Gas-phase reactions of protonated chlorine, Cl2H+, with H2(D2) and CH4. A mass spectrometric and theoretical study

Article abstract of DOI:10.1016/S0009-2614(99)00313-9

FT-ICR (Fourier Transform Ion Cyclotron Resonance) experiments showed that the reaction of Cl₂H⁺ with H₂ (rate coefficient 6.3±1.0 cm³ s^-1 molecule^-1 at 298 K) yields ClH₂⁺ ions, identified as protonated hydrochloric acid by CAD (Collisionally Activated Decomposition) mass spectrometry. Calculations performed at the B3LYP and CCSD(T) levels of theory show that the reaction involves insertion of the terminal Cl atom of Cl₂H⁺ into the H-H bond, which accounts for the experimental observation that ClD₂⁺ but not ClHD⁺ is formed from the reaction of Cl₂H⁺ with D₂. The same Cl⁺ insertion mechanism characterizes the reaction of Cl₂H⁺ with CH₄, whose product was identified as protonated methyl chloride, CH₃ClH⁺, by CAD mass spectrometry.

Full text of DOI:10.1016/S0009-2614(99)00313-9

3
0 April 1999
Chemical Physics Letters 304 Ž1999. 191–196
q
ž /
Gas-phase reactions of protonated chlorine, Cl H , with H D
2
2
2
and CH . A mass spectrometric and theoretical study
4
Giulia de Petris a,), Federico Pepi , Marzio Rosi
a
b
a
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente AttiÕe - UniÕersit a` di Roma ‘‘La Sapienza’’, P.le Aldo
Moro, 5, 00185 Rome, Italy
b
Istituto per le Tecnologie Chimiche and Centro di Studi CNR ‘‘Calcolo IntensiÕo in Scienze Molecolari’’, UniÕersit a` di Perugia, 06100
Perugia, Italy
Received 3 August 1998; in final form 25 February 1999
Abstract
FT-ICR ŽFourier Transform Ion Cyclotron Resonance. experiments showed that the reaction of Cl H with H2 Žrate
q
2
3
y1
y1
q
coefficient 6.3"1.0 cm s molecule at 298 K. yields ClH2 ions, identified as protonated hydrochloric acid by CAD
ŽCollisionally Activated Decomposition. mass spectrometry. Calculations performed at the B3LYP and CCSDŽT. levels of
q
theory show that the reaction involves insertion of the terminal Cl atom of Cl H into the H–H bond, which accounts for
2
q
q
q
q
the experimental observation that ClD2 but not ClHD is formed from the reaction of Cl H with D . The same Cl
2
2
q
insertion mechanism characterizes the reaction of Cl H with CH , whose product was identified as protonated methyl
chloride, CH ClH , by CAD mass spectrometry. q 1999 Elsevier Science B.V. All rights reserved.
2
4
q
3
1
. Introduction
Polyhalogen cations have been extensively studied in condensed phases, owing in particular to their role in
the halogenation reactions promoted by cationic complexes of molecular halogens with strong acids w1–3x. We
have recently extended the study to gaseous polyhalogen cations preparing and characterizing one of the most
q
simple species, the Cl H ion, by the joint application of mass spectrometric and theoretical methods w4x. As a
2
q
natural extension, we have undertaken a study of the Cl H reactivity towards representative molecules,
2
utilizing the same combination of experimental and theoretical tools, and here report the electrophilic
q
chlorination by Cl H of H and CH , the most simple inorganic and organic molecules.
2
2
4
2
. Methodology
Density functional theory, using the hybrid w5x B3LYP functional w6x, was used to localize the stationary
points of the investigated systems and to evaluate the vibrational frequencies. Single-point energy calculations at
)
Corresponding author. E-mail: depetris@axrma.uniroma1.it
0
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž99.00313-9

1
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G. de Petris et al.rChemical Physics Letters 304 (1999) 191–196
the optimized geometries were performed using the coupled-cluster single and double excitation method w7x with
a perturbational estimate of the triple excitations wCCSDŽT. approachx w8x. Zero-point energy corrections
evaluated at the B3LYP level were added to the CCSDŽT. energies. The 0 K total energies of the species of
interest were corrected to 298 K by adding translational, rotational and vibrational contributions. The absolute
entropies were calculated by using standard statistical-mechanistic procedures from scaled harmonic frequencies
and moments of inertia relative to B3LYPr6-311qqGŽ3df, 3pd. optimized geometries. All the B3LYP
calculations were performed using the 6-311qqGŽ3df, 3pd. basis set w9–12x, while the CCSDŽT. calculations
were done using both the 6-311qqGŽ3df, 3pd. basis set w9–12x and for the smaller systems the correlation-
consistent polarized valence set cc-pVQZ developed by Dunning and co-workers w13–15x. The choice among
the basis sets was made on the ground of their performances in previous calculations on the proton affinities of
Cl2 and ClF w4x. All calculations were performed using Gaussian 94 w16x. The FT-ICR experiments were
performed utilizing an APEX 47e Spectrometer ŽBruker Spectrospin. equipped with an external ion source, a
q
cylindrical ‘infinity’ cell w17x and a pulsed valve. The Cl H ions generated in the external source and driven
2
y5
into the cell were thermalized by collision with Ar, admitted in a 20 ms pulse to a pressure up to 10 Torr,
then mass selected by ‘soft’ ejection techniques and allowed to react with H , D or CH , present at stationary
2
2
4
y8
y7
pressures from 10
to 10
Torr. The readings of the Bayard–Alpert ionization gauge were calibrated and
corrected according to established procedures w18x. The CAD spectra were recorded using a ZAB-Spec oa-TOF
mass spectrometer ŽVG Micromass., under the following typical conditions: accelerating voltage 8 kV; emission
corrent, 1 mA; repeller voltage, 0 V; electron energy, 40 eV; CI ŽChemical Ionization. source temperature,
1
708C; collision gas, He.
3
. Results and discussion
q
3
.1. Cl H reactions with H and D
2
2
2
Isolated and thermalized Cl H isotopomers undergo Clq transfer to hydrogen
q
2
q
q
Cl H qH ™ ClH qHCl ,
1
Ž .
2
2
2
whose rate coefficient is 6.3"1 cm moleculey1 sy1 at 298 K, corresponding to a collisional efficiency of
3
6
.5%, deduced from the ion–molecule collision rate calculated utilizing the average dipole orientation ŽADO.
theory w19x or the trajectory algorithm w20x.
q
Replacing H with D , the Cl transfer
2
2
q
q
Cl H qD ™ ClD qHCl
Ž
1a
.
2
2
2
is somewhat slower Žks3.9"0.5 cm molecule sy1. which corresponds to a k rk kinetic isotope effect
3
y1
H
D
q
of 1.6"0.3. Most remarkably, no ClHD ions are formed, and the absence of isotopic scrambling is
mechanistically informative, suggesting that reaction Ž1. is a clean Clq transfer to hydrogen. This contrasts with
q
q
the complex mechanism of the deceptively similar reaction of F H with D , which is not a simple F
2
2
q
q
q
transfer, yielding a mixture of FHD and FD2 ions w21x. The ClH ions from Ž1. can confidently be assigned
2
the H–Cl–H connectivity, based on the comparison ŽTable 1. of their CAD spectra with those of model ions
o
y1
obtained by CH rCl of HCl, according to the mildly exothermic process, D H sy3.2 kcal mol w4,22x
4
q
q
CH qHCl ™ ClH qCH .
2
Ž .
5
2
4
The assignment of the wH–Cl–Hxq connectivity is supported by the reactivity pattern of the ClHq2 ions
outlined by FT-ICR experiments, whose results characterize the cation as a pure Brønsted acid, devoid of the

G. de Petris et al.rChemical Physics Letters 304 (1999) 191–196
193
Table 1
CAD spectra of ClH and ClD2 ions from various sources
3
7
q
37
q
2
Fragment
Source of the ions
species
mrz
reaction Ž1.
20.2a
79.8
reaction Ž2.
reaction Ž1a.
3
3
3
7
7
7
q
Cl
37
38
39
18.0
82.0
18.4
81.6
q
q
ClH
ClD
a
Relative intensity normalized to the sum of the fragments intensities. Standard deviation of relative intensities "5%.
chlorinating ability expected for a species of the alternative wCl–H–Hxq connectivity. Thus, proton transfer, but
q
q
not Cl transfer is observed, even to basesrnucleophiles of relatively high Cl affinity
Consistent with the experimental picture, the theoretical analysis of reaction Ž1. provides useful mechanistic
insight. Fig. 1 illustrates the optimized geometries of the species involved and the two consecutive steps Ž4. and
Ž6. of the overall reaction Ž1., whereas Table 2 summarizes their enthalpy changes at 298 K.
q
According to the theoretical description, the interaction of Cl H with H2 yields the intermediate 2, which
2
is formed by the insertion of the terminal Cl atom of 1 into the s bond of H and, based on its large Cl–Cl
2
separation, can be regarded as a ŽHCl PPP 4. cluster. The latter species at 0 K is stable with respect to
dissociation into HCl and 4, but evolves towards the more stable species 3, i.e. the proton-bound HCl dimer,
whose connectivity does not allow the occurrence of reaction Ž1.. At 298 K, however, taking into account the
entropic contributions, intermediate 2 is unstable with respect to dissociation into the products of reaction Ž1..
q
The overall process can be described as a clean Cl transfer to H or D , and in the latter case consistent
2
2
with the experimental results,it is not expected to cause isotopic mixing.
o
Finally, the D H change computed at the CCSDŽT. level of theory compares well with that calculated from
1
y1
experimental data w4,22x since the difference between the two results, y48.0 and y45.9 kcal mol , falls well
within their combined uncertainty range.
q
3
.2. Cl H reaction with methane
2
q
FT-ICR experiments show that thermalized Cl H ions react with methane yielding chlorinated products, the
2
major process, being the electrophilic chlorination
q
q
Cl H qCH ™ CH Cl qHCl .
7
Ž .
2
4
4
The structure of the charged products was probed by CAD spectrometry, by comparing its dissociation
q
pattern with those of model wCH –Cl–Hx ions obtained by CH rCl of CH Cl, according to the equation
3
4
3
q
q
CH qCH Cl ™ CH ClH qCH
Ž
Ž
8
9
.
.
5
3
3
4
and by CH FrCl of HCl, according to the equation
3
q
q
Ž
CH3 .2F qHCl ™ CH ClH qCH F .
3
3

1
94
G. de Petris et al.rChemical Physics Letters 304 (1999) 191–196
Fig. 1. Relevant reactions of the species of interest and their optimized geometries, see text.
The results, summarized in Table 3, show that the spectra of the ions from processes Ž7.–Ž9. are
q
indistinguishable within experimental error, which allows one to assign the CH ClH structure to the charged
3
y1
product from chlorination Ž7.. Accordingly, the process can be calculated to be exothermic by 46.2 kcal mol
w4,22x. The structural assignment is consistent with the results of a FT-ICR survey of the reactivity of the
q
CH Cl ion from Ž7., showing that it behaves as a Brønsted acid, e.g. towards CH CN, or a methylating agent,
4
3
e.g. towards H O, but never as a chlorinating agent.
2
Table 2
y1
Energetics Žkcal mol . at 298 K of relevant processes calculated at the CCSDŽT. level of theory
Processa
D H2o
98
1
2
2
1
qH ™ 2
y54.9
y13.9
4.7
2
™ 3
™ 4qHCl
b
qH ™ 4qHCl
y50.2
2
a
The species are labeled as in Fig. 1.
The value is y48.0 at CCSDŽT.rcc-pVQZ level of theory.
b

G. de Petris et al.rChemical Physics Letters 304 (1999) 191–196
195
Table 3
CAD spectra of CH4 Cl ions from various sources
3
7
q
Fragment
Source of the ions
species
mrz
reaction Ž7.
reaction Ž8.
reaction Ž9.
q
24.3a
17.1
7.2
8.3
3.0
12.1
6.3
16.6
3.7
CH Cl
52
51
50
49
39
38
37
15
14
13
26.5
13.1
7.2
8.9
3.2
13.1
6.8
15.2
4.3
25.0
14.7
7.0
8.5
3.3
12.8
6.3
16.5
4.3
3
q
CH Cl
2
q
CHCl
q
CCl
q
ClH
2
q
ClH
q
Cl
CH
CH
q
3
q
2
q
CH
1.4
1.7
1.6
a
Relative intensity normalized to the sum of the fragments intensities. Standard deviation of relative intensities "5%.
By analogy with the reaction with hydrogen, and based on the structure of the charged product, the
q
mechanism of reaction Ž7. most likely involves the insertion of the terminal chlorine atom of Cl H into one of
2
the s bonds of methane, followed by loss of HCl according to the scheme
Support for the above mechanism, and extension of the study to other chlorination processes, is expected
from the results of theoretical and experimental work currently under way in our laboratories.
4
. Conclusions
The study of the gas-phase reactivity of Cl H towards simple molecules such as H2 and CH4 has
q
2
demonstrated its remarkable ability to promote electrophilic chlorination according to a mechanism whereby the
terminal Cl atom of the cation inserts into a s bond of the neutral molecule.
Acknowledgements
The authors are grateful to Professors F. Cacace and A. Sgamellotti for their interest in this work and to F.
Angelelli who performed the FT-ICR measurements. Financial support from the Universities of Rome and
Perugia, and from CNR is gratefully acknowledged.
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