AM1 AND ELECTRON IMPACT MASS SPECTROMETRY STUDY OF THE FRAGMENTATION OF 4-ACYL ISOCHROMAN-1,3-DIONES

ABSTRACT. Recently, in electron impact mass spectrometry (EIMS), it has been found a good correlation between the fragmentation processes of coumarins and the electronic charges of the atoms of their skeleton. In this paper, the same analytical method has been applied to 4-acyl isochroman-1,3-diones, whose mass spectra had been studied earlier and described for the very first time from our laboratory. Results obtained are in good agreement with the last study.


INTRODUCTION
4-Acyl isochroman-1,3-diones 1 are very interesting compounds studied in our laboratories.It has been found to have several properties for these compounds.They exhibit fluorescence [1][2], anti-oxidant [3] and anti-bacterial properties [4], among others.Some of their spectral properties, nuclear magnetic resonance (NMR) [5], mass [6], UV-Visible [3], and their reactivity [3,[7][8][9], have been described.Their crystal structures have also been investigated [10][11][12].Recently a new approach of analysis of fragmentation processes of coumarins in electron impact mass spectrometry has been described to give a good correlation with the electronic charges of their atoms [13][14][15].It has been found that fragmentation processes in electron impact mass spectrometry (EIMS) takes place mainly at the level of atoms bearing high negative charge.It seems to be possible to predict and explain most of their fragmentations, using electronic charges of atoms.So, as a part of our continuing investigations about 4-acyl isochroman-1,3-diones 1, the fragmentation processes of these compounds in electron impact mass spectrometry (EIMS) was successfully analyzed by the calculated AM1 electronic charges of atoms.It has been found that the method presents a good reliability.The electronic charges of atoms become a valuable instrument for understanding the fragmentation processes of organic compounds in MS.This study is the second investigation about mass spectra of these compounds but the very first one with both experimental and theoretical investigations.

EXPERIMENTAL
The title compounds 1 have been obtained as described in the literature [1], by action of acid chlorides or acid anhydrides on homophthalic anhydride in the presence of a base, as shown below:

RESULTS AND DISCUSSION
The acylated isochroman-1,3-diones 1 have been obtained with more than 70% yield [1].They have been identified by their melting points and by their 1 H, 13 C and 17 O NMR spectra [3,5].Mass spectra.All the mass spectra were obtained by electron impact mass spectrometry (EIMS) on a CPG-JSM AX505 apparatus at 70 eV and the data are given in Table 1.
AM1 calculation of electronic charges of atoms.The electronic charges of atoms were performed by Austin Model 1 (AM1) semi empirical method [16] using "Chem3D Ultra8" software on a Pentium 4 Computer.Results are given in Table 2.
Table 1.Mass spectra (EIMS) of compounds 1.  Tautomery of compounds 1.The three tautomeric forms A, B, C (Scheme 3) had been suspected and described as possible structures for these compounds [8,9] but the only one form C has been really observed both in solution [5] and in crystalline form [10][11][12].
Formation of molecular ion.In all the precedents descriptions of the structure of compounds 1 using NMR [5] or crystallographic methods [10][11][12], only the tautomeric form C had been found, whatever the nature of R (aliphatic or aromatic).AM1 calculation of heat of formation (∆H) of 1 gave positive values also the same results [5].The behaviour of these compounds in mass spectrometry has been reported for the very first time since a few years [6].It had been observed only the two enolic forms, B and C, of this tautomery.Nevertheless, this tautomery takes place only after formation of molecular ion.According to the above theory, the molecular ion must take place at the level of O 4 and/or O 3 , as oxygen bearing the highest negative charges.
Fragmentations must then take place at the level of C The M-18 fragmentation takes place after transposition of aliphatic hydrogen from C 12 onto O 4 atom, to produce water.The M-17 fragmentation takes place by direct lost of the hydroxyl group, when R is an aromatic moiety.In this case, there is no hydrogen on C 12 to remove from this aromatic group.It must be noticed that for compounds 1a and 1b, atoms C 12 and O 4 exhibit high negative charges.
Formation of the fragments m/z = 162, 145 and 134.Due to the proximity of the charges values of C 4 and O 4 , the transposition of aliphatic hydrogen described above in the formation of water, can also take place onto C 4 , leading to another type of fragmentation shown below.It is then obtained the fragments m/z = 162 (Scheme 6), 145 and 134 (Scheme 7).The following mechanism had been described to explain the formation of these fragments, obtained exclusively when R is an aliphatic moiety (R = CH 3 or C 2 H 5 ).However, the difference of stability of this fragment m/z = 162, 50% for 1a and only 8% for 1b, can be explained by the difference between the charges value of their C 12 atom: -0.351 for 1a and -0.254 for 1b.So, the transfer of the aliphatic hydrogen from this C 12 onto C 4 is less favoured for 1b than for 1a.
This fragment presents several mesomeric forms as shown below and lead to the fragments m/z =145 and 134.In all these last fragmentations, atoms which put a lot into the fragmentations processes are C 9 , C 4 , C 12 , O 1 , O 2 and O 3 .On the other hand, it is then possible to understand why the compound 1b, (R = CH 3 -CH 2 ), has its base peak at m/z = 118.Its formation take place from the fragment m/z = 162 by the follow mechanism, after losing CO 2 (Scheme 8).It is, once, noticeable that these atoms exhibit high negative electronic charge.Formation of fragments m/z = 105, 77 and 51.An original behaviour has been signalled the presence of three unusual fragments m/z = 105, 77 and 51 in the spectrum of fused bicyclic heterocyclic compounds.These fragments had been observed both for 1a and 1b (R aliphatic) [6].Usually, these fragments are characteristic of mono substituted phenones and mono substituted derivatives of benzoic acid.If the presence of these fragments has been naturally obtained and described above for the compound 1c (Ar = C 6 H 5 ), in the present case, this phenomenon has been explained after considering the fragment m/z = 145, as their parent peak because only these two compounds can form these fragments.In this case, the follow process had been described.After a transfer of hydrogen atom from the fragment m/z = 145, the ion with the m/z = 105 is obtained, which leads to other fragments 77 and 51 by the usual process of fragmentation (Scheme 9) [6].This process is driven by C 4 , C 7 and C 9 which are bearers of high negative electronic charges.

CONCLUSIONS
After the application of analysis of fragmentations of three series of coumarins through the electronic charges of their atoms, title compounds had also been investigated by electron impact mass spectrometry, using the electronic charges of atoms obtained by AM1 semi empirical method.The method appeared as a useful one for understanding and explaining the processes of the fragmentations.In this study, it has been found logic and good explanations of most of fragmentations obtained: formation of molecular ions of the compounds and most of their fragments take place mainly at the level of atoms bearing high positive charge.So loss of water (H 2 O) or hydroxyl (OH .) group and formation of fragments m/z = 162, 145, 134, 105, 77 and 51 can be justified by exploiting electronic charges of atoms.The correlation between charges of atoms and fragmentations processes of these compounds had been found to be very suitable.So, the method seemed to be applicable to every family of organic compounds.

1 . 5 :
Scheme 4. Formation of molecular ion and of fragments R-CO + or Ar-CO + .Other fragmentations processes of 4-acyl isochroman-1,3-diones 1.Formation of M-18 and M-17 fragments.It has been observed that compounds 1, with an aliphatic R, loose one molecule of water (M-18) while the compounds with arylic R loose

Table 2
. Electronic charges of atoms of compounds 1.