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Reaction of carbon atoms with H2Cl+: an ab initio study of a possible interstellar process
THEO CHEM Journal of Molecular Structure (Theochem) 363 (1996) 319-331
Reaction of carbon atoms with H2C1+: an ab initio study of a possible interstellar process Victor M. Ray&,
Carmen
Barrientos’,
Antonio
Large”*
Departamento de Quimica Fisica J Analitica, Facultad de Quimica, Universidad de Oviedo, 33006 Oviedo, Spain
Received 21 July 1995; accepted IO November 1995
Abstract An ab initio study of the reaction of carbon atoms with H2C1+ ions has been carried out at the Gl level. The (H&Cl)+ species on the triplet surface have been characterized and their molecular structure discussed. It is found that the approach through a hydrogen atom is favored over approach toward chlorine. The former leads in a first step to a CHClH+ species, which finally leads to the more stable HCClH+ and H&Cl+ species. These isomers are quite stable
relative to the reactants, about 64 and 79 kcal mol-‘, respectively. Contrary to what was previosuly assumed. the CH’ + HCl channel is not possible in the interstellar medium, since it is clearly endothermic, whereas production of Ccl+ + Hz is only slightly exothermic. We predict that the dominant channels should be the production of HCCl+ + H and of Cl + CH:, which are both exothermic (by more than 42 and 20 kcal mol-‘, respectively, at the Gl level) and proceed without an activation barrier. The HCCl+ ion may be a precursor of interstellar CC1 through dissociative recombination. Keword.s: Ab initio calculation; Chlorine; Interstellar chemistry;
1. Introduction Until quite recently, most of the known interstellar molecules consisted of first-row elements and only a few sulfur and silicon compounds had been detected. Nowadays, the number of molecules containing second-row elements has increased significantly [l-3]. Furthermore, it is expected that once the gas-phase chemistry of second-row elements is understood, other molecules could be detectable in space.
* Corresponding author. ’ Present address: Departamento de Quimica Fisica, Facultad de Ciencias. Universidad de Valladolid, 47005 Valladolid, Spain.
Ion-molecule
reaction;
Molecular
structure
Little is known about the interstellar chemistry of chlorine. As one of the more volatile second-row elements, its abundance in dense clouds is expected to remain high. Laboratory studies of ionmolecule reactions involving chlorine, together with kinetic modeling, indicate that chlorine should be present mainly as atomic chlorine and in the form of HCl [4]. So far, HCl is the only chlorinecontaining molecule detected in the interstellar medium, whereas a number of metal chlorides (NaCl, AlCl and KCl) have been observed in circumstellar shells [5]. Nevertheless, there are indications that carbon-chlorine compounds could be produced under interstellar conditions. One of the major depletion mechanisms for HCl is the reaction with C+ [6]. Laboratory studies
0166-1280/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0166-1280(95)04451-5
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V.M. Ray& et al./Journal of Molecular Structure (Theochem)
[7-91have determined reaction
the rate constant
C++HCl+CCl++H
for the
(1)
and its dynamics studied by crossed-beam experiments [lo]. In addition, a theoretical calculation of the rate constant has been carried out [l 11, as well as an ab initio study of the energetics of this reaction [12]. One of the most important conclusions of these studies is that production of Ccl+ from the reaction of C+ with HCl is feasible in the interstellar medium. From the study of Blake et al. [4] it is clear that Ccl+ is a relatively stable species, exhibiting reactivity with a few interstellar molecules (NHs, H2CO) and oxygen atoms. The dominant depletion mechanism in dense interstellar clouds would be the reaction with atomic oxygen and recombination with electrons. These characteristics, as pointed out by Gruebele et al. [13], suggest that detection of Ccl+ could provide information about not only the chlorine chemistry but also the chemical constitution of dense clouds. Nevertheless, there are other possible synthetic routes, apart from (1) for the production of Ccl+. In their study of the chemistry of chlorine in dense interstellar clouds, Blake et al. [4] included the reaction of C with H2C1+ as one of the most significant reactions. H2Cl+ is produced from protonation of HCl. HCl + H: + H&l+
+ H2
(2)
which is found to proceed with a rate constant of 3.5 x 10e9 cm3 s-i [4]. There is another synthetic route initiated by chlorine atoms [4] Cl+H+HC1++H2 HCl+ + H2 4 H&l+
(3) + H
(4)
but their rate constants have not been measured. For the reaction of carbon atoms with protonated hydrogen, Blake et al. [4] assumed two reaction channels, C + H2Cl+ + Ccl+
+ Hz
(5)
C + H2Cl+ -+ HCl + CH+
(6)
one of them leading to Ccl+. However, there is, in principle, a third possible channel corresponding to
363 (1996) 319-331
elimination
of a hydrogen
C + H$l+
+ (HCCl)+
atom: + H
(7)
This channel is particularly interesting since subsequent dissociative recombination of (HCCl)+ could lead to neutral Ccl: (HCCl)+
+ e- + CC1 + H
(8)
The purpose of this paper is to carry out a theoretical study of the reaction of carbon atoms with H2C1+, in order to ascertain whether this process is feasible under interstellar conditions and therefore can serve as a source of carbon-chlorine compounds in space.
2. Computational
methods
Ab initio molecular orbital computations were carried out using the program code GAUSSIAN 92 v41. Equilibrium geometries were obtained using second-order Msller-Plesset theory (MP2) with the split-valence plus polarization 6-3 lG** basis set [15,16]. All electrons were included in these MP2 calculations (“full” electron correlation). Harmonic vibrational frequencies have been estimated at the MP2(fu11)/6-31G* level using analytical derivatives. Electronic energies have been computed using fourth-order Marller-Plesset (MP4) theory [ 17,181 with the MC-311G** basis set, which is made up from the 6-3 11G basis set [ 191 for carbon and hydrogen and the McLean and Chandler basis set for chlorine [20], supplemented with polarization functions. In these calculations the “frozen-core” approximation was employed (inner-shell molecular orbitals were not included for computing correlation energies). Additional calculations employing a slightly modified version of the socalled Gl method [21] were carried out. Gl theory is a composite procedure which assumes that some higher-level corrections are additive. To overcome limitations of the basis set, MP4 calculations with diffuse functions, using the MC-31 l+G** basis set, and with additional polarization functions, employing the MC-31 lG(2df,p) basis set, were carried out. Since we will be dealing with open-shell
V.M. Raydn et aLlJournal of Molecular
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Structure (Theochem) 363 (1996) 319-331
states, instead of substracting the MP4/MC311G** values from the MP4/MC-311+G** and MP4/MC-31 lG(2df,p) ones, as in the original Gl method, we use the projected MP4 (PMP4) values in order to avoid spin-contamination problems [22,23]. Quadratic configuration interaction calculations at the QCISD(T) level were also carried out to account for deficiencies in the MollerPlesset method. The last modification with respect to the original Gl method is the use of scaled [24] MP2(fu11)/6-3 1G* vibrational frequencies instead
of the HF/6-31G* ones to estimate zero-point vibrational energy (ZPVE) corrections. Finally, a higher-level correction is included in Gl theory, AE(HLC) = -O.l9n, - 5.95n,, where no and na are the number of (Y and /3 valence electrons, respectively, and AE(HLC) is in millihartrees. A detailed analysis of the bond-breaking processes showed that consideration of singleconfiguration-based methods is not an important limitation in this case. The characteristics of the binding for the 0.734
H-H
H
(‘c,+)
1.293 Cl
/
\
3
95.9
1.556
Cl
C
(‘Al)
H
C-
1.117
H
(3m
(31-o
1.268
Cl
H
($+)
C H 1.298 ‘O’-Or
J
C
(2A’)
/ Cl
1.826
H 131.8 1.099
\
7
fi
Cl
b
t2A’)
1.548 Fig. 1. Optimized structures at the MP2(fu11)/6-31G** given in Angstriims and angles in degrees.
level for the reactant
and products
of the reaction
of C with HlCl+.
Distances
are
V.M. Raydn et al./Journal of Molecular Structure (Theochem)
322
products and intermediates of the reaction of C with H2Cl+ have been analyzed by means of Bader’s topological analysis [25] of the electronic charge density, p(r), using the AIMPAC series of programs [26]. We have employed the MP2/MC31 lG** wavefunctions to build up the electron density and compute several properties of bond critical points, such as p(yc), V2p(rc), and the energy density H(Y,). The energy density at the bond critical point indicates that a bond is covalent if H(r,) < 0 and ionic if H(r,) > 0 [27]. 3. Results and discussion Starting from the reactants in their ground states, the reaction of carbon atoms with H2Cl+ should take place, in principle, on the triplet surface. Since charge transfer is not a competitive process (C+ + H2Cl lie about 121 kcal mol-’ above the reactants at the Gl level), we have considered the following processes: ,+ CCl+(311) + H*(‘Cg+) HCI(‘C+) C(3P)
(9)
The MP2(fu11)/6-31G** optimized geometries of the reactants and products are shown in Fig. 1, and the MP2(fu11)/6-31G* frequencies are given in Table 1. Fig. 2 shows the geometrical parameters of the triplet (H2CCl)+ species and the corresponding MP2(fu11)/6-31G* vibrational frequencies are given in Table 2. The approach of carbon toward chlorine could take place, in principle, along C2, symmetry, and C(3P) splits into three spatially degenerate states corresponding to 3B2 (. . . al1 b2’), 3B1 (. . . al1 b, ‘) and 3A2 (. b2’ bt ‘) symmetries. Nevertheless, the interaction of any of these states with H2Cl+ (with two electron pairs occuping al and bt molecular orbitals) results in at least a three-electron
Table 2 MP2/6-31G* harmonic vibrational triplet (H2CC1)+ species
L
+ CH;(2A,)
(HCCl)+(‘A’)
vibrational
frequencies
Vibrational
H$I+(‘A,)
Cl-H Cl-H bend
asym stretch sym stretch
2861 2853 1306
H2C1 Cp’)
H-H
stretch
4536
ccI+(
CC1
stretch
1165
C-H
stretch
2919
CH+(3TI)
’ C’)
CILH stretch
3048
CClH+(‘A’)
Cl-H bend C-Cl
stretch
2781 768 547
HCCl+( ‘A’)
CH stretch C-Cl stretch bend
3208 1219 989
CH:(2A,)
C-H asym stretch C-H sym stretch bend
3395 3143 1015
stretch
Frequency
br b2 a, b2 a, aI
Out-of-plane bend CIH2 rock Cl-H, sym stretch Cl-H2 asym stretch CC1 stretch H-H stretch
41 61 151 206 1162 4494
3 CHClH+(3A’)
a’ a’ a” a’ a’ a’
H-Cl stretch CHCl bend Out-of-plane bend HClH bend C-H stretch Cl&H stretch
362 468 715 1014 1301 2930
4 HCCIH+(3A)
HCCIH bend (torsion) CC1 stretch HClC bend CICH bend Cl-H stretch C-H stretch
350 743 907 1050 2788 3281
5 H2CCl+(3A”)
a’ a’ a” a’ a’ a’
608 717 841 1341 3100 3254
(12) H2(3B2)
of the
Frequency
Species
%I)
mode
(cm-‘)
of the
142 i 109 163 1302 2849 2858
2 Ccl+. Table 1 MP2/6-3lG* harmonic reactant and products
mode
(cm-‘)
ClH2 rock CC1 stretch Out-of-plane bend ClH2 bend CIH2 sym stretch ClH2 asym stretch
C1H;(3Bz)
(11)
+ H(‘S)
frequencies
b2 at br a, a, b2
1 C.
+ H2Cl+(‘A,) Cl(‘P)
HCI(
Vibrational
Species
(lo)
+ CH+(‘II)
363 (1996) 319-331
CH2 wag Cl-C stretch Asym bend Sym bend CH2 sym stretch CH2 asym stretch
V.M. Ray& et al./Journal of Molecular
Structure (Theochem)
2.993
__--
1.557
Cl _‘-‘,‘f
C
e-
I ---_
--
’
13B2)
2
t3B2)
323
H 0.736
;4:1- ---_
363 (1996) 319-331
H
176.8
c
4-c
1.284
1.364
98.2
3
(3A’)
4
t3A)
\
H
1.737 (
Cl
Fig. 2. Optimized angles in degrees.
structures
ly&f+f; _-C
1.698
at the MP2(fu11)/6-31G**
5 (‘A”)
1.088
level for the triplet (H*CCl)+
species. Distances
are given in Angstriims
and
324
V.M. Ray& et al./Journal of Molecular Structure (Theochem)
repulsive interaction, and therefore it is expected that high energy barriers should be encountered along this path. The 3A2 state is repulsive and dissociated into C(3P) + H2Cl+(‘Al). In the case of the 3B2 and 3Bi states we have found stationary points at long C-Cl distances, the 3B2 state lying lower in energy than the 3B1 state. For this reason we will only discuss the 3B2 state, which corresponds to structure 1 in Fig. 2. From the geometrical parameters shown in Fig. 2 this can be viewed as an atom-molecular ion complex with an electrostatic bond, C . . ClHl. The C1H2 moiety preserves essentially the geometry of the H2Cl+ cation (shown in Fig. l), whose vibrational frequencies are easily recognized in the vibrational frequencies of structure 1. Nevertheless, as can be seen in Table 2, structure 1 is not a true minimum since it has an imaginary bz frequency (the same is observed for the corresponding stationary point of the 3Bi electronic state; furthermore, inclusion of polarization functions for hydrogen atoms has no effect on this feature, since both states still have an imaginary frequency at the MP2(fu11)/6-3 1G** level). We have tried to find a true minimum following this mode, but all attempts collapsed into structure 3 (see below). However, a true minimum of C,, symmetry (3B2 electronic state) can be reached, after pushing over a barrier, in the region of small C-Cl distances. The geometrical parameters of this minimum (structure 2 in Fig. 2) suggest that this structure can be described as a Ccl+. . Hz complex, since both C-Cl and H-H distances are almost identical to those found for the fragments at the same level of theory (see Fig. 1). Furthermore, the corresponding CC1 and H-H stretching frequencies are nearly the same as those found for CClf(311) and H2(’ C,‘). On the contrary, the symmetric and asymmetric stretching frequencies are very small (with values of 151 and 206 cm-‘, respectively). The approach of the carbon atom to H*Cl’ could also take place toward one of the hydrogen atoms. In fact, the atomic charges of chlorine and hydrogen in H2Clf are nearly the same (+0.332 for Cl and +0.334 for H from Mulliken population analysis), and there should be no electrostatic preference for the approach toward chlorine or one of the hydrogen atoms. This approach results in the
363 (1996) 319-331
rather unusual planar structure labelled 3 in Fig. 2, which corresponds to a 3A’ electronic state and shows C-H and H-Cl bond distances which are both lengthened compared with the corresponding distances in CH+ (1.117 A) and H$l+ (1.293 A). In fact, bothOdistances are nearly equally increased (about 0.25 A). It is also interesting to note that the / CHCl bond angle is close to180”, indicating an almost linear arrangement for these atoms. There is also, of course, a 3A” state which lies higher in energy than the 3A’ state and which has a similar optimized geometry, the main difference being the C-H bond distance which is 1.372 A for the 3A” state, the other geometrical parameters being the same as for the 3A’ state. We shall refer in what follows to the 3A’ state. Since the unpaired electrons for structure 3 are mainly located at the carbon atom (the spin density at carbon is 1.824, whereas that of chlorine is 0.155) the interaction with carbon is established through its lone pair. The partial donation of electronic charge from carbon toward the H2C1+ unit results in atomic charges of $0.382 for carbon, +0.168 for the hydrogen atom bonded to carbon, +0.150 for chlorine, and f0.300 for the other hydrogen atom (which remains nearly unchanged). These values, as well as the bond distances, suggest that the interaction must have a certain degree of covalency, which is also supported by the relatively high bond energy of 15 kcal mall’ (see below, Fig. 4) which is certainly greater than typical hydrogen bonds. We have also studied the structure resulting from insertion of carbon into an H-Cl bond, structure 4 in Fig. 2. This HCClH+ isomer deviates significantly from planarity (the dihedral angle is 97.8”) and shows a relatively long C-Cl bond distance which is closer to that found in CClH+ than to the value found in HCCl+. This is also reflected in the C-Cl stretching frequency which is close to that of CClH+. Both unpaired electrons are located at carbon and the bonding in this species is intermediate between an electrostatic interaction (HC+ + ClH) and a covalent one (with significant dative bonding from one of the chlorine lone pairs to carbon). Finally, migration of the hydrogen bonded to chlorine in structure 4 results in triplet H&Cl+.
V.M. Raydn et al./Journal of Molecular Structure (Theochem)
In Czv symmetry the lowest-lying state is 3A2, but it shows an imaginary frequency for out-of-plane bending. Following this mode a pyramidal structure is obtained, corresponding to a 3A” electronic state (structure 5 in Fig. 2). Pyramidalization is obviously due to the unpaired electron occupying a b, molecular orbital which is essentially a carbon 2p orbital (the other unpaired electron is located at chlorine, which formally bears the positive charge). The C-Cl bond distance is much longer than that found for the HCCl+ species (see Fig. l), as a consequence of the absence of the favorable interaction due to delocalization of the electrons occupying the 3p,(Cl) orbital into the vacant 2pJC) orbital (which is now occupied by a single electron in structure 5) which contributes to strengthening the C-Cl bond in HCCl+. The 3A’ state corresponding to the same connectivity lies higher in energy (about 15 kcal mall’ at the Gl level). The most interesting results from the topological analysis of the electronic charge density are collected in Table 3. We provide the bond length and the electronic density (p), Laplacian of p and energy density at the bond critical point for the C Cl bond in the different species (except in the case of structure 3, where the properties of the C-H and H-Cl bonds are given). The values shown in Table 3 confirm the previous qualitative discussion about the bonding in these species. Thus, structure 1, C. ClH:, is the only species which shows the characteristics of a C-Cl ionic bond, since H(r,)
Table 3 Bond properties
of the C-Cl
ccl+(sP) HCCl+(*A’) CClH+(*A’) 1 C...CIH’(3B2) 2 CCI’ H2(3Bz) 3 CHCIH+(3A’) C-H H-Cl 4 HCCIH+(‘A) 5 H2CCI+(3A”)
bonda for Ccl’,
(CHCI)+
363 (1996) 319-331
325
takes a positive value [27]. The positive value of V*p(r,) is also indicative of a closed-shell interaction [25] between carbon and the ClHi unit. In all other cases the negative values of H(r,) indicate that some covalency is enhancing the binding. Structure 2 has electronic charge density properties very similar to those found for Ccl+, indicating that essentially the same C-Cl bond is already present in that species. On the contrary both structures 4 and 5 show properties for the C-Cl bond which suggest that some changes should still occur in the C-Cl electron density distribution to reach the HCCl+ product. A particularly interesting case is structure 3, since it has an unusual bonding through a hydrogen atom. The values of H(r,) for the C-H and HCl bonds in this species support the covalent character of these bonds. Furthermore, the negative values of 2p(rc) confirm a shared rather than a closed-shell interaction in both cases. In this respect we should recall that hydrogen bonds are characterized by small values of p(r,) and positive values of V2p(rc) [25]. Nevertheless, the (X,(/X3 ratio, Xi being the highest (in absolute value) of the two negative curvatures and X3 the positive curvature at the bond critical point, is lower than unity for both C-H and H-Cl bonds in CHClH+. The values of the ratio 1X11/X3for typical shared interactions are normally greater than unity, denoting that perpendicular contractions are dominating the binding region [25]. However, in typical ionic interactions the values of IX,I/X, are
and (H2CCI)’
speciesb
R
p(rd
v*P(rC)
H(rd
1x1I/X3
1.556 1.548 1.826 3.321 1.557 1.364 1.564 1.737 1.698
0.2749 0.2933 0.1641 0.0070 0.2743 0.1344 0.1234 0.1943 0.2108
-0.4964 -0.6525 -0.1531 0.0256 -0.4928 -0.2138 -0.1306 -0.3064 -0.3881
-0.2768 -0.3126 -0.1095 0.0014 -0.2757 -0.0915 -0.0771 -0.1610 -0.1979
1.4038 1.7284 0.7796 0.1405 1.3912 0.7946 0.6811 1.0993 1.3575
a In the case of the CHClH+ species the bond properties of the C-H b R is in Angstroms. p(r,), V’p(r,) and H(r,) are in atomic units.
and H-Cl
bonds are given.
326
V.M. Raydn et al.lJournal of Molecular Structure (Theochem)
363 (1996) 319-331
less than unity, denoting that the contraction of p toward the nuclei is dominating the binding region. Therefore, the C-H and H-Cl bonds of CHClH+ exhibit characteristics of covalency (negatives values of 2p(r,) and H(r,)), as well as of ionic interactions (small values of p(r,), IX,I/X, ratios smaller than unity), and one could classify them as corresponding to intermediate interactions. There is also a compound with a C-Cl bond where the IX,i/X, ratio is less than unit (apart from structure 1 which has an ionic C-Cl bond), namely CClH’(2A’), which together with its low value of p(r,) (the smallest of all C-Cl bonds with the exception of structure 1) suggests a high ionic character for this bonding. In fact, this compound can be viewed as the Cf . . . ClH ion-molecule complex [ 121. Once we know the structure of the triplet (H,CCl)+ species we can discuss the reaction of ground-state carbon atoms with H2CI’. The total energies at the MP2(full)~6-3lG*~ and Cl levels of the reactants, products, intermediates and transition states involved in this reaction are collected in Table 4, along with the S* expectation values. As can be seen, spin contamination is very low for most of the species, the worst case being transition state TS2 for which {S”) = 2.189. Therefore, MP theory should be quite reliable for computation of electronic energies on the (H2CCl)+ triplet surface,
especially when using projected MP values. The geometries of the transition states are given in Fig. 3. For all transition states we checked that the imaginary frequency corresponds to the vibrational mode associated with the desired reaction path. Finally, the relative energies at the Gl and MP2(full)/6-31G** (in parentheses) levels for all species involved in the reaction are shown in Fig. 4, From the energies shown in Table 4 it can be concluded that, as expected, the lowest-lying triplet (HlCCl)+ species is H,CCl+ with both hydrogen atoms bonded to carbon. In fact, this species is quite stable relative to Ci3P) + H2Clt(‘A,) (nearly 80 kcal mol-I). The next isomer in stability order is HCClH’ (about 15.5 kcal molY’ higher) and then follows CHCIHS at much higher energy (about 64 kcal mol-’ above H2CClf). The purely electrostatically bonded species C. . . ClH: and Ccl+ . . . Hz lie only slightly below the reactants (2 and 5 kcal mol-I, respectively). It is readily seen in Fig. 4 that production of CClH+(‘A’) as well as of CH+(311) -I-HCl(‘Cl) is endothermic by more than 15 and 12 kcal rnol~-~ ’ , respectively. The endothermicity of these processes should preclude their viability under interstellar conditions. However, production of HCCl+(*A’) is clearly exothermic by more than 40 kcal mol-I, whereas formation of CCl+(311)
Table 4 Total energies (in hartrees) of the reactants, 3 lG** and G 1 levels of theory
points
products
and stationary
for the C(jP) + HQ+(‘A
Species
MP2
Gl
is’)
C(3P) + H2CI+(‘A,) 1 C..-CIH;(3B,) 2 CCI+ H2(3Bz) 3 CHCIHrrf3A’) 4 HCCIH+(‘A) 5 H$CI+ ( 3A”) TSI TS2 TS3 TS4 TS5 HCC1+(2A’) + H CCIH+(‘A’) + H CH+(311) + HCI(‘C+) CCl+(311) + HZ(‘Cg+) Cl(‘P) + CH;(“A,)
-498.17290 -498.17672 -498.17571 -498.19598 -498.28086 -498.30557 -498.18373 --498.22393 -498.24413 -498.23949 -498.09428 -498.23933 -498.13484 -498.16135 -498.17419 -498.21558
-498.33396 -498.33763 -498.34205 -498.35808 -498.43587 -498.46042 -498.34740 -498.40240 -498.40840 -498.40192 -498.27828 -498.40200 -498.30985 -498.31380 -498.34034 -498.36622
2.005 2.005 2.081 2.010 2.012 2.014 2.005 2.189 2.033 2.009 2.033 0.768 + 0.750 0.759 + 0.750 2.002 2.081 0.755 i 0.751
, ) reaction at the MPZ(full)/6-
V.M. Raydn et al./Journal
Structure
of Molecular
(Theochem)
321
363 (1996) 319-331
106.4 2.776
C
39.1
. ,f
\ 1.149
-A” ,c’
\
.
-*
1.280
.
#~ \
‘”
..=
#’ 2.019
TSl DIH=86.4
H 117.3
\ l-Ego
-r\
Cl
’
TS2 C 1.567
1.088 DIH=91.O
H
H
1.086 1.741
\
..
\
TS3
Y DIHz124.8
l;.402
1.464 ‘\\ “”
.=
176.9 1.557
Cl
TS4 DIH-131.7
‘H _ce
1.529 2.165
C
i
Cl ::$-w --
Fig. 3. Optimized structures at the MP2(full)/6-31G** in Angstriims and angles in degrees.
level for the transition
appears to be only slightly exothermic. The fact that the exothermicity of the latter increases at higher levels of theory sugests that this process is very likely to be thermodynamically favored. There
H
---
0.922
TS5
;I
states of the reaction
of C with HZ@.
Distances
are given
is a third competitive process, namely the C1(2P) + CH:(2A1) channel, since it is exothermic by more than 20 kcal molF’. This channel implies elimination of chlorine from the H2CC1+ isomer.
V.M. Ruydn et al.jJournal of Molecular Structure (Theochem)
328
__I..L---
u)
$
343 (19961 319-331
V.M. Ruydn et al.lJournal
of Molecular
Because of the physical conditions in the interstellar medium (low temperature, low density), only exothermic reactions which proceed without activation energy, or at most a very small one, are important processes for the production of interstellar molecules. For this reason, it is of utmost interest to determine not only the energetics but also the activation barriers for possible interstellar reactions. Once the CHClH+ intermediate is formed (a process which does not imply an activation barrier), two different mechanisms may be proposed for the production of HCCl+: (a) conversion into the HCClH+ isomer and direct hydrogen abstraction from this species; (b) further isomerization of HCClH+ into the global minimum on the triplet surface, H*CCl+, followed by elimination of one of the hydrogen atoms. (a)
C + H&l+--+ z
(b)
CHClH+ % HCClH+
HCCl+ + H
C + H$l+
+
CHClH+ z
% H2CC1+ 2
HCClH+
HCCl+ + H
The first step in both cases implies isomerization of the CHClH+ complex into HCClH+ through transition state TS 1. This isomerization involves an energy barrier (from CHClH’) of 6.7 kcal mall’ at the Gl level. However, TSl lies more than 8 kcal mall’ below the reactants and, therefore, the system has enough energy to reach the HCClH+ minimum. In addition, the other transition states involved in paths (a) and (b) (TS2, TS3 and TS4) all lie below C + HZ&. Therefore both paths (a) and (b) are barrier free, indicating that both can take place under interstellar conditions. Which one of the paths is preferred depends essentially on the relative energies of the transition states. Since TS2 (abstraction of a chlorine-bonded hydrogen atom) and TS4 (abstraction of a hydrogen atom from carbon) are very close in energy, there should be no severe preference for any of the two mechanisms. Furthermore, it seems that in fact TS2 and TS4 could even disappear at higher levels of theory. At the MP2(fu11)/6-31G** level both lie above the reaction products (11.1 and 1.3
Structure
(Theochem)
363 (1996) 319-331
329
kcal mall’, respectively), whereas at the Gl level they are nearly isoenergetic with HCCl+, indicating that quite likely there should be no other energy barrier, apart from its endothermicity, for the hydrogen abstraction from both HCClH+ and H&Cl+. For the Ccl+ + H2 channel there are also two possible mechanisms: (c) formation of Ccl. . Hl (structure 2) followed by hydrogen molecule abstraction from this species; (d) the system may fall into the lowest-lying triplet H&Cl+ isomer and the subsequent loss of a hydrogen molecule from this species would lead to Ccl+. The formation of Ccl+ ... H2 involved in path (c) implies a transition state, TS5 in Fig. 3, which lies more than 46 kcal mall’ above the reactants at the MP2(fu11)/6-3 1G ** level. Even if this relative energy is substantially reduced at the Gl level (about 35 kcal mol-’ at this level of theory), it is clear that this process will be subject to a significant barrier which will prevent its viability under interstellar conditions. The presence of this barrier is in agreement with the qualitative discussion at the beginning of this paper. Abstraction of the hydrogen molecule from H&Cl+, however, proceeds without an activation barrier. We have searched for a possible transition state involved in this process, but all optimizations we tried led directly to the dissociation products. Nevertheless, to confirm the absence of any barrier for this process (apart from its endothermicity) we have followed this process in detail. We carried out geometry optimizations for HzCClt at different fixed C-X bond lengths (X being the middle point between both hydrogen atoms) at the MP2 level with the 3-21G* basis set, followed by singlepoint calculations with the 6-31G** basis set, followed by single-point calculations with the 631G** basis set. The results of this scan at the MP2(full)/6-3lG**//MP2(full)/3-2lG* level are shown in Fig. 5, which clearly indicates that there is no transition state involved in the elimination of the hydrogen molecule at both projected and unprojected MP2 levels. A similar conclusion has been reached for the elimination of chlorine from H,CCl+. Since all searches for a transition state associated to this process led to C1(2P) + CHz(2AI) , we followed
V.M.
330 -498.16 2
Ray&
et al./Journal
of Molecular
1
Structure
I agaIIaa
-498.18
zl -498.2 0 -498.22 i
I
-498.24 t
0
-498.26 t 0
-498.28
0
MP2
f!
-498.3 -498.32
~ 0
1
2
4
3
5
r(c-x)/A Fig. 5. Energy profile at the MP2(fu11)/6-31G**//MP2(fu11)/321G* level for hydrogen molecule abstraction from triplet HzCClt to give CClf(3fI) + H,(‘Cg+), with respect to the CX distance (X being the middle point between both hydrogen atoms).
the same procedure as for elimination of a hydrogen molecule. The results of the scan for different Cl-C distances are depicted in Fig. 6, which clearly shows the absence of an activation energy for this process. -498.2 2 :
-498.22
-498.24
0
-498.26
a -498.28
0 0
-498.3
-
.
0 -498.32 1.5
MP2
0 PMP2
I
I
I
I
I
I
I
2
2.5
3
3.5
4
4.5
5
5.5
(Theochem)
363
(1996)
319-331
4. Conclusions An ab initio study of the molecular structure of the different triplet (H&Cl)+ species has been carried out. The lowest-lying triplet is the H2CClf species, which has a pyramidal structure. A nonplanar HCClH+ species follows in stability order, lying about 15.5 kcal mol-’ above H$Cl+. The C,, approach of carbon toward H$l+ results in species of very low stability, whereas the approach toward a hydrogen atom is more favorable. The latter approach gives an unusual CHClH+ species, with a hydrogen atom acting as a bridge between carbon and chlorine atoms, which lies more than 15 kcal molP1 below C + H&l+, but lies more than 64 kcal molP1 higher than H$Cl+. The reaction of carbon atoms with H&l+ ions has been studied as a possible reaction of interest in interstellar chlorine chemistry. One of the two previously assumed channels, production of CH+ + HCl, is shown to be clearly endothermic and, therefore, quite unlikely to be feasible under interstellar conditions. The second one, namely production of Ccl+ + HZ, is predicted to be slightly exothermic (about 4 kcal molP1 at the Gl level) and to proceed without an activation barrier. Nevertheless, there are two other competitive processes which should be predominant: production of Cl + CHZ and HCCl+ + H. The exothermicity of these processes is 20.3 and 42.6 kcal molP’, respectively, at the Gl level. For the production of Cl + CH: the system must fall into the HzCCl+ minimum, which proceeds through transition states below the reactants, and then fragmentation of chlorine takes place without an activation barrier. For the synthesis of HCCl+ there are two different barrier-free mechanisms: hydrogen abstraction from HCClH+ and from H&Cl+. Therefore, the reaction of carbon atoms with H$l+ is predicted to lead to the HCCl+ ion, as a major product, under interstellar conditions. This cation may be a precursor of interstellar CC1 through dissociative recombination.
r(CI-Q/A Fig. 6. Energy profile at the MP2(fu11)/6-31G**//MP2(fu11)/321G* level for chlorine abstraction from triplet H&Cl+ to give CI(‘P) + CH:(‘At), at different Cl-C distances.
Acknowledgment This
research
has
been
supported
by
the
V.M. Ray& et al./Journal of Molecular Structure (Theochem)
Ministerio de Educacicin y Ciencia of Spain (DGICYT, Grants PB9 l -0207-CO2-02 and PB94- 1314CO3-02).
References [l] W.W. Duley and D.A. Williams, Interstellar Chemistry, Academic Press, London, 1984. [2] B.E. Turner, Space Sci. Rev., 51 (1989) 235. [3] D. Smith, Chem. Rev., 92 (1992) 1473. [4] G.A. Blake, V.G.Anicich and W.T. Huntress, Astrophys. J., 300 (1986) 415. [5] J. Cernicharo and M. Guehn, Astron. Astrophys., 183 (1987) LlO. [6] E.F. van Dishoeck, M.C. Hemert and A. Dalgarno, J. Chem. Phys., 77 (1982) 3693. [7] V.G. Anicich, W.T. Huntress and J. Futrell, Chem. Phys. Lett., 40 (1976) 233. [8] V.G. Anicich and W.T. Huntress, Astrophys. J. Suppl., 62 (1986) 553. [9] C. Rebrion, J.B. Marquette, B.R. Rowe and D.C. Clary, Chem. Phys. Lett., 143 (1988) 130. [lo] D.M. Sonnenfroh and J.M. Farrar, Astrophys. J., 335 (1988) 491. [ll] C.E. DateoandDC. Clary, J.Chem.Phys.,90(1989)7216. [12] C. Barrientos, A. Largo, P. Redondo, F. Pauzat and Y. Ellinger, J. Phys. Chem., 97 (1993) 173. [13] M. Gruebele, M. Polak, G.A. Black and R.J. Saykally, J. Chem. Phys., 85 (1986) 6276.
363 (1996) 319-331
331
P.M.W. Gill, v41 M.J. Frisch, G.W. Trucks, M. Head-Gordon, M.W. Wong, J.B. Foresman, B.G. Johnson, H.B. Schlegel, M.A. Robb, ES. Replogle, R. Gomperts, J.L. Andres, K. Raghavachari, J.S. Binkley, C. Gonzalez, R.L. Martin, D.J. Fox, D.H. DeFrees, J. Baker, J.J.P. Stewart and J.A. Pople, GAUSSIAN 92, Gaussian Inc., Pittsburgh, PA, 1992. and J.A. Pople, Theor. Chim. Acta, 28 [I51P.C. Hariharan (1973) 213. W.J. Pietro, W.J. Hehre, J.S. Binkley, [I61M.M. Francl, M.S. Gordon, D.J. DeFrees and J.A. Pople, J. Chem. Phys., 77 (1982) 3654. Int. J. Quantum Chem., 14 [I71J.A. Pople and R. Krishnan, (1978) 91. U81 R. Krishnan, M.J. Frisch and J.A. Pople, J. Chem. Phys., 72 (1980) 4244. J.S. Binkley, R. Seeger and J.A. Pople, [I91R. Krishnan, J. Chem. Phys., 72 (1980) 650. J. Chem. Phys., 72 WI A.D. McLean and G.S. Chandler, (1980) 5639. D.J. Fox, K. Raghavachari I211J.A. Pople, M. Head-Gordon, and L.A. Curtis, J. Chem. Phys., 90 (1989) 5622. WI H.B. Schlegel, J. Chem. Phys., 84 (1986) 4530. [231H.B. Schlegel, J. Chem. Phys., 92 (1988) 3075. [241D.J. DeFrees and A.D. McLean, J. Chem. Phys., 82 (1985) 333. WI R.F.W. Bader, Atoms in Molecules. A Quantum Theory, Clarendon Press, Oxford, 1990. R.F.W. Bader and T.H. Tang, WI F.W. Biegler-Konig, J. Comput. Chem., 27 (1980) 1924. u71 D. Cremer and E. Kraka, Croat. Chem. Acta, 57 (1984) 1259.
Reaction of carbon atoms with H2C1+: an ab initio study of a possible interstellar process Victor M. Ray&,
Carmen
Barrientos’,
Antonio
Large”*
Departamento de Quimica Fisica J Analitica, Facultad de Quimica, Universidad de Oviedo, 33006 Oviedo, Spain
Received 21 July 1995; accepted IO November 1995
Abstract An ab initio study of the reaction of carbon atoms with H2C1+ ions has been carried out at the Gl level. The (H&Cl)+ species on the triplet surface have been characterized and their molecular structure discussed. It is found that the approach through a hydrogen atom is favored over approach toward chlorine. The former leads in a first step to a CHClH+ species, which finally leads to the more stable HCClH+ and H&Cl+ species. These isomers are quite stable
relative to the reactants, about 64 and 79 kcal mol-‘, respectively. Contrary to what was previosuly assumed. the CH’ + HCl channel is not possible in the interstellar medium, since it is clearly endothermic, whereas production of Ccl+ + Hz is only slightly exothermic. We predict that the dominant channels should be the production of HCCl+ + H and of Cl + CH:, which are both exothermic (by more than 42 and 20 kcal mol-‘, respectively, at the Gl level) and proceed without an activation barrier. The HCCl+ ion may be a precursor of interstellar CC1 through dissociative recombination. Keword.s: Ab initio calculation; Chlorine; Interstellar chemistry;
1. Introduction Until quite recently, most of the known interstellar molecules consisted of first-row elements and only a few sulfur and silicon compounds had been detected. Nowadays, the number of molecules containing second-row elements has increased significantly [l-3]. Furthermore, it is expected that once the gas-phase chemistry of second-row elements is understood, other molecules could be detectable in space.
* Corresponding author. ’ Present address: Departamento de Quimica Fisica, Facultad de Ciencias. Universidad de Valladolid, 47005 Valladolid, Spain.
Ion-molecule
reaction;
Molecular
structure
Little is known about the interstellar chemistry of chlorine. As one of the more volatile second-row elements, its abundance in dense clouds is expected to remain high. Laboratory studies of ionmolecule reactions involving chlorine, together with kinetic modeling, indicate that chlorine should be present mainly as atomic chlorine and in the form of HCl [4]. So far, HCl is the only chlorinecontaining molecule detected in the interstellar medium, whereas a number of metal chlorides (NaCl, AlCl and KCl) have been observed in circumstellar shells [5]. Nevertheless, there are indications that carbon-chlorine compounds could be produced under interstellar conditions. One of the major depletion mechanisms for HCl is the reaction with C+ [6]. Laboratory studies
0166-1280/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0166-1280(95)04451-5
320
V.M. Ray& et al./Journal of Molecular Structure (Theochem)
[7-91have determined reaction
the rate constant
C++HCl+CCl++H
for the
(1)
and its dynamics studied by crossed-beam experiments [lo]. In addition, a theoretical calculation of the rate constant has been carried out [l 11, as well as an ab initio study of the energetics of this reaction [12]. One of the most important conclusions of these studies is that production of Ccl+ from the reaction of C+ with HCl is feasible in the interstellar medium. From the study of Blake et al. [4] it is clear that Ccl+ is a relatively stable species, exhibiting reactivity with a few interstellar molecules (NHs, H2CO) and oxygen atoms. The dominant depletion mechanism in dense interstellar clouds would be the reaction with atomic oxygen and recombination with electrons. These characteristics, as pointed out by Gruebele et al. [13], suggest that detection of Ccl+ could provide information about not only the chlorine chemistry but also the chemical constitution of dense clouds. Nevertheless, there are other possible synthetic routes, apart from (1) for the production of Ccl+. In their study of the chemistry of chlorine in dense interstellar clouds, Blake et al. [4] included the reaction of C with H2C1+ as one of the most significant reactions. H2Cl+ is produced from protonation of HCl. HCl + H: + H&l+
+ H2
(2)
which is found to proceed with a rate constant of 3.5 x 10e9 cm3 s-i [4]. There is another synthetic route initiated by chlorine atoms [4] Cl+H+HC1++H2 HCl+ + H2 4 H&l+
(3) + H
(4)
but their rate constants have not been measured. For the reaction of carbon atoms with protonated hydrogen, Blake et al. [4] assumed two reaction channels, C + H2Cl+ + Ccl+
+ Hz
(5)
C + H2Cl+ -+ HCl + CH+
(6)
one of them leading to Ccl+. However, there is, in principle, a third possible channel corresponding to
363 (1996) 319-331
elimination
of a hydrogen
C + H$l+
+ (HCCl)+
atom: + H
(7)
This channel is particularly interesting since subsequent dissociative recombination of (HCCl)+ could lead to neutral Ccl: (HCCl)+
+ e- + CC1 + H
(8)
The purpose of this paper is to carry out a theoretical study of the reaction of carbon atoms with H2C1+, in order to ascertain whether this process is feasible under interstellar conditions and therefore can serve as a source of carbon-chlorine compounds in space.
2. Computational
methods
Ab initio molecular orbital computations were carried out using the program code GAUSSIAN 92 v41. Equilibrium geometries were obtained using second-order Msller-Plesset theory (MP2) with the split-valence plus polarization 6-3 lG** basis set [15,16]. All electrons were included in these MP2 calculations (“full” electron correlation). Harmonic vibrational frequencies have been estimated at the MP2(fu11)/6-31G* level using analytical derivatives. Electronic energies have been computed using fourth-order Marller-Plesset (MP4) theory [ 17,181 with the MC-311G** basis set, which is made up from the 6-3 11G basis set [ 191 for carbon and hydrogen and the McLean and Chandler basis set for chlorine [20], supplemented with polarization functions. In these calculations the “frozen-core” approximation was employed (inner-shell molecular orbitals were not included for computing correlation energies). Additional calculations employing a slightly modified version of the socalled Gl method [21] were carried out. Gl theory is a composite procedure which assumes that some higher-level corrections are additive. To overcome limitations of the basis set, MP4 calculations with diffuse functions, using the MC-31 l+G** basis set, and with additional polarization functions, employing the MC-31 lG(2df,p) basis set, were carried out. Since we will be dealing with open-shell
V.M. Raydn et aLlJournal of Molecular
321
Structure (Theochem) 363 (1996) 319-331
states, instead of substracting the MP4/MC311G** values from the MP4/MC-311+G** and MP4/MC-31 lG(2df,p) ones, as in the original Gl method, we use the projected MP4 (PMP4) values in order to avoid spin-contamination problems [22,23]. Quadratic configuration interaction calculations at the QCISD(T) level were also carried out to account for deficiencies in the MollerPlesset method. The last modification with respect to the original Gl method is the use of scaled [24] MP2(fu11)/6-3 1G* vibrational frequencies instead
of the HF/6-31G* ones to estimate zero-point vibrational energy (ZPVE) corrections. Finally, a higher-level correction is included in Gl theory, AE(HLC) = -O.l9n, - 5.95n,, where no and na are the number of (Y and /3 valence electrons, respectively, and AE(HLC) is in millihartrees. A detailed analysis of the bond-breaking processes showed that consideration of singleconfiguration-based methods is not an important limitation in this case. The characteristics of the binding for the 0.734
H-H
H
(‘c,+)
1.293 Cl
/
\
3
95.9
1.556
Cl
C
(‘Al)
H
C-
1.117
H
(3m
(31-o
1.268
Cl
H
($+)
C H 1.298 ‘O’-Or
J
C
(2A’)
/ Cl
1.826
H 131.8 1.099
\
7
fi
Cl
b
t2A’)
1.548 Fig. 1. Optimized structures at the MP2(fu11)/6-31G** given in Angstriims and angles in degrees.
level for the reactant
and products
of the reaction
of C with HlCl+.
Distances
are
V.M. Raydn et al./Journal of Molecular Structure (Theochem)
322
products and intermediates of the reaction of C with H2Cl+ have been analyzed by means of Bader’s topological analysis [25] of the electronic charge density, p(r), using the AIMPAC series of programs [26]. We have employed the MP2/MC31 lG** wavefunctions to build up the electron density and compute several properties of bond critical points, such as p(yc), V2p(rc), and the energy density H(Y,). The energy density at the bond critical point indicates that a bond is covalent if H(r,) < 0 and ionic if H(r,) > 0 [27]. 3. Results and discussion Starting from the reactants in their ground states, the reaction of carbon atoms with H2Cl+ should take place, in principle, on the triplet surface. Since charge transfer is not a competitive process (C+ + H2Cl lie about 121 kcal mol-’ above the reactants at the Gl level), we have considered the following processes: ,+ CCl+(311) + H*(‘Cg+) HCI(‘C+) C(3P)
(9)
The MP2(fu11)/6-31G** optimized geometries of the reactants and products are shown in Fig. 1, and the MP2(fu11)/6-31G* frequencies are given in Table 1. Fig. 2 shows the geometrical parameters of the triplet (H2CCl)+ species and the corresponding MP2(fu11)/6-31G* vibrational frequencies are given in Table 2. The approach of carbon toward chlorine could take place, in principle, along C2, symmetry, and C(3P) splits into three spatially degenerate states corresponding to 3B2 (. . . al1 b2’), 3B1 (. . . al1 b, ‘) and 3A2 (. b2’ bt ‘) symmetries. Nevertheless, the interaction of any of these states with H2Cl+ (with two electron pairs occuping al and bt molecular orbitals) results in at least a three-electron
Table 2 MP2/6-31G* harmonic vibrational triplet (H2CC1)+ species
L
+ CH;(2A,)
(HCCl)+(‘A’)
vibrational
frequencies
Vibrational
H$I+(‘A,)
Cl-H Cl-H bend
asym stretch sym stretch
2861 2853 1306
H2C1 Cp’)
H-H
stretch
4536
ccI+(
CC1
stretch
1165
C-H
stretch
2919
CH+(3TI)
’ C’)
CILH stretch
3048
CClH+(‘A’)
Cl-H bend C-Cl
stretch
2781 768 547
HCCl+( ‘A’)
CH stretch C-Cl stretch bend
3208 1219 989
CH:(2A,)
C-H asym stretch C-H sym stretch bend
3395 3143 1015
stretch
Frequency
br b2 a, b2 a, aI
Out-of-plane bend CIH2 rock Cl-H, sym stretch Cl-H2 asym stretch CC1 stretch H-H stretch
41 61 151 206 1162 4494
3 CHClH+(3A’)
a’ a’ a” a’ a’ a’
H-Cl stretch CHCl bend Out-of-plane bend HClH bend C-H stretch Cl&H stretch
362 468 715 1014 1301 2930
4 HCCIH+(3A)
HCCIH bend (torsion) CC1 stretch HClC bend CICH bend Cl-H stretch C-H stretch
350 743 907 1050 2788 3281
5 H2CCl+(3A”)
a’ a’ a” a’ a’ a’
608 717 841 1341 3100 3254
(12) H2(3B2)
of the
Frequency
Species
%I)
mode
(cm-‘)
of the
142 i 109 163 1302 2849 2858
2 Ccl+. Table 1 MP2/6-3lG* harmonic reactant and products
mode
(cm-‘)
ClH2 rock CC1 stretch Out-of-plane bend ClH2 bend CIH2 sym stretch ClH2 asym stretch
C1H;(3Bz)
(11)
+ H(‘S)
frequencies
b2 at br a, a, b2
1 C.
+ H2Cl+(‘A,) Cl(‘P)
HCI(
Vibrational
Species
(lo)
+ CH+(‘II)
363 (1996) 319-331
CH2 wag Cl-C stretch Asym bend Sym bend CH2 sym stretch CH2 asym stretch
V.M. Ray& et al./Journal of Molecular
Structure (Theochem)
2.993
__--
1.557
Cl _‘-‘,‘f
C
e-
I ---_
--
’
13B2)
2
t3B2)
323
H 0.736
;4:1- ---_
363 (1996) 319-331
H
176.8
c
4-c
1.284
1.364
98.2
3
(3A’)
4
t3A)
\
H
1.737 (
Cl
Fig. 2. Optimized angles in degrees.
structures
ly&f+f; _-C
1.698
at the MP2(fu11)/6-31G**
5 (‘A”)
1.088
level for the triplet (H*CCl)+
species. Distances
are given in Angstriims
and
324
V.M. Ray& et al./Journal of Molecular Structure (Theochem)
repulsive interaction, and therefore it is expected that high energy barriers should be encountered along this path. The 3A2 state is repulsive and dissociated into C(3P) + H2Cl+(‘Al). In the case of the 3B2 and 3Bi states we have found stationary points at long C-Cl distances, the 3B2 state lying lower in energy than the 3B1 state. For this reason we will only discuss the 3B2 state, which corresponds to structure 1 in Fig. 2. From the geometrical parameters shown in Fig. 2 this can be viewed as an atom-molecular ion complex with an electrostatic bond, C . . ClHl. The C1H2 moiety preserves essentially the geometry of the H2Cl+ cation (shown in Fig. l), whose vibrational frequencies are easily recognized in the vibrational frequencies of structure 1. Nevertheless, as can be seen in Table 2, structure 1 is not a true minimum since it has an imaginary bz frequency (the same is observed for the corresponding stationary point of the 3Bi electronic state; furthermore, inclusion of polarization functions for hydrogen atoms has no effect on this feature, since both states still have an imaginary frequency at the MP2(fu11)/6-3 1G** level). We have tried to find a true minimum following this mode, but all attempts collapsed into structure 3 (see below). However, a true minimum of C,, symmetry (3B2 electronic state) can be reached, after pushing over a barrier, in the region of small C-Cl distances. The geometrical parameters of this minimum (structure 2 in Fig. 2) suggest that this structure can be described as a Ccl+. . Hz complex, since both C-Cl and H-H distances are almost identical to those found for the fragments at the same level of theory (see Fig. 1). Furthermore, the corresponding CC1 and H-H stretching frequencies are nearly the same as those found for CClf(311) and H2(’ C,‘). On the contrary, the symmetric and asymmetric stretching frequencies are very small (with values of 151 and 206 cm-‘, respectively). The approach of the carbon atom to H*Cl’ could also take place toward one of the hydrogen atoms. In fact, the atomic charges of chlorine and hydrogen in H2Clf are nearly the same (+0.332 for Cl and +0.334 for H from Mulliken population analysis), and there should be no electrostatic preference for the approach toward chlorine or one of the hydrogen atoms. This approach results in the
363 (1996) 319-331
rather unusual planar structure labelled 3 in Fig. 2, which corresponds to a 3A’ electronic state and shows C-H and H-Cl bond distances which are both lengthened compared with the corresponding distances in CH+ (1.117 A) and H$l+ (1.293 A). In fact, bothOdistances are nearly equally increased (about 0.25 A). It is also interesting to note that the / CHCl bond angle is close to180”, indicating an almost linear arrangement for these atoms. There is also, of course, a 3A” state which lies higher in energy than the 3A’ state and which has a similar optimized geometry, the main difference being the C-H bond distance which is 1.372 A for the 3A” state, the other geometrical parameters being the same as for the 3A’ state. We shall refer in what follows to the 3A’ state. Since the unpaired electrons for structure 3 are mainly located at the carbon atom (the spin density at carbon is 1.824, whereas that of chlorine is 0.155) the interaction with carbon is established through its lone pair. The partial donation of electronic charge from carbon toward the H2C1+ unit results in atomic charges of $0.382 for carbon, +0.168 for the hydrogen atom bonded to carbon, +0.150 for chlorine, and f0.300 for the other hydrogen atom (which remains nearly unchanged). These values, as well as the bond distances, suggest that the interaction must have a certain degree of covalency, which is also supported by the relatively high bond energy of 15 kcal mall’ (see below, Fig. 4) which is certainly greater than typical hydrogen bonds. We have also studied the structure resulting from insertion of carbon into an H-Cl bond, structure 4 in Fig. 2. This HCClH+ isomer deviates significantly from planarity (the dihedral angle is 97.8”) and shows a relatively long C-Cl bond distance which is closer to that found in CClH+ than to the value found in HCCl+. This is also reflected in the C-Cl stretching frequency which is close to that of CClH+. Both unpaired electrons are located at carbon and the bonding in this species is intermediate between an electrostatic interaction (HC+ + ClH) and a covalent one (with significant dative bonding from one of the chlorine lone pairs to carbon). Finally, migration of the hydrogen bonded to chlorine in structure 4 results in triplet H&Cl+.
V.M. Raydn et al./Journal of Molecular Structure (Theochem)
In Czv symmetry the lowest-lying state is 3A2, but it shows an imaginary frequency for out-of-plane bending. Following this mode a pyramidal structure is obtained, corresponding to a 3A” electronic state (structure 5 in Fig. 2). Pyramidalization is obviously due to the unpaired electron occupying a b, molecular orbital which is essentially a carbon 2p orbital (the other unpaired electron is located at chlorine, which formally bears the positive charge). The C-Cl bond distance is much longer than that found for the HCCl+ species (see Fig. l), as a consequence of the absence of the favorable interaction due to delocalization of the electrons occupying the 3p,(Cl) orbital into the vacant 2pJC) orbital (which is now occupied by a single electron in structure 5) which contributes to strengthening the C-Cl bond in HCCl+. The 3A’ state corresponding to the same connectivity lies higher in energy (about 15 kcal mall’ at the Gl level). The most interesting results from the topological analysis of the electronic charge density are collected in Table 3. We provide the bond length and the electronic density (p), Laplacian of p and energy density at the bond critical point for the C Cl bond in the different species (except in the case of structure 3, where the properties of the C-H and H-Cl bonds are given). The values shown in Table 3 confirm the previous qualitative discussion about the bonding in these species. Thus, structure 1, C. ClH:, is the only species which shows the characteristics of a C-Cl ionic bond, since H(r,)
Table 3 Bond properties
of the C-Cl
ccl+(sP) HCCl+(*A’) CClH+(*A’) 1 C...CIH’(3B2) 2 CCI’ H2(3Bz) 3 CHCIH+(3A’) C-H H-Cl 4 HCCIH+(‘A) 5 H2CCI+(3A”)
bonda for Ccl’,
(CHCI)+
363 (1996) 319-331
325
takes a positive value [27]. The positive value of V*p(r,) is also indicative of a closed-shell interaction [25] between carbon and the ClHi unit. In all other cases the negative values of H(r,) indicate that some covalency is enhancing the binding. Structure 2 has electronic charge density properties very similar to those found for Ccl+, indicating that essentially the same C-Cl bond is already present in that species. On the contrary both structures 4 and 5 show properties for the C-Cl bond which suggest that some changes should still occur in the C-Cl electron density distribution to reach the HCCl+ product. A particularly interesting case is structure 3, since it has an unusual bonding through a hydrogen atom. The values of H(r,) for the C-H and HCl bonds in this species support the covalent character of these bonds. Furthermore, the negative values of 2p(rc) confirm a shared rather than a closed-shell interaction in both cases. In this respect we should recall that hydrogen bonds are characterized by small values of p(r,) and positive values of V2p(rc) [25]. Nevertheless, the (X,(/X3 ratio, Xi being the highest (in absolute value) of the two negative curvatures and X3 the positive curvature at the bond critical point, is lower than unity for both C-H and H-Cl bonds in CHClH+. The values of the ratio 1X11/X3for typical shared interactions are normally greater than unity, denoting that perpendicular contractions are dominating the binding region [25]. However, in typical ionic interactions the values of IX,I/X, are
and (H2CCI)’
speciesb
R
p(rd
v*P(rC)
H(rd
1x1I/X3
1.556 1.548 1.826 3.321 1.557 1.364 1.564 1.737 1.698
0.2749 0.2933 0.1641 0.0070 0.2743 0.1344 0.1234 0.1943 0.2108
-0.4964 -0.6525 -0.1531 0.0256 -0.4928 -0.2138 -0.1306 -0.3064 -0.3881
-0.2768 -0.3126 -0.1095 0.0014 -0.2757 -0.0915 -0.0771 -0.1610 -0.1979
1.4038 1.7284 0.7796 0.1405 1.3912 0.7946 0.6811 1.0993 1.3575
a In the case of the CHClH+ species the bond properties of the C-H b R is in Angstroms. p(r,), V’p(r,) and H(r,) are in atomic units.
and H-Cl
bonds are given.
326
V.M. Raydn et al.lJournal of Molecular Structure (Theochem)
363 (1996) 319-331
less than unity, denoting that the contraction of p toward the nuclei is dominating the binding region. Therefore, the C-H and H-Cl bonds of CHClH+ exhibit characteristics of covalency (negatives values of 2p(r,) and H(r,)), as well as of ionic interactions (small values of p(r,), IX,I/X, ratios smaller than unity), and one could classify them as corresponding to intermediate interactions. There is also a compound with a C-Cl bond where the IX,i/X, ratio is less than unit (apart from structure 1 which has an ionic C-Cl bond), namely CClH’(2A’), which together with its low value of p(r,) (the smallest of all C-Cl bonds with the exception of structure 1) suggests a high ionic character for this bonding. In fact, this compound can be viewed as the Cf . . . ClH ion-molecule complex [ 121. Once we know the structure of the triplet (H,CCl)+ species we can discuss the reaction of ground-state carbon atoms with H2CI’. The total energies at the MP2(full)~6-3lG*~ and Cl levels of the reactants, products, intermediates and transition states involved in this reaction are collected in Table 4, along with the S* expectation values. As can be seen, spin contamination is very low for most of the species, the worst case being transition state TS2 for which {S”) = 2.189. Therefore, MP theory should be quite reliable for computation of electronic energies on the (H2CCl)+ triplet surface,
especially when using projected MP values. The geometries of the transition states are given in Fig. 3. For all transition states we checked that the imaginary frequency corresponds to the vibrational mode associated with the desired reaction path. Finally, the relative energies at the Gl and MP2(full)/6-31G** (in parentheses) levels for all species involved in the reaction are shown in Fig. 4, From the energies shown in Table 4 it can be concluded that, as expected, the lowest-lying triplet (HlCCl)+ species is H,CCl+ with both hydrogen atoms bonded to carbon. In fact, this species is quite stable relative to Ci3P) + H2Clt(‘A,) (nearly 80 kcal mol-I). The next isomer in stability order is HCClH’ (about 15.5 kcal molY’ higher) and then follows CHCIHS at much higher energy (about 64 kcal mol-’ above H2CClf). The purely electrostatically bonded species C. . . ClH: and Ccl+ . . . Hz lie only slightly below the reactants (2 and 5 kcal mol-I, respectively). It is readily seen in Fig. 4 that production of CClH+(‘A’) as well as of CH+(311) -I-HCl(‘Cl) is endothermic by more than 15 and 12 kcal rnol~-~ ’ , respectively. The endothermicity of these processes should preclude their viability under interstellar conditions. However, production of HCCl+(*A’) is clearly exothermic by more than 40 kcal mol-I, whereas formation of CCl+(311)
Table 4 Total energies (in hartrees) of the reactants, 3 lG** and G 1 levels of theory
points
products
and stationary
for the C(jP) + HQ+(‘A
Species
MP2
Gl
is’)
C(3P) + H2CI+(‘A,) 1 C..-CIH;(3B,) 2 CCI+ H2(3Bz) 3 CHCIHrrf3A’) 4 HCCIH+(‘A) 5 H$CI+ ( 3A”) TSI TS2 TS3 TS4 TS5 HCC1+(2A’) + H CCIH+(‘A’) + H CH+(311) + HCI(‘C+) CCl+(311) + HZ(‘Cg+) Cl(‘P) + CH;(“A,)
-498.17290 -498.17672 -498.17571 -498.19598 -498.28086 -498.30557 -498.18373 --498.22393 -498.24413 -498.23949 -498.09428 -498.23933 -498.13484 -498.16135 -498.17419 -498.21558
-498.33396 -498.33763 -498.34205 -498.35808 -498.43587 -498.46042 -498.34740 -498.40240 -498.40840 -498.40192 -498.27828 -498.40200 -498.30985 -498.31380 -498.34034 -498.36622
2.005 2.005 2.081 2.010 2.012 2.014 2.005 2.189 2.033 2.009 2.033 0.768 + 0.750 0.759 + 0.750 2.002 2.081 0.755 i 0.751
, ) reaction at the MPZ(full)/6-
V.M. Raydn et al./Journal
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321
363 (1996) 319-331
106.4 2.776
C
39.1
. ,f
\ 1.149
-A” ,c’
\
.
-*
1.280
.
#~ \
‘”
..=
#’ 2.019
TSl DIH=86.4
H 117.3
\ l-Ego
-r\
Cl
’
TS2 C 1.567
1.088 DIH=91.O
H
H
1.086 1.741
\
..
\
TS3
Y DIHz124.8
l;.402
1.464 ‘\\ “”
.=
176.9 1.557
Cl
TS4 DIH-131.7
‘H _ce
1.529 2.165
C
i
Cl ::$-w --
Fig. 3. Optimized structures at the MP2(full)/6-31G** in Angstriims and angles in degrees.
level for the transition
appears to be only slightly exothermic. The fact that the exothermicity of the latter increases at higher levels of theory sugests that this process is very likely to be thermodynamically favored. There
H
---
0.922
TS5
;I
states of the reaction
of C with HZ@.
Distances
are given
is a third competitive process, namely the C1(2P) + CH:(2A1) channel, since it is exothermic by more than 20 kcal molF’. This channel implies elimination of chlorine from the H2CC1+ isomer.
V.M. Ruydn et al.jJournal of Molecular Structure (Theochem)
328
__I..L---
u)
$
343 (19961 319-331
V.M. Ruydn et al.lJournal
of Molecular
Because of the physical conditions in the interstellar medium (low temperature, low density), only exothermic reactions which proceed without activation energy, or at most a very small one, are important processes for the production of interstellar molecules. For this reason, it is of utmost interest to determine not only the energetics but also the activation barriers for possible interstellar reactions. Once the CHClH+ intermediate is formed (a process which does not imply an activation barrier), two different mechanisms may be proposed for the production of HCCl+: (a) conversion into the HCClH+ isomer and direct hydrogen abstraction from this species; (b) further isomerization of HCClH+ into the global minimum on the triplet surface, H*CCl+, followed by elimination of one of the hydrogen atoms. (a)
C + H&l+--+ z
(b)
CHClH+ % HCClH+
HCCl+ + H
C + H$l+
+
CHClH+ z
% H2CC1+ 2
HCClH+
HCCl+ + H
The first step in both cases implies isomerization of the CHClH+ complex into HCClH+ through transition state TS 1. This isomerization involves an energy barrier (from CHClH’) of 6.7 kcal mall’ at the Gl level. However, TSl lies more than 8 kcal mall’ below the reactants and, therefore, the system has enough energy to reach the HCClH+ minimum. In addition, the other transition states involved in paths (a) and (b) (TS2, TS3 and TS4) all lie below C + HZ&. Therefore both paths (a) and (b) are barrier free, indicating that both can take place under interstellar conditions. Which one of the paths is preferred depends essentially on the relative energies of the transition states. Since TS2 (abstraction of a chlorine-bonded hydrogen atom) and TS4 (abstraction of a hydrogen atom from carbon) are very close in energy, there should be no severe preference for any of the two mechanisms. Furthermore, it seems that in fact TS2 and TS4 could even disappear at higher levels of theory. At the MP2(fu11)/6-31G** level both lie above the reaction products (11.1 and 1.3
Structure
(Theochem)
363 (1996) 319-331
329
kcal mall’, respectively), whereas at the Gl level they are nearly isoenergetic with HCCl+, indicating that quite likely there should be no other energy barrier, apart from its endothermicity, for the hydrogen abstraction from both HCClH+ and H&Cl+. For the Ccl+ + H2 channel there are also two possible mechanisms: (c) formation of Ccl. . Hl (structure 2) followed by hydrogen molecule abstraction from this species; (d) the system may fall into the lowest-lying triplet H&Cl+ isomer and the subsequent loss of a hydrogen molecule from this species would lead to Ccl+. The formation of Ccl+ ... H2 involved in path (c) implies a transition state, TS5 in Fig. 3, which lies more than 46 kcal mall’ above the reactants at the MP2(fu11)/6-3 1G ** level. Even if this relative energy is substantially reduced at the Gl level (about 35 kcal mol-’ at this level of theory), it is clear that this process will be subject to a significant barrier which will prevent its viability under interstellar conditions. The presence of this barrier is in agreement with the qualitative discussion at the beginning of this paper. Abstraction of the hydrogen molecule from H&Cl+, however, proceeds without an activation barrier. We have searched for a possible transition state involved in this process, but all optimizations we tried led directly to the dissociation products. Nevertheless, to confirm the absence of any barrier for this process (apart from its endothermicity) we have followed this process in detail. We carried out geometry optimizations for HzCClt at different fixed C-X bond lengths (X being the middle point between both hydrogen atoms) at the MP2 level with the 3-21G* basis set, followed by singlepoint calculations with the 6-31G** basis set, followed by single-point calculations with the 631G** basis set. The results of this scan at the MP2(full)/6-3lG**//MP2(full)/3-2lG* level are shown in Fig. 5, which clearly indicates that there is no transition state involved in the elimination of the hydrogen molecule at both projected and unprojected MP2 levels. A similar conclusion has been reached for the elimination of chlorine from H,CCl+. Since all searches for a transition state associated to this process led to C1(2P) + CHz(2AI) , we followed
V.M.
330 -498.16 2
Ray&
et al./Journal
of Molecular
1
Structure
I agaIIaa
-498.18
zl -498.2 0 -498.22 i
I
-498.24 t
0
-498.26 t 0
-498.28
0
MP2
f!
-498.3 -498.32
~ 0
1
2
4
3
5
r(c-x)/A Fig. 5. Energy profile at the MP2(fu11)/6-31G**//MP2(fu11)/321G* level for hydrogen molecule abstraction from triplet HzCClt to give CClf(3fI) + H,(‘Cg+), with respect to the CX distance (X being the middle point between both hydrogen atoms).
the same procedure as for elimination of a hydrogen molecule. The results of the scan for different Cl-C distances are depicted in Fig. 6, which clearly shows the absence of an activation energy for this process. -498.2 2 :
-498.22
-498.24
0
-498.26
a -498.28
0 0
-498.3
-
.
0 -498.32 1.5
MP2
0 PMP2
I
I
I
I
I
I
I
2
2.5
3
3.5
4
4.5
5
5.5
(Theochem)
363
(1996)
319-331
4. Conclusions An ab initio study of the molecular structure of the different triplet (H&Cl)+ species has been carried out. The lowest-lying triplet is the H2CClf species, which has a pyramidal structure. A nonplanar HCClH+ species follows in stability order, lying about 15.5 kcal mol-’ above H$Cl+. The C,, approach of carbon toward H$l+ results in species of very low stability, whereas the approach toward a hydrogen atom is more favorable. The latter approach gives an unusual CHClH+ species, with a hydrogen atom acting as a bridge between carbon and chlorine atoms, which lies more than 15 kcal molP1 below C + H&l+, but lies more than 64 kcal molP1 higher than H$Cl+. The reaction of carbon atoms with H&l+ ions has been studied as a possible reaction of interest in interstellar chlorine chemistry. One of the two previously assumed channels, production of CH+ + HCl, is shown to be clearly endothermic and, therefore, quite unlikely to be feasible under interstellar conditions. The second one, namely production of Ccl+ + HZ, is predicted to be slightly exothermic (about 4 kcal molP1 at the Gl level) and to proceed without an activation barrier. Nevertheless, there are two other competitive processes which should be predominant: production of Cl + CHZ and HCCl+ + H. The exothermicity of these processes is 20.3 and 42.6 kcal molP’, respectively, at the Gl level. For the production of Cl + CH: the system must fall into the HzCCl+ minimum, which proceeds through transition states below the reactants, and then fragmentation of chlorine takes place without an activation barrier. For the synthesis of HCCl+ there are two different barrier-free mechanisms: hydrogen abstraction from HCClH+ and from H&Cl+. Therefore, the reaction of carbon atoms with H$l+ is predicted to lead to the HCCl+ ion, as a major product, under interstellar conditions. This cation may be a precursor of interstellar CC1 through dissociative recombination.
r(CI-Q/A Fig. 6. Energy profile at the MP2(fu11)/6-31G**//MP2(fu11)/321G* level for chlorine abstraction from triplet H&Cl+ to give CI(‘P) + CH:(‘At), at different Cl-C distances.
Acknowledgment This
research
has
been
supported
by
the
V.M. Ray& et al./Journal of Molecular Structure (Theochem)
Ministerio de Educacicin y Ciencia of Spain (DGICYT, Grants PB9 l -0207-CO2-02 and PB94- 1314CO3-02).
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