PHOTOIONIZATION MASS SPECTROMETRY OF ALIPHATIC MONOTERPENE ALCOHOLS L. V. Kravchenko and G. R. Rik

UDC 543.51 : 541.141.7 : 668.532

Recently, considerable advances have been achieved in the application of ultraviolet radiation to the formation of ions in the source of a m a s s s p e c t r o m e t e r . Thanks to a number of basic and methodical advantages, photoionization m a s s s p e c t r o m e t r y can be used successfully to investigate the m e c h a n i s m of ionization p r o c e s s e s [1, 2]. The aim of the p r e s e n t work was to determine the basic p r o c e s s e s of dissociative photoionization of monoterpene alcohols and to c o m p a r e them with the p r o c e s s e s taking place on e l e c t r o n impact. Figures 1-3 give the photoionization m a s s s p e c t r a of geraniol, nerol, and linalool, and also the corresponding spect r a obtained by the e l e c t r o n - i m p a c t method. Geraniol. On photoionization, the splitting out of w a t e r in the decomposition of the m o l e c u l a r ion leads to the appearance of an ion with mite 136 as the strongest in the m a s s s p e c t r u m . It is difficult to determine a c c u r a t e l y the s t r u c t u r a l formula of the radical cation formed in this p r o c e s s : apparently we are dealing with a set of distinct i s o m e r i c s t r u c t u r e s . The splitting out of water may be accompanied by cleavage of the C - C bonds of the side chains. The elimination of a methyl radical gives an ion with m / e 121, and the splitting out of an isopropyl radical an ion with m / e 93. Of the cleavages not induced by an oxygen atom, the most probable is the cleavage of the allyl bond. This bond is in the fl position to two probable points of localization of the charge - the q u a t e r n a r y c e n t e r s of the s t r u c t u r e with double bonds - and is readily broken on e l e c t r o n impact [3] with the formation of an ion with m / e 69 {M+ ~ 6 9 + + 8 5 ) , which decomposes with the ejection of ethylene and gives a strong peak of an allyl cation (mite 41). On photoionization, the ion with m / e 69 has an insignificant intensity, but together with it an ion with m / e 84 appears which has a low intensity on e l e c t r o n impact. In the field of low m a s s numbers, the photoionization m a s s s p e c t r u m of geraniol shows a s t r o n g p e a k of an ion with m / e 59. Since in the e l e c t r o n - i m p a c t m a s s s p e c t r a of monoterpene hydrocarbons no ion with such a value of the m a s s n u m b e r is observed [4], it may be regarded as a homolog of the hydroxonium ion. No f r a g m e n t a r y ions in the regions of m a s s numbers below 59 are formed in the photoionization of geraniol. This is apparently due to the low excitation energy of the heavy f r a g m e n t a r y ions. A c h a r a c t e r i s t i c feature of the photoionization m a s s s p e c t r u m of geraniol is the increase in the r e l a tive intensities of the c h a r a c t e r i s t i c ions with high m a s s n u m b e r s . The relative magnitude of the peak of the m o l e c u l a r ion in photoionization is 25 times g r e a t e r than on e l e c t r o n impact. The intensity of the c h a r a c t e r i s t i c ion with m / e 136 i n c r e a s e s 20-fold. The c h a r a c t e r i s t i c s of the photoionization m a s s s p e c t r a p e r m i t the question to be put of the possibility of the effective analysis of i s o m e r i c compounds. Nerol. Nerol is a s t e r e o i s o m e r of geraniol. As is known [5], trans i s o m e r s are m o r e stable to photon and e l e c t r o n impacts. This is confirmed by differences in the photoionization m a s s s p e c t r a of g e r a niol, which is a t r a n s i s o m e r , and nerol, which is the c o r r e s p o n d i n g cis i s o m e r . The ratio of the intensities of the peaks of the m o l e c u l a r ions of the cis and t r a n s i s o m e r s that we studied is I c i s / I t r a n s = 1 : 4. The intensity of the m o l e c u l a r ion as a fraction of the total ion c u r r e n t is 6% Agrophysical S c i e n t i f i c - R e s e a r c h Institute. Translated f r o m Khimiya P r i r o d n y k h Soedinenii, No. 6, pp. 723-726, N o v e m b e r - D e c e m b e r , 1974. Original article submitted September 27, 1973. ©19 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

746

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Fig. 1. Mass s p e c t r a of geraniol: a} on photoionization; b} on e l e c t r o n impact.

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Fig. 2. Mass s p e c t r a of nerol: a) on photoionization; b) on e l e c t r o n .impact.

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Fig. 3. Mass s p e c t r a of linalool: a) on photoionizatlon; b) on e l e c t r o n impact.

747

f o r nerol and 14% f o r geraniol, i.e., the stability of the cis i s o m e r is 2.3 t i m e s s m a l l e r than f o r the t r a n s isomer. On ionization with e l e c t r o n s having an energy of 40 eV, the ratio of stabilities of the m o l e c u l a r ions of geraniol and nerol is 1.4. It can be s e e n f r o m this that in the c a s e of photoionization we have a m o r e c h a r a c t e r i s t i c value of the stability f o r each of the i s o m e r s than on e l e c t r o n impact. Linalool. In c o n t r a s t to the m o n o t e r p e n e alcohols that we have c o n s i d e r e d above, linalool is a t e r t i a r y alcohol. In addition to this s t r u c t u r a l difference it lacks a readily-cleaved doubly allyl bond. The m o l e c u l a r ions of t e r t i a r y alcohols a r e c h a r a c t e r i z e d by low stability [6], and t h e r e f o r e the peak of the m o l e c u l a r ion on photoionization is considerably i n f e r i o r in intensity to the peak of the m o l e c u l a r ion in the m a s s s p e c t r a of a p r i m a r y alcohol. The relative intensities of the m o l e c u l a r ions for geraniol and linalool a r e , r e s p e c t i v e l y , 14 and 1~. The relative intensities of the ions M +. and ( M - 18) +. f o r m e d in the photoionization of linalool a r e , r e s p e c t i v e l y , 25 and 20 t i m e s g r e a t e r than on e l e c t r o n impact. On photoionization, the p r o c e s s leading to the f o r m a t i o n of the ion with m / e 71 which a r i s e s in the decomposition of the m o l e c u l a r ion at the C 5 - C 6 bond is unlikely. It m u s t be mentioned that, in the photoionization m a s s s p e c t r u m , ions a r e o b s e r v e d the f o r m a t i o n of w h i c h i s c o n n e c t e d with the m i g r a t i o n of hydrogen (ions with m / e 136, 111, 95, 80, 59). At the s a m e time, ions with m / e 71 and 69, the a p p e a r a n c e of which takes place with simple bond cleavage and the e n e r g e t i c favorability of which is obvious, a r e absent f r o m the m a s s s p e c t r a . EXPERIMENTAL The photoionization m a s s s p e c t r a were r e c o r d e d on an a p p a r a t u s constructed on the b a s i s of a MI1301 m a s s s p e c t r o m e t e r . F o r the photoionlzation of the vapors of the compounds under investigation, unresolved light was introduced into the ionization c h a m b e r through a window f o r m e d f r o m a lithium fluoride c r y s t a l . The s o u r c e of vacuum u l t r a v i o l e t radiation was a high-voltage hydrogen lamp [7]. The m a x i m u m e n e r g y of the photons p a s s i n g into the ionization c h a m b e r was limited by the t r a n s m i s s i o n of the window and did not exeeed 11.8 eV. On photoionization, the ion c u r r e n t s did not exceed 10 -44 A, and they were r e c o r d e d by m e a n s of an ion-optical c o n v e r t e r . The e l e c t r o n - i m p a c t m a s s s p e c t r a w e r e taken on the s a m e a p p a r a t u s with an ionization voltage of 40 V. The t e m p e r a t u r e of the inlet s y s t e m and of the ionization c h a m b e r was 100°C in both c a s e s . SUMMARY 1. The dissociative processes observed in the photoionlzation of monoterpene alcohols have been investigated. It has been established that the relative intensities of the peaks of the heavy ions, including the molecular ions, are higher on photoionization than on electron impact. 2. It has been found that the mass spectra of the isomeric alcohols obtained on photoionization differ from one another to a greater degree than the electronic mass spectra.

LITERATURE l.

2. 3. 4.

5. 6.

7.

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CITED

M. E. Akopyan, Usp. Fotonlki, No. 1, 46 (1969). N. W. Reid, Int. J. Mass Spectr. Ion Phys., 6, 1 (1971). R. I. Reed, in: Mass S p e c t r o m e t r y of Organic Ions (ed. F. W. McLafferty), A c a d e m i c P r e s s , New York (1963), Ch. 13. A. F. T h o m a s and B. Wilhalm, Helv. Chim. Acta, 4__77,475 (1964). P. Natalis, in: Applications of Mass S p e c t r o m e t r y of Organic C h e m i s t r y (ed. R. I. Reed), Academic P r e s s , London (1965), p. 379. R. A. Friedel, J. L. Shults, and A. G. Sharkey, Anal. Chem., 28, 926 (1956). F. L Vilesov, P r i b o r y i Tekhn. ]~ksperim., No. 4, 89 (1958).