216 J. CHEM. RESEARCH (S), 1997 J. Chem. Research (S), 1997, 216–217† Quantum Chemical Study of the Hydrogen-bonded C4H2...HCl Complex† Asit Kumar Chandra* and Minh Tho Nguyen Laboratory of Quantum Chemistry, Department of Chemistry, University of Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium Ab initio molecular orbital calculations at the MP2 full level and density functional calculations using 6-31++G(d,p) basis functions are performed on the C4H2 ...HCl system and it is observed that formation of a weak complex is possible with a p...HCl type hydrogen bond between the C4H2 and HCl molecules.The traditional view of H-bonding interactions has been expanded to include many weak interactions.1–3 Significant among these is the interaction of hydrogen halides with systems having p-electrons (e.g. C2H2 ...HX). The XH...p interaction is also important in the context of crystal engineering and molecular recognition.4,5 The p-electron rich system behaves as an electron donor in these complexes. Theoretical studies play an important role because it is often not possible to determine precisely the minimum energy structure from the experiment alone, particularly when there are many possible forms of the complexes with comparable energies.The purpose of the present communication is to predict the structure and energetics of the hitherto unknown weak hydrogenbonded complex between C4H2 and HCl molecules. To the best of our knowledge, there is no reported experimental or theoretical study on this system and thus we feel the present calculation will be useful for providing basic information on this complex.Method The geometries of the molecules C4H2 and HCl and of the complexes were fully optimised at the MP2=full level and also with density functional theory (DFT) calculations using B3LYP6,7 and B3PW918 exchange-correlation (XC) functionals. 6-31++G(d,p) basis set was used for all the calculations.The DFT calculations with B3LYP and B3PW91 XC functionals are henceforth simply referred to as B3LYP and B3PW91. The Gaussian-94 program package9 was used for all the calculations involving DFT whereas the HONDO program package was used for the MP2 calculations. Harmonic vibrational frequencies are calculated at the MP2 level and DFT with B3LYMP. Results and Discussion We have already mentioned that to the best of our knowledge there is no reported experimental evidence for the complex between C4H2 and HCl.Recently, Chandra et al.10 reported a theoretical study on the C4H2 ...HF complex at the MP2 level of theory.10 It was observed from the theoretical calculations that the most stable complex between diacetylene and HF forms when the H-atom of HF interacts with the p-electron cloud of diacetylene (Lp complex). A weak sigma complex is another possibility in which the acidic hydrogen atom of the diacetylene forms a hydrogen bond with the fluorine atom of HF.It was not possible, however, to separate these two possible forms from the experimental results.11 In the case of HCl, we observed that the sigma complex is very weak and the interaction energy is only 0.1 kcal at the MP2 level. The p complex (Fig. 1) was found to be the most stable complex between C4H2 and HCl and optimisations were carried out both at the MP2 level and at the B3LYP and B3PW91 levels for this complex. The results obtained are summarised in Tables 1 and 2.It is evident from Table 1 that the monomer geometries do not change significantly upon complex formation. This indicates that the stability and structure of the complex are determined predominantly by longrange electrostatic interactions. The importance of electrostatic interactions for determining the structure of the H-bonded complex was emphasized earlier by Buckingham and Fowler.12 However, small increments in the C�C (at which H-bonding takes place) and H·Cl bond lengths have been observed in the complex. Although individual bond lengths are different at MP2 and DFT the increments observed are nearly the same.The hydrogen bond length (Rh, the distance between the centre of the C�C bond and the hydrogen atom of HCl) increases and the dissociation energy decreases when going from MP2 to B3LYP and B3PW91 levels. The geometrical parameters obtained from B3LYP and B3PW91 are almost identical but the dissociation energy decreases from B3LYP to B3PW91.It should be pointed out that methods using finite basis expansions suffer from the basis set superposition error (BSSE). However, in view of the quality of the basis set used in the present calculations, we did not perform any BSSE corrections. Moreover, the BSSE correction by the counterpoise method has been questioned many times13,14 and it has been argued that it overcorrects the BSSE. The hydrogen bond length Rh obtained for the C4H2 ...HCl complex is larger when compared to the corresponding HF complex and/or the HCl complex with acetylene. 10,15 For example, the hydrogen bond lengths obtained *To receive any correspondence. †This is a Short Paper as defined in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1997, Issue 1]; there is therefore no corresponding material in J. Chem. Research (M). Fig. 1 Schematic diagram of the diacetylene complexes with HCl Table 1 The optimised geometrical parameters of the C4H2–HCl complex.Values in parentheses correspond to isolated molecules. R/Å MP2 B3LYP B3PW91 H·Cl C·C C·H C�C C�Ca Rh 1.274 (1.269) 1.373 (1.372) 1.064 (1.063) 1.226 (1.225) 1.228 (1.225) 2.439 1.294 (1.287) 1.369 (1.369) 1.067 (1.066) 1.214 (1.214) 1.216 (1.214) 2.468 1.292 (1.283) 1.367 (1.367) 1.068 (1.066) 1.214 (1.214) 1.216 (1.214) 2.481 aC�C bond involved in the hydrogen bonding. Table 2 Total energies (a.u.) at the stationary points, binding energies and dipole moments of the C4H2–HCl complex.Values in parentheses indicate the binding energies after zero point energy correction Method C4H2 HCl C4H2–HCl DE/kcal molµ1 m/Da MP2 B3LYP B3PW91 µ153.03773 µ153.49587 µ153.42427 µ460.21853 µ460.80328 µ460.74973 µ613.25978 µ614.30212 µ614.17662 2.20 (1.31) 1.87 (1.05) 1.65 (0.83) 2.02 2.11 2.16 aDipole moments of HCl obtained from the MP2, B3LYP and B3PW91 calculations are 1.45, 1.46 and 1.49 D respectively.J. CHEM. RESEARCH (S), 1997 217 at the MP2 level are 2.22 and 2.43 Å for the HF and HCl complexes of C4H2 respectively, which is, of course, expected from the strength of the hydrogen bond.The rotational constants calculated from the B3LYP optimised geometry are 5920.17, 1303.02 and 1067.96 MHz. The dipole moments of the complex obtained from MP2, B3LYP and B3PW91 calculations are 2.02, 2.11 and 2.16 D, respectively. Considering the dipole moments of the isolated HCl molecule, it is clear that complex formation between C4H2 and HCl introduces a significant amount of induced dipole moment in the system.The magnitude of the induced dipole moment is nearly the same as that in the C2H2 ...HCl complex.15 A low frequency shift, compared to the isolated molecule values, of the intramolecular hydrogen chloride stretching vibration was observed from the MP2 and DFT calculations (see Table 3). The magnitude of the shifts are 69 and 99 cmµ1 at the MP2 and B3LYP levels, respectively. The p...HCl hydrogen bond is weaker in the diacetylene complex compared to the corresponding acetylene complex and thus a smaller frequency shift of HCl in the diacetylene complex is expected. The same trend was observed from the present theoretical calculations.The HCl frequency shifts observed at the MP2 level for the C2H2 ...HCl and C4H2 ...HCl are 73 and 69 cmµ1, respectively and with B3LYP 126 and 99 cmµ1, respectively. Bearing in mind the strength of the hydrogen bond, it seems that the B3LYP level estimates vibrational frequencies more accurately than the MP2 level.Recently Geerlings and co-workers also made the same observation.16 The intermolecular vibrational frequencies for the C4H2 ...HCl complex are given in Table 4. Intermolecular stretching vibrational frequencies obtained from the B3LYP calfor the C4H2 and C2H2 complexes with HCl are 80 and 93 cmµ1, respectively, which is, of course, expected from the strength of the hydrogen bond of the two complexes.Fig. 2 shows the intermolecular potential curves for the C4H2 ...HCl complex. The variation of the dipole moments of the C4H2 ...HCl complex and isolated HCl with the change in HCl bond length are presented in Fig. 3 which shows that the dipole moment changes more rapidly for the complex. It is, therefore, expected that the intensity of the H·Cl stretching vibration will be increased upon complex formation with diacetylene. Conclusion It has been observed from MP2 and DFT calculations with B3LYP and B3PW91 XC functionals that the formation of a weak molecular complex is possible between diacetylene and HCl with a p...HCl type hydrogen bond.The hydrogen bond lengths obtained from MP2, B3LYP and B3PW91 are 2.439, 2.468 and 2.481 Å, respectively. The binding energy of the C4H2 ...HCl complex should be around 1 kcal molµ1 and the intensity of the H·Cl stretching vibration should increase upon complex formation. The authors are grateful to the Fund for Scientific Research (F.W. O.-Vlaanderen) for financial support. Received, 23rd December 1996; Accepted, 24th February 1997; Paper E/6/08549K References 1 A. C. Legon and D. J. Millen, Chem. Rev., 1986, 86, 635. 2 P. Hobza and R. Zahradnik, Chem. Rev., 1988, 88, 871. 3 J. J. Dannenberg and R. Rios, J. Phys. Chem., 1994, 98, 6714. 4 G. A. Jeffrey and W. Saenger, Hydrogen Bonding in Biological Structures, Springer, Berlin, 1991. 5 G. R. Desiraju, Crystal Engineering: The Design of Organic Solids, Elsevier, Amsterdam, 1989. 6 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785. 7 A. D. Becke, J. Chem. Phys., 1993, 98, 5648. 8 J. P. Perdew and Y. Wang, Phys. Rev. B, 1992, 45, 13244. 9 Gaussian 94, Revision C.3, M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J.B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C. Gonzalez and J. A. Pople, Gaussian Inc., Pittsburgh, PA, 1995. 10 A. K. Chandra, S. Pal, A. C. Limaye and S. R. Gadre, Chem. Phys. Lett., 1995, 247, 95. 11 K. O. Patten and L. Andrews, J. Phys. Chem., 1986, 90, 3910. 12 A. D. Buckingham and P. W. Fowler, Can. J. Chem., 1985, 63, 2018. 13 M. J. Frisch, J. E. Del Bene, J. S. Binkley and H. F. Schaeffer III, J. Chem. Phys., 1986, 84, 2279. 14 D. W. Schwenke and D. G. Truhlar, J. Chem. Phys., 1985, 82, 2418. 15 A. K. Chandra and M. T. Nguyen, unpublished results. 16 F. D. Proft, J. M. L. Martin and P. Geerlings, Chem. Phys. Lett., 1996, 250, 393. 17 S. A. McDonald, G. L. Johnson, B. W. Keelan and L. Andrews, J. Am. Chem. Soc., 1980, 102, 2892. Fig. 2 Intermolecular potential curves for the C4H2 ...HCl complex calculated at MP2 level with 6-31++G(d,p) basis set Fig. 3 Variation of dipole moment with the change in H·Cl bond length from the equilibrium value, in (I) the C4H2 ...HCl complex and (II) isolated HCl Table 3 Hydrogen halide vibrational frequencies and low frequency complex shifts (v/cmµ1) MP2 B3LYP Expt.a Molecule HCl 3120 2949 2888 C4H2–HCl 3051 2850 Shift 69 99 aRef.17. Table 4 Intermolecular harmonic frequencies for C4H2–HCl (cmµ1) Method Stretch In-plane Out-of-plane MP2 B3LYP 101 80 28 31 293 298 234 244