Diagnostic fragmentations of adducts formed between carbanions and carbon disulfide in the gas phase. A joint experimental and theoretical study

Article abstract of DOI:10.1039/b916477d

Selected carbanions react with carbon disulfide in a modified LCQ ion trap mass spectrometer to form adducts, which when collisionally activated, decompose by processes which in some cases identify the structures of the original carbanions. For example (i) C₆H₅^- + CS ₂→ C₆H₅CS₂^-→ C₆H₅S^- + CS, occurs through a 3-membered ring ipso transition state, and (ii) the reaction between C₆H ₅CH₂^- and CS₂ gives an adduct which loses H₂S, whereas the adduct(s) formed between o-CH ₃C₆H₅^- and CS₂ loses H₂S and CS. Finally, it is shown that decarboxylation of C ₆H₅CH₂CH₂CO₂^- produces the β-phenylethyl anion (PhCH₂CH₂ ^-), and that this thermalized anion reacts with CS₂ to form C₆H₅CH₂CH₂CS₂ ^- which when energized fragments specifically by the process C ₆H₅CH₂CH₂CS₂ ^-→ C₆H₅CH₂^-CHC(S)SH → [(C₆H₅CH₂CHCS) ^-SH] → C₆H₅CH₂CCS^- + H₂S. Experimental findings of processes (ii) and (iii) were aided by deuterium labelling studies, and all reaction profiles were studied by theoretical calculations at the UCCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory unless indicated to the contrary.

Full text of DOI:10.1039/b916477d

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Diagnostic fragmentations of adducts formed between carbanions and carbon
disulfide in the gas phase. A joint experimental and theoretical study†
Micheal J. Maclean, Scott Walker, Tianfang Wang, Peter C. H. Eichinger, Patrick J. Sherman and
John H. Bowie*
Received 11th August 2009, Accepted 24th September 2009
First published as an Advance Article on the web 23rd October 2009
DOI: 10.1039/b916477d
Selected carbanions react with carbon disulfide in a modified LCQ ion trap mass spectrometer to form
adducts, which when collisionally activated, decompose by processes which in some cases identify the
structures of the original carbanions. For example (i) C6H5 + CS2 → C6H5CS2 → C6H5S- + CS,
-
-
-
occurs through a 3-membered ring ipso transition state, and (ii) the reaction between C6H5CH2 and
CS2 gives an adduct which loses H2S, whereas the adduct(s) formed between o-CH3C6H5 and CS2 loses
H2S and CS. Finally, it is shown that decarboxylation of C6H5CH2CH2CO2- produces the b-phenylethyl
-
-
-
anion (PhCH2CH2 ), and that this thermalized anion reacts with CS2 to form C6H5CH2CH2CS2 which
-
-
when energized fragments specifically by the process C6H5CH2CH2CS2 → C6H5CH2 CHC(S)SH →
-
-
= =
[(C6H5CH2CH C S) SH] → C6H5CH2CCS + H2S. Experimental findings of processes (ii) and (iii)
were aided by deuterium labelling studies, and all reaction profiles were studied by theoretical
calculations at the UCCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory unless indicated to
the contrary.
of the aromatic ring by electron-withdrawing groups. Heavy atom
Introduction
(13C and 18O) labelling shows that the product ion PhO- from
PhO(CH2)2O- and products PhO- and PhS- from PhS(CH2)2O- are
formed exclusively from Smiles intermediates A (X = Y = O or X =
S, Y = O).17 [For the degenerate process where X = Y = O, calcu-
lations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31+G(d) level
of theory indicate that ipso intermediate A (DE = +285 kJ mol-1) is
reached via a transition state 289 kJ mol-1 above PhO(CH2)2O-].18
In contrast, the formation of PhO- from PhO(CH2)3O- occurs by
competitive Smiles (85%) and SNi reactions (15%), while PhO-
is formed from PhO(CH2)4O- solely by an SNi process.17 18O
Labelling shows that when there is a substituent in the ortho
position, the gas phase Smiles rearrangement competes with an
ortho cyclization process as shown in eqn (2).19 The classical
[PhNO2]-. to [PhONO]-. gas phase rearrangement is probably an
ipso process,20 and gas-phase Smiles processes have been proposed
for 2-hydroxybenzyl-N-pyrimidinylamine,21 phenoxy-N-phenyl
acetamide anions,22 deprotonated 2-(4,6-dimethoxypyrimidine-2-
ylsulfanyl)-N-phenylbenzamide23 and other systems.24–26
A classical (condensed phase) Smiles rearrangement1 is shown in
eqn (1). This nucleophilic ipso attack normally requires an electron
withdrawing group (e.g. nitro, sulfonyl or halogen) either in the
ortho or para position on the aromatic ring; generally X is a good
leaving group, Y is a strong nucleophile and Z is shown as a para
substituent in eqn (1). The anionic Smiles rearrangement has been
used extensively synthetically (see e.g.2–9 for some recent examples),
and radical Smiles rearrangements have also been reported.10–14
The Truce–Smiles rearrangement (involving attack of a carbanion
centre at an ipso electrophilic centre) has also been used as a
synthetic method.15,16
(1)
The gas phase Smiles rearrangement has not been studied in
such depth as that in the condensed phase. The Smiles rearrange-
ment occurs in the gas phase without the necessity for activation
(2)
Department of Chemistry, The University of Adelaide, South Australia, 5005,
Australia. E-mail: john.bowie@adelaide.edu.au
† Electronic supplementary information (ESI) available: Tables S1 and
S1(a): Anion geometries and energies of reaction pathway for the CS2
addition to the singlet phenyl anion. Tables S2 and S2(a): Anion geometries
and energies of reaction pathway for the collision induced dissociation of
the singlet PhCOS- anion. Tables S3 and S3(a): Anion geometries and
energies for the possible reaction pathway for the CS2 addition to the
singlet Benzyl anion. Tables S4 and S4(a): Anion geometries and energies of
reaction pathway for the interconversion of the singlet anion o-CH3(C6H4)-
We wished to revisit our gas-phase ipso rearrangement studies in
order to determine whether reactions of CS2 with aryl carbanions
give adducts cf.27,28 which undergo collision-induced cleavages
[perhaps ipso (Smiles) rearrangements] diagnostic of the structures
of the reacting carbanions. The systems we have chosen to react
-
to singlet PhCH2-. Tables S5 and S5(a): Anion geometries and energies
with CS2 are (a) the phenyl anion, (b) the isomers PhCH2 and
-
-
-
of reaction pathway for the interconversion of the singlet PhCH2CH2
.
[ortho-CH3C6H4 ] and (c) the b-phenylethyl anion (PhCH2CH2 )
Tables S6 and S6(a): Potential energy surface for the dissociation of
-
together with some other isomeric anions C8H9 . In (c), we wished
-
the singlet PhCH2CH2CS2 anion. Tables S7 and S7(a): Potential energy
-
to determine whether the reaction with CS2 can settle the old
debate29–31 as to whether the b-phenylethyl anion is stable or
surface of dissociation of the singlet PhCH2CH2CO2 anion. See DOI:
10.1039/b916477d
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undergoes side chain rearrangement via an ipso, or some other
intermediate.
Results and discussion
1. Reaction of the phenyl anion with CS2
The phenyl anion was formed in the LCQ mass spectrometer by
decarboxylation of the benzoate anion.32 Reaction of C6H5 with
-
CS2 forms C6H5CS2- and collisional activation of this species gives
C6H5S- (m/z 109) as the only observable fragment ion. Reduction
in the pressure of the CS2 reagent gas in this system results in
-
the detection of a minor peak corresponding to m/z 77 (C6H5 )
accompanying m/z 109.
Since anions can react with CS2 in the gas phase either at C or S,33
the collision induced negative ion mass spectrum of deprotonated
dithiobenzoic acid (C6H5CS2H) was measured using a Waters
QTOF2 mass spectrometer operating in negative ion electrospray
ionization mass spectrometry (ESI MS/MS) mode. We chose the
QTOF2 (rather than the LCQ) for this purpose since the QTOF2
uses an efficient gas collision CID MS/MS process for accelerated
ions, whereas the LCQ uses a less efficient collisional process in
Fig. 1 The ipso rearrangement of PhCS2-. Energies at the UCCSD(T)/
6-31+G(d,p)//B3LYP/6-31+G(d,p)] level of theory. Relative free energies
(DG) in kJ mol-1. Full details of geometries and energies of minima and
transition states are recorded in Supplementary table 1.†
-
the mass analyser. Two fragment ions, m/z 77 [(C6H5 ) 100%] and
m/z 109 [(C6H5S-) 64%] were observed. This is consistent with the
adduct of the ion molecule reaction being C6H5CS2- and suggests
-
that fragment anion C6H5 is produced in the LCQ, but reacts
immediately with CS2 on formation.
In order to compare the reactions of C6H5CS2 with those of
-
oxygen analogues, we have measured the negative ion CID ESI
MS/MS of [C6H5COS]- and C6H5CO2 using a Waters QTOF2
-
mass spectrometer. The spectrum of [C6H5COS]- shows m/z 77
(100%), 93 (C6H5O-) 15% and 109 (C6H5S-) 55%, while that of the
-
benzoate anion gives m/z 77 (C6H5 ) as the only fragment anion.
-
So C6H5CO2 does not undergo the ipso rearrangement, whereas
[C6H5COS]- undergoes two competitive ipso rearrangements.
The reaction coordinate profiles of the possible ipso re-
arrangements described above have been investigated at the
UCCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory.
All relative energies indicated in the text and in the Figures are DG
(free energy) values (as requested by a reviewer rather than DE).
The supplementary tables list both DE and DG values. Results for
the ipso processes of C6H5CS2- and C6H5COS- are summarized in
Fig. 1 and 2, with full details of geometries and energies listed in
supplementary tables 1 and 2.† No ipso intermediates are identified
Fig. 2 The ipso rearrangements of [PhCOS]-. Energies at the UCCSD(T)/
6-31+G(d,p)//B3LYP/6-31+G(d,p)] level of theory. Relative free energies
in kJ mol-1. Full details of geometries and energies of minima and
transition states are recorded in Supplementary table 2.†
-
in any case. In the case of C6H5CS2 (Fig. 1), reaction proceeds
through an ipso transition state (+316 kJ mol-1) to [C6H5SCS]-,
which is energized and may decompose to C6H5S- and CS in an
overall endothermic reaction (+193 kJ mol-1).‡
The theoretical results are consistent with experiments, where the
relative abundance of the peak due to C6H5S- is greater than that
of C6H5O-.
The two competitive rearrangements of [C6H5COS]- are
shown in Fig. 2. Formation of C6H5S- and CO is endother-
mic (+26 kJ mol-1) proceeding via an ipso transition state
(+300 kJ mol-1). In comparison, the competitive formation of
C6H5O- and CS is more endothermic (+245 kJ mol-1) with a
transition state marginally higher in energy (at +306 kJ mol-1).
2. Reactions of the benzyl and ortho-tolyl anions with CS2
The reactions of these two anions with CS2 give adducts whose
characteristic fragmentations differentiate between the structures
of the original reacting carbanions. The precursors of the two
anions were produced following electrospray ionization of phenyl-
acetic acid and ortho-toluic acid to yield the two carboxylate
anions which were decarboxylated following collisional activation
‡ A reviewer has asked whether electron detachment of the precursor
anion competes with the ipso rearrangement to form C6H5S-. The electron
∑
affinity of C6H5CS2 is calculated to be 3.345 eV (332 kJ mol-1) [using the
G3B3 level of theory (see Experimental section)] so the process shown in
Fig. 1 is more energetically favourable than electron loss.
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to yield the benzyl and ortho-tolyl anions respectively. The reaction
of these two anions with CS2 gave adducts whose fragmentations
were probed using the CID MS4 scanning procedure. Decom-
-
position of PhCH2CS2 gave only one fragmentation, yielding
a pronounced peak corresponding to the [PhCH2CS2 - H2S]-
-
ion (m/z 133). When this procedure was repeated with the D
-
-
labelled species PhCD2 , the adduct PhCD2CS2 lost only D2S.
-
This process is shown in eqn (3): decomposition of PhCH2CS2
gives PhCCS- and H2S, an endothermic process (+120 kJ mol-1)
at the UCCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of
theory (see Supplementary Table 3†).
-
-
= =
PhCH2CS2 → Ph·CH(=S)SH → [(PhCH C S) SH] →
-
≡
PhC CS + H2S
(3)
The situation with the ortho-tolyl anion is more complex:
the adduct formed with CS2 when collisionally activated shows
competitive losses of H2S and CS. The loss of CS is simply the
ipso rearrangement (cf. Fig. 1) of the expected adduct ortho-
-
CH3C6H4CS2 . But how is H2S lost? This was uncovered when
the reaction between the D labelled ortho-CD3C6H4- and CS2 was
studied. The decarboxylation process formed ortho-CD3C6H4- and
if this anion was allowed to react immediately with CS2, the adduct
lost only CS by a standard ipso rearrangement. If, in contrast,
-
the ortho-CD3C6H4 anion was allowed to remain in the trap for
10 microseconds, it back exchanged two D for H (reaction with
residual H2O from the electrospray ionization process). This ion
formed an adduct with CS2 which, on collisional activation, lost
H2S exclusively. This can only be due to the process shown in
eqn (4): namely D transfer to the ortho position followed by D/H
exchange of the two remaining benzylic deuteriums by water (a
process of the type first reported by DePuy et al 27,28) to yield
the D1 adduct B which then loses H2S (see eqn (4) and cf. eqn
(3)). The interconversion of the ortho-tolyl and benzyl anions is an
exothermic process (-53 kJ mol-1) with a barrier of 179 kJ mol-1
at the UCCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of
theory (see Fig. 3 and Supplementary table 4†).
Fig. 3 The interconversion of the benzyl and ortho-tolyl anion. Energies
at the UCCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p)] level of theory.
Relative free energies in kJ mol-1. Full details of geometries and energies
of minima and transition states are recorded in Supplementary table 4.†
(4)
3. The reactions of the b-phenylethyl anion and other isomers
with CS2
-
The question as to whether the b-phenylethyl anion (PhCH2CH2 )
is stable has been a matter of debate for almost 20 years. Nibbering
et al.30 presented data which showed that the b-phenylethyl anion
rearranges to the cyclised ipso form on collisional activation in
a conventional mass spectrometer, while Squires and Graul31
proposed that the b-phenylethyl anion was stable in a flowing
afterglow instrument, because the ion formed by decarboxylation
of PhCH2CH2CO2- was thermalised by the helium carrier gas, and
had a gas phase basicity (DG◦acid) of 1699 13 kJ mol-1, a value
-
Fig. 4 The interconversions of PhCH2CH2 (A), Ph-CHCH3 (D),
-
ortho-C2H5–C6H4 (C) and the cyclised species (B). Energies at the
UCCSD(T)/6-31+G(d,p)//B3LYP/6-31+G(d,p)] level of theory. Relative
free energies in kJ mol-1. Full details of geometries and energies of minima
and transition states are recorded in Supplementary table 5.†
PhCH2CH2 can H+ transfer from the ortho position to give
-
ortho-C2H5C6H4- (C) [barrier (+49 kJ mol-1), reaction exothermic
-
consistent with that expected for PhCH2CH2 .
∑
The reaction coordinate calculations [at the UCCSD(T)/
6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory] of relevant
(-59 kJ mol-1)]. However, the electron affinity of PhCH2CH2 at
the G3B3 level of theory is calculated to be 0.215 eV (21 kJ mol-1),
so these two H transfer processes are unfavourable compared with
-
C8H9 isomers provide some interesting results (see Fig. 4 and
-
-
Supplementary table 5†). First, PhCH2CH2 (A) can undergo a
electron loss from PhCH2CH2 . Finally, the data shown in Fig. 4
1,2-H transfer over a transition state (+145 kJ mol-1) to yield
indicates that the conversion of PhCH2CH2 (A) to the cyclic
-
Ph-CHCH3 (D) in an exothermic reaction (-104 kJ mol-1). Second,
isomer (B) is endothermic (+15 kJ mol-1) and has a modest barrier
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to the transition state of 38 kJ mol-1. This interconversion and
-
electron loss from PhCH2CH2 should therefore be competitive.
These theoretical data are consistent with previous studies, namely
-
(i) Nibbering et al. finding rearrangement of PhCH2CH2 on
collisional excitation,30 and (ii) Squires and Graul suggesting that
PhCH2CH2- is stable when the ion is efficiently thermalized by the
helium carrier gas in the flowing afterglow drift tube.31
The collision induced mass spectrum of the adduct formed
-
-)
between “PhCH2CH2 ” (by decarboxylation of PhCH2CH2CO2
and CS2 shows pronounced loss of H2S. The analogous spectra
-
-
of Ph(CH3)CHCS2 and ortho-C2H5–C6H4-CS2 do not exhibit
loss of H2S. Ortho-C2H5C6H4-CS2 eliminates CS by the standard
-
ipso rearrangement (data not shown but cf. Fig. 1)], while
Ph(CH3)CHCS2 yields Ph(CH3)CH- at low CS2 pressures. It can
-
-
therefore be concluded that PhCH2CH2 neither isomerizes to
-
Fig. 5 The CID MS/MS of PhCH2CD2CS2 using electrospray ioniza-
tion with a Waters QTOF2 mass spectrometer. The collisional activation
process in the QTOF2 produces more energized anions than those formed
Ph-CHCH3 nor to ortho-C2H5C6H4 prior to or during reaction
-
with CS2 in the LCQ mass spectrometer.
-
The two
D
labelled species PhCH2CD2CS2
and
in the LCQ. Thus the peaks observed at m/z 92 and 76 in Fig. 5 are not
observed in Fig. 7. Since H2S is lost exclusively from PhCD2CH2CS2
the small peaks at m/z 148 and 149 (losses of HDS and H2S respectively)
shown in this Figure are produced by undefined reactions favoured by the
-
-
PhCD2CH2CS2 were formed in the Waters QTOF2 mass
spectrometer [by deprotonation under electrospray ionization of
PhCH2CD2CS2H and PhCD2CH2CS2H in methanol]. These two
dithiocarboxylate anions are those which would be formed by
the ion molecule reactions between the appropriately D-labelled
b-phenylethyl anion and CS2 in the LCQ spectrometer. CID
,
operation of a primary deuterium isotope effect occurring for the loss of
-
D2S from PhCH2CD2CS2
.
-
MS/MS of PhCH2CD2CS2 shows loss of D2S (see Fig. 5), while
through the D2 analogue of cyclic (B) (Fig. 4) prior to or during
reaction with CS2, the subsequent losses of H2S : D2S should
be the same from both labelled adducts (assuming no deuterium
isotope effect is operating). If there is no cyclization then specific
losses of H2S and D2S should be observed, as seen in the QTOF2
spectra described above. The LCQ data are as follows. The adduct
PhCD2CH2CS2- loses H2S. Loss of H2S from PhCH2CH2CS2- has
been shown by ab initio calculations to occur as shown in Fig. 6
(also Supplementary table 6†).
-
-
The two D labelled species “PhCD2CH2 ” and “PhCH2CD2 ”
were prepared by decarboxylation of their respective carboxylate
anions in the LCQ mass spectrometer. If these anions interconvert
-
formed from “PhCD2CH2 ” and CS2 loses H2S, while that from
-
Fig. 6 Reaction coordinate profile for the dissociation of PhCH2CH2CS2 to PhCH2CCS- + H2S. Energies at the UCCSD(T)/6-31+G(d,p)//B3LYP/
6-31+G(d,p)] level of theory. Relative free energies in kJ mol-1. Full details of geometries and energies of minima and transition states are recorded in
-
-
Supplementary table 6.† The reaction PhCH2CH2 + CS2 → PhCH2CH2CS2 is exothermic by 330 kJ mol-1.
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-
“PhCH2CD2 ” and CS2 loses D2S (see Fig. 7). The spectrum shown
Experimental
in Fig. 7 does not change when the trapping time of the reactant
ion is increased to 10 msec.
Mass spectrometry
-
-
All ion molecule reactions were carried out using electrospray
ionization with a Finnigan LCQ ion trap mass spectrometer,
modified36 to allow ion-molecule reactions to be carried out by
the incorporation of additional inlets for introduction of an extra
reagent gas or liquid (in this case CS2). The reagent CS2 was
injected into the system by syringe at a rate of 5 mL min-1. Typical
electrospray conditions involved a needle potential of 4.0 to 5.0 kV
and a heated capillary temperature of 180 ◦C. Ions undergoing ion-
molecule reactions in the LCQ have been essentially shown to be at
room temperature.34,35 These experiments were investigated using
the MSn capability of the LCQ instrument. As an example, CID
of C6H5CO2- to C6H5- and CO2 utilises an MS/MS scan, reaction
PhCH2CH2CO2 → PhCH2CH2 + CO2
(5)
-
of C6H5 with CS2 (MS/MS/MS) and finally, decomposition of
-
the C6H5 plus CS2 adduct utilises an MS/MS/MS/MS scan.
Electrospray CID MS/MS spectra of ArCO2 , ArCOS- and
-
-
ArCS2 anions were measured using a Waters QTOF2 hybrid
orthogonal acceleration time-of-flight mass spectrometer (Wa-
ters/Micromass, Manchester, UK) with a mass range to m/z
10,000. The QTOF2 is fitted with an electrospray (ES) source
in an orthogonal configuration with a Z-spray interface. Samples
were dissolved in acetonitrile–water (1 : 1 v/v) and infused into the
ES source at a flow rate of 5 mL min-1. Experimental conditions
were as follows: capillary voltage 3.1 kV, source temperature 80 ◦C,
Fig. 7 The CID MS/MS/MS/MS scan of “PhCH2CD2CS2-” formed by
the reaction of CS2 with “PhCH2CD2-” (formed from PhCH2CD2CO2-).
Electrospray ionization of PhCH2CD2CO2H using a modified36 Finnigan
LCQ mass spectrometer.
The decarboxylation reaction shown in eqn (5) is calcu-
lated to be endothermic (+231 kJ mol-1) at the UCCSD(T)/
6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory (see Supple-
mentary table 7†). Thus PhCH2CH2CO2- on collisional activation
must produce a precursor with sufficient energy (231 kJ mol-1)
in order to effect decarboxylation. Collisional processes will
produce energized precursor ions with a range of excess energies.
◦
desolvation temperature 150 C, and cone voltage 50 V. Tandem
mass spectrometry (MS/MS) data were acquired using argon as
the collision gas and the collision energy was set to give maximum
fragmentation.
Theoretical methods
-
The consequence of this is that some product ions PhCH2CH2
may have excess energy. The theoretical results shown in Fig. 4
Geometry optimizations were carried out with the B3LYP/6-
31+G(d,p) basis set [energies at UCCSD(T)/6-31+G(d,p) within
the GAUSSIAN 03 suite of programs.37 Stationary points were
characterized as either minima (no imaginary frequencies) or
transition states (one imaginary frequency) by calculation of the
frequencies using analytical gradient procedures. The minima
connected by a given transition structure were confirmed by
intrinsic reaction coordinate (IRC) calculations.38 The calculated
frequencies were also used to determine zero-point vibrational
energies.
-
indicate that PhCH2CH2 (A) needs only 38 kJ mol-1 of excess
energy in order to effect conversion to ipso species B. The
-
experiments already carried out for the reactions of PhCH2CD2
and PhCD2CH2- with CS2 show that there is no evidence of the ipso
anion (B) (see Fig. 4) reacting with CS2 during the ion molecule
reaction, or of B being involved in an equilibrium process with A
preceding or accompanying the ion molecule reaction with CS2.
This indicates that only thermalised ions A (or more correctly,
anions with less than 38 kJ mol-1 of excess energy) are reacting
with CS2 in the LCQ ion trap (cf. Gronert34,35).
The G3B3 level of theory39 was used to calculate electron
affinities (the difference in energy between the appropriate anion
and the corresponding radical). Three levels of theory were used
Conclusions
∑
∑
for chosen standards CH3CO2 and HCO2 . The computed values
obtained were compared with experimental values as shown
below:-
-
1. The reaction between C6H5 and CS2 in the gas phase gives an
-
adduct C6H5CS2 which, when energized, rearranges via an ipso
∑
∑
transition state to yield C6H5S- and CS.
Level of theory CH3CO2 HCO2
2. The respective reactions between C6H5CH2 and ortho-
Experimental 3.470 eV40 3.541 eV41
-
CH3C6H5 with CS2 give adducts which can be readily distin-
G242 3.499 eV 3.715 eV
-
guished by their collision induced dissociations; i.e. the adduct
with C6H5CH2 loses H2S, while that with ortho-CH3C6H5 loses
G3B3 3.319 eV 3.555 eV
UCCSD//B3LYP 3.082 eV 3.474 eV
-
-
both H2S and CS2, and
The G3B3 level of theory was used for calculating the following
electron affinities because of reasonable accuracy (see above)
-
3. Decarboxylation of C6H5CH2CH2CO2- gives C6H5CH2CH2
∑
which reacts with CS2 without rearrangement, to yield
and efficiency of computer time usage:- PhCS2 3.345 eV and
∑
-
C6H5CH2CH2CS2 .
PhCH2CH2 0.215 eV.
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-
All calculations were carried out using the South Australian
Partnership for Advanced Computing (SAPAC) Facility.
the gas phase ion chemistry experiments to form PhCH2CD2CS2
in the QTOF2 mass spectrometer.
3,3-Dideutero-3-phenylpropane dithiolic acid. 2-Phenyl-2,2-
dideutero-1-bromoethane in THF was allowed to react with Mg in
THF, followed by addition of CS2 and a catalytic amount of CuBr
in THF at -50 ◦C.43 3,3-Dideutero-3-phenylpropanedithiolic acid
was isolated as an unstable red–orange oil [(M - H)- m/z 183;
25% yield, d2 = 95%]. This was used immediately in the gas phase
Materials/synthesis
The following were purchased from Sigma-Aldrich and were used
without purification:- (i) argon gas, carbon disulfide, benzoic
acid, thiobenzoic acid, phenylacetic acid, ortho-toluic acid, 1-
phenylpropionic acid, 2-phenylpropionic acid, ortho-ethylbenzoic
acid, and 1,2-dibromobenzene; (ii) CD3I (d3 > 99.5%).
-
experiment in the QTOF2 to form PhCD2CH2CS2 .
Dithiobenzoic acid was made by a standard Grignard reaction
between bromobenzene and carbon disulfide43 [(M - H)- m/z 153;
m.p. dec. > 200 ◦C; lit44 208 ◦C).
Dithiophenyl acetic acid (benzene ethane dithioic acid) was
prepared by a standard Grignard reaction usi◦ng benzyl chloride
and carbon disulfide.43 (yield 57%; m.p. 17–19 C, lit.45 20 ◦C
Acknowledgements
We thank (i) the Australian Research Council for funding our
negative ion mass spectrometry projects, and for Ph.D. (MJM) and
research associate (SW, PCHE) stipends, (ii) the South Australian
Partners for Advanced Computing (eResearch, The University
of Adelaide) for generous provision of time on the Aquila
supercomputer, and (iii) Professor R.A.J. O’Hair (University
of Melbourne) for providing details of the modification of the
Finnigan LCQ mass spectrometer to allow the study of ion–
molecule reactions.
◦
◦
o-Methyl(d3)benzoic acid [m.p. 103–104 C (lit.46 104–105 C;
d3 = 99.5%) was made by a Grignard sequence using d3-methyl
iodide (d3 = 99.5%) and commencing with the diethylacetal of
o-bromobenzaldehyde.47
2,2-Dideuterophenylacetic acid [m.p. 77–78 ◦C (lit.48 78 ◦C; d2 =
95%] was prepared by two cycles of exchange of phenylacetic acid
with deuterium oxide using a standard method.48
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