peaks at m/z 207, 177 and 53. Additional peaks occur in the spectra of the ethyl derivatives 24 and 25 at m/z 246, 217 and 129, whereas the fluoro derivatives 29 and 30 yield peaks at

m/z 250,221 and 133. The spectra of compounds 27 and

28 contain additional peaks at m/z 271 and 245.

The mass spectra of four bisalkylcyclohexylphenyl- pyrimidine derivatives 31-34 (class 111) are presented in Table 5. Reports on the mass spectra of this class of compounds were not found in the literature (for three cyanophenylpyrimidinylcyclohexane derivatives, how- ever, the masses of the base peaks have been reported5).

As expected with cyclohexylphenyl derivatives, the spectra contain the vinyl-type ions [R'C4N,H2C,H4CH=CH2]+' at m/z (181

+

R') and their satellites at

5 13 u.' The latter peaks, at m/z

(194

+

R') and m/z (168

+

R'), correspond to the (iso-) propenyl [R'C,N2H,C,H,C(CH2)=CH2]+ and tropylium- or benzyl-like [R1C,N2H2C6H,CH,] + ions,

respectively. The ions at m/z (168

+

R') and m/z (181

+

R1) are accompanied by peaks 1 u higher in mass.

Loss of Cn-1H2n-1 radicals (from R' = C,H2,,,) from the ions at m/z (181

+

R') and at f 13 u results in another set of three ions at m/z 208 ([H,CC,N,H, C,H4C(CH2)=CH2] +'), 195 ([H2CC4N2H,C6H4 CH-CH,] +) and 182 ([H,CC,N,H,C,H,CH,]+'). The occurrence of the protonated ions

Table 4. Relative abundances (YO) of characteristic ions in the EI mass spectra of biphenylpyrimidine derivatives (class 11)'

miz

230

+ R ' 244 244 232 217 203 178 177 176 151 150 115 Base No. R' R' RMM M+. + A C + R 2 271 + A C + A C + A c + A c 207 + A ' + A ' + A ' 177 + A C +A' + A C 53 43 41 peak" 24 E t BU 316 100 45 43 - 14 6 - 5 20 57 8 14 6 14 4 3 6 9 12 316 4 6 18 14 344 26 Bu Pr 330 73 100 11 - 2 9 - - - 4 68 - 15 7 16 4 3 5 11 14 301 27 Hx Pe 386 82 100 11 7 36 3d 1 2 1 60

-

10 4 11 2 1 3 20 17 329 28 Hx Hp 414 99 100 10 10 46 3' 1 3 1 63 7 10 4 12 2 2 4 37 23 343 30 Pr F 292 76 100 -

'

3' 25 10' 4 52 16 8 3 11 10 3' 3 - 5 263 aEt=ethyl; Pr=n-propyl; Bu =n-butyl; Pe=n-pentyl; Hx=n-hexyl; Hp=n-heptyl.

cA=14(CH,,compounds2~28)or19(F,29and30).

'At 1 u lower in mass.

'

See m/z (244 + R2).

25 Et HX 344 100 49 31 3 12 7 - 5 18 52 7 11 5 11 -

29 Et F 278 100 64 -

'

8' 16 6' 3 68 23 11 4 15 15 4' 8

-

- 278

m/z of base peak.

(5)

138 P. A. LECLERCQ A N D H. M. van den BOGAERT

Table 5. Relative abundances (YO) of characteristic ions in the EI mass spectra of cyclohexylphenylpyrimidine derivatives (class 111)'

m h 194 >82 181 169 168 No. R ' R 2 RMM M + ' [ M - 1 ] + [ M - 1 5 ] + [ M - 2 9 ] + [ M - 4 3 ] + [ M - 5 7 ] + [M-711' + R ' + R ' + R ' + A ' + R ' 100 29 60 14 30 31 Et Et 294 83 21 6 9 4 9 32 Et Pr 308 54 12 - 7 8 8 - 100 40 84 21 43 100 43 77 25 48 33 Pr Pr 322 78 17 3 13 10 8 34 Bu Pr 336 100 20 4 11 17

-

7 86 35 60 20 36 - - mi2 156 Base No. R ' R' RMM M + ' 208 195 182 + R ' 168 155 142 129 116 115 103 67 55 53 43 41 peakb 31 Et Et 294 83 9 21 14 13 10 3 8 23 17 15 7 6 18 15 11 35 223 32 Et Pr 308 54 11 25 16 13 10 3 9 27 17 13 7 7 21 15 19 39 223 33 Pr Pr 322 78 21 55 29 23 22 6 13 19 22 15 7 12 27 11 27 60 237 34 Bu Pr 336 100 21 57 36 24 23 6 14 19 23 13 6 10 28 14 39 58 336

a Et =ethyl; Pr =n-propyl; Bu =n-butyl.

bmfz of base peak.

[R'C,N,H,C,H,]+ at m/z (156

+

R')

is interesting. CH=CH,]+' a t m / z 129 and [NCC6H4CH2]+ The remaining ions are independent of the individual at m/z 116. Finally, these spectra contain ions formed alkyl substituents: [C4N2H,C,H4CH2]+' at m/z 168, by losses of small alkyl radicals from the molecular [NCC,H4C(CH2)=CH2] + at m/z 142, [NCC,H4 ions.

REFERENCES

1. P. A. Leclercq and H. M. van den Bogaert, Org. Mass Spectrom. 4. R. L. Hubbard, in Proceedings of the Symposium on the

25,550 (1 990) ; 25,683 (1 990). Physics and Chemistry of Liquid Crystal Devices, ed. by G . J. 2. M. Schadt, Liq. Cryst. 5, 57 (1989). Sprokel, p. 331. Plenum Press, New York (1 979).

3. A. Boller, M. Cereghetti, M. Schadt and H. P. Scherrer. Mol. 5. J. CermBk, I. Kolb and 1. NAdhernB, Chem. Prum. 38, 407

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