Energetic Diagrams and Structural Properties of Monohaloacetylenes

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Energetic Diagrams and Structural Properties of Monohaloacetylenes HC#CX (X=F, Cl, Br). Dorra Khiri, Majdi Hochlaf, and Gilberte Chambaud J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b04504 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 16, 2016

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The Journal of Physical Chemistry

Energetic Diagrams and Structural Properties of Monohaloacetylenes HC≡CX (X = F, Cl, Br) D. Khiri,† M. Hochlaf,† and G. Chambaud∗,‡ †Université Paris-Est, Laboratoire Modélisation et Simulation Multi-Echelle MSME, UMR 8208 CNRS, 5 Bd Descartes, 77454, Marne-la-Vallée, France ‡Université Paris-Est, Laboratoire Modélisation et Simulation Multi-Echelle MSME, UMR 8208 CNRS 5, Bd Descartes, 77454, Marne-la-Vallée, France, Tel:+33160957303 E-mail: [email protected]

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Abstract Highly correlated electronic wavefunctions within the Multi Reference Configuration Interaction (MRCI) approach are used to study the stability and the formation processes of the monohaloacetylenes HCCX and monohalovinylidenes C2 HX (X = F, Cl, Br) in their electronic ground state. These tetra-atomics can be formed through the reaction of triatomic fragments C2 F, C2 Cl, C2 Br with an hydrogen atom or of C2 H with halogen atoms via barrierless reactions whereas the reactions between the diatomics [C2 + HX] need to overcome barriers of 1.70 eV, 0.89 eV and 0.58 eV for X = F, Cl, Br. It is found that the linear HCCX isomers, in singlet symmetry, are more stable than the singlet C2 HX iso-forms by 1.995 eV, 2.083 eV and 1.958 eV for X = F, Cl, Br. The very small isomerization barriers from iso to linear forms are calculated 0.067 eV, 0.044 eV and 0.100 eV for F, Cl and Br systems. The dissociation energies of the HCCX systems (without ZPE corrections), resulting from the breaking of the CX bond, are calculated to be 5.647 eV, 4.691 eV and 4.129 eV for X = F, Cl, Br respectively. At the equilibrium geometry of the X1 Σ+ state of HCCX, the vertical excitation energies in singlet and triplet symmetries are all larger than the respective dissociation energies. Stable excited states are found only as 3 A′ , 3 A′′ and 1 A′′ monohalovinylidene structures.

I-Introduction The monohaloacetylenes HC2 F, HC2 Cl and HC2 Br have been known and prepared in the gas phase for a rather long time and hence are considered as stable molecules. The formation of HC2 Br was mentioned already in 1865 during the decomposition of dibromomuconic acid 1 and the molecule was obtained as a pure product several years later. Heating the barium salt of ββ-dichloroacrylate with barium hydroxide leads to the formation of HC2 Cl. 2 The last of these three monohaloacetylenes to be discovered was the fluoroacetylene which was obtained by pyrolysis of fluoromaleic anhydride and also by reaction of magnesium with


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2-bromo-1,1-difluoro-ethylene. 3 The first direct observation of HC2 F has been conducted by Gilles et al. 4 using negative ion photodetachement spectroscopy. They found that the barrier for the isomerization of the iso-form to the linear one by H-migration is very small (0.086 eV) and concluded that this iso-form is not stable. Several investigations based on their chemical properties supported a linear stable structure for the three monohaloacetylenes. 5,6 Electron diffraction measurements 7 showed the linearity of chloroacetylene and bromoacetylene, and microwave spectroscopy confirmed the linear structure of chloroacetylene. 8 A similar linear structure has been proposed for fluoroacetylene through IR spectroscopy. 9,10 The electronic structures of HC2 F and HC2 Cl have been first calculated by Moreau et al. 11 Later, the spectroscopic constants and the equilibrium geometry of HC2 Cl have been determined using the coupled electron pair approximation (CEPA) by Horn et al. 12 Electric multipole moments and (hyper)polarizabilities of the haloacetylenes (HC2 X, X = F, Cl, Br and I) have been reported by Maroulis 13 using finite-field self-consistent field, MøllerPlesset perturbation theory and confirmed later by Harrison 14 using coupled cluster calculations with optimized gaussian basis sets. The current active interest for the determination of the energetics and spectroscopic properties of these systems stems from their wide range of applications such as in atmospheric chemistry, combustion or ultra-fast chemistry. This requires their accurate identification. In this work, we have constructed the energy diagrams including the different di-, tri- and tetra-atomic species involved in the formation of the stable monohaloacetylenes. The long range interactions of the hydrogen halides with C2 have been recently studied, 15 showing that the formation of molecular systems HC2 X and C2 HX via such reactions is exothermic, but requires activation barriers of 1.70 eV, 0.89 eV and 0.58 eV for X = F, Cl and Br, respectively. An other path to obtain HC2 X and C2 HX is via the reaction of hydrogen or halogen atoms with the triatomic radicals C2 F, C2 Cl and C2 Br or C2 H. The relative stability of both HC2 X and C2 HX isomers is also considered in this contribution, together with the isomerization path. In addition, the relative stability of excited states has been investigated



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in singlet and triplet symmetries for the linear and iso-forms.

II-Electronic Computational Details In this work, electronic structure calculations have been performed at the MRCI level of theory with the MOLPRO program package 16 to calculate the energies and some properties of low-lying electronic states of [C2 HX] systems. In the MRCI calculations, the molecular orbitals have been optimized in a preceding full-valence CASSCF step. Configurations with a coefficient larger than 0.001 in the CASSCF wavefunction expansion were included in the reference set for the MRCI step in which all the valence electrons have been correlated, yielding a dimension of approximately 2x107 contracted configurations for the MRCI matrix. The Davidson correction (MRCI+Q) has been applied and the basis set superposition error (BSSE) has been corrected at all geometries of the long range interactions in the C2 + HX systems according to the Boys and Bernardi counterpoise scheme. 17 Even though the MRCI method is not size-consistent, such an ansatz should provide reliable data for the determination of an energetic diagram. In order to validate the stationary points on the potential energy surfaces, IRC and frequency analysis have been performed when necessary. For the halogen atoms, we used the core pseudo-potentials ECPnMWB, 18 with n = 2, 10, 28 for F, Cl and Br respectively in order to treat the same number of effective electrons for the three molecular systems. For the carbon, hydrogen, fluorine and chlorine atoms, the aug-cc-pVQZ (aVQZ in the following) basis sets of Dunning 19 have been used. For the bromine atom, we used the ECP28MWB-aVQZ 20 basis set corresponding to (15s, 11p, 4d, 3f, 2g) / (5s, 5p, 4d, 3f, 2g).

III-Data on the triatomic fragments The two lowest electronic states 2 Σ+ and 2 Π of the four triatomic radicals C2 H, C2 F, C2 Cl and C2 Br have been studied at the MRCI level of theory using aVQZ basis sets. These two 4 ACS Paragon Plus Environment

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electronic states are close energetically for C2 H (∆E = 0.44 eV) and even very close for the three C2 X systems. In the X 2 Σ+ state, the CC bond length equals approximately 1.2 ˚ A, characteristic of a triple CC bond and it is longer in the 2 Π state, with approximately 1.3 ˚ A. For the C2 H radical, the ground state is the X 2 Σ+ and the first excited state is the 2 Π. Both states have a linear equilibrium geometry; their geometrical parameters and relative energies (∆Ee ) are given in Table 1 in good agreement with experimental data deduced from ro-vibrational absorption spectroscopy. 21–23 For C2 F, C2 Cl and C2 Br the ground state is the bent X2 A′ electronic state, correlated adiabatically at linearity to the 2 Σ+ state, with very small barriers to linearity calculated to be 0.077 eV, 0.084 eV and 0.093 eV for F, Cl and Br systems respectively. For the three systems, the floppy bent X2 A′ state corresponds mainly to the 2 A′ component of the 2 Π state, which lies higher in energy than the 2 Σ+ state at linearity, giving an avoided crossing very close to linearity. The equilibrium CC bond length in the X2 A′ state is intermediate between that of the 2 Σ+ and 2 Π states showing that this state results from a strong interaction between both very close electronic states. The two other electronic components, 2 A′ and 2 A′′ in this energy domain have linear equilibrium geometry. The calculated geometries and relative energies (∆Ee ) of these low lying electronic states are given in Table 2 and they are also compared there with previous calculated data for C2 F, 24,25 C2 Cl 26 and C2 Br 27 respectively. Since the C-H bond is stronger than the C-X bonds in the tetra-atomics, the [C2 H + X] dissociation limits are lying energetically below the [C2 X + H] ones, as shown in Table 3 where, for comparison, all energies are given relative to the energy at equilibrium of the ground state of the corresponding linear tetra-atomic molecule.

IV-Energetic diagrams The energetic diagrams of the [C2 HX] systems have been investigated at the MRCI+Q/aVQZ level, showing the different formation pathways in singlet symmetry, either via the reaction



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of the C2 and HX diatomics or via the reaction of triatomic radicals C2 F, C2 Cl, C2 Br with an hydrogen atom or of C2 H with halogen atoms. For the lowest electronic state, the isomerization paths between the two tetra-atomic isomers, the linear HC2 X and the iso form C2 HX are included in the diagrams. The energies of the dissociation limits into diatomic fragments C2 + HX in singlet and triplet symmetries are given in Table 3 relative to the energies of the ground state of the corresponding linear tetra-atomic molecules. At long distances between the fragments, weakly bound structures have been found for different relative orientations of the diatomic fragments (parallel, T-shape, perpendicular). 15 These shallow minima, referred as (C2 +HX)Para, (C2 +HX)T-shape and (C2 +HX)Perp in Table 3 are separated from the stable linear forms HCCX by barriers of 1.70 eV, 0.89 eV and 0.58 eV for HF, HCl and HBr with respect to the corresponding lowest singlet dissociation limits [C2 (X1 Σ+ g ) + HX]. The energetically lowest transition states corresponding to these barriers are referred as C2 HX(TSpath ) in Tables 3, 4, 5 and 6. The lowest paths to the corresponding iso-forms are the same as for the stable linear forms. The existence of such barriers is hindering the direct reaction of the diatomic fragments to give the tetra-atomic molecules without providing additional energy to the systems. The triplet dissociation limits [C2 (a3 Πu ) + HX] are very close energetically to the singlet ones, but the triplet states are located higher in energy than the singlet ones at all geometries and do not provide an easier path for the formation of the tetra-atomic systems. For the reactions of triatomic radical C2 H with halogen atoms, which correspond to the lowest asymptotic limits, and also for the reactions of the C2 F, C2 Cl, C2 Br radicals with an hydrogen atom, the minimum energy paths have been investigated in linear and bent geometries for the singlet states with the atom approaching the triatomic toward the external C of C2 X. There is no energy barrier along these paths and the systems evolve directly to the stable 1 Σ+ linear isomer HCCX, whatever X. This smooth energetic behavior stems from the very similar geometric structures of the triatomics C2 H/C2 X and the tetra-atomics XC2 H. For the lowest triplet 3 A’ and 3 A” states, which are found stable in iso-forms and not in linear


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geometries (see below), we have investigated the approach of the halogen/hydrogen atom toward the central carbon atom: because there are complex electronic interactions between the two lowest 3 A’ states arising from the first dissociation asymptote [X(2 P)+C2 H(X 2 )], they have been averaged together during the computations. For the 3 A” the situation is much simpler and the path was studied for the lowest 3 A” state alone. For these three triplet states, one can observe a complex geometric rearrangement corresponding to a simultaneous lengthening of the CC bond and bending of the CC-H/CC-X bonds when the halogen/hydrogen atom are approaching the internal carbon atom. This is achieved via barriers along the reaction paths leading to the lowest 3 A’ and 3 A” states. In the present study, these barriers have not been determined precisely since our concern is the main reactive paths issued from these asymptotes which are clearly barrierless paths via singlet states. At the respective equilibrium geometries of the linear singlet ground states, it is found that the linear forms of the tetra-atomics are more stable than the iso-forms with stabilization energies (without including the Zero-Point-Energy (ZPE) corrections) 1.995 eV, 2.083 eV and 1.958 eV for [C2 HF], [C2 HCl] and [C2 HBr] (cf. Tables 4, 5 and 6). These values are very close to the stabilization energy in acetylene/vinylidene, calculated 1.973 eV (45.5 kcal/mol, 1 eV=23.061 kcal/mol) (without ZPE). 28 The structural data of both isomers, given in Tables 4, 5 and 6 show similar characteristics: there is an increase of all bond lengths when passing from the linear to the iso-form. In the linear forms, the CC bond lengths are very close to 1.2 ˚ A, when in the iso-forms they are close or larger than 1.3 ˚ A, as also observed for acetylene/vinylidene isomers. 28 In the isomerization processes the CH bond lengths only increase by 0.02 ˚ A, for the three [C2 HX], whatever X as also observed for the [C2 H2 ] system. 28 The CX bond lengths are slightly larger in the iso-forms than in the linear forms for the three [C2 HX], whatever X. These structural parameters evidence stronger bonds in the linear forms than in the iso-forms, resulting in a larger ZPE for the linear than for the iso-forms. This is in perfect agreement with the stabilization energy, including ZPE correction, close to 0.087 eV (2.0 kcal/mol) of the iso-form calculated in the acetylene/vinylidene isomerization



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studied by Lee et al. 28 Equivalently, a previous study of the isomerization of C2 HF to HCCF by Pople et al. 29 showed that inclusion of ZPE correction in hydrogen atom migration leads to a stabilization of the transition state relative to C2 HF.

Figure 1: Labels of the atoms in the iso and linear forms of [C2 HX] system

The isomerization of C2 HX to HCCX can proceed via the migration of hydrogen or halogen atom. In singlet symmetry, the optimized geometrical parameters and corresponding energies of the stationary points on the C2 HX → HCCX isomerization surface have been determined using - the labels of the different atoms of these structures are defined in Figure 5. It is found that the hydrogen migration, accompanied by a shortening of the CC bond, requires much less energy than the halogen migration; the corresponding quantities are noted C2 HX(TSH ) and C2 HX(TSX ) in Tables 4, 5 and 6 for the [C2 HF], [C2 HCl] and [C2 HBr] systems respectively. Because of these very small isomerization barriers (0.067 eV, 0.044 eV and 0.100 eV for F, Cl and Br systems respectively) the iso-forms C2 HX are not very stable and it was not relevant to include the ZPE, estimated close to 0.4 eV, and the eventual vibrational levels in the energetic diagrams when such small energies are involved. A small barrier of 0.087 eV (2.0 kcal/mol) in the isomerization of C2 HF was pointed out in the experimental works of Gilles et al. 4 in good agreement with our calculations. A more recent accurate determination of the isomerization in C2 H2 molecule 30 shows that the isomerization path from iso to linear forms exhibits a complex structure resulting from the strong geometric modification of the molecule, with two transition states and a local minimum: the height 8 ACS Paragon Plus Environment

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of the resulting highest barrier is calculated 0.104 eV (2.41 kcal/mol), very similar to the barriers obtained in the present work with halo-systems. In our investigations we did not evidence complex double transition states for the H migration as observed in C2 H2 system.

Figure 2: Energetic diagram of the [C2 HF] system

Relative energies of the [C2 HX] systems calculated at the MRCI+Q/aVQZ level have been summarized in Table 3 where the energies of the X 1 Σ+ states of the linear isomers HC2 X are taken as references. These data have been used to construct the energetic diagrams drawn in Figures 2, 3 and 4 for the [C2 HF], [C2 HCl] and [C2 HBr] systems respectively. These diagrams show similar features for the three monohalosystems as follows: i) the direct reactions of triatomic C2 H + halogen atom leading to the stable linear singlet ground state are possible and barrierless, ii) on contrary, there are potential barriers in the reactions of C2 + HX , iii) in singlet symmetry, the linear HCCX state is stable but the iso-form is not. At the same level of theory (MRCI+Q/aVQZ), we have studied the low-lying excited states in singlet and triplet symmetries, correlated adiabatically to the first dissociation limits. Vertical energies, calculated at the equilibrium geometry of the X 1 Σ+ ground state 9 ACS Paragon Plus Environment


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Figure 3: Energetic diagram of the [C2 HCl] system

Figure 4: Energetic diagram of the [C2 HBr] system


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of the linear HCCX are given in Table 7. For the three halogen-systems, the lowest vertical excited states are the 3 Σ+ states, followed by the 3 Π states and then by the singlet states, all of them lying above their lowest dissociation limits into [C2 H + X]. This is in agreement with the fact that the HCCX systems do not absorb in the visible domain and hence are not colored. It was not possible to find an equilibrium geometry at linearity for these 3 Σ+ and 3

Π states because they evolve directly to the iso-forms with one stable 3 A′ and one stable

3

A′′ states. The second 3 A′ states are found higher in energy and above the corresponding

dissociation limits. The energies and optimized geometries of these 3 A′ and 3 A′′ components are given in Tables 4, 5 and 6. Among the three halogen systems, the largest spin-orbit interactions occur when the bromine atom is concerned: in the dissociation limit [C2 H + Br] the spin-orbit splitting in the Br (2 P) state is as large as 3685 cm−1 (0.457 eV). This induces a lowering of the spin-orbit ground state by 1228 cm−1 (0.15 eV) compared to the average limit. By contrast, in the 3 A” state of the iso-form the spin-orbit interactions have been calculated very small even for bromine systems (less than 0.1 cm−1 for C2 HBr) but the equilibrium energy of this 3 A” state is still lying below its dissociation limit and the state can thus be considered as potentially stable. The comparison with previous calculated values of these triplet states for C2 HF 34,36 shows good agreement and can validate our results for the other halogen-systems. In these triplet states, the CC and the CX bond lengths are found larger than in the linear X 1 Σ+ ground state. For the three systems, the triplet states are calculated higher than the X 1 Σ+ linear state by more than 3 eV and higher by more than 1.5 eV than the corresponding 1 A′ iso-forms as can be seen in Table 3. At the equilibrium geometry of the X 1 Σ+ state the first singlet excited states are found in 1 Π symmetry, located energetically between 6.1 eV and 7.5 eV above the ground state depending on X. As in triplet symmetry it was not possible to find a stable linear excited structure in singlet symmetry, since the systems evolve directly to the iso-forms: a stable 1 A′′ component is found which characteristics are given in Table 4, 5 and 6 for the three systems. These excited 1 A′′ states are lying energetically below (for F and Cl compounds), and slightly above (for Br) the



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lowest dissociation limit [C2 H(X 2 Σ+ )+X(2 P )] to which they are adiabatically correlated. The second components of 1 Π states, namely the first excited 1 A′ states, are found higher in energy and above the corresponding dissociation limits. Several conclusions can be extracted from the data in Table 3: the breaking of the C-X bond in HCCX is easier for Br than for Cl and F, and this bond is stronger by approximately 1 eV than in the corresponding C2 X radical as can be seen by comparison with the energetics of the dissociation into [C2 + H + X] in this Table; the dissociation of the C-H bond is approximately the same for the three systems (5.9 - 6.0 eV) and does not depend on the halogen, even though the polarity of the three molecules is different. The dipole moments of the linear tetra-atomics, calculated within the Finite-Field approach, are given in Table 8 (0.289 a.u., 0.179 a.u. and 0.091 a.u. for HCCF, HCCCl and HCCBr respectively), in perfect agreement with previous known values. On the basis of these diagrams, one can estimate the dissociation energies of the monohaloacetylenes (without ZPE corrections), resulting from the breaking of the CX bond, to be 5.647 eV, 4.691 eV and 4.129 eV for HC2 F, HC2 Cl and HC2 Br respectively.

Conclusions Based on MRCI+Q/aVQZ calculations, the energy diagrams of the [C2 HX] molecular systems, in singlet symmetry, have been constructed for the three halogens F, Cl and Br. From these diagrams we can conclude that the formations of the stable linear HCCX molecules are possible through direct reactions without energy barrier via the triatom+atom paths. We could evaluate the rather large dissociation energies of these tetra-atomic molecules, equal to 5.647 eV, 4.691 eV and 4.129 eV for X = F, Cl, Br systems respectively resulting from the breaking of the C-X bond. In singlet symmetry, the linear tetra-atomic HC2 X structures are more stable than the C2 HX iso-forms and the barrier (from iso to linear forms) are so small that we could conclude that the iso-forms are not stable. Concerning the excited


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states, we could show for the first time that there is no stable excited linear monohaloacetylene states below the first dissociation limits. However, stable excited states are found in monohalovinylidene structures as 3 A′ and 3 A′′ symmetries for the three halogens, located approximately 3 eV above the linear singlet ground state. For fluorine and chlorine compounds an excited 1 A′′ state is also found in a vinylidene structure, located approximately 4 eV above the linear singlet ground state. This work should help in understanding the photochemistry of HCCX and the spectroscopy of these molecules close and above the dissociation barriers.

Acknowledgements This research was supported by a European COST action 2015-2019, MOLIM (MOLecules In Motion) (CM1405).

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Monosubstituted Forms of CCH. Mol. Phys. 1989, 66, 955-960. (22) Yan, W. B; Dane, C. B; Zeitz. D; Hall, J. L; Curl, R. F. Color Center Laser Spectroscopy of C2 H and C2 D . J. Mol. Phys. 1987, 123, 486-495. (23) Yan, W. B; ; Hall, J. L; Stephens, J. W; Richnow, M. L; Curl, R. F. Color Center Laser Spectroscopy of Vibrationally Excited C2 H. J. Chem. Phys. 1987, 86, 1657-1661. (24) Tarroni, R. Structural and Spectroscopic Effects of Vibronic Coupling in the C2 F Radical. Chem. Phys. Lett. 2003, 380, 624-631. (25) Tarroni, R.; Khriachtchev, L.; Domanskaya, A.; Räsänen, M.; Misochko, E.; Akimov,



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A. Infrared Spectrum of Elusive C2 F Radical: A Matrix-Isolation and Computational Study. Chem. Phys. Lett. 2010, 493, 220-224. (26) Tarroni, R.; Carter, S. Ab Initio Prediction of the Infrared-Absorption Spectrum of the C2 Cl Radical. J. Chem. Phys. 2005, 123, 014320-014327. (27) Tarroni, R.; Carter, S. Ab Initio Prediction of the Infrared Absorption Spectrum of the C2 Br Radical. Mol. Phys. 2006, 104, 2821-2828. (28) Lee, H.; Baraban, J. H.; Field R. W.; Stanton J. F. High-accuracy estimates for the vinylidene-acetylene izomerisation energy and the ground state rotational constants of :C=CH2. J. Phys. Chem. A. 2013, 117, 11679-83. (29) Frisch, M. J.; Krishnan, R.; Pople, J. A.; Schleyer, P. V. R. The Stability of Fluorovinylidene and Difluorovinylidene. Chem. Phys. Lett. 1981, 81, 421-423. (30) Han H.; Li A.; Guo H. Toward spectroscopically accurate global ab initio potential energy surface for the acetylene-vinylidene isomerization. J. Chem. Phys. 2014, 141, 244312-22. ˇ M.; Noga, J.; Jacquemin, D.; Perpète.E. A. Longitudinal NLO Properties (31) Medved, of C2 H2 , HCCF, and C2 F2 : Electron Correlation and Vibration Effects. Int. J. Quant. Chem. 2005, 102, 209-223. ¨ Oswald, R. Ab Initio Spectroscopic (32) Botschwina, P.; Oswald, M.; Fl˜ ugge, J.; Heyl, A.; Constants and the Equilibrium Geometry of HCCF. Chem. Phys.Lett. 1993, 209, 117-125. (33) Borro, A. F.; Mills, I. M.; Mose, A. Overtone Spectra and Anharmonic Resonances in Haloacetylenes. Chem. Phys. 1995, 190, 363-371. (34) DeLeeuw, B. J.; Fermann, J. T.; Xie, Y.; Schaefer, H. F. Substitution Effects on the Properties of Unsaturated Carbenes: Fluorovinylidene (HFC=C:). J. Am. Chem. Soc. 1993, 115, 1039-1047. 16 ACS Paragon Plus Environment

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(35) Kakkar, R.; Pathak, M.; Chadha, P. Theoretical Study of Unimolecular Rearrangements of Vinylidenes to Acetylenes. J. Quant. Chem. 2005, 102, 189-199. (36) Zhi-Heng, L.; Field, R. W. Contrasting Origins of the Isomerization Barriers for Vinylidene, Fluorovinylidene, and Difluorovinylidene. J. Chem. Phys. 2003, 118, 4037-4044. (37) Le Guennec, M.; Wlodarczak, G.; Demaison, J. The Millimeterwave Spectrum and Structure of Chloroacetylene. J. Mol. Spectrosc. 1993, 158, 357-362. (38) Parthiban, S.; Martin, J. M. L.; Liebman, J. F. The Heats of Formation of the Haloacetylenes XCCY [X, Y = H, F, Cl]: Basis Set Limit Ab Initio Results and Thermochemical Analysis. J. Mol.Phys. 2002, 100:4, 453-464. (39) Jones, H.; Owen, N. L.; Sheridan, J. Dipole Moment and Microwave Spectrum of Bromo-acetylene. Nature. 1967, 213, 175. (40) Jones, H.; Sheridan, J.; Stiefvater O. L. The Microwave Spectrum of Bromoacetylene ; rs-spectrum, Dipole Moment, Quadrupole Coupling Constants and Excited Vibration States. Z. Naturforsch. 1977, 32a, 866-875. (41) Tanaka, T.; Yamada, C.; Hirota, E. Laser Stark and Laser Microwave Double Resonance Spectroscopy of Fluoroacetylene with the CO2 Laser. J. Mol. Spectrosc. 1976, 63, 142-151. (42) Ebenstein, W. L.; Hanning, C.; Shostak, S. L.; Muenter, J. S. Radio Frequency Spectra of Chloroacetylene in v=0 and v=1 of the CH Stretching Vibration. J. Chem. Phys. 1987, 87, 1948-1951. (43) Bass, S. M.; DeLeon, R. L.; Muenter, J. S. Electric Dipole Moment and Hyperfine Properties of Bromoacetylene in the Ground and First Excited C-H Stretching Vibrational States. J. Chem. Phys. 1990, 92, 71-75.



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Figures caption: Figure 1: Labels of the atoms in the iso and linear forms of [C2 HX] system Figure 2: Energetic diagram of the [C2 HF] system Figure 3: Energetic diagram of the [C2 HCl] system Figure 4: Energetic diagram of the [C2 HBr] system

Tables caption: Table 1: MRCI+Q/aVQZ calculations of the two lowest electronic states of C2 H; excitation energies ∆Ee (in eV) and equilibrium geometries. Table 2: MRCI+Q/aVQZ calculations of the lowest electronic states of C2 X, X = F, Cl and Br; excitation energies ∆Ee (in eV) and equilibrium geometries. Table 3: Calculated relative energies ∆E (in eV) in the [C2 HX] systems. Table 4: Optimized geometries and energies ∆E of the stationary points on the [C2 HF] reactive surface. Table 5: Optimized geometries and energies ∆E of the stationary points on the [C2 HCl] reactive surface. Table 6: Optimized geometries and energies ∆E of the stationary points on the [C2 HBr] reactive surface. Table 7: Calculated vertical energies, ∆E of the lowest excited states of the linear HCCX molecules in triplet and singlet symmetries. Table 8: Dipole moments, µ (a.u.) of the HCCX linear molecules.


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.

Table 1: MRCI+Q/aVQZ calculations of the two lowest electronic states of C2 H; excitation energies ∆Ee (in eV) and equilibrium geometries. State X 2 Σ+ A2 Π

Method this work expta this work expta

RCC 1.212 1.219 1.292 1.289

RCH 1.064 1.050 1.067 1.060

θHCC 180 180 180 180

∆Ee (eV)* 0.0 0.0 0.44 0.45

*The excitation energy is given relative to the energy of the X 2 Σ+ ground state, calculated E= -76.490339 Hartree. Equilibrium distances in (˚ A), angles in deg, optimized at the MRCI+Q/aVQZ level. (a) ref 21–23



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Table 2: MRCI+Q/aVQZ calculations of the lowest electronic states of C2 X, X = F, Cl and Br; excitation energies ∆Ee (in eV) and equilibrium geometries. State X 2A

′

Method

RCC

this work a b

2 Σ+

this work a

θXCC

∆Ee (eV)*

1.268 1.271 1.269 1.203 1.245

RCX C2 F 1.272 1.276 1.277 1.274 1.277

154.2 165.0 154.8 180 180

1.296 1.302

1.271 1.271

180 180

0.0 0.0 0.0 0.077 0.034 0.105 0.080

b 2Π

this work a b

X 2A

′

this work c

2 Σ+

this work

0.144 1.262 1.263 1.211

C2 Cl 1.638 1.639 1.638

156.6 156.4 180

c 2Π

this work c

X 2A

′

this work d

2 Σ+ 2Π

this work this work

1.295 1.296 1.261 1.262 1.213 1.296

1.618 1.624 C2 Br 1.787 1.796 1.798 1.776

180 180 156.2 156.1 180 180

0.0 0.0 0.084 0.059 0.094

0.0 0.0 0.093 0.108 ′

*The excitation energies are given relative to the energy of the X 2 A ground state of C2 X, calculated E= -100.128074 Hartree, -90.850388 Hartree and -89.235832 Hartree for X = F, Cl and Br. Equilibrium distances in (˚ A), angles in deg, optimized at the MRCI+Q/aVQZ level. (a) (MRCI/aVTZ) 24 ; (b) (MRCI/aVQZ) 25 ; (c) (MRCI/aVQZ) 26 ; (d) (MRCI/aVQZ) 27 .


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Table 3: Calculated relative energies ∆E (in eV) in the [C2 HX] systems.

X(2 P)+C2 H(X 2 Σ+ ) X(2 P)+C2 H(A2 Π) ′ H(2 S)+C2 X(X 2 A ) H(2 S)+C2 X(2 Σ+ ) H(2 S)+C2 X(2 Π) HCCX(1 Σ+ ) C2 HX(TSH ) C2 HX(TSX ) ′ C2 HX(1 A ) ′ C2 HX(3 A ) ′′ C2 HX(3 A ) ′′ C2 HX(1 A ) C2 HX(TSpath ) (C2 +HX)Para (C2 +HX)T-shape (C2 +HX)Perp 1 + C2 (X 1 Σ+ g )+HX(X Σ ) C2 (a3 Π)+HX(X 1 Σ+ ) 2 2 C2 (X 1 Σ+ g )+H( S)+X( P) 3 2 2 C2 (a Π)+H( S)+X( P)

∆E (eV) C2 HF C2 HCl 5.648 4.691 6.086 5.126 6.037 5.975 6.115 6.060 6.117 6.070 0.0 0.0 2.061 2.127 3.651 2.579 1.994 2.083 3.410 3.728 3.609 3.914 4.051 4.279 6.332 6.024 4.613 5.124 4.603 5.081 4.487 5.040 4.632 5.134 4.740 5.243 10.744 9.785 10.852 9.893

C2 HBr 4.130 4.565 5.933 6.027 6.042 0.0 2.059 2.112 1.959 3.668 3.837 4.198 5.792 5.201 5.025 5.142 5.212 5.321 9.224 9.332

∆E is the MRCI+Q/aVQZ energy relative to the energy of the 1 Σ+ ground state of the corresponding linear HCCX isomer, calculated E= -100.84991 Hartree, -91.569947 Hartree and -89.953855 Hartree for X = F, Cl and Br.



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Table 4: Optimized geometries and energies ∆E of the stationary points on the [C2 HF] reactive surface. State HCCF(1 Σ+ g)

Method this work a b

exptc exptd e g h

C2 HF(TSpath ) this work State Method 1 ′ C2 HF( A ) this work e f g

C2 HF(TSH )

this work e g

C2 HF(TSF )

this work f g

3

′

C2 HF( A )

this work e

C2 HF(3 A′′ )

this work e

C2 HF(1 A′′ )

this work

RC a C b 1.199 1.1990 1.1969 1.1962 1.198 1.1846 1.191 1.2174 1.247 RC a C b 1.328 1.3154 1.313 1.313 1.270 1.2638 1.281 1.287 1.287 1.287 1.296 1.2986 1.379 1.4038 1.413

RC b H 1.060 1.0544 1.0586 1.0603 1.053 1.0542 1.060 1.0582 1.906 RC a H 1.084 1.0803 1.097 1.097 1.152 1.1761 1.150 1.063 1.071 1.071 1.071 1.0737 1.080 1.0790 1.078

RC a F 1.274 1.2785 1.2762 1.2764 1.279 1.2753 1.276 1.2876 1.906 RC a F 1.324 1.3262 1.327 1.326 1.286 1.2880 1.300 1.823 1.823 1.823 1.320 1.3451 1.292 1.3169 1.308

θF C a C b 180.0

θC a C b H 180.0

∆E* (eV) 0.0

85.2

85.2

θF C a C b 129.2 128.0 132.7 133.0 156.2 164.3 157.0 54.9 57.5 57.5 122.7 122.6 123.5 122.1 121.1

θC b C a H 113.6 114.8 110.2 109.8 81.9 71.8 81.5 178.7 178.2 178.3 124.1 124.8 126.5 126.8 126.6

6.332 ∆E (eV) 1.994 1.893

2.061 2.088 3.651

3.410 3.065 3.609 3.098 4.051

Equilibrium distances R in (˚ A), angles θ in deg, optimized at the MRCI+Q/aVQZ level. ∆E is the energy relative to the ground state energy of the linear HCCF. (a) (CCSD(T)/aug-cc-pVTZ) 31 ; (b) (CCSD(T)/aug-cc-pVQZ) 32 ; (c) Vibrational spectroscopy 33 ; (d) Microwave spectroscopy 10 ; (e) (CISD/TZ2P) 34 ; (f) (B3LYP/6-311++G(3df,3pd)) 35 ; (g) (B3LYP/6-311++G(3df,3pd)) 36 ; (h) (MP2) 13 .


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Table 5: Optimized geometries and energies ∆E of the stationary points on the [C2 HCl] reactive surface. State HC2 Cl(1 Σ+ g)

Method RC a C b this work 1.206 expta 1.2034 b 1.2046 c 1.2299 d 1.2063 C2 HCl(TSpath ) this work 1.247 State Method RC a C b 1 ′ C2 HCl( A ) this work 1.320 C2 HCl(TSH ) this work 1.271 C2 HCl(TSCl ) this work 1.261 C2 HCl(3 A′ ) this work 1.296 3 ′′ C2 HCl( A ) this work 1.373 1 ′′ C2 HCl( A ) this work 1.413

RC b H 1.062 1.0549 1.0611 1.0599 1.0626 2.300 RC a H 1.085 1.143 1.063 1.070 1.078 1.078

RC a Cl 1.639 1.6380 1.6487 1.6461 1.6412 2.300 RC a Cl 1.719 1.670 2.157 1.741 1.722 1.691

θClC a C b 180.0

θC a C b H 180.0

∆E (eV) 0.0

90.53 θClC a C b 121.4 159.6 59.9 123.1 124.0 121.2

90.53 θC b C a H 119.5 77.4 174.3 123.0 125.8 125.2

6.024 ∆E (eV) 2.083 2.127 2.579 3.728 3.914 4.279

Equilibrium distances R in (˚ A), angles θ in deg, optimized at the MRCI+Q/aVQZ level. ∆E is the energy relative to the ground state energy of the linear HCCCl. (a) Rotational spectroscopy 37 ; (b) (CEPA-1/cc-pVTZ) 12 ; (c) (MP2) 13 ; (d) (CCSD(T)/cc-pVQZ+1) 38 . Table 6: Optimized geometries and energies ∆E of the stationary points on the [C2 HBr] reactive surface. State HC2 Br(1 Σ+ g)

Method RC a C b this work 1.208 expta 1.204 b expt 1.2038 c 1.2343 C2 HBr(TSpath ) this work 1.247 State Method RC a C b 1 ′ C2 HBr( A ) this work 1.300 C2 HBr(TSH ) this work 1.267 C2 HBr(TSBr ) this work 1.256 C2 HBr(3 A′ ) this work 1.294 C2 HBr(3 A′′ ) this work 1.364 1 ′′ C2 HBr( A ) this work 1.406

RC b H 1.062 1.055 1.0553 1.0629 2.401 RC a H 1.088 1.149 1.064 1.070 1.076 1.078

RC a Br 1.796 1.791 1.7916 1.7888 2.401 RC a Br 1.882 1.816 2.275 1.900 1.901 1.855

θBrC a C b 180.0

θC a C b H 180.0

∆E (eV) 0.0

92.18

92.18 θC b C a H 128.8 75.2 172.6 123.2 126.9 126.2

5.792 ∆E (eV) 1.959 2.059 2.112 3.668 3.837 4.198

θBrC a C b 111.1 161.6 61.7 122.9 123.6 120.8

Equilibrium distances R in (˚ A), angles θ in deg, optimized at the MRCI+Q/aVQZ level. ∆E is the energy relative to the ground state energy of the linear HCCBr. (a) Microwave spectroscopy 39 ; (b) Microwave spectroscopy 40 ; (c) MP2 13 23 ACS Paragon Plus Environment


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Table 7: Calculated vertical energies, ∆E of the lowest excited states of the linear HCCX molecules in triplet and singlet symmetries.

3

Σ+ 3 Π 1 Π 1 − Σ 1 ∆ 1 + Σ

∆E (eV) HCCF HCCCl 6.326 5.559 7.265 6.824 7.544 7.317 7.708 6.895 8.004 7.162 11.187 9.835

HCCBr 5.492 5.681 6.144 6.804 7.048 9.208

The MRCI+Q/aVQZ ∆E are calculated at the equilibrium geometry of the electronic ground state of the linear HCCX.

Table 8: Dipole moments, µ (a.u.) of the HCCX linear molecules. a

this work HCCF 0.289 HCCCl 0.179 HCCBr 0.091

µ(a.u.) MP2b CCSD(T)c 0.256 0.286 0.161 0.187 0.123 0.129

exp 0.283 d 0.173 e 0.090 f

All calculated dipole moments have been obtained at equilibrium geometry of the linear molecules, using the Finite-Field approach. (a) MRCI+Q/aVQZ; 13 (b) ref ; (c) ref 14 ; (d) Microwave spectroscopy 41 ; (e) Radio frequence spectroscopy 42 ; (f) Vibrational spectroscopy 43


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Figure 5: Abstract Figure


Energetic Diagrams and Structural Properties of Monohaloacetylenes

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