A 'meta effect' in the fragmentation reactions of ionised alkyl phenols and alkyl anisoles

Article abstract of DOI:10.1002/jms.2977

The competition between benzylic cleavage (simple bond fission [SBF]) and retro-ene rearrangement (RER) from ionised ortho, meta and para RC ₆H₄OH and RC₆H₄OCH₃ (R = n-C₃H7, n-C₄H₉, n-C₅H₁₁, n-C₇H₁₅, n-C₉H₁₉, n-C ₁₅H₃₁) is examined. It is observed that the SBF/RER ratio is significantly influenced by the position of the substituent on the aromatic ring. As a rule, phenols and anisoles substituted by an alkyl group in meta position lead to more abundant methylene-2,4-cyclohexadiene cations (RER fragmentation) than their ortho and para homologues. This 'meta effect' is explained on the basis of energetic and kinetic of the two reaction channels. Quantum chemistry computations have been used to provide estimate of the thermochemistry associated with these two fragmentation routes. G3B3 calculation shows that a hydroxy or a methoxy group in the meta position destabilises the SBF and stabilises the RER product ions. Modelling of the SBF/RER intensities ratio has been performed assuming two single reaction rates for both fragmentation processes and computing them within the statistical RRKM formalism in the case of ortho, meta and para butyl phenols. It is clearly demonstrated that, combining thermochemistry and kinetics, the inequality (SBF/RER) _meta<(SBF/RER)_ortho<(SBF/RER)_para holds for the butyl phenols series. It is expected that the 'meta effect' described in this study enables unequivocal identification of meta isomers from ortho and para isomers not only of alkyl phenols and alkyl anisoles but also in other alkyl benzene series. Copyright

Full text of DOI:10.1002/jms.2977

Research Article
Received: 23 December 2011
Revised: 10 February 2012
Accepted: 11 February 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/jms.2977
A ‘meta effect’ in the fragmentation reactions
of ionised alkyl phenols and alkyl anisoles
Guy Bouchoux,a* Michel Sablier,a Tetsuo Miyakoshib and Takashi Hondab
The competition between benzylic cleavage (simple bond fission [SBF]) and retro-ene rearrangement (RER) from ionised ortho,
meta and para RC6H4OH and RC6H4OCH3 (R = n-C3H7, n-C4H9, n-C5H11, n-C7H15, n-C9H19, n-C15H31) is examined. It is observed
that the SBF/RER ratio is significantly influenced by the position of the substituent on the aromatic ring. As a rule, phenols
and anisoles substituted by an alkyl group in meta position lead to more abundant methylene-2,4-cyclohexadiene cations
(RER fragmentation) than their ortho and para homologues. This ‘meta effect’ is explained on the basis of energetic and kinetic
of the two reaction channels. Quantum chemistry computations have been used to provide estimate of the thermochemistry
associated with these two fragmentation routes. G3B3 calculation shows that a hydroxy or a methoxy group in the meta
position destabilises the SBF and stabilises the RER product ions. Modelling of the SBF/RER intensities ratio has been performed
assuming two single reaction rates for both fragmentation processes and computing them within the statistical RRKM
formalism in the case of ortho, meta and para butyl phenols. It is clearly demonstrated that, combining thermochemistry
and kinetics, the inequality (SBF/RER)meta < (SBF/RER)ortho < (SBF/RER)para holds for the butyl phenols series. It is expected that
the ‘meta effect’ described in this study enables unequivocal identification of meta isomers from ortho and para isomers not
only of alkyl phenols and alkyl anisoles but also in other alkyl benzene series. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: electron ionisation; substituted alkyl benzenes; fragmentation competition; retro-ene rearrangement; benzylic cleavage;
quantum chemistry computation; RRKM modelling
If the cases of alkyl benzenes are well described, less information is
available on substituted alkyl benzenes where competition between
INTRODUCTION
Competitions between simple bond fission (SBF) and rearrangement
are ubiquitous events in gas phase ion chemistry.[1] Prototypes of
such competition are the allylic cleavage/retro-ene fragmentations
of ionised alkenes and the homologue benzylic cleavage/retro-ene
fragmentations of ionised alkyl benzenes (Scheme 1).
SBF and RER may be influenced by the nature of the substituent and
by its position relative to the alkyl chain on the aromatic ring. In the
present study, we explore the fragmentation patterns of molecular
cations produced by electron ionisation of ortho, para and meta
substituted alkyl phenols and alkyl anisoles. It will be shown that it
is possible to distinguish the substitution sites in these series by
using the SBF/RER ratio, in particular a ‘meta effect’ is evidenced.
An explanation of the experimental observation is provided by
examining the fragmentation energetic of the two competitive
reactions channels using quantum chemical computation and by
modelling the reaction kinetic using statistical rate calculations to
take into account internal energy and entropy effects.
In this latter series, the two competitive channels are (i) the C–C
bond elongation leading to the benzyl cation (m/z 91 in Scheme 1)
plus a n-propyl radical (SBF fragmentation, Scheme 1) and (ii) the
retro-ene rearrangement (RER) producing ionised methylene-2,4-
cyclohexadiene (m/z 92 in Scheme 1) and propene (RER fragmenta-
tion, Scheme 1). Probably the most extensively studied example of
competition between SBF and RER fragmentations during the last
decades is ionised butyl benzene.[2–22] Experiments on this system
involved ion-beam[2,3,6,8,9] and trapped-ion[4] photodissociation,
charge exchange ionisation,[10,11] collisional activation,[11–14] chem-
ical and thermal activation[15] and photoelectron–photoion coinci-
dence technique.[7] These experiments allowed to explore a range
of internal energy, E, of the butyl benzene molecular ion extending
from ~2 to ~6 eV. A strong dependence of the ratio of ion abun-
dances [m/z 91]/[m/z 92] with internal energy E has been observed
and quantitatively ascertained thus leading to call the butyl benzene
radical cation a ‘thermometer ion’. For this type of competition, the
SBF fragmentation is usually characterised by a high critical energy
and a loose transition structure while the RER fragmentation passes
through a more congested transition structure but presents a lower
critical energy. As a consequence, the two rate constants associated
with these fragmentations exhibit a crossing point. Under these
circumstances, it is understandable why the ratio of peak intensities
[m/z 91]/[m/z 92] passes from zero at low internal energy E to a value
significantly higher than unity at high E values for butyl benzene.
EXPERIMENTAL AND COMPUTATIONAL
SECTION
Computational details
Quantum chemistry calculations have been conducted using the
GAUSSIAN03 suite of programs[23] within the density functional
*
Correspondence to: Guy Bouchoux, DCMR, Ecole Polytechnique, 91128,
Palaiseau Cedex, France. E-mail: bouchoux@dcmr.polytechnique.fr
a Laboratoire des Mécanismes Réactionnels, Ecole Polytechnique, CNRS, 91128
Palaiseau, France
b School of Science and Technology, Meiji University, Higashi-Mita 1-1-1, Tama-ku
Kawasaki, 214-8571, Japan
J. Mass. Spectrom. 2012, 47, 539–546
Copyright © 2012 John Wiley & Sons, Ltd.

G. Bouchoux et al.
Demethylation of 2-alkyl anisoles with BBr3 in CHCl3 gives the
corresponding 2-alkyl phenols in ~95% yields[28]
:
Simple Bond Fission
+
SBF
m/z 91
H
Retro Ene Rearrangement
+
RER
H
Starting with 3-anisyl bromide, similar reactions provide 3-alkyl
anisoles and 3-alkyl phenols.[29]
m/z 92
Scheme 1. Competition between simple bond fission (SBF) and retro-ene
rearrangement (RER).
RESULTS AND DISCUSSION
theory. Geometries were optimised at the B3LYP/6-31 + G(d,p)
level of theory, and unscaled vibrational frequencies calculated
at this level were used to calculate vibrational contribution to
enthalpy at 0 K and 298 K. Although geometrical parameters
are well reproduced at this level of theory, limitations of B3LYP/
6-31 + G(d,p) computations to evaluate precise reaction energies
or heats of formation were documented in recent reviews.[24]
Means to correct for these deficiencies consist to perform single
point energy calculations with enlarged basis set or(and) with
different correlated methods. A popular procedure, which was
applied to the various systems examined here, uses a simple
extension of the basis set size such as B3LYP/6-311++G
Experimental evidence for a meta effect
The 70-eV electron ionisation mass spectra of ortho and meta
substituted alkyl phenols and alkyl anisoles synthetised as
described earlier were recorded, and the essential data are
presented in Table 1 together with some previous literature
results.[30] The SBF and the RER reactions (Scheme 1) cause m/z
107 and 108 fragment ions in the case of alkyl phenols, and these
signals are obviously displaced to m/z 121 and 122 for alkyl anisoles.
An essential observation that emerges from examination of
Table 1 is that the SBF/RER ratio is always lower for the meta
isomers than for the ortho or para counterparts for a given side
chain length. Thus, the meta substituted alkyl phenols and alkyl
anisoles exhibit a distinct behaviour allowing us to speak of a ‘meta
effect’. The data presented in Table 1 also show that the ratio SBF/
RER decreases markedly when the number of carbon atoms of the
alkyl chain increases from C3 to C5 until attaining a constant limiting
value of ~ 0.5 (0.3) for meta and ~7 (22) for ortho hydroxyl
(methoxy) isomers as illustrated on Fig. 1. Note that the few
examples of para isomers closely follow the ortho behaviour.
To understand the origin of this meta effect on the fragmenta-
tions of ionised alkyl phenols and alkyl anisoles, we first examined
the thermochemistry of these reactions using both known
thermochemical data and quantum chemistry calculations.
Because it will be seen that thermochemistry is insufficient to
explain the experimental observations, a reaction rates modelling
on the basis of the butyl benzene case has been further
undertaken to include internal energy effect.
(3df,2p)//B3LYP/6-31 + G(d,p) single point calculation.
A
second possibility involves more expensive composite
methods such as the Gn procedures. We used here the G3B3
composite method[25] to estimate several crucial heats of
formation values.
GC/MS analytical conditions
The gas chromatography/mass spectrometry measurements were
carried out using an Agilent 6890 N/5975 GC/MS system (Agilent
tec., Ltd). The samples of 2-alkyl phenol (1) and 3-alkyl phenol (2)
were dissolved in acetone (4.0mg in 1 ml). Approximately 1 ml of
each solution was injected in the stainless steel capillary column
(0.25 mm i.d. Â 30 m) coated with Ultra Alloy PY-1(HT/MS) (100%
methyl silicone) of the gas chromatograph. The gas chromatograph
oven was programmed to provide a constant temperature increase
of 12ꢀC per min from 40ꢀC to 320ꢀC then held for 10 min at 320 ꢀC.
The injection and the interface temperatures were equal to 280ꢀC.
The flow rate of helium gas was 1 ml/min. All products were
identified by mass spectrometry in the electron ionisation mode
(EI-mode) using ionisation energy of 70 eV.
Theoretical results: thermochemistry
The thermochemistry of the SBF and RER reaction products of alkyl
benzene, alkyl phenols and alkyl anisoles bearing an alkyl chain
ranging from C3 to C9 is summarised in Tables 2 and 3. The
difference in enthalpies between the two sets of fragmentation
Synthesis of alkyl phenols and alkyl anisoles
products, ΔHꢀSBF-RER = Σ ΔfHꢀ(SBF products)
À
Σ
ΔfHꢀ(RER
One of us previously developed efficient method for synthesising
urushiol analogues.[26] In the present study, this experimental
procedure has been followed to synthesise ortho and meta
substituted alkyl phenols and alkyl anisoles using Grignard
reagents. Briefly, the cross-coupling reaction of 2-anisylmagnesium
bromide with alkyl bromide in presence of copper (I) chloride in
products), is presented in Table 3. The corresponding values were
determined using the individual 298 K heats of formations
computed using the atomisation energies obtained at the G3B3
level of theory (Table 2).
The general feature, underlined in the Introduction section, of a
larger endothermicity of the SBF with respect to the rearrangement
is clearly apparent in Table 3. For all the alkyl benzenes and alkyl
phenols and all but one alkyl anisoles examined here, the term
THF gave 2-alkyl anisoles in 75% to 86% yields[27]
:
ΔHꢀ
is positive. Moreover, ΔHꢀ
seems to be only
SBF-RER
SBF-RER
slightly influenced by the length of the alkyl chain. According to
the data reported in Table 3, a noticeable increase in ΔHꢀSBF-RER is
observed when passing from the propyl to the butyl benzene
series. However, increasing the chain length above C4 does not
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J. Mass. Spectrom. 2012, 47, 539–546

A meta effect
Table 1. Competition between SBF and RER in a 70-eV electron ionisation mass spectra of some alkyl benzene derivatives XC6H4R
SBF%a
RER%a
RER%corr
SBF/RERcorr
b
b
X
R
H
n-C3H7
n-C4H9
n-C5H11
n-C7H15
100
100
100
100
100
100
100
100
53
10
54.6
72
2.3
46.9
64.3
81.8
2.3
43 Æ 24
2.1 Æ 0.3
1.6 Æ 0.2
1.2 Æ 0.2
43 Æ 24
89.5
10
ortho
meta
para
n-C3H7
n-C4H9
n-C5H11
49
41.3
1.3
2.4 Æ 0.3
77 Æ 70
9
ortho
meta
para
12.8
100
10
5.1
19.6 Æ 5.8
95.9
2.3
0.60 Æ 0.06
43 Æ 24
100
100
45
ortho
meta
para
22
14.3
96.5
7.3
7.0 Æ 1.2
OH
100
15
0.50 Æ 0.05
13.7 Æ 3.2
6.6 Æ 1.1
100
100
36.4
100
52.7
100
39.7
100
100
100
32.9
100
36
ortho
meta
ortho
meta
ortho
meta
ortho
meta
ortho
meta
ortho
meta
para
22.9
100
33.2
100
18.5
100
9.2
15.2
97.2
25.5
95.9
10.8
96.9
1.5
n-C7H15
n-C9H19
n-C15H31
n-C3H7
0.37 Æ 0.04
3.9 Æ 0.6
0.55 Æ 0.06
9.3 Æ 1.8
0.41 Æ 0.05
67 Æ 53
83.6
13.3
100
12
75.9
5.6
1.3 Æ 0.2
17.9 Æ 5.0
0.30 Æ 0.04
23.3 Æ 7.8
0.40 Æ 0.04
43 Æ 24
n-C4H9
97.5
4.3
n-C5H11
100
10
97.2
2.3
100
100
29.5
100
31.2
100
33.4
100
100
100
37.2
100
OCH3
ortho
meta
ortho
meta
ortho
meta
ortho
12
4.3
23.3 Æ 7.8
0.30 Æ 0.03
19.6 Æ 5.8
0.30 Æ 0.04
21.3 Æ 6.7
0.30 Æ 0.04
6.5 Æ 1.1
n-C7H15
n-C9H19
n-C15H31
100
12.8
100
12.4
100
23
97.7
5.1
97.6
4.7
97.4
15.3
53.3
7.2
CH2CH2OH meta
ortho
61
1.9 Æ 0.2
14.9
100
9.1
13.9 Æ 3.3
0.40 Æ 0.04
71 Æ 61
NH2
n-C6H13
meta
para
97.1
1.4
aIn italic, from NIST mass spectra database.[30]
bCorrected for the 13C contribution of the SBF signal to the RER ion abundances. Error calculated assuming 10% relative error on the measured peak
intensities.
change anymore this enthalpy difference. This observation parallels
that of a limiting SBF/RER ratio value when the alkyl chain of
phenols or anisoles is larger than C4 as observed in Fig. 1.
and 98 kJ/mol for para, ortho and meta isomers, respectively (values
that compare closely with the 298 K G3B3 computed ΔHꢀSBF-RER of
20, 50 and 99kJ/mol; Table 3).
The essential observation from Table 3 is certainly the clear
The origin of the clear influence of the substitution site,
evidenced in Fig. 2 for butyl phenols and in Table 3 for a larger
panel of molecules, is obviously that the stabilities of the SBF and
RER fragment ions are strongly influenced by the position of the
hydroxyl group. More specifically, the benzyl cation is stabilised
by OH (or OCH3) group in position para or ortho while, by contrast,
ionised methylene-2,4-cyclohexadiene is more stable when the
hydroxyl (or methoxy) group is in position meta. Insight on these
questions is provided by quantum chemistry calculations of charge
and spin densities.
increase in the difference in enthalpies ΔHꢀ
when passing
SBF-RER
from para to ortho and to meta isomers for phenols or anisoles of
a given alkyl chain length. It is noteworthy that ΔHꢀSBF-RER is as high
as 85–100 kJ/mol for the meta isomers, whereas it never exceeds 20
or 50 kJ/mol for its para and ortho counterparts. We chose, as a
representative example, the case of ortho, meta and para ionised
butyl phenols to illustrate this behaviour. This system was
investigated at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31 + G(d,p)
level of theory. The resulting 0 K energy profiles associated with
SBF and RER fragmentations of these molecular cations are
presented in Fig. 2. At the B3LYP/6-311++G(3df,2p)//B3LYP/
Natural bond orbital and Atomic Polar Tensor population
analysis show that, as expected, benzyl cation may be described
by the four canonical forms A, B, C and C′ (Scheme 2).[31]
6-31 + G(d,p) level, the 0 K ΔHꢀ
values are equal to 19, 48
SBF-RER
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G. Bouchoux et al.
Consequently, substitution by a p-donor group in position para or
ortho would provide a significant stabilisation to the corresponding
species. This is indeed what is observed when the computed ΔfHꢀ
of the three hydroxy benzyl cations are considered (Table 2). At
298 K, the para and ortho hydroxy benzyl cations are more stable
than their meta homologue by 53 and 34 kJ/mol, respectively
(G3B3 calculations[32]; Table 2). Similar Hꢀ298 differences of 52 and
39 kJ/mol are observed for the methoxy derivatives (Table 2).
Concerning ionised methylene-2,4-cyclohexadiene, Mulliken
population analysis shows a major spin density on the carbon atom
of the exocyclic methylene (I, Scheme 3) and a minor contribution
on carbon C(5) (II, Scheme 3). Therefore, the set of canonical forms I
would mainly describe the electronic configuration of this radical
cation (Scheme 3). Considering the two electronic structures IA
and IB, it is evident that the presence of an OH group in positions
C(3) (meta) or C(5) (meta′) would stabilise the radical cation.
Accordingly, the two hydroxy 2,4-cyclohexadiene substituted by
the OH group in positions C(3) and C(5) are indeed more stable
than their homologues ortho and para (by 15 and 26 kJ/mol,
respectively, G3B3 calculations[32]; Table 2). Comparable gain in
stability is obtained for the methoxy homologues (17 and 34 kJ/mol,
respectively, G3B3 calculations, Table 2).
para
100
Alkyl phenols
7
6
5
4
3
2
10
7
6
5
ortho
4
3
2
1
7
6
5
meta
4
4
6
8
10
12
14
number of carbon atoms in the side chain
100
para
Alkyl anisoles
7
6
5
4
3
ortho
2
10
7
6
5
Theoretical results: kinetic modelling of reactions rates.
4
According to the data presented in Fig. 2 and in Table 3, it would be
tempting to predict that the RER fragmentation should be always
dominant because, whatever the substitution site, it is the process
of lower energy. This is in conflict with the fact that the SBF
fragmentation is dominant in the 70-eV mass spectra of para and
ortho alkyl phenols and alkyl anisoles. Thermochemistry alone is
consequently unable to explain the experimental observations.
Consideration of the nature of the transition structure and of the
internal energy effect on SBF and RER reaction rates is
consequently essential to understand the clear-cut character of
the observed meta effect. For this purpose, we performed statistical
modelling of the reaction kinetics on the ortho, meta and para butyl
3
2
1
7
6
5
meta
4
3
4
6
8
10
12
14
number of carbon atoms in the side chain
Figure 1. SBF/RER ratio as a function of the alkyl chain length for substi-
tuted phenols (up) and substituted anisoles (down).
a
ꢀ
Table 2. Heats of formation (ΔfH298 ) of the various species involved in the SBF and RER fragmentations of alkyl benzene, alkyl phenols and alkyl
anisoles (kJ/mol)
Species
H
OH
OCH3
Benzyle+
912.7
ortho 699b
meta 733b
para 680b
ortho 732b
meta 717b
para 743b
ortho 700
meta 739
para 681
ortho 741
meta 724
para 758
•+
Methylene-2,4-cyclohexadiene
947.4
•
C2H5
120.6
51.5
C2H4
•
n- C3H7
103.5
20.0
CH3CH = CH2
•
n- C4H9
81.9
CH3CH2CH = CH2
0.4
•
n- C6H13
40.1
CH3(CH2)3CH = CH2
À42.0
À1.9
À84.1
•
n- C8H17
CH3(CH2)5CH = CH2
aG3B3 calculations using atomisation energies.
bFrom Bouchoux.[32]
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A meta effect
SBF
Table 3. Enthalpy differences (kJ/mol) between SBF and RER
products originating from competitive fragmentations of ionised
benzene derivatives XC6H4R (G3B3 computations)
HO
+
a
SBF-RER
X
R
ΔHꢀ
213
RER
H
n-C3H7
n-C4H9
n-C5H11
n-C7H15
n-C9H19
34
49
47
48
47
36
85
7
179
160
HO
+
°
H
0 (kJ/mol)
141
131
115
ortho
n-C3H7 meta
para
meta
ortho
para
ortho
50
99
20
48
97
19
50
99
19
49
98
19
28
84
16
11
0
n-C4H9 meta
para
ortho
n-C5H11 meta
para
HO
OH
ortho
Figure 2. Zero Kelvin energy profiles for SBF and RER fragmentations
of ortho, meta and para butyl phenols (B3LYP/6-311++G(3df,2p)//B3LYP/
6-31 + G(d,p) calculations).
n-C7H15 meta
para
ortho
n-C9H19 meta
para
ortho
n-C3H7 meta
para
À8
43
99
7
ortho
n-C4H9 meta
para
A
B
C
C'
OCH3
ortho
41
97
5
Scheme 2. Canonical forms describing benzyl cation.
n-C5H11 meta
para
ortho
41
97
5
n-C7H15 meta
para
I
ortho
41
97
5
n-C9H19 meta
para
A
B
C
aΔHꢀSBF-RER = Σ ΔfHꢀ(SBF products) À Σ ΔfHꢀ(RER products), individual
G3B3 ΔfH298 taken from Table 2.
phenol series using the 0 K enthalpy profiles presented in Fig. 2.
Before presenting the results of this investigation, it is important to
recall the major characteristics of the competitive fragmentations
of ionised butyl benzene itself.
II
As indicated in the Introduction section, the ratio of ion
abundances [91]/[92] (i.e. SBF/RER) originating from butyl benzene
radical cation has been determined precisely as function of internal
energy by a variety of techniques. A graphical representation of
these essential data is reported in Fig. 3a. It is noteworthy that in
the electron ionisation (70 eV) mass spectrum of butyl benzene,
the ratio [91]/[92] is equal to 2.1, thus pointing to an average
internal energy centred around 4.6eV for this system (value
indicated by a vertical bar in Fig. 3a). Photoelectron/photoion
coincidence experiments on two different apparatus[7] provided
rate constant k(92) values between 105 and 106 sÀ1, whereas
photodissociation mass analysed ion kinetic energy spectroscopy
was used to explored the 108 sÀ1 region.[8] These two sets of
experimental rate constant, k(92), values cover a range of internal
A
B
C
Scheme 3. Canonical forms describing methylene-2,4-cyclohexadiene
radical cation.
energy E situated between 2 and 5 eV. Obviously, using these data
and assuming that the [91]/[92] ion abundances ratio is equal to the
reaction rates ratio, one may also deduce experimental k(91) values
as a function of E (markers in Fig. 3b). These experimental results
have been used in several kinetic modelling studies using single
rate coefficient for each, m/z 91 and 92, fragmentation routes, the
critical energy Eꢀ of each process being treated as an adjustable
parameter but generally approaching the corresponding
endothermicities of the fragmentations.[7,8,14,15,22] Figure 3b
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G. Bouchoux et al.
1012
1011
1010
109
108
107
106
105
104
103
102
101
100
10-1
10-2
10-3
10
1
(a)
(b)
Butylbenzene
91/92 ratio
Butylbenzene
m/z 91 and m/z 92 rate constants
m/z 92 (RER)
m/z 91 (SBF)
0.1
Experiments from:
Baer et al. [7]
Oh et al. [8]
Dunbar et al. [5]
Experiments from:
Baer et al. [7]
Oh et al. [8]
0.01
0.001
Kinetic modeling:
Kinetic modeling:
k(RER) E°=1.10 eV
k(91)/k(92)
k(SBF) E°=1.57 eV
1
2
3
4
5
6
1
2
3
4
5
6
Internal energy E (eV)
Internal energy E (eV)
Figure 3. Experimental [m/z 91]/[m/z 92] ratio (a) and overall rate constants k(91) and k(92) (b) versus internal energy of butyl benzene molecular ion.[5,7,8]
The fitting curves are obtained after kinetic modelling of k(91) and k(92).[7,8,14,21,22] Vertical bar in panel a corresponds to the average internal energy of
molecular ions formed by a 70-eV electron ionisation.
presents one example of this kinetic modelling obtained using the
statistical RRKM formalism and Eꢀ = 1.10 and 1.57 eV to model k(92)
and k(91) reaction rates, respectively. Figure 3b illustrates clearly
the fact that the SBF fragmentation presents a high critical energy
and a loose transition structure whereas the RER fragmentation
possesses a low critical energy but passes through a tight transition
structure. The combination of these characteristics results in
the crossing of the two rate constants associated with these
fragmentations and explain why the m/z 92 signal decreases
dramatically with respect to m/z 91 when increasing internal energy.
It should be underlined that more sophisticated statistical rate
treatment[15,21] or consideration of more complete mechanistic
scheme[22] does not fundamentally alter the overall results
illustrated by Fig. 3, particularly for internal energy higher than
~3 eV. Thus, we decided to extend the statistical kinetic treatment
used for butyl benzene to ortho, meta and para butyl phenols. A
modelling of the SBF/RER intensities ratio has been performed
assuming two single reaction rates for both processes and
computing them within the statistical RRKM formalism exactly as
already performed for butyl benzene itself.[7,8,14] The essential
parameters necessary for these calculations are the vibrational
frequencies of the reactants and transitions states as well as the
critical energies. To compute the rate constants k(107) and k(106)
corresponding to SBF and RER fragmentations of butyl phenols, we
used frequencies calculated at the B3LYP/6-31 + G(d,p) level for the
molecular ions. Modification of some of these vibrational frequencies
based on previous treatments of ionised butyl benzene[7,8,14,22]
allowed us to describe the transition structures. For the SBF reaction,
a C–C stretching mode (900 cmÀ1) has been taken as reaction
coordinate, and four deformation modes were reduced by a factor
two and one torsional mode becomes a free rotation. Concerning
the RER fragmentation, a C–H stretching mode (3100 cmÀ1) has been
taken as reaction coordinate, and six deformations and internal
rotations modes were increased to reproduce the tight character of
this transition structure. Finally, the critical energy values used in
the rate constant calculations were the 0 K endothermicities of each
fragmentation computed at the B3LYP/6311++G(3df,2p)//B3LYP/
6-31 + G(d,p) + ZPE level (Fig. 2). Results obtained using this
approach, in the 0- to 6-eV internal energy range, are summarised
in Figs 4 and 5.
Figure 4 illustrates the SBF and the RER rate constants evolutions
with internal energy for the three isomers ortho, meta and para
1011
1010
109
108
107
106
105
104
103
1011
1010
109
108
107
106
105
104
103
1011
ortho-butyl phenol
meta-butyl phenol
para-butyl phenol
1010
109
108
107
106
105
104
103
k(RER) E°=1.15 eV
k(SBF) E°=1.70 eV
k(RER) E°=1.10 eV
k(SBF) E°=2.00 eV
k(RER) E°=1.50 eV
k(SBF) E°=1.70 eV
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
Internal energy E (eV)
Internal energy E (eV)
Internal energy E (eV)
Figure 4. Individual rate constants calculated for the SBF and RER fragmentations of ortho, meta and para butyl phenols (using B3LYP/6-311++G
(3df,2p)//B3LYP/6-31 + G(d,p) critical energies (Eꢀ), and B3LYP/6-31 + G(d,p) vibrational frequencies).
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J. Mass. Spectrom. 2012, 47, 539–546

A meta effect
100
is not sufficient to interpret the data. Because the SBF fragmenta-
tion is characterised by a loose transition structure, its rate constant
may overcome that of the RER fragmentation at high energy. This is
so for ortho and para isomers but not for the meta derivative where
the SBF critical energy is too high to be counterbalanced by the
looseness of the transition structure. As a consequence, in a 0- to
6-eV internal energy range, k(SBF) is always lower than k(RER) and
the ratio of peaks intensities [SBF]/[RER] lower than unity for the
meta isomers.
Butyl phenols
107/108 ratio
10
1
ortho-OH
para-OH
meta-OH
It may be noted that this meta effect would be also expected
for p-donor substituents other than OH or OCH3. Accordingly, this
effect also appears with donor substituents such as NH2 or with
alkyl chains different from n-alkyl groups as exemplified in the
last lines of Table 1.
Finally, the importance of the meta effect in the analytical
aspect of mass spectrometry should be underlined. It provides
a useful means to distinguish this isomer in complicated mixtures
as recently evidenced in lacquer samples and pyrolysis products
of lignins and polyphenols.[33]
0.1
Butyl benzene
[91]/[92]
0.01
0.001
1
2
3
4
5
6
Internal energy E (eV)
Figure 5. Calculated ratio k(107)/k(108) for ortho, meta and para butyl
phenols and k(91)/k(92) of butyl benzene itself versus internal energy of
the molecular ions. The vertical bar corresponds to the expected average
internal energy of molecular ions formed by a 70-eV electron ionisation.
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