Fahmey, M A

Ghonime, Hoda S

Selim, Ezzat T M

English

IACS Institutional Repository

Indian ]. Phys. 6SB (2), 171-179 (1991)Mass spectrometric study of molecular ions of methyl- phenol isomers using electron impact techniqueEzzat T  M Selim, M A  Fahmey and Hoda S GhonlmeNuclear Physics Department, Nuclear Research Centre,Atomic Energy Authority, Cairo, EgyptReceived 29 June 1990, accepted 14 November 1990Abstract: The mass spectra of 2-, 3- and 4-methylphenol isomers at 70 and 14 eV have been recorded and discussed. Substituent effects of CH, and OH groups on the relative abundance of the prominent ions produced from melhyl- phenols as well as the mechanistic pathways for the formation of some major ions are reported and discussed. The ionization efficiency curves for the molecular ions of methylphenol isomers, using deconvoluted first differential technique are reported and investigated The values of ionization energies at threshold are measured at 8.46 ±  0.06, 8.36 ±0.11 and 8.31 ± 0 .0 8  eV for 2-, 3- and 4- methylphenol, respectively and are explained as due to ionization of 1 e,  ^electrons from 773(6 , component) orbitals of the corresponding methylphenol isomers. Molecular energy levels have been detected in the curves and some of these levels are tentatively explained.Keywords : Mass spectra, methylphenol isomers, electron impact technique, substituent effects, ionization efficiency curves, molecular energy levels.PACS Nos : 34.80 Gs, 35.20. WgI • IntroductionThe abundance of the molecular ion, [M ]+, depends mainly on its stability and the amount of energy needed to ionize the molecule. Particular structural features tend to sho\N characteristic values of these properties, so that the magnitude of [M]+ provides an indication of the structure of the molecule (McLafferty 1973). On the other hand, the mass spectra of positional isomers are essentially similar, their spectra often exhibit differences in the relative abundances of some of the fragment ions (McLafferty 1973). However, some isomers could not be distin­guished by electron impact mass spectra only because the relative intensities of the fragments are so similar (Mclver 1975), therefore, it may be necessary to correlate between mass spectra and energy consideration.The aim of the present work is to study the effect of the relative position of substituent groups C H 3 and OH on the relative abundance of the main fragment ions obtained from methylphenol isomers. Moreover, the ionization efficiency (IE)t71curves for the molwcular ions of methylphenol isomers (for about 4 e V  above threshold) are reported using electron impact technique in order to measure the ionization energies and molecular energy levels for these isomers. It might be noted that valence shell orbital energies for the methylphenol isomers were investi­gated previously by Palmer et al (1979) using photoelectron spectra and ab initio MO calculation.The directly measured IE data are treated with the deconvoluted first differen­tial (DFD) technique (Selim 1985). The results (Selim and Helal 1982, Selim et al 1984, Selim 1985 and Selim et al 1987) obtained by this technique are reliable and sufficiently accurate for comparison with similar values in the literature.2. ExperimentalThe IE curves were obtained using an Atlas C H -4  single focusing mass spectro­meter with a normal electron impact (A N -4 ) ion source. The conditions of measurements and the method of calibration of the energy scale were the same as previously reported (Selim 1976). The electron energy was varied in 0.10 eV steps using a 10-turn helipot potentiometer and was measured with a digital multimeter Model VR-3511 of Hitachi. The energy scale was calibrated using the 15.76 eV threshold of argon.The directly measured IE curves were smoothed using a 5-point smoothing (Savitzky and Golay 1964) computer technique in order to decrease the random noise in the raw data. Once again using a computer technique the smoothed data were treated to obtain the first derivative curves, and then the inverse convolution technique of Vogt and Pascual (1972) was applied to obtain the DFD IE curves. The complete details of the application of this combined technique had been reported elsewhere (Selim 1985) together with merits and limitations of the technique.The mass spectra of methylphenol isomers were scanned at 70 and 14 eV electron energy and the intensity of prominent ions was calculated relative to the total ion intensity. Also, the metastable peaks associated with different fragmenta­tion processes have been detected and reported.3. Results and discussionThe mass spectra at 70 eV as well as 14 eV for the three molecules are recorded. Table 1 presents the relative intensities of the different peaks in the mass spectra of the three isomers relative to the total peaks intensity ( % ^  ). The decomposi-. 70,14tion pathways for the formation of the principal ions obtained from methylphenol isomers are illustrated in Figure 1. The DFD IE curves (corrected) for [C^HgO]* molecular ions obtained from methylphenol isomers for about 4 eV above threshold are given in Figure 2. The values of ionization energies at threshold as well as molecular energy levels for the three isomers are reported in Table 2 together with1 7 2  Ezzat TM Selim M A  Fahmey and Hoda S G h o n im eMass spectrometric study of molecular ions etc 1 7 3similar results reported previously by other authors using different techniques. The values reported in Table 2 are the average of six measurements and the errors quoted are the standard deviations. Only, reproducible values for molecular energy levels are reported in the Table.Table I. Mass spectra of 2-, 3- and 4-methylphenol at 70 and 14 eV.R.|*Ionformulam/z 2 -methyl-phenol70 eV 14 eV3-methyl-phenol70 eV U e V4-methyl-phenol70 eV 14 eVC ,H .O - C,H ;0^C,Hlc ,h ;c «h :,CoH?C , H tc .h :C.HJc ,h ;C.H iC .H iC.Ht108107106929190898079787765646354535251503930.0322.600.720.332.516.472.182.727.841.797.150.840.451.111.38 2.54 1.59 2.75 1.713.3863.5 23 10.872.164.870.633.371.832.5429.2225.070.761.963.42 1.29 0,2 2.837.482.43 6  93 1.02 0.56 1.371.492.721.72 3.3 1.784.4462.7725.30.752.454.521.02.5729.0828.850.811.633.081.250.352.857.592.157.330.730.521.021.392.651.802.471.283.1760.5327.720.782.425.380.852.30*The intensities are cular ion ( % ^ 7 0 f S 4 ) -given as a percentage of total ion current from m/z -39 up to the moleI. M ass spectra of 2-, 3- and 4-methylpbenol :The relatively large peak at 70 eV and at 14 eV due to formation of [M -H ^O ]^ fragment ion (m/z= 9 0 ) (Table 1) from 2-methylphenol, in comparison with relatively low intensity peaks for the same ion obtained from the other two isomers is an example of the ''ortho effect" (McLafferty 1973).H^Cl 7 '  OHreorr Hh- o-"h^' W HjO( m/2s90>The (C t H rO];^ molecular ions obtained from the three isomers of methylphenol represent the most intense peaks in the presently measured mass spectra (base174 Ezzat T M S e lim  M A Fahmey and Hoda S G h o n im eCo3OooBoc0)JTaEI-^t-oc(0CMOiC(i>>CD<DCd>3OE-ocCO(/>o’&0c0con01^  ^  r*n (3>00 00 00inCOS 1oinodlO -H «CO CM 00CM ID o>00 00 GO'e T-001 ^ COo00UJ 00 d1 ^T— '1—CMT“Q.o$0jco-HCOCO00^  CO T- OiCO COCDo64^COCMCM CM CD 00»- CMr-00o 00o O(J)o o r-d d d d d d41 -H fl 41 41 HCO O)CMCOCM CDOCDin CD00 D) d r-CMCDT- IDT~ 00q0>O 0>Od d d d dH-l ■H 4^ 4H 4^o 00 CO o>CO COd d T— «r- CM*— r - r - T"■oc0COr-O)co0s*oc00co0cx:oCDCDCJ>0ca>CO00o>oIDCMT—001CD COq00 d 1 ^ T-CM00oIDr- 00o00o r**od d d d d41 4i 41 4H 4^o CD o CD o>CO CM q COd d •— CM<r“ T- <r-3ocCMCDo>2  C0—  ® D“O(1) 01 1a. oM o s s  s p e c tro m e tr ic  study of molecular ions etc 1 7 5peaks) at 70 eV and 14 eV. The relative intensities of the ions are much higher at 14 eV than at 70 eV (Table 1) indicating a higher stability for molecular ions at the lower energy. The relatively higher stability of the molecular ion of 2-methylphenol (more intense peak) (Table 1) in comparison with the other two isomers, may be due to interaction between methyl group and oxygen atom (of OH group) as a result of proximity of the two groups which assists to increase the stability of the molecular ion of the 2-methylphenol more than in the case of the other two isomers. The bonding interaction may take the form ;HI0 -.C H ,The [C ,H ,0 ]+  ions represent the second most intense peaks in the mass spectra of the three isomers at the higher and the lower energies (Table 1). Labelling studies (Russell et al 1983) revealed that the loss of H occurs predominantly (70 v..) at the methyl group. The order of decreasing intensity (stability) is 4-methyl phenol > 3-methylphenol > 2 -methylphenol. The relatively higher stability in case of 1 ,4  position was discussed by Dewar and Landman (1977) as a result of strong Interaction between the ring and hydroxyl group which is consistent with diquinoid structure. Also, they suggested that hydroxybenzyl ion may actually be more stable than hydroxytropylium structure. One may suggest the formation of diquinoid structure (Russell et al 1983) as :CHiThe [C 7H 7]* fragment ions have a relatively low intensity in the mass spectra of methylphenoi isomers at 70 eV as well as 14 eV (Table 1). The only possible way for forming ions from methylphenols is the fracture of hydroxy-phenylbond. As a result of the great stability of this bond [Aczel and Lumpkin (1960) discussed the great stability of phenol molecular ion as attributed to the resonance structure involving the unshared electrons on the oxygen atom], one expects that the probability for forming ion will be minimized. The relatively lowerintensity of [C 7H 7]+ ion obtained from 2 -methylphenol may be a result of interac­tion between methyl and hydroxyl groups as a result of proximity of the two groups which assists to increase further the stability of hydroxy-phenyl bond.The [C #H7]+ fragment ions are the third most intense peak in the mass spectra of methylphenoi isomers. Tw o  metastable peaks observed at 57.8 and 758.3 may indicate the formation of [CnH,]* ions from [M]+ as well as [C ,H tO]* ([M -1]^) ions. The formation of ions from [C^H^O]^ ions (hydroxy,benzyl structure) demands that the latter ions must bo in a keto-form, while the formation of [C bH t]* ions directly from molecular ions of methylphenol isomers occurs by CHO loss from enol-form of molecular ion. The mechanisms of formation may be illustrated :176 Ezzat T M Selim M A Fohmey and Hoda S Ghonime II • • I '  -c -  / '..-I /It is clear from the relative intensities at 70 eV and 14 eV (Table 1) that [CeH^]^ ions formed from the three isomers at 70 eV have almost equal probability for formation, while at 14 eV a significant differences between the relative intensities of the ions produced from the three isomers are observed.The ions are the fourth most intense fragment ion in the massspectra of methylphenol isomers. The relative intensities of the ions obtained from the isomers at 70 eV as well as 14 eV are almost equal.Finally, the fragmentation pathways may be confirmed by the detection of metastable peaks in the mass spectra of the three isomers at m * equal to : 106, 58.3, 57.8, 75.05, 56.5, 55.4, 46-4, 34.7 and 33.8 (Figure 1) indicating[ c . b Jm =3^ 7L -* m*=33B *• -*r t T ^ 56.5Figure I. Fragmentation pathways for the formation of the principal ions obtained from methylphenol isomers.similar fragmentation pathways, independent of the relative position of the OHand CH, substituents.M o s s  spectrometric study of molecular Ions etc 1 7 7II, Molecular ions :The three molecular ions under investigation in the present [work are the isomeric molecular ions of methylphenol having the formula108) molecular ions obtained from 2-, 3- and 4-methylphenol :The DFD IE curves for the molecular ions of 2-, 3 - and 4-methylphenol show a slow rise (for about 1 eV above threshold) which may indicate that the adiabatic ionization energy does not coincide with vertical ionization energy. The ionization energies reported at threshold are 8.46 ±  0.06 eV (2-methylphenol), 8.36 ±  0.11 eV (3-methylphenol) and 8.31 ± 0 .0 8  eV (4-methylphenol). The difference between the ionization energies of 2 -methylphenol and 4 -methylphenol is equal to 0.15 eV. This is in good agreement with the observation of Gibson (1973) that for disubstituted benzenes, the difference in ionization energies of the three isomers is less than 0.20 eV. The presently measured values of ionization energies of methylphenols at threshold are in good agreement with the first ionization energies of the same molecular ions measured by Palmer et al (1979) using photo­electron spectroscopy. The previously reported electron impact values by RussellFigure 2. The deconvoluted first differential IE curves for [C 7H.O]’ (m/z=108) molecular ions obtained from (a) 2-methylphenol. (b) 3-methylphenol and (c) 4 -methylphenol.et a /(1983) for the ionization energies at threshold of methylphenol isomers are lower than the presently measured values by 0.32, 0.13 and 0.18 eV for 2-, 3 - and 4-methylphenol, respectively. The early reported electron impact values for the ionization energies by Crable and Kearns (1962) at 8.93 eV (2 -methylphenol), 8.98 eV (3-methylphenol) and 8.97 eV (4 -methylphenol) are very probably not corresponding to the first ionization energies of the three isomers. On the other hand, the ionization energy values at threshold measured by Pignataro et al (1966)178 Ezzat T M Selim M A  Fohmey and Hoda S G honim e(8.52 eV) for the meta isomer and by Johnstone and Mellon (1973) (8.34 eV) for the para isomer using electron impact are in good agreement with the presently measured values. Recently, Mehler and Gerhards (1985) calculated the ionization energies and higher energy levels of the isomers of methylphenol by ab initio molecular«orbital calculations ; the calculated values are higher to high extent from thexjorresponding values measured in the present work.Comparing the ionization energy at threshold for benzene (9.25 eV) (Palmer et al 1979) with that of phenol (8.56 eV) (Palmer et al 1979) and toluene (8.80 eV) (Selim et o/1982) indicates that the introduction of OH substituent in benzene lower the ionization energy of benzene by 0.69 eV while the introduction of CH3 substituent lower the ionization energy by 0.45 eV. The additivity of substituent effects • requires that the first ionization energy of benzene to be lowered by 0.69+0.45 -1.14 eV in case of the introduction of the OH and CH3 substituents. However, the present results indicate that the ionization energies for methylphenol isomers at threshold are lowered-relative to benzene by 0.79, 0.89 and 0.94 eV for 2-, 3- and 4-methylphenol, respectively.One expects that the degeneracy of ground state for benzene positive ion is removed (Dewar and Worley 1969) by the disubstitution of electron-donating groups ’OH and 'CHa. Consequently, one may assume that the first ionization energies at 8.46, 8.36 and 8.31 eV for 2-, 3- and 4-methylphenol isomers, respectively are due to ionization of le^j electrons from ttj (B  ^ component) orbitals of the corresponding methylphenol isomers. Hence the molecular energy levels measured at 9.40 eV (2-methylphenol), 9.30 eV (3-methylphenol) and 9.29 eV (4-methylphenol) are tentatively considered as due to ionization of electrons from TTg (Aj component) orbitals of methylphenol isomers.The two lowest ionization energies of methylphenol isomers differ in energy by 0.94, 0.94 and 0.98 eV for 2-, 3- and 4-methylphenol, respectively. These almost equal differences which correspond to the splitting of tTs/tt,  orbital energy levels for methylphenols may be compared with differences 0.70 eV for phenol and 0.83 eV for anisole (Baker et al 1968). It is worthwhile to mention that the previous photoelectron spectroscopy results (Turner et al 1970) indicate that in disubstituted benzenes, where both substituents contribute to the moving apart of the and levels, the actual splittings are greater in the case of the para disubstituted compounds than in the corresponding ortho and meta derivatives.Palmer et al (1979) had concluded, using ab initio MO calculations and photoelectron spectroscopy that the third ionization energy in 2- and 3-methylphenol is to be o-orbital rather than tr-type. On the other hand, they (Palmer et al 1979) considered the third ionization energy of 4-methylphenol to correspond to nr-orbital. Following the conclusion of Palmer et al (1979) the present authors assume that the higher energy levels at 11.20eV (2-methylphenol) and 11,14 eV (3-methyl­phenol) are due to removal of electron from the corresponding c-orbitals. The higher energy level for 4-methylphenol at 11.06 eV is tentatively considered to be JM a s s  s p e c tro m e tr ic  study of molecular ions e tc 1 7 9due to one of the two lower rr-electrons (O j„). Palmer et al (1979) also measured a higher energy level at 11.31 eV in the photoelectron spectrum of 3 -methylphenol and a similar one at 11.24 eV for 4-methylphenol. No equivalent levels have been detected in the presently reported ionization efficiency curves for 3 - and 4-methylphenol.Three higher molecular energy levels had been measured by us at 11.76 eV (2-methylphenol), 11.73 eV (3-methylphenol) and 11.56 eV (4-methylphenol). These levels have similarities to those reported by Palmer et al (1979) at 11.76 eV, 11.74 eV and 11.67 eV for 2 -, 3 - and 4-methylphenol, respectively and explained by them as due to removal of an electron from the corresponding n, o and a orbitals, respectively.The molecular energy levels measured by the present authors at 12.39 eV (2-methylphenol), 12.19 eV (3-methylphenol) and 12.61 eV (4-methylphenol) have similar levels reported by Palmer et al (1979) using photoelectron spectroscopy technique at 12.33 eV, 12.40 eV and 12.84 eV for 2 -, 3- and 4-methylphenol, respectively. Palmer et al (1979) had assigned the latter energy levels to removal of an electron from the corresponding a orbitals for methylphenol isomers.Finally, molecular energy levels have been measured at 10.36 eV (2 -methyl­phenol), 10.38 eV (3-methylphenol) and 10.23 eV (4-methylphenol). No equivalent levels wore  reported in the literature using either photoelectron spectroscopy or electron impact techniques.ReferencesAczel T and Lumpkin H E 1960 Anal. Chem. 33 3 8 6Baker A  D, May D P and Turner 0 W 1968 ) .  C h e m .  S o c .  ( B )  22Crable G E and Kearns G L 1962 J . P h y s .  C h e m .  66 436Dewar M J S and Worley S D 1969;. C h e m .  P h y s .  SO 654Dewar M J S  and Landman D 1977/. Am. C h e m .  S o c .  9 9  4633, 7439Gibson H W 1973 C o n . J .  C h e m .  51 3065Johnstone R A W  and Mellon F A  1973 J .  C h e m .  S o c .  F a r a d a y  T r a n s .  I I 69 36Mclver (Jr) R T 1975 O r g .  M a s s  S p e c t r o m .  10 396Mehler E L and Gerhards J 1985 J. Am. Chem. Soc. 107 5856McLafferty F W 1973 In t e r p r e t a t io n  o f  m a s s  spectra (Benjamin ; London)Palmer M H, Moyes W and Speirs M 1979 J .  M o l .  S t r u c t .  52 293Pignataro S, Foffani A, Innorta G and Dislefano G 1966 Z. P h y s .  C h e m .  (Frankfurt) Neue Folge, Bd 49 291Russell D H, Freiser B S, McBay E H and Canada D C 1983 O r g .  M a s s  Spectrom. 18 474 Savitzky A  and Golay M J E 1964 A n a l .  C h e m .  36 1 627 Selim E T M 1976 I n d i a n  ] .  P u r e  A p p l.  P h y s . 14 547 1985 Egypt./. P h y s . 16 249Selim E T M and Helal A  I 1982 O r g .  Mass S p e c t r o m .  17 539Salim E T M. Helal A  I and Zahran N F 1984 P ro c .  I n d i a n  A c a d .  S c i .  ( C h e m .  S c i . )  93 87 Selim E T M, Rabbih M A. Fahmey M A  and Rezk A  M H 1987 Org. Moss S p e c t r o m .  2 2  381 turner D W, Baker C, Baker A  D and Brundle C R 1970 Mo/ecu/or photoefectron spectroscopy (London : Wiley Interscience)Vogt J and Pascuat C 1972 I n t . J .  M a s s  Spectrom. Io n  P h y s .  9  441

Mass Spectrometric study of Molecular Ions of Methylphenol Isomers using Electron Impact Technique

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Mass Spectrometric study of Molecular Ions of Methylphenol Isomers using Electron Impact Technique

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