J. Phys. Chem. 1995,99, 6812-6818
6812
Mechanisms for the Reaction of Triplet Ps with XH (X = C1, F). Formation of the P-X Bond Xabier Lopez,* Mirari Ayerbe, Jesus M. Ugalde,* and Fernando P. Cossio Kimika Fakultatea, Euskal Herriko Unibertsitatea, P.K. 1072, 20080 Donostia, Euskadi, Spain Received: August IO, 1994; In Final Form: January 7, 1995@
Ab initio molecular orbital calculations on the ion-molecule reactions of the triplet P+ with CIH and FH have been performed at the G1*, G2* (G1 and G2 methodologies with MP2/6-31G(d,p) base geometries), MCSCF, and QCISD(T) levels of theory. Two reaction mechanisms leading to PX+ H (X = C1, F) products have been investigated. Both of them proceed through the formation of an ion-molecule complex, in the first step of the reaction, without energy barrier. Reaction pathway I involves the abstraction of hydrogen from the X atom. A significant configuration mixing is found to take place for this abstaction, and we have found that it is energetically unfavored with respect to the reactants. Reaction pathway I1 consists of the transposition of the hydrogen from the X atom to P, to form the HPX+ intermediate, and then abstraction of the hydrogen from P. Reaction pathway 11, which is energetically more favorable than pathway I, could be a likely source of P-CI (X = F, Cl) bond containing compounds in the interstellar media; however, it is predicted to be an unlikely source for P-F compounds.
+
1. Introduction The investigation of ion-molecule reaction mechanisms is of paramount importance in the understanding of the chemistry developed inside dense interstellar clouds. These kinds of processes have been claimed’ to be at the origin of the various chemical species detected in these clouds. It is specially important to determine what exothermic mechanism occurs without energy barrier, since these kinds of chemical processes are the only ones likely to take place under the extreme interstellar conditions of low temperature and density. Detection of the P v and PC3phosphorus compounds in these clouds has renewed interest in the gas phase chemistry of the phosphorus cation. Several experimental studies using cyclotron resonance techniques4 and selected ion flow tube techniques5 of the reaction of P+ and PHZ with various neutrals pointed out that PO would be the most abundant interstellar phosphorus compound. Efficient gas phase synthesis for interstellar PN has also been proposed.6 Theoretical studies concerning the gas phase chemistry of phosphorus have also been published. Thus, Maclagan has reported a theoretical study of some phosphorus compounds? namely, PN, PO, PS, and PCH, along with an ab initio study of the PC,H: ion.8 Recently, Y&iez et al. have published a series of papers concerning the thermochemistry of singlet and triplet phosphorus with elemental molecules?-’2 Our group has also carried out several theoretical studies about the gas phase chemistry of the phosphorus cation in its triplet ground state, with several small common molecules in the interstellar medium such as NH3,OH2, C&, HCN, SH2, and C2.l3-l8 In the present paper we extent our previous studies to ClH and FH molecules; thus, the ion-molecule reactions of triplet P+ with ClH and FH under interstellar conditions are discussed. The volatility of chlorine suggests that its gas phase abundance in the molecular clouds should be high. However, its abundance, although rather high, has been found to be lower than e x p e ~ t e d . ’ ~Photodissociation .~~ of HC1 and the reaction of C+ with C1H have been claimed as the more important degradation mechanisms for ClH.*’ A recent ab initio study supports these considerations.22We shall show in this paper that reaction with @Abstractpublished in Advance ACS Abstracts, April 15, 1995.
0022-3654/95/2099-6812$09.00/0
P+ is also one more likely C1H degradation reaction which decreases the ClH abundance. Theoretical studies concerning the PF and PCl species in their neutral and ionic form can be found in the l i t e r a t ~ r e . ~The ~,~~ ground state for these species has been determined to be a 32for PX and 211for PX+, with ionization potentials of 9.41 and 8.98 eV for PF and PC1, respectively. A remarkable fact is that ionization is found to shrink the P-X bond for both systems. This behavior is in agreement with earlier studies on PF+ and NC1+ species by vacuum ultraviolet photoelectron spe~troscopy,~~ in which a shrinking of the bond is inferred from the spectroscopic data. Studies of the hydrogenated HPF+ species in its neutral26and cationic27species can also be found. For the neutral form, a ,A” ground state has been characterized with PH and PF bonds very similar to the corresponding diatomics. For the cationic species a shrinking of the PF bond is also observed due to an appreciable n donation. 2. Methods
All the structures were optimized at the MP2/6-31G(d,p), CASSCF/6-3lG(d,p), and QCISD(T)/6-31lG(d,p) levels of theory, for the (PClH)+ system (Figure l), whereas for the (PFH)+ system, we optimized at the MP2/6-31G(d,p) and QCISD(T)/6-31 lG(d,p) levels (Figure 2). The active space of the CASSCF wave function is formed by all the valence electrons and orbitals except the highest occupied s orbital of the X atom. The active space formed in this way comprises a total of 1512 C1 symmetry configuration state functions (CSFs). It should be pointed out that we have not been able to fully optimize the TS1 for the (PFH)+ system, at both MPU6-31G(d,p) and CASSCF/6-3 lG(d,p) levels of theory. However, when we go to the QCISD(T)/6-311G(d,p) level, we have got an optimum geometry. In our opinion, this is an effect of the basis set. In order to assess this point further, we carried out an extra calculation, trying to optimize the structure at the QCISD(T)/ 6-31G(d,p) level of theory. We found the same oscillating behavior as for the MP2 and CASSCF methods. A significant configuration mixing was found for the transition state TS1, which implies the abstraction of the hydrogen from the X atom. A plot of the variation of the energy of the three lowest electronic states versus the X-H distance has been calculated 0 1995 American Chemical Society
Reaction of Triplet P+ with
XH (X = C1, F) 2.061 (2.039)
P-
J. Phys. Chem., Vol. 99,No. 18, 1995 6813
c1
c1-
1.281 (1.269) 11.2671
I@ H
P-
l@ I
101.27 (102.35)
1.928 (1.913) r1.9471
(117.86)
P
1.954 ( 1.935 )
(2.283) f2.3221
11.9681
IM
,
JNT
A
8
/ I
**
0
8*t
I
l@
1.629 (1.621) 11.6441
.
88
/'
l@
115.20
H 11.4231
1.578 (1.563) r1.6011
c1
121.59 ,H (123.39) [118.841 ,*'2.080 ,#'
t%\ t t
P'
P
'cl
2.135 (2.087) 12.1621
4' ;;E; 1.965 (1.936) [1.9961 TSl
TS2
Figure 1. Geometries at the QCISD(T)/6-31lG(d,p), MP2/6-3 lG(d,p) (in parentheses), and CASSCF/6-3 lG(d,p) (in brackets) levels of theory for the stationary points of the potential hypersurface of the triplet (PClH)+ species.
P-
1.531 (1.539)
I@ F
1.622
P
( 1.622)
F
-F
0.913 (0.921)
l@
114.08
128.13 (122.28HH0.935 F (0.949) 2.019 (1.958)
l@ '
1.542 (1S49) JNT
LM
l@ H I
, I
135.55
1.559
8
P'
F 1.702 (1.729) TS2
1.610 TSl
Figure 2. QCISD(T)/6-31 lG(d,p) and MP2/6-31G(d,p) geometries, in parentheses, for the stationary points of the potential hypersurface of the triplet (PFH)+ species.
using SA-CASSCF wave functions, in order to show the cross of various electronic state involved in this process (Figure 4 and 5 ) . For the other stationary points, no significant configuration mixing was found and the main configuration always shows a coefficient greater than 0.95. At the CASSCF/6-31G(d,p) geometries, we made a single-point calculation with a better basis set, MC-31 lG(d,p). We will refer to this CASSCF/
MC-311G(d,p)//CASSCF/6-31G(d,p) energy as the CASSCF energy. All the calculations concerning multiconfigurational wave functions were accomplished using the GAMESS package of programs.** At the MP2/6-31G(d,p) geometries, the energies were also refined, by means of the G1 and G2 methodol~gies.*~-~~ We will call these energies the G1* and G2* energies, in order to
Lopez et al.
6814 J. Phys. Chem., Vol. 99, No. 18, I995
10.00
-
:
-+
Pathway I X=F - Pathway I1 X=F
0.00 ‘ P + + X H -10.00 1 -20.00
-30.00 1 -40.00
< IM
INT
-50.00
Figure 3. Energy profiles for reaction pathway I and I1 of (PCIH)+ and (PFH)’ systems, calculated at the QCISD(T)/6-31 l+G(3df,2p)//QCISD(T)/6-31 lG(d,p) level of theory. ZPVE corrections are also included, but they have been evaluated at the MP2/6-311G(d,p) level.
100.00 2
80.00 -
60.00 40.00 -
20*oo 0.00
I
-20.00
1
]
9 3
,P CI-H v,
0
v,
3
c3
e4
9 m
v! m
Figure 4. Potential energy curves for the abstraction of hydrogen from the chlorine atom of the ion-molecule complex PClH+. The avoided crossing is indicated by dashed lines.
150.00
1
o
3A’ 3A”
100.00 50.00
0.00 -50.00
F-H
vf
9
v!
v!
9
9
cr, Figure 5. Potential energy curves for the abstraction of hydrogen from the fluorine atom of the ion-molecule complex PFH+. The avoided crossing is indicated by dashed lines. 0
3
distinguish them from the normal G1 and G2 energies which imply optimizations at the MP2/6-3 1G(d) level. We have considered the MP2/6-31G(d,p) frequencies in order to account for the zero-point vibrational energy (ZPVE) corrections. The G1 and G2 methodologies have been proven to have a high reliability for a huge amount of chemical species. G1 theory gives energies (bond dissociation energies, ionization potentials,
e4
e4
3
electron affinities, and proton affinities) accurate to about f 3 kcal/mol for compounds containing second-row atoms, while G2 theory improves on such figures. In general, we have found a good agreement between the G1* and G2* relative energies; however, for some cases (the TS1 of the (PClH)+ system and the PCl+ H products) the relative energies are very small, but not negligible, and it is not clear whether higher levels of
+
’
J. Phys. Chem., Vol. 99, No. 18, 1995 6815
Reaction of Triplet P+ with XH (X = C1, F)
TABLE 1: Total Energies (in hartrees) and.Relative Energies (in k d m o l ) of the Stationary Points for the (PClH)+ System at the MP4, CASSCF, G1*, G2*, and QCISD(T) Levels of Theory“ species MP4 CASSCF G1* G2* QCISD(T) (9) P+ (3P)
+ ClH (‘2)
PClH+ (3A”) HPC1+ (3A”) TSl (3A”) TS2 (3A”)
+ PCl (3Z) + H+
PCl+ (211) H (2S)
-800.462 542 (0.00) -800.503 924 (-25.97) -800.502 827 (-25.28) -800.435 546 (16.94) -800.464 263 (- 1.08) -800.450 889 (7.31)
-800.665 892 (0.00) -800.716 165 (-31.55) -800.702 904 (- 23.23) -800.635 029 (19.37) -800.672 993 (-4.46) -800.645 386 (12.87) -800.476 556 (1 18.81)
-800.771 (0.00) -800.839 (-42.82) -800.836 (-41.10) -800.775 (-2.43) -800.803 (-20.35) -800.778 (-4.15) -800.614 (98.51)
474 7 13 973 342 905 092 496
-800.773 745 (0.00) -800.839 076 (-41 .00) -800.836 319 (-39.27) -800.774 454 (-0.44) -800.802 927 (- 18.31) -800.775 932 (- 1.37) -800.612 029 (101.48)
-800.749 498 (0.00) -800.814 651 (-40.88) -800.811 616 (-38.98) -800.745 147 (2.73) -800.778 189 (- 18.00) -800.751 407 (- 1.20) -800.587 630 (101.57)
2.008 2.019 2.013 2.119 2.056 0.765
+ 0.750
2.021
Spin contamination is also given. TABLE 2: Total Energies (in hartrees) and Relative Energies (in kcaYmol) of the Stationary Points for the (PFH)+ System at the G1*, G2*, and QCISD(T) Levels of TheorJP
species
G1*
P+ (3P) FH (IZ)
-440.781 020 (0.00) -440.827 627 (-29.25) -440.820 269 (-24.63)
+
PFH+ (3A”) HPF+ (3A”)
G2* -440.783 939
(0.00) -440.829 981 (-28.84) -440.821 227 (-23.40)
TSl (3A”) TS2 (3A”)
-440.766 884 (10.70) -440.766 010 (1 1.25) -440.618 375 (103.89)
-440.765 506 (9.74) -440.766 290 (9.24) -440.619 528 (101.34)
+ PF (3Z)+ H+
PF+ (211) H (2S)
(9)
QCISD(T) -440.757 789
2.008
(0.00) -440.804 (-29.07) -440.796 (-24.11) -440.713 (27.48) -440.740 (10.91) -440.741 (10.49) -440.592 (103.63)
108
2.020
217
2.009
991
2.034
405
2.061
071
0.764
647
2.025
+ 0.750
Spin contamination is also reported. TABLE 3: Bond Properties of the P-X Bond for PX and PXH Systems with X = C1, P PF+ (211) PF (32) PCl+ (TI) PCl(32) (PFH)+ (3A”) (PHF)+ (3A”) (HPF)+ (3A”) (PClH)+ (3A”) (PHCl)’ (3A”) (HPCl)+ (3A”)
1.539 1.622 1.913 2.039 1.958 1.702 1.549 2.283 2.087 1.935
0.1673 0.1388 0.1529 0.1233 0.0690 0.1216 0.1659 0.0750 0.1122 0.1522
1.2618 0.8015 0.1510 -0.0818 0.091 1 0.4677 1.163 0.0066 -0.0742 -0.0009
-0.0722 -0.0668 -0.1398 -0.1057 -0.0341 -0.0738 -0.0760 -0.03 13 -0.0832 -0.1446
0.0978
o.oO0o 0.1873 0.0000 0.0598 0.1381 0.1052 0.0574 0.2782 0.2064
R is in angstroms. e(rc),Ve(r,),H(rc),and E are in atomic units.
theory would change the exo-/endothermicity of these processes. Hence, we decided to perform more accurate calculations, improving the quality of the method and the basis set in the optimization and going beyond the additive assumptions in the refinement of the energetics for the stationary points. Thus, we carried out extra calculations at the QCISD(T)/6-31l+G(3df,2p)//QCISD(T)/6-31lG(d,p) level of theory. We also report the spin contamination, which is negligible for all the stationary points, except for the TS 1 of the (PClH)+ system. The energies calculated in that way will be referred to as the QCISD(T) energies (Tables 1 and 2). All these calculations have been carried out using the GAUSSIAN 92 program package.32 The characteristics of the binding for the various stationary points of the reaction pathway 11have been analyzed by means of the Bader’s topological analysis of @(r)and - v ~ ( r ) , 3using ~ the AIMPAC series of programs.34 We have employed the MP2/6-3 1(d,p)//MP2/6-31G(d) wave functions to build up the electron density. Plots of the Laplacian of the stationary points
for the reaction pathway I1 are depicted in Figure 6. Properties of the bond critical point (rc)such as @(rc),V @ ( r c the ) , value of the energy density H(rc),and the ellipticity ( E ) , can be found in Table 3. The energy density at the bond critical point indicates a bond to be covalent if H(rc) < 0 and ionic if H(rJ > 0,35whereas E is indicative of the 7t character of the bond. Binding analysis was also complemented by means of the natural bond orbital (NBO) analysis of the HF/6-31g(d,p)//MP2/ 6-31G(d,p) wave function, and the results are summarized in Table 4. Natural atomic charges have also been evaluated, but at one correlated level of theory, Le., MP2/6-3 lG(d,p)//MP2/ 6-31G(d,p). These analyses have been carried out using the NBO program,36as implemented in the GAUSSIAN92 series of programs.32
3. Results and Discussion
+
The possible products of the P+ (3P) XH ( ‘ E )ion-molecule reaction leading to the formation of P-X bonds are PX (3Z) H+ and PX+ (TI) H (2S). As we can see in Tables 1 and 2, the dissociation channel of lowest energy clearly corresponds to the latter, the former being much higher in energy (101.57 kcal/mol for X = C1 and 103.63 kcaymol for X = F, at the QCISD(T) level of theory). Hence, we can conclude that P+ XH systems will primarily dissociate into PX+ H species. This channel is clearly endothermic for the fluorine (10.49 kcaY mol) and slightly exothermic for the chlorine (- 1.20 kcaymol). These figures clearly indicate that high levels of theory are needed to arrive at meaningful conclusions on the ex04 endothermicity of these reactions. Nevertheless, a word of caution is necessary. Namely, it cannot be ruled out that better calculations will not change the energy profile of these reactions, particularly that of the P+ CIH; in fact, at the G1* level of
+
+
+
+
+
Lopez et al.
6816 J. Phys. Chem., Vol. 99, No. 18, 1995
I
............
.....
PClH'
PFH'
.......................
PHCl+
PHF' ........................
i
.....
........................
HPC1' HPF' Figure 6. Contour maps of the V2@calculated from the MP2/6-31G(d,p)//MP2/6-3lG(d,p) wave functions for the ion-molecule complexes PXH+ and the intermediates HPXf, along with the transition state connecting them, PHX', with X = F, C1. Negative values of V 2 q are denoted by solid contours, positive values by dot contours.
TABLE 4: Natural Changes and Wiberg Bond Indexes for the Stationary Points of the Reaction Pathway I1 and NBO Analysis for the P-X Interaction in the Stable Species ~~
natural charges"
bond indexes"
~
natural bond orbital analysisb
species
QP
Qx
QH
P-X
X-H
P-H
PFH+ PHF'(TS2) HPF+ PClH+ PHC1+ (TS2) HPCl+
0.852 1.069 1.375 0.649 0.741 0.892
-0.504 -0.422 -0.446 -0.015 0.103 0.021
0.652 0.353 0.071 0.366 0.157 0.087
0.248 0.521 0.720 0.544 0.803 1.089
0.533 0.406 0.024 0.816 0.343 0.028
0.004 0.212 0.795 0.003 0.479 0.830
bond
interaction
EZ''
nF
37.00
CXd
occupancy
Cp
~pcl
1.995 1.999
20.02 14.43
79.98 85.57
UPCI
1.991
38.40
61.61
-.(3p.J~
UPF
Evaluated using the MP2/6-3lG(d,p)//MP2/6-31G(d,p)wave function. From the analysis of the HF/6-3 lG(d,p)//MP2/6-31C(d,p)wave function. and X atom (CX). Interaction energy in kcaymol. Polarization coefficients for phosphorus (CP) a
theory the reaction is exothermic by a larger amount, Le. -4.15 kcawmol, and with the G2* and QCISD(T) levels of theory this exothermicity is decreased to our best estimate of 1.2 kcaumol. Two reaction pathways can lead to the PX+ H products. For both of them, in the first step of the reaction, an ionmolecule complex is formed (IM structure of Figures 1 and 2). In accordance with standard ion-molecule structures, large P-X distances are encountered in the ion-molecule complexes (2.297 and 2.019 8, for C1 and F, respectively, at the QCISD(T) level of theory), while the X-H bond properties vary only slightly with respect to the values of the free molecules (see Table 3). As it has been reported previou~ly,~'the P-X bond in such
+
complexes involves both electrostatic and electronic interactions. The electrostatic binding is predominantly due to the attraction between the positive charge of the phosphorus and the dipole moment of the XH molecules. Electronic interactions are mainly attributable to the electron donation from a lone pair of the X atom to the empty 3p, orbital of the phosphorus lying on the bond axis. This donation is greater for C1 than for F. Thus, for PClH+ a apcl natural bond orbital highly polarized toward chlorine is found, while for PFH+ no OPF natural bond orbital appears, although an nF (3p,)p high-energy interaction is present, namely, of 37.0 kcdmol. The higher covalent character of the PCl bond is also observed through the Laplacian of e,
-
Reaction of Triplet P+ with XH (X = C1, F) shown in Figure 6. For both systems, PClH+ and PFH+, Hrc has negative values, indicating that some covalency is enhancing the binding. Despite the more negative values of Hr, for the P-F bond, other bond properties, along with the shape of the Laplacian, show higher covalency for the P-Cl bond than for the P-F one. This is not surprising, due to simple electronegativity arguments. Thus, in view of its higher covalency, ClH is bound more strongly to P+ than FH, -40.88 kcal/mol for the former and -29.07 kcal/mol for the latter. This precludes the greater stability of the P-Cl bond compared with the P-F bond, which permits the former to show a more exothermic channel than the latter. No energy barrier was found at the MP2/6-31G(d,p) level of theory for the formation of these ion-molecule complexes. This is a common behavior with other ion-molecule reactions, for which the lack of energy barrier for the formation of the ionmolecule complex, along with a high stability of these complexes, provides the system with an excess of kinetic energy which can be employed to overcome further energy barriers of the reaction mechanism. Two pathways seem to be feasible for the formation of a P-X bond. The former, starting from the PXH+ complex, consists of hydrogen abstraction from the X atom (pathway I). The latter takes place firstly through a transportation of the hydrogen over the P, followed by an abstraction of the H from the P (pathway 11). 3.1. Pathway I. Abstraction of the H from X leads to a crossing of three different electronic states (YZ, HZ,YH; see Figures 4 and 5). This requires the use of multiconfigurational wave functions to allow for the necessary configurationmixing. So, we have explored this reaction pathway by means of CASSCF wave functions. The YZ configuration corresponds to the situation in which the unpaired electrons are in the 3p, and 3p, orbitals of the phosphorus atom, the x axis being the one determined by the P-X u bond. This is the electronic configuration found for the ion-molecule complex, and if we neglect all configuration mixing, this one would result in the heterolytic breaking of the X-H bond, leading to the PX (32) H+ dissociation channel. The remaining two configurations localize one of the unpaired electrons on a 3p orbital of the phosphorus and the other on the 1s orbital of the hydrogen (at short XH distances, this is equivalent to occupying the antibonding U*CIH).Depending on the orientation of the occupied 3p orbital with respect to the P-X internuclear axis, we will obtain the HZ configuration (3p, orbital occupied) or the YH configuration (3pyorbital occupied). Both configurations lead to the PX+ H dissociation channel and, of course, at long X-H distances become degenerate, since 3p, and 3p, are indistinguishable. However, the only configuration compatible with that of the ion-molecule complex is the HZ configuration, for it has the same electronic symmetry (3A”) of the YZ configuration. Thus, YZ and HZ crossing is avoided, and at intermediate X-H distances the electronic state will be a mixture of these two configurations. The YH configuration wil not mix with the other two, since it belongs to a different electronic symmetry (3A’). The MP2/6-3 lG(d,p), CASSCFl6-3lG(d,p), and QCISD(T)/ 6-3 1lG(d,p) properly characterized optimized geometries for the TS1 transition state of the PCIHf system can be found in Figure 1. Surprisingly, the geometries are in good agreement at all levels of theory, despite the well-known deficiency of perturbational methods to describe properly a configurationmixing situation. A long C1-H distance, along with a short P-Cl one, reveals that it is a late transition state. As was expected, the electronic state corresponds to a mixture of the YZ (coefficient = 0.79) and HZ (coefficient = -0.38)
+
+
J. Phys. Chem., Vol. 99, No. 18, 1995 6817 configurations. The electron rearrangement occumng for this reaction can be summarized as follows: (a) homolytic breaking of the UCIH bond, (b) withdrawing of the (3py)p electron by the partially occupied (3p& orbital, and (c) transformation of the donating (3pJcl into a opcl bond. Remarkably, for the (PFH)+ system, we have not been able to arrive at a stationary point with the MP2/6-31G(d,p) and CASSCF/6-31G(d,p) levels of theory. However with the QCISD(T)/6-31lG(d,p) we have found it, having analogous features to that of the (PClH)+ system. Namely, a late transition state is found with a P-F distance close to the distance of the PF+ product. Table 2 and Figure 3 show the relative energies and the energy profile, respectively, for this reaction pathway at the QCISD(T) level of theory. For the (PFH)+ system the TS1 is clearly above the reactants level (27.48 kcal/mol), and it is 56.55 kcal/mol above the ion-molecule complex. However, for (PClH)+ the differences are not so big, the TS1 lies only 2.73 kcal/mol above the reactants level at the same level of theory, and at lower levels of theory such as G1* and G2* it lies even more below, -2.43 and -0.44 kcal/mol, respectively. With respect to the ion-molecule complex the barrier is now reduced to 43.61 kcal/mol. This is a very high energy barrier, and it is indicative of the difficulty for the system to perform the above described electron rearrangement, which involves the homolytic dissociation of a highly polarized bond and the removal of an electron from the P+ cation. These two processes are of high energy cost. The end result is that a substantial energy barrier arises for both systems, and in the case of (PFH)+ this barrier could hardly be overcome under intertellar conditions, while it remains uncertain for (PClH)+ system. 3.2. Pathway II. The other way of extracting hydrogen from the PXH+ system would consist of the two-step reaction pathway 11. Thus, from the ion-molecule complex, a transposition of the hydrogen from X to P occurs through a transition state of the TS2 form, leading to an intermediate of the JNT form. Then, the hydrogen is abstracted from the phosphorus atom. The transposition reaction involves the following orbital rearrangement: 3 s ~ UPH;UCIH (3pY)c1;(3PX)c1+ UPX. Notice that this process leads to the formation of a UPX bond in the HPX+ intermediate. It is clear from the shape of the Laplacians and from the values of the P-X bond properties that, as the transposition process goes on, the P-X bond is strengthened. Hence, Hrcbecomes more negative, denoting that the concentration of electronic charge between P and X basins increases. Thus, the electron density at the bond critical point is increased by 0.0969 and 0.0772 au for X = F and X = C1, respectively. This is particularly so for the HPC1+ intermediate, for which the shape of the Laplacian suffers a substantial change to a situation in which P and C1 basins are linked by zones of electronic charge concentration. Also, it should be pointed out that now the P-X bonds are reinforced by a n donation, which is reflected in the values of the ellipticity (6, Table 4). According to this behavior of the electron density, the P-X bond distances decrease as the H shift proceeds, from 2.297 and 2.019 A at the ion-molecule complexes to 1.965 and 1.542 8, at the intermediates, for C1 and F, respectively. Despite this strengthening of the P-X bond, the HPX+ species became less stable than the ion-molecule complexes at all levels of theory, a fact indicative of the weaker P-H bonds compared with the X -H bonds. With respect to the energies of the TS2 transition states, we can point out that the TS2 transition states correspond to a much lower energy barrier than the TS1 ones (see Figure 3 and Tables 1 and 2). At the QCISD(T) level of theory, the TS2 of the PClH+ system is -18.00 kcal/mol below the reactants and 20.73 +
+
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Lopez et al.
kcal/mol more stable than TS1. However, for the (PFH)+ system TS2 is still above the reactants level by 10.91 kcal/mol. It is clear that under interstellar conditions this barrier can be overcome for the (PClH)+ system, but it should be very unlikely for (PFH)'. The reason for the greater stability of TS2 compared to TS1 lies in the lower energy cost of the electronic rearrangement that should occur to reach the intermediate (INT). Namely, the 3sp OPH electron rearrangement is favored by the high extension of this orbital and the ringlike structure of the TS2 transition state; the OclH (3p& takes advantage of the polarization of the ffClH bond toward C1, and finally the (3p& OW]electron donation is eased by the positive charge of the phosphorus. Therefore, the fiist step of the reaction pathway I1 is likely to be energetically allowed under interstellar conditions for the (PClH)+ system. Finally, to reach the products, abstraction of the hydrogen from the phosphorus remains. No energy barrier at the MP2/6-31G(d,p) level of theory has been found for such a process. This can be somewhat surprising, since a high energy barrier is found for the abstraction of the hydrogen from the X atom, while the abstraction from phosphorus does not exhibit any energy barrier. To understand such a different behavior, a careful inspection of the properties of the electron density of the HPX+ complexes is helpful (Table 3). At a first glance, it is remarkable that the bond properties of the P-X bond in the HPX+ species are very similar to that of the PX+ product, whereas the same is not true for PXH+ species. Thus, it is clear that no great changes should occur in the P-X subsystem to go from HPX+ intermediates to the products, while for PXH+ substantial changes need to be accomplished. In other words, for hydrogen abstraction from the phosphorus the only electron rearrangement concerns the homolytic breaking of the PH bond and the pairing of the resultant phosphorus electron with the 3p, unpaired electron centered on the phosphorus, whereas abstraction of hydrogen from the X atom implies, in addition, a great change of the P-X electron density distribution, and consequently, it should be less favored.
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4. Concluding Remarks The reaction pathways that can lead to the interstellar production of PCl and PF compounds from P+ ClH and P' FH reactions have been characterized. For FH no exothermic channel leading to PF bond containing compounds was found, so the ion-molecule reaction is thermodynamically forbidden under interstellar conditions. For ClH we have found one PX+ H exothermic dissociation channel, although the exothermicity is too low to be conclusive. Two reaction mechanisms can lead to these products. For both of them, a highly stable ionmolecule complex is formed in the first step of the reaction. The abstraction of hydrogen from the X atom is found to be energetically unfavored, and it seems not to be feasible under interstellar conditions. However, the more indirect reaction pathway 11, which involves a hydrogen transposition from X to P and then hydrogen abstraction from the phosphorus atom, is kinetically favored, and for C1 it is likely to occur under interstellar conditions. The only problem could be the exothermicity of the reaction; thus, from the data reported here we cannot be definite about the exothermicity of the reaction, and it could be thermodynamically forbidden, although not kinetically. So we can conclude that the P+ C1H ion-molecule reaction could be a source of PCl compounds in the interstellar medium, whereas the P+ FH reaction is not a likely source of PF compounds.
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Acknowledgment. This research has been supported by the Spanish Office of Science and Education (Ministerio de
Educacion y Ciencia) Grant No. PB91-0456, and the Basque Provincial Govemment of Guipuzcoa (Gipuzkoako Foru Aldundia). One of us (X.L.) wishes to thank the Basque Govemment (Eusko Jaurlaritza) for a grant. References and Notes (1) Duley, W. W.; Williams, D. A. Interstellar Chemistry; Academic Press: 1984. (2) Turner, B. E.; Bally, J. Astrophys. J. 1987, 321, L75. (3) Guelin, M.; Cernicharo, J.; Paubert, G.; Turner, B. E. Astron. Astropys 1990, 230, L9. (4) Thorne, L. R.; Anicich, V. G.; Huntress, W. T. Chem. Phys. Lett. 1984, 280, 139. ( 5 ) Smith, D.; McIntosh, B. J.; Adams, N. G. J. Chem. Phys. 1989, 90, 6213. (6) Millar, T. J.; Bennet, A.; Herbst, E. Mon. Not. R. Astron. SOC.1987, 229, 41. (7) Maclagan, R. G. A. R. J. Phys. Chem. 1990, 94, 3373. (8) Maclagan, R. G. A. R. Chem. Phys. Lett. 1989, 163, 349. (9) Esseffar, M.; Luna, A,; M6, 0.;YBtiez, M. Chem. Phys. Lett. 1993, 209, 557. (10) Esseffar, M.; Luna, A.; M6, 0.:Yifiez, M. J . Phys. Chem. 1993, 97, 6607. (11) Esseffar, M.; Luna, A,; M6, 0.;Yifiez, M. J . Phys. Chem. 1994, 98, 8679. (12) Esseffar, M.; Luna, A.; M6,O.; Yhiez, M. Chem. Phys. Lett. 1994, 223, 240. (13) Largo, A,; Flores, J. R.; Banientos, C.; Ugalde, J. M. J. Phys. Chem. 1991, 95, 170-175. (14) Largo, A.; Redondo, P.; Banientos, C.; Ugalde, J. M. J . Phys. Chem. 1991,95, 5443-5445. (15) Largo, A.; Flores, J. R.; Banientos, C.; Ugalde, J. M. J . Phys. Chem. 1991, 95, 6553-6557. (16) Largo, A.; Barrientos, C. J. Phys. Chem. 1991, 95, 9864. (17) Lopez, X.; Ugalde, J. M.; Barrientos, C.; Largo, A.; Redondo, P. J . Phys. Chem. 1993, 97, 1521-1525. (18) Largo, A.; Bamentos, C.; Lopez, X.; Ugalde, J. M. J. Phys. Chem. 1994, 98, 3985-3988. (19) Blake, G. A.; Anicich, V. G.; Huntress, W. T. Astrophys J . 1986, 300, 415. (20) Blake, G. A.; Keene, J.; Phillips, T. G. Astrophys. J . 1985, 295, 501. (21) Van Dishoeck, E. F.; Van Hemert, M. C.; Dlagamo, A. J. Chem. Phys. 1982, 77, 3693. (22) Banientos, C.; Largo, A.; Redondo, P.; Pauzat, F.; Ellinger, Y. J . Phys. Chem. 1993, 97, 173-176. (23) Nguyen, M. T. Mol. Phys. 1986, 59, 547-558. (24) Peterson, K. A.; Woods, R. C. J . Chem. Phys. 1990, 93, 18761888. (25) Butcher, V.; Dyke, J. M.; Lewis, A. E.; Moms, A,; Ridha, A. J. Chem. Soc., Faraday Trans. 2 1988, 84, 299-310. (26) Schiirmann,B. L.; Knowles, D. B.; Hirsch, G.; Buenker, R. J. Chem. Phys. Lett. 1988, 145, 529-536. (27) Harrison, J. F. J. Am. Chem. Soc. 1981, 103, 7406-7413. (28) (a) Dupuis, M.; Spangler, D.; Wendoloski, J. J. NRCC Software Catalog; University of California: Berkeley, CA, 1980; Program QGOl. (b) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A,; Jensen, J. H.; Koseki, S.; Gordon, M. S.; Nguyen, K. A.; Windus, T. L.; Elbert, S . T. QCPE Bulletin 1990, 10, 42-54. (29) Pople, J. A.; Head-Gordon, M.; Fox, D. J.; Raghavachari, K.; Curtiss, L. A. Chem. Phys. 1989, 90, 5622-5629. (30) Curtiss, L. A.; Jones, C.; Trucks, G. W.; Raghavachari, K.; Pople, J. A. J . Chem. Phys. 1990, 93, 2537-2545. (31) Curtiss, L. A.; Raghavachari, K.; Trucks, G. W.; Pople, J. A. J. Chem. Phys. 1991, 94, 7221-7230. (32) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S . ; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S . ; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92, Revision C; Gaussian, Inc.: Pittsburgh, PA, 1992. (33) Bader, R. F. W. Atoms in Molecules. A Quantum Theory; Clarendon Press: Oxford, 1990. (34) Biegler-Konig, F. W.; Bader, R. F. W.; Tang, T. H. J. Comput. Chem. 1980, 27, 1924. (35) Cremer, D.; Kraka, E. Croat. Chem. Act. 1984, 57, 1259-1281. (36) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO Version 3.1. (37) Lopez, X.; Irigoras, A.; Ugalde, J. M.; Cossio, F. P. J. Am. Chem. SOC.1994, 116, 10670-10678. JW42 122H
